Multimodal Assay of Arsenite Contamination in Environmental

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Multimodal Assay of Arsenite Contamination in Environmental Samples with Improved Sensitivity through StimuliResponse of Multi-Ligands Modified Silver Nanoparticles Shao-Hua Wen, Ru-Ping Liang, Li Zhang, and Jian-Ding Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04934 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Sustainable Chemistry & Engineering

Multimodal Assay of Arsenite Contamination in Environmental Samples with Improved Sensitivity through Stimuli-Response of Multi-Ligands Modified Silver Nanoparticles Shao-Hua Wen, Ru-Ping Liang, Li Zhang, Jian-Ding Qiu*

College of Chemistry, Nanchang University, 999 Xuefu Road, Nanchang 330031, China *Corresponding authors. Tel/Fax: +86-791-83969518. E-mail: [email protected].

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ABSTRACT: Arsenic has an open circulatory system in the nature and is high-toxic to both humans and ecosystem. This study describes a multimodal assay of arsenite (As(III)) in environmental samples via stimuli-response of multi-ligands functionalized silver nanoparticles (Ag NPs). The multi-ligands functionalized Ag NPs are prepared through chemical reduction using asparagine (Asn) as the capping ligand and further modification with

reduced

glutathione

(GSH)

and

dithiothreitol

(DTT).

The

obtained

GSH/DTT/Asn-Ag NPs can be used as multifunctional probes for multimodal assay of As(III) owing to the excellent plasmonic property and unique electrochemical activity. The multimodal method can not only detect As(III) by naked-eye colorimetric and spectrophotometric assay, but also by turn-on and high-sensitive electrochemical assay. The strategy features with convenient operation, short analytical time, on-site assay, high sensitivity and low cost. The As(III)-responsive multimodal sensor demonstrates excellent performance for detection of low trace As(III) in various environmental water and juice samples. The results are in accordance with the ones obtained by ICP-MS method. Besides, the plasmonic metal nanoparticles-based multimodal sensing method can be used to ameliorate the analytical performance of sensors for toxic targets, which will be very conducive to facilitate the practical application of sensors related to environmental and food monitoring. KEYWORDS:

Arsenite,

Silver

nanoparticles,

As-S

Electrochemical assay, Real samples.

2


bond,

Spectral

assay,

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INTRODUCTION As a toxic metalloid, arsenic (As) ubiquitously distributed in the crust of earth, which can circulate through the complex combination of natural processes such as wind erosion of soil, weathering reactions, volcanic eruption and biological processes as well as through a sequence of anthropogenic activities such as the usage of arsenical pesticides, the combustion of fossil fuels and mining activity.1-3 Among the various sources of arsenic in the environment, the arsenic contamination of groundwater and drinking water probably poses the enormous threat to the eco-system and human health.1,4 It generally occurs in natural waters as the arsenate (H3AsO4, As(V)) and arsenite (H3AsO3, As(III)) forms.4 However, As(III) is proved to be more pernicious about ~60 times and more arduous to remove from water than As(V).4,5 Long-term exposure to the arsenic contaminated water can cause adverse health problems and has been proven carcinogens in humans, ranging from skin lesions, various vascular diseases, diabetes mellitus, respiratory disease and many types of cancers.6,7 The literatures have shown that nearly 20 million population are under the risk of the consumption of arsenic-tainted groundwater in some regions of China, where the level of arsenic exceeds the stated threshold of 10 ppb per liter of the World Health Organization (WHO) and Chinese standard guideline for drinking water.8,9 Furthermore, arsenic transportation from the sources to water is a result of accumulation and can never be permanently eliminated.3,8 Obviously, arsenic monitoring in water of new environmental situation puts forward new demands on the analytical techniques with rapid, facile, on-site, and cost-effective analytical performance. 3



The conventional techniques for laboratory analysis like hydride generation and atomic fluorescence spectrometry (HG-AFS),10 inductively coupled plasma mass spectrometry (ICP-MS),11 hydride generation (HG) with ICP-MS,12 laser-induced desorption/ionization mass spectrometry (LDI-MS),13 and high-performance liquid chromatography with ICP-MS (HPLC-ICP-MS)14 have been utilized for As(III) detection in water samples or foodstuffs at trace level. However, these techniques need cumbersome equipment, highly trained technicians, tedious sample handling, time-consuming and even high cost.15,16 Besides, these methods are neither capable of on-site field detection nor readily available in developing countries.16 To date, several efficient and convenient techniques and strategies have been developed for sensitive determination of arsenic in water, such as UV−vis spectrophotometry,17-19 electrochemical techniques20,21 and bacterial biosensor.22 In view of the simplicity, and on-site detection, colorimetric assays of metal nanoparticles (MNPs)-based have been used as one of the most versatile methods for various targets.23 Various kinds of liquid-phase colorimetric sensors have been developed through surface plasmon resonance (SPR) changes of MNPs,

e.g. Au NPs and Ag NPs24. The assay

principle is generally based on targets-induced aggregation of plasmonic MNPs along with the color change of solutions, further causing dramatic changes of SPR absorption bands of MNPs. Ray’s group has reported a colorimetric assay of arsenic based on multifunctional ligands modified Au NPs with good selectivity and sensitivity.17 Zhou et al. developed a sensitive colorimetric sensor for arsenic determination based on 4


