Reaction-Induced Fluorescence Switching of an Indole-Based Schiff Base for Selective Cyanide Sensing

Area of Science:

• Analytical Chemistry
• Organic Chemistry
• Materials Science

Background:

• Schiff base ligands are versatile tools in chemical sensing.
• Developing selective and sensitive methods for cyanide detection remains a challenge.
• Fluorescence-based sensing offers high sensitivity and real-time monitoring capabilities.

Purpose of the Study:

• To synthesize and characterize a Schiff base ligand for cyanide ion detection.
• To investigate the mechanism of cyanide recognition and fluorescence response.
• To evaluate the sensing performance, including selectivity, sensitivity, and detection limit.

Main Methods:

• Synthesis of a Schiff base ligand from 4-(aminomethyl)indole and 2,3-dihydroxybenzaldehyde.
• Fluorescence spectroscopy to monitor the response to cyanide ions.
• Nuclear Magnetic Resonance (NMR) titration and Electrospray Ionization Mass Spectrometry (ESI-MS) for mechanistic studies.
• Density Functional Theory (DFT) calculations to understand electronic structure changes.

Main Results:

• The Schiff base ligand exhibits a significant fluorescence enhancement in the presence of cyanide ions.
• Cyanide recognition occurs via nucleophilic addition at the imine center, altering the electronic structure.
• DFT calculations confirmed an increased HOMO-LUMO energy gap, correlating with fluorescence activation.
• High selectivity for cyanide over other tested anions was observed.
• An association constant of 5.66 × 10⁴ M⁻¹ and a detection limit of 0.055 µM were determined.

Conclusions:

• The Schiff base ligand acts as an effective probe for selective cyanide sensing.
• Reaction-induced structural transformation is key to the fluorescence activation mechanism.
• The developed method provides a sensitive and reliable approach for cyanide detection.