Many methods and protocols are available for latent fingerprint development and imaging. Yet, imaging become challenging when the background has complex patterns or is fluorescent. Fingerprints developed with NIR (800nm) emitting upconversion nanocrystals stand out, because the imaging technique strongly suppresses the background. This is due to the fact that most organic dyes and pigments found in printed materials have little or no absorbance in the NIR region and do not show any fluorescence upon 980 nm excitation. Aside from ‘zero’ background imaging, we demonstrate several additional advantages of using the 800nm emitting (48%Yb/2%Tm: β-NaYF<sub>4</sub>) upconversion nanocrystals for latent fingerprint development. The advantages include: 1) the nanocrystals are capped with oleate, which helps in fingerprint selectivity; 2) 800 nm luminescent image can be captured in room light with the help of bandpass filters; and 3) high internal quantum yield of the NIR emitting upconversion nanocrystals (2.5% at 1.6 W/cm<sup>2 </sup>power). Further, we evaluate the effect of the size of the nanocrystals on the quality of fingerprint development. The internal metrics, brightness, contrast, and resolution were used for fingerprint quality evaluation. Preliminary results suggest that the larger nanocrystals (~240nm) exhibit improved brightness and contrast without compromising the resolution of the fingerprint compared to the smallest nanocrystals used (~70nm).

Latent fingerprint development and imaging with NIR(980nm)-to-NIR(800nm) upconversion nanocrystals

2019 Spring National Meeting

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