molecules

Systematic Review
Systematic Review on Biosynthesis of Silver Nanoparticles
and Antibacterial Activities: Application and
Theoretical Perspectives
Shafqat Qamer 1,2 , Muhammad Hibatullah Romli 3,4 , Fahrudin Che-Hamzah 5 , Norashiqin Misni 1 ,
Narcisse M. S. Joseph 1 , Nagi A. AL-Haj 6 and Syafinaz Amin-Nordin 1, *

1 Department of Medical Microbiology, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia; gs56387@student.upm.edu.my (S.Q.); norashiqin@upm.edu.my (N.M.);
narcissems@upm.edu.my (N.M.S.J.)
2 Department of Basic Medical Science, College of Medicine, Prince Sattam Bin Abdulaziz University,
Alkharj 11942, Saudi Arabia; s.ahmed@psau.edu.sa
3 Department of Rehabilitation Medicine, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia; mhibatullah@upm.edu.my
4 Malaysian Research Institute on Ageing (MyAgeing), Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia
5 Orthopaedic Department, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia; fahrudinch@upm.edu.my
6 Department of Medical Microbiology, Faculty of Medicine and Health Sciences, Sana’a University,
Sana’a 009671, Yemen; naji2005@gmail.com
* Correspondence: syafinaz@upm.edu.my







Citation: Qamer, S.; Romli, M.H.;

Abstract: The biosynthesis of silver nanoparticles and the antibacterial activities has provided
Che-Hamzah, F.; Misni, N.; Joseph, enormous data on populations, geographical areas, and experiments with bio silver nanoparticles’
N.M.S.; AL-Haj, N.A.; Amin-Nordin, antibacterial operation. Several peer-reviewed publications have discussed various aspects of this
S. Systematic Review on Biosynthesis subject field over the last generation. However, there is an absence of a detailed and structured
of Silver Nanoparticles and framework that can represent the research domain on this topic. This paper attempts to evaluate
Antibacterial Activities: Application current articles mainly on the biosynthesis of nanoparticles or antibacterial activities utilizing the
and Theoretical Perspectives. scientific methodology of big data analytics. A comprehensive study was done using multiple
Molecules 2021, 26, 5057. https://
databases—Medline, Scopus, and Web of Sciences through PRISMA (i.e., Preferred Reporting Items
doi.org/10.3390/molecules26165057
for Systematic Reviews and Meta-Analyses). The keywords used included ‘biosynthesis silver
nano particles’ OR ‘silver nanoparticles’ OR ‘biosynthesis’ AND ‘antibacterial behavior’ OR ‘anti-
Academic Editor: Rita Cortesi
microbial opposition’ AND ‘systematic analysis,’ by using MeSH (Medical Subject Headings) terms,
Received: 8 August 2021
Boolean operator’s parenthesis, or truncations as required. Since their effectiveness is dependent
Accepted: 15 August 2021 on particle size or initial concentration, it necessitates more research. Understanding the field of
Published: 20 August 2021 silver nanoparticle biosynthesis and antibacterial activity in Gulf areas and most Asian countries
also necessitates its use of human-generated data. Furthermore, the need for this work has been
Publisher’s Note: MDPI stays neutral highlighted by the lack of predictive modeling in this field and a need to combine specific domain
with regard to jurisdictional claims in expertise. Studies eligible for such a review were determined by certain inclusion and exclusion
published maps and institutional affil- criteria. This study contributes to the existence of theoretical and analytical studies in this domain.
iations. After testing as per inclusion criteria, seven in vitro studies were selected out of 28 studies. Findings
reveal that silver nanoparticles have different degrees of antimicrobial activity based on numerous
factors. Limitations of the study include studies with low to moderate risks of bias and antimicrobial
effects of silver nanoparticles. The study also reveals the possible use of silver nanoparticles as
Copyright: © 2021 by the authors. antibacterial irrigants using various methods, including a qualitative evaluation of knowledge and a
Licensee MDPI, Basel, Switzerland. comprehensive collection and interpretation of scientific studies.
This article is an open access article
distributed under the terms and Keywords: data analytics; silver nanoparticles; biosynthesis; antibacterial activities
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).

Molecules 2021, 26, 5057. https://doi.org/10.3390/molecules26165057 https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 5057 2 of 21

1. Introduction
Silver nanoparticles Ag-NPs have shown exceptional properties. There are many ap-
plications of the Ag-NPs in bio-medical fields. The most significant advantage of Ag-NPs
is the production of antibiotics. Currently, biosynthesized Ag-NPs have been extensively
studied in the last ten years [1]. The size of Ag-NPs is adjusted according to the specific
application [2]. For instance, the Ag-NPs that are prepared for drug delivery are greater
than 100 nm in size. Furthermore, Ag-NPs are significantly used in antimicrobial applica-
tions since they have shown antimicrobial properties. These unique characteristics of silver
nanoparticles enabled their use in the fields of nanomedicine, pharmacy, biosensing, and
biomedical engineering.
Ag-NPs have many practical applications, such as antibacterial and anticancer therapeu-
tics, diagnostics, and optoelectronics, water disinfection, and other clinical/pharmaceutical
applications. Silver is known to have fascinating properties. In addition, they are cost-
effective, are abundantly found in nature, and silver nanoparticles have undeniable po-
tential compared with stable gold nanoparticles [2]. Currently, antibacterial resistance has
become very frequent and is considered as the most complex and global health challenge.
There is an urgent need to discover and synthesize new biomedicines or find natural alter-
natives, as it is of utmost priority and high demand in the present circumstances. Globally,
it has become the most challenging situation to deal with the hospital and community-
acquired infections that are particularly caused by multidrug-resistant bacteria [3]. More-
over, the death rate has been significantly increasing due to these multidrug-resistant
bacterial pathogens. Multiple studies suggest that the extensive and improper use of antibi-
otics resulted in the formation of new multidrug bacterial resistance genes [3]. Therefore,
it is essential to reduce the extensive intake of antibiotics. It is important to discover and
develop novel warfare schemes to fight against multidrug-resistant bacteria and reduce
chemical drug intake complications to treat microbial infections.
Antibacterial materials are primarily divided into inorganic, organic, and natural
materials. Natural antibacterial materials have a limited application range, while organic
antibacterial materials with low heat resistance are prone to causing bacterial drug resis-
tance. In terms of durability, heat resistance, and the emergence of antibiotic resistance in
bacterial strains, inorganic antibacterial materials outperform organic antibacterial materi-
als. As a result, it has received a lot of attention in past years [4]. For example, Ag+ ions are
a broad-spectrum antibacterial agent that may effectively stop bacteria, fungi, and algae
from growing [5]. Many inorganic nanoparticles (NPs) have shown severe cytotoxicity,
indicating that a new generation of bactericidal materials could be developed [6]. Silver
NPs, specifically, have been intensively researched due to their great antibacterial action
while causing minimum disruption to human cells [7]. For example, silicon nanowires
coated with silver nanoparticles exhibit effective antibacterial action [8]. However, most
studies [9,10] have concentrated on the antibacterial performance and mechanisms of
Ag-NPs, with little effort devoted to the development of environmentally friendly and bio-
compatible preparation techniques. Ag-NPs have different physicochemical and biological
properties than their counterparts related to the higher surface to volume ratio [11].
Multiple pieces of research have been conducted to demonstrate the impact of shapes
of Ag-NPs on their antibacterial activity. The number and position of surface plasmon
resonance (SPR) peaks are dependent on the shape of Ag-NPs. For example, spherical
particles show a single scattering peak. In contrast, anisotropic shapes such as rods,
triangular prisms, and cubes show multiple scattering peaks in the visible wavelengths due
to highly localized charge polarizations at corners and edges [12,13]. As a result, achieving a
size-tunable synthesis of Ag-NPs with substantial surface area and surface activity and poor
stability, and a strong aggregation potential is a considerable problem [14,15]. Several forms
of Ag-incorporated nanomaterials have been discovered, and Ag-NPs immobilized on
various inorganic and organic substrates have been found to have increased and extended
antibacterial properties [16].
Molecules 2021, 26, 5057 3 of 21

At present, it is still unclear how silver inhibits bacterial growth through its antibac-
terial properties [17]. According to some researchers, the antibacterial impact of Ag-NPs
could be explained by three different hypotheses: contact action, the formation of reactive
oxygen species (ROS), and release of Ag+. For example Morones [18] claims that Ag-NPs
can be linked to the surface of a bacteria’s cell membrane to disrupt the cell membrane’s
functions and enter through the bacteria to induce cytoplasm leakage and eventually result
in the bacteria’s death. The antibacterial activity of Ag-NPs, according to Wang [19], is due
to an increase in ROS concentration. They believe that ROS causes bacterial mortality by
causing intracellular oxidation, membrane potential changes, and the release of cellular
contents [20,21].
Ag nanoparticles (NPs) are gaining more popularity in a variety of industries, in
health (medication, medical tools, pharmacology, biotechnology) as well as non-health
(textiles, food, consumer goods, telecommunications, technology, power, magnetism, and
environmental remediation) sectors [22,23]. Mittal discovered that synthesizing metallic
nanoparticles with plant extracts was cost-effective and environment friendly and sup-
ported various analytical techniques [24]. Alongside the advantages, it is necessary to
point out the problems associated with the green synthesis of Ag nanoparticles. Some
major drawbacks of using plant extracts for synthesizing Ag nanoparticles include the slow
rate of synthesis, the limited number of sizes and shapes of nanoparticles; and compared
to conventional methods, plants produce a low yield of secreted proteins. Plants cannot
be manipulated due to the choice of nanoparticles either through optimized synthesis or
through genetic engineering [25].
Antimicrobial, anti-rheumatic, antiviral, bacterial, diuretic, expectorant, hypertensive,
and insecticide activities of many natural products have been published in many types
of research. Biogenic nanoparticle synthesis using plant extracts aiming for antimicrobial
potency may synergistically impact on organisms [26]. In other words, the biological
synthesis of silver nanoparticles has many promising aspects, varying by productivity and
by ease of processing to become automatically secure with such a single-phase method for
large-scale nanoparticle synthesis.
There are very few documented overviews on Ag-NPs biosynthesis and antibacterial
activity. The size, shape, and surface morphology of Ag-NPs are important factors in
determining their characteristics. The antibacterial activities of Ag-NPs are linked to the
oxidation and release of Ag+ ions into the environment, making it an excellent antibacterial
agent [27]. Compared to pure silver metal, Ag-NPs are projected to have a high surface
area to volume ratio and a high fraction of surface atoms, resulting in high antibacterial
activity [28]. Additionally, the smaller size of Ag-NPs eases the penetration through
the cell membrane and alters the intercellular processes of the bacterial cell. Most Ag
nanoparticles prepared by green synthesis are investigated for their antibacterial properties
or for cancer treatment. Recent studies have shown that Ag-NPs with diameters ranging
from 38 to 72 nm and 17 to 29 nm may be synthesized using Chrysanthemum indicum [29] or
Acacia leucophloea extract [30]. Both samples had excellent antibacterial effects. Similarly,
Ganoderma neojaponicum Imazeki was utilized to synthesize Ag- -NPs, which could be used
as chemotherapeutics against breast cancer cells [31].
The absence of predictive analytics in this field and the need to incorporate domain-
specific expert knowledge highlights the current study’s need. However, a fragmented
view of a theoretical and functional implementation of silver nanoparticle biosynthesis and
antibacterial activities leads to various conclusions. As a result, the present study aims to
know the progress of biogenic Ag-NPs. One of the objectives of this study was to look into
the most advanced methods used in the biosynthesis of nanoparticles and also how they
can be used as a possible treatment drug.

2. Methodology
Several literature reviews about the biosynthesis of metallic nanoparticles with antimi-
crobial properties were searched and collected. Many reports were found on the subject,
Molecules 2021, 26, 5057 4 of 21

and a detailed review was essential to combine all the results of the reports to drive a
conclusion and avoid any conflict of information, ambiguity, and misunderstanding. This
overview, which aims to highlight and classify bio-silver nanoparticles’ antibacterial activ-
ity, focuses exclusively on aggregating all such systematic reviews, as recommended by
Cooper and Koenka [32]. PRISMA (i.e., Preferred Reporting Items for Systematic Reviews
and Meta-Analyses) checklist was adopted to document this summary.

