RESUMO
Upconversion nanoparticles (UCNPs) are an excellent choice to construct security features against counterfeiting, owing to their unique NIR-to-VIS upconversion luminescence (UCL) characteristics. However, the application of upconversion materials is limited, due to their single and invariant emission colors. Herein, the temperature-dependent UCL properties of NaGdF4:Yb/Ho (or Tm) UCNPs in the solid state have been investigated. An anomalous UCL enhancement at higher temperatures has been demonstrated for these small-sized (<10 nm) UCNPs and the underlying mechanism is discussed herein. Meanwhile, effective UCL with tunable multicolor emissions has been realized by the rational incorporation of Ho3+ and Tm3+ emitters into a single nanostructure. The emission colors of these Ho/Tm co-doped Na(Gd,Yb)F4 UCNPs can be tuned by changing the laser power or temperature, due to the different spectral sensitivities of the Tm3+ and Ho3+ emitters to the excitation power density and temperature. The power- and temperature-responsive color shifts of these Ho/Tm co-doped UCNPs are favorable for immediate recognition by the naked eye, but are hard to copy, offering the possibility of designing more secure anti-counterfeiting patterns.
RESUMO
Upconversion nanoparticles (UCNPs) provide an ideal platform for achieving multifunction, such as multimodal imaging, sensing, therapy, etc., mainly by combining with other nanomaterials to construct complicated heterogeneous nanostructures. Multifunctional integration on a simple single-phase structure still is an open question and poses a big challenge. Here we show that small-sized NaGdF(4):Yb(3), Er(3+) UCNPs (~7.5 nm) can simultaneously possess upconversion luminescence (UCL), temperature sensing, paramagnetic and photothermal conversion properties, endowing them great potential for photothermal treatments with real-time imaging and temperature monitoring. Effects of Yb(3+) concentrations, nanoparticle sizes and core/shell structures on the light-to-heat conversion capability of UCNPs were also investigated, and the results were discussed on the basis of the variation in absorption rates and non-radiative relaxation probabilities of UCNPs. There is a competition between UCL and light-to-heat conversion processes. Higher UCL efficiency and enhanced photothermal conversion properties can be realized on UCNPs with the active-core/active-shell structure due to enhanced absorption rates.
