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

Abstract

Novel 1,8-naphthalimide-based fluorescent probes <b>NI-1</b> and <b>NI-2</b> were designed and screened for use as chemosensors for detection of heavy metal ions. Two moieties, methylpyridine (<b>NI-1</b>) and hydroxyphenyl (<b>NI-2</b>), were attached via piperazine at the C-4 position of the napthalimide core resulting in a notable effect on their spectroscopic properties. <b>NI-1</b> and <b>NI-2</b> are pH sensitive and show an increase in fluorescence intensity at around 525 nm (switch "on") in the acidic environment, with p<i>K</i>_a 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 <b>NI-1</b> 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 <b>NI-1</b> 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, <b>NI-2</b> 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 <b>NI-2</b> into the "on" state. <b>NI-2</b> 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.