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aggregation of Au NPs, which is controlled by arsenic-binding aptamer and cationic polymer.18 Ghodake et al. used amino acid asparagine-modified Au NPs as colorimetric reporters for selective spectral sensing of As(III), but with a higher detection level from 100 ppb to 2000 ppb.19 Alternatively, electrochemical methods have been considered as one of the most accessible and powerful techniques in achieving sensitive and rapid detection of trace heavy metal ions, featured with rapid analysis time, high sensitivity and low cost.25,26 Recently, electrochemical methods, particularly stripping voltammetry, for the detection of trace As(III) in water have made great progress.20,26,27 The principle of stripping voltammetry assay is that As(III) is first pre-concentrated and further reduced to As(0) deposited on the working electrode surface, and then the stripping process is performed in a anodic scan mode to re-oxidize As(0) to As(III) with a specific stripping current. Various functionalized working electrodes have been reported for stripping voltammetry detection of As(III), such as Fe3O4 microsphere modified screen-printed carbon electrode (SPCE),28 Au NPs decorated glassy carbon electrode (GCE),29 ruthenium nanoparticles (Ru NPs) modified GCE,30 Au NPs-Fe3O4 microsphere modified SPCE,31 bimetallic Fe-noble metal NPs modified GCE,32 and Au NPs-embedded carbon films.33 Most of the developed electrochemical methods require an acidic or strongly acidic condition strongly,29,30,32 which may raise the cost and restrict the actual application for assay of natural water (usually weak alkaline). Another major challenge is the competition of some other metal cations coexisted in “unknown” natural environmental water and their 5



interferential stripping potentials, in particular the existence of Cu(II).20 Therefore, it is of great significance and challenge to seek a reliable and sensitive electrochemical method for As(III) assay in real natural water to detect As(III) with high selectivity under mild condition.

Scheme 1. Schematic illustration of the GSH/DTT/Asn-Ag NPs-based dual-modal assay for As(III) detection. Illustration of (A) the synthesis of GSH/DTT/Asn-Ag NPs used as colorimetric probe for As(III) detection and (B) the electrochemical As(III) sensor based on GSH/DTT/Asn-Ag NPs as redox signal probe. The motivation of this study is to construct a convenient, high-sensitive, low-cost and on-site assay of As(III) in real natural water and juice samples with spectrophotometric and electrochemical sensing method as illustrated in Scheme 1. To accomplish this goal, a multi-ligand (GSH/DTT/Asn) modified Ag NPs probe is first designed. The specific interaction via As–O and As–S linkages between GSH/DTT/Asn-Ag NPs and As(III) results in the aggregation of the silver NPs, which enables to colorimetric and UV-vis 6


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assay As(III) with high sensitivity and good selectivity based on the plasmonic property. In addition, the As(III)-induced aggregation of GSH/DTT/Asn-Ag NPs as the electrochemical signal probe allows to detect As(III) with markedly improved sensitivity and detection limit. The analytical performance of this multimodal sensing method in practical application of different natural water and juice samples has been further validated by the ICP-MS standard method.

EXPERIMENTAL SECTION Chemicals and Reagents. Silver nitrate (AgNO3, 99.8%) was obtained from Tianjin Damao Chemical Reagent Co. Ltd. (Tianjin, China). Reduced glutathione (GSH, 99%), sodium arsenate dibasic heptahydrate (Na2HAsO4·7H2O) and sodium (meta) arsenite (NaAsO2, As(III)) were provided by J&K Scientific Ltd. (Shanghai, China). Dithiothreitol (DTT), glucose, bovine serum albumin (BSA), adenosine 5′-triphosphate disodium salt hydrate (ATP), humic acid (HA) and lactic acid were purchased from Sigma−Aldrich (St. Louis, USA). L-Asparagine (Asn), sodium borohydride (NaBH4), ferric chloride (FeCl3·6H2O, 99.0%), potassium ferricyanide (K3Fe(CN)6, 99.0%) and ethylenediamine-N,N,N′,N′-tetraacetic acid disodium (EDTA), urea, vitamin C and Tris(hydroxymethyl)aminomethane (Tris) were purchased from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). All metal ions, anionic salts and other chemicals were used as received from suppliers without any purification. Ultrapure water (>18.2 MΩ cm) was obtained from a Milli-Q purification system for preparing solutions 7



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throughout the experiments. Instrumentation and Characterization. UV-vis absorption spectra through all experiments were collected on a spectrophotometer (UV-2450, Shimadzu). Fourier transform infrared (FT-IR) spectra for the surface modification of nanoparticles were obtained using a FT-IR spectrometer (TENSOR II, Bruker). The morphologies of Ag NPs and As(III)-induced aggregation of GSH/DTT/Asn-Ag NPs were investigated using JEOL JEM-2010 transmission electron microscope with an accelerating voltage of 200 kV. Differential pulse voltammetry (DPV) was performed on a CHI 630C electrochemical workstation (Shanghai Chenhua instrument Co. Ltd.). A modified gold (Au) electrode (Φ=2 mm), a saturated calomel electrode (SCE),

and a platinum (Pt)

wire as the working electrode, the reference electrode, and the counter electrode, respectively, were used. All electrochemical assays were performed using a three-electrode system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on an IviumStat electrochemical workstation (Ivium Technologies) in 1 mL HEPES solution (20 mM, pH 7.2) containing 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (molar ratio 1:1) and 0.1 M KCl. CV was scanned from -0.3 V to 0.6 V with scan rate at 100 mV/s. EIS curves were carried out in a frequency range from 0.01 Hz to 1 x 105 Hz. The DPV measurement conditions were taken: the potential range from -0.2 to 0.2 V, pulse width 0.05 s, modulation amplitude 0.05 V, and sample width 0.0167 s. The concentration of As(III) in real water samples was detected by ICP-MS (Varian 820-MS, USA). 8


