Design and synthesis of “off-on” fluorescent probes with rapid near-infrared time responsivity for Aβ protein

The utilization of near-infrared fluorescent probes for the detection of amyloid-β (Aβ) in Alzheimer's disease (AD) has been extensively investigated. Nevertheless, the development of probes that can simultaneously meet the requirements for rapid, accurate, and visual detection remains challenging. In this study, a novel Aβ-targeted fluorescent probe, designated NQM-FN , has been designed based on a D-π-A structural framework. NQM-FN incorporates quinoline-malononitrile (QM) as the fluorescent component and N,N- dimethylmino as the recognition unit. By utilizing a push-pull electronic conjugation system (with N,N- dimethylmino as electron donor; sulfonic acid and cyanide groups as electron acceptors), NQM-FN induces near-infrared emission and enhances blood-brain barrier (BBB) permeability. Fluorescence analysis indicated that NQM-FN is non-emissive in aqueous environments due to aggregation-caused quenching (ACQ), yet it demonstrates robust fluorescence in organic solvents, such as ethanol. When bound to Aβ aggregates, conformational rigidification suppresses twisted intramolecular charge transfer (TICT), facilitating rapid ‘off-on’ detection with increased emission intensity. NQM-FN interacts with Aβ42 aggregates in PBS solution, showing a rapid “off-on” switch (approximately 10 s). MTT assays revealed that the NQM-FN probe has good cellular compatibility, sustaining over 80 % cell viability. Confocal laser scanning microscopy (CLSM) imaging confirmed the successful real-time, sensitive, and visual detection of Aβ42 aggregates within live cells using the NQM-FN probe. This research introduces an innovative method for synthesizing fluorescent probes for the detection of Aβ42 aggregates, and the developed NQM-FN probe shows great promise as a tool for fluorescent detection.