molecules

Review
Nanotechnology–General Aspects: A Chemical Reduction
Approach to the Synthesis of Nanoparticles
Paulina Szczyglewska * , Agnieszka Feliczak-Guzik * and Izabela Nowak

Faculty of Chemistry, Adam Mickiewicz University in Poznań, Uniwersytetu Poznańskiego 8,

61-614 Poznań, Poland; nowakiza@amu.edu.pl
* Correspondence: paulina.debek@amu.edu.pl (P.S.); agaguzik@amu.edu.pl (A.F.-G.); Tel.: +48-61-829-1748 (P.S.);
+48-61-829-1747 (A.F.-G.)

Abstract: The role of nanotechnology is increasingly important in our society. Through it, scientists
are acquiring the ability to understand the structure and properties of materials and manipulate them
at the scale of atoms and molecules. Nanomaterials are at the forefront of the rapidly growing field of
nanotechnology. The synthesis of nanostructured materials, especially metallic nanoparticles, has
attracted tremendous interest over the past decade due to their unique properties, making these
materials excellent and indispensable in many areas of human activity. These special properties
can be attributed to the small size and large specific surface area of nanoparticles, which are very
different from those of bulk materials. Nanoparticles of different sizes and shapes are needed for
many applications, so a variety of protocols are required to produce monodisperse nanoparticles
with controlled morphology. The purpose of this review is firstly to introduce the reader to the
basic aspects related to the field of nanotechnology and, secondly, to discuss metallic nanoparticles
in greater detail. This article explains the basic concepts of nanotechnology, introduces methods
for synthesizing nanoparticles, and describes their types, properties, and possible applications. Of
many methods proposed for the synthesis of metal nanoparticles, a chemical reduction is usually
preferred because it is easy to perform, cost-effective, efficient, and also allows control of the structural
parameters through optimization of the synthesis conditions. Therefore, a chemical reduction method
is discussed in more detail—each factor needed for the synthesis of nanoparticles by chemical
Citation: Szczyglewska, P.; reduction is described in detail, i.e., metal precursors, solvents, reducing agents, and stabilizers. The
Feliczak-Guzik, A.; Nowak, I. methods that are used to characterize nanomaterials are described. Finally, based on the available
Nanotechnology–General Aspects: A
literature collection, it is shown how changing the synthesis parameters/methods affects the final
Chemical Reduction Approach to the
characteristics of nanoparticles.
Synthesis of Nanoparticles. Molecules
2023, 28, 4932. https://doi.org/
Keywords: nanotechnology; metal nanoparticles; chemical reduction
10.3390/molecules28134932

Academic Editors: Yunchao Xie,

Chi Zhang and Hongxing Dong

Received: 31 May 2023

1. Basic Aspects of Nanotechnology
Revised: 15 June 2023 1.1. The Era of Nanomaterials
Accepted: 20 June 2023 As known from history, different periods in the history of civilization have their
Published: 22 June 2023 specific names; for instance, the Stone Age refers to the period from the appearance of the
first stone tools used by man, the Bronze Age refers to when tools made from copper-tin
alloy were used, and the Iron Age was when iron became the main raw material for making
tools [1]. Human dreams combined with imagination often give birth to new sciences—
Copyright: © 2023 by the authors.
which is how nanotechnology was born. Since nanotechnology is unquestionably one of
Licensee MDPI, Basel, Switzerland.
the top buzzwords of the new millennium and has so far significantly raised the standard
This article is an open access article
distributed under the terms and
of living, this period might be called the era of nanomaterials. Some researchers even
conditions of the Creative Commons
go so far as to say that the impact of nanotechnology will be so great that the term will
Attribution (CC BY) license (https://
be used to describe a new era of global economic growth [2]. It is predicted that the
creativecommons.org/licenses/by/ level of nanotechnology will determine a country’s position in the global economy. Thus,
4.0/).

Molecules 2023, 28, 4932. https://doi.org/10.3390/molecules28134932 https://www.mdpi.com/journal/molecules

 Molecules 2023, 28, x FOR PEER REVIEW 2 of 39
Molecules 2023, 28, 4932 2 of 38

country’s position in the global economy. Thus, the products of nanotechnology are
the products
expected of nanotechnology
to make are expected
a significant contribution to make a significant
to the problem-solving process contribution
through the to the
problem-solving process
use of smaller materials andthrough
systemsthe[3]. use of smaller materials and systems [3].
The topics
The topicsrelated
relatedtotonanotechnology
nanotechnology havehave
recently attracted
recently a greatadeal
attracted greatof deal
attention
of attention
from scientists
from scientists around
aroundthe theworld.
world.Over
Overthe the
pastpast
20 years, the number
20 years, of literature
the number items items
of literature
related to
related to this
thissubject
subjecthashas experienced
experienced tremendous
tremendous growth, as shown
growth, in the chart
as shown in thebelow
chart below
(Figure 1). The number of scientific papers with the keyword “nano”
(Figure 1). The number of scientific papers with the keyword “nano” is increasing is increasing over
over time
time in an approximately linear fashion, and it is predicted that in 2025 there will be more
in an approximately linear fashion, and it is predicted that in 2025 there will be more than
than 30,000 of them. This article provides an overview of the basic aspects of
30,000 of them. This article provides an overview of the basic aspects of nanotechnology,
nanotechnology, introduces the broad topic of transition metal nanoparticles, discussing
introduces the and
their properties broad topic of
methods of synthesis
transition andmetal nanoparticles,
characterization. discussing
The next their properties
section develops
and methods of synthesis and characterization. The next section
the topic of the synthesis of transition metal nanoparticles by chemical reduction develops the and
topic of the
synthesis
provides an extensive review of relevant literature. The last section also includes extensive
of transition metal nanoparticles by chemical reduction and provides an a
review of relevant
discussion literature.
of the effects The factors
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finalincludes a discussion
nanoparticles producedofby thetheeffects of
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of chemical final nanoparticles produced by the method of chemical reduction.
reduction.

35000
35,000

30000
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25,000

20000
20,000

15,000
15000
10,000
10000
5000
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00
2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022 2024
Year

Figure 1. Number of publications in the field of nanotechnology according to the Web of Science
Figure 1. Number of publications in the field of nanotechnology according to the Web of Science
(literature
(literature items thatinclude
items that include the
the prefix
prefix “nano”
“nano” in topic);
in the the topic); data
data as asMay
of 20 of 202023.
May 2023.

1.2.
1.2. What Is Nanotechnology?
What Is Nanotechnology?
The answertotothis
The answer thisquestion
question is not
is not straightforward.
straightforward. It istobest
It is best starttothe
start the explanation
explanation
with the prefix “nano”, which comes from the Greek word “nanos”,
with the prefix “nano”, which comes from the Greek word “nanos”, which translates which translates
into into
“dwarf”. Therefore,nanotechnology
“dwarf”. Therefore, nanotechnology cancan simply
simply meanmean technology
technology relatedrelated
to smallto small things.
things.
However,
However, thetheprefix
prefix“nano”
“nano” also
also hashas another
another meaning—in
meaning—in scientificscientific
language, language,
it means 1it means
1billionth
billionthofof
a meter,
a meter,referred to as
referred toaas“nanometer” [2–5].[2–5].
a “nanometer” GivenGiven
this, nanotechnology
this, nanotechnologycan be can be
viewed as being related to technologies operating at the nanometer level. In 1994,the
viewed as being related to technologies operating at the nanometer level. In 1994, the Royal
Royal Society/Royal
Society/Royal Academy Academy of Engineering
of Engineering Working
Working Group Group adopted
adopted the following
the following definitions
todefinitions to help distinguish
help distinguish between nanoscience
between nanoscience and nanotechnology,
and nanotechnology, namely namely
[3,6]:[3,6]:
• nanoscience—is the study of structures and molecules at the atomic, molecular, and
• nanoscience—is the study of structures and molecules at the atomic, molecular, and
macromolecular scales, whose properties are significantly different from those
macromolecular scales, whose properties are significantly different from those occur-
occurring on a larger scale,
• ring on a larger scale,
nanotechnology—is technology that uses nanoscience in practical applications, such
• nanotechnology—is
as a variety of devicestechnology
or systems, that uses nanoscience
by controlling shape andinsize
practical
at the applications,
nanometer such
as a variety
scale. of devices or systems, by controlling shape and size at the nanometer
scale.
Nanotechnology is thus an interdisciplinary field that integrates science and technol-
ogy [1,7]. However, it is a very broad discipline, and putting it into a single definition can
be misleading. Moreover, there is no limit to the areas of research or end products that will
fall under the definition of nanotechnology. So it is worth looking at the basic concepts in
the field of nanotechnology as cited below [8]:
• nanoscale—a scale having one or more dimensions of the order of 100 nm or less,
Molecules 2023, 28, 4932 3 of 38

• nanomaterial—a material with one or more external dimensions or internal structure

that can exhibit novel properties compared to the same material without nanoscale
features,
• nanoparticle—a particle with at least one dimension at the nanoscale,
• nanocomposite—a composite in which at least one of the phases has at least one
dimension at the nanoscale,
• nanostructured materials—having a structure at the nanoscale.
The National Nanotechnology Initiative has clarified the definition of nanotechnology
and defined nanotechnology as a field that includes [7,9]:
• research and development of technologies in the 1–100 nm range,
• creation of small structures with novel properties,
• controlling and manipulating structures at the atomic scale.
Nanotechnology, therefore, refers to materials and systems whose structures and com-
ponents exhibit new and significantly changed physical, chemical, and biological properties
due to their size at the nanoscale. Materials made of structures of sizes in the range of
about 10−9 –10−7 m (1–100 nm) show important changes in the characteristics compared to
those of isolated molecules (1 nm) or bulk materials [10,11]. Although nanotechnology is a
new word, it is not an entirely new field. Nature has created many objects and processes
that operate on the micro to nano scale. The new behavior of nanoparticles at the nanoscale
is not necessarily predictable from that observed at large scales. Important changes in
behavior are not only caused by order of magnitude reduction but also by new phenomena
such as size limitation, the prevalence of interfacial phenomena, quantum mechanics, and
Coulomb blocking [10]. It is worth going back a few decades to realize how this amazing
field of science, now called nanotechnology, has developed.

1.3. Short History of Nanotechnology

The development of nanoscience can be traced as far back as the ancient Greeks in the
fifth century BC, when they pondered whether the matter was continuous and, therefore,
divided into smaller parts or whether it consisted of small, indivisible particles, which
are referred to as atoms [3]. Nevertheless, the term “nanotechnology” was first used by
Japanese scientist Norio Taniguchi, who delivered a lecture in 1974 describing how the
dimensional accuracy with which things are created has improved over time (1940s–1970s).
The scientist correctly predicted that by the late 1980s, techniques would evolve to the point
where dimensional accuracies greater than 100 nm would become possible. He advocated
that nanotechnology involved the processing, separation, consolidation, and deformation
of materials by a single atom or molecule [3,12].
On the other hand, even earlier, in 1959, physicist Richard Feynman (winner of the
1965 Nobel Prize in Physics) gave his famous lecture on “There is plenty of room at
the bottom” at a meeting of the American Physical Society, outlining the prospects for
atomic engineering concerning structures at a level of single atoms or molecules. Although
what he talked about was not explicitly called “nanotechnology,” the ideas he presented
were prophetic indeed, and this area is what we call “nanotechnology” today. Among
other things, Professor Feynman predicted the development of techniques that could
be used to create large integrated circuits and the revolutionary effects that the use of
these circuits would produce. For these reasons, he is considered the father of modern
nanotechnology [13].
The golden era of nanotechnology did not begin until the 1980s, when the miniatur-
ization of instruments using microfabrication technology began [10]. The greatest fruit of
miniaturization and, at the same time, a very big step in the development of nanotechnol-
ogy as a scientific field was the construction of two types of microscopes: the Scanning
Tunneling Microscope in 1982 (Gerd Binnig and Heinrich Rohrer) [14,15] and the Atomic
Force Microscope in 1986 (Gerd Binnig, Calvin F. Quate, Christoph Gerber) [16,17], which
enabled researchers to see and manipulate atoms for the first time. To this day, many oppor-
tunities to study nanostructures have arisen, thereby generating excitement in the scientific
Molecules 2023, 28, 4932 4 of 38

community. Scientists from almost all disciplines are eagerly pursuing the production and
measurement of nanostructures to see what new interesting phenomena are happening on
this scale [10]. In everyday life, although we do not think about it, we may perceive the
huge impact of very small-scale processes on our world.
At the beginning of the 21st century, interest in the new fields of nanoscience and
nanotechnology increased. In the United States, for example, Feynman’s attitude and his
concept of manipulating matter at the atomic level played an important role in shaping
national scientific priorities. President Bill Clinton advocated funding research into this
emerging technology in a speech at Caltech in 2000. Three years later, President George W.
Bush signed into law the 21st Century Nanotechnology Research and Development Act.
The legislation made nanotechnology research a national priority and created the National
Technology Initiative [13].

1.4. Application of Nanotechnology Products

It is not an exaggeration to say that nanotechnology can be a threat to many industries
due to the fact that it often offers smaller, cheaper, and faster devices with superior func-
tionality while using less raw materials and energy [1,18]. It has the potential to affect the
production of virtually every man-made object, from automobiles, electronics, advanced
diagnostics, and surgery, to advanced medicines and tissue/bone substitutes. Today’s
nanotechnology takes advantage of current advances in chemistry, physics, materials sci-
ence, biotechnology, and electronics to create new materials that have unique properties.
Some of the applications of nanotechnology have already made their mark on the market,
while others are being intensively researched for solutions to humanity’s biggest problems:
diseases, clean energy, clean water, etc. The products of advanced nanotechnology that will
be available in the coming decades promise even more revolutionary applications than the
products of current and near-future nanotechnology [12]. The results of exciting projects
are being published at a steady pace in the form of scientific papers, review articles as well
as patents, but it is impossible to list or discuss all the areas of activity that would involve
the use of nanotechnology products. Nevertheless, the remainder of this literature review
describes in very general terms selected areas of application of nanotechnology products.
In addition, Figure 2 collects the applications most frequently mentioned in the literature
Molecules 2023, 28, x FOR PEER REVIEW 5 of 39
without distinguishing between those already on the market and those in the research
phase.

Figure2.
Figure 2. Potential
Potential applications
applications of
ofnanotechnology
nanotechnologyproducts.
products.

1.4.1. Energy and Environment

The uncertain future of traditional energy sources and increasingly new
environmental policies have contributed to a global movement to develop and implement
alternative methods of energy extraction and efficient energy use. Nowadays,
Molecules 2023, 28, 4932 5 of 38

1.4.1. Energy and Environment

The uncertain future of traditional energy sources and increasingly new environmental
policies have contributed to a global movement to develop and implement alternative
methods of energy extraction and efficient energy use. Nowadays, nanotechnological
inventions are penetrating into various subsystems of the entire energy system, starting
from energy acquisition, conversion, distribution, and storage and ending with energy
utilization [19,20]. The application of nanotechnology in the energy field, which includes
lithium-ion batteries, fuel cells, light-emitting diodes, ultracapacitors, and solar cells, among
others, is a hot topic in many scientific studies. Unfortunately, the development of this
field of nanotechnology has been hampered by significant production costs compared
to that of conventional technologies [14]. Today, thanks to nanomaterials, photovoltaic
cells are increasing their efficiency while reducing the cost of electricity production at an
unprecedented rate. The production, storage, and conversion of hydrogen into electricity
in fuel cells benefit from more efficient water-splitting catalysts, better-nanostructured
materials for higher hydrogen adsorption capacity, and cheaper yet simpler fuel cells [19].

