Abstract
The stability, sensitivity, and volatility of essential oils are some of their most serious limitations, and nanoencapsulation has been considered one of the most effective techniques for solving these problems. This research aimed to investigate the incorporation of Salvia rosmarinus Speen and Cedrus atlantica Manetti (MEO) essential oil mixture in Gum Arabic (GA) and to evaluate nanoencapsulation’s ability to promote antifungal activity against two brown rot fungi responsible for wood decay Gloeophyllum trabeum and Poria placenta. The optimization of encapsulation efficiency was performed using response surface methodology (RSM) with two parameters: solid-to-solid (MEO/GA ratio) and solid-to-liquid (MEO/ethanol). The recovered powder characterization was followed by various techniques using a scanning electron microscope (SEM), X-ray diffractometry (XRD), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), and thermo-gravimetric analysis (TGA). The optimal nanoencapsulating conditions obtained from RSM were ratios of MEO/GA of 1:10 (w/w) and MEO/ethanol of 10% (v/v), which provided the greatest encapsulation efficiency (87%). The results of SEM, XRD, DLS, FTIR, and TGA showed that the encapsulation of MEO using GA modified particle form and molecular structure and increased thermal stability. An antifungal activity assay indicated that an effective concentration of MEO had an inhibitory effect on brown rot fungi. It had 50% of the maximal effect (EC50) value of 5.15 ± 0.88 µg/mL and 12.63 ± 0.65 µg/mL for G. trabeum and P. placenta, respectively. Therefore, this product has a great potential as a natural wood preservative for sustainable construction and green building.
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Introduction
The wood industry is currently experiencing an increase in demand for a safe, and natural wood preservative1,2. Finding antifungal compounds in nature is crucial because of growing worries about the negative effects of commercial fungicides3. Researchers and the industry are paying increasingly more attention to natural anti-wood-decay fungi due to their ability to preserve the quality and safety of wood products4.
Essential oils are well-known viable alternatives for use in the wood preservation sector since they are non-toxic to humans and the environment5,6,7,8. However, the stability, water solubility, and bioavailability of the active components of essential oils are recognized as serious problems9, and encapsulation technology is used to overcome them10. The encapsulation of bioactive components using natural biopolymers has piqued researchers’ interest. Among the biopolymers, gum arabic has been extensively employed as an encapsulating material because of its stability, biodegradability, bioavailability, effectiveness, low toxicity, high-water solubility, and low solution viscosity, all of which enhance its ability to incorporate compound11,12.
Gum Arabic is commonly used in the textile, paint, printing, pharmaceutics, and food industries as a bulking agent, stabilizer, emulsifier, shelf-life increaser, satiating agent, and encapsulating agent for bioactive components13. It is a natural product of acacia exudate from the trees Acacia senegal and Acacia seyal, which belong to the Fabaceae family. This dried sap, which has a high commercial value, is a globally used product, mostly harvested in Africa and Western Asia14. Gum Arabic is a complex mixture of polysaccharides and proteins, composed mainly of 90% arabinogalactan, 10% arabinogalactan proteins and 1% glycoproteins15,16. Arabinogalactan protein is an excellent emulsifier due to its interfacial property, which ensures emulsification17. The arabinogalactan protein’s structure includes polypeptide chains and hydrophilic carbohydrate blocks, giving the protein favorable emulsifying property18.
The nanoencapsulation preparation process involves techniques such as ionic gelation, coacervation, emulsification, freeze-drying, and spray drying19. Freeze-drying is a commonly used method for essential oil nanoencapsulation20. The nanoencapsulation process is influenced by different parameters during the molecular inclusion phase. Preparation technique, volume ratio, reaction time, stirring rate, time, pH, type, the concentration of wall material, and cross-linking agents are the main parameters that can influence the complexation process19. Therefore, more significance should be granted to these parameters to improve nanoencapsulation and, subsequently, its application in various industrial systems.
