Surface-Engineered Nitrogen-Doped Carbon Dots with Quaternary Ammonium for Fluorescent Sensing, Antioxidant, and Antibacterial Applications.

Nitrogen-doped carbon dots (CDs) have attracted extensive attention as promising nanomaterials because of their tunable fluorescence, excellent biocompatibility, and the ability in biofunctional modification. In this work, CDs were prepared via hydrothermal treatment using table sugar as the carbon source and polyethylenimine (PEI) as both a nitrogen dopant and a passivation agent. The multifunctional CDs were subsequently modified with the quaternary ammonium salt, benzalkonium chloride (BZC), to enhance surface reactivity and stability for sensing and biomedical applications. The obtained CDs-BZC displayed intense blue emission with a maximum peak at 453 nm under the excitation of 360 nm, accompanied by a high fluorescence quantum yield (37.9%) and an excitation-dependent bathochromic shift. The CDs-BZC were highly photostable and selective to Fe3+ ions with μM-level detection via fluorescence quenching. The CDs-BZC also exhibited remarkable antibacterial activity toward Staphylococcus aureus and Escherichia coli, as well as dose-dependent antioxidant activity according to DPPH and ABTS assays. These results indicated that CDs-BZC could become a promising multifunctional nanoplatform for integrated fluorescent sensing, antimicrobial, and antioxidant applications.