Self-assembled CNTs/CdS/dehydrogenase hybrid-based amperometric biosensor triggered by photovoltaic effect

Abstract

A novel multi-components hybrid material, self-assembled quantum dots (CdS) and glutamate dehydrogenase (GDH) onto multiwall carbon nanotubes (CNTs), was designed for amperometric biosensing system. The ζ-potential and transmission electron microscopy (TEM) analyses confirmed the uniform growth of the CdS/GDH onto carboxyl-functionalized CNTs. Compared with the single CdS, the resulting hybrid material showed more efficient generation of photocurrent upon illumination. The incident light excites CdS and generates charge carriers, and then CNTs facilitates the charge transfer. For dehydrogenase-based biosensor, normally, the cofactor of β-nicotinamide adenine dinucleotide (NAD⁺) or β-nicotinamide adenine dinucleotide phosphate (NADP⁺) is necessary. Furthermore, we found the photovoltaic effect of CNTs/CdS/GDH can trigger the dehydrogenase enzymatic reaction in the absence of the NAD⁺ or NADP⁺ cofactors. The electrochemical experiment results also demonstrate that the cofactor-independent dehydrogenase biosensing system had series attractive characteristics, such as a good sensitivity (11.9 nA/μM), lower detection limit (up to 50 nM), an acceptable reproducibility and stability. These studies aid in understanding the combination of the semiconductor nanohybrids (CNTs/QDs, etc.) and biomolecules (enzymes, etc.), which has potential for the applications in biosensor, biofuel cell, biomedical and other bioelectronics field.

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.