Pt Nanoparticles Inserting in Carbon Nanotube Arrays

Jul 10, 2009 - A facile strategy has been developed to prepare carbon nanotubes loading Pt nanoparticle (Pt-CNT) composites. The method involves the ...
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Pt Nanoparticles Inserting in Carbon Nanotube Arrays: Nanocomposites for Glucose Biosensors Zhenhai Wen,†,‡ Suqin Ci,† and Jinghong Li*,‡ School of EnVironment and Chemical Engineering, Nanchang Hangkong UniVersity, Nanchang 330063, China, and Department of Chemistry, Key Lab of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua UniVersity, Beijing 100084, China ReceiVed: March 29, 2009; ReVised Manuscript ReceiVed: June 14, 2009

A facile strategy has been developed to prepare carbon nanotubes loading Pt nanoparticle (Pt-CNT) composites. The method involves the polymerization reaction of glucose and the reduction deposition of a platinum source in the pores of anodic alumina membranes (AAMs) under hydrothermal conditions. The Pt-CNT nanocomposites can be obtained through the subsequent carbonization and removal of the AAM template. Through transmission electron microscopy and field-emission scanning electron microscopy, it is observed that the nanocomposites possess a stable hierarchical structure, in which the Pt nanoparticles are uniformly entrapped on the surface of CNTs. Additionally, the Pt-CNT nanocomposites contain large amounts of oxygen-rich groups that are beneficial to improving its solubility in water and biocompatibility for retaining the bioactivity of glucose oxidase. The nanocomposites electrode is successfully used as a sensitively amperometric sensor for low-potential determination of H2O2. The Pt-CNT-based glucose biosensor is fabricated by mixing the composites with the glucose oxidase, displaying a wide linear calibration range nearly 3 orders of magnitude of glucose concentrations (0.16-11.5 mM) and a low detection limit of 0.055 mM. Furthermore, the biosensor exhibits some other excellent characteristics, such as high sensitivity and selectivity, short response time, and long-term stability. 1. Introduction Glucose biosensors, since the development by Clark and Lyons,1 have received considerable attention and continuous research interest, which aims to generate reliable methods for in vitro or in vivo glucose measurement because of their potential applications in biotechnology, process control, and clinical diagnosis.2-5 Because the glucose oxidase (GOD) can identify glucose target molecules quickly and accurately even in complicated systems, most of glucose biosensors are based on the GOD enzymatic reaction, in which GOD catalyzes the oxidation of glucose to gluconolactone and H2O2 with the assistance of oxygen. GOD

glucose + O2 98 gluconolactone + H2O2

(1)

Accordingly, the concentration of glucose can be indirectly monitored through electrochemical determination of the consumed dissolving oxygen or the liberated H2O2 from an enzymatic reaction.6-11 As well-known, the dissolving oxygen is facilely affected by the change of environment, which might result in a measureless deviation for the detecting result. On the other hand, there exists a rather high overvoltage for the oxidation or reduction of H2O2 at common solid electrodes, leading to interferences from various electroactive species such as ascorbic acid (AA), uric acid (UA), etc. It is therefore highly desired to design and prepare a functional material for the * To whom correspondence should be addressed. Tel: +86 10 6279 5290. Fax: +86 10 6277 1149. E-mail: [email protected]. † Nanchang Hangkong University. ‡ Tsinghua University.

modification of the electrode surface; one of the purposes is to efficiently lower the H2O2 oxidation/reduction overvoltage, and the other is for immobilization of GOD and maintenance of the highly enzymatic activity of GOD.12,13 The emerging nanotechnology offers great opportunities to improve the sensitivity, long-term stability, and anti-interference ability of the biosensor systems.14-16 On one hand, some nanostructured materials, due to their unique properties such as large specific surface area, nontoxicity, and biocompatibility, will be beneficial by improving the sensitivity of sensors and maintaining the bioactivity of bimolecules when used as electrode materials of an electrochemical biosensor.17-19 On the other hand, the unique porous structures of nanomaterials can greatly enhance the active surface area available for protein binding, leading to a shorter response time and long-term stability at the nanomaterials-based biosensor.20-22 Ever since its discovery in 1991 by Iijima, carbon nanotubes (CNTs) have been of tremendous interest for novel materials and devices due to their remarkable thermal, electronic, photonic, and unique structural features. Additionally, CNTs can behave electrically as a metal or as a semiconductor and possess a high chemical stability and high surface-to-volume ratio, which render tremendous opportunities for potential applications in biosensors, fuel cells, and lithium ion batteries. For CNT-based biosensors, there are still some developmental challenges to be addressed. One major barrier is the insolubility of CNTs in most solvents, which may greatly limit their application in biosensor systems.23-26 To improve water solubility of CNT, it is necessary to functionalize CNTs through an oxidative treatment that involves extensive ultrasonic treatment in a mixture of concentrated nitric and sulfuric acids, which may decorate the ends and sidewalls of the CNTs with carboxyl groups.27 Recent implementations suggest that further deposition of platinum (Pt)