1.4.2. Electronics
Miniaturization of electronic devices is considered a key factor in achieving higher
efficiency and speed of information exchange. This achievement is now available mainly
due to great advances in nanolithography techniques. Thanks to the nanotechnology-based
approach, the performance of traditional semiconductors has increased, and new possibili-
ties have emerged [19]. Creating more efficient computer processors is another advantage
of nanoelectronics, which can improve the capabilities of electronic components in several
ways: (i) by improving the screens displayed on electronic devices, which includes reduc-
ing power consumption while reducing the weight and thickness of the screens, (ii) by
increasing the density of memory chips, and (iii) by reducing the size of transistors used
in integrated circuits [21]. Products of nanotechnology are also used in supercapacitors—
which are a type of electrochemical capacitors and have been recognized as some of the
most reliable and efficient energy storage and conversion devices. For example, nanowires
are being produced and used as electrode material for supercapacitors [22,23].

1.4.3. Agri-Food Production

Nanotechnology has been recognized as a valuable tool in modern agriculture, as
well as in all areas of the food industry, which include: improved food quality and safety,
reduced agricultural inputs, enriched nutrient absorption from the soil, pathogen detection,
food processing, and packaging, delivery of bioactive compounds to destinations, and
production of nanotechnology-based food additives. New approaches based on nanotech-
nology lead to increased safety and nutritional value of food products, reduced chemical
spreading, minimized nutrient losses in fertilization, and increased yields through pest
control [24]. In addition, new agrochemicals and new delivery mechanisms have been
developed based on nanotechnological devices to improve crop yields and reduce pesticide
use [19,24,25].

1.4.4. Cosmetics
Applications of nanotechnology and nanomaterials can be found in many cosmetic
products, including moisturizers, hair care products, makeup products, and sunscreens
[26,27]. Novel nanocarriers with various formulations, such as liposomes, niosomes, na-
noemulsions, microemulsions, solid lipid nanoparticles, nanostructured lipid carriers, and
nanospheres, have replaced the use of conventional delivery systems. These novel nanocar-
riers have the advantages of increased skin penetration (vitamins and other antioxidants),
controlled and prolonged drug release, greater stability (unsaturated fatty acids, vitamins,
antioxidants encapsulated in nanoparticles), site targeting, high entrapment efficiency, and
improved aesthetics of the final product (e.g., in mineral sunscreens, making the parti-
cles of the active mineral smaller; they can be applied without leaving a noticeable white
Molecules 2023, 28, 4932 6 of 38

film) [19,25]. However, it should be mentioned that nanotoxicological studies have shown
concern about the impact of the increased use of nanoparticles in cosmetics, as there is the
potential for them to penetrate the skin and cause health risks [28,29], which is described in
more detail later in this article.

1.4.5. Medicine
Since several components of living cells are nanoscale in size, it was foreseeable that
nanotechnology would be useful in biology and medicine. Today, some nanoscale materials
have found clinical applications, and the field of nanomedicine has emerged as a result. As
one of the main parts of nanotechnology, nanomedicine aims to provide more efficient tools
for preventing and treating various diseases through the interaction of nanomaterials with
biological molecules. Strategies based on nanomedicine have opened up new horizons
for biomedical engineers as well as clinicians in the prevention, diagnosis, and treatment
of serious diseases. With nanotechnology applied to medicine, significant improvements
have been witnessed in drug delivery systems, protein detection, cancer treatment, medical
imaging and diagnostic platforms, implantable materials, and tissue regeneration strategies,
among others [19,30–32]. It should be mentioned that various nanosystems have already
been used for the benefit of human health, and a larger number of medical projects are
underway.

1.4.6. Military and Security

Looking at nanotechnology in the light of national security, it is the latest means by
which military weapons can be reduced in size and weight while maintaining the same per-
formance or even increasing it. The potential use of nanotechnology in military applications
encompasses almost every aspect, from civilian applications such as ultralight clothing and
footwear for the individual warrior to advanced information-processing electronics that
can be embedded in a variety of materials [33]. It is certain that nanotechnology will also
be used in the military field to design new weapons with unique properties, such as small
self-steering mini-robot missiles and the creation of devices that can collect water in all
conditions. With all this, it is important to remember that nanotechnological products have
a powerful impact on the world, but if they are used for the wrong purposes, the damage
could be irreversible, for dealing with nanotechnology is about testing our humanity, ethics,
and knowledge [34].

1.5. Risk Associated with Nanotechnology

In addition to the many advantages they offer, it is clear that nanotechnological
products are not free of drawbacks. The described field of science involves many, often the
same problems as in the introduction of any new technology. Manipulation of matter at the
nanoscale often adversely affects the environment and human life [1,8,21]. In their work,
many scientists point out the risks, toxicity, and many other hazards that are associated
with nanotechnology.
Every person is exposed to nanometer-sized particles; we breathe them in with every
breath and ingest them with every drink/meal. In fact, every organism on Earth con-
stantly encounters nanometer-sized entities. The vast majority cause little ill effect and
go unnoticed, but sometimes nanometer-sized materials can cause significant harm to
the body. The interactions between nanomaterials and cells, animals, humans, and the
environment are complex, and much research needs to be done to understand in detail
how the physical, chemical, and other properties of nanomaterials affect these interactions,
and thus determine the ultimate impact of nanomaterials on health and the environment.
Research in recent years has confirmed that nanoscale materials exhibit unexpected toxicity.
Nanoparticles are more likely to react with cells and various biological components, such
as proteins, and travel through organisms, increasing their chances of interacting to trigger
inflammatory and immune responses [35,36].
Molecules 2023, 28, 4932 7 of 38

Thus, it is safe to say that today nanotechnology affects human life every day. The
potential benefits are many and varied; however, due to the high human exposure to
nanoparticles, there is considerable concern about potential health and environmental risks.
This concern has even led to the emergence of additional scientific disciplines, including
nanotoxicology and nanomedicine. Nanotoxicology involves the study of the potential
negative health effects of nanoparticles. Nanomedicine, which includes sub-disciplines
such as tissue engineering, biomaterials, biosensors, and bioimaging, has been developed
to study the benefits and risks of nanomaterials used in medicine and medical devices.
Some of the potential medical benefits of nanomaterials include improved drug delivery,
production of antimicrobial coatings for medical devices, reduced inflammation, improved
healing of surgical tissues, and detection of circulating cancer cells. However, due to
the lack of reliable toxicity data, the potential impact on human health remains a major
concern [13].
As a result, new laws and regulations are emerging all the time regarding the risks of
nanoproducts. In Europe, the Registration, Evaluation, and Authorization of Chemicals
legislation has been in effect since 2007. New European cosmetics regulations require
products containing nanomaterials to be listed on the product labels of finished products.
In the United States, on the other hand, the U.S. Food and Drug Administration has issued
draft guidelines to help manufacturers determine whether their products use nanomaterials,
while the Environmental Protection Agency has issued draft guidelines for nanomaterials
use [35].

2. Nanoparticles
2.1. Unique Features of Nanoparticles
The definition of nanoparticles can vary for different fields and different materials.
From the theoretical point of view, they are often called nanoclusters or simply clusters,
which are defined as a combination of millions of atoms or molecules, which can be of
the same or different types. Nanoparticles can be amorphous or crystalline, and their
surfaces can act as carriers. NPs exhibit properties between those of bulk material and
atomic or molecular structures. They should be considered a distinct state of matter, such
as crystalline forms of nanoparticles (fullerenes and carbon nanotubes) and traditional
crystalline solid forms (graphite and diamond) [37]. Nanoparticles are everywhere and
of great scientific interest. A bulk material has constant physical properties regardless of
its size, but at the nanoscale, this is often not the case. It has been proven many times
that several well-characterized bulk materials have the most interesting properties when
tested at the nanoscale [38]. Nanotechnology thus produces products with completely
new properties, often superior to those of the starting material. Nanoparticles have a high
percentage of atoms on the surface, and this is the main feature that differentiates their
properties from those of bulk material [35,39]. The most important features of NPs that
make them so attractive in materials chemistry and chemical engineering are generally
related to the following properties [4,5,40–42]:
• a high volume-to-surface ratio, one result of which is the high reactivity of nanometer-
sized materials, in which interactions between molecules may easily occur,
• the presence of the surface plasmon resonance effect (this aspect is generally important
in optical applications),
• different physical properties with respect to the starting metal; surface energy and
melting point are particularly sensitive to nanoparticle size,
• a large number of low-coordination sites on the surface relative to that in the starting
material, with remarkable effects on chemical reactivity and catalytic properties,
• easy surface functionalization, which makes them very attractive, especially in
nanomedicine for selective drug transport in target organs and tissues.
Molecules 2023, 28, 4932 8 of 38

2.2. Types of Nanoparticles

Various classification systems have been created to organize nanometer-sized materi-
als. Nanoparticles can be classified based on their properties, such as shape, size, activity,
or the type of materials they are made of, among others [19,32]. The following is a clas-
sification of nanoparticles based on the materials from which they are made. Using this
criterion, nanoparticles are divided into inorganic (carbon-based, metal and metal oxide,
semiconductor, ceramic) and organic (polymeric, derived from biomolecules) [43,44]. Thus,
nanomaterials can be classified into the following categories:

2.2.1. Inorganic Nanoparticles

• carbon-based nanoparticles
Nanoparticles of this type are composed entirely of carbon, taking the form of a hollow
ellipsoid or tube [10]. This class of materials includes single- and multiwalled carbon
nanotubes, graphene, fullerenes, nanofibers, fluorescent carbon quantum dots, and carbon
dots. The aforementioned materials are widely used in many scientific fields due to their
unique physical, chemical, mechanical, and thermal properties. They are characterized by
large surface area, good biocompatibility, low toxicity, and low manufacturing costs, and
they may be obtained by using greener synthesis routes [44–46].
• metal and metal oxide nanoparticles
Metal nanoparticles, as the name implies, consist solely of metal precursors (the metal
atom determines the properties of these nanoparticles), while metal oxide nanoparticles
consist of a metal precursor combined with oxygen [37]. The materials in question have
a unique and wide range of physicochemical properties. They have improved chemical,
electrical, optical, thermal, mechanical, electromagnetic, and surface properties compared
to bulk materials. In addition, they offer large surface areas, controlled size and morphology,
and simple surface modification [44,45]. For these and many other reasons, they have found
applications in such areas as biomedicine, catalysis, and energy harvesting (more on the
applications of metal nanoparticles is in the following subsections). Metals commonly used
to synthesize such nanoparticles include gold (Au), silver (Ag), platinum (Pt), palladium
(Pd), copper (Cu), iron (Fe), lead (Pb), and zinc (Zn) [10,44].
• semiconducting nanoparticles and quantum dots
Semiconducting nanostructures exhibit both metallic and non-metallic properties and
are characterized by the occurrence of the quantum confinement (quantum confinement)
effect, most often when the particle size is smaller than 10 nm [44,47]. The semiconduct-
ing nanostructures in which the aforementioned phenomenon occurs are called quantum
wells, where the confinement occurs in one dimension, while the nanostructures that are
confined in two dimensions are called quantum wires. In contrast, a third type of semicon-
ductor nanostructures that are constrained in three dimensions are quantum dots [46,48].
Semiconducting nanoparticles have found applications in biological research for labeling
DNA, cells, and proteins. They are an alternative to natural fluorophores, and their optical
properties are controlled by many factors, such as shape, size, doping, and the surrounding
environment [44,45].
• ceramic nanoparticles
Ceramic nanoparticles are inorganic solids synthesized by sintering and subsequent
cooling [45,46]. They mainly consist of oxides, carbides, phosphates, and carbonates of
metals and metalloids, such as calcium, titanium, silicon, etc. Most ceramic nanoparticles
are composed of silica or alumina. The porous nature of nanoparticles contributes to their
physical protection against degradation and degranulation. Nanophase ceramics can be
divided into nanoparticles, nanoshells, and nanoclay [44]. Their special properties, such
as high heat resistance, chemical inertness, remarkable mechanical strength, exceptional
pH resistance, high loading capacity, and ease of incorporation into hydrophobic and
hydrophilic systems, enable their use in various areas, such as catalysis, photocatalysis,
Molecules 2023, 28, 4932 9 of 38

and dye photodegradation. They are particularly widely studied in biomedical applica-
tions, especially in drug delivery, thanks to their controlled size, surface functionalization,
porosity, and surface area-to-volume ratio. However, their disadvantages include low
biodegradability, high density, and potential toxicity [44].

2.2.2. Organic Nanoparticles

• polymeric nanoparticles
Polymeric nanoparticles are solid colloidal particles of sizes in the range of 10 nm–1 µm.
They are usually made of biodegradable and biocompatible, naturally occurring poly-
mers [44,47]. As a result, they are very often used as drug carriers on whose surface the
former can be adsorbed physically or chemically. They are excellent carriers thanks to their
small size, water solubility, non-toxicity, long shelf life, and excellent stability. There are
two types of polymeric nanoparticles: nanospheres—nanoparticles in which the drug is
uniformly dispersed on the matrix, and nanocapsules—nanoparticles in which the drug
is embedded in a cavity and surrounded by a polymeric membrane. Natural polymers,
such as proteins or polysaccharides, as well as synthetic polymers, are often used for the
synthesis of polymeric nanoparticles. Such nanoparticles are extremely susceptible to
surface modification through chemical processes and thus have excellent pharmacokinetic
control. The most notable nanoparticles of this type are polymers made of polylactic acid,
gelatin, poly(lactic and glycolic acid) copolymer, and chitosan. Moreover, such polymers
can also be coated on the surface of other types of nanoparticles [44,45,49].
• biomolecule derived nanoparticles
Biomolecules such as proteins, nucleic acids, lipids, and polysaccharides have unique
characteristics and can be used to prepare nanoparticles. The biomolecule-derived nanopar-
ticles are increasingly in demand mainly because they are biocompatible and biodegradable.
In addition, they are readily available and non-immunogenic. In addition to their own
unique functions, bioparticles can conjugate with other inorganic nanoparticles to form
Molecules 2023, 28, x FOR PEER REVIEW 10 of 39
special hybrids.

3. Metallic Nanoparticles
3. Metallic
3.1. General Methods Nanoparticles
for Preparing Metallic Nanoparticles
3.1. General Methods for Preparing Metallic Nanoparticles
There are a number of approaches to fabricating nanostructures, which are broadly
Thereclasses,
divided into two main are a number
namelyof approaches
bottom-uptomethods
fabricating nanostructures,
and which areas
top-down methods, broadly
divided into two main classes, namely bottom-up methods and top-down methods, as
schematically presented in Figure 3 [10–12,38,40,48,50,51]. These approaches are further
schematically presented in Figure 3 [10–12,38,40,48,50,51]. These approaches are further
subdivided into different subclasses based on operation, response conditions, or other
subdivided into different subclasses based on operation, response conditions, or other
protocols adopted.
protocols adopted.

Figure
Figure 3. Schematic 3. Schematic representation
representation of nanostructure
of nanostructure fabrication based
fabrication methods, methods,
on based
[50]. on [50].

