Chemically modified microelectrodes designed for the electrochemical determination of nitric oxide in biological systems

Since the identification of nitric oxide (NO) as an endothelial-derived relaxing factor, several research groups are now working to develop an understanding of the mechanism by which this free-radical gas is synthesized, diffused and reacted from and within various cells and different biological tissues. Thus, it appears that measuring NO in biological models is very difficult because of its low concentration and fleeting existence. Indeed, NO directly reacts extremely fast with superoxide and other very active free radicals of biological importance. Most of the techniques for assaying NO release use indirect methods for estimating endogeneous NO, relying on measurements of secondary species. In fact, NO in biological systems may be directly assayed by three nonelectrochemical strategies (EPR, spectrophotometry and chemiluminescence) but these strategies suffer from being ex situ detection techniques with poor selectivity or sensitivity. The desire to measure a small amount of NO release in situ has led to an active area of research involving the design of microsensors and new amperometric microelectrode probes are now developed to detect NO. The use of electrochemistry as a potential way to do so is very promising and the reality is that surface electrode modification is needed to make the electrode material selective for NO. Therefore, the design of modified electrode surfaces using organized layers combined with microelectrodes is attractive as possible selective microsensors for the electrochemical detection of NO. A number of new amperometric approaches have reached this goal. This article consists of a critical review of electrochemical methods for the detection and determination of nitric oxide in biological materials to clarify some aspects of the reported microsensors.