Valence States Modulation Strategy for Picomole Level Assay of Hg2+

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Valence States Modulation Strategy for Picomole Level Assay of Hg in Drinking and Environmental Water by Directional Self-Assembly of Gold Nanorods 2+

Lu Chen, Linlin Lu, Sufan Wang, and Yunsheng Xia ACS Sens., Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

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Valence States Modulation Strategy for Picomole Level Assay of Hg2+ in Drinking and Environmental Water by Directional Self-Assembly of Gold Nanorods Lu Chen, Linlin Lu, Sufan Wang, and Yunsheng Xia* Key Laboratory of Functional Molecular Solids, Ministry of Education; College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, China.

ABSTRACT: In this study, we present a valence states modulation strategy for picomole level assay of Hg2+ using directional self-assembly of gold nanorods (AuNRs) as signal readout. Hg2+ ions are first controllably reduced to Hg+ ions by appropriate ascorbic acid, and the reduced Hg+ ions react with the tips of the pre-added AuNRs and form gold amalgam. Such Hg+ decorated AuNRs then end-to-end self-assemble into one-dimensional architectures by the bridging effects of lysine based on the high affinity of NH2-Hg+ interactions. Correspondingly, the AuNRs’ longitudinal surface plasmon resonance is gradually reduced and a new broad band appears at 900-1100 nm region simultaneously. The resulting distinctly ratiometric signal output is not only favorable for Hg2+ ions detection but competent for their quantification. Under optimal conditions, the linear range is 22.8 pM to 11.4 nM, and the detection limit is as low as 8.7 pM. Various transition/heavy metal ions, such as Pb2+, Ti2+, Co2+, Fe3+, Mn2+, Ba2+, Fe2+, Ni2+, Al3+, Cu2+, Ag+, Au3+ do not interfere with the assay. Because of ultrahigh sensitivity and excellent selectivity, the proposed system can be employed for assaying ultra-trace of Hg2+ containing in drinking and commonly environmental water samples, which is difficult to be achieved by conventional colorimetric systems. These results indicate that the present platform possesses specific advantages and potential applications in the assay of ultra-trace amounts of Hg2+ ions.

KEYWORDS: Hg2+ ions; Gold nanorods; Self-assembly; Ratiometric sensing; Valence States Modulation

Mercury, one of the most toxic heavy metal elements, is not biodegradable and tends to accumulate in living organisms. It can lead to serious damage to many organs such as kidney, 1

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brain, and deleterious effects on human body in the forms of tremors, vegetative nerve functional disturbance, etc.1,2 In order to avoid/decrease mercurialism, the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) define that the maximum allowable level of inorganic mercury in drinking water is no more than 30 and 10 nM, respectively.3,4 Thus, great efforts have been devoted to explore various approaches for monitoring ionic mercury (Hg2+), the most stable and widespread inorganic form in environment and organisms.5 Currently, atomic absorption spectrometry, inductively coupled plasma mass spectrometry and atomic fluorescence spectrometry (AFS) are widely used for the detection of Hg2+.6-9 However, these techniques are expensive and necessitate great instrumentations, which is difficult to be popularized, especially for remote places. Therefore, it is significant to develop rapid, cost-effective and easy techniques that can be used to sense trace amounts of Hg2+ ions in variety of water samples. In recent years, gold nanoparticles (AuNPs) have been well employed as colorimetric reporters for sensing of various target analytes including Hg2+ ions, due to that AuNPs possess particle distance dependent plasmonic band, very high extinction coefficient, versatile surface modification, as well as naked eye-distinguishable readout.10-13 For example, a variety of thymine (T)-containing DNA/DNAzyme functionalized AuNPs has been well designed and employed for assaying Hg2+ based on the analytes modulated T-T mismatch configuration.14,15 In addition to DNA, other molecules including amino acids, proteins, aptamers, etc. have also be adopted as modifying/binding agents in the fabrication of AuNPs based Hg2+ sensors.16-19 Despite these substantial achievements, there are a few issues and even potential problems still be concerned. First of all, most of the used AuNPs are spherical or quasi-spherical, and the signal readout is based on analytes modulated AuNP dispersion/aggregation. This strategy is simple and possesses 2


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distinct color change (wine ↔ purple). However, on the one hand, the isotropous surface of nanospheres would evenly interact with analytes. So, more analytes are needed to alter the particle surface properties (surface structures, charges, and so on) and cause the transformation of NPs’ aggregative states, which is obviously against higher sensitivity. On the other hand, the random aggregation is not highly specific. It is known many other factors (pH, ionic strength, non-specific complexation, etc.) can also affect AuNP stability. Then, during the surface modification, the multi-step centrifugation purification processes not only are costly and tedious, but often lead to irreversible agglomeration of the NPs.20 So, it is desirable to develop specific, highly sensitive, cost-effective AuNPs based colorimetric platforms for Hg2+ assay. Gold nanorods (AuNRs) are one of the most extensively studied quasi one-dimensional metal NPs due to their special optical properties and various potential applications. Especially, AuNRs synthesized by colloidal method possess anisotropic surface properties. Their side face is densely capped by a double layer of cetyltrimethyl ammonium bromide (CTAB) molecules and the tips are only less capped, which causes that most of the AuNRs’ surface is inert and only very limited two tips are active. As a result, the sensing systems based on AuNR tips’ reactions/interactions possess intrinsic high sensitivity.21 Previously, Andres et al. proposed an etching method for sensing of Hg2+ ions using unmodified AuNRs as reporter.22 With the assistance of NaBH4, Hg2+ ions can well etch and shorten the AuNRs, which causes the dramatic blue shift of the AuNRs’ longitudinal SPR band. Then, Paul and co-workers presented an improved approach for the assay Hg2+ ions by combination of the above NaBH4 etching and subsequent self-assembly.23 Due to self-assembly induced coupling effect, the SPR signal output is amplified and results in a higher sensitivity. However, NaBH4 itself can also shorten the AuNRs even without Hg2+ ions.22 Furthermore, 3


