Colorimetric Signal Amplification Assay for Mercury ... - ACS Publications

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Colorimetric Signal Amplification Assay for Mercury Ions Based on the Catalysis of Gold Amalgam Zhengbo Chen,*,† Chenmeng Zhang,† Qinggang Gao,† Guo Wang,*,† Lulu Tan,†,‡ and Qing Liao*,†,§ †

Department of Chemistry, Capital Normal University, Beijing, 100048, China College of Resources Environment and Tourism, Capital Normal University, Beijing, 100048, China § Beijing Key Laboratory for Optical Materials and Photonic Devices, Capital Normal University, Beijing, 100048, China ‡

S Supporting Information *

ABSTRACT: Mercury is a major threat to the environment and to human health. It is highly desirable to develop a user-friendly kit for on-site mercury detection. Such a method must be able to detect mercury below the threshold levels (10 nM) for drinking water defined by the U.S. Environmental Protection Agency. Herein, we for the first time reported catalytically active gold amalgam-based reaction between 4-nitrophenol and NaBH4 with colorimetric sensing function. We take advantage of the correlation between the catalytic properties and the surface area of gold amalgam, which is proportional to the amount of the gold nanoparticle (AuNP)bound Hg2+. As the concentration of Hg2+ increases until the saturation of Hg onto the AuNPs, the catalytic performance of the gold amalgam is much stronger due to the formation of gold amalgam and the increase of the nanoparticle surface area, leading to the decrease of the reduction time of 4-nitrophenol for the color change. This sensing system exhibits excellent selectivity and ultrahigh sensitivity up to the 1.45 nM detection limit. The practical use of this system for Hg2+ determination in tap water samples is also demonstrated successfully.

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sensors based on organic molecules,26,27 oligonucleotides,28−30 DNAzymes,31−33 proteins,34 and genetically engineered bacteria,35,36 have been designed for Hg2+ detection in aqueous solutions. Unfortunately, most of these methods suffer from poor sensitivity and selectivity, low water solubility, a complex synthesis procedure, and time-consuming DNA probe preparation. It is well-known that a Au surface exhibits a strong affinity for Hg2+. Thus, after the reduction of Hg2+ with citrate or NaBH4, the Hg(0) thus generated is strongly bonded onto the surface of Au-based nanomaterials to form a solid amalgam-like structure.37−42 The adsorption of Hg to silver or gold particles surfaces shifted the surface plasmon resonance spectroscopy of the nanoparticles to shorter wavelengths.43 On the basis of this feature, ionic or elemental mercury species was determined by adsorption of mercury vapor to gold surfaces.44,45 In addition, the recent discovery of the inherent peroxidase-like activity of gold nanoparticles (AuNPs) offer new opportunities for biosensor development, as this nanozyme activity (nanoparticles’ enzyme mimicking activity) is independent of the change in surface plasmon resonance (SPR) caused by nanoparticle aggregation.46−50 We present herein a colorimetric detection system for Hg2+ that is facile, aqueous-based, highly sensitive, and selective and thus addresses the limitations of many existing systems. In our work, AuNPs could catalyze the reduction of Hg2+ by surface-

ercury, present in a variety of different forms (metallic, inorganic, and organic), is a widespread bioaccumulative pollutant. Mercury contamination in the environment and in living organisms continues to be a critical issue of concern on a global scale. Solvated mercuric ion (Hg2+), one of the most stable inorganic forms, is a highly reactive agent1−5 that causes grisly immunotoxic,6 genotoxic, and neurototoxic7 effects, including damage to the central nervous system,8 endocrine system,7 kidney,9 and other organs.4,7,10−12 Mercury poisoning, through food-chain accumulation or high dose exposure, can result in several diseases, including acrodynia, Hunter-Russell syndrome, and Minamata disease.13 Studies have shown higher mercury concentrations in brains of deceased and in blood of living patients with Alzheimer’s disease.8 Therefore, the development of new Hg2+ detection methods that are sensitive, low-cost, and applicable to aqueous systems has become an urgent need.14 To date, a plethora of methods, including cold vapor technique with atomic absorption spectrometry,15,16 inductively coupled plasma atomic emission spectrometry,17 inductively coupled plasma mass spectrometry,18,19 surface-enhanced Raman scattering,20,21 impedance spectrometry,22,23 fluorescence,24 and colorimetry,25 have been applied to detect Hg2+. Although these methods offer excellent sensitivity, the bulky instrumentation and long-term sample pretreatment processes limits their use for on-site analysis, bringing the shortcomings of being complicated, rather costly, time-consuming, and nonportable. In response to these shortcomings, developing new approaches for the handiness and quickness detection and quantification of Hg2+ is highly desirable. To date, various Hg2+ © XXXX American Chemical Society

