Hindawi

Journal of Chemistry
Volume 2017, Article ID 6897960, 9 pages
https://doi.org/10.1155/2017/6897960

Research Article
Application of Starch-Stabilized Silver Nanoparticles as
a Colorimetric Sensor for Mercury(II) in 0.005 mol/L Nitric Acid

Penka Vasileva,1 Teodora Alexandrova,1 and Irina Karadjova2

1
Department of General and Inorganic Chemistry, Faculty of Chemistry and Pharmacy, Laboratory of Nanoparticle Science
and Technology, University of Sofia St. Kliment Ohridski, 1 J. Bourchier Blvd., 1164 Sofia, Bulgaria
2
Department of Analytical Chemistry, Faculty of Chemistry and Pharmacy, University of Sofia St. Kliment Ohridski,
1 J. Bourchier Blvd., 1164 Sofia, Bulgaria

Correspondence should be addressed to Penka Vasileva; pvasileva@chem.uni-sofia.bg

Received 12 December 2016; Revised 13 March 2017; Accepted 28 March 2017; Published 13 April 2017

Academic Editor: Roberto Comparelli

Copyright 2017 Penka Vasileva et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A sensitive and selective Hg2+ optical sensor has been developed based on the redox interaction of Hg2+ with starch-coated silver
nanoparticles (AgNPs) in the presence of 0.005 mol L1 HNO3 . The relative intensity of the localized surface plasmon absorption
band of AgNPs at 406 nm is linearly dependent on the concentration of Hg2+ with positive slope for the concentration range
012.5 g L1 and negative slope for the concentration range 25500 g L1 . Experiments performed demonstrated that metal ions
(Na+ , K+ , Mg2+ , Ca2+ , Pb2+ , Cu2+ , Zn2+ , Cd2+ , Fe3+ , Co2+ , and Ni2+ ) do not interfere under the same conditions, due to the absence
of oxidative activity of these ions, which guarantees the high selectivity of the proposed optical sensor towards Hg2+ . The limits of
detection and quantification were found to be 0.9 g L1 and 2.7 g L1 , respectively, and relative standard deviations varied in the
range 912% for Hg content from 0.9 to 12.5 g L1 and 59% for Hg levels from 25 to 500 g L1 . The method was validated by
analysis of CRM Estuarine Water BCR505. A possible mechanism of interaction between AgNPs and Hg2+ for both concentration
ranges was proposed on the basis of UV-Vis, TEM, and SAED analyses.

1. Introduction instrumental methods and techniques have been developed

for Hg determination at low environmentally relevant con-
Monitoring of toxic metals in aquatic ecosystems is an impor- centrations like atomic absorption/emission spectrometry
tant analytical task as far as these contaminants adversely (AAS/AES) [3], atomic fluorescence spectrometry (AFS) [4,
affect the environment and have serious medical effects on 5], and high-performance liquid chromatography (HPLC)
human health. One of the most harmful pollutants among [6, 7]. In spite of being very sensitive and precise for Hg deter-
them is Hg, which is still released in the environment and mination, these methods often require a time-consuming
widely distributed in air, water, and soil [1]. At very low sample preparation step as well as expensive instrumentation.
concentrations, Hg affects humans health, causing a variety Various colorimetric assays (based on the use of sensitive
of diseases to the heart, kidneys, brain, and nervous and chromophores or fluorophores [811], polymers [12, 13],
endocrine systems [2] Naturally occurring levels of mercury oligonucleotides [14, 15], DNA [16, 17], and metal nanopar-
in groundwater and surface water are less than 0.5 g L1 , ticles [1820]) have been developed and reported in the
although local mineral deposits may produce higher levels literature as convenient and simple alternative methods for
in groundwaters. Essential quality standard for Hg maximum the detection of target analytes without the requirement of
permissible limit of 1 g L1 has been adopted at EU level and sophisticated apparatus.
requires regular monitoring of Hg content in drinking waters. Metal nanoparticles have unique properties and applica-
It is well known that Hg exists in natural waters as different tions in numerous fields, which are attributed to the collective
species: Hg0 , methyl-Hg, and inorganic Hg(II); however, the dipole oscillation known as Surface Plasmon Resonance
dominant toxic species in drinking waters is Hg(II). Various (SPR) [21]. This phenomenon makes them very desirable
2 Journal of Chemistry