2.1. Step 1: Formulating the Problem

In the first step, the problem was formulated about how the biogenic Ag-NP was
assessed and established during examination of the antibacterial activity. This research
question was further elaborated to examine whether Ag-NP could serve as smart weapons
against multiple drug-resistant microorganisms, and replace the antibiotics, or if these
Ag-NP could be used as a possible treatment drug to act as an effective antimicrobial agent.

2.2. Step 2: Literature Searches for Research Syntheses

In the second step, a systematic literature review was initially conducted in December
2019. A review of the related literature was carried out on three electronic databases,
namely ScienceDirect, MEDLINE, and Scopus. A blend of specific terms was used in the
research study, taken from the title, abstract, and keywords such as (‘biosynthesis silver
nanoparticles’ OR ‘silver nanoparticles’ OR ‘biosynthesis’ OR ‘green synthesis’) AND
(‘antibacterial activity’ OR ‘antimicrobial resistance’) AND (‘systematic review’), through
the use of MeSH (Medical Subject Headings) terms, Boolean operators, parentheses, and
insertions when necessary. During the searching process, a search chain or query links were
created for each database. It was necessary to vary the syntax of each search criterion or
query according to the database. Operational definitions of each keyword were also found
in the selected articles. The final analysis included articles that contained facts, summaries,
and reviewers’ remarks.
The English language was the only language that was selected for the literature search.
Inclusion and exclusion criteria were defined for both the phases of the search procedure:
The inclusion criteria consisted of the following: (i) articles related to biosynthesized silver
nanoparticles and antibacterial effects, (ii) experimental study design, (iii) in vitro studies,
and (iv) published in English. The exclusion criteria dealt with (i) reports that did not
perform a systematic search (literature review), (ii) were not peer-reviewed, (iii) were not
centered on biosynthesis, (iv) were not linked to antibacterial-dependent outcomes, and
(v) were not published in English. Prior to the selection process, duplicate papers were
also excluded. The papers published in two journals with the same title, same first author,
same study design, sample size, and the same number of in-text citations or references
were referred to as duplicate papers. We performed thorough research to identify any
such paper and did not include in our review. The principal author’s name was used for
reviewing every retrieved research paper for inclusion and exclusion. Moreover, full texts
were also retrieved for all such authors until a resolution was reached. The papers included
in the research were searched again to find more research articles. The papers that were
retrieved again passed through the same screening procedure to ensure and increase the
authenticity of the research.

2.3. Step 3: Data Extraction

A matrix table has been used to display each selected paper. A descriptive analysis
was done to review each article. After the descriptive analysis was done, the articles were
then compared as Cooper and Koenka [32] suggested. The authors’ names, the study’s
objectives, the number of studies with the feedbacks, the identified number of criteria, and
the results and feedback were retrieved from the literature reviews. While the analysis was
carried out, the data were collected to verify and validate the solutions and results. This
analysis was done with the help of identification and description of the issues for every
keyword.
Molecules 2021, 26, 5057 5 of 21

2.4. Step 4: Evaluate the Quality of Primary Data and Research Synthesis
Detailed information was not available on the antimicrobial properties. The objective
of the current study is to collect the data and information regarding the biosynthesis
and the antibacterial properties of the Ag-NPs. The content of each article was assessed
and evaluated by using a tool, i.e., A Measurement Tool to assess Systematic Reviews
(AMSTAR). This tool is particularly used to assess and evaluate the validity and reliability
of systematic reviews [33]. In addition, it is also best for the observation [34].

2.5. Step 5: Meta-Analyzing and Integrating Their Outcomes of the Synthesis

The selected articles were organized into specific domains of Ag-NPs such as their
sizes, shapes, applications, and chemical compositions. The articles regarding the synthesis
and functions of silver nanoparticles were grouped together. The articles selected regard-
ing the synthesis of silver nanoparticles were purely based on the silver nanoparticles’
biosynthesis, and they also exhibited effective antibacterial properties. A few steps were ad-
ditionally taken to analyze the antibacterial properties of the synthesized Ag-NPs. Initially,
the author conducted an electronic literature search regarding the silver nanoparticles
to find out the original research. The articles selected by the author that documented
the biosynthesis of the Ag-NPs were the most relevant, authentic, original, and recently
published. Next, the corresponding or first author of the initial article was included, since
several articles were present on the antibacterial effects of Ag-NPs. The choice of corre-
sponding or the first author was recommended by Costas and Bordons [35], who are most
responsible for preparing the report or developing the idea. After the completion of 5 steps,
the review required integration of the results of the reviewed articles, especially to check
whether they showed disagreeing findings, and it was required to conduct a second-order
meta-analysis before recording the results or findings before the final step. The current
study results showed the absence of any discordant results; hence, the meta-analysis was
not required. The next section presents the results of the current study.

3. Results
There were about 4327 publications found, the majority of which were from the
identified records. A total of 2703 articles were left for testing after the removal of duplicates.
After their initial screening, only 83 papers were selected for a full-text examination.
Then, their eligibility was checked as per inclusion–exclusion criteria, and 28 articles
were selected for the final examination. Figure 1 shows the screening process. During
the process, the biosynthesis of silver nanoparticles and antibacterial activities were the
main focus. The selected systematic reviews were particularly dealt with the biosynthesis
of Ag-NPs through leaves and plants and the antibacterial spectrum they had. Some
systematic reviews had also investigated potential antibacterial mechanisms in Ag-NPs-
and examined their optimization in orthopedic implants. Some papers were subjected
to full-text screening to make decisions quickly; for instance, two studies [36,37] were
subjected to screening for their eligibility analysis. The main objective was to find out a
methodology that could be used to resolve any disagreement and to achieve consensus.
Table 1 illustrates the AMSTAR procedure; it shows the lowest score, i.e., 4, and the
highest score, i.e., 8. The higher the score is, the better is the quality of the review (e.g.,
score between 8 and 11 stands for good quality; between 4 and 7 stands for moderate
quality; between 0 and 3 read as lower quality) [38]. These scores are based on the 11-item
eligibility instrument where the presence of each item in each cited article was given a
score of 1 or otherwise 0. Certain citations were categorized as Can’t Answer (CA) or Not
Applicable (NA) (See Table 1).
 Molecules2021,
Molecules 26,x 5057
2021,26, FOR PEER REVIEW 6 of2221
6 of

PRISMA(Preferred
Figure1.1.PRISMA
Figure (PreferredReporting
ReportingItems
Itemsfor
forSystematic
SystematicReviews
Reviewsand
andMeta-Analysis).
Meta-Analysis).

Table
Table 1 illustrates AMSTAR table.
the1.AMSTAR procedure; it shows the lowest score, i.e., 4, and the
highest score, i.e., 8. The higher the score is, the better is the quality of the review (e.g.,
Quality Assessment Tool AMSTAR
score between 8 and 11 stands for good quality; between 4 and 7 stands for moderate
Citation quality; between 1 0 and2 3 read3 as lower
4 quality)
5 6
[38]. 7
These 8
scores 9 based
are 10 on the
11 11-item
Score
1 Khan et al. 2018eligibility
[39] 1
instrument CAwhere1 the presence
1 0of each
1 itemNA in each
1 1 article
cited 1 was1 given 8 a
2 Ahmed et al.2019score
[40] of 1 or otherwise
1 0 0. Certain
1 1 0 1 NA 1 1 1 1
citations were categorized as Can’t Answer (CA) or Not 8
3 Mishra et al. 2019 [41] 1 0 0 1 0 1 NA 1 1 1 1 8
Applicable (NA) (See Table 1).
4 Roy et al. 2019 [42] 1 0 1 1 0 1 1 1 NA 1 1 8
5 Nasrollahzadeh et al. 2019 [43] 1 0 1 1 0 1 1 1 NA 1 1 8
6 Zafar et al. 2019 [44] 1 Table
0 1. AMSTAR
1 1table. 0 1 NA 1 NA 1 1 7
7 Nisar et al. 2019 [45] 1 0 1 1 0 1 NA 1 NA 1 1 7
Quality Assessment Tool AMSTAR
8 Some et al. 2018 [46] 1 0 1 CA 0 1 NA 1 NA 1 1 6
9 FahimiradCitation
et al. 2019 [47] 1 1 0 2 1 3 CA 4 50 6 1 7 NA 8 1 9 NA 10 1 11 1 Score
7
1 10 Khan et al.2020
ElShafey 2018[48]
[39] 1 1 0 CA 1 1 CA 1 00 1 1 NANA 1 1 1 1 1 0 1 1 86
2 11 Kumar et
Ahmed et al. 2018 [49]
al.2019 [40] 1 1 0 0 1 1 CA 1 00 1 1 NANA 1 0 1 NA 1 1 1 1 85
3 12 Singh et al. 2020 [37]
Mishra et al. 2019 [41] 1
1 0
0 1
0 1
1 00 1 1 NANA 1 1 1 NA 1 1
1 1
87
13 Salleh et al. 2020 [50] 1 0 1 1 0 1 NA 1 NA 0 1 6
4 14 RoyQing
Yun’an et al.et2019 [42][36]
al. 2018 1 1 CA 0 1 1 11 00 1 1 1 NA 1 1 NANA 1 CA 1 1 86
5 15 Nasrollahzadeh
Ferdous 2020et al. 2019 [43]
[51] 1 1 CA 0 1 1 11 00 1 1 1 NA 1 1 NANA 1 0 1 1 86
6 16 Yin et
Zafar etal.
al.2020
2019[52]
[44] 1 1 CA 0 1 1 11 00 1 1 NANA 1 1 NA 1 1 1 1 1 76
7 17 Ahmadetetal.
Nisar al.2019
2019 [45]
[53] 1 1 0 0 11 11 00 1 1 NANA 1 1 NANA 1 1 1 1 77
18 Gumel et al. 2019 [25] 1 0 1 1 CA 1 NA 1 NA 1 1 7
8 19 Some et al. 2018
Escárcega-González et al.[46]
2018 [54] 1 1 0 0 11 CA
0 00 1 1 NANA 1 1 NANA 1 1 1 1 66
9 Fahimirad et al. 2019 [47] 1 0 1 CA 0 1 NA 1 NA 1 1 7
10 ElShafey 2020 [48] 1 0 1 CA 0 1 NA 1 1 0 1 6
Molecules 2021, 26, 5057 7 of 21

Table 1. Cont.

Quality Assessment Tool AMSTAR

Citation 1 2 3 4 5 6 7 8 9 10 11 Score
20 Nagar et al. 2018 [55] 1 CA 1 0 0 1 NA 1 NA 0 1 5
21 Mikhailov et al. 2018 [56] 1 0 1 1 0 1 NA 0 NA 1 1 6
22 Hamelian et al 2018 [57] 1 0 0 0 0 1 NA 1 NA 0 1 4
23 Zulfiqar et al. 2018 [58] 1 0 0 1 0 0 NA 0 NA 1 1 4
24 Haqq 2018 [59] 1 0 0 0 0 1 NA 0 NA 1 1 4
25 Ishak et al. 2019 [60] 1 0 CA 0 0 1 NA 1 NA 1 1 5
26 de Aragao et al. 2019 [61] 1 0 CA 0 0 1 NA 1 NA 1 1 5
27 Hasnain Met al. 2019 [62] 1 0 CA 1 0 1 NA 1 NA 1 1 6
28 Das et al. 2020 [63] 1 0 CA 1 0 1 NA 1 NA 1 1 6
1 = Yes; 0 = No; CA = Can’t Answer; NA = Not Applicable (Source: Sharifetal. 2013) [38] Characteristics of Included Studies.

Table 2 shows the 11 items that determined the eligibility criteria of each citation
illustrated in Table 1 [38].

