# Radiation-Assisted Synthesis of Polymer-Based Nanomaterials
## 3 Metal-Based Nanomaterials
### Quantum Dots  
Quantum Dots (QDs) are fluorescence type semiconductor nanosized particles which are composed of core material enclosed within a shell of another semiconductor material. They are made up of either heavy metal or inorganic materials with size range from 2 to 10 nm. Semiconductor materials in the outer shell of QDs facilitates the surface for bioconjugation. This provides effective surface area for binding of drugs with the target molecule. Functionalization and modification of the surface makes them suitable for applications in the pharmaceutical field, such as biomedical imaging, drug delivery and drug release studies. The semiconductor outer shell of QDs is made of heavy metals or inorganic materials such as cadmium, selenium, zinc oxide, silica, etc., which are coated with shell material. Figure 1 shows the ideal structure of a semiconductor quantum dot. Semiconductor QDs are synthesized through high temperature, gamma irradiation, and microwave and sol-gel techniques [19]. In the synthesis via gamma irradiation, the precursor is irradiated with \(\gamma\)-rays to different doses to form a colloidal product. This method is extensively used in the preparation of metal and metal oxide QDs as exemplified below. Cadmium selenide (CdSe) QDs are one the most intensively studied among all QDs because of its wide applications as sensors, biomarkers, LEDs and photovoltaics. Radiation-chemical methods have been preferred among various routes of synthesis of CdSe QDs. Chitosan-coated CdSe QDs were successfully synthesized in aqueous system through a gamma irradiation method at room temperature and ambient pressure. The diameter of the resulting QDs was about 4 nm with narrow size distribution [20]. Polymer films of poly (vinyl alcohol) containing CdSe QDs of various sizes from 3 to 5 nm have been synthesized by a radiation-chemical route using CdSO\({}_{4}\) and Na\({}_{2}\)SeSO\({}_{3}\) as the starting materials under a reducing condition. The 2 MeV electron beam and \(\gamma\)-radiation were used for the synthesis. The sizes of QDs were found to be strongly dependent on the dose rate of radiation and initial precursor concentrations. The films containing these QDs were quite stable for months [21]. In a more recent work, CdSe QDs were synthesized in aqueous solutions using equimolar cadmium sulfate and sodium seleno sulfate through 7 MeV electron beam irradiation in the presence of different saccharides. Starch was found to have a better capping property, whereas glucose was found to reduce their cytotoxicity effect. The mechanism of radiolytic synthesis of these QDs was investigated by pulse radiolysis studies [22]. SnSe QDs have been mainly prepared by chemical vapor deposition, vacuum evaporation, electrodeposition and hydro- or solvo-thermal methods. In these methods, the growth temperatures are high and temperature gradient has to be carefully controlled in addition to the problem of using toxic organometallic precursors. A fast and easy method of ionizing radiation was exploited for the synthesis of SnSe QDs at room temperature with the aid of CTAB as the surfactant. The QDs obtained with an average diameter of 4 nm showed strong blue luminescence [23]. The synthesis of size-controlled, stable ZnSe QDs with narrow size distribution in aqueous environments by conventional chemical methods remains to be a challenge. Stable, monodisperse and luminescent ZnSe QDs capped with chelating ethylene diamine were synthesized under ambient conditions. The concentration of precursors, zinc salt, selenium source, ethylene diamine and absorbed dose (7-90 kGy) were optimized for obtaining good quality particles. Selective quenching of luminescence of as-synthesized QDs by Cu\({}^{2+}\) ions showed a detection limit of cupric ions in nano-molar range [24]. Figure 1: Semiconductor quantum dots. Reprinted with permission from Ref. [19]. Copyright 2021 Elsevier. CdS QDs of 1.3-5.6 nm have been synthesized in micellar solutions by gamma irradiation.