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Article

Fungal Colonization of Weathered Radiata Pine Surfaces Protected with Inorganic Nanoparticles and Coatings

by
Vicente A. Hernandez
1,2,*,
Nicole Sagredo
2,
Javiera Riquelme
1,
Romina Romero
3 and
Philip D. Evans
4
1
Facultad de Ciencias Forestales, Departamento de Manejo de Bosques y Medio Ambiente, Universidad de Concepción, Victoria 631, Concepción 40730386, Chile
2
Centro de Biotecnología, Universidad de Concepción, Barrio Universitario S/N, Concepción 40730386, Chile
3
Facultad de Ciencias Químicas, Departamento de Química Analítica e Inorgánica, Universidad de Concepción, Edmundo Larenas 129, Concepción 4070371, Chile
4
Department of Wood Science, Faculty of Forestry, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada
*
Author to whom correspondence should be addressed.
Forests 2023, 14(7), 1338; https://doi.org/10.3390/f14071338
Submission received: 1 June 2023 / Revised: 23 June 2023 / Accepted: 26 June 2023 / Published: 29 June 2023
(This article belongs to the Special Issue Preservation, Protection, and Modification of Wood and Wood Byproducts)
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Abstract

:
Photoactive nanoparticles are used to reduce microbial colonization and self-clean surfaces of materials such as glass and ceramics. To test whether such an approach is feasible for wood surfaces, we treated radiata pine samples with TiO2 (rutile and anatase) or ZnO nanoparticles and then coated the samples with different finishes. Coated samples and uncoated controls were exposed outdoors for six months. After exposure, fungi colonizing wood surfaces were identified using molecular techniques and microscopy, and colour changes in the wood samples were also measured. Treatment of uncoated surfaces with nanoparticles reduced the discolouration of wood during weathering but had little effect on colonization of wood by black mould fungi. In contrast, pretreatment of samples with titanium dioxide nanoparticles increased the number and diversity of fungi including basidiomycetes colonizing coated samples, whereas zinc oxide nanoparticles had the opposite effect. Zinc oxide nanoparticles, however, were less effective than rutile titanium dioxide nanoparticles at reducing the discolouration of coated samples exposed to natural weathering. We conclude that none of the photoactive nanoparticles on their own are able to reduce microbial colonization and discoloration of samples. This suggests that it may be difficult to create self-cleaning wood surfaces using photoactive nanoparticles.