3.1.1. The Top-Down Approach

3.1.1. The Top-Down Approach
In the top-downIn the top-downthe
approach, approach, the corresponding
corresponding macroscopic macroscopic
materialmaterial is reduced
is reduced by by
physicalmeans.
physical or chemical or chemical means.
In this In this
method, method,
the the blocks
building building blocksoratoms
atoms or molecules
molecules are are
carefully
carefully deposited deposited
through through
controlled controlled
reactions, andreactions, and the self-organization
the self-organization that takes
that takes place
place at a later stage can lead to the formation of nanostructures.
at a later stage can lead to the formation of nanostructures. The main disadvantage of The main disadvantage
of the top-down approach is the final imperfection of the surface structure. Such defects
the top-down approach is the final imperfection of the surface structure. Such defects in
in surface structure can significantly affect the final physical properties and surface
chemistry of metallic nanoparticles due to their high aspect ratio [10,12,32,38,52,53].

3.1.2. The Bottom-Up Approach

The bottom-up approach refers to the construction of an atom-by-atom, molecule-by-
molecule, or cluster-by-cluster structure, in which the metal ion is brought to a zero-valent
Molecules 2023, 28, 4932 10 of 38

surface structure can significantly affect the final physical properties and surface chemistry
of metallic nanoparticles due to their high aspect ratio [10,12,32,38,52,53].

3.1.2. The Bottom-Up Approach

The bottom-up approach refers to the construction of an atom-by-atom, molecule-
by-molecule, or cluster-by-cluster structure, in which the metal ion is brought to a zero-
valent state, and the atoms aggregate further to form nanoparticles [32,40]. It is achieved
through processes of self-assembly of individual atoms into larger clusters and their fur-
ther aggregation into final nanoparticles. In this approach, nanostructured blocks are
initially formed and then assembled using chemical or biological procedures. A distinct
advantage of the bottom-up approach is the increased possibility of obtaining metallic
nanoparticles with a much smaller number of defects and a more homogeneous chemical
composition [10,12,38,52,53].
The proposed methods for producing nanostructures are the first rough classification.
In order to make the viewer aware of how many methods of nanoparticle synthesis have
already been developed, the methods for synthesis of metallic nanoparticles by chemical,
physical as well as biological routes have been further divided, which is presented later in
this paper.

3.2. Methods for Producing Metallic Nanoparticles

3.2.1. Chemical Methods
Chemical methods for the synthesis of metallic nanoparticles are the most widespread,
the most numerous, and at the same time, the most efficient ones. These methods are
described as easy, convenient, inexpensive (for large-scale production), and quick to carry
out while not requiring the use of complex apparatus. Moreover, the final nanoparticles
can be stored for long periods of time without significant loss in stability [54]. Although the
production of nanoparticles by chemical methods has many advantages, it is pointed out
that the use of toxic chemicals necessary for nanoparticle stabilization, as well as solvents,
is environmentally unfriendly [47]. In addition, contamination of the final nanoparticles
with chemicals is often observed, and significant amounts of hazardous by-products are
produced [5,38,55].

3.2.2. Physical Methods

Physical methods for the synthesis of metallic nanoparticles are based on the use of
microwaves, ultrasound, irradiation, or mechanical grinding, among others, to obtain the
desired product, which involves enormous energy consumption (the need to maintain high
pressures and temperatures in most techniques) [5,38,54]. In physical methods, there is no
problem of contamination of the final nanoparticles with solvent, which is the case in most
chemical methods. Moreover, hazardous materials and chemical reagents are not used in
procedures of this type. To sum up, the production rate of nanoparticles is quite low, while
the production cost is high [38,54,55].

3.2.3. Biological Methods (Biochemical)

Biological methods are based on the use of a number of biological systems that have
the ability to convert metal ions into metal nanoparticles due to the reduced abilities of
proteins and metabolites present in these systems [47,54]. Biological methods for the syn-
thesis of nanoparticles involving microorganisms, natural plant extracts, bacterial extracts,
enzymes, and plants or plant extracts have been proposed as possible ecological alterna-
tives to chemical and physical methods [56–59]. Interactions between microorganisms
and metals have already been well documented, and the ability of microorganisms to
extract/accumulate metals has already been employed in many biotechnological processes,
such as bioleaching and bioremediation [60]. Synthesis of nanoparticles by biological
methods is easy, cost-effective, energy-efficient (most bioprocesses occur under normal
pressure and at ambient temperature), utilizes natural resources, and, with all this, they are
 extracts, enzymes, and plants or plant extracts have been proposed as possible ecological
alternatives to chemical and physical methods [56–59]. Interactions between
microorganisms and metals have already been well documented, and the ability of
microorganisms to extract/accumulate metals has already been employed in many
biotechnological processes, such as bioleaching and bioremediation [60]. Synthesis of
Molecules 2023, 28, 4932 nanoparticles by biological methods is easy, cost-effective, energy-efficient (most 11 of 38

bioprocesses occur under normal pressure and at ambient temperature), utilizes natural
resources, and, with all this, they are environmentally friendly methods that do not
require the use of harsh,
environmentally toxic,
friendly and expensive
methods that do notchemicals
require the[38,55,57,61,62].
use of harsh, toxic, and expensive
chemicals
As shown [38,55,57,61,62].
in Figure 4, the above-mentioned types of methods for nanoparticle
As shown
production, i.e., inphysical,
Figure 4, chemical,
the above-mentioned types of
and biological, aremethods
furtherfor nanoparticle
divided produc-
into many
tion, i.e., physical,
subgroups. In addition, chemical,
thereandare biological, are further
still methods divided
that cannot be into many in
included subgroups.
the threeIn
addition,groups—they
described there are stillhave methods
beenthat cannot
collected inbe included
a group in theasthree
labeled “otherdescribed
methods”.groups—
It is
they have been collected in a group labeled as “other methods”. It is
impossible to discuss each of the listed methods for synthesizing metallic nanoparticles.impossible to discuss
each
The of the listed
following partmethods
of this for
papersynthesizing
focuses on metallic
chemicalnanoparticles.
reduction, The following
which part of
is the most
this paperused
commonly focuses
method on chemical reduction,
for producing which
metallic is the most commonly used method for
nanoparticles.
producing metallic nanoparticles.

• Ball milling
• Thermal evaporation • Sol-gel process
• Lithography • Chemical vapour deposition
• Vapor phase and vaporization • Chemical reduction
• Laser ablation • Solution-based synthesis
• Ultrasonication • Solvothermal
• Photoirradiation Physical Chemical • Reverse micelle
• Spray pyrolysis • Co-precipitation
• Radiolysis
methods methods

Biological Other
• Bacteria methods methods
• Vitamins
• Archaea • Enzymes
• Actinomecete • Monosacharides and
• Fungi polysacharides
• Algae • Proteins and amino acids
• Plants and phytochemicals • Microwave-assisted synthesis
• Viruses

Figure 4. Selected important examples of chemical, physical, biological, and other methods for the
preparation of metallic nanoparticles based on [54].

3.3. Synthesis of Metallic Nanoparticles by Chemical Reduction

Metallic nanoparticles have gained a lot of attention because of their unusual proper-
ties, which are different from metal in their standard form (bulk metal). The chemical and
physical properties of metal nanoparticles strongly depend on their size as well as their
structure, shape, and size distribution. Therefore, control of these parameters is crucial
and is often achieved by varying synthesis methods, reducing agents, and stabilizers. Pre-
cise control of the above parameters allows the achievement of the desired physical and
chemical properties changes in nanoparticles.
Chemical reduction, as the name implies, uses chemical-reducing agents. This method
can be further classified according to the energy source used or the device used for the
reaction, giving a wide range of possibilities for their manufacture [63]. Of the wide range
of synthesis methods by chemical means, chemical reduction is the most common and
simplest method for synthesizing nanoparticles. The substrates in this method can be either
natural compounds from plants or microorganisms or reagents/chemicals that have the
ability to cause a reduction in an oxidized state. Moreover, nanoparticles synthesized from
natural compounds are less toxic than those prepared using chemicals. An important aspect
of the synthesis of nanoparticles by the chemical reduction method is that their size can
be strictly controlled, allowing the synthesis of nanoparticles with different morphologies.
Further, the method is cost-effective and can be easily scaled up for large-scale preparation
without the need for high pressure, energy, and temperature [64].
Because of its simplicity, the main chemical method for the synthesis of metal nanopar-
ticles is the reduction in metal ions in solution (chemical reduction method) [65]. As early
as 1857, Michael Faraday was the first to report systematic research on the synthesis of
colloidal gold using the chemical reduction method [12]. The chemical reduction process
can be carried out in both aqueous and organic solvents, the latter being the preferred
Molecules 2023, 28, 4932 12 of 38

choice since metal nanoparticles are particularly sensitive to oxidation [40]. By control-
ling the reaction parameters, it is possible to produce metal nanoparticles with designer
sizes, shapes, and particle size distributions [65]. Parameters affecting the final shape, size,
stability, or aggregation state of metallic nanoparticles are listed in Table 1.

Table 1. Parameters affecting the morphology of metallic nanoparticles, based on [65].

Synthesis Parameters Reaction Conditions

type and concentration of metal salts reaction environment
type and concentration of stabilizer temperature
type and concentration of reducer pH
molar ratio of stabilizer to metal salt stirring
molar ratio of reducer to metal salt synthesis time

The production of metal nanoparticles by chemical reduction involves the reduction

of salts of a selected metal by a reducing agent in the presence of a stabilizer [37]. The role
of the stabilizer is to protect the metal nanoparticles from assembling into larger aggregates.
A typical synthesis of nanoparticles consists of three major steps, as shown in Figure 5. In
the first step, a redox reaction takes place, in which electrons from the reducing agent are
transferred to the metal atoms, resulting in the formation of free metal atoms. The equation
Molecules 2023, 28, x FOR PEER REVIEW 13 of 39
describing the transfer of electrons from the reducing agent to the metal is as follows [66]:

mMen+ + nRed → mMe0 + nOx (1)

Figure 5.
Figure Scheme of
5. Scheme of nanoparticle
nanoparticleformation
formation(using silver
(using as an
silver as example) by chemical
an example) reduction;
by chemical based
reduction;
on [60].
based on [60].

In the
In the second
second stage,
stage, aa nucleation
nucleation process
process takes
takes place,
place, in
in which
which free
free metal
metal atoms
atoms
collidewith
collide withone oneanother,
another,leading
leadingto tothe
theformation
formationof ofstable
stablenuclei.
nuclei.InInthe
thelast—third
last—thirdstage,
stage,
stabilizers are added, which prevents the aggregation of
stabilizers are added, which prevents the aggregation of nanoparticles.nanoparticles.
Metalatoms
Metal atomsproduced
producedby bythe
the reduction
reduction inin homogeneous
homogeneoussolutions
solutionsareareessentially
essentially
insoluble in the liquid and, therefore, gradually aggregate into clusters called
insoluble in the liquid and, therefore, gradually aggregate into clusters called nuclei. The nuclei. The
nuclei are dynamic entities involved in a continuous dissociation-condensation
nuclei are dynamic entities involved in a continuous dissociation-condensation process. process.
Whilenew
While newmetal
metalatoms
atomsare
areproduced
producedin inthe
thesystem,
system,thethenuclei
nucleireach
reachaacritical
criticalsize
sizeand
and
separate from the solution as solid particles (nuclei). The number
separate from the solution as solid particles (nuclei). The number and size of nuclei and size of nuclei
produceddepend
produced dependon onaanumber
numberof ofreaction
reactionparameters,
parameters,suchsuchasassolute
soluteconcentration,
concentration,the the
redox potential of the reduction reaction, temperature, nature and concentration of the
redox potential of the reduction reaction, temperature, nature and concentration of the
surfactant, solvent viscosity, and surface tension. Nucleation is rarely the final step in the
surfactant, solvent viscosity, and surface tension. Nucleation is rarely the final step in the
formation of metal particles unless special steps are taken (e.g., by adding special agents). It
formation of metal particles unless special steps are taken (e.g., by adding special agents).
should be noted that after further addition of metal atoms, nuclei grow to primary particles
It should be noted that after further addition of metal atoms, nuclei grow to primary
(nanosize); however, these are generally unstable [66]. In order to produce stable end
particles (nanosize); however, these are generally unstable [66]. In order to produce stable
products, the aggregation process must be stopped at the early stages of particle formation,
end products, the aggregation process must be stopped at the early stages of particle
which can be achieved in a number of ways, namely by electrostatic, steric, electrosteric,
formation, which can be achieved in a number of ways, namely by electrostatic, steric,
electrosteric, and hydration stabilization mechanisms. The following is a description of
the necessary components involved in the formation of transition metal nanoparticles,
such as metal precursor, solvent, reductant, and stabilizer, giving their role in the process
and listing the most commonly used chemical compounds performing the above-
Molecules 2023, 28, 4932 13 of 38

and hydration stabilization mechanisms. The following is a description of the necessary

components involved in the formation of transition metal nanoparticles, such as metal
precursor, solvent, reductant, and stabilizer, giving their role in the process and listing the
most commonly used chemical compounds performing the above-described functions.

3.3.1. Metal Precursor

A precursor is a molecule containing the metal atoms from which a nanoparticle
will be built and thus is a key factor determining the final nanomaterial. According to
a commonly accepted technique, the metal precursor dissolved in a suitable solvent is
mixed with both a suitable reducing agent and a suitable stabilizing agent in a well-stirred
reactor in an inert atmosphere [40]. The composition of the overall mixture depends on
the operating conditions under which the reaction takes place [40]. Based on a review of
literature reports in the area of transition metal nanoparticle synthesis, the most common
metal precursors used in the synthesis of the most popular metallic nanoparticles, such as
Pd, Pt, Ag, Au, Ru, and Cu, are summarized in Table 2.

Table 2. Most commonly used precursors in the synthesis of selected transition metal nanoparticles.

Metal Metal Precursor References

Pd(OAc)2 [67,68]
Na2 PdCl4 [69–73]
Pd(NO3 )2 ·2H2 O [74,75]
Pd
K2 PdCl4 [76]
H2 PdCl4 [77,78]
PdCl2 [79–82]
H2 PtCl6 ·6H2 O [67,73,74,79,83]
K2 PtCl6 [84,85]
Pt
Na2 PtCl4 [86]
Pt(acac)2 [87]
AgNO3 [63,69,88–96]
AgBF4 [97]
Ag
AgBF6 [97]
AgClO4 [98]
HAuCl4 ·3H2 O [69,74,79,85,99–101]
Au
KAuCl4 [102,103]
RuCl3 ·3H2 O [75,104–107]
Ru
Ru(NO)(NO3 )3 [108]
CuSO4 ·5H2 O [109–111]
Cu CuCl2 ·H2 O [112,113]
Cu(NO3 )2 ·3H2 O [114,115]

3.3.2. Solvent
In the context of nanoparticle synthesis, solvents are widely used as a medium for
dissolving metal precursors, transferring heat and reactants, and dispersing the resulting
nanoparticles. The most commonly used solvents include ethanol, toluene, 1-octadecene,
and dimethylformamide. However, current synthesis methods predominantly rely on the
use of organic solvents with high toxicity. Thus, it is important to design environmentally
friendly alternative solvents to reduce and eliminate environmental risks. Alternative
solvents include, e.g., water [111], supercritical fluids [116], and ionic liquids [117,118]. The
use of water is beneficial to the environment because it is non-toxic, non-flammable, widely
available, and in addition, has a low price with abundant reserves. However, the intensive
energy inputs involved in water-using production make its use a challenge in view of the
demand for energy-efficient production. The interest in using supercritical solvents, such as
supercritical water and supercritical carbon dioxide, has been increasing. At temperatures
Molecules 2023, 28, 4932 14 of 38

above the critical point, ordinary solvents transform into supercritical fluids with a much
larger void volume and greater compressibility. This endows them with a number of unique
physical properties, including density, diffusion coefficient, and thermal conductivity [119].
In contrast, ionic liquids, which consist of charged pairs of inorganic and organic ions,
are characterized by their liquid state at room temperature. Due to their very low vapor
pressure, ionic liquids are considered promising alternative solvents for replacing toxic
volatile organic solvents to reduce environmental risks. A major advantage of using ionic
liquids as solvents is that the use of stabilizing agents is generally unnecessary, further
simplifying reaction systems and reducing material consumption [119].