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Preparation and Modified of Ag NPs. All the glassware were soaked in aqua regia (Caution: Be particularly careful when using!) for a certain time and then thoroughly washed with fresh water and finally rewashed with ultrapure water and air dried. The Asn-Ag NPs were prepared by chemical reduction of silver nitrate with sodium borohydride according to the previous study.34 Asn (10 mM, 1 mL) and AgNO3 (10 mM, 1 mL) were orderly added into ultrapure water (98 mL) under vigorously stirring. After fully mixing, 2 mL fresh NaBH4 (0.1 M) was added dropwise with vigorous stirring in the dark conditions, producing a light yellow colored solution. The obtained mixture was kept violently stirring for 1 h at room temperature. The mixture solution of GSH and DTT with certain ratio was slowly dropped into the above Asn-Ag NPs solution and continuously stirred for 2 h at room temperature. Afterwards, the solution was kept at 4 °C in dark and left for 24 h. The synthesized GSH/DTT/Asn-Ag NPs were centrifuged and repeatedly washed with isopropanol at 12000 rpm for three times. The purified GSH/DTT/Asn-Ag NPs was re-dispersed in 50 mL ultrapure water as stock solution at 4 °C for further experiments. The bare Ag NPs were prepared by directly chemical reduction of AgNO3 with NaBH4 without any ligand. The every ligand modified Ag NPs were synthesized in the similar way. The GSH/Asn-Ag NPs were further synthesized in the same route. Selective Investigation. To ensure that the modified Ag NPs probe can specifically detect As(III), the common metal ions in water were incubated with Ag NPs nanoprobes (GSH/Asn-Ag NPs or GSH/DTT/Asn-Ag NPs) in tubes, and the UV-vis absorbance ratio 9



(A512/A394) of the nanoprobes was recorded. The selective experiments were divided into two stages. (1) A comparative experiment between two probes GSH/Asn-Ag NPs and GSH/DTT/Asn-Ag NPs was conducted at the same level of different metal ions. Briefly, in a 2 mL tube, 100 µL HEPES buffer (pH 7.2, 20 mM) was first added into 300 µL solution of Ag NPs probe (GSH/Asn-Ag NPs or GSH/DTT/Asn-Ag NPs). After thoroughly mixing, a series of different metal ions (100 ppb) were mixed with the above solution and incubated at ambient temperature for 1 h, respectively. The total volume of all solutions was 500 µL. The UV-vis intensity after 10 min was recorded. (2) Next, we further investigated the GSH/DTT/Asn-Ag NPs probe toward the target As(III). The operation procedure was similar to the stage (1). The difference was that the concentration of As(III) used was 10 ppb, whereas the concentration of other metal ions were 20 times higher for using. The UV-vis intensity after 10 min was recorded. Colorimetry and UV-vis Spectra for As(III) Detection. The colorimetric assay was performed using the GSH/DTT/Asn-Ag NPs as sensing probes. First, As(III) solutions with different concentrations were prepared with HEPES buffer (20 mM, pH 7.2). The As(III) solutions with different concentrations were sufficiently mixed with 300 µL GSH/DTT/Asn-Ag NPs solutions. The total volume of all solutions were finally kept as 500 µL with ultrapure water and then incubated for 10 min at ambient temperature. The obtained solutions with changed color were detected by UV-vis spectrophotometer. Fabrication of the Electrochemical Sensor for As(III) Detection. A gold electrode was first chemically pretreated using a piranha solution (98% H2SO4 : 30% H2O2 = 3 : 1, 10


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v/v) for 5 min and then washed thoroughly by ultrapure water. (Caution: Piranha solution reacts with organic solvents violently and should be handled with extravagant care!). Subsequently, Au electrode was successively polished with 1, 0.3, and 0.05 µm alumina slurry on a polishing pad to a mirror-like surface. Using ultrapure water to wash thoroughly, the polished electrode was ultrasonically cleaned consecutively in ethanol, 25% HNO3 and ultrapure water for 10 min before modification. Finally, the Au electrode was electrochemically scanned with H2SO4 (0.5 M) via CV to remove any remaining impurities, then washing with ultrapure water. After drying with nitrogen gas, the cleaned Au electrode was immersed into the mixture solution contained 0.5 mM GSH and 0.5 mM DTT for 6 h at room temperature to achieve the formation of self-assembled monolayer on the electrode surface. Then, using ultrapure water to rinse, the modified electrode was blown with ultrapure nitrogen gas to remove physically adsorbed species before use. The GSH/DTT modified Au electrode was first immersed into HEPES (20 mM, pH 7.2) solution containing different concentrations of target As(III) at ambient temperature for 5 min. Then, 50 µL GSH/DTT/Asn-Ag NPs probe solution was added into As(III) solutions, which were employed to react with the electrodes for another 10 min. After being rinsed with ultrapure water, the obtained gold electrode was used as the working electrode for signal output and then measured by DPV in 0.1 M KCl electrolyte. Preparation and Assay of Real Water and Juice Samples. (i) Real Water Samples. The real water samples were collected from three different natural basins — Ganjiang River, Runxi Lake and well water. The Ganjiang River, located in the southeast (mainly 11



