SWCNTs in comparison to MWCNTs. Overall these results clearly suggested that the SWCNTs exhibited a greater antimicrobial property than MWCNTs. The mechanism of action involved the partial penetration of CNTs and subsequent membrane damage. These effects of SWCNTs are attributed to the diameter (size) of the nanotubes where the smaller diameter aided in better penetration of CNTs into bacterial cells. Penetration was followed by membrane damage affecting the metabolic activity and altered stress‐related gene expressions (Kang et al., 2008).
Zhang et al. (2008) prepared different metallic silver and gold NPs by in situ reduction and stabilized with poly(amidoamine) with terminal dimethylamine groups [HPAMAM‐N(CH3)2]. The size and dispersity of the Ag (7.1–1 nm) and Au (7.7–3.9 nm) NMs can be changed by changing the molar ratio of metal with stabilizer. The antimicrobial property of these series of NMs was tested against Gram‐positive bacteria, Gram‐negative bacteria, and fungi. In these cases, the smallest particles with high surface‐to‐volume ratio exhibited the maximum antimicrobial activity against bacteria and fungi. Along with the size, the cationic terminal groups on surface contributed to a certain amount through interaction with the negative bacterial surface (Zhang et al., 2008).
Apart from individual particle size, experimental or physiological size of the materials does matter in terms of antimicrobial activity. In general, NPs tend to aggregate in experimental and physiological conditions due to their high reactivity. There is a greater chance that NMs that are exposed to bacterial cells at physiological conditions will aggregate rather than existing as individual particles. In such cases, the antibacterial activity obtained may be attributed to agglomerated units rather than the specified size. Neglecting this factor generally results in the misinterpretations of the data. In order to address this issue, a study was conducted with three photosensitive materials such as TiO2, SiO2, and ZnO to analyze their antimicrobial properties in water suspension. The authors reported that the experimental size of NPs was not the same as the true particle size as resulted from the potential aggregation of the NMs. Even though the antibacterial activity of the agglomerated system was similar to that of the material at the same concentration (Zhang et al., 2008). However, antimicrobial activity would have significantly higher in the case of individual units in comparison to the aggregates. It suggests that the size of NM is a very important factor that dictates the physicochemical property of NMs; however, it cannot be considered a general phenomenon in all the cases.
1.12 Influence of Shape on the Antibacterial Activity and Mechanism of Action of Nanomaterials
In addition to size, surface chemistry, composition, and shape also affect the functionality or activity of NMs. Shape plays a crucial role with regard to the interaction and the toxic effects on bacterial cell. Notably, the shape and size of the NMs dictate the physicochemical characteristics such as optical, electromagnetic, catalytic, and the crucial biological properties of the NMs. Taking the aforementioned factors into consideration, researchers attempted to develop various synthesis processes to gain precise control over the physicochemical factors such as size and shape (Bansal et al., 2010; Mulvaney, 1996; Narayanan & El‐Sayed, 2004). In addition to size, the shape of the NM also determines the surface area of the material where even same materials with the same size will have different surface areas because of a change in shape. Next to shape of NM, crystalline nature of the nanostructures also plays an important role. It is generally defined as the relative abundance of particular crystallographic planes where each of them shows specific properties and reactivity. Conclusively, the shape and crystalline nature are important parameters, next to size, that play a significant role in the nano‐bio interaction. Various studies have documented that the shape and crystallinity of NM have a great influence over the behavior of NMs and their biological activity such as antibacterial activity, and their uptake rate. In an earlier study, it was observed that spherical NPs had higher cellular uptake than nanorods (Chithrani, Ghazani, & Chan, 2006). Yang et al. (2016) showed that gold nanorods of different aspect ratio illustrated a significant variation in the cellular uptake rate. A significant increase in the internalization rate was observed with increase in the aspect ratio from 1 to 2 (AR2) where further increase did decrease the cellular internalization rate (Yang et al., 2016).
