In this study, a tetradentate Schiff base ligand L was synthesized by condensing o-phenylenediamine with two moles of pyridoxal. The solution of L in EtOH/DMSO showed aggregation-induced emission (AIE) behaviour with the addition of water. The weakly emissive L in EtOH/DMSO at 474/442 nm showed a blue-shifted fluorescence enhancement at 380 nm with the addition of water/HEPES buffer. The aggregation of L was supported by DLS and SEM analyses. The blocking of CN isomerization due to the restriction of intramolecular rotation (RIR) upon aggregation converts the weakly emissive L into a highly blue-fluorescent L. The aggregates of L in a mixed EtOH:HEPES buffer (5% EtOH, pH = 7.4) medium was explored for the fluorescence sensing of metal ions. The addition of Cu²⁺ formed a complex with L in a 1 : 1 binding ratio, which quenched the fluorescence of L due to the paramagnetic nature of Cu²⁺ and the inhibition of ESIPT. The binding constants estimated for the complex L–Cu²⁺ by UV-vis and fluorescence techniques were 1.36 × 10⁴ M⁻¹ and 3.63 × 10⁴ M⁻¹, respectively. The quenched fluorescence of L was restored with the addition of a strong chelating agent, EDTA. The reversible fluorescence turn-off sensor L can be applied to detect Cu²⁺ down to 5.31 × 10⁻⁷ M. The test paper strips of L were developed for the visual detection of Cu²⁺. The practical applicability of L was demonstrated by monitoring Cu²⁺ in real environmental water samples. Furthermore, the L–Cu²⁺ complex generated in situ was applied for the selective detection of sulphide anions among the other examined anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, CN⁻, N₃⁻, NO₂⁻, ClO₄⁻, HSO₄⁻, and H₂PO₄⁻).