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 higher antibacterial activity against E. coli and P. aeruginosa in comparison to bigger spherical NPs (30–200 nm) and triangular NMs (edge length 150 nm). This observation was in contrast with previous studies of Pal et al. (2007) and Van Dong Ha, Binh, and Kasbohm (2012), where the triangle‐shaped silver NMs exhibited better antimicrobial property than spherical NMs. The explanation of enhanced antibacterial effect of triangular silver nanostructures was based on the presence of highly reactive and dense atomic crystal facets (111). But here the XRD study revealed that the spherical silver NPs had a strong diffraction peak at 2θ 38.5° from (111) facets. This suggested that spherical NPs were made up of top basal plane with the reactive (111) crystal facets, which could have enhanced the ROS production in bacterial cell and so their antimicrobial property (Raza et al., 2016). In a recent study Cheon et al. (2019) showed a shape‐dependent antimicrobial property of Ag NPs. Antimicrobial property of differently shaped Ag NPs was studied using S. aureus, E. coli, and P. aeruginosa. The zone of inhibition studies showed that Ag spherical NP exhibited highest antibacterial property followed by Ag NM disks and triangular plate Ag NMs (Cheon et al., 2019). The difference in the antimicrobial property was attributed to the release of Ag+ ions from NMs. Considering the surface area, spheres had the highest surface area (1307 ± 5 cm2) followed by disk (1104 ± 109 cm2) and triangular plate (1028 ± 35 cm2). The higher the surface area the higher the release of Ag+ ions, which could be the plausible reason for the highest antimicrobial property of spherical nanospheres followed by disk and triangular plate. The released Ag+ ions interact with bacterial proteins or enzymes through sulfhydryl groups, thereby inactivating or destabilizing the cellular components. Ag+ ions also bind to the membrane proteins that are involved in the ATP generation and ion transport across the cell membrane. Further, the released Ag+ ions interact with nucleic acids and disrupt the H‐bonds of DNA strands, thereby preventing the division and growth of the bacteria. Additionally, they induce the production of ROS, which in turn oxidizes the cellular components such as proteins and DNA. Gao et al. (2013) showed that the antibacterial activity of the Ag nanospheres is higher than that of nanoplates. The results suggested that nanospheres with higher surface area might have had greater contact with bacterial surface in comparison to nanoplates (Gao et al., 2013). In another recent study, Acharya et al. (2018) compared the antibacterial property of Ag nanorods with spherical NPs against E. coli, P. aeruginosa, S. aureus, and B. subtilis. The study revealed that both nanospheres and nanorods were very effective against both the Gram‐positive and Gram‐negative bacteria. The enhanced antibacterial property of both spheres and rods has been attributed to the presence of (111) plane. In general, plane (111) possesses high atomic density, which is one of the factors that determines the antibacterial activity of the Ag NMs (Acharya et al., 2018).
In a similar study, Sharma, Agarwal, and Balani (2016) studied the effect of shapes of ZnO nanostructures on their bactericidal property. Here the author synthesized ZnO microrods and microdisks from ZnO NPs of size (<100 nm) using hydrothermal route. The antimicrobial activity of ZnO structures against Gram‐positive S. aureus and Staphylococcus epidermidis revealed that minimum inhibitory concentration (MIC) of all the three materials was within 0.5 μg/ml, whereas the same for Gram‐negative bacteria E. coli was in the range of 70–76 μg/ml. The mechanism of action involved the production of H2O2, Zn2+ ions release, and the presence of surface oxygen vacancies. In general, the release of metal ion from a material depends on the plane area of the material. Notably, the ZnO microdisks had the highest basal plane area, which led to the highest release of Zn2+ ions from the surface (75.3 ± 14.6 μg/l in MilliQ water and 631.3 ± 17.3 μg/l in LB medium) in 48 hours at 37 °C. Hence the mechanism of action of ZnO microdisks and microrods were suggested to be through release of Zn2+ ions whereas the release of H2O2 and Zn2+ along with cellular internalization was in case of ZnO particles (Sharma et al., 2016). Cha et al. (2015) studied the effect of ZnO NMs over a model enzyme β‐galactosidase (GAL), which can be extrapolated to similar bacterial enzymes. The inhibition mechanisms of three different shapes of ZnO NMs – pyramid, plates, and spheres – were studied using: Michaelis Menten, Lineweaver Burk, and Eadie–Hofstee kinetics models. Among the three different shapes, nanoplates exhibited competitive inhibition over GAL whereas the ZnO NMs of pyramid shape exhibited noncompetitive mechanism. Such variation in the inhibitory effects has been explained by the ability of particular shapes to partially enter the grooves of the active site and inhibit the catalytic reaction by interfering in the reconfiguration of enzyme during substrate binding (Cha et al., 2015). The sharp edges and the apexes of nanopyramids could have been a better geometrical match to the surface of enzyme. These factors further determine the association