Table 2. Eligibility criteria of citations.

S. No Eligibility Item
1. Provided with the prior design.
2. Extraction of data and selection of duplicate study is made.
3. Literature has been searched comprehensively.
4. The publication has passed the inclusion criteria.
5. An index of articles (with inclusion and/or exclusion) is given.
6. Features of included articles are provided.
7. The scientific quality of the included studies was evaluated before documenting.
8. The scientific quality of the included studies was used to formulate conclusions.
9. Appropriate methods were used to combine the studies’ findings.
10. The likelihood of publication bias was assessed.
11. The conflict of interest was stated.

3.1. Characteristics of Included Studies

The information in Table 3 from the 28 primary studies and reviews seven of which are
in vitro studies support that Ag-NPs were biosynthesized successfully. Various biological
sources were used for the green synthesis of Ag-NPs, including plants (n = 25), algae (n = 2),
and fungi (n = 1). However, the general approach referred to a herbal-mediated fabrication
of Ag-NPs (89.28% of studies). These studies utilized different green plants extracts, such
as leaf extracts, stem bioresources, and others.

Table 3. Characteristics and observations from the systematic reviews.

Author Year Objective of Study Summary Finding

• To determine the potential antibacterial • To avoid implant-related infection and show
Yun’an Qing et al. mechanisms of Ag-NPs. how Ag-NPs with high antibacterial efficacy
2018 • To elaborate methods to enhance the are commonly used in implant surface
[36]
biocompatibility of Ag-NPs. modification.
• To elucidate the factors such as the size, shape
scale, surface chemistry, and stability.
• How the defined structural factors such as size,
• To examine how Ag-NPs’ antibacterial
Ferdous [51] 2020 shape scale, surface chemistry, and stability
activities are influenced by structural factors,
affect the antibacterial mechanism of Ag-NPs.
which could aid in the development of more
effective Ag-NPs.
Molecules 2021, 26, 5057 8 of 21

Table 3. Cont.

Author Year Objective of Study Summary Finding

• It is centered on the recent data on
• To gather the most up-to-date information on
Ag-NP-based nanostructures’ biomedical
Yin et al. [52] 2020 the biomedical applications of Ag-NP-based
applications, and parameters such as toxicity,
nanostructures.
physiochemical, and bio-functional properties,
• In the field of nanotechnology, green
synthesized Ag-NPs have unrivaled
• To assess the green synthesis, characterization,
significance.
Ahmad et al. [53] 2019 and biological activities of Ag-NPs using a
• Ag-NPs have a broad range of pharmacological
variety of biological sources.
operations, and their cost-effectiveness makes
them a viable alternative to local medicines.
• Thymus Kotschyanus extract was used in this
study to synthesize Ag-NPs in an
• To focus on Thymus-based green silver environmentally friendly, healthy, and practical
nanoparticle synthesis. way. There were no chemical substances
Hamelian et al. [57] 2018 • To investigate an antibacterial, antioxidant, and involved.
cytotoxic effects of synthesized nanoparticles. • Silver nanoparticles with a diameter of 50 nm
in this herb have a strong antibacterial and
antioxidant impact.
• The antimicrobial properties of silver
• To learn more about silver nanoparticle
nanoparticles and plant extracts, such as
Gumel et al. [26] 2019 biogenesis and the mechanisms that underpin
antibacterial and antifungal properties, are
their antimicrobial efficacy.
demonstrated in this report.
• To develop a green one-pot synthesis process
Escárcega-González for Ag-NP production that incorporates the • The results show that the Ag-NPs used in this
2018 Acacia rigidula extract as a therapeutic agent to study can destroy pathogenic bacteria.
et al. [54]
treat pathogens.
• The biosynthesized Ag-NPs is classified using
• To investigate whether a leaf broth of A. indica a variety of instrumental techniques. The
Nagar et al. [55] 2018 can be used as a reducing and capping agent to particles were described as crystalline average
synthesize Ag-NPs. size cubical particles with a high level of
stability.
• The synthesis of MNPs is influenced by
• This analysis focuses on the synthesis of temperature, incubation time, and pH. This
biological MNPs by plants and microbes, as study found that biologically synthesized
Ahmad et al. [40] 2019 well as their cellular uptake, biocompatibility, MNPs had higher biocompatibility than MNPs
cytotoxicity, and biomedical applications. synthesized using different physicochemical
methods.
• Ag-NPs are regarded as a crucial expansion in
the continuum of nanomaterials due to the
• The aim of this research was to look into
versatile qualities it offers in terms of
current Ag-NP biosynthesis trends; and
Mishra et al. [41] 2019 application in various fields of study.
• To find out if they have antimicrobial activity
• It is likely that NP synthesis will be used in the
and any biotechnological potential as well.
future to make antimicrobial compounds in
biomedical nanotechnology.
• At this time, experimentally determining the
• To synthesize Ag-NP using different scale, form, etc. and feasibility of
physicochemical methods. biosynthesized Ag-NP dispersion.
Mikhailov et al. [56] 2018 • To look at a certain synthetic method that use • Implementing silver nanoparticles NP
biological objects to make elemental silver biosynthesis and predefined parameters will
nanoparticles. eventually necessitate the development of new
concepts and methods.
• Nanoparticles appear to be able to cross the
• To address recent developments in green
membrane and cause damage. Loading drugs
synthesis of silver nanoparticles, while the
Roy et al. [42] 2019 on the nanoparticle surface can increase the
mechanism of antimicrobial action underpins
efficiency of biocidal motion in addition to
their use as antimicrobial agents.
disrupting the membrane.
• According to morphological and structural
characteristics, the study found Ag-NPs as
• To determine if plant extract Fagonia cretica
hugely crystalline, averaging 16 nm size, and
could be used as a reducing and stabilizing
Zulfiqar et al. [58] 2019 the presence of active bio-reducing and
agent in the synthesis of Ag-NPs and to see
stabilizing agents in the Fagonia cretica extract.
how effective the extract is against bacteria.
• Ag-NPs revealed antibacterial activity against
a few other plant extracts
Molecules 2021, 26, 5057 9 of 21

Table 3. Cont.

Author Year Objective of Study Summary Finding

• To examine whether by reducing Ag+ ions
• This research looked into the green synthesis of
Nasrollahzadeh et al. (plant extracts) and controlling the size of the
2019 Ag-based nano catalysts such as Ag-NPs,
[43] NPs, the Ag-based nanoparticles can be
AgPD NPs, and AU Ag-NPs.
produced.
• To emphasize the importance of plant extracts • Ag-NPs have been shown to be effective in
in the bio-fabrication of nanoparticles as a treating M. Incognita.
Zafar et al. [44] 2019 renewable, non-toxic, and environmentally • Silver nanoparticles are used in food packaging
friendly process. to increase the shelf life of the product.
• Several antimicrobial green-base nanoparticles
have been successfully developed from a
• To assess the antimicrobial properties of variety of biological sources, the most
Nisar et al. [45] 2019 various biosynthesized metal nanoparticles, as prominent of which are plants.
well as the mechanisms by which they work. • These bio-nanomaterials have proved to be
effective against bacterial and fungi that cause
disease (both plant and human).
• Biomolecules act as both reducing and
• The goal of this work is to look into the stabilizing agents in the green pathway,
synthesis and characterization of resulting in biocompatible NPs.
Some et al. [46] 2019 biomolecule-capped Ag-NPs; and • In the literature, promising findings on
• To evaluate antimicrobial properties in the Ag-NPs’ antimicrobial activity against a variety
presence of human and plant pathogens. of pathogenic microorganisms have been
recorded.
• The potential of Ag-NPs to perform in a
• To examine the biological activities of Ag-NP number of bioassays has also been lauded.
Haqq et al. [59] 2018 and plant-mediated green synthesis. • This study would help researchers create new
Ag-NP-based drugs using green technology.
• To share the latest research on metal and metal
• Plant extracts have attracted a lot of interest
oxide nanoparticles, including silver and gold
because of their ability to minimize and
Ishak et al. 2019 [60] 2019 nanoparticles; and to issue directions and
stabilize metal nanoparticles in a single phase
implementations for green synthesis methods
using their unique natural properties.
based on plant extracts.
• The environmentally sustainable and general
approach can be extended to a number of
• To focus on MNP and Monps biosynthesis
therapeutic and scientific uses, as well as other
procedures, including a comparison of green
noble metals such as Ag and Pd.
synthesis and conventional chemistry methods,
El Shafey [48] 2020 • The low cost and ease of synthesis of
as well as several new directions for green
antimicrobial nanoparticles using local plant
synthesis of nanoparticles from various plant
extracts without the use of a toxic chemical
parts, particularly plant leaf extract.
reducer are the main advantages of the greener
preparation methods.
• Ag-NPs were tested for antimicrobial activity
against Gram-negative and Gram-positive
• To make silver nanoparticles, researchers use a
Escherichia coli and Staphylococcus aureus, and
natural polysaccharide derived from red
both samples showed antimicrobial activity
marine algae (Gracilaria birdiae).
de Aragao et al. [61] 2019 against E. coli.
• To monitor the antimicrobial activity of the
• The Ag-NPs were made using natural sources
synthesized NPs against representative strains
such as red algae, which have favorable
of Staphylococcus aureus and Escherichia coli.
properties, in a simple, fast, and one-step
process.
• According to antibacterial activity testing,
purple heart plant extracts primarily resulted
in the removal of silver ions and the
• Synthesis of stability in silver nanoparticles stabilization of silver nanoparticles.
Hasnain et al. 2019 from extract of purple heart plant leaves using • These purple heart plant leaves
2019 a biological reduction technique. extract-mediated synthesized silver
[62]
• To see if it is successful against bacteria. nanoparticles have antibacterial activity
against E. coli and S. aureus at a concentration
of 100 µg/mL, which is much better than the
extract concentration.
Molecules 2021, 26, 5057 10 of 21

Table 3. Cont.

Author Year Objective of Study Summary Finding

• To provide basic information about medicinal
plants and silver nanoparticles and show • Promising non-chemicals have no effect on
whether they have antiviral, bactericidal, and adult bees (plant extracts).
Khan et al. [39] 2018 fungicidal properties. • This study attempted to determine the current
• To demonstrate how medicinal plants can be state of medicinal plant science worldwide.
used in a wide range of applications.
• According to the current study, the novel
G-Ag-NPs demonstrated strong antibacterial
• The aim of this study was to plan, classify, and
properties against both Gram-negative and
Kumar et al. [49] 2019 evaluate the potential of G-Ag-NPs as a wound
Gram-positive bacterial strains, suggesting that
treatment against human pathogenic bacteria.
they have a lot of potential for treating
pathogen-infected wounds.
• To gain a thorough understanding of the
Ag-NPs synthesis process as it is mediated by • These nanoparticles were found to be non-toxic
Fahimirad et al. [47] plants. to normal human cells at therapeutic
• To evaluate antimicrobial and cytotoxic concentrations.
properties, as well as their implementations.
• Nanoparticles interact with DNA, enzymes,
• To research into the green synthesis of different
ribosomes, and lysosomes, influencing cell
metal NPs that have been verified.
Singh et al. [36] 2020 membrane permeability, oxidative stress, gene
• To assess the antibacterial properties’ different
expression, protein activation, and enzyme
modes and mechanisms.
activation.
• To discuss the synthesis of Ag-NPs by plants • Ag-NP biosynthesis has been investigated
Das et al. [63] 2020 and algae, as well as their use as an using a number of plants and algae, as well as
antimicrobial agent. Ag-NP reduction using biological components.
• To learn more about the mechanisms that cause • Ag-NPs’ specific physicochemical properties
Salleh et al. [50] 2020 Ag-NPs to have antiviral and antibacterial are influenced by a variety of factors, including
effects on microorganisms. scale, surfactant, and structure morphology.

