Electrode materials for nitric oxide detection

Nitric oxide oxidation signals were compared for uniform test electrodes of platinum, iridium, palladium, rhodium, ruthenium, gold, graphite, and a nickel-porphyrin on graphite in deaerated phosphate-buffered saline (pH 7.0) at 35 degrees C. All tested materials detected NO(*) amperometrically. Current densities (A/M/cm(2) +/- S.D.) were Ir (0.021 +/- 0.002), Rh (0.088 +/- 0.012), graphite (0.117 +/- 0.018), Pd (0.118 +/- 0.033), Au (0.149 +/- 0. 039), Pt (0.237 +/- 0.117), Ni (II)-tetra(3-methoxy-4-hydroxyphenyl) porphyrin on graphite (0.239 +/- 0.009), and Ru (0.680 +/- 0.058). NO(*) oxidation current on ruthenium was maximal at 675 mV (vs Ag/AgCl), nearly three times that on the next-best materials, platinum and Ni-porphyrin on graphite poised at 800 mV. The measured limit of detection for NO(*) on Ru was below 3 nM. Enhanced NO(*) oxidation current on ruthenium is apparently due to formation of nitrosyl- or chloronitrosyl-ruthenium complexes at the electrode surface. At fixed potentials above 675 mV, ruthenium exhibited an even larger NO(*) response, characterized by current flow opposite in polarity to an oxidation, which we hypothesize reflects suppression of the oxidative background current (presumably due to chloride oxidation or to the electrolysis of water) by a film consisting of nitrosyl- or chloronitrosyl-ruthenium complexes. The sensitive response of the ruthenium electrode to the direct oxidation of NO(*) may be useful in sensors for biomedical applications.