A Cu²⁺-specific colorimetric reversible fluorescent receptor HL was designed and synthesized which showed a naked eye observable colour change from colourless to pink on addition of an aqueous buffer (pH 7.4) solution of 30 ppb Cu²⁺. Short response time (≤5 s) and low detection limit (nearly 3 ppb) make HL suitable as a reliable “dip-in” open eye sensor for Cu²⁺. Bio-imaging application in live brine shrimp Artemia enabled HL to detect Cu²⁺ at as low as 10 ppb exposure.