Surface functionalization of cellulose fibers with titanium dioxide nanoparticles and their combined bactericidal activities

Abstract

A well-adherent surface of titanium oxide nanoparticles was produced on cellulose fibers at low temperature from an aqueous titania sol that was obtained via hydrolysis and condensation reactions of titanium isopropoxide in water. SEM investigations of the formed titania films revealed a semi-spherical particle morphology with grain size about 10 nm in diameter. The coated substrates showed substantial bactericidal properties under UV radiation, ambient fluorescent white light and dark conditions. The possible mechanisms for the antibacterial activity are discussed. The stability of the titania coatings was investigated by comparing the UV transmission profiles of coated fibers before and after repeated washing.

Introduction

Extensive efforts have been made to control emerging disease infections in the past several decades [1]. Nevertheless, the spreading of antibiotic resistant pathogens is still a growing concern globally [2]. The ability of microorganisms to survive on environmental surfaces makes infection transmission a critical issue, and studies have shown that some infectious bacteria can survive on the surface of various polymeric and textile materials for more than 90 days [3]. Antimicrobial surfaces not only provide protection against infectious diseases but also against odor, staining, deterioration, and allergies [4]. As a preventive measure, antimicrobial surfaces should be necessary features of daily used materials, especially in some high-risk environments, such as medical and related health-care and hygienic applications. Therefore, new strategies are required for conferring common materials, such as glass, plastics and textiles with bactericidal properties. Due to the poor heat resistance of plastics, wood and textiles, low temperature strategies are essential. The application of several antibacterial surfaces to polymeric and textile materials has been discussed [5], [6], [7], [8], [9], [10].

In recent years, great interests in the bactericidal activity of TiO₂ photocatalyst have been growing [11], [12]. Several techniques such as sol–gel [13], spray pyrolysis [14], chemical vapor deposition [15] can be used to prepare titanium dioxide thin films. Among these preparation techniques, the relatively simple sol–gel method is the most widely used. However, the disadvantage of all these techniques is that a high temperature process is required to produce highly photocatalytic thin films. The formation of photocatalytic titania films at low temperatures is required for any practical application of titania in thermosensitive materials. In addition, the destructive effect of strong acids, used in the sol–gel process to keep aqueous sols in the peptized state, on such substrates at elevated temperature hinders such applications. A third obstacle is the need for UV radiation as titania is reported to perform as a photocatalyst in their antibacterial application [11], [12]. Several attempts have been made to sensitize titanium dioxide for the much longer visible range [16], [17], [18]. In presence of light, TiO₂ is excited with energy equal to (or higher than) its band gap (≈3.2 eV) and electrons transfer from the valence band (O 2p) to the conduction band (Ti 3d) occurs. As a result, electron/hole pairs are formed in the conduction/valence band region. In the presence of oxygen and/or H₂O superoxide (O₂) and/or hydroxyl (OH) radicals are formed. These radicals attack adsorbed organic species on the surface of TiO₂ and decompose them. Although, the radical mechanism of the bactericidal activity of TiO₂ in presence of UV light has been extensively studied [19], [20], dark catalysis of titanium dioxide is reported to result in dehydrogenation and dehydration of organic substances at elevated temperatures [21]. The destruction of some oral microorganisms in presence of TiO₂ powder was also discussed and found to be equally achieved in light and dark [22].

Recently, we have succeeded in forming nanostructured photocatalytic titania from an alcohol-based sol at low temperatures [23], [24]. However, for practical applications, the use of water as a medium may offer several economical and ecological advantages over alcohols.

In this contribution, we describe a novel method to form an antibacterial titania surface on organic cellulose fibers from an aqueous TiO₂ sol at low temperatures. The antibacterial activity was performed under various conditions, and a high level of bactericidal effect of titania coated cellulose substrates was observed. Although it is generally accepted that TiO₂ is bactericidally inactive in absence of light, we found that the cellulose-TiO₂ composite possesses a bactericidal property in presence and absence of light with different rates of cell destruction. Titania coating on a low heat resistance material such as cellulose is a potential process in functionalizing thermosensitive common materials with self-cleaning and antibacterial properties for the decomposition of organic dirt, environmental pollutants and harmful microorganisms.