1. Introduction

The presence of fungi on wood surfaces exposed to natural weathering was first documented during the 19th and early 20th centuries [1,2]. However, identification of fungi colonizing weathered wood did not occur until much later [3]. Fungi colonizing weathered wood surfaces are mainly heavily pigmented filamentous fungi, commonly referred to as black moulds, that can withstand UV radiation, desiccation, and high temperatures [3,4]. Moulds contribute to the grey colour of weathered wood in addition to discoloration caused by solar radiation and the accumulation of dust and dirt [5,6,7,8,9].
Coated wood is also susceptible to fungal colonization during weathering. Strains of the filamentous fungi Aureobasidium sp., Phoma sp., Alternaria sp., Cladosporium sp., and Stemphylum sp. have been found colonizing weathered coated wood [3,10]. These black moulds can even grow into the coating film by using it as a carbon source [3,10]. Several theories have been proposed to explain the colonization of coated wood by black moulds. The most plausible are those postulating that fungal spores land on wood surfaces prior to the application of coatings, and then germinate if growth conditions are favourable. Fungal colonization beneath wood coatings can also occur via small imperfections in the film [3,10]. The growth of fungal hyphae under the coating can result in blistering and fracture of the film, eventually leading to coating failure [3,11]. Filamentous fungi can also colonize chemically and thermally modified wood exposed to weathering. Species of Hormonema and Mucor have been isolated from thermally modified Norway spruce (Picea abies (L) H. Karst.) [12]. Similarly, Phoma spp. and Cladosporium spp. have been isolated from acetylated Scots pine (Pinus sylvestris L.). Wood modified with DMDHEU (1,3-dimethylol-4,5-dihydroxyethylene urea), amino-alkyl-functional oligomeric siloxane, or sodium water glass can be colonized by black moulds such as Trichoderma sp., Epicoccum spp., and Aureobasidium spp. [13]. Hence, there is great interest in developing effective ways of inhibiting the colonization of uncoated and coated wood surfaces by black moulds.
One possible way for inhibiting the colonization of wood by black moulds is to treat the wood with photocatalytic metallic nanoparticles. Such nanoparticles are able to create self-cleaning materials (glass and ceramics), as reactive species produced by the nanoparticles under UV excitation can degrade microorganisms and also pollutants [14,15,16,17,18,19]. However, this use of metallic nanoparticles for wood protection has received little attention. In contrast, copper nanoparticles are the most common biocidal treatment used to protect wood from fungal decay [20]. In addition, cerium oxide, titanium dioxide, and zinc oxide nanoparticles have been widely tested as a means at protecting wood against photodegradation and improving the longevity of clear or lightly pigmented coating systems on wood [21,22]. Metal oxide nanoparticles are attractive in these applications because of their low cost, preservative capacity, chemical stability, low toxicity, and high oxidation rate [23]. When used as protective layers, nanoparticles exhibit transparency and relatively high intermolecular energy [24]. However, it is unclear if inorganic nanoparticles can self-clean wood.
In this work, we treated radiata pine samples with six different types of titanium dioxide and zinc oxide nanoparticles that varied in the extent to which they form reactive species under UV excitation or possess biocidal capacity (zinc oxide vs. titanium dioxide) [22]. Samples were left uncoated or coated with commercial finishes and exposed to natural weathering for six months. Following natural weathering, we examined the colonization of samples by fungi and the discoloration of wood surfaces. We hypothesize that the photoprotective and photocatalytic effect of TiO2 and ZnO nanoparticles alter the fungal colonization of wood samples, due to the possible deleterious action of radical species on fungal cells. We were particularly interested in detecting inhibition of fungal colonization of samples by photocatalytic nanoparticles.

2. Materials and Methods

2.1. Wood Samples

Radiata pine (Pinus radiata D. Don) samples (n = 210) were sawn from parent wood boards that had been conditioned at 20 ± 1 °C and 65 ± 5% relative humidity to achieve a moisture content of 12%. The samples were 10 × 40 × 80 mm3 in size with their grain oriented longitudinally. Average air-dry density and number of growth rings per cm of samples were 0.496 g/cm3 and between 1 and 2 (approx.), with a standard deviation of 0.031 g/cm3 and 0.21, respectively. These parameters were measured using 20 supplemental samples prior to treatment. Wood samples were treated in groups of three with a unique combination of nanoparticles and coating to form 70 different treatments groups (Table S1).

2.2. Impregnation with Nanoparticles

The nanoparticles used to impregnate wood samples were obtained from US Research Nanomaterials, Inc. (Houston, TX, USA). Their purity was 99%, had a white appearance, and were almost spherical. The type of nanoparticle and their density and sizes are shown in Table 1. Information on their photocatalytic activity can be found in Hernandez et al. (2022) [22].
Dispersions of nanoparticles in ethanol at a concentration of 1% (w/v) were prepared and wood samples were vacuum impregnated (30 min, −0.1 MPa) with the dispersions. Prior impregnation, dispersions were sonicated for 30 min at room temperature to avoid agglomerations. After impregnation, treated and untreated controls were dried in a fume-hood at room temperature for 24 h, and then stored in a conditioned room (as above) for two weeks. In previous work, the reliability of this method for impregnation of zinc oxide and titanium dioxide nanoparticles into radiata pine samples was confirmed by scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis [22]. This approach was necessary because weight gains due to the presence of nanoparticles were negligible in all cases. In the current work, presence of nanoparticles in the wood samples was confirmed by SEM (images are available upon request).

2.3. Coating Application

Nine commercial transparent and semitransparent coatings intended for outdoor use were purchased from a retail store. The coatings selected ranged from specialized products such as marine varnishes to more generic finishes. Nevertheless, the coatings are representative of those on the market in South America. Selected coatings were applied onto the surface of untreated samples and ones treated with nanoparticles. Uncoated and untreated samples acted as controls. Two coats of finish were applied onto the samples with an interval of 48 h between coats, thus ensuring the first coat was dried before the second coat was applied. Coatings and their composition, according to information provided by each manufacturer, are shown in Table 2.

