Multicolor N, P Co-Doped Carbon Quantum Dots for Sensing, Fingerprint Imaging, and Fruit Preservation

Multi-emissive carbon quantum dots (CQDs) are attracting widespread attention across fields. Three kinds of nitrogen and phosphorus co-doped CQDs (NPCQDs) were synthesized through a two-step method. Firstly, phosphorus-doped CQDs (PCQDs) with blue fluorescence were rapidly prepared using maleic acid and disodium hydrogen phosphate as the carbon and phosphorus sources by microwave method. Subsequently, through the hydrothermal method, NPCQDs were successfully obtained using 3-aminophenylboronic acid, diethylenetriamine, and o-phenylenediamine as dopants and reacting with PCQDs. Their emission peaks are located at 437 nm exhibiting blue fluoresce (B-NPCQDs), 550 nm (yellow, Y-NPCQDs), and 600 nm (red, R-NPCQDs), respectively. Compared with PCQDs (11.7%), the QYs are high values of 17.32%, 41.37%, and 37.32% for B-NPCQDs, Y-NPCQDs, and R-NPCQDs, respectively. Y-NPCQDs exhibited superior fluorescence stability against most interferents, while B-NPCQDs and R-NPCQDs served as selective fluorescent probes for Fe²⁺ and Ag⁺/Cu²⁺ detection, respectively. For fingerprint imaging, Y-NPCQDs@Al₂O₃ composites achieved clear visualization of fingerprint first-, second-, and third-level features. In comparison, B-NPCQDs@Al₂O₃ only revealed first-level features with insufficient detail, while R-NPCQDs@Al₂O₃ led to blurred images due to excessive adhesion to the substrate. In fruit preservation, 0.5% Y-NPCQDs/PVA film showed the optimal effect on jujubes, maintaining good fruit color and plumpness for 13 d and significantly delaying ripening.