Fluorescent Sensor Based on 1H-Pyrazolo[3,4-b]quinoline Derivative for Detecting Zn²⁺ Cations

The photophysical and sensory properties of the donor-acceptor pyrazoloquinoline derivative (PQPc) were investigated using absorption, steady-state, and time-resolved fluorescence measurements. The compound synthesized from commercial, readily available substrates exhibited absorptions in the UV-Vis range, with a maximum of the longwave band around 390 nm. The maximum fluorescence was around 460-480 nm, depending on the solvent. The quantum yield was between 12.87% (for n-hexane) and 0.75% (for acetonitrile) and decreased with increasing solvent polarity. The PET mechanism was implicated as the cause of fluorescence quenching. Divalent ions such as Zn²⁺, Pb²⁺, Cd²⁺, Ca²⁺, Mg²⁺, Co²⁺, Ni²⁺, and Cu²⁺ were introduced to study the fluorescent response of PQPc. A 13-times increase in fluorescence quantum yield was observed after the addition of Zn²⁺ ions. Detailed research was carried out for the PQPc-Zn²⁺ system in order to check the possibility of analytical applications of PQPc as a fluorescent sensor. A detection limit of Zn²⁺ was set at the value level 1.93 × 10^-7 M. PQPc-Zn²⁺ complexes had a stoichiometry of 1:1 with a binding constant of 859 M^-1. Biological studies showed that the sensor was localized in cells near the membrane and cytoplasm and may be used to detect zinc ions in eukaryotic cells.