Multi-Emissive Solid State Carbon Dots Modified With Ligands for Recognition of Latent Fingerprints

Solid-state fluorescent materials with tunable visible emission are highly desirable for high-contrast latent fingerprint (LFP) visualization; however, most carbon dots (CDs) undergo aggregation-caused quenching in the solid state. In this study, we present a ligand-directed approach to engineer aggregation-induced emission (AIE)-active CD powders with tunable solid-state fluorescence (SSF) for forensic applications. The hydrothermally synthesized nitrogen-doped humic acid-derived carbon dots (N-hCDs) were functionalized with two aromatic aldehydic ligands via Schiff-base chemistry, producing CDs@I and CDs@D that display distinct light-red and yellow solid-state emission. The emission tuning arises from ligand-induced modulation of surface states, restricted intramolecular motion, and ordered lamellar self-assembly, which together suppress nonradiative decay in the aggregated state. The SSF powders enable high-contrast visualization of LFPs on diverse nonporous substrates, resolving level-2 and level-3 ridge features under UV illumination. Moreover, the materials exhibit excellent thermal stability, photostability, and aging resistance, underscoring their robustness for practical forensic applications. This work demonstrates a versatile surface-engineering strategy for color-tunable solid-state emissive CDs, broadening their utility in LFPs detection.