1388-2481/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII S1388-2481 ( 99 ) 00056-9 www.elsevier.nl/locate/elecom Electrochemistry Communications 1 (1999) 274–277 Oxidation of NO mediated by water-soluble iron porphyrin Jingyuan Chen a, *, Osamu Ikeda a,1 , Takashi Hatasa a , Akira Kitajima b , Mikio Miyake b , Atsushi Yamatodani c a Department of Chemistry, Faculty of Science, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan b Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsuno kuchi, Ishikawa 923-1100, Japan c Faculty of Medicine, Osaka University, 1-7 Yamadaoka, Suita, Osaka 565-0871, Japan Received 6 May 1999; received in revised form 7 June 1999; accepted 9 June 1999 Abstract An anodic voltammetric wave of NO catalyzed by meso-tetra(N-methyl-4-pyridyl) iron(III) pentachloride ([Fe III (TMPyP)] 5q ) was found in a phosphate buffer solution (pH 7.4). The current was 10 times larger than the diffusion-controlled current of NO without the iron porphyrin. The current can be used to detect NO in aerobic physiological environments. Spectroelectrochemical measurements suggested the formation of iron-nitrosyl complex, which is responsible for the catalytic oxidation of NO. The intermediate of the catalytic oxidation was detected by spectroelectrochemistry. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Nitric oxide; Iron porphyrins; Nitrosyl complexes; Catalytic oxidation; Spectroelectrochemistry; Water-soluble porphyrins 1. Introduction Nitric oxide in vivo plays regulatory roles in physiological processes, such as smooth muscle relaxation, anti-platelet aggregation and neurotransmission [1,2]. In order to get biological functions of NO in perspective, analytical tech- niques for detecting NO have been developed in biological systems [3–10]. An interesting and powerful method is the electrochemical technique by use of a Clark-type electrode, which has the possibility of in vivo monitoring of NO [11– 13]. Since NO is reduced at potentials more negative than the reduction potential of oxygen, it is difficult to detect it under aerobic conditions. Shibuki [14,15] found that NO was detected by electrochemical oxidation through a thin polymer membrane distinguished from oxygen. Unfortu- nately, there are drawbacks in physiological application; the nonlinear relation between the current and NO concentration as well as a lack of stability to prolonged NO exposure [16,17]. The drawbacks can be overcome by taking advantage of metal porphyrins as catalysts [18–23]. Although porphyrin catalysts have been applied to the detection of NO in blood, they were unstable in use as sensors [21–24]. Hayon et al. [13] used [Fe III (TMPyP)] 5q as an immobilized catalyst in * Corresponding author. Tel.: q81-76-264-5769; fax: q81-76-264-5988; e-mail: tsuen@sgkit.ge.kanazawa-u.ac.jp 1 Also corresponding author. the Nafion film for detecting NO. Bedioui et al. [25] studied the reactivity of [Fe III (TMPyP)] 5q -modified electrode with NO. However, the reaction mechanism is still unclear. In this work, we report the electrooxidation mechanism of NO cat- alyzed by [Fe III (TMPyP)] 5q in a phosphate buffer solution by the spectroelectrochemical technique. 2. Experimental 2.1. Materials All aqueous solutions were prepared from twice-distilled water. Salts for phosphate buffer solution (PBS) were of analytical reagent purity. [Fe III (TMPyP)]Cl 5 was prepared by the methods of Pasternack et al. [26] and Bedioui et al. [25]. The product was purified by chromatography with a help of an anion exchange resin. UV–Vis data; l max (log e): 422 (4.82), 490 (3.78) and 597 (3.66) nm in 50 mM PBS of pH 7.4. All the solutions were deoxygenated by bubbling ultra- pure argon gas for 15 min. Saturated NO solutions were prepared by bubbling a mixture of argon and NO gas for 30 min into a deoxygenated solution before each electrochemical and spectroscopic run. The molar concentration of NO in the solution was evaluated from Ostwald’s solubility coefficient [27] for a given partial pressure of NO.