Starch-based near-infrared organic fluorophores for the imaging of latent fingerprints

Visualization of latent fingerprints (LFPs) is important in the field of forensic identification and criminal investigation and requires fluorescent materials with near-infrared (NIR) emission to suppress the background fluorescence, with simultaneously high selectivity for fingerprints; however, this remains a challenge. Herein, we rationally designed and prepared a new NIR-emissive composite material composed of an NIR fluorophore, i.e., (E)-4-(7-(4-bromophenyl)imino) methyl)benzo[C][1,2,5]thiadiazol-4-yl)-N,N-dimethylaniline (BrBTN), and porous starch through a simple grinding method. The BrBTN powder exhibited strong NIR emission with a high quantum yield of 0.209. Porous starch enabled high adsorption ability for fingerprints. Using the BrBTN@starch composite, clear images of the tertiary structure of a fingerprint with NIR emission on various substrates were obtained under 405 nm light excitation within 10 s using a mobile phone alone after a simple dust removal treatment. Benefiting from its strong NIR emission and high selectivity, the imaging resolution of the fingerprint fluorescent patterns based on the NIR composite was 3-times higher than that of commercial fingerprint powder. This work provides a novel strategy for the design of highly emissive NIR fingerprint powder with the advantages of reduced background fluorescence, easy operation, high photostability, low cost and low toxicity to developers, indicating its great potential in the field of trace inspection and biological imaging.