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NaBH4 is not stable and tends to decompose at ambient conditions. In terms of the improved etching-assembly method, in addition to the inherent problems of NaBH4, the assembly is uncontrollable (end-to-end (EE), side-by-side self-assemblies, and even random aggregation proceed simultaneously), because it is modulated by non-specific electrostatic interactions (triggered by PO43- anions). Conceivably, these problems would cause the confusion in the sensing, especially for extremely low concentrations of Hg2+ ions. Herein we present a simple but precise sensing system for Hg2+ ions with picomole level sensitivity using well-defined EE self-assembly of AuNRs as signal readout. The assay principle is based on our discovery of the unique chemical properties of mercurous ions (Hg+). Different from Hg2+ and Hg0, Hg+ ions, on the one hand, can react with Au and form gold amalgam; on the other hand, have high affinity with NH2 groups by coordination effect. In the assay processes (Scheme 1), Hg2+ ions are first controllably reduced to Hg+ ions by appropriate ascorbic acid (AA), and the resulting Hg+ ions react with the tips of the pre-added AuNRs and form gold amalgam, which then EE self-assembly each other by bridging effects of lysine based on the high affinity of NH2-Hg+ interactions. Correspondingly, the AuNRs’ longitudinal SPR is gradually reduced and a new SPR band appears at 900-1100 nm region simultaneously. The resulting distinctly ratiometric signal output is not only favorable for Hg2+ ions detection but competent for their quantification. Under optimal conditions, the linear range is 22.8 pM to 11.4 nM, and the detection limit is as low as 8.7 pM. Various transition/heavy metal ions, such as Pb2+, Ti2+, Co2+, Fe3+, Mn2+, Ba2+, Fe2+, Ni2+, Al3+, Cu2+, Ag+, Au3+ do not interfere with the assay. Because of ultrahigh sensitivity and excellent selectivity, the proposed system can be employed for assaying ultra-trace of Hg2+ in drinking and river water samples, which is difficult to be achieved by conventional colorimetric 4


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systems. These results indicate that the present platform possesses specific advantages and potential applications in the assay of ultra-trace amounts of Hg2+ ions.

EXPERIMENTAL SECTION Instruments and Characterizations. The absorption spectra were recorded with a Hitachi-U-3010 spectrometer. Characterizations of scanning electron microscopy (SEM) was carried out on Hitachi S-4800 under the accelerating voltage of 5 kV. All pH values were measured with a model pHs-3c meter. High-resolution TEM (HRTEM) was carried out on Tecnai G2 F20 U-Twin (FEI) under the accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed by an ESCALAB 250 Xi XPS system of Thermo Scientific, where the analysis chamber was 1.5 × 10−9 mbar and the X-ray spot was 500 µm. AFS was

.

measured by using SuoKun SK series AFS instrument

Materials. HAuCl4·3H2O was purchased from Sigma-Aldrich. CTAB, AA, NaBH4, AgNO3, HCl were acquired from Shanghai Reagent Company. Mg(NO3)2·6H2O, Pb(NO3)2, Ti(SO4)2, CoCl2·6H2O, FeCl3·6H2O, MnCl2·4H2O, BaCl2·2H2O, FeSO4·7H2O, NiCl2·6H2O, AlCl3·6H2O, CuCl2·2H2O, CdCl2·2.5H2O, KCl, CaCl2, Na2SO4, NaHCO3, KNO3, NaCl, HgCl2 were acquired from Shanghai Chemical Reagent Co. All solutions were prepared with double deionized water. Synthesis and Purification of the AuNRs. AuNRs were prepared by our developed “large volume” synthesis system21, which is a modified seeded growth method.24-26 Specifically, the seed solution was first made by the addition of 0.01 M HAuCl4 solution (0.25 mL) into 0.1 M CTAB solution (9.75 mL) in a 50 mL erlenmeyer flask. A freshly prepared, ice-cold 0.01 M NaBH4 solution (0.6 mL) was then injected quickly into the mixture solution, followed by magneton rapid 5


