Protein-Promoted Synthesis of Pt Nanoparticles on Carbon Nanotubes

ARTICLE pubs.acs.org/JPCC

Protein-Promoted Synthesis of Pt Nanoparticles on Carbon Nanotubes for Electrocatalytic Nanohybrids with Enhanced Glucose Sensing Gang Wei,† Fugang Xu,‡ Zhuang Li,‡ and Klaus D. Jandt*,† † ‡

Institute of Materials Science & Technology (IMT), Friedrich-Schiller-University Jena, D-07743 Jena, Germany State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022, Changchun, P. R. China ABSTRACT: A facile and effective strategy to create carbon nanotube (CNT)based platinum nanoparticle (PtNP) nanohybrids is reported in this work. First, we prepared toluene-soluble CNTs by modifying CNTs with a bifunctional linker, Z-glycine N-succinimidyl ester (Z-Gly-OSu), and then we synthesized superwater-soluble CNT
hemoglobin (HGB) hybrid nanofibers (about 5 mg/mL) by phase-transferring the Z-Gly-OSu-modified CNTs from toluene to HGB aqueous solution. The prepared CNT
HGB nanofibers were used as templates to bind PtCl62
ions by electrostatic interaction between proteins and negatively charged ions. CNT
PtNP nanohybrids were successfully synthesized by chemically reducing the PtCl62
ions with NaBH4. The results indicate that PtNPs with uniform size and shape were created on the side wall of CNTs with high dispersion, and the loading of PtNPs on CNTs was improved with the increase of protein concentration used for the preparation of CNT
HGB nanofibers. A nonenzymatic amperometric sensor for highly sensitive and selective detection of glucose was successfully fabricated as demonstrating based on the synthesized CNT
PtNP nanohybrids. Under optimal conditions, selective detection of glucose in a linear concentration range of 28.0 μM
46.6 mM (R = 0.996) was obtained, which reveals a lower limit of detection and wider linear response compared to some previously reported glucose sensors.

1. INTRODUCTION Carbon nanotube (CNT)-based nanohybrids have attracted significant research attention due to their potential applications in the fields of catalysis,1
3 biosensors,4
6 hydrogen storage,7 drug delivery,8 biomaterials,9,10 and nanoelectronics.11,12 A challenge in this research field is that the applications to utilize CNTs require surface functionalization to enhance their dispersion and ability to participate in chemical and physical reactions.13,14 Effective functionalization of CNTs is needed in many CNTbased applications.15
20 For example, in materials science and nanotechnological fields, CNTs have been functionalized to separate the as-produced CNT bundles into individual CNTs for further preparation of CNT
nanoparticle (NP) hybrids.15
17 For the applications in sensors and catalysis, CNTs have been functionalized to provide specific recognition sites for analytes and strong interfacial interaction with metallic NPs.1
6,18 In biological fields, CNTs have been functionalized to enhance their solubility and biocompatibility.19,20 Many strategies have been utilized to functionalize CNTs. With physical approaches, CNTs were successfully functionalized using both nonreactive and reactive gases and ion-beam treatment.21,22 Chemical methods were also widely applied to functionalize CNTs by covalent and noncovalent interactions. The covalent functionalization includes strong acids oxidation, r 2011 American Chemical Society

direct reaction with fluorine, and addition of radicals and nitrenes.23
26 The noncovalent functionalization of CNTs involves the reaction with benzyl mercaptan, polymers, cations, surfactants, polyelectrolytes, DNA, peptides, and proteins.2,17,18,27
35 It is known that the noncovalent functionalization has the advantages of both improving the solubility and preserving the integrity and electronic characteristics of CNTs, making it suitable to produce CNT
NP hybrids under mild conditions.17,18 For the production of CNT
NP hybrids, two potential strategies have been used. In one strategy, the CNTs were first chemically functionalized and then interacted with NPs to form hybrids.3,11,17,27,36 In another strategy, CNTs were functionalized with peptides and proteins first, and the bound peptides and proteins inspired the formation of NPs on the CNTs.2,37 For instance, Bale et al. reported a facile strategy to functionalize CNTs with poly(L-lysine), bovine serum albumin, soybean peroxidase, and R1-acid glycoprotein and found that the adsorbed proteins can control the growth of silver NPs on CNTs.2 In their work, selection of proteins that can directly functionalize the single-walled CNTs (SWCNTs) was required, and their Received: March 11, 2011 Revised: May 2, 2011 Published: May 25, 2011 11453

dx.doi.org/10.1021/jp202324q | J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C Scheme 1. Model Presentation of Protein-Directed Synthesis of CNT
PtNP Nanohybrids

