# Exploitation of Antimicrobial Nanoparticles and Their Applications in Biomedical Engineering
## 2 Nanomaterials
### Nanomaterials with Antimicrobial Properties
#### 2.3.1 Mono-Metallic Nanoparticles  
Antibacterial metal has a long history. Ag NPs have been recently applied in various fields, such as dentistry, pharmaceutical and biomedical industries, due to their low toxicity and high antimicrobial efficacies [72]. It has been reported that Ag NPs with an average diameter of 21 nm and concentration of \(>\) 75 \(\upmu\)g/mL inhibit a wide range of Gram-negative bacteria species (i.e., _Escherichia coli_, _Vibrio cholerae_, _Salmonella typli_ and _Pseudomonas aeruginosa_) [73]. The authors hypothesised possible mechanisms of such bacterial inhibition involve. First, Ag NPs disrupting cellular metabolic activity and inflicting membrane damage to the cells, which both subsequently trigger the generation of ROS and DNA damage [74]. In addition, according to other reports, Ag NPs can also induce pits and gaps in the bacterial membrane, which assist the further accumulation of Ag NPs and form free radicals causing cell death [75]. The release of Ag\({}^{+}\)/Ag\({}^{2+}\) ions is also considered as the main bactericidal mechanism, which interact with disulphide or sulphydryl groups of enzymes, leading to disruption of cellular metabolic processes [76; 77]. In another study, Ag NPs with a 12.4 nm diameter were found to reduce bacterial growth on agar plates and at concentrations of 10-100 \(\upmu\)g/mL were required to act against _E. coli_ at 10\({}^{5}\) colony forming units (CFUs) [75]. As expected, increased concentrations of Ag NPs, increased inhibitory effects on bacteria. For instance, at a concentration of 10 \(\upmu\)g/mL, the Ag NPs only inhibited 70% CFU of the _E. coli_ populations, whereas increasing the NPs concentration to 50 \(\upmu\)g/mL or above completely inhibited _E. coli_ growth. In microbiology, the minimum inhibitory concentration (MIC) is defined as the lowest concentration of nanoparticles that inhibits the growth of a microorganism, and it is a method that is commonly used to assess the susceptibility of microorganisms to antimicrobials. Ag NPs solutions that were synthesised in situ via the reduction of silver nitrate showed to have directly inhibited the growth of bacteria. It was found that the MIC\({}_{50}\) of Ag NPs against _E. coli_ was estimated to be between 3.3 nM and 6.6 nM, whilst MIC\({}_{100}\) was achieved at the concentration of 33 nM. In the case of Gram-positive species, _S. aureus_, MIC\({}_{100}\) was not detected as it appeared the required concentration to be well beyond the raw concentration of the synthesised materials [78]. In a separate experiment, [PERSON] et al. investigated how antibacterial activities relate to the shape of Ag NPs [79]. They found that triangular nanoparticles with an average edge length of 40 nm displayed the strongest bactericidal activity, reducing _E. coli_ viability more extensively when compared to spherical (mean sizes = 39 nm) and rod-shaped (mean sizes = 133 \(\times\) 16 nm) Ag NPs. These triangular-shaped Ag NPs predominated, with a {111} surface, with high-atom-density facets, and they were found to contain saturated Ag\({}^{+}\)/Ag\({}^{2+}\) ions. The bacterial colony was completely inhibited by 1 ug of triangular Ag NPs, whilst more than 12.5-50 ug of spherical-shaped nanoparticles were required to inhibit the bacterial growth significantly. In contrast, 100 ug of rod-shaped nanoparticles and AgNO3 were unable to achieve the same effect. Another study concerning the size- and shape-dependent antimicrobial activity proposed the smaller size of Ag NPs proved the better performance of antimicrobial against Gram-positive (_Staphylococcus epidermidis_ and _Bacillus megaterium_) and Gram-negative bacteria (_P. aeruginosa_). The authors concluded Ag NPs are potentially applicable in clinical wound dressing, bio-adhesives, biofilms and the coating of biomedical materials [80]. Moreover, certain Ag NPs have been found to inhibit activity against antibiotic-resistant species also. For example, [PERSON] et al. found Ag NPs of 100 nm had antimicrobial activity against erythromycin-resistant _S.