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inversion for 2 min. The resultant seed solution was kept at 30 °C for 2 h before use. To grow AuNRs, 0.01 M HAuCl4 (7.5 mL) and 0.01 M AgNO3 (1.2 mL) were first mixed with 0.1 M CTAB (150 mL) in a 250 mL plastic tube. 1.0 M HCl (3 mL) was then added for adjusting the pH of the growth solution, followed by the addition of 0.1 M ascorbic acid (1.2 mL). After the growth solution was mixed by inversion, the CTAB-stabilized seed solution (0.21 mL) was rapidly injected. The resultant solution was gently mixed for 10 s and left undisturbed 12 h in 30 °C water-bath. For purification of the AuNRs, 5 mL of the AuNRs sample was collected by centrifugation at 8000 r/min for 10 min, and it was further washed two times by water. The obtained precipitate was re-dispersed in water, and the AuNRs’ concentration was estimated by Lambert-Beer Law.27 Procedures for Hg2+ Sensing. AuNRs (69 pM, 2.0 mL) and different concentrations of Hg2+ (200 µL), AA (0.01 M, 30 µL), phosphate buffer solution( PBS, 2 mM, 200 µL) were orderly added into a series of 5 mL colorimetric tubes. 5 minutes later, lysine (9 mM, 200 µL) solution was introduced. After 20 min incubation, the mixed solutions were transferred separately into a 1 cm quartz cuvette, and their absorption spectra were recorded at ambient conditions. Procedures for Hg2+ Sensing in Real Samples. Tap water (from Lab) and river water (from Yangtze River) samples were first filtrated three times using 0.22 µm filters, respectively. Then, to avoid the potential interference of Fe3+, F- ions were pre-added as masking agents (In 10 mL various water samples, NaF solutions (0.1 M, 10 µL) were added, and the final concentrations of F- ions were 100 µM). Third, for the measurements of the recovery rates, the pre-treated water samples were added a few Hg2+ standard solutions (In 3 mL various pre-treated water samples, 10 and 20 µL of Hg2+ standard solutions (0.75 µM) were introduced, and the final concentrations of 6


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the added Hg2+ were 2.5 and 5.0 nM, respectively.). During the sensing, 0.525 mL of various water samples, AuNRs (82 pM, 1.675 mL), AA (0.01 M, 30 µL), PBS (2 mM, 200 µL) were orderly added into a series of 5 mL colorimetric tubes. 5 minutes later, lysine (9 mM, 200 µL) solution was introduced. 20 min later, the mixed solutions were transferred separately into a 1 cm quartz cuvette, and their absorption spectra were recorded at ambient conditions.

RESULTS AND DISCUSSION The Mechanism on Valence States Modulated Directional Self-Assembly of Gold Nanorods for Hg2+ Assay. As shown in Figure 1A, the introduction of Hg2+ ions (7.6 µM) has almost no any effects on both the absorption profile and morphology of the AuNRs, which probably indicates that the two species do not interact with each other. In contrast, in the Hg2+-AuNRs solution, the addition of some reducing agent, namely AA (0.114 mM), causes a slight hypochromatic shift (from 744 to 733 nm) of the AuNRs’ longitudinal SPR peak, although their length and morphology are almost invariable (Figures 1E and 1F). In terms of the resulting products, XPS characterization (Figure 1G) indicates some Hg+ and Hg0 are deposited onto the AuNRs, and the former is dominant (~ 70 %). Conceivably, both Hg+ and Hg0 should result from the pre-added Hg2+ ions, which are reduced by AA and then deposited on the AuNRs. To investigate the interactions of Hg+/Hg0 and the AuNRs, such reduced Hg2+-AuNRs (in the presence of 0.114 mM of AA) system was characterized by high resolution transmission electron microscopy (HRTEM). As shown in Figure 2, the as-prepared AuNRs exhibit highly consistent lattice fringe, indicating their monocrystalline structure. Furthermore, the original products possess the lattice spacing distance with 0.206 nm 7


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throughout the AuNRs, which corresponds to (200) facet of gold materials. After addition of Hg2+ ions and moderate reduction by appropriate AA (0.114 mM), the morphology (Figure 1E) and the crystal lattice (Figure 2B) of the AuNRs keep almost unchanged. However, the lattice spacing distance at the AuNRs’ tips slightly increase from 0.206 to 0.211 nm, which should be attributed to the formation of gold amalgam by the deposition of Hg+ (Hg0) on the AuNRs.28 Due to higher activity, such interactions selectively occur at AuNRs’ tips, which facilitates to higher sensitivity for Hg2+ assay (see below). As the dosage of the used AA increases to 0.570 mM, more Hg0 atoms (76.19 %) deposition is observed (Figure 1K). At the same time, the longitudinal SPR peak shift from 744 to 716 nm, and the AuNRs’ length is decreased from 53 to 47 nm. According to literature, the decrease of the AuNRs’ length with the hypochromatic shift of the SPR peak results from the strong interactions of Hg0 and the AuNRs.29 In terms of the above three reaction systems, their interactions with lysine molecules (one of bridging agents due to its two NH2 groups at both ends of the molecule) were further investigated, respectively. As shown in Figure 3A and 3E, for unreduced (without AA) and sufficiently reduced (in the presence of 0.570 mM of AA) Hg2+-AuNRs systems, the absorption profiles of the AuNRs keep changeless with the addition of lysine (0.684 mM). Interestingly, the moderately reduced (0.114 mM AA) one shows a remarkable modulation of the AuNRs’ SPR profiles: the intensity of the longitudinal peak at 744 nm well decreased, and a new broad band appears at longer wavelength (900-1100 nm). As demonstrated by SEM (Figure 3D), such evolution of the absorption results from the EE assembly of the AuNRs. Although lysine modulated aggregation/assembly of (gold NPs-Hg2+) systems have been reported previously,30,31 the exactly chemical mechanism has remained an open question. Herein, in 8


