Forensic applications of rare earths: Anticounterfeiting materials and latent fingerprint developers

When considering interdisciplinary applications of rare earth (RE: Sc, Y, Ln, with Ln = lanthanides = La-Lu) research, forensic science likely is not the first discipline that springs to mind. Yet despite this, the unique optical properties of RE materials are excellently suited to address several forensic challenges. Forensic science involves the use of scientific methods and techniques to analyze, interpret and understand the remains of past criminal activity; and RE materials hold promise for two areas of forensic science: materials to frustrate counterfeiting, and developers of latent fingerprints. Opening with common luminescent processes, this chapter describes up-conversion (UC), down-conversion (DS), and a selection of less-common processes like mechanoluminescence (ML), thermoluminescence (TL) and long persistence luminescence (LPL). Next, recent developments in the application of RE materials to frustrate the actions of counterfeiters are detailed. Many RE materials are built into security inks, dyes, labels, taggants and physical keys, to exploit difficult-to-replicate luminescent profiles. Newer materials, such as security resins for 3D printing, and strategies for encryption, such as QR codes, physical unclonable function (PUF) keys and Code-93 barcodes, are also discussed. Recent developments applying RE materials as fingerprint developers, are then outlined, focusing on nanoscale powders, suspensions, solutions, and vapors. In addition to surveying the present state of RE fingerprint developers, suggestions for improving the authenticity of validation studies are provided with the aim of more clearly conveying the value of new and established RE materials in forensic applications. With a goal of boosting engagement and developing clearer discourse, this chapter seeks to spur further innovation and accelerate the uptake of RE materials within the forensic community.