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Cite this: J. Anal. At. Spectrom., 2016,
31, 2285
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Speciation analysis of silver sulfide nanoparticles in
environmental waters by magnetic solid-phase
extraction coupled with ICP-MS†
Xiaoxia Zhou,ab Jingfu Liu,*ab Chungang Yuanc and Yongsheng Chen*d
The growing production and widespread application of silver nanoparticles (AgNPs) have led to their release
into the environment, where they are mostly transformed to silver sulfide nanoparticles (Ag2S NPs). Thus,
speciation analysis of Ag2S NPs in environmental matrices is essential for understanding the
environmental process and toxic effects of AgNPs. Herein, we report the use of aged iron oxide
magnetic particles (IOMPs) as magnetic solid-phase extraction adsorbents for speciation analysis of Ag2S
NPs. It was found that IOMPs are excellent adsorbents for the selective extraction of silver-containing
nanoparticles (AgCNPs) including Ag2S NPs, AgNPs and AgCl NPs in the presence of Ag+. More
importantly, Ag2S NPs can be distinguished from the other AgCNPs by sequential elution. After preeluting AgNPs and AgCl NPs as Ag+ with 2% (v/v) acetic acid and IOMPs as the sacrificial oxidants, Ag2S
NPs were completely eluted as a Ag(I) complex by 10 mM thiourea in 2% (v/v) acetic acid and quantified
directly by inductively coupled plasma mass spectrometry (ICP-MS). While the extraction of Ag+ was
negligible, the maximum extraction of AgCNPs by IOMPs was attained at pH 4.9–6.2, and the
interference of humic acid in the AgCNP extraction can be efficiently eliminated by adding Ca2+. Under
optimized conditions, the method provides low detection limit (0.068 mg L1) and high reproducibility
Received 8th July 2016
Accepted 20th September 2016
(relative standard deviations < 5.1%) for Ag2S NPs. By simultaneously spiking 0.16–10.3 mg L1 AgNPs and
0.53–14.8 mg L1 Ag2S NPs, the Ag2S NP recoveries were in the range of 69.6–100.2% for the tap, river
and lake waters; and in the range of 106.2–149.9% for the WWTP effluent due to the part sulfidation of
DOI: 10.1039/c6ja00243a
AgNPs to Ag2S NPs. Our method is valid for the speciation analysis of Ag2S NPs in water samples, which
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provides an efficient approach for studying the sulfidation of AgNPs and Ag+.
1. Introduction
As a broad-spectrum antimicrobial agent, silver nanoparticles
(AgNPs) are now widely incorporated into consumer and
medical products with an estimated production of 500 tons per
year.1,2 The growing production and widespread application of
AgNPs have led to their release into the environment, and
therefore increased public concerns on their potential risks to
general populations and to the ecosystem,3–6 due to AgNPs'
proven toxicity to various organisms including microorganisms,
algae, plants, animals and human cells.7
a
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center
for Eco-Environmental Science, Chinese Academy of Sciences, Beijing 100085, China.
E-mail: j[email protected]; Fax: +86-10-62849192; Tel: +86-10-62849192
b
University of Chinese Academy of Sciences, Beijing 100049, China
c
School of Environmental Science & Engineering, North China Electric Power
University, Baoding 071000, China
d
School of Civil and Environmental Engineering, Georgia Institute of Technology,
Atlanta, Georgia 30332, USA. E-mail: [email protected]; Fax: +1- 404894-2278; Tel: +1-404-894-3089
† Electronic supplementary information (ESI) available: Additional results are
provided. See DOI: 10.1039/c6ja00243a
This journal is © The Royal Society of Chemistry 2016
Once released into the environment, AgNPs undergo various
chemical transformations, such as chemical oxidation to
release Ag+, which further reacts with Ag-complexing ligands
(organic matter, sulde, chloride) existing in the environment
to form various Ag-containing NPs (AgCNPs) like AgCl NPs and
Ag2S NPs.7–10 It was reported that about 80% of the AgNPs were
transformed to Ag2S NPs in waste water treatment plants.11
Previous studies have demonstrated that Ag2S NPs have lower
toxicity in comparison to AgNPs,12–16 while oxidation is likely to
increase AgNP toxicity by the released Ag+;13,17 thus suldation is
expected as an antidote process of AgNPs by decreasing the Ag+
release.13–16 Therefore, an accurate speciation analysis of Ag2S
NPs in environmental and biological matrices is essential for
the insightful understanding of the environmental process and
the toxic effects of AgNPs.
