This study used 976 nm illumination to generate 800 nm luminescent fingerprint images. NIR-to-NIR UCNP were demonstrated to have significant advantages over NIR-to-visible UCNP in developing latent fingerprints. NIR-to-NIR UCNP were significantly brighter than NIR-to-green -NaYF4:2%Er, 18%Yb UCNP of comparable size, so that lower irradiance was required to obtain high-quality images. The increased brightness was 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 reduced 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 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 were estimated for the nanomaterials used in this study at typical irradiance levels used 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 in this project produces fingerprint images of high resolution, as evidenced by the high number of minutiae which can be identified. (publisher abstract modified)
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