Analytica Chimica Acta 825 (2014) 51–56

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Development of silver/gold nanocages onto indium tin oxide glass as a reagentless plasmonic mercury sensor Daodan Huang a , Tingting Hu a , Na Chen a , Wei Zhang b, **, Junwei Di a, * a The Key Lab of Health Chemistry and Molecular Diagnosis of Suzhou, College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou 215123, PR China b Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

 A reagentless, sensitive and selective optical sensor for detection of Hg(II) was developed.  Silver–gold nanocages were prepared on the transparent indium tin oxide coated glass surface.  The nanomaterials could act as optical sensing probe as well as reducing agent.  The plasmonic sensor could be used to detect mercury ions in field analysis.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 January 2014 Received in revised form 25 March 2014 Accepted 26 March 2014 Available online 29 March 2014

We demonstrate the utilization of silver/gold nanocages (Ag/Au NCs) deposited onto transparent indium tin oxide (ITO) film glass as the basis of a reagentless, simple and inexpensive mercury probe. The localized surface plasmon resonance (LSPR) peak wavelength was located at 800 nm. By utilizing the redox reaction between Hg2+ ions and Ag atoms that existed in Ag/Au NCs, the LSPR peak of Ag/Au NCs was blue-shifted. Thus, we develop an optical sensing probe for the detection of Hg2+ ions. The LSPR peak changes were lineally proportional to the concentration of Hg2+ ions over the range from 10 ppb to 0.5 ppm. The detection limit was 5 ppb. This plasmonic probe shows good selectivity and high sensitivity. The proposed optical probe is successfully applied to the sensing of Hg2+ in real samples. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Localized surface plasmon resonance Mercury Optical sensor Silver/gold nanocage

1. Introduction Gold and silver nanoparticles exhibit unique localized surface plasmon resonance (LSPR) properties, which have attracted great

* Corresponding author at: Soochow University, Department of Chemistry, Suzhou Industrial Park, Suzhou, China. Tel.: +86 512 65880354; fax: +86 512 65880089. ** Corresponding author. Tel.: +86 23 63061455. E-mail addresses: [email protected] (W. Zhang), [email protected], [email protected] (J. Di). http://dx.doi.org/10.1016/j.aca.2014.03.037 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

attention from the bioanalytical field [1–6]. In the recent years, great progress has been achieved in the development of LSPRbased biosensors, such as Staphylococcus aureus enterotoxin B [7], phosphopeptides [8], Alzheimer’s tau protein [9], hepatitis B virus [10], and salbutamol [11]. Moreover, the LSPR spectroscopy can be monitored using a basic UV–vis spectrophotometer. Thus, LSPRbased biosensors have been developed as a fast, simple, and low cost technology. Herein, we further extended the scope of LSPR-based sensors to detect mercury ions. Mercury is regarded as a highly toxic and widely dispersed in the environment, which can lead to damage of

