Short communicationSelf-assembled CNTs/CdS/dehydrogenase hybrid-based amperometric biosensor triggered by photovoltaic effect
Introduction
The organization of functional semiconductor nanoparticles (QDs) on surfaces attracts more and more research efforts in recent years (Ma et al., 2005, Sapsford et al., 2004, Daniel and Astruc, 2004). For application of the photoactive materials, photochemical excitation of QDs to generate an electron–hole pair is the fundamental process (Feeman et al., 2007). Therefore, the hybrid or composite semiconductor systems were developed to behave as the matrices, which can retard the electron–hole recombination and improve photocurrents (Zaban et al., 2000, Liu and Kamat, 1993, Subramanian et al., 2001). For instance, the preparation of semiconductor-CNTs hybrid, such as SiO2- (Fu et al., 2002), TiO2- (Banerjee and Wong, 2002), SnO2- (Han and Zettl, 2003), CdSe- (Ravindran et al., 2003) and CdS-CNTs (Shi et al., 2004), has been pursued as an effective route. Of special interest is the CdS-CNTs composite, which is capable of generating photocurrent from visible light with unusually high efficiency (Shi et al., 2004, Sheeney-Haj-Ichia et al., 2005).
For developing amperometric biosensors or biofuel cell, the most fundamental principle is electrical contacting of redox-enzymes with electrodes surface (Willner and Katz, 2000, Katz et al., 1999). Different methods have been used to assemble integrated, electrically contacted enzyme electrodes. Conductive CNTs and quantum dots (QDs) usually have high values of conductivity and transmittance, which allow efficient electron transfer between the matrix electrode and redox proteins (enzymes) (Gooding, 2005, Joseph, 2005, Zhao et al., 2002, Zhou et al., 2005). Moreover, some hybrid complexes with QDs and redox proteins, such as AChE-CdS (Katz et al., 2004), formaldehyde dehydrogenase (FDH)-CdS (Vastarella and Nicastri, 2005) and hemoglobin (Hb)-TiO2 were also employed to assay the enzyme activities and to develop different biosensors (Willner et al., 2007, Zhang et al., 2007). To our best knowledge, the use of CNTs/QDs/enzyme hybrid system application in electrochemical biosensor, however, is little explored (Liu et al., 2007).
For the dehydrogenase-based biosensors, a critical issue is that their cofactors NAD(P)+ are recycled only at high overpotential at most electrodes, resulting in the loss of functionality and stability (Schmakel et al., 1975; Prieto-Simón and Fabregàs, 2006). In previous studies, semiconductor nanocrystals represent a possible route to enhance the electron/hole transfer efficiency, overcoming the problems of the NAD(P)+-dependence (Vastarella and Nicastri, 2005, Zhang et al., 2007). Herein, we described a novel NAD(P)+-independent dehydrogenase biosensor based on CNTs/CdS assembly. The hybrid system showed good generation effects upon illumination, and the photoelectrochemical response can trigger the dehydrogenase reaction without using the cofactors NAD(P)+. Thus, dehydrogenase-based reagentless biosensors can be afforded by combination of the photovoltaic effect with QDs.
Section snippets
Materials and chemicals
Glutamate dehydrogenase (GDH, EC 1.4.1.3), l-glutamate (monosodium salt, 99%) and β-nicotinamide adenine dinucleotide (NAD+), Poly (diallyldimethylammonium chloride) (PDDA) were purchased from Sigma–Aldrich. Multiwall CNTs were obtained from Shenzhen Nanotech Port Co. Ltd. (China). The monodisperse CdS-PDDA clusters with diameter about 6 nm were synthesized as the previously described (Zhang et al., 2006). Phosphate buffer was used for immobilization and electrochemical measurements 0.2 M
LbL self-assembly CdS and GDH on carbon nanotubes
The layer-by-layer (LbL) technique based on electrostatic or other molecular forces creates an advantageous approach to construct different types of assembled materials. It has been used to construct polyelectrolyte/CNTs multilayer to improve solubility and mechanical properties and retain the enzyme molecular activity (Tang et al., 2007, Xu et al., 2007). Herein, GDH and CdS-PDDA were alternating adsorbed onto the surface of the CNTs. Fig. 1a shows weak negatively charged (uncoated) CNTs gave
Conclusions
In summary, a novel hybrid nanostructure was successfully prepared by LbL self-assembly GDH and CdS onto surface of CNTs. The CNTs/CdS/GDH hybrid system shows an efficient photovoltaic effect under visible light irradiation, which can trigger the dehydrogenase-based enzymatic reaction without the NAD(P)+. The resultant biosensor behaved good electrochemical performance to glutamate with well sensibility, low detect limit. Thus, the overall procedure is shown to successfully combine the
Acknowledgments
The authors gratefully acknowledge the National Natural Science Foundation of China (20676038) and the Key Project of Science and Technology for Ministry of Education (107045) for financial supports.
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