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Fabrication and electrochemical behaviour of vertically aligned boron-doped diamond nanorod forest electrodes
Electrochemistry Communications 11 (2009) 1093–1096
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Fabrication and electrochemical behaviour of vertically aligned boron-doped diamond nanorod forest electrodes Daibing Luo a,b, Jinfang Zhi a,* a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, No. 2, Beiyitiao, Zhong-guan-cun, Haidian District, Beijing 100190, China b Graduate University of Chinese Academy of Sciences, No. 2, Beiyitiao, Zhong-guan-cun, Haidian District, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 20 January 2009 Received in revised form 16 March 2009 Accepted 18 March 2009 Available online 26 March 2009 Keywords: Boron-doped diamond nanorod forest Electrochemistry Amperometric biosensor
a b s t r a c t Vertically aligned boron-doped diamond nanorod forests (BDDNF) were successfully fabricated by depositing a diamond film onto silicon nanowires (SiNWs) using hot filament chemical vapor deposition (HFCVD). The boron-doped diamond nanorods were characterized by Raman spectroscopy and scanning electron microscopy (SEM). The BDDNF obtained from the SiNWs on the silicon wafer could be directly used as an electrode and its electrochemical behaviour is discussed here. Compared to a flat boron-doped diamond (BDD) electrode, the BDDNF electrode showed high sensitivity in the amperometric detection of adenine. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Boron-doped diamond (BDD) is an alternative to traditional carbon electrodes that provides superior chemical stability, low background currents, good compatibility, and a very wide potential window of water stability [1]. The initial research of BDD electrochemistry was reported by Pleskov et al. [2]. From then on, BDD has attracted considerable interest in various fields, ranging from electroanalysis [3] and electrosynthesis [4] to electrochemical pollutant treatment [5]. Owing to these outstanding properties, BDD plays an important role in biosensors [6]. In order to detect some biomolecules, varieties of metals or molecules have been used as electron transfer mediators, via modification on the BDD surface [7]. However, modified mediators usually bring a diffusion impediment to electrolyte to the electrode surface and therefore influence the electrode reaction kinetics. Moreover, some kinds of metals or molecules modified on the electrode surface are not stable in rigorous environments. These disadvantages have restricted the applications of modified BDD electrodes. Therefore, it will be of great significance to obtain better electrochemical properties by only changing the surface morphologies or areas of BDD electrodes. Recently, nanostructured electrode materials have been employed as a powerful means to improve the electrochemical per-
* Corresponding author. Tel.: +86 10 82543537. E-mail address: [email protected] (J. Zhi). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.03.011
formance of electrodes. High-aspect-ratio carbon nanomaterials have been shown to be ideal for biosensor applications since they are conductive, biocompatible, and possess very large surface areas [8]. Therefore, it can be expected that the hybrid of conductive diamond and its nanostructures may afford BDD electrodes some novel properties. A few examples of diamond-based nanostructures have been fabricated by Baik et al. We can cite the literatures of diamond nanowhiskers obtained by radio frequency air plasma on polycrystalline diamond films [9], and well-aligned diamond nanocylinders produced by microwave plasma CVD with a porous alumina template [10]. However, nanostructured conductive diamond films used in electrochemical applications have rarely been studied. In this paper we demonstrate the fabrication of a boron-doped diamond nanorod forest (BDDNF) electrode and its particularly electrochemical performance compared with a flat BDD electrode. The results indicate that the BDDNF electrodes may be a preferable choice in practical electrochemical applications. 2. Experimental 2.1. Regents Si wafers (p-type, (1 0 0) surface) were purchased from Aldrich. Adenine and trimethyl borate were obtained from Wako Pure Chemical Industries. All chemicals were used ‘‘as received” without further purification. All solutions were prepared with ultrapure water (18.2 MX cm).
