A new water-soluble NBD-based probe for rapid and off-on visualization of latent fingerprints

Fingerprint identification is vital in forensic investigation. However, conventional chemical reagents often suffer from poor water solubility, organic solvent toxicity, and potentially lengthy and complex imaging procedures. Herein, we report a water-soluble NBD-based fluorescent probe (NDH) for real-time off-on visualization of LFPs. NDH operates via an intramolecular charge transfer (ICT) mechanism, showing weak fluorescence in water but strong yellow emission upon contact with fingerprint residues due to reduced polarity and hydrophobic interaction. The quaternary ammonium cation ensures excellent water solubility and electrostatic affinity to sebaceous components. A simple two-step synthesis supports cost-effective preparation and potential scale-up. Using a 200 μM NDH aqueous solution, clear LFP images were obtained within 15 s from the onset of spraying on various substrates, with Level 1-3 fingerprint details clearly resolved. The probe exhibits high stability for aged fingerprints (up to 30 days) and prolonged reagent shelf life (≥ 2 months), and allows reliable tape replication. NDH exhibits low cytotoxicity and is compatible with subsequent forensic DNA analysis. Its simple synthesis, low cost, good aqueous solubility without organic co-solvents, and rapid performance make NDH a potentially practical reagent for rapid on-site LFP visualization.