Lignin Nanoparticles with Entrapped Thymus spp. Essential Oils for the Control of Wood-Rot Fungi

Abstract

After decades of utilization of fossil-based and environmentally hazardous compounds for wood preservation against fungal attack, there is a strong need to substitute those compounds with bio-based bioactive solutions, such as essential oils. In this work, lignin nanoparticles containing four essential oils from thyme species (Thymus capitatus, Coridothymus capitatus, T. vulgaris, and T. vulgaris Demeter) were applied as biocides in in vitro experiments to test their anti-fungal effect against two white-rot fungi (Trametes versicolor and Pleurotus ostreatus) and two brown-rot fungi (Poria monticola and Gloeophyllum trabeum). Entrapment of essential oils provided a delayed release over a time frame of 7 days from the lignin carrier matrix and resulted in lower minimum inhibitory concentrations of the essential oils against the brown-rot fungi (0.30–0.60 mg/mL), while for the white-rot fungi, identical concentrations were determined compared with free essential oils (0.05–0.30 mg/mL). Fourier Transform infrared (FTIR) spectroscopy was used to assess the fungal cell wall changes in the presence of essential oils in the growth medium. The results regarding brown-rot fungi present a promising approach for a more effective and sustainable utilization of essential oils against this class of wood-rot fungi. In the case of white-rot fungi, lignin nanoparticles, as essential oils delivery vehicles, still need optimization in their efficacy.

Spray Coating of Wood with Nanoparticles from Lignin and Polylactic Glycolic Acid Loaded with Thyme Essential Oils

“…On average, the DLC for the EOLs were higher than for the EOPs, but the DLC of EOP-TV was in the same range as the one for EOL-TV. The results for the DLE and DLC of the two EOLs samples were lower than those reported in an earlier study [59], indicating that the EO concentrations determined there using an UV spectroscopy method were over-estimated compared with the EO concentrations determined by the more exact HPLC method applied in the present study. The DLE determined for the EOPs were comparable with results reported by Zhu et al [95] where thymol was entrapped into PLGA microspheres and a maximum DLE of around 40% was achieved.…”

“…An SEM analysis of the aqueous EOP dispersions revealed particles with a smooth surface with a rather uniform size distribution, and size measurements resulted in particle diameters in the range from 100 to 700 nm (Figure 1). Interestingly, the EOPs did not show surface pores and hollow structures as was observed for the EOLs [59], an observation that was also reported elsewhere by Zhu et al [95], while Moeini et al [84] detected crystals of the loaded neem oil in the surface of the PLGA nanospheres. SEM images of the EOPs further revealed densely packed particles, indicating a certain tendency for agglomeration which was especially observed in the case of unloaded EOPs (Figure 1G-I).…”

“…In EOL-TV, the particle size fractions with the highest frequency were between 600 nm and 1000 nm while the most abundant fractions in EOL-TC were between 1200 nm and 1800 nm. Based on the SEM images of the EOPs as well as SEM images of the EOLs published earlier [59], particle size measurements were conducted using Adobe Photoshop software (version 25.5.1). Interestingly, the EOPs showed much smaller average particle diameters than the EOLs and while unloaded PLGA NPs had a bigger average particle diameter than the EOPs, it was the opposite for unloaded LNPs (LNPs alone), which showed smaller average particle diameters than the loaded EOLs (Table 2).…”

“…Other options to make wood protection more environmentally friendly is to replace fossil-based ingredients in traditional wood protection formulations with bio-based additives like lignin or to use lignin nanoparticles (LNPs) as coatings or as carriers of bioactives [37,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60]. Lignin coatings were investigated in a wide range of applications [61][62][63], including as water vapor and oxygen barriers in fiber-based packaging solutions [64], as protective coatings for natural fibers for textile production [65], or for the surface modification of paper products [66,67].…”

“…In earlier works, the authors of this study prepared LNPs with entrapped cinnamon and thyme EOs for the subsequent delayed release of the incorporated EOs, and the LNPs with entrapped Thymus spp. EO gave promising results for the protection of pine seedlings against Phytophtora cactorum as well as against wood-rot fungi [31,39,59].…”