Upconversion Nanoparticle Composites

In recent years, the design and application of multifunctional nanocomposites have attracted extensive research interest from scientists. Combining two or more materials through a specific route to construct a new type of material not only overcomes the limitations of a single component itself, but also exhibits dual or multifunctional properties. Rare earth ion-doped upconversion nanoparticles (UCNPs) are favored by researchers in various fields due to their unique physical and chemical properties. UCNPs are combined with other functional materials to achieve synergistic effects, and the resulting nanocomposites show great application potential in biomedicine, anti-counterfeiting and photocatalysis.

Rare earth-doped Upconversion Nanoparticles (UCNPs)

Upconversion luminescence is the luminescence process of absorbing two or more low-energy photons and radiating one high-energy photon, which is, converting long-wave radiation into short-wave radiation. It is an anti-Stokes luminescence. Rare earth-doped UCNPs have the advantages of narrow emission band, long fluorescence lifetime, low toxicity, large anti-Stokes shift, adjustable luminescence color, no spontaneous fluorescence of biological tissues, no photobleaching and flickering, etc., and show great application potential in biomedicine, three-dimensional stereoscopic display, anti-counterfeiting technology and solar spectrum conversion.

Construction Strategies of Upconversion Nanoparticles -based Nanocomposites

At present, many strategies have been developed to integrate UCNPs and other functional materials into a nanosystem. The research on the construction strategies and synthesis methods of UCNPs nanocomposites mainly includes self-assembly (electrostatic adsorption, specific recognition and covalent bonding), in situ growth and epitaxial growth. Self-assembly is the most commonly used method for constructing UCNPs-based nanocomposites. It usually requires the preparation of various monomer components in advance, and then self-assembles to form a nanosystem through electrostatic adsorption, specific recognition or covalent bonding. In situ growth usually requires polymer modification with special functional groups to form UCNPs to form precursors, which are used as nucleation and growth centers to induce other nanodots to further grow on their surface to form a core-satellite structure. Epitaxial growth first synthesizes monodisperse core nanocrystals, and then introduces shell precursors to achieve orderly growth of core-shell structures by regulating composition, relative arrangement, exposed crystal faces and interfaces. Usually, some strict requirements need to be met, such as matching of crystal structure and lattice parameters, similar reaction temperature and synthesis conditions, etc.

Applications of Upconversion Nanoparticles-based Nanocomposites

With the increasing interest in research on UCNPs-based nanocomposites, it has been widely used in some emerging fields.

1. Bioimaging is an important biological analysis and diagnostic tool. Common imaging techniques include optical imaging, magnetic resonance imaging (MRI), computed tomography X-ray scanning imaging (CT), single photon emission computed tomography (SPECT) and photoacoustic imaging (PAI). Although molecular probes have been used for imaging for many years, the imaging contrast provided by traditional molecular probes is insufficient and can usually only be used in a single imaging mode. UCNPs have good chemical and optical stability and good biocompatibility, and near-infrared excitation light can effectively avoid the interference of biological background fluorescence and achieve high signal-to-noise ratio biological imaging. At the same time, the combination of UCNPs with other nanofunctional materials as multimodal bioimaging probes has also been widely studied, and its imaging performance has been verified in cell and small animal models.

2. How to achieve effective treatment of cancer has always been a difficult problem and research hotspot in the medical field. Due to the complexity and diversity of the tumor environment, single-mode treatment cannot completely eradicate malignant tumors. The combination of multiple treatment methods can overcome the limitations of a single treatment method and achieve synergistic and effective cancer treatment. This article focuses on the application potential of UCNPs-based nanocomposites in various treatment systems, including chemotherapy, photothermal therapy, photodynamic therapy, radiation therapy, chemodynamic therapy, gas therapy, immunotherapy, and synergistic treatment of various treatment methods. The combination of multiple treatment methods can overcome the weaknesses and limitations of a single treatment method, thereby effectively inhibiting tumor growth, recurrence and metastasis.

