From Blue to Green: Demonstration of a pH-Dependent Fluorescence Shift of Carbon Quantum Dots

Carbon quantum dots (CQDs) have emerged as versatile, water-soluble nanomaterials with strong photoluminescence and tunable optical properties, making them promising candidates for applications in sensing, bioimaging, and optoelectronics. While inorganic quantum dots have long been established in educational settings, organic CQDs, particularly with functional responsiveness, remain underrepresented in K-12 science education. In this paper, we present a simple, safe, and low-cost bottom-up synthesis of fluorescent, pH-sensitive CQDs using ascorbic acid and sodium acetate, which were heated directly over a gas burner. The procedure takes less than 5 min and requires minimal equipment. The resulting CQDs exhibit bright blue-to-turquoise fluorescence under UV light. Under basic conditions, a bathochromic shift to green fluorescence occurs because of deprotonation of surface functional groups, accompanied by self-absorption. This pH-dependent behavior is also visible under daylight, showing a color shift from yellow to orange resulting from absorption and transmission. The experiment provides a tangible demonstration of surface chemistry, electronic transitions, and nanoscale structure–property relationships. Learners perceived the experiment as an easy procedure that contributes to a basic understanding of the nanoscale, as an accompanying assessment shows (n = 20). By combining low-cost materials, rapid synthesis, and vivid visual outcomes, we give an outlook on how this work will contribute to nanoscience education by offering a readily accessible and engaging pathway into the world of functional organic nanomaterials.