Nonenzymatic Amperometric Glucose Sensing of Platinum, Copper

Jan 5, 2009 - Nanoparticle-Carbon Nanotube Hybrid Nanostructures. Yoon Myung, Dong Myung Jang, Yong Jae Cho, Han Sung Kim, and Jeunghee Park*...
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J. Phys. Chem. C 2009, 113, 1251–1259

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Nonenzymatic Amperometric Glucose Sensing of Platinum, Copper Sulfide, and Tin Oxide Nanoparticle-Carbon Nanotube Hybrid Nanostructures Yoon Myung, Dong Myung Jang, Yong Jae Cho, Han Sung Kim, and Jeunghee Park* Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea

Jong-Ung Kim and Youngmin Choi AdVanced Materials DiVision, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea

Cheol Jin Lee School of Electrical Engineering, Korea UniVersity, Seoul 136-701, Korea ReceiVed: July 26, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008

We fabricated highly sensitive nonenzymatic amperometric glucose biosensors using platinum (Pt), copper sulfide (Cu2S), and tin oxide (SnO2) nanocrystal- (NC) carbon nanotube (CNT) hybrid nanostructures, where the NCs were grown in situ on the CNTs by the solvothermal method. The double-walled CNT series respond more sensitively than the multiwalled CNT series. The relative sensitivity of the NCs follows the order Pt > Cu2S > SnO2. The highest sensing ability of the Pt NC-CNT nanostructures guarantees a sensitivity of 280 µAcm-2 mM-1 over a wide concentration range from 0.2 µM to 12 mM at pH ) 7.2 in phosphate buffer saline solution. The synergetic combination of the electrocatalytic activity of the NCs and the electrical network formed through their direct binding with the CNTs enhances the sensing ability of the NC-CNT hybrid nanostructures. The sensitivity, selectivity, and stability of these NC-CNT hybrid nanostructures demonstrated their potential for use as novel nonenzymatic glucose sensors. 1. Introduction Since their discovery in 1991, carbon nanotubes (CNTs) have attracted a tremendous amount of attention due to their extraordinary physical, chemical, and mechanical properties.1 Their large surface area offers abundant reactive sites to generate Faradic currents, and the reactions on CNT-modified biosensor (e.g., glucose sensors) electrodes, which belong to the surfacecontrolled process, exhibit direct electron transfer properties.2-4 Lately, significant interest has been directed toward the design of nanocrystals (NC) and CNT composites, to improve the enzymatic electrochemical performance. The key to this strategy is the utilization of the active NCs to optimize the electrical contact with the backing electrode through the conductive CNTs. The use of novel metal (e.g., Pt, Pd) or semiconductor (e.g., CdS, CdS-ZnS, CdTe, Cu2S) NCs resulted in an increase in the stability and sensitivity of enzymatic glucose sensors based on glucose oxidase (GOx).5-12 However, the greatest drawback of these enzymatic glucose sensors is their lack of stability originating from the nature of the enzyme, which is difficult to overcome. Although GOx is quite stable compared with other enzymes, GOx-based glucose sensors are always exposed to possible thermal and chemical deformation. To address this problem, many attempts have been made to develop glucose sensors without using enzymes. Numerous nanostructured materials have been reported, and their novel characteristics certainly provide new opportunities to develop innovative nonenzymatic glucose sensors.13 For instance, glucose detection has been reported using mesoporous Pt, and the mesoporous surface retains sufficient sensitivity in the presence of excessive chloride ions (0.1 M KCl).14 Nanotubular Pt array * Corresponding author. E-mail: [email protected].

