Molecular recognition of carbonate ion using a novel turn-on fluorescent probe

A novel turn-on fluorescent probe 3 was synthesized by condensing salicylaldehyde and nicotinic hydrazide for the selective detection of CO₃^2- in aqueous medium. Probe 3 exhibited a turn-on fluorescence response toward CO₃^2- with excellent selectivity, sensitivity (DL = 2.76 μM), and good reversibility. The binding constant (K) of probe 3 with CO₃^2- was calculated to be 5 × 10³ M^-1 (log K 3.69). The 1:1 stoichiometry of the complex between probe 3 and CO₃^2- ions was confirmed by Job's plot and ESI-MS spectra. Deprotonation and hydrogen-bonding interactions are involved in the recognition of CO₃^2- ion, which was also suggested by ¹H NMR, ESI-MS spectra, and Density Functional Theory (DFT) calculations. Moreover, an INHIBIT type molecular logic gate was constructed by using 3:CO₃^2- and CH₃COOH as inputs and current signal as output. Owing to the practical applications, probe 3 demonstrated its efficiency in quantifying CO₃^2- ion in real water samples through standard addition method, thus showcasing its potential in real environment. Further, the MTT assay indicated very low cytotoxicity (IC₅₀ = 1 mM) of probe 3 and also the cell imaging experiments demonstrated the effective sensing of CO₃^2- ions with probe 3 in the biological systems.