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l -Cysteine modified flexible PDMS–gold electrode for sensing ascorbic acid and copper
Sensors and Actuators B 161 (2012) 1124–1128
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
l-Cysteine modified flexible PDMS–gold electrode for sensing ascorbic acid and copper Dahe Fan a , Lianhua Bi a,b , Fan Tang a , Huxiang Zheng a , Qin Xu b,∗ , Wei Wang a,∗∗ a Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, School of Chemical and biological Engineering, Yancheng Institute of Technology, Yancheng 224051, China b School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
a r t i c l e
i n f o
Article history: Received 20 September 2011 Received in revised form 17 October 2011 Accepted 1 November 2011 Available online 6 November 2011 Keywords: Flexible poly(dimethylsioxane) (PDMS)-based gold electrode l-Cysteine modified gold electrode Copper Ascorbic acid Portable device
a b s t r a c t Applications of flexible poly(dimethylsioxane) (PDMS)-based gold electrode were expanded, modification of l-cysteine on the electrode was carried out, and applied for sensing ascorbic acid and copper. Electrochemical properties of proposed modified electrode were similar to that of conventional electrode. Copper(II) was accumulated on the l-cysteine modified electrode and measured with cyclic voltammetry, reductive currents were linear with concentrations of copper(II) in range of 10−8 to 1.5 × 10−6 M and limit of detection was 5.0 × 10−9 M. This proposed sensor could give quick response to ascorbic acid (AA) (<5 s) and the currents were linear with concentrations of AA in range of 6.6 × 10−7 to 2.6 × 10−3 M and limit of detection was 2 × 10−7 M. The performance was better than previous reports. The devices are simple, low-cost, easy-to-fabricate and portable. Therefore, conventional modification methods could be applied on the flexible poly(dimethylsioxane) (PDMS)-based gold electrode for point-of-care applications in environmental monitoring, public health, and food safety. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polydimethylsiloxane (PDMS) is an attractive polymeric matrix due to its many favorable properties such as chemical inertness, biocompatibility, mechanical flexibility, nontoxic, stability, high dielectric constant, optical clarity in the visible and ultraviolet region and importantly, ease of processing [1]. Since PDMS–gold nanoparticles (AuNPs) composite films and patterned Au/PDMS substrate with chemical gold plating method on native PDMS was introduced [2,3]. Our group and Wu et al. have demonstrated the gold layer could be applied as electrode for electrochemical applications [4,5]. The advantages of self-assembled monolayer (SAM) include their ease of preparation, their stability and the possibility of introducing different chemical functionalities. The incorporation of appropriate chemical functionality with some molecular level control into the highly ordered monolayers allows the preparation of surfaces with tailor-made properties. Modification of gold electrode with SAM of alkane-thiol is of research interests [6]. The high organization of the monolayer assures cooperative and homogeneous behavior of the entire electrode surface. Therefore, the
∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 515 88298848; fax: +86 515 88298191. E-mail addresses: [email protected] (Q. Xu), [email protected], [email protected] (W. Wang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.001
self-assembly technique is a convenient method due to its versatility in modifying surfaces in a controllable manner. It has been shown that organothiol molecules upon adsorption at gold lose the hydrogen from the thiol group and that an S–Au bond is formed [7,8]. Amino acids and peptides have been employed as the recognition element for sensing based on self-assembled monolayers. l-Cysteine has been applied as selective ligand for chemical accumulation of metal ions [9–13], and as a promoter for increasing the rate of electron transfer [14,15]. A cysteine monolayer has the ability to have either a net positive or a net negative charge depending on the solution pH, this property could be leveraged to electrostatically deposit either polycationic or polyanionic polymers onto a cysteine monolayer by adjusting the bulk solution pH [16]. In this research, we demonstrated extensive applications of flexible poly(dimethylsioxane) (PDMS)-based gold electrode. Modification of l-cysteine on the electrode was carried out, and applied for sensing ascorbic acid and copper. Electrochemical properties of proposed modified electrode were similar to that of conventional electrode. Copper(II) was accumulated on the l-cysteine modified electrode and measured with cyclic voltammetry, reductive currents were linear with concentrations of copper(II) in range of 10−8 to 10−6 M and limit of detection was 5.0 × 10−9 M. This proposed sensor could give quick response to ascorbic acid (AA) (<5 s) and the currents were linear with concentrations of AA in range of 6.6 × 10−7 to 2.6 × 10−3 M and limit of detection was 2 × 10−7 M. The devices were simple, low-cost, easy-to-fabricate and portable.
D. Fan et al. / Sensors and Actuators B 161 (2012) 1124–1128
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2. Experimental 2.1. Materials and instrumentation Poly(dimethylsiloxane) (PDMS) was obtained from Dow Corning (midland, MI, USA). l-Cysteine (C3 H7 NO2 S), glucose and chloroauric acid (HAuCl4 ·4H2 O) were from Sigma–Aldrich. Ascorbic acid (AA), potassium chloride (KCl), and copper nitrate (Cu(NO3 )2 ) were from Shanghai Chemical Reagent Company (Shanghai, China). Sodium acetate (CH3 COONa) was purchased from Northeast Pharmaceutical Group Co., Ltd, acetic acid (CH3 COOH) and potassium hydrogen carbonate (KHCO3 ) were obtained from Shanghai Shenxiang Chemical Reagent Company (Shanghai, China). Supporting electrolytes were 0.1 M acetate buffer and 0.05 M phosphate buffer. All solutions were prepared with deionized water. All reagents were of analytical grade. All electrochemical experiments were performed with a CHI840C electrochemical workstation (Shanghai Chenhua Company, Shanghai, China).
