A novel and simple fluorometric probe for mercury (II) ions based on chelation-enhanced emission and aggregate...

This study explored the molecular structure, photophysical properties, and sensing performance of ampicillin (Amp) as a fluorometric probe for detecting Hg²⁺ ions. The UV-Vis absorption spectrum of Amp solution in ultrapure water exhibited typical absorption bands at 215 and 260 nm, with optimal fluorescence (FL) emission at 430 nm when excited at 350 nm and a bright blue color under UV irradiation. The FL behavior was excitation wavelength-independent but sensitive to emission intensity, and remained stable at near-neutral pH values. Upon interaction with Hg²⁺, a noticeable enhancement in FL emission intensity was observed at low Hg²⁺ concentrations (≤30 μM), followed by gradual FL quenching at higher Hg²⁺ concentrations (≥30 μM) with aggregation and visible precipitation. The system shows good selectivity for Hg²⁺ over a wide range of other competing analytes, and a linear relationship between the FL intensity ratio (F/F₀) and Hg²⁺ concentrations was observed over the range of 0-10 μM, with an LOD of 5.9 nM. Mechanistic studies reveal that the FL enhancement at low Hg²⁺ concentrations is associated with chelation-enhanced FL (CHEF), whereas the emission quenching at higher concentrations arises from the heavy-atom effect and aggregation-induced processes. These results were supported by several spectroscopic and analytical evaluations for the interaction of the AmpHg²⁺ system, such as ¹H NMR, FTIR, UV-Vis, FL lifetime, binding constant calculation, Job's plot analysis, reversible binding study, visual observation, and morphological characterization. This study demonstrates that Amp solution can be used as a fluorometric probe for Hg²⁺ detection in aqueous systems with good sensitivity and selectivity.