Since the discovery of nitric oxide (NO) as a vasodilatory messenger, in particular after its identification as an endothelial-derived relaxing factor (EDRF), there has been a mushroom effect in the research of NO in biological systems. Monitoring of NO in biological samples in vivo and in real time is desirable in many fields of NO research. Although several techniques are available for measurement of NO, the electrochemical method is most advantageous because of its speed and sensitivity. Since the first commercially available electrochemical NO detection system, many NO electrodes have been developed with dimensions from μm to cm, and with detection limits in the low nanomolar range. However, there is still a continuing demand for new NO sensors with lower detection limits. An electrochemical sensor to detect nitric oxide gas dissolved in solution as well as in gas phase is described as well as a fabrication method for the electrode. The sensor is based on the selective oxidation of nitric oxide by a multielectrode array of microelectrodes created on activated carbon, which has been deposited on a silicon chip substrate. The array, in turn, is further modified with several layers of cationic ion exchanger then the subsequent addition of NO selective membranes. The microchip NO sensor, is characterized by a linear response to concentrations of NO up to 100 μM, a response time of a few seconds, and a detection limit of less than 0.3 nM. In biological samples the sensor discriminates against substances such as nitrite, dopamine and ascorbic acid. Moreover, compared with present NO sensors, this sensor is significantly less temperature sensitive, resulting in significantly improved sensor detectivity as compared with present electrochemical sensors. Major applications for measurement of NO concentration are in chemical media, biological tissue, cell cultures, or in blood.