ABSTRACT
Upconversion (UC) is a nonlinear process in which the excitation energy is absorbed by two or more photons that leads to the emission of light at shorter wavelength than the excitation wavelength. In this case, near-infrared (NIR) light, that is, 980 nm (lower energy, longer wavelength) is upconverted to visible light, 400-700 nm (higher energy, shorter wavelength) by UC material. In contrast to other emission processes, UC has high internal quantum
CONTENTS
10.1 Introduction ................................................................................................ 291 10.2 Upconverting Nanoparticles .................................................................... 292 10.3 Upconversion Security Printing Prospective ......................................... 293 10.4 Synthesis, Functionalization, and Scale-up ........................................... 298
10.4.1 Hansen Solubility Parameters ..................................................... 299 10.5 Toxicity Assessment .................................................................................. 299 10.6 Security Printing Applications ................................................................ 301
10.6.1 Red, Green, and Blue Upconversion Printing ............................302 10.6.2 Micro Upconverting QR Code Tags ............................................304 10.6.3 Color-Coded Multilayer Photopatterned Microstructures ......305 10.6.4 Micro Barcodes ...............................................................................305 10.6.5 Tunable Lifetime Multiplexing of UCNPs ..................................305 10.6.6 NIR-to-NIR Upconversion Security Printing ............................308 10.6.7 Scanning Laser Imaging of Upconversion Printing ................. 312 10.6.8 Multicolor Nano Barcoding in a Single
Upconversion Crystal ................................................................ 313 10.6.9 Upconversion Polymeric Nanobers .......................................... 314
10.7 Readability of Upconversion Prints ........................................................ 314 10.8 Conclusion .................................................................................................. 315 Acknowledgments .............................................................................................. 316 References ............................................................................................................. 316
efciency (Haase and Schäfer 2011). In the 1950s, the UC effect was rst discovered and studied intensively during the 1960s but up until 2003 it was observed only in bulk, transition metals or rare earth-doped phosphors which were suggested to have applications in the eld of detectors, display, and laser materials (Bünzli and Eliseeva 2011). Since then UC materials have found their application in solar cells, waveguide ampliers and later, they became applicable in rewritable optical storage nondestructive optical memory and many more potential scientic applications; and in the last decade, UC materials became well known in the biological eld for bio sensing, drug delivery, etc. (Ho et al. 2003; Kim et al 2009; Shalav et al. 2005; Suyver et al. 2005; Vennerberg and Lin 2011; Wang and Bass 2004; Wang and Liu 2009; Wang et al. 2010; Zhou et al. 2015).
