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.