High-Performance Colorimetric Detection of Hg2+ Based on

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High-performance colorimetric detection of Hg based on triangular silver nanoprisms 2+

Ningyi Chen, Yujie Zhang, Hongyu Liu, Xiaoxia Wu, Yonglong Li, Lijing Miao, Zheyu Shen, and Aiguo Wu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00001 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016

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High-performance colorimetric detection of Hg2+ based on triangular silver nanoprisms

Ningyi Chena,b,#, Yujie Zhanga,#, Hongyu Liu a,b, Xiaoxia Wua, Yonglong Lia, Lijing Miaoa, Zheyu Shena,*, Aiguo Wua,*

a

Key Laboratory of Magnetic Materials and Devices & Key Laboratory of Additive Manufacturing

Materials of Zhejiang Province & Division of Functional Materials and Nanodevices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China. b

Nano Science and Technology Institute, University of Science and Technology of China, Suzhou,

Jiangsu, 215123, China.

1

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ABSTRACT Mercury ion (Hg2+) arising from a variety of natural sources and industrial wastes has been widely recognized as one of the most hazardous pollutants. It’s very important to develop highly selective and sensitive probe for rapid detection of Hg2+ in aquatic ecosystems. Here we propose a new strategy for high-performance colorimetric detection of Hg2+, i.e. anti-etching of silver nanoprisms (AgNPRs). In the absence of Hg2+, the AgNPRs can be etched by I− inducing an obvious color change from blue to red. However, in the presence of Hg2+, the formation of Ag-Hg nanoalloy can protect the AgNPRs from I− etching and the color remains blue. This mechanism is verified by UV-vis, TEM, DLS and EDS. Our AgNPRs-based colorimetric probe exhibits excellent selectivity for Hg2+. The limit of detection (LOD) of Hg2+ is 30 nM by eye-vision and 3 nM by UV-vis spectroscopy, which is lower than the mercury toxic level defined by US Environmental Protection Agency (10 nM). A good linear relationship (R2=0.993) between the wavelength shift and Hg2+ concentrations indicates that our probe can be used for the quantitative assay of Hg2+. The results of Hg2+ detection in real environmental samples indicate the feasibility and sensitivity of our probe for application in complicated environmental samples.

Key words: Hg2+ colorimetric probe, triangular silver nanoprisms, anti-etching, selectivity, sensitivity.

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Heavy metal ions with marked characteristics, such as severe toxicity, strong bioaccumulation, non-natural degradation and complicated sources, have caused serious damage to the ecological environment and human health.1 Mercury ion (Hg2+) arising from a variety of natural sources and industrial wastes has been widely recognized as one of the most hazardous pollutants.2 Toxicological studies confirmed that Hg2+ can cause great damage to the brain, kidney, central nervous system, immune system and endocrine system.3−6 Thus it has become a sparked interest in recent years to detect the levels of toxic Hg2+ in aquatic ecosystems. Currently, many instrumental techniques with high sensitivity can be used to detect Hg2+, such as atomic absorption spectrometry (AAS),7 fluorescence-based sensor membranes,8 gas chromatography-inductively coupled plasma-mass spectrometry,9 inductively coupled plasma mass spectrometry (ICP-MS),10 electrochemical method.11 However, they have obvious disadvantages including time-consuming, expensive costs and complicated sample preparation process, which limit their applications for on-site analysis. Compared with the above methods for Hg2+ detection, colorimetric sensors based on noble metal nanoparticles have attracted more and more attentions due to their distinct variation in color associated with morphology change.12−15 They offer advantages of convenience, efficiency, low cost and no requirement of any sophisticated instrumentation. The noble metal nanoparticles with some surface modifications have been widely applied for the detection of heavy metal ions,16−21 especially for Hg2+. The primary adopted strategies are triggering aggregation or etching of the gold nanoparticles (AuNPs) or silver nanoparticles (AgNPs) with different recognition units such as oligonucleotides,22 polymers,23 fluorophores,24 protein,25 and small thiolate ligand.26 Silver nanoprisms (AgNPRs) are rarely used for rapid colorimetric detection compared with AuNPs and AgNPs. The AgNPRs possess high surface energy, particularly at the tip and edge portion where the silver atoms are easily etched by oxidation. The etching may result in color change of the AgNPR dispersion and shift or disappearance of the UV-vis absorption, which provide the feasibility of Hg2+ detection by AgNPRs.27 3