electrodes possess high sensitivity and selectivity, due to their high surface-roughness factor and structure.15 CuO nanowire array electrodes exhibit high sensitivity, selectivity, and immunity to chloride poisoning.16 The immobilization of CNTs17,18 and metallic NCs including Pt,19 Pt-Pb alloy,20,21 Ni,22,23 Au,24,25 and Cu,26-29 with/without CNTs on the electrode has also been an important strategy in the construction of nonenzymatic glucose sensors. However, most of these electrodes were designed to work under high pH conditions, which inevitably cause surface degradation. Herein, we report a study on the nonenzymatic glucose sensing of Pt, copper sulfide (β-Cu2S), and tin oxide (SnO2) NCs attached to double-walled (DWCNTs) and multiwalled CNTs (MWCNTs). The high-density NCs were grown in situ on the CNTs using the solvothermal method. We fabricated highly sensitive, stable, and fast amperometric glucose sensors operating at pH ) 7.2 in phosphate buffer saline (0.15 M NaCl) solution. The modification of a glassy carbon (GC) electrode with these NC-CNT hybrid nanostructures increases its active area and promotes the electron transfer for the glucose oxidation reaction via the CNTs. We compared hydrogen peroxide (H2O2) and nonenzymatic glucose sensing ability of the NC-CNT hybrid nanostructures and discuss the electrocatalytic activity of the NCs. The p-type semiconductor, β-Cu2S (Eg ) 1.2 eV), and n-type SnO2 (Eg ) 3.6 eV) NCs were selected in conjunction with the well-known, effective catalyst, Pt NCs, since these NCs (attached to the MWCNTs) demonstrated excellent enzymatic electrocatalyticactivitytowardglucoseanduricacid,respectively.12,30 The electronic structures of the hybrid nanostructures were investigated using X-ray photoelectron spectroscopy (XPS). This systematic study provided valuable information on the binding states of the NCs with the CNTs, which is requisite to build

10.1021/jp806633j CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

1252 J. Phys. Chem. C, Vol. 113, No. 4, 2009 more sensitive biosensor platforms. We also demonstrated that these NC-CNTs hybrid nanostructures are insensitive to potential interfering agents such as ascorbic acid and uric acid. 2. Experimental Section Purified (acid-functionalized) DWCNTs (Unidym Inc.) and MWCNTs (produced by CVD method, Sigma-Aldrich) were used as ligands and templates to grow the NCs. The synthesis of the Pt NCs was performed using the solvothermal reaction with PVP/ethylene glycol.31 6.2 × 10-2 mmol of chloroplatinic acid hexahydrate (H2PtCl6 · 6H2O 99.9%, Sigma-Aldrich), 1.24 × 10-3 mmol of PVP (polyvinylpyrrolidone, MW 10000, Sigma-Aldrich), and 18 mg of CNTs were dissolved in 10 mL of ethylene glycol. The resulting solution was heated to 180 °C and the NCs were allowed to grow under vigorous stirring for 1 h. Acetone was added at 50 °C to precipitate the products, which were washed with distilled water and isolated by centrifugation. The free Pt NCs were synthesized under the same conditions, but without the CNTs. The Cu2S-CNT hybrid nanostructures were synthesized by the solvothermal method, as described elsewhere.12 As the typical procedure used to synthesize the Cu2S-MWCNT hybrid nanostructures, 10 mg of the acid-functionalized MWCNTs were dispersed in 10 mL of dry oleylamine by ultrasonication, and then 1 mmol (0.27 g) of copper acetylacetonate (Cu(acac)2) and 0.5 mmol (0.015 g) of elemental sulfur (S) were added to a slightly heated solution of oleylamine. The resulting solution was heated to 190 °C and the NCs were allowed to grow under vigorous stirring for 10 min. In the subsequent purification, ethanol was added at 50 °C to precipitate the product. The free Cu2S NCs were synthesized under the same conditions, but without the CNTs. The SnO2-CNTs hybrid nanostructures were synthesized by the procedure reported by Han and Zettl.32 One gram of tin chloride (anhydrous 99%, Sigma-Aldrich) was put into 40 mL of distilled water, and then 0.7 mL of HCl (38%) and 10 mg of CNTs were added. This solution was sonicated for 5 min and then stirred for 1 h at room temperature. The treated nanotube sample was then rinsed with distilled water and isolated by centrifugation. The products were characterized by field-emission transmission electron microscopy (FE TEM, FEI TECNAI G2 200 kV and Jeol JEM 2100F) and high-voltage transmission electron microscopy (HVEM, Jeol JEM ARM 1300S, 1.25 MV). Raman spectroscopy measurements (Horiba Jobin-Yvon HR-800 UV) were recorded using an Ar ion laser (λ)514.5 nm). Highresolution X-ray diffraction (XRD) patterns were obtained using the 8C2 beam line of the Pohang Light Source (PLS) with monochromatic radiation (λ ) 1.54520 Å). X-ray photoelectron spectroscopy (XPS) was measured using the 8A1 beam line of the PLS and a laboratory-based spectrometer (ESCALAB 250, VG Scientifics) using a photon energy of 1486.6 eV (Al KR). Thermal gravimetric analysis (TGA, TA Instrument, SDT 2960) was used to measure the wt % of NCs in the hybrid nanostructures. Glassy carbon (GC) electrodes (3 mm diameter, BAS) were carefully polished with a nylon/diamond pad and 3 µm diamond polishing suspension, rinsed with distilled water and ethanol, and then dried under ambient nitrogen gas. 2-15 mg of the NC-CNT hybrid nanostructures were dissolved in a mixture of 0.1 mL of Nafion-perfluorosulfonated ion-exchange resin (5 wt %, Sigma-Aldrich) and 0.9 mL of distilled water. About 60 min of ultrasonication was necessary to obtain uniformly dispersed NC-CNT hybrid nanostructures. After dropping 10 µL of the NC-CNT hybrid nanostructures solution on the electrode surface, the electrode was dried in air.

