Construction, Mechanism, and Forensic Application of Green-Light-Excited Fluorescent Carbon Dots/Diatomite Composites

Latent fingerprints are invisible impressions that need to be developed before being used for individual identification. To advance fingerprint powdering, which is the most common method of developing latent fingerprints on nonporous substrates, two types of green-light-excited carbon dots (CDs) were synthesized hydrothermally using rhodamine 6G (R6G) and rhodamine B (RB) as the precursors. Under the physical effects of capillary attraction and coffee-ring effect, they were dispersed into hierarchical porous micron-sized diatomite (DE) in a facile, cost-effective, and eco-friendly way. The evolution of fluorescence characteristics from water-dispersed CDs to solid CDs/DE composite powders is clarified from the perspectives of physical properties and microstructures of the CDs and diatomite matrix. Optimized green fluorescent R6G-CDs/DE and yellow fluorescent RB-CDs/DE can be excited by visible light (green region) and show very high performance in fingerprint development under the luminescence modes, which has low phototoxicity and helps to preserve chemical information in fingerprint residues. Furthermore, image enhancement protocols, called "digital filters", established based on qualitative and quantitative analyses of red, green, and blue (RGB) channel images of fluorescent fingerprints developed with the CDs/DE composite powders are very efficient in eliminating background interferences of challenging surfaces such as multicolored or patterned substrates. The as-prepared new materials for fingerprint development and the created methods for image enhancement show great promise in practical application scenarios.