Stripping Voltammetric Detection of Mercury(II) Based on a Bimetallic

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Anal. Chem. 2010, 82, 567–573

Stripping Voltammetric Detection of Mercury(II) Based on a Bimetallic Au-Pt Inorganic-Organic Hybrid Nanocomposite Modified Glassy Carbon Electrode Jingming Gong,*,† Ting Zhou,† Dandan Song,† Lizhi Zhang,*,† and Xianluo Hu*,‡ Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China, and State Key Laboratory of Material Processing and Die & Mould Technology, College of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China A new, highly sensitive and selective sensor for the electrochemical assay of Hg(II) by anodic stripping voltammetry has been developed, whereby a glassy carbon electrode is modified with a novel inorganic-organic hybrid nanocomposite, namely, bimetallic Au-Pt nanoparticles/organic nanofibers (labeled as Au-PtNPs/NFs). The sensor possesses a three-dimensional (3D) porous network nanoarchitecture, in which the bimetallic Au-Pt NPs serving as metal NP-based microelectrode ensembles are homogenously distributed in the matrix of interlaced organic NFs. The surface structure and composition of the sensor were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Its electrochemical performance was systematically investigated. Our results show that such a newly designed, Au-PtNPs/NF nanohybrid modified electrode provides remarkably improved sensitivity and selectivity for the stripping assay of Hg(II). The detection limit is found to be as low as 0.008 ppb (S/N ) 3) that is much below the guideline value from the World Health Organization (WHO). Interferences from other heavy metal ions such as Cu(II), Cr(III), Co(II), Fe(II), Zn(II), and Mn(II) ions associated with mercury analysis are effectively inhibited. Toward the goal for practical applications, the sensor was further evaluated by monitoring Hg(II) in tap and river water specimens. Mercury(II) (Hg2+), widely distributed in air, water, and soil, is considered to be one of the highly toxic heavy metal ions. It is a hazardous pollutant with recognized accumulative character in the environment and biota.1-4 High exposure to mercury * To whom correspondence should be addressed. E-mail: jmgong@ mail.ccnu.edu.cn (J.M. Gong). Phone/Fax: +86-27-6786 7535 (J.M.). † Central Normal University. ‡ Huazhong University of Science and Technology. (1) Miller, J. R.; Ros00dwland, J.; Lechler, P. J.; Desilets, M.; Hsu, L. C. Water, Air, Soil Pollut. 1996, 86, 373–388. (2) Eisler, R. Environ. Geochem. Health 2003, 25, 325–345. (3) Wang, Q. R.; Kim, D.; Dionysiou, D. D.; Sorial, G. A.; Timberlake, D. Environ. Pollut. 2004, 131, 323–336. (4) Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. Inc. Environ. Toxicol. 2003, 18, 149–175. 10.1021/ac901846a  2010 American Chemical Society Published on Web 12/16/2009

would induce serious health problems like kidney and respiratory failure, damage in the gastrointestinal tract and nervous system, and impairment of speech, hearing, and working.5 Because of the increasing threat of global mercury release into the environment, there has been a significant worldwide concern regarding the environment, healthiness, and food safety.6,7 Rapid and reliable determination of trace Hg(II) has become increasingly desirable. A variety of methods, such as gas chromatography-atomic fluorescence spectrometry,8 atomic absorption spectrometry,9-12 atomic fluorescence spectrometry,13 and isotope dilution cold vapor inductively coupled plasma mass spectrometry14 have been reported for monitoring Hg(II). Most of these strategies, however, have either limitations with respect to sensitivity, selectivity, and simplicity or the need of expensive instruments, laboratory setup, and high operating cost. Therefore, there is a growing need for developing an efficient sensing system that is not only sensitive and reliable but also simple, economical, and practical. The electrochemical methods, particularly stripping voltammetry, have attracted significant interest for trace analysis of heavy metals, due to their excellent sensitivity, short analysis time, low power consumption, and cheap equipment.15-20 The stripping voltam(5) McKeown-Eyssen, G. E.; Ruedy, J.; Neims, A. Am. J. Epidemiol. 1983, 118, 470–479. (6) Kim, Y. J.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165–167. (7) Rex, M.; Hernandez, F. E.; Campiglia, A. D. Anal. Chem. 2006, 78, 445– 451. (8) D’Ulivo, A.; Loreti, V.; Onor, M.; Pitzalis, E.; Zamboni, R. Anal. Chem. 2003, 75, 2591–2600. (9) Yan, X. P.; Yin, X. B.; Jiang, D. Q.; He, X. W. Anal. Chem. 2003, 75, 1726– 1732. (10) Christopher, S. J.; Long, S. E.; Rearick, M. S.; Fassett, J. D. Anal. Chem. 2001, 73, 2190–2199. (11) Vil’pan, Y. A.; Grinshtein, I. L.; Akatove, A. A.; Gucer, S. J. Anal. Chem. 2005, 60, 45–51. (12) Kopysc, E.; Pyrzynska, K.; Garbos, S.; Bulska, E. Anal. Sci. 2006, 16, 1309– 1312. (13) Xu, H.; Zeng, L. P.; Xing, S. J.; Shi, G. Y.; Chen, J. S.; Xian, Y. Z.; Jin, L. T. Electrochem. Commun. 2008, 10, 1893–1896. (14) Naka, K.; Ando, D.; Wang, X.; Chujo, Y. Langmuir 2007, 23, 3450–3454. (15) Song, Y.; Swain, G. M. Anal. Chem. 2007, 79, 2412–2420. (16) Cheng, K. C.; Chen, P. Y. Electroanalysis 2008, 20, 207–210. (17) Salau ¨ n, P.; van den berg, C. M. G. Anal. Chem. 2006, 78, 5052–5060. (18) Dai, X.; Wildgoose, G. G.; Salter, C.; Crossley, A.; Compton, R. G. Anal. Chem. 2006, 78, 6102–6108. (19) Pan, D. W.; Wang, Y.; Chen, Z. P.; Lou, T. T.; Qin, W. Anal. Chem. 2009, 81, 5088–5094.

