A new 'naked-eye' quinoline-based 'reactive' ratiometric fluorescent probe was prepared. The reactive stoichiometry of the probe with Hg²⁺ ion was 2:1. The probe exhibited high selectivity towards Hg²⁺ ion to other metal ions with a 410-fold increase in absorbance intensity ratio (A₄₀₂/A₃₄₀) in aqueous solution over a wide-range pH value (2-12), accompanied by a resonance color change from colorless to pale yellow visible to naked-eye.