Core/Shell Upconversion Nanocrystal Hybrids with Temperature‐Dependent Emission Color Changes for Multilevel Anticounterfeiting Applications

Abstract

terfeiting nanomaterials with highlevel security features have attracted increasing interest in recent years, taking advantages of diverse luminescence characteristics of UCNCs, such as multicolor emission, [2a,3] excitation power dependence, [4] dual-mode fluorescence, [5] tunable luminescence lifetime, [6] and plasmonic coupling effects. [7] These novel UCNCs can produce color-coded or color-tunable anticounterfeiting patterns, authenticated by changing the excitation wavelength/power or utilizing the timeresolved scanning method. Although the extra anticounterfeiting features make these novel UC nanomaterials or nanocomposites very hard to duplicate, they typically require complex fabrication processes and costly reading instruments, such as ultrahigh power lasers and time-gated decoding instrumentation. Hence, the exploration of novel UC nanomaterials with high-level security, simple fabrication procedures, and convenient authentication methods is highly desired.Luminescent materials generally suffer emission loss at higher temperatures, which is well known as the thermal quenching. However, Yb-sensitized core-only UCNCs have been found to show the dramatic upconversion luminescence (UCL) increase at elevated temperatures by our group [8] and also by Zhou et al. very recently. [9] This thermally induced UCL increase phenomenon has been ascribed to temperaturedependent surface effects of UCNCs. Zhou et al. proposed a surface phonon-assisted UCL model and suggested that the heat-favorable phonons existing at the NC surface increased the energy transfer from Yb 3+ to emitting ions and induced the UCL increase at elevated temperatures. [9] Nevertheless, deep spectral analysis at various temperatures and circumstances indicated that the UCL increase should be due to the gradually attenuated quenching effect of surface-adsorbed water molecules with increasing temperature. [10] The exact mechanism behind this totally different light-heat interplay behavior at the nanoscale need to be further elucidated. [11] On the other hand, this thermally induced UCL increase phenomenon provides a novel strategy to design UC anticounterfeiting nanomaterials with strengthened security features. In the present work, based on quite different UCL temperature-dependences of active-core@inert-shell NCs (thermal quenching) and active-core@active-shell ones (thermally induced increase), This work presents a novel and highly secure anticounterfeiting strategy based on core/shell upconversion nanocrystal (UCNC) hybrids with temperature-responsive multicolor emissions. Opposite luminescent temperaturedependences are found for active-core@inert-shell (thermal quenching) and active-core@active-shell (thermally induced enhancement) UCNCs. Accordingly, their hybrids are designed to show obvious color changes with increasing temperature under 975 nm excitation. Various color-shifting pathways (from white to green, blue to green, etc.) are achieved by adjusting the core/shell NC combinations in the hybrids. Moreover, color changes of the prin...

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

“…This category of RGB inks is based on microparticles with very large dimensions, which was used in inkjet printing without any problems such as clogging of the printer head due to different morphology of the hollow microtubes compared to the previous UCNPs. Shao and co-workers reported the synthesis of the core–shell upconverting nanocrystal hybrids with temperature-responsive multicolor emissions as a type of anticounterfeiting material based on UCNPs, where the multicolor emissions resulted from opposite temperature responsivity of the two series of core–shell nanoparticles with a mean particle size of 14–15 nm . The first series are named as active/core–inert/shell showing thermal quenching of upconversion luminescence, and the second sample is active/core–active/shell, which has thermally induced enhancement of upconversion luminescence (under excitation with laser light of 975 nm for both of the samples).…”

Thermal enhancement of upconversion emission in nanocrystals: a comprehensive summary

“…54 Through rational design and controlled synthesis, Hu et al recently presented thermochromic nanocomposite inks based in the combination of small-sized UCNPs with inert-shell or Yb 3+doped active-shell coatings. 68 Because of different thermally induced luminescence variation tendencies of adopted shell coating UCNPs, the emission colour of nanocomposite inks could shift throughout the chromaticity diagram following the increase in temperature. (Fig.…”

“…(d) Temperature-dependent UCL of Yb 3+ /Er 3+ NaGdF4 UCNPs with inert-shell or Yb 3+ -doped active-shell coating, and temperature-responsive colour changes of Chinese characters printed with the corresponding hybrid inks. Reproduced from ref 68. with permission from the Wiley-VCH.…”

Inflection in size-dependence of thermally enhanced up-conversion luminescence of UCNPs

“…Fortunately, the unique anomalous luminescence thermal enhancement recently discovered in UCNPs has been found to be size-dependent as well, 7–9 which initials great opportunities for applications such as optical thermal sensing and anti-counterfeiting. 10–15…”