results indicate that there are only small amounts of proteins bound onto CNTs. To increase the loading efficiency of proteins on CNTs and prepare CNT
NP hybrids with high metallic NP coating, another strategy is needed. The aim of this study was to create CNT
protein hybrid nanofibers with high solubility and high protein coating on CNTs and further to synthesize functional nanohybrids based on CNTs and high-density platinum nanoparticles (PtNP). To achieve this aim, a synthesis process was utilized, as shown in Scheme 1. First, the bifunctional linker, Z-glycine N-succinimidyl ester (Z-Gly-OSu), was used to noncovalently functionalize CNTs to create toluene-soluble CNTs. After that, super-watersoluble CNTs were created by phase-transferring the toluenesoluble CNTs from toluene to hemoglobin (HGB) aqueous solution. The CNT
HGB hybrid nanofibers were used as precursors to prepare functional metallic nanohybrids. CNT
HGB
PtCl62
hybrids were successfully created by electrostatic interaction between proteins with PtCl62
ions, and CNT
PtNP nanohybrids were synthesized by chemically reducing the created CNT
HGB
PtCl62
hybrids with NaBH4. The synthesized CNT
PtNP nanohybrids displayed higher electrocatalytic activity in glucose sensing compared to that with either pristine CNTs or PtNPs. This work provides a simple and useful strategy to create super-water-soluble CNT
protein hybrid nanofibers, which will extend the potential applications of CNTs in nanotechnology and biotechnology. At the same time, our study is helpful to understand the interactions between CNTs and proteins as well as NPs.

2. EXPERIMENTAL SECTION 2.1. Materials. Bovine plasma HGB, multiwalled CNTs (90% pure, 10
15 nm in diameter and 1
10 μm in length), Z-GlyOSu (C4H14N2O6, g95.0% pure, CAS number: 2899-60-7), K2PtCl6, NaBH4, and phosphotungstic acid (PTA, 95% pure) were purchased from Sigma-Aldrich Chemie GmbH (Taufkirchen, Germany). Methanol (99%) and toluene (g99.5%) were obtained from VWR International GmbH (Darmstadt, Germany). β-D-Glucose, ascorbic acid (AA), and uric acid (UA) were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents were used as received without further purification. The water used was purified through a Millipore system (∼18.2 MΩ 3 cm). Carbon-coated Cu grids for TEM characterization were purchased from Plano GmbH (Wetzlar, Germany). 2.2. Synthesis of CNT
HGB Hybrid Nanofibers by PhaseTransfer Process. CNTs were first noncovalently functionalized with the bifunctional linker, Z-Gly-OSu, to create N-hydroxysuccinimide (NHS) ester groups on the sidewall of CNTs. In

ARTICLE

brief, 20 mg of CNTs was dispersed in 20 mL of methanol and sonicated for 1 h to form a stable suspension. After that, 10 mL of this CNT suspension was mixed with 10 mL of Z-Gly-OSu (50 mM) dissolved in methanol, and the mixed solution was sonicated for 30 min. After vigorous stirring at room temperature for 24 h, 4 mL of mixed solution was taken and the Z-Gly-OSufunctionalized CNTs were separated from the mixed solution by centrifugation (10 000 rpm, 15 min) twice. Toluene-soluble CNTs with a concentration of 0.1 mg/mL were produced by dissolving the centrifugal CNTs with 20 mL of toluene. CNT
HGB hybrid nanofibers were prepared by a phasetransfer approach as introduced in our previous work.38 In brief, 5 mL of CNT toluene solution was sonicated for 1 h and then mixed with 5 mL of HGB aqueous solution with different concentration (0.2, 0.5, 1.0, and 10.0 mg/mL). After vigorous stirring for 24 h, the black CNT was moved from the upper toluene layer to the bottom water layer of the liquid. The bottom layer of solution was taken, and the CNT
HGB hybrid nanofibers were separated from the superfluous proteins by centrifugation (10 000 rpm, 15 min) twice and redispersed in 10 mL of deionized water. 2.3. Synthesis of HGB-Protected PtNPs and CNT
PtNP Nanohybrids. The preparation of HGB-protected PtNPs was performed according to our previous work.17 First, 1 mL of K2PtCl6 (10 mM) was mixed with 1 mL of HGB aqueous solution (1 mg/mL) and 18 mL of H2O under vigorous stirring. Thirty minutes later, freshly prepared NaBH4 aqueous solution (1%, m/v) was added to the mixed solution drop by drop to produce HGB-protected PtNPs. CNT
PtNP nanohybrids were prepared by a two-step chemical reduction process. In the first step, 2 mL of water-soluble CNT
HGB nanofibers was mixed with 2 mL of K2PtCl6 (10 mM) under stirring, and the pH of mixed solution was adjusted to 3. CNT
HGB
PtCl62
hybrids were created after 12 h by the electrostatic interaction between protein and PtCl62
. In the next step, CNT
HGB
PtCl62
hybrids were separated from the mixed solution by centrifugation (10 000 rpm, 15 min) twice and redispersed in 4 mL of deionized water. 60 μL of freshly prepared NaBH4 aqueous solution (1%, m/v) was added once to produce CNT
PtNP nanohybrids. The CNT
PtNP nanohybrids produced with 1.0 mg/mL HGB were used for the electrocatalytic experiments. 2.4. Preparation of CNT-, PtNP-, and CNT
PtNP-Modified Electrodes. The glassy carbon electrode (GCE, 2.0 mm in diameter) was polished before each experiment with 1, 0.3, and 0.05 μm alumina slurry, respectively, and then successively washed with diluted nitric acid, acetone, and distilled water in an ultrasonic bath. 4 μL of sample solution (CNT, PtNP, and CNT
PtNP) was dropped onto the pretreated GCE and dried in air. After that, 2 μL of Nafion (0.1%) was cast onto the electrode to avoid the leakage of the materials. Such CNT-, PtNP-, and CNT
PtNP-modified electrodes were used for the cyclic voltammetry (CV) and amperometric response experiments. 2.5. Experimental Techniques. TEM images were taken by a JEOL 3010 electron microscope (JEOL Ltd., Tokyo, Japan) at 300 kV coupled with energy dispersive X-ray (EDX) spectroscopy. For the TEM characterization of CNT
HGB nanofibers, 15 μL of CNT
HGB solution was dropped onto a carboncoated Cu grid and stained with 1% PTA for 90 s. A UV
vis spectrometer (Perkin-Elmer, Cambridge, UK) was utilized to measure the adsorption intensity of protein at ∼277 nm. X-ray 11454

