Multi-stimuli-responsive and dynamic color tunable security ink for multilevel anticounterfeiting.

Luminescent security features have been used for anticounterfeiting for a long time. However, constant effort is required to strengthen these security features to be ahead of counterfeiters. Here, we developed a multi-stimuli-responsive luminescent security ink containing Tb(ASA) 3 Phen, K 2 SiF 6 :Mn 4+, and NaYF 4 :Yb 3+ /Er 3+ luminescent materials in PVC gold medium. Tb(ASA) 3 Phen complex shows a broad excitation band in the UV region; upon UV light radiation it shows strong greenish emission of Tb 3+ ions through the antenna effect. K 2 SiF 6 :Mn 4+ , on the other hand, has three excitation bands with maxima at 248, 354, and 454 nm which emit red light after excitation through these bands. NaYF 4 :Yb 3+ /Er 3+ is used as an upconverting nanophosphor showing green emission under 976 nm laser excitation. Thus, the multi-stimuli-responsive luminescent security ink shows greenish, red, and green emissions under 367 nm, 450 nm, and 976 nm excitations, respectively. Furthermore, the distinct lifetimes of the activators in Tb(ASA) 3 Phen and K 2 SiF 6 :Mn 4+ , i.e. 0.1708 ms and 8.165 ms, respectively, under 380 nm excitation make this ink suitable for dynamic anticounterfeiting as well. The ink shows a change in the emission color with time delay, after the removal of the 380 nm excitation source, from greenish yellow (at 0 delays) to reddish color after a delay of 7.5 ms. These unique optical features along with excellent photo-, chemical- and environmental stability make this ink useful for advanced-level anticounterfeiting.