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2.2. Preparation of the BDDNF electrode SiNWs were firstly prepared by electroless metal deposition (EMD) technique as described in Ref. [11]. A piece of silicon wafer with as-grown SiNWs was used as the substrate for diamond deposition. The deposition of boron-doped diamond on the as-grown SiNWs was carried out in a HFCVD reactor (purchased from Shanghai JiaoYou Diamond Coating Co. Ltd.). An acetone solution containing trimethyl borate (0.5% atoms ratio) was served as the carbon source. The carbon source/H2 flow ratio was 200:50 and the growth duration was 60 min. The bias voltage was set at 4.5 V. The substrate was heated by Tantalum wires with an ac power supply at the voltage of 20 V and the current of 60 to 70 A, and the deposition temperature was kept at about 600 °C. 2.3. Measurements The surface morphology of the sample was investigated by a scanning electron microscopy (Hitachi Ultra-High-Resolution S4300). The Raman spectroscopy was obtained using a Renishaw 1000 Raman spectrometer (Renishaw Ltd., UK). The electrochemical experiments were performed with a potentiostat/galvanostat (Model 263 A, Princeton, USA). The impedance spectroscopy measurements were taken with the same potentiostat/galvanostat device connected to a FRD 100 frequency response detector (Princeton, USA). The electrochemical measurements were carried out in a three-electrode system with a single compartment cell using a saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. A normal flat BDD electrode prepared under the same deposition condition was used for comparison. The electrochemical experiments were performed in a solution of 10 mL volume. The geometric area of the working electrode exposed to the solution was 0.08 cm2. 3. Results and discussion 3.1. Characterization of the BDDNF electrode From the SEM image of Fig. 1a, a high density of diamond crystallites coating the SiNWs like a nanorod forest vertically aligned on the original silicon wafer can be clearly observed. The SEM image taken at a high magnification (Fig. 1b) of the nanorods, reveals that the polycrystalline diamond coverage is complete and continuous along the whole length of the SiNWs with crystallite diameters ranging approximately from 50 nm to 80 nm. The quality of the diamond nanorods is studied by Raman spectroscopy. As shown in Fig. 1c, two peaks at 1332 cm1 and 1520 cm1 can be observed. The peak at 1332 cm1 is related to the presence of crystalline diamond. The broadening of this peak is caused by the finite domain size of the nanocrystalline diamond grains. The peak at 1520 cm1 is attributed to the disordered carbon and sp2-bonded carbon at the grain boundaries of nanocrystalline diamond [12]. A shoulder at 1130 cm1 is often identified as a signature for high quality of nanocrystalline diamond [13]. 3.2. Electrochemical behaviour of the BDDNF electrode 3.2.1. Characterization of impedance spectroscopy We have measured the faradic impedance of the electrodes taking FeðCNÞ63=4 as a model species in 0.1 M KCl solution. The corresponding Nyquist plots obtained of the BDDNF electrode and the flat BDD electrode with a frequency range of 1 Hz–100 kHz at a potential of 0.3 V (vs. SCE) are shown in Fig. 2. In general, for the equivalent circuit, the Nyquist plots shows a semicircle and a linear
region, the semicircle at higher frequencies corresponds to an electron-transfer-limited process, and the linear portion at lower frequencies corresponds to a diffusion-limited process. In our experiment, the Nyquist plots of the BDDNF electrode exhibit two semicircles rather than one (Fig. 2a), while only one semicircle is observed on the flat BDD electrode (Fig. 2b). Obviously, the appearance of the two semicircles should be directly related to the surface morphology of the BDDNF electrode. We consider that the high frequency (HF) semicircle is related to the porosity of the electrode and independent of the kinetics of the faradic process, and the low frequency (LF) semicircle is related to the kinetics of the faradic reaction [14]. The diameter of the HF semicircle indicates the charge transfer resistance related to the geometry of the electrode, while the diameter of the LF semicircle indicates the charge transfer resistance related to the kinetics of the faradic reaction. The present result of the BDDNF electrode is quite similar to the impedance behaviour of porous electrodes, on which two semicircles can be observed in the Nyquist plots for a charge transfer reaction as described in Ref. [15]. To obtain more detailed information, we have further examined the concentration dependence of the impedance behaviour of the two electrodes and compared the results. A typical faradic impedance result can be observed at the flat BDD electrode, i.e., only one semicircle is obtained and its diameter decreases with increasing 3=4 (Fig. 2b). At the BDDNF electrode, concentrations of FeðCNÞ6 two semicircles are observed and the diameter of the LF semicircle 3=4 , while the decreases with increasing concentrations of FeðCNÞ6 HF semicircle does not show obvious change (Fig. 2a). These results further confirm that there is diffusion restriction of the reactant among the nanorods due to the existence of the gaps, and this property will have an effect on the electrochemical performance of the BDDNF electrode. 