Selective Sensing of Human Serum Albumin by a D···π···A-Based Self-Assembled Nanoprobe via a Disassembly Mechanism: Quantification and Live Cell Imaging Applications

Human serum albumin (HSA) serves as a vital biomarker for diagnosing kidney and liver diseases. Herein, we describe the detailed synthetic procedure of a D···π···A-based fluorescent probe HPQM [(E)-2-(2-(8-(3-hydroxypropoxy)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium], which forms weakly fluorescent nanoaggregates in aqueous environments due to the aggregation-caused quenching phenomenon. Nevertheless, in the presence of HSA, it becomes highly fluorescent (∼25-fold enhancement), facilitating the disassembly of nanoaggregates into its monomer. The study reveals that noncovalent interactions between HPQM and HSA promote the disassembly of the nanoaggregates and trapping of the HPQM monomer in the HSA cavity, resulting in red fluorescence by prohibiting the free rotation of the probe molecules owing to the substantial steric hindrance and low polarity environment present inside the HSA cavity. Multiple circular dichroism spectral analyses of HSA in the absence and presence of the probe revealed HPQM-induced conformational changes in the secondary and tertiary structures of HSA. The 3σ/slope technique yielded a limit of detection value of 29.8 nM (1.98 mg/L) in aqueous solutions, highlighting its high sensitivity toward HSA. Also, HPQM offers some practical utility including live cell imaging of HSA and the detection and estimation of HSA content in human blood and urine.