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brain, nervous system, kidneys, and immune system. Plasmonic metal nanoparticles-based colorimetric detection is a highly attractive approach for the development of on-field analysis or rapid screening [12]. For the detection of Hg(II) ions, there are two major sensing mechanisms for metallic nanoparticles: (1) aggregation-dependent shifts in plasmon peak wavelength. Many gold nanoparticle probes for the colorimetric determination of Hg(II) have been developed, which is based on oligonucleotides [13,14], oligopeptides [15,16], and other functional molecules [17–19]. (2) Local refractive index-dependent shifts in plasmon peak wavelength. For example, mercury ions can be reduced by reducing agents (mostly sodium borohydride) to deposit it onto the surface of metal nanoparticles. Mercury adsorption would induce a blue shift of the LSPR peak of metal nanoparticles. This changes in the LSPR band can be used for the detection of Hg(II) concentration [20–24]. Guo et al. developed a test strip for Hg2+ detection based on DNA-functionalized gold nanoparticles. However, DNA is expensive and instable during storage. Recently, silver nanoparticles were used as both sensing platform and reducing agent for the detection of Hg(II) [25–27]. This method has good selectivity, high sensitivity and requires no additional reagents. However, it is well known that silver nanoparticles are not very stable for storage, which is limited in the application. It has been reported that bimetallic nanostructures of Ag/Au either in alloy or multilayer form can tune the electronic, optical and chemical properties [28,29]. For example, a charge compensation mechanism leads to increase electron density within the Ag layer yielding a negative oxidation state. Thus the oxidation resistivity is enhanced and the chemical stability is improved [29]. In this paper, we first deposited silver nanoparticles on transparent indium tin oxide (ITO) glass by electrochemical method. Subsequently, these nanoparticles are used as templates to grow thin gold shells with holes due to the diffusion of the dissolution of Ag across the shells. Stopping the replacement reaction between silver and gold salt at intermediate stage, a bimetallic gold–silver nanoshell with several pinholes and a partial hollow centre was produced, which was defined as Ag/Au nanocage (Ag/Au NC). Herein, we demonstrate the analytical potential of Ag/Au NCs for detecting Hg(II) ions. This method holds high selectivity, good sensitivity, and requires no additional reagents.

2. Experimental 2.1. Chemical and materials Indium tin oxide film coated glass (1.1 mm of thickness, less than 100 V) was purchased from Suzhou NSG Electronics Co., Ltd. (Suzhou, China). Tetrachloroauric acid (HAuCl44H2O), silver nitrate (AgNO3), KNO3, Hg(NO3)2, Na2HPO4, and citric acid were obtained from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals were of analytical grade and used without further purification. 0.1 mol L1 phosphate–citric acid buffer solution (pH 2.0) was prepared by mixing stock standard solution of Na2HPO4 and citric acid. All solutions were made up with Millipore water. 2.2. Apparatus The deposition of silver nanoparticles on ITO substrate surface and cyclic voltammograms were performed using CHI 830 electrochemical workstation (CH Instruments, Shanghai, China). UV-visible absorption spectra were recorded with a TU-2810 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). Scanning electron microscopy (SEM) was obtained using S-4700 scanning electron microscope (Hitachi, Japan). The topography images of these nanoparticles in solution were obtained with transmission electron microscopy (TEM) (FEI Company Tecnai G2 20, Holand). X-ray diffraction (XRD) analysis was recorded by X’Pert-Pro MPD (Panalytical, Holland). 2.3. Fabrication of Au/Ag NCs The ITO electrode (0.6  3.0 cm2) was cleaned using NH3–H2O (1:20), ethanol, and distilled water for 10 min sequentially in an ultrasonic bath. Then the electrode immersed in the 0.2 mmol L1 AgNO3 and 0.3 mol L1 KNO3 solutions. Prior to electrolysis the electrolyte was deoxygenated by bubbling highly pure N2 for about 15 min and maintained under N2 atmosphere during experiment. Silver nanoparticles were electrodeposited by applying a cyclic voltammogram (CV) in the potential range 0.2 to 0.5 V at 0.05 V s1 for 100 cycles at 30  C. After rinsed with water, the Au/Ag

Fig. 1. SEM image of silver nanoparticles deposited onto ITO substrate (A) with its high resolution image (B) and SEM image of Ag/Au NCs formed by reacting these silver nanoparticles with an HAuCl4 solution (C) with its high resolution image (D).

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Fig. 3. UV-visible absorption spectra of silver nanoparticles deposited on ITO substrate before (a) and after addition of HAuCl4 for different time (b–f), incubation of 15 min and treated with 0.5 mM of Fe(NO3)3 solution (g).

Fig. 2. TEM image of Ag/Au NCs peeled from ITO substrate.

