Naphthalimide-Piperazine Derivatives as Multifunctional “On” and “Off” Fluorescent Switches for pH, Hg(2+) and Cu(2+) 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 pK(a) values at 4.98 and 2.91, respectively. Amongst heavy metal ions only Cu(2+) and Hg(2+) had a significant effect on the spectroscopic properties. The fluorescence of NI-1 is quenched in the presence of either Cu(2+) or Hg(2+) which is attributed to the formation of 1:1 metal-ligand complexes with binding constants of 3.6 × 10(5) and 3.9 × 10(4), respectively. The NI-1 chemosensor can be used for the quantification of Cu(2+) 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 Hg(2+) 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 Hg(2+) ions. Here, the formation of a complex with Hg(2+) (binding constant 8.3 × 10(3)) turns the green fluorescence of NI-2 into the “on” state. NI-2 showed remarkable selectivity towards Hg(2+) ions, allowing for determination of Hg(2+) concentration over a linear range of 1.3 μM to 25 μM and a limit of detection of 4.1 × 10(−7) M.