an amperometric glucose biosensor fabricated with pt nanoparticle-decorated

Sensors and Actuators B 137 (2009) 134–138 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb An amperometric glucose biosensor fabricated with Pt nanoparticle-decorated carbon nanotubes/TiO 2 nanotube arrays composite Xinyu Pang, Dongmei He, Shenglian Luo, Qingyun Cai ∗ State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan University, Changsha 410082, People’s Republic of China article info Article history: Received 14 August 2008 Received in revised form 27 September 2008 Accepted 30 September 2008 Available online 15 October 2008 Keywords: Carbon nanotube Titania nanotube Platinum nanoparticles Glucose biosensor abstract Carbon nanotubes (CNTs)-modified titania nanotube (NT) arrays are prepared by vapor-growing CNTs in the inner of titania NTs. Pt nanoparticles of ∼3 nm in diameter are uniformly decorated on the as synthesized titania-supported CNTs (TiO 2 /CNTs) electrode, showing remarkably improved catalytic activ- ities for the oxidation of hydrogen peroxide. The consequent glucose biosensor fabricated by modifying TiO 2 /CNT/Pt electrode with glucose oxidase (GOx) presents a high sensitivity of 0.24 ␮AmM −1 cm −2 to glucose in the range of 0.006 mM to 1.5 mM with a response time of less than 3 s and a detection limit of 5.7 ␮M at 3 signal/noise ratio. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Since the development of the first glucose biosensor, much attention has been focused on the improvement of the response performances of enzyme electrodes for biosensor research [1–8]. Wireless glucose biosensors were developed by using a pH- responding polymer [4] or a glucose-responding mass-changing polymer [9] as the sensing coating. Significant research and efforts concerning preparation of glucose sensor for blood glucose mon- itoring have been reported [2–4] and most of them are based on the determination of hydrogen peroxide [5–8]. Carbon nanotubes (CNT) have attracted increasing research interests due to their unique electrical, geometrical, and mechanical properties [10] that make them excellent materials for the construction of ultrasensi- tive electrochemical biosensors which has led more recently to an upsurge of research on incorporating CNTs into biosensing plat- forms [7,8]. It has been demonstrated that electrodes based on the combination of metal nanoparticles and CNT exhibits highly sensitive and selective responses to hydrogen peroxide generated by enzymatic reactions [7,8,11]. Wang et al. [11] reported that multi-wall CNTs dissolved in Nafion could be applied to construct amperometric sensor for hydrogen peroxide. Electrochemists have also successfully taken advantage of CNT for accelerating the elec- ∗ Corresponding author. Tel.: +86 731 8822170; fax: +86 731 8821848. E-mail address: qycai0001@hnu.cn (Q. Cai). tron transfer reaction involving electrocatalytic activities toward H 2 O 2 , NADH, cysteine, ascorbic acid, nucleic acids, and homocys- teine [12,13]. Enzyme immobilization is a key step in fabrication of a sen- sitive and stable biosensor. Generally in biosensors enzymes are immobilized to the sensor surface by either cross-linking with, e.g. glutaraldehyde [14] or being protected with a thin gel or poly- mer layer of, e.g. Nafion [15,16] to avoid the loss of enzymes. In particular, new materials and methods were researched for get- ting more active and stable biosensors in immobilizing enzyme [17,18]. Nano-architectured TiO 2 has also attracted considerable interest due to the superior properties such as large specific sur- face area, high uniformity, and excellent biocompatibility [19–25], and has been applied in a variety of fields including highly effi- cient photocatalysis [20,21], fuel cells [22], biosensors [23], and hydrogen sensor [24,25]. Liu and Chen [23] fabricated a biosen- sor for measuring H 2 O 2 utilizing a TiO 2 nanotube array on which horseradish peroxidase and thionine were adsorbed. The high uni- formity and superior semi-conductivity of the TiO 2 NT array makes it a sensitive hydrogen gas-responsive material [24,25].TiO 2 film has also been utilized in the immobilization of proteins for bio- analytical applications due to its stability and biocompatibility [26–28]. Topoglidis et al. reported the adsorption of protein on nanocrystalline TiO 2 films based on the electrostatic interactions between the negatively and positively charged groups of them [28]. These successful applications indicate that nanocrystalline TiO 2 materials are promising ideal functional materials for biosensor substrate. 