Cyanide-Triggered Fluorescence Turn-On in an Indole-Based Chemodosimeter

The development of fluorescence-based systems capable of the selective recognition of cyanide remains important due to the high toxicity and widespread environmental relevance of this anion. In this work, an indole-derived Schiff base probe (P), obtained from 4-(aminomethyl)indole and 2-hydroxy-3-methoxybenzaldehyde, was evaluated as a fluorescent probe for cyanide (CN⁻) sensing in a CH₃CN/H₂O (7:3, v/v) medium. The addition of CN⁻ triggered a pronounced fluorescence enhancement at 483 nm, resulting in an approximately five-fold increase in emission intensity, whereas other common anions produced negligible spectral changes. Quantitative fluorescence titration studies yielded an apparent association constant of 3.63 × 10⁴ M⁻¹ and a detection limit of 0.117 µM. The binding stoichiometry was determined to be 1:1 from Job's method of continuous variation and was further supported by ESI-MS analysis. Spectroscopic investigations established that CN⁻ recognition occurs through nucleophilic attack at the azomethine carbon, leading to disruption of the conjugated Schiff base framework, as evidenced by ¹H NMR titration experiments. Density functional theory (DFT) calculations further supported the proposed sensing process by revealing substantial changes in frontier molecular orbital distributions and an increase in the HOMO-LUMO energy gap from 4.29 to 5.03 eV following cyanide addition. Collectively, the experimental investigations and DFT analysis demonstrate that P functions as a sensitive and selective fluorescence turn-on chemodosimeter for CN⁻ detection, exhibiting an instant fluorescence response under the employed experimental conditions and successful validation in real water samples.