⁶⁰Co is a radioactive isotope of ⁵⁹Co and is widely used in industrial and medical fields such as nondestructive testing, food storage and radiotherapy. However, radioisotopes can cause serious environmental and human health problems. Therefore, detection of radioactive isotopes such as ⁶⁰Co is very important. In this study, a simple and easy-to-use hydrogel sensor for the colorimetric detection of cobalt ions was prepared. The PVA-based hydrogels were prepared by repeated freeze-and-thaw cycles of a mixed solution containing PVA, 5-dimethylamino-2-(2-pyridylazo)phenol and tetraethylorthosilicate. The concentration of Co ions in the aqueous solution was measured using the color changes of the hydrogels. An orange-colored hydrogel turned purple when exposed to cobalt ions. The color changes of the hydrogels were quantified through RGB values obtained by Photoshop software.