Currently, there is a lack of methods for the speciation
analysis of trace Ag2S NPs in the presence of other AgCNPs and
Ag+.18–21 Although Ag K-edge X-ray absorption near edge spectroscopy (XANES) was applied to analyze the chemical transformation products of AgNPs including Ag2S NPs,22–24 it suffered
from a high detection limit of high mg L1 range, high cost and
rare availability of the instrument. Inductively coupled plasma-
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mass spectrometry (ICP-MS) showed an excellent qualitative
analysis ability in the trace and ultratrace determination of
element species, which has been a powerful tool for the detection of metal-containing NPs.25–27 However, the technique only
allowed for the determination of the total metal concentration
in fractions of NPs, rather than discriminating different metalcontaining NPs, such as Ag2S NPs in the presence of AgNPs and
AgCl NPs.
Due to the extremely low concentration of Ag2S NPs in the
environments, extraction and preconcentration of Ag2S NPs
followed by detection with ICP-MS are a promising approach for
its speciation analysis. Considering the lack of a straightforward
method for selective extraction and specic detection of Ag2S
NPs, a rational strategy for its speciation is to extract all the
AgCNPs and then distinguish them by sequential elution. The
large difference in solubility products (Ksp), e.g., 1.8 1010 for
AgCl and 6.0 1051 for Ag2S,7 makes it possible to sequentially
elute different AgCNPs as complexes of Ag+ with appropriate
eluents.
Magnetic solid-phase extraction (MSPE) with Fe3O4 magnetic
particles (termed as iron oxide magnetic particles, IOMPs) is
a promising technique for extraction and preconcentration of
Ag2S NPs. IOMPs show excellent performance in the extraction
of various analytes, such as high recovery and enrichment
factor, simple operation, easy isolation and surface modication.28,29 The reported high enrichment factor (up to 250-fold)
for AgNPs30 also suggests its great potential in extracting NPs.
Furthermore, the Fe3+ constituent in the IOMPs makes it
possible to function as the sacricial oxidants in eluting AgNPs.
Herein, we found that IOMPs are capable of selectively
extracting the predominant AgCNPs in the environment
including AgNPs, AgCl NPs and Ag2S NPs in the presence of Ag+;
and the extracted Ag2S NPs can be distinguished from the other
AgCNPs by sequential elution, i.e., aer pre-eluting AgNPs and
AgCl NPs by acetic acid, Ag2S NPs can be eluted by a mixture of
thiourea/acetic acid. Based on these ndings, we proposed an
approach for the speciation of Ag2S NPs in environmental
waters: (i) MSPE of AgCNPs with IOMPs as adsorbents; (ii) aer
pre-eluting AgNPs and AgCl NPs as Ag+ with 2% (v/v) acetic acid,
Ag2S NPs were completely eluted as the Ag(I) complex by 10 mM
thiourea in 2% (v/v) acetic acid and quantied directly by
inductively coupled plasma mass spectrometry (ICP-MS).
2.
2.1
Materials and methods
Chemicals and reagents
An aqueous dispersion of AgNPs with an unknown coating was
purchased from Shanghai Huzheng Nanotechnology Co., Ltd.
(Shanghai, China). Citrate coated AgNPs with the particle sizes
of 10, 20, 40, 60, and 100 nm, respectively, were obtained from
Sigma-Aldrich (St. Louis, MO). Silver nitrate, poly (vinyl
alcohol)-124 (PVA-124), sodium borohydride and hydroxylammonium chloride were purchased from Sinopharm
Chemical Reagent Co. (Beijing, China). Polyvinylpyrrolidone
(PVP-58, MW ¼ 58 000), polyvinylpyrrolidone (PVP-3.5, MW ¼
3500) and gum arabic (GA) were procured from Aladdin
Chemistry Co., Ltd. (Shanghai, China). Thiourea was purchased
2286 | J. Anal. At. Spectrom., 2016, 31, 2285–2292
from Sigma-Aldrich (St. Louis, MO). Suwannee River humic acid
from an aquatic source was purchased from the International
Humic Substances Society (IHSS, St. Paul. MN). The other
reagents were obtained from Beijing Chemicals (Beijing,
China). All the reagents were used as obtained without further
purication. Ultrapure water (18.2 MU) produced with a Milli-Q
Gradient System (Millipore, Bedford) was used throughout the
experiments.
