Jeevan Meruga

Security printing of covert quick response codes using upconverting nanoparticle inks

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.

Red-green-blue printing using luminescence-upconversion inks

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 highly-resolved 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 primary-color 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.

A NIR-to-NIR upconversion luminescence system for security printing applications

A covert print-and-read system is demonstrated based on NIR-to-NIR upconversion luminescence. Inks activated with Yb3+/Tm3+ doped β-NaYF4 upconversion nanoparticles were used to print covert features on various substrates, including paper, epoxy resin, and circuit boards. The Yb3+/Tm3+ doping concentrations were optimized to maximize the brightness of 800 nm upconversion emission excited with 980 nm light, while simultaneously minimizing unwanted blue upconversion. Images printed with the NIR-optimized inks are invisible to the naked eye under ambient lighting or under 980 nm excitation. NIR-to-NIR images are easily captured, however, using an inexpensive, modified point-and-shoot CCD camera, even at modest excitation power densities (1.5 W cm−2). It is demonstrated that the latent images can also be read through select hard or soft coatings which are opaque to visible light, such as black inkjet print, or dyed epoxy resin, without significant attenuation of brightness. The ability to protect the printed images with durable, opaque coatings increases the tamper-resistance and the covertness of the system; removes the requirement that the print be invisible on the bare substrate; and blocks any visible emission that might be present, even under very high excitation power densities.

Hansen Solubility Parameters of Surfactant-Capped Silver Nanoparticles for Ink and Printing Technologies

Optimal ink formulations, inclusive of nanoparticles, are often limited to matching the nanoparticles capping agent or surface degree of polarity to the solvent of choice. Rather than relying on this single attribute, nanoparticle dispersibility was optimized by identifying the Hansen solubility parameters (HSPs) of decanoic-acid-capped 5 nm silver nanoparticles (AgNPs) by broad spectrum dispersion testing and a more specific binary solvent gradient dispersion method. From the HSPs, solvents were chosen to disperse poly(methyl methacrylate) (PMMA) and nanoparticles, give uniform evaporation profiles, and yield a phase-separated microstructure of nanoparticles on PMMA via film formation by solvent evaporation. The goal of this research was to yield a film that is reflective or transparent depending on the angle of incident light (i.e., optically variable). The nanoparticle HSPs were very close to alkanes with added small polar and hydrogen-bonding components. This led to two ink formulations: one of 90:10 vol % toluene/methyl benzoate and one containing 80:10:10 vol % toluene/p-xylene/mesitylene, both of which yielded the desired final microstructure of a nanoparticle layer on a PMMA film. This approach to nanoparticle ink formulation allows one to obtain an ink that has desirable dispersive qualities, rheology, and evaporation to give a desired printed structure.

QR code antenna for wireless and security applications

This paper presents the first QR code antenna. The QR code itself represents a specific code based on any text, URL, and any alphanumeric contents. However it can also serve as the antenna that is described in this paper, or any other QR code-shaped antenna. This antenna has a return loss of 8.5 dB at 2.43 GHz and gain of 1.25 dBi, representative of a good receiver. These antennas can be used as an additional security feature and/or replacement for RFID tag antennas.

Stable Inks Containing Upconverting Nanoparticles Based on an Oil-in-Water Nanoemulsion

An oil-in-water nanoemulsion capable of dispersing upconverting nanoparticles (UCNPs) for 7 months was investigated. Negative staining transmission electron microscopy shows that the UCNPs reside in the oil phase of the nanoemulsion. Dynamic light scattering measurements indicate that the majority of the oil volume is contained in droplets less than 1 μm in diameter. The system studied could be used to inkjet print UCNPs at least 7 months after the ink was first formulated. Nanoemulsion stability was tested in the short term, over 11 days, using an ink stability test developed for this research. It was found that after an initial loss of UCNPs, the majority of the UCNPs remained well-dispersed in solution. The UCNP dispersion was stable for longer periods under storage at 333 K compared to storage at 277 K.

Multi-layered covert QR codes for increased capacity and security

Quick Response (QR) codes, used in marketing, warehouse management, product tracking, and other applications, are usually only comprised of visible black and white modules. The goal of this research is the creation of covert, color QR codes for increased data capacity, and security. This goal was met by layering QR codes, each of a unique color, printing the covert versions of the color QR codes, upconverting the covert codes into visible codes and unlayering them to take advantage of the various color channels present. It was shown that color layering of the QR codes effectively increased the data capacity by three times that of a traditional QR code, while the covert nature of the code provides added security. Finally, it was also demonstrated that a layered QR code with six base colors (using color intensity variations) could further increase QR code data capacity.

QR code antennas for WiFi/WLAN/Bluetooth applications

This paper demonstrates the suitability and successful implementation of QR (Quick Response) code structures as antennas for wireless communication applications. QR code antennas can have different visual shape for the same URL, based on different error correction level. These antennas can be engineered to radiate at the same frequency. Here, the simulations of three QR code antennas are presented. The antennas have low return loss at 2.4 GHz and omnidirectional pattern, ideal for receivers. These antennas can also be used as an advanced and hard-to-replicate security feature for WiFi/WLAN/Bluetooth applications or other.