<p>A single-step procedure was used to create a colorimetric sensor based on barbituric acid, which was used to detect metal ions (Hg²⁺). When Hg²⁺ ions was incorporated to CH₃OH medium of sensor <b>1</b>, a decrease in absorption intensity peak at 405&#xa0;nm accompanied with colour variation from colourless to dark yellow in visible light was noticed. The association coefficient was found to be of the order of 10⁶ M^− 1. Also, that there was steady growth in absorbance band at 405&#xa0;nm with the incorporation of cysteine in the solution of <b>1</b> + Hg²⁺ complex. The sensor <b>1</b> was also utilized for the recognition of Hg²⁺ ions in actual water samples. Also, owing to colorimetric alterations in visible light, the sensor <b>1</b> was employed in imperceptible ink applicability and smartphone Colorimetric App.</p>