Abstract
Introduction
The process of photocatalysis is activated in natural purification of aqueous systems by sunlight (the ultraviolet [UV] rays), initiating the breakdown of organic molecules; however, the antimicrobial effect of this natural purification system is limited. The utilization of semiconductors and the introduction of catalysts to promote specific redox processes on semiconductor surfaces was introduced in 1970. 2 Since then, intensive studies performed in this field have confirmed that semiconductors (with a primary focus on TiO2) could enhance the purification process.
Semiconductors act as sensitizers for UV light-induced redox processes because of their electronic structure. 3 Following illumination, the semiconductor photocatalyst absorbs photons with energy equal or higher than its band gap energy. This excites an electron from the valence band to the conduction band of the semiconductor, producing an electron hole pair. At the surface, holes are trapped by hydroxyl groups and form hydroxyl radicals that can participate in subsequent chemical reactions. Similarly, electrons can be trapped by adsorbed oxygen to form the adsorbed superoxide anion radical.
Nanotechnology is a fast growing field; products using nanoparticles (NPs) can be found in various industrial, medical, personal, and military applications. 4 Intensive studies suggested that nanomaterials have greater activity relative to their bulk form. 5,6 These favorable characteristics are the result of the small size of the crystallites, which defines the surface area available for adsorption and decomposition of organic materials. The use of metal oxide NPs became a very attractive field. The higher surface-to-volume ratio of these ultrafine particles could favor the transfer of photogenerated charge carriers (i.e., e and h+) to the adsorbed metal oxide. 7 Since the earliest report on the use of nano-photocatalysts in 1980s, additional nanocrystalline semiconductor systems have been developed and characterized. The most frequently studied nanopaticles include but are not limited to oxides, sulphides, and selenides, such as TiO2, ZnO, WO3, V2O5, Ag2O, CdS, and many others. 8
Research in the field of photocatalytic disinfection has been very diverse, with the NPs TiO2/UV process being shown to successfully inactivate many microorganisms, including bacteria and fungi such as Escherichia coli, Micrococcus luteus, Bacillus subtilis (cells and spores), Staphylococcus aureus, 9 Streptoccocus faecalis, 10 Lactobacillus acidophilus, 11 Candida albicans, 9 and others. Moreover, activation of TiO2 with UV was found to be effective against parasites such as Giardia intestinalis and Acanthamoeba castellani cysts. 12
Although the majority of the research focused on TiO2. Other nanoparticles that generate reactive oxygen species (ROS) such as Ag2O 13 and ZnO 14 –18 were widely investigated for their strong antibacterial activity upon irradiation with UV light. It is assumed that even in ordinary room light, with a total light intensity of ∼10 μW cm2, the intensity of UV light, which is ∼1 μW cm2, is enough to induce ROS formation. 19
The mechanism of the UV-induced photocatalytic action of TiO2 has been widely studied. 20 –23 Most of the studies agree that ROS generated by the metal oxide NPs are responsible for their activity. 24 –26 Although UV-excited TiO2 had been shown to have excellent activity, it possesses several disadvantages; UV light (wavelength <400 nm), accounts for only a small proportion of solar light (3–5%). In addition, the high energy UV radiation can induce serious damage to tissues and cells. This is the reason that the potential application of TiO2 substrates is greatly restricted in the public environment.
For the abovementioned reasons, attempts have been made to use visible light for inducing the photocatalytic process in metal oxide NPs.
In the following section, techniques for using visible light for inducing antibacterial activity in metal oxide nanoparticles will be discussed.
Doped metal oxide NPs
Attempts to shift metal oxide NP absorption into the visible light region have mainly focused on their doping with transition metals ions. 3,27 However, shortcomings of doped metal oxide NPs such as thermal instability, 28 the tendency to form charged carrier recombination centers, and the expensive facilities required for ion implantation, make metal-doped metal oxides impractical for industrial applications. 29 During the last decade, anion doping of metal oxides was successfully performed using anions of N. 30 C, 31 S, 32 , P, 33 and F. 34 Among these anion dopants, nitrogen seems to be the most effective because of its similar size to oxygen, metastable AX center formation, and small ionization energy. 35
The major effect of N-doping on the absorption spectra is the greatly enhanced absorption at long wavelength regions (>700 nm) in N-doped TiO2 and ZrO2, which might be useful in enhancing the photocatalytic visible light activities of these materials. 36 The active wavelength of TiO2-x N x , within the visible spectrum promises a wide range of antibacterial applications under visible lightening. 30
Nanocomposites
An additional approach is to chemically attach the metal oxide NPs to an organic molecule, subsequently enhancing its activity in the visible and near infrared (NIR) range. For example; Li et al., 37 observed that ZnO/graphene oxide nanocomposites exhibited a remarkably enhanced photocatalytic efficiency following irradiation with visible light compared with that of graphene oxide sheets and flower-like ZnO particles alone. In another study, 38 it was found that the absorption band of TiO2 nanoparticles is significantly red shifted with the addition of cysteine. The higher visible light activity of the NPs was suggested to be the result of improved separation efficiency of the photoinduced charge carriers of TiO2, as was shown by photovoltage spectroscopy (SPS) results.