for colorimetric sensing of Hg2+ ions because the inter- NiCl2 , CdCl2 , CoCl2 , and FeCl3 ) (from Merck, Germany),
action between the nanoparticles and the analyte changes and pharmaceutical grade D-(+) glucose (from Alfa Aesar,
the intensity and/or position of the absorption band in the Germany) were used. Stock Hg standard solution, Trace
visible spectrum, which often might be observed with the CEPT, 998 g mL1 in 2 mol L1 HNO3 (Sigma-Aldrich,
naked eye [22]. The limitations observed for these systems are USA), was used to prepare a working standard solution of
mainly connected with poor selectivity, high detection limit 1000 g L1 Hg2+ in 0.01 mol L1 HNO3 . Standard solutions
for Hg(II), complicated synthesis of the probe materials, or for Hg within the concentration range of 01000 g L1
complicated analytical procedures. were prepared weekly by serial dilution of this solution in
In this study, we present a new colorimetric assay for 0.01 mol L1 HNO3 . All diluted Hg solutions were stored in
Hg ions in 0.005 mol L1 HNO3 using starch-stabilized
2+
dark glass flasks and kept refrigerated at 4 C.
silver nanoparticles (AgNPs). A change in the absorbance
strength is expected as a result of the redox interaction 2.3. Synthesis and Characterization of Silver Nanoparticles.
between AgNPs and either Hg2+ ions or NO3 ions. The Hg The synthesis of AgNPs follows a green synthetic procedure as
concentration determines which of these two redox reactions described in our previous study [34]. The silver nanoparticles
dominates as the two oxidants compete with each other for were obtained through a reduction reaction of silver nitrate
Ag oxidation. This way, detection of very low environmentally with D-glucose as a reducing agent in the presence of starch
relevant Hg contents is possible. Several sensing systems as a stabilizer and suitable sodium hydroxide amount as a
have been already reported based on the interaction between reaction catalyst. Briefly, 24 mL of 0.001 M AgNO3 and 48 mL
AgNPs and Hg(II) ions [2332]; however, detailed study of of 0.2% solution of starch were mixed and left for at least 15
Hg behavior in the presence of another competitive oxidant minutes to form a complex under an ultrasonic treatment
is rarely performed and discussed. A dual functional sensor (ultrasonic bath, power 100 W, frequency 38 MHz). After
for determination of Hg and H2 O2 has been developed that, 720 L of 0.1 M D-glucose was added and sonicated
based on a similar approach: addition of H2 O2 to a mixture for 5 minutes. The reaction was started by the addition of
of AgNPs and Hg(II) ions [33]. The method presented in 3.6 mL of 0.1 M NaOH and continued for one hour at a
this study, however, differs not only as a mechanism of constant temperature (30 C) in an ultrasonic bath to ensure
the process, but also as a behavior of Hg2+ ions at very the homogeneous formation of the silver nanoparticles.
low concentrations (below 25 g L1 ) towards AgNPs in the The as-prepared AgNPs were purified and concentrated
presence of NO3 ions as a second oxidant. A simple and three times by ultracentrifugation (90 min, 14,000 rpm). The