Myung et al. Cyclic voltammetry (CV) and chronoamperometry (CA) measurements were performed using an electrochemical analyzer (Epsilon, BAS Inc.). A Pt-wire auxiliary electrode and Ag/AgCl standard reference electrode (Sigma-Aldrich) were used as the counter and reference electrodes, respectively. Using the modified-GC working electrode, the CV measurement was carried out in a solution of 20 mM K4Fe(CN)6 and 0.2 M potassium chloride (KCl) as the supporting electrolyte. Then, the CV data were measured in a mixture (30 mL) of 1 mM of H2O2 or 1 mM of D-(+)-glucose (Sigma-Aldrich 97%, remainder primarily anomer) and 10 mM of phosphate buffer saline solution (PBS, pH 7.2). The constant-potential amperometry measurements required the prepositioning (∼300 s) and operation of the electrode at a constant applied potential of 0.5 V versus Ag/ AgCl (3 M NaCl). For the CA measurement, various concentrations of H2O2 or glucose were prepared in pH 7.2 PBS solution. Once the current reached the baseline in the absence of H2O2 (or glucose), H2O2 (or glucose) was added every 1 min thereafter. The CA data were also measured for ascorbic acid (AA, Sigma-Aldrich), uric acid (UA, Sigma-Aldrich), and fructose (Sigma-Aldrich) in PBS solution. 3. Results The TEM image of the Pt-DWCNT hybrid nanostructures is displayed in Figure 1a. All of the CNTs are decorated homogeneously with the NCs. The average diameter of the DWCNTs is 3 ( 0.3 nm, the average diameter of the bundles is 10 ( 5 nm, and the average size of the Pt NCs is 3 ( 0.3 nm. The spherical NCs are tightly bound to the graphite layers of the DWCNTs. The Pt-MWCNT hybrid nanostructures were also successfully synthesized, as shown in Figure 1b. All of the MWCNTs (average diameter ) 40 ( 10 nm) are decorated homogeneously with the high-density 3 nm Pt NCs. Parts c and d of Figure 1 show the TEM images of the Cu2S-DWCNT and -MWCNT hybrid nanostructures, respectively. The size of the spherical Cu2S NCs is 5-8 nm with an average value of 7 ( 1 nm. Parts e and f of Figure 1 show the TEM images of the SnO2-DWCNT and -MWCNT hybrid nanostructures, respectively, where the average size of the NCs is 2 ( 0.2 nm. The lattice-resolved TEM images of the DWCNTs, Pt-, Cu2S-, and SnO2 NC-CNT hybrid nanostructures are displayed in the Supporting Information, Figure S1, revealing the crystalline phase of the NCs. The EDX analysis validates the composition of the NCs (Supporting Information, Figure S2). The XRD pattern confirms the composition and crystal structure of the NCs in the NC-DWCNT hybrid nanostructures, as shown in Figure 2. The peak position is consistent with that of the reference values (JCPDS No. 04-0802, cubic a ) 3.890 Å for Pt; JCPDS No. 26-1116, hexagonal a ) 3.961 Å, c ) 6.722 Å for β-Cu2S; JCPDS No. 41-1445, tetragonal a)4.755 Å, c)3.199 Å for SnO2). The avg. sizes of the NCs were estimated using the Debye-Scherrer equation, and found to be 3, 8, and 2 nm for the Pt, Cu2S, and SnO2 NCs, respectively, which is consistent with the TEM images. Figure 3a shows the fine-scanned XPS C 1s spectra for the DWCNTs and their Pt, Cu2S, and SnO2 NC hybrid nanostructures, using a photon energy of 630 eV. As the NCs are deposited, the full-width at half-maximum (fwhm) of the asymmetric band, centered at 284.5 eV, increases significantly from 0.7 to 1.8 (Pt), 1.2 (Cu2S) or 1.7 eV (SnO2). This asymmetric band could be resolved into two bands at 284.5 (PC1) and 285.5 (PC2) by fitting it into the Voigt function.33 The binding energy of the C atoms bonded to the defects through dangling bonds appear at a higher energy relative to those of

