Shape and Size Dependent Bactericidal Activity of Photoirradiated TiO2 Nanostructures

Abstract

The most widely used anatase-TiO2 (P25, Degussa, Germany, size 30-50 nm) photocatalyst of spherical shape has been found to display high bacterial killing effect under UV light irradiation. The mechanism involves the production of electron-hole (e-h+) pair by the photoexcited TiO2, which are responsible for the oxidation and reduction of the adsorbed species on the TiO2 surface and the strongly oxidizing positive holes decompose the bacterial cells. Recently it has been discovered that the photocatalytic activity of TiO2 is significantly changed with change in size, shape and surface structural morphology. However, till date, there are no reports available in the literature about the bacterial killing effect of different shapes of TiO2 nanostructures. To our knowledge, this is the first report about the comparative bactericidal activity of different shapes of TiO2 nanoparticles. In this work, we prepared TiO2 nanosphere, nanorod and nanotube, characterized their surface structural dissimilarity and finally investigated their photocatalytic antibacterial properties with respect to killing of prokaryotic gram-negative bacterium Agrobacterium tumefaciens LBA4404. The effect of Au, Ag and Cu co-catalysts photodeposition onto TiO2 for the enhanced photokilling efficiency and their relative co-catalytic activity was also verified. The XRD structural study, TEM size & shape analysis and surface area measurement confirmed the formation of TiO2 spherical (size = 30-50 nm & surface area = 55 m2/g), nanorod (length = 90-114 nm, width = 8.1-11.5 nm, aspect ratio = length/width = 8.5-13.5 & surface area = 69 m2/g) and nanotube (length = 93- 118 nm, width = 9.8-15 nm, aspect ratio = 6.99-13.66 & surface area = 176 m2/g) crystal structure and surface morphology. It has been found that TiO2 nanotube and nanorod showed higher bactericidal activity than spherical shape particles. The TiO2 nanotube displayed highest bacterial killing effect because of its highest surface area (176 m2/g) & surface to volume ratio (0.32-0.44 nm-1) among the studied photocatalysts. The Au and Ag deposited TiO2 nanoparticles exhibited better efficiency in bacterial death as compared to Cu loading and bare TiO2 photocatalysts. This difference in antimicrobial properties can be explained on the basis of their structural morphology & surface area difference, variation in energetics of photoexcited charge species, photogenerated electron transfer rate and their redox ability. Atomic Absorption Spectroscopy elemental analysis also confirmed the potassium ions, K+ leakage during photocatalytic killing of bacterial cells during light illumination over TiO2 nanostructures.