2.4. Weathering Exposure

The wood samples were exposed to natural weathering in three 0.8 × 1 m racks, at an inclination of 45°, equatorially oriented (facing north). The exposure site was located at University of Concepción, Concepción, Chile (36°50′32″ S, 73°01′31″ W), and the samples were exposed between September and February (spring–summer).

2.5. Isolation and Identification of Fungi

The isolation of filamentous fungi was carried out using methods described by Lim et al. (2005) [25]. A small piece of wood was excised from each sample, under sterile conditions, using a scalpel. In each case, the surface layer at the sampling area was carefully removed with the scalpel, to permit subsequent removal of a small portion of wood from the cleaned portion of the sample. This allowed the isolation of filamentous fungi growing inside the wood cells, close to the exposed surface, and avoided the inclusion of fungal spores that could have landed at the surface of the sample. The excised piece was placed directly onto malt extract agar (1%) in a Petri dish. After several days of cultivation, all fungal colonies developing in the plate were subcultivated in new plates. Pure cultures of the filamentous fungi developing on each plate were obtained by simple replication or by single spore isolation [26]. Pure cultures were identified using molecular techniques, which were complemented by observations using microscopy [27]. Molecular techniques involved the extraction of fungal ribosomal DNA (rDNA) using the kit, GeneJete #K0721. Amplification of the rDNA used the primers ITS1–ITS4, while purification and sequencing of PCR products were performed at Macrogen (Seoul, Republic of Korea). Sequences were cross-referenced at the GeneBank database (http://www.ncbi.nlm.nih.gov/blast/Blast.cgi, accessed on 7 June 2022). Fungi were identified to genus or species level depending on the information available.

2.6. Measurement of Colour Changes

The colour of surfaces exposed to weathering was measured before, during, and after exposure by using a spectrophotometer Konica-Minolta CM-5 and the CIE L*a*b* colour system. The colour change (ΔE) was calculated for every sample using the CIE colour components measured before and after exposure (Equation (1)). After colour measurements, digital images of the surfaces were obtained using a desktop scanner (Epson Perfection V370) at a scale and resolution of 1:1 and 96 dpi, respectively.
ΔE = [(L*2 − L*1)2 + (a*2 − a*1)2 + (b*2 − b*1)2]1/2
In the equation above, ΔE corresponds to the CIE L*a*b* colour difference between the surface before and after exposure. The colour components before (1) and after exposure (2) are L* (lightness), a* (greenness–redness) and b* (yellowness–blueness).

2.7. Statistical Analyses of Data

Genstat V22 software was used to analyze fungal isolation data and colour changes. An analysis of variance (ANOVA) using a subroutine (convstrt) was used to compare the difference in colour of untreated and coated samples versus treated and coated samples, and differences between treated and coated samples. The same ANOVA was used to compare the lightness of untreated and uncoated samples with treated and uncoated samples. Significant results are presented in graphs and errors bars in graphs derived from Fisher’s least significant difference (LSD) test, and can be used to estimate whether differences between individual means are significant (p < 0.05).

3. Results

3.1. Effect of Treatment with Nanoparticles on Colour and Fungal Colonization of Uncoated Samples

Treatment of wood samples with nanoparticles without subsequent coating of treated samples was of interest in terms of the ability of nanoparticles on their own to self-clean surfaces. Both untreated and treated samples became darker during weathering. Photographs of the samples before and after weathering show the extent of darkening (Figure 1). Darkening of untreated samples was greater than that of samples treated with nanoparticles when averaged across all treatments (Delta L* = 16.7 for untreated versus 13.1 for treated). Analysis of variance (ANOVA) employing a contrast between untreated versus treated samples indicated that this difference in change in lightness was close to statistical significance (p = 0.06) even though replication was low, suggesting a relatively large effect [28] (Table S3). ANOVA revealed no effect (p = 0.61) of different nanoparticle treatments on darkening of uncoated samples during weathering.
The darkening (greying) of wood during natural weathering is associated with the colonization of wood surfaces by highly melanized moulds. Such moulds were the most common fungi isolated from both untreated and treated samples, accounting for 79% of all fungal isolates. Six fungal isolates were obtained from untreated controls, five ascomycetes, and one basidiomycete fungus (Table 1). As expected, dominant fungi were the melanized moulds Aureobasidium sp. (three isolates), Hormonema sp. (one isolate) and Cladosporium sp. (one isolate) (Figure 2 and Figure 3). The basidiomycete was identified as the cobalt crust fungus Terana caerulea (Lam.) Kuntze. In uncoated samples treated with nanoparticles, the number of fungal isolates was always equal to or greater than that of samples without nanoparticles. Melanized moulds predominated as was the case for untreated samples, but larger numbers of basidiomycete fungi were isolated from samples treated with TiO2 (anatase) (Table 3). Sequences accession numbers for the fungi isolated from wood samples are shown in Table S2.