fast analytical procedure for determination of Hg in drinking dispersion obtained was denoted as a stock solution of AgNPs
waters is developed and verified by the analysis of a certified and used in the experiments for colorimetric determination
reference material. of Hg2+ . The AgNPs stock solution was kept in a dark
glass flask at room temperature and was homogenized in an
ultrasonic bath for 30 min prior to each experiment.
2. Materials and Methods
2.1. Apparatus. UV-Vis absorption spectra were recorded on 2.4. Colorimetric Detection of Hg2+ Ions. The colorimetric
an Evolution 300 spectrometer (Thermo Scientific, USA) detection of Hg2+ ions via starch-stabilized silver nanopar-
within the 200800 nm range using quartz cuvettes with 1 cm ticles was conducted as follows: an aliquot of 200 L AgNPs
optical path length. High-purity water was used as a ref- stock solution and 300 L high-purity water were consecu-
erence sample for background absorption. The morphology tively added to a small quartz cuvette, followed by addition
and particle sizes were examined using a high-resolution of 500 L Hg2+ solution with varying concentrations. The
transmission electron microscope (TEM, JEOL JEM-2100 resulting mixture was equilibrated by stirring on Vortex for an
operating at an accelerating voltage of 200 kV). A volume optimum incubation time and then the UV-Vis spectrum in
of 5 L AgNPs suspension was placed on a carbon-covered the wavelength range of 200800 nm was recorded. In order
copper grid for TEM and air-dried. The histogram of AgNPs to investigate the sensitivity of the colorimetric assay towards
size distribution and the mean diameter of nanoparticles other ions, starch-stabilized AgNPs were allowed to interact
were determined by counting at least 200 nanoparticles under the same conditions with 50 mol L1 solutions of
from the different TEM images using ImageJ software. Some alkali (Na+ , K+ ), alkaline earth (Mg2+ , Ca2+ ), Pb2+ , and
structural details of the nanoparticles were analyzed using transition-metal ions (Cu2+ , Zn2+ , Cd2+ , Fe3+ , Co2+ , and
the high-resolution TEM image and SAED pattern. The zeta Ni2+ ) (separately for each ion). The resulting solutions were
() potential of nanoparticles was measured with a ZetaSizer monitored by optical absorption spectroscopy.
Nano ZS (Malvern) instrument.
2.5. Determination of Hg in Tap/Underground Water. Tap/
2.2. Chemicals. All chemicals used were of analytical-reagent underground water sample (20 mL) was filtered through a
grade and all aqueous solutions were prepared in high-purity 0.45 m filter and acidified with HNO3 until reaching pH
water (Millipore Corp., Milford, MA, USA). Silver nitrate in the range 22.3. Sample aliquot of 500 L was transferred
(AgNO3 , 99.8%), soluble starch, sodium hydroxide (NaOH, to a quartz cuvette, and 200 L stock solution of AgNPs
99%), nitric acid (HNO3 , 65%), salts of the different cations was added and the mixture was stirred by the Vortex. After
studied (NaCl, KCl, MgCl2 , CaCl2 , Pb(NO3 )2 , ZnCl2 , CuCl2 , the incubation time of 5 min, the UV-Vis absorbance was
Journal of Chemistry 3