3.3.3. Reducer
The selection of a suitable reducing agent is also a key factor since the size, shape, and
particle size distribution strongly depend on its nature. The introduction of a reducing
agent initiates the reduction in the metal precursor. The reduction in metal salts requires
matching the reactivity of the reducing agent to the redox potential of the metal. In addition,
it has been proved that if the reaction rate is too high during the synthesis process, a large
number of metal nuclei are rapidly formed, and too small particles are formed. On the other
hand, particle agglomeration occurs if the reaction rate is too slow [63]. As for the choice
of a reducing agent, it is very wide and depends on the specific redox thermodynamics.
Additionally, often the choice of the most suitable reducing agent is determined experi-
mentally, and its introduced volume is greater than the stoichiometric requirement [119].
In many cases, the activity of reducing agents is strongly determined by the pH of the
solution [40]. The most commonly used reducing agents include [119,120]: ethylene glycol,
sodium borohydride, oleyl amine, formaldehyde, carbon oxide(II), hydrazine, ethanol,
oxalic acid, hydrogen peroxide, vitamin C, citric acid (Table 3).
Most of the reducing agents used are toxic, requiring significant safety protection
during laboratory testing and industrial production. Moreover, as mentioned earlier, in
nanoparticle synthesis, the reducing agent is introduced in excess, so as a result, the remain-
ing amount of highly reactive reducing agents in the final products is large. Therefore, the
great challenge in nanoparticle synthesis is to reduce and eliminate the use of hazardous
reducing agents replaced by environmentally viable alternatives. Because of their non-
toxicity, polysaccharides are considered green reducing agents for nanoparticle synthesis.
The undoubted advantage of using polysaccharides, in addition to the benefits of green
chemistry, is that the hydroxyl groups in the structures of polysaccharides provide them
with the ability to reduce metal precursors at the same time as allowing polysaccharides
to dissolve in water, further avoiding the use of dangerous organic solvents. In addition,
the weak chemical interactions between polysaccharides and nanoparticles ensure that
the resulting nanoparticles can be easily separated from the reaction mixtures, making
the production more energy-efficient. In some cases, polysaccharides can act as both
reducing and capping agents [119]. Many research groups have also successfully used
glucose [79,121,122], fructose [79,123], or sucrose [79] as reducing agents in the synthesis of
metallic nanoparticles.

3.3.4. Stabilizer
Since nanoparticles are essentially finely divided bulk materials, they are usually
thermodynamically unstable because of the agglomeration phenomenon. Consequently,
they need to be kinetically stabilized, and this is usually realized with a stabilizer. Stabiliz-
ing agents are widely used in nanoparticle synthesis not only for their protection against
aggregation but also for their ability to limit growth and control the morphology of the
products [119,136]. Stabilization is achieved by hydration forces, electrostatic forces, steric
forces, or a combination thereof (electrostatic forces). Electrostatic stabilization involves
only manipulating the balance between attractive and repulsive forces. The hydrative
stabilization mechanism is quite powerful for hydrophilic nanoparticles of solids but is
less effective in stabilizing metal particles. Steric and electrosteric stabilization involve
Molecules 2023, 28, 4932 15 of 38

the adsorption of surfactant, polymer, or polyelectrolyte molecules onto metal particles,

resulting in the control of unbalanced van der Waals attraction forces [64]. The stabilizer is
usually introduced during nanoparticle formation. The interaction between the stabilizer
and the nanoparticle surface is highly dynamic, and its strength and nature often control the
long-term stability of the nanoparticle dispersion. The formation of nanoparticles stabilized
by the most common stabilizers (surfactants, polymers and dendrimers, organic ligands) is
discussed below, and Table 4 summarizes the types of stabilizers most commonly used in
the synthesis of metallic nanoparticles.

Table 3. A review of reductants used in the synthesis of Ag and Au nanoparticles.

Reducer Type of Nanoparticles References

Glucose Au [124]
Foeniculum vulgare extract Au [125]
Capsicum annum extract Au [126]
Sodium citrate Au [127]
Sodium citrate Au [128]
Sodium borohydride Ag [129]
Sodium borohydride
Glucose Ag [130]
Formaldehyde
Glucose
Sucrose Ag [131]
Dextran
Glucose
Ag [132]
Dextrin
Hydrazine Au [133]
Citrate
Ascorbic acid Au [134]
Hydrogen peroxide
Cetyltrimethylammonium
Au [135]
chloride

(a) surfactants—The use of salts/surfactants is a popular route to stabilize metal

nanoparticles (Figure 6a). It is believed that surfactant-stabilized nanoparticles strongly
adsorb a layer of anions on the metal surface, which in turn are surrounded by a layer
(countercations) to maintain electrical neutrality. Both surfactant components play a key role
in protecting the metal from agglomeration. Varying the nature of the cationic component
allows nanoparticles to disperse in organic or aqueous environments. As far as the ionic
surfactants are concerned, a typical approach is to match the opposite charges of metal ions
and surfactant ions to perform chemical reduction [137,139]. The choice of surfactant is a
key point in all chemical synthesis methods, as its molecular structure, its concentration, and
even its mixing time in the reaction medium have a fundamental impact on the kinetics and
geometry of metallic nanophases. Non-ionic surfactants are characterized by the absence of
dissociative groups on the hydrophilic part of their molecules, such as ethoxylated alcohols,
ethoxylated amines, amine oxides, and thiols. They are less sensitive to electrolytes than
ionic surfactants, so they can be used even at high concentrations of dissolved salts. Ionic
surfactants, on the other hand, are grouped into anionic and cationic types according to
the sign of the charges located on the hydrophilic part of their molecule. Alkyl sulfates,
and alkylammonium salts are the most common anionic and cationic surfactants used in
Molecules 2023, 28, 4932 16 of 38

the synthesis of metal nanoparticles, respectively. In both cases, their hydrophilicity is

controlled by changing the length of the alkyl chain [39].

Table 4. Main categories of frequently used stabilizers, based on [119,120,137,138].

Class Components
N-terminated:
oleyl amine
octadecylamine
dodecylamine
O-terminated:
Organic ligands oleic acid
linoleic acid
P-terminated:
Molecules 2023, 28, x FOR PEER REVIEW 17 of 39
triphenyl phosphine
tri-n-octylphosphine
S-terminated:
concentration, and even its mixing time in the reaction medium have a fundamental
thiols
impact on the kinetics and geometry of metallic nanophases. Non-ionic surfactants are
polyvinyl pyrrolidone
characterized by the absence of dissociative groups on the polyvinyl alcohol part of their
hydrophilic
molecules, such aspolymers
ethoxylated alcohols, ethoxylated amines, amine oxides,
polyethylene glycol and thiols.
polypropylene
They are less sensitive to electrolytes than ionic surfactants, so they can glycol
be used even at
polyacrylic acid
high concentrations of dissolved salts. Ionic surfactants, on the other hand, are grouped
polyphenylene oxide
into anionic and cationic types according to the sign of the charges located on the
hydrophilic part of polyamido(amine)
their molecule. Alkyl sulfates, and alkylammonium salts are the most
dendrimers
poly(propyleneimine)
common anionic and cationic surfactants used in the synthesis of metal nanoparticles,
respectively. In both cases, their hydrophilicity is hexadecyltrimethylammonium
surfactants controlled by changing the lengthbromide
of the
alkyl chain [39]. tetra-N-alkylammonium halides

Figure 6. Schematic representation of the ways to stabilize metal nanoparticles (using silver as an
Figure 6. Schematic representation of the ways to stabilize metal nanoparticles (using silver as an
example) by using stabilizers: (a) surfactants, (b) polymers, (c) ligands; based on [137].
example) by using stabilizers: (a) surfactants, (b) polymers, (c) ligands; based on [137].
(b) steric stabilizers (polymers and dendrimers)—stabilization of nanosystems can also
(b) stericbystabilizers
be achieved (polymers
incorporating and an
them into dendrimers)—stabilization
organic matrix, which canofbe nanosystems can
either a flexible
also be achieved by incorporating them into an organic matrix, which
polymer or a more organized dendritic structure (Figure 6b). The steric weight of this can be either a
flexible polymer or a more organized dendritic structure (Figure 6b). The
class of stabilizing agents prevents the agglomeration of nanoparticles. Polymers provide steric weight of
this class of of
stabilization stabilizing agents
nanoparticles prevents
through theirthe agglomeration
binding of nanoparticles.
affinity to the surface and alsoPolymers
through
provide stabilization
the spatial of nanoparticles
mass of their through
three-dimensional their binding
structure affinity to the
[119]. Dendrimers aresurface and also
hyperbranched
through the spatial
macromolecules with mass
a highofconcentration
their three-dimensional structure
of functional groups [119].
whose Dendrimers
cavities are
behave like
hyperbranched macromolecules with a high concentration of functional
molecular boxes that can trap and stabilize metal nanoparticles, especially if heteroatoms groups whose
cavities
are insidebehave like molecular
the dendrimer. boxesretain
Dendrimers that the
canguest
trap molecule
and stabilize metal
through nanoparticles,
covalent bonding,
especially if heteroatoms are inside the dendrimer. Dendrimers retain the
electrostatic forces, or van der Waals forces [140]. Polymers such as polyvinyl pyrrolidoneguest molecule
through covalent
and polyvinyl bonding,
alcohol electrostatic
are widely used to forces,
protect or van der Waals
nanoparticles forces
because [140].commercial
of their Polymers
such as polyvinyl pyrrolidone and polyvinyl alcohol are widely used to protect
nanoparticles because of their commercial availability at relatively low cost and their
solubility in a range of solvents, including water [137,139]. The main advantages of using
dendrimers include:
Molecules 2023, 28, 4932 17 of 38

availability at relatively low cost and their solubility in a range of solvents, including
water [137,139]. The main advantages of using dendrimers include:
• the possibility of obtaining monodisperse nanoparticles due to the high homogeneity
and porosity of the dendrimers,
• prevention of nanoparticle agglomeration due to the steric effect of the dendrimers,
• possibility of application in catalysis, among other things, since the nanoparticles are
only partially surrounded by the dendrimer,
• acting as a “nanofilter” to control access of small molecules to the attached nanoparti-
cles (depending on types of functional groups and solvents),
• the possibility of changing the solubility between the hydrophilic dendrimer and
hydrophobic metal molecules due to the fact that the end groups of the dendrimer can
be combined with other functional groups.
(c) organic ligands—one of the most common methods of stabilizing metal nanoparti-
cles is the addition of an organic ligand, which usually contains a heteroatom with a free
electron pair (Figure 6c). The organic chain of the ligand prevents agglomeration, while the
heteroatom binds strongly to the metal surface. The most commonly used ligands include
those based on sulfur, phosphorus, oxygen, and nitrogen [137].
In the literature on the synthesis of metallic nanoparticles, one can increasingly often
encounter the use of naturally occurring chemicals as stabilizing agents. As already men-
tioned when describing alternative reducing agents, polysaccharides can act as reducing
agents but also as stabilizers. It is not surprising, for with the growing efforts to mini-
mize or completely eliminate waste and implement sustainable processes by adopting the
12 basic principles of Green Chemistry, it is desirable to develop biological and biomimetic
approaches to the preparation of advanced materials. Most commonly, chitosan [141,142],
starch [143], heparin [144,145], and also chemical compounds belonging to the group of
biomolecules are used as natural stabilizing agents.

3.4. Characterization of Metallic Nanoparticles

Nanostructures have attracted tremendous interest as a rapidly growing class of
materials suitable for many applications. Numerous techniques exist to characterize the
size, crystal structure, elemental composition, and a range of other physical properties of
nanoparticles. Certain physical properties can be assessed by more than one technique.
Different advantages and limitations of each technique complicate the selection of the most
appropriate method, and in fact, quite often, a broader characterization of nanoparticles
is necessary, requiring a comprehensive approach by combining techniques in a comple-
mentary manner. In this context, it is desirable to know the limitations and advantages of
different techniques in order to know whether, in some cases, the use of only one or two of
them is sufficient to provide reliable information when studying a specific parameter [146].
It is worth mentioning that the characterization of nanoparticles is carried out using vari-
ous techniques, mainly taken from materials engineering—most of these techniques are
summarized in Table 5 [5].

Table 5. Characteristic parameters of nanoparticles and the corresponding characterization techniques

[11,146].

Entity Characterized Suitable Characterization Techniques

TEM, HRTEM, SEM, AFM, XRD, DLS, SLS,
Size (structural properties) NTA, SAXS, EXAFS, FMR, DCS, ICP-MS,
UV-Vis, MALDI, NMR, TRPS, EPLS
DCS, DLS, SAXS, NTA, ICP-MS, FMR, DTA,
Size distribution
TRPS, SEM
Surface area, specific surface area BET, liquid NMR
Molecules 2023, 28, 4932 18 of 38

Table 5. Cont.

Entity Characterized Suitable Characterization Techniques

Surface charge Zeta potential, EPM
TEM, HRTEM, AFM, EPLS, FMR,
Shape
3D-tomography
XRD, XPS, ICP-MS, ICP-OES, SEM-EDX, NMR,
Elemental-chemical composition
MFM, LEIS
Crystal structure XRD, EXAFS, HRTEM, electron diffraction
Concentration ICP-MS, UV-Vis, RMM-MEMS, DCS, TRPS
Zeta potential, DLS, DCS, UV-Vis, SEM,
Agglomeration state
Cryo-TEM, TEM
Chemical state-oxidation state XAS, EELS, XPS, Mössbauer
Density DCS, RMM-MEMS
3D visualization 3D-tomography, AFM, SEM
Optical properties UV-Vis-NIR, PL, EELS-STEM
SQUID, VSM, Mössbauer, MFM, FMR, XMCD,
Magnetic properties
magnetic susceptibility
AFM—atomic force microscopy, BET—Braunauer–Emmet–Teller, Cryo-TEM—cryogenic transmission electron
microscopy, DLS—dynamic light scattering, DSC—differential scanning calorimetry, DTA—differential thermal
analysis, EELS—electron energy-loss spectroscopy, EELS-STEM—electron energy loss spectroscopy with scanning
transmission electron microscope, EPLS—electrophoretic light scattering, EPM—electrophoretic mobility, EXAFS—
extended X-ray absorption fine structure, FMR—ferromagnetic resonance, HRTEM—high resolution transmission
electron microscopy, ICP-MS—inductively coupled plasma mass spectrometry, ICP-OES—inductively coupled
plasma optical emission spectroscopy, LEIS—low-energy ion scattering, MALDI—matrix-assisted laser desorp-
tion/ionization, MFM—magnetic force microscopy, NMR—nuclear magnetic resonance, NTA—nanoparticle track-
ing analysis, PL—photoluminescence, RMM-MEMS—resonant mass measurement with micro electro-mechanical
systems, SAXS—small-angle X-ray scattering, SEM -scanning electron microscopy, SEM-EDX—scanning electron
microscopy with energy dispersive X-ray spectroscopy, SLS—static light scattering, SQUID—superconducting
quantum interference device, TEM—transmission electron microscopy, TRPS—tunable resistive pulse sensing,
UV-Vis—ultraviolet-visible spectroscopy, VSM—value stream mapping, XAS—X-ray absorption spectroscopy,
XMCD—X-ray magnetic circular dichroism, XPS—X-ray photoelectron spectroscopy, XRD—X-ray diffraction.