2.2
Instrumentation
The experiments were carried out using an ICP-MS instrument
(Agilent 7700cs). The experimental conditions are listed in
Table 1. 115In as an internal standard element was used to
compensate matrix effect and signal dri.
2.3
Synthesis and characterization of AgCNPs
PVP-capped AgNPs (including PVP-3.5-AgNPs and PVP-58AgNPs), PVA-protected AgNPs, GA-protected AgNPs, AgCl NPs,
Ag2S NPs and Ag & Ag2S NPs (prepared by the reaction of AgNPs
with S2, containing mainly Ag2S NPs, and some Ag@Ag2S NPs
that have a Ag core and a Ag2S shell) were prepared according to
the details described in the ESI.† Transmission electron
microscopy (TEM) was carried out with an H-7500 (Hitachi,
Japan) operated at 80 kV. TEM samples were prepared by
placing 5 mL aliquots of an AgCNP aqueous sample onto
a carbon-coated grid and drying at room temperature under
a vacuum. Images and size distribution obtained by TEM of
AgCNPs are shown in Fig. S1.† The zeta potential of AgCNPs in
the range of pH 3–10 was determined by dynamic light scattering (DLS) with a Malvern Nano ZS (Malvern, UK). All the
AgCNP stock dispersions were kept in the dark and quantied
with ICP-MS (Agilent 7700cs).
2.4
Synthesis and characterization of IOMPs
Preparation of IOMPs was performed through a solvothermal
reaction with low cost and the batch process modied the literature.28,31 Briey, 2.7 g of FeCl3$6H2O was dissolved in 100 mL of
ethylene glycol under vigorous stirring to form a clear solution.
Then, 7.2 g of sodium acetate were added with constant stirring
for 20 min. The obtained homogeneous yellow solution was
transferred into a Teon-lined stainless-steel autoclave and
sealed to maintain at 200 C for 10 h. The product was collected
with a permanent hand-held magnet and washed several times
Table 1
Experimental parameters of the ICP-MS
Parameter
Value
Instrument
RF power
Sampling depth
Carrier gas
Integration time
Monitored isotopes
Internal standard element
Agilent 7700cs ICP-MS
1500 W
10 mm
1.0 L min1
0.3 s
107
Ag+, 109Ag+
115
In
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with ethanol and ultrapure water in sequence to remove the
nonmagnetic byproducts, and then dried in a vacuum at 60 C for
6 h. Finally, the dried microspheres were aged overnight at 90 C
and stored in a glass bottle, which was able to preserve its
extraction performance for at least 6 months. Our experience
showed that 4.2 g of IOMPs were obtained when using 9 Teonlined stainless-steel autoclaves at one time, which can satisfy over
210 sample analyses without reusing. TEM and zeta potential
characterization of IOMPs were performed with the same
instruments as those used for the AgCNPs. The magnetization
measurements were conducted with a vibrating sample magnetometer (VSM, Lakeshore 735, OH).
2.5
Extraction and sequential elution of AgCNPs
A dened volume of AgCNP suspension placed in a polypropylene centrifuge tube or a glass container was adjusted to
pH 7.0 with a 1.0 M acetic acid/sodium acetate (HAc/NaAc)
buffer solution. For the extraction of AgCNPs from environmental samples and aqueous solutions containing humic acid,
however, an extra solution of 1.0 M Ca(NO3)2 was added to
a nal concentration of 10 mM aer the pre-equilibrium of
AgCNPs with HA. Then, 20 mg of the as-prepared aged IOMPs
(200 mL of 100 mg mL1 IOMP dispersion) were added with the
accompanying pH change from 7.0 to 6.2. The mixture was
mechanically shaken (300 rpm) for 1 h at room temperature.
The extraction efficiency, calculated by eqn (1), was adopted to
evaluate and optimize the extraction procedure.
Extraction (%) ¼ 100 (C0 Cf)/C0
(1)
1
in which C0 and Cf are the concentrations of AgCNPs (mg L ) in
the initial suspension and in the sample aer extraction
measured by ICP-MS, respectively.