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C

ARTICLE

Figure 1. XPS spectra of CNTs (a, c) and Z-Gly-OSu functionalized CNTs (b, d) at the peak C 1s and N 1s.

photoelectron spectroscopy (XPS) measurements were conducted on a PHI Quantum 2000 spectrometer (PHI Co., Chanhassen, MN) employing a monochromatic Al KR radiation as the X-ray source (hν = 1486.6 eV). All electrochemical experiments were performed on a CHI 660A electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China). A conventional three-electrode system was employed with a bare or modified GCE as working electrode, a Pt wire as auxiliary electrode, and a KCl saturated Ag/AgCl electrode as reference electrode.

3. RESULTS AND DISCUSSION 3.1. Noncovalent Functionalization of CNTs with Bifunctional Linker. Noncovalent functionalization of CNT side walls

is important to the biocompatibility and bioapplications of CNTs because this functionalization does not disrupt the conjugated electronic structure in nanotubes.39 It is also important to keep the structure of CNTs for further applications because the reactivity of CNT is directly related to its electronic properties.1,39 Z-Gly-OSu is a bifunctional linker with a phenyl group on one end and an NHS group on the other end (shown in Scheme 1). This type of linker can provide not only the noncovalent functionalization of CNTs via π
π interaction but also the reactive group to interact with protein molecules via nucleophilic substitution of NHS by the amine group on protein and finally form an amide bond.17,18 In this current work, XPS was utilized to measure the functionalization of CNTs with Z-Gly-OSu. In a previous study,

Yang et al. investigated the π
π interaction between benzyl mercaptan and multiwalled CNTs by XPS measurement.36 Their study revealed that the π
π interaction can be demonstrated by the spectral changes. Figure 1 shows the XPS spectra of pristine CNTs and the Z-Gly-OSu-functionalized CNTs at the peaks of C 1s and N 1s. Before functionalization with Z-Gly-OSu, the binding energy of C 1s peak of CNTs is located at 284.0 eV (Figure 1a). After functionalization, two C 1s peaks can be found in the XPS spectrum, as shown in Figure 1b. One peak is located at 284.8 eV, which reveals a small shift compared to that of pristine CNTs.36 The other peak located at 288.5 eV could be assigned to N
CdO from the amide bond, which originates from the
OSu group.40 The change of the N 1s peak can also identify the functionalization of CNTs with Z-Gly-OSu. It is clear that there is no N 1s peak for the CNT sample without noncovalent functionalization (Figure 1c), but for the functionalized CNTs, there is an N 1s peak at 399.8 eV (Figure 1d), which can be ascribed to the N atoms of Z-Gly-OSu linker. On the basis of the XPS results, we conclude that the CNTs were successfully functionalized with Z-Gly-OSu in our experiments. 3.2. Preparation and Characterization of CNT
HGB Hybrid Nanofibers. After functionalization with Z-Gly-OSu, the CNTs can be dissolved in toluene solution with high concentration by sonication for several minutes, but the solubility of functionalized CNTs in water is very poor, as shown in Figure 2a,b. Therefore, a conjugation of water-soluble proteins with Z-Gly-OSu-functionalized CNTs was performed to increase the water solubility and possible bioapplications of CNTs. In this present work, CNT
HGB hybrid nanofibers were prepared 11455