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Figure 1. TEM and HRTEM images of the (a) Pt-DWCNT, (b) Pt-MWCNT, (c) Cu2S-DWCNT, (d) Cu2S-MWCNT, (e) SnO2-DWCNT, and (f) SnO2-MWCNT hybrid nanostructures. The average sizes of the spherical Pt, Cu2S, and SnO2 NCs are 3 ( 0.3, 7 ( 1, and 2 ( 0.2 nm, respectively.

the graphite C atoms. The PC1 band can be assigned to the C atoms binding to the graphite network, and the PC2 band corresponds to the C atoms at the defect sites. As the NCs are deposited, the fraction of the PC2 band significantly increases, following the sequence SnO2 > Pt > Cu2S. The width and fraction of the resolved PC1 and PC2 bands are listed in Table 1. Figure 3b shows the fine-scanned XPS C 1s spectra for the MWCNTs and their NC hybrid nanostructures, using a photon energy of 630 eV. Table 1 also lists the width and fraction of the resolved PC1 and PC2 bands. The NC deposition increases the fraction of the PC2 band, showing the same behavior as that of the DWCNT series. The fine-scanned Pt 4f5/2 and 4f7/2 peaks of the free Pt NCs and Pt-DWCNT hybrid nanostructures are displayed in Figure 3c. The peak shifts to the higher-energy region (0.4 eV); 4f7/2

peak moves from 70.4 to 70.8 eV, as the Pt NCs are deposited on the DWCNTs. The fine-scanned Cu 2p1/2 and 2p3/2 peaks of the Cu2S-DWCNTs are displayed in Figure 3d, along with those of the free Cu2S NCs. These 2p peaks shift to the higher energy region (0.3 eV) when the Cu2S NCs are grown on the DWCNTs. We also measured the Raman spectra in order to confirm that the NC deposition produces defects on the CNT surface, as shown in the Supporting Information, Figure S3. The intensity ratio of the D band to the G band (ID/IG) is listed in Table 1, showing the larger ratio of ID/IG for the more defective graphite layers (SnO2 > Pt > Cu2S). The MWCNT series show a higher ID/IG ratio than the DWCNT series. The thermal gravimetric analysis (TGA) reveals that the weight percent of the NCs is about 20 ( 10% (Supporting Information, Figure S4).

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Figure 2. XRD patterns of Pt-, Cu2S-, and SnO2-DWCNT hybrid nanostructures

For the development of amperometric biosensors, the CNTs and NC-CNT hybrid nanostructures (with the same weight) were solubilized in Nafion, a perfluorosulfonated polymer, to facilitate the modification of the GC electrode surface. Parts a and b of Figure 4 represent the steady-state CVs for the modified GC electrodes with the DWCNTs and MWCNTs, respectively, and their NC hybrid nanostructures, in 20 mM Fe(CN)64- and 0.2 M KCl, at a scan rate of 50 mV s-1. All of the modified electrodes exhibit well-defined oxidation (at 0.35 V) and reduction (at 0.22 V) peaks toward the redox couple; Fe(CN)63+ e- f Fe(CN)64-. The sensitivity is in the order Pt-CNTs > Cu2S-CNTs > SnO2-CNTs > CNTs for both the DWCNTs and MWCNTs. The amperometric detection of H2O2 was performed, because H2O2 is released during the oxidation of glucose by GOx in the presence of oxygen. For the detection of glucose, no enzyme is immobilized on the modified GC electrode. The two-electron oxidation of glucose produces glucono lactone, which is hydrolyzed into stable gluconic acid as the final product.14 Figure 4c represents the steady-state CV for the modified GC electrodes with the Pt-DWCNT hybrid nanostructures, in 1 mM H2O2/ PBS and 1 mM glucose/PBS solutions, at a scan rate of 50 mV s-1. For H2O2, the current plateau above 0 V is due to the electrochemical oxidation of adsorbed H2O2 (Pt-OOH-) to O2.34 The electro-oxidation current peak at 0.44 V (in the forward scan) is ascribed to the oxidation reaction of electro-sorption of H2O2, following the formation of free Pt active sites.35 The CV curve with forward and reverse scans in the range of -1.0 to +1.0 V, is shown in the Supporting Information, Figure S5. In the case of glucose, there is main oxidation current peak appearing at 0.37 V, which is due to the electro-oxidation of glucose molecules. At a potential of ∼0.3 V, the Pt-OH surface species start to form and oxidize the poisoning intermediates derived from the electro-sorption of glucose, releasing free Pt active sites for the direct oxidation. Then the electro-oxidation current of glucose increases, forming a current peak at 0.37 V. The decreased current at potentials positive with respect to this peak could be due to the formation of platinum oxide, which inhibits the direct electro-oxidation. The CV curve with forward and reverse scans in the range of -1.0 to +1.0 V, is also shown in the Supporting Information, Figure S5. The CV curve feature is in good agreement with the previous findings of other research groups.15,36,37 Figure 4d represents the steady-state CV for the GC electrodes modified with the Cu2S-DWCNT hybrid nanostructures, in 1 mM H2O2/PBS and 1 mM glucose/PBS solutions, at a scan rate of 50 mV s-1. For both H2O2 and glucose, the CV curve exhibits