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.09.051 X. Pang et al. / Sensors and Actuators B 137 (2009) 134–138 135 Here, an electrocatalytic enzyme biosensor based on Pt nanoparticles-decorated TiO 2 /CNT composite substrate was devel- oped. The Electrochemical catalytic activity of the as-prepared TiO 2 /CNT/Pt electrode in response to H 2 O 2 was investigated. An amperometric glucose oxidase (GOx) biosensor was fabricated by modifying GOx on the as-prepared electrode, and a sensitive response to glucose was achieved. 2. Experimental 2.1. Reagents and apparatus Titanium foil (99.8%, 0.127 mm thick) was purchased from Aldrich (Milwaukee, WI). Sodium fluoride, citric acid, hexachloro- platinic (IV) acid and glucose of analytical reagent grade were purchased from commercial sources and used as supplied. GOx (type: X-S, Aspergillus niger, 127 units/mg) were obtained from Aldrich–Sigma (St. Louis, MO). The supporting electrolyte was 0.067 M pH 7.2 phosphate buffer solutions (PBS). Double distille d water was used throughout the experiments. The catalyst topology was characterized using a field-emission scanning electron microscope (FE–SEM) operating at 5 kV (JSM 6700F; JEOL, Tokyo, Japan). An energy dispersive X-ray (EDX) spec- trometer fitted to the scanning electron microscope was used for elemental analysis. Transmission electron microscopy (TEM) images were obtained using a JEM 3010 (JEOL; Tokyo, Japan) operating at 300 kV. Cyclic voltammetry (CV) and amperometric measurements were performed in a standard three-electrode con- figuration with the modified electrodes as working electrode, a platinum flake auxiliary electrode, and Ag/AgCl reference electrode (saturated by KCl) by an electrochemical working station (CHI660B; CH Instruments, Inc., Austin, TX). 2.2. Preparation of TiO 2 /CNT/Pt/GOx electrode The construction process of the electrode is schemati- cally shown in Fig. 1. Prior to anodization, the titanium foil (3 mm× 15 mm) was degreased by sonication in acetone and then ethanol. The cleaned titanium ribbon was anodized at anodiza- tion voltages of 15 V in an electrolyte containing 0.2 M citric acid and 0.1 M NaF at room temperature for 5 h in a conventional two- electrode system with a platinum cathode. The titanium sample was only partially immersed in the electrolyte, with the upper un-anodized portion used as an electrical contact. The efficient geometrical area of the anodize d part (both sides) is 0.6 cm 2 . The resulting nanotube arrays were ∼90 nm in diameter. CNTs were prepared by chemical vapor deposition (CVD)accord- ing to paper [29]. Nickel cations were introduced into the TiO 2 nanotubes by capillary action and electric field, and electrochemi- cally reduced to form Ni nanoparticles as the catalyst for the growth of CNTs. CNTs were grown from the inner of TiO 2 nanotubes through catalytic decomposition of acetylene over Ni nanoparticles in a vac- uum tube furnace for 1 h at 700 ◦ C. On the CNT-modified TiO 2 NTs, Pt nanoparticles were electode- posited using chronopotentiometry at a current density of 5 mA/s in astandard three-electrode system with a TiO 2 /CNT working elec- trode, a platinumwire auxiliary electrode, and an Ag/AgCl reference electrode. The catalytic activity of TiO 2 /CNT/Pt electrode was deter- mined by cyclic voltammetry with 1 mM H 2 O 2 as the testing probe at a scan rate of 100 mV s −1 in a solution containing 10 mL pH 7.2 (measured with Mettler-Toledo Delta 320 pH meter) phosphate buffer solution (PBS) and 0.1 M NaCl. The enzyme solution was prepared by dissolving 36 mg of GOx in 1 mL of 0.067 M PBS(pH 7.2) containing 0.1 M NaCl. A TiO 2 /CNT/Pt electrode was loaded with 20 ␮L enzyme solution, and dried at 4 ◦ C overnight. The resultant electrode was rinsed with PBS to remove free enzymes. When not in use, the enzyme biosensor was stored in phosphate buffer (pH 7.2) at 4 ◦ C in a refrigerator. Amperometry of glucose were performed in 10 mL phosphate buffer solution under stirring at ambient temperature. 