This process is accompanied by the emission of radiation, which can be measured to give the band gap size of a semiconductor. Recombination occurs when an electron from a higher energy level relaxes to a lower energy level and recombines with an electron hole. The wavelength of photon emissions depends not only on the material from which the dot is made but also its size the smaller the size of the QDs, the larger the band gap energy and QDs emit blue light, larger QDs having smaller band gaps emit the larger wavelength. When the electron returns to a lower energy level, a narrow, symmetric energy band emission occurs. When the QD is hit by incident light, it absorbs a photon with a higher energy than that of the band gap of the composing semiconductor. If, however, an external stimulus is applied, an electron may move from the valence band to the conduction band, i.e., those energy levels above the band gap. Most electrons occupy energy levels below this band gap in the area known as the valence band, indeed most energy levels in the valence band are occupied. However, there are some energy levels that the electrons cannot occupy, which are collectively known as band gap. The quantum confinement means that the energy levels that the electrons inhabit become discrete, with a finite separation between them. QDs are defined as particles with physical dimensions smaller than the exciton Bohr radius. An exciton Bohr radius is the distance in an electron-hole pair in a bulk semiconductor. The quantum confinement effects occur when size of nanoparticle smaller than exciton Bohr radius. The small size of QDs lead to what is known as “quantum confinement”. The limitation of heavy metal–containing QDs stimulates extensive research interests in exploring alternative strategies for the design of fluorescent nanocrystals with high biocompatibility. The potential toxicity of the QDs is a cause for concern because they are made of heavy metals. Most of the current studies were designed to ask questions concerning the physicochemical properties of novel QD products, not QD toxicity per se. To make them useful for biomedical applications, QDs need to be conjugated to biological molecules without disturbing the biological function of these molecules. They are necessarily made water soluble by surface modifying them with various bifunctional surface ligands or caps to promote aqueous solubility and enhancing biocompatibility. Hydrophobic QDs are insoluble in aqueous solution and cannot be directly employed in biomedical applications. These core-shell QDs are hydrophobic and only organic soluble as prepared. Highly luminescent QDs are prepared by coating the core with another material, resulting in core-shell quantum dots that are more stable in various chemical environments. These nanoparticles have size-tunable emission, strong light absorbance, and very high levels of brightness and photostability. Superior optical properties are a promising alternative to organic dyes for fluorescence biomedical applications. Quantum dots (QDs) are novel class of inorganic fluorophore with superior photophysical properties.
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