# Exploitation of Antimicrobial Nanoparticles and Their Applications in Biomedical Engineering
## 2 Nanomaterials
### Nanoparticle Synthesis and Processing Methods
#### 2.1.1 Bottom Up  
Within the bottom-up approach, the synthesis of nanoparticles can be classified into the following categories:  
1. Gas-phase synthesis: This is when nanoparticles are generated through the interaction of gaseous precursor components over a catalyst or prepared surface. For instance, chemical vapour deposition (CVD) is a method involving the deposition of a thin film of gaseous reactants onto a substrate. A thin film of product is generated on the substrate surface when it is heated at ambient temperature by combining gas molecules [24]. The advantages of CVD are in producing highly pure, uniform, hard and strong nanoparticles. However, CVD requires special equipment, and the gaseous by-products can be toxic [25]. Still, CVD is a widely used technique to deposit metallic nano silver layers on the substrate surface heated to the temperature 573 K at atmospheric pressure. Nano metal layers generated using this method were proved to be active against a wide spectrum of bacterial strains [26]. For example, the antimicrobial property studies of AgNPs and CuNPs deposited on the surface of such biomedical materials, such as titanium, TiAlNb alloy and steel (317 L), confirmed the inhibitory effect against _S. aureus_[27]. [PERSON] et al. reported wound dressings that were functionalised with Ag NPs using the CVD technique were found to exhibit a strong antibacterial effect against both _S. aureus_ and _Klebsiella pneumoniae_ strains with a low concentration of the silver coating [28]. 2. Liquid-phase synthesis: Two common processes are used to synthesise nanomaterials via liquid-phase synthesis; they are the'sol-gel' and'microwave assist' methods. Nanomaterials are formed via the sol-gel process. Although often limited to producing metal oxides (i.e., ZnO, TiO\({}_{2}\)), it is sometimes preferred over using the gas-phase synthesis due to its simplicity and lower processing temperature required to produce nanoparticles at a higher rate [29]. Sol-gel synthesis typically uses metal alkoxides as the precursors or any other reactants that would form a homogeneous medium with the applied solvent. The process first undergoes hydrolysis/polycondensation reactions to form a colloidal suspension ('sol'). This is then followed by complete solvent evaporation (or calcination) to allow nano products to be formed via precipitation/recrystallisation. By controlling the parameters and drying conditions during the precipitation process, different forms of Nano products, such as gel-film, uniform particle coat, or nano-fibre, can be manufactured using this method. Nanopowder (i.e., iron oxide Fe\({}_{3}\)O\({}_{4}\)) can also be obtained using the sol-gel method by simply filtering the precipitated colloidal products [30; 31]. [PERSON] et al. reported how raising the calcination temperature influenced the sizes and agglomeration in the ZnO NPs that were formed using the sol-gel process [32]. [PERSON] et al. claimed to have used the sol-gel method to produce thorn-like ZnO nanoparticles, which showed good antimicrobial and antifungal activities against _Bacillus subtilis_, _Escherichia coli_ and _Candidia albicans_[33]. Research data suggest there has been an increased interest in utilising microwave radiation for nanoparticle synthesis. The electromagnetic energy produced by microwave power enables localised heating to be delivered to the reaction media. [34; 35; 36; 37]. For instance, [PERSON] and co-workers synthesised environment-friendly Ag NPs using a microwave reactor, where Silver nitrate (AgNO\({}_{3}\)) was heated and irradiated in water at 150 \({}^{\circ}\)C whilst L-Lysine/L-arginine was respectively added as reducing agents and surfactants [38]. This reaction produced uniform and monodispersed Ag NPs with average particle sizes between 26.3 and 26.7 nm in diameter in ten seconds. The manipulation of microwave power and radiation time can also control the morphology of nanoparticles produced. [PERSON] et al.