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Up to now, to the best of our knowledge, little research has been done to employ AgNPRs as a colorimetric probe for Hg2+ detection except that one relative study was reported by Chen et al.28 They developed a simple colorimetric probe for Hg2+ detection, which is based on the Hg2+-induced morphology transition of 1-dodecanethiol (C12H25SH)-capped AgNPRs upon the presence of iodide ion (I−). This colorimetric probe has a good selectivity for Hg2+. However, the limit of detection (LOD) of Hg2+ by UV-vis spectroscopy is 10 nM, which is just equal to the mercury toxic level defined by US Environmental Protection Agency. In addition, the influence of the mixed ions on the selectivity and the LOD of Hg2+ by the naked eyes are not clarified. Herein, we propose a new strategy for high-performance colorimetric detection of Hg2+ (i.e. anti-etching of AgNPRs), which is shown in Scheme 1. In the absence of Hg2+, cysteamine (CA)-capped AgNPRs cannot be integrally protected from I− etching because the silver atoms at the corners and edges of AgNPRs are active and easy to coordinate with I− (Ksp[AgI]= 8.49×10-17) 29 resulting in dissociation of the silver atoms from its original nanostructure. The morphology transition from nanoprism to nanodisk induces an obvious color change of the AgNPR dispersion from blue to red. However, in the presence of Hg2+, it is easily combined with -SH of CA inducing partial nudity of the corners and edges of AgNPRs. Then a redox reaction between Ag0 and Hg2+ would lead to the formation of Ag-Hg nanoalloy, which can protect the corners and edges of AgNPRs from I− etching and keep the morphology frozen. In this scenario, the color of the AgNPR dispersion could remain blue. Based on the anti-etching mechanism, Hg2+ could be detected quickly in 15 minutes with excellent selectivity and high sensitivity. The mixed ions have no influence on the selectivity. The LOD is 30 nM by the naked eyes and 3.0 nM by UV-vis spectroscopy, which is much lower than the mercury toxic level defined by US Environmental Protection Agency.

EXPERIMENTAL SECTION Materials Cysteamine (2-Aminoethanethiol, CA) was purchased from J&K Scientific Ltd. Hydrogen 4


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peroxide (H2O2), silver nitrate (AgNO3), trisodium citrate dehydrate (C6H5Na3O7·2H2O), MgCl2·6H2O, CaCl2, BaCl2·2H2O, MnCl2·4H2O, CdCl2·2.5H2O, Pb(NO3)2, CuCl2·2H2O, HgCl2, NiCl2·6H2O, CoCl2·6H2O, FeSO4·7H2O, FeCl3, CrCl3·6H2O and K2Cr2O7 were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Poly(vinylpyrrolidone) (PVP, MW~58000g/mol), sodium borohydride (NaBH4) and potassium iodide (KI) were purchased from Aladdin-regent Co., Ltd. (Shanghai, China). All the chemical reagents were used as received without further purification. The glasswares were cleaned thoroughly with aqua-regia (HCl : HNO3 = 3 : 1 (V/V)) and well rinsed with Milli-Q water prior to use.

Apparatus Transmission electron microscopy (TEM) images and Energy-dispersive X-ray spectroscopy (EDS) mapping were performed using a JEOL2100 instrument operated at an accelerating voltage of 200 kV. Dynamic light scattering (DLS) data were carried out on Zetasizer Nano ZS instrumentation (Malvern Instruments Ltd.). The inductively coupled plasma-atomic emission spectrometry (ICP-MS) data were made on an Optima 2100DV ICP instrument for real water sample. UV-vis spectroscopy was performed using a T10CS instrument (Beijing Purkinje General Instrument Co., Ltd, China).

