A simple epoxy based polymer 1 bearing a triphenylamine unit is shown to be an on–off type fluorescent sensor for Hg²⁺ ions in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0) with high sensitivity and selectivity. In the sensing process, other metal ions such as Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ag⁺ did not interfere.