Recent progress in background-free latent fingerprint imaging

Owing to their unique pattern and abundant chemical composition, latent fingerprints (LFPs) can serve as "ID cards" and "information banks" of donors and therefore are valuable for forensic investigation, access control, and even medical diagnosis. LFP imaging has attracted considerable attention, and a great variety of contrast agents has been developed. In LFP imaging, background signals such as background fluorescence from the underlying surface can seriously blur the LFP images and decrease imaging sensitivity; thus, great efforts have been made to eliminate background interference. Here, we stratify the recent progress in background-free LFP imaging by making use of the difference in properties between contrast agents and background compounds. For example, near-infrared (NIR) light-activatable contrast agents can efficiently remove background signals in LFP imaging because the background compounds cannot be excited by NIR light, showing that the difference in excitation properties between contrast agents and background compounds can be employed to eliminate background interference. This review is organized around background-free LFP imaging based on the difference in optical properties between contrast agents and background compounds: (i) different excitation wavelengths, (ii) different emission wavelengths, (iii) different luminescence lifetime values, (iv) different plasmonic properties, (v) different photothermal properties, and (vi) different electrochemiluminescence properties.