The physicochemical properties of nanomaterials largely depend on their three-

dimensional morphology (size, shape, and surface topography), the surrounding media,
and their distribution in space. The correlation of these parameters with the correspond-
ing physical and chemical properties is a fundamental requirement for discovering new
properties and applications, as well as for advancing the fundamental and practical knowl-
edge required to design and manufacture new materials [35]. In order to characterize
nanoparticles, it is useful to first find answers to the three fundamental questions [147]:
• What does the material look like (size, size distribution, shape, topography, degree of
agglomeration, aggregation)?
• What is the material made of (chemical composition, crystal structure, purity, impurity
level, elemental composition, chemical composition, and phase composition)?
• Which factors affect the material’s interaction with its environment (specific surface
area, surface chemistry, surface charge)?
Although there are many different information needs for nanoparticles and other
nanostructured materials, considerable efforts have been made to identify the most nec-
essary or required property measurements, which are shown in Figure 7. Although the
determination of the size, distribution, and particle shape is vital, the surface properties
of nanoparticles are also important (specific surface area, surface charge, and degree of
agglomeration) [147]. Since the proportion of atoms and molecules near the surface in-
creases significantly for nanostructured materials, it is not surprising that surface properties
play a significant role in determining the behavior of such materials. The most important
 nanostructured materials, considerable efforts have been made to identify the most
necessary or required property measurements, which are shown in Figure 7. Although the
determination of the size, distribution, and particle shape is vital, the surface properties
of nanoparticles are also important (specific surface area, surface charge, and degree of
Molecules 2023, 28, 4932 agglomeration) [147]. Since the proportion of atoms and molecules near the surface 19 of 38
increases significantly for nanostructured materials, it is not surprising that surface
properties play a significant role in determining the behavior of such materials. The most
important parameters
parameters that providethat provide
sufficient sufficient information
information about the nanoparticles
about the nanoparticles in question arein
question are
given below. given below.

Figure 7.7.Schematic
Figure Schematicrepresentation of parameters
representation needed
of parameters to be determined
needed to characterize
to be determined nanoparti-
to characterize
cles, based on [147].
nanoparticles, based on [147].

3.4.1. Size
As mentioned on the first pages of this paper, nanoscale objects are all materials with
at least one dimension smaller than 100 nm. The size of nanoparticles is one of the most
important factors affecting their physicochemical and functional properties [148]. Thus,
it should be the first parameter to be measured when characterizing these materials. Size
refers to the spatial extent of an object. For a spherical object, it can be unambiguously
described by a single dimension, but since many particle systems have an irregular shape,
usually, the particle size is expressed as an equivalent spherical diameter [149]. For non-
spherical objects, several dimensions are needed to fully define the actual extension of the
object in space. While measuring size may seem trivial for macroscopic-scale objects, at
the nanoscale, it takes on different meanings according to the technique used to measure
it. In the case of nanoparticles, size can refer to (i) its overall physical dimension defined
by its atomic structure, (ii) the effective size of a particle in a specific matrix according to
its diffusion/sedimentation behavior, (iii) the effective size of a nanoparticle-based on its
mass/electron distribution. This variety of size definitions reflects the broad spectrum of
physical approaches that can be used to characterize nanoparticles [150]. In principle, there
is a variety of analytical instruments that can be used to measure the size of nanoparticles;
however, many require specialized equipment that is not widely available for general
use [148]. Some of the simplest techniques with which to measure the size and morphology
of nanoparticles at the single particle level at sub-nanometer resolution are high-resolution
microscopy techniques. Methods based on light scattering, diffusion, and sedimentation
are commonly used to analyze colloidal suspensions. However, it is usually not possible to
obtain direct information about the shape of nanoparticles from these approaches, and the
equivalent diameter corresponding to that of a sphere behaving in the same way as the sam-
ple under study is usually referred to as the characteristic size. To translate this information
into the actual dimensions of NPs, knowledge of their shape is required. Finally, static
scattering methods, using light or X-rays, provide information about the mass/electron
distribution of nanoparticles and, therefore, indirectly about their shape [150].

3.4.2. Size Distribution

Typically, the goal of nanoparticle synthesis is to obtain a monodisperse population;
however, it should be kept in mind that the actual sample always exhibits a certain degree
of variability. Therefore, the size distribution of nanoparticles is an intrinsic measure of
Molecules 2023, 28, 4932 20 of 38

the control and quality of the synthesis procedure, while the estimated size value refers
only to the averaged amount obtained from this distribution. There are many techniques
for measuring particle size distribution based on a variety of physical principles; some
examples include laser diffraction, dynamic light scattering, microscopy, and surface area
measurements [149]. Typically, the size properties of nanoparticles are represented by the
particle size distribution in combination with measures of central tendency (such as mean
diameter) and width of distribution (such as polydispersity index). When reporting particle
size data, it is always important to specify whether the particle dimensions are given as
radius or diameter [148].

3.4.3. Shape
Although particles are often assumed to be spherical, they actually have a wide va-
riety of geometric shapes and can also take on spherical, planar, cylindrical, tubular, or
conical shapes. Nanoparticles can also have complex internal structures, such as homo-
geneous or core-shell. They may occur as isolated particles or may be organized into
different types of structural arrangements, such as chains or flocs. The morphology and
organization of nanoparticles play an important role in determining their physicochemical
and functional properties, so it is important to know analytical methods for measuring
these properties [148]. It has been proven that particles with the same composition and
similar dimensions can exhibit drastically different behavior due to their different shape.
The shape is usually characterized using high-resolution microscopy techniques, which
allow the morphology of sub-nanometer particles to be determined. However, electron
microscopy typically provides a 2D projection of a particle’s shape onto a plane, which in
the special cases of highly anisotropic particles, can lead to erroneous estimates of particle
morphology. To circumvent this limitation, the characterization of particles exhibiting
pronounced 3D anisotropy can be performed by acquiring projections of a large number of
randomly oriented identical particles to reconstruct their spatial arrangement or electron
tomography. Information on particle shape and anisotropy can also be obtained by using
scattering-based techniques that can be more easily applied in solution, for example, by
combining static and dynamic light scattering techniques. However, such particle analysis
only allows quantitative inference of the particle’s anisotropy coefficient, and detailed
study of particle morphology remains limited to high-resolution microscopy. Nevertheless,
qualitative shape information obtained by scattering-based characterization methods is
often necessary to confirm microscopy results since sample preparation and evaluation by
electron microscopy can affect the agglomeration state of the sample or induce damage to
the particle framework [150].

3.4.4. Surface Charge

The surface charge of nanoparticles is important because it determines their interaction
with other charged particles in their environment. The boundary between the solid and
liquid phases is a dynamic environment, and numerous phenomena contribute to the
appearance of charge on the surface of nanoparticles [150], including the presence of
ionized components on their surfaces, such as ionic surfactants, phospholipids, proteins
or polysaccharides [148]. This charge not only influences the behavior of nanoparticles
in different environments but also controls their tendency to aggregate since electrostatic
repulsion between molecules is a key factor promoting the stability of colloidal solutions.
In particular, in an electrolyte solution, mobile charges are attracted by static charges on
the surface of nanoparticles, effectively leading to electric potential shielding, which can
eventually lead to particle aggregation [150]. The electrical characteristics of nanoparticles
depend on the type, concentration, and location of functional groups on their surface,
as well as the ionic composition and physical properties of the surrounding liquid [148].
A typical measure of surface charge and colloidal stability is the zeta potential. Several
parameters affect the zeta potential of particles in solution, namely the ionic strength of
the solvent, the presence of charged or uncharged particles that can adsorb to the particle
Molecules 2023, 28, 4932 21 of 38

surface, and the pH of the solution [150]. The electrical properties of nanoparticles are also
often characterized by surface charge density and surface electric potential [148]. To sum
up, the electrical charge of nanoparticles is a key characteristic that is extremely important
in determining colloidal stability, particle self-organization, chemical catalysis, and various
biomedical applications [151].

3.4.5. Surface AREA/Porosity

Surface area is another significant factor in characterizing nanoparticles. The surface
area-to-volume ratio of a nanoparticle has a huge impact on its performance and properties.
The ability to synthesize nanoparticles containing porous frameworks has greatly expanded
the application range of nanomaterials. Porosity provides nanoparticles with a dramatically
increased surface-to-volume ratio, which can exceed that of solid particles of equal size by
several orders of magnitude. To permit the development and characterization of porous
nanoparticles, it is necessary to study porosity at different levels, namely (i) the size of the
pore opening, (ii) the dimensions and volume of the porous cavity, (iii) the connectivity
of the porous structure, (iv) the specific surface area (the sum of the inner and outer
surfaces), (v) the surface-to-volume ratio, and (vi) the functionalization of the inner and
outer surfaces [150]. The most commonly used and one of the simplest methods for
measuring surface area is BET analysis.

3.4.6. Concentration
The total concentration of nanoparticles present in a sample after they have been
manufactured is a very important issue. Sometimes the total concentration of nanoparticles
is known due to the nature of the ingredients and the processing used to make them, but in
other cases, it may be necessary to measure the concentration of particles, so appropriate
analytical methods are needed. Many analytical methods are available for determining
the total concentration of particles in a suspension. If there are no significant levels of
solutes in the surrounding liquid, the nanoparticle suspension can simply be dried and
weighed. If solutes are present, the nanoparticles can be separated by centrifugation,
dialysis, filtration, or selective precipitation, then dried and weighed. If the nanoparticles
scatter light significantly, then the concentration can be determined by simply measuring
the turbidity after establishing an appropriate turbidity calibration curve as a function of
particle concentration. The main limitation of this method is that the turbidity of a colloidal
dispersion depends on the size of the particles as well as their concentration. This method
may therefore be most suitable when the size of the nanoparticles is already known or can
be measured independently. In addition, many nanoparticles may only scatter light weakly
due to their small size relative to the wavelength of light, and thus, may not cause a large
change in turbidity with particle concentration [148].

3.4.7. Composition
The most appropriate method for determining the composition of nanoparticles de-
pends on their exact nature, i.e., the type and amount of their various components [148]. In
principle, determination of the purity of nanomaterials can be achieved by analyzing their
chemical composition, for it is the chemical or elemental composition that determines the
purity and, therefore, the performance of nanoparticles. The presence of higher secondary
or undesirable elements in a nanoparticle can reduce its performance and lead to secondary
reaction and contamination. Analysis of the chemical composition of nanomaterials is more
complicated than that of a single unit [152]. Among a wide variety of methods, the char-
acterization of nanomaterial’s composition is usually carried out by X-ray photoelectron
spectroscopy [148].

3.4.8. Agglomeration State

The dispersion state of a particulate system describes the degree of agglomeration
of particles that coexist in groups or clusters through the action of various inter-particle
Molecules 2023, 28, 4932 22 of 38

forces, the most fundamental of which are van der Waals forces. The magnitude of these
forces is a function of the fundamental atomic and molecular properties of the surface
atoms, the surface morphology, and the proximity of these surfaces [149]. For most mea-
surement techniques, the size distribution of existing agglomerates is quantified. Thus,
the measured size distribution is highly dependent on the dispersion state of the system.
Due to attractive forces, particles will tend to agglomerate in suspension unless they are
stabilized by equivalent repulsive forces, such as surface charge or steric effects. However,
the smaller the particle size, the greater the relative attractive forces per unit mass. This
means that once agglomerated, it becomes increasingly difficult to disperse materials at the
nanoscale as the size decreases [149]. The dispersion of nanoparticles in solution is usually
measured using dynamic light scattering, as this technique provides measurements of a
large number of particles in solution and provides a robust and quantitative assessment
of narrow size distributions. A technique that complements dynamic light scattering is
transmission electron microscopy which is mainly used to measure the primary size of
nanoparticles [153].

3.5. Examples from Literature

As has been mentioned several times, the method for the synthesis of metal nanoparti-
cles by chemical reduction allows the production of nanomaterials with controlled morphol-
ogy. To illustrate how changing various synthesis parameters affects the final nanoparticles,
selected examples from the literature are described below.
Researchers from the Suriati research group [63] have synthesized silver nanoparticles
by chemical reduction, which turned out to be a simple, inexpensive, and partly green
method as they used ascorbic acid (C6 H8 O6 ), which is one form of vitamin C, as a surfactant.
The metal precursor was silver nitrate (AgNO3 ), while the reducing agent was sodium
citrate (C6 H5 O7 Na3 ). The concentrations of sodium citrate and ascorbic acid were varied
to observe the effects of these parameters, particularly on the size and morphology of the
silver nanoparticles.
• The effect of the reducer concentration
At the reducing agent concentrations of 4.0–8.0 mM, all the nanoparticles produced
had a quasi-spherical shape. In contrast, it was observed that the nanoparticle sizes showed
a decreasing trend with increasing concentration of the reducer, from 38.53 nm at 4.0 mM
C6 H5 O7 Na3 to 36.32 at 8.0 mM C6 H5 O7 Na3 . Moreover, with increasing concentration of
the reducer, a narrowing of the particle size distribution was observed from 20–65 nm to
20–50 nm. A possible reason for this phenomenon could be that the rate of the reaction is
directly proportional to the concentration of the reactant according to the law of action of
masses, from which it follows that as the concentration of trisodium citrate increased, the
rate of the reaction increased. It has been concluded that as the reaction rate increased, the
silver ions were consumed faster, leaving less room for particle size growth.
• The effect of the stabilizer concentration
The TEM observations have shown that the average size of silver nanoparticles in-
creased as the concentration of ascorbic acid increased from 1.0 mM to 4.0 mM. The average
sizes of silver nanoparticles produced at 1.0 mM, 2.0 mM, 3.0 mM, and 4.0 mM ascorbic
acid were 37.24 nm, 43.04 nm, 45.85 nm, and 47.28 nm, respectively. It seemed likely
that ascorbic acid was able to kinetically control the growth rate of various surfaces by
selectively adsorbing on these surfaces. Because of the acidic properties of ascorbic acid,
the addition of a high concentration of ascorbic acid subsequently lowered the pH of the
solution.
The work reported by Chou and co-workers [94] investigated the synthesis of silver
nanoparticles by the chemical reduction method using silver nitrate (AgNO3 ) as a metal
precursor, formaldehyde (CH2 O) as a reducing agent, and polyvinylpyrrolidone/poly(vinyl
alcohol), (PVP/PVA) as stabilizing agents. A solution of either sodium carbonate (Na2 CO3 )
Molecules 2023, 28, 4932 23 of 38

or sodium hydroxide (NaOH) was used to determine the preferred pH. The effect of the
amount of alkaline solution on the morphology of the final nanoparticles was examined.
• The effect of pH
Although high pH is preferred in nanoparticle synthesis due to its higher reducing
power, Chou’s research group has shown an adverse effect of high pH on particle size.
When more NaOH was added to the reaction system, the silver colloids settled at the bottom
of the solution. Thus, NaOH was replaced with a weak base, Na2 CO3, in order to release
hydroxyl ions only when the pH fell below certain values. The effect of the amount of
sodium carbonate on the average particle size and also the standard deviation of the particle
size distribution is shown in Figure 8. As shown in this figure, the optimal conditions
were at a ratio of Na2 CO3 /AgNO3 between 1.0 and 1.5, at which smaller nanoparticle sizes
were obtained. When more Na2 CO3 was added, the pH of the solution increased, which
adversely affected the stability of the silver colloids; moreover, the size of the nanoparticles
Molecules 2023, 28, x FOR PEER REVIEW
increased from 10–20 nm to 80–100 nm, as did the size distribution, as indicated by24the of bar
39

shown in the figure below.