NCs were fabricated by the electrode incubated in 0.02 mM HAuCl4 solution for 15 min at 50  C. 2.4. LSPR assay for the detection of Hg2+ The proposed sensor was incubated in the buffer solution (pH 2.0) for 0.5 h prior to measurement. After rinsed throughout by water and dried by nitrogen gas, the LSPR band was recorded. Next, this sensor was incubated in Hg2+ and the phosphate–citric acid buffer solution (pH 2.0) for 80 min at 30  C. Finally, after cleanness and dryness, the LSPR band was recorded again. Thus, the changes of LSPR peak of the sensor was obtained for detection of Hg(II) concentration. 3. Results and discussion 3.1. Characterization of Ag/Au NCs The fabrication of Ag/Au NCs on transparent ITO substrate involved two major steps including the deposition of silver nanoparticles and the galvanic replacement reaction between silver and HAuCl4 solution, which was similar to previous reports [30–32]. The silver nanoparticles were electrodeposited onto ITO surface as the sacrificial template. Because the standard electrode potential of the AuCl4 couple (0.99 V) is higher than that of the Ag + /Ag couple (0.80 V), a redox reaction can occur between zerovalent silver and gold salt [33]. 3AgðsÞ þ Au3þ ðaqÞ ! AuðsÞ þ 3Agþ ðaqÞ

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nanoparticles, the pinholes in Ag/Au NCs could be observed clearly on the SEM images (Fig. 1B and D). This indicated that porous nanostructures had been formed. In order to further characterize nanostructures, the nanoparticles were released into ethanol solution upon sonication. Fig. 2 illustrates the TEM image of Ag/Au NCs peeled off from ITO substrate. It is clearly observed that nanocage structures were formed. The thickness of gold shell was obtained to be 5–6 nm. Since gold and silver nanostructures exhibit distinctive LSPR peaks in the visible region, conveniently, monitor the spectrum to study the metal nanostructures. Fig. 3 shows the UV-visible absorption spectra of silver nanoparticles on ITO substrate before and after treatments. The LSPR peak wavelength of silver nanoparticles was located at 460 nm. After reacting with an aqueous HAuCl4 solution for 5 min, this peak was decreased largely and a new peak around 800 nm was appeared. The new LSPR peak was attributed to deposition of gold on the surface of silver nanoparticles. It slowly but continuously red-shifted with the increasing incubation time. This phenomenon was consistent with previous reports [30–32]. The intensity of LSPR peak about

(1)

Then, the element gold was deposited on the surface of silver nanoparticles after addition of gold salt solution. At the same time, the Ag(0) core began to oxide and generated cave on the surface. The gold shells were incomplete structure in the initial stage because Ag(I) could continuously diffuse across this shell. By controlling the reaction time, an Ag/Au NC (a bimetallic gold/silver nanoshell with several pinholes and a partial hollow centre) was produced. Fig. 1A and B shows SEM images of silver nanoparticles electrodeposited on ITO substrate. These solid particles were welldistributed on the ITO surface with a mean size of 70 nm. Fig. 1C and D exhibits SEM images of the product after the silver nanoparticles were treated with gold salt solution for 15 min. The morphologies of Ag/Au NCs were similar to that of silver templates (Fig. 1A and C). However, comparing with solid silver

Fig. 4. Cyclic voltammograms of silver nanoparticles (a) and Ag/Au NCs (b) on ITO substrate in 0.05 mol/L H2SO4 at potential scan rate of 0.05 V s1.

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Fig. 5. UV-visible absorption spectra of the Ag/Au NC probe before (a) and after incubation of phosphate–citric acid buffer solution (b) and addition of 0.5 ppm of Hg(II) ions.

