Colorimetric and fluorimetric sensing of trace amount of Hg²⁺ in real samples by a new rhodamine B-based Schiff base probe: Smartphone, living cell applications and DFT calculation

In this study, a Schiff base-based probe, named as 2-((methyl benzoate-4-yl)methyleneamino)-3′,6′-bis(diethylamino)spiro[isoindoline-1,9′-xanthene]-3-one (2), was successfully synthesized and characterized by various spectral techniques besides single-crystal x-ray diffraction. The investigation was focused on the analyzing the selectivity of the probe towards some common metal ions. The study was conducted in an aqueous medium, and the results indicated that the probe showed high selectivity towards the Hg²⁺ ion. The utilization of mass analysis revealed that the optical emission originating from rhodamine ethyl ester can be generated through the spirolactam ring/Schiff base hydrolysis facilitated by the Hg²⁺ ion. The Limit of Detection (LOD) for the Hg²⁺ ions by colorimetry and fluorometry were determined to be 3.0 × 10^-8 M and 9.4 × 10^-9 M, respectively. A technique using a smartphone was quickly applied to analyze Hg²⁺ in real samples with good recovery rates (96.5%-101.5%). To explain their 3-dimensional geometries and the electronic properties, the DFT calculations were also performed on 2, its spiro bond opened Hg-complex (3-Hg), Hg²⁺ uncomplexed product (3) and the rhodamine ethyl ester hydrolysis product. Moreover, mercury detection potency of the probe at the cellular level was investigated on adenocarcinoma alveolar basal epithelial (A549) cancer cells, and the result showed that the probe exhibits high potential for living cell imaging.