in Jiangxi Province) of mainland China, is situated in the middle and lower reaches of the Yangtze River. The Runxi Lake is a small one located on the campus of Nanchang University, China. The well water was collected from a well in local village (Xinjian County of Jiangxi Province, China). Briefly, all natural water samples were first centrifuged at 12 000 rpm for 5 min to detach solid impurities, and then filtered twice by a 0.22 µm microfiltration membrane. The treated water samples were used five-fold dilution method to compensate for matrix effects of the natural water samples. Finally, the diluted water samples were spiked with As(III) at different concentration levels, which were then analyzed with the above-mentioned two methods. The detection results were also compared with those obtained by ICP-MS method. (ii) Real Juice Samples. Three kinds of fruits (grapefruit−Guangxi; orange−Jiangxi; and sugarcane−Yunnan, China) were purchased from the local market. These kinds of fruits are in season now and often used to make juice. After removing the pericarp, fruit juice was collected into a clean and pasteurized bottle by strong mechanical extrusion. The juice samples were filtered three by a 0.22 µm microfiltration membrane. In order to eliminate the matrix effects of the juice, the original juice samples were diluted five-fold for further test. To investigate the practicability of the present sensors in juice samples on the response of As(III), the samples were spiked with different concentrations of As(III) (2 and 10 ppb) solutions. ICP-MS assay with the diluted juice samples were also employed to verify the sensor tests.

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RESULTS AND DISCUSSION Characterization of the GSH/DTT/Asn-Ag NPs. Synthesis of GSH/DTT/Asn-Ag NPs was based on liquid phase reactions and mild reaction conditions. The morphology of GSH/DTT/Asn-Ag NPs was observed with a transmission electron microscope (TEM, Figure 1A). The TEM images show the discrete Ag NPs with an average size of ~10.6 nm. The concentration of the GSH/DTT/Asn-Ag NPs is 0.25 nM, which is calculated with Beer-Lambert’s law and extinction coefficient (ε) of 5.06 × 109 M-1 cm-1 (Molar extinction coefficient of the obtained Ag NPs can be calculated with specific equation ln ε = 1.4418 ln D + 18.955).35 The surface charge of GSH/DTT/Asn-Ag NPs was determined by the zeta potential (Figure S1). The GSH/DTT/Asn-Ag NPs is negatively charged (-17.2 mV) due to deprotonation of its surface groups. The decreased zeta potential compared with Asn-Ag NPs (-12.6 mV) indicates that GSH carrying negative charges are modified on the Asn-Ag NPs. The results of zeta potential are in line with the isoelectric point of Asn (5.41) and GSH (5.93) at the pH 7.2. The stability of the prepared GSH/DTT/Asn-Ag NPs was investigated via monitoring UV-vis absorbance intensity at 394 nm. No obvious change in absorbance at 394 nm (Figure S2A) is observed when the pH value of the system varies from 4 to 10, which is attributed to the multi-ligands on the surface of Ag NPs. Besides, the time-dependent stability was also examined by monitoring the absorbance band at 394 nm. The absorbance intensity at 394 nm only occurs a little change over a period of 60 days (Figure S2B). The presence of surface negative charges imparts good stability under nearly neutral pH conditions. The results 13



indicate that the obtained GSH/DTT/Asn-Ag NPs are suitable for the assay of natural water samples. FT-IR experiments were performed to investigate the various functional groups on GSH/DTT/Asn-Ag NPs. Figure 1B shows the FT-IR spectra of pure Asn, GSH, DTT and the GSH/ DTT/Asn modified Ag NPs, respectively. Comparing the FT-IR spectra of pure ligands with modified Ag NPs, some significant features appear: the 3000–3600 cm−1 peaks correspond to the –NH and the –OH vibrations, the characteristic peak at ~1780 cm−1 attributed to the stretching vibration of the –C=O, and the characteristic peak for the stretching vibration peak of –SH at ~2600 cm−1. These main characteristic absorption peaks are also clearly observed for GSH/DTT/Asn-Ag NPs. The results indicate that the functional groups like -COOH and -SH are successfully appended to the surface of Ag NPs. However, the peaks center of –NH and –OH vibrations shifted from ~3300 cm−1 to ~3260 cm−1 due to the effects of Ag NPs (curve d, Figure 1B).36 For another, when reacted with As(III), the stretching vibration peak of –SH on GSH/DTT/Asn-Ag NPs was not observed clearly due to the As-S linkages.37 Besides, the characteristic peak at ~1780 cm−1 of –C=O was hugely diminished attributed to As-O linkages.38 To better understand surface functional groups of the bare Ag NPs and every individual molecule modified Ag NPs, we further investigated their FT-IR spectra changes. The detailed FT-IR results and analysis are shown in Figure S3 and Supporting Information.