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C

ARTICLE

Figure 3. Loading efficiency of HGB with different concentration on CNTs (0.1 mg/mL). The inset is the calibration plot of adsorption intensity at 277 nm vs concentration of HGB. The adsorption intensity of HGB at 277 nm was detected with a UV
vis spectrometer. Figure 2. (a) Photographs of CNT and protein solutions before and after phase transfer. The concentration of CNTs is 0.1 mg/mL, and the concentration of HGB is 0.2, 0.5, and 1.0 mg/mL for I
III, respectively. (b) Optical images of Z-Gly-OSu-modified CNTs (IV) and CNT
HGB nanofibers (V) with a concentration of 5.0 mg/mL in water. (c, d) Typical TEM images of (c) Z-Gly-OSu functionalized CNTs and (d) CNT
HGB hybrid nanofibers in bottle III after phase transfer. The scale bars in (c) and (d) represent 50 nm.

by phase-transferring Z-Gly-OSu-functionalized CNTs from toluene to HGB aqueous solution to cause the covalent binding of proteins along CNTs. Phase-transfer methods have been used to transfer NPs and quantum dots from an aqueous phase into an organic phase or from an organic phase into an aqueous phase.41
43 Figure 2a reveals the typical optical images of Z-Gly-OSufunctionalized CNTs in toluene (top layer) and HGB aqueous solutions with different concentration (bottom layer) before and after phase-transfer treatment. Before the phase-transfer treatment, Z-Gly-OSu-functionalized CNTs were dispersed in the top toluene phase. After stirring the mixed solution of CNTs and HGB (0.2, 0.5, and 1.0 mg/mL for sample I
III, respectively) for 24 h, it was found that the black CNT layers were moved from the top toluene phase to the lower protein aqueous phase by phase transfer. We suggest that the protein molecules act as catalysts to promote the transfer of CNTs in this process by the covalent conjugation with Z-Gly-OSu-functionalized CNTs. It should be noted that the concentration of HGB is crucial for the final formation of CNT
HGB hybrid nanofibers. For example, when the concentration of CNTs was 0.1 mg/mL and the concentration of HGB was 0.1 mg/mL, the CNT layer could not be moved to the HGB aqueous layer even if the reaction period was increased to 48 h. When the concentration of HGB was increased to 10 mg/mL, CNT layer with a concentration of 5.0 mg/mL could be transferred to the HGB layer and CNT
HGB hybrid nanofibers with high concentration (∼5 mg/mL) was created. These CNT
HGB hybrid nanofibers reveal super-water-solubility compared to the Z-Gly-OSu-functionalized CNTs, as shown in Figure 2b. TEM images identify the formation of CNT
HGB hybrid nanofibers. Figure 2c is a typical TEM image of Z-Gly-OSufunctionalized CNTs before phase transfer, and the CNTs reveal

a clear tube-like structure with a diameter of 10
15 nm. After the protein conjugation, a necklace-like structure was formed, and the black dots along the CNT template were thought to be HGB molecules (Figure 2d). It should be noted that this strategy may be used to prepare many other water-soluble CNT
protein hybrid nanofibers. We created CNT
fibronectin,38 CNT
fibrinogen, and CNT
γ-globulin hybrid nanofibers previously with the same strategy and investigated the potential applications of the created hybrid nanofibers. The loading efficiency of HGB on functionalized CNTs (0.1 mg/mL) was measured by a UV
vis spectrometer. The principle is based on measuring the adsorption intensity of HGB solution at 277 nm, and the inset in Figure 3 gives a calibrated plot of adsorption intensity vs concentration of HGB. Before phase transfer, the adsorption intensity of HGB was measured as I1. After phase-transfer treatment, the CNT
HGB nanofibers were separated by centrifugation from the mixed aqueous solution, and the intensity of left solution was detected with a UV
vis spectrometer to label as I2. The loading efficiency is the ratio of (I1
I2)/I1. A plot of loading efficiency vs HGB concentration is shown in Figure 3. When the concentration of HGB was 0.2 mg/mL, the loading efficiency of HGB on CNTs was ∼99.0%. With the increase of HGB concentration from 0.2 to 0.5, 1.0, 5.0, and 10.0 mg/mL, the loading efficiency decreased from 99.0% to 92.7%, 67.9%, 14.4%, and 6.8%, respectively. It can be concluded that 1 mg/mL HGB is sufficient to conjugate with Z-Gly-OSu-functionalized CNTs with a concentration of 0.1 mg/mL, and there will be a large amount of proteins not to conjugate with CNTs when the HGB concentration is higher than 5 mg/mL. 3.3. Controlled Synthesis of CNT
PtNP Nanohybrids. Proteins can provide binding sites for negatively and positively charged metallic ions based on their isoelectic points and pH value of solution.2,44 In this work, CNT
HGB
PtCl62
nanohybrids were first prepared by the electrostatic interaction between positively charged CNT
HGBnþ nanofibers and negatively charged PtCl62
ions, and then the CNT
PtNP nanohybrids were prepared by the chemical reduction of CNT
HGB
PtCl62
hybrids with NaBH4. The HGB molecules conjugated onto the CNTs can not only direct the 11456

