Pt–Pb nanowire array electrode for enzyme-free glucose detection
Introduction
For a long time, the electrocatalytic oxidation of glucose has been a focal subject of many investigations, because of the great importance of sugar sensing in human blood, and their potential use for fuel cell applications (Clark et al., 1962, Reach and Wilson, 1992, Turner et al., 1999, Katz et al., 1999, Mano et al., 2003). Glucose oxidase as enzymatic catalyst has been widely used for glucose biosensor fabrication (Heller, 1990, Liaudet et al., 1990, Badia et al., 1993). Good selectivity and high sensitivity have been achieved for glucose detection by such enzymatic sensors, however, the poor stability of the enzymatic sensors due to the intrinsic nature of enzymes and interfering effects of some other electro-oxidizable species also remain as problems for real sensor applications (Wilson and Turner, 1992, Shoji and Freund, 2001, Park et al., 2006).
Direct electrocatalytic oxidation of glucose at an enzyme-free electrode would exhibit conveniences and advantages to avoid the drawbacks of the enzyme electrode. Early researches have focused on the use of noble metals Pt and Au for developing enzyme-free sensors (Vassilyev et al., 1985, Azdic et al., 1989, Beden et al., 1996, Hsiao et al., 1996). However, these electrodes often have drawbacks of low sensitivity and poor stability caused by surface poisoning from the adsorbed intermediates (Ernst et al., 1979, Bae et al., 1991). Additionally, such conventional electrodes often suffer from the influence during electrochemical detection of glucose coming from some electroactive species such as ascorbic acid (AA), uric acid (UA), and p-acetamidophenol (PP) under physiological conditions (Wang et al., 1997, Celej and Rivas, 1998).
Efforts have been attempted to overcome these drawbacks by modifying the substrate electrode with ad-metals (Aoun et al., 2003, Aoun et al., 2004, Sakamoto and Takamura, 1982, Kokkinidis and Xonoglou, 1985, Wittstock et al., 1998, Zhang et al., 1997) and nanoparticles (Jena and Raj, 2006, Tominaga et al., 2005, Xu et al., 2006, Farrell and Breslin, 2004) or by using different kinds of electrode materials (Park et al., 2003, Song et al., 2005, Yuan et al., 2005, Sun et al., 2001, Yeo and Johnson, 2000, Male et al., 2004, Cui et al., 2006, Cui et al., 2007), since the key factor that affects both the sensitivity and selectivity of glucose detection is the electrocatalytic activity of the electrode materials. Sun et al. (2001) demonstrated that Pt–Pb alloy (Pt2Pb) electrodes generated more stable reproducible and larger responses compared with pure Pt electrode. Additionally, electro-oxidation of glucose on such Pt2Pb surfaces took place at remarkably negative potentials, thus, interference could be effectively avoided by detecting glucose at a low potential. Sheu and coworkers (Cui et al., 2006) have constructed a Pt–Pb alloy nanoparticle/carbon nanotube nanocomposite electrode and investigated its electrocatalytic behavior towards glucose oxidation. In addition, they also demonstrated that the modification of the nanocomposite electrode with Nafion coating followed by electrodeposition of a second layer of Pt–Pb nanoparticles could effectively improve the sensitivity and selectivity of the sensor towards glucose oxidation (Cui et al., 2007). Macroporous platinum (Park et al., 2003, Song et al., 2005) and platinum-nanotubule array (Yuan et al., 2005) electrodes with high electrode surface roughness were developed to enhance discriminatively the amperometric response of glucose oxidation and to effectively lower the interference coming from the electroactive species. The results of the above reports show that both the component and the surface structure of the electrode play important role in catalytic oxidation of glucose.