3.2. Effect of Treatment with Nanoparticles on Colour and Fungal Colonization of Coated Samples

Analysis of variance revealed significant (p < 0.001) effects of treatment with nanoparticles and also coating type on the colour changes in samples exposed to natural weathering (Table S4). There was no significant (p = 0.134) interaction of treatment and coating type on colour change in the samples. Significant differences (p < 0.05) in the colour changes in samples treated with different nanoparticles are apparent in Figure 4.
The colour changes in samples treated with rutile titanium dioxide (TiR50 and TiR100) were significantly (p < 0.05) lower than that of all other samples, with the exception of the untreated control and sample treated with 100 nm diameter anatase titanium dioxide. Discolouration of the samples treated with zinc oxide nanoparticles was significantly (p < 0.05) greater than that of the samples treated with rutile titanium dioxide.
Discolouration of samples was affected by coating type, as expected (Figure 5). This figure plots the discolouration of untreated and treated samples, although there was no significant (p = 0.134) interaction of treatment and coating type on discolouration. The major differences in discolouration of the coated samples is the greater discolouration of samples treated with coating B5 (Table 2) and the lower discolouration of samples treated with coating B3 (wood protector based on natural oils, Table 2). Discolouration of the samples treated with the other coating types was similar (Figure 5).
Table 4 shows the fungal isolates on coated samples treated with nanoparticles and exposed to the weather for 6 months. This table shows that less fungal colonization occurred on the samples treated with zinc oxide nanoparticles, particularly those treated with ZnO 35–45 nm. In addition, samples treated with 35–45 nm zinc oxide nanoparticles were only colonized by melanized moulds. Overall, Aureobasidium sp. and Cladosporium sp. were still the dominant fungi colonizing coated samples, albeit at a lower level than on uncoated samples, but Hormonema sp., which was commonly found on uncoated samples, was only isolated once from coated samples. Another difference between colonization of uncoated and coated samples is the greater diversity and numbers of ascomycetes and basidiomycetes isolated from coated samples, with the exception of those treated with zinc oxide nanoparticles, as mentioned above.