Received: July 25, 2015 Accepted: October 5, 2015

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DOI: 10.1021/acs.analchem.5b02812 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry capped citrate. The effect of Hg2+ uptake implies amalgam formation, which leads to remarkable enhancement of the peroxidase-like activity of citrate-capped AuNPs. On the basis of this effect, a colorimetric mercury sensor was established. To signal colorimetrically this gold amalgamation event, 4nitrophenol was employed as the visible signaling reporter. AuNPs and gold amalgam were used to catalyze the reduction of 4-nitrophenol by NaBH4. Importantly, for the signal readout, the assay is based only on naked-eye recognition and does not require any complicated instrumentation. The strong catalytic efficiency of gold amalgam, combined with the high specificity of gold amalgamation and the simplicity of detection, provides a new and efficient approach for developing a highly sensitive and handy alternative for to the monitoring of Hg2+ in environmental or biological samples.

Scheme 1. Principle of Hg−Au Alloy-Based Colorimetric Assay for Hg2+

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EXPERIMENTAL SECTION Materials. 4-Nitrophenol, chloroauric acid (HAuCl4), and sodium citrate were obtained from Sigma-Aldrich. Standard mercuric ion in 2−5% nitric acid was purchased from AccuStandard (Beijing, China). All other reagents are of analytical reagent grade. All solutions were prepared with MilliQ water (18.2 MΩ cm−1) from a Millipore system. Instrumentation. The UV−vis spectra were obtained by an UV-2550 spectrophotometer (Shimadzu Corporation). Transmission electron microscope (TEM) images were obtained on a Hitachi (H-7650, 80 kV) transmission electron microscope. Size distribution measurements were performed on a Malvern Zetasizer Nano-ZS90 (ZEN3590). An Agilent 7500 inductively coupled plasma mass spectrometry (ICPMS) system (Agilent Technologies, Santa Clara, CA) was used to detect the Hg2+ concentration of the blood samples and the contaminated water. Synthesis of Gold Nanoparticles. The AuNPs with a mean diameter of 15 nm were synthesized by the citrate reduction method.51 Briefly, a 25 mL solution containing 38.8 mM trisodium citrate was added to 250 mL of a boiling 1 mM HAuCl4 solution and consistently stirred and heated for an additional 15 min until the color of the mixture turned from light yellow to dark red. After cooling to room temperature, the concentration of the AuNP colloid was estimated according to the Beer−Lambert law to analyze the absorbance spectra.52 The obtained AuNPs were characterized by UV−vis spectrophotometry, DLS, and TEM (Figure S1). Hg2+ Assay Protocols. For Hg2+ detection, Hg2+ solution with various concentrations and volume was added to 10 μL of AuNP solution (5 nM), and the mixture was incubated for 1 min at room temperature. Then 20 μL of 4-nitrophenol solution (10 mM) was injected into the mixture. Following that, a freshly prepared NaBH4 solution (50 μL, 0.2 M) was added into the above mixture. The color of the resulting mixture was observed every minute and the absorption spectra in the range of 200−800 nm were recorded.

any complicated instrumentation. In the absence of Hg2+, yellow 4-nitrophenol freely approaches to the catalytic AuNP surface and turns to colorless 4-aminophenol. In the presence of Hg2+, because of the high affinity between Au and Hg,37−42 the reduction of Hg2+ with citrate resulted in the formation of the Hg−Au alloy. As the concentration of the Hg2+ increases, the catalysis of the Hg−Au alloys becomes stronger, which in turn leads to the decrease of the reduction time of 4nitrophenol for the color change (Scheme 1A). When the deposition of Hg onto the Au surface reaches saturation, the residual Hg2+ causes the AuNPs to aggregate under conditions of high ionic strength, leading to smaller specific area of the AuNPs, which results in poorer catalytic ability of the AuNPs and increases the reduction time of 4-nitrophenol for the color change again (Scheme 1B). Principle Calculations. The principle calculations are performed in order to explain the enhanced catalytic activity produced by low concentrations of Hg2+. The diameter of the AuNPs is around 15 nm. Considering the Au−Au bond of 0.288 nm in bulk,53 the number of gold atoms in diameter is estimated to be 52 and the total number of gold atoms in a AuNP should be on the order of 105. The gold atoms are too many to be included in first principle calculations. Instead, a four layer model is constructed to investigate the interaction between gold and adsorbed 4-nitrophenol. The close-packed Au (111) surface is selected since it is the most stable surface. A 7 × 7 hexagonal supercell as shown in Figure 1A is used. In this case, the distance between image molecules is bigger than 10 Å so that the interaction between the molecules can be neglected. The two layers in the bottom shown in Figure 1B are kept fixed to reflect the effect from the inner gold atoms, while the other atoms are allowed to move during geometrical optimization. The Perdew−Burke−Ernzerhof (PBE) functional54−,56, with a long-range dispersion correction53 is used to describe the van der Waals interactions between 4-nitrophenol and gold. The projector augmented wave (PAW) basis57,58 with an energy