Addition of polyaniline (PANI) to TiO2 39 and ZnO 40 resulted in a broad band absorption, at 230–900 nm. Moreover, PANI acted as an effective template to stabilize TiO2 particles at nanoscale through interfacial chemical bonds. 39 Also, coating TiO2 with graphite oxide and fullerene enabled TiO2 nanoparticles to be active by visible light. 41 Finally, it is worth mentioning that it is possible to modify TiO2 nanoparticles with carotenoids 42,43 in order to shift their absorption band to the red region of the spectrum.
The advantages of the organic molecules/metal oxide nanocomposites are as follows: (1) They show better light and thermal stability, which ensures their recycling 44,45 for prolong usage without a significant loss of activity. (2) The organic conjugates largely extend the absorption spectra of the NPs to the red/NIR region, 46 –49 unlike the relatively small extension (up to 500 nm) achieved when they are coated with transition metals. 30,50 –52
Metaloxide nanoparticles with absorption bands in the visible range
Recently it has been demonstrated that some metal oxide NPs (uncoated) absorb visible light. For example, ZnO and TiO2 NPs have an absorption peak in UV-A, with marked absorption in the blue region, as well. 53 –55 This led us to examine ROS production and antimicrobial action of ZnO and TiO2 nanoparticles as a function of the wavelengths in the visible range. 53,54,56,57
We found that nanoparticles of ZnO and TiO2 produced different types of ROS in their water suspension, depending upon the crystal phase of the nanoparticle.
Using the electron spin resonance (ESR) spin trap technique, we showed that both hydroxyl (OH·) and superoxide anion (·O2
−) radicals were present in water suspension of TiO2 rutile and anatase NPs. We further showed that the amount of ROS is higher in the anatase phase. Illumination of the nanoparticle suspensions with visible light caused an elevation in the production of
In both TiO2 and ZnO, only the blue part of the visible spectrum enhanced ROS formation. 53,54 This is consistent with the marked absorption of ZnO and TiO2 in the blue region. As expected, the rutile TiO2 with a smaller energy gap of 3 eV=415 nm, were found to be more sensitive to blue light than the anatase form, with a gap of 3.2 eV=387nm. 54
The effect of blue light on the antibacterial activity of ZnO and TiO2 was also demonstrated; 56 combination of illumination with NPs resulted in a marked increase in the reduction of bacterial viability compared with treatment with visible light or NPs alone. ZnO NPs showed increased activity compared with TiO2 NPs. 56
An enhanced antibacterial activity of visible light excited metal oxide NPs was also shown in a recent work by Sapkota et al. 58 In this work, it was found that the antibacterial activity of ZnO nanorods in the presence of visible light, 500–700 nm, was two times higher than under dark conditions.
The size and shape of the NPs have been known to have an effect on the NPs' activity. Recently, it was also proposed that the method of preparation can affect the biological activity of metal oxides. For example, it was found 59 that low-frequency ultrasound irradiation of commercial Degussa P25 TiO2 nanoparticles increased their absorbance in the visible light by increasing the number of oxygen vacancies.
Metal oxides used on a micro scale have a rich history, with applications in food, materials, and biological studies; however, it is well known that the shape, size, and morphology of a compound can also play a significant role in its biotoxicity. For example, TiO2 has been previously classified as biologically inert, both in animals and in humans, 60,61 and this material was, therefore, considered very safe. However, there have been several recent studies claiming that metal oxide NPs, which easily penetrate the skin, can cause adverse effects on organ, tissue, cellular, and subcellular levels because of their unusual physicochemical properties, that is, small size, high surface-to-volume ratio, electronic properties, and surface structure reactivity. 62,63
To avoid NP toxicity on the host tissue, we suggest depositing NPs on transparent textiles (bandages), which will be loaded on the wound previous to illumination. Several recent studies demonstrated high antimicrobial activity of such NP-coated textiles. No leaching of the nanoparticles from the coated textiles was detected; 64 –66 therefore, the fabrics were safe to use.
Summary
The use of metal oxide NPs combined with visible light irradiation opens new possibilities for surface decontamination. As described, it is possible to extend the absorption region of the NPs to the red/NIR by doping them with transition metals ions or organic molecules. In cases in which the NPs might be toxic, it is possible to coat them on a vast variety of surfaces.
Footnotes
Author Disclosure Statement
No competing financial interests exist.