Table 3 comprises 28 primary studies and reviews that support Ag-NPs produced by
green synthesis successfully, seven of which are in vitro studies. Various biological sources
were used for the green synthesis of AG-NPs, including plants (n = 25, 89%), algae (n = 2,
7.1%), and fungi (n = 1, 3.5%); however, the general approach referred to herbal-mediated
fabrication of Ag-NPs (89.28% of studies).
The majority of these studies contained TEM analysis of nanoparticles that revealed the
existence of Ag and confirming the presence of Ag-NPs. These studies have indicated that
there are interactions among metallic ions and biomolecules, including proteins, peptides,
and amino acids, and some have a major impact on metallic NPs’ therapeutic ability.

3.2. Silver Nanoparticles Reduction through Plant Extract as Reducing Agents

The availability of reducing agent in plants, which aid in the synthesis of biocompatible
Ag-NPs, was a common aspect of all 28 studies. Secondary metabolites in the extract, such
as terpenoids, flavonoids, phenols, alkaloids, and proteins, only operate as reducing
agents. These research studies revealed details about such plants, and also the shape and
size of plants created through the usage of elemental Ag-NPs This study shows how a
flavonoids and phenol compound can be widely utilized for plant extract preparation as a
bio-stabilizing agent and bio-reducing agent for the synthesis of zero valent Ag-NPs [50,63].
Proteins may also be used as a bio-reducing agent for silver ions, although they have both
benefits and drawbacks [37].
There are also many studies on naturally active ingredients in Aloe vera leaves. These
include pectin, hemicellulose, and lignin, which can be utilized to lessen Ag ions. These
studies show that Ag nanoparticles can be prepared to utilize Aloe vera leaves extract as a
potential reducing agent. Aloe vera leaves reduce Ag salts into Ag nanoparticles, because
they contain polyphenols groups [26].
Molecules 2021, 26, 5057 11 of 21

3.3. Synthesis
Several factors such as the method used for synthesis, pH, temperature, pressure,
time, particle size, pore size, environment, and proximity greatly influence the quality and
quantity of the synthesized nanoparticles and their characterization and applications [63].
The reviewed articles also suggest that the bottom–up techniques were predominantly
used in the synthesis of Ag-NPs relative to the top–down techniques (<5% of reviewed
articles). This is mainly attributed to the surface imperfection of formed particles used in
the top–down approach [63]. It should also be noted that while Ag-NPs used a variety
of plants and particles of various sizes and shapes, Azadiractha indica terrestrial plant
leaves and Sargassum wightii among them were the most commonly used (29% and 26%
respectively). The marine algae (7.1%) were also reported to have been subjected to the
synthesis of Ag-NPs antibacterial potential, according to these researchers.
Hence, it is evident that various plants or their extracts are clearly involved throughout
the biosynthesis of Ag-NPs of different size and shape and discussed in various publica-
tions [30,42]. It has also been established that Ag-NPs can be made in a cubic shape from
plant extracts [23,34,35,42] (see Table 3 for details).
Silver nitrate (AgNO3 ) is the most widely used salt precursor accounting for almost
83% of those reported 28 articles using specific synthesis methods. The dominant use of
AgNO3 is attributed to its low cost and chemical stability when compared to other types
of silver salts [64]. The preparation method of Ag nanoparticles utilizing green-reducing
agents is also appealing. Extracts of Azadirachta indica (Neem), Ocimum senuiflorum,
Elephantopus scaber, and Carica papaya (Papaya) are also prepared to reduce 1 mM aqueous
AgNO3 solutions synthesizing Ag nanoparticles.

3.4. Size in Relation with Antibacterial Activity

The bactericidal properties of Ag-NPs synthesized from plant extracts have been
demonstrated in almost all these 28 studies. This was owing to the ground nature and
small scale, which caused the volume ratio to rise upon the surface. The sphere-shaped
Ag-NPs are found to be more effective against the Klebsiella and E. coli bacterial strains than
the rod-shaped Ag-NP. The sphere-shaped Ag-NPs exhibit stronger antibacterial activity
than rod-shaped and wire-shaped Ag-NPs with similar diameters, suggesting that the
shape effect on antibacterial activity is due to the specific and large surface area and facet
reactivity, as reported by Raza Maet [65].
Table 4 presents an overview of various elements of different plants and synthesis of
their components such as leaf extracts, their shape and size, components used for synthesis,
the antibacterial activity and the bacterial impact, and the bio-functionalizing compounds.
Other features such as the shape, chemical properties, and size are also mentioned. The
particle size of biosynthesized Ag-NPs below 100 nm with various shapes are reported in
all these investigations, while the majority of the studies report spherical morphology for
biogenic Ag-NPs (60.8%).

Table 4. Plant extracts that were used for biosynthesis of silver nanoparticles.

Used Component Size Bio-Functionalizing

Plant Shape Bacterial Impact Reference
for the Synthesis (nm) Compounds
Thymus Escherichia coli and
Extract 50–60 Spherical Protein [58]
kotschyanus Staphylococcus aureus
Juglans regia Cubic and
Leaf 15–30 Streptococcus mutans Flavonoids [63]
(Bark) smooth
Antibacterial action against
Seaweed ulva
Extract 42–83 Spherical Escherichia coli, Protein [44]
flexuosa
Staphylococcus aureus
Escherichia coli, Bacillus
Acacia rigidula Extract 8–60 Spherical Phenol compound [45]
subtilis, P. aeruginosa
Azadirachta indica Leaf 48 Cubic E. coli Protein [46]
Molecules 2021, 26, 5057 12 of 21

Table 4. Cont.

Used Component Size Bio-Functionalizing

Plant Shape Bacterial Impact Reference
for the Synthesis (nm) Compounds
Ocimum
Extract 25–40 Linear E. coli and B. subtilis Flavonoids [47]
tenuiflorum
Elephantopus B. subtilis, L. lactis, P.
Extract 37 Spherical Proteins [50]
scaber aeruginosa, A. penicillioides
Proteus vulgaris,
Hydroxyl and
Fagonia cretica Extract 16 Spherical Escherichia coli and [51]
secondary amines
Klebsiella Pneumoniae
Carica papaya Escherichia coli,
Leaf 60–80 Spherical Proteins [52]
(Papaya) Staphylococcus aureus
Argemone Leaf proteins and
Leaf 30 Cubic E. coli and B. subtilis [53]
mexicana metabolites
Datura
Leaf 15–20 Spherical Streptococcus mutans Flavonoids, terpenoids [54]
stramonium
E. coli, P. aeruginosa and
Cola nitida pod Extract 12–80 Cubic Protein [52]
Klebsiella
Taraxacum Cubic and Xanthomonas axonopodis Flavonoids, terpenoids,
Leaf 15 [26]
officinale hexagonal and P. syringae and triterpenes
P. aeruginosa and Bacillus
Rosa indica Leaf 1–100 spherical Polyphenol [55]
subtilis
Polyphenols, lipids, and
Phoenix dactylifera 15–40 Cubic E. coli [53]
fatty acids
Mangosteen Extract 30 Spherical E. coli and S. aureus Flavonoids [58]
Rheum palmatum
Extract 121 Cubic S. aureus and P. aeruginosa Flavonoids, terpenoids [64]
root
Prunus japonica Extract 26 Spherical Proteus vulgaris Protein [59]
Boerhaaviadiffusa 25 F. ranchiophilum Phenol [60]
Banana peel Extract 23.7 Cubic E. coli, P. aeruginosa Lips and fatty acids [61]
Aloe vera leaf 15.2 Cubic S. aureus Protein [62]
Pelargonium
graveolens Leaves 16–40 Spherical E. coli, P. aeruginosa Flavonoids [63]
(Geranium)
Sargassum wightii Extract 68.04 Cubic S. aureus and P. aeruginosa Protein [26]

According to the data of this work, the average size of Ag-NPs is 30–60 nm, which
are spherical in shape along with flavonoid and phenol compounds. All have excellent
antibacterial activity against E. coli and S. aureus.

4. Discussion
The findings of this review show that the antimicrobial properties of Ag-NPs and
their green synthesis are essential because they are linked to improving human health.
Currently, bacterial antibiotic resistance is found to be a significant source of concern in
most research studies, the antimicrobial action of NPs being the most effective antibacterial
agent. Furthermore, Ag-NPs are used as nanocarriers for antibiotics and drugs, assisting
the enhancement of antibiotic activity toward resistant microbes [66]. Several studies have
examined plants, having been used as a source of synthesis in many experiments, with
benefits such as plant material availability, low price, high availability for mass production,
purgative properties, and secondary metabolites. For both the synthesis of Ag-NPs, a
correct and controlled need for biological entities would result in well-characterized and
highly stable NPs [67]. One of the major benefits of utilizing is identifying the genesis of
Ag-NPs existing in plants and discovering the non-toxicity and metabolites causing silver
reduction.
 Molecules 2021, 26, 5057 13 of 21

Molecules 2021, 26, x FOR PEER REVIEW

4.1. Ag-NPs Application and Mechanism of Action 14 of 22
ules 2021, 26, x FOR PEER REVIEW 14 of 22
Previous studies have attributed Ag-NPs antibacterial activity to the release of silver
ions, which can be produced and introduced by AG-NP oxidative dissolution in the pres-
suchofasoxygen
ence sulfhydryl,
[68,69]. amino, imidazole,
Similarly, phosphate,
silver ions strongly and carbonyl
attract groups found
electron-donating groupson mem-
such
such as sulfhydryl,
branes
as amino,
sulfhydryl, imidazole,
and proteins
amino, [70,71].phosphate,
imidazole, ionsand
cancarbonyl
Silverphosphate, form groupsAS-Ag
andpersistent
carbonyl foundbonds
groups on mem-
found with protein thiol
on membranes
branes and proteins
groups
and [70,71].
(ASH),
proteins Silver
[70,71]. ionsthe
altering
Silver can 3D
ions form
can persistent
structure AS-Ag
of proteins
form persistent bonds with protein
and blocking
AS-Ag bonds withactive thiol
binding
protein thiolsites [72].
groups
groups (ASH), Asaltering
(ASH),a result,thesilver
altering 3Dthestructure
ions
3D can of
structureproteins
prevent of the and blocking
movement
proteins active
and
and blocking binding
release sites [72].
of potassium
active binding (K+)[72].
sites ionsAsfroma
As a result, silver ions
microbial can prevent
cells, as welltheasmovement
the creation and
of release
adenosine of potassium
triphosphate (K+)
result, silver ions can prevent the movement and release of potassium (K+) ions from micro- ions
(ATP) from
[73]. Furthermore,
microbial cells, ascells,
silver
bial well
ionsas well
as the creation
may easily
as of adenosine
combine
the creation ofwith triphosphate
various
adenosine (ATP) [73].
biomolecules,
triphosphate (ATP) Furthermore,
including DNA, RNA,
[73]. Furthermore, and
silver
silver ions mayions easily
peptides, combine
may easilygenerating with
combine various
insoluble
with various biomolecules,
complexes including
that prevent
biomolecules, DNA,
cell DNA,
including RNA,
division RNA,andand
and reproduction
peptides,
peptides, generating
[74,75]. insoluble
generating insolublecomplexes
complexesthat thatprevent
prevent cell
cell division and andreproduction
reproduction[74,75].
[74,75]. Possible
Possiblemechanisms
mechanismsofofaction action ofof
AG-NP
AG-NP areare
shown
shown in Figures
in Figures 2 and 3 [76,77].
2 and Figure
3 [76,77]. Figure2
Possibleillustrates
mechanisms the of action
action of AG-NP
mechanism ofare shown
Ag-NP in
againstFigures 2
bacterial and 3
cells,[76 ,77].
activities
2 illustrates the action mechanism of Ag-NP against bacterial cells, activities of which in- Figure
of which include
2 illustrates the action
membrane
clude mechanism
damagedamage
membrane inoftheAg-NP
in theagainst
form ofform bacterial
inhibition
of cells,
of cell
inhibition of activities
multiplication of which in-
or Reactive
cell multiplication Oxidative
or Reactive Oxi-
clude membrane damage
Species
dative (ROS)
Species in the form
formation of inhibition
seen as cell
(ROS) formation of
damage
seen as cell multiplication
ordamage
destruction. or Reactive
The formerThe
or destruction. Oxi-
action of inhibiting
former action of
dative Species (ROS)
the
inhibiting formation
the cell seen
cell multiplication as cell
is due
multiplication damage
to the dueortodestruction.
association
is the Ag-NPThe
ofassociation former
withof DNAAg-NP action
or withofbiomolecules,
other DNA or other
inhibiting thewhile the latterwhile
cell multiplication
biomolecules, actionistheoflatter
due cell destruction
to the of cellisdestruction
association
action aofresult
Ag-NP of is
the
with interaction
DNAoforthe
a result of
other enzymes of
interaction anden-
biomolecules,molecules.
while the latter
zymes and molecules. action of cell destruction is a result of the interaction of en-
zymes and molecules.

