Binary temporal upconversion codes of Mn2+-activated nanoparticles for multilevel anti-counterfeiting

Abstract

Optical characteristics of luminescent materials, such as emission profile and lifetime, play an important role in their applications in optical data storage, document security, diagnostics, and therapeutics. Lanthanide-doped upconversion nanoparticles are particularly suitable for such applications due to their inherent optical properties, including large anti-Stokes shift, distinguishable spectroscopic fingerprint, and long luminescence lifetime. However, conventional upconversion nanoparticles have a limited capacity for information storage or complexity to prevent counterfeiting. Here, we demonstrate that integration of long-lived Mn²⁺ upconversion emission and relatively short-lived lanthanide upconversion emission in a particulate platform allows the generation of binary temporal codes for efficient data encoding. Precise control of the particle's structure allows the excitation feasible both under 980 and 808 nm irradiation. We find that the as-prepared Mn²⁺-doped nanoparticles are especially useful for multilevel anti-counterfeiting with high-throughput rate of authentication and without the need for complex time-gated decoding instrumentation.Luminescent materials that are capable of binary temporal coding are desirable for multilevel anti-counterfeiting. Here, the authors engineer nanoparticles that produce binary color codes on different timescales by combining the long-lived luminescence of Mn²⁺ with the relatively short-lived emission of lanthanides.

Fig. 1

Rational design of binary temporal upconversion codes through Mn²⁺ codoping. a Structure design of a multilayer nanoparticle for simultaneously displaying short- and long-lived upconversion emissions. Noted that NaGdF₄:Mn(30 mol%) and NaGdF₄:Yb/Tm(49/1 mol%) are exploited as core and first shell (S₁) of the multilayer nanoparticle to achieve long-lived Mn²⁺ luminescence through Gd-sublattice-mediated energy migration. Other characteristic lanthanide emissions from the nanoparticle can be realized by further coating additional shell layers (S_n) with different lanthanide compositions: strategy i S₂ = NaYF₄, ii S₂ = NaYF₄:A (A = Eu³⁺, Eu³⁺/Tb³⁺ or Tb³⁺), iii S₂ = NaYF₄, S₃ = NaYF₄:Yb/Er (5/0.05, 20/2 or 50/0.05 mol%), iv S₂ = NaYF₄:Nd (20 mol%), S₃ = NaYF₄:Nd/Yb/Er(1/30/0.5 or 2/10/1 mol%), and v S₂ = NaYF₄, S₃ = NaYF₄:Nd (20 mol%), S₄ = NaYF₄:Nd/Yb/Er(2/10/1 mol%). b Proposed energy transfer pathway between the core and shell layers accounting for the simultaneous generation of short- and long-lived upconversion luminescence under excitation at 980 nm

Fig. 2

Characterization of multilayer nanoparticles of NaGdF₄:Mn (30 mol%)@NaGdF₄:Yb/Tm (49/1 mol%)@NaYF₄. a TEM image of the as-prepared upconversion nanoparticles, scale bar, 50 nm. (Inset: high-resolution TEM imaging of a single core-shell-shell nanoparticle, scale bar, 5 nm). b Emission profile of the as-prepared nanoparticles under excitation at 980 nm (power density: 30 W cm⁻²). c Time-resolved spectrum of the as-prepared Mn²⁺-doped upconversion nanoparticles. d Lifetime comparison of Mn²⁺ emission (550 nm, ⁴T₁ → ⁶A₁) and Tm³⁺ (475 nm, ¹D₂ → ³F₄) of the as-prepared Mn²⁺-doped nanoparticles recorded in aqueous solution at room temperature. e Power density dependence of the upconverted Tm³⁺ and Mn²⁺ emissions. Note that the slopes of power-dependent emission centered at 345 nm for Tm³⁺ and at 550 nm for Mn²⁺ are measured to be 3.34 and 3.36, respectively. f Proposed cross-relaxation (CR) processes between two neighboring Tm³⁺ ions, accounting for the experimentally observed lower exponential power dependence of the Tm³⁺ and Mn²⁺ emissions