Synthesis of Silver Nanoprisms (AgNPRs) The AgNPRs were prepared according to a reported thermal method with minor modifications.30,31 Typically, aqueous solutions of AgNO3 (20 mM, 0.5 mL), C6H5Na3O7·2H2O (30 mM, 6.0 mL), PVP (0.7 mM, 6.0 mL) and H2O2 (30 wt%, 240 µL) were mixed with 99.5 mL of Milli-Q water and vigorously stirred at room temperature. Fresh NaBH4 solution (100 mM, 1.0 mL) was then rapidly added to the aforementioned mixed solution with stirring to generate a pale yellow colloid. The colloid was then placed in the dark for 2 h (the color changed from pale yellow to blue) and subsequently stored at 4 oC. 5


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Sensing detection of Hg2+ The sensing detection of Hg2+ was carried out in aqueous solution at room temperature. First, 50 µL of CA solutions (ranging from 0.10 to 1.2 µM) were separately added into 850 µL of AgNPRs solutions. 50 µL of Hg2+ solutions with various concentrations were then added to the mixtures. After that, 50 µL of KI aqueous solution (ranging from 10 to 70 µM) was added to each mixture. The incubation time varied from 1.0 to 33 min. Finally, the color change and UV-vis absorption spectra were recorded respectively.

Selective detection of Hg2+ To verify the selectivity of our proposed colorimetric probe based on AgNPRs, other ions including Cd2+, Mn2+, Co2+, Cu2+, Ba2+, Pb2+, Ni2+, Hg2+, Cr3+, Cr2O72-, Fe2+, Fe3+, Mg2+, F-, SO32-, S2O42-, CO32-, PO43-,SO42- and NO3- were also tested in a similar manner as mentioned above. In addition, the influence of the mixed ions on the selectivity was also studied.

Detection of real water samples The real water samples of tap water and pond water obtained in our institution were first filtered through syringe filters with 0.2 µm of membrane for further use, and then directly spiked with standard Hg2+ solutions to different concentrations as stock solutions. After that, these spiked samples were analyzed using our colorimetric probe or ICP-MS.

RESULTS AND DISCUSSION Mechanism of the colorimetric probe based on AgNPRs for Hg2+ detection The proposed mechanism of the AgNPRs-based colorimetric probe for Hg2+ detection is anti-etching of the AgNPRs as shown in Scheme 1, which was verified by UV-vis, TEM , DLS and EDS. Figure 1 shows UV-vis spectra and the corresponding colorimetric response of the AgNPR 6


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dispersions at different conditions. It is found that the blank and CA-stabilized AgNPR dispersions (without I− and Hg2+) are blue and have three absorption peaks in the UV-vis spectra. The peak wavelengths respectively locate at 350, 420 and 680 nm, and the absorption peaks at 680 nm are strongest. However, the color of AgNPR dispersion becomes red and the peak wavelength of the UV-vis absorption at 680 nm shifts to 482 nm after I− etching for 15 min. The I− can etch the AgNPRs because the silver atoms at the corners and edges of AgNPRs prefer to coordinate with I− (Ksp[AgI] = 8.49 × 10−17)29 causing the tip to be truncated or completely dissolved and thus evoking the blue shift of the absorption peak. With stabilization of CA, the color change of the AgNPR dispersion and the corresponding peak wavelength shift still happen after I− etching. This result indicates that the modification of CA (i.e. the -SH of CA binds to the surface of AgNPRs through formation of Ag-S bonds) can enhance the stability of the AgNPRs, but cannot prevent the AgNPRs from I− etching. When the AgNPRs are incubated with I− in the presence of CA and Hg2+, the color of the AgNPR dispersion remains blue and the corresponding peak wavelength only has a tiny blue shift. That’s because the redox reaction between Ag0 and Hg2+ would lead to the formation of Ag-Hg nanoalloy, which can protect the corners and edges of AgNPRs from I− etching and keep the morphology frozen (Scheme 1). The redox reaction between Ag0 and Hg2+, which is given below as eq. 1, could be confirmed from the standard electrode potentials (Eo(Hg2+/Hg) = 0.852 V, Eo(Ag2+/Ag) = 0.799 V.32,33 Agn + Hg2+ ⇋ Agn-2Hg + 2Ag+