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addition to XPS and HRTEM detection, Density functional theory (DFT) calculations were further performed to better understand the self-assembly modulation behaviors. As we know, Hg is one of typical soft metal elements, which can interact with Au and form gold amalgam. Furthermore, Hg salts have high affinity with some soft Lewis basic species, such as amino and thiol groups. Because the exact properties of metal elements closely related to their valence electrons, we first employ DFT to calculate the interactions of differently valent Hg element with Au atoms and amino groups, respectively. As shown in Table 1, the binding energies of Au0-Hg0 and Au0-Hg+ are 10.46 and 52.71 kcal/mol, while Hg2+ cannot effectively bond with Au0. On the other hand, the binding energies of Hg0, Hg+, and Hg2+ with NH2 are 1.677, 49.85 and 162.3 kcal/mol, respectively. These results indicate that mercurous ions (Hg+) can interact with both Au0 and NH2 groups. Based on the experimental results and DFT calculations, the present lysine modulated AuNR EE self-assembly can be understood as: in the presence of moderately reductive conditions, Hg2+ ions are mainly reduced to Hg+ ions and then deposit on the tips of the pre-added AuNRs; then, the added lysine molecules interact with the deposited Hg+ ions by NH2-Hg+ coordination effect, which causes the EE self-assembly of the AuNRs (Scheme 1B). Because the interaction of Hg0 and NH2 is too weak, the AuNRs in the sufficiently reductive system cannot be induced to assemble each other by lysine molecules. Factors Affecting Hg2+ Sensing. Conceivably, the present lysine modulated self-assembly of AuNRs has potential application for Hg2+ sensing. For better analytical performances, the experimental conditions were optimized. First, as described above, the amounts of the used AA are critical for the assembly modulation. Too few AA cannot effectively reduce Hg2+, while too many reducing agents would directly reduce Hg2+ to Hg0. We found that the major products were 9


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mercurous ions in the presence of 0.114 mM AA, which was employed for the assay. Then, the reaction time for the AA reduction was investigated. As shown in Figures 1D, 0.114 mM of AA can lead to a slight blue shift (~ 11 nm) of the AuNRs’ longitudinal SPR band during moderate reduction, which is employed for the investigation of the reduction time. It is found that such blue shift can be accomplished within 5 min (Figure 4A). As a result, a 5 min reduction time was used. Third, the assembly behaviors were studied at pH 6.0-8.0. As shown in Figure 4B, distinct EE assembly signals are only observed as pH > 6.5. The reason is that: at lower pH values, NH2 groups will protonize and form NH3+ cations, which hinders their binding with Hg+ ions. So, a neutral condition (pH 7.0) was chosen for the assay. Third, the dynamics of the lysine modulated AuNR EE assembly was studied. As shown in Figure 4C, the EE self-assembly goes faster in the first 10 min, and then gradually reaches an equilibrium after 20 min reaction. Based on this observation, 20 min was chosen for the lysine modulated EE assembly. Finally, we studied the effects of salt concentration on the responses of Hg2+ ions. As we know, in terms of T-Hg-T based assay systems, the coexisting anions can strongly affect and even disturb the assay.32 This problem was then solved by chemical reaction based methods.33 As shown in Figure S2, the AuNRs can well disperse as NaCl concentrations below to 8 mM, and higher-salt medium leads to the aggregation of the AuNRs. As a result, the responses of Hg2+ were investigated in the presence of 0-8 mM NaCl. As described in Figure 4D, the coexisting salt has very limited effects on the Hg2+ modulated AuNRs EE assembly, which suggests that the present assay system possess rather robust signal readout. Analytical Performances for Hg2+ Ions Assay. Under the optimal conditions discussed above, the EE assembly of the AuNRs vs. the concentrations of Hg2+ ions was measured. As shown in 10