4. Discussion

The application of commercially available coatings to untreated wood and wood treated with titanium dioxide nanoparticles generally increased the number of fungal isolates and the diversity of fungi that were isolated from samples exposed to natural weathering for 6 months. Melanized moulds that are commonly isolated from weathered wood [3,4] were the dominant species in both uncoated and coated samples. However, in coated samples less highly pigmented fungi became recurrent, increasing the diversity of fungal species isolated. One exception was coated samples treated with zinc oxide nanoparticles. In these samples, black moulds remained dominant and there was little increase in the diversity of fungi observed. This may be explained by the biocidal action of zinc oxide ions and possibly by the higher level of photocatalytic activity of ZnO nanoparticles compared to TiO2 [22]. Since clear coatings allow UV radiation to reach wood surfaces, ZnO nanoparticles located under the film can be excited by this radiation to produce ions and radical species that can act on living cells present under the film coating film.
Colour changes at the uncoated and coated surfaces indicated that UV radiation affected wood surfaces. Rutile titanium dioxide nanoparticles (50 and 100 nm) reduced colour changes in the coated samples, in accord with our previous work on the use of these nanoparticles for the photoprotection of radiata pine surfaces [22]. Conversely, and unexpectedly, greater numbers of hyaline fungi were isolated from coated samples pretreated with titanium dioxide nanoparticles (Table 4). In addition, these samples showed lower prevalence of black moulds and increased fungal diversity. Such effects might be associated with photoprotection provided by titanium dioxide nanoparticles, which absorb UV while scattering and reflecting visible light that could pass through clear coatings, acting as a shield not only for wood surfaces, but also for fungi that do not have the evolutionary adaptations to withstand high levels of UV radiation. On the other hand, the lower level of photoprotection provided by zinc oxide nanoparticles compared to anatase and rutile titanium dioxide nanoparticles [22], and the biocidal activity of zinc oxide, may explain the smaller numbers of fungal isolates from these samples, and why hyaline fungi were not successful colonizers in samples treated with zinc oxide nanoparticles. Thus, contrary to the original hypothesis of this study, the ability of nanoparticles and coatings to act as a photoprotective shield for fungi colonizing wood seems more important than any destructive photocatalytic effect of the nanoparticles on fungi.
Dominant fungi in uncoated and coated control samples were species of the ubiquitous black moulds Aureobasidium sp., Hormonena sp., Cladosporium sp., and Penicillium sp. These fungi have been frequently isolated from weathered wood, both coated and uncoated [4,29,30,31,32,33,34]. They produce highly melanized mycelia and spores, which is a desirable attribute for a microorganism exposed to UV radiation, high temperatures, and lack of moisture at weathered wood surfaces [35,36,37,38]. Microorganisms with these characteristics are well adapted to weathered wood surfaces. In addition to the dominant fungal species isolated in control samples (without nanoparticles), strains of Alternaria sp., Coniochaeta sp., Epicoccum nigrum Link, and Valsa sp., which have also been reported in weathered wood previously, were isolated in the coated and uncoated samples, treated and untreated with nanoparticles [4,39]. These fungi can either generate pigmented mycelia and spores or produce specialized structures like pycnidium to promote their survival under the severe conditions encountered at weathered wood surfaces [27,39,40]. Other ascomycetes isolated from the samples were Botrytis cinerea Pers., Clypeosphaeria sp., Cytospora eucalypticola Van der Westh., Heterotruncatella sp., and Ramularia sp. These fungi are less common on wood surfaces, but have mould-like development and are common inhabitants of wood debris in softwood and hardwood forests [39,41,42,43,44,45]. Fungal colonization of preservative-treated and chemically modified wood surfaces exposed outdoors has been described in several studies [12,13,46]. However, colonization of wood surfaces protected with nanoparticles exposed outdoors has received little attention. Nanoparticles have been shown to protect wood from weathering, specifically to decrease colour changes due to UV photodegradation [22]. Nevertheless, nanoparticles vary in their ability to restrict development of moulds at wood surfaces. For example, Terzi et al. (2016) [47] tested Scots pine sapwood treated with nano zinc oxide, boron oxide, titanium dioxide, cerium oxide, tin oxide, and copper oxide, against Aspergillus niger van Tieghem, Trichoderma harzianum Rifai, and Penicillium pinophilum Thom. Their results showed that only zinc oxide and boron oxide were able to decrease mould development at wood surfaces under controlled conditions in the absence of solar radiation. This corresponds to some extent with our results, which showed that of all the nanoparticles we tested, zinc oxide was the most successful at suppressing fungal development in presence of coatings, probably due to its ability to produce ions, and radical species using the UV radiation that passed through the clear coatings.
In addition to ascomycetes, basidiomycete fungi were also isolated from the wood samples. Three species of basidiomycete were isolated from control samples and 12 different species were isolated from samples treated with nanoparticles (with and without coatings). Basidiomycete fungi have been mentioned before as colonizers of weathered wood surfaces [25,48], although they are not as frequent as highly melanized ascomycetes. Basidiomycete fungi can produce white, brown, and simultaneous wood decay [49]. In this study, white-rot fungi, which can degrade and modify lignin, were the most frequent basidiomycetes isolated. These included Armillaria sp., Bjerkandera sp., Ganoderma resinaceum Murrill, Hyphodermella sp., Junghuhnia nitida (Pers.) Ryvarden, a Phanerochaetaceae fungus, Phlebia sp., Sistotrema sp., Stereum sp., and Trametes versicolor (L.) Lloyd [50,51,52,53,54,55,56,57]. Trechispora nivea (Pers.) K. H. Larss. was the only brown-rot decay fungus isolated from the samples [52]. Other basidiomycetes isolated were Burgella sp., Byssomerulius corium (Pers.) Parmasto, Skvortzovia pinicola (J.Erikss.) G.Gruhn & Hallenb., and T. caerulea. Basidiomycete fungi found growing in the samples may degrade wood when the moisture content at wood surfaces is above the fibre saturation point. These conditions may occur at wood surfaces exposed outdoors in wet conditions. Thus, it is possible that basidiomycete fungi contribute to deterioration of weathered wood surfaces accelerating checking, which would increase the depth of wetting and hence favour further microbial activity and survival of other microorganisms. This suggestion is supported by the fact that most basidiomycete isolates collected from the samples can degrade lignin, whose deterioration encourages checking [58]. Lignin is the component of wood that is most rapidly degraded during weathering by UV radiation and it is possible that the availability of aromatic photodegradation products favour colonization of weathered wood by white-rot fungi [58,59].
Digital image and colour change results revealed that in coated wood the main changes were associated with photodegradation of wood with lower influence of fungal colonization and fungal stains. In contrast, uncoated samples, both treated with nanoparticles and untreated, showed colour changes associated fungal staining and photodegradation. It is possible that nanoparticles were leached by rain in uncoated control samples, decreasing any effects that the treatment had on fungi. It has been shown that one of the most prevalent fungi detected in this work, Aureobasidum sp., can melanize in the presence of UV radiation, thus contributing to the greying of wood surfaces during weathering [9].
The photocatalytic activity of nanoparticles has been advantageously used to create self-cleaning glass and ceramic surfaces, because the reactive species they produce under UV radiation can destroy pollutants and undesirable microorganism [14,15,17]. The results of our work show that the prospect of using the reactive species produced by photocatalytic nanoparticles to decrease fungal colonization is difficult to achieve with the current treatments. Apparently, the photoprotective shielding effect of the nanoparticles prevails over the production of free radicals that could degrade fungi. Also, fungal colonization easily extends several millimetres under the wood surface, and at such distance the microorganisms can avoid the presence of short-lived free radical generated near the surface. Consequently, further research is necessary to test other approaches to produce self-cleaning solid wood surfaces.