(1)

These significant changes were confirmed by TEM images of the AgNPRs at different conditions as shown in Figure 2. It is found that sharp corners can be observed in the TEM images of the blank AgNPRs (Figure 2a), CA-stabilized AgNPRs (Figure 2c) and CA-stabilized AgNPRs in the presence of Hg2+ (Figure 2e), and the morphologies of the AgNPRs are all triangular. However, the AgNPRs or CA-stabilized AgNPRs incubated with I− (Figure 2b or d) are etched from nanoprisms to nanodisks. These results demonstrate that I− can etch the AgNPRs inducing a morphology change, but CA and Hg2+ cannot etch the AgNPRs and CA cannot prevent the etching of the AgNPRs by I−. 7


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The morphologies of the CA-stabilized AgNPRs incubated with I− in the presence of 2.5 µM Hg2+ (Figure 2f) remain triangular nanoprisms, which indicates that Hg2+ can prevent the etching of the AgNPRs by I− due to the redox reaction between Ag0 and Hg2+ and the formation of Ag-Hg nanoalloy. The above obvious changes observed by TEM images could be further verified by the size distributions of AgNPRs at different conditions (Figure S1). It can be seen that whether the surface of AgNPRs is covered by CA or not (Figure S1a,c), the AgNPRs could be etched once the I− is added (Figure S1b,d) because the particle sizes become smaller. However, in the presence of Hg2+, the size distributions of the CA-stabilized AgNPRs with or without I− are very similar (Figure S1e,f) because the triangle shape of AgNPRs is passivated via formation of the protective layer of Hg-Ag nanoalloy. In order to reconfirm the formation of the Hg-Ag nanoalloy, the structure and composition of the AgNPRs in the presence of CA and I− incubated with or without Hg2+ are respectively analyzed by HRTEM and EDS (Figure S2). It is found that the structure of the Hg-Ag nanoalloy is too thin to be observed by HRTEM. In addition, the Ag and Hg elements are uniformly distributed in the edge area of the AgNPRs with incubation of Hg2+, but the Hg element is not found in the edge area of the AgNPRs without incubation of Hg2+. This result reconfirms the formation of the Hg-Ag nanoalloy in the edge area of the AgNPRs. The above results consist with our proposed strategy for Hg2+ detection (Scheme 1) and the UV-vis spectra and the color change of the AgNPR dispersions (Figure 1).

Optimization of experimental conditions The sensitivity of the AgNPRs-based colorimetric probe for Hg2+ detection could be influenced by three aspects: (1) concentration of the surface modification reagent CA; (2) concentration of the chemical etching reagent I−; (3) time of the anti-etching process. To obtain the maximum response to Hg2+, the above three experimental conditions are respectively optimized according to the sensing effect of our colorimetric probe (Figure S3). The color of the CA-stabilized AgNPR dispersions incubated with 40 µM of I− and 2.5 µM of 8


