Covalently Coupling Carbon Quantum Dots for Photoluminescence Red Shift Response to pH

Conventional fluorescent pH sensors, despite offering high sensitivity and rapid response, are limited by their reliance on fluorescence intensity changes, hindering applications requiring precise wavelength control. Here, we present a pH sensing strategy based on cross-linked carbon quantum dots (CCL-CQDs) displaying a remarkable pH-dependent red shift in the fluorescence emission wavelength. Amino- and carboxyl-functionalized CQDs were synthesized via a one-step hydrothermal method and further assembled into CCL-CQDs through the condensation reaction between amino groups and glutaraldehyde. The CCL-CQDs displayed excellent pH sensitivity, with their fluorescence emission wavelength exhibiting a linear red shift upon increasing pH (from 2.29 to 7.16). The results of mechanism exploration revealed that H⁺ induced the cleavage of C═N bonds in the CCL-CQD structure, leading to the formation of -COOH groups and increased surface-oxidized carbon content. This enhanced oxidation generated more surface defects, triggering a wavelength shift in surface-state-related fluorescence emission. This study demonstrates the successful synthesis of pH-sensitive CCL-CQDs with an excellent fluorescence detection performance.