To evaluate the analytical performance of the NO sensors, cyclic voltammetric and amperometric measurements were performed using a CH Instruments 730B bipotentiostat (Austin, TX). The electrode assembly (3-electrode configuration) consisted of a xerogel-modified Pt working electrode (2-mm diameter), a Pt-coiled counter electrode (0.6-mm diameter), and a Ag/AgCl reference electrode (3.0 M KCl; CH Instruments).
Two standard NO solutions (1.9 mM and 41 nM) were prepared by purging phosphate-buffered saline (PBS; 0.01 M, pH 7.4) with Ar for 30 min to remove oxygen, then NO (99.5% and 24.1 ppm) for 30 min. (seeSupporting Information for detailed calculations).31 (link),40 (link) The NO gas was purified before use by passing it through a column packed with KOH pellets to remove trace NO degradation products. The CO solution (0.9 mM) was similarly prepared by successively purging PBS with Ar for 30 min and CO (99.5%) for another 30 min.56 (link) (Caution! The NO and CO purging process must be carried out in a fume hood since NO and CO gases are toxic!)45 Solutions of NO and interfering species (e.g., nitrite, ascorbic acid, uric acid, acetaminophen, dopamine, ammonia/ammonium, and carbon monoxide) were prepared fresh every 2 d and stored at 4 °C. All sensors were pre-polarized for at least 30 min and tested in deoxygenated PBS (prepared by purging with N2) at room temperature with constant stirring. Electrooxidation currents of NO and interfering species were recorded at an applied potential of +0.8 V (vs Ag/AgCl) (see Supporting Information ). Sensors were stored in PBS at room temperature between measurements.
To determine the resistance of the xerogel film, AC impedance spectroscopy was performed in PBS (0.01 M, pH 7.4) using a xerogel (10-μm thick)-modified Pt working electrode (2-mm diameter) and a Ag/AgCl reference electrode (3.0 M KCl). A Ensman Instrumentation 400 Potentiostat (Bloomington, IN) was used to apply a 1000 Hz, 20 mV sinusoidal wave to the working electrode. Potentiometry measurements (CH Instruments 730B Biopotentiostat) were also conducted to examine the influence of lipophilic cations and anions on the boundary potential generated at the xerogel/sample interface. The potential between the xerogel-coated Pt and reference electrodes was monitored before and after the addition of lipophilic cations or anions (i.e., tetrabutylammonium, cholate, and thiocyanate) at concentrations up to 1 mM in PBS (0.01 M, pH 7.4).
Two standard NO solutions (1.9 mM and 41 nM) were prepared by purging phosphate-buffered saline (PBS; 0.01 M, pH 7.4) with Ar for 30 min to remove oxygen, then NO (99.5% and 24.1 ppm) for 30 min. (see
To determine the resistance of the xerogel film, AC impedance spectroscopy was performed in PBS (0.01 M, pH 7.4) using a xerogel (10-μm thick)-modified Pt working electrode (2-mm diameter) and a Ag/AgCl reference electrode (3.0 M KCl). A Ensman Instrumentation 400 Potentiostat (Bloomington, IN) was used to apply a 1000 Hz, 20 mV sinusoidal wave to the working electrode. Potentiometry measurements (CH Instruments 730B Biopotentiostat) were also conducted to examine the influence of lipophilic cations and anions on the boundary potential generated at the xerogel/sample interface. The potential between the xerogel-coated Pt and reference electrodes was monitored before and after the addition of lipophilic cations or anions (i.e., tetrabutylammonium, cholate, and thiocyanate) at concentrations up to 1 mM in PBS (0.01 M, pH 7.4).
