Photon Upconversion Nanomaterials

Upconversion nanoparticles can emit ultraviolet/visible/near-infrared light under near-infrared excitation (anti-Stokes emission). This unique optical property precludes background fluorescence and light scattering from biological materials. The emission of multiple and narrow emission lines is an additional hallmark of upconversion nanoparticles that opens up new avenues for optical applications. The unique optical properties make the upconversion nanoparticles ideal for bio-imaging applications with attractive advantages such as no auto-fluorescence from bio-tissues and a large penetration depth. In this chapter, we give a general introduction to the upconversion luminescence materials from the aspects of energy transfer mechanism, category, chemical composition, nanosized upconversion materials, and so on.

Development of facile synthesis strategies for high-quality lanthanide-doped upconversion nanoparticles with controlled composition, crystalline phase, shape, and size is crucial in tuning their chemical and optical properties and exploring their potential applications in diverse fields. This chapter focuses primarily on the formation mechanism of the crystallization (including nucleation and growth) for monodisperse nanocrystals and various synthetic procedures for the upconversion nanoparticles. Finally, various optical, chemical, and structural characterizations of upconversion nanoparticles are also summarized.

The upconversion luminescence is one of the most significant parameters of upconversion nanoparticles. It is very important for various practical applications, such as bio-imaging, bio-sensing, multiplexed encoding, and so on. This chapter focuses on the upconversion luminescence of the lanthanide-doped nanocrystals, including the emission color tunability and the enhancement of the upconversion luminescence of upconversion nanocrystals.

In recent years, the development of multi-functional nanomaterials with fantastic physical, chemical, and biological properties has become an attractive research topic, demonstrating high potential in biomedicine, catalysis, energy conversion, water treatment adsorbents, and so on. As one of the most important luminescence nanomaterials, upconversion nanoparticles (UCNPs) were also used to fabricate multi-functional nanocomposites (UCNPs-X). In this chapter, we summarize recent advanced upconversion nanoparticles-based nanocomposites, including UCNPs-mSiO₂ (mSiO₂ = mesoporous SiO₂), UCNPs-MNPs (MNPs = magnetic nanoparticles), UCNPs-metal, and UCNPs-semiconductor nanocomposites.

One of the most important applications of upconversion nanoparticles (UCNPs) is bioapplication. A crucial issue facing the use of hydrophobic UCNPs for bioapplications is to render them water soluble and provide reactive groups for subsequent bioconjugation to various biomolecules. Surface modification can act as the bridge between the synthesis and biomedical applications of UCNPs. Surface modification of the UCNPs should not only improve the photostability of the nanoparticles with desirable interfacial properties, but also provide a potential platform for attaching biological molecules and other conjugated materials for various biomedical applications.

Rare-earth upconversion (UC) nanoparticles, when excited by continuous-wave near-infrared (NIR) light, exhibit a unique narrow photoluminescence with higher energy. Such special UC luminescence makes UCNPs promising as bio-imaging probes with attractive features, such as no auto-fluorescence from biological samples and deep penetration depth. As a result, UCNPs have emerged as novel imaging agents for small animals. In this chapter, recent reports regarding the biomedical imaging investigation using UCNPs are summarized. The applications of UCNPs for small-animal imaging, including optical imaging, optical tomography, and multimodal imaging are reviewed in detail. The recent techniques for upconversion imaging and toxicity assessment of UCNPs are also presented. Finally, we discuss the challenges and opportunities in the development of UCNPs for biomedical imaging.

Highly sensitive detection and accurate analysis of diverse biomolecules are critical in biology, medicine, and life science. Due to the unique nature of upconversion nanoparticles (UCNPs), there is no background luminescence significantly decreasing the limit of detection (LOD). Upconversion (UC) materials feature large anti-Stokes shifts and long lifetimes, leading to largely reduced background noise. Their optical properties are independent of the environment. Thereby, efficient bioassay methods have been developed based on UCNPs in the past decade. In this chapter, we provide a comprehensive survey on the advances of UCNPs’ applications in homogeneous detection, heterogeneous detection, and encoding for multiplexed detection of biomolecules and tumor markers.