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Figure 1. (A) TEM image and (Inset) size distribution histogram of the GSH/DTT/Asn-Ag NPs. (B) FT-IR spectra of (a) Asn, (b) GSH, (c) DTT (d) GSH/DTT/Asn-Ag NPs, and (e) GSH/DTT/Asn-Ag NPs reacted with 100 ppb As(III). Sensing mechanism for As(III) assay. It has been reported that As(III) can strongly interact with active –COOH and –SH through As–O and As–S bonds, respectively.17,39 Generally, each As(III) ion can bind with three GSH or Asn or DTT molecules through free –COOH or –SH groups (Figure S4). Firstly, we investigated every individual functional molecule-modified Ag NPs for the preliminary assay of As(III) detection. The comprehensive experimental results are summarized in Table S1. The Ag NPs solution exhibits an absorption band at ~394 nm (A394) (Figure S5A and curve a of Figure 2A), which is attributed to the localized surface plasmon resonance absorption (LSPR) of the Ag NPs.40 Figure S5A shows that the absorption spectrum of bare Ag NPs reacted with 200 ppb As(III) for 60 min keeps almost unchanged compared to original bare Ag NPs, which results from no active binding groups. When only Asn was used to modify Ag NPs, Asn interact with Ag+ through carboxyl and hydroxyl groups during the formation 15



process of Ag NPs.41 So the Asn-Ag NPs is insensitive to As(III) due to few active carboxyl sites for As–O linkage (Figure S5B). The GSH-Ag NPs is a little sensitive to As(III) because GSH has -COOH and -SH functionality (Figure S5C). However, the active groups, particularly –SH, can be greatly consumed by Ag NPs due to Ag-S bonds during modification process.40 So the main effective binding sites of GSH-Ag NPs towards As(III) are –COOH functionality. Because DTT molecule have two active –SH groups, DTT modified Ag NPs could remain more active –SH groups that can conducive to bind with As(III) via As–S linkage. As shown in Figure S5D, DTT-Ag NPs is more sensitive to As(III) than other individual molecule modified Ag NPs. More importantly, the presence of As(III) can induce the aggregation of DTT-Ag NPs accompanied by an obvious new absorption peak near 512 nm owing to the coupling effect of the aggregated Ag NPs. Finally, we tried to introduce more functional groups including active -COOH and –SH groups to improve the binding ability of Ag NPs toward As(III). So we further investigated the multi-ligand (GSH/DTT/Asn) modified Ag NPs as probe for As(III) assay. UV−vis absorption spectra of the solutions of GSH/DTT/Asn-Ag NPs without and with As(III) were further investigated. The Ag NPs solution only exhibits an absorption band at 394 nm (A394) (black curve, Figure2B).Addition of As(III) causes a distinct decrease in the absorption intensity at 394 nm for the Ag NPs and the occurrence of a new absorption band at ~512 nm. Compared to other individual molecule modified Ag NPs, the absorption intensity ratio (A512/A394) of the multi-ligand GSH/DTT/Asn-Ag NPs 16


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as probe to 200 ppb As(III) is calculated about 0.8681 (Table S1), which is the highest absorption change and suggests the highest sensitivity among these Ag NPs probe. From the inset in Figure 2A, an obvious color change occurs from yellow (tube a) of blank Ag NPs solution to light purple (tube b) of As(III)-induced aggregation of Ag NPs. In consideration of the reaction time and sensitivity to target As(III), therefore, we choose the multi-ligand containing Asn, GSH and DTT modified Ag NPs as a promising colorimetric and plasmonic probe for the determination of As(III). As further demonstrated by the TEM characterizations, the Ag NPs remain well dispersed state (Inset of Figure 2B). When As(III) is present, aggregation of the Ag NPs occurs as shown in Figure 2B due to the strong interaction of GSH/DTT/Asn-Ag NPs with As(III). All these results well illustrates that the multi-ligand modified Ag NPs can be used as an effective probe for As(III) detection.

Figure 2. (A) UV−vis absorption spectral changes of the GSH/DTT/Asn-Ag NPs in the (a) absence or (b) presence of 200 ppb As(III). Inset shows the corresponding photos of the GSH/DTT/Asn-Ag NPs solutions in the (a) absence or (b) presence of 200 ppb As(III). (B) TEM image of the GSH/DTT/Asn-Ag NPs reacted with 20 ppb As(III). Inset is the 17



TEM image of GSH/DTT/Asn-Ag NPs without As(III). Selective assay toward As(III). The environmental water often have various heavy metal ions as listed in Table S2, which also show the guidelines of common metal ions in drinking water regulated by China and WHO. In order to investigate the binding ability of two main differently modified Ag NPs, various metal ions were added to the GSH/Asn-Ag NPs and GSH/DTT/Asn-Ag NPs solutions, respectively. The UV-vis absorbance intensities of these solutions were recorded and the absorbance ratios A512/A394 are shown in Figure 3. When GSH is only used to modify the Asn-Ag NPs, the assay shows negligible responses toward Na(I), K(I), Ca(II), Mg(II), Zn(II), Ba(II), Al(III) and Cr(III) (all at 200 ppb, blue column). However, a change in absorbance ratios (A512/A394) with different degrees occur in the presence of As(III), Fe(II), Pb(II), Cd(II), Ni(II), Fe(III), Hg(II) and Cu(II) in solution. When the Ag NPs are modified with three ligands Asn, GSH, and DTT, the absorbance ratios A512/A394 of the obtained GSH/DTT/Asn-Ag NPs do not change significantly upon addition of Fe(II), Pb(II), Cd(II), Ni(II), Fe(III) or Cu (II), respectively, although a small change is observed with addition of Hg(II). We can see that the GSH/DTT/Asn-Ag NPs shows the most prominent response to As(III) at the same concentration level. This can be explained by the stability constants between As(III) and the binding ligands, logK = 32.0 (GSH) and 37.8 (DTT). The stability constants (logK) between DTT and other heavy-metal ions are 17.6 (Hg), 15.3 (Cu), 14.6 (Cd), 13.89 (Pb), 10.7 (Ni).17,42 Because the binding stability constant of 18


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the DTT with As(III) is about 30 orders of magnitude higher than As(III) with other interfering metal ions, the GSH/DTT/Asn-Ag NPs show negligible responses toward most of the interfering species. However, the GSH/DTT/Asn-Ag NPs show an obvious response toward Hg(II) due to its higher stability constant and affinity ability with DTT.