Aer extraction, the aged IOMPs were collected by using an
external magnet and rinsed with water. Then, the aged IOMPs
were mixed with 1 mL of 2% (v/v) HAc and shaken (300 rpm) for
2 h at room temperature to facilitate the dissolution of AgNPs
and AgCl NPs to release Ag+, while Ag2S NPs still remained on
the surface of IOMPs, and the obtained solution was collected
for the quantication of AgNPs and AgCl NPs by ICP-MS.
Subsequently, the IOMPs were washed with 1 mL of ultrapure
water three times and transferred into 1 mL of a mixed solution
containing 10 mM thiourea and 2% (v/v) HAc aqueous solution
to incubate for another 90 min under shaking (300 rpm).
Aerwards, the obtained solution was tested by ICP-MS for Ag2S
NP concentration. The recovery of AgCNPs, calculated by eqn
(2), was adopted to evaluate and optimize the entire sample
pretreatment procedure including extraction and elution.
Recovery (%) ¼ 100 CeVe/C0V0
(2)
in which Ce is the Ag concentration (mg L1) in the eluent, V0 is
the initial sample volume, and Ve is the eluent volume.
2.6
Water sample collection
Public tap water was collected directly from our laboratory.
River and lake water samples were collected manually from
Beijing, China. In particular, river water was collected from the
Jingmi River, lake water was taken from the Gaobeidian Lake,
and the municipal sewage effluent was retrieved from the
Qinghe wastewater treatment plant (WWTP). All of the water
samples were collected in glass bottles, which were rinsed
several times with the sample rst, and ltered through
a 0.45 mm cellulose acetate membrane before use. The pH of the
samples was measured using an ORION 4 STAR pH ISE
benchtop (Thermo Fisher Scientic, Waltham, MA). The dissolved organic carbon, DOC, of humic acid solution was
determined with a Phoenix 8000 UV-persulfate total organic
carbon analyzer (Tekmar-Dohrmann, Cincinnati, OH).
3.
Results and discussion
3.1 Preparation and characterization of pristine and aged
IOMPs
The synthesized pristine and aged IOMPs were characterized by
a series of methods, and the TEM images and magnetization
curves are shown in Fig. 1. Aer aging, no signicant changes in
morphology were observed from the TEM images, and the IOMPs
were monodispersed in size with an average diameter, obtained
by counting more than 120 particles, of 223 20 nm for the aged
IOMPs. However, the color of IOMPs changed from black to
brown, and the saturation magnetization of the aged IOMPs (64.4
emu g1) was slightly lower than that of the pristine IOMPs (64.9
emu g1), suggesting that part of the Fe3O4 was oxidized to
a-Fe2O3 under heating in air.32,33 Nevertheless, the saturation
magnetization is high enough to ensure their separation from the
Fig. 1 Characterization of pristine and aged IOMPs. (A) TEM image of pristine IOMPs without aging. (B) TEM image and size distribution of aged
IOMPs. (C) Magnetization curves of pristine IOMPs and aged IOMPs.
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aqueous solution in a few minutes with a permanent hand-held
magnet. The DLS analysis showed that the zero point charge pH
(pHpzc) of the aged IOMPs was between 6 and 7 (Fig. 2A), which is
close to 6.8 reported in a previous literature.34
Since IOMPs act as both adsorbents in the extraction of
AgCNPs and oxidants in the elution of AgNPs, it is essential to
optimize the aging time in IOMP preparation, which is relevant
to the amount of Fe(III) and, therefore, the oxidization capability
of IOMPs. Experiments showed that while both the pristine and
aged IOMPs can effectively adsorb AgCNPs from an aqueous
solution, the AgNP recovery from the aged IOMPs was markedly
higher than that from the pristine IOMPs. The AgNP recovery
increased with aging time of the IOMPs (in air at 90 C) in the
studied range of 0–12 h. The respective AgNP recovery
percentages at various aging times (shown in parentheses) were
29.8 0.5 (0 h), 49.3 1.1 (1.5 h), 61.6 6.8 (6 h), and 96.4 0.9
(overnight, 12 h), indicating that an aging time of 12 h is
necessary for the complete recovery of AgNPs. This was attributed to the fact that with the increase of aging time, the Fe(III)
amount on the surface of IOMPs increased, which enhanced the
oxidization of AgNPs to Ag+, and therefore increased the
recovery of adsorbed AgNPs.
We further evaluated the effect of aging on the adsorbent
stability by testing the Fe leaching of pristine and aged IOMPs
(aged in air at 90 C, 12 h) in various solutions (see Table S1†).
