Ratiometric fluorescent detection of Cu(II) in semi-aqueous solution using a two-fluorophore approach

Schiff base sensor 1, containing naphthalene and naphthalimide fluorophores with separate and distinct emission wavelengths, showed good selectivity for Cu(II) over other tested physiological and environmentally important cations through changes in its fluorescence spectra in THF/H2O (9:1) HEPES buffered solution. By taking the ratiometric change of the emissions at 435 nm (naphthalene–Schiff base) and 510 nm (naphthalimide) good linearity was observed in the 0–10 μM range. The enhancement of the 435 nm emission upon binding Cu2+ was attributed to a prevention of the rapid CN isomerisation that otherwise leads to non-radiative decay, while the quenching of the naphthalimide emission was attributed to electron transfer between the excited naphthalimide fluorophore and the redox active Cu2+.