A well‐known antibacterial material, TiO2 is generally constituted by three crystalline phases such as anatase, rutile, and brookite. In this case, the anatase phase possesses the highest photocatalytic activity, which is due to its high charge transport properties and the presence of highly reactive (001) crystal facet. In comparison to the less‐reactive crystal facet (101), (001) facet produced more efficient electron–hole pairs and reduced their recombination rates. Hence, the antimicrobial property of TiO2 can be enhanced by increasing the ratio of exposed anatase (001) crystal facet. The anatase crystal of TiO2 is generally present in a truncated octahedral bipyramid shape, which is composed of eight less‐reactive (101) facets in the sides added with two highly reactive (001) facets in the top and bottom. However, during synthesis, the highly reactive facets tend to reduce their surface area to minimize the surface free energy. The use of capping agents such as hydrofluoric acid in the synthesis can bind and stabilize the reactive facets (Ong et al., 2014).
Tong et al. (2013) prepared different shapes of TiO2 NMs such as nanorods, nanotubes, and nanosheets with exposed high‐reactive (001) facets. All the nanostructures with more exposed (001) facets produced high hydroxyl radicals in comparison to classical TiO2 NPs P25 (25 nm). Though it enhanced the photocatalytic activity, the antimicrobial property of nanostructures did not follow the same trend where P25 exhibited the highest antimicrobial activity followed by nanorods, nanosheets, and nanotubes, respectively. This has been attributed to the aspect ratio of the nanostructures where the interaction of bacteria and NMs depends on the surface area of the NM. The low antimicrobial profile of the elongated structures such as nanotubes, nanorods, and nanosheets could be attributed to reduced or limited exposure of ROS producing surface to bacterial cells. Since, the elongated nanostructures generally tend to stack over each other due to their strong van der Waals attraction forces (Tong et al., 2013).
Such a direct correlation of the active facets and antimicrobial property has also been found true in the case of silver NPs. In the case of silver, facet (111) is a highly atomic dense lattice that interacts with bacterial cell surface directly and causes membrane damage in comparison to less atomic dense (100) facets. Pal et al. (2007) studied the antibacterial activity of the silver NPs of different shapes against E. coli. Silver NMs with truncated triangular nanoplates exhibited higher antibacterial activity in comparison to spherical and rod‐shaped NMs. Bacterial cell treated with the triangle‐shaped nanoplates having (111) lattice plane showed drastic changes in the membrane, which caused rupture and cell death. This study clearly indicated in addition to the nano‐size of the material, the morphology of NM having (111) lattice plane enhanced the antimicrobial property of silver NMs (Pal et al., 2007).
Gilbertson et al. (2016) studied the antibacterial activity of CuO NMs as a function of their shape. In this study, the author synthesized nanopowders (<50 nm) and nanosheets (~250–1000 nm2 × 15 nm thick) of CuO and compared their antimicrobial property with bulk of CuO material (500 nm–3 μm). The nanosheets of CuO illustrated the highest antimicrobial activity against the tested E. coli followed by nanopowders and bulk CuO. The difference in their level of antimicrobial activity was clearly attributed to their shape. As we discussed in the mechanism of action of NMs, the mechanism of action of NMs can be physical or chemical. The TEM studies revealed that CuO nanosheets oriented parallel to bacterial surface similar to other 1D NMs. This could have led to better interaction of CuO nanosheets with E. coli. Similarly, biochemical reactivity of NMs was evaluated using glutathione oxidation assay. It was observed from the study that the nanosheets exhibited higher oxidation of glutathione than the other two materials. Other studies such electrochemical and catalytic surface reactivity assay also revealed that CuO nanosheets had higher reactivity in comparison to other samples. The above results pertained to the high catalytic reactivity of CuO nanosheets, which produces oxidative stress‐related species and activates the pathways for cellular death (Gilbertson et al., 2016).
Raza et al. (2016) showed that spherical silver NPs of smaller size (15–50 nm) had