800 nm was first increased and then decreased slowly with increasing the replacement time. This is resulted from continuous deposition of gold and dissolution of silver. The maximum intensity of peak was obtained when the replacement time was 15 min, which is more favorable for LSPR sensing. If the incubation time is long enough, the gold shell becomes complete [32,33]. This is not desirable because it blocks the reaction between silver atoms and Hg2+ ions for detection of mercury. Thus, we selected 15 min as incubation time in the next experiments. Furthermore, when the Ag–Au NCs was treated with aqueous Fe(NO3)3 solution by dissolving Ag [34], the LSPR peak was further red-shifted in wavelength and decreased in intensity. The results suggested the Ag–Au bimetallic nanoshells with partial hollow cavity were formed. Fig. 4 displays the cyclic voltammograms of silver nanoparticles and Ag/Au NCs in 0.05 mol L1 H2SO4. The redox properties that come from silver were clearly observed for the electrode of silver nanoparticles deposited on ITO substrate (curve a in Fig. 4). In contrast, a new pair of gold redox peaks was also observed for the Ag/Au NCs in the oxidative scan for the Au oxidation and the Au oxide reduction in the reductive scan (curve b in Fig. 4). The experimental results further demonstrated that metal nanoparticles were composed of silver and gold. 3.2. Plasmonic sensing of Hg(II) Fig. 5 displays the UV-visible absorption spectra of Ag/Au NCs before and after incubation in buffer solution and Hg(II) solution. The LSPR spectrum of the sensor remains constant after immersing in phosphate–citric acid buffer solution (pH 2.0) for 3 h, indicating that the sensor was very stable. However, after incubation of the strip in Hg(II) and phosphate–citric acid buffer solution, a marked blue shift was observed. The change of LSPR band can be attributed to the oxidation of Ag atoms in the Ag/Au NCs by Hg(II) ions to form a shell of mercury with or without amalgamation on the surface of the Ag/Au NCs. The redox reaction can be described as follows [25,35]: AgðsÞ þ Hg2þ ðaqÞ ! Agþ ðaqÞ þ HgðsÞ

(2)

This galvanic replacement reaction process is similar to that between silver and gold salt. The holes in the surface of Ag/Au NCs provided the paths of the diffusion of Ag(I) ions.

Fig. 6. TEM image of the Ag/Au NCs obtained after the addition of Hg2+ ions (0.5 ppm) and then peeled off from ITO substrate (A). The XRD pattern obtained for the Ag/Au NCs after their exposure to Hg2+ ions (B).

We used TEM to examine the nanomaterials before and after mercury exposure. There was no marked difference in the nanostructures between TEM image (Fig. 6A) and Fig. 2. This suggests that mercury exposure induce little change of the Ag/Au NC nanostructures and cannot be distinguished by TEM images. We also used XRD pattern to characterize the Ag/Au NCs before and after mercury exposure. Only one peak corresponding to Ag/Au (111) crystallographic planes (2u = 38.3 ) was observed in the XRD pattern of Ag/Au NCs. After exposure of mercury, a new weak band appeared at 31.3 which corresponded to the Hg (11 0) planes [26]. This indicates that polyhedral nanostructures with zerovalent Hg were deposited on the surface [26,36]. However, the formation of amalgam between Hg and Au/Ag NCs cannot be completely ruled out because of their high affinity towards mercury [26,37]. Our experimental results agree with that of the reports [21,36] where a core shell structure with a layer of mercury on Ag/Ag NC surface is deposited which is related to a change in the polarization of the near surface region. To obtain the response time of the prepared sensor upon addition of Hg(II) ions, the peak wavelength shifts at different time

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Fig. 7. Response time of the LSPR sensor with 0.5 ppm of Hg2+ ions and phosphate– citric acid buffer solution.