3=4 on the BDDNF elecThe voltammetric responses to FeðCNÞ6 trode and the flat BDD electrode have also been investigated. As shown in Fig. 2c, the DEp of 190 mV for the BDDNF electrode and 230 mV for the flat BDD electrode are observed to 4 mM FeðCNÞ63=4 solution, respectively, which suggests that the BDDNF electrode, with large electro-active surface area, exhibits higher electron transfer kinetics than the flat BDD electrode. 3.2.2. Biosensing characteristics of the BDDNF electrode Normal flat BDD electrodes have been used as electrochemical biosensors due to their good stability and sensitivity. The research effort here is to compare the biosensing characteristics of the BDDNF electrode with the flat BDD electrode in amperometric detection of biomolecules. We used adenine, a fundamental compound in biological systems, as a biomolecule sample. Generally, amperometric detection of adenine is difficult on bare metal or carbon electrodes due to the overlap of oxygen evolution coming from the decomposition of water. As shown in Fig. 3a, well-defined oxidation current peaks for adenine in 0.1 M ammonium acetate buffer solution (pH 4.28) could be observed at a high potential (1.4 V vs. SCE) at the two electrodes. However, their oxidation peak currents are different. The typical amperometric responses of the two electrodes to concentration changes of adenine at the potential of 1.4 V (vs. SCE) have been examined and the corresponding results are shown in Fig. 3b. As expected, the BDDNF electrode displays an amplified response contrast with the flat BDD electrode. The sensitivity is 0.205 lA/lM for the BDDNF electrode and 0.024 lA/lM for the BDD electrode. Meanwhile, the value of lowest detection concentration (S/N = 3) for adenine is 2.5 ± 0.4 lM on the BDDNF electrode and 5.0 ± 0.5 lM on the BDD electrode, respectively. When interpreting the particularly electrochemical performance of the BDDNF electrode, we mainly consider the large
D. Luo, J. Zhi / Electrochemistry Communications 11 (2009) 1093–1096
1095
Fig. 1. SEM images from (a) top, and (b) amplificatory view. (c) Raman spectrum of the boron-doped diamond nanorods.
Fig. 2. The impedance changes with the concentrations of FeðCNÞ3=4 at (a) the BDDNF electrode, and (b) the flat BDD electrode at 0.3 V (vs. SCE). (c) Comparison of the cyclic 6 voltammograms in 0.1 M KCl solution containing 4 mM FeðCNÞ63=4 with a scan rate of 50 mVs1 at the BDDNF electrode and the flat BDD electrode, respectively.
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Fig. 3. (a) Cyclic voltammograms of 0.5 mM adenine in 0.1 M ammonium acetate buffer solution (pH 4.28) at the flat BDD electrode and the BDDNF electrode, respectively, with a scan rate of 50 mVs1. (b) Steady-state amperometric responses of the flat BDD electrode and the BDDNF electrode in a stirred solution (the same buffer) at an applied potential of 1.4 V (vs. SCE).
surface area and mass transport characteristics of the nanorod forest, i.e., primarily governed by the size and geometric spacing of the nanorod array. An electrode with such nanorods would experience various diffusion styles, i.e., a hemispherical diffusion mixed with planar diffusion profile [16,17]. Consequently, the BDDNF electrode with the additional charge transfer due to its large electro-active surface area and different diffusion styles results in higher amperometric responses than the flat BDD electrode. The electrochemical behaviour of the BDDNF electrode is far from completely understood, but the results reported here demonstrate the favorable possibilities for the applications of this kind of electrode in electroanalysis field. 4. Conclusion In summary, we have demonstrated a feasible method for fabricating of vertically aligned boron-doped diamond nanorod forests. Our method for the preparation of the BDDNF electrode is simple, convenient, and low-cost. The BDDNF electrode exhibits excellent electrochemical performance due to its large electro-active surface area and particular nanostructure. The impedance behaviour of the BDDNF electrode like that of porous electrodes 3=4 has been discussed. And a better biosensing activity in FeðCNÞ6 of the BDDNF electrode towards adenine than the BDD electrode has been observed. Based on these, the BDDNF electrode may be expected to be a novel electrode for practical applications in electrochemical detection of biomolecules.
