Although photon upconversion was first studied in bulk crystals and optical fibers, it became better known with the development of
nanomaterials. This happened due to the many ways in which nanostructures with photon upconversion properties can be applied. This new class of materials may broadly be referred to as
upconverting nanoparticles or UCNPs.
Lanthanide-doped nanoparticles Lanthanide-doped nanoparticles emerged in the late 1990s owing to the increasing focus on nanotechnology. Although their optical transitions essentially resemble those in bulk materials, the nanostructure amenable to surface modifications results in improved or new characteristics. Besides, the small size of the particles allow their use as alternatives to molecular
fluorophores for biological applications. Their unique optical properties, such as large Stokes shift and the lack of blinking, have enabled them to rival conventional luminescent probes in challenging tasks including single-molecule tracking and deep tissue imaging. In the case of bioimaging, as lanthanide-doped nanoparticles can be excited with near-infrared light, they can reduce
autofluorescence of biological samples and thus improve the contrast of the image. Lanthanide-doped nanoparticles are nanocrystals of a transparent material (more often the fluorides NaYF4, NaGdF4, LiYF4, YF3,
CaF2 or oxides such as
Gd2O3) doped with lanthanide ions. The most common lanthanide ions used in photon upconversion are the pairs erbium-ytterbium (Er3+,Yb3+) or thulium-ytterbium (Tm3+, Yb3+). In such combinations ytterbium ions are added as antennas, to absorb light at around 980 nm and transfer it to the upconverter ion. If this ion is erbium, then a characteristic green and red emission is observed, while when the upconverter ion is thulium, the emission includes near-ultraviolet, blue and red light. Despite the promising aspects of these nanomaterials, one urgent task that confronts materials chemists lies in the synthesis of nanoparticles with tunable emission, which is essential for applications in multiplexed imaging and sensing. The development of a reproducible, high yield synthetic route that allows controlled growth of rare earth halide nanoparticles has enabled the development and commercialization of upconversion nanoparticles in many different bioapplications. The recent progress in this direction includes the synthesis of structured nanocrystals crystals, such as particles with a core/shell structure, allowing upconversion through
interfacial energy transfer (IET).
Semiconductor nanoparticles Semiconductor nanoparticles or
quantum dots have often been demonstrated to emit light of shorter wavelength than the excitation following a
two-photon absorption mechanism, not photon upconversion. However, recently the use of semiconductor nanoparticles, such as CdSe, PbS and PbSe as sensitizers combined with molecular emitters has been shown as a new strategy for photon upconversion through triplet-triplet annihilation. and blue light to ultraviolet. This technique benefits from a very high upconverting capability. Especially, these materials can be used to capture the infrared region of sunlight to electricity and enhance the efficiency of photovoltaic solar cells. Upconversion (UC)-based bioimaging can overcome these limitations as their excitation occurs at lower frequencies and the emission at higher frequencies. Kwon et al. developed multifunctional silica-based nanocapsules, synthesized to encapsulate two distinct triplet-triplet annihilation UC
chromophore pairs. Each nanocapsule emits different colors, blue or green, following a red light excitation. These nanocapsules were further conjugated with either antibodies or peptides to selectively target breast or colon cancer cells, respectively. Both in vitro and in vivo experimental results demonstrated cancer-specific and differential-color imaging from single wavelength excitation as well as far greater accumulation at targeted tumor sites than that due to the
enhanced permeability and retention effect. This approach can be used to host a variety of chromophore pairs for various tumor-specific, color-coding scenarios and can be employed for diagnosis of a wide range of cancer types within the heterogeneous tumor microenvironment. == See also ==