Mean diameter 15.4 nm Hg2+ in the presence of 0.005 mol L1 nitric acid was followed
3.0
St. dev. 3.9 nm within one hour by measurements of UV-Vis absorbance.
2.5
Typical evolution of UV-Vis absorbance spectrum with time,
Absorbance (a.u.)

2.0
1.5
due to the interaction of AgNPs with 400 g L1 Hg2+ and
1.0 respective color change of the AgNPs dispersion, is shown in
0.5 Figure 2.
0.0
200 400 600 800
The changes that occurred in the LSPR absorption band
Wavelength (nm)
5 9 13 17 21 25 of AgNPs are reflected on the color of the samples, which can
Particle diameter (nm) be seen even with the naked eye. It is seen that the sensors
response is significant during the first five minutes of the
reaction process and a negligible change in the absorption
intensity is observed over time. This fact allows convenient
20 nm
analytical detection of Hg2+ within only five minutes.
5 nm
As a next step, the sensitivity and applicability of starch-
Figure 1: TEM image of starch-stabilized silver nanoparticles; coated AgNPs for quantitative determination of Hg2+ ions
insets: UV-Vis absorption spectrum and digital photographs (left), under the defined optimal conditions were studied. The col-
high-resolution TEM image of single nanoparticles, and histogram orimetric response and LSPR band behavior were monitored
of nanoparticle size distribution (right). as a function of Hg2+ concentrations, ranging from 0 to
500 g L1 in the presence of 0.005 mol L1 HNO3 (Figure 3).
As seen from the UV-Vis absorbance spectra (5-minute
incubation time), the addition of 0.005 mol L1 HNO3 results
measured at 407 nm. Parallel sample aliquot of 250 L is in a considerable decrease of the intensity of AgNPs charac-
diluted twice with 0.005 mol L1 HNO3 and passed through teristic plasmon band at 407 nm accompanied by a slight blue
the procedure described above. The response of this sample shift (Figure 3(a)). In addition, a shoulder band appears at
(increase or decrease, related to the original one, Figure 4) is the wavelength range of 450600 nm. The increase of Hg2+
used to distinguish the low from the high linear concentration concentration from 0 to 12.5 g L1 in 0.005 mol L1 HNO3
range of Hg and to choose an appropriate calibration curve. leads to a gradual increase of the intensity of the characteristic
plasmon band of AgNPs at 407 nm and its value gradu-
3. Results and Discussion ally approximates to the absorption intensity of the blank
nanoparticle solution (without both NO3 and Hg2+ ). In
3.1. Characterization of AgNPs. The UV-Vis absorption spec- addition, the intensity of the shoulder band decreases along
trum of starch-stabilized AgNPs, recorded at 25 C, is shown with increasing intensity of the main plasmon absorbance
in Figure 1 (inset). A single and sharp SPR band appears at band. The spectra show a clear isosbestic point at 445 nm
407 nm, which indicates the formation of nanometer-sized upon addition of Hg2+ in 0.005 mol L1 HNO3 , demon-
particles. This is further confirmed by the TEM observation strating that the aggregation of AgNPs is directly related to
and size distribution histogram, shown in Figure 1. the concentration of Hg2+ . Contrariwise, a gradual decrease
The spherical-like AgNPs exhibit a relatively narrow of the intensity of the characteristic plasmon band of the
size distribution with a mean diameter of 15.4 3.9 nm. AgNPs at 407 nm is observed for the Hg concentration range
In addition to the nanospheres, some typical polyhedral from 25 to 500 g L1 . The spectra presented in Figure 3(b)
nanoparticles (multiple twined nanocrystals) can be easily also show that the decrease of intensity of the absorbance
observed. The crystalline nature of AgNPs is clearly observed maximum at 407 nm is accompanied with a slight blue
on the HRTEM image in Figure 1 (inset) and proved by shift, which is strengthened for the higher concentrations of
the lattice characterization (e.g., the spacing between the Hg2+ . This phenomenon is already reported and described
individual lattice fringes of 0.235 nm, which corresponds to as a change of the refractive index of the particles and the
(111) plane lattice spacing of pure silver). The colloidal stability formation of a mercury layer around AgNPs, yielding an
of starch-coated AgNPs is confirmed by the -potential value
amalgam-like structure [25, 35, 36]. It might be suggested
of 25.3 1.3 mV measured in 0.001 mol L1 KCl at pH 6.8.
that, for the first Hg concentration range (012.5 g L1 ), a
redox reaction proceeds between zero-valent silver (Ag0 ) and
3.2. The Optimization of Colorimetric Sensing of Hg2+ . Sev-
either Hg2+ or NO3 ions. The values of standard electrode
eral parameters were investigated systematically in order
to establish optimal conditions for the direct colorimetric potentials of the components in the system confirm this
detection of Hg2+ . As a first step, the pH value was adjusted suggestion: 0 (Ag+ /Ag0 ) = 0.799 V; 0 (Hg2+ /Hg0 ) = 0.854 V;
taking into account the analysis of real samples and HNO3 0 (NO3 /NH4 + ) = 0.864 V. Because the standard electrode
which is typically used for water sample preservation. The potential of NO3 /NH4 + is commeasurable with that of
experiments performed showed that 0.005 mol L1 HNO3 Hg2+ /Hg0 , two competitive oxidizing agents are involved in
ensured the highest sensitivity and could be accepted as an the studied sensing system. The most probable explanation
optimal sample medium. In order to evaluate the optimum for the decrease of LSPR band intensity in the presence of
contact time, the kinetic of interaction between AgNPs and 0.005 mol L1 HNO3 and further increase upon addition of
4 Journal of Chemistry