10.1021/jp902830z CCC: $40.75  2009 American Chemical Society Published on Web 07/10/2009

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Figure 1. Schematic illustration of the synthetic procedure of Pt-CNTs: (1) Pt deposition and glucose polymerization in the AAM nanochannel under hydrothermal conditions and (2) liberation of Pt-CNTs from the AAM template.

nanoparticles on the surfaces of CNTs can significantly improve the catalytic activity for H2O2 electrochemical oxidation/ reduction and lower the overvoltage, which are beneficial in fabricating biosensors with high sensitivity and selectivity.28,29 However, it is relatively difficult to uniformly embed Pt nanoparticles in CNTs and simultaneously realize the special function of the nanohybrids. In this study, a one-pot strategy has been developed to prepare Pt-CNT nanocomposites, in which the Pt nanoparticles can be uniformly entrapped on the surfaces of CNTs. In this strategy, the hydrothermal polymerization of glucose and reductants of H2PtCl6 is confined in nanochannels of anodic alumina membranes (AAMs). The Pt-CNT nanocomposites can be obtained through carbonization treatment and subsequent removal of AAMs. According to electrochemical investigation, the Pt-CNT nanocomposite system shows an excellent electrocatalytic activity toward H2O2 electroreduction and can realize lowpotential amperometric detection of H2O2. Meanwhile, the glucose biosensor based on the Pt-CNT-GOD composites could efficiently retain the bioactivity of the GOD and display significant improved performances associated with excellent selectivity, high sensitivity, and good stability. 2. Experiments 2.1. Reagents. GOD (from Aspergillus niger, E.C. 1.1.3.4) and Nafion (5 wt % in lower aliphatic alcohols) were purchased from Sigma. All other reagents were of analytical grade and used without further purification. All solutions were prepared with pure water, which was purified with a Millipore-Q purification system to a specific resistance over 18.0 MQ cm. 2.2. Synthesis. Several AAMs (Whatman, United States) with 0.2 µm pore and 60 µm thickness were first immersed in 25 mL of aqueous solution containing 0.001 M H2PtCl6 and 0.5 M glucose for 0.5 h. Then, the mixture containing AAM templates was transferred to a 30 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 5-10 h. After the system was allowed to cool to room temperature, the AAM templates were carefully taken out from the autoclave. The black slurry overlaying the surface of the AAM was removed through discrete scrape. For the carbonization, the AAM templates were pylorized at 750 °C under nitrogen flowing for 3 h. The Ptsupported CNT composites (Pt-CNTs) were liberated by dissolving the resulting AAM templates with 10% HF solution. The pure phase CNTs were fabricated through the same way, just no H2PtCl6 was added in the reaction autoclave. 2.3. Characterization. X-ray diffraction (XRD) was performed in the 2θ range from 10 to 80° on a Bruker D8-Advance

