A simple design of carbon dots for dual-mode pH sensing and monitoring food freshness.

A pH-responsive carbon dot (CD) was synthesized using o-phenylenediamine and tyrosine as precursors via a one-step solvothermal method. The synthesized CDs were characterized using multiple analytical techniques, revealing UV-visible absorbance and excitation-independent fluorescence emission, diverse hydrophilic functional groups, crystalline structure with notable defects, and excellent optical stability, which is a key feature for their use as robust and reliable sensing material. The resulting CDs demonstrated visible absorbance and fluorescent intensity changes across the full pH range from 1 to 14, with a notable color change in solution, which has rarely been reported in the field of pH sensing by CDs. This pH responsiveness was further validated through tests on real solution samples, including fruit and vegetable juices, vinegar, and water, demonstrating the CDs’ reliability and practicality as a pH sensor in diverse environments. Additionally, the integration of these CDs into a paper strip enabled straightforward and visible-light-based monitoring of seafood freshness, like fish and shrimp. This work offers a cost-effective and sensitive platform for pH detection applications in different fields.