Functionalized Schiff-base containing phenothiazine and cholic acid as a paper-based fluorescence turn-off sensor for Sn(ii) ion detection and its application

Abstract

Employing a one-pot Schiff-base condensation approach, the derivative “cholyl hydrazide Schiff base of phenothiazine” (ChHSB-PTZ) based on cholic acid has been synthesized and characterized via standard procedures. ChHSB-PTZ demonstrates exceptional sensitivity towards Sn(II) metal ion detection through fluorescence “turn-off” behavior based on chelation-enhanced fluorescence quenching (CHEQ) even in the presence of various cations and anions. Through modified Stern–Volmer and Stern–Volmer graph analyses, the binding constant (K_a) and limit of detection (LOD) were found to be 2.8 × 10¹⁰ M⁻¹ and 2.2 nM, respectively. A 1 : 1 stoichiometry between ChHSB-PTZ and the Sn(II) metal ion was found via Job's plot. Moreover, the distribution of electronic states in the ChHSB-PTZ Schiff base and the sensing mechanism (CHEQ) were clarified through the use of ¹H NMR titration spectroscopy and density functional theory (DFT) calculations. ChHSB-PTZ test paper strips are developed as an essential tool to improve the analytical applicability of the device and enable the rapid, accurate, and affordable detection of Sn(II) metal ions in environmental analysis.