including antibiotic‐resistant bacteria where the antimicrobial agent needs to have a broad spectrum of antimicrobial property. In this regard, NMs have been exploited in the preparation of wound dressing materials. Silver NPs along with polyvinyl alcohol and chitosan have been used in the synthesis of fiber mat for wound healing. Nanosilver with high antimicrobial property significantly inhibited the growth of bacteria and along with mat fiber enhanced the wound healing rate (Li et al., 2013). In addition, NMs with antimicrobial properties have been also used in bone and dental implants. Bone cement containing polymethyl methacrylate (PMMA) with silver‐doped silica glass powder significantly inhibited the formation of biofilm over the implants (Miola et al., 2015). TiO2 mixed with prosthesis retarded bacterial growth and prevented biofilm formation over light exposure (Aboelzahab et al., 2012). Among various NMs, the most commonly exploited antibacterial NMs include nanometals, metal oxides, carbonaceous materials, and cationic polymers.
1.7.1 Nanometals
Metals over a threshold concentration exhibit toxicity on the biological system. Among them, heavy metals such as arsenic, cadmium, chromium, lead, and mercury are highly toxic to the living system (Tchounwou et al., 2012). The property of metal toxicity has been exploited to prepare antibacterial nanometals that are toxic to bacterial cells. An additional advantage of nanometals is the possibility of tuning the properties of the system during the synthesis phase.
Metal NMs include silver NPs, copper NPs, and iron NPs. Silver NPs are one of the well‐known antimicrobial materials that have been used in various healthcare, industrial, and medicinal sectors, their mechanism of action being the toxic effect of Ag+ ions exerted by interacting with sulfhydryl groups in proteins. Further, silver ions also inhibit DNA replication and also affect the cell membrane integrity, thus compromising the permeability (Feng et al., 2000). Wigginton et al. (2010) showed that the Ag NPs have high affinity for tryptophanase protein of E. coli. It is evident from the literature that the enzyme tryptophanase is crucial for the production of indole from tryptophan amino acid where indole plays a crucial role behind the multidrug exporters and also acts as a signaling molecule in biofilm formation. It was clearly evident from the study that the binding of Ag NPs deactivated the enzyme which could make the bacteria prone to effect of drugs and other antimicrobial materials (Wigginton et al., 2010). Ag NPs exhibited bactericidal activity by inactivating the enzyme phosphomannose isomerase in bacteria, which converts mannose 6 phosphate to fructose 6 phosphate. This conversion is one of the crucial steps in glycolysis of bacterial sugar metabolism (Dakal et al., 2016; Sundar & Kumar Prajapati, 2012). In another study, Elechiguerra et al. (2005) showed that the interaction of Ag NPs with HIV‐1 was not random but based on the structure of virus envelope. The envelope consists of a protruding gp120 glycoprotein connected to intracellular matrix protein p17 through a transmembrane glycoprotein gp41. Importantly, HIV‐1 binds to CD4 receptor sites on the host cell using gp120 protein knobs. It was observed in this study that Ag NPs attach to the HIV‐1 surface by sulfur‐containing residues of the gp120 protein (Elechiguerra et al., 2005). Next to silver NPs, copper NPs have gained attention in the past few years as cost‐effective alternatives to the former. Copper NPs also exert a mechanism similar to that of silver. They act by damaging the bacterial cell wall and disrupting the DNA structure (Raffi et al., 2010).
1.7.2 Metal Oxides
Similar to metal NMs, oxides of metal also exhibit antimicrobial activity through the metal ions released from the system. The mechanism of action is the ROS production induced by metal ions or by the accumulation of NPs inside the cells. Most metal oxides are semiconductors in nature with larger bandgap energy. Among the metal oxides, ZnO (3.2 eV) has the highest bandgap energy similar to TiO2, which gets activated at a wavelength of about 390 nm to induce ROS production. CdO has a bandgap energy of about 2.1 eV with activation wavelength at 590 nm (Table 1.2).
Among the metal oxides, the most widely used are TiO2, ZnO, CuO, and MgO for antibacterial activity. A vastly studied metal oxide is TiO2, which is used in various industrial and environmental applications such as in sunscreen preparation, implant coatings, and removal of water and air contaminants. The antimicrobial efficiency of a system depends on the wavelength of light used for activation, the intensity of light, concentration, interaction time, temperature, and target microbes (Markowska‐Szczupak, Ulfig, & Morawski, 2011). Efficacy of the TiO2 system depends on the photoinduced generation of ROS, which happens effectively at the oxide anatase phase. When light of energy equal to or higher than the bandgap energy (3.22 eV, anatase phase) is exposed, photocatalytic activation produces an energy‐rich electron–hole pair. The electron produced is transferred to reducible species such oxygen to generate free radicals like superoxides O2−. In a similar way, it induces the production of hydroxyl radicals and hydrogen peroxide in acidic conditions (Kühn et al., 2003). A schematic representation of photoactivated ROS generation and antimicrobial property of NMs is given in Figure 1.2 as described by Gardini et al. (2018)
Table 1.2 Bandgap energy and activation wavelength for various metal oxide NMs (Gardini et al., 2018).
| S. No | Material | Bandgap energy (eV) | Activation wavelength (nm) |
|---|---|---|---|
| 1 | CdO | 2.1 | 590 |
| 2 | Fe2O3 | 2.2 | 565 |
| 3 | WO3 | 2.8 | 443 |
| 4 | TiO2 | 3.2 | 387 |
| 5 | ZnO | 3.2 | 390 |
Figure 1.2 Schematic representation of photoactivated ROS generation and antimicrobial property of NMs.
In a similar way, ZnO, a semiconductor with larger bandgap energy, is applied in coatings, paints, and sunscreens. Upon exposure of UV light, photocatalysis induces the ROS production, which is responsible for its antimicrobial effect. Here, the roughness of the system depends on the surface defects. Efficacy of the ZnO NP system increases with decrease in size where the roughness of the particle along with its high surface area causes the disruption of the microbial cell wall (Padmavathy & Vijayaraghavan, 2008). ZnO NMs have also been reported to interact with some disease target proteins. Chatterjee et al. (2010) studied the effect of ZnO NPs over periplasmic domain structure of ToxR protein of Vibrio cholerae. ToxR protein plays a critical role in the regulation of expression of many virulence factors of the bacteria. It was observed that the binding of the protein ToxR to ZnO NPs' surface reduced the stability of protein where it was more susceptible to denaturation. Further, significant change in the structure of the protein was also observed (Chatterjee et al., 2010).
1.7.3 Carbonaceous NMs
At the nanoscale, carbon forms different allotropes, which include graphene, carbon nanotubes, fullerenes, and nanodiamonds where