Highly selective 4-amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) chemosensors for Zn(II) under physiological pH conditions

The design and synthesis of two novel fluorescent sensors based on the photoinduced electron transfer ( PET) concept, 1 and 2, for the detection of zinc under competitive media is described. These sensors are based on the 4-amino-1,8-naphthalimide fluorophore, which has an absorption band centred at 450 nm and emits in the green with lambda(max) similar to 550 nm. By functionalizing the chromophore with a simple benzyl or ethyl-aryl based iminodiacetate receptor at the 4-position, both high selectivity and sensitivity were achieved for the sensing of Zn(II) over other competitive transition and Group I and II metal ions. These sensors were also shown to be pH independent, with a pK(a) of 2.3 being determined for 1, which allows these to be used in highly competitive pH media. Upon sensing of Zn( II) the fluorescence emission spectrum is 'switched on' demonstrating the suppression of PET from the receptor to the fluorophore. For 1, the sensing of Zn( II) was achieved with K-d = 4 nM when measured in pH 7.4 buffered solution, in the presence of 1.1 mM of EGTA.