Synthesis of zinc oxide nanoparticles on graphene–carbon nanotube hybrid for glucose biosensor applications
Introduction
The applications of biocompatible nanomaterials for enzymatic glucose sensors were explored as they help the enzymes to retain its activity and to augment direct electron transfer between the active site of the enzyme and the electrode (Umar, 2010). Carbon nanotubes (CNTs) and graphene (GR) are the most widely used nanomaterials in electrochemical sensors due to their conductivity, chemical stability, flexibility and low cost. GR is a two dimensional layer of graphite with sp2 carbon atoms arranged in a hexagonal lattice. It has attracted tremendous research interest among theoretical and experimental scientists due to its unique electrical properties and rich surface chemistry (Geim and Novoselov, 2007). The distinct properties of GR have led to its application in developing devices such as field-effect transistors (Li et al., 2008a, Li et al., 2008b), photovoltaic devices (Williams and Kamat, 2009), capacitors (Stoller et al., 2008) and electrochemical sensors (Zhou et al., 2009). The GR sheets have low polarity, hydrophobic surfaces with high π-electron density. These properties make GR to be useful for immobilizing molecules through covalent (Si and Samulski, 2008) and non-covalent interactions (Stankovich et al., 2006). GR can be produced through chemical or thermal reduction of graphene oxide (GO) (Shao et al., 2010). The chemical reduction paves the way for deposition of GR from solution, enabling devices to be fabricated on any surface (Eda et al., 2008). The hydrophilic properties of GO make it an exceptional material to suspend the CNTs in aqueous medium. The hybridization of GO and CNTs is expected to have synergistic effects as they have similar structural and physical properties (Sharma et al., 2012). On the other hand, CNTs are the rolled version of GR which are single walled (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). CNTs are widely used in electrochemical sensors and biosensors applications due to their distinct electrochemical properties such as high electrocatalytic activity and high electron-transfer rate (Lin et al., 2004). Thus, CNTs and GR have a great potential as supporting materials for enzymes. Along with carbon supports, metal oxide nanostructures can be used to adsorb enzymes due to their high specific surface area, biocompatability and chemical stability. The nanostructured metal oxides coupled with different redox enzymes were used for fabricating biosensors (Wan et al., 2004). Among different metal oxides, ZnO nanostructures have been used in glucose sensors as they have advantages such as non-toxic, electrochemical activity and high electron transfer rate (Zang et al., 2007).
The electrochemical glucose sensors are most widely used for measuring blood glucose levels in diabetic patients because of their high sensitivity, selectivity, rapidness, fidelity, portability and low cost (Oliver et al., 2009). The major challenge in the fabrication of electrochemical glucose sensors is the immobilization of model enzyme, glucose oxidase (GOx) on the electrode surface through effective electrical communication between GOx and the transducer without compromising the mechanical stability and catalytic activity of the enzyme (Chen et al., 2013). To attain these key features, various materials and methods have been used for the surface modification of electrodes (Liu et al., 2013, Periasamy et al., 2011). Glucose oxidase (GOx) is highly selective for glucose in biological samples.
In this study, we utilized CNTs, GR and ZnO to immobilize GOx for the development of a voltammetric gluose sensor. GO–CNT composite was prepared by a simple solution based approach, then the GR–CNT–ZnO composite was obtained by the simultaneous reduction of GO and zinc acetate using sodium borodihydride (NaBH4). GOx was successfully immobilized on the GR–CNT–ZnO composite and it was employed for the fabrication of a voltammetric glucose sensor. The practicality of the proposed GR–CNT–ZnO/GOx sensor was successfully demonstrated in human serum sample.
Section snippets
Preparation of GR–CNT–ZnO composite
The preparation of GR–CNT–ZnO composite is shown in Scheme 1. Briefly, graphite oxide was prepared by modified Hummer's method (Hummers and Offeman, 1958) and subsequently exfoliated to GO via ultrasonication for 2 h. Then, GO was centrifuged at 3000 rpm for 30 min and homogenous dispersion of GO was collected. About 10 mg of CNTs was added to 5 mL GO (1 mg mL−1) and ultrasonicated for 2 h. The yellowish brown color of GO changed to black color, indicating the formation of GO–CNT composite. GO–CNT was
Surface morphological study of GR–CNT–ZnO composite
Fig. S1 shows the SEM images of GO (A) and GO–CNT hybrid (B). The SEM image of GO clearly indicates characteristic wrinkled sheet like morphology. SEM image of the GO–CNT hybrid portrays the CNTs networks decorated with GO sheets. The strong π–π non-covalent interaction between hydrophobic surface of GO and tubular structure of CNTs is the critical driving force towards the formation of a stable GO–CNT hybrid (Mani et al., 2013). The SEM image of the GR–CNT–ZnO composite (Fig. 1A) shows that
Conclusion
We prepared the GR–CNT–ZnO composite by an one-pot chemical reduction route. The GR–CNT–ZnO composite was characterized by SEM, EDX, TEM, XRD, AT-FTIR and electrochemical techniques. GOx was immobilized on the composite without using any binders and cross-linking agents. Direct electrochemistry of GOx was observed on the prepared composite with fast electron transfer (ks=5.544 s−1). The GOx immobilized modified electrode showed good electrocatalytic activity towards determination of glucose in
Acknowledgments
This study was supported by the grants from National Taipei University of Technology (NTUT-100-140-01) to K.-Y. Hwa. Our sincere thanks to Prof. Chang Shu-Mei for helping us to perform the FTIR spectroscopy. The authors like to thank Dr. Binesh Unnikrishnan, Dr. Arun Prakash Periasamy and Dr. Veerappan Mani for their help throughout this project. We also thank Ms. Sasipriya Kathirvel and Mr. Rajkumar Devasenathipathy for their assistance in this work.
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