Figure 3. Histogram of the UV-vis absorbance ratio (A512/A394, where A394 and A512 represent the absorbance intensities of GSH/DTT/Asn-Ag NPs at 394 and 512 nm, respectively) of the GSH/Asn-Ag NPs probe and GSH/DTT/Asn-Ag NPs probe toward different ions. The concentrations of all metal ions were 200 ppb. In the previous studies, EDTA was used to chelate with several interfering metal ions, such as Hg(II) and Cu(II).29,43 To further improve the selectivity of the GSH/DTT/Asn-Ag NPs to As(III), chelating ligand EDTA was utilized to eliminate the interference from Hg(II) due to its much larger complexation constant with EDTA than As(III). As shown in Figure 4A, the absorbance ratios (A512/A394) of the GSH/DTT/Asn-Ag NPs solutions toward these interfering metal ions (except for Hg(II)) 19



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are roughly consistent with the blank value, even the concentrations of these interfering metal ions are 20-fold higher than that of As(III). The results indicate that EDTA can effectively mask the interference from Hg(II), whereas EDTA does not show any influence

on

the

assay

of

As(III).29,43

Furthermore,

the

A512/A394

of

the

GSH/DTT/Asn-Ag NPs toward commonly interfering organic compounds (acetate, Tirs, EDTA, HA, urea), biomolecules (glucose, vitamin C, lactic acid, ATP, BSA), and anions (Cl-, CO32-, SiO32-, SO42-, PO43-) changes negligibly, even the concentrations of these interfering substances are 20-fold higher than that of As(III) (Figure 4B). Thus, GSH/DTT/Asn-Ag NPs can be used as a highly selective probe for assay of As(III) in real samples.

Figure 4. Histograms of the UV-vis absorbance ratios (A512/A394) of GSH/DTT/Asn-Ag NPs probe toward (A) different metal ions and (B) commonly interfering organic compounds, biomolecules, and anions. The concentrations of (A) all other metal ions and (B) interfering substances were 200 ppb. The concentration of target As(III) was 10 ppb.

20


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Colorimetric and UV-vis absorption assay of As(III). To further investigate the analytical performance of the constructed colorimetric method, a series of arsenic samples containing various concentrations of target As(III) were tested under optimal experimental conditions (Figure S6–S7). As shown in Figure 5A, the color of the GSH/DTT/Asn-Ag NPs solutions gradually changes from yellow to light pink to light purple with the increase of As(III) concentrations, suggesting an aggravation in the aggregation of Ag NPs. It should be pointed out that the obvious color change for the target As(III) can be clearly distinguished only when the concentration of As(III) increases to a certain level. More importantly, the colorimetric assay provides a favorable mode for the rapid and on-site monitoring of environmental water with high-level As(III) contamination in those rural regions. Meanwhile, the UV-vis spectra (Figure 5B and Figure S8) show that the absorbance peak at 394 nm gradually decreases, while the absorbance peak at 512 nm increases with the gradual increment (from 0 to 200 ppb) of target As(III) concentration. Figure 5C presents the ratio A512/A394 as a function of As(III) concentration. The resulting calibration curve exhibits a good linear relationship (R2 = 0.9921) between the absorbance ratio and the concentration of As(III) within a dynamic range from 0.4 to 20 ppb. The method limit of detection (LOD) is calculated to be 0.36 ppb (3σ, S/N = 3). The limit of detection is comparable to or even lower than those reported colorimetric methods as listed in Table S3. As the maximum allowable level of As(III) in drinking water is 10 ppb in the WHO’s guidelines for drinking water quality, the present colorimetric method holds great potential for on-site analysis with its 21



naked-eye readout, simple operation and low costs.

Figure 5. (A) Photographs of the GSH/DTT/Asn-Ag NPs solution incubated with different concentrations (0, 0.2, 0.4, 0.8, 1, 1.2, 1.5, 2, 4, 5, 10, 15, 20, 40, 80, 150, 200 ppb) of As(III) for 10 min. (B) UV−vis absorption spectra of the GSH/DTT/Asn-Ag NPs solutions in the presence of increasing As(III) concentration (0, 0.2, 0.4, 0.8, 1, 1.2, 1.5, 2, 4, 5, 10, 15, 20, 40, 80, 150, 200 ppb). The enlarged absorption spectra around 394 nm (red dashed line) and 512 nm (blue dashed line) are shown in Figure S8. (C) Calibration curve for the absorbance intensity ratio (A512/A394) vs As(III) concentrations from 0.4 to 200 ppb. Error bars represent ± standard deviation of 5 individual replicates. Electrochemical assay of As(III). As reported before, Ag NPs show excellent plasmonic property and unique electrochemical activity as well. A lot of sensors have been fabricated for various targets based on either surface plasmon resonance44 or electrochemical redox activity of Ag NPs.45 Nowadays, multiple mode assay strategies 22