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C

ARTICLE

Figure 4. (a
c) TEM images of PtNPs formed on CNT
HGB hybrid nanofibers by adjusting the concentration of HGB to (a) 0.2, (b) 0.5, and (c) 1.0 mg/mL. The inset in (c) is the EDX spectrum of CNT
PtNP nanohybrids. (d) TEM image of HGB-protected PtNPs. The scale bars represent 50, 50, 100, and 10 nm for (a) to (d), respectively.

formation of PtNPs along CNTs but also prevent the aggregation of PtNPs. Figure 4a
c shows representative TEM images of prepared CNT
PtNP nanohybrids based on CNT
HGB nanofibers with different HGB concentration. When the HGB concentration was 0.2 mg/mL, there is only a small amount of PtNPs formed on the side wall of CNTs (Figure 4a). It is clear that there were not enough protein molecules conjugated onto the CNTs when the protein concentration was too low, such as 0.2 mg/mL. When the HGB concentration was increased to 0.5 and 1.0 mg/mL, the density of PtNPs on CNTs was greatly increased (Figure 4a,b), and PtNPs with a diameter of ∼3.2 nm were uniformly dispersed on the side walls of CNTs. The inset in Figure 4c demonstrates the EDX spectrum of corresponding CNT
PtNP nanohybrids. The existence of elemental Pt can be clearly identified. As suggest by the previous studies, the dispersion of PtNPs on CNTs is critical to their further application as catalysts because they can provide large available surfaces and enhance the electrocatalytic activity toward the analytes.45,46 To compare the catalytic activity of CNT
PtNP nanohybrids with PtNPs, dispersed PtNPs were prepared, as shown in Figure 4d. The PtNPs were chemically reduced by the protection of HGB molecules as the strategy introduced in our previous work.17 It was found that the diameter of PtNPs was ∼3 nm. To further confirm the formation of metallic PtNPs on CNTs, the XPS technique was used to characterize the prepared CNT
PtNP nanohybrids. Figure 5a,b shows the typical XPS spectra of CNT
PtNP nanohybrids shown in Figure 4c at the peak areas of C 1s and Pt 4f, respectively. The C 1s spectrum reveals two peaks at 285.5 and 289.6 eV, respectively (Figure 5a). These values are almost 0.7 and 0.9 eV higher than that of the Z-Gly-OSu-functionalized CNTs shown in Figure 1b. These shifts can be ascribed to the binding of proteins and PtNPs on CNTs.

Figure 5. XPS spectra of CNT
PtNP nanohybrids at the peak area of (a) C 1s and (b) Pt 4f.

Pt 4f spectrum has a doublet structure, Pt 4f7/2 and Pt 4f5/2. As seen in Figure 5b, two binding energy peaks of Pt 4f were located at 73.0 and 76.3 eV, respectively. Our presented data are approximately 1.7 and 1.6 eV higher than that of the polycrystalline PtNPs (71.3 and 74.7 eV for 4f7/2 and 4f5/2). According to the explanation of Yang et al.,36 we suggest that these shifts are ascribed to two possible effects. One effect is the larger finalstate effects caused by the small size of PtNPs, and the other is the PtNP charging due to the chemical reduction. On the basis of the TEM and XPS results, it can be concluded that the PtNPs with high dispersion were created onto the CNTs, and 1-D CNTPtNP nanohybrids were successfully prepared. 3.4. Application of CNT
PtNP Nanohybrids in Glucose Sensing. The accurate and rapid detection of glucose is of great importance in many fields, such as the detection of diabetes, wastewater treatment in food industries, and implantable biofuel cells.47,48 The nonenzymatic electrochemical glucose sensor has attracted more interest due to its low cost and high stability compared with enzyme-based biosensors.5,46
51 Considering the good catalytic performance of PtNPs and the wide applications of CNTs, the prepared CNT
PtNP nanohybrids are expected to have better performance than either CNTs or PtNPs in the analytical field, such as the electrochemical sensing of glucose. To test this hypothesis, the prepared CNT
PtNP 11457

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C

ARTICLE

Figure 7. Current
time response of CNT
PtNP-modified GCE in 0.1 M NaOH solution with successive addition of 0.02 mM UA, 0.1 mM AA, and 2.0 mM glucose at an applied potential of (a) 0 V, (b)
0.15 V, and (c)
0.4 V, respectively.