Figure
Figure 8.
8. Effect
Effect of
of Na
Na22CO
CO3/AgNO
3 /AgNO 3 ratio ononthe
3 ratio thesilver
silveraverage
averagesize
sizeand
andits
itsstandard
standarddeviation.
deviation.
Reproduced
Reproducedwith
withpermission
permissionfrom
from [94].
[94].

Liguo’s
Liguo’sresearch
researchgroup
group[95]
[95]prepared
prepared silver
silvernanoparticles
nanoparticlesby bythe
thechemical
chemicalreduction
reduction
method,
method,using
using hexadecyltrimethylammonium
hexadecyltrimethylammoniumbromide bromide(CTAB)
(CTAB)toto prevent
prevent nanoparticle
nanoparticle
agglomeration,
agglomeration,silver
silvernitrate
nitrate(AgNO
(AgNO33) )as asthe
themetal
metal source
source and
and formaldehyde
formaldehyde (CH (CH22O)
O)as
as
the
thereducing
reducingagent.
agent.The
ThepHpHofofthe
thesolution
solutionwaswasadjusted
adjustedby byadding
addingnitric
nitricacid
acid(V)
(V)(HNO
(HNO33) )
orsodium
or sodiumhydroxide
hydroxide(NaOH)
(NaOH) to to the
the solution
solution mixture
mixture at
at the
the nanoparticle
nanoparticle synthesis
synthesis stage.
stage.
•• The
Theeffect
effect of
of the
the stabilizer
stabilizer concentration
concentration
The
The addition of a small amountof
addition of a small amount ofCTAB
CTABto tothe
thesolution
solutionresulted
resultedin
inthe
theformation
formationof of
particles
particles of large size, irregular shape, and significant agglomeration degree(Figure
of large size, irregular shape, and significant agglomeration degree (Figure9a).
9a).
As
As thethe amount
amount of of CTAB
CTAB increased,
increased, the
the particle
particle sizes
sizes were better dispersed.
were better dispersed. When
When the
the
CTAB/AgNO
CTAB/AgNO 3 ratio
3 ratiowas
was 0.8, nanosilver
0.8, nanosilver particles
particles of
of20–40
20–40 nm
nm were
wereobtained
obtained (Figure
(Figure 9b),
9b),
while
while when
when this ratio reached
this ratio reached1.2,1.2,then
thenparticles
particlessmaller
smallerthan
than
1010nmnm were
were formed
formed (Figure
(Figure 9c).
9c). It has been shown that CTAB has an inhibitory effect on the process of particle growth
and agglomeration, which was explained by the effect of long carbon chains of CTAB, which
may reduce the possibility of collisions between silver particles. In addition, CTAB, due to
its structure, exhibits a strong steric effect. Thus, at low concentrations, it had little effect
on the particle size and agglomeration, and as its concentration increased, better dispersion
was achieved. On the other hand, an excessive amount of CTAB retards the growth of
particles, as they are wrapped inside CTAB molecules, which prevents the growth of nuclei
resulting in the formation of particles of several nanometers in size.

Figure 9. TEM morphologies of nano-silver particles with different CTAB amounts, CTAB/AgNO3:
 • The effect of the stabilizer concentration
The addition of a small amount of CTAB to the solution resulted in the formation of
particles of large size, irregular shape, and significant agglomeration degree (Figure 9a).
As the amount of CTAB increased, the particle sizes were better dispersed. When the
CTAB/AgNO3 ratio was 0.8, nanosilver particles of 20–40 nm were obtained (Figure 9b),
Molecules 2023, 28, 4932 24 of 38
while when this ratio reached 1.2, then particles smaller than 10 nm were formed (Figure
9c).

Figure9.9. TEM morphologies

Figure morphologies ofof nano-silver
nano-silverparticles
particleswith
withdifferent
differentCTAB
CTABamounts,
amounts,CTAB/AgNO
CTAB/AgNO33::
Molecules 2023, 28, x FOR PEER REVIEW
(a) 0.5, (b) 0.8, (c) 1.2. Reproduced with permission from [95]. 25 of 39
(a) 0.5, (b) 0.8, (c) 1.2. Reproduced with permission from [95].

• The effect
It has beenofshown
temperature
that CTAB has an inhibitory effect on the process of particle growth
and As
agglomeration,
As established by
established which
Liguowas explained
al., the
et al., by thetemperature
the reaction
reaction effect of long
temperature carbon
affects
affects thechains
the of CTAB,
morphology
morphology of
whichparticles
silver may reduce the possibility
in two
two ways. On ofthe
ways. On the one collisions
one between
hand, the silver
rate ofparticles.
reduction In addition,
in silver CTAB,
nitrate
silver nitrate
due to itson
depends structure,
temperature,exhibits
temperature, a strong
while
while on
onthe thesteric
other effect.
otherhand,
hand, Thus, at low concentrations,
temperature
temperature affects
affectsthethe it had little
interactions,
interactions, in
effect
this
in on
case,
this the
case, particle
between
betweenCTAB size
CTABandand agglomeration,
andAgAg. As
+ + . Asaaresult, and
result, both as its concentration increased,
both temperature-dependent processes better
processes
dispersionaffect
ultimately was achieved.
affect the morphology
the On theof
morphology other
of hand, an excessive
the nanosilver
the nanosilver particles. amount
particles. of CTAB retards
The morphological responsethe
growth
of
of particles
particlesof prepared
particles, at
prepared asdifferent
at they aretemperatures
different wrapped inside
temperatures CTAB molecules,
was examined.
was examined. TEM images
TEM which
images prevents
of the obtainedthe
growth of nuclei
nanoparticles resulting
at the reactionin the formation of
temperatures of particles
C, 40of◦°C,
20 ◦°C, several
C, and nanometers
60 ◦°C in size. in
presented
C are presented in
◦ C, heterogeneous
•
Figure
Figures 10a,
The 10a,b, and c,
effectb,of andrespectively. When the reaction temperature was
c, respectively. When the reaction temperature was 20 °C,
temperature 20
particles of about
heterogeneous 100 nmof
particles inabout
size were
100 obtained.
nm in sizeWith
wereincreasing
obtained. reaction temperature,
With increasing the
reaction
particles
temperature,becamethe smaller—their
particles became size was close to size
smaller—their 30 nm 40 ◦ C,
wasatclose to while
30 nmatat60 40◦°C,
C, itwhile
was
about 10 nm.
at 60 °C, it was about 10 nm.

Figure 10. TEM

Figure 10. TEM morphologies
morphologies of nano-silver
nano-silver particles at different
different reaction temperatures: (a) 20 ◦°C,
reaction temperatures: C,
(b) 40
(b) ◦
40 °C, ◦
C, (c) 60 °C. from [95].
C. Reproduced with permission from [95].

These
These observations
observations were interpreted as follows. follows. When the reaction temperature is
low (20 ◦°C),C), CTAB dissolves poorly, and in in addition,
addition, precipitation is often easier, which which
ultimately
ultimately leads
leads toto a small amount of CTAB participating in the reaction, further resulting
in
in aa reduced
reducedeffect
effectononinhibiting
inhibitingparticle
particlegrowth.
growth.On Onthe other
the otherhand,
hand,thethe
reducing
reducing ability of
ability
formaldehyde is very weak at low temperatures. As a result, the reaction
of formaldehyde is very weak at low temperatures. As a result, the reaction is slow, and is slow, and the
initial nuclei
the initial can then
nuclei can consume
then consumemost of the of
most reduced silver atoms.
the reduced The number
silver atoms. of further
The number of
forming nuclei isnuclei
further forming smallerisand largeand
smaller particles
large with a wide
particles sizea distribution
with are eventually
wide size distribution are
formed.
eventually 40 ◦ C, CTAB
Atformed. At 40 is°C,fully
CTAB dissolved, and the amount
is fully dissolved, and the of amount
silver nitrate decreases
of silver nitrate
at
decreases at a moderate rate. The nanosilver particles obtained exhibit a When
a moderate rate. The nanosilver particles obtained exhibit a flake morphology. flake
the temperature
morphology. Whenreaches 60 ◦ C, the reduction
the temperature reaches 60in°C, silver cations isin
the reduction faster.
silverA large is
cations number
faster.
of
A silver atoms are
large number of generated
silver atomsin aare
short time, and
generated in athe nucleation
short time, and ratethe
is greatly increased.
nucleation rate is
The nucleation process consumes most of the silver reserves, which
greatly increased. The nucleation process consumes most of the silver reserves, which inhibits the growth of
newly formed particles, so small particles are formed.
inhibits the growth of newly formed particles, so small particles are formed.
•• The The effect
effect of
of pH
pH
The
The morphology
morphology ofof silver
silver nanoparticles
nanoparticles at
at different
different pH
pH was
was also
also examined.
examined. As shown
in Figure 11, at pH of 3, the particle size was
Figure 11, at pH of 3, the particle size was less than 20 nm; at pH
pH increased
increased to 9,
9, the
the
agglomeration of the particles formed was advanced; while at pH of 6, the particles
showed a flaky structure and good dispersion. The explanation proposed was that at a
low pH value, the reduction ability of formaldehyde is weak, resulting in a low reaction
rate. On the other hand, the presence of a large number of H + in the solution inhibits the
Molecules 2023, 28, 4932 25 of 38

agglomeration of the particles formed was advanced; while at pH of 6, the particles showed
a flaky structure and good dispersion. The explanation proposed was that at a low pH
value, the reduction ability of formaldehyde is weak, resulting in a low reaction rate. On
the other hand, the presence of a large number of H+ in the solution inhibits the reaction
Molecules 2023, 28, x FOR PEER REVIEW 26 of 39
and causes an incomplete reaction of silver cations.

Figure11.
Figure 11.TEM
TEMimages
imagesofofnano-silver
nano-silverparticles
particles under
under different
different pHpH conditions:
conditions: (a)(a)
pHpH
= 3,= (b)
3, (b)
pHpH
= 6,=
6, (c) pH = 9. Reproduced with permission from
(c) pH = 9. Reproduced with permission from [95]. [95].

Inaddition,
In addition,after
afterthe
thereactions
reactionswere
werecomplete,
complete,the thecontents
contentsofof
AgAg+ with Cl−−at
+ with Cl atdifferent
different
pHwere
pH werechecked.
checked.At AtpHpHofof3,3,the
thereduction
reductionininAg +
Ag was
+ wasincomplete
incompletebecause
becausefewer
fewersilver
silver
atomsexisting
atoms existingin inthe
thesolution
solutionparticipated
participatedin in the
the nucleation
nucleation and and particle
particle growth
growth processes,
processes,
eventuallyleading
eventually leadingto tosmaller
smallernanosilver
nanosilverparticle
particleformation.
formation.At At weakly
weakly acidic
acidicpH,
pH,aasmall
small
amount
amountof ofHH++ in
in solution
solution isis beneficial
beneficial for
for maintaining
maintaining the the stability
stability of
of the
the bilayer
bilayer surface,
surface,
which
whichfavors
favorsthetheformation
formationofofstable
stablenanoparticles
nanoparticleswith withgood
gooddispersion.
dispersion.
Guzmán
Guzmán and co-workers [91] have synthesized silver nanoparticlesby
and co-workers [91] have synthesized silver nanoparticles bythe
thechemical
chemical
reduction
reductionmethod
methodusing
usingsilver
silvernitrate
nitrate(AgNO
(AgNO33)) as as the
the metal
metal precursor,
precursor,hydrazine
hydrazinehydrate
hydrate
(N
(N22H
H44), and/or
and/or sodium citrate (Na (Na33C6HH55OO77) )as
as the
the reducing agent and and sodium
sodiumdodecyl
dodecyl
sulfate
sulfate(SDS)
(SDS)and/or
and/or sodium
sodium citrate
citrate(Na
(Na33CC66H H55O77)) for stabilizing the the whole
whole system.
system. The
The
effect
effectof
ofthetheconcentration
concentration of ofsodium
sodium citrate
citrate and and the
the type
type ofofreducing
reducing agent
agent used
used on
onthe
the
morphology of the final particles was investigated.
morphology of the final particles was investigated.
•• The
Theeffect
effectof
ofthe
thereducer
reducerconcentration
concentration
TEMstudies
TEM studiesindicated
indicatedaacorrelation
correlationbetween
betweenthe theconcentration
concentrationof ofsodium
sodiumcitrate
citrateand
and
themorphology
the morphology of the the final
finalnanoparticles.
nanoparticles.For Forhydrazine
hydrazine at at
a concentration
a concentration of 2.0
of mM and
2.0 mM
sodium
and sodiumcitrate at 1.0
citrate mM,
at 1.0 mM, thethe
silver nanoparticles
silver nanoparticleswere wereobtained
obtainedinin thethe form of small,
small,
significantly agglomerated grains
significantly grains (Figure
(Figure12A,12A,left).
left).InIn
addition,
addition, thethe
obtained
obtained particle size
particle
histogram
size histogram showed
showed the the
sizesize
range
rangeof of
thethesilver
silvernanoparticles
nanoparticlesfrom from77to to 20 with an
20 nm with an
average diameter
average diameter of of99nmnm(Figure
(Figure12A,12A,right).
right).In
Incontrast,
contrast,the thenanoparticles
nanoparticlesobtained
obtainedat at
twice
twicethe
theconcentration
concentration of sodium
of sodium citrate and the
citrate same
and theconcentration of hydrazine
same concentration assumed
of hydrazine
aassumed
spherical ashape and were
spherical shape additionally
and werecharacterized
additionallybycharacterized
good dispersion by (Figure 12B, left).
good dispersion
From the12B,
(Figure histogram,
left). From it was deduced that,
the histogram, in this
it was case,that,
deduced the silver
in thisparticles
case, theassumed sizes
silver particles
ranging
assumed from
sizes7 to 20 nmfrom
ranging and 22 to20
7 to 35 nm
nm,andwith22antoaverage diameter
35 nm, with of 11 nm
an average (Figure of
diameter 12B,
11
right). The described relationship was also confirmed by UV-Vis
nm (Figure 12B, right). The described relationship was also confirmed by UV-Vis studies, studies, which revealed
typical plasmonic
which revealed absorption
typical maxima
plasmonic at 405 nm
absorption and 406
maxima nm nm
at 405 when andthe
406sodium
nm when citrate
the
solution
sodium of 1.0 mM
citrate and 2.0
solution of mM,
1.0 mMrespectively,
and 2.0 mM,was used. The observed
respectively, was used.differences in the
The observed
position andin
differences shape of plasmonic
the position absorption
and shape are due to
of plasmonic differences
absorption areindue
the to
particle size, shape,
differences in the
and dielectric
particle constant
size, shape, andof dielectric
the surrounding
constantmedium.
of the surrounding medium.
Molecules 2023, 28, 4932 26 of 38

Figure 12. TEM image and particle size distribution of silver nanoparticles obtained at 1.0 mM (A)
and 2.0 mM (B) of citrate of sodium solution. Reproduced with permission from [91].

