**6.3. Quantum dots**

Quantum dots are tiny semiconductor crystals of luminescent nanocrystals with rich surface chemistry and unique optical properties with the size of 1–10 nm made up of compounds from group II to VI and III to V, for example, Ag, Cd, Hg, Ln, P, Pb, Se, Te, Zn, and so on. QDs have distinctive characteristics such as size-tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous excitation of multiple fluorescence colors.

Depending on their size by laser, the quantum dots glow brightly in different colors, such as Adirondack Green (520nm), Blue (514 nm), Greenish blue (544 nm), Green (559 nm), Yellowish green (571 nm), Yellow (577 nm), Yellowish orange (581 nm), Fort Orange (600nm), Orange (610 nm), and Maple Red-Orange (620nm).

QDs are nearly spherical semiconductor particles with core-shell structure. Colloidal core/shell QDs, such as CdSe/ZnS, CdSe/CdS/ZnS, CdTe/CdSe, and InP/ZnS, are commonly synthesized for biomedical applications, whereas CdSe/ZnS, CdTe/ZnS, and CdSe/CdS/ZnS have been commonly used.

Quantum dots are made up of three parts, that is, core, shell, and cap.

**Core** is made up of CdSe, which is a semiconductor material. Core is surrounded by shell which is made up of ZnS for improving its optical properties and cap encapsulates the double layer quantum dots by different materials like silica which helps in improving solubility in aqueous buffers. Structure of quantum dot is shown in **Figure 3**.

**Figure 3.** Structure of a quantum dot.

The semiconducting nature and the size-dependent fluorescence of these nanocrystals have been successfully applied for in vitro, in vivo transfection and for diagnosis of various diseases. One of the most important emerging applications of QDs appears to be *traceable* drug delivery, because it has the potential to elucidate the pharmacokinetics and pharmacodynamics of drug candidates and to provide the design principles for drug carrier engineering.

In gene technology, the quantum dot can be conjugated with oligonucleotide sequences (attached via surface carboxylic acid groups) may be targeted to bind with DNA or mRNA. Gene-associated drugs can be loaded within a QD core or attached to the surface of these nanoparticles through direct conjugation or electrostatic complexation by which QDs can protect the gene from degradation by nucleases. This property has been utilized for an assay of single nucleotide polymorphism (SNP). Due to concerns about long-term *in vivo* toxicity and degradation, QDs are currently limited to cell and small animal uses [30, 31, 77–101].
