Quantum dot nanoparticles: Properties, surface functionalization, and their applications in biosensoring and imaging

Colloidal semiconductor nanocrystals, also known as Quantum Dots (QDs), posses unique properties due to their nanometric size. They have broad absorption spectra and narrow emission bands that are related to the materials used and to their size. QDs represent a new class of fluorescent probes that are gradually substituting traditional organic dyes since they present many advantages compared to them, first of all improved photostability. Furthermore, since QDs have broad absorption spectra, it is possible to excite many QDs using just one wavelength. Much progress has been made in the last years in the synthesis of QDs, which now can yield highly homogeneous and highly crystalline QDs. Many strategies are also available nowadays to make QDs water soluble and biocompatible, the most common being the ligand exchange and polymer coating. The surface passivation of the QDs to make them water soluble also allows for further functionalization. As an example, if biological ligands are attached to the outer shell of the nanocrystals, they can selectively recognize specific targets. This approach can be exploited for numerous applications, among them biosensoring and imaging. Biosensors are a class of probes developed for biomarkers detection on a real-time or continuous basis in a complex mixture. This easy to use and low cost technique perfectly combines with the optical properties of QD. For example, the high photostability of QDs can allow for continuous monitoring of a signal over time. Furthermore, simultaneous detection of several specific receptors can be also achieved if many QDs with different emission colors are combined into a single structure, effectively behaving as an optical barcode. Optical imaging, in particular fluorescence imaging, is an area where QDs are gaining increasing popularity. Near-infrared wavelengths are of key importance for biological analysis since in this region biological tissues absorb only weakly. Few traditional organic dyes are available for such spectral window and in addition they suffer from photobleaching. On the other hand, QDs can be tuned to the desired emission wavelength by adjusting their composition and size. QDs have been already extensively used for cell imaging in vitro; however, the full potential of QDs can be appreciated only when they are employed in in vivo imaging. The preparation of multicolor probes which are highly stable in biological buffers and can be followed over long periods of time can be achieved by exploiting the QDs properties. Although just in its infancy, biosensoring and imaging by means of QDs has already proved to be of paramount importance in biomedicine and future developments in QDs synthesis and functionalization will probably yield nano-tools of priceless value for medical application, e.g. for the early detection of diseases, such as in cancer diagnosis. In this chapter, we will try to give to the reader a general overview on the many aspects of QDs, mainly of their physical properties that are relevant for biological applications and on the strategies followed to make them biocompatible. Then the main biological applications of QDs will be reviewed, their implementation in biosensoring and imaging, both in vitro and in vivo, including their exploitation in photodynamic therapy. Finally, we will give an overview on the toxicity issues and on the upcoming new generations of QDs that should solve those issues.