Acknowledgements This work was supported by the Major Research Development Program of China (No. 2006CB933000) and National Natural Science Foundation of China (207731500). References [1] T.N. Rao, A. Fujishima, Diamond Relat. Mater. 9 (2000) 384. [2] Y.V. Pleskov, A.Y. Sakharova, M.D. Krotova, L.L. Bouilov, B.P. Spitsyn, J. Electroanal. Chem. 228 (1987) 19. [3] F. Bouamrane, A. Tadjeddine, J.E. Butler, R. Tenne, C. Lévy-Clément, J. Electroanal. Chem. 405 (1996) 95. [4] M.A. Kulandainathan, C. Hall, D. Wolverson, J.S. Foord, S.M. MacDonald, F. Marken, J. Electroanal. Chem. 606 (2007) 150. [5] A. Kapałka, B. Lanova, H. Baltruschat, G. Fóti, C. Comniellis, Eletrochem. Commun. 10 (2008) 1215. [6] Y.L. Zhou, R.H. Tian, J.F. Zhi, Biosens. Bioelectron. 22 (2007) 822. [7] Y. Song, G.M. Swain, Anal. Chem. 79 (2007) 2412. [8] T.E. McKnight, A.V. Melechko, D.W. Austin, T. Sims, M.A. Guillorn, M.L. Simpson, J. Phys. Chem. B 108 (2004) 7115. [9] E.-S. Baik, Y.-J. Baik, S.W. Lee, D. Jeon, Thin Solid Films 377 (2000) 295. [10] H. Masuda, T. Yanagishita, K. Yasui, K. Nishio, I. Yagi, T.N. Rao, A. Fujishima, Adv. Mater. 13 (2001) 247. [11] K.Q. Peng, Y.J. Yan, S.P. Gao, J. Zhu, Adv. Mater. 14 (2002) 1164. [12] E.C. Almeida, A.V. Diniz, V.J. Trava-Airoldi, N.G. Ferreira, Thin Solid Films 485 (2005) 241. [13] A.C. Ferrari, J. Robertson, Phys. Rev. B 63 (2001) 121405. [14] C. Hitz, A. Lasia, J. Electroanal. Chem. 500 (2001) 213. [15] A. Lasia, J. Electroanal. Chem. 397 (1995) 27. [16] T. Watanabe, T.A. Ivandini, Y. Makide, A. Fujishima, Y. Einaga, Anal. Chem. 78 (2006) 7857. [17] K.L. Soh, W.P. Kang, J.L. Davidson, Y.M. Wong, D.E. Cliffel, G.M. Swain, Diamond Relat. Mater. 17 (2008) 240.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Fabrication and electrochemical behaviour of vertically aligned boron-doped diamond nanorod forest electrodes Daibing Luo a,b, Jinfang Zhi a,* a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, No. 2, Beiyitiao, Zhong-guan-cun, Haidian District, Beijing 100190, China b Graduate University of Chinese Academy of Sciences, No. 2, Beiyitiao, Zhong-guan-cun, Haidian District, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 20 January 2009 Received in revised form 16 March 2009 Accepted 18 March 2009 Available online 26 March 2009 Keywords: Boron-doped diamond nanorod forest Electrochemistry Amperometric biosensor
a b s t r a c t Vertically aligned boron-doped diamond nanorod forests (BDDNF) were successfully fabricated by depositing a diamond film onto silicon nanowires (SiNWs) using hot filament chemical vapor deposition (HFCVD). The boron-doped diamond nanorods were characterized by Raman spectroscopy and scanning electron microscopy (SEM). The BDDNF obtained from the SiNWs on the silicon wafer could be directly used as an electrode and its electrochemical behaviour is discussed here. Compared to a flat boron-doped diamond (BDD) electrode, the BDDNF electrode showed high sensitivity in the amperometric detection of adenine. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Boron-doped diamond (BDD) is an alternative to traditional carbon electrodes that provides superior chemical stability, low background currents, good compatibility, and a very wide potential window of water stability [1]. The initial research of BDD electrochemistry was reported by Pleskov et al. [2]. From then on, BDD has attracted considerable interest in various fields, ranging from electroanalysis [3] and electrosynthesis [4] to electrochemical pollutant treatment [5]. Owing to these outstanding properties, BDD plays an important role in biosensors [6]. In order to detect some biomolecules, varieties of metals or molecules have been used as electron transfer mediators, via modification on the BDD surface [7]. However, modified mediators usually bring a diffusion impediment to electrolyte to the electrode surface and therefore influence the electrode reaction kinetics. Moreover, some kinds of metals or molecules modified on the electrode surface are not stable in rigorous environments. These disadvantages have restricted the applications of modified BDD electrodes. Therefore, it will be of great significance to obtain better electrochemical properties by only changing the surface morphologies or areas of BDD electrodes. Recently, nanostructured electrode materials have been employed as a powerful means to improve the electrochemical per-
* Corresponding author. Tel.: +86 10 82543537. E-mail address: [email protected] (J. Zhi). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.03.011
