**2. Upconversion Luminescence Nanomaterials**

Upconversion nanomaterials can emit high-energy lights when excited with two or more low-energy photons. They can produce ultraviolet (UV)-visible or near-infrared (NIR) light upon excitation with NIR light, depending on size or dopants, owing to their unique properties, such as good optical stability, narrow emission band, large anti-Stokes spectral shift, high levels of light penetration in biological tissues, long luminescence lifetime, and high signal-to-noise ratio. The review paper by Dr. Jigang Wang and his collaborators systematically introduced the physical mechanism of upconversion luminescence nanomaterials and their potential applications in bioimaging, detection, photodynamic therapy, and therapeutics [1].

Upconversion nanocrystals converting near-infrared light into high-energy UV emissions may provide many exciting opportunities for drug release, photocatalysis, photodynamic therapy, and solid-state lasing. However, a key challenge is their low conversion efficiency. For that, Dr. Chuanyu Hu's team [2] proposed and developed dye-sensitized and heterogeneous core–shell lanthanide nanostructures for ultraviolet upconversion improvement. They systematically investigated the main factors on ultraviolet upconversion emission. Interestingly, they found a method for a largely promoted multiphoton upconversion, which provides more opportunities for applications in biomedicine, photo-catalysis, and environmental science.

#### **3. Luminescence for Solid-State Lighting, Displays, and Anti-Counterfeiting**

White-light-emitting diodes show grea<sup>t</sup> promise for replacing traditional lighting devices because of their high efficiency, low energy consumption, and long lifetime. Metal

**Citation:** Chen, W.; Cao, D. Luminescence Nanomaterials and Applications. *Nanomaterials* **2023**, *13*, 1047. https://doi.org/10.3390/ nano13061047

Received: 24 February 2023 Accepted: 7 March 2023 Published: 14 March2023

**Copyright:** © 2023 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/).

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organic frameworks (MOFs) are good materials for white-light emissions. The encapsulation of organic dyes is a simple way to obtain luminescent MOFs. In a review, Dr. Derong Cao and his collaboration team summarized the recent research on the design and construction of dye-encapsulated MOFs phosphors and their potential applications [3].

Dr. Dangli Gao and her collaboration team reported the optical properties of Zn3Ga2GeO8:Mn phosphors that could be modified by different preparation methods, including a hydrothermal method and solid-state diffusion combined with a non-equivalent ion-doping strategy [4]. Consequently, Mn-doped Zn3Ga2GeO8 phosphors prepared by a hydrothermal method showed an enhanced red emission at 701 nm and a green persistent luminescence at 540 nm, while the phosphors prepared by solid-state diffusion in combination with hetero-valent doping only exhibited an enhancement in the single-band red emission. Furthermore, the substitution of hetero-valent dopant ion Li+ into different sites can change the emission colors. These fantastic phenomena were discussed in detail in the paper [4].

The study of room-temperature phosphorescent carbon quantum dots is important for various applications. Dr. Hongmei Yu et al. [5] successfully fabricated matrix-free carbonized polymer dots (CPDs) that can produce green room-temperature phosphorescence under dual-mode visible- and ultraviolet-light excitations. Hydrogen bonding can provide a space protection and stably excite the triplet state. This self-matrix structure effectively avoids the non-radiative transition by blocking the intramolecular motion of CPDs. The long lasting room-temperature phosphorescence is good for applications in anti-counterfeiting.

#### **4. Particle Based Sensing Technology**

Early cancer detection is important, and plenty of sensors are being explored. Dr. Liu and her collaboration team reported novel lanthanide-upconverted NaYF4:Yb,Tm fluorescence probes, which can detect cancer-related specific miRNAs in very low concentrations [6]. The detection is based on emissions at 345, 646, and 802 nm upon excitation at 980 nm. The two common proteins, miRNA-155 and miRNA-150, were captured by the designed fluorescent probes. The probes can effectively distinguish miRNA-155 from partialand complete-base mismatched miRNA-155, which is critical for early cancer detection.

Sulfur quantum dots (SQDs) are considered potential green nanomaterials because they have no heavy metals. Dr. Yang [7] and his collaboration team prepared SQDs by a microwave-assisted method using sulfur powder as a precursor. SQDs show the highest emission at 470 nm when excited at 380 nm and have a good sensitivity and selectivity in alkaline phosphatase detection.

Radiation detection and dosimetry are old questions but pose new challenges for security and safety. As such, Dr. Abd Khamim Ismail et al. [8] examined the thermoluminescence dosimetry behaviors of Ag-ZnO films. The dose–response revealed high linearity up to 4 Gy. The proposed sensitivity was 1.8 times higher than the TLD 100 dosimeters.
