Preventing fungal growth in wood by titanium dioxide nanoparticles
Graphical abstract
Introduction
The term nano refers to materials whose dimensions are less than 100 nm. Nanomaterials are characterized by a high surface/volume ratio, which gives them larger activities in surface related phenomena (e.g. adsorption, reaction rates, electronic conductivity, etc.) compared to bulky systems with same mass (De Filpo et al., 2006, De Filpo et al., 2010). Particular environmental remediation applications are based on redox reactions from semiconductors, especially titanium dioxide, which photo-catalytically degrade organic compounds into harmless inorganic compounds (Herrmann, 1999, Nicoletta et al., 2012). When a semiconductor catalyst, e.g. titanium dioxide, in contact with water and oxygen molecules adsorbs some radiation with an intensity of energy that is larger than the characteristic band-gap, electrons are promoted from valence band to conduction band, creating free electrons and electron holes pairs (Hoffmann et al., 1995, Minero, 1995, Rauf et al., 2007). Both electrons and holes can move to the semiconductor surface and produce reactive oxygen species like superoxide anions and hydroxyl radicals, which can oxidize organic compounds, whereas electrons can reduce them (Mahmoodi et al., 2006), as sketched in the following reactions:
A lot of research has been recently devoted to the modification of semiconductor band-gap in order to shift their photo-activity to the visible region (Han et al., 2009). In particular, semiconductors have been modified by doping with noble metals (e.g. Au, Ag, and Pt), transition elements (such as Fe3+, Mo5+, Ru3+, V+4, Rh3+), lanthanides (including Eu, Ce, Nd, Er, Pr, Sm, and La), alkaline metals (such as Li, K, and Na), CdS, non-metals (e.g. N, F, S, B, and C), and dyes (Asahi et al., 2001, Yu et al., 2005, Wong et al., 2006, Hu et al., 2007, Lan et al., 2007, Li et al., 2007, Kubacka et al., 2009).
Titanium dioxide is a semiconductor with a band-gap of 3.2 eV (absorption wavelength less than 380 nm) characterized by long term stability, and UV photo-activity (Keller et al., 2010). In addition to the mineralization of organic compounds, this last property ensures to TiO2 nanoparticles/films antibacterial and antifungal abilities due to the production of reactive redox species. In fact, the TiO2-generated hydroxyl radicals , superoxide anions , and hydrogen peroxide molecules (H2O2) damage cell membrane (Cho et al., 2004) and can inactivate a wide range of organisms (Huang et al., 2000) (bacteria, viruses, fungi, and algae). Consequently, photocatalysis has been suggested as an alternative technique for surface sterilization and water purification (Fujishima et al., 2000). Since the early works of Matsunaga et al., 1985, Matsunaga et al., 1988 on the inactivation of Escherichia coli, several authors have reported the biocidal properties (disinfection) of pure titanium dioxide upon UV light irradiation (Matsunaga and Okochi, 1995, Koizumi and Taya, 2002). Such biocidal activity can be enhanced by co-doping nanoparticles with silver, carbon, and sulphur (Hamal et al., 2010).
Environmental pollution can cause deterioration of stone materials and favour biological attacks by microorganisms. The biological decay of cultural heritage buildings is a serious problem considering the cleaning and repairing costs and, eventually, the cultural losses (Chen and Blume, 2002). Following the works on construction and building materials (Chen and Poon, 2009, Maury Ramirez et al., 2010), Fonseca et al. (2010) have recently proposed, for the first time, an alternative application of titanium dioxide for preventing bio-deterioration of mortars in cultural heritage buildings. Both in lab- and in situ- (Palácio Nacional da Pena, Sintra, Portugal) treatments showed the biocidal and preventing bio-deterioration properties of titanium dioxide (pure and Fe3+-doped anatase form) against lichens and other phototropic microorganisms. Furthermore, the TiO2 treatments resulted to be more effective than other conventional biocides, which, generally, suffer from short term protection and toxicity towards environment and health (Chen et al., 2009, Shabir Mahr et al., 2013). Nevertheless, it should be mentioned that a risk assessment for TiO2 nanoparticles has not been published yet even if they are a common additive in many foods and personal care products (Weir et al., 2012).
Recently, it has been assessed that titanium dioxide nanoparticles inhibit Aspergillus niger colonization of limestone and Carrara marble, when they are dispersed over protective organic coatings and applied to stone samples (La Russa et al., 2012).
White- and brown-rots are fungi that break down lignin and (hemi)cellulose in wood, respectively, leading to a fast and diffuse decay in wood and artistic handworks even far from the surroundings of the fungal attack. Consequently, the wood shrinks, shows a white-/brown-discolouration and, eventually, cracks into small pieces. Hypocrea lixii and Mucor circinelloides are, respectively, white- and brown-rot fungi, attacking wood and causing its deterioration and degradation (Fig. 1).
In this work we have treated eight different types of wood, some of which are commonly used in the field of cultural heritage, with a solution of TiO2 nanoparticles. It is expected that, due to their very small size, nanoparticles can penetrate deeply into the wood pores (Mantanis and Papadopoulos, 2010) causing a photo-catalytic protection against external and internal fungal colonization. Treated and untreated samples have been placed in contact with H. lixii (white-rot fungus) and M. circinelloides (brown-rot fungus) and the fungal growth has been followed by optical microscope observation. In addition, scanning electron microscopy has been used to investigate the distribution of TiO2 nanoparticles throughout wood pores of specimens.
Section snippets
Materials and methods
The wood species used in this work are listed in Table 1, where their resistance to rot fungal attacks is reported. The choice of these particular wood species is due to the fact that they are commonly used in the production of cultural heritage handworks. Samples show different resistance to fungal attack and impregnation degree for softwood and hardwood.
TiO2 has photo-activity against all kinds of microbes under light excitation with energy above its band gap (3.2 eV). P25 titanium dioxide
Analysis of microorganism growth
As expected, both model fungi grow differently according to the infected wood sample. Following as an example the microorganism growth on Sessile oak samples, we observed that white-rot fungus (Fig. 2, on the left) starts its attack after three days from inoculation and shows a fast propagation reaching a level of colonization of around 80%. In a similar manner, brown-rot fungus attacks Sessile oak samples (Fig. 2, on the right), but colonization evolves in a slower manner than white-rot fungus
Discussion and conclusions
It is known that the integrity of wood handworks can be lost as a consequence of a biological damage. Fungi, moulds and insects can easily attack and bio-deteriorate wood samples: the final degree of damage is dependent on wood morphology, environmental conditions, and application of consolidating or preservative products (Sterflinger, 2010).
Even if the TiO2 antimicrobial performance has been well assessed with bacteria, its antifungal activity has been shown to be much weaker due, mainly, to
Acknowledgements
MIUR, the Italian Ministry for University, is acknowledged for financial supports (grants PRIN and EX 60%).
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