We developed a multimode Hg²⁺ selective sensor system based on carbazole-barbituric acid conjugate (CBA). Its colorimetric and fluorescent behavior for Hg²⁺ ions in aqueous medium was investigated. Upon Hg²⁺ binding, CBA exhibited significant change in the absorption spectrum along with an enhanced emission at 593 nm. This allows the selective and sensitive detection of Hg²⁺ without any interference from other metal ions.