• The effect of the type of reducer

In addition, the effect of the type of reducing agent on the particle size was examined.
When hydrazine was used as a reducing agent, the average diameter of the particles
obtained was about 30 nm. On the other hand, when a mixture of hydrazine and sodium
citrate was used as a reducing agent, the average particle diameter was in the range of
15–48 nm, which, compared to the particle size obtained with hydrazine alone, indicates a
slight increase in the average particle diameter.
Song and co-workers [89] have synthesized silver nanoparticles using silver nitrate
(AgNO3 ) as the precursor while sodium borohydride (NaBH4 ) and sodium dodecyl sulfate
(SDS) as the reducing agent and stabilizing agents, respectively. The effects of several vari-
ables, i.e., the concentration of AgNO3 , NaBH4, and SDS, on the final silver nanoparticles
were examined.
• The effect of metal precursor concentration
Figure 13 shows the UV-Vis spectra of colloidal silver nanoparticles prepared at
different initial concentrations of AgNO3 (0.0001 M, 0.0002 M, 0.0005 M, and 0.001 M). The
nanoparticles were synthesized at the NaBH4 /AgNO3 molar ratio of 10 and SDS/AgNO3
weight ratio of 2. The color of the solutions depended on the concentration of AgNO3
added. As the initial concentration of AgNO3 increased, the color of the solution changed
from yellow to brown. The absorption peak at about 400 nm was attributed to plasmonic
excitation by silver nanospheres, indicating the formation of silver nanoparticles. At low
concentrations of AgNO3 , the maximum weak absorption of surface plasmon peaks was
observed at 400 nm, indicating that silver nanoparticles were produced at relatively low
concentrations. As the AgNO3 concentration increased, the intensity of the maximum
plasmonic peak increased, indicating that higher concentrations of silver nanoparticles
were formed.
Molecules2023,
Molecules 2023,28,
28,4932
x FOR PEER REVIEW 28 39
27 of 38

Figure 13.UV-Vis
Figure13. UV-Visabsorption
absorptionspectra
spectraofofthe silver
the nanoparticles
silver prepared
nanoparticles viavia
prepared reduction in AgNO
reduction 3 at3
in AgNO
different initial
at different concentrations.
initial Reproduced
concentrations. Reproduced with permission
with from
permission [89].
from [89].

•• The
Theeffect
effectof ofreducer
reducerconcentration
concentration
To
To understand the effectof
understand the effect ofNaBH
NaBH44 concentration,
concentration, the the reduction
reduction reaction
reactionwas
was studied
studied
at different NaBH /AgNO
at different NaBH44/AgNO33 molar ratiosmolar ratios (0.5–15), at the
the initial AgNO33 concentration of
initial AgNO concentration of
0.001
0.001 M,
M, and
andthe theSDS/AgNO
SDS/AgNO3 weight ratio of 2. The corresponding UV-Vis UV-Vis spectra
spectra are
are
shown
shown inin Figure
Figure 14.14. At
Atthe
thelowest
lowestNaBHNaBH 4 /AgNO
4/AgNO 3 molar
3 molar ratio,
ratio, a weak
a weak plasmonic
plasmonic peakpeak
was
was observed
observed at 400at 400
nm,nm, indicating
indicating a arelatively
relativelylow lowconcentration
concentration of of silver
silver nanoparticles
nanoparticles
formed,
formed,thethereason
reasonbeing
being insufficient
insufficientreduction.
reduction. It isItknown
is known that that
the UV-Vis absorption
the UV-Vis peak
absorption
can also provide information about the degree of dispersion of silver
peak can also provide information about the degree of dispersion of silver nanoparticles. nanoparticles. The
narrower it is, the better the dispersion degree of nanoparticles is obtained.
The narrower it is, the better the dispersion degree of nanoparticles is obtained. At molar At molar ratios
of 2 andof5,2the
ratios and absorption peak at 400
5, the absorption nm was
peak at 400 broad,
nm indicating
was broad, that the silverthat
indicating nanoparticles
the silver
were aggregated, while when the molar ratios were 10 and
nanoparticles were aggregated, while when the molar ratios were 10 and 15, narrow 15, narrow absorption peaks
were obtained,
absorption peaksindicating that the silver
were obtained, nanoparticles
indicating that thewere well
silver dispersed. The
nanoparticles reason
were well
for this, according to Song et al., was the use of too little NaBH 4 so that
dispersed. The reason for this, according to Song et al., was the use of too little NaBH 4 so boron hydroxide
B(OH) 3 (produced
that boron hydroxide by hydrolysis of NaBH4by
B(OH)3 (produced , see Equationof
hydrolysis (2)) was absorbed
NaBH into the silver
4, see Equation (2)) was
nanoparticles, reducing the electron density and causing deep
absorbed into the silver nanoparticles, reducing the electron density and causing aggregation. On the other
deep
hand, when an excessive amount of NaBH4 was used, a thick layer of BH4− prevented the
aggregation. On the other hand, when an excessive amount of NaBH 4 was used, a thick
absorption of4− boron hydroxide onto the surfaces of silver nanoparticles, resulting in well-
layer of BH prevented the absorption of boron hydroxide onto the surfaces of silver
dispersed nanoparticles. These results indicate that NaBH4 acted not only as a reducing
nanoparticles, resulting in well-dispersed nanoparticles. These results indicate that
agent but also as a stabilizer protecting against the aggregation of silver nanoparticles.
NaBH4 acted not only as a reducing agent but also as a stabilizer protecting against the
aggregation of silver Ag nanoparticles.
+
+ BH4− + 3H O → Ag0 + B(OH) + 3.5H (2)
2 3 2
Ag+ + BH4− + 3H2O → Ag0 + B(OH)3 + 3.5H2 (2)
• The effect of stabilizer concentration
The main purpose of introducing SDS into the solution was to prevent the growth and
aggregation of silver nanoparticles. Figure 15 shows UV-Vis spectra of silver nanoparticles
with different SDS/AgNO3 weight ratios (0.5–20). The nanoparticles were synthesized
under conditions of an initial AgNO3 concentration (0.001 M) and a NaBH4 /AgNO3
molar ratio of 4. As the SDS concentration increased, the color of the solutions changed
from brown to yellow. At high SDS/AgNO3 weight ratios (5, 20), narrow plasmonic
absorption peaks were observed at 400 nm, confirming the nanocrystalline nature and
well-dispersed state of the silver particles. However, when the weight ratios were low
(0.5, 2), the absorption peaks became broad, indicating that the silver nanoparticles were
aggregated. These results imply that with the right amount of SDS, it absorbs the surface of
silver nanoparticles and protects them from steric growth and aggregation.
Molecules 2023,28,
Molecules2023, 28,4932
x FOR PEER REVIEW 28 of 38
29 39

Figure 14. UV-Vis absorption spectra of the silver nanoparticles prepared at different NaBH4/AgNO3
molar ratios. Reproduced with permission from [89].

• The effect of stabilizer concentration

The main purpose of introducing SDS into the solution was to prevent the growth
and aggregation of silver nanoparticles. Figure 15 shows UV-Vis spectra of silver
nanoparticles with different SDS/AgNO3 weight ratios (0.5–20). The nanoparticles were
synthesized under conditions of an initial AgNO3 concentration (0.001 M) and a
NaBH4/AgNO3 molar ratio of 4. As the SDS concentration increased, the color of the
solutions changed from brown to yellow. At high SDS/AgNO3 weight ratios (5, 20), narrow
plasmonic absorption peaks were observed at 400 nm, confirming the nanocrystalline
nature and well-dispersed state of the silver particles. However, when the weight ratios
were low (0.5, 2), the absorption peaks became broad, indicating that the silver
nanoparticles were aggregated. These results imply that with the right amount of SDS, it
Figure14.
Figure 14.UV-Vis
UV-Visabsorption
absorptionspectra
spectraofofthe
thesilver
silvernanoparticles
nanoparticlesprepared
preparedatatdifferent
different NaBH
NaBH 4/AgNO
4 /AgNO 33
absorbs the surface of silver nanoparticles and protects them from steric growth and
molar ratios. Reproduced with permission from
molar ratios. Reproduced with permission from [89]. [89].
aggregation.
• The effect of stabilizer concentration
The main purpose of introducing SDS into the solution was to prevent the growth
and aggregation of silver nanoparticles. Figure 15 shows UV-Vis spectra of silver
nanoparticles with different SDS/AgNO3 weight ratios (0.5–20). The nanoparticles were
synthesized under conditions of an initial AgNO3 concentration (0.001 M) and a
NaBH4/AgNO3 molar ratio of 4. As the SDS concentration increased, the color of the
solutions changed from brown to yellow. At high SDS/AgNO3 weight ratios (5, 20), narrow
plasmonic absorption peaks were observed at 400 nm, confirming the nanocrystalline
nature and well-dispersed state of the silver particles. However, when the weight ratios
were low (0.5, 2), the absorption peaks became broad, indicating that the silver
nanoparticles were aggregated. These results imply that with the right amount of SDS, it
absorbs the surface of silver nanoparticles and protects them from steric growth and
aggregation.

Figure 15.UV-Vis
Figure15. UV-Visabsorption
absorptionspectra
spectraofofthe
thesilver
silvernanoparticles
nanoparticlesprepared
preparedwith
withdifferent
differentSDS/AgNO
SDS/AgNO
33
weight
weightratios.
ratios.Reproduced
Reproducedwith
withpermission
permissionfrom from[89].
[89].

Researchers
Researchers from
from the
theAlqadi
Alqadi research
research group
group [96][96]synthesized
synthesized silver
silver nanoparticles,
nanoparticles,
where they controlled their size by changing the pH value of the
where they controlled their size by changing the pH value of the reaction system. reaction system.
ForFor
the
the synthesis,
synthesis, they
they used
used silvernitrate
silver nitrate(AgNO
(AgNO 3 ),which
3), whichwas
was reduced
reduced with
with ascorbic
ascorbic acid
acid
(C
(C6 6H
H88O
O66),
), while
whilesodium
sodium citrate (Na(Na
citrate 3 C63H O75O
C56H ) was used as a stabilizer. The pH was manip-
7) was used as a stabilizer. The pH was
ulated with the addition of either sodium hydroxide
manipulated with the addition of either sodium hydroxide (NaOH) or citric
(NaOH) oracid
citric(Cacid
6 H8(C
O76).H8O7).
•• The
Theeffect
effectofofpH
pH
The changes in the size of silver nanoparticles in response to a change in the pH of the
solution were monitored. Absorption spectra recorded at different pH values are shown in
Figure 16. Based on the plasmon resonance peaks obtained, it was found that at high pH,
silver nanoparticles of smaller sizes were obtained (the plasmon resonance peak shifted
towards the short wavelength region as well as increasingly narrow peaks) compared to
Figure 15. UV-Vis absorption spectra of the silver nanoparticles prepared with different SDS/AgNO3
those
weightobtained at low pHwith
ratios. Reproduced values. The difference
permission from [89]. was attributed to the different rates of
precursor reduction.
Researchers from the Alqadi research group [96] synthesized silver nanoparticles,
where they controlled their size by changing the pH value of the reaction system. For the
synthesis, they used silver nitrate (AgNO3), which was reduced with ascorbic acid
(C6H8O6), while sodium citrate (Na3C6H5O7) was used as a stabilizer. The pH was
manipulated with the addition of either sodium hydroxide (NaOH) or citric acid (C6H8O7).
• The effect of pH
 The
Thechanges
changesin inthe
thesize
sizeofofsilver
silvernanoparticles
nanoparticlesininresponse
responseto toaachange
changein inthe
thepH
pHof of
the
the solution
solution were
were monitored.
monitored. Absorption
Absorption spectra
spectra recorded
recorded atat different
different pH
pH values
values are
are
shown
shownin inFigure
Figure16.
16.Based
Basedon onthe
theplasmon
plasmonresonance
resonancepeaks
peaksobtained,
obtained,ititwas
wasfound
foundthat
thatatat
high
highpH,
pH,silver
silvernanoparticles
nanoparticlesof ofsmaller
smallersizes
sizeswere
wereobtained
obtained(the
(theplasmon
plasmonresonance
resonancepeakpeak
shifted
shifted towards
towards thethe short
short wavelength
wavelength region
region as
as well
well as
as increasingly
increasingly narrow
narrow peaks)
peaks)
Molecules 2023, 28, 4932 29 of 38
compared
compared to to those
those obtained
obtained at at low
low pH
pH values.
values. The
The difference
difference was
was attributed
attributed toto the
the
different
differentrates
ratesofofprecursor
precursorreduction.
reduction.

Figure
Figure16. Absorptionspectra
16.Absorption
Absorption spectraof
spectra ofof AgNPs
AgNPs
AgNPs atatat different
different
different pHpH
pH values.
values.
values. Reproduced
Reproduced
Reproduced withwith
with permission
permission
permissionfrom
from
from
[96]. [96].
[96].

InInaddition
In additionto
addition tothe
to theinverse
the inverseproportionality
inverse proportionalitybetween
proportionality between particle
betweenparticle size
particlesize and
sizeand
andpHpH
pHvalue,
value, ititis
value,it isis
clear
clear that
clearthat increasing
thatincreasing the
increasingthe pH
thepH value
pHvalue yields
valueyields spherical
yieldsspherical nanoparticles,
sphericalnanoparticles, while
nanoparticles,while at
whileat low
atlow pH,
lowpH, rods
pH,rods
rods
and
andtriangular
and triangularparticle
triangular particleshapes
particle shapeswere
shapes wereformed,
were formed,as
formed, asshown
as shownin
shown inFigure
in Figure17.
Figure 17. The
17.The irregularity
irregularityof
Theirregularity ofthe
of the
the
particle
particle shape
particleshape was
shapewas attributed
wasattributed to
attributedto the
totheslow
theslow reduction
slowreductionrate of
reductionrate the
rateof precursor,
ofthe as
theprecursor, well
precursor,as as the
aswell
wellaspoor
as the
the
balance
poor between
poorbalance nucleation
balancebetween and
betweennucleation growth
nucleationand processes.
andgrowth
growthprocesses.
processes.

Figure 17.
17.TEM
Figure17.
Figure TEMimage
TEM imageof
image ofnanoparticles
of nanoparticlesformed
nanoparticles formedat
formed atatdifferent pH
differentpH
different values.
values.Reproduced
pHvalues. with
Reproducedwith
Reproduced permission
withpermission
permission
from
from[96].
[96].
from [96].

Beyribey and co-workers [83] have synthesized platinum nanoparticles by the chemical
reduction in hexachloroplatinic acid (H2 PtCl6 ) with hydrazine (N2 H4 ). The purpose of the
experiment was to study the effect of temperature and pH on the structure of platinum
particles.
• The effect of temperature and pH
The synthesis of platinum nanoparticles was carried out at pH equal to 4, 7, or 10
and at 25 ◦ C, 40 ◦ C, or 50 ◦ C. At the low pH of the solution, no characteristic structures of
platinum nanoparticles formed; they were only observed when the pH of the solution was
 purpose of the experiment was to study the effect of temperature and pH on the structure
of platinum particles.
• The effect of temperature and pH
The synthesis of platinum nanoparticles was carried out at pH equal to 4, 7, or 10 and
Molecules 2023, 28, 4932 30 of 38
at 25 °C, 40 °C, or 50 °C. At the low pH of the solution, no characteristic structures of
platinum nanoparticles formed; they were only observed when the pH of the solution was
10. A significant difference in the morphological distribution of platinum particles at high
10. A significant
temperatures anddifference
pH was in theobserved
also morphological distribution
compared to that ofofthe
platinum particlesobtained
nanoparticles at high
temperatures and pH was also observed compared to that of the nanoparticles
at lower temperatures and lower pH values, as shown in the photographs below (Figureobtained at
lower
18). temperatures and lower pH values, as shown in the photographs below (Figure 18).

Figure 18.
Figure 18. SEM
SEM micrographs
micrographs at
at different
different temperatures
temperatures and
and pH
pH values.
values. Reproduced
Reproduced with
with permission
permission
from [83].
from [83].

