Fabrication of biosensors using vinyl polymer-grafted carbon nanotubes

A biosensor is commonly defined as a device incorporating a bioreceptor connected to a transducer, which converts an observed response into a measurable signal proportional to analyte concentration which then is conveyed to a detector (Eggins, 1996). As demonstrated in Fig. 1, a biosensor consists of a bio-element and a sensor-element. A specific bio-element, including enzyme, antibody, microorganism, cell, and DNA, recognizes a specific analyte, and a sensor element transduces the change in the biomolecules into an electrical signal. Biosensors can be classified either by their bioreceptor or their transducer. Biosensors are known as enzymatic biosensors (enzymes), genosensors (DNAs), immunosensors (antibodies), etc. depending on the bioreceptors used. Biosensors can also be divided into several categories based on the transduction process, such as electrochemical, optical, piezoelectric, and thermal/calorimetric. Among these, electrochemical biosensors are the most widespread, numerous and successfully commercialized devices of biomolecular electronics (Dzyadevych et al., 2008). Much literature on carbon nanotube (CNT)-based biosensors has been published over the past several years because CNTs have the following advantages: (1) small size with large surface area, (2) high sensitivity, (3) fast response time, (4) enhanced electron transfer and (5) easy protein immobilization on CNT-modified electrodes, coupled with the fact that several methods have been developed (J. Wang & Musameh, 2003a; J. Wang et al., 2003b; Y. Saito et al., 1993). These properties make CNTs ideal for use in electrochemical biosensors and nanoscale electronic devices. Such potential applications would greatly benefit from CNTs in promoting the electron-transfer reaction of biomolecules, including catecholamine neurotransmitters (J. Wang et al., 2002a), cytochrome c (J. Wang et al., 2002b), ascorbic acid (Z. H. Wang et al., 2002), NADH (Musameh et al., 2002), and hydrazine compounds (Zhao et al., 2002). The insolubility of CNTs in most solvents is a major barrier for developing such CNT-based biosensing devices. Therefore, surface modification is necessary for CNT materials to be biocompatible and to improve solubility in common solvents and selective binding capability to biotargets. There are two main approaches for surface modification of CNTs: a non-covalent wrapping or adsorption and covalent chemical tethering. The non-covalent approach includes surfactant modification, polymer wrapping, and polymer absorption via various adsorption forces, such as van der Waals and π-stacking interactions. The advantage of non-covalent modification is that the structures and mechanical properties of CNTs remain intact.