Quantum Dots and Their Clinical Applications

Jaydeep      Das; Padma      Thiagarajan

doi:10.2174/22106812112029990005

Abstract

Quantum dots (QDs) are a landmark development in the field of nanotechnology. These zero-dimensional semiconductors, unlike their physical attributes, have enormous capacity to store energy within themselves. This energy is contained in several thousands of electrons that constitute the nanoparticles. QDs are synthesized by both organic and inorganic methods, the choice being dependent upon the desired efficacy of the resultant product. The process of synthesis is auto regulated by the phenomenon of nucleation threshold, whilst quality control occurs through ostwald ripening, given by Gibbs Thompson equation. The optical and electronic properties of these particles are dependent upon the transitions of constituent electrons between the valence and conductance bands and this phenomenon is exploited in synthesizing QDs to be employed as biomarkers. Fluorescence is obtained when electrons return to the ground state from the lowest vibrational level of an excited state, emitting quanta of radiation in the process. As wavelength varies with each energy state, a rainbow of fluorescence is seen with quantum dots of different sizes and shapes. The properties of these nanoparticles are largely attributed to their high surface area to volume ratio, with the former parameter being subjected to modifications like capping and functionalization. This not only enhances their quantum yield but also multiplies their fluorescing abilities. As biomarkers, these particles find many clinical applications, in an array of in vivo and in vitro techniques, ranging from drug delivery systems to interstitial target tracking. However, one special clinical application of QDs that demands reverence for their utility, is their specific and selective cancer cell imaging activity. Photosensitizers may be linked to QDs, bound to monoclonal antibodies, and targeted to cancer cells, where they may generate reactive oxygen species (ROS) upon exposure to light. This could help in the treatment of metastatic malignancies and form a part of photodynamic therapy. On the other side lies the toxic effect of these particles that can be traced to their heavy metal crystalline cores. However, pursuit is on, to overcome this drawback, so that QDs can be used more effectively in vivo for a longer period of time.

Keywords: Biomarkers, clinical applications, photodynamic therapy, tumor targeting, quantum dots.