Nitric oxide, a reactive free radical, regulates a variety of biological processes. The absence of tools to detect NO directly, rapidly, specifically, and selectively motivated us to synthesize metal-based fluorescent probes to visualize the presence of NO. We prepared and investigated Co(II), Fe( II), Ru(II), Rh( II), and Cu(II) complexes as turn-on fluorescent NO sensors. Our exploration has provided insight into how the interaction of transition-metal centers with nitric oxide can be utilized for NO sensing.