The fabrication of a colloidal gold–carbon nanotubes composite film on a gold electrode and its application for the determination of cytochrome c
Introduction
Au colloids display unusual physical and chemical properties because of their unique size and shape. They are widely used as catalysts for many chemical reactions because of their high catalytic activities. Electrochemical behavior and applications of Au colloids in an electrochemical field have witnessed a significant growth in the past few years [1], [2], [3], [4], [5], [6], [7]. In particular, Au colloids possess biocompatibility and proteins bound to colloidal Au electrostatically can retain biological activity [8]. It has been demonstrated that several enzymes could maintain their enzymatic and electrochemical activity when immobilized on colloidal Au [9], [10]. The immobilization of a redox protein on Aunano can help the protein to keep a favored orientation or to make possible conducting channels between the prosthetic groups and the electrode surface, thus reducing the effective electron transfer distance and facilitating electron transfer between electrode and enzyme [11].
Carbon nanotubes, consisting of cylindrical graphite sheets with nanometer diameter, combine in unique properties of high electrical conductivity, high chemical stability and extremely high mechanical strength and modulus. Their subtle electronic properties suggest that carbon nanotubes have the ability to promote electron transfer reactions when used as electrode materials in electrochemical reactions. Multi-wall carbon nanotubes (MWNTs) were used first by Hill and co-workers [12] to realize the direct electrochemistry of proteins. Since that time, MWNTs have opened up new possibilities for studying the electrochemical processes of various proteins and enzymes [13], [14], [15], [16].
The key idea of this paper is to combine Aunano and MWNTs to modify gold electrodes in order to improve their electroactivity for cytochrome c. MWNTs offer a cubic network for Aunano and the gelatinization of Aunano in DHP ensures that the composite film remains stable on the surface of the gold electrode. It is demonstrated that Aunano are in electrical contact with the gold electrode through the MWNTs, enabling the composite film to be used as an electrode. The composite film on the gold electrode surface was characterized by electrochemical impedance spectroscopy and cyclic voltammetry (CV). It was found that the composite film promoted the direct electron transfer of horse heart cytochrome c. The optimum loading of MWNTs and Aunano in the composite film was investigated and the optimized composite film was used to detect cytochrome c successfully.
Section snippets
Reagents
Horse heart cytochrome c was obtained from Sigma and used as received. Dihexadecylphosphate was purchased from Fluka. All the other chemicals were of analytical grade and were used without further purification. Doubly distilled water was used throughout and the supporting electrolyte was usually phosphate buffer containing 0.06 M Na2HPO4 and NaH2PO4.
The multi-wall carbon nanotubes, MWNTs (obtained from the Institute of Nanometer Materials, Central China Normal University, China), were
Assembly of Aunano on MWNTs
Pristine carbon nanotubes are very hydrophobic and most metals do not adhere to the carbon nanotubes [19]. The oxidation of the carbon nanotube surface is required to create functional groups and facilitate metal deposition on the carbon nanotubes [20]. Our method for the deposition of Aunano on MWNTs is simple and effective. First, dihexadecylphosphate, a negatively charged surfactant, was dispersed in colloidal gold and then MWNTs were solubilized in the dispersion. Aunano were assembled on
Conclusion
In summary, an easy and effective method is described to deposit Aunano on MWNTs. The Aunano combined with a MWNTs dispersion in DHP was immobilized successfully on a gold electrode surface. The composite layer on the gold electrode surface was characterized by electrochemical impedance spectroscopy and CV. The modified electrode combined the excellent electroactivity of Aunano and MWNTs, resulting in high response to cytochrome c.
Acknowledgment
The financial support from the National Natural Science Foundation of China (Nos. 30370397 and 60171023) is gratefully acknowledged.
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