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metry provides an efficient and reliable way to detect mercury at low concentration. Many solid electrodes have been so far reported for the detection of Hg(II), such as gold,21,22 gold wire,17 platinum,23 and chemically modified electrodes.18,20,24,25 Gold was found to be a superior substrate as a working electrode because of its high affinity for mercury, which could enhance the preconcentration effect. However, the limitation of gold-based electrodes is that the undesirable gold amalgam may be formed and destroy the surface feature of the electrodes.26 To achieve reproducibility, complex electrochemical and mechanical pretreatments are often necessary to refresh the gold electrodes. This is very timeconsuming.27 Therefore, it is highly desirable to develop new alternative electrode materials for stripping analysis to meet the fast-growing demands for on-site environmental monitoring of trace mercury ions. In recent years, nanotechnology has become one of the most active areas in analytical chemistry.28 Nanosized metal particle (especially Au nanoparticles (AuNPs) assembled on various supports) modified electrodes have emerged as a promising alternative for the electroanalysis of Hg(II).18,24,25,29 AuNP modified electrodes possess both high surface area and renewable surface. Meanwhile, metal NP modified electrodes may serve as random arrays of microelectrodes.18,30-34 They show distinct advantages over the conventional macroelectrodes, such as increased mass transport, decreased influence of the solution resistance, low detection limit, and better signal-to-noise ratio.35-37 In addition, AuNP-based microelectrode ensembles have been proven to be a promising approach to the detection of heavy metals.18,24,25,29 Nowadays, a key bottleneck is how to fabricate efficient AuNP-based ensembles for monitoring Hg(II). In this regard, Compton et al. loaded AuNPs onto glassy carbon microspheres and then electrically wired the AuNP modified glassy carbon microsphere film to the underlying macroelectrode using carbon nanotubes.18 Raj et al. constructed AuNP ensembles onto the thiol functionalized sol-gel silicate network by a colloidal (20) Dai, X.; Nekrassova, O.; Hyde, M. E.; Compton, R. G. Anal. Chem. 2004, 76, 5924–5929. (21) Ordeig, O.; Banks, C. E.; del Campo, J.; Mun ˜oz, F. X.; Compton, R. G. Electroanalysis 2006, 18, 573–578. (22) Bonfil, Y.; Brand, M.; Kirowa-Eisner, E. Anal. Chim. Acta 2000, 424, 65– 76. (23) Uhlig, A.; Schnakenberg, U.; Hintsche, R. Electroanalysis 1997, 9, 125– 129. (24) Abollino, O.; Giacomino, A.; Malandrino, M.; Piscionieri, G.; Mentasti, E. Electroanalysis 2008, 20, 75–83. (25) Jena, B. K.; Raj, C. R. Anal. Chem. 2008, 80, 4836–4844. (26) Welch, C.; Nekrassova, O.; Dai, X.; Hyde, M.; Compton, R. G. Chem. Phys. Chem. 2004, 5, 1405–1410. (27) Brainina, K. Z.; Stozhko, N. Y.; Shalygina, Z. J. Anal. Chem. 2004, 59, 753–759. (28) Murray, R. W. Chem. Rev. 2008, 108, 2688–2720. (29) Xu, H.; Zeng, L. P.; Xing, S. J.; Shi, G. Y.; Xian, Y. Z.; Jin, L. T. Electrochem. Commun. 2008, 10, 1839–1843. (30) Davies, T. J.; Compton, R. G. J. Electroanal. Chem. 2005, 585, 63–82. (31) Davies, T. J.; Banks, C. E.; Compton, R. G. J. Solid State Electrochem. 2005, 9, 797–808. (32) Simm, A. O.; Banks, C. E.; Ward-Jones, S.; Davies, T. J.; Lawrence, N. S.; Jones, T. G. J.; Jiang, L.; Compton, R. G. Analyst 2005, 130, 1303–1311. (33) Simm, A. O.; Ward-Jones, S.; Banks, C. E.; Compton, R. G. Anal. Sci. 2005, 21, 667–671. (34) Cheng, W.; Dong, S.; Wang, E. Langmuir 2002, 18, 9947–9952. (35) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625–2630. (36) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1984, 161, 247–268. (37) Cassidy, J.; Ghoroghchian, J.; Sarfarazi, F.; Smith, J. J.; Pons, S. Electrochim. Acta 1986, 31, 629–636.