While there is no signicant difference in Fe leaching between
these two types of IOMPs in natural and tap water (all are below
0.074%), the leaching of Fe from aged IOMPs in acidic solutions is
generally lower than that of the pristine ones. In particular, in
0.001 M HCl, the Fe leaching percentage of pristine IOMPs (1.14%)
was almost 3.5-folds that of the aged ones (0.33%). Thus, it was
speculated that the surface-oxidized layer of the IOMPs, formed by
the oxidization of Fe3O4 to a-Fe2O3 under heating in air,29,30
protects the IOMPs from dissolution under acidic conditions.35
Hence, it can be concluded that the process of aging improves
both the stability and oxidation activity of the IOMPs under acidic
conditions. Aged IOMPs were used in the following studies.
3.2
Factors inuencing the extraction of AgCNPs
To enhance the extraction efficiency, parameters that
commonly affect extraction were investigated according to
previous studies,21,25,36 such as sample pH, extraction time,
humic acid, particle size and coatings. Four typical AgCNP
dispersions (about 1 mg L1) were used as models, including
AgNPs, AgCl NPs, Ag2S NPs (synthesized by the direct reaction
between Ag+ and S2), and Ag & Ag2S NPs (prepared by the
reaction of AgNPs with S2, containing mainly Ag2S NPs and
Ag@Ag2S NPs).
3.2.1. Sample pH. Since pH affects both the surface charge
and the dispersion state of NPs in aqueous solutions, it plays
a key role in the extraction of NPs.37 The extraction of the four
AgCNPs was optimized in the range of pH 3–10, and the highest
extraction efficiency was obtained at about pH 7.0 (Fig. 2B). At
pH # 4.9, the extraction efficiencies of AgNPs, AgCl NPs, and Ag
& Ag2S NPs decreased with pH due to their dissolution in acidic
solutions, while the extraction efficiency of Ag2S NPs remained
unchanged as they were not dissolved. At pH $ 6.2, however,
the reduced extraction efficiencies of AgNPs, Ag2S NPs and Ag &
Ag2S NPs were attributed to the electrostatic repulsion between
the negative charges of these AgCNPs and the aged IOMPs
(Fig. 2A), and the constant extraction efficiency of AgCl NPs was
ascribed to their precipitation under basic conditions.
It is very interesting to note that Ag+ was not extracted in the
entire studied pH range (Fig. 2B). Electrostatic interactions play
a key role in the selective extraction of AgCNPs resulting from
the zeta potential of AgCNPs and IOMPs at pH 6.2 for the
positive charge on the IOMPs and the negative charge on the
AgCNPs, but other interactions could be also involved, such as
van der Waals interaction and coprecipitation. Nevertheless, pH
6.2 was employed as the optimum value in the following
studies.
3.2.2. Extraction time. The effect of extraction time was
studied in the range of 10–120 min. The results revealed that the
extraction efficiency of all the AgCNPs increased with time up to
60 min and then remained constant (Fig. 2C), indicating that
the extraction reached equilibrium within 60 min, which was
adopted in the following studies.
3.2.3. Humic acid. Since NPs can be stabilized by interaction with the dissolved organic matter such as humic acid that
is common in the environmental waters,38,39 it is essential to
investigate the potential effects of humic acid on the extraction
efficiency. As shown in Fig. 3A, the increased humic acid
concentration reduced the extraction efficiencies of AgNPs, Ag2S
NPs and Ag & Ag2S NPs, which might be ascribed to the coating
Fig. 2 (A) Zeta potential of AgCNPs and aged IOMPs at different pH levels. (B) Effect of pH on the extraction of AgCNPs and Ag+. (C) Effect of
shaking time on the extraction of AgCNPs. All error bars represent standard deviation (n ¼ 3).
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of humic acid on the aged IOMPs and the AgCNPs. It is noteworthy that humic acid has no signicant effect on the extraction of AgCl NPs, but the reason remains unclear. While
increasing the concentration of aged IOMPs does not improve
the extraction of these AgCNPs, a signicantly increased
extraction efficiency was obtained by the addition of Ca2+. In the
presence of humic acid (10 mg L1 dissolved organic carbon,
DOC), the extraction efficiency of all the AgCNPs exceeds 93%
with the addition of 10 mM Ca2+ (Fig. 3B). The excellent capability of Ca2+ in suppressing the interference of humic acid can
be attributed to the cation bridging between carboxyl groups on
humic acid structures,40,41 which enhanced the aggregation of
humic acid, and therefore reduced the coating of humic acid on
the AgCNPs and IOMPs.