5. Conclusions

We conclude that the treatment of wood with photoactive titanium dioxide nanoparticles does not restrict the colonization of either uncoated or coated wood by surface moulds. In particular, less pigmented mould fungi and basidiomycetes were more common in wood treated with these nanoparticles. In contrast, zinc oxide nanoparticles were able to reduce fungal colonization in coated wood, but they were less effective than rutile titanium dioxide nanoparticles at reducing the discolouration of coated samples exposed to natural weathering. We explain our observations and propose that the photoprotective action of nanoparticles and coatings allow filamentous fungi, which are not naturally adapted to the extreme conditions at wood surfaces exposed outdoors, to colonize wood surfaces. Hence, the photoprotective effect of titanium dioxide nanoparticles and biocidal activity of zinc oxide nanoparticles prevail over the influence that free radicals, generated by photocatalysis of the nanoparticles, have on fungal growth. Therefore, it is unlikely that photocatalytic nanoparticles can ‘clean’ wood surfaces as they are able to do with glass and ceramic surfaces.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f14071338/s1, Table S1: Distribution of nanoparticle and coating treatments; Table S2: Accession number for sequences in GeneBank of fungi isolated from wood samples; Table S3: ANOVA table for lightness changes; Table S4: Analysis of variance for colour changes.

Author Contributions

Conceptualization, V.A.H., R.R. and P.D.E.; methodology, V.A.H., J.R. and N.S.; formal analysis, V.A.H. and P.D.E.; writing—original draft preparation, V.A.H. and R.R.; writing—review and editing, V.A.H. and P.D.E.; supervision, V.A.H. and P.D.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by ANID, Fondecyt 11180030, and NSERC Discovery 327380–2013.

Data Availability Statement

The data presented in this study are available on request to the corresponding author.

Acknowledgments

V.A.H. acknowledges support from PAI Convocatoria Nacional Subvención a Instalación en la Academia 2018, 77180054.