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chemical approach.25 These studies demonstrated that AuNPbased microelectrode ensembles combined with stripping voltammetry could well meet the requirements of decentralized pointof-care tests or field detections of Hg ions in the environment. Despite these outstanding achievements obtained, effective construction of AuNP-based microelectrode ensembles onto a certain support for monitoring Hg(II) is still challenging. Over the past decade, one-dimensional (1D) inorganic-organic hybrid nanomaterials have received much interest because of their intriguing properties and potential applications in chemical or biochemical sensors, catalysis, and nanodevices.38-45 Decoration of organic nanowires with metal NPs could be an attractive route to fabricate inorganic-organic hybrid nanomaterials without compromising the functions of the nanowires or nanoparticles.40 In this work, we report a new approach to construct AuNP-based ensembles by the use of inorganic-organic hybrid nanomaterials. 3, 3′, 5, 5′-Tetramethylbenzidine (TMB) has an excellent coordination ability with transition metal ions.46 Here, TMB-based organic nanofibers (NFs) doped with Pt(II) were synthesized through a facile wet-chemical process,47 followed by AuNPs that were electrodeposited onto the 3D interlaced network of organic nanofibers. Interestingly, a part of Pt(II) ions in TMB-based organic nanofibers were simultaneously reduced to metallic Pt during the decoration of AuNPs onto the organic NFs. This brings about the formation of bimetallic Au-PtNP/NF inorganic-organic hybrid nanostructures. Recently, bimetallic NPs are extensively investigated on account of their extraordinary properties, such as good conductivity, and better catalytic activities than their monometallic counterparts.48-54 It is worth noting that the asformed composite matrix in our work possesses a 3D porous network structure with a large effective surface area and high catalytic activity and behaves like microelectrode ensembles. This (38) Briseno, A. L.; Mannsfeld, S. C. B.; Formo, E.; Xiong, Y. J.; Lu, X. M.; Bao, Z. N.; Jenekhe, S. A.; Xia, Y. N. J. Mater. Chem. 2008, 18, 5395–5398. (39) Yoshida, T.; Zhang, J.; Komatsu, D.; Sawatani, S.; Minoura, H.; Pauporte´, T.; Lincot, D.; Oekermann, T.; Schlettwein, D.; Tada, H.; Wohrle, D.; Funabiki, K.; Matsui, M.; Miura, H.; Yanagi, H. Adv. Funct. Mater. 2009, 19, 17–43. (40) Milliron, D. J.; Gur, I.; Alivisatos, A. P. MRS Bull. 2005, 30, 41–44. (41) Briseno, A. L.; Mannsfeld, S. C. B.; Liu, X.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Nano Lett. 2007, 7, 668–675. (42) Lu, G.; Li, C.; Shen, J.; Chen, Z.; Shi, G. J. Phys. Chem. C 2007, 111, 5926– 5931. (43) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080. (44) Polleux, J.; Gurlo, A.; Barsan, N.; Weimar, U.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2006, 45, 261–265. (45) Fo ¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10 (3), 195–217. (46) Holland, V. R.; Saunders, B. C.; Rose, F. L.; Walpole, A. L. Tetrahedron 1974, 30, 3299–3302. (47) Yang, J. H.; Wang, H. S.; Lu, L. H.; Wang, Y. B.; Shi, W. D.; Zhang, H. J. Synth. Met. 2008, 158 (4), 572–576. (48) Wang, L. Y.; Shi, X.; Kariuki, N. N.; Schadt, M.; Wang, G. R.; Rendeng, Q.; Choi, J.; Lu, S.; Zhong, C. J. J. Am. Chem. Soc. 2007, 129, 2161–2170. (49) Ghosh, S. K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Appl. Catal., A 2004, 268, 61–66. (50) Liu, J. H.; Wang, A. Q.; Chi, Y. S.; Lin, H. P.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 40–43. (51) Nutt, M. O.; Hughes, J. B.; Wong, M. S. Environ. Sci. Technol. 2005, 39, 1346–1353. (52) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362–365. (53) Lee, Y. W.; Kim, N. H.; Lee, K. Y.; Kwon, K.; Kim, M.; Han, S. W. J. Phys. Chem. C 2008, 112, 6717–6722. (54) Roy, K.; Lahiri, S. Anal. Chem. 2008, 80, 7504–7507.

