Upconversion nanoparticles (UCPNs) capable of emitting near-infrared (NIR) light under NIR excitation attract much attention for bioapplications. However, the low upconversion efficiency and emission intensity significantly limits the progress in utilizing UCPNs. The present work is dedicated to the enhancement of upconversion efficiency and NIR luminescence intensity.

The theoretical model that describes the changes in the populations of the excited states participating in the upconversion process was developed using a rate equations system for Yb³⁺–Tm³⁺ ions in hexagonal NaGdF₄. According to the modeling results, by increasing of Yb³⁺ content to 80%, the intensity of NIR upconversion luminescence corresponding to ³H₄–³H₆ transition could be increased up to 3.5 times in comparison to 30:1.5 Yb–Tm concentration ratio which was calculated to be optimal and up to 9.5 times in comparison to often used 30:0.5 Yb–Tm concentration. The optimal doping content regions were determined. For 80% Yb³⁺ content the optimal Tm³⁺ content will be 2.5%, for 30% Yb³⁺ content—1.5%.

Modeling results were confirmed experimentally by the synthesis of NaGdF₄ nanoparticles doped with Yb³⁺ and Tm³⁺ with the concentration ratios of 80:2, 80:3, 80:4, and 80:5. To obtain single-phase hexagonal samples at high doping concentrations, higher synthesis temperatures were required. A single-phase sample NaGdF₄: Yb³⁺–Tm³⁺ 80:5% was obtained at 320 C synthesis temperature. According to the spectroscopic study of the obtained nanoparticles, the optimal concentration ratio for intense NIR luminescence obtaining was 80:3.

The modeling results were in good agreement with the literature data and the results of our experiments. The developed model allows determining the optimal doping concentrations for obtaining effective NIR-to-NIR converters, based on NaGdF₄:Yb³⁺–Tm³⁺ nanoparticles.

能够在近红外（NIR）激发下发射近红外（NIR）光的上转换纳米粒子（UCPNs）在生物应用中引起了很多关注。然而，低的上转换效率和发射强度显着限制了使用UCPNs的进展。目前的工作致力于提高上转换效率和近红外发光强度。

使用速率方程系统对六角形NaGdF _4中的Yb ³⁺ -Tm ³⁺离子建立了描述参与上转换过程的激发态总体变化的理论模型。根据建模结果，通过将Yb ³⁺含量提高到80％，与30：1.5 Yb–Tm浓度相比，与³ H ₄ – ³ H ₆跃迁相对应的NIR上转换发光强度可以提高到3.5倍。与通常使用的30：0.5 Yb–Tm浓度相比，该比率被计算为最佳，最高可达9.5倍。确定了最佳掺杂含量区域。对于80％Yb³⁺含量，最佳Tm ³⁺含量为2.5％，Yb ³⁺含量为1.5％。

通过合成掺杂浓度为80：2、80：3、80 ：4和80：5的Yb ³⁺和Tm ³⁺的NaGdF ₄纳米颗粒，实验确定了建模结果。为了获得高掺杂浓度的单相六角形样品，需要更高的合成温度。在320℃的合成温度下获得了单相样品NaGdF ₄：Yb ³⁺ -Tm ³⁺ 80：5％。根据所得纳米颗粒的光谱研究，获得强NIR发光的最佳浓度比为80：3。

建模结果与文献数据和我们的实验结果非常吻合。基于NaGdF ₄：Yb ³⁺ –Tm ³⁺纳米粒子，开发的模型可以确定获得有效NIR到NIR转换器的最佳掺杂浓度。