Spacer-Acceptor Synergy in Cyanide Detection: Leveraging Dual-Site Reactivity to Overcome Sensitivity-Range Trade-Offs

The persistent trade-off between sensitivity and linear concentration range in reaction-based chemosensors remains a critical limitation for rapid analyte detection with high precision. This significantly limits the practical deployment of these reaction-based chemosensors, especially in the detection of hazardous ions such as cyanide. Addressing this challenge, we rationally developed donor-π-acceptor fluorescent probes, TRMN and TπRMN, featuring triphenylamine donors, conjugated rhodanine acceptors, and differentiated π-spacers. Concurrently, their parent aldehydes were investigated to understand their photophysical baseline, wherein the spacer rigidity suppresses the nonradiative process. Both TRMN and TπRMN exhibit rapid (<20 s) and selective recognition of cyanide. However, the flexible vinyl-bridged TRMN achieves a superior sensitivity of 0.39 nM with a narrow linear range (2-5.5 μM). In contrast, TπRMN exhibits dual-site reactivity, enabling cyanide addition at both the vinyl β-carbon and the β-carbon of the pyrrolo[3,2-<i>b</i>]indole spacer. Additionally, the rigid spacer design constrains the conformational freedom upon adduct formation, suppressing the nonradiative decay processes. These factors collectively enable TπRMN to achieve an extended linear range of 8-34 μM with a detection limit of 4.2 nM. On the other hand, the probes exhibit excellent solid-state fluorescence with a pronounced bathochromic shift relative to their solution state and thus have been employed for latent fingerprint imaging. Overall, this work establishes spacer-acceptor synergy engineering as a generalizable strategy to overcome fundamental sensitivity-range trade-offs, providing a comprehensive framework for next-generation reaction-based chemosensors in environmental monitoring.