should greatly facilitate the rapid, stable, and sensitive detection of Hg(II). To the best of our knowledge, this is the first report on the determination of Hg(II) by the use of bimetallic Au-PtNP/ NF inorganic-organic hybrid nanomaterials. Encouragingly, our findings demonstrate that such a novel Au-PtNP/NF hybrid film modified electrode offers remarkably improved sensitivity and selectivity for stripping measurements of Hg(II). EXPERIMENTAL SECTION Apparatus and Chemicals. Electrochemical measurements were performed on a CHI 660A electrochemical workstation (CHI, USA) with a conventional three-electrode system comprising a platinum wire as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference, and the modified or unmodified glass carbon electrode (GCE) as the working electrode. The general morphology of the products was characterized by the scanning electron microscopy (SEM, JSM-5600). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermoelectron instrument (VG multilab 2000). An inductively coupled plasma mass spectrometer (ICPMS, Elan Drc-E, PerkinElmer, USA) was used for the measurements. The Hg(II) concentration was determined from a response curve generated using a series of standard solutions of known Hg(II) concentration. The response curve for calibration was generated using peak intensities. TMB and H2PtCl6 were purchased from Aladdin Co. (Shanghai, China). HAuCl4 was obtained from Alfa Aesar. All other chemicals were of analytical-reagent grade and used without further purification. Deionized water (18.2 MΩ resistance) was used throughout. All experiments were carried out at ambient temperature. The colloidal gold nanoparticles (AuNPs) were prepared according to the literature. (See Supporting Information for preparation details.)25 Electrode Preparation. Prior to modification, the basal GCE was polished to a mirror finish using 1.0, 0.3, and 0.05 µm alumina slurries. After each polishing, the electrode was sonicated in ethanol and doubly distilled water for 5 min, successively, in order to remove any adsorbed substances on the electrode surface. Finally, it was dried under a nitrogen atmosphere until it was ready for use. TMB-based NFs were prepared according to the previous work.47 Briefly, 1.25 mL of 4 mM aqueous H2PtCl6 solution was added into 2 mL of 2.5 mM ethanol TMB solution at room temperature. Several minutes later, doped nanofibers (NFs) were formed with a large amount of blue-violet precipitate observed. Then, the precipitate was collected by centrifugation, washed several times with water, and dried at 50 °C. The suspension of TMB-based NFs (0.75 mg mL-1) was prepared by dispersing the resulting powdered NFs into the ethanol solution under ultrasonication for 2 h. Subsequently, 10 µL of the NF dispersion was dropped onto the surface of the cleansed GCE and was kept at room temperature until dry (labeled as NFs/GCE). The further modification of AuNPs onto NFs/GCE was conducted by cyclic volammetry (CV) scanning from 0.2 to -1.0 V in 0.1 M KCl solution containing 0.5 mM HAuCl4 at a scan rate of 50 mV s-1 for 12 cycles. During the electrodeposition process, a part of doped Pt(II) ions were simultaneously reduced to Pt atoms, thus leading to a bimetallic Au-PtNP/ NF inorganic-organic hybrid nanocomposite. After modification, the electrode (denoted as Au-PtNPs/NFs/GCE) was

Figure 1. High-resolution XPS spectra of (a) Au4f and (b) Pt4f on Au-PtNP/NF composite modified GCE.

thoroughly rinsed with water and kept at room temperature for further use. For comparison, the modification of AuNPs was also carried out by immersion of NFs/GCE into a suspension of colloidal gold NPs to obtain AuNPs/NFs/GCE, where the doped Pt(II) ions were unchanged. Stripping Voltammetric Detection of Mercury. Square wave anodic stripping voltammetry (SWASV) was applied for the successive determination of Hg(II) under the optimized conditions. Mercury was deposited at the potential of 0.5 V for 100 s in 1 M HCl by the reduction of Hg(II) to Hg(0). The anodic stripping of electrodeposited Hg(0) was performed at 0.1-0.7 V with optimized parameters (frequency, 40 Hz; amplitude, 20 mV; potential increment, 4 mV). Multiple successive SWASV scanning was used to remove the deposited mercury until the anodic stripping response disappeared. Also, the regeneration of the sensor surface was achieved. RESULTS AND DISCUSSION Characterization of Au-PtNPs/NFs/GCE Surface. Figure 1 shows the high-resolution XPS spectra for NFs/GCE modified with AuNPs by CV scanning. The Au4f7/2 and Au4f5/2 peaks are present at 84.0 and 87.7 eV, respectively, indicating the successful electrodeposition of metallic Au onto the surface of NFs. The spectrum of Pt4f could be divided into two doublets, assigned to two different Pt species with characteristic binding energies of Pt(II) and Pt(0) states. The Pt4f signal is composed of four individual peaks. Peaks (1) and (2) can be attributed to Pt4f7/2 (71.5 eV) and Pt4f5/2 (75.2 eV) lines of Pt(II), while peaks Analytical Chemistry, Vol. 82, No. 2, January 15, 2010

