Improved Planar Amperometric Nitric Oxide Sensor Based on Platinized Platinum Anode. 1. Experimental Results and Theory When Applied for Monitoring NO Release from Diazeniumdiolate-Doped Polymeric Films

An improved miniature amperometric nitric oxide sensor design with a planar sensing tip (ranging from 150 μm to 2 mm in diameter) is reported. The sensor is fabricated using a platinized platinum anode and a Ag/AgCl cathode housed behind a microporous poly(tetrafluoroethylene) (PTFE; Gore-tex) gas-permeable membrane. Platinization of the working platinum electrode surface dramatically improves the analytical performance of the sensor by providing ∼10-fold higher sensitivity (0.8-1.3 pA/nM), ∼10-fold lower detection limit (≤1 nM), and extended (at least 3-fold) stability (>3 d) compared to sensors prepared with bare Pt electrodes. These improvements in performance arise from increasing the kinetics and lowering the required potential for the 3-electron oxidation of NO to nitrate, relative to that observed using a nonplatinized working electrode. The outer porous PTFE membrane provides complete selectivity for NO over nitrite ions (up to 10 mM nitrite). The new sensor is applied for surface measurements of NO released from diazenium-diolate-loaded silicone rubber films (SR-DACA-6/N₂O ₂). The effects of sensor size (for sensor dimensions of 0.15-, 1-, and 2-mm o.d.) and the distance of the sensor from the surface of the NO-emitting polymer film are investigated via experiments as well as theoretical calculations. A significant analyte trapping effect is demonstrated, the degree of which depends on the sensor size and its distance from the surface. It is further demonstrated that surface NO concentrations for fresh SR-DACA-6/N₂O₂ loaded films are also influenced by the polymer film thickness, with thicker films generating higher surface concentrations of NO.