A rhodamine-based fluorescent sensor for rapid detection of Hg2+ exhibiting aggregation induced enhancement of emission (AIEE) in aqueous surfactant medium

Department of Chemistry, Jadavpur University, Jadavpur, Kolkata-700 032, India E-mail : m_ali2062@yahoo.com Fax : 91-33-24146223 Department of Molecular & Human Genetics Division, CSIR-Indian Institute of Chemical Biology, 4, Raja S. C. Mallick Road, Kolkata-700 032, India Manuscript received 20 June 2017, accepted 25 June 2017 An easily synthesizable rhodamine-based chemosensor, L₂ , selectively recognizes Hg²⁺ and Al³⁺ ions in the presence of all biologically relevant and toxic heavy metal ions. Very low detection limit (47 nM for Hg²⁺) along with cell permeability and negligible cytotoxicity provides a good opportunity towards cell imaging of Hg²⁺. SEM studies reveal rod-like microstructure for L₂ in water, which changes to a porous microstructure in the presence of Hg²⁺. It was interesting to note that the presence of SDS solubilized the otherwise insoluble probe in pure aqueous medium. In case of Al³⁺ it was astonishingly observed a fluorescence quenching on increasing the SDS concentration, while a ~33-fold enhancement of FI of [L₂ -Hg²⁺] complex was observed compared to that in the absence of SDS, making the prove selective towards Hg²⁺ over Al³⁺ in the aqueous SDS medium. In SDS/water system, there is a steep rise in FI, reaches a maximum at ~7 mM of SDS and then fluorescence intensity decreases gradually with the increase in [SDS] up to 28 mM. These observations clearly signify the SDS-assisted formation of polymer aggregates of the complex on the surface of monolayer of SDS formed in pre-micellar concentrations with higher FI, which is converted to the monomer being trapped inside the micellar cavity beyond the critical micellar concentration (cmc) with comparatively lower FI, indicating an interesting AIEE phenomenon. This proposition is further supported by the dependence of fluorescence anisotropy (r) on [SDS].