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Figure 3. Cyclic voltammograms of 5 mM K3[Fe(CN)6] at (a) bare GC, (b) NFs/GCE, and (c) Au-PtNPs/NFs/GCE in 0.1 M KCl. Scan rate: 100 mV s-1.

Figure 2. Typical SEM images: (a) the as-synthesized TMB-based NFs; (b) Au-PtNP/NF inorganic-organic hybrid nanocomposite onto GCE. Inset: corresponding magnified SEM image.

(3) and (4) can be assigned to Pt4f7/2 (72.5 eV) and Pt4f5/2 (75.5 eV) lines of metallic Pt(0), respectively. It is concluded that a part of Pt(II) ions doped in NFs were reduced to Pt(0) during the further electrodeposition of AuNPs by CV scanning. Thus, the bimetallic Au-PtNP/NF inorganic-organic hybrid nanocomposite is obtained. The representative SEM images of the as-synthesized TMBbased NFs and Au-PtNP/NF inorganic-ogranic hybrid nanocomposite onto GCE are shown in Figure 2. It can be seen that the as-prepared NFs have diameters of ∼200 nm and lengths up to several tens of micrometers (Inset of Figure 2a). These nanofibers interlaced together. After the subsequent deposition process, one can see that uniform Au-PtNPs with an average diameter of ∼120 nm aligned along the surface of those NFs. The generated NPs were homogenously distributed in the matrix of interlaced NFs, constructing a 3D interlaced network. To characterize the conductivity of NFs/GCE and Au-PtNPs/ NFs/GCE, the cyclic voltammetric experiments were carried out by the use of Fe(CN)63- as the electrochemical probe. Figure 3 shows the CV curves of GC modified with different materials. It can be seen that Fe(CN)64-/3- redox reaction at the bare GCE gave a couple of well-behaved CV peaks at Em of 0.210 V with ∆Ep of 61 mV (curve a). At NFs/GCE, only a small sigmoid waveform was observed, indicating that TMB-based NFs film onto GCE and dramatically inhibit electron transfer between the redox probe and GCE. Whereas, after the decoration of metallic NPs, peak-shaped voltammograms appeared again, and 570

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the peak currents were apparently increased. According to the Randles-Sevcik equation,55 the surface area of the Au-PtNPs/ NFs/GCE electrode was calculated to be 0.0674 cm2, 5 times larger than that of NFs/GCE (0.0128 cm2). This observation indicates that the electrode/solution interface becomes accessible to electrochemical probes with the modification of NPs, consistent with the SEM results. The generated NPs were homogenously distributed along the interlaced NFs, constructing individual elements of the electrode. Owing to the small spacing between each individual “microelectrode” in comparison with the size of the NPs, the construction of electrode interface having micro/nanoscale chemical architectures could result in the overlapping diffusion zones and resultant planar diffusion to the array.18 Herein, a peak-shaped voltammogram was observed again at Au-PtNPs/NFs/GCE, indicating the overlap of diffusion layers at the individual elements of the electrode. Obviously, we successfully constructed AuNP-based microelectrode ensembles by the use of inorganic-organic hybrid nanomaterials. Because the ratio of faradic to the capacitive current is significantly high at a microelectrode ensemble, the electroanalytical detection limits will be much lower than that at an analogous macrosized electrode.35-37 As the supporting matrix, this inorganic-organic hybrid nanocomposite of Au-PtNPs/NFs provides an efficient and advantageous platform for sensing applications. Electrochemical Behavior of Hg(II) at Au-PtNPs/NFs/ GCE. The electrochemical response of the Au-PtNPs/NFs/GCE toward Hg(II) was first examined with cyclic voltammetry. Figure 4 displays the CV curves of Hg(II) scanning from -0.45 to 0.80 V at the scan rate of 100 mV s-1 in 1 M HCl. No peak was observed at bare GCE (curve a) and NFs/GCE (curve b) in 2 ppb Hg(II). Whereas, with the further modification of NPs, at Au-PtNPs/ NFs/GCE, a pair of well-defined redox peaks (Epc ) 0.522 V; Epa ) 0.556 V) were observed (curve c). Such a voltammetric response has not been observed at Au-PtNPs/NFs/GCE in the absence of Hg(II). It can be ascribed to the reduction of Hg(II) to Hg(0) first during the cathodic sweep and then to the reoxidation of Hg(0) to Hg(II) again during the reversal anodic sweep. Another peak was observed at about -0.22 V at (55) Bard, A. J.; Faulkner, L. R. Electrochemical Methods-Fundamentals and Applications: John Wiley and Sons; New York, 2000.

