# Radiation-Assisted Synthesis of Polymer-Based Nanomaterials
## 4 Polymer-Based Nanomaterials
### Protein-Based Nanoparticles  
In medical applications, surface potential of nanoparticles significantly affects their accumulation in the cell. In addition to particle size and surface potential, the presence of amino acids in particle structure is an important factor for cell adhesion. This invoked interest in radiation modification of proteins. Protein-based drug carriers are promising alternatives to traditional polymeric drug delivery systems due to their intrinsic biocompatibility and biodegradability. Albumin nanoparticles are known for their effectiveness and safety as drug carriers. Numerous functions and properties of serum albumin have attracted the interest of scientists for the preparation of its nanoparticles. Classical methods of chemical crosslinking or thermal gelation may cause undesirable reactions and protein denaturation. Hence, as a room temperature process not requiring chemical crosslinking agents, ionizing radiation has been found to be quite advantageous for the preparation of albumin nanoparticles. It has been shown that gamma irradiation of ethanol solution of Bovine Serum Albumin (BSA) allowed the formation of nanoparticles within the size range of 20-40 nm [33]. The mechanism of radiation-induced formation of albumin nanoparticles has been elaborated and explained by a two-step process. First, protein molecules aggregate in the water/ethanol mixture. Changing the concentration of ethanol provides a certain level of size control of the aggregates prior to the second step of irradiation. Irradiation of albumin solution leads to the formation of corresponding nanoparticles via intramolecular crosslinking within the coil. The optimization of parameters of cosolvent concentration and irradiation dose allowed fine tuning of nanoparticle size from 7-70 nm [34]. Radiation-induced synthesis of other protein-based nanoparticles has been demonstrated also as a technique capable of inducing protein crosslinking and conferring controllable particle size at nanoscale, while preserving their biological properties. Papain nanoparticles of \(7.7+/-0.9\) nm and 85% residual proteolytic activity were synthesized using phosphate buffer and ethanol as cosolvent irradiated to 10 kGy dose [35]. Basically, the technique combines, as in the case of albumin, protein aggregation by means of protein desolvation and radiation-induced intramolecular crosslinking. This approach has been recently advanced by the same group to have a better understanding of crosslinking process and the possible contribution of bittyrosine linkages and disulfide bridges on the formation of nanoparticle assembly. The feasibility of radiation sterilization of nanoparticles during their synthesis was also checked [35].  
In order to establish a nanoparticle platform suitable for diagnostic and drug delivery applications, peptides composed of aromatic amino acid residues were designed and synthesized by means of radiation crosslinking mechanism of proteins. The peptide nanoparticles with particle size of \(\sim\)100 nm produced by \(\gamma\)-irradiation (5-15 kGy dose range) displayed a positive surface potential, maintained biodegradability and were stable in water and phosphate buffer solution during actual diagnosis [36].