A Polarity-Sensitive Dicyanoisophorone Based Fluorescent Probe for Targeted Human Serum Albumin Detection in Blood and Urine With Cellular Imaging.

Protein biomarkers serve as a crucial prognostic indicator for life-threatening disorders in the era of personalized medicine. In this context, we developed a polarity-sensitive fluorescent probe, DQDM, for the sensitive detection and quantification of human serum albumin (HSA). It operates through an intramolecular charge-transfer (ICT) mechanism showing weak fluorescence in ~100% PBS. Notably, upon binding to HSA, a substantial (~40-fold) fluorescence enhancement at ~650 nm was observed, which arises from its encapsulation within the low-polarity, hydrophobic binding cavity of the protein resulting in a restricted conformational flexibility which suppress nonradiative relaxation pathways of the probe. The strong encapsulation of DQDM at subdomain IB of HSA was confirmed through both site marker displacement assays and molecular docking. The low dissociation constant [ K d = (1.02 ± 0.02) μM] indicates a high binding affinity. CD spectroscopy analysis revealed that the DQDM probe induces a significant conformational change in the secondary structure of HSA. With a rapid response time (~30 s) and a relatively low LOD of 27 nM, calculated using the 3σ/slope method, DQDM enables sensitive quantification of HSA in urine and blood samples. Beyond solution-phase sensing, DQDM demonstrates effective intracellular fluorescence, supporting HSA-dependent cell imaging and highlighting its potential for diagnostic applications.