Near Infrared-to-Near Infrared Upconversion Nanocrystals for Latent Fingerprint Development

The advantages of near infrared (NIR)-to-visible upconversion nanoparticles (UCNP) for latent fingerprint development have been previously documented. In the present study, the use of NIR-to-NIR UCNP, composed of β-NaYF4:2%Tm, 48%Yb, is evaluated for latent fingerprint analysis. Here, 976 nm illumination is used to generate 800 nm luminescent fingerprint images. NIR-to-NIR UCNP are demonstrated to have significant advantages over NIR-to-visible UCNP in developing latent fingerprints. NIR-to-NIR UCNP are significantly brighter than NIR-to-green β-NaYF4:2%Er, 18%Yb UCNP of comparable size, so that lower irradiance is required to obtain high-quality images. The increased brightness is due mainly to the much higher internal quantum yield of the NIR-to-NIR UCNP at the irradiance levels used for imaging. Imaging at 800 nm often significantly reduces the background interference from substrates with complex printed patterns because many inks do not absorb appreciably at 800 nm. In most instances, imaging can be performed in full room lighting without significant degradation of the image because modern lighting produces very little output in the NIR. Using β-NaYF4:2%Tm, 48%Yb@NaYF4 core–shell nanoparticles, fingerprints can be imaged easily using excitation irradiance levels below 100 mW·cm–2. The intrinsic quantum yields of the NIR-to-NIR upconversion are estimated for the nanomaterials used in this study at typical irradiance levels used here to image fingerprints. It is shown that the method for processing as-synthesized UCNP into powders has significant impact on the effective particle size in fingerprint development and on how the particles coat the fingerprint residue. The method demonstrated here produces fingerprint images of high resolution, as evidenced by the high number of minutiae which can be identified.