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Hg2+ is compared with that of the dispersions without Hg2+ (Figure S3a). It is found that the stabilizer CA cannot be used too much (i.e. higher than 0.25 µM) according the color change of the AgNPR dispersions. That’s because excess CA may completely cover the surface of AgNPRs, protect it from I− etching and then reduce the sensing effect of our colorimetric probe to Hg2+. According the color change as shown in Figure S3a, the maximum CA concentration can be fixed at 0.25 µM as an optimized condition for the following experiments. The color of the AgNPR dispersions in the presence of 0.25 µM of CA incubated with I− (10-70µM) and Hg2+ (2.5 µM) is also compared with that of the dispersions without Hg2+ (Figure S3b). The chemical etching reagent I− also cannot be used too much (i.e. higher than 50 µM) according the color change of the AgNPR dispersions because excess I− may destroy the structure of AgNPRs with Hg-Ag nanoalloy. Therefore, 50 µM is chosen as an optimal concentration of I− for the subsequent experiments. Influence of the time of anti-etching process on the sensing effect of our colorimetric probe to Hg2+ is also investigated via analysis of the UV-vis absorption. The wavelength shift is calculated between the peak wavelengths of the AgNPR dispersions incubated with I− (50.0 µM) in the presence of CA (0.25 µM) and Hg2+ (0 or 2.5 µM) and those of the AgNPR dispersions without I−, and plotted as a function of the incubation time (i.e. the time of anti-etching process) (Figure S3c). The wavelength shift of CA-stabilized AgNPRs dispersion with Hg2+ is much smaller than that without Hg2+ at any time, and the difference arrived at the maximum value when the incubation time is longer than 15 minutes, which is chosen as an optimal condition for the following experiments.

Selectivity of the Hg2+ colorimetric probe At the optimized conditions, the selectivity of the AgNPRs-based colorimetric probe for Hg2+ is evaluated by comparing with 13 kinds of metal ions (i.e. Cd2+, Mn2+, Co2+, Cu2+, Ba2+, Pb2+, Ni2+, Hg2+, Cr3+, Cr2O72-, Fe2+, Fe3+ and Mg2+) and 7 kinds of anions (i.e. S2O42-, SO32-, F-, SO42-, CO32- , 9


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NO3- and PO43-). The color of the AgNPR dispersions incubated with 50.0 µM of I− in the presence of CA (0.25 µM) and single metal ion (2.5 µM for Hg2+ , Fe2+ and Cr3+, 25 µM for other metal ions) is compared each other (Figure S4a). It is found that only the AgNPR dispersion with Hg2+ is blue and others are all red. Figure 3a shows the wavelength shift between the peak wavelengths of AgNPR dispersions incubated with 50 µM of I− in the presence of CA (0.25 µM) and single metal ion (2.5 µM for Hg2+ Fe2+ and Cr3+,25 µM for other metal ions) and that in the absence of metal ions. The wavelength shift of Hg2+ is much larger than that of other metal ions. These results demonstrate that the anti-etching process can only induced by Hg2+ due to the formation of Ag-Hg nanoalloy. The influence of other metal ions on the Hg2+ sensing effect is also investigated to verify the selectivity of our AgNPRs-based colorimetric probe. The color of the AgNPR dispersions incubated with 50.0 µM of I− in the presence of CA (0.25 µM) and metal ions (2.5 µM of Hg2+ plus 2.5 µM of Fe2+ or Cr3+, 25 µM of other metal ions) is shown in Figure S4b. It is found that the AgNPR dispersions with Hg2+ are all blue and other metal ions do not affect the color. Figure 3b shows the wavelength shift between the peak wavelengths of the AgNPR dispersions incubated with 50 µM of I− in the presence of CA (0.25 µM) and metal ions (2.5 µM of Hg2+ plus 2.5 µM of Fe2+ or Cr3+, 25 µM of other metal ions) and that in the absence of metal ions. The wavelength shift of Hg2+ plus other metal ions are all comparable to that of Hg2+ without other metal ions. These results indicate that the above various other metal ions have no influence on the Hg2+ sensing effect of our AgNPRs-based colorimetric probe. The error bars in Figure 3 are not big, which indicates that the reproducibility and accuracy of the experiments are not bad. The AgNPRs-based colorimetric probe is also used for distinguishing the interference of anions, including S2O42-, SO32-, F-, SO42-, CO32- , NO3- and PO43-. The wavelength shift between the peak wavelengths of AgNPR dispersions incubated with 50 µM of I− in the presence of CA (0.25 µM) and single ion (2.5 µM for Hg2+, 25 µM for other anions) and that of the control (in the absence of Hg2+ and anions) is shown in Figure S5a. It can be seen that the wavelength shift of Hg2+ is also 10


