An apparatus utilizes miniaturized surface plasmon resonance (SPR) and ion-selective self-assembled monolayer (SAM) and hydrogel chemistry to measure metal ion concentrations in liquids. The SPR optical system is packaged in a compact and cost-effective form factor. An electronic circuit drives the optical system. The SPR system utilizes an optical window that is coated with the SAM layer or hydrogel material. The SAM layer and hydrogel materials are highly selective to a specific metal ion of interest. The miniaturized SPR sensor is situated in an optical-fluidic cell or an optical-fluidic manifold with the SAM layer or hydrogel material in contact with the liquid. Metal ions selectively attach to the SAM layer or hydrogel material, thereby affecting the SPR signal. Changes in the SPR signal are used to accurately determine the metal ion concentration in the liquid. The liquids may be either static or dynamic.