Recently, the nanoencapsulation of essential oils in biopolymers has been widely studied in various fields, such as food preservation and pharmacology19,21,22. No study, however, has evaluated the potential for the nanoencapsulation of a mixture of essential oils to prevent wood against wood-decay fungi. Therefore, the current work aimed to interest in the incorporation of a Salvia rosmarinus and Cedrus atlantica essential oils mixture (MEO), which had previously been shown to exhibit optimal antifungal activity23, in Gum Arabic (GA) and to evaluate nanoencapsulation’s ability to promote antifungal activity against two brown rot fungi responsible for wood decay. Cedrus atlantica, is a significant forest tree species found in Northern Africa. This tree, also known as Atlas cedar, holds great economic and ecological importance in Morocco's Mediterranean mountains as it belongs to the Pinaceae family. Numerous studies have examined the various bio-functions of C. atlantica, including anti-inflammatory24, anticancer, antioxidant25, antimicrobial, insecticidal, and analgesic26 properties. On the other hand, rosemary, a globally renowned medicinal plant, is of paramount significance in Morocco. The essential oil derived from rosemary (previously named Rosmarinus officinalis) is extensively employed for the treatment of diverse ailments due to its pharmacological attributes. Multiple investigations have demonstrated that S. rosmarinus essential oil possesses antioxidant27, antimicrobial28, antifungal29, anti-inflammatory30, insecticidal31, and antiparasitic32 qualities. Its efficacy against fungal phytopathogens can be attributed to its high content of monoterpenes, including 1,8-cineole, camphor, and α-pinene. The nanoencapsulation condition was optimized utilizing response surface methodology (RSM), focusing on the independent parameters of the solid-to-liquid ratio of MEO/ethanol and the solid-to-solid ratio of MEO/GA and using encapsulation efficacy as dependent responses. The efficacy of this MEO-GA complex was evaluated in terms of its ability to enhance antifungal activity against brown rot fungi, including Gloeophyllum trabeum and Poria placenta.
Material and methods
Plant material and essential oils extraction
Leaves from S. rosmarinus and C. atlantica sawdust were collected in April from the Talsint area (Oriental region, Morocco) and the Azrou Forest (Middle Atlas Mountains, Morocco), respectively. The botanical identification was performed by Professor Satrani Badr at the Forestry Research Center Laboratory, Rabat, Morocco. The current study conformed to all applicable institutional, national, and international guidelines and regulations. Regarding collecting purposes, no specific permission is required. The MEO was extracted by simultaneous hydrodistillation utilizing a Clevenger-type apparatus as previously reported23. A mixture of 55% S. rosmarinus (110 g) and 45% C. atlantica (90 g), or 200 g of plants, was used. The plants were placed in two layers in the distillation flask, with C. atlantica wood sweats on the top and S. rosmarinus on the bottom. One liter of water was added to the distillation flask, and the mixture was boiled for 4 h. The MEO was stored at 4°C until further use.
Chromatographic analysis
The chemical analysis of the EO mixture was performed by GC–MS (Gas chromatography coupled to mass spectrometry), and GC-FID (flame ionization detector). Component identification was accomplished using GC/MS analysis, and component quantification was performed using GC-FID analysis.
Gas chromatography/mass spectrometry analysis
The chemical analysis was carried out by a Hewlett-Packard (HP6890) gas chromatography coupled to a mass spectrometer (HP 5973), using a 30 m HP-5MS column ([cross-linked 5% PHME siloxane] 0.25 mm I.D., 0.25 µm film thickness). The temperature of the column was fixed at 50 °C and gradually raised to 250 °C at a rate of 2 °C/min. Helium was employed as a carrier gas, flowing at a rate of 112 mL/min and 1.5 mL/min with a split mode ratio of 1/74.7. The NIST 98 spectrum library was used to confirm the components’ MS identification. The scan mass range was between 35 and 450 m/z, the ion source temperature was 230 °C, and the ionization voltage was 70 eV.
Gas chromatography (GC-FID) analysis
A Hewlett-Packard gas chromatograph (HP 6890) coupled to an HP-5 capillary column was utilized to analyze the samples, an injector set to 275 °C, and an FID detector. The temperature of the oven was calibrated at 50 °C for 5 min before it was increased to 250 °C at a rate of 4 °C/min. Nitrogen was utilized as the carrier gas (1.8 mL/min).