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Figure 5A, the SPR absorption intensities of the AuNRs at 744 nm gradually decrease as the concentration of the analytes is increased; while the change of the SPR band at 900-1100 nm is just reversed. It should be noted that the SPR band shows a distinct EE assembly behavior even the concentration of Hg2+ is as low as 22.8 pM. Then, the transverse SPR band at 510 nm is kept well, which demonstrates that the AuNRs do not randomly aggregate in the sensing. The ratio of the absorbance values at 1030 and 744 nm, namely, A1030 nm/A744 nm, linearly increases as Hg2+ concentrations change from 22.8 pM to 11.4 nM (R = 0.993). The detection limit can low to 8.7 pM. Generally, the sensitivity for Hg2+ assay often ranges from micromole to nanomole levels, using spherical AuNPs as reporters.31,34-48 In contrast, AuNRs possess anisotropic morphology and surface chemical properties, and the resulting very limited active sites and directional EE self-assembly behaviors facilitates to a higher sensitivity. Compared with previous AuNRs based Hg2+ assay systems,22,23 the proposed valence states modulation based strategy not only well retains a higher sensitivity but provides a more robust signal readout (The present assay system avoids

both

extremely

strong

reducing

agent

and

salts

induced

non-specific

self-assembly/aggregation). To assess the selectivity of the present method for Hg2+ ions, the effects of common transition/heavy metal ions, including Pb2+, Ti2+, Co2+, Fe3+, Mn2+, Ba2+, Fe2+, Ni2+, Al3+, Cu2+, Ag+, Au3+, on the sensing system were first examined. As shown in Figure 6A, these cations have very little effect on the sensing system at micromole level, even for their mixture. As we know, some rather soft cations (such as Cu2+, Cd2+, Ag+, and so on) can also interact with NH2 groups by coordination effect. Herein, to avoid their potential interference, a higher concentration (0.684 mM) of lysine was employed. We then investigated the interference of eight common ions in 11


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environmental water samples. As shown in Figure 6B, these ions have very little effects on the absorption of the AuNRs. These data indicate that the proposed method has excellent selectivity for Hg2+ sensing. Real Sample Assay. To evaluate the applicability of the present assay to real samples, various water samples, namely tap water and river water were tested, respectively. Although micromole level of Fe3+ ions do not interfere with the assay (Figure 6A), higher concentration ones can cause the aggregation of the AuNRs. To avoid the potential interferences, masking agents of F- ions were pre-added. As shown in Figure 7 and Figure S3 in Supporting Information, the AuNRs’ SPR profile keep almost invariable in the presence of the two water samples, indicating that the coexisting ions and other substances do not interfere with the assay. After conducting AA reduction and lysine bridging processes, different results were observed for the two water samples. In terms of the (AuNRs-tap water) system, the AuNRs’ band keeps almost invariable (green curve, Figure 7A), especially, the SPR band at 900-1100 nm completely overlaps with that of the dispersed AuNRs. While for the (AuNRs-river water) one, the AuNRs’ SPR band exhibits an observable change with the characteristic of AuNR EE assembly behaviors: namely the longitudinal SPR band decreases and the band at 900-1100 nm increases (green curve, Figure 7B). These experimental data indicate that the proposed system can probably detect ultratrace Hg2+ (~1-2 nM) in the water sample from Yangtze River. To demonstrate its validity, AFS measurements for the two water samples were further performed. As shown in Table 2, for the two samples, the concentrations of Hg2+ determined by our method and AFS are in agreement with each other on the whole. It should be noted that the present valence states modulation strategy can directly detect ultra-trace “intrinsic” (non-spiked) Hg2+ ions in environmental water samples, 12


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which is difficult to be achieved by previous AuNPs based colorimetric systems (Table S2 in Supporting Information). Finally, recovery tests of standard addition method show that their recovery rates range from 92.0 % to 112.0 %. Due to excellent analytical performance (high sensitivity and selectivity), the proposed system is promising for monitoring Hg2+ content in drinking water, even with regard to the standard of U.S. EPA.

CONCLUSION In summary, we herein presented a AuNR directional EE assembly system for assaying Hg2+ ions at picomole level by means of the unique chemical properties of mercury ions. Compared with previous gold NP based sensing systems, the proposed one is not simpler and more cost-effective, but possesses more precise and reliable signal output. Due to high sensitivity and excellent anti-interference capacity, it can quantify Hg2+ content even when their concentrations are as low as 10 nM in various water samples, demonstrating its potential corresponding applications.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: The following files are available free of charge. Figure S1: The absorption spectra, SEM and size distribution of the as prepared AuNRs. Figure S2: The absorption spectra of the as prepared AuNRs dispersed in different concentrations of NaCl solution. Figure S3: The absorption spectra of the as prepared AuNRs dispersed in double deionized water, tap water and river water. Table S1: The DFT calculation results. Table S2: Comparing the analytical performances of Hg2+ sensing between the present method and previous AuNPs based systems. 13


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AUTHOR INFORMATION Corresponding Author *Fax: +86-553-3869303. Phone: +86-553-3869303. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21422501) and Foundation for Innovation Team of Bioanalytical Chemistry.

