Naphthalimide-Piperazine Derivatives as Multifunctional “On” and “Off” Fluorescent Switches for pH, Hg2+ and Cu2+ Ions
2023
Kristina Pršir | Mislav Matić | Marlena Grbić | Gerhard J. Mohr | Svjetlana Krištafor | Ivana Murković Steinberg
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 &ldquo:on&rdquo:) 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 &mu:M and a detection limit of 1.5 ×: 10&minus:8 M. The linear range for the determination of Hg2+ is from 2 &mu:M to 10 &mu:M, with a detection limit of 8.8 ×: 10&minus: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 &ldquo:on&rdquo: state. NI-2 showed remarkable selectivity towards Hg2+ ions, allowing for determination of Hg2+ concentration over a linear range of 1.3 &mu:M to 25 &mu:M and a limit of detection of 4.1 ×: 10&minus:7 M.
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