Upconverting Nanoparticles (UCNPs)

Upconverting luminescence (UCL), is an anti-Stokes shift phenomenon. UCNPs have narrow emission spectra, high chemical stability, as well as strong brightness. Upon anti-Stokes shift feature, the near-infrared (NIR)-to-visible UNCPs emit higher energy in visible light wavelength by absorbing the lower energy, the NIR laser energy. This results in the absence of blinking effects, and zero autofluorescence.

Upconversion is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. It holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a mean to surpass the Shockley-Queisser efficiency limit.

The fluorescence emission band of UCNPs overlaps the absorption band of QDs; resulting UCNPs transfer their emission energy to QDs via a fluorescence resonance energy transfer (FRET) or energy re-absorption. The way to transfer energy from UCNPs to QDs greatly depends on the distance between UCNPs and QDs. If the distance is less than 100 Å, the preferred procedure is FRET, which transfers energy in the form of phonons or lattice vibrations. Otherwise, the alternative is re-absorption, which transfers energy as photons.

Lanthanide-doped upconversion nanoparticles are the most common one and have been recently developed as a new generation of luminescent probes, and possessing considerable merits. The lanthanide-doped quantum dots offers low autofluorescence, high penetration depth large anti-stokes shifts, high photostability, sharp emission bandwidth and low toxicity.

In addition due to doping of other types of lanthanide ions into nanoparticles, the upconversion nanoparticles can be featured further with other imaging modalities, such as magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), single-photon emission tomography (SPECT). Due to several properties the upconversion quantum dots can be used in various applications such as biosensing, solar cells and bioimaging therapy.