Myung et al. distinctive oxidation peaks at around 0.4 V in the forward scan. The CV curve was measured in the range of -1.0 to +0.8 V, as shown in the Supporting Information, Figure S5. This oxidation peak can be surely ascribed to the oxidation of H2O2 (or glucose) molecules. Parts e and f of Figure 4 represent the steady-state CVs for the modified GC electrodes with the SnO2-DWCNT hybrid nanostructures and the DWCNTs, respectively, measured under the same conditions. They show no sharp oxidation or reduction peaks, which distinguishes them from the Pt and Cu2S NCs. Figure 5a displays the linear amperometric response (µA/ cm2; electrode area ) 0.0707 cm2) of the GC electrodes modified with the Pt-, Cu2S-, and SnO2-DWCNT (also MWCNT) hybrid nanostructures upon the addition of H2O2 solution (up to 1 µM). The working potential is set at 0.5 V, where the maximum positive current is obtained from the CA data of the Cu2S-DWCNT modified electrode (Supporting Information, Figure S6). The reproducibility of the current signal for 5 repeated injections of H2O2 was within 10%. The repeated use of the electrodes did not affect their long-term stability, as long as the measurement was not performed at a high concentration of H2O2 (>20 mM). The response time is about 3 s. Figure 5b shows that the response is linear for H2O2 concentrations in the range between 50 nM and 2.5 µM. The linear response provides the sensitivity, as listed in Table 1. The detection limit (S/N ) ∼ 3) is also listed. The sensitivities are 12, 12, and 1.1 mA cm-2 mM-1 and the detection limits are 10, 10, and 50 nM for the Pt-, Cu2S-, and SnO2-DWCNTs, respectively. In the case of the MWCNTs, the sensitivities are 1.2, 2.4, and 0.13 mA cm-2 mM-1 and the detection limits are 50, 20, and 70 nM for the Pt, Cu2S, and SnO2 NCs, respectively. The sensitivities are about 1/8 (on average) of that of the DWCNTs. The relative sensitivity of the NC-CNT hybrid nanostructure follows the order Pt ≈ Cu2S > SnO2 on average. Figure 5c displays the nonenzymatic amperometric response of the Pt-, Cu2S-, and SnO2-DWCNT (also MWCNT) modified GC electrodes upon the addition of glucose solution. The working potential is also set at 0.5 V. The response is linear for glucose concentrations in the range between 5 µM and 50 µM. Figure 5d displays the linearly fitted response data, showing sensitivities of 280, 35, and 9.9 µA cm-2 mM-1 for the Pt-, Cu2S-, and SnO2-DWCNTs, respectively. The corresponding detection limits (S/N ) ∼ 3) are 0.2, 1, and 3 µM, respectively. For the Pt-, Cu2S-, and SnO2-MWCNT hybrid nanostructures, the sensitivities are 110, 5.0, and 1.4 µA cm-2 mM-1 and the detection limits are 0.5, 5, and 10 µM, respectively, which are about 1/5 (on average) of those of the DWCNT series. The detection limits and sensitivities are listed in Table 2. Parts e and f of Figure 5 (and insets) correspond to the linear response data of the Pt- and Cu2S-DWCNTs for glucose concentrations up to 11.7 mM, respectively. The nonenzymatic detection limit (280 µA cm-2 mM-1) of the Pt-DWCNT-based GC electrode is higher than the highest value (31.3 µA cm-2 mM-1) reported for a nonenzymatic Pt nanotube array electrode at pH ) 9.18.15 The sensitivity sequence is Pt > Cu2S > SnO2 for both the DWCNTs and MWCNTs, which is consistent with that of the Fe(CN)63-/Fe(CN)64- redox reaction. Figure 6a presents the selectivity testing results of the Pt-DWCNT modified electrode with successive additions of 5 µM ∼ 50 µM glucose, AA, UA, and fructose in 10 mM PBS solution at +0.5 V. Figure 6b displays the response of the Cu2S-DWCNT modified electrode upon the additions of glucose, UA, AA, and fructose. These two electrodes produce lower current signals for both common interfering agents (∼3

