Upconverting Nanoparticles for Sensing

Rare earth doped nanoparticles have the uncharacteristic ability to (up)convert near-infrared (NIR) excitation light to higher energies spanning the UV, visible, and shorter wavelength NIR regions via a multiphoton process known as upconversion. The ability to stimulate luminescent nanoparticles with NIR light has made possible their use in a plethora of biological and medical applications. In fact, the biggest impact of these upconverting nanoparticles (UCNPs) would be in the field of disease diagnostics and therapeutics, now commonly referred to as theranostics. UCNPs that can be excited by low levels of NIR light but that emit higher-energy emission offer an attractive alternative to conventional fluorophores, both as detectable species for labeling and as energy donors for FRET-based bioassays. UCNPs can be excited selectively in the presence of very high levels of conventional fluorophores. Relative to visible and UV radiation, NIR photons have greater tissue penetration and cause much less damage to the specimens under study because NIR radiation is not absorbed strongly by chromophores commonly found in tissues. Thus, NIR excitation of UCNPs in a biological sample does not give rise to background autofluorescence and consequently detection sensitivity can, in principle, be very high. Upconverting labels also have unique potential advantages as energy donors in FRET-based bioassays because NIR excitation will only excite the donor, and not the acceptor or any fluorescent impurities in the sample. Here, we present the synthesis and surface functionalization of various NIR excited UCNPs and demonstrate their potential use in sensing applications.