Revisiting the classification of NIR-absorbing/emitting nanomaterials for in vivo bioapplications

With the development of nonlinear optics and new imaging methods, near-infrared (NIR) light can excite contrast agents to probe biological specimens both functionally and structurally with a deeper imaging depth and a higher spatial resolution than linear optical approaches. There is considerable and growing interest in how biological specimens respond to NIR light. Moreover, the visible absorption band of most functional nanomaterials becomes NIR-excitable through multiphoton processes, thus allowing multifunctional imaging and combined therapy with noble metal and magnetic nanoparticles both in vitro and in vivo. A groundbreaking example is the use of different laser techniques to excite single-type NIR-absorbing/emitting nanomaterials to produce multiphoton emission by femtosecond lasers using either a remote control system for photodynamic therapy or photo-induced chemical bond dissociation. These techniques provided superior anatomical resolution and detection sensitivity for in vivo tumor-targeted imaging than those offered by conventional methods. Here we summarize the most recent progress in the development of smart NIR-absorbing/emitting nanomaterials for in vivo bioapplications. Nanomaterials that absorb or emit near-infrared light are increasingly being used to probe the structures and functions of biosystems. In particular, they are valuable for in vivo monitoring of drug nanocarriers, which offer many advantages over conventional drug-delivery methods. João Conde of Massachusetts Institute of Technology in the USA and co-workers give a timely review of recent progress in this area. They consider the various types of near-infrared absorbing and emitting nanomaterials. The scientists assess nine kinds of nanoparticles that are useful for multiphoton microscopy. They then overview toxicity studies of these materials. Finally, the researchers provide a roadmap for future research by listing ten properties that ideal near-infrared bioprobes would possess. Specifically, they note that ideal bioprobes should be biocompatible, versatile and multifunctional. A huge interest shows that optical contrast agents to probe biological specimen both functionally and structurally with deeper action depth and better spatial resolution is closely related with the development of near-infrared (NIR) light excitation and nonlinear optics and new imaging methods. This review will go beyond the state-of-the-art by revisiting the up-to-date progress in developing smart NIR-to-NIR and upconverted nanomaterials for in vivo bioapplications. The latest advances in the development of novel NIR-to-NIR linear and nonlinear optical nanomaterials and their potential applications in cancer targeting, diagnosis and therapeutics and deep-tissue imaging are described.