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


have been demonstrated to be more reliable, more efficient and higher sensitive than the single mode method.46-48 Among these dual-mode sensors, electrochemical technique is usually used as a powerful technique in achieving the highly sensitive, low cost and on-site detection of trace targets. Particularly, Ag NPs have been successfully used as both colorimetric sensing probe and electrochemical redox probe for the analysis of heavy metal ions.49 As has been proved that solid-state voltammetric determination of Ag NPs as electrochemical labels has displayed a distinct advantage in measurement due to its peculiarity and simplicity, because it involves the determination of the electrochemical signal stemming from the valent-state transformation between one surface-confined solid to another surface-confined solid.50 As shown in Scheme1B, the GSH/DTT was first assembled onto a clean gold electrode through Au-thiol bonds. In the absence of target As(III), almost no GSH/DTT/Asn-AgNPs probe can be captured onto the electrode surface without the joint points of As–O/As–S. When the GSH/DTT modified gold electrode

is

immersed

in

a

mixture

solution

containing

As(III)

and

GSH/DTT/Asn-AgNPs probe, As(III) can induce the aggregation of Ag NPs probe and further lead to more Ag NPs tethered on the electrode surface. Two well obvious current peaks appear on the cyclic voltammogram in 0.1 M KCl electrolyte, which are ascribed to the oxidation of Ag atom to AgCl and followed reduction of AgCl back to Ag atom, respectively (Figure S9). The charge efficiency between the oxidation and subsequent reduction is almost 100%, indicating reversible transformation of the Ag NPs probe to the silver chloride solid phase during the anodic scan and then subsequent conversion back to 23



Ag atom during the cathodic scan.50 No detectable current response without As(III) indicates an excellent signal-to-background ratio for the electrochemical assay of As(III). The successful modification of the electrode surface was characterized by cyclic voltammograms (CVs) (Figure S10A) and electrochemical impedance spectroscopy (EIS) (Figure S10B). Incubation procedure of the GSH/DTT modified electrode with As(III) and GSH/DTT/Asn-AgNPs was also investigated, and results show that two-step treatment can significantly increase the current output of Ag NPs (Figure S11). For evaluation of the analytical performance of the electrochemically amplified sensing method, the peak current of Ag NPs was recorded with the increase of As(III) concentration. As shown in Figure 6A, the peak current responses of Ag NPs gradually increase when the concentration of As(III) increases from 0 to 50 ppb. The calibration plots show a good linear relationship between the intensity of peak current and the concentration of As(III) in the range of 0.01−40 ppb with the square of correlation coefficient (R2) at 0.9916 (Figure 6B). The LOD of the electrochemical amplified method is down to 5.2 ppt at 3σ (S/N = 3). The linear range and detection limit are better than or at least comparable to the recently reported electrochemical and other techniques for As(III) detection (Table S3). Based on the strategy of target As(III)-induced Ag NPs aggregation on the electrode and highly characteristic electrochemical redox behavior for Ag NPs-mediated signal amplification,45 the electrochemical method can acquire significantly higher sensitivity and lower detection limit. Moreover, reproducibility of the As(III) electrochemical method was investigated through testing 2 ppb As(III) solution on 24


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


seven parallel prepared gold electrodes. The relative standard deviation (RSD) of the values calculated from these different electrodes is 5.6 %. Finally, the prepared electrode for As(III) assay was measured after being stored at 4 °C for 1 week and 95.1% of the original current signal remains, indicating satisfactory stability of the proposed electrochemical sensing method.

Figure 6. (A) DPV response curves obtained by GSH/DTT-modified Au electrodes after incubation with mixtures of GSH/DTT/Asn-AgNPs and different concentrations of As(III) (0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.4, 0.8, 1, 2, 4, 8, 10, 20, 40, 50 ppb). (B) Calibration curve for the peak current value (at 0.095 V) vs the logarithm value of As(III) concentration from 0.01 to 40 ppb. All data reported are averages of at least three individual experiments. Error bars represent ± standard deviation of 5 individual replicates. The electrolyte was 0.1 M KCl solution; the scan rate was 100 mV/s.

Multimodal analysis of As(III) in environmental water and fruit juice. The applicability of the multimodal sensor were evaluated by the detection of As(III) collected from different environmental waters, such as Ganjiang River, Runxi Lake and 25



Page 26 of 34

well water. After centrifugal treatment, the collected water samples were diluted 5-fold with ultrapure water to avoid the matrix effects. First, the concentrations of As(III) in these real water samples were detected by ICP-MS standard method. As shown in Table 1, there is no As(III) detectable in the Ganjiang River and well water samples by ICP-MS, while As(III) in the lake water is detected at 1.212 ppb. More importantly, the electrochemical method can detect the trace level (0.0812 ppb) of As(III) in the well water, which is below the LOD of the ICP-MS method. Besides, different concentrations of As(III) were added into the diluted water samples and then measured by employing the proposed dual-modal methods and ICP-MS. The testing data are summarized in Table 1. The concentration of As(III) measured by the dual-modal methods are in good accordance with the results obtained by the ICP-MS standard method. One point to note is that the percentages of relative error (RE) of the proposed UV-vis absorption and electrochemical methods with respect to the values detected by ICP-MS standard method for all samples are quite satisfactory. These results confirm that the present dual-modal can be applicable for As(III) detection, especially the electrochemical method is suitable for the trace level of As(III) detection in natural water samples. Table 1. Detection of As(III) in Various Water Samples.

