Visible and ultraviolet light excitable room-temperature phosphorescent carbon dot-based materials for background-free latent fingerprint visualization

Latent fingerprint visualization on complex substrates typically suffers from interference from background patterns and fluorescence. Carbon dots (CDs) with room-temperature phosphorescence (RTP) provide an effective route to eliminate background interference due to their long-lived emission. However, most fingerprint reagents based on phosphorescent CDs are limited to single ultraviolet (UV) lamp excitation, resulting in poor operational flexibility and restricted anti-interference performance on complex substrates. While visible light excitation is safer and more flexible, current visible-light-excited phosphorescent fingerprint powders require tedious multi-step preparation. In this study, CDs with alumina matrix (CDs@Al 2 O 3 ) composite emitting RTP under both visible and UV light excitation is synthesized in situ through one-step calcination. The prepared composite features visible RTP lasting for 12 s and 6 s to the naked eye after 365 nm UV and visible light excitation, respectively. With dual-mode excitation properties, the CDs@Al 2 O 3 is applied to imaging of latent fingerprints (LFPs) on various complex substrates. Additionally, background interference from different substrates is effectively eliminated by flexibly switching the light source, which significantly enhances the contrast and detail resolution of fingerprint images. This strategy simplifies the preparation procedure while greatly improving the applicability and reliability of latent fingerprint development in complex scenarios, providing a facile and versatile approach for background-free visualization of LFPs.