Near Infrared-Emitting Bioprobes for Low-Autofluorescence Imaging Techniques

One of the biggest drawbacks of in vivo optical fluorescence imaging is the undesired emission by endogenous tissues and molecules of the subject under study, known as autofluorescence. In the worst case this obfuscates the signal of the actual bioprobe and in better cases it results in low signal-to-noise ratios and requires therefore large doses of the materials used as imaging/contrast agents, which rapidly becomes a toxicity issue. Hence, over roughly the last decade approaches were developed that enable autofluorescence-free imaging. These techniques can broadly be divided into two groups that will be discussed in this chapter. The first group includes a variety of nanomaterials that can either be imaged at wavelengths in the infrared above the autofluorescence signals (nanoparticles like Ag2S, PbS-based quantum dots but also carbon-based nanoparticles and organic polymers) while blocking the autofluorescence through filters or it is based on nanomaterials with long lifetimes, filtering autofluorescence through a time delay. The second group of approaches avoids the occurrence of autofluorescence either through the absence of excitation radiation (bioluminescence, chemiluminescence) or through irradiation before the in vivo application, which is the case for long-persistent luminescent or afterglow nanoparticles in the infrared.