Samples

Spiked As(III) (ppb)

As(III)/ppb

Gan River 1

0

−

−

−

−

−

−

−

−

Gan River 2

5

4.90

0.52

5.21

0.92

104.2

4.91

4.09

98.2

Gan River 3

20

19.86

0.36

20.86

0.75

104.3

20.49

3.32

102.5

ICP-MS

RSD (%)

UV-vis

RSD (%)

Recoverya (%)

26


Electrochemistry

RSD (%)

Recoverya (%)

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Well water 1

0

−

−

−

−

−

−

−

−

Well water 2

5

4.92

0.49

4.86

1.12

97.2

4.94

4.54

98.8

Well water 3

20

19.91

0.38

20.90

0.72

104.5

20.38

3.85

101.9

Runxi Lake 1

0

1.21

0.81

1.61

1.08

−

1.12

5.01

−

Runxi Lake 2

5

6.12

0.75

6.49

1.01

105.6

6.31

4.86

102.0

Runxi Lake 3

20

21.15

0.68

21.87

0.87

103.3

21.72

3.62

102.6

(aRecovery (%) = [(cFound - cBlank)/cAdded] × 100)

We also used the present multimodal method to detect As(III) in the juice samples. The original diluted juice samples were first detected by both the present sensor and ICP-MS method. There is no As(III) detectable in all the original juice samples. So the spiked juice samples were further analyzed for the recovery tests. The testing results are summarized in Table 2. Quantitative recovery of As(III) can be attained for all juice samples, which are also compared with the results of ICP-MS. For UV-vis assay, spiked recoveries for As(III) are found to be in the range of 96.0%−106.1%. Whereas using electrochemical assay, the recoveries for As(III) are detected in the range of 95.5%−103.2%. For further investigation of the reliability, all juice samples were also detected by ICP-MS (Table 2). The t-test shows that the analytical results obtained by the proposed sensor are not obviously different from the results obtained by ICP-MS at a confidence level of 95%. Table 2. Detection of As(III) in Various Fruit Juice Samples.

Samples Grapefruit 1

Spiked As(III) (ppb) 0

As(III)/ppb ICP-MS

RSD (%)

UV-vis

RSD (%)

Recoverya (%)

Electrochemistry

RSD (%)

Recoverya (%)

−

−

−

−

−

−

−

−

27



Page 28 of 34

Grapefruit 2

2

1.93

0.91

1.92

1.39

96.0

1.94

4.12

97.0

Grapefruit 3

10

9.76

0.76

10.39

1.12

103.9

10.21

3.75

102.1

Orange 1

0

−

−

−

−

−

−

−

−

Orange 2

2

1.89

1.02

2.09

1.29

104.5

1.91

5.01

95.5

Orange 3

10

9.62

0.85

10.61

1.08

106.1

10.32

4.06

103.2

Sugarcane 1

0

−

−

−

−

−

−

−

−

Sugarcane 2

2

1.95

1.06

2.12

1.17

106.0

1.92

5.02

96.0

Sugarcane 3

10

9.69

0.82

10.52

1.26

105.2

10.28

3.97

102.8

a

( Recovery (%) = [(cFound - cBlank)/cAdded] × 100)

CONCLUSIONS In summary, this study proposes a multimodal sensing strategy based on multi-ligands modified plasmonic Ag NPs for reliable and highly sensitive detection of As(III) in natural water and juice samples. The results indicate that multi-ligands can vastly improve the selectivity toward the target As(III). More importantly, the colorimetric assay of solution phase based on the target induced aggregation of Ag NPs can be converted into the electrode surface-accumulated electrochemical analysis. With target-induced increasing amounts of Ag NPs gathered onto the electrode surface, the formation of Ag NPs network structure can be used as electrochemical redox probes for signal amplification, significantly improving the sensitivity of target arsenite detection. The results clearly indicate that target-induced the aggregation of multi-ligands functionalized Ag NPs can be used as multifunctional colorimetric, spectrophotometric and electrochemical sensing elements, which can effectively improve the selectivity and sensitivity. The present strategy provides a facile, high-sensitive, on-site and low-cost way for practical application of the sensor toward toxic As(III) in environmental water 28


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and juice samples.

ASSOCIATED CONTENT Supporting Information Additional information as noted in text. Guidelines of metal ions in drinking water regulated by China and WHO; Zeta potentials of different Ag NPs; Stability of the Ag NPs; Schematic of the chemical reactions between As(III) and Asn, DTT, or GSH; Optimization of ligands and pH; Optimization of reaction time; Cyclic voltammetric curves; Typical CVs and Nyquist plots for the different electrodes; Current responses of the gold electrodes with different incubation ways; Comparison of the analytical performance of different methods.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21675078) and the Jiangxi Province Natural Science Foundation (20165BCB18022). Notes The authors declare no competing financial interest.

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Accumulation:

Conversion

of

Liquid-Phase

Colorimetric

Assay

into

Enhanced

Surface-Tethered Electrochemical Analysis. J. Am. Chem. Soc. 2015, 137 (28), 8880-8883. 50. Singh, P.; Parent, K. L.; Buttry, D. A., Electrochemical Solid-State Phase Transformations of Silver Nanoparticles. J. Am. Chem. Soc. 2012, 134 (12), 5610-5617.

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Page 34 of 34

For Table of Contents Use Only

Synopsis

Multimodal

assay

of

arsenite

in

environmental

samples

through

stimuli-response of multi-ligands modified silver nanoparticles as multifunctional probe.

34


Multimodal Assay of Arsenite Contamination in Environmental

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