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RESULTS AND DISCUSSION Sensing Mechanism. Our sensor design relies on the controlled reaction time of NaBH4 with 4-nitrophenol through the catalysis of gold amalgam to induce the yellow-to-colorless phase change, which is quantitatively correlated with the surface coverage as determined by the AuNP-bound target Hg2+ (Scheme 1). Most importantly, for the signal readout, the assay is based only on naked-eye detection and does not require B

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temperature. The pH of the solution played an important role in the reaction of NaBH4 and 4-nitrophenol (Figure S3). Because of the instability of NaBH4 in acid conditions, the decomposition of hydrogen was produced. Under alkaline conditions, however, the AuNPs were easy to aggregate, which resulted in poor catalytic ability of the AuNPs and increased the reduction time of 4-nitrophenol. Thus, the solution with pH 7.0 was taken as the optimal reaction solution. The optimum size of AuNPs was optimized by changing different AuNP sizes (5, 15, 30, and 50 nm) to assess the catalytic ability of the AuNPs toward the reduction of 4-nitrophenol (Figure S4). The CCT of the solution containing 5 and 15 nm AuNPs was 5 and 8.75 min, respectively. Whereas the mixture containing 30 and 50 nm AuNPs still did not turn colorless in 24 h. The results indicated that the catalytic performance of 5 nm AuNPs was the best, and that of 30 and 50 nm AuNPs was the poorest. The catalytic ability of 5 nm AuNPs was superior to that of 15 nm AuNPs; however, all further experiments employed 15 nm AuNPs to catalyze the reaction of 4-nitrophenol by NaBH4 due to instability of 5 nm AuNPs. Sensitivity. To test the colorimetric signal amplification effect of AuNPs, we investigated the Hg2+ concentrationdependent response of the sensor based on the catalysis of gold amalgam. Aliquots of standard solutions containing concentrations of Hg2+ ranging from 0 to 1000 nM were tested, and the UV−vis absorption spectra are presented in the Supporting Information, Figures S5 and S6. As the color-change time (CCT) of the solution containing Hg2+ at every concentration increases, the UV−vis absorbance band at 400 nm, which can be assigned to 4-nitrophenol, decreased gradually and finally became flat, proving the complete conversion of 4-nitrophenol to 4-aminophenol during the catalytic reaction. Interestingly, as shown in Figure 2A, it was observed that the CCT decreases linearly with the logarithm of Hg2+ concentration at lower concentrations (1−100 nM) (line a), whereas the CCT increased with higher concentrations (100−1000 nM), reaching a saturated signal beyond a certain Hg2+ concentration (800 nM) (line c), with a linear range of 100−800 nM (line b), which indicated the strong quantitative nature of the Hg2+ detection system. The color change of the detection solution in the presence of different Hg2+ concentrations ranging from 0 to 1000 nM at different CCTs was shown in Figure 2B. At the beginning of the reaction, all the solutions displayed yellow color, indicative of the incomplete catalytic reduction of 4nitrophenol. As the catalytic reaction proceeded, the solution containing 100 nM Hg2+ first turned colorless. Then, the other solutions gradually became colorless. Whereas the solutions containing 0, 1, 800, and 1000 nM of Hg2+ turned colorless finally, which further verified the time-dependent gradual color changes as a function of Hg2+ concentration. Experimental evidence showed that the reduction of 4-nitrophenol to 4aminophenol by NaBH4 was too slow in absence of Hg2+ and in the presence of a high concentration of Hg2+ (500 nM) due to aggregate formation (Figure S7). Direct evidence for the nonaggregation and aggregation of AuNPs induced by Hg2+ with low concentrations (0−100 nM) and high concentrations (300−800 nM) was supported by TEM measurement. As shown in Figure 3A−D, AuNPs were well dispersed in the presence of Hg2+ (5−1000 nM) due to formation of the gold amalgamation. The size of the aggregates increased gradually with the increase of the Hg2+ concentrations (300−800 nM) due to the saturation of Hg deposited onto the Au surfaces (Figure 3E−G). Importantly, the sensing strategy visualized the