Fig. 1. Cyclic voltammogram for the reductive desorption of PDMS–AuNPs–LC electrode in 0.5 M NaOH solution (deaerated) at the scan rate of 100 mV s−1 .
2.2. Fabrication of flexible PDMS based electrode 3. Results and discussion PDMS film was prepared similar as described procedure in Refs. [3–5]. Briefly, PDMS monomer and the curing agent were firstly mixed in a proportion of 1:0.1 and cured at 120 ◦ C for 30 min. Chemical gold plating solution containing 0.03 M HAuCl4 , 2 M KHCO3 , and 9.9 M glucose (v/v, 2:1:1) was prepared just before use. Plexiglass frame with inner area 3 cm × 4 cm was sandwiched between two native 2-mm-thick PDMS films named ‘cover chip’ and ‘base chip’, and then the solution for chemical gold plating was injected into this sandwich architecture. It was then incubated at room temperature for 4 h to ensure the elemental gold deposit completely. All resulting PDMS–gold films were washed with deionized water for three times, and stored at room temperature when not in use. A piece of PDMS–gold film of 2.5 cm × 0.4 cm was cut as working electrode, working electrode area was defined as 1.5 cm × 0.4 cm controlled by transparent adhesive tape and connected by indium tin oxide (ITO) conductive glass. The counter electrode is Pt and reference electrode is Ag/AgCl. 2.3. Modification of l-cysteine on PDMS–AuNPs electrode for detection of Cu(II) Gold electrode mentioned above was immersed in phosphate buffer containing 10 mM l-cysteine for 30 min at room temperature for self-assembly of cysteine. Chemical accumulation of Cu2+ on the electrode was carried out at open circuit by dipping the electrode into 10 mL of a stirred, buffered (pH 5.5) solution of copper nitrate for 5 min. Then the electrode was removed from the cell, washed thoroughly with purified water, and transferred to a cell containing phosphate buffer solution (pH 5.5). Cyclic voltammograms were recorded at a scan rate of 100 mV s−1 and the potential range from −0.4 to 0.7 V. 2.4. Modification of l-cysteine on PDMS–AuNPs electrode for detection of AA Gold electrode mentioned above was immersed in 0.1 M HAc–NaAc buffer containing 20 mM l-cysteine for 24 h at room temperature for self-assembly of cysteine. Chemical measurements were performed in HAc–NaAc buffered solution (pH 4.5). Cyclic voltammograms were recorded in the potential range from −0.4 to 0.7 V. Prior to each experiment, the solution was deoxygenated with nitrogen and kept in nitrogen atmosphere at room temperature.
3.1. Electrochemical desorption of l-cysteine from a modified gold electrode l-Cysteine adsorbs onto gold electrodes via the thiol side-chain of amino acid. N-alkanethiols adsorbed on gold and silver surfaces are both oxidatively and reductively desorbed, and the potential for the reductive desorption process is dependent on both the strength of the gold–sulfur interaction and the extent of the intermolecular interactions between adjacent adsorbates [17,18]. Thiols are reductively desorbed by the following reaction. Au–SR + e− = Au + RS−
(1)
The cyclic voltammogram (Fig. 1) of the l-cysteine modified electrode in a deaerated NaOH solution (0.5 M) from −0.2 to −1.4 V (vs. Ag/AgCl) exhibits two reduction peaks, it is similar to the literature reported [17,19]. We propose that the first peak (at −0.7 V) is due to the cleavage of the gold–sulfur bond (having a shape characteristic of an adsorbed species) and the second peak, having more diffusion-like character is possibly due to a similar field-induced rearrangement of cysteine clusters after cleavage. 3.2. Adsorption of copper at a gold electrode modified with l-cysteine Voltammograms of a gold electrode modified with l-cysteine before and after adsorption from buffer containing 10 M Cu2+ for 5 min were shown in Fig. 2. In the absence of copper(II) ions, there were no peaks in the gold electrode modified with l-cysteine. After copper accumulation for 5 min in a 10 M Cu2+ solution and washing, the voltammogram was recorded in buffer solution that was free of copper. A peak appeared at 0.115 V during Cu(II) was reduced to Cu(I). Cu(I) was reoxidized during the subsequent positive sweep with a peak potential of +0.359 mV. Repeated cycling did not lead to a significant loss of signal, which indicated that copper remained strongly adsorbed in both oxidation states within the potential range of the experiment. Fig. 3 showed electrochemical behavior of our PDMS–gold electrode immersed in 1 M Cu(II) solution. The insert showed anodic and cathodic peak currents were directly proportional to the square root of the scan rate between 10 and 500 mV s−1 . The solid lines represented a linear fit to both oxidation current and reduction current with the regression equation: upper data, Y = 8.2663 + 0.2131X
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Fig. 2. Cyclic voltammograms of a PDMS–AuNPs–LC electrode in phosphate buffer (pH 5.5) and after subsequent immersion in 1 M Cu(II) for 5 min and return to the phosphate buffer. Scan rate: 100 mV s−1 .
Fig. 4. Cyclic voltammograms of PDMS–AuNPs–LC electrode immersed in 0.01, 0.1, 0.5, 1.0, 5.0, 10 and 15 M Cu(II) solutions for 5 min. Inset: calibration plot of the dependence of the measured current on Cu(II) concentrations. Scan rate: 100 mV s−1 .