Figure 2. Mechanism of action of silver nanoparticles against bacterial cells [76].

Figure 2. Mechanism of action of silver nanoparticles against bacterial cells [76].
Figure 2. Mechanism of action of silver nanoparticles against bacterial cells [76].

Figure 3. Segregation of AG-NPs silver ions and proteins disrupting the general funciton of the
Figure 3. Segregation of AG-NPs silver ions and proteins disrupting the general funciton of the
cell [77].
Figure 3. Segregation of AG-NPs silver ions and proteins disrupting the general funciton of the cell [77].
cell [77].
Figure33summarizes
Figure summarizeshow howAG-NP
AG-NPisisattached
attachedtotocell
cellmembranes,
membranes,membrane
membraneproteins,
proteins,
Figure 3and
summarizes
andDNA
DNAbases,howdisrupting
bases, AG-NP is attached
disrupting normal to cell
functionmembranes,
(blue membrane
arrows. This proteins,
generates silver
normal function (blue arrows. This generates silver ions ionsand
and
and DNA bases, disrupting
affects normal
membranes, function
DNA, and (blue
proteinsarrows.
(red This generates
arrows. This silver ions
accumulates and
reactive
affects membranes, DNA, and proteins (red arrows. This accumulates reactive oxidative oxidative
affects membranes,
speciesDNA, andwhich
(ROS), proteins
may(red arrows.
also affect This
DNA, accumulates reactiveand
cell membranes, oxidative
membrane proteins
species (ROS),(black
which may also affect
dotted line) [77]. DNA, cell membranes, and membrane proteins
(black dotted line) [77].
Molecules 2021, 26, 5057 14 of 21

Molecules 2021, 26, x FOR PEER REVIEW 15 of 22

species (ROS), which may also affect DNA, cell membranes, and membrane proteins (black
dotted line) [77].
4.2. ROS-Based
4.2. ROS-Based Antibacterial
Antibacterial Effects
Effects ofof Silver
Silver Ions
Ions
The specific
The specificmechanism
mechanismofofsilver silvernanoparticles’
nanoparticles’ antibacterial
antibacterial properties
properties cancan be fur-
be further
ther studied
studied through
through variousvarious antibacterial
antibacterial actions actions It is well-known
It is well-known that Ag-NPs
that Ag-NPs continu-
continuously
ously discharge silver ions, which are a potential microbe-killing
discharge silver ions, which are a potential microbe-killing mechanism [78]. These silver mechanism [78]. These
silver ions can adhere to the cell wall and cytoplasmic membrane
ions can adhere to the cell wall and cytoplasmic membrane due to electrostatic attraction due to electrostatic at-
traction and affinity to sulfur proteins. The adhered ions can
and affinity to sulfur proteins. The adhered ions can improve the cytoplasmic membrane improve the cytoplasmic
membrane permeability
permeability and result in and result in of
disruption disruption
the bacterialof the bacterial[79].
envelope envelope [79]. The res-
The respiratory en-
piratory
zymes canenzymes can alsoonce
also be disabled be disabled
free silver once
ionsfree
entersilver
into ions
cells, enter intoincells,
resulting resulting
reactive oxygen in
reactive(ROS)
species oxygen butspecies
no ATP (ROS) but noresults
synthesis ATP synthesis
[80]. ROSresults
however [80].canROS however
trigger can trigger
cell membrane
cell membrane
rupture and DNA rupture and DNA
alteration. Sincealteration.
sulfur and Since sulfur and
phosphorus arephosphorus
key components are keyofcompo-
DNA,
nents of DNA, the interaction of silver ions with these elements
the interaction of silver ions with these elements can create issues with DNA replication, can create issues with
DNA
cell replication, cell
reproduction, andreproduction,
even microorganism and even microorganism
death. Furthermore, death.
silverFurthermore,
ions can prevent silver
ions canproduction
protein prevent protein production
by denaturing by denaturing
ribosomes ribosomes[81].
in the cytoplasm in the cytoplasm [81].
Figure 4 exhibits aa few
Figure few antibacterial
antibacterialactions actionsofofsilver
silvernanoparticles
nanoparticles (Ag-NPs)
(Ag-NPs) such
such as
Cell
as wall
Cell andand
wall cytoplasmic
cytoplasmic membrane
membrane are disrupted
are disrupted and silver nanoparticles
and silver nanoparticlesrelease silver
release
ions (Ag+),
silver whichwhich
ions (Ag+), attachattach
to or to
pass through
or pass the cell
through the wall and and
cell wall cytoplasmic
cytoplasmic membrane.
membrane. Ri-
bosome denaturation
Ribosome denaturation such
suchas silver
as silver ionsions
alsoalso
denature
denature ribosomes,
ribosomes, inhibiting
inhibitingprotein syn-
protein
thesis. Inhibition
synthesis. of adenosine
Inhibition of adenosine triphosphate
triphosphate (ATP) production
(ATP) production too results in silver
too results ions
in silver
ions deactivating
deactivating respiratory
respiratory enzymes enzymes
in the in the cytoplasmic
cytoplasmic membrane, membrane,
preventing preventing
ATP produc-ATP
production.
tion. Membrane Membrane
disruptiondisruption
is also is seenalsotoseen to be caused
be caused by reactive
by reactive oxygenoxygen
speciesspecies
(ROS)
(ROS)
producedproduced by a disrupted
by a disrupted electron
electron transport
transport chain:chain:
that is,that
ROSis, produced
ROS produced by a broken
by a broken elec-
electron transport chain can induce membrane disruption. It is
tron transport chain can induce membrane disruption. It is also seen that silver and reac- also seen that silver and
reactive
tive oxygen species bind to deoxyribonucleic acid (DNA) and hinder replication andand
oxygen species bind to deoxyribonucleic acid (DNA) and hinder replication cell
cell proliferation
proliferation while while
silversilver nanoparticles
nanoparticles can becan seenbeaccumulating
seen accumulating in cell
in cell wall wall
pits, pits,
causing
causing
membrane membrane denaturation.
denaturation. As a result,As membrane
a result, membrane
perforation perforation
is causedisand caused
silverand silver
nanopar-
nanoparticles
ticles penetrate penetrate the cytoplasmic
the cytoplasmic membrane membrane
directly directly
and release and organelles
release organelles
from thefrom cell.
the cell. These are evidence of the antibacterial actions of silver
These are evidence of the antibacterial actions of silver nanoparticles (Ag-NPs), causing nanoparticles (Ag-NPs),
causing
disruption disruption and its variants
and its variants within awithincell. a cell.

Figure
Figure4.4.Antibacterial
Antibacterialactions
actionsof
ofsilver
silvernanoparticles
nanoparticles(Ag-NPs).
(Ag-NPs).

Plant extracts are favored due to having a slow rate of nanoparticle synthesis using
microbes as well as their simplicity, performance, and feasibility. Plants’ capacity to ab-
sorb and detoxify heavy metals has been well established [82,83]. Furthermore, Ag-NPs
are made from plants and could be easily prepared from plant extracts or perhaps even
Molecules 2021, 26, 5057 15 of 21

Plant extracts are favored due to having a slow rate of nanoparticle synthesis using
microbes as well as their simplicity, performance, and feasibility. Plants’ capacity to absorb
and detoxify heavy metals has been well established [82,83]. Furthermore, Ag-NPs are
made from plants and could be easily prepared from plant extracts or perhaps even from
the entire plant [84]. Plants containing reducing and stabilizing agents are of a great help
in the development of biocompatible Ag-NPs as well as secondary metabolites inside the
extracts such as terpenoids, flavonoids, phenols, alkaloids, proteins, and carbohydrates,
which often function as reducing agents. [36,48,58,85] discover that for the production of
silver nanoparticles using plant extracts, Thymus kotschyanus leaf extract was specifically
used to synthesize Ag-NPs. The spherical Ag-NP particles were obtained with the size
varied in the range of 50–60 nm. Later, a group of researchers [63] made use of spherical
silver nanoparticles using different extracts viz., Juglans regia (bark), seaweed Ulva flexuosa,
and Phoenix dactylifera with sizes varying from 15 to 60 nm.
Similar results were achieved by [44,45]. in which Acacia rigidula and Azadirachta
indica leaves extract were used to synthesize Ag-NPs, with shape being spherical and the
size varying in the range 8–60 nm and 48 nm. The spherical silver nanoparticles were
also utilized in other studies [47] using Ocimum senuiflorum extract with sizes from 25
to 40 nm and Asmaa [48] who utilized Pelargonium graveolens of size ranging from 16
to 40 nm. Das [63] used Ag-NP in cubic form from plant extracts to prepare elemental
silver nanoparticles using the leaf extract of Banana peel and Aloe vera, with the synthesized
Ag-NPs varying in size of about 23 nm and 15 nm, respectively.
Ag-NPs made from natural extracts with a shape other than spherical have been
reported in a very limited number of articles [57]. Seven studies found a cubic shape, and
two others examined the synthesis of trigonal Ag-NPs with a cubical form [86,87]. Only
one study [88] used Acorus calamus extracts to classify the spherical shapes of Ag-NPs with
a size of 83 nm. Most of these studies utilized synthesized nanoparticles spherical with
a shape of less than 100 nm. Each of these studies has also reported strong antibacterial
activity toward pathogenic microorganisms including Staphylococcus and Pseudomonas.
The antimicrobial activity of the synthesized Ag-NPs has also been significantly reported
against the growth of Proteus vulgaris, Escherichia coli, and Klebsiella pneumoniae strains with
different concentrations while treating Fagonia cretica that were spherical in shape [51].

4.3. In Vitro Cytotoxicity

A phenomenon noticed in a few studies was the discussion of toxic effects of Ag-
NPs on different cell lines, including macrophages (RAW 264.7), bronchial epithelial
cells (BEAS-2B), alveolar epithelial cells (A549), hepatocytes (C3A, HepG2), colon cells
(Caco2), skin keratinocytes (HaCaT), human epidermal keratinocytes (HEKs), erythrocytes,
neuroblastoma cells, embryonic kidney cells (HEK293T), porcine kidney cells (Pk 15),
monocytic cells (THP-1), and stem cells [22,89]. The exposure of A549 cells was also found
in increasing concentrations of Ag-NPs for 24 h and causes morphological changes such as
cell shrinkage, cellular extensions, a specific spreading pattern, and cell death in a dose-
dependent manner [58,63]. The results of in vitro studies further indicate that Ag-NPs are
toxic to the mammalian cells that are derived from the skin, the liver, the lung, the brain, the
vascular system, and reproductive organs [36]. The cytotoxicity of Ag-NPs depends on their
size, shape, surface charge, coating/capping agent, dosage, oxidation state, agglomeration,
and type of pathogens against which their toxicity is investigated [52,63]. Despite these
studies, the toxicological nature of Ag-NPs mechanism is still unclear.