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Figure 3. XPS C 1s spectra of (a) DWCNTs and Pt-, Cu2S-, and SnO2-DWCNT hybrid nanostructures; (b) MWCNTs and Pt-, Cu2S-, and SnO2-MWCNT hybrid nanostructures, using a photon energy 630 eV. The data points (open circles) of the C 1s band are fitted by two Voigt functions, PC1 and PC2 (dotted lines). Fine-scanned XPS of (c) Pt 4f5/2 and 4f7/2 of free Pt NCs and Pt NC-DWCNT hybrid nanostructures, and (d) Cu 2p1/2 and 2p3/2 of free Cu2S NCs and Cu2S NC-DWCNT hybrid nanostructures.

TABLE 1: Fwhm and Area % of Resolved Bands from the XPS C1s Peak for the NC-DWCNT and -MWCNT Nanostructures, and Raman Spectroscopy Data Fitting Parameters PC1 (graphite) CNT

NC

DWCNT Pt Cu2S SnO2 MWCNT Pt Cu2S SnO2

C1s fwhm (eV) 0.7 1.8 1.2 1.7 0.8 1.8 1.5 2.0

a

PC2 (defects)

fwhm (eV)

area%

fwhm (eV)

area%

Raman ID/IGb

0.7 0.7 0.7 0.7 0.6 0.6 0.9 0.7

65 30 50 22 62 34 48 22

2.0 1.9 1.5 1.9 1.9 2.5 1.9 1.9

35 70 50 78 38 66 52 78

0.04 0.2 0.15 0.2 0.45 0.81 0.81 0.85

a

The XPS C1s spectra were measured using 630 eV synchrotron radiation. spectra with an error of 5%.

times less). There was a 10% decrease in the sensitivity of the current response to the added glucose if both of these interfering species were present. 4. Discussion The TEM images reveal that the average sizes of the Pt, β-Cu2S, and SnO2 NCs grown on the CNTs are 3, 7, and 2 nm, respectively, and that their density increases as the size decreases. The fine-scanned XPS C 1s spectrum shows that the fraction of the defect band (PC2) is directly correlated with the size/density of the NCs, as shown in the TEM images (Figure 1). More defects of CNTs are produced by the deposition of the smaller size and higher density NCs, following the sequence SnO2 > Pt > Cu2S. The Raman spectroscopy measurement supports such a correlation. The higher energy shift of the Pt

b

The intensity ratios of the D versus G bands in the Raman

4f and Cu 2p peaks resulting from the deposition of the NCs on the CNTs may be caused by the binding interaction of the electronic states of the Pt NCs (or Cu2S NCs) with those of the more electronegative C atoms in the graphite layers. The stronger interaction of the Pt NCs with the CNTs than that of the Cu2S NCs results in the larger peak shift. The results suggest that the stronger interaction of the electronic states of NCs and CNTs is related to an increase in the number of defect sites of the CNTs. Therefore, the degree of interaction would decrease following the order SnO2 > Pt > Cu2S. In the present work, we first compared the amperometric sensing ability of the Pt, Cu2S, and SnO2 NC-DWCNT (and MWCNT) hybrid nanostructures. All of the CV and CA data consistently show that the DWCNT series exhibit the higher sensitivity than the MWCNT series, even for the Fe(CN)63-/

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Figure 4. CV performance of the modified GC electrodes with Pt, Cu2S, and SnO2 NC (a) -DWCNT and (b) -MWCNT hybrid nanostructures, toward the Fe(CN)64-/ Fe(CN)63- redox couple using 20 mM Fe(CN)64- and 0.2 M KCl, at 50 mV s-1. CV performance of (c) Pt-DWCNT, (d) Cu2S-DWCNT, (e) SnO2-DWCNT, and (f) DWCNT modified GC electrodes in amperometric detection of 1 mM H2O2 and 1 mM glucose in pH ) 7.2 PBS solution vs Ag/AgCl reference electrode at a scan rate of 50 mV s-1.

