S-nitrosothiol species (RSNOs) occur naturally in the body in very low concentrations. These stable species can be catalyzed to produce nitric oxide (NO), a biologically active molecule. Nitric oxide has important biological functions such as the inhibition of platelet activation and aggregation, and vasodilation. However, to continue studies on the function of RSNOs in the body, an easy way to measure the amount in the blood needs to be devised. Screen printed disposable carbon electrodes are utilized to amperometrically detect RSNO and NO. It is found that the current responses are proportional to the amount of NO added in testing solutions; however, they are also significantly influenced by the presence of interfering molecules (e.g. sodium ascorbate and sodium nitrate). To enhance the selectivity of such sensors, the surface of electrodes is modified with various polymeric coatings (e.g. Teflon) drop wise to create a selective membrane. The addition of Teflon AF is found to produce reproducible membrane while still maintaining an average detection limit for NO of below 35 nM compared to around 170 nM without modification. The selectivity for interfering species also increases. Initial studies are being conducted to detect RSNO by attaching an additional polymeric layer containing catalytic selenium over the electrode of improved selectivity to convert the RSNO into NO for detection. This will eventually lead to electrodes being able to detect the amount of endogenous RSNOs in an individual's blood possibly providing an early diagnostic system for those at risk for heart attack and stroke.