4.4. Ag-NPs Application

One of the most significant advantages of nano silver-based biomaterials is their
inherent anti-pathogenic properties, which can be seen in both planktonic and biofilm-
organized microorganisms. Antimicrobial peptides such as polymyxin B are used to
functionalize nanoparticles and functionalizing molecules such as sodium borohydride,
which are used to create effective biocompatible Ag-NPs with high penetration ability [63].
Molecules 2021, 26, 5057 16 of 21

There is also a lot of factual information about Ag-bio-NP’s applications [90–92], such
as that there is the chance of Ag-NPs having antimicrobial activity, as evidenced by the
information reported [30,35,42].
Due to the large surface area and small nano-size, the processed zirconia nanoparticles
show strong antibacterial activity against Salmpnella typhi, Bacillus subtilis, and Escherichia
coli. Researchers also reveal the antibacterial activity of biogenic silver nanoparticles where
biosynthesized Ag-NPs and use of the leaf extract of a plant Protium serratum are reported in
detail [93]. These reports show that the formulated Ag-NPs displayed strong antibacterial
activity toward food-borne pathogens such as Pseudomonas aeruginosa, Escherichia coli, and
Bacillus subtilis [94,95]. One of the first papers in this list examined a leaf extract of Cucumis
sativus to make elemental Ag-NPs. Using extracts of the leaf of Ocimum tenuiflorum for
“green synthesis,” the researchers [96] observed the presence of prismatic silver nano
peptides whose size range from 25 to 40 mm. Another research [97] used Capsicum annuum
from 50 to 70 nm to make extracts with spherical-sized silver nanoparticles. Singh [66]
suggested that Cannabis sativa can also be used to make Ag-NPs, with extract of leaf of
Argemone mexicana acting as a capping and reducing agent when added to an aqueous
solution of silver nitrate. These findings confirm that silver nanoparticles may have
antibacterial activity against E. coli [46].
Ag-NPs made from natural extracts having other than spheroid shapes have been
reported in a very limited number of articles. The synthesis of trigonal Ag-NPs with a
cubical form has been examined in two articles [91,92]. One article used Acorus calamus
extracts to classify the Ag-NPs having spherical shapes and sizes of 83 nm [93]. The synthe-
sized nanoparticles have demonstrated powerful antimicrobial activity toward pathogenic
microorganisms such as Staphylococcus and Pseudomonas in various other research papers.
UV-Vis spectrometer, X-ray diffractometer (XRD), scanning electron microscopy (SEM),
and Fourier Transmission Infrared (FTIR) spectrophotometer are also used to investigate
the properties of NPs. It is emphasized that geometric forms such as oval or spheri-
cal structures are conglomerates of the smaller embryonic elemental silver particles [98].
Higher-resolution images of spherical silver nanoparticles obtained from SEM would
further authenticate it [99]
Hence, in this review, we intended to provide a brief overview of Ag-NP biosynthesis
from plant extracts in order to investigate their antibacterial capacity. It was premised
that Ag-NPs were commonly used during electronics and photonics as biosensors in
biocatalysis, protein coagulation, and drug delivery due to their oxidation stability. Silver
nanoparticles have been used to coat cutlery, door handles, and a computer keyboard and
mouse; these are also used to make new coatings and cosmetics and air-conditioning filters,
lakes, toilets, and other areas [57].
The environmental and economic problems associated with most Ag-NPs synthesis
methods have led to a search for many other environmentally and economically beneficial
options. Usually, due to its multiple health, economic, financial, and medicinal advantages,
the biological process of synthesis utilizing plant resources has been considered suitable for
the production of Ag-NPs. For example, Ag-NPs synthesized with plant extracts have such
unique properties that have also boosted their use in agriculture for fertilizers, pesticides,
and fumigants. Phyto synthesized Ag-NPs have found widespread use in the manufacture
of disinfectant, antifungal, anticancer, antioxidant, anti-inflammatory, and antidiabetic
agents in medicine and pharmacy [100]. Other plant extracts (as raw material for Ag-NPs
synthesis) of Brassica oleracea, Brassica oleracea, Ocimum tenuiflorum, and Cola nitida pod
plants are also suggested based on findings. These extracts have a substantial impact on
E. coli, Pseudomonas, and other Gram-negative rods. Leaf extracts are also given preference
during the synthesis of Ag-NPs, which can be seen in Table 2, although some sections of
related plants—flowers, seeds, fruits, and so on—could also be used in some instances.
Molecules 2021, 26, 5057 17 of 21

5. Limitations of the Study

There are a few limitations to conduct a literature review of this nature. For instance,
it was difficult to extrapolate the results directly of such experimental studies to a clinical
setting due to skills and instruments deficiency. The testing of both clinical and standard
reference strains leads to achieving more realistic results. Although 28 (n) research papers
were reviewed systematically in view of study objectives, simultaneous characteristics (e.g.,
application of silver nanoparticles, anti-biofilm, antibacterial activity) in each one of them
were not present. Therefore, the variation of the number of included studies for all of these
characteristics was different. However, despite these limitations, this study will serve as a
good source of reference for finding relevant studies and building a point of view on silver
nanoparticles and their applications in preventing biofilm formations.

6. Conclusions
The analysis focuses on research that has been done mostly on the biosynthesis of
Ag-NPs utilizing different plant extracts, as well as the elimination utilizing biological
components. Biogenic NPs have been shown to have a significant antibacterial effect
for all or the majority of the bacteria, such as Proteus vulgaris, Pseudomonas aeruginosa,
Staphylococcus aureus, Vibrio cholera, and Klebsiella spp. While biosynthesis becomes less
toxic or environmentally sustainable, reducing compounds necessitates more research
to comprehend surface chemistry fully. A review of many studies confirmed this trend.
Furthermore, Ag-NPs, made from a variety of plants and algae, has a powerful antibacterial
ability. This area has yielded several studies. We hope that this approach of green synthesis
will aid researchers in determining the long-term advantages of Ag-NPs generated by
biosynthesis in their various potential applications. Ag-NPs are appealing because they
are harmless to humans at low concentrations and have antibacterial activity throughout a
broad spectrum. Nevertheless, before applying the applications of Ag-NPs, the potential
toxicological effects must be investigated.

Author Contributions: Conceptualization, S.Q., M.H.R. and S.A.-N.; Methodology, all authors;
Resources, all authors; Writing—original draft preparation, S.Q.; Writing—review and editing, S.Q.,
M.H.R. and F.C.-H.; Visualization, N.M., N.M.S.J. and N.A.A.-H.; Supervision, S.A.-N., M.H.R. and
F.C.-H. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: No new data were created or analyzed in this study. Data sharing is
not applicable.
Acknowledgments: The authors wish to acknowledge the help received from individual, colleagues
and fellow scholars.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Rao, M.L.; Savithramma, N. Biological synthesis of silver nanoparticles using Svensonia Hyderabadensis leaf extract and
evaluation of their antimicrobial efficacy. J. Pharm. Sci. Res. 2011, 3, 1117.
2. Lee, S.H.; Jun, B.H. Silver nanoparticles: Synthesis and application for nanomedicine. Int. J. Mol. Sci. 2019, 20, 865. [CrossRef]
[PubMed]
3. Yousaf, H.; Mehmood, A.; Ahmad, K.S.; Raffi, M. Green synthesis of silver nanoparticles and their applications as an alternative
antibacterial and antioxidant agent. Mater. Sci. Eng. C 2020, 112, 110901. [CrossRef] [PubMed]
4. Dastjerdi, R.; Montazer, M. A review on the application of inorganic nano-structured materials in the modification of textiles:
Focus on anti-microbial properties. Colloids Surf. B Biointerfaces 2010, 79, 5–18. [CrossRef]
5. Kamat, P.V. Photophysical, photochemical and photocatalytic aspects of metal nanoparticles. J. Phys. Chem. B 2002, 106, 7729–7744.
[CrossRef]
Molecules 2021, 26, 5057 18 of 21

6. Lu, X.; Rycenga, M.; Skrabalak, S.E.; Wiley, B.; Xia, Y. Chemical synthesis of novel plasmonic nanoparticles. Annu. Rev. Phys.
Chem. 2009, 60, 167–192. [CrossRef] [PubMed]
7. Sotiriou, G.A.; Pratsinis, S.E. Antibacterial activity of nanosilver ions and particles. Environ. Sci. Technol. 2010, 44, 5649–5654.
[CrossRef]
8. Lv, M.; Su, S.; He, Y.; Huang, Q.; Hu, W.; Li, D.; Fan, C.; Lee, S.T. Long-term antimicrobial effect of silicon nanowires decorated
with silver nanoparticles. Adv. Mater. 2010, 22, 5463–5467. [CrossRef]
9. Ghosh, S.; Saraswathi, A.; Indi, S.S.; Hoti, S.L.; Vasan, H.N. Ag@ AgI, core@ shell structure in agarose matrix as hybrid: Synthesis,
characterization, and antimicrobial activity. Langmuir 2012, 28, 8550–8561. [CrossRef]
10. Xiu, Z.M.; Zhang, Q.B.; Puppala, H.L.; Colvin, V.L.; Alvarez, P.J. Negligible particle-specific antibacterial activity of silver
nanoparticles. Nano Lett. 2012, 12, 4271–4275. [CrossRef]
11. Zhang, X.F.; Liu, Z.G.; Shen, W.; Gurunathan, S. Silver Nanoparticles: Synthesis, Characterisation, Properties, Applications, and
Therapeutic Approaches. Int. J. Mol. Sci. 2016, 17, 1534. [CrossRef] [PubMed]
12. Sun, Y.; Xia, Y. Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298, 2176–2179. [CrossRef] [PubMed]
13. Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Shape-controlled synthesis of metal nanostructures: The case of silver. Chem. A Eur. J. 2005,
11, 454–463. [CrossRef]
14. Abou El-Nour, K.M.; Eftaiha, A.A.; Al-Warthan, A.; Ammar, R.A. Synthesis and applications of silver nanoparticles. Arab. J. Chem.
2010, 3, 135–140. [CrossRef]
15. Wu, Y.; Yang, Y.; Zhang, Z.; Wang, Z.; Zhao, Y.; Sun, L. A facile method to prepare size-tunable silver nanoparticles and its
antibacterial mechanism. Adv. Powder Technol. 2018, 29, 407–415. [CrossRef]
16. Ni, Z.; Gu, X.; He, Y.; Wang, Z.; Zou, X.; Zhao, Y.; Sun, L. Synthesis of silver nanoparticle-decorated hydroxyapatite (HA@ Ag)
poriferous nanocomposites and the study of their antibacterial activities. RSC Adv. 2018, 8, 41722–41730. [CrossRef]
17. Kim, J.S.; Kuk, E.; Yu, K.N.; Kim, J.H.; Park, S.J.; Lee, H.J.; Kim, S.H.; Park, Y.K.; Park, Y.H.; Hwang, C.Y.; et al. Antimicrobial
effects of silver nanoparticles. Nanomed. Nanotechnol. Biol. Med. 2007, 3, 95–101. [CrossRef]
18. Morones, J.R.; Elechiguerra, J.L.; Camacho, A.; Holt, K.; Kouri, J.B.; Ramírez, J.T.; Yacaman, M.J. The bactericidal effect of silver
nanoparticles. Nanotechnology 2005, 16, 2346. [CrossRef] [PubMed]
19. Wang, G.; Jin, W.; Qasim, A.M.; Gao, A.; Peng, X.; Li, W.; Feng, H.; Chu, P.K. Antibacterial effects of titanium embedded with
silver nanoparticles based on electron-transfer-induced reactive oxygen species. Biomaterials 2017, 124, 25–34. [CrossRef]
20. Le Ouay, B.; Stellacci, F. Antibacterial activity of silver nanoparticles: A surface science insight. Nano Today 2015, 10, 339–354.
[CrossRef]
21. Wang, X.; Zhu, S.; Liu, L.; Li, L. Flexible antibacterial film based on conjugated polyelectrolyte/silver nanocomposites. ACS Appl.
Mater. Interfaces 2017, 9, 9051–9058. [CrossRef] [PubMed]
22. Akter, M.; Sikder, M.T.; Rahman, M.M.; Ullah, A.A.; Hossain, K.F.; Banik, S.; Hosokawa, T.; Saito, T.; Kurasaki, M. A systematic
review on silver nanoparticles-induced cytotoxicity: Physicochemical properties and perspectives. J. Adv. Res. 2018, 9, 1–6.
[CrossRef] [PubMed]
23. Schröfel, A.; Kratošová, G.; Šafařík, I.; Šafaříková, M.; Raška, I.; Shor, L.M. Applications of biosynthesized metallic nanoparticles—
A review. Acta Biomater. 2014, 10, 4023–4042. [CrossRef] [PubMed]
24. Mittal, A.K.; Chisti, Y.; Banerjee, U.C. Synthesis of metallic nanoparticles using plant extracts. Biotechnol. Adv. 2013, 31, 346–356.
[CrossRef]
25. Kharissova, O.V.; Dias, H.R.; Kharisov, B.I.; Pérez, B.O.; Pérez, V.M. The greener synthesis of nanoparticles. Trends Biotechnol.
2013, 31, 240–248. [CrossRef]
26. Gumel, A.M.; Surayya, M.M.; Yaro, M.N.; Waziri, I.Z.; Amina, A.A. Biogenic synthesis of silver nanoparticles and its synergistic
antimicrobial potency: An overview. J. Appl. Biotechnol. Bioeng. 2019, 6, 22–28.
27. SivaKumar, T.; Rathimeena, T.; Thangapandian, V.; Shankar, T. Silver nanoparticles synthesis of Mentha arvensis extracts and
evaluation of antioxidant properties. J. Biosci. Bioeng. 2015, 1, 22–28.
28. Prabu, H.J.; Johnson, I. Antibacterial activity of silver nanoparticles synthesized from plant leaf extract of Cycas circinalis, Ficus
amplissima, Commelina benghalensis and Lippia nodiflora leaves. J. Chem. Pharm. Res. 2015, 7, 443–449.
29. Arokiyaraj, S.; Arasu, M.V.; Vincent, S.; Prakash, N.U.; Choi, S.H.; Oh, Y.K.; Choi, K.C.; Kim, K.H. Rapid green synthesis of silver
nanoparticles from Chrysanthemum indicum L and its antibacterial and cytotoxic effects: An in vitro study. Int. J. Nanomed. 2014,
9, 379. [CrossRef]
30. Murugan, K.; Senthilkumar, B.; Senbagam, D.; Al-Sohaibani, S. Biosynthesis of silver nanoparticles using Acacia leucophloea
extract and their antibacterial activity. Int. J. Nanomed. 2014, 9, 2431.
31. Gurunathan, S.; Raman, J.; Abd Malek, S.N.; John, P.A.; Vikineswary, S. Green synthesis of silver nanoparticles using Ganoderma
neo-japonicum Imazeki: A potential cytotoxic agent against breast cancer cells. Int. J. Nanomed. 2013, 8, 4399.
32. Cooper, H.; Koenka, A.C. The overview of reviews: Unique challenges and opportunities when research syntheses are the
principal elements of new integrative scholarship. Am. Psychol. 2012, 67, 446. [CrossRef]
33. Shea, B.J.; Grimshaw, J.M.; Wells, G.A.; Boers, M.; Andersson, N.; Hamel, C.; Porter, A.C.; Tugwell, P.; Moher, D.; Bouter, L.M.
Development of AMSTAR: A measurement tool to assess the methodological quality of systematic reviews. BMC Med. Res.
Methodol. 2007, 7, 1–7. [CrossRef] [PubMed]
Molecules 2021, 26, 5057 19 of 21

