Gadolinium-Doped Carbon Dots with Dual Enzyme Activity for Visual Detection of Cu²⁺ and Antibacterial Applications

The preparation and property studies of long-wavelength-emitting carbon dots (CDs) nanozymes with multifunctions like multienzyme activity nanozymes, antibacterial, and antioxidation have promising prospects in biochemical detection and clinical fields. This paper reports the fast preparation of dual enzyme-like active yellow-green carbon quantum dots (4-ASA/Gd-CDs) through a one-step hydrothermal route with the rare-earth element Gd doping, 4-aminosalicylic acid (4-ASA), and polyethyleneimine (PEI) as raw materials. The prepared CDs nanozymes can produce O-2(center dot-) under the irradiation of UV light and rapidly oxidize the colorless substrate o-phenylenediamine (OPD) into yellow 2,3-diaminophenazine (oxOPD); meanwhile, the heavy metal Cu2+ can strongly coordinate with CDs, leading to electron transfer and the inhibition of CDs-like oxidative enzyme activity, and finally realize the real-time colorimetric detection to Cu2+. The limit of detection was as low as 0.38 mu M (3 sigma/k) with a dynamic range from 0 to 38.4 mu M. Moreover, the portable hydrogels (CDs-OPD-PSA) were also prepared, and the visual detection of Cu2+ was achieved with the assistance of a smartphone. The experiment showed that the prepared 4-ASA/Gd-CDs nanozymes exhibited excellent antibacterial ability and superoxide dismutase (SOD)-like activity. The antibacterial mechanisms of CDs against gram-negative/positive bacteria were investigated through the potential value of 4-ASA/Gd-CDs and its own antioxidant activity. The MICs of 4-ASA/Gd-CDs against G(+) Staphylococcus aureus and Bacillus subtilis are 30 and 17 mu g/mL, respectively, and those against G(-) Escherichia coli and Pseudomonas aeruginosa are 210 and 35 mu g/mL, respectively. This research would provide an excellent theoretical and practical basis for the synthesis of dual enzyme-like active CDs and their application in antibacterial and antioxidation fields.