Naphthalimide-Piperazine Derivatives as Multifunctional 'On' and 'Off' Fluorescent Switches for pH, Hg2+ and Cu2+ Ions

Novel 1,8-naphthalimide-based fluorescent probes NI-1 and NI-2 were designed and screened for use as chemosensors for detection of heavy metal ions. Two moieties, methylpyridine (NI-1) and hydroxyphenyl (NI-2), were attached via piperazine at the C-4 position of the napthalimide core resulting in a notable effect on their spectroscopic properties. NI-1 and NI-2 are pH sensitive and show an increase in fluorescence intensity at around 525 nm (switch “on”) in the acidic environment, with pKa values at 4.98 and 2.91, respectively. Amongst heavy metal ions only Cu2+ and Hg2+ had a significant effect on the spectroscopic properties. The fluorescence of NI-1 is quenched in the presence of either Cu2+ or Hg2+ which is attributed to the formation of 1:1 metal-ligand complexes with binding constants of 3.6 × 105 and 3.9 × 104, respectively. The NI-1 chemosensor can be used for the quantification of Cu2+ ions in sub-micromolar quantities, with a linear range from 250 nM to 4.0 μM and a detection limit of 1.5 × 10−8 M. The linear range for the determination of Hg2+ is from 2 μM to 10 μM, with a detection limit of 8.8 × 10−8 M. Conversely, NI-2 behaves like a typical photoinduced electron transfer (PET) sensor for Hg2+ ions. Here, the formation of a complex with Hg2+ (binding constant 8.3 × 103) turns the green fluorescence of NI-2 into the “on” state. NI-2 showed remarkable selectivity towards Hg2+ ions, allowing for determination of Hg2+ concentration over a linear range of 1.3 μM to 25 μM and a limit of detection of 4.1 × 10−7 M.