formance of electrodes. High-aspect-ratio carbon nanomaterials have been shown to be ideal for biosensor applications since they are conductive, biocompatible, and possess very large surface areas [8]. Therefore, it can be expected that the hybrid of conductive diamond and its nanostructures may afford BDD electrodes some novel properties. A few examples of diamond-based nanostructures have been fabricated by Baik et al. We can cite the literatures of diamond nanowhiskers obtained by radio frequency air plasma on polycrystalline diamond films [9], and well-aligned diamond nanocylinders produced by microwave plasma CVD with a porous alumina template [10]. However, nanostructured conductive diamond films used in electrochemical applications have rarely been studied. In this paper we demonstrate the fabrication of a boron-doped diamond nanorod forest (BDDNF) electrode and its particularly electrochemical performance compared with a flat BDD electrode. The results indicate that the BDDNF electrodes may be a preferable choice in practical electrochemical applications. 2. Experimental 2.1. Regents Si wafers (p-type, (1 0 0) surface) were purchased from Aldrich. Adenine and trimethyl borate were obtained from Wako Pure Chemical Industries. All chemicals were used ‘‘as received” without further purification. All solutions were prepared with ultrapure water (18.2 MX cm).
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2.2. Preparation of the BDDNF electrode SiNWs were firstly prepared by electroless metal deposition (EMD) technique as described in Ref. [11]. A piece of silicon wafer with as-grown SiNWs was used as the substrate for diamond deposition. The deposition of boron-doped diamond on the as-grown SiNWs was carried out in a HFCVD reactor (purchased from Shanghai JiaoYou Diamond Coating Co. Ltd.). An acetone solution containing trimethyl borate (0.5% atoms ratio) was served as the carbon source. The carbon source/H2 flow ratio was 200:50 and the growth duration was 60 min. The bias voltage was set at 4.5 V. The substrate was heated by Tantalum wires with an ac power supply at the voltage of 20 V and the current of 60 to 70 A, and the deposition temperature was kept at about 600 °C. 2.3. Measurements The surface morphology of the sample was investigated by a scanning electron microscopy (Hitachi Ultra-High-Resolution S4300). The Raman spectroscopy was obtained using a Renishaw 1000 Raman spectrometer (Renishaw Ltd., UK). The electrochemical experiments were performed with a potentiostat/galvanostat (Model 263 A, Princeton, USA). The impedance spectroscopy measurements were taken with the same potentiostat/galvanostat device connected to a FRD 100 frequency response detector (Princeton, USA). The electrochemical measurements were carried out in a three-electrode system with a single compartment cell using a saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. A normal flat BDD electrode prepared under the same deposition condition was used for comparison. The electrochemical experiments were performed in a solution of 10 mL volume. The geometric area of the working electrode exposed to the solution was 0.08 cm2. 3. Results and discussion 3.1. Characterization of the BDDNF electrode From the SEM image of Fig. 1a, a high density of diamond crystallites coating the SiNWs like a nanorod forest vertically aligned on the original silicon wafer can be clearly observed. The SEM image taken at a high magnification (Fig. 1b) of the nanorods, reveals that the polycrystalline diamond coverage is complete and continuous along the whole length of the SiNWs with crystallite diameters ranging approximately from 50 nm to 80 nm. The quality of the diamond nanorods is studied by Raman spectroscopy. As shown in Fig. 1c, two peaks at 1332 cm1 and 1520 cm1 can be observed. The peak at 1332 cm1 is related to the presence of crystalline diamond. The broadening of this peak is caused by the finite domain size of the nanocrystalline diamond grains. The peak at 1520 cm1 is attributed to the disordered carbon and sp2-bonded carbon at the grain boundaries of nanocrystalline diamond [12]. A shoulder at 1130 cm1 is often identified as a signature for high quality of nanocrystalline diamond [13]. 3.2. Electrochemical behaviour of the BDDNF electrode 3.2.1. Characterization of impedance spectroscopy We have measured the faradic impedance of the electrodes taking FeðCNÞ63=4 as a model species in 0.1 M KCl solution. The corresponding Nyquist plots obtained of the BDDNF electrode and the flat BDD electrode with a frequency range of 1 Hz–100 kHz at a potential of 0.3 V (vs. SCE) are shown in Fig. 2. In general, for the equivalent circuit, the Nyquist plots shows a semicircle and a linear