(R = 0.9966), lower data, Y = −9.4674 − 0.2018X (R = 0.9964). The linearity indicated the mass transfer in this system was a diffusion controlled process. The peak shape of CVs showed a typical reversible electrochemical reaction in which the rate of reaction was governed by diffusion of the electroactive species to the surface of a planar electrode. Under the optimum analytical conditions, the linear range of Cu(II) detection was studied. Fig. 4 showed the cyclic voltammograms of various concentrations of Cu(II) at the PDMS–AuNPs–LC electrode. The dependence of reduction peak current on the concentration of Cu(II) was in a linear relationship in the range of 0.01–1.5 M. The linear regression equation was expressed as Y = 14.0798 + 10.2185X (R = 0.9977). The detection limit of Cu(II) is 0.005 M (based on the signal-to-noise ratio of 3). RSD was less than 3.4% for 5 parallel measurements. The performance was consistent with the conventional electrode [17], in which the linear range was 0.08–10.0 M and the detection limit was less than 0.005 M. The sensor could be regenerated in 0.1 M perchloric acid, for detection of 0.1 M Cu(II), some metal ions with 100 times
which could interact with L-cysteine such as zinc, lead and manganese will increase the response signal [17]. When a higher concentration of Cu(II) 100 M to 0.1 M was applied in the research, a second redox process appeared at a more negative potential in the cyclic voltammogram. We tested cyclic voltammetry at the PDMS–AuNPs–LC electrode that had been immersed in 100 M Cu2+ (data not shown). The phenomenon could be explained as that the new peaks seen after immersion in high concentration of Cu2+ resulted from the electrochemistry of Cu+ (possibly as Cu2 S) at the electrode surface after breakdown of part of the cysteine monolayer [17,20].
Fig. 3. Cyclic voltammograms of PDMS–AuNPs–LC electrode accumulated in 1 M Cu(II) solution at different scan rates (from inner to outer: 10, 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mV s−1 ), The insert is the relationship of redox currents and the square root of the scan rates. Solid lines represent a linear fit to both oxidation current and reduction current with the regression equation: upper data, Y = 8.2663 + 0.2131X (R = 0.9966); lower data, Y = −9.4674 − 0.2018X (R = 0.9964).
3.3. Electrochemical detection of AA at a PDMS–AuNPs electrode modified with l-cysteine Fig. 5 showed cyclic voltammograms of PDMS–AuNPs and PDMS–AuNPs–LC electrodes with the potential range of 0.1–0.7 V in a deaerated HAc–NaAc buffer containing 1.0 mM AA at a scan rate of 100 mV s−1 . There was a small and broad anodic peak appeared at the PDMS–AuNPs electrode at 0.45 V approximately, however, a sharp well-defined anodic peak appeared at the PDMS–AuNPs–LC electrode at 0.32 V. These phenomena suggested that the oxidation of AA was more favorable at the PDMS–AuNPs–LC electrode.
Fig. 5. Cyclic voltammograms of 1.1 mM AA at PDMS–AuNPs electrode (a) and PDMS–AuNPs–LC electrode (b) in HAc-NaAc buffer solution (pH 4.5) at scan rate of 100 mV s−1 .
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2.7% for 5 parallel measurements. The performance is better than unmodified electrode [4], in which the linear range was 2.3 × 10−5 to 7.0 × 10−3 M, and detection limit was 8.0 × 10−6 M. The response time was about 5 s, which indicated a generally fast electrontransfer process in this PDMS-based gold electrode. Stability of the electrode was investigated after it was stored in refrigerator (4 ◦ C) for two weeks, amperometric response signal was equal to that of freshly prepared PDMS-based gold electrode. 4. Conclusion
Fig. 6. Cyclic voltammograms of PDMS–AuNPs–LC electrode at different scan rates (from inner to outer: 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mV s−1 ). Insert: plots of the dependence of the anodic peak current on square root of scan rates.
The reason was that l-cysteine self-assembled film could act as a promoter to increase the rate of electron transfer and lower the overpotential of AA [15]. Electrochemical behavior of PDMS–gold electrode modified with LC in 0.16 mM AA was shown in Fig. 6. The insert showed anodic peak currents were directly proportional to the square root of the scan rate between 50 and 500 mV s−1 . The solid line represented a linear fit to both oxidation current with the regression equation Y = −5.4647 + 3.7082X (R = 0.9972). The linearity indicated the mass transfer in this system was a diffusion controlled process. Usually, dopamine is coexistent composition and will give interference in the determination of AA, according to Ref. [21], AA can be distinguished at different potential. Current–time response curves were observed at 0.3 V (vs. Ag/AgCl) with subsequent spiking of 1.0 mM AA in 50 mL of 0.1 M HAc–NaAc (pH 4.5). The i–t calibration curve and relationship between catalytic current and concentration of AA were shown in Fig. 7. The linear relationship between catalytic current and concentration was observed in the range of 6.6 × 10−7 to 2.6 × 10−3 M. The linear regression equation were expressed as Y = −0.0205 + 54.2691X (R = 0.9987), with the detection limit of 2.0 × 10−7 M at a signal-to-noise ratio of 3. RSD was less than
Fig. 7. i–t curves of successive additions of 1.0 mM AA in 50 mL of 0.1 M HAc–NaAc (pH 4.5) with PDMS–AuNPs–LC electrode. Insert: plots of the dependence of concentrations of AA on the catalytic of peak currents.
