(308a) Micromachined Multielectrode Microprobes for Neurotransmitters with An On-Probe Iridium Oxide Reference

We are designing, producing, and implementing micromachined, multielectrode microprobes for the monitoring of neurotransmitter concentration changes in near real-time in the brains of live rodents.  Our devices may be used to investigate a variety of neurological disorders in vivo.  For example, glutamate and dopamine are neurotransmitters whose imbalance has been linked to Huntington’s disease and Parkinson’s disease as well as addiction.  We construct 150 microprobe devices simultaneously on four-inch silicon wafers with four, micron-sized electrodes per probe using micro-electro-mechanical-systems (MEMS) fabrication technologies.  One or more microelectrode sites per probe is chemically modified with permselective polymer films and immobilized glutamate oxidase for the electroenzymatic detection of glutamate with sensitivity of 120 nA × uM^-1 × cm^-2, detection limit of 2 uM, and 1 sec response time.  A second site may be modified with a thin layer of electrodeposited overoxidized polypyrrole for sensing of dopamine (DA) with sensitivity of 3250 nA × uM^-1 × cm^-2, detection limit of 60 nM and 1 sec response time.  Both sensors are highly selective against the array of electrooxidizable species normally present in brain extracellular fluid.  The electrodeposition of an iridium oxide (IrOx) film on a third site provides an on-probe reference.  Such an on-probe reference eliminates the need for a separate reference electrode, which traditionally has been a macroscopic Ag/AgCl wire electrode that causes inflammation of living tissue.  We have observed the additional benefit of a ~60% decrease in signal noise in vivo using the on-probe IrOx reference.  Although the IrOx reference exhibited pH-dependence of potential corresponding to -77 mV/pH, the extracellular pH in brain tissue normally is maintained tightly over the range of ~7.35 to 7.45.  Thus, our microprobes with IrOx reference were able to detect successfully a physiologically relevant range of glutamate concentrations in the presence of interferents (e.g., ascorbic acid, dopamine) both in vitro and in vivo.