Investigation of Changes in the Surface Chemistry and Morphology of Platinum Microelectrodes Subjected to a Dielectrophoretic Field

Aytug Gencoglu1, Emily F. Cotten1, S. Anell Pullen1, Sarah Thompson2, B. Selin Tosun3, and Adrienne R. Minerick1. (1) Dave C. Swalm School of Chemical Engineering, Mississippi State University, Box 9595, Mississippi State, MS 39762, (2) Mississippi School for Math and Science, Mississippi University for Women, 1100 College Street, Columbus, MS 39701, (3) Metallurgical and Materials Engineering Department, Istanbul Technical University, ITU Kimya-Metalurji Fakultesi, PK: 34469, Maslak, Istanbul, Turkey

Previous studies have suggested that the performance of platinum microelectrodes, a metal widely regarded as inert at microdevice operating voltages, tended to improve in performance in medical microdevices as the microelectrodes aged. Early studies of our microelectrodes were carried out over 4 hours in nonuniform AC fields similar to those utilized in our group's medical microdevices in approximately 0.5 mL of a phosphate saline buffer solution (PBS) at pH 6.7. The nonuniform electric field was generated by application of a 1kHz, 6 VPP signal. Morphological changes that could be pitting or deposition of material were visualized by SEM and gas formation at the electrodes was observed in real time by optical microscopy. Oxidation of platinum on the microelectrode surface was detected by XPS. Oxidation of platinum has been reported before, and recent studies have reported the reduction of platinum by SECM-generated radical anions. However, the previous studies characterized regions whose area was 9 microns squared and 10000 microns squared whereas our work has looked at the macroscopic scale at the order of mm2 on 100 micron diameter platinum microelectrodes 3 cm in length. Here we present the findings of systematic experiments aimed at finding out the source of the chemical and morphological changes on platinum microelectrode surfaces. The morphological and chemical changes were determined as a function of the frequency of the electric field, the exposure duration of the microelectrodes in the electric field, and the chemical species present in the buffer solution. These effects were systematically explored by running fresh microdevices under various conditions and comparing the surface chemical compositions of the platinum microelectrodes by XPS and EDS and their surface morphologies by SEM and XRD.