Figure 1. (A) Top view and (B) side view of 4-nitrophenol absorbed on gold surface, (C) top view and (D) side view of 4-nitrophenol absorbed on the mercury−gold surface.

cutoff 400 eV is adopted. The calculations are performed with the Vienna ab initio simulation package (VASP) program.59 When 4-nitrophenol is adsorbed on the gold surface, there should be a charge transfer between 4-nitrophenol and gold according to their different chemical potentials. A reduction reaction is a process involving a gain of electrons. The electron transfer from gold atoms to 4-nitrophenol should help the reduction process. For this reason, an important parameter, Bader charge,60−62 is calculated based on the optimized geometries. The calculation indicates that there is a 0.017 e transfer from gold atoms to 4-nitrophenol. This should assist the reduction of the molecule. However, the charge transfer is too weak, so the catalytic activity is not so good. Considering the low concentration mercury, a model with a layer of mercury atoms above the gold surface is constructed. Mercury atoms are situated at the stable 3-fold hollow sites63 in the starting geometry. The calculation method is similar to that of the pure gold model. After geometrical optimization, mercury atoms move slightly toward the first gold layer as shown in Figure 1C. The mercury atoms at the adsorption sites deviate from the flat plane as shown in Figure 1D. The deviation is more obvious than that of pure gold as shown in Figure 1B. This implies that the interaction between 4-nitrophenol and mercury atoms is stronger than that between 4-nitrophenol and gold atoms. Bader charge analysis shows that there is 0.141e transfer from mercury atoms to 4-nitrophenol. The electron transfer is about 8 times as large as that for pure gold model. In fact, the absolute electronegativity of mercury is smaller than that of gold.64 The electron transfer from mercury atoms to the adsorbed 4nitrophenol is much easier than from gold atoms. This should be responsible for the enhanced catalytic activity introduced by low concentration mercury. Optimization of Experimental Parameters. To achieve high sensitivity without compromising specificity during Hg2+ detection, a number of experiments were performed to optimize experimental parameters such as the reaction temperature, the pH value of the solution, as well as the size of AuNPs. The catalytic activity of AuNPs was assessed at four different temperatures (10, 25, 30, and 40 °C) after complete reaction by measuring CCT with the naked eye (Figure S2). Among these, 25 °C showed the fastest degree of reduction of 4-nitrophenol. Thus, 25 °C was chosen as the optimal reaction C

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Figure 4. CCT as a function of Hg2+ concentration within the range of 0−5 nM.

permitted in drinking water by the United States Environmental Protection Agency (EPA). Selectivity. The high specificity of Hg−Au interaction provides excellent selectivity toward Hg2+ over other metal ions. We first chose 13 kinds of interfering ions including Ni2+, Sr2+, Sn2+, Mg2+, Co2+, Pb2+, Cd2+, Fe3+, Sb3+, Ba2+, Cr2+, Zn2+, and Ag+ (each 50 nM, 10 times of the concentration of Hg2+) as interfering ions. As shown in Figure 5A, compared with the

Figure 5. (A) Effects of different cations on the catalytic ability of citrate-capped AuNPs. (B) Colorimetric response of the sensor to Hg2+ (5 nM) in the complex media that contains all of the interfering ions (each 5 nM).

Figure 2. (A) Plots of the CCT vs the logarithm of Hg 2+ concentration. (B) Color changes of the detection solution containing Hg2+ with various concentrations as the catalytic reaction proceeds.

12 min of CCT of the detection system at 5 nM of Hg2+, the CCT of the mixtures containing the other cations were more than 13 min, which indicated that only Hg2+ ions can significantly enhance the catalytic activity of AuNPs, whereas other metal ions did not show a distinct enhancement effect, which explains the complete conversion of yellow 4-nitrophenol to colorless 4-aminophenol owing to the presence of Au−Hg alloys with very strong catalytic properties. The colorimetric response of the sensor to Hg2+ (5 nM) in the complex media that contains all of the interfering ions (each 5 nM) also corroborated the fact. As demonstrated in Figure 5B, the CCT of the solution containing Hg2+ and other interfering ions was 12.5 min, there was no significant difference compared to the CCT (12 min) of the solution containing only Hg2+, indicating the selective binding of Hg2+ to the surface of AuNPs. Analytical Application. Encouraged by the outstanding sensitivity and selectivity of this colorimetric method, the practicality of this method for the detection of Hg2+ in tap water was evaluated. The tap water samples were collected from the lab’s tap in our institute, and there is no detectable Hg2+ existing in the tap water samples. Thus, using the standard addition method, different concentrations of Hg2+ (1, 5, 10, 50, 100, 300, 500, 800, and 1000 nM) were, respectively, spiked in tap water samples and then analyzed with the method. As shown in Figure 6, the CCTs of the solution in tap water were in good agreement with the values obtained in deionized water