Figure 6. CVs of (a) CNT-, (b) PtNP-, and (c) CNT
PtNP-modified GCE in 0.1 M NaOH in the absence (dashed curve) and presence (solid curve) of 20 mM glucose at a scan rate of 0.1 V/s.

nanohybrids shown in Figure 4c were utilized to modify the GCE, and the CNT
PtNP-modified GCE in glucose sensing was investigated in this work. Figure 6 shows the CVs of CNT-, PtNP-, and CNT
PtNPmodified GCE in the absence (dash curve) and presence (solid curve) of 20 mM glucose at a scan rate of 0.1 V/s. As can be seen in Figure 6a, the CNT-modified GCE almost has no response to the injection of glucose. Although a peak current was observed

for the PtNP-modified GCE after glucose was added, the response signal was relatively weak (Figure 6b). For the CNTPtNP-modified GCE, large current responses and several welldefined current peaks were observed, as shown in Figure 6c. In the positive potential scan, four anodic current peaks at
0.75,
0.44,
0.25, and þ0.07 V were observed, which are attributed to the oxidation of glucose and resulting intermediates.46,50 In the negative potential scan, two cathodic current peaks appear. The peak at
0.35 V could be due to the reduction of platinum oxide that was formed in the positive potential scan and the peak at
0.70 V could be due to the reduction of the oxidized products of glucose or desorption of the oxidation products.46 The present CV results indicate that the CNT
PtNP-modified GCE has better electrocatalytic activity than either CNT-modified GCE or PtNP-modified GCE toward the oxidation of glucose in alkaline solution. Some electroactive molecules such as UA and AA could affect the electrochemical detection of glucose.49 Therefore, the selective detection of glucose from the UA and AA at an optimal applied potential is needed. Figure 7a
c shows the amperometric responses of CNT-PtNP-modified GCE to glucose in the presence of UA (0.02 mM) and AA (0.1 mM) at an applied potential of 0,
0.15, and
0.4 V, respectively. It was found that the glucose response was overlapped by the contribution of UA and AA at potentials of 0 V (Figure 7a), and the use of a lower potential of
0.15 V greatly decreased the contribution of UA (Figure 7b). At an applied potential of
0.4 V, no interference was observed (Figure 7c), which indicates that the response of UA and AA could be ignored at this applied potential. Under an applied potential of
0.4 V, amperometric determination of glucose was carried out. Figure 8a shows the amperometric response of glucose with successive increased concentration at CNT
PtNP-modified GCE. It can be found that upon the injection of glucose the fabricated sensor could reach 95% of the maximum steady current response within 2 s, which displays a rapid response. The rapid response may be ascribed to the good catalytic performance of PtNPs and the high conductivity of CNTs. This sensor was very stable over a long time injection, and a wide-range linear response was obtained, as shown in Figure 8b. According to this corresponding calibration curve, it can be found that the linear response of this sensor to 11458

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C

ARTICLE

nanohybrids on glucose sensing was investigated. CNT-, PtNP-, and CNT
PtNP-modified GCEs were prepared in this work. The CV results indicate that the CNT
PtNP-modified GCE reveals better electrocatalytic activity to glucose than either the CNT-modified GCE or PtNP-modified GCE. It is expected that our presented approaches will expand the applications of CNTs in many fields, such as materials science, tissue engineering, and biomedicine. For example, it is helpful to prepare novel nanomaterials for bone implantation and to investigate the interactions between CNTs and cells.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ49 3641 947730.

’ ACKNOWLEDGMENT G. Wei and K. D. Jandt gratefully acknowledge the financial support of the Carl-Zeiss Foundation via a postdoctoral stipend and the project “Innovations- und Gr€underlabor f€ur neue Werkstoffe (Biomaterialien) und Verfahren (IGWV) an der Friedrich-Schiller-Universit€at Jena” (Grant 03GL0026). F. Xu and Z. Li appreciate the financial support from the National Natural Science Foundation of China (No. 20775077) and the National Basic Research Program of China (973 Program, No. 2010CB933600). ’ REFERENCES

Figure 8. (a) Amperometric I
T response of CNT
PtNP-modified GCE with successive addition of 0.2 mM (1 time), 0.4 mM (1 time), and 2 mM (N times) glucose to 0.1 M NaOH solution at an applied potential of
0.4 V. (b) The corresponding calibration plot for glucose. The linear range can be up to 46.6 mM (indicated as P).

glucose was up to 46.6 mM, which is much higher than the 15 mM required for the practical use in the detection of blood glucose.49 The limit of detection of this CNT-PtNP-based sensor was ∼0.028 mM with a signal-to-noise ratio of 3. The glucose sensor presented in our work showed a lower limit of detection and wider linear response (0
44.6 mM) compared to some previously reported glucose sensors based on CNTs, mesoporous carbon, and PtNPs.5,46
51 This improved sensitivity is ascribed to the super-water-solubility of Z-Gly-OSu-functionalized CNTs and the high-density PtNPs with small size on CNTs.