Conflicts of Interest

The authors declare no conflict of interest. The funding agencies had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Effect of six months of natural weathering on the appearance of untreated control and samples treated with different titanium dioxide and zinc oxide nanoparticles.
Figure 1. Effect of six months of natural weathering on the appearance of untreated control and samples treated with different titanium dioxide and zinc oxide nanoparticles.
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Figure 2. Fungal isolates growing on malt extract agar 2%. Ascomycetes: (A) Aureobasidum sp., (B) Cladosporium sp., (C) Hormonema sp., (D) Alternaria sp., (E) Coniochaeta, (F) Clypeosphaeria sp., (G) Epicoccum sp., and (H) Botrytis sp.; Basidiomycetes: (I) Ganoderma sp., (J) Junjhuhnia sp., (K) Bjerkandera sp., and (L) Trametes sp.
Figure 2. Fungal isolates growing on malt extract agar 2%. Ascomycetes: (A) Aureobasidum sp., (B) Cladosporium sp., (C) Hormonema sp., (D) Alternaria sp., (E) Coniochaeta, (F) Clypeosphaeria sp., (G) Epicoccum sp., and (H) Botrytis sp.; Basidiomycetes: (I) Ganoderma sp., (J) Junjhuhnia sp., (K) Bjerkandera sp., and (L) Trametes sp.
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Forests 14 01338 g003 550
Figure 3. Light microscopy images of several fungi isolated from samples (40×). (A) Aureobasidum sp., (B) Hormonema sp., (C) Cladosporium sp., (D) Alternaria sp., (E) Penicillium, and (F) Phlebia sp.
Figure 3. Light microscopy images of several fungi isolated from samples (40×). (A) Aureobasidum sp., (B) Hormonema sp., (C) Cladosporium sp., (D) Alternaria sp., (E) Penicillium, and (F) Phlebia sp.
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Figure 4. Average colour change in samples treated with different nanoparticles and exposed to the weather for six months (results averaged across different coatings).
Figure 4. Average colour change in samples treated with different nanoparticles and exposed to the weather for six months (results averaged across different coatings).
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Figure 5. Average colour change (ΔE) of samples finished with different coatings and exposed to the weather for six months (results averaged across different NP-treatments).
Figure 5. Average colour change (ΔE) of samples finished with different coatings and exposed to the weather for six months (results averaged across different NP-treatments).
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Table
Table 1. TiO2 and ZnO nanoparticles used to treat samples.
Table 1. TiO2 and ZnO nanoparticles used to treat samples.
LabelCompositionSizeBulk Density (g/cm3)
R50Rutile—TiO250 nm0.31
R100Rutile—TiO2100 nm0.37
A40Antase—TiO240 nm0.42
A100Antase—TiO2100 nm0.65
Z35ZnO35–45 nm0.48
Z80ZnO80–200 nm0.48
Table
Table 2. Coatings selected for testing with nanoparticles.
Table 2. Coatings selected for testing with nanoparticles.
LabelDescriptionComposition Informed by the Manufacturer
B1Wood stabilizerLight hydrotreated paraffinic distillates from petroleum, heavy aromatic solvent from naphtha, phosphoric acid ethylxilo esters, naphthalene, paraffinix wax, copper 8-quinolinol, 4,5-dichloro-2-octyl-3(2H)-isothiazolone
B2VarnishUV protector, water repellent, fungicide
B3Wood protectorSunflower, soy, and linseed oils, desiccant agents, propiconazole, solvent, aliphatic hydrocarbons
B4Marine varnishAcrylic emulsion, organic solvents, special additives for UV protection, wide-spectrum biocide
