27.4.7 Other Metal-Based ENMs

In addition to the conventional metal-based ENMs, there has been great interest in

using other metals such as iron (Fe), palladium (Pd), platinum (Pt), tellurium (Te) and

selenium (Se) based ENMs for antimicrobial applications.

Among these metallic NMs, Fe-based ENMs are widely explored for their

biocidal properties with proven efficacy owing to their higher sensitivity to visible

light for the formation of radicals, low cost of production, chemical stability (stable

across a broad range of pH), ease of fabrication non-toxicity, abundance, and

reasonable cost. Basnet et al. (2013) reported the fabrication of α-Fe2O3

nanocolumns and nanorods for visible light antimicrobial applications against

E. coli bacteria. Under visible light exposure, the nanorod arrays were observed to

be more efficient at inactivating E. coli than thin film samples. The increased

biocidal effects of nanorods were attributed to their morphology, which promoted

the longer contact times between bacteria and α-Fe2O3 surface and thus increased the

probability that E. coli could be inactivated by direct photochemical oxidation of the

intra cellular coenzyme A.

Lee and co-workers (2008) reported the high bactericidal activity of zero-valent

Fe (Fe⁰) NPs in aqueous solution against E. coli. A strong bactericidal effect of Fe⁰

NPs was found under deaerated conditions, with a linear correlation between log

inactivation and Fe⁰ NPs dose (0.82 log inactivation/mg/L nano-Fe⁰ h). The inacti-

vation of E. coli under air saturation required much higher Fe⁰ NPs doses due to the

corrosion and surface oxidation of Fe⁰ NPs by dissolved O2.

Polyvinyl alcohol (PVA), often referred as green polymer, is among the most

commonly used synthetic polymers for biomedical applications due to its solubility

pattern and easy degradability and biocompatibility. Tran and colleagues (2010)

studied the biocidal effect of PVA-stabilized Fe2O3 NPs on S. aureus. The results

provided evidence that Fe2O3 NPs inhibited the growth of S. aureus and the

antimicrobial inhibition behaviour was concentration-dependent. In addition, all

cells were not adversely impacted in the presence of Fe2O3 NPs, especially

osteoblasts (bone-forming cells), whose growth was observed to be enhanced.

These studies have shown that Fe2O3 NPs can have a dual therapeutic role which

could boost bone growth and inhibit bacterial infection as well. Finally, this research

suggested that, with an appropriate external magnetic field, Fe2O3 magnetic NPs

could be guided to destroy bacteria as required in the body.

Other than Au and Ag, precious metals such as platinum (Pt), palladium (Pd),

rhenium (Re), etc. have also been studied for their antimicrobial activity. For

instance, research has shown that platinum NPs have the capability to pass into the

cell which makes it a potentially good candidate for antimicrobial therapy (Rice et al.

2019). Tahir et al. (2017) examined the antimicrobial activity of biosynthesized Pt

NPs (2–7 nm) against GP (B. subtilis) and GN (P. aeruginosa) bacteria. The results

showed the high antimicrobial activity of Pt NPs for both the bacteria.

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