Cu ion modification of baicalin derived carbon dots with red emission for selective histidine sensing

Fluorescent carbon dots (CDs) derived from traditional Chinese medicine (TCM) face limitations in water solubility and tunable emission, hindering their biomedical application. To address this, we synthesized baicalin-based CDs and engineered their surface with cetyltrimethylammonium chloride (CTAC) and Cu ions. This sequential modification first shifted the emission from blue to orange by introducing N/O-containing functional groups that reduced the energy gap. Subsequent Cu incorporation created Cu-related energy levels, further narrowing the gap to achieve red emission at 660 nm alongside enhanced solubility. In addition, the resulting Cu/CTAC-CDs served as a selective fluorescent probe for histidine, enabling its sensitive detection in complex matrices such as pig blood and milk. This work provides a viable route to fabricate water-soluble, emission-tunable TCM-derived CDs and offers fundamental insights into surface-state-controlled luminescence, advancing their development for biomedical applications.