Morphology-Directing Synthesis of Rhodamine-Based Fluorophore Microstructures and Application toward Extra- and Intracellular Detection of Hg²⁺

A new, easily synthesizable rhodamine-based chemosensor with potential N₂O₂ donor atoms, L³, has been characterized by single-crystal X-ray diffraction together with ¹H NMR and high-resolution mass spectrometry (HRMS) studies. L³ was found to bind selectively and reversibly to the highly toxic Hg²⁺ ion. The binding stoichiometry and formation constant of the sensor toward Hg²⁺ were determined by various techniques, including UV–vis, fluorescence, and Job’s studies, and substantiated by HRMS methods. None of the biologically relevant and toxic heavy metal ions interfered with the detection of Hg²⁺ ion. The limit of detection of Hg²⁺calculated by the 3σ method was 1.62 nM. The biocompatibility of L³ with respect to its good solubility in mixed organic/aqueous media (MeCN/H₂O) and cell permeability with no or negligible cytotoxicity provides good opportunities for in vitro/in vivo cell imaging studies. As the probe is poorly soluble in pure water, an attempt was made to frame nano/microstructures in the absence and in the presence of sodium dodecyl sulfate (SDS) as a soft template, which was found to be very useful in synthesizing morphologically interesting L³ microcrystals. In pure water, micro-organization of L³ indeed occurred with block-shaped morphology very similar to that in the presence of SDS as a template. However, when we added Hg²⁺ to the solution of L³ under the above two conditions, the morphologies of the microstructures were slightly different; in the first case, a flowerlike structure was observed, and in second case, a simple well-defined spherical microstructure was obtained. Optical microscopy revealed a dotlike microstructure for L³–SDS assemblies, which changed to a panicle microstructure in the presence of Hg²⁺. UV–vis absorption and steady-state and time-resolved fluorescence studies were also carried out in the absence and presence of Hg²⁺, and also the SDS concentration was varied at fixed concentrations of the receptor and guest. The results revealed that the fluorescence intensity increased steadily with [SDS] until it became saturated at ∼7 mM SDS, indicating that the extent of perturbation to the emissive species increases with the increase in [SDS] until it becomes thermodynamically stable. There was also an increase in anisotropy with increasing SDS concentration, which clearly manifests the restriction of movement of the probe in the presence of SDS.