Tuning Fluorescence Properties of BIM Probes via Clay Integration: Targeting Rifampicin with Improved Selectivity

The incorporation of photoactive organic dyes into layered inorganic materials substantially improves the optical and chemical properties, rendering them highly suitable for sensing applications. Therefore, we integrated Bisindolyl methane (BIM)-based neutral probes with bentonite clay, an inorganic layered material. Probes 1 and 2 were unoxidized and oxidized BIM-derivatives respectively, which typically exhibited quenched luminescence due to intramolecular rotations in the solution. By embedding probe 1 within bentonite interlayers or adsorbing it onto the clay surface, the molecular conformation of the probe was immobilized. Furthermore, the restricted intramolecular rotation via molecular flattening (planarization), resulted in enhanced fluorescence emission in the 1. clay composite. On the other hand, it was observed that the unoxidized rigid probe 2 was predominantly adsorbed on the clay surface, consequently resulting in fluorescence quenching. Detailed photophysical analysis revealed that the intercalation of probe 1 into the clay induces a planarization that differs from its behavior in solution. Moreover, the 1. clay composite was successfully utilized for the selective detection of rifampicin. The fluorescence intensity of the 1. clay composite was quenched, with the appearance of a new red-shifted band in the presence of rifampicin. The quenched band originally attributed to the probe, suggests that rifampicin displaced probe molecules from the clay interlayer into the solution, likely due to strong hydrogen bonding between hydroxyl groups of rifampicin and the clay interlayers. Additionally, the probe demonstrated higher selectivity for rifampicin over other tuberculosis drugs. This finding highlights the potential application of the 1. clay in developing a highly selective and efficient sensing platform for rifampicin detection, which could be employed in clinical diagnostics or therapeutic drug monitoring.