Quantum Dots

Quantum dots are nanosized crystals that act as semiconductors depending on the temperature and purity of the material. The tight size range (2—10 nm) of quantum dots manifests quantum mechanics—like properties [3]. Quantum dots are researched as contrast agents for diagnosis and detection, due to narrow emission spectra, high light stability, and wide and continuous absorption spectra. Additionally, the energy spectrum of these nanosystems can also be tailored by controlling the size, shape, and energy levels. Smaller quantum dots require high energy for electrons to enter an excited state. It results in high-energy frequency and smaller wavelength, emitting light at the blue end of the spectrum. Conversely, large quantum dots emit light at the red end of the spectrum [3]. The structure of quantum dots comprises a semiconductor core made of heavy metals [like cadmium selenide (CdSe), lead selenide (PbSe), or indium arsenide (InAs)] and an outer shell [zinc sulfide (ZnS), cadmium sulfide (CdS)] to prevent toxicity. CdSe/ZnS quantum dots are currently the most commonly available commercial products [7]. Numerous methods have been utilized to synthesize quantum dots, i.e., plasma synthesis, electrochemical assembly, and viral assembly. Colloidal synthesis is still the most common technique [8].

Quantum dots are very stable against photobleaching and can be considered as alternatives for traditional fluorescent indicator dyes and proteins. These materials exhibit tunable luminescence and electrical properties that are employed in various biological applications such as imaging, DNA detection, and cell sorting, labeling, and tracking [8]. Despite their size, quantum dots can be combined with other nanosystems for widening their applications [3].