Photon upconversion

Such processes can be observed in materials with very different sizes and structures, including optical fibers, bulk crystals or nanoparticles, as long as they contain any of the active ions mentioned above.

[3] An early proposal (a solid-state IR quantum counter) was made by Nicolaas Bloembergen in 1959[4] and the process was first observed by François Auzel in 1966.

These surface properties can be realized through designs of photonic crystals, and theories and experiments have been demonstrated on thermophotovoltaics and passive radiative cooling.

[8][9] Under best criterion, energy conversion efficiency from solar radiation to electricity by introducing up-converter can go up to 73% using AM1.5D spectrum and 76% considering sun as a black body source at 6,000 K for a single-junction cell.

[14] 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.

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.

[17][18][19] 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.

Kwon et al. developed multifunctional silica-based nanocapsules, synthesized to encapsulate two distinct triplet-triplet annihilation UC chromophore pairs.

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.