We demonstrate here that flexible poly(dimethylsioxane) (PDMS)-based gold electrode is suitable for modification according to conventional methods. With l-cysteine modification, the electrode is applied for copper and ascorbic acid detection, the characterizations of modified electrode are similar to that of conventional electrode. This research indicates that flexible PDMS–AuNPs electrode have the potential applications for pointof-care devices in environmental monitoring, public health, and food safety. Acknowledgements We greatly appreciate the financial support of National Natural Science Foundation of China (20875080 and 20705030), Foundation of International Cooperation of Jiangsu Province (BZ2010053), Foundation of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (AE201017), and Scientific Project of Yancheng Institute of Technology (XKY2009010, XKY2009016 and XKY2011003). References [1] M.L. Van Poll, F. Zhou, M. Ramstedt, L. Hu, W.T.S. Huck, A self-assembly approach to chemical micropatterning of poly(dimethylsiloxane), Angew. Chem. Int. Ed. 46 (2007) 6634–6637. [2] Q. Zhang, J.J. Xu, Y. Liu, H.Y. Chen, In-situ synthesis of poly(dimethylsiloxane)–gold nanoparticles composite films and its application in microfluidic systems, Lab Chip 8 (2) (2008) 352–357. [3] H.J. Bai, M.L. Shao, H.L. Gou, J.J. Xu, H.Y. Chen, Patterned Au/poly(dimethylsiloxane) substrate fabricated by chemical plating coupled with electrochemical etching for cell patterning, Langmuir 25 (2009) 10402–10407. [4] Q. Xu, L. Bi, H. Zheng, D. Fan, W. Wang, PDMS-based gold electrode for sensing ascorbic acid, Colloids Surf. B: Biointerfaces 88 (2011) 362–365. [5] W.Y. Wu, X.Q. Zhong, W. Wang, J.J. Zhu, Flexible PDMS-based three-electrode sensor, Electrochem. Commun. 12 (2010) 1600–1604. [6] Z.H. Wang, Y.M. Wang, G.A. Luo, A selective voltammetric method for uric acid detection at -cyclodextrin modified electrode incorporating carbon nanotubes, Analyst 127 (2002) 1353–1358. [7] Y.Z. Li, J.Y. Huang, R.T. McIver, J.C. Hemminger, Characterization of thiol selfassembled films by laser desorption Fourier-transform mass-spectrometry, J. Am. Chem. Soc. 114 (1992) 2428–2432. [8] G.Z. Hu, Y.G. Ma, Y. Guo, S.J. Shao, Electrocatalytic oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode, Electrochim. Acta 53 (2008) 6610–6615. [9] M. Koneswaran, R. Narayanaswamy, l-Cysteine-capped ZnS quantum dots based fluorescence sensor for Cu(2+) ion, Sens. Actuators B 139 (2009) 104–109. [10] A.C. Liu, D.C. Chen, C.C. Lin, H.H. Chou, C.H. Chen, Application of cysteine monolayers for electrochemical determination of sub-ppb copper (II), Anal. Chem. 71 (1999) 1549–1552. [11] W. Stricks, I.M. Kolthoff, Polarographic investigations of reactions in aqueous solutions containing copper and cysteine (cystine). 1. Cuprous copper and cysteine in ammoniacal medium—the dissociation constant of cuprous cysteinate, J. Am. Chem. Soc. 73 (1951) 1723–1727. [12] Y.Z. Fu, L.L. Wang, Q. Chen, J. Zhou, Enantioselective recognition of chiral mandelic acid in the presence of Zn(II) ions by l-cysteine-modified electrode, Sens. Actuators B 155 (2011) 140–144. [13] W. Yang, J.J. Gooding, D.B. Hibbert, Redox voltammetry of sub-parts per billion levels of Cu2+ at polyaspartate-modified gold electrodes, Analyst 126 (2001) 1573–1577. [14] S.K. Moccelini, S.C. Fernandes, I.C. Vieira, Bean sprout peroxidase biosensor based on l-cysteine self-assembled monolayer for the determination of dopamine, Sens. Actuators B 133 (2008) 364–369. [15] C.G. Fu, C.H. Su, R.F. Shan, Electrochemical properties of l-cysteine selfassembled membrane modified gold electrode, Acta Phys. Chim. Sin. 20 (2004) 207–210.
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[16] W. Sanders, M.R. Anderson, Electrostatic deposition of polycations and polyanions onto cysteine monolayers, J. Colloid Interface Sci. 331 (2009) 318–321. [17] W.R. Yang, J.J. Gooding, D.B. Hibbert, Characterisation of gold electrodes modified with self-assembled monolayers of l-cysteine for the adsorptive stripping analysis of copper, J. Electroanal. Chem. 516 (2001) 10–16. [18] M.M. Walczak, D.D. Popenoe, R.S. Deinhammer, B.D. Lamp, C. Chung, M.D. Porter, Reductive desorption of alkanethiolate monolayers at gold—a measure of surface coverage, Langmuir 7 (1991) 2687–2693. [19] A.J. Tudos, D.C. Johnson, Dissolution of cold electrodes in alkaline media containing cysteine, Anal. Chem. 67 (1995) 557–560. [20] S. Steinberg, Y. Tor, A. Shanzer, I. Rubinstein, “Ion-selective monolayer membranes based on self-assembling tetradentate ligand monolayers on gold electrodes: nature of the ionic selectivity”, Thin Films, Academic Press, New York, 1995, Vol. 20, pp. 183–205. [21] G.Z. Hu, Y.C. Liu, J. Zhao, S.Q. Cui, Z.S. Yang, Y.Z. Zhang, Selective response of dopamine in the presence of ascorbic acid on l-cysteine self-assembled gold electrode, Biochemistry 69 (2006) 254–257.
Lianhua Bi received her BS degree from Huaian Normal Institute (China) in 2008. She entered the MS course in Yangzhou University and Yancheng Institute of Technology (China) in 2008, majored in analytical chemistry. Now, she is engaged in microfluidics. Fan Tang received his BS degree from Jiangsu Teachers University of Technology (China) in 2009. He entered the MS course in Anhui University of Science and Technology and Yancheng Institute of Technology (China) in 2009, majored in analytical chemistry. Now, he is engaged in microfluidics. Huxiang Zheng received his BS degree from Nanjing Xiaozhuang University (China) in 2008. He entered the MS course in Changzhou University and Yancheng Institute of Technology (China) in 2008, majored in analytical chemistry. Now, he is engaged in microfluidics.