The zeta
The zetapotential,
potential, informing
informing aboutabout the physical
the physical stability
stability of emulsions of and
emulsions and
suspensions,
suspensions, was also measured. If all the particles in a suspension have a
was also measured. If all the particles in a suspension have a high negative or positive zetahigh negative
or positivethen
potential, zetathey
potential, theneach
will repel theyother
will repel eachnot
and will other
tendand will not tend
to flocculate. to flocculate.
However, if the
However,
particles if the
have lowparticles have low
zeta potential zeta then
values, potential
therevalues, thentothere
is no force is nothe
prevent force to prevent
particles from
the particles
clumping from clumping
together. The general together.
dividingThe linegeneral
between dividing lineunstable
stable and betweensuspensions
stable and
unstable
is usuallysuspensions
taken at +30ismV usually
or −30taken
mV.atParticles
+30 mV with
or −30a mV.
zeta Particles
potentialwith
more a zeta potential
positive than
more positive than +30 mV or more negative than −30 mV are usually
+30 mV or more negative than −30 mV are usually considered stable. It has been shown considered stable.
It has
that been shown
platinum that synthesized
particles at 25 ◦ C
platinum particles synthesized at 25 °C
in the pH range in the
of 4–7 arepH range of
unstable 4–7 are
(Table 6).
unstable (Table 6).
Table 6. The average zeta potential of platinum particles was obtained at various temperatures and
Table
pH 6. The
values. average zeta
Reproduced potential
with of platinum
permission particles was obtained at various temperatures and
from [83].
pH values. Reproduced with permission from [83].
Zeta Potential Analysis (mV) T = 25 ◦ C T = 40 ◦ C T = 50 ◦ C
Zeta Potential Analysis (mV) T = 25 °C T = 40 °C T = 50 °C
pH = 4 Pt + water −14.5 −43.8 −29.4
pH==74
pH PtPt
+ +water
water −14.5
−20.7 −43.8
−50.8 −−29.4
55.7
pH==107
pH PtPt
+ +water
water −20.7
−78.1 −50.8
−45.4 −−55.7
52.4
pH = 10 Pt + water −78.1 −45.4 −52.4
An equally interesting study was conducted by scientists from the Patharkar re-
search group [105], who synthesized ruthenium nanoparticles by the chemical reduction in
ruthenium chloride (RuCl3 ) using sodium borohydride (NaBH4 ) as a reducing agent and
sodium dodecyl sulfate (SDS) as a stabilizer (other stabilizers such as PVP, CTAB, and AOT
were also used). The influence of changes in such parameters as the molar ratio (MR) of
SDS/RuCl3 , NaBH4 /RuCl3, or the type of stabilizer used on the Ru nanoparticles size and
their size distribution was established.
• The effect of stabilizer concentration
The SDS/RuCl3 molar ratio was changed from 1 to 40, keeping the RuCl3 concentration
at 0.2 mM and the NaBH4 /RuCl3 molar ratio at 30. As shown in Figure 19a, the particle
 sodium dodecyl sulfate (SDS) as a stabilizer (other stabilizers such as PVP, CTAB, and
AOT were also used). The influence of changes in such parameters as the molar ratio (MR)
of SDS/RuCl3, NaBH4/RuCl3, or the type of stabilizer used on the Ru nanoparticles size and
their size distribution was established.
Molecules 2023, 28, 4932 • The effect of stabilizer concentration 31 of 38
The SDS/RuCl3 molar ratio was changed from 1 to 40, keeping the RuCl 3
concentration at 0.2 mM and the NaBH4/RuCl3 molar ratio at 30. As shown in Figure 19a,
the
sizeparticle sizeas
decreased decreased
the MR ofasSDS/RuCl
the MR of3 increased
SDS/RuClto 3 increased to 20. The
20. The diameter diameter
of Ru of Ru
nanoparticles
nanoparticles
was found towas found
be 90 nm atto MR
be 90= 1nm at 20
and MRnm = 1atand
MR20 = nm at MR
20. At MR =>20.20,At
theMR > 20, the
particle size
particle sizeasincreased
increased as the3 ratio
the SDS/RuCl SDS/RuCl 3 ratioThe
increased. increased. The
large size of large size of thenanoparticles
the ruthenium ruthenium
nanoparticles
formed at MRformed = 1 wasatinterpreted
MR = 1 as wasdueinterpreted
to a higherasdegree
due to a higher degree
of agglomeration, of
which
was the result which
agglomeration, of an insufficient
was the resultamount
of anofinsufficient
stabilizingamount
agent inofthe system. agent
stabilizing On theinother
the
hand, an
system. Onincreasingly higher
the other hand, ansurfactant concentration
increasingly increased
higher surfactant the viscosityincreased
concentration of the system,
the
which led
viscosity of to
thea decrease in the led
system, which migration rate of in
to a decrease thethe
surfactant
migration and/or
rate ofa decrease in the
the surfactant
diffusion
and/or rate of the
a decrease inmicelles and a decrease
the diffusion in electrostatic
rate of the micelles and repulsion, which
a decrease in had the effect
electrostatic
of promoting
repulsion, which thehad
agglomeration
the effect ofprocess of the
promoting particles
the and so larger
agglomeration processnanoparticles were
of the particles
ultimately
and so largerformed.
nanoparticles were ultimately formed.

(a) (b)
Figure
Figure19.
19.The
Theeffects
effectsof
of (a)
(a) SDS/RuCl
SDS/RuCl3 3MR
MRand
and(b)
(b)NaBH
NaBH4/RuCl 3 MR on the particle size using a
4 /RuCl3 MR on the particle size using a
particle
particlesize
sizeanalyzer.
analyzer.Reproduced
Reproducedwithwithpermission
permissionfrom
from[105].
[105].

•• The
Theeffect
effectofofreductant
reductantconcentration
concentration
The
The researchers decided totostudy
researchers decided studythe the
effecteffect of NaBH
of NaBH 4 concentration (MR
4 concentration (MR NaBH4 /
NaBH
RuCl43/RuCl 3 = 10–30) on the size of Ru nanoparticles, holding other parameters constant
= 10–30) on the size of Ru nanoparticles, holding other parameters constant
(RuCl
(RuCl3 = 0.2 mM,MR
3 = 0.2 mM, MRSDS/RuCl
SDS/RuCl 3 = 20). The scientists observed that at a lower NaBH4/RuCl3
3 = 20). The scientists observed that at a lower NaBH4 /RuCl3
molar
molar ratio (MR = 10), the size of the nanoparticleswas
ratio (MR = 10), the size of the nanoparticles waslarger
largerdueduetotoinsufficient
insufficientreduction
reduction
ininRuCl
RuCl3 (Figure 19b). However, as the molar ratio increased from 15toto30,
3 (Figure 19b). However, as the molar ratio increased from 15 30,narrow
narrowpeaks
peaks
were
wereobtained,
obtained,suggesting
suggestingthatthatthetheRu Runanoparticles
nanoparticlesproduced
producedwere weresmaller
smallerininsize.
size.Based
Based
ononthe
theresults,
results,the theresearchers
researchersconcluded
concludedthat thataalower
lowerconcentration
concentrationof ofNaBH
NaBH4 4produces
produces
boron
boronhydroxide
hydroxidethrough
throughhydrolysis
hydrolysisofofNaBH NaBH4.4 .The
Theboron
boronhydroxide
hydroxidewas wasthen
thenabsorbed
absorbed
into
intothe
theRu Runanoparticles,
nanoparticles,reducing
reducingthe theelectron
electrondensity
densityofofthe
thesurface
surfaceandandcausing
causingthetheRu
Ru
nanoparticles
nanoparticles to aggregate, resulting in a larger nanoparticle size. On the other hand,aa
to aggregate, resulting in a larger nanoparticle size. On the other hand,
higher
higherconcentration
concentrationofofNaBH NaBH4 4increased
increasedthe theconcentration
concentrationofofboron
boronhydroxide,
hydroxide,which
which
formed
formedaathick thickBH BH4−4−layer,
layer,preventing
preventingboron boronhydroxide
hydroxidefromfrombeing
beingabsorbed
absorbedintointothe
the
surface
surfaceofofRu Runanoparticles,
nanoparticles,resulting
resultingininwell-dispersed
well-dispersedyet yetsmaller
smallernanoparticles.
nanoparticles.
•• The
Theeffect
effectofofstabilizer
stabilizertype
type
In order to establish the effect of different types of stabilizing agents on nanoparticle
size, a series of syntheses were performed in which PVP, SDS, CTAB, or AOT were used
as stabilizers, and the final materials were subjected to particle size testing (RuCl3 con-
centration = 0.2 mM, MR surfactant/RuCl3 = 20, MR NaBH4 /RuCl3 = 30). The smallest
particle size was obtained when using PVP (~20 nm) and SDS (~25 nm), which was much
smaller than that measured when AOT and CTAB were used as stabilizers (Figure 20). The
explanation was that PVP, due to its structure, can act as both a stabilizer and a reducing
agent, which resulted in the formation of particles of small size. In contrast, in the presence
of CTAB, which is a cationic surfactant, the Ru nanoparticles were attracted to the positive
 size, a series of syntheses were performed in which PVP, SDS, CTAB, or AOT were used
as stabilizers, and the final materials were subjected to particle size testing (RuCl3
concentration = 0.2 mM, MR surfactant/RuCl3 = 20, MR NaBH4/RuCl3 = 30). The smallest
particle size was obtained when using PVP (~20 nm) and SDS (~25 nm), which was much
smaller than that measured when AOT and CTAB were used as stabilizers (Figure 20).
Molecules 2023, 28, 4932 32 of 38
The explanation was that PVP, due to its structure, can act as both a stabilizer and a
reducing agent, which resulted in the formation of particles of small size. In contrast, in
the presence of CTAB, which is a cationic surfactant, the Ru nanoparticles were attracted
charge of the surfactant,
to the positive charge of where they agglomerated
the surfactant, where theynear the outer surface
agglomerated ofouter
near the the micelles,
surface
resulting in the formation
of the micelles, ofthe
resulting in larger nanoparticles.
formation of larger nanoparticles.

Figure20.
Figure 20.Study
Studythe
theeffect
effectofofdifferent
different types
types of stabilizing
of stabilizing agents
agents on particle
on particle size size
usingusing a particle
a particle size
size analyzer.
analyzer. Reproduced
Reproduced with with permission
permission from from
[105].[105].

4.
4. Conclusions
Conclusions and and Future
Future Perspectives
Perspectives of of Nanoparticles
Nanoparticles
In this review article, we have tried to introduce
In this review article, we have tried to introduce the reader
the to to
reader thethe
basic issues
basic issuesrelated to
related
the field of nanotechnology and transition metal nanoparticles. This
to the field of nanotechnology and transition metal nanoparticles. This technology is atechnology is a growing
field in thefield
growing current
in theglobal market
current globaland has a wide
market range
and has of applications
a wide in every industrial
range of applications in every
sector due to its unique properties. The global market
industrial sector due to its unique properties. The global market for thisfor this advanced technology
advancedis
growing at a very fast pace, and thus, the use of nanomaterials
technology is growing at a very fast pace, and thus, the use of nanomaterials and and nanoproducts is
expected to increase worldwide in the coming years. However,
nanoproducts is expected to increase worldwide in the coming years. However, adverse adverse effects of some
nanomaterials on human health and the environment have been documented. Therefore,
effects of some nanomaterials on human health and the environment have been
there is an urgent need to develop new approaches and standardized procedures to study
documented. Therefore, there is an urgent need to develop new approaches and
the potentially hazardous effects of nanoparticles on human health and the environment.
standardized procedures to study the potentially hazardous effects of nanoparticles on
Nanotechnology has so far already played a key role in the development of various fields.
human health and the environment. Nanotechnology has so far already played a key role
In particular, nanoparticles are widely used in a number of branches of industry, from
in the development of various fields. In particular, nanoparticles are widely used in a
biomedicine to engineering, due to their unique size-dependent physical and chemical
number of branches of industry, from biomedicine to engineering, due to their unique
properties (e.g., high surface-to-volume ratio).
size-dependent physical and chemical properties (e.g., high surface-to-volume ratio).
The use of nanotechnology in medicine is having a huge impact on human health in
The use of nanotechnology in medicine is having a huge impact on human health in
terms of diagnosis, prevention, and treatment of diseases. The development of metallic
terms of diagnosis, prevention, and treatment of diseases. The development of metallic
nanoparticles is very rapid and multidirectional, giving these materials the potential as new
nanoparticles is very rapid and multidirectional, giving these materials the potential as
tools for future therapeutic drug delivery methods, especially in the treatment of cancer,
new tools for diabetes,
inflammation, future therapeutic
and antiviral drug delivery
therapy [32].methods,
Key issues especially in the treatment
that are associated of
with the
cancer, development
clinical inflammation,ofdiabetes, and antiviral
nanoparticles therapy
are biological [32]. Keylarge-scale
difficulties, issues thatmanufacturing,
are associated
biocompatibility, and cost-effectiveness compared to current therapies [154]. large-scale
with the clinical development of nanoparticles are biological difficulties, Therefore,
manufacturing,
extensive research biocompatibility, and cost-effectiveness
in the field of nanomedicine, especially compared to current
in drug delivery therapies
systems, is
[154]. Therefore, extensive research in the field of nanomedicine,
urgently needed. The rapid development of nanotechnology provides practical tools especially in drug
for
delivery
using thissystems, is urgently
fascinating needed.
technology, The rapid
especially development
in the of nanotechnology
aspect of biofuels. As alreadyprovides
proven,
practical
the use oftools for using
different typesthis
of fascinating technology, especially
nanoparticles—especially metallic inones,
the aspect of production
for the biofuels. As
of various types of biofuels (biodiesel, biohydrogen, biogas, and bioethanol) can improve
productivity and production efficiency [155]. However, high production, separation, and
reuse costs limit the practical application of biocatalysts for biofuel production. The use
of a nanobiocatalyst can overcome the disadvantages, mainly stability and reusability,
thus reflecting the importance of biomass-based biorefinery to make it cost-effective and
sustainable [156].
With the development of nanotechnology, a tendency toward the miniaturization of
various goods has strengthened. For this reason, technology has changed from semi to
milli and from micro to nano. It will not be a surprise if picotechnology soon appears
Molecules 2023, 28, 4932 33 of 38

on the market, but this will not happen suddenly. At the current rate of nanotechnology
development, almost all branches of engineering, from electronics and medicine to robotics,
are expected to rely on it for their efficiency, durability, reliability, and reproducibility.
Therefore, may dare to claim that nanotechnology is the engineering of the future. With all
this said, it is important to remember that nanotechnology, which can provide us with a
wide range of capabilities that must be used thoughtfully and responsibly, is a tool with the
power to change the world into a better place to live in, only if it is used for good purposes.

Author Contributions: Conceptualization, A.F.-G.; writing—original draft preparation, P.S.; writing—

review and editing, A.F.-G. Furthermore, I.N.; visualization, P.S.; supervision, I.N.; funding acquisi-
tion, I.N. All authors have read and agreed to the published version of the manuscript.
Funding: The work was carried out within the project “Advanced biocomposites for tomorrow’s
economy BIOG-NET” funded by Foundation for Polish Science from the European Regional
Development Fund (POIR.04.04.00-00-1792/18-00) and National Science Centre (MINIATURA 4;
2020/04/X/ST5/00491). P. Szczyglewska, who received financial support from National Science Cen-
tre (MINIATURA 4; 2020/04/X/ST5/00491), thanks MDPI for the discount to publish the manuscript
in the journal Molecules.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Not applicable.

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