intervals were recorded (Fig. 7A). After addition of 0.5 ppm Hg(II) ions, blue shifts of the LSPR peak were observed within 80 min and then the peak remained constant with time. Thus, the incubation time of 80 min was used in subsequent detection. Because the sensing action is involved in a chemical process in heterogenerous phase, we have followed the replacement reaction between Ag(0) and 0.5 ppm Hg(II) using the spectroscopic method in the temperature range 20–50  C. As expected, the response time is found to decrease with the increasing temperature. However, when the incubation temperature was fixed at 50  C, the LSPR peak was first blue-shift and then red-shift. This may be attributed to reconstructing its wall into highly crystalline structures via processes such as Ostwald ripening [33] at high temperature. Therefore, we selected incubation temperature at 30  C for detection of Hg(II) in the next experiments. The sensitivity of Ag/Au NCs probe towards Hg2+ was investigated. UV-visible absorption spectra of Ag/Au NC sensor were recorded before and after addition of Hg(II) with various concentrations. The blue shifts of the LSPR peak as a function of the

Fig. 8. Relationship between the blue shift of LSPR peak and Hg2+ concentration.

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Fig. 9. Comparison of the peak shift of the Ag–Au NCs film sensor before and after incubation of different metal ions.

concentration of Hg2+ (0.01–0.8 ppm) was plotted in Fig. 8. The LSPR peak wavelength was blue-shifted upto 0.6 ppm with the increase of the concentration of Hg(II). A linear relationship was observed from 0.01 to 0.5 ppm of Hg2+, with a correlation coefficient of 0.9970. The detection limit of 5 ppb is obtained at a signal-to-noise ratio of 3. Table 1 shows a comparison between our simple reagentless LSPR sensor with some optical methods for detection of mercury. To test the selectivity of Ag–Au NCs film probe towards Hg2+ (E (Hg2+/Hg) = 0.85 V), other metal ions, including Au3+ (E (AuCl4/ Au) = 0.99 V), Cu2+ (E (Cu2+/Cu) = 0.34 V), Mg2+ (E (Mg2+/Mg) = 2.36 V), Ca2+ (E (Ca2+/Ca) = 2.87 V), Co2+ (E (Co2+/Co) = 0.28 V), Pb2+ (E (Pb2+/Pb) = 0.13 V), Cd2+ (E (Cd2+/Cd) = 0.40 V), Mn2+ (E (Mn2+/Mn) = 0.28 V), Fe2+ (E (Fe2+/Fe) = 0.44 V), Ag+ (E (Ag + /Ag) = 0.80 V), Ni2+ (E (Ni2+/Ni) = 0.25 V), Al3+ (E (Al3+/Al) = 1.66 V), Zn2+ (E (Zn2+/Zn) = 0.76 V) were examined under optimized conditions. Fig. 9 illustrates the LSPR peak shift of Ag–Au NCs film sensor after incubation of various metal ions. As expected, other metal ions have no evident effect on the LSPR peak shift. Because Au3+ can also oxidize Ag, the LSPR peak of the sensor exhibited redshift markedly at high concentration of HAuCl4 (>1 ppm). The results indicate that the Ag/Au NCs film sensor has very high selectivity towards Hg2+. The high selectivity of this probe is derived from gold selectivity of the galvanic replacement reaction and high affinity of Hg on Au surface. The stability of the Ag/Au NCs has also been evaluated by examining the blue-shift of the sensor. After the Ag/Au NCs film sensor was kept in desiccators for 1 month, its response was not markedly change for the detection of Hg(II). This indicates that the Ag/Au NCs film sensor has good stability. The application of the plasmonic sensor was evaluated for the detection of Hg(II) in real samples. Tap water and lake water sample was collected from our laboratory and Dushu Lake in Suzhou, respectively. The lake water sample was pretreated by filtration. A sample of paste coated paper (used in battery) was obtained from National Battery Inspection and Testing Center (Suzhou, China). A piece of paste coated paper was weighed (1 g) and torn into small pieces. Then they were put into beaker and added 10 mL of HNO3 (1:1) solution. After adding water to the sample immersed completely in solution and covering the surface plate, the sample was heated to near boiling for 10 min. Next, the extracted solution was filtered. The filtrate was collected in a 100 mL volumetric flask

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Table 1 Comparison of optical methods for mercury detection. Methods

Specific materials

Linear range

Detection limit

Ref.

