Colorimetric detection of Co ions by a poly(vinyl alcohol)-based hydrogel using color coordinate

An organic-inorganic hybridized, poly(vinyl alcohol) (PVA)-based hydrogel was used to detect Co ions in aqueous medium. As a sensing probe, 5-dimethylamino-2-(2-pyridylazo)phenol (DMAP) was immobilized in the hydrogel. The hydrogel was prepared by the freeze-thawing method after a sol-gel reaction of DMAP, tetraethyl orthosilicate, and PVA. The color of the DMAP-PVA hydrogel changed from yellow to purple upon exposure to Co ions, the color change of the hydrogel was quantified through colorimetric analysis according to the concentrations of Co ions. The red, green, and blue (RGB) values were extracted from the photographic images of the hydrogel obtained by a stereoscopic microscope, and the RGB% and H values were calculated through the RGB values. The RGB% of the DMAP-PVA hydrogel increased with increasing Co ion concentration. In addition, the DMAP-PVA hydrogel did not show significant changes in RGB% values upon exposure to other metal ions, indicative of selectivity. For quantitative measurement of the Co ion concentration, the H value was used because it decreased linearly with increasing Co ion concentration. The DMAP-PVA hydrogel could easily detect Co ions with good specificity, easy synthesis, and portability. The intense color change was visually detected upon exposure to Co ions, even on a dried solid surface, as well as in aqueous solutions, indicating potential use in a nuclear facility.