Recently, intense interest has been focused on three-dimensional nanostructured arrays for their broad applications ranging from the development of microbatteries (Long et al., 2004) and DNA sensors (Lapierre et al., 2003, Gasparac et al., 2004) and even for biosensors development (Chen et al., 2003, Delvaux et al., 2004, Koehne et al., 2004, Delvaux et al., 2005). Ugo and coworkers (Leo et al., 2007) fabricated the three-dimensional nanoelectrode ensembles (3D-NEEs) and investigated their electroanalytical peculiarities. Electrochemical performance of redox species characterized by fast and slow heterogeneous electron transfer kinetics was studied to understand the factors influencing voltammeric responses at the 3D-NEEs. As a result, for fast redox couples, faradaic currents are not influenced by the change of electrode structures and only depend on the overall geometric area of the ensemble; however, this is not true for slow redox probes for which faradaic currents are significantly dependent on the active area of 3D-NEEs. For very fast redox couples, faradaic currents are insensitive to the dimension of the electrode since the heterogeneous kinetics is always very fast and total overlap conditions remain always operative. On the other hand, for redox couples characterized by slow heterogeneous kinetics, each electrode behaves individually and, because of strong kinetic limitations, faradaic currents depend significantly on the increase in active area. They also demonstrated that the high surface area of 3D-NEEs could be best exploited to increase the voltammetric signals for electroactive species that diffused from the bulk solution, but are characterized by slow heterogeneous electron transfer kinetics.
It is well known that direct glucose oxidation on metal substrates undergoes a sluggish kinetic process, and the faradaic current for glucose oxidation depends significantly on the active area of electrode. Combining the advantageous features of the Pt–Pb alloy and the three-dimensional nanostructured arrays in electrocatalytic oxidation of glucose, we developed an enzyme-free glucose sensor based on the Pt–Pb nanowire array electrode (Pt–PbNAE), which was simply synthesized by electrochemical deposition of Pt–Pb alloy into the pores of track-etched polycarbonate host membrane, followed by chemical etching of the template. The electrochemical catalytic activity of the sensor toward glucose oxidation was studied by comparing with that of nanowire array electrodes of different electrode materials. The interference coming from AA was also investigated, and a suitable glucose detection condition was chosen.
Section snippets
Materials
Nuclepore track-etch polycarbonate (PC) membranes (0.2 μm) were provided by Whatman (Anodisc 25 mm). Chloroplatinic acid (H2PtCl6·6H2O), hydrogen tetrachloroaurate(III) hydrate (HAuCl4·3H2O), glucose and Pb(NO3)2 were purchased from Beijing Chemical Company. l-Ascorbic acid was obtained from Bio Basic Inc. A 0.1 M phosphate buffer solution (PBS) was prepared using Na2HPO4 and NaH2PO4 and adjusted to pH 7.4 by NaOH. All of the other chemicals were of analytical grade and used as received. All
SEM characterization of the Pt–Pb nanowires
A Pt–PbNAE platform for glucose detection was generated by electrodeposition of Pt–Pb alloy into the pores of a polycarbonate membrane and subsequent chemical etching of the template. Fig. 1 shows the scanning electron micrographs of the structure of the as-prepared electrode, and confirmed the formation of the Pt–PbNAE. From Fig. 1A, we can see that the Pt–Pb nanowires stand well on the substrate surface, and the diameter is rather uniformly distributed around (230 ± 20 nm). As expected, these
Conclusion
In summary, an enzyme-free glucose sensor based on the Pt–PbNAE was synthesized by electrochemical template deposition of Pt–Pb alloy into the pores of track-etched polycarbonate host membrane and the following chemical remove of the template. SEM images showed that the Pt–Pb nanowires exhibited well-alined three-dimensional structure and the XPS results confirmed that the Pt and Pb were in their alloy form coexisted in the Pt–PbNAE. Direct glucose oxidation on such Pt–PbNAE was investigated in
Acknowledgements
This work is supported by the National Natural Science Foundation (20175007) of China, and State Key Laboratory for Supramolecular Structure and Materials, Jilin University.
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