region, the semicircle at higher frequencies corresponds to an electron-transfer-limited process, and the linear portion at lower frequencies corresponds to a diffusion-limited process. In our experiment, the Nyquist plots of the BDDNF electrode exhibit two semicircles rather than one (Fig. 2a), while only one semicircle is observed on the flat BDD electrode (Fig. 2b). Obviously, the appearance of the two semicircles should be directly related to the surface morphology of the BDDNF electrode. We consider that the high frequency (HF) semicircle is related to the porosity of the electrode and independent of the kinetics of the faradic process, and the low frequency (LF) semicircle is related to the kinetics of the faradic reaction [14]. The diameter of the HF semicircle indicates the charge transfer resistance related to the geometry of the electrode, while the diameter of the LF semicircle indicates the charge transfer resistance related to the kinetics of the faradic reaction. The present result of the BDDNF electrode is quite similar to the impedance behaviour of porous electrodes, on which two semicircles can be observed in the Nyquist plots for a charge transfer reaction as described in Ref. [15]. To obtain more detailed information, we have further examined the concentration dependence of the impedance behaviour of the two electrodes and compared the results. A typical faradic impedance result can be observed at the flat BDD electrode, i.e., only one semicircle is obtained and its diameter decreases with increasing 3=4 (Fig. 2b). At the BDDNF electrode, concentrations of FeðCNÞ6 two semicircles are observed and the diameter of the LF semicircle 3=4 , while the decreases with increasing concentrations of FeðCNÞ6 HF semicircle does not show obvious change (Fig. 2a). These results further confirm that there is diffusion restriction of the reactant among the nanorods due to the existence of the gaps, and this property will have an effect on the electrochemical performance of the BDDNF electrode. 3=4 on the BDDNF elecThe voltammetric responses to FeðCNÞ6 trode and the flat BDD electrode have also been investigated. As shown in Fig. 2c, the DEp of 190 mV for the BDDNF electrode and 230 mV for the flat BDD electrode are observed to 4 mM FeðCNÞ63=4 solution, respectively, which suggests that the BDDNF electrode, with large electro-active surface area, exhibits higher electron transfer kinetics than the flat BDD electrode. 3.2.2. Biosensing characteristics of the BDDNF electrode Normal flat BDD electrodes have been used as electrochemical biosensors due to their good stability and sensitivity. The research effort here is to compare the biosensing characteristics of the BDDNF electrode with the flat BDD electrode in amperometric detection of biomolecules. We used adenine, a fundamental compound in biological systems, as a biomolecule sample. Generally, amperometric detection of adenine is difficult on bare metal or carbon electrodes due to the overlap of oxygen evolution coming from the decomposition of water. As shown in Fig. 3a, well-defined oxidation current peaks for adenine in 0.1 M ammonium acetate buffer solution (pH 4.28) could be observed at a high potential (1.4 V vs. SCE) at the two electrodes. However, their oxidation peak currents are different. The typical amperometric responses of the two electrodes to concentration changes of adenine at the potential of 1.4 V (vs. SCE) have been examined and the corresponding results are shown in Fig. 3b. As expected, the BDDNF electrode displays an amplified response contrast with the flat BDD electrode. The sensitivity is 0.205 lA/lM for the BDDNF electrode and 0.024 lA/lM for the BDD electrode. Meanwhile, the value of lowest detection concentration (S/N = 3) for adenine is 2.5 ± 0.4 lM on the BDDNF electrode and 5.0 ± 0.5 lM on the BDD electrode, respectively. When interpreting the particularly electrochemical performance of the BDDNF electrode, we mainly consider the large
D. Luo, J. Zhi / Electrochemistry Communications 11 (2009) 1093–1096
1095
Fig. 1. SEM images from (a) top, and (b) amplificatory view. (c) Raman spectrum of the boron-doped diamond nanorods.
Fig. 2. The impedance changes with the concentrations of FeðCNÞ3=4 at (a) the BDDNF electrode, and (b) the flat BDD electrode at 0.3 V (vs. SCE). (c) Comparison of the cyclic 6 voltammograms in 0.1 M KCl solution containing 4 mM FeðCNÞ63=4 with a scan rate of 50 mVs1 at the BDDNF electrode and the flat BDD electrode, respectively.