3.2.4. Coatings and particle sizes of Ag2S NPs. It is important to evaluate the effects of coatings and particle sizes of Ag2S
NPs on extraction. It was reported that AgNPs with different
coatings are immediately covered with humic acid upon their
entry into the environment;7 thus, the Ag2S NPs, transformed
from AgNPs with different coatings, were likely coated with
humic acid in the environment. As shown in Fig. 3, humic acid
has no effect on the extraction of Ag2S NPs with the addition of
Ca2+. Additionally, experiments showed that both Ag2S NPs
(4.3 0.9 nm) and Ag & Ag2S NPs (5.0 1.0 nm & 17.5 3.0 nm)
were quantitatively extracted, with the extraction efficiencies of
89.5 0.2% and 94.2 0.7%, respectively. Therefore, it can be
concluded that coatings and particle sizes have a limited effect
on the Ag2S NP extraction. To further verify this conclusion, the
effects of coatings and particles were also tested using AgNPs as
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substitutes. As shown in Fig. 4, the extraction efficiencies of
AgNPs with different particle coatings like poly(vinyl alcohol)124 (PVA-124), polyvinylpyrrolidone (PVP 58, MW ¼ 58 000),
polyvinylpyrrolidone (PVP 3.5, MW ¼ 3500), and gum arabic
(GA) showed no signicant difference (>97.7%); and the citrate
capped AgNPs with particle sizes between 10 and 100 nm were
all successfully extracted at relatively high rates ranging from
67.9 0.4% to 95.6 0.7%.
3.3
Selectivity of extraction
To investigate the selectivity of extraction, each individual
AgCNP and Ag+ were extracted, respectively, under the aforementioned optimized extraction conditions. As shown in
Fig. S2,† all the AgCNPs were extracted with efficiencies over
90%, whereas the Ag+ extraction efficiency was below 5%. Due to
humic acid's observed ability to complex with noble metal ions
and associate with magnetic particles, we further studied the
effects of humic acid on the Ag+ extraction efficiency. The
results showed that the extraction efficiency of Ag+ was 4.65% in
the presence of humic acid (10 mg L1 DOC), indicating that the
proposed method was able to selectively extract AgCNPs even in
the presence of humic acid, which is consistent with the results
from previous literature.27
3.4
Sequential elution of AgCNPs
Although MSPE is suitable for the extraction and preconcentration of AgCNPs from water samples, it is unable to selectively
extract a specic AgCNP species. Hence, a sequential elution
Effects of humic acid and Ca2+ on the extraction of AgCNPs. (A) Effect of humic acid on the extraction of AgCNPs. (B) Effect of Ca2+ on the
extraction of AgCNPs in the presence of humic acid (10 mg L1 DOC). All error bars represent standard deviation (n ¼ 3).
Fig. 3
Fig. 4 Effect of coatings (A) and particle sizes (B) on the extraction of AgNPs. The resulting solution was then mechanically shaken at 300 rpm for
1 h. All error bars represent standard deviation (n ¼ 3).
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procedure was developed to distinguish Ag2S NPs from AgNPs
and AgCl NPs. As shown in Fig. 5, by using 2% (v/v) acetic acid
(HAc) as an eluent, the respective recoveries were >90% for
AgNPs and AgCl NPs, <2% for Ag2S NPs, and 20% for Ag & Ag2S
NPs which contain part of AgNPs, and the elution reached
a maximum within 120 min. While using 10 mM thiourea
prepared in a 2% (v/v) HAc solution as an eluent, recoveries of
all the AgCNPs were >85%, and the elution reached equilibrium
within 90 min.
Based on the differential elution capability of the two eluents
to the AgCNPs, a sequential elution procedure was developed
for the selective elution of the studied AgCNPs. In particular,
2% (v/v) HAc was rst added into the aged IOMPs with extracted
AgCNPs, and the mixture was shaken at 300 rpm for 120 min for
the elution of AgNPs and AgCl NPs. Aer magnetic separation,
the aqueous solution was collected for the quantication of
total AgNPs and AgCl NPs as Ag; while the IOMPs were treated
with a solution of 10 mM thiourea in 2% (v/v) HAc for eluting
Ag2S NPs, which can be quantied by the ICP-MS determination
of the Ag content in the aqueous solution aer a second
magnetic separation. It is noteworthy that AgNPs were not
oxidized under weak acidic conditions during extraction.