34. Pieper, D.; Mathes, T.; Eikermann, M. Impact of choice of quality appraisal tool for systematic reviews in overviews. J. Evid. Based
Med. 2014, 7, 72–78. [CrossRef] [PubMed]
35. Costas, R.; Bordons, M. Do age and professional rank influence the order of authorship in scientific publications? Some evidence
from a micro-level perspective. Scientometrics 2011, 88, 145–161. [CrossRef]
36. Yun’an Qing, L.C.; Li, R.; Liu, G.; Zhang, Y.; Tang, X.; Wang, J.; Liu, H.; Qin, Y. Potential antibacterial mechanism of silver
nanoparticles and the optimization of orthopedic implants by advanced modification technologies. Int. J. Nanomed. 2018, 13, 3311.
[CrossRef] [PubMed]
37. Singh, A.; Gautam, P.K.; Verma, A.; Singh, V.; Shivapriya, P.M.; Shivalkar, S.; Sahoo, A.K.; Samanta, S.K. Green synthesis of
metallic nanoparticles as effective alternatives to treat antibiotics resistant bacterial infections: A review. Biotechnol. Rep. 2020,
25, e00427. [CrossRef]
38. Sharif, M.O.; Janjua-Sharif, F.N.; Ali, H.; Ahmed, F. Systematic reviews explained: AMSTAR-how to tell the good from the bad
and the ugly. Oral Health Dent. Manag. 2013, 12, 9–16.
39. Khan, S.U.; Anjum, S.I.; Ansari, M.J.; Khan, M.H.; Kamal, S.; Rahman, K.; Shoaib, M.; Man, S.; Khan, A.J.; Khan, S.U.; et al.
Antimicrobial potentials of medicinal plant’s extract and their derived silver nanoparticles: A focus on honey bee pathogen. Saudi
J. Biol. Sci. 2019, 26, 1815–1834. [CrossRef]
40. Ahmad, F.; Ashraf, N.; Ashraf, T.; Zhou, R.B.; Yin, D.C. Biological synthesis of metallic nanoparticles (MNPs) by plants and
microbes: Their cellular uptake, biocompatibility, and biomedical applications. Appl. Microbiol. Biotechnol. 2019, 103, 2913–2935.
[CrossRef]
41. Mishra, P.; Singh, L.; Mishra, S. Biosynthetic silver nanoparticles-current trends and future scope: An overview. IOSR J. Pharm.
Biol. Sci. 2019, 14, 37–43.
42. Roy, A.; Bulut, O.; Some, S.; Mandal, A.K.; Yilmaz, M.D. Green synthesis of silver nanoparticles: Biomolecule-nanoparticle
organizations targeting antimicrobial activity. RSC Adv. 2019, 9, 2673–2702. [CrossRef]
43. Nasrollahzadeh, M.; Mahmoudi-Gom Yek, S.; Motahharifar, N.; Ghafori Gorab, M. Recent developments in the plant-mediated
green synthesis of Ag-based nanoparticles for environmental and catalytic applications. Chem. Rec. 2019, 19, 2436–2479. [CrossRef]
44. Zafar, A.; Rizvi, R.; Mahmood, I. Biofabrication of silver nanoparticles from various plant extracts: Blessing to nanotechnology.
Int. J. Environ. Anal. Chem. 2019, 99, 1434–1445. [CrossRef]
45. Nisar, P.; Ali, N.; Rahman, L.; Ali, M.; Shinwari, Z.K. Antimicrobial activities of biologically synthesized metal nanoparticles: An
insight into the mechanism of action. JBIC J. Biol. Inorg. Chem. 2019, 24, 929–941. [CrossRef] [PubMed]
46. Some, S.; Sen, I.K.; Mandal, A.; Aslan, T.; Ustun, Y.; Yilmaz, E.Ş.; Katı, A.; Demirbas, A.; Mandal, A.K.; Ocsoy, I. Biosynthesis of
silver nanoparticles and their versatile antimicrobial properties. Mater. Res. Express 2018, 6, 012001. [CrossRef]
47. Fahimirad, S.; Ajalloueian, F.; Ghorbanpour, M. Synthesis and therapeutic potential of silver nanomaterials derived from plant
extracts. Ecotoxicol. Environ. Saf. 2019, 168, 260–278. [CrossRef]
48. El Shafey, A.M. Green synthesis of metal and metal oxide nanoparticles from plant leaf extracts and their applications: A review.
Green Process. Synth. 2020, 9, 304–339. [CrossRef]
49. Kumar, S.S.; Houreld, N.N.; Kroukamp, E.M.; Abrahamse, H. Cellular imaging and bactericidal mechanism of green-synthesized
silver nanoparticles against human pathogenic bacteria. J. Photochem. Photobiol. B Biol. 2018, 178, 259–269. [CrossRef]
50. Salleh, A.; Naomi, R.; Utami, N.D.; Mohammad, A.W.; Mahmoudi, E.; Mustafa, N.; Fauzi, M.B. The potential of silver nanoparticles
for antiviral and antibacterial applications: A mechanism of action. Nanomaterials 2020, 10, 1566. [CrossRef]
51. Ferdous, Z.; Nemmar, A. Health impact of silver nanoparticles: A review of the biodistribution and toxicity following various
routes of exposure. Int. J. Mol. Sci. 2020, 21, 2375. [CrossRef]
52. Yin, I.X.; Zhang, J.; Zhao, I.S.; Mei, M.L.; Li, Q.; Chu, C.H. The antibacterial mechanism of silver nanoparticles and its application
in dentistry. Int. J. Nanomed. 2020, 15, 2555. [CrossRef]
53. Ahmad, S.; Munir, S.; Zeb, N.; Ullah, A.; Khan, B.; Ali, J.; Bilal, M.; Omer, M.; Alamzeb, M.; Salman, S.M.; et al. Green
nanotechnology: A review on green synthesis of silver nanoparticles—An ecofriendly approach. Int. J. Nanomed. 2019, 14, 5087.
[CrossRef] [PubMed]
54. Escárcega-González, C.E.; Garza-Cervantes, J.A.; Vazquez-Rodríguez, A.; Montelongo-Peralta, L.Z.; Treviño-Gonzalez, M.T.;
Castro, E.D.; Saucedo-Salazar, E.M.; Morales, R.C.; Soto, D.R.; González, F.T.; et al. In vivo antimicrobial activity of silver
nanoparticles produced via a green chemistry synthesis using Acacia rigidula as a reducing and capping agent. Int. J. Nanomed.
2018, 13, 2349. [CrossRef] [PubMed]
55. Nagar, N.; Devra, V. Green synthesis and characterization of copper nanoparticles using Azadirachta indica leaves. Mater. Chem.
Phys. 2018, 213, 44–51. [CrossRef]
56. Mikhailov, O.V.; Mikhailova, E.O. Elemental silver nanoparticles: Biosynthesis and bio applications. Materials 2019, 12, 3177.
[CrossRef] [PubMed]
57. Hamelian, M.; Zangeneh, M.M.; Amisama, A.; Varmira, K.; Veisi, H. Green synthesis of silver nanoparticles using Thymus
kotschyanus extract and evaluation of their antioxidant, antibacterial and cytotoxic effects. Appl. Organomet. Chem. 2018, 32,
e4458. [CrossRef]
58. Zulfiqar, H.; Zafar, A.; Rasheed, M.N.; Ali, Z.; Mehmood, K.; Mazher, A.; Hasan, M.; Mahmood, N. Synthesis of silver
nanoparticles using Fagonia cretica and their antimicrobial activities. Nanoscale Adv. 2019, 1, 1707–1713. [CrossRef]
Molecules 2021, 26, 5057 20 of 21

