Microorganisms like bacteria, viruses and moulds are present all around us and have colonized all of the habitats in the world, including wood and wooden surfaces. If environmental conditions are suitable, these organisms can remain vital and transmitable for a considerable number of days if not months (Van Doremalen et al. 2020). The fungal spores in living environment are spread around mostly from one person to another through the air flow and through contact with surfaces, like doors, tabletops, sittings, handles and similar. In general, the growth of moulds usually does not affect the mechanical properties of the wood, but if the wood remains moist long time enough, further development of fungi can cause the formation of soft rot, which can deteriorate the mechanical properties of the wood (Uzunovic et al. 2008). Moreover, as reported by Møller and co-workers (2017) and Viitanen and co-workers (2010), the presence of fungi and moulds like Aspergillus sp. (e.g. Aspergillus fumigatus), Aureobasidium pullullans, Alternaria alternata, Cladosporium sp. (e.g. Cladosporium herbarum, Cladosporium sphaerospermum), Mucor sp., Penicillium sp. (e.g. Penicillium brevicompactum), Stachybotrus sp. (e.g. Stachybotrus atra), and Fusarium solani, growing on wooden elements installed in living environment can pose a serious threat for human health. Therefore, the antimicrobial protection of surfaces helps to prevent the spread of infection and diseases caused by microorganisms in the human population (Elashnikov et al. 2021).
The susceptibility of wood-based materials for infections with microbes can be determined by monitoring the growth of moulds and fungi on either uncoated or coated surfaces (Gobakken et al. 2012). In general, each coating asset for surface treatment of wooden elements must meet several basic requirements. In order to successfully provide the expected functions of surface protection and a desired aesthetic appearance, the proper interactions between the wooden substrate and the coating are of crucial importance (Miklečić et al. 2022). This way, good wetting of the wooden substrate, sufficient spilling and penetration of the coating in the wood structure defines the mechanical anchoring and adhesion of the coating film to the substrate. At the same time, the physical performance and resistance properties of the formed surface system are indicated by the surface hydrophobicity, resistance to cold liquids and changes in the surface appearance caused by the irradiation of ultra-violet (UV) light.
In the last decades, the trends in the development of new coating systems for wood are focused on searching and investigating the coating formulations with new binders and nanoparticles (e.g. TiO2, SiO2, ZnO, Al2O3, nanocellulose, graphene) as additives for improvement of various mechanical and physical properties (Kaboorani et al. 2017), as well as for preservation purposes (Tomak et al. 2018). Moreover, the need for effective and cost-affordable fungicidal agents (biocides) with long-acting and without risk to human health and the environment is extremely high.
Molybdenum trioxide (MoO3) was reported as one of the promising antimicrobial active substances (Zollfrank et al. 2012; Krishnamoorthy et al. 2013). In contrast to some other fungicidal compounds, the health risks in the case of MoO3 are minimal, as Mo is one of the essential elements in trace amounts and is a cofactor for the formation of enzymes in human bodies (Yoshida et al. 2006). Otherwise, the 1 h-exposure of human keratinocyte cells (HaCaT) to MoO3 nanowires in concentration up to 1 mg × mL–1 after does not show any effect on cell survival (Božinović et al. 2020).
Minubaeva (2007) attributed the antimicrobial efficacy of MoO3 to its dissolution in water and the formation of antimicrobial molybdic acid (H2MoO4) (1):
$$\text{Mo}{\text{O}}_{\text{3}}\text{+}{\text{H}}_{\text{2}}\text{O =}{\text{ }\left[\text{HMo}{\text{O}}_{\text{4}}\right]}^{\text{–}}\text{+}{\text{H}}^{\text{+}}\text{ }\text{↔}{\text{ }\text{H}}_{\text{2}}\text{Mo}{\text{O}}_{\text{4}}\text{ }$$
1
Then, the H2MoO4 dissolves also into oxonium (H3O+) and molybdate (MoO42–) ions, as follows (2):
$${\text{H}}_{\text{2}}\text{Mo}{\text{O}}_{\text{4}}\text{+}{\text{2H}}_{\text{2}}\text{O ↔ }{\text{2H}}_{\text{3}}{\text{O}}^{\text{+}}\text{+Mo}{\text{O}}_{\text{4}}^{\text{2–}}$$
2
,
while the protonation of the \(\text{Mo}{\text{O}}_{\text{4}}^{\text{2‒}}\) ions takes place simultaneously.
In principle, the diffusion of H3O+ ions through the cell wall of microbes causes an imbalance in pH value and the cell's enzyme and transport system. Therefore, for the antimicrobial action of MoO3 nanoparticles on the surface, the rate of MoO3 solubility in water (brought on the surface by microbes, air humidity, or cleaning) is of a critical in this process. Because that antimicrobial action is triggered by the presence of water in the liquid or vapour phase, this type of MoO3-containing nanocomposite can be an environmentally friendly sustainable solution because the use of detergents and disinfectants can be reduced.
The incorporation of MoO3 into polymeric binder allows the application of such polymer as a coating on various solid substrates, with controlled release rates and long-term use. Shafaei and co-authors (2016) reported the effect of different MoO3 crystal structures of micrometre size embedded in different polymer composites on antimicrobial activity against the bacteria S. aureus, E. coli and P. aeruginosa. The authors concluded that direct contact between the bacteria and MoO3 is needed that the H3O+ ions can diffuse through the bacterial membrane. All tests so far have been done with commercially available MoO3 with a micrometer size. Because the rate of solubility and consequent onset of antimicrobial activity depend on the specific surface area of MoO3, the nanoparticles (i.e. nanowires and nanotubes) of MoO3 have an advantage over the materials with MoO3 of micrometer size. In this manner, the high antimicrobial efficacy of MoO3 nanowires incorporated into poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and water-soluble polyvinylpyrrolidone (PVP) polymers was proven in our recent studies (Gradišar Centa et al. 2020; Gradišar Centa et al. 2021).
Several examples of the use of coatings containing nanoparticles with antimicrobial properties well illustrate the potential use of such coating on furniture surfaces (Wei et al. 2014; Gerullis et al. 2018). The aim of this research was to introduce the MoO3 nanoparticles into PVDF-HFP/PVP polymer composite acting as a binder to obtain a completely new type of coating for wood with improved physical properties and antimicrobial functionalities. Firstly, the basic properties (solids content and surface tension) of the synthetized liquid PVDF-HFP/PVP/MoO3 composite coating were determined. As the coating film thickness greatly influences the properties of the coating films and the properties of the surface systems, the coating was applied in three different wet-film thickness, namely 90 µm, 180 µm and 360 µm. The relation between the coating film thickness and its hardness was determined with a pendulum damping test. The wettability of wood with the coating was studied with contact angle (CA) measurements and by monitoring the spilling of the coating droplets over the surface after application on wood. After application of the coating on the surface of a wood, several properties of the coated wood were determined: Microstructure and surface roughness, hydrophobicity, stability against ultra-violet (UV) light irradiation, resistance properties, and the coating adhesion. Finally, the ability of the developed coating to prevent the growth of blue-stain fungi and moulds on the wood was tested.