Fig. 3

Synthetic strategies for tuning short-lived upconversion emission. a Doping of lanthanide activators (A = Eu³⁺, Eu³⁺/Tb³⁺ or Tb³⁺) into the outmost shell (S₂) of NaYF₄ to realize interfacial energy transfer. In contrast to energy migration strategy, interfacial energy transfer mainly occurs at the interface and has less effect on the Mn²⁺ luminescence. b Representative emission profiles of the as-prepared NaGdF₄:Mn(30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄:A (A = Eu³⁺, Eu³⁺/Tb³⁺ or Tb³⁺) nanoparticles. c Strategy involving the addition of NaYF₄ (S₂) and NaYF₄:Yb/Er (S₃) layers. Note that the emission dependence of the NaYF₄:Yb/Er shell on the relative doping level of Yb/Er provides an additional means to modulate the emission color of the nanoparticles. d Emission profiles of the as-prepared NaGdF₄:Mn(30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄@NaYF₄:Yb/Er(5/0.05, 20/2 or 50/0.05) nanoparticles. e Doping of Nd³⁺ into the NaYF₄ shell (S₂) allows the excitation to be carried out either under 980 or 808 nm. Notably, the small-sized and curved arrow between S₁ and S₂ layers is used to represent a much weaker interfacial energy transfer from Gd³⁺ to Nd³⁺ relative to energy migration from Gd³⁺ in the S₁ layer to Mn²⁺ in the core. f Emission spectra of NaGdF₄:Mn(30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄:Nd(20 mol%) nanoparticles and the corresponding multilayer nanoparticles passivated with NaYF₄:Nd/Yb/Er (x mol%) (x = 1/30/0.5 or 2/10/1) under excitation at 980 (red curve) and 808 nm (black curve). The pump powers of the 980 and 808 nm lasers were fixed at 1 and 4 W for spectral measurement, respectively. g Luminescence photographs showing multicolour tuning of the steady upconversion of the as-prepared nanoparticles under excitation at 980 or 808 nm. The energy transfer from Nd³⁺  → Yb³⁺  → Tm³⁺  → Gd³⁺  → Mn²⁺ under excitation at 808 nm can be largely suppressed by growth of an inert shell between NaYF₄:Nd (20 mol%) and NaGdF₄:Yb/Tm(49/1 mol%), and thus different color outputs can be generated upon excitation at 980 and 808 nm (Supplementary Fig. 19)

Fig. 4

Multilevel anti-counterfeiting application with Mn²⁺-activated core-shell nanoparticles. a General design of the 2D patterns (i to vi) made with nanoparticles of different composition. Steady irradiation with a 980 nm laser (6 W cm⁻²) leads to multicolor features of the patterns, while dynamic scanning of the patterns with a focused laser beam (64 W cm⁻²) gives rise to a different scenario. Unlike purely lanthanide-doped nanoparticles with only a bright spot of emission emerging from pattern i under the dynamic scanning, the as-prepared Mn²⁺-doped nanoparticles show a bright spot of emission with a green-colored tail from pattern v. b Emission profiles of the patterns recorded under different irradiation conditions. Each pattern varies significantly in emission color under dynamic scanning at 980 nm. Steady irradiation or dynamic scanning at 808 nm of the patterns iv and v, made with Nd³⁺-sensitized nanoparticles, can provide a similar level of readout to that on 980 excitation. Nanoparticles used for generating the patterns: i NaYF₄:Yb/Er (20/2 mol%); ii NaGdF₄:Mn (30 mol%)@NaGdF₄:Yb/Tm (49/1 mol%)@NaYF₄@NaYF₄:Yb/Er (50/0.05 mol%); iii NaGdF₄:Mn (30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄:Eu(20 mol%); iv NaGdF₄:Mn(30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄:Nd(20 mol%)@NaYF₄:Nd/Yb/Er(2/10/1 mol%); v NaGdF₄:Mn(30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄:Nd(20 mol%); and vi NaGdF₄:Mn(30 mol%)@NaGdF₄:Yb/Tm(49/1 mol%)@NaYF₄@NaYF₄:Yb/Er(5/0.05 mol%)