REFERENCES (1) Onyido, I.; Norris, A. R.; Buncel, E. Biomolecule-mercury interactions: modalities of DNA base-mercury binding mechanisms. remediation strategies. Chem. Rev. 2004, 104, 5911–5929. (2) Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. Review: environmental exposure to mercury and its toxicopathologic implications for public health. Environ. Toxicol 2003, 18, 149−175. (3) Burin, G. J.; Becking, G. C. The World-Health-Organization (WHO) guidelines for drinking-water quality: a global perspective on trace contaminants of drinking-water. Trace Substances in Environmental Health－XXIV 1991, 207−219. (4) Aragay, G.; Pons, J.; Merkoci, A. Recent trends in macro-, micro-, ̧and nanomaterial-based tools and strategies for heavy-metal detection. Chem. Rev. 2011, 111, 3433−3458. (5) Lin, Y. H.; Tseng, W. L. Ultrasensitive sensing of Hg2+ and CH3Hg+ based on the fluorescence 14


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quenching of lysozyme type vistabilized gold nanoclusters. Anal. Chem. 2010, 82, 9194−9200. (6) Hsu, I. H.; Hsu, T. C.; Sun, Y. C. Gold-nanoparticle-based graphite furnace atomic absorption spectrometry amplification and magnetic separation method for sensitive detection of mercuric ions. Biosens. Bioelectron. 2011, 26, 4605–4609. (7) Erxleben, H.; Ruzicka, J. Atomic absorption spectroscopy for mercury, automated by sequential injection and miniaturized in lab-on-valve system. Anal. Chem. 2005, 77, 5124−5128. (8) Hong, Y. S.; Rifkin, E.; Bouwer, E. Combination of diffusive gradient in a thin film probe and IC-ICP-MS for the simultaneous determination of CH3Hg+ and Hg2+ in toxic water. J. Environ. Sci. Technol. 2011, 45, 6429–6436. (9) Yuan, C. G.; Lin, K.; Chang, A. Determination of trace mercury in environmental samples by cold vapor atomic fluorescence spectrometry after cloud point extraction. Microchim Acta 2010, 171, 313–319. (10) Wang, Z.; Ma, L. Gold nanoparticle probes. Coord. Chem. Rev. 2009, 253, 1607−1618. (11) Du, J.; Zhu, B.; Peng, X.; Chen, X. Optical reading of contaminants in aqueous media based on gold nanoparticles. Small 2014, 10, 3461−3479. (12) Song, Y.; Wei, W.; Qu, X. Colorimetric biosensing using smart materials. Adv. Mater. 2011, 23, 4215−4236. (13) Saha, K.; Agasti, S. S.; Kim, C.; Li, X.; Rotello, V. M. Gold nanoparticles in chemical and biological sensing. Chem. Rev. 2012, 112, 2739−2779. (14) Tanaka, Y.; Oda, S.; Yamaguchi, H.; Kondo, Y.; Kojima, C.; Ono, A.

15

N-15N J-coupling

across HgII: direct observation of HgII-mediated T-T base pairs in a DNA duplex. J. Am. Chem. Soc. 2007, 129, 244–245. 15


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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

Page 16 of 30

(15) Xue, X.; Wang, F.; Liu, X. One-step, room temperature, colorimetric detection of mercury (Hg2+) using DNA/nanoparticle conjugates. J. Am. Chem. Soc. 2008, 130, 3244–3245. (16) Chai, F.; Wang, C.; Wang, T.; Ma, Z.; Su, Z. L-cysteine functionalized gold nanoparticles for the colorimetric detection of Hg2+ induced by ultraviolet light. Nanotechnology 2010, 21, 1–6. (17) Guan, J.; Wang, Y. C.; Gunasekaran, S. Using L-arginine-functionalized gold nanorods for visible detection of mercury(II) ions. J. Food Sci. 2015, 80, 828–833. (18) Wegner, S. V.; Okesli, A.; Chen, P.; He, C. Design of an emission ratiometric biosensor from merR family proteins: a sensitive and selective sensor for Hg2+. J. Am. Chem. Soc. 2007, 129, 3474–3475. (19) Ye, B. C.; Yin, B. C. Highly sensitive detection of mercury(II) ions by fluorescence polarization enhanced by gold nanoparticles. Angew. Chem. Int. Ed. 2008, 47, 8386−8389. (20) Medintz, I. L.; Berti, L.; Pons, T.; Grimes, A. F.; English, D. S.; Alessandrini, A.; Facci, P.; Mattoussi, H. A reactive peptidic linker for self-assembling hybrid quantum dot-DNA bioconjugates. Nano Lett. 2007, 7, 1741−1748. (21) Lu, L.; Xia, Y. Enzymatic reaction modulated gold nanorod end-to-end self-assembly for ultrahigh sensitively colorimetric sensing of cholinesterase and organophosphate pesticides in human blood. Anal. Chem. 2015, 87, 8584–8591. (22) Rex, M.; Hernandez, F. E.; Campiglia, A. D. Pushing the limits of mercury sensors with gold nanorods. Anal. Chem. 2006, 78, 445–451. (23) Huang, H.; Qu, C.; Liu, X.; Huang, S.; Xu, Z.; Zhu, Y.; Chu, P. K. Amplification of localized surface plasmon resonance signals by a gold nanorod assembly and ultra-sensitive detection of mercury. Chem. Commun. 2011, 47, 6897–6899. 16