B5Wood protectorHydrocarbons C11-C14, C10-C13, isocyanates, aromatic compounds, zirconium salts, IPBC, acid 2 ethyl-hexanoic, and others
B6Marine varnishModified alkyd resins and special additives
B7Wood protectorSynthetic resins, mineral spirit, solar filters, water repellents, biocides
B8Marine varnishGlycerol phthalic resin in aliphatic solvents, Stoddard, zircon oxide, and others
B9Protective coating for wood productsAlkyd resin with additives, aliphatic solvents, organometallic complex, inorganic pigments, solar filters protection factor 70, heavy naphtha, benzene, and others
Table
Table 3. Fungal isolates on uncoated samples treated with nanoparticles following six months of outdoor exposure.
Table 3. Fungal isolates on uncoated samples treated with nanoparticles following six months of outdoor exposure.
GeneraDivisionUncoated SamplesTotal Isolates *
No-NPA40A100R50R100ZnO35ZnO80
Aureobasidium sp.Ascomycete332233319 (40.4)
Cladosporium sp.Ascomycete1 11 14 (8.5)
Hormonema sp.Ascomycete122332114 (29.8)
Alternaria sp.Ascomycete 1 1 (2.15)
Stereum sp.Ascomycete 2 2 (4.3)
Epicoccum nigrumAscomycete 1 1 (2.15)
Sistotrema sp.Ascomycete 1 1 (2.15)
Cytospora eucalypticolaAscomycete 1 1 (2.15)
Armillaria sp.Basidiomycete 1 1 (2.15)
Burgella sp.Basidiomycete 1 1 (2.15)
Byssomerulius coriumBasidiomycete 11 (2.15)
Terana caeruleaBasidiomycete1 1 (2.15)
Total number of isolates 678677647
Diversity of genera isolated 436334427
* Percentage in parentheses.
Table
Table 4. Fungal isolates on coated samples treated with nanoparticles following six months of outdoor exposure.
Table 4. Fungal isolates on coated samples treated with nanoparticles following six months of outdoor exposure.
GeneraDivisionCoated SamplesTotal Isolates *
No-NPA40A100R50R100ZnO35ZnO80
Aureobasidium sp.Ascomycete24 242115 (23.4)
Cladosporium sp.Ascomycete221312314 (21.8)
Hormonema sp.Ascomycete1 1 (1.56)
Alternaria sp.Ascomycete 1111 4 (6.25)
Penicillium sp.Ascomycete1 21 4 6.25)
Botrytis cinereaAscomycete 111 3 (4.7)
Coniochaeta spAscomycete12 3 (4.7)
Bjerkandera sp. Ascomycete 11 13 (4.7)
Stereum sp.Ascomycete 11 (1.56)
Epicoccum nigrumAscomycete 1 1 (1.56)
Phlebia sp.Ascomycete 22 (3.12)
Sistotrema sp.Ascomycete 1 1 (1.56)
Trametes versicolorAscomycete1 1 2 (3.12)
ClypeosphaeriaI sp.Ascomycete 1 1 (1.56)
Heterotruncatella sp.Ascomycete 1 1 (1.56)
Ramularia sp.Ascomycete 1 1 (1.56)
Valsa sp.Ascomycete 1 1 (1.56)
Ganoderma resinaceumBasidiomycete 1 1 (1.56)
Hyphodermella sp.Basidiomycete1 1 (1.56)
Junghuhnia nitidaBasidiomycete 1 1 (1.56)
Phanerochaetaceae sp.Basidiomycete 1 1 (1.56)
Skvortzovia pinicolaBasidiomycete 1 1 (1.56)
Trechispora niveaBasidiomycete 1 1 (1.56)
Total isolates 913613114864
Diversity of genera isolated 786982545
* Percentage in parentheses.
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Hernandez, V.A.; Sagredo, N.; Riquelme, J.; Romero, R.; Evans, P.D. Fungal Colonization of Weathered Radiata Pine Surfaces Protected with Inorganic Nanoparticles and Coatings. Forests 2023, 14, 1338. https://doi.org/10.3390/f14071338

AMA Style

Hernandez VA, Sagredo N, Riquelme J, Romero R, Evans PD. Fungal Colonization of Weathered Radiata Pine Surfaces Protected with Inorganic Nanoparticles and Coatings. Forests. 2023; 14(7):1338. https://doi.org/10.3390/f14071338

Chicago/Turabian Style

Hernandez, Vicente A., Nicole Sagredo, Javiera Riquelme, Romina Romero, and Philip D. Evans. 2023. "Fungal Colonization of Weathered Radiata Pine Surfaces Protected with Inorganic Nanoparticles and Coatings" Forests 14, no. 7: 1338. https://doi.org/10.3390/f14071338

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