Figure 4. Cyclic voltammograms of (a) bare GCE, (b) NFs/GCE, and (c) Au-PtNPs/NFs/GCE in the presence of 2 ppb Hg(II) in 1 M HCl. Scan rate: 100 mV s-1. The inset is stripping voltammograms of (a) Au-PtNPs/NFs/GCE in the absence of Hg(II), (b) AuNPs/NFs/ GCE, and (c) Au-PtNPs/NFs/GCE in the presence of 2 ppb Hg(II) in 1 M HCl. SWV conditions: scanning potential range, 0.1-0.7 V; frequency, 40 Hz; potential increment, 4 mV; amplitude of the squarewave, 20 mV.

Au-PtNPs/NFs/GCE. It can be attributed to the characteristic peaks of the hydrogen adsorption and desorption (Had/Hde) onto PtNPs, further confirming the existence of Pt(0) in the composite. It agrees well with the XPS results. The stripping voltammetric technique is known to be one of the most sensitive techniques in the electroanalysis of trace metals in different samples. It involves two steps for the detection of Hg(II): (i) deposition of Hg(0) at an optimized potential for a certain duration of time and (ii) anodic stripping of deposited Hg(0). The anodic stripping signal has been used to monitor the concentration of Hg(II) in solution. The deposition potential and the time were optimized to be 0.50 V and 100 s, respectively. As shown in the inset of Figure 4, no anodic stripping peak was observed at Au-PtNPs/NFs/GCE in 1 M HCl (curve a). Evidently, a very sharp and well-defined stripping peak at a potential of about 0.56 V vs SCE (curve c) appeared in the presence of 2 ppb Hg(II). For comparison, AuNPs directly modified NFs nanocomposited onto GCE (AuNPs/NFs/GCE) were also fabricated by directly immersing the NFs/GCE into the colloidal gold suspension for ∼10 min (typical SEM images shown in Figure S-1, Supporting Information). Compared with the stripping peak response of Hg(II) at AuNPs/NFs/GCE, the stripping peak current at Au-PtNPs/NFs/GCE was greatly enhanced. Obviously, the presence of the bimetallic Au-Pt inorganic-organic hybrid nanomaterials plays a great role, providing an advantageous and high-performance platform for the sensing of Hg. Optimization for the Detection of Hg(II) at Au-PtNPs/ NFs/GCE. We further optimized the experimental parameters to get high-performance stripping analysis of mercury. Since the bimetallic Au-PtNP inorganic-organic hybrid nanomaterial plays an important role in the performance of the sensor, the effect of the amounts of Au-PtNPs onto the electrode was investigated by controlling the cycles of CV scanning (in 0.5 mM HAuCl4 + 0.1 M KCl). The potential scanning cycles would directly affect the size and the composition of the bimetallic Au-PtNPs produced. As shown in Figure 5a, the stripping peak current of Hg(II) rises with the cycles at first

Figure 5. Effects of (a) potential scanning cycles in 0.5 mM HAuCl4 + 0.1 M KCl, (b) deposition potential, and, (c) deposition time on the stripping responses of 10 ppb Hg(II) at Au-PtNPs/NFs/GCE in 1 M HCl. Electrochemical stripping detection conditions are the same as Figure 4.

up to 12 cycles and then decreases. It is likely related to the nanostructural platform ensembled by Au-PtNPs onto electrode. Owing to the increased surface area caused by the modification of metal NPs, the available electrode surface sites for Hg deposition increase with the increase of the CV cycles up to 12 cycles. With the further increase of the potential scanning cycles, the metal NPs would be continuously generated. However, the aggregation of metal NPs may occur, leading to a decrease of the sensing performance. So 12 potential cycles were used to prepare Au-PtNP/NF modified GCE. Analytical Chemistry, Vol. 82, No. 2, January 15, 2010

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Table 1. Effects of Various Metal Ions on the Electrochemical Stripping Signals of Hg2+ at Au-PtNPs/NFs/GCE (2 ppb Hg2+ and 20 ppb each for Cu2+, Cr3+, Co2+, Fe2+, Zn2+, As3+, and Mn2+)a interference 2+

Cu Cr3+ Co2+ Fe2+ Zn2+ As3+ Mn2+

contribution (%) (ip.Hg(II) ) 100%) +0.24 ± 0.01 +12.65 ± 0.50 +9.23 ± 0.35 -1.71 ± 0.14 -3.87 ± 0.18 -1.28 ± 0.11 -2.75 ± 0.15

a SWV conditions: scanning potential range, 0.1-0.7 V; frequency, 40 Hz; potential increment, 4 mV; amplitude of the square-wave, 20 mV.

