Viscosity-sensitive NIR probe for detecting cellular state transitions in Normal, Cancer and apoptotic cells

Intracellular viscosity serves as a biomarker for various diseases, as it is linked to multiple biological processes. Measuring intracellular viscosity precisely is essential for disease diagnosis and evaluating treatment efficacy. To address this, a viscosity-responsive, deep-red-emitting fluorescent probe (T5) has been synthesized using thiazolidinone and dimethylaminocinnamaldehyde as the acceptor and donor, respectively. The probe exhibits a "turn on" fluorescence response with high selectivity in highly viscous medium. The emission intensity of the probe at 641 nm increases progressively with increasing solvent viscosity. The working mechanism of the developed probe is primarily based on the restriction of intramolecular rotation (RIR) in a highly viscous medium, as further supported by DFT and TD-DFT analyses. The probe exhibits long-wavelength emission, which is advantageous for achieving deeper tissue penetration and higher sensitivity in bio-imaging applications. The practical applicability of the probe was evaluated by monitoring micro-environmental viscosity variations in a cell-based model comprising normal, cancerous and apoptotic MDA-MB-231 cells. Furthermore, the interaction of the probe with Bovine serum albumin (BSA) was investigated using UV-vis and fluorescence spectroscopy. Experimental results revealed that the probe binds within the hydrophobic cavity of BSA, a finding corroborated by molecular docking and dynamics studies. Overall, the developed sensor demonstrates strong potential not only as a sensitive detector of microenvironment viscosity in biological systems but also as a valuable tool for monitoring conformational changes in protein structures.