3. Counterfeit and shoddy currency, drugs and valuables are increasingly damaging the market economy, causing immeasurable losses to consumers and copyright owners. Rare earth-doped UCNPs are ideal anti-counterfeiting materials in the field of fluorescent anti-counterfeiting due to their rich intermediate energy levels and distinguishable spectral characteristics. However, traditional single-light source excitation and single-mode fluorescence greatly limit their application. By combining UCNPs with other luminescent materials to develop new nanocomposites, specific fluorescence signals can be emitted over a wide spectral range to achieve multi-color dual-mode fluorescence anti-counterfeiting and information storage.

4. The development of photocatalysts with broad-spectrum absorption characteristics (ultraviolet to near-infrared light region) to achieve the effective use of solar energy in various fields (such as photocatalytic hydrogen production, elimination of environmental pollutants, antibacterial, etc.) has always been a hot topic of research. UCNPs can absorb near-infrared light and convert it into ultraviolet/visible light. Therefore, the nanocomposite constructed by combining UCNPs and semiconductor materials can be excited by near-infrared light to generate photogenerated electrons (e⁻) and holes (h⁺), thereby making full use of sunlight and improving photocatalytic efficiency.

Prospects and Challenges of Upconversion Nanoparticles Composites

As one of the most groundbreaking frontier technologies, nanotechnology has promoted the cross-disciplinary integration and development, as well as the design and construction of nanocomposites. In recent years, rare earth ion-doped UCNPs have aroused research interest due to their unique physical and chemical properties. UCNPs combined with other functional materials to construct nanocomposites and achieve synergistic effects can be used as candidate materials with more powerful functions, providing better upconversion luminescence effects and thus exerting greater application potential. It is worth noting that despite the good progress made in the synthesis and application of UCNPs-based nanocomposites, there are still challenges in the following aspects.

The existing synthesis methods based on UCNPs nanocomposites still have many shortcomings. The self-assembly method has the disadvantages of being time-consuming, easy to aggregate, and weakly adsorbed, and the structure is easily destroyed under the action of certain solvents. The in-situ growth method also has certain limitations. The surface of UCNPs must be modified with a polymer with special functional groups to form a precursor, which is used as a nucleation and growth center to induce other nanodots to grow further on the surface. This has prompted us to develop more novel modified materials to ensure that the luminescence of UCNPs will not be excessively quenched, and it can also well ensure that other materials grow uniformly on the surface of UCNPs. The epitaxial growth method often uses toxic and expensive precursors or organic solvents, the product is hydrophobic, and the synthesis temperature is relatively high. Heteroepitaxial growth requires more stringent conditions, and it is difficult to track the reaction process in situ and thus it is difficult to accurately elucidate its reaction mechanism. Therefore, other simple and outstanding synthetic methods still need to be developed and explored.

The biological application research based on UCNPs nanocomposites is still in its infancy, and there are still many problems to be solved. It is of great significance to quantitatively load functional molecules into nanocomposites and achieve controlled release. It cannot be ignored that reducing biological toxicity, improving metabolic efficiency in vivo, and ensuring the repeatability of diagnostic and therapeutic effects are prerequisites for the future use of UCNPs-based nanocomposites in biomedicine.

By combining UCNPs with other luminescent substances, tunable, multicolor, and multimodal luminescent nanocomposites are obtained, which greatly improves the level of fluorescent anti-counterfeiting. Although multiple anti-counterfeiting materials can be used simultaneously to give them multiple security features, the implementation process is complicated, resulting in low production efficiency. This has prompted researchers to develop new nanocomposites with photostability, multiple anti-counterfeiting features and identification methods, and easy processing to promote their practical application.

Although great progress has been made in constructing nanocomposites with certain photocatalytic activity in the ultraviolet to near-infrared region by enhancing the upconversion luminescence of UCNPs, introducing semiconductors, and constructing heterogeneous structures, the utilization rate of light is still not high. The photocatalytic mechanism of nanocomposites needs to be further studied, and the photocatalytic efficiency needs to be further improved.


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