Biographies
Qin Xu received her BS and MS degree in analytical chemistry from Yanzhou University (China) in 2000 and 2003, respectively. and PhD degree in analytical chemistry from Nanjing University (China) in 2006. Now she is an associate professor of Yangzhou University (China). Her research interests are in the areas of electrochemical detection.
Dahe Fan received his MS degree in chemistry from Nanjing University of Technology (China) in 2000. Now he is a full professor of Yancheng Institute of Technology (China). His research interests are in the areas of chromatography.
Wei Wang received his BS and PhD degree in analytical chemistry from Nanjing University (China) in 1991 and 2007, respectively. Now he is a full professor of Yancheng Institute of Technology (China). His research interests are in the areas of microfluidics and electrochemical detection.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
l-Cysteine modified flexible PDMS–gold electrode for sensing ascorbic acid and copper Dahe Fan a , Lianhua Bi a,b , Fan Tang a , Huxiang Zheng a , Qin Xu b,∗ , Wei Wang a,∗∗ a Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, School of Chemical and biological Engineering, Yancheng Institute of Technology, Yancheng 224051, China b School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
a r t i c l e
i n f o
Article history: Received 20 September 2011 Received in revised form 17 October 2011 Accepted 1 November 2011 Available online 6 November 2011 Keywords: Flexible poly(dimethylsioxane) (PDMS)-based gold electrode l-Cysteine modified gold electrode Copper Ascorbic acid Portable device
a b s t r a c t Applications of flexible poly(dimethylsioxane) (PDMS)-based gold electrode were expanded, modification of l-cysteine on the electrode was carried out, and applied for sensing ascorbic acid and copper. Electrochemical properties of proposed modified electrode were similar to that of conventional electrode. Copper(II) was accumulated on the l-cysteine modified electrode and measured with cyclic voltammetry, reductive currents were linear with concentrations of copper(II) in range of 10−8 to 1.5 × 10−6 M and limit of detection was 5.0 × 10−9 M. This proposed sensor could give quick response to ascorbic acid (AA) (<5 s) and the currents were linear with concentrations of AA in range of 6.6 × 10−7 to 2.6 × 10−3 M and limit of detection was 2 × 10−7 M. The performance was better than previous reports. The devices are simple, low-cost, easy-to-fabricate and portable. Therefore, conventional modification methods could be applied on the flexible poly(dimethylsioxane) (PDMS)-based gold electrode for point-of-care applications in environmental monitoring, public health, and food safety. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polydimethylsiloxane (PDMS) is an attractive polymeric matrix due to its many favorable properties such as chemical inertness, biocompatibility, mechanical flexibility, nontoxic, stability, high dielectric constant, optical clarity in the visible and ultraviolet region and importantly, ease of processing [1]. Since PDMS–gold nanoparticles (AuNPs) composite films and patterned Au/PDMS substrate with chemical gold plating method on native PDMS was introduced [2,3]. Our group and Wu et al. have demonstrated the gold layer could be applied as electrode for electrochemical applications [4,5]. The advantages of self-assembled monolayer (SAM) include their ease of preparation, their stability and the possibility of introducing different chemical functionalities. The incorporation of appropriate chemical functionality with some molecular level control into the highly ordered monolayers allows the preparation of surfaces with tailor-made properties. Modification of gold electrode with SAM of alkane-thiol is of research interests [6]. The high organization of the monolayer assures cooperative and homogeneous behavior of the entire electrode surface. Therefore, the
∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 515 88298848; fax: +86 515 88298191. E-mail addresses: [email protected] (Q. Xu), [email protected], [email protected] (W. Wang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.001
self-assembly technique is a convenient method due to its versatility in modifying surfaces in a controllable manner. It has been shown that organothiol molecules upon adsorption at gold lose the hydrogen from the thiol group and that an S–Au bond is formed [7,8]. Amino acids and peptides have been employed as the recognition element for sensing based on self-assembled monolayers. l-Cysteine has been applied as selective ligand for chemical accumulation of metal ions [9–13], and as a promoter for increasing the rate of electron transfer [14,15]. A cysteine monolayer has the ability to have either a net positive or a net negative charge depending on the solution pH, this property could be leveraged to electrostatically deposit either polycationic or polyanionic polymers onto a cysteine monolayer by adjusting the bulk solution pH [16]. In this research, we demonstrated extensive applications of flexible poly(dimethylsioxane) (PDMS)-based gold electrode. Modification of l-cysteine on the electrode was carried out, and applied for sensing ascorbic acid and copper. Electrochemical properties of proposed modified electrode were similar to that of conventional electrode. Copper(II) was accumulated on the l-cysteine modified electrode and measured with cyclic voltammetry, reductive currents were linear with concentrations of copper(II) in range of 10−8 to 10−6 M and limit of detection was 5.0 × 10−9 M. This proposed sensor could give quick response to ascorbic acid (AA) (<5 s) and the currents were linear with concentrations of AA in range of 6.6 × 10−7 to 2.6 × 10−3 M and limit of detection was 2 × 10−7 M. The devices were simple, low-cost, easy-to-fabricate and portable.