However, they were dissolved as Ag(I) complexes due to the
reaction of Ag0 with Fe(III) to form Ag(I) under strong acidic
conditions during elution, and the complex reaction of Ag(I)
with Ac; the Ag(I) NPs (AgCl NPs and Ag2S NPs) were also dissolved through the complex reaction with Ag(I), which was
enhanced by the addition of a thiourea complex agent for Ag2S
NPs. This dissolution of AgCNPs was conrmed by the absence
of both the NPs in the TEM observation and the surface plasma
resonance peak of AgNPs in UV-vis spectroscopy of the collected
aqueous solutions aer elution (data not shown). As IOMPs
Paper
were cheap and partly sacriced as oxidants during the elution
of AgNPs, they were used as disposable MSPE adsorbents.
It is important to note that different AgCNPs may exist in real
environments. As shown in Table 2, the Ksp of Ag2S is extremely
low. Though other Ag-containing compounds including AgCl,
Ag2CO3 and Ag2O have similar Ksp, AgCl NPs are the most
thermodynamically favored and environmentally relevant,
considering that the Ksp of AgCl is dened by the product of
activities of Ag+ and Cl, while others (i.e. Ag2CO3, Ag2O and
Ag2SO4) are dened by the product of activities of Ag+ squared
and divalent anions. The thermodynamics of the dissolution of
other potential AgCNPs including Ag2O NPs, Ag2CO3 NPs and
Ag2SO4 by Ac is favored over that of AgCl NPs. Accordingly, it is
safe to conclude that only Ag2S NPs remained on the surface of
IOMPs aer pre-elution with 2% (v/v) HAc.
3.5
Method evaluation
The performance of the method was evaluated by determining
parameters including linear range, calibration curve, correlation coefficient (r2), relative standard deviation (RSD), and limit
of detection (LOD). As shown in Table S2,† by extracting 50 mL
of aqueous standard solutions spiked with increasing
Table 2
Solubility products (Ksp) of some typical AgCNPs7
Compound
Formula
Ksp
Silver oxide
Silver carbonate
Silver chloride
Silver sulde
Silver sulfate
Ag2O
Ag2CO3
AgCl
Ag2S
Ag2SO4
4.00 1011 (mol L1)3
8.46 1012 (mol L1)3
1.77 1010 (mol L1)2
5.92 1051 (mol L1)3
1.52 105 (mol L1)3
Optimization of the elution conditions for AgCNPs. (A) Effect of HAc concentration on the elution of AgCNPs; (B) effect of incubation time
on the elution of AgNPs and AgCl NPs by 2% (v/v) HAc; (C) effect of thiourea concentration in 2% (v/v) HAc on the elution of AgCNPs; and (D)
effect of incubation time on the elution of AgCNPs by 10 mM thiourea in 2% HAc. All error bars represent standard deviation (n ¼ 3).
Fig. 5
2290 | J. Anal. At. Spectrom., 2016, 31, 2285–2292
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Table 3
JAAS
Speciation analysis of Ag2S NPs by the proposed method after spiking both AgNPs and Ag2S NPs in water samples (mean n, n ¼ 3)a
Spiked (mg L1)
Sample
AgNPs
Ag2S NPs
Enrichment factorc
Recovery of Ag2S NPs (%)
Tap water
1.0
1.7b
10.3
0.85
1.7b
8.5
0.77
1.7b
7.7
7.4
0.16b
1.7b
1.48
4.6b
14.8
0.84
4.6b
8.3
0.84
4.6b
8.3
7.7
0.53b
4.6b
100
50
50
100
50
50
100
50
50
50
100
50
69.6 91.9 90.9 92.1 93.6 100.2 93.3 98.7 93.0 149.9 114.4 106.2 River water
Lake water
WWTP effluent
2.7
6.4
6.2
3.0
3.7
1.9
8.6
2.0
4.3
1.3
7.3
3.1
a
Mixtures of AgNPs and Ag2S NPs were spiked, unless otherwise specied. b Spiked with Ag & Ag2S NPs, the corresponding spiked amounts of
AgNPs and Ag2S NPs were determined by spiking the same amount of Ag & Ag2S NPs into pure water and determined by the proposed method.