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Fig. 3. (a) Cyclic voltammograms of 0.5 mM adenine in 0.1 M ammonium acetate buffer solution (pH 4.28) at the flat BDD electrode and the BDDNF electrode, respectively, with a scan rate of 50 mVs1. (b) Steady-state amperometric responses of the flat BDD electrode and the BDDNF electrode in a stirred solution (the same buffer) at an applied potential of 1.4 V (vs. SCE).
surface area and mass transport characteristics of the nanorod forest, i.e., primarily governed by the size and geometric spacing of the nanorod array. An electrode with such nanorods would experience various diffusion styles, i.e., a hemispherical diffusion mixed with planar diffusion profile [16,17]. Consequently, the BDDNF electrode with the additional charge transfer due to its large electro-active surface area and different diffusion styles results in higher amperometric responses than the flat BDD electrode. The electrochemical behaviour of the BDDNF electrode is far from completely understood, but the results reported here demonstrate the favorable possibilities for the applications of this kind of electrode in electroanalysis field. 4. Conclusion In summary, we have demonstrated a feasible method for fabricating of vertically aligned boron-doped diamond nanorod forests. Our method for the preparation of the BDDNF electrode is simple, convenient, and low-cost. The BDDNF electrode exhibits excellent electrochemical performance due to its large electro-active surface area and particular nanostructure. The impedance behaviour of the BDDNF electrode like that of porous electrodes 3=4 has been discussed. And a better biosensing activity in FeðCNÞ6 of the BDDNF electrode towards adenine than the BDD electrode has been observed. Based on these, the BDDNF electrode may be expected to be a novel electrode for practical applications in electrochemical detection of biomolecules.
Acknowledgements This work was supported by the Major Research Development Program of China (No. 2006CB933000) and National Natural Science Foundation of China (207731500). References [1] T.N. Rao, A. Fujishima, Diamond Relat. Mater. 9 (2000) 384. [2] Y.V. Pleskov, A.Y. Sakharova, M.D. Krotova, L.L. Bouilov, B.P. Spitsyn, J. Electroanal. Chem. 228 (1987) 19. [3] F. Bouamrane, A. Tadjeddine, J.E. Butler, R. Tenne, C. Lévy-Clément, J. Electroanal. Chem. 405 (1996) 95. [4] M.A. Kulandainathan, C. Hall, D. Wolverson, J.S. Foord, S.M. MacDonald, F. Marken, J. Electroanal. Chem. 606 (2007) 150. [5] A. Kapałka, B. Lanova, H. Baltruschat, G. Fóti, C. Comniellis, Eletrochem. Commun. 10 (2008) 1215. [6] Y.L. Zhou, R.H. Tian, J.F. Zhi, Biosens. Bioelectron. 22 (2007) 822. [7] Y. Song, G.M. Swain, Anal. Chem. 79 (2007) 2412. [8] T.E. McKnight, A.V. Melechko, D.W. Austin, T. Sims, M.A. Guillorn, M.L. Simpson, J. Phys. Chem. B 108 (2004) 7115. [9] E.-S. Baik, Y.-J. Baik, S.W. Lee, D. Jeon, Thin Solid Films 377 (2000) 295. [10] H. Masuda, T. Yanagishita, K. Yasui, K. Nishio, I. Yagi, T.N. Rao, A. Fujishima, Adv. Mater. 13 (2001) 247. [11] K.Q. Peng, Y.J. Yan, S.P. Gao, J. Zhu, Adv. Mater. 14 (2002) 1164. [12] E.C. Almeida, A.V. Diniz, V.J. Trava-Airoldi, N.G. Ferreira, Thin Solid Films 485 (2005) 241. [13] A.C. Ferrari, J. Robertson, Phys. Rev. B 63 (2001) 121405. [14] C. Hitz, A. Lasia, J. Electroanal. Chem. 500 (2001) 213. [15] A. Lasia, J. Electroanal. Chem. 397 (1995) 27. [16] T. Watanabe, T.A. Ivandini, Y. Makide, A. Fujishima, Y. Einaga, Anal. Chem. 78 (2006) 7857. [17] K.L. Soh, W.P. Kang, J.L. Davidson, Y.M. Wong, D.E. Cliffel, G.M. Swain, Diamond Relat. Mater. 17 (2008) 240.