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2. Experimental 2.1. Materials and instrumentation Poly(dimethylsiloxane) (PDMS) was obtained from Dow Corning (midland, MI, USA). l-Cysteine (C3 H7 NO2 S), glucose and chloroauric acid (HAuCl4 ·4H2 O) were from Sigma–Aldrich. Ascorbic acid (AA), potassium chloride (KCl), and copper nitrate (Cu(NO3 )2 ) were from Shanghai Chemical Reagent Company (Shanghai, China). Sodium acetate (CH3 COONa) was purchased from Northeast Pharmaceutical Group Co., Ltd, acetic acid (CH3 COOH) and potassium hydrogen carbonate (KHCO3 ) were obtained from Shanghai Shenxiang Chemical Reagent Company (Shanghai, China). Supporting electrolytes were 0.1 M acetate buffer and 0.05 M phosphate buffer. All solutions were prepared with deionized water. All reagents were of analytical grade. All electrochemical experiments were performed with a CHI840C electrochemical workstation (Shanghai Chenhua Company, Shanghai, China).
Fig. 1. Cyclic voltammogram for the reductive desorption of PDMS–AuNPs–LC electrode in 0.5 M NaOH solution (deaerated) at the scan rate of 100 mV s−1 .
2.2. Fabrication of flexible PDMS based electrode 3. Results and discussion PDMS film was prepared similar as described procedure in Refs. [3–5]. Briefly, PDMS monomer and the curing agent were firstly mixed in a proportion of 1:0.1 and cured at 120 ◦ C for 30 min. Chemical gold plating solution containing 0.03 M HAuCl4 , 2 M KHCO3 , and 9.9 M glucose (v/v, 2:1:1) was prepared just before use. Plexiglass frame with inner area 3 cm × 4 cm was sandwiched between two native 2-mm-thick PDMS films named ‘cover chip’ and ‘base chip’, and then the solution for chemical gold plating was injected into this sandwich architecture. It was then incubated at room temperature for 4 h to ensure the elemental gold deposit completely. All resulting PDMS–gold films were washed with deionized water for three times, and stored at room temperature when not in use. A piece of PDMS–gold film of 2.5 cm × 0.4 cm was cut as working electrode, working electrode area was defined as 1.5 cm × 0.4 cm controlled by transparent adhesive tape and connected by indium tin oxide (ITO) conductive glass. The counter electrode is Pt and reference electrode is Ag/AgCl. 2.3. Modification of l-cysteine on PDMS–AuNPs electrode for detection of Cu(II) Gold electrode mentioned above was immersed in phosphate buffer containing 10 mM l-cysteine for 30 min at room temperature for self-assembly of cysteine. Chemical accumulation of Cu2+ on the electrode was carried out at open circuit by dipping the electrode into 10 mL of a stirred, buffered (pH 5.5) solution of copper nitrate for 5 min. Then the electrode was removed from the cell, washed thoroughly with purified water, and transferred to a cell containing phosphate buffer solution (pH 5.5). Cyclic voltammograms were recorded at a scan rate of 100 mV s−1 and the potential range from −0.4 to 0.7 V. 2.4. Modification of l-cysteine on PDMS–AuNPs electrode for detection of AA Gold electrode mentioned above was immersed in 0.1 M HAc–NaAc buffer containing 20 mM l-cysteine for 24 h at room temperature for self-assembly of cysteine. Chemical measurements were performed in HAc–NaAc buffered solution (pH 4.5). Cyclic voltammograms were recorded in the potential range from −0.4 to 0.7 V. Prior to each experiment, the solution was deoxygenated with nitrogen and kept in nitrogen atmosphere at room temperature.
3.1. Electrochemical desorption of l-cysteine from a modified gold electrode l-Cysteine adsorbs onto gold electrodes via the thiol side-chain of amino acid. N-alkanethiols adsorbed on gold and silver surfaces are both oxidatively and reductively desorbed, and the potential for the reductive desorption process is dependent on both the strength of the gold–sulfur interaction and the extent of the intermolecular interactions between adjacent adsorbates [17,18]. Thiols are reductively desorbed by the following reaction. Au–SR + e− = Au + RS−
(1)
The cyclic voltammogram (Fig. 1) of the l-cysteine modified electrode in a deaerated NaOH solution (0.5 M) from −0.2 to −1.4 V (vs. Ag/AgCl) exhibits two reduction peaks, it is similar to the literature reported [17,19]. We propose that the first peak (at −0.7 V) is due to the cleavage of the gold–sulfur bond (having a shape characteristic of an adsorbed species) and the second peak, having more diffusion-like character is possibly due to a similar field-induced rearrangement of cysteine clusters after cleavage. 3.2. Adsorption of copper at a gold electrode modified with l-cysteine Voltammograms of a gold electrode modified with l-cysteine before and after adsorption from buffer containing 10 M Cu2+ for 5 min were shown in Fig. 2. In the absence of copper(II) ions, there were no peaks in the gold electrode modified with l-cysteine. After copper accumulation for 5 min in a 10 M Cu2+ solution and washing, the voltammogram was recorded in buffer solution that was free of copper. A peak appeared at 0.115 V during Cu(II) was reduced to Cu(I). Cu(I) was reoxidized during the subsequent positive sweep with a peak potential of +0.359 mV. Repeated cycling did not lead to a significant loss of signal, which indicated that copper remained strongly adsorbed in both oxidation states within the potential range of the experiment. Fig. 3 showed electrochemical behavior of our PDMS–gold electrode immersed in 1 M Cu(II) solution. The insert showed anodic and cathodic peak currents were directly proportional to the square root of the scan rate between 10 and 500 mV s−1 . The solid lines represented a linear fit to both oxidation current and reduction current with the regression equation: upper data, Y = 8.2663 + 0.2131X
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Fig. 2. Cyclic voltammograms of a PDMS–AuNPs–LC electrode in phosphate buffer (pH 5.5) and after subsequent immersion in 1 M Cu(II) for 5 min and return to the phosphate buffer. Scan rate: 100 mV s−1 .