X-ray powder diffractometer with monochromatized Cu KR radiation (λ ) 1.5418 Å). The microscopic features of the samples were observed by using a Hitachi model H-800 transmission electron microscope (TEM), a JEM 2010 highresolution transmission electron microscope (HRTEM) opened at an accelerating voltage of 120 kV, field emmission scan electron micrographs (FESEM) (JSM 7401F), and a Sirion200 FESEM equipped with an energy dispersive spectroscopy analyzer. Fourier transform IR (FTIR) measurements were carried out through a Perkin-Elmer spectrophotometer operating in the infrared domain between 500 and 4000 cm-1 by using a KBr matrix. All of the samples characterized were prepared with a hydrothermal time of 5 h except a special claim. 2.4. Electrochemical Measurements. Electrochemical measurements were carried out with a CHI 660 electrochemical workstation (CHI Inc., United States) at room temperature in one three-electrode cell by using a Ag/AgCl electrode and Pt wire as the reference and the counter electrodes, respectively. Without a special statement, 0.1 M phosphate buffer solution (PBS), pH 7.0, was used as the electrolyte in all experiments. A glass carbon (GC) electrode was polished using 0.3 and 0.05 µm alumina slurries; after it was washed with water and acetone, the GC was subjected to ultrasonic agitation for 2 min in ultrapure water and dried. The Pt-CNT and CNT electrodes were prepared by a simple casting method. Typically, a homogeneous solution containing 0.5 mg mL-1 Pt-CNTs or CNTs was prepared by adding appropriate samples into the aqueous solution of 0.5% Nafion. A 4.0 µL aliquot of this solution was uniformly pipetted onto the surface of a freshly polished GC electrode by using a syringe. A beaker was covered over the electrode so that water could evaporate slowly in air and a uniform film electrode could be formed. For preparing the Pt-CNT-GOD electrode, the 0.5 mg mL-1 Pt-CNT was mixed with the same amount volume of GOD solution (10 mg/mL GOD); 4.0 µL of the solution was cast onto the surface of a GC electrode to obtain Nafion-Pt-CNT-GOD film-modified electrode. This enzyme-modified electrode was stored at 4 °C in a refrigerator when not in use. Before electrochemical measurements, all of the as-prepared film electrodes were immersed in pH 7.0 PBS for 20 min to remove residual composites. 3. Results and Discussion 3.1. Preparation of Pt-CNT Nanocomposites. The process for fabricating Pt-CNT nanocomposites is schematically shown in Figure 1. It is well-known that H2PtCl6 can be reduced to Pt by the glucose with the assistance of heating and that the glucose

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Figure 2. SEM images of the Pt-CNTs (a and b), TEM images (c-e), and EDS spectroscopy of Pt-CNTs.

will polymerize into carbonaceous polymers under certain hydrothermal conditions. In this reaction systems, the hydrothermal temperature (180 °C) is enough to induce the above two reactions. Initially, Pt sources (H2PtCl6) were reduced to Pt nanoparticles by glucose that simultaneously engendered carbonaceous polymer films on the surface of AAM nanochannels through a series of complicated chemical reactions.30 During the process, Pt nanoparticles would entrap in the carbonaceous polymer films since both of them were produced nearly at the same time. Afterward, the Pt-CNT nanocomposites could be obtained after carbonization and dissolution of AAM. In comparison with our previous work and conventional methods,24 the present strategy can provide several advantages. First of all, the synthetic procedure is more facile, simple, and convenient since the deposition of Pt nanoparticles and the formation of carbonaceous polymers were one-pot produced, which is completely converse to most of the previous work, which needs at least two steps to fulfill this procedure. On the second hand, the present method would make Pt nanoparticles possibly entrap on the surface of CNTs uniformly since the nucleation and growth of the Pt nanoparticles and the glucose polymerization reaction almost simultaneously occurred. Finally, the as-prepared Pt-CNTs would possess more stable hierarchical structures as a result, as the Pt nanoparticles have been embedded into the CNTs, which are significantly of importance for their implicated applications. 3.2. Characterization of Pt-CNT Nanocomposites. The morphology of the Pt-CNTs was first investigated by FESEM. Figure 2a,b shows the FESEM images of a typical sample from different visual angles, respectively, in which two basic pieces of information can be concluded; one is that the array of aligned Pt-CNT nanocomposites was successfully prepared, and the other is that the Pt nanoparticles were actually distributed on the surfaces of CNTs. The detailed information about structures and morphologies of the Pt-CNT nanocomposites can be observed through a TEM image, as shown in Figure 2c-e. The as-prepared Pt-CNTs are about 200 nm in diameter and have cylinders with a thin wall thickness of about 10 nm (Figure 2c,d), and it affords additional evidence that the Pt