All samples were dissolved in methanol to a ratio of 1/50, and an injection volume of 1 L was used in a split mode at a flow rate of 72.1 mL/min. Peak area normalization was utilized to determine the percentages representing the relative proportions of the EO’s components. The HP-5 MS column’s retention indices were determined using a homologous series of C8–C28 alkanes. The components were also recognized by comparing the retention indices with the retention indices mentioned in the literature.
Encapsulation process of S. rosmarinus and C. atlantica essential oil mixture in gum arabic
The MEO-GA complexes were prepared using the freeze-drying technique described by Hu et al.33 A quantity of Gum Arabic (GA) was diluted in 10 mL of distilled water under a stirring magnetic field and stirred at 40 °C for 1 h, followed by filtration (0.25 µm) to eliminate the insoluble part. The MEO was then diluted in ethanol and added to the aqueous GA solution, as shown in Table 1. The mixture was then cooled to 25 °C and agitated gently in the dark. Finally, the suspension was freeze-dried at − 80 °C for 48 h according to Muhoza et al.34 methodology and kept at 4 °C for further analysis (Fig. 1).
Encapsulation process illustration of MEO using GA.
Encapsulation efficiency
Encapsulation efficiency (EE) was performed based on Ez-Zoubi et al.35 methodology. A calibration curve was produced utilizing various EO concentrations diluted in ethanol at λmax = 210 nm (BK-D580 Spectrophotometer). The sample was then diluted in ethanol and placed in an ultrasonic water bath at 50 °C, and the EE was calculated by Eq. (1).
Characterization of S. rosmarinus and C. atlantica essential oil mixture in gum arabic
Morphology and particles size analysis
A morphological examination of the wall material (GA) and MEO inside GA (MEO/GA) was performed by scanning with an electron microscope (JSM-IT500HR), as described by Karrar et al.36. The samples were placed at a magnitude of 500× and a voltage of 16 kV under a high vacuum. After suspending 5 mg of MEO-GA in 20 mL of deionized water, the particle size of the sample was assessed by dynamic light scattering (Litesizer 500), following Al-Maqtari et al.37 methodology.
X-ray powder diffraction analysis
The crystallinity of MEO after the encapsulation process was achieved by X-ray diffraction (Xpert-Pro). The measurements were determined by Cu radiation to be from 5° to 50° (2θ) at room temperature, with generator settings of 40 kV and 30 mA.
Fourier transform infrared spectroscopy analysis
The Fourier transform infrared spectroscopy spectra (VERTEX 70 – BRUKER) of the samples above and the MEO were processed using the KBr-pellet method at a resolution of 4 cm−1, and 32 scans, ranging from 400 to 4000 cm−1, were made38.
Thermogravimetric analysis
Thermogravimetric analysis (TGA) of the samples was performed by LINSEIS STA PT1600 in an air atmosphere and heated at 10°C per minute from 20 to 600 °C, following Paula et al.39 methodology.
Antifungal activity
The antifungal activities of gum arabic (GA) and encapsulated mixture essential oil (MEO-GA) against two brown rot fungi were performed according to the Eos dispersion methodology with minor modifications40,41. Two brown rot fungi Gloeophyllum trabeum (G. trabeum) (Persoon ex Fries) Murril (ATCC 11539), and Poria placenta (P. placenta) (Fries) Cooke sensu J. Eriksson (ATCC 9891) were provided by the Microbiology Mycotheque Culture Collections and the Mycology Laboratory at the Forestry Research Center in Rabat, Morocco.
To conduct the experiments, the MEO and GA were serially diluted in a 0.2% sterile agar solution, and 20 mL of solid medium malt extract were added to each dilution to attain the desired concentrations (7.81, 15.63, 31.25, 62.5, 125, 250, 500, and 1000 g/L [w/v]) after the mixtures were poured into Petri dishes. The positive control (Nystatin) was prepared using the same procedure. The negative control was produced by replacing the MEO-GA and GA with a 0.2% agar solution. Each Petri dish has a 1 cm3 diameter piece in the center. After that, the Petri dishes were incubated for 7 days at 25 °C. Each assay was performed in triplicate. The antifungal activity of the MEO-GA and GA was assessed by determining their concentration for a 50% inhibition of mycelial growth (EC50) or the effective concentration for 50% of mycelium growth (EC50). The EC50 was calculated with the following equation (Eq. 2):
where C is the average mycelial growth zone diameter (mm) for control, and T is the average mycelial growth zone diameter of the experiment (mm). Probit analysis was used to calculate the EC50 values (effective concentration for 50% of mycelium growth) for the studied samples (MEO-GA and GA)42.