Figure 3. TEM images of AuNPs in the presence of Hg2+ with various concentrations: (A) 0, (B) 5 nM, (C) 50 nM, (D) 100 nM, (E) 300 nM, (F) 500 nM, and (B) 800 nM in the solution.

color signal with the same intensity regardless of Hg2+ concentration, resulting in easy discrimination of the colorimetric response by the naked eye even at very low concentration of Hg2+ near the limit of detection (LOD). We further investigated the quantitative response of the sensing strategy to low Hg2+ concentrations ranging from 0 to 5 nM (Figure 4), it got to a plateau at 1 nM Hg2+ and yielded an excellent LOD (1.45 nM) as calculated in terms of the 3σ rule, which is much lower than the maximum level (10 nM) of Hg2+ D

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demonstrating the excellent performance of this sensor in practical application.

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CONCLUSIONS In summary, the specific AuNPs/mercury interaction to form gold amalgam was utilized for colorimetric detection of Hg2+ by employing 4-nitrophenol as the signal reporter. The process involves the catalytic reaction between 4-nitrophenol and NaBH4 induced by the catalytically active AuNPs. Upon Au− Hg2+ → Au−Hg alloys conversion, the gold amalgam enhances the gold catalysis, and amplifies the visual signal. The present approach can be engineered in ways that offer unique advantages and capabilities that are not available from conventional Hg2+ sensing systems. (1) The method can sensitively and selectively measure Hg2+ with detection limits of 1.45 nM, which are superior to most current approaches for metal ion analysis. Its excellent selectivity results from the wellknown gold amalgam process that occurs specifically between Au and mercury atoms, and the high sensitivity is contributed to the catalytic signal amplification induced by gold and gold amalgam. (2) The catalytic activity of the AuNP surfaces is systematically controlled by increasing the AuNP surfaces area with various concentrations of Hg2+. Our detection system visualizes the color signal with the same intensity regardless of the Hg2+ concentration, which allows one to easily recognize the colorimetric response even at very low concentrations of Hg2+ near the LOD. (3) The method is simple, convenient, and cost-effective, without specific design of the Hg2+ recognition probe such as aptamers, only requiring the mixing of several solutions (AuNP, Hg2+, NaBH4, and 4-nitrophenol solution) at room temperature to detect Hg2+. The Hg2+ in natural media is accurately analyzed using the simple method, which is comparable to the conventional Hg2+ sensing approach (ICPMS). This detection scheme has a high potential to be extended to the on-site colorimetric detection of proteins and other metal ions by masking the nanoparticle surfaces with aptamer-target binding.

Figure 6. (A) Plots of the CCT vs the logarithm of Hg 2+ concentration. (B) Color changes of the detection solution containing Hg2+ with various concentrations as the catalytic reaction proceeds in tap water.

under the same conditions. The UV−vis spectra of the detection mixtures containing Hg2+ with various concentrations in tap water were shown in Figure S8. Furthermore, the correlation of the analysis results obtained from the developed colorimetric method and the classic ICPMS method in the clinical laboratory was examined by detecting Hg2+ in real blood samples and the contaminated water (Figure 7 and

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ASSOCIATED CONTENT

S Supporting Information *

Additional spectra and data analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02812. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-010-68903047. E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

Figure 7. Correlation of the detection results of Hg2+ in real blood samples between the developed colorimetric method and the ICPMS method.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors gratefully acknowledge the financial support of the Natural Science Foundation of China (Grant No. 21371123) and the Scientific Research Project of Beijing Educational Committee (Grant KM201410028006).

Figure S9). The samples collected were first filtered through a column (packed with an anionic-exchange resin) to remove oils and other organic impurities. The regression equation for the detection result for Hg2+ was obtained with correlation coefficients of 0.996. The result revealed high consistency in determination of Hg2+ in blood samples and the contaminated water by present approach and conventional instrument,

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DOI: 10.1021/acs.analchem.5b02812 Anal. Chem. XXXX, XXX, XXX−XXX