We introduce a general design to construct fluorescence-switching probes. Upon the interaction of the ligand with the protein, the crowded surroundings restrict the bond rotation of the fluorescent molecular rotor to trigger a strong fluorescence signal, which is reduced upon the addition of a competitive ligand or after protein degradation.

In this communication, we report a simple albumin probe based on a fluorescent molecular rotor for the detection of trace albumin levels in urine. In the presence of albumin, the probe exhibits remarkable 400-fold fluorescence enhancement with high selectivity and sensitivity. The probe was successfully applied in the quantitative detection of urinary albumin.

Currently most of the fluorogenic probes are designed for the detection of enzymes which work by converting the non-fluorescence substrate into the fluorescence product via an enzymatic reaction. On the other hand, the design of fluorogenic probes for non-enzymatic proteins remains a great challenge. Herein, we report a general strategy to create near-IR fluorogenic probes, where a small molecule ligand is conjugated to a novel γ-phenyl-substituted Cy5 fluorophore, for the selective detection of proteins through a non-enzymatic process. Detail mechanistic studies reveal that the probes self-assemble to form fluorescence-quenched J-type aggregate. In the presence of target analyte, bright fluorescence in the near-IR region is emitted through the recognition-induced disassembly of the probe aggregate. This Cy5 fluorophore is a unique self-assembly/disassembly dye as it gives remarkable fluorescence enhancement. Based on the same design, three different fluorogenic probes were constructed and one of them was applied for the no-wash imaging of tumor cells for the detection of hypoxia-induced cancer-specific biomarker, transmembrane-type carbonic anhydrase IX.