Current methods for monitoring nitrogen monoxide (NO) and new strategies for designing NO probes are reviewed. The production of endogenous NO must be monitored if we are to understand the physiological and pathological roles of NO. Many methods have been reported for this purpose, including colorimetry, fluorometry, electrochemical methods, and electron paramagnetic resonance (EPR). Among these methods, one of the most promising for practical use is EPR using (dithiocarbamato)iron(II) complex as a spin trap. In the structure-sensitivity relationship, it was found that an electron-donating group on the dithiocarbamato ligand is favorable for enhancing the sensitivity of NO detection due to the stabilization of the Fe(II) complex form. A new concept, "radical-exchange", is also introduced to solve the issue of low sensitivity in EPR methods. This concept has been used to develop two types of NO probe: spin trap and fluorescent probes. These reagents detect NO to a detection limit of 10 nM-sub-μM. This strategy, using radical exchange in conjunction with EPR, is potentially useful for the design of sensitive probes that can detect NO directly.
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