Fabrication and Applications of Nanoelectrodes using Carbon Nanotubes

Abstract

This Ph. D. thesis can be divided into three parts. In first part, the fabrication of carbon nanotube (CNT) nanoelectrodes, consisted of many individual single-walled nanotubes (SWNTs), is described. In the second part, the modification of the CNT electrodes for neural application is discussed, along with its advantages that include improved sensor selectivity and sensitivity. Finally, liquid pumping of an ionic solution using CNT nanoelectrodes is presented. In first part, CNT micro- and nanoelectrodes were successfully fabricated on a sharpened tungsten tip using dielectrophoresis (DEP) and surface tension. Through electrodeposition, Au was coated successfully onto CNT nanoelectrodes to obtain the desired properties. Organic matters that had accumulated between SWNTs during the dispersion process was minimized by conducting SWNT dispersion in a clean room environment, or removed via plasma treatment or acetone cleaning. The diameter of a single SWNT was on the order of several tens of nanometers, with numerous pores between the SWNTs. In second part, commercial tungsten microelectrodes were modified with CNTs to enhance their performance. Fabricated CNT-modified electrodes (CNEs) have much smaller opened surface than the commercial tungsten microelectrodes. But, the impedance with the respect to surface area of the CNEs is much less than that of tungsten microelectrodes because of their large electrochemical surface area (ESA). In addition, the noise level of neural signals recorded by CNEs is significantly less. Thus, the SNR is greater than that obtained using tungsten microelectrodes. Importantly, when applied in a mouse brain in vivo, the CNEs can detect action potentials five times more efficiently than tungsten microelectrodes. This technique provides a significant advance in the recording of neural signals, especially in brain regions with sparse neuronal densities. In second part, CNT wire electrodes (CWEs) and sharpened tungsten wire electrodes (TWEs) were fabricated having various diameters to study liquid pumping of an ionic solution. The ionic solution was pumped along the electrode by a DC electric field of several volts, and the direction of liquid flow headed to the electrode having a negative bias. This phenomenon is caused by a movement of electric double layer (EDL) near the electrode and resulting flow is electro-osmotic flow (EOF). According to the electric field analysis, the z-directional electric field around meniscus of small electrode is more concentrated than that of big electrode. Thus, the pumping effect may be increased as the diameter of the electrode decreased. Interestingly in the CWEs, the initiating voltage for liquid pumping was not changed with increasing the diameters up to 20 μm and the voltage magnitude of the CWEs was lower than that of the TWEs. About CWEs and TWEs having various diameters, we classified into three categories of ‘No pumping zone’, ‘Recommended (R.) pumping zone’, and ‘Not recommended (N. R.) pumping zone’ according to an initiating voltage and a faradaic reaction. The flow rates of transported liquid along CWEs were about picolitres per minute and followed the Helmholtz-Smoluchowski slip velocity. The liquid pumping using the CWEs could provide new method to biological studies with adoptable flow rates and larger ‘R. pumping zone’.