Colorimetry Colorimetry Colorimetry LSPR sensor Fluorescence Fluorescence Hyper-Rayleigh scattering SERS LSPR sensor

AuNPs-DNA AuNRs-L-cysteine AgNPs-starch AgNPs-PVA DNA-AuNPs DNA-AuNPs MPA-HCys-PDCA-AuNPs Thiol-AgNPs Ag-AuNCs

0.1–10 mM 0.5–250 mM 0.01–1 ppm 0.01–1 ppm 0.02–1.0 mM 0.05–2.5 mM 5–100 ppb 0.01–2 mM 0.01–0.5 ppm

60 nM 0.06 mM 5 ppb 1 ppb 16 nM 25 nM 5 ppb 2.4 nM 5 ppb (25 nM)

[13] [17] [27] [25] [38] [39] [40] [41] This work

Table 2 Determination of Hg2+ in real samples using the prepared sensor (n = 3). Samples

Detection (ppm)

Added (ppm)

Found (ppm)

Recovery (%)

RSD (%)

CVAASa (ppm)

Coated paper Lake water Tap water

0.278 Not detected Not detected

0.2 0.2 0.2

0.214 0.208 0.202

107 104 101

1.4 1.8 3.2

0.295 – –

a

Cold vapor atomic absorption spectrometry.

and diluted to the mark. A suitable volume of the as-prepared samples was analyzed according to the proposed general procedure. The results were shown in Table 2. The recovery was in the range of 101–107% and the relative standard deviation (RSD) was in the range of 1.4–3.2%. The fact showed that the analytical results of the samples were satisfactory. 4. Conclusions In conclusion, the Ag/Au NCs were prepared by the electrodeposition of silver nanoparticles on ITO substrate and the galvanic replacement reaction between silver and HAuCl4 solution. Hg2+ ions in aqueous solution were recognized by the simple redox reaction between silver and Hg2+ ions. The blue-shift in LSPR peak of Ag/Au NCs film sensor was directly proportional to the concentration of Hg2+ ions. This method exhibits high sensitivity, good selectivity, and requires no any additional reagents. The Ag/ Au NCs could be used as an excellent optical sensing probe in the field analysis of Hg(II) ions. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21075086), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207). References [1] K.M. Mayer, J.H. Hafner, Chemical Reviews 111 (2011) 3828–3857. [2] M.E. Stewart, C.R. Anderton, L.B. Thompson, J. Maria, S.K. Gray, J.A. Rogers, R.G. Nuzzo, Chemical Reviews 108 (2008) 494–521. [3] A.J. Haes, R.P. Van Duyne, Expert Review of Molecular Diagnostics 4 (2004) 527–537. [4] B. Sepúlveda, P.C. Angeloméb, L.M. Lechuga, L.M. Liz-Marzán, Nano Today 4 (2009) 244–251. [5] H. Chen, T. Ming, L. Zhao, F. Wang, L. Sun, J. Wang, C. Yan, Nano Today 5 (2010) 494–505. [6] N.J. Halas, Nano Letters 10 (2010) 3816–3822. [7] S. Zhu, C. Du, Y. Fu, Optical Materials 31 (2009) 1608–1613. [8] J. Chen, Y. Chen, Analytical and Bioanalytical Chemistry 399 (2011) 1173–1180. [9] M. Vestergaard, K. Kerman, D. Kim, H.M. Hiep, E. Tamiya, Talanta 74 (2008) 1038–1042. [10] S. Zheng, D. Kim, T.J. Park, S.J. Lee, S.Y. Lee, Talanta 82 (2010) 803–809.

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gold nanocages onto indium tin oxide glass as a reagentless plasmonic mercury sensor.

We demonstrate the utilization of silver/gold nanocages (Ag/Au NCs) deposited onto transparent indium tin oxide (ITO) film glass as the basis of a rea...
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