Fe(CN)64- redox reaction. The lower ID/IG ratio of the DWCNTs than that of the MWCNTs is directly correlated with the better crystallinity of graphitic layers, which results in the higher conductivity. Note that the Fe(CN)63-/Fe(CN)64- redox reaction belongs to a diffusion-controlled reaction, so the sensitivity can depend directly on the conductivity of the electrode. Therefore, the result indicates that the conductivity of the CNTs plays an important role in determining the sensitivity. The experiment was also performed using single-walled CNTs (synthesized by CVD, Aldrich-Sigma), supporting this conclusion (Supporting Information, Figures S7-S9). For both the DWCNTs and MWCNTs, the deposition of the NCs enhances the sensitivity of the H2O2 and glucose detection in the order Pt > Cu2S > SnO2. In the case of H2O2 detection, the relative sensitivity shows the sequence Pt ≈ Cu2S > SnO2. These relative sensitivities (both H2O2 and glucose) do not follow the degree of binding interaction between the NCs and the CNTs. Therefore, the glucose sensing ability of the NC-CNT hybrid nanostructures can be considered to result from the combination of two factors: one is the electrocatalytic activity of the NCs and the other one is the electrical network formed through their direct binding with the CNTs. The highest sensitivity of Pt NCs certainly comes from their excellent electrocatalytic power. The higher density would induce more active sites for the catalytic redox reaction and bring about an efficient electrical network through their direct binding with the CNTs, which would greatly increase the sensitivity.

However, the SnO2 NCs consistently show the lowest sensing ability, although they have the highest density. Furthermore, despite their having the lowest density, the Cu2S NCs exhibit higher sensitivity than the SnO2 NCs. For the detection of H2O2, the Cu2S NCs exhibit surprisingly high sensitivity, which is comparable to that of the Pt NCs. They also contribute in increasing the sensitivity toward the Fe(CN)63-/Fe(CN)64- redox reaction. The conductivity of the bulk SnO2 is known to be higher than that of the bulk Cu2S, since the carrier mobilities of the SnO2 and Cu2S single crystals are 150-260 cm2 V-1 s-1 and 3.02-4.75 cm2 V-1 s-1, respectively.38 However, the XPS and Raman data reveal that the deposition of SnO2 NCs deteriorates the crystallinity of the CNTs, which can reduce significantly the conductivity of the CNT hybrid structures. The higher band gap of the SnO2 NCs would further contribute to the loss of their catalytic activity toward the redox reaction. On the other hand, the higher sensitivity of the Cu2S NCs might be due to their higher electrocatalytic activity toward the redox reaction of the target analytes. We can offer no firm explanation for the high electrocatalytic activity of the Cu2S NCs. Nevertheless, one possible explanation is that the p-Cu2S NCs would act as an efficient electronacceptor catalyst toward the oxidation reaction of H2O2 (i.e., H2O2 f O2 + 2H+ + 2e-) or glucose molecules. The potential energy diagram of the valence/conduction bands suggests the allowed hole transfer between the valence band of Cu2S and the oxidation reaction of H2O2 (Supporting Information, Figure S10). It was also reported that the

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Figure 5. (a) CA response of Pt-, Cu2S-, and SnO2-DWCNT (and -MWCNT) modified GC electrodes for H2O2 solution (50 nM∼2.5 µM), in PBS solution, at 0.5 V, and (b) their linear fitted plot of the response data as a function of the H2O2 concentration. (c) CA response of Pt-, Cu2S-, and SnO2-DWCNT (and MWCNT):Nafion modified GC electrodes for glucose (5-50 µM), in PBS solution, at 0.5 V, and (d) their linear fitted plot of the response data as a function of the glucose concentration. The response of the (e) Pt- and (f) Cu2S-DWCNT modified GC electrodes is linear for glucose concentrations between 50 µM and 11.7 mM. Insets represent the linear fit of the data points.

