A low affinity nanoparticle based fluorescent ratiometric probe for the determination of Zn( ii ) concentrations in living cells

A ratiometric polymeric fluorescent probe for Zn( ii ) was developed comprising a Zn( ii ) responsive naphthalimide sensing unit and a Zn( ii ) insensitive rhodamine based calibration fluorophore attached to a common poly(methyl methacrylate) backbone. The Zn( ii ) responsive unit was designed to operate according to the photoinduced electron transfer (PET) mechanism with the naphthalimide fluorescence intensity increasing with increasing Zn( ii ) concentration. The amphiphilic polymeric probe was found to self-assemble in aqueous solution to generate nanoparticles with a hydrodynamic diameter of 10.61 ± 7.61 nm. Good selectivity for Zn( ii ) was observed when tested against a range of physiological relevant cations and excellent linearity ( R 2 = 0.99) was demonstrated in the 0–5.0 mM Zn( ii ) range when the ratiometric intensity ( I 527 / I 580 ) was plotted as a function of Zn( ii ) concentration. The probe was also capable of discriminating between resting and high Zn( ii ) levels in living cells using confocal fluorescence microscopy and exhibited no evidence of cellular toxicity. Combined, these results highlight the potential of the nanoparticle probe as a low-affinity ratiometric sensor for Zn( ii ) compatible with use in living cells.