4. CONCLUSIONS In summary, we demonstrated a facile and effective strategy to prepare functional CNT
PtNP nanohybrids by the mediation of protein molecules. We prepared super-water-soluble CNT
HGB nanofibers and then synthesized CNT
PtNP nanohybrids by chemically reducing the PtCl62
ions that electrostatically adsorbed onto the CNT
HGB nanofibers. The results indicate that protein molecules can direct the formation of PtNPs with uniform size and high dispersion along the side wall of CNTs, and the protein concentration is crucial for the water-solubility of CNTs and the density of PtNPs formed on CNTs. The electrocatalytic activity of prepared CNT
PtNP

(1) Eder, D. Chem. Rev. 2010, 110, 1348–1385. (2) Bale, S. S.; Asuri, P.; Karajanagi, S. S.; Dordick, J. S.; Kane, R. S. Adv. Mater. 2007, 19, 3167–3170. (3) Yang, W.; Wang, X.; Yang, F.; Yang, C.; Yang, X. Adv. Mater. 2008, 20, 2579–2587. (4) Allen, B. L.; Kichambare, P. D.; Star, A. Adv. Mater. 2007, 19, 1439–1451. (5) Yang, J.; Zhang, R.; Xu, Y.; He, P.; Fang, Y. Electrochem. Commun. 2008, 10, 1889–1892. (6) Wang., J. Electroanalysis 2005, 17, 7–14. (7) Skowro nski, J. M.; Scharff, P.; Pf€ander, N.; Cui, S. Adv. Mater. 2003, 15, 55–57. (8) Pastorin, G.; Wu, W.; Wieckowski, S.; Briand, J. P.; Kostarelos, K.; Prato, M.; Bianco, A. Chem. Commun. 2006, 1182–1184. (9) Zhao, B.; Hu, H.; Mandal, S. K.; Haddon, R. C. Chem. Mater. 2005, 17, 3235–3241. (10) Harrison, B.; Atala, A. Biomaterials 2007, 28, 344–353. (11) Kurppa, K.; Jiang, H.; Szilvay, G. R.; Nasibulin, A. G.; Kauppinen, E. I.; Linder, M. B. Angew. Chem., Int. Ed. 2007, 46, 6446–6449. (12) Han, J.; Kim, H.; Kim, D. Y.; Jo, S. M.; Jang, S. Y. ACS Nano 2010, 4, 3503–3509. (13) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105–1136. (14) Britz, D. A.; Khlobystov, A. N. Chem. Soc. Rev. 2006, 35, 637–659. (15) Hull, R. V.; Li, L.; Xing, Y.; Chusuei, C. C. Chem. Mater. 2006, 18, 1780–1788. (16) Li, J.; Tang, S.; Lu, L.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 9401–9409. (17) Wei, G.; Pan, C.; Reichert, J.; Jandt, K. D. Carbon 2010, 48, 645–653. (18) Chen, R. C.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838–3839. (19) Zhang, D. H.; Kandadai, M. A.; Cech, J.; Roth, S.; Curran, S. A. J. Phys. Chem. B 2006, 110, 12910–12915. 11459