3.0
407 nm
400
g L1 Hg2+
2.5

2.0 399 nm

Absorbance (a.u.)
1.5

1.0

0.5

0.0
300 400 500 600 700 800
Wavelength (nm)
Ag blank 30 min
5 min 45 min
15 min 60 min
(a) (b)
Figure 2: (a) Evolution of UV-Vis absorbance spectrum of AgNPs and (b) color change of the AgNPs dispersion upon the addition of
400 g L1 Hg2+ in the presence of 0.005 mol L1 HNO3 .

3.0 3.0

Hg2+
2.5 2.5

2.0 2.0
Absorbance (a.u.)
Absorbance (a.u.)

2+
Hg
1.5 1.5

1.0 1.0

0.5 0.5

0.0 0.0
300 400 500 600 700 800 300 400 500 600 700 800
Wavelength (nm) Wavelength (nm)
AgNPs blank 2.5
g L1 AgNPs blank 200
g L1
0
g L1 12.5
g L1 25
g L1 300
g L1
50
g L1 400
g L1
100
g L1 500
g L1
(a) (b)

Figure 3: UV-Vis absorption responses of starch-stabilized AgNPs recorded 5 min after the addition of Hg2+ with various concentrations: (a)
012.5 g L1 Hg2+ and (b) 25500 g L1 Hg2+ in the presence of 0.005 mol L1 HNO3 .

Hg2+ (Figure 3(a)) is that, at low Hg2+ concentrations, the of positively charged Hg2+ on the surface of negatively
oxidative effect of NO3 ions towards surface silver atoms is charged AgNPs followed by amalgamation. In this way, the
dominant. In the presence of higher concentrations of Hg2+ surface of AgNPs is inaccessible for oxidation by NO3 .
(Figure 3(b)), the surface of nanoparticles is protected by the Evidently, within the range of 25500 g L1 Hg2+ , the main
layer of Ag-Hg-amalgam due to the sorption and reduction redox interaction is between the AgNPs and Hg2+ . Such
Journal of Chemistry 5

0.96
1.00
0.94
0.96
0.92
0.92
0.90

A t /A 0 at 407 nm
A t /A 0 at 407 nm

0.88 0.88

0.86 0.84
0.84 0.80
0.82 0.76
0.80 0.72
0.78
0.68
0.76
0.0 2.5 5.0 7.5 10.0 12.5 0 100 200 300 400 500
Hg2+ (
g L1 ) Hg2+ (
g L1 )

Experimental data Experimental data

Linear fit Linear fit
(a) (b)

Figure 4: Plot of At / 0 as a function of the Hg2+ concentration over the ranges of (a) 012.5 g L1 and (b) 25500 g L1 in the presence of
0.005 mol L1 HNO3 .

behavioral dissimilarities of the analyte (Hg2+ ) for different 0.30

concentration ranges have not been observed and reported
0.25
in the previous studies on the AgNPs-based optical sensing
system for Hg2+ colorimetric detection. We have to point
(A 0 A t )/A 0

0.20
out, however, that none of these reports mention the acidity 0.15
of the reaction media, which most probably determines the
oxidizing power of reagents in the system. 0.10
For quantitative determination of Hg2+ , the change of 0.05
the intensity of LSPR band maximum of silver nanoparti-
cles at 407 nm upon the addition of analyte with various 0.00
K(I)

