Planar ruthenium oxide sensors for cell-on-a-chip metabolic studies

While the use of iridium oxide electrodes for neuronal stimulation and pH sensing is continuously increasing, ruthenium oxide is a material less familiar in biomedicine. However, ruthenium oxide deposited by reactive sputtering glass or ceramic substrate with a platinum contact layer is shown to yield sensors well suitable for a extracellular recording of cell-mediated pH changes in cell culture media. Ruthenium oxide was directly grown with MCF-7 and L 929 cells and is fully biocompatible. The pH sensitivity ranges between 52 and 58 mV/pH, it is approximately linear between pH 5.5 and pH 11. The drift typically ranges between I and 2 mV h^-1. With increasing thickness of the ruthenium oxide layer, drift decreases and response time increases. A practicable tradeoffis a thickness of 1-2 μm. A pronounced redox cross-sensitivity of ruthenium oxide is a limiting factor in situations where concentration of dissolved oxygen is not constant or cannot be determined independently. The observed sensitivity to dissolved oxygen is 0.2 mV/hPa. Ruthenium oxide spots on ceramic sensor chips were used for an exemplary cell based assay with MCF-7 cells and showed a response of extracellular acidification rate to the alkaloid drug cytochalasin B.