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Counterfeiting of valuable documents, currency and branded products is a challenging problem that has serious economic, security and health ramifications for governments, businesses and consumers all over the world. It is estimated that counterfeiting represents a multi-billion dollar underground economy with counterfeit products being produced on a large scale every year. Counterfeiting is an increasingly high-tech crime and calls for high-tech solutions to prevent and deter the acts of counterfeiting. The present review briefly outlines and addresses the key challenges in this area, including the above mentioned concerns for anti-counterfeiting applications. This article describes a unique combination of all possible kinds of security ink formulations based on lanthanide doped luminescent nanomaterials, quantum dots (semiconductor and carbon based), metal organic frameworks as well as plasmonic nanomaterials for their possible use in anti-counterfeiting applications. Moreover, in this review, we have briefly discussed and described the historical background of luminescent nanomaterials, basic concepts and detailed synthesis methods along with their characterization. Furthermore, we have also discussed the methods adopted for the fabrication and design of luminescent security inks, various security printing techniques and their anti-counterfeiting applications.

In this paper, the synthesis of multi-stage excitable (379 nm, 980 nm and 1550 nm), highly luminescent3+ nanorods by a hydrothermal method which can emit both hypersensitive green (562 nm) and strong red (660 nm) in a single host lattice and which can be used for a transparent security ink application is reported. Furthermore, these luminescent nanorods also exhibit both down-shift (1127 nm) as well as upconversion (562 nm) features at a single excitation near infrared wavelength of 980 nm. The characterization of these luminescent nanorods was explored by studying their structure/microstructure, using photoluminescence and time-resolved spectroscopic techniques. Furthermore, these luminescent nanorods exhibited the tuning of emission colours from red to green by controlling the sintering temperature. It was demonstrated that these novel luminescent nanorods offer new opportunities for making high-end, multi-stage excitable transparent security ink. The state-of-art production process of ink is easy and it is most suitable for bulk production at an economical cost. Thus, the security inks could be used globally in applications for protection against counterfeiting.

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The duplicity of important documents has emerged as a serious problem worldwide. Therefore, many efforts have been devoted to developing easy and fast anticounterfeiting techniques with multicolor emission. Herein, we report the synthesis of multicolor luminescent lanthanide-doped YO nanorods by hydrothermal method and their usability in designing of unclonable security codes for anticounterfeiting applications. The spectroscopic features of nanorods are probed by photoluminescence spectroscopy. The YO:Eu, YO:Tb, and YO:Ce nanorods emit hypersensitive red (at 611 nm), strong green (at 541 nm), and bright blue (at 438 nm) emissions at 254, 305, and 381 nm, respectively. The SEM and TEM/HRTEM results reveal that these nanorods have diameter and length in the range of 80-120 nm and ∼2-5 μm, respectively. The two-dimensional spatially resolved photoluminescence intensity distribution in nanorods is also investigated by using confocal photoluminescence microscopic technique. Further, highly luminescent unclonable security codes are printed by a simple screen printing technique using luminescent ink fabricated from admixing of lanthanide doped multicolor nanorods in PVC medium. The prospective use of these multicolor luminescent nanorods provide a new opportunity for easily printable, highly stable, and unclonable multicolor luminescent security codes for anti-counterfeiting applications.

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