12 nm at the same concentration not only limited the growth of the bacteria but also killed them completely. Here, the mechanism of action involved ROS production and the accumulation of nano‐sized particles in the cytoplasm of S. aureus (Raghupathi et al., 2011).
In another study, size‐dependent antimicrobial activity of cobalt ferrite core/shell NPs was demonstrated. Antimicrobial property of three different sizes of NPs (1.65, 5, and 15 nm) was studied against Saccharomyces cerevisiae and Candida parapsilosis. Notably, against S. cerevisiae 1.65 nm exhibited 12 and 25% higher killing than 5 and 15 nm particles, respectively. Similarly, in the case of C. parapsilosis, the same trend was reported whereas 1.6 nm particles showed 15 and 44% higher killing efficiency in comparison to 5 and 15 nm particles, respectively. The antimicrobial activity of the cobalt ferrite NPs at size of 7–8 nm was suggested to be due to intracellular diffusion with subsequent interaction with cell membrane causing oxidative stress and finally DNA damage. It was also suggested that with decrease in size, the cobalt content of the shell might have increased, which in turn improved the interaction or binding efficiency of particle with bacterial cell (Žalnėravičius et al., 2016).
Morones et al. (2005) investigated the effect of size of silver NPs in the range of 1–100 nm against four different Gram‐negative bacteria (E. coli, V. cholera, P. aeruginosa, and Scrub typhus). High‐angle annular dark‐field (HAADF) scanning transmission electron microscopy (STEM) technique showed that silver NP in range of 1–10 nm was able to attach to bacterial cell membrane, which altered its permeability and respiration. Further the NPs that have penetrated caused intracellular damage by interacting with sulfur and phosphorous‐containing substances such as proteins and DNA. Through this study the author confirmed that the size of silver NPs does play a crucial role in the antibacterial effect (Morones et al., 2005).
Later, Adams et al. (2014) demonstrated a size‐dependent antimicrobial activity of the palladium NPs with a small difference in the particle size (<1 nm). Three different self‐contained sub‐10 nm particles (2.0 ± 0.1, 2.5 ± 0.2, and 3.0 ± 0.2 nm) were tested against Gram‐negative and Gram‐positive bacteria (i.e. E. coli and S. aureus). In the case of E. coli it was observed that smallest particles had higher bacterial killing effect followed by medium‐sized and larger particles. Interestingly, in the case of S. aureus, middle‐sized particles exhibited higher effect than the smallest and larger particles. This study clearly suggested that even a small difference in size (i.e. <1 nm) affects the antimicrobial property of the NPs, which also depends on the strains used (Adams et al., 2014).
In a similar study, the effect of size was explained in terms of change in diameter of carbon nanotubes: a well‐known antibacterial material. In this study, the antibacterial activity of single‐walled (SWCNTs) and multi‐walled carbon nanotubes (MWCNTs) with outer diameter of about 0.9 and 30 nm respectively was considered for assessing the effect of size. Scanning electron microscopy studies showed that E. coli cells attached to SWCNTs exhibited higher degree of cellular damage than those attached to MWCNTs. It was also observed that the E. coli cells treated with SWCNTs got inactivated (80 ± 10%) at a higher percentage than those treated with MWCNTs (24 ± 4%). Similarly, a metabolic activity study also suggested that the cells attached to SWCNTs had lesser metabolic activity than the cells with MWCNTs. Further, the measurement of cytoplasmic content efflux and gene expression of stress and DNA‐related products of CNTs‐treated bacterial cells confirmed the superior toxicity of 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
