Reversible enhancement of fluorescence in acidic pH driven by tryptophan stabilized copper nanoclusters and its application in bioimaging

Aqueous soluble tryptophan templated copper nanoclusters (Trp-CuNCs) are synthesized in one step utilizing eco-friendly precursors and employed in pH sensing as pH acts as an indispensable tool in various fields like chemistry, biology, agriculture etc. Intriguingly, Trp-CuNCs manifested reversible enhancement in emission on moving from basic to acidic pH (9.0–3.0) while turn-on emission in acidic medium is uncommon in reported literatures. The observed peculiarity was due to the presence of indole group in tryptophan which caused aggregation induced quenching (AIQ) in basic medium and it was corroborated by Dynamic Light Scattering (DLS). The role of indole group in aggregation was further substantiated by carrying out control experiments. The type of quenching encountered was identified as static quenching by PL-lifetime experiments. To extend the pragmatic applicability of Trp-CuNCs, bioimaging of HeLa cells under different pH and real sample analysis (water and an antacid product) were performed.