Li ions affect the upconversion efficiency by changing the local crystal field of the luminescent center. Herein, in order to improve the upconversion efficiency of NaYF₄:Yb³⁺/Eu³⁺, a series of NaYF₄:Yb³⁺/Eu³⁺ micro-particles with different Li⁺ doping concentrations were synthesized by the hydrothermal synthesis method, respectively. Firstly, the structure and morphology of NaYF₄:Yb³⁺/Eu³⁺ upconversion micro-particles (UCMPs) with different doping concentrations were analyzed by X-ray diffraction and a scanning electron microscope (SEM). SEM results show that the UCMPs are not only highly crystallized, but also have hexagons with different Li⁺ concentrations of NaYF₄:Yb³⁺/Eu³⁺. X-ray diffraction shows that the crystal field around Eu³⁺ changes with the increase of Li⁺ concentration. Then, the fluorescence spectrum of NaYF₄:Yb³⁺/Eu³⁺ was studied under the irradiation of a 980 nm laser. The results show that the fluorescence intensity of NaYF₄:Yb³⁺/Eu³⁺ with 2% Li⁺ is the strongest, which is twice the intensity of NaYF₄:Yb³⁺/Eu³⁺ without Li⁺. Finally, the fluorescence imaging analysis of NaYF₄:Yb³⁺/Eu³⁺ with 2% Li⁺ concentration was carried out. The UCMPs are used to screen printing to evaluate the imaging effect on different sample surfaces. The results show NaYF₄:Yb³⁺/Eu³⁺ (with 2% Li⁺) has great application prospects in anti-counterfeiting recognition.

In this Letter, a miniature wearable Raman spectroscopy system is developed. A wearable fiber-optic probe is employed to help the stable and convenient collection of Raman spectra. A nonlinear partial least squares model based on a multivariate dominant factor is employed to predict the glucose level. The mean coefficients of determination are 0.99, 0.893, and 0.844 for the glucose solution, laboratory rats, and human volunteers. The results demonstrate that a miniature wearable Raman spectroscopy system is feasible to achieve the noninvasive detection of human blood glucose and has important clinical application value in disease diagnosis.