Figure 6. Stripping voltammograms of increasing mercury concentrations (from bottom to top: 0, 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 ppb, respectively). The inset (a) is stripping voltammograms of Au-PtNPs/NFs/GCE toward the lower concentrations of Hg(II) (from bottom to top: 0.02, 0.08, 0.1, and 0.25 ppb, respectively); The inset (b) shows the calibration curve. Electrochemical stripping detection conditions are the same as those in the inset of Figure 4.

Effects of the deposition potential and time on the Hg stripping responses were also investigated. As shown in Figure 4, the reduction of Hg(II) to Hg(0) occurred at Epc ) 0.522 V vs SCE. We explored the effects of the deposition potential (from -0.5 to +0.5 V) on the stripping signal (Figure 5b). The best stripping signal is obtained at -0.1 V, and second best is obtained at 0.5 V. The stripping voltammetric technique involves first deposition of metal(0) at an optimized potential and then anodic stripping. To use a more positive deposition potential to lower the interferences is a facile approach.17 So a deposition potential of 0.5 V was selected, a balance between maximum signal and the interference from other heavy metals avoided. Figure 5c shows the relationship between the Hg stripping signal and the deposition time at 0.5 V for Au-PtNPs/NFs/GCE. The peak current increased proportionally with time between 50 and 100 s. When the deposition time was longer than 100 s, the curve leveled off, indicating a saturation of the available electrode surface sites for Hg deposition. It is supposed that no more Hg can be deposited once Hg covers the entire Au-PtNPs/NFs/GCE surface. Analytical Performance for the Detection of Hg(II). Under the optimized experimental conditions, the Au-PtNPs/NFs/GCE was applied for the successive determination of Hg(II) by squarewave anodic stripping voltammetry (SWASV). Figure 6 shows the SWASV response of the Au-PtNPs/NFs/GCE toward Hg(II) at different concentrations in 1 M HCl. A very good linearity of peak current versus Hg(II) concentration was obtained with a correlation coefficient of 0.9982. From the slope of the calibration plot, the sensitivity of the electrode was 7.993 µA/ppb, higher than that obtained by the previously reported methods.18,24,25 The calibration plot was linear up to 10 ppb, and the detection limit of 0.008 ppb (8 ppt) was obtained with the calculation based on a signal-to-noise ratio equal to 3 (S/N ) 3). This detection limit is significantly lower than 0.3 ppb (5 min deposition) at a polyviologen modified electrode by the use of stripping analysis16 and lower than those reported with AuNP-based ensemble modified electrodes.25,29 Such a high sensitivity and low detection limit obtained is most likely ascribed to the ensemble behavior of the nanostructured platform. The bimetallic Au-PtNP/NF inorganic572

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organic hybrid composite constructs a high-performance platform for the sensing of Hg(II). Our results demonstrate that the proposed Au-PtNP/NF composite is reliable for the detection of Hg(II). As the detection limit achieved is well below the guideline value of Hg(II) in drinking water (1 ppb) set by the World Health Organization (WHO),56 it is expected that the Au-PtNPs/ NFs/GCE can be used for real samples analysis. The relative standard deviation was 1.2% for 10 replicate determinations of 2 ppb Hg(II), indicating the acceptable reproducibility. Long-term stability of the electrode is very essential for the in situ monitoring mercury level in water. The stability of the Au-Pt/NFs/GCE stored at room temperature was studied. No observable change in the magnitude of the stripping current was observed in the first 10 day storage. After a 30 day storage period, the sensor retained 85% of its initial current response. Selectivity. Selective detection of Hg(II) in the solution is a challenging task, as the other metal ions are commonly present in the real sample. They can be codeposited and stripped off under the experimental condition used for the detection of Hg(II). In our work, the possible interferences arising from Cu(II), Co(II), Cr(III), Fe(II), Zn(II), and Mn(II) ions were used to evaluate the selectivity of the Au-PtNPs/NFs/GCE to Hg(II). Table 1 shows the electrochemical stripping signals of mercury in the presence of 10-fold concentrations of each element in the solution with respect to Hg(II). One can see that the stripping peak current of 2 ppb Hg(II) varies slightly, indicating that these metal ions do not interfere with the detection of mercury. Evidently, the selectivity of the sensor could be remarkably improved by the use of a Au-PtNP/NF modified electrode. It is suggested that the nanostructured platform composed of a bimetallic Au-PtNP/NF inorganic-organic hybrid should be responsible for the satisfactory selectivity toward the detection of Hg(II). It is likely due to the strong interaction between bimetallic Au-PtNPs/NFs and mercury ions. Another reason is that the high deposition potential of 0.50 V applied could efficiently avoid the interference from those metal ions. Under the present deposition potential (0.5 V), the stripping responses from other metal ions will be greatly diminished, due to their deposition potentials far below 0.5 V. For example, when the deposition potential was set on -0.1 V vs SCE, the interference from Cu(II) become apparent (Figure S-2, Supporting Information). To use a more positive deposition potential to lower the interferences is a facile approach.17 (56) http://www.who.int/water sanitation health/dwq/chemicals/mercury/ en.