Experimental design and data analysis
A central composite design (CCD) was conducted to investigate optimum encapsulation conditions. The effect of the two parameters (independent variables)—the solid-to-solid (MEO/GA ratio) and solid-to-liquid (MEO/ethanol ratio)—on the output response was inclusion encapsulation efficiency (Y), which maximized the response value. The levels of the independent parameters in the experiment were listed in Tables 1 and 2. All experiments were performed in triplicate. Notably, the design involved four runs at the square's corners, four runs in the center, and four axial runs. The corners of the square for the coded variables were (X1, X2) = (1, 1), (1, 1), (1, 1), (1, 1); the center points were (X1, X2) = (0, 0); and the axial runs were (X1, X2) = (1.21, 0), (1.21, 0), (0, 1.21). (0, 1.21) (Fig. 2).
Central composite design for two parameters.
To correlate the dependent variable (Y) with the independent parameters (Xi, i = 1, 2), a second-order multiple linear regression model was utilized as follows:
where Y is the dependent variable measured (in this study, Y represents encapsulation efficiency), and a0, ai, and aii indicate the regression coefficients for the constant term, second order, and interaction effects, respectively. Xi and Xj represent the coded independent studied parameters, and k and ɛ represent the total number of the optimized parameters as well as the error associated with the experiments, respectively. Multiple regressions to the fitted model were used to assess the experiments’ data. The significance of differences between the independent variables was determined using the analysis of variance test (ANOVA). The coefficients of determination (R2 and adjusted R2) were used to assess the performance of the postured models and their predictions43,44. The fitted model’s surface responses and contour plots were displayed using Design Expert version 13 software (Stat-Ease Society, Minneapolis, MN, USA) to visualize the relationship between the independent and dependent variables.
Statistical analysis
Tree replicated per treatments were performed for all experiments. The means and standard deviations were determined. The significant differences among samples were examined by Tukey post hoc’s test (p < 0.05) utilizing OriginPro 2021 software (OriginLab, Northampton, Massachusetts, USA).
Results and discussion
Chemical composition
The chemical compositions of the two essential oils, S. rosmarinus and C. atlantica, alone and in combinations (55%/45%), were determined using gas chromatographic analysis and mass spectrometry (GC–MS), as shown in Table 3.
Twenty-six compounds were identified in the S. rosmarinus EO, representing a total of 100%. The Monoterpenes were predominant in S. rosmarinus EO (99.04% ± 0.65%). There was also a significant amount of oxygenated monoterpenes (80.33% ± 3.21%). The percentage of monoterpene hydrocarbons was 18.71% ± 1.14%, but the quantities of sesquiterpenes were almost nonexistent (0.96% ± 0.02%). The major components were 1,8-cineole (50.20% ± 3.80%), camphor (18.47% ± 2.1%), α-pinene (9.96% ± 2.21%), α-terpineol (3.78% ± 1.15%), borneol (3.56% ± 1.41%), and camphene (3.38% ± 1.52%). These compounds were similar to those found in many studies23,45. According to Hannour et al.46, S. rosmarinus EO from the oriental region in Morocco, the Middle Atlas Mountains, and northern Morocco are characterized by a 1,8-cineole component, whereas camphor was the most abundant compound in S. rosmarinus EO from Loukkos47.