3. Results and discussion 3.1. Characterization of TiO 2 /CNT/Pt electrode To investigate the microstructure of the as synthesized TiO 2 /CNT/Pt electrode, it was characterized by SEM and TEM. Fig. 2 shows the topography of the TiO 2 nanotubes and configuration of TiO 2 /CNT/Pt electrode. Fig. 2a shows that uniform TiO 2 nanotubes were formed with a pore size of about 90 nm in diameter and a length of about 320nm (shown in the inset of Fig. 2a). Fig. 2(b) shows that long CNTs with an average diameter of about 50 nm are grownonTiO 2 NTs. The CNTs were grownfrom the inside of the TiO 2 NTs [30] as the Ni catalyst was electrodeposited inside the TiO 2 NTs. To confirm this speculation the surface CNTs was removed by son- ication, and a CNTs-embedded porous annealed TiO 2 substrate is seen as shown inFig. 2(c), where the TiO 2 NTs were collapsed during the growth process of CNTs. Since the CNTs were forme d at around 650 ◦ C [31] before the collapse of TiO 2 NTs at around 680 ◦ C [32], Fig. 1. Schematic showing the construction of the Pt nanoparticle-decorated CNT/TiO 2 NT electrode, step (a): electrodeposition of Ni nanoparticles into TiO 2 NTs; step (b): CNTs development from the inner of TiO 2 by chemical vapor deposition; and step (c): electrodeposition of Pt nanoparticles on CNTs. 136 X. Pang et al. / Sensors and Actuators B 137 (2009) 134–138 Fig. 2. SEM micrographs showing the morphology of TiO 2 /CNT/Pt electrode: (a) TiO 2 NTs prepared at 15 V with 90 nm in diameter and 300 nm in length (the inset), (b) Pt nanopaticles-deposited CNTs grown from the inner of TiO 2 NTs; the corresponding TEM image is shown in the inset and the corresponding EDX pattern is shown in (d). (c) The SEM image of TiO 2 /CNT/Pt electrode after removing the surface CNTs. CNTs were embedded in TiO 2 substrate which existed in the mix- ture of rutile and anatase phase based on the former results [19,32]. The CNTs-embedded TiO 2 substrate facilitates the electron trans- fer due to its enhanced conductivity, and the disorderly distributed CNTs provide a large surface area for Pt nanoparticles dispersion and a conductive network for electrons transfer. The TEM imaging (the inset in Fig. 2b) shows that Pt nanoparticles at an average size ∼3 nm are uniformly dispersed within the CNT network. The corre- sponding EDX spectrum given in Fig. 2(d) shows the presence of Ti, Pt, C, and O, with a Ti:O atomic ratio of 1:2. The uniformly dispersed small Pt nanoparticles are essential to a high catalytic activity. 3.2. Detection of hydrogen peroxide While the quantification of glucose is based on the electro- chemical detection of the enzymatically liberated H 2 O 2 , the sensor sensitivity is dependent on the electrochemical response ofthe sen- sor to H 2 O 2 ; electrodes with high catalytic efficiency to H 2 O 2 would achieve high sensitivity to glucose. The electrochemical response of the TiO 2 /CNT/Pt electrode to H 2 O 2 was firstly investigated. Fig. 3 shows the cyclic voltammograms of the TiO 2 /CNT/Pt electrode in the absence (curve d) and in the presence of increasing H 2 O 2 con- centration (curves a–c). The response of the TiO 2 /CNT/Pt electrode in 0.067 M pH 7.2 PBS displays a distinct background current, which indicates that the incorporation of CNT with TiO 2 NTs facilitates the electron transfer between electrode and H 2 O 2 . A pair of well- defined redox peaks (Epa, 0.45 V;Epc, 0.0 V) of H 2 O 2 were observed, and the H 2 O 2 oxidation peak potential at the TiO 2 /CNT/Pt electrode is at 0.45 V, which is 230 mV lower than that at the Pt bulk electrode (0.67 V). This suggests the catalytic activity of Pt nanopartilces to the oxidation of H 2 O 2 [14] and