Table 2. Determination of Hg(II) in Real Water Samples Using the Proposed Method (n ) 3) concentration sample

taken (ppb)

found (ppb)

RSD (%)

recovery (%)

tap water 1 tap water 2 tap water 3 tap water 4 tap water 5 tap water 6 tap water 7 river water 1 river water 2 river water 3 river water 4 river water 5 river water 6 river water 7

0.02 0.08 0.10 0.25 0.80 2.00 7.50 0 0.20 0.50 3.00 5.00 6.50 8.00

0.019 0.079 0.105 0.26 0.79 2.07 7.35 0.52 0.70 1.07 3.59 5.46 7.06 8.68

1.50 0.45 0.32 1.53 0.63 0.58 1.13 3.50 1.40 1.00 1.87 0.28 0.71 0.02

95.0 98.7 105.0 100.4 98.8 103.5 98.0 97.2 104.9 102.0 98.9 100.5 101.9

Therefore, such a deposition potential of 0.5 V applied could favorably avoid the interference from other heavy metal ions. Not only is the detection limit of our Au-PtNP/NF modified electrode below the guideline value of Hg(II) set by the WHO but also it does not suffer from the interference. The performance of our novel electrode is superior to the existing ones.16,25,29 Analysis of the Real Samples. To further demonstrate the practicality of the present electrode, it was evaluated by processing tap and river water samples. A river water sample was collected from Yangtze River (Wuhan, China) and treated through a standard 0.45 µm filter. All the water samples were spiked with Hg(II) at different concentration levels, which were prepared on the basis of possible metal ions present in the environmental water and then analyzed with the proposed method (summarized in Table 2). The original mercury concentration in the river water sample was tested to be 0.52 ppb, well below the guideline value of Hg(II) in drinking water (1 ppb) set by the WHO. The accuracy of the method was also assessed by comparing the electrochemical results with those obtained by ICPMS. A river water sample collected from Yangtze River was analyzed using both methods. A value of 0.52 ± 0.02 ppb was found electrochemically, and a

value of 0.47 ± 0.02 ppb was obtained by ICPMS, showing a difference of 10.6%. The results indicated that the proposed method was highly accurate, precise, and reproducible. It can be used for direct analysis of relevant real samples. CONCLUSIONS In this work, a high-performance and sensitive platform for the stripping analysis of Hg(II) has been successfully built. This platform is based on a bimetallic Au-PtNP/NF inorganic-organic hybrid nanocomposite. Bimetallic Au-PtNPs are homogenously distributed in the interlaced NF matrix, constructing a 3D porous network. Such a 3D porous nanostructured composite film greatly facilitates electron-transfer processes and the sensing behavior for Hg(II) detection, leading to a remarkably improved sensitivity and selectivity. The resulting biosensor exhibits fine applicability for the detection of Hg(II) in practical water samples. We believe that the sensor modified with the bimetallic inorganic-organic hybrid nanomaterials will be attractive to detect toxic heavy metal ions. Further study is being undergone. ACKNOWLEDGMENT This work was supported by National Science Foundation of China (Grant 20803026), Natural Science Foundation of Hubei Province (Grant 2008CDB032), Natural Science Foundation of Central China Normal University, Program for Innovation Team of Hubei Province (Grant 2009CDA048), and the Key Project of Ministry of Education of China (Grant 108097). SUPPORTING INFORMATION AVAILABLE The preparation of colloidal gold nanoparticles, SEM images of AuNPs/NFs/GCE, and stripping voltammograms of 2 ppb Hg(II) and 2 ppb Hg(II) + 20 ppb Cu(II) (deposition at 0.5 V for 100 s and deposition at -0.1 V for 100 s, respectively) in 1 M HCl. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review August 16, 2009. Accepted November 27, 2009. AC901846A

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