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much larger than that of other anions. Figure S5b shows the corresponding color of the AgNPRs dispersions. It is easy to distinguish Hg2+ and other anions because the color of the dispersions containing anions changes from blue to red, while Hg2+ remains blue. Therefore, it can be concluded that our AgNPRs-based colorimetric probe for Hg2+ exhibits excellent selectivity.

Sensitivity of the Hg2+ colorimetric probe The colorimetric response and UV-vis spectra are both used to evaluate the sensitivity of our AgNPRs-based colorimetric probe for Hg2+. Figure 4 shows the photographic image of the AgNPRs-based colorimetric probe at the optimized conditions with various Hg2+ concentrations (25-800 nM). When the Hg2+ concentration is higher than 30 nM, the color of the AgNPRs-based colorimetric probe gradually changes from red to blue with increasing of Hg2+ concentration. Compared with the control (without Hg2+ incubation), the color change of the AgNPRs-based colorimetric probe incubated with Hg2+ higher than 30 nM can be readily read out, but that with 25 nM of Hg2+ cannot. Thus, the limit of detection (LOD) of our Hg2+ colorimetric probe is 30 nM by the naked eyes, which is much lower than most of the reported LOD values by eye-vision based on noble metal nanoparticles for rapid detection of Hg2+.32,34-37 Figure 5a shows the UV-vis absorption spectra of the AgNPR dispersions incubated with 50.0 µM of I− in the presence of Hg2+ with various concentrations. We can find that the UV-vis spectrum shifts towards red with increasing of Hg2+ concentration from 0 to 5.0 µM. That’s because the morphology change of AgNPRs induced by I− is prevented by Hg2+ due to the formation of Ag-Hg nanoalloy. The wavelength shift calculated between the peak wavelengths of the AgNPR dispersions incubated with I− (50.0 µM) in the presence of Hg2+ and that in the absence of Hg2+ is subsequently used for the quantitative analysis of Hg2+. In addition, the data presented in Figure 1e and Figure 5a (C16 = 2.5 µM) are different. That’s because the reaction temperature (i.e. room temperature) may be slightly different and it may minorly influence the results. Figure 5b shows the plot of the wavelength shift as a function of Hg2+ concentration ranging from 0 to 5.0 µM, and the 11


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inset plot shows the wavelength shift versus different Hg2+ concentrations in the range of 3.0-500 nM. We can get a good linear relationship (R2=0.993) between the wavelength shift and Hg2+ concentrations from 3.0 nM to 500 nM. Therefore, it can be concluded that our AgNPRs-based colorimetric probe can be used for the quantitative assay of Hg2+ and the LOD is 3.0 nM by UV-vis spectroscopy, which is much lower than the mercury toxic level defined by US Environmental Protection Agency (10 nM or 2 ppb). The 3.0 nM of LOD is far lower than most of the reported LOD based on the noble metal nanoparticles for rapid detection of Hg2+.32,34,38-40

Detection of real samples In order to evaluate the practicality of our AgNPRs-based colorimetric probe for on-site analysis and real-time detection of Hg2+, a certain concentration of Hg2+ was added into the real water samples including tap water and lake water adopting standard addition method. Figure 6 shows the photograph of the AgNPRs-based colorimetric probe incubated with 0.1 or 0.4 µM of Hg2+ in real water samples. The detailed results determined by our AgNPRs-based probe using UV-vis spectroscopy are shown in Table 1 compared with those measured by ICP-MS. The results analyzed by both methods are very close and the recoveries (calculated from: (observed value with Hg2+ addition – observed value without Hg2+ addition) / added value) are in the satisfying range from 75 to 82 %. Consequently, the potential application of our AgNPRs-based colorimetric probe in real environmental samples is promising.

