Synthesis of bright upconversion submicrocrystals for high-contrast imaging of latent-fingerprints with cyanoacrylate fuming

Upconversion particles (UCPs) as a new generation of imaging agent are gaining prominence due to their unique optical properties. Herein, we report the synthesis of bright UCPs for high-contrast imaging of latent-fingerprints with cyanoacrylate-fuming (CA-fuming). The hexagonal-phase and rod-shaped NaYF4:Yb,Er/Ce submicrocrystals (0.5 x 1.0 mu m in dimension) coated with polyethylenimine (PEI) are synthesized using a dopant-controlled strategy and exhibit much stronger UC fluorescence than their nanosized analogues. The appropriate particle size and good surface properties of the UCPs make them easily come into the holes formed on fibrous layers of fingerprint ridges after fuming by CA. Compared with downconversion fluorescent materials, the UCPs exhibit the ability to suppress background fluorescence interference for obtaining high-contrast fingerprint images under near-infrared (NIR) light irradiation. Our results indicate that this strategy can successfully be applied to detect latent-fingerprints on various surfaces including non-porous and porous surfaces, and the fingerprints from different people can be identified. All of these benefits ensure this strategy is an important tool in fingerprint detection and will find wide-ranging applications in forensic investigations and medical diagnostics.