Counterfeiting costs governments and private industries billions of dollars annually due to loss of value in currency and other printed items. This research involves using lanthanide doped β-NaYF(4) nanoparticles for security printing applications. Inks comprised of Yb(3+)/Er(3+) and Yb(3+)/Tm(3+) doped β-NaYF(4) nanoparticles with oleic acid as the capping agent in toluene and methyl benzoate with poly(methyl methacrylate) (PMMA) as the binding agent were used to print quick response (QR) codes. The QR codes were made using an AutoCAD file and printed with Optomec direct-write aerosol jetting(®). The printed QR codes are invisible under ambient lighting conditions, but are readable using a near-IR laser, and were successfully scanned using a smart phone. This research demonstrates that QR codes, which have been used primarily for information sharing applications, can also be used for security purposes. Higher levels of security were achieved by printing both green and blue upconverting inks, based on combinations of Er(3+)/Yb(3+) and Tm(3+)/Yb(3+), respectively, in a single QR code. The near-infrared (NIR)-to-visible upconversion luminescence properties of the two-ink QR codes were analyzed, including the influence of NIR excitation power density on perceived color, in term of the CIE 1931 chromaticity index. It was also shown that this security ink can be optimized for line width, thickness and stability on different substrates.

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.

Two methods of direct-write printing for producing highly resolved features of a polymer impregnated with luminescent upconversion phosphors for security applications are presented. The printed polymer structures range in shape from features to text. The thin polymer features were deposited by direct-write printing of atomized material as well as by screen-printing techniques. These films contain highly luminescent lanthanide-doped, rare-earth nanocrystals, β-NaYF₄:3%Er, 17%Yb, which are capped with oleic acid. This capping agent allows the nanocrystals to disperse throughout the films for full detailing of printed features. Upconversion of deposited features was obtained using a 980 nm wavelength laser with emission of upconverted light in the visible region at both 540 and 660 nm. Features were deposited onto high bond paper, Kapton®, and glass to demonstrate possible covert and forensic security printing applications, as they are printed in various features and invisible to 'naked-eye' viewing at low concentrations of nanocrystals.

Nonequilibrium electrophoretic-mobility measurements by laser-Doppler electrophoresis are reported for alkali halide particles. These measurements have been made for the first time and allow for the sign of the surface charge to be predicted for alkali halides in their saturated brines. In general, the results can be explained from the simplified lattice ion hydration theory; however, potassium chloride is, at least, one noteworthy exception. On the basis of this analysis, particle dispersion/ aggregation behavior in saturated brines can be explained. Further, the nature of collector (surfactant) adsorption and the flotation behavior of the alkali halide salts from their saturated brines can now be described more accurately.

In this study silver nanoparticles were synthesized with short chain (C6-C10) carboxylic acids as capping agents and prepared as conductive inks for fabricating electrically conductive patterns using direct write technologies. The structural characterization of as-synthesized nanoparticles revealed that the particles are spherical in shape with narrow size distribution (4.1 to 4.7 nm) and have face centered cubic crystal structure. Silver-particle-based inks were prepared by dispersing the particles in toluene and separating non-dispersing particles from the inks. The loading of silver particles in the solvent was increased with the increasing chain length of capping agents. As a result, inks with a wide range of nanoparticulate concentrations ($3 to 66 wt%) were able to be prepared and most of these inks were stable for at least a month. All the inks exhibited shear thinning behavior and this shear thinning became more prominent for higher concentration inks. As the nanoparticle concentration of the inks was increased, surface tension was decreased and the contact angles of the inks with the KaptonÒ and glass were increased. Printing of microelectrodes, lines and films was carried out using aerosol jet printing and ultrasonic spray coating. The conductivity of printed microelectrodes was 10 to 87% of the bulk silver conductivity with the sintering temperatures as low as 130 to 250 C depending on the ink used.

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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.