Background-Free Development of Latent Fingerprints on Fluorescent Substrates

In order to significantly increase the signal-to-noise ratio of fingerprint development, in this paper, the background-free development of latent fingerprints based on NaYbF4:Ho upconversion luminescent powders (UCLPs) and SrAl2O4:Eu, Dy afterglow luminescent powders (AGLPs) were proposed, and the contrast between developed fingerprint (signal) and background substrate (noise) was also quantified using spectral analysis. First, NaYbF4:Ho UCLPs and SrAl2O4:Eu, Dy AGLPs were synthesized via solvothermal and combustion approaches , respectively. Then, the morphologies, crystal structures, absorption spectra, and luminescent properties of the above two powders were characterized. NaYbF4:Ho UCLPs were cylindrical shaped nanomaterials with hexagonal crystal structure, had a maximum near-infrared (NIR) absorption wavelength of 976 nm, and could emit green upconversion luminescence at the wavelength of 539 nm under 980 nm NIR excitation. While, SrAl2O4:Eu, Dy AGLPs were polyhedral shaped micromaterials with monoclinic crystal structure, had a maximum ultraviolet (UV) absorption wavelength of 331 nm, and could emit green afterglow luminescence at the wavelength of 515 nm under 365 nm UV excitation. Finally, latent fingerprints on various fluorescent substrates were stained using NaYbF4:Ho and SrAl2O4:Eu, Dy dry powders, followed by fluorescently enhanced via upconversion luminescence mode and afterglow luminescence mode, respectively, and thus the background-free development of fingerprints was achieved. In addition, the signal-to-noise ratio in fingerprint development was subjectively evaluated and objectively analyzed by vision effect and spectral analysis, respectively. Fingerprint development results showed that developed fingerprints could emit bright green luminescence in the dark field, producing enough color contrast between the fingerprint and the substrate. The aid of upconversion luminescence mode or afterglow luminescence mode could avoid the interference of background noise. Spectral analysis showed that the intensity contrast between the developing signal and the background noise was significant, resulting a high signal-to-noise ratio. Compared with the normal fluorescence mode based on traditional fluorescent powders, the upconversion luminescence mode and afterglow luminescence mode had an outstanding advantage of ultra-high signal-to-noise ratio in latent fingerprint development. Our proposed UCLP-based upconversion luminescence mode and AGLP-based afterglow luminescence mode have achieved the background-free development of latent fingerprints, which will not only expand the applications of rare earth luminescent powders but also broaden the innovative ideas for fingerprint development.