*Article* **Dye Sensitization for Ultraviolet Upconversion Enhancement**

**Mingkai Wang 1, Hanlin Wei 1, Shuai Wang 1, Chuanyu Hu 2,3,4,\* and Qianqian Su 1,\***


**Abstract:** Upconversion nanocrystals that converted near-infrared radiation into emission in the ultraviolet spectral region offer many exciting opportunities for drug release, photocatalysis, photodynamic therapy, and solid-state lasing. However, a key challenge is the development of lanthanidedoped nanocrystals with efficient ultraviolet emission, due to low conversion efficiency. Here, we develop a dye-sensitized, heterogeneous core–multishelled lanthanide nanoparticle for ultraviolet upconversion enhancement. We systematically study the main influencing factors on ultraviolet upconversion emission, including dye concentration, excitation wavelength, and dye-sensitizer distance. Interestingly, our experimental results demonstrate a largely promoted multiphoton upconversion. The underlying mechanism and detailed energy transfer pathway are illustrated. These findings offer insights into future developments of highly ultraviolet-emissive nanohybrids and provide more opportunities for applications in photo-catalysis, biomedicine, and environmental science.

**Keywords:** lanthanide nanoparticles; ultraviolet upconversion; dye sensitization; heterogeneous nanoparticles; energy transfer; luminescence enhancement