59. Haqq, S.M.; Pandey, H.I.; Gerard, M.A.; Chattree, A.M. Bio-fabrication of silver nanoparticles using Chrysanthemum coronarium
flower extract and It’s in vitro antibacterial activity. Int. J. Appl. Pharm. 2018, 10, 209–213. [CrossRef]
60. Ishak, N.M.; Kamarudin, S.K.; Timmiati, S.N. Green synthesis of metal and metal oxide nanoparticles via plant extracts: An
overview. Mater. Res. Express 2019, 6, 112004. [CrossRef]
61. de Aragao, A.P.; de Oliveira, T.M.; Quelemes, P.V.; Perfeito, M.L.; Araujo, M.C.; Santiago, J.D.; Cardoso, V.S.; Quaresma, P.; de
Almeida, J.R.; da Silva, D.A. Green synthesis of silver nanoparticles using the seaweed Gracilaria birdiae and their antibacterial
activity. Arab. J. Chem. 2019, 12, 4182–4188. [CrossRef]
62. Hasnain, M.S.; Javed, M.N.; Alam, M.S.; Rishishwar, P.; Rishishwar, S.; Ali, S.; Nayak, A.K.; Beg, S. Purple heart plant leaves
extract-mediated silver nanoparticle synthesis: Optimization by Box-Behnken design. Mater. Sci. Eng. C 2019, 99, 1105–1114.
[CrossRef] [PubMed]
63. Das, C.A.; Kumar, V.G.; Dhas, T.S.; Karthick, V.; Govindaraju, K.; Joselin, J.M.; Baalamurugan, J. Antibacterial activity of silver
nanoparticles (biosynthesis): A short review on recent advances. Biocatal. Agric. Biotechnol. 2020, 27, 101593. [CrossRef]
64. Lee, K.J.; Lee, Y.I.; Shim, I.K.; Jun, B.H.; Cho, H.J.; Joung, J.W. Large-scale synthesis of polymer-stabilized silver nanoparticles. In
Solid State Phenomena; Trans Tech Publications Ltd.: Freienbach, Switzerland, 2007; Volume 124, pp. 1189–1192.
65. Raza, M.A.; Kanwal, Z.; Rauf, A.; Sabri, A.N.; Riaz, S.; Naseem, S. Size-and shape-dependent antibacterial studies of silver
nanoparticles synthesized by wet chemical routes. Nanomaterials 2016, 6, 74. [CrossRef]
66. Singh, H.; Du, J.; Singh, P.; Yi, T.H. Ecofriendly synthesis of silver and gold nanoparticles by Euphrasia officinalis leaf extract and
its biomedical applications. Artif. Cells Nanomed. Biotechnol. 2018, 46, 1163–1170. [CrossRef] [PubMed]
67. Patil, S.; Chandrasekaran, R. Biogenic nanoparticles: A comprehensive perspective in synthesis, characterization, application and
its challenges. J. Genet. Eng. Biotechnol. 2020, 18, 1–23. [CrossRef]
68. Dibrov, P.; Dzioba, J.; Gosink, K.K.; Häse, C.C. Chemiosmotic mechanism of antimicrobial activity of Ag+ in Vibrio cholerae.
Antimicrob. Agents Chemother. 2002, 46, 2668–2670. [CrossRef] [PubMed]
69. Reidy, B.; Haase, A.; Luch, A.; Dawson, K.A.; Lynch, I. Mechanisms of silver nanoparticle release, transformation and toxicity:
A critical review of current knowledge and recommendations for future studies and applications. Materials 2013, 6, 2295–2350.
[CrossRef]
70. Bragg, P.D.; Rainnie, D.J. The effect of silver ions on the respiratory chain of Escherichia coli. Can. J. Microbiol. 1974, 20, 883–889.
[CrossRef]
71. Holt, K.B.; Bard, A.J. Interaction of silver (I) ions with the respiratory chain of Escherichia coli: An electrochemical and scanning
electrochemical microscopy study of the antimicrobial mechanism of micromolar Ag+. Biochemistry 2005, 44, 13214–13223.
[CrossRef]
72. Klueh, U.; Wagner, V.; Kelly, S.; Johnson, A.; Bryers, J.D. Efficacy of silver-coated fabric to prevent bacterial colonization and
subsequent device-based biofilm formation. J. Biomed. Mater. Res. Off. J. Soc. Biomater. Jpn. Soc. Biomater. Aust. Soc. Biomater.
Korean Soc. Biomater. 2000, 53, 621–631. [CrossRef]
73. Yamanaka, M.; Hara, K.; Kudo, J. Bactericidal actions of a silver ion solution on Escherichia coli, studied by energy-filtering
transmission electron microscopy and proteomic analysis. Appl. Environ. Microbiol. 2005, 71, 7589–7593. [CrossRef]
74. Jung, W.K.; Koo, H.C.; Kim, K.W.; Shin, S.; Kim, S.H.; Park, Y.H. Antibacterial activity and mechanism of action of the silver ion
in Staphylococcus aureus and Escherichia coli. Appl. Environ. Microbiol. 2008, 74, 2171–2178. [CrossRef] [PubMed]
75. Lopez-Carballo, G.; Higueras, L.; Gavara, R.; Hernández-Muñoz, P. Silver ions release from antibacterial chitosan films containing
in situ generated silver nanoparticles. J. Agric. Food Chem. 2013, 61, 260–267. [CrossRef] [PubMed]
76. Siddiqi, K.S.; Husen, A.; Rao, R.A. A review on biosynthesis of silver nanoparticles and their biocidal properties. J. Nanobiotechnol-
ogy 2018, 16, 1–28. [CrossRef] [PubMed]
77. Tang, S.; Zheng, J. Antibacterial activity of silver nanoparticles: Structural effects. Adv. Healthc. Mater. 2018, 7, 1701503. [CrossRef]
78. Bapat, R.A.; Chaubal, T.V.; Joshi, C.P.; Bapat, P.R.; Choudhury, H.; Pandey, M.; Gorain, B.; Kesharwani, P. An overview of
application of silver nanoparticles for biomaterials in dentistry. Mater. Sci. Eng. C 2018, 91, 881–898. [CrossRef]
79. Khorrami, S.; Zarrabi, A.; Khaleghi, M.; Danaei, M.; Mozafari, M.R. Selective cytotoxicity of green synthesized silver nanoparticles
against the MCF-7 tumor cell line and their enhanced antioxidant and antimicrobial properties. Int. J. Nanomed. 2018, 13, 8013.
[CrossRef] [PubMed]
80. Ramkumar, V.S.; Pugazhendhi, A.; Gopalakrishnan, K.; Sivagurunathan, P.; Saratale, G.D.; Dung, T.N.; Kannapiran, E. Biofabrica-
tion and characterization of silver nanoparticles using aqueous extract of seaweed Enteromorpha compressa and its biomedical
properties. Biotechnol. Rep. 2017, 14, 1–7. [CrossRef]
81. Durán, N.; Nakazato, G.; Seabra, A.B. Antimicrobial activity of biogenic silver nanoparticles, and silver chloride nanoparticles:
An overview and comments. Appl. Microbiol. Biotechnol. 2016, 100, 6555–6570. [CrossRef]
82. Hamdan, S.; Pastar, I.; Drakulich, S.; Dikici, E.; Tomic-Canic, M.; Deo, S.; Daunert, S. Nanotechnology-driven therapeutic
interventions in wound healing: Potential uses and applications. ACS Cent. Sci. 2017, 3, 163–175. [CrossRef] [PubMed]
83. Naraginti, S.; Kumari, P.L.; Das, R.K.; Sivakumar, A.; Patil, S.H.; Andhalkar, V.V. Amelioration of excision wounds by topical
application of green synthesized, formulated silver and gold nanoparticles in albino Wistar rats. Mater. Sci. Eng. C 2016, 62,
293–300. [CrossRef]
84. Bose, D.; Chatterjee, S. Biogenic synthesis of silver nanoparticles using guava (Psidium guajava) leaf extract and its antibacterial
activity against Pseudomonas aeruginosa. Appl. Nanosci. 2016, 6, 895–901. [CrossRef]
Molecules 2021, 26, 5057 21 of 21

85. Ahsan, A.; Farooq, M.A.; Ahsan Bajwa, A.; Parveen, A. Green synthesis of silver nanoparticles using Parthenium hysterophorus:
Optimization, characterization and in vitro therapeutic evaluation. Molecules 2020, 25, 3324. [CrossRef] [PubMed]
86. Adelere, I.A.; Lateef, A.; Aboyeji, D.O.; Abdulsalam, R.; Adabara, N.U.; Bala, J.D. Biosynthesis of Silver Nanoparticles Using
Aqueous Extract of Buchholzia Coriacea (Wonderful Kola) Seeds and Their Antimicrobial Activities. Ann. Food Sci. Technol. 2017,
18, 671–679.
87. Saratale, R.G.; Benelli, G.; Kumar, G.; Kim, D.S.; Saratale, G.D. Bio-fabrication of silver nanoparticles using the leaf extract of an
ancient herbal medicine, dandelion (Taraxacum officinale), evaluation of their antioxidant, anticancer potential, and antimicrobial
activity against phytopathogens. Environ. Sci. Pollut. Res. 2018, 25, 10392–10406. [CrossRef] [PubMed]
88. Nakkala, J.R.; Mata, R.; Gupta, A.K.; Sadras, S.R. Biological activities of green silver nanoparticles synthesized with Acorous
calamus rhizome extract. Eur. J. Med. Chem. 2014, 85, 784–794. [CrossRef] [PubMed]
89. Jiang, X.; Lu, C.; Tang, M.; Yang, Z.; Jia, W.; Ma, Y.; Jia, P.; Pei, D.; Wang, H. Nanotoxicity of silver nanoparticles on HEK293T cells:
A combined study using biomechanical and biological techniques. ACS Omega 2018, 3, 6770–6778. [CrossRef]
90. Franci, G.; Falanga, A.; Galdiero, S.; Palomba, L.; Rai, M.; Morelli, G.; Galdiero, M. Silver nanoparticles as potential antibacterial
agents. Molecules 2015, 20, 8856–8874. [CrossRef]
91. Singh, P.; Kim, Y.J.; Singh, H.; Wang, C.; Hwang, K.H.; Farh, M.E.; Yang, D.C. Biosynthesis, characterization, and antimicrobial
applications of silver nanoparticles. Int. J. Nanomed. 2015, 10, 2567.
92. Abdelghany, T.M.; Al-Rajhi, A.M.; Al Abboud, M.A.; Alawlaqi, M.M.; Magdah, A.G.; Helmy, E.A.; Mabrouk, A.S. Recent advances
in green synthesis of silver nanoparticles and their applications: About future directions. A review. BioNanoScience 2018, 8, 5–16.
[CrossRef]
93. Mohanta, Y.K.; Panda, S.K.; Bastia, A.K.; Mohanta, T.K. Biosynthesis of silver nanoparticles from Protium serratum and
investigation of their potential impacts on food safety and control. Front. Microbiol. 2017, 8, 626. [CrossRef] [PubMed]
94. Agarwal, P.; Mehta, A.; Kachhwaha, S.; Kothari, S.L. Green synthesis of silver nanoparticles and their activity against Mycobac-
terium tuberculosis. Adv. Sci. Eng. Med. 2013, 5, 709–714. [CrossRef]
95. Thakur, N.; Gaikar, V.G.; Sen, D.; Mazumder, S.; Pandita, N.S. Phytosynthesis of silver nanoparticles using walnut (Juglans regia)
bark with characterization of the antibacterial activity against Streptococcus mutans. Anal. Lett. 2017, 50, 690–711. [CrossRef]
96. Patil, R.S.; Kokate, M.R.; Kolekar, S.S. Bioinspired synthesis of highly stabilized silver nanoparticles using Ocimum tenuiflorum
leaf extract and their antibacterial activity. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2012, 91, 234–238. [CrossRef] [PubMed]
97. Agarwal, P.; Bairwa, V.K.; Kachhwaha, S.; Kothari, S.L. Green synthesis of silver nanoparticles using callus extract of Capsicum
annuum L. and their activity against microorganisms. Int. J. Nanotechnol. Appl. 2014, 4, 1–8.
98. Krutyakov, Y.A.; Kudrinskiy, A.A.; Olenin, A.Y.; Lisichkin, G.V. Synthesis and properties of silver nanoparticles: Advances and
prospects. Russ. Chem. Rev. 2008, 77, 233. [CrossRef]
99. Cai, X.; Zhai, A. Preparation of microsized silver crystals with different morphologies by a wet-chemical method. Rare Met. 2010,
29, 407–412. [CrossRef]
100. Akintelu, S.A.; Olugbeko, S.C.; Folorunso, F.A.; Oyebamiji, A.K.; Folorunso, A.S. Characterization and pharmacological efficacy
of silver nanoparticles biosynthesized using the bark extract of Garcinia kola. J. Chem. 2020, 2020, 2876019. [CrossRef]