Page 17 of 30

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

ACS Sensors

(24) Nikoobakht, B.; El-Sayed, M. A. Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method. Chem. Mater. 2003, 15, 1957−1962. (25) Xia, Y.; Song, L.; Zhu, C. Turn-on and near-infrared fluorescent sensing for 2,4,6-trinitrotoluene based on hybrid (gold nanorod)-(quantum dots) assembly. Anal. Chem. 2011, 83, 1401−1407. (26) Song, L.; Wang, S.; Kotov, N. A.; Xia, Y. Nonexclusive fluorescent sensing for l/d enantiomers enabled by dynamic nanoparticle-nanorod assemblies. Anal. Chem. 2012, 84, 7330−7335. (27) Orendorff, C. J.; Murphy, C. J. Quantitation of metal content in the silver-assisted growth of gold nanorods. J. Phys. Chem. B 2006, 110, 3990−3994. (28) Chaudhary, A.; Dwivedi, C.; Chawla, M.; Gupta, A.; Nandi, C. K. Lysine and dithiothreitol promoted ultrasensitive optical and colorimetric detection of mercury using anisotropic gold nanoparticles. J. Mater. Chem. C 2015, 3, 6962–6965. (29) Wang, J. G.; Fossey, J. S.; Li, M.; Xie, T.; Long, Y. T. Real-time plasmonic monitoring of single gold amalgam nanoalloy electrochemical formation and stripping. ACS Appl. Mat. Interfaces 2016, 8, 8305–8314. (30) Wang, X. X.; Liu, J. M.; Jiang, S. L.; Jiao, L.; Lin, L. P.; Cui, M. L.; Zhang, X. Y.; Zhang, L. H.; Zheng, Z. Y. Non-aggregation colorimetric sensor for detecting vitamin C based on surface plasmon resonance of gold nanorods. Sens. Actuators, B 2013, 182, 205–210. (31) Sener, G.; Uzun L.; Denizli, A. Lysine-promoted colorimetric response of gold nanoparticles: a simple assay for ultrasensitive mercury(II) detection. Anal. Chem. 2014, 86, 514−520. (32) Kiy, M. M.; Zaki, A.; Menhaj, A. B.; Samadi, A.; Liu, J. Dissecting the effect of anions on 17


ACS Sensors

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

Page 18 of 30

Hg2+ detection using a FRET based DNA probe. Analyst 2012, 137, 3535−3540. (33) Huang, P. J. J.; Wang, F.; Liu. J. Cleavable molecular beacon for Hg2+ detection based on phosphorothioate RNA modifications. Anal. Chem. 2015, 87, 6890−6895. (34) Lin, C. Y.; Yu, C. J.; Lin, Y. H.; Tseng, W. L. Colorimetric sensing of silver(I) and mercury(II) ions based on an assembly of tween 20-stabilized gold nanoparticles. Anal. Chem. 2010, 82, 6830−6837. (35) Li,

D.;

Wieckowska,

A.;

Willner,

I.

Optical

analysis

of

Hg2+

ions

by

oligonucleotide-gold-nanoparticle hybrids and DNA-based machines. Angew. Chem. Int. Ed. 2008, 47, 3927−3931. (36) Ding, N.; Zhao, H.; Peng, W.; He, Y.; Zhou, Y.; Yuan, L.; Zhang, Y. A simple colorimetric sensor based on anti-aggregation of gold nanoparticles for Hg2+ detection. Colloids Surf., A 2012, 395, 161−167. (37) Du, J.; Yin, S.; Jiang, L.; Ma, B.; Chen, X. A colorimetric logic gate based on free gold nanoparticles and the coordination strategy between melamine and mercury ions. Chem. Commun. 2013, 49, 4196−4198. (38) Annadhasan, M.; Muthukumarasamyvel, T.; Badu V. R. S.; Rajendiran, N. Green synthesized silver and gold nanoparticles for colorimetric detection of Hg2+, Pb2+, and Mn2+ in aqueous medium. ACS Sustain. Chem. Eng. 2014, 2, 887−896. (39) Chen, X.; Zu, Y.; Xie, H.; Kemas, A. M.; Gao, Z. Coordination of mercury(II) to gold nanoparticle associated nitrotriazole towards sensitive colorimetric detection of mercuric ion with a tunable dynamic range. Analyst 2011, 136, 1690−1696. (40) You, J.; Hu, H.; Zhou, J.; Zhang, L.; Zhang, Y.; Kondo, T. Novel cellulose 18