Fig. 4. Cyclic voltammograms of PDMS–AuNPs–LC electrode immersed in 0.01, 0.1, 0.5, 1.0, 5.0, 10 and 15 M Cu(II) solutions for 5 min. Inset: calibration plot of the dependence of the measured current on Cu(II) concentrations. Scan rate: 100 mV s−1 .
(R = 0.9966), lower data, Y = −9.4674 − 0.2018X (R = 0.9964). The linearity indicated the mass transfer in this system was a diffusion controlled process. The peak shape of CVs showed a typical reversible electrochemical reaction in which the rate of reaction was governed by diffusion of the electroactive species to the surface of a planar electrode. Under the optimum analytical conditions, the linear range of Cu(II) detection was studied. Fig. 4 showed the cyclic voltammograms of various concentrations of Cu(II) at the PDMS–AuNPs–LC electrode. The dependence of reduction peak current on the concentration of Cu(II) was in a linear relationship in the range of 0.01–1.5 M. The linear regression equation was expressed as Y = 14.0798 + 10.2185X (R = 0.9977). The detection limit of Cu(II) is 0.005 M (based on the signal-to-noise ratio of 3). RSD was less than 3.4% for 5 parallel measurements. The performance was consistent with the conventional electrode [17], in which the linear range was 0.08–10.0 M and the detection limit was less than 0.005 M. The sensor could be regenerated in 0.1 M perchloric acid, for detection of 0.1 M Cu(II), some metal ions with 100 times
which could interact with L-cysteine such as zinc, lead and manganese will increase the response signal [17]. When a higher concentration of Cu(II) 100 M to 0.1 M was applied in the research, a second redox process appeared at a more negative potential in the cyclic voltammogram. We tested cyclic voltammetry at the PDMS–AuNPs–LC electrode that had been immersed in 100 M Cu2+ (data not shown). The phenomenon could be explained as that the new peaks seen after immersion in high concentration of Cu2+ resulted from the electrochemistry of Cu+ (possibly as Cu2 S) at the electrode surface after breakdown of part of the cysteine monolayer [17,20].
Fig. 3. Cyclic voltammograms of PDMS–AuNPs–LC electrode accumulated in 1 M Cu(II) solution at different scan rates (from inner to outer: 10, 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mV s−1 ), The insert is the relationship of redox currents and the square root of the scan rates. Solid lines represent a linear fit to both oxidation current and reduction current with the regression equation: upper data, Y = 8.2663 + 0.2131X (R = 0.9966); lower data, Y = −9.4674 − 0.2018X (R = 0.9964).
3.3. Electrochemical detection of AA at a PDMS–AuNPs electrode modified with l-cysteine Fig. 5 showed cyclic voltammograms of PDMS–AuNPs and PDMS–AuNPs–LC electrodes with the potential range of 0.1–0.7 V in a deaerated HAc–NaAc buffer containing 1.0 mM AA at a scan rate of 100 mV s−1 . There was a small and broad anodic peak appeared at the PDMS–AuNPs electrode at 0.45 V approximately, however, a sharp well-defined anodic peak appeared at the PDMS–AuNPs–LC electrode at 0.32 V. These phenomena suggested that the oxidation of AA was more favorable at the PDMS–AuNPs–LC electrode.
Fig. 5. Cyclic voltammograms of 1.1 mM AA at PDMS–AuNPs electrode (a) and PDMS–AuNPs–LC electrode (b) in HAc-NaAc buffer solution (pH 4.5) at scan rate of 100 mV s−1 .
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2.7% for 5 parallel measurements. The performance is better than unmodified electrode [4], in which the linear range was 2.3 × 10−5 to 7.0 × 10−3 M, and detection limit was 8.0 × 10−6 M. The response time was about 5 s, which indicated a generally fast electrontransfer process in this PDMS-based gold electrode. Stability of the electrode was investigated after it was stored in refrigerator (4 ◦ C) for two weeks, amperometric response signal was equal to that of freshly prepared PDMS-based gold electrode. 4. Conclusion
Fig. 6. Cyclic voltammograms of PDMS–AuNPs–LC electrode at different scan rates (from inner to outer: 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mV s−1 ). Insert: plots of the dependence of the anodic peak current on square root of scan rates.
The reason was that l-cysteine self-assembled film could act as a promoter to increase the rate of electron transfer and lower the overpotential of AA [15]. Electrochemical behavior of PDMS–gold electrode modified with LC in 0.16 mM AA was shown in Fig. 6. The insert showed anodic peak currents were directly proportional to the square root of the scan rate between 50 and 500 mV s−1 . The solid line represented a linear fit to both oxidation current with the regression equation Y = −5.4647 + 3.7082X (R = 0.9972). The linearity indicated the mass transfer in this system was a diffusion controlled process. Usually, dopamine is coexistent composition and will give interference in the determination of AA, according to Ref. [21], AA can be distinguished at different potential. Current–time response curves were observed at 0.3 V (vs. Ag/AgCl) with subsequent spiking of 1.0 mM AA in 50 mL of 0.1 M HAc–NaAc (pH 4.5). The i–t calibration curve and relationship between catalytic current and concentration of AA were shown in Fig. 7. The linear relationship between catalytic current and concentration was observed in the range of 6.6 × 10−7 to 2.6 × 10−3 M. The linear regression equation were expressed as Y = −0.0205 + 54.2691X (R = 0.9987), with the detection limit of 2.0 × 10−7 M at a signal-to-noise ratio of 3. RSD was less than
Fig. 7. i–t curves of successive additions of 1.0 mM AA in 50 mL of 0.1 M HAc–NaAc (pH 4.5) with PDMS–AuNPs–LC electrode. Insert: plots of the dependence of concentrations of AA on the catalytic of peak currents.