a faster electron transfer rate at the TiO 2 /CNT/Pt electrode. Hall et al. [33] investigated the mecha- nism of electrochemical oxidation of H 2 O 2 at platinum electrodes, concluding that the oxidation reaction is an adsorption-controlled Fig. 3. Cyclic voltammograms of TiO 2 /CNT/Pt electrode in 0.067 M pH 7.2 phosphate buffer containing: (a) 15, (b) 5, (c) 2.5, and (d) 0 mM H 2 O 2 . Scan rate: 100 mV/s. X. Pang et al. / Sensors and Actuators B 137 (2009) 134–138 137 Fig. 4. Ampermetric responses of TiO 2 /CNT/Pt electrode to continuously injection of 1 ␮mol/L H 2 O 2 (pH 7.2, 0.067 mol/L PBS). Working potential: 400 mV (vs. Ag/AgCl); the inset shows the calibration curve. mechanism, which depends on the potential, temperature, phos- phate buffer, pH value, and chloride concentration. The mechanism should be applicable to the oxidation of H 2 O 2 at the TiO 2 /CNT/Pt electrode; the high adsorbability of TiO 2 and CNT to H 2 O 2 facil- itate the oxidation reaction of H 2 O 2 . Such electrocatalytic action facilitates low-potential amperometric measurements of hydrogen peroxide. The redox currents of H 2 O 2 increase with the increase of H 2 O 2 concentration. The peak potential difference is 0.45V, indicating an irreversible redox reaction of H 2 O 2 at the TiO 2 /CNT/Pt electrode. The catalytic activity toward H 2 O 2 of the TiO 2 /CNT/Pt electrode was assessed by quantitative analysis of the amperometric response to successive injection of 1 ␮MH 2 O 2 . As shown in Fig. 4, a current response (i)of0.145␮A/␮M was achieved at applied poten- tial of 400 mV, with a liner response in the range of 0.001 mM to 2 mM with a slope of 0.134 ␮A/␮M(R 2 = 0.997) and a limit of detection (LOD) of 1 ␮M at a signal-to-noise ratio of 3 as shown in the inset. The achieved LOD is much lower than what was obtained at a horseradish peroxidase-modified multi-wall carbon nanotubes/chitosan sensor (10␮M) [34], and at the mesoporous Pt microelectrode (4.5␮M) [35], indicating that the TiO 2 /CNT/Pt electrode is with a superior electrocatalytic activity toward the oxidation of H 2 O 2 . 3.3. Detection of glucose A glucose biosensor was fabricated by modifying GOx onto the as prepared TiO 2 /CNT/Pt electrode which detects H 2 O 2 gen- erated by enzymatic reactions. Fig. 5 shows the amperometric responses of the glucose sensor to the successive additions of 1 mM glucose at applied potential of 400 mV vs. Ag/AgCl reference elec- trode. An evident current response (i)of0.138␮A was achieved. Successive addition of 1 mM glucose (n = 7) showed a relative stan- dard deviation (R.S.D.) of 3.2% verifying a good repeatability of the sensor. As shown in the inset, the nanocomposite biosensor exhibits a linear response from 0.01 mM to 1.5mM with a response slope of 0.146 ␮AmM −1 (R 2 = 0.998). A sensitivity of as high as 0.24 ␮AmM −1 cm −2 was achieved. The LOD was 5.7 ␮M at a signal- to-noise ratio of 3, and the response time was less than 3 s. The proposed sensor represents a simple and fast approach to the detec- tion of glucose. The inter-sensor reproducibility was investigated at a glucose concentration of 0.01 mM. Three independently made GOx sensors showed an acceptable reproducibility with a variation coefficient of Fig. 5. Ampermetric responses of TiO 2 /CNT/Pt/GOx sensor to continuously injec- tion of 1 mmol/L glucose (pH 7.2, 0.067 mol/L PBS). Working potential: 400 mV (vs. Ag/AgCl); the inset shows the calibration curve. 4.8% (n = 5) for the current determination at 0.01 mM g lucose. The sensor response to 0.01 mM glucose was decreased by 18% after one month mostly due to the decrease in the enzyme activity. 