Na(I)

Mg(II)

Ca(II)

Cu(II)

Zn(II)

Pb(II)

Cd(II)

Fe(III)

Co(II)

Ni(II)

Hg(II)
concentrations was estimated as a ratio At / 0 , where 0
corresponds to the intensity of the absorbance maximum
of blank AgNPs solution (without both NO3 and Hg2+ Figure 5: Colorimetric response of starch-stabilized AgNPs
ions) and At corresponds to the intensity of the absorbance recorded 5 min after the addition of 5 105 mol L1 metal ions.
maximum of silver nanoparticles 5 min after the addition of
Hg2+ standard solutions (Figure 4).
As indicated in Figures 4(a) and 4(b), linear correlations
exist between the relative value of the absorbance maximum to define the selectivity of the proposed system for colori-
intensity and the concentration of Hg2+ over the concentra- metric Hg2+ determination. This has been evaluated through
tion ranges 012.5 g L1 (A = 0.7814 + 1.30 102 c with the response of the assay to various environmentally relevant
2 = 0.995) and 25500 g L1 (A = 0.991 5.90 104 c metal ions including Na+ , K+ , Mg2+ , Ca2+ , Pb2+ , Cu2+ , Zn2+ ,
with 2 = 0.993), respectively. As a conclusion, the optical Cd2+ , Fe3+ , Co2+ , and Ni2+ under the same conditions as in
sensor studied using starch-stabilized AgNPs ensures a linear the case of Hg2+ . The optical response of AgNPs to the tested
response over the concentration range from 0.9 to 12.5 g L1 ions (concentration level of 50 mol L1 ) after 5 min of their
which covers all environmentally relevant concentrations addition (separately for each ion) is illustrated in Figure 5. For
of Hg and might be used for fast screening of Hg in the comparison, the optical response of AgNPs to the Hg2+ ions
aquatic environment. The second concentration range from at a concentration level of 2.5 mol L1 is also presented.
25 to 500 g L1 Hg2+ can be successfully applied for the It is easy to observe that all other metal ions produce
determination of Hg in highly contaminated and rarely found a much weaker signal (almost at baseline level) except Fe3+
industrial wastewaters. which shows modest interference. The reason is that only
Hg2+ can be reduced by surface atoms of AgNPs to form
3.3. Selectivity of Hg2+ Optical Sensing by Starch-Coated stable Ag-Hg amalgam. The addition of Fe3+ resulted in a tiny
AgNPs. From an analytical point of view, it is very important intensity decrease and red shift of the absorption band. This
6 Journal of Chemistry

20 nm

(a) (b)
Figure 6: (a) TEM image (scale bar is 20 nm) and (b) corresponding SAED pattern of starch-coated silver nanoparticles after treatment by
Hg2+ solution at a concentration of 500 g L1 in the presence of 0.005 mol L1 HNO3 .

Ag Hg2+
e e

(1) Redox process

(2) Amalgamation

Metal bridging force

Aggregation

Hg2+ ions Ag+ ions

AgNPs Ag n Hg m or/and mercury atoms
Figure 7: The proposed mechanism of the interaction between starch-coated AgNPs and Hg2+ solution.

