Boosting responses of fluorescent imaging probes toward sulfur dioxide through engineering side chain length

Development of fluorescent probes with high sensitivity and specificity is always desirable, yet, challenging. Conventionally, improving the responses of probes relied on optimizing the reactivities of recognition sites, either by increasing the binding affinity or reaction rates. Herein, we found the sensitivity and response kinetics could be improved by changing the aggregation behaviors of probes. As a proof-of-concept, benzothiazole derivatives with different side chain lengths were prepared and the enhanced responses toward sulfur dioxide (SO₂) were observed for the probe with longer side chain. We demonstrated that the long side chain facilitates formation of tightly aggregates, which possessed higher positive charges and susceptible recognition sites as compared with probes possessing short side chains, resulting in better sensitivity and faster responses. In addition, we also demonstrated the generality of such design protocols with probes displaying aggregation induced emission (AIE) properties. Thus, the proposed side chain engineering strategy provides new paradigm for probe design.