Figure 3. FTIR spectrum of the Pt-CNTs.

nanoparticles are well-distributed and embedded into the CNTs uniformly (Figure 2e). According to energy-dispersive X-ray spectroscopy (EDS), it is semiquantitatively revealed that the Pt-CNTs were comprised of 80.8 wt % C, 4.3 wt % O, and 14.9 wt % Pt, respectively (Figure 2f), indicating that the AAM templates were completely removed by HF. It should be noted that the existence of O could be attributed to some groups on the surfaces of CNTs. According to FTIR measurements (Figure 3), one can observe an intensive peak at about 1700 cm-1, corresponding to the vibration of the CdO group, suggesting the existence of large amounts of the CdO group in the PtCNT samples. Additionally, the peaks at approaching 1400 and 3000 cm-1, respectively, can be assigned to the characteristic vibrations of the C-OH stretching bond and the COOH group. These oxygen-containing groups not only can significantly improve the uniform dispersion or solubility of Pt-CNT nanocomposites in water but also are beneficial to improve its biocompatibility for retaining bioactivity. It should be noted that these functional groups were just derived from the fabricating process, suggesting that the troublesome procedure of oxidative treatment or chemical modification could be avoided.31,32 The crystalline nature of the Pt-CNT nanocomposites was recorded by an X-ray powder diffraction (XRD) spectrum in the range of 10-80° (Figure 4). One can observe three

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Figure 4. XRD patterns of the Pt-CNTs.

characteristic diffraction peaks at 39.7, 46.2, and 67.4° corresponding to the (111), (200), and (220) crystalline planes of face central cubic (fcc) Pt, respectively, indicating that the Pt nanoparticles were composed of pure crystalline Pt. Additionally, a feeble and broad peak can be observed at about 25°, which can be attributed to the (002) planes of carbonized carbon. 3.3. Detection of Hydrogen Peroxide. Pt has been extensively investigated for its use as an electrode material for the detection of hydrogen peroxide and for the fabrication of various biosensors. Usually, the detection of H2O2 was operated at relatively high potential around 0.6 V for most biosensors based on Pt electrode, which easily suffered from interferences from electroactive materials.33-35 Therefore, we expect that the present Pt-CNT electrode could provide the advantage of lowering the potential for the detection of H2O2. For comparison, a CNT electrode was also prepared through the same way. It should be noted that the CNTs were prepared in a similar route to PtCNTs (see the Experiments); the SEM and TEM images of CNTs are shown in Figure S1 of the Supporting Information. We have first studied the electrochemical behaviors of H2O2 at Pt-CNT electrodes, CNT electrodes, and bulk Pt electrodes, respectively, through a cyclic voltammetry (CV) technique, which was conducted in 0.1 M PBS (pH 7.0). Figure 5a shows CVs obtained at the Pt-CNT electrode in the absence (dashed line) and presence (solid line) of 0.12 mM H2O2 between -0.5 and 0.3 V at a scan rate of 50 mV s-1. No current peak can be observed in the PBS solution without H2O2. However, a significant increase of current can be observed in the PBS containing a concentration of 0.12 mM H2O2, and the H2O2 electroreduction on Pt-CNTs occurs within a wide potential range between 0.15 and -0.1 V with a peak maximum of about 0.02 V. In contrast, there is no obvious cathodic peak in Figure 5b obtained at the CNT-modified electrode at the same potential range no matter whether H2O2 was added into the PBS or not. For the bulk Pt electrode (Figure 5c), only a slight and broad cathodic peak can be also observed in PBS containing 0.12 mM H2O2. The electrochemical reduction of H2O2 was also investigated at different concentrations (not shown); it is found that the peak current increased with the sequential addition of H2O2, indicating a stable relationship between the reduction current and the H2O2 concentration. The peak currents of H2O2 at the Pt-CNT nanocomposite-modified GC electrode are much larger than those at the CNTs and bulk Pt electrodes, indicating that Pt-CNT nanocomposites exhibited much better electrocatalytic activity than the bulk Pt electrode. The improved electrocatalytic activity should be attributed to their unique nanostructures, high surface-to-volume ratio, and the synergistic effect provided by CNTs and Pt nanoparticles, which were of significance for highly selective and highly sensitive detection of H2O2.