effect could be interpreted in terms of Fe(III) complexation 65-3156) and Ag (PDF 89-3722) as main phases in the
with oxidized species of carbohydrates (starch and glucose) aggregated mass formed during the interaction of starch-
which are sorbed on the surface of silver nanoparticles [37]. coated AgNPs with Hg2+ . Some impurities of metallic Hg
(PDF 01-1017) are also detected.
3.4. Mechanism of Interaction between AgNPs and Hg2+ . To On the basis of TEM/SAED results, a multistep inter-
elucidate the mechanism of sensing activity of the starch- action of Hg2+ with the silver nanoparticles could be
coated AgNPs towards Hg2+ , the nanoparticles were exam- inferred. The interaction involves (i) the electrostatic attrac-
ined before and after Hg2+ exposure using TEM with SAED tion between negatively charged silver nanoparticles and
observations. Figure 6 shows TEM micrograph with the positively charged Hg2+ species, decreasing the distance
corresponding SAED pattern obtained from the agglomerate between nanoparticles; (ii) adsorption of Hg2+ on the surface
formed during interaction of AgNPs with Hg2+ solution at a of AgNPs and their reduction to Hg0 by the surface Ag atoms
concentration of 500 g L1 . (simultaneously obtained Ag+ diffuse into the solution);
As can be seen from Figure 6(a), the nanoparticles are (iii) amalgamation of the freshly generated mercury atoms
of varying sizes and there is a large distribution after Hg2+ with the surface Ag atoms [25, 32, 39]; (iv) the interaction
exposure. The TEM image shows a larger particle, which is of Hg2+ with AgNPs which decreases surface charges of
surrounded by smaller particles. It seems that larger particles nanoparticles, leading to their destabilization and aggrega-
are undergoing Ostwald ripening. A similar observation is tion. The latter one is confirmed by the shape evolution of
already reported for gold nanoparticles utilized for mercury AgNPs observed in Figure 6(a). The suggested mechanism of
removal from drinking water [38] and for colorimetric detec- optical sensing of Hg2+ by starch-coated silver nanoparticles
tion of Hg2+ using the AgNPs embedded in cyclodextrin- is illustrated in Figure 7.
silicate composite [39].
The data from the analysis of SAED pattern (Figure 6(b)) 3.5. Analytical Application. In order to test the applicability
are summarized with interpretation accuracy of 1% in Table 1. of the sensor developed for Hg2+ and total Hg determination,
The analysis shows the existence of Ag2 Hg3 amalgam (PDF samples of tap water (Sofia) and mineral water (Gorna Bania,
Journal of Chemistry 7

Table 1: SAED data of AgNPs aggregates formed after exposure of silver nanoparticles to Hg2+ at concentration of 500 g L1 in the presence
of 0.005 mol L1 HNO3 . ()f : double electron diffraction effects; SAED interpretation: accuracy 1%.

Ag
Ag Ag2 Hg3
PDF 87-0598
PDF 89-3722 PDF 65-3156
d (A) Relative intensity a = 2.8862 A,
a = 4.0855(1) A a = 10.0506 A
c = 10.000 A
SG Fm3m SG I23
P63 /mmc
2.438 s 101 (410)f
2.136 s 332
1.506 s 622
1.287 s (310)f 237
1.057 w 205 930
0.979 w (410)f 059
s: strong; w: weak.

Table 2: Comparison of different methods using silver nanoparticles as a colorimetric sensing probe for Hg2+ determination.

Sensing probe Linear concentration range Detection limit Ref.

Starch-stabilized AgNPs 50 nmol L1 5000 nmol L1 25 nmol L1 [23]
Unmodified AgNPs stabilized with
10100 mol L1 2.2 mol L1 [26]
extract of soap-root plant
1
50 nmol L
Gum kondagogu-stabilized AgNPs 50500 nmol L1 [31]
(LOQ)
Citrate-capped AgNPs 0.02 nmol L1 0.9 mol L1 [33]
1-Dodecanethiol-capped Ag nanoprisms
104000 nmol L1 3.3 nmol L1 [40]
upon the presence of iodides
1 1 1
Poly(vinylpyrrolidone)-stabilized AgNPs 1 nmol L 30 mol L 1 nmol L [41]
Carrageenan-functionalized Ag/AgCl
1100 mol L1 1 mol L1 [42]
NPs
Starch-coated AgNPs in the presence of
4.52500 nmol L1 4.5 nmol L1 This work
0.005 mol L1 HNO3

