Nanodiamond macroelectrodes and ultramicroelectrode arrays for bio-analyte detection

Abstract

Properties such as high electrical conductivity; chemical and electrochemical stability over a wide range of conditions; rapid electron transfer kinetics for different redox systems; and reproducible electrical, microstructural and chemical properties are essential pre-requisites for electrochemical electrodes. Nanodiamond is one such electrode material which has been shown to be suitable for electroanalytical applications. In this dissertation, MPECVD (Microwave Plasma Enhanced Chemical Vapor Deposition) process and conventional silicon microfabrication technology has been used for synthesis and fabrication of nanodiamond thin-film macroelectrodes and ultramicroelectrode arrays (UMEAs) with different geometries- pyramidal, planar and columnar. This work combines the excellent material properties of nanodiamond with the benefits of using UMEAs- higher analyte flux density, reduced ohmic losses and higher temporal resolution. Scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy were used for material characterization. Electrochemical performance was analyzed using ferri/ferrocyanide redox couple and essential bio-analytes such as Dopamine, Ascorbic Acid (AA) and Uric Acid (UA) in phosphate buffered saline (PBS). To simulate true physiological conditions, detection and quantification of dopamine in presence of interferants such as AA and UA was also examined, in-vitro. In addition, effects of the input gas mixture, i.e., hydrogen, methane and nitrogen, on nanodiamond macroelectrode properties were evaluated and optimized. Increase in nitrogen gas flow produced distinct changes in the nanodiamond microstructure and electrochemical response. The fabrication processes and electroanalytical performance, under steady state conditions, of the UMEAs with different geometries were compared to identify their respective drawbacks and new fabrication processes were developed to overcome them in order to consistently produce more reliable bio-sensors. The sensitivity of the UMEAs varied inversely with their dimensions. Neurotransmitter concentrations vary at sub-second time-scale and can only be monitored and quantified by using an electrode with high temporal resolution and fast electron transfer kinetics. Hence, ‘background subtracted fast scan cyclic voltammetry’ for Dopamine detection using planar nanodiamond UMEA was implemented. All the results from electrochemical characterization were achieved without any surface functionalization and/or modification.