CONCLUSIONS In summary, we propose a new strategy for high-performance colorimetric detection of Hg2+ (i.e. anti-etching of AgNPRs). In the absence of Hg2+, the AgNPRs can be etched by I− inducing an obvious color change of the AgNPR dispersion from blue to red. In the presence of Hg2+, the formation of Ag-Hg nanoalloy can protect the AgNPRs from I− etching and the color of the AgNPR dispersion remains blue. The Hg2+ sensing mechanism of our AgNPRs-based colorimetric probe is 12


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verified by UV-vis, TEM and DLS. According to the sensing effect of the proposed AgNPRs-based colorimetric probe to Hg2+, the concentration of the stabilizer CA, the concentration of the chemical etching reagent I− and the time of the anti-etching process are optimized to be 0.25 µM, 50 µM and 15 min, respectively. At the optimized conditions, the selectivity of the AgNPRs-based colorimetric probe for Hg2+ is evaluated by comparing with 13 kinds of metal ions and 7 kinds of anions. The results indicate that our AgNPRs-based colorimetric probe exhibits excellent selectivity for Hg2+ because Hg2+ can protect the AgNPRs from I− attack via formation of Ag-Hg nanoalloy, but other ions cannot. The LOD of Hg2+ is 30 nM by eye-vision and 3 nM by UV-vis spectroscopy, which is much lower than the mercury toxic level defined by US Environmental Protection Agency (10 nM or 2 ppb). A good linear relationship (R2=0.993) between the wavelength shift and Hg2+ concentrations indicates that our AgNPRs-based colorimetric probe can be used for the quantitative assay of Hg2+. The results of Hg2+ detection in real environmental samples indicate the feasibility and sensitivity of our proposed Hg2+ colorimetric probe for application in complicated environmental samples.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1: The size distributions of AgNPRs at different conditions; Figure S2: EDS and HRTEM analysis; Figure S3: Optimization of the experimental conditions; Figure S4: Selectivity of the probe for Hg2+ compared with other metal ions; Figure S5: Selectivity of the probe for Hg2+ compared with other anions (PDF).

AUTHOR INFORMATION # These authors equally contributed to this work. Corresponding authors 13


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*E-mail: [email protected]; or [email protected] Tel: +86 574 86685039, or +86 574 87617278; Fax: +86 574 86685163. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Authors acknowledge the financial support by the Project for Science and Technology Service of Chinese Academy of Sciences (KFJ-SW-STS-172 & KFJ-EW-STS-016), the aided program for Science and Technology Innovative Research Team of Ningbo Municipality (Grant No. 2014B82010, and 2015B11002), Natural Science Foundation of China (Grants Nos. 31128007), Zhejiang Provincial Natural Science Foundation of China (Grant No. R5110230), and Hundred Talents Program of Chinese Academy of Sciences (2010-735).

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Scheme 1. Mechanism scheme of the AgNPRs-based colorimetric probe for Hg2+ detection. In the absence of Hg2+, I− could attach and etch the corners and edges of AgNPRs via formation of AgI (Ksp = 8.49×10-17) resulting in the morphology transition from nanoprism to nanodisk. Hg2+ is easily combined with -SH of CA inducing partial nudity of the corners and edges of AgNPRs. Then a redox reaction between Ag0 and Hg2+ would lead to the formation of Ag-Hg nanoalloy, which can protect the corners and edges of AgNPRs from I− etching and keep the shape frozen.

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Figure 1. UV-vis spectra of the AgNPR dispersions at different conditions (the inset images correspond to the colorimetric response). (a): AgNPRs (control); (b): AgNPRs incubated with 50 µM of I−; (c): AgNPRs in the presence of CA (0.25 µM); (d): AgNPRs incubated with 50 µM of I− in the presence of CA (0.25 µM); (e): AgNPRs incubated with 50 µM of I− in the presence of CA (0.25 µM) and Hg2+ (2.5 µM).