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polyampholyte-gold nanoparticle-based colorimetric competition assay for the detection of cysteine and mercury(II). Langmuir 2013, 29, 5085−5092. (41) Liu, D.; Qu, W.; Chen, W.; Zhang, W.; Wang, Z.; Jiang, X. Highly sensitive, colorimetric detection of mercury(II) in aqueous media by quaternary ammonium group-capped gold nanoparticles at room temperature. Anal. Chem. 2010, 82, 9606−9610. (42) Wang, Y.; Yang, F.; Yang, X. Colorimetric biosensing of mercury(II) ion using unmodified gold nanoparticle probes and thrombin-binding aptamer. Biosens. Bioelectron. 2010, 25, 1994−1998. (43) Chansuvarn, W.; Imyim, A. Visual and colorimetric detection of mercury(II) ion using gold nanoparticles stabilized with a dithia-diaza ligand. Microchim. Acta 2012, 176, 57−64. (44) Hung, Y. L.; Hsiung, T. M.; Chen, Y. Y.; Huang, Y. F.; Huang, C. C. Colorimetric detection of heavy metal ions using label-free gold nanoparticles and alkanethiols. J. Phys. Chem. C 2010, 114, 16329−16334. (45) Li, Y.; Wu, P.; Xu, H.; Zhang, Z.; Zhong, X. Highly selective and sensitive visualizable detection of Hg2+ based on anti-aggregation of gold nanoparticles. Talanta 2011, 84, 508−512. (46) Lou, T.; Chen, L.; Zhang, C.; Kang, Q.; You, H.; Shen, D.; Chen, L. A simple and sensitive colorimetric method for detection of mercury ions based on anti-aggregation of gold nanoparticles. Anal. Methods 2012, 4, 488−491. (47) Lou, T.; Chen, Z.; Wang, Y.; Chen, L. Blue-to-red colorimetric sensing strategy for Hg2+ and Ag+ via redox-regulated surface chemistry of gold nanoparticles. ACS Appl. Mat. Interfaces 2011, 3, 1568−1573. (48) Zhang, F.; Zeng, L.; Yang, C.; Xin, J.; Wang, H.; Wu, A. A one-step colorimetric method of 19


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analysis detection of Hg2+ based on an in situ formation of Au@HgS core-shell structures. Analyst 2011, 136, 2825−2830.

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Scheme 1. Schematic Illustration of the Valence States Modulation Based Strategy for Assay of Hg2+ by Directional Self-Assembly of AuNRs.

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Figure 1. (A) The interactions of the AuNRs with Hg2+ ions at different reducing conditions. For A, B and C, in the absence of AA. For D, E, F and G, in the presence of appropriate AA (0.114 mM). For H, I, J, and K, in the presence of excess AA (0.570 mM). The four rows are absorption spectra (A, D, H), SEM (B, E, I), histograms of the AuNRs’ length and XPS detection. The concentrations of the AuNRs and Hg2+ are 52 pM and 7.6 µM, respectively.

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Figure 2. (A) HRTEM image of the AuNRs. (B) HRTEM image of the (AuNRs–Hg2+) reduced by 0.114 mM of AA. The concentrations of the AuNRs and Hg2+ are 52 pM and 7.6 µM, respectively.

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Figure 3. (A) The unreduced (A, B), moderately reduced (C, D) and sufficiently reduced (E, F) (AuNRs-Hg2+ ions) systems in the presence of lysine. The two rows are absorption spectra (A, C, E) and SEM images (B, D, F). The concentrations of the added AA for the three systems are 0 (A, B), 0.114 mM (C, D), and 0.570 mM (E, F), respectively. The concentrations of the AuNRs, Hg2+, and lysine are 52 pM, 7.6 µM, and 0.684 mM, respectively.

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Figure 4. (A) Effects of reaction time on the AA (0.114 mM) reduction. (B) Effects of pH on the lysine induced AuNR EE self-assembly. (C) Effects of reaction time. (D) Effects of salt concentrations on the lysine induced AuNR EE self-assembly. The concentrations of the AuNRs, AA, and lysine are 52 pM, 0.114 mM, and 0.684 mM, respectively. For, A, B, and C, the added Hg2+ ions are 7.6 µM; for D, the added Hg2+ ions are 1.52 µM.

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Figure 5. (A) Absorbance response of the AuNRs based assay system to Hg2+. (B) Linear plots of A1030/A744 vs. the concentration of the analytes, R = 0.993. The concentrations of the AuNRs, AA and lysine are 52 pM, 0.114 mM and 0.684 mM, respectively.

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Figure 6. Absorption responses of the AuNRs based assay system to some transition/heavy metal ions (A) and eight common ions in environmental water samples (B). The concentrations of the AuNRs, AA and lysine are 52 pM, 0.114 mM and 0.684 mM, respectively. The concentrations of different metal ions are all 7.6 µM.

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Figure 7. Sensing of Hg2+ in tap (A) and river (B) water samples by the proposed AuNRs based system. The concentrations of the AuNRs, AA and lysine are 52 pM, 0.114 mM and 0.684 mM, respectively. It was noted that the water samples were finally diluted 5 times during the assay.

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Table 1. The Binding Energy Values (kcal/mol) of Au Atom and -NH2 Group and Mercury Ion with Different Valence States.

e

Hg0

Hg+

Hg2+

Au0

10.46

52.71

a

NH2

1.677

49.85

162.3

-

- mean “can not react each other”

Table 2. Hg2+ Assay in Various Water Samples. b

a

samples

tap water

river water

a

AFS Hg /(nM)

0 2.5 5 0 2.5 5

not found 2.63 5.52 1.10 3.71 6.01

2+

d

this method Hg2+/(nM)

recovery (％)

not found 2.81 5.16 1.83 4.12 7.11

e

112 103 e 92 106

The water samples were finally diluted 5 times during the assay.

b,c,d e

c

added Hg2+/(nM)

Herein, all concentrations of Hg2+ ions corresponded to the original water samples (without dilution).

- mean “not detectable”

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For TOC only

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Valence States Modulation Strategy for Picomole Level Assay of Hg2+

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