Development of electrochemical sensors based on carbon interdigitated array nanoelectrodes for high current amplification

Abstract

The feasibility of carbon interdigitated array (IDA) nanoelectrodes as low-cost and highly sensitive electrochemical sensing devices was developed and the functionality of the sensors was tested in this study. The carbon nanoelectrodes were fabricated using Carbon MEMS (C-MEMS) process. This versatile microfabricaton technology enables the simple fabrication of carbon nanostructures using only conventional batch microfabrication processes such as photolithography and pyrolysis. C-MEMS process has advantages including mass production, simple process, low cost and controllable design. This thesis consists of two main research topics. First, the fabrication and electrochemical sensing capability of 1:1 aspect ratio carbon IDA nanoelectrodes (width ~ 300 nm, thickness ~ 340 nm) will be demonstrated. The electrochemical sensing capability of IDA electrodes can be evaluated by measuring amplification factor and collection efficiency in bulk solution. The architecture of tightly spaced nanoelectrodes enables recycling of redox species between two sets of electrodes resulting in electrochemical current signal enhancement. 25 of maximum amplification factor and 98% of collection efficiency were achieved by the 1:1 aspect ratio carbon IDA nanoelectrodes. The effects of the geometry of IDA electrodes on the current amplification were investigated. The electrode-to-electrode gap, width, height of carbon IDA nanoelectrodes are dominant parameters determining the efficiency of redox recycling. The diffusion of the redox species, which are reacted at the electrode surfaces, to the bulk solution depends on the gap and height of the electrodes. The width of the electrode determines density of the electrodes. In the second part of thesis, the effect of the confinement of redox species in a microchannel on the electrochemical sensing capability of carbon IDA nanoelectrodes embedded in the channel will be discussed. Carbon IDA nanoelectrodes (width~650 nm, thickness~650 nm) were simply integrated in a polydimethylsiloxane (PDMS) microchannel by simply bonding a PDMS layer with a channel slot on top of the substrate containing the IDA nanoelectrodes. The diffusion of the redox species confined in the microchannel was simulated for the study of redox cycling efficiency. The simulation showed that the diffusion-limited current collected from the IDA nanoelectrodes in a microchannel does not change significantly as the microchannel height reduces as long as the channel is high enough not to hinder the radial diffusion of the nanoelectrodes. This simulation study was confirmed by measuring redox currents of the carbon IDA nanoelectrodes embedded in microchannels of various sizes. The current could be amplified up to 250 and 1116 times in cyclic voltammetry and chronoamperometry respectively because of deficiency of redox species in the single mode where no redox cycling effect occurs. The feasibility of the carbon IDA carbon nanoelectrodes as biosensors was tested by measuring the concentration of dopamine.