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Figure 2. TEM images of the AgNPRs at different conditions. (a): AgNPRs (control); (b): AgNPRs incubated with 50 µM of I−; (c): AgNPRs in the presence of CA (0.25 µM); (d): AgNPRs incubated with 50 µM of I− in the presence of CA (0.25 µM); (e): AgNPRs in the presence of CA (0.25 µM) and Hg2+ (1.0 µM); (f): AgNPRs incubated with 50 µM of I− in the presence of CA (0.25 µM) and Hg2+ (2.5 µM).

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Figure 3. Selectivity of the AgNPRs-based colorimetric probe for Hg2+ compared with other metal ions (Mean ± SE, n = 3). (a): Wavelength shift between the peak wavelengths of AgNPR dispersions incubated with 50 µM of I− in the presence of CA (0.25 µM) and single metal ion (2.5 µM for Hg2+ , Fe2+ and Cr3+, 25 µM for other metal ions) and that in the absence of metal ions; (b): Wavelength shift between the peak wavelengths of the AgNPR dispersions incubated with 50 µM of I− in the presence of CA (0.25 µM) and metal ions (2.5 µM of Hg2+ plus 2.5 µM of Fe3+ or Cr3+, 25 µM of other metal ions) and that in the absence of metal ions. The incubation time is 15 min.

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Figure 4. Photographic image of the AgNPRs-based colorimetric probe incubated with 50.0 µM of I− in the presence of Hg2+ with various concentrations (25-800 nM). The detection system incubated with 50.0 µM of I− in the absence of Hg2+ is used as a control. The concentration of CA is 0.25 µM. The incubation time is 15 min.

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Figure 5. Sensitivity of the AgNPRs-based colorimetric probe for Hg2+ detection. (a): UV-vis absorption spectra of the AgNPR dispersions incubated with 50.0 µM of I− in the presence of Hg2+ with various concentrations; (b): Plot of the wavelength shift as a function of Hg2+ concentration ranging from 0 to 5.0 µM. The wavelength shift is calculated between the peak wavelengths of the AgNPR dispersions incubated with I− (50.0 µM) in the presence of Hg2+ and that in the absence of Hg2+. The inset plot shows the wavelength shift versus different Hg2+ concentrations in the range of 3.0-500 nM.

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Figure 6. Photograph of the AgNPRs-based colorimetric probe (containing 0.25 µM of CA and 50.0 µM of I−) incubated with different concentrations of Hg2+ in real water samples. The concentrations of Hg2+ in the tap water and lake water are respectively 0.1 and 0.4 µM determined by ICP-MS.

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Table 1. Detection of Hg2+ in real water samples by our AgNPRs-based probe or ICP-MS. ICP-MSb AgNPRs-based probec Recoveryd Sample Added (µM)a (µM) (µM) (mean±E, n=3) (%) Tap water Lake water a

0.100

0.104

0.082±0.002

82.0

0.400

0.405

0.300±0.005

75.0

0.100

0.111

0.076±0.005

76.0

0.400

0.420

0.306±0.005

76.5

2+

The added amount of Hg in the real water samples; The Hg2+ concentration in the real water samples determined by ICP-MS; c The Hg2+ concentration in the real water samples determined by our AgNPRs-based probe using UV-vis spectroscopy; d Calculated from the equation: (observed value with Hg2+ addition – observed value without Hg2+ addition) / added value.

b

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Graphical Abstract

A new strategy for high-performance colorimetric detection of Hg2+ in aquatic ecosystems is developed based on anti-etching of silver nanoprisms (AgNPRs) because Hg2+ can protect the AgNPRs from I− etching via formation of Ag-Hg nanoalloy, but other ions cannot.


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High-Performance Colorimetric Detection of Hg2+ Based on

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