Variables affecting the antibacterial properties of nano and pigmentary titania particles in suspension

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Abstract

The antibacterial activity of photoactivated nano and pigmentary titania particles in suspension was evaluated using Escherichia coli as the target organism. The antibacterial activity of nanotitanium particles was determined more by their intrinsic ability to generate radicals than to particle size. Indeed there was an inverse relationship between particle size and activity. The antibacterial activity of the particles was affected by multiple experimental parameters. The reliability and variability of the results were affected by the physiological status of the bacterial cells, the initial cell concentration, and the set up of the irradiation system and were also improved if the cell–particle mixture was stirred during irradiation. The development of appropriate in vitro testing methods is essential in the determination of antimicrobial effectiveness of these particles and this is examined here coupled with the use of the microwave spectroscopic method for determining the photoactivity of the pigments in terms of carrier generation.

Introduction

When light energy of wavelength below 400 nm is utilised by the photocatalytic action of titanium dioxide, the free radicals produced can decompose or destroy foreign materials. An antibacterial effect has been demonstrated, with several groups describing antimicrobial effects of suspensions of particles, or thin films [1], [2], [3], [4], [5], [6]. Most of this work has been carried out using the anatase pigment P25 (Degussa Chemical Company, Germany).

The thermal and photocatalytic activity of a selection of nano versus micron-sized anatase and rutile titania pigments in polyethylene and alkyd based paint films has already been assessed in our laboratories [7], [8], [9], [10]. Preliminary experiments demonstrated an antibacterial effect of the pigments by irradiation of a mixture of bacterial cell suspensions and pigment mixtures [9], [10], [11]. Further work was carried out to clarify the relative activities of the pigment series in terms of antibacterial effects, and to define experimental variables which may affect a demonstrable in vitro antibacterial activity. We believe that this is the first study using a range of nanoparticle TiO2 pigments with different particle sizes for monitoring the inactivation/destruction of bacteria. Coupled within this study is the use of the photodielectric microwave method for analysis and comparing the titania activities for cross-referencing [12], [13].

Section snippets

Materials

A range of anatase and rutile pigments (supplied by Millenium Inorganic Chemicals, Grimsby, UK) were used in the study (Table 1). The most active photocatalysts are formulations based on the anatase crystal phase, and most work [15], [16] has been carried out on pigment B (Degussa P25). In contrast to the remaining pigments, which were supplied as powders, pigment C is a colloidal dispersion of nanoparticles (colloidal suspension of TiO2, 14–16%, stabilised with an amine, with a pH of 10–11.5).

Microwave analysis

In this technique microwaves are directed, via a waveguide, through an aperture into a cavity. At certain frequencies the cavity abstracts appreciable power, since the oscillating electric and magnetic fields of the microwave energy reach a maximum when they are resonant with the cavity. Typical microwave spectra for a selection of the anatase and rutile pigments used here with respect to that for the cavity have been illustrated earlier [12], [13]. The amount of energy stored by a microwave

Conclusions

The overall effect of activity will depend on whether more TiO2 is activated as a consequence of increased surface area, or whether less TiO2 is activated because less light passes through the suspension due to light scattering. Larger aggregates of particles sediment in a liquid system, and an increased concentration of pigments shows less of an antimicrobial effect since less light passes through the suspension if the cell–particle mixture is not stirred. Conversely, Calgon milled pigments

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