# Exploitation of Antimicrobial Nanoparticles and Their Applications in Biomedical Engineering
## 2 Nanomaterials
### Intrinsic Properties and Characteristics of Nanoparticles  
Nanomaterials have been increasingly studied because of their unique physical, chemical, optical, magnetic and electrical properties in comparison to traditional bulk materials [1]. The two primary factors which provide nanomaterials with their unique properties include surface effect and quantum confinement effect [51]. Nanomaterials have a much greater surface area to volume ratio than their conventional forms. Compared to bulk materials, a large portion of the atoms reside on the surface than those in the core of the particle, and they are also found to be less stable (hence more reactive) due to lower coordination and the presence of unsatisfied bonds in their atomic structures [52]. In particular, where edge and corner atoms are often associated with the minimum energy interactions and binding with foreign atoms [51]. The Quantum confinement effect is associated with the discrete energy levels present in the atomic structure. In other words, when the particle sizes decrease to their nanoscale, the percentage of atoms on the surface increases, resulting in amplified activities and many special properties [51; 53]. These factors affect the chemical reactivity of materials as well as leading to unique physical, chemical, optical, electrical and magnetic behaviours. As an example, it was reported that silver Ag NP with average-sized 3.5 nm have a melting temperature of approximately 112 \({}^{\circ}\)C, which is much lower in comparison to the corresponding bulk Ag (960 \({}^{\circ}\)C) [54]. Similarly, the melting point of the nano Cu (47 nm) was reported to be 670 \({}^{\circ}\)C lower than its equivalent bulk sample [55]. Furthermore, Zinc Oxide nanoparticles (ZnO NP) have also been widely studied due to their unique catalytic activities in chemical reactions and diverse organic syntheses [56; 57]. Certain nanomaterials, such as gold (Au) and silver (Ag), are known to own unique optical properties called the Surface Plasmon Resonance (SPR), which is an oscillation of conductive electrons that resonate on the metal surface when it is being excited by a specific wavelength of light. The phenomenon and incidences of SPR found in nanomaterials have been receiving tremendous interest in biomedical research in the last decade [58; 59; 60]. The tuneable optical properties of noble nanoparticles (especially Au NPs) are highly dependent on their particle size and shape. Ag NPs with a diameter of 70 nm are known to scatter green light (\(\sim\lambda_{\text{max}}\) 530 nm) and transmit to orange. However, when Nano gold (Au NPs) are added, the absorption band of Ag-Au alloy NPs shifts to longer wavelengths [61]. The first and very successful human clinical trial that involved the utility of Gold nano shells was performed in 2019 in photothermal cancer therapy. Gold nano shells were injected intravenously under human skin, irradiated by a near-infrared source and converted into heat energy, inducing highly localized hyperthermia (a photothermal reaction) and resulting in highly effective cell death and tumour remission [62]. This breakthrough has proven the safe administration of certain nanomaterials, the extended life expectancy of the patients who underwent this clinical trial and promoted a big step in utilising nanomaterials for biomedical and healthcare applications. It is worth noting that the same concept has already been intensively studied using Gold nanorods and triangular AgNPs in photothermal treatment for wound infection caused by _P. aeruginosa_ and resistant bacteria in mice [58; 63]. Similar to the melting, catalytic and optical properties, the antimicrobial activity of nanoparticles is influenced by their size-related physical behaviours. Although smaller nanoparticles have shown an increase in antimicrobial activity, smaller nanoparticles have also shown to have increased cytotoxic effects on mammalian cells.