We demonstrate here that flexible poly(dimethylsioxane) (PDMS)-based gold electrode is suitable for modification according to conventional methods. With l-cysteine modification, the electrode is applied for copper and ascorbic acid detection, the characterizations of modified electrode are similar to that of conventional electrode. This research indicates that flexible PDMS–AuNPs electrode have the potential applications for pointof-care devices in environmental monitoring, public health, and food safety. Acknowledgements We greatly appreciate the financial support of National Natural Science Foundation of China (20875080 and 20705030), Foundation of International Cooperation of Jiangsu Province (BZ2010053), Foundation of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (AE201017), and Scientific Project of Yancheng Institute of Technology (XKY2009010, XKY2009016 and XKY2011003). References [1] M.L. Van Poll, F. Zhou, M. Ramstedt, L. Hu, W.T.S. Huck, A self-assembly approach to chemical micropatterning of poly(dimethylsiloxane), Angew. Chem. Int. Ed. 46 (2007) 6634–6637. [2] Q. Zhang, J.J. Xu, Y. Liu, H.Y. Chen, In-situ synthesis of poly(dimethylsiloxane)–gold nanoparticles composite films and its application in microfluidic systems, Lab Chip 8 (2) (2008) 352–357. [3] H.J. Bai, M.L. Shao, H.L. Gou, J.J. Xu, H.Y. Chen, Patterned Au/poly(dimethylsiloxane) substrate fabricated by chemical plating coupled with electrochemical etching for cell patterning, Langmuir 25 (2009) 10402–10407. [4] Q. Xu, L. Bi, H. Zheng, D. Fan, W. Wang, PDMS-based gold electrode for sensing ascorbic acid, Colloids Surf. B: Biointerfaces 88 (2011) 362–365. [5] W.Y. Wu, X.Q. Zhong, W. Wang, J.J. Zhu, Flexible PDMS-based three-electrode sensor, Electrochem. Commun. 12 (2010) 1600–1604. [6] Z.H. Wang, Y.M. Wang, G.A. Luo, A selective voltammetric method for uric acid detection at -cyclodextrin modified electrode incorporating carbon nanotubes, Analyst 127 (2002) 1353–1358. [7] Y.Z. Li, J.Y. Huang, R.T. McIver, J.C. Hemminger, Characterization of thiol selfassembled films by laser desorption Fourier-transform mass-spectrometry, J. Am. Chem. Soc. 114 (1992) 2428–2432. [8] G.Z. Hu, Y.G. Ma, Y. Guo, S.J. Shao, Electrocatalytic oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode, Electrochim. Acta 53 (2008) 6610–6615. [9] M. Koneswaran, R. Narayanaswamy, l-Cysteine-capped ZnS quantum dots based fluorescence sensor for Cu(2+) ion, Sens. Actuators B 139 (2009) 104–109. [10] A.C. Liu, D.C. Chen, C.C. Lin, H.H. Chou, C.H. Chen, Application of cysteine monolayers for electrochemical determination of sub-ppb copper (II), Anal. Chem. 71 (1999) 1549–1552. [11] W. Stricks, I.M. Kolthoff, Polarographic investigations of reactions in aqueous solutions containing copper and cysteine (cystine). 1. Cuprous copper and cysteine in ammoniacal medium—the dissociation constant of cuprous cysteinate, J. Am. Chem. Soc. 73 (1951) 1723–1727. [12] Y.Z. Fu, L.L. Wang, Q. Chen, J. Zhou, Enantioselective recognition of chiral mandelic acid in the presence of Zn(II) ions by l-cysteine-modified electrode, Sens. Actuators B 155 (2011) 140–144. [13] W. Yang, J.J. Gooding, D.B. Hibbert, Redox voltammetry of sub-parts per billion levels of Cu2+ at polyaspartate-modified gold electrodes, Analyst 126 (2001) 1573–1577. [14] S.K. Moccelini, S.C. Fernandes, I.C. Vieira, Bean sprout peroxidase biosensor based on l-cysteine self-assembled monolayer for the determination of dopamine, Sens. Actuators B 133 (2008) 364–369. [15] C.G. Fu, C.H. Su, R.F. Shan, Electrochemical properties of l-cysteine selfassembled membrane modified gold electrode, Acta Phys. Chim. Sin. 20 (2004) 207–210.
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Lianhua Bi received her BS degree from Huaian Normal Institute (China) in 2008. She entered the MS course in Yangzhou University and Yancheng Institute of Technology (China) in 2008, majored in analytical chemistry. Now, she is engaged in microfluidics. Fan Tang received his BS degree from Jiangsu Teachers University of Technology (China) in 2009. He entered the MS course in Anhui University of Science and Technology and Yancheng Institute of Technology (China) in 2009, majored in analytical chemistry. Now, he is engaged in microfluidics. Huxiang Zheng received his BS degree from Nanjing Xiaozhuang University (China) in 2008. He entered the MS course in Changzhou University and Yancheng Institute of Technology (China) in 2008, majored in analytical chemistry. Now, he is engaged in microfluidics.
Biographies
Qin Xu received her BS and MS degree in analytical chemistry from Yanzhou University (China) in 2000 and 2003, respectively. and PhD degree in analytical chemistry from Nanjing University (China) in 2006. Now she is an associate professor of Yangzhou University (China). Her research interests are in the areas of electrochemical detection.
Dahe Fan received his MS degree in chemistry from Nanjing University of Technology (China) in 2000. Now he is a full professor of Yancheng Institute of Technology (China). His research interests are in the areas of chromatography.
Wei Wang received his BS and PhD degree in analytical chemistry from Nanjing University (China) in 1991 and 2007, respectively. Now he is a full professor of Yancheng Institute of Technology (China). His research interests are in the areas of microfluidics and electrochemical detection.