Modified electrode approaches for nitric oxide sensing

Three different methods for the determination of nitric oxide (NO) in solution are described. These are based, respectively, on the use of a horseradish peroxidase (HRP) biosensor or on electrodes modified with films of redox-active transition metal complexes. In the case of the biosensor the enzyme was electrochemically immobilized onto a glassy carbon (GC) electrode. The activity of HRP is inhibited in presence of NO. Thus, the decrease in activity is correlated to the concentration of NO present in solution. The biosensor responds linearly over the range of 2.7x10(-6)-1.1x10(-5) M NO with a detection limit (5% inhibition) of 2.0x10(-6) M. In the case of chemically modified electrodes, particular emphasis is placed on materials capable of catalyzing the oxidation of NO. In terms of electrocatalyst, the discussion will centre on electrodeposited films of 6,17-diferrocenyldibenzo[b,i]5,9,14,18-tetraaza[14]annulen]-nickel(II) and indium(III) hexacyanoferrate(III). The resulting sensors exhibited potent and persistent electroacatalytic activity towards the oxidation of NO with low detection limits (1 muM) and good linear relationship between the catalytic current and NO concentrations. In addition, interference due to the presence of nitrate and nitrite have been significantly reduced. According to these results, the described modified electrodes have been used as sensors for the determination of NO generated by decomposition of a typical NO-donor, such as S-nitroso-N-acetyl-d,l-penicillamine (SNAP). A critical comparison of the various methodologies employed is made.