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460

The Journal of Physical Chemistry C

ARTICLE

(20) Khang, D.; Kim, S. Y.; Liu-Snyder, P.; Palmore, G. T. R.; Durbin, S. M.; Webster, T. J. Biomaterials 2007, 28, 4756–4768. (21) Plank, N. O. V.; Cheung, R.; Andrews, R. J. Appl. Phys. Lett. 2004, 85, 3229–3231. (22) Yang, D. Q.; Rochette, J. F.; Sacher, E. Langmuir 2005, 21, 8539–8545. (23) Wang, Z.; Shirley, M. D.; Meikle, S. T.; Whitby, R. L. D.; Mikhalovsky, S. V. Carbon 2009, 47, 73–79. (24) Shofner, M. L.; Khabashesku, V. N.; Barrera, E. V. Chem. Mater. 2006, 18, 906–913. (25) Liu, Y.; Yao, Z.; Adronov, A. Macromolecules 2005, 38, 1172–1179. (26) Gao, C.; He, H.; Zhou, L.; Zheng, X.; Zhang, Y. Chem. Mater. 2009, 21, 360–370. (27) Yang, G. W.; Gao, G. Y.; Zhao, G. Y.; Li, H. L. Carbon 2007, 45, 3036–3041. (28) Wu, T.; Lin, Y.; Liao, C. Carbon 2005, 43, 734–740. (29) Sainbury, T.; Fitzmaurice, D. Chem. Mater. 2004, 16, 2174– 2179. (30) Wenseleers, W.; Vlasov, I. I.; Goovaerts, E.; Obraztsova, E. D.; Lobach, A. S.; Bouwen, A. Adv. Funct. Mater. 2004, 14, 1105–1112. (31) Paloniemi, H.; Aaritalo, T.; Laiho, T.; Like, H.; Kocharova, N.; Haapakka, K.; Terzi, F.; Seeber, R.; Lukkari, J. J. Phys. Chem. B 2005, 109, 8634–8642. (32) Hu, C.; Zhang, Y.; Bao, G.; Zhang, Y.; Liu, M.; Wang, Z. J. Phys. Chem. B 2005, 109, 20072–20076. (33) Balavoine, F.; Shultz, P.; Richard, C.; Mallouh, V.; Ebbesen, T. W.; Mioskowski, C. Angew. Chem., Int. Ed. 1999, 38, 1912–1915. (34) Wang, S.; Humphreys, E. S.; Chung, S. Y.; Delduco, D. F.; Lustig, S. R.; Wang, H.; Parker, K. N.; Rizzo, N. W.; Subramoney, S.; Chiang, Y. M.; Jagota, A. Nature Mater. 2003, 2, 196–200. (35) Ortiz-Acevedo, A.; Xie, H.; Zorbas, V.; Sampson, W. M.; Dalton, A. B.; Baughman, R. H.; Draper, R. K.; Musselman, I. H.; Dieckmann, G. R. J. Am. Chem. Soc. 2005, 127, 9512–9517. (36) Yang, D. Q.; Hennequin, B.; Sacher, E. Chem. Mater. 2006, 18, 5033–5038. (37) Pender, M. J.; Sowards, L. A.; Hartgerink, J. D.; Stone, M. O.; Naik, R. R. Nano Lett. 2006, 6, 40–44. (38) Wei, G.; Zhang, J.; Xie, L.; Jandt, K. D. Carbon 2011, 49, 2216–2226. (39) Lu, F.; Gu, L.; Meziani, M. J.; Wang, X.; Luo, P. G.; Veca, L. M.; Cao, L.; Sun, Y. P. Adv. Mater. 2009, 21, 139–152. (40) Rosario-Castro, B. I.; Contes, E. J.; Perez-Davis, M. E.; Cabrera, C. R. Rev. Adv. Mater. Sci. 2005, 10, 381–386. (41) Cheng, W.; Wang, E. J. Phys. Chem. B 2004, 108, 24–26. (42) Yu, W. W.; Chang, E.; Sayes, C. M.; Drezek, R.; Colvin, V. L. Nanotechnology 2006, 17, 4483–4487. (43) Liu, W.; Choi, H. S.; Zimmer, J. P.; Tanaka, E.; Frangioni, J. V.; Bawendi, M. J. Am. Chem. Soc. 2007, 129, 14530–14531. (44) Wei, G.; Wang, L.; Sun, L.; Song, Y.; Sun, Y.; Guo, C.; Yang, T.; Li, Z. J. Phys. Chem. C 2007, 111, 1976–1982. (45) Mu, Y.; Liang, H.; Hu, J.; Jiang, L.; Wan, L. J. Phys. Chem. B 2005, 109, 22212–22216. (46) Rong, L. W.; Yang, C.; Qian, Q. Y.; Xia, X. H. Talanta 2007, 72, 819–824. (47) Su, C.; Zhang, C.; Lu, G.; Ma, C. Electroanalysis 2010, 22, 1901–1905. (48) Zhao, J.; Wang, F.; Yu, J.; Hu, S. Talanta 2006, 70, 449–454. (49) Lin, Y.; Lu, F.; Tu, Y.; Ren, Z. Nano Lett. 2004, 4, 191–195. (50) Bo, X.; Ndamanisha, J. C.; Bai, J.; Guo, L. Talanta 2010, 81, 339–345. (51) Xu, F.; Cui, K.; Sun, Y.; Guo, C.; Liu, Z.; Zhang, Y.; Shi, Y.; Li, Z. Talanta 2010, 82, 1845–1852.

11460

dx.doi.org/10.1021/jp202324q |J. Phys. Chem. C 2011, 115, 11453–11460