Kniagevo) were spiked at levels close to the permissible on silver nanoparticles as a colorimetric sensing probe. It is
limit (drinking water) of 1 g L1 . Total Hg content in these evident that the proposed method ensures higher or equal
samples was defined preliminarily by ICP-MS and results for sensitivity with those of earlier reported colorimetric AgNPs-
all samples were below 0.05 g L1 Hg. Recoveries achieved based sensors [23, 26, 31, 33, 4042].
using the described procedure are in the range 9397%, For partial validation of the procedure, CRM Estuarine
thus confirming the possibility of fast Hg2+ screening in Water BCR505 was analyzed after solid phase extraction (10-
drinking waters using the proposed sensor based on starch- fold Hg enrichment) [43]. Three sample aliquots of 800 L
coated AgNPs. The limits of detection (LOD) and limits of were analyzed according to the proposed analytical proce-
quantification (LOQ) were evaluated on the basis of repeated dure. The result of 0.73 0.08 nmol L1 Hg was in reasonable
analysis of blank (AgNPs). The calculations were based on agreement with the (additional material information) value
3 and 10 criteria using the linear regression equations of 0.69 nmol kg1 Hg (138 g L1 ).
and slopes of calibration graphs for Hg2+ (Figure 4). The
defined values for LOD (0.9 g L1 ) and LOQ (2.7 g L1 ) 4. Conclusions
show that the proposed sensor is not suitable for surface
water monitoring but might be successfully used for fast A simple, fast, and low cost analytical procedure is developed
on-site control of the quality of sources for drinking water. for easy and sensitive quantification of Hg2+ in the presence
Within-batch precision strongly depends on the analyte of 0.005 mol L1 HNO3 by using starch-coated AgNPs as a
concentration in the measuring solution: 912% for Hg2+ in LSPR-based optical sensor. The Hg2+ sensing is based on
the range 0.912.5 g L1 and 59% for Hg2+ in the range over the optical response (change in the absorbance strength of
25500 g L1 . Table 2 further summarizes the linear ranges LSPR band) of silver nanoparticles depending on the Hg2+
and detection limits of various Hg2+ detection methods based concentration. Possible mechanism of interaction between
8 Journal of Chemistry

AgNPs and Hg2+ was proposed. An accurate and reliable spirolactam by replacing one atom: design of rhodamine B
determination of Hg is achieved in two concentration ranges: thiohydrazide for recognition of Hg(II) in aqueous solution,
0.912.5 g L1 and 25500 g L1 . The limits of detection Organic Letters, vol. 8, no. 5, pp. 859861, 2006.
and quantification achieved were 0.9 g L1 and 2.7 g L1 , [10] Y. Zhao and Z. Zhong, Tuning the sensitivity of a foldamer-
respectively, and relative standard deviations varied in the based mercury sensor by its folding energy, Journal of the
range 912% for Hg content from 0.9 to 12.5 g L1 and 59% American Chemical Society, vol. 128, no. 31, pp. 99889989,
2006.
for Hg levels from 25 to 500 g L1 . The LSPR-based optical
sensor for Hg(II) might be used for simple and fast on-site [11] H. Wang, Y. Wang, J. Jin, and R. Yang, Gold nanoparticle-based
screening of sources for abstraction of drinking water and for colorimetric and turn-on fluorescent probe for mercury(II)
ions in aqueous solution, Analytical Chemistry, vol. 80, no. 23,
Hg determination in wastewaters.
pp. 90219028, 2008.
[12] X. Liu, Y. Tang, L. Wang et al., Optical detection of mercury(II)
Conflicts of Interest in aqueous solutions by using conjugated polymers and label-
The authors declare that there are no conflicts of interest free oligonucleotides, Advanced Materials, vol. 19, no. 11, pp.
regarding the publication of this paper. 14711474, 2007.
[13] I.-B. Kim and U. H. F. Bunz, Modulating the sensory response
of a conjugated polymer by proteins: an agglutination assay
Acknowledgments for mercury ions in water, Journal of the American Chemical
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