3.4. Performances of biosensors The functions of the CNT and Pt nanoparticles were inves- tigated by fabricating GOx sensors with (a) bare TiO 2 NTs, (b) Pt-deposited TiO 2 NTs (TiO 2 /Pt), (c) Pt-deposited TiO 2 NTs/CNT composite (TiO 2 /CNT/Pt), and (d) the electrode c but the sur- face CNT was removed (TiO 2 /(CNT)/Pt) as substrate, respectively. The Pt loading was 0.126 mg/cm 2 . Fig. 6 shows the amperomet- ric responses of the four sensors upon subsequent additions of 1 mM glucose. The TiO 2 /CNT/Pt/GOx sensor exhibits the highest electrocatalytic activity toward glucose (Fig. 6c), while the bare TiO 2 NT/GOx sensor exhibits negligible response (Fig. 6a), indicat- ing the catalytic activity of Pt nanoparticles. The current response (i)atTiO 2 /CNT/Pt/GOx sensor (Fig. 6c) is ten times that at the TiO 2 /Pt/GOx sensor (Fig. 6b), indicating that the CNT enhances the catalytic activity of Pt due to the large surface area of the CNT net- Fig. 6. Amperometric responses of sensors: (a) TiO 2 /GOx, (b) TiO 2 /Pt/GOx, (c) TiO 2 /CNT/Pt/GOx, and (d) TiO 2 (CNT)/Pt/GOx in 10 mM PBS (pH 7.2) to continu- ous injection of 1 mM glucose at 400 mV working potential (vs. Ag/AgCl). Scan rate: 100mVs −1 . 138 X. Pang et al. / Sensors and Actuators B 137 (2009) 134–138 work which increases the Pt loading and facilitates the electron transfer. In addition, CNT network provides superior adsorption ability toward glucose. The interesting result is that even the sur- face CNT has been removed (TiO 2 (CNT)/Pt/GOx sensor), the current response (Fig. 6d) is still three times that at the TiO 2 /Pt/GOx sen- sor (Fig. 6b), confirming again that the CNTs were grown from the inner of TiO 2 NTs, and the CNTs-embedded TiO 2 showed sensitive responses to glucose. As for practical applications, we can remove the surface CNTs to obtain a stable electrode structure. There would be no potential bio-hazards resulted from the lose of CNTs. 4. Conclusions A Pt nanoparticle-decorated electrode was fabricated by using the TiO 2 /CNT composite as substrate. The electrode exhibits a high catalytic efficiency to the oxidation of hydrogen peroxide with a slop of 0.134 ␮A/mM. The resultant GOx sensor by modifying GOx on the as-prepared electrode displays a good response to glucose with a sensitivity of 0.24 ␮AmM −1 cm −2 and a LOD of 5.7 ␮M. The sensor-to-sensor reproducibility was measured on three indepen- dently made GOx sensors with a variation coefficient of 4.8%. The high catalytic activity is ascribed to the large surface area and good conductivity of CNT network, the highly dispersed Pt nanoparticles, and the excellent biocompatibility of TiO 2 to H 2 O 2 . Acknowledgment Funding for this work by the National Science Foundation of China under Grants No. 20775024, and the Specialized Research Fund for the Doctoral Program of Higher Education under grant 20050532024 is gratefully acknowledged. Professor Shenglian Luo gratefully acknowledges partial support of this work by the National Science Foundation for Distinguished Young Scholars under Grant No. 50725825. References [1] O.A. Raitman, E. Katz, A.F. Buckmann, I. 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Denuault, Detection ofhydrogen peroxide at mesoporous platinummicroelectrodes,Anal. Chem. 74 (2002) 1322–1326. Biographies Xinyu Pang received BS degree in chemistry from Hunan University, PR China in 2006. Now he is pursuing his MS degree in analytical chemistry at Hunan University in the research group of professor Qingyun Cai. Dongmei He received BS degree in chemistry from Henyang Normal University, PR China in 2005 and MS degree in analytical chemistry from Hunan University, PR China in 2008. She is currently an assistant researcher at Kangde Solar Energy Int. Co., Dongguan, Guandong, PR, China. Qingyun Cai received BA degree in 1983 and MS degree in 1986, both in chemistry from Hunan University, PR China. Since then he has been on the faculty at Hunan University. He earned the PhD in chemistry in 1996 from Hunan University. He is cur- rently a full-time professor in the Department of Chemistry at Hunan University, PR China. His primary research interests concern the chemo/biosensors and functional (nano) materials. . www.elsevier.com/locate/snb An amperometric glucose biosensor fabricated with Pt nanoparticle-decorated carbon nanotubes/TiO 2 nanotube arrays composite Xinyu Pang, Dongmei. September 2008 Accepted 30 September 2008 Available online 15 October 2008 Keywords: Carbon nanotube Titania nanotube Platinum nanoparticles Glucose biosensor abstract Carbon