TABLE 2: Sensitivity and Detection Limit of Amperometric H2O2 and Nonenzymatic Glucose Biosensors H2O2 CNTs

NCs

DWCNTs

Pt Cu2S SnO2 Pt Cu2S SnO2

MWCNTs

a

-2

sensitivity (mAcm

-1

mM )

Nonenzymatic a

detection limit (nM)

12 12 1.1 1.2 2.4 0.13

10 10 50 50 20 70

-2

sensitivity (µAcm 280 35 9.9 110 5.0 1.4

mM-1)

detection limit (µM) 0.2 1 3 0.5 5 10

The detection limit is determined for S/N ) ∼ 3.

complexes of Cu(I) ions with various ligands (especially sulfur ligation) present high activity toward the decomposition of H2O2.39,40 The efficient catalytic reaction of the Cu2S NCs would involve in the H2O2 oxidation reactions, which increases the sensitivity of the Cu2S-CNT nanostructures to the level at which it is equal to that of the Pt-CNT nanostructures. The avoidance of endogenous interfering species is a big challenge in nonenzymatic glucose detection, because several structurally similar organic substances (for instance, AA, UA,

fructose, and acetoamidophenol) are also simultaneously oxidized along with glucose at the electrode surface and, hence, give interfering electrochemical signals. The present results show that the Pt- and Cu2S NC-CNTs hybrid nanostructures are less sensitive to AA, UA, and fructose. It is noteworthy that since Nafion is negatively charged, this anti-interfering effect of the electrode is due to the NC-CNTs hybrids or due to Nafion is not clear. Nevertheless, we conclude that the NC-CNT nanostructures have outstanding features of sensitivity and selectivity, which

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Figure 6. CA response of (a) Pt- and (b) Cu2S-DWCNT modified GC electrodes (at 0.5 V) upon subsequent addition of 5-50 µM glucose, AA, UA, and fructose, in pH ) 7.2 PBS solution.

enable the monitoring of trace levels of glucose in biological systems. Furthermore, most nonenzymatic sensors were developed for operation under high PH conditions. In contrast, our sensor system demonstrates excellent performance without surface contamination in neutral buffer saline solution (even with an excess chloride concentration), which gives them good potential for use as enzyme-free chemical sensors.

ITRC support program supervised by the IITA (IITA-2008C1090-0804-0013), and BK21. Y.C. would like to acknowledge the support from the MKE (10027910). The SEM, HVEM, XRD, and XPS measurements were performed at the Korea Basic Science Institute in Seoul, Daejeon, Taegu, and Pusan, respectively. The experiments at the PLS were partially supported by MOST and POSTECH.

5. Conclusion

Supporting Information Available: Figures showing HRTEM, EDX data, Raman data, TGA data, CV curve, and CA data of NC-CNT hybrid nanostructures, and valence/conduction band potential diagram of Cu2S. This material is available free of charge via the Internet at http://pubs.acs.org.

High-quality Pt, β-Cu2S, and SnO2 NCs were grown in situ on acid-functionalized DWCNTs and MWCNTs by the solvothermal method. The average sizes of the Pt, β-Cu2S, and SnO2 NCs were 3, 7, and 2 nm, respectively. We investigated the electronic structures of the free NCs, CNTs, and hybrid nanostructures using XPS, and the results showed that the higher density NCs induce a greater level of binding interaction with the CNTs, following the order SnO2 > Pt > Cu2S. The GC electrodes modified with these NC-CNT hybrid nanostructures and Nafion acted as highly sensitive, stable, and fast amperometric glucose sensors operating at pH ) 7.2 in PBS solution. The DWCNT series responded more sensitively than the MWCNT series. The relative sensitivity of the NC-CNT hybrid nanostructures follows the order Pt > Cu2S > SnO2, for nonenzymatic glucose sensing. The highest sensing ability of the Pt NC-DWCNTs guarantees a level of sensitivity of 280 µA cm-2 mM-1 over a wide concentration range from 0.2 µM to 12 mM, with a detection limit (S/N ) 3) of 0.2 µM. In contrast, for H2O2, the sensitivity follows the order Pt ≈ Cu2S > SnO2. We suggest that the unusually high electrocatalytic activity of the Cu2S NCs toward the target analytes (especially H2O2) would enhance their sensitivity, although their binding interaction with the graphitic layers is weaker than that of the other NCs. On the other hand, the SnO2 NCs exhibit the lowest electrocatalytic activity toward the analytes. Therefore, the combination of the electrocatalytic activity of the NCs and the electrical network formed through their direct binding with the CNTs determines the sensing ability of the NC-CNT nanostructures. The NC-CNT hybrid nanostructures are less sensitive to AA, UA, and fructose. Our sensor system demonstrates excellent sensitivity, selectivity, and stability without surface contamination in neutral buffer saline solution. Acknowledgment. This work was supported by the KRF (R14-2003-033-01003-0; R02-2004-000-10025-0; KRF-2008314-C00175),theKOSEF(R01-2008-000-10825-0;M1080300121808), the MKE (The Ministry of Knowledge Economy) under the

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