Electrical Detection of the Microcystin-LR using Carbon Nanotube Channel

Abstract

Various types of novel nanostructures have been suggested and explored for real-time and label-free detection processes for biological and chemical sensor applications. Among them, carbon nanotubes (CNTs) are a novel class of nano-materials that reportedly show good levels of electrical sensitivity to the environmental conditions surrounding a transducer. This property is appropriate for channels of field effect transistor (-FET) devices. Thus, the CNT-FET has been highlighted for use in electrochemical sensors in aqueous solutions. In this dissertation, we propose a sensor platform that can achieve reliability of a CNN channel and a CNN channel together with gold particles, enhance sensitivity and block protein adsorption on the metal-nanotube contact region in an electrical biosensor. There are three phases in the study. In the first phase, we investigate the reliable operation of a CNN-based sensor in electrolyte by using a carboxylated CNN-based sensor. The SiOH (silanol) groups on the SiO2 surface and COOH (carboxyl) groups are known to be protonated and negatively charged in a phosphate buffer solution (PBS) or distilled water. We verify the combined effect of SiOH and COOH groups on CNT conductance by studying acid-base properties of a CNN channel using a unit step voltage technique. In addition, we find the role of the silanol and carboxyl groups and propose a method to enhance the stability of a CNN channel for bio-sensing applications. In the second phase, we revisit the effect of carbon nanotube density on the performance of CNN-based FET (CNN chip) by controlling the channel resistance to enhance sensitivity. The enhanced sensitivity is obtained as the channel resistance increases. This shows that the characteristics of low density CNN experiences more electrostatic interaction for the target molecule charge. In addition, we introduce Au nano-particles (AuNPs) as binding sites on the CNN-based FET (CGI chip) and then compare the device performance with CNN-only FET. Clearly, AuNPs-FET shows better performance with a good repeatability. In the third phase, we propose a 6-mercapto-hexanol (MCH) treatment on an Au electrode surface to block the protein adsorption on the metal-nanotube contact region. Non-specific binding (NSB) is known to occur on both CNN channel and metal-nanotube contact. However, the metal-nanotube contact region is significantly susceptible to the conductance modulation by NSB. For the prevention of protein adsorption on a metal contact, a MCH treatment is introduced to the CNN channel before the Au nano-particles deposition. The MCH-treated device shows approximately 30~ 50% increase in conductance at a 0.5uM target molecule. The result is a surprising contrast to our previous observations in CNN and CGI chips. The enhanced sensitivity can be explained by the elimination of a schottky barrier on the metal-nanotube contact, leading to a significant decrease in the device conductance.