c
Calculated by V0/Ve, in which V0 is the initial sample volume, and Ve is the eluent volume.
concentrations of Ag2S NPs, and eluted with 1 mL of eluents for
ICP-MS detection, the linear correlation coefficient (r2) was
0.9967, and the reproducibility (relative standard deviation,
RSD) obtained by determining ve water samples (50 mL)
spiked with 100 mg L1 Ag2S NPs was 3.0%. The detection limits,
dened as three times the baseline noise (S/N ¼ 3), was 0.068 mg
L1. It is worth mentioning that the method LOD was closely
related to the extraction volume. Considering that the model
predicted the concentration of AgNPs as 0.080 mg L1 in environmental water and that 80% of AgNPs can be transformed
to Ag2S NPs, the detection limit of our method is very low and
could satisfy the requirement for detecting Ag2S NPs in environmental waters.
3.6
Application in real water sample analysis
To assess the applicability of the proposed method, four water
samples including tap water, river water, lake water and WWTP
effluent were analyzed by simultaneously spiking 0.16–10.3 mg
L1 AgNPs and 0.53–14.8 mg L1 Ag2S NPs. The basic characteristics of the water samples are given in Table S3,† and the
contents of AgNPs and Ag2S NPs were both below the detection
limits of the present study. To study the recoveries of different
Ag2S NPs in real environmental samples, both the Ag & Ag2S NPs
and the mixture of AgNPs and Ag2S NPs were tested. As shown
in Table 3, the obtained recoveries of Ag2S NPs ranged from 69.6
to 100.2% for the tap, river and lake waters, indicating that the
inorganic ions and naturally occurring suspended solids had
a limited inuence on the extraction. For the WWTP effluent,
however, the spiked recoveries were in the range of 106.2–
149.9%, which can be attributed in part to the transformation of
AgNPs to Ag2S NPs that commonly occur in WWTP.10,14–16
Overall, the results were good in view of the low spiking levels
and matrix complexity, indicating that the proposed method is
capable of the speciation analysis of Ag2S NPs in environmental
samples. However, further studies are needed to obtain more
satisfactory recoveries.
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The results from the batch spiking experiments suggested
that suldation of AgNPs may be anticipated in the real environmental water samples. Therefore, the obtained Ag2S NP
concentrations were higher than those of the spiked ones when
AgNPs were spiked together with Ag2S NPs into samples containing sulde, such as WWTPs. Though only a fraction of AgNPs
transformed to Ag2S NPs, such a phenomenon will be obvious
when the concentration of AgNPs is similar to or greater than that
of Ag2S NPs. It is noteworthy that the rate of suldation strongly
depends on the environmentally relevant conditions.
4. Conclusions
For the rst time, the aged IOMPs were found to be excellent
adsorbents for the selective extraction of AgCNPs and sacricial
oxidants in the speciation analysis of Ag2S NPs in environmental water samples. While AgCNPs of different sizes and
coatings were adsorbed onto magnetic particles through electrostatic adsorption, Ag+ still remained in the sample, which
enabled the selective extraction of AgCNPs. Ca2+ was added into
the extraction system to eliminate the interference of humic
acid. The sequential elution of AgNPs and AgCl NPs with 2%
(v/v) HAc, and Ag2S NPs with a mixture of 10 mM thiourea and
2% (v/v) HAc, makes it possible to distinguish the Ag2S NP
content from other AgCNPs. Our method provides a novel
approach for the speciation analysis of Ag2S NPs in water
samples, which can contribute to understanding the environmental process, fate and toxicity of AgCNPs in the environment.
It seems clear that the developed method requires a relatively
long analysis time (4 h); therefore, further studies are necessary to shorten the analysis time, such as investigating the effect
of increasing temperature or elution with the aid of ultrasonication. Additionally, appropriate analytical methods able
to preserve the particle size and shape are necessary to provide
a better insight into the potential hazards of AgCNPs in the
environment.
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JAAS
Acknowledgements
This work was partially supported by the National Key Research
and Development Program of China (2016YFA0203102), the
Science Foundation of China (21337004 and 21227012), and the
U.S. National Science Foundation (USNSF Grant No. CBET1235166).
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