Recent advances in producing pre-defined 2D patterns of upconversion nanophosphors via photolithography and printing techniques present new opportunities for the use of these materials in security applications. Here, we demonstrate an RGB additive-color printing system that produces highlyresolved pre-defined patterns that are invisible under ambient lighting, but which are viewable as luminescent multi-color images under NIR excitation. Patterns are generated by independent deposition of three primary-color (red, green and blue) upconverting inks using an aerosol jet printer. The primarycolor inks are printed as isolated and overlapping features to produce images that simultaneously emit red, green, blue, cyan, magenta, yellow and white upconversion luminescence. The dependence of the chromaticity of certain secondary colors (cyan and magenta) and white on NIR excitation power density can be exploited as an additional authentication feature. The development of an RGB upconversion printing system paves the way for an entirely new arena in security printing.

The advantages of near infrared (NIR)-to-visible upconversion nanoparticles (UCNP) for latent fingerprint development have been previously documented. In the present study, the use of NIR-to-NIR UCNP, composed of β-NaYF4:2%Tm, 48%Yb, is evaluated for latent fingerprint analysis. Here, 976 nm illumination is used to generate 800 nm luminescent fingerprint images. NIR-to-NIR UCNP are demonstrated to have significant advantages over NIR-to-visible UCNP in developing latent fingerprints. NIR-to-NIR UCNP are significantly brighter than NIR-to-green β-NaYF4:2%Er, 18%Yb UCNP of comparable size, so that lower irradiance is required to obtain high-quality images. The increased brightness is 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 reduces 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 β-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 are estimated for the nanomaterials used in this study at typical irradiance levels used here 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 here produces fingerprint images of high resolution, as evidenced by the high number of minutiae which can be identified.

A covert print-and-read system is demonstrated based on NIR-to-NIR upconversion luminescence.

We demonstrate a highly sensitive and selective sensing platform for the electrochemical detection of Hg 2+ in aqueous media. A graphene oxide (GO) and silver nanowire (AgNW) nanocomposites modified platinum (Pt) electrode has been applied to determine Hg 2+ by using square-wave anodic stripping voltammetry (SWASV). The synergistic effect of graphene oxide and conductive AgNW greatly facilitates faster electron transport and sensing behavior for Hg 2+ . Under the optimum conditions, the sensor shows a high sensitivity of ∼0.29 μA/nM and a linear response in the range 1−70 nM toward Hg 2+ . The detection limit of the GO−AgNW nanocomposites modified electrode toward Hg 2+ is ∼0.1 nM, which is significantly less than the safety limit defined by the World Health Organization. The sensor has an excellent selective response to Hg 2+ against other interfering heavy metal ions such as Pb 2+ , Cd 2+ , Cu 2+ , Na + , and Ag + . In addition, the sensor exhibits a high repeatability and reproducibility. The sensor is employed for the detection of Hg 2+ in tap water samples with an outstanding performance, suggesting it is a very promising platform for on-site monitoring of Hg 2+ in water.