Figure 5. Cyclic voltammograms of porous Pt-CNTs (a), CNTs (b), and bulk Pt (c) electrodes in the absence (dashed line) and the presence (solid line) of 0.120 mM H2O2 in 0.1 M PBS solution (pH 7.0) at a scan rate of 50 mV s-1.

Figure 6 depicts the typical amperometric response at the PtCNT electrode for successive addition of different amounts of H2O2 at a constant applied potential of -0.1 V. The Pt-CNT electrode exhibits a fast and sensitive response to the addition of H2O2 with steady-state current reached within 5 s. Additionally, with the addition of different amounts of H2O2, the PtCNT electrode exhibits a corresponding proportion of amperometric response. According to the measurement, the linear range of the Pt-CNT electrode toward H2O2 is from 5.0 × 10-6 to 2.5 × 10-2 M, covering nearly 4 orders of magnitudes with a sensitivity of 0.14 a.m.-1 cm-2. On the basis of S/N ) 3, a detection limit of 1.5 × 10-6 M was obtained. A 0.1 mM concentration of H2O2 was measured continuously 12 times, and a relative standard deviation (RSD) of 3.6% was obtained. Furthermore, repeated utilization of the as-prepared Pt-CNT electrode showed a slight effect on its long-term stability. The satisfactory results suggest the reliability of the Pt-CNT electrode for the determination of H2O2 since the Pt-CNT electrode displays the excellent combination of the low detection potential with wide linear range and high sensitivity.

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Figure 8. Calibration curve of steady-state currents vs concentrations of glucose. Data are from Figure 7. Figure 6. Typical current-time response curves of the Pt-CNTs electrode upon successive additions of H2O2 into pH 7.0 PBS solution. Applied potential: -0.1 V (vs Ag/AgCl).

the same method, and a RSD of 5.13% was observed toward 1 mM glucose, indicating the reliability of the method. When the concentration of glucose reached as high as 30.0 mM, the calibration curve tended to level off, demonstrating the typical characteristic of Michaelis-Menten kinetic at the enzyme-based electrode. The apparent Michaelis-Menten constant Km is usually used to evaluate the biological activity of immobilized enzyme since it indicates the enzyme-substrate kinetics for the enzyme electrode. The apparent MichaelisMentenconstantcanbecalculatedaccordingtotheLineweaver-Burk equation:36

Km ) (Imax - I)/I × c Figure 7. Typical current-time response curves of the Pt-CNTs-GOD electrode upon successive additions of different concentrations of glucose into pH 7.0 PBS solution. Applied potential: -0.1 V (vs Ag/ AgCl).

3.4. Detection of Glucose. The wide linear range and high sensitivity of the Pt-CNT electrode toward H2O2 detection suggest that it may provide an ideal transduction for the fabrication of oxidase-based biosensors. Additionally, large amounts of enzymes are envisaged to be immobilized in the nanocomposites due to the large surface area, unique nanostructures, and surface functional properties of the Pt-CNT array. These excellent characteristics are also of significance for improving the performance of the corresponding designed biosensor. Figure 7 shows a typical amperometric response of the Pt-CNT-GOD electrode for successive addition of different amounts of glucose to a stirred PBS at a constant applied potential of -0.1 V. It should be noted that, in this study, -0.1 V was selected as the applied potential because such a low applied potential would be beneficial to decrease the background current and minimize the responses of common interference species. The Pt-CNT-GOD biosensor exhibits a good linear relationship with the concentration of glucose in the range of 0.16-11.5 mM; the regression equation with the calibration plot (Figure 8) is C (mM) ) 0.16I + 2.05 with a correlation coefficient of 0.994 (n ) 12). The response time for the addition of glucose was about 5 s, and the detection limit was determined to be 0.055 mM glucose based on S/N ) 3. In addition, for the Pt-CNT-GOD glucose biosensor, a set of 10 different amperometric measurements for 1.0 mM glucose with a single biosensor yield a RSD of 4.72%, indicating a good repeatability of the as-prepared Pt-CNTs biosensor. To investigate the reproducibility of the biosensor, three biosensors were fabricated through

