**1. Introduction**

Lanthanide-doped upconversion nanoparticles can absorb near-infrared (NIR) laser light and emit visible and ultraviolet light, with potential applications in bioimaging [1–5], biotherapy [6–12], and so on. In particular, the applications of these nanoparticles in optogenetic [13,14], photothermal [15,16], and photodynamic [17–19] therapy could be achieved via ultraviolet (UV) light emission under NIR excitation. Although UV light can be obtained by Nd3+- and Yb3+-sensitized upconversion [17,18,20,21], it is challenging to realize the high luminescence intensity needed to satisfy the minimum requirement of biological applications. This obstacle can be addressed in several ways: by controlling dopant composition [22], nanoparticle phase and size [23], excitation beam pulse width [24], and nanoparticle core–shell design [21,25–29]. Very recently, our group has made significant progress in overcoming the difficulty using an upconverted excitation lock-in (UCEL) strategy [30].

Hybrid systems are composed of inorganic nanoparticles and an organic dye, which can significantly strengthen the absorbance and expand the absorbance spectra of inorganic nanoparticles [31], leading to enhancement of their emission intensities. It has been demonstrated that NIR dye can effectively enhance the upconversion emission of lanthanide-doped nanoparticles [14,32–34]. However, previous studies have mainly focused on the analysis of visible upconversion emission. Little effort has been made to develop a hybrid nanoparticle with enhanced UV luminescence.

**Citation:** Wang, M.; Wei, H.; Wang, S.; Hu, C.; Su, Q. Dye Sensitization for Ultraviolet Upconversion Enhancement. *Nanomaterials* **2021**, *11*, 3114. https://doi.org/10.3390/ nano11113114

Academic Editors: Marcin Runowski and Julia Pérez-Prieto

Received: 20 October 2021 Accepted: 11 November 2021 Published: 18 November 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In this study, we developed IR-806-loaded upconversion nanoparticles (Gd-CSYS2S3@ IR-806) with enhanced upconversion emission in the UV spectral region. The key factors that influence upconversion emission were studied, including dye concentration, excitation wavelength, and distance between the dye and the sensitizer Nd3+ (Scheme 1). We also demonstrated the dominant effect of ligand loading on multiphoton upconversion. In addition, the upconversion mechanism and the energy transfer pathway in Gd-CSYS2S3@IR-806 hybrid nanoparticles were carefully studied. This study provides new insights into the mechanistic understanding of UV upconversion luminescence in hybrid nanoparticles and enables new opportunities for these nanomaterials in a broad range of applications.

**Scheme 1.** Schematic illustration of the key factors that influence UV enhancement in IR-806- loaded upconversion nanoparticles, including dye concentration, excitation wavelength, and dye– sensitizer distance.

## **2. Materials and Methods**