The last few decades have witnessed the emergence and rapid development of nanoscience and nanotechnology including nanochemistry, nanophysics, nanomedicine, and nanobiomaterials, which allows for the continuous exploration of facile, inexpensive, sensitive, and specific biological analytical techniques by employing nanobioprobes. Recently, upconversion nanoparticles (UCNPs) demonstrated their great potential in many fields of biological science including cells and tissue labeling for bio-imaging, bio-detection, therapy, and multiplexed analysis. In this chapter, we mainly summarize recent advanced UCNPs-based nanosensors with different energy acceptors, including organic dyes, gold nanoparticles, carbon nanomaterials, and MnO₂ nanosheets for various analytes (ions, gas molecules, biomolecules, etc.).

Light-activated therapy (LAT) has attracted enormous attention over the past decades because light enables noninvasive activation or release of various therapeutic compounds, such as photosensitizers, chemotherapeutic drugs, proteins, and genes with high spatial and temporal resolution. However, most currently used photosensitive compounds in LAT are sensitive to ultraviolet (UV) or visible light which suffers from some limitations such as low tissue penetration and photodamage to living organisms. Upconversion nanoparticles (UCNPs) that can convert near-infrared (NIR) light to visible/UV light have been proven to be effective for LAT in vivo in recent years, owing to the high tissue penetration and minimal photocytotoxicity of NIR light. Furthermore, hydrophilic UCNPs with targeting moieties can transport hydrophobic drugs in biological media and achieve active targeting, thus enhancing the therapeutic efficacy. In this chapter, we review the recent advances in the field of UCNP-based LAT with focus on photodynamic therapy (PDT) and NIR light-triggered release and uncaging.

Thermometer has been developed for several centuries from contact to non-contact. With the development of nanotechnology, it becomes possible to use nanothermometry to detect the temperature within a single cell. Upconversion nanoparticle (UCNPs) is one of the promising candidates for developing non-contact nanothermometry. There are several parameters that define the luminescence properties which are dependent on temperature: intensity, band shape, spectral position, bandwidth, polarization, and lifetime. Among these parameters, band shape is the most widely used one. So we introduce the FIR theory which is used to figure out the relationship between the band shape and temperature. We also compare UCNPs with quantum dots, gold nanoparticles (GNPs), and green fluorescent protein (GFP) to show each other’s merits and demerits. We can foresee that UCNPs could be very useful in the near future even though there are still many challenges need to conquer.

Besides various bio-applications such as biomedicine, bioassay, photon dynamic treatment, and drug delivery, UCNPs have also been used in other applications. They can be explored as a NIR absorber in solar cells because they can absorb NIR light which has been wasted in traditional solar cells. They can be used as sensitizer in photocatalysis due to the energy transfer from UCNPs to the quantum dots or organic dyes. They are considered as next generation anti-counterfeiting materials due to their unique optical properties. In this chapter, the recent progress on other applications (besides bio-application) based on UCNPs is reviewed.

Lanthanide ion-doped upconversion luminescent nanomaterials are emerging as one of the most important members in the luminescence materials family. It has been well developed and pushed forward with unprecedented speed toward diverse applications in recent years, especially biological applications. The upconversion luminescence-based bio-probes inherit the unique optical properties of lanthanide ions, such as long-lived luminescence, large Stokes or anti-Stokes shifts, narrow emission bands, high resistance to photo-bleaching, and low toxicity. Most importantly, compared with the conventional biological labels, upconversion nanoparticles exhibit virtually zero auto-fluorescence background, less excitation light scattering, and deep tissue penetration because of excitation being in the NIR region that is within the optical transparency window. By taking advantage of the distinct lanthanide-based upconversion luminescence, a series of efficient analytical methods, such as heterogeneous time-resolved upconversion luminescence assays, homogeneous FRET-based upconversion luminescence assays, bio-imaging, PDT, targeted drug delivery, and optical–magnetic multimodal bio-sensing techniques, have been established based on these novel upconversion nanoparticles in the past decade.