where I is the steady-state response current when substrates are added, Imax is the maximum current measured in saturated substrate conditions, and c represents the bulk concentration of the substrate. In our work, The Km value for the Pt-CNT-GOD biosensor is estimated to be 2.6 mM according to the Lineweaver-Burk equation. The Km value determined with the present biosensor is much smaller than those of the previously reported values and earlier reported value for free enzyme,29,33,37,38 indicating that the GOD entrapped in the nanocomposite possesses a higher enzyme activity due to preconcentration of glucose through hydrogen bond interaction with the oxygenrich groups in the nanocomposites or higher intrinsic affinity of the inmmobilized GOD toward glucose. The stability of the glucose biosensor has been investigated through the amperometric response to 1 mM glucose at -0.1 V in PBS at intervals over several days, and the Pt-CNT-GOD electrode was stored at 4 °C in a refrigerator when not in use. The results show that the electrode remained relatively about 94% of the original value over the first 20 days, and it decreased to 90% after 1 month (use more than 100 times), suggesting that the stability and biocompatibility of the Pt-CNTs for enzyme immobilization render substantial improvement in long-term stability of the glucose biosensor. The relatively good storage stability indicated that the existence of the functional groups (oxygen-rich groups) from the Pt-CNT nanocomposites was very beneficial for improving its biocompatibility and retaining the bioactivity of GOD. The general problem in the electrochemical detection of glucose is the interference from physiological species such as AA, UA, and dopamine (DA), etc. We have studied the selectivity of the present glucose biosensor against these possible interfering species through measuring the amperometric response to successive addition of physiological levels of various

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J. Phys. Chem. C, Vol. 113, No. 31, 2009 13487 and enzymes and may find wide potential applications in biosensors, biocatalysis, and biomedical devices. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20675044 and 20628303), National Basic Research Program of China (No. 2007CB310500), and Key Program Foundation of Jiangxi Educational Committee (GJJ09019). Supporting Information Available: SEM and TEM images of the CNTs and cyclic voltammograms of 1 mM DA, UA, and AA and 1 mM glucose. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Typical current-time response curves of the Pt-CNTs-GOD electrode to several interference species (0.1 mM DA, UA, and AA) and 1 mM glucose in pH 7.0 PBS solution. Applied potential: -0.1 V (vs Ag/AgCl).

interfering species (0.1 mM AA, 0.1 mM UA, and 0.1 mM DA) and 1 mM glucose at the potential of -0.1 V. As shown in Figure 9, the three interfering species generated a completely negligible current response as compared to the current responses to 1.0 mM glucose, indicating a high selectivity for the as-prepared glucose biosensor. Further CV experiments reconfirmed that the Pt/CNTs had a good selectivity for glucose determination (Figure S2 of the Supporting Information), in which one can find that there was not any obvious peak in the CVs of UA and AA. Although a slight peak can be found in DA, however, the height was much smaller than that from the glucose. The high selectivity could be attributed to the following two reasons: On one hand, the relative low potential for detection could greatly minimize the responses of common electroactive interference. On the other hand, the presence of Nafion in the Pt-CNTs also facilitated to diminish the interferences since the negatively charged sulfonate (SO3-) groups in Nafion would prevent the negatively charged interfering species (e.g., AA and UA) from permeating into the electrode surface. 4. Conclusion In summary, we have developed a facile route to fabricate unique nanostructured Pt-CNT nanohybrids by combining the template method with a hydrothermal technique. It is observed that Pt nanoparticles were uniformly entrapped into the CNTs, which suggests that the nanocomposites have a more stable nanostructure. The carbonized CNT showed an excellent stability and high catalytic activity for catalyzing H2O2 electroreduction, which could be attributed to highly distributed Pt nanoparticles, a high surface area of the catalysts, and the unique and stable nanostructures formed through the synthetic route. Investigation shows that the as-prepared Pt-CNT nanohybrids not only exhibit efficiency for enzyme immobilization but also are of great importance for the fabricating of oxidase-based biosensors due to their unique and excellent properties. The asprepared Pt-CNT-based glucose biosensor displayed a series of excellent features such as good sensitivity and reproducibility, wide linear range, fast response, and good long-term stability. It is also envisaged that the unique Pt-CNT nanohybrids could provide a good biosensing platform for other redox proteins

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