Thirty-nine compounds were identified in the C. atlantica sawdust EO, representing a total of 100%. The sesquiterpenes were dominant (98.70% ± 1.72%), and there were large amounts of oxygenated sesquiterpenes (67.25% ± 2.52%) and sesquiterpene hydrocarbons (31.6% ± 0.89%). The monoterpenes were found in low percentages (1.30% ± 0.16%). The major component of the EO was β-himachalene (11.23% ± 1.32%), followed by (E)-α-atlantone (10.33% ± 2.41%), deodarnone (10.52% ± 2.92%), himachalol (10.13% ± 2.04%), α-himachalene (6.83% ± 1.02%), E(E)-γ-atlantone (6.14% ± 2.01%), (Z)-α-atlantone (5.32% ± 2.75%), and γ-himachalene (3.28% ± 1.22%). This EO was characterized by a himachalene chemotype in three isomers: α, β, and γ48,49. However, Zrira et al.26 found that α-(E)-atlantone and le β-himachalène were the most abundant components.
The chemical composition of the combination of 55% S. rosmarinus and 45% C. atlantica was determined under the same identification conditions as the previous EO. Forty-four compounds were identified, representing a total of 99.55%. Chromatography analysis showed that the 55% S. rosmarinus and 45% C. atlantica EO was mainly composed of monoterpenes (70.84% ± 1.56%), with a significant percentage of oxygenated monoterpenes (58.78% ± 1.24%) and fewer monoterpene hydrocarbons (12.06% ± 1.13%). In addition, the MEO consisted of sesquiterpene compounds (28.71% ± 1.51%) with oxygenated sesquiterpenes (19.63% ± 1.17%) and a small percentage of sesquiterpene hydrocarbons (9.08% ± 1.45%). The amount of monoterpene was higher than that of sesquiterpenes, which may be because the quantity of S. rosmarinus was higher than that of C. atlantica in the binary combination (55% of S. rosmarinus and 45% of C. atlantica). The EO composed of 55% S. rosmarinus and 45% C. atlantica mainly comprised 1,8-cineole (36.25% ± 1.67%), camphor (13.87% ± 1.54%), α-pinene (6.70% ± 1.52%), myrtenal (3.04% ± 0.25%), β-himachalene (3.85% ± 1.14%), and borneol (3.01% ± 1.21%). These compounds were detected in small amounts in binary combination, even though they were presented in the individual oil in a relatively high percentage. Simultaneous hydrodistillation led to a new component in the binary mixtures: a myrtenal compound (3.04% ± 0.25%). This finding can be explained by the interaction between the different compounds, the effect of the temperature during simultaneous hydrodistillation, or the appearance of compounds that already existed in trace amounts in the individual oils.
Results of the binary mixture's chemical composition indicated that the main compounds were nearly the same as those identified in previous studies23. Yet they differed in percentages due to the number of plants used in the binary combination during simultaneous hydrodistillation. In accordance with our study, Kharraf et al.50 found that the major components detected in EO mixtures were the same as those presented in the individual EOs. However, Muturi et al.51 findings differed from ours in that the number of components in the EO mixtures were lower than that in individual EOs. A new component in the chemical composition of EOs was found when mixing A. sativum (bulbs) and C. paradisi (leaves)52. Wangrawa et al.53 found that components such as isopiperitone, p-cymene, b-elemol, carvacrol, and caryophylenne were presented in the Lippia multiflora + Cymbopogon schoenanthus combination.
Optimization of S. rosmarinus and C. atlantica essential oil mixture encapsulation condition in gum arabic
To select the parameters that most impacted encapsulation efficiency, preliminary studies were performed based on literature data to optimize the encapsulation procedure in GA56,57,58. The parameters were a solid-to-solid ratio of MEO/GA and a solid-to-liquid ratio of MEO/ethanol. Due to the low water solubility of EOs, ethanol was required as a co-solvent in the encapsulation process59. An ethanol concentration of greater than 50%, though, leads to low encapsulation efficiency60. To minimize dissipation during the inclusion process, the ratios of MEO/ethanol and MEO/GA were varied in the range of 5.8–54.2% (v/v) and 1/0.05–1/10.945, respectively33,56.
A central composite design (CCD) was performed to optimize the encapsulation parameters. By employing CCD, a matrix of 12 experiments was generated. Table 4 shows the observed response values for each experiment’s results.
The results of an ANOVA test conducted to validate the regression models were presented in Table 5. The results revealed that the quadratic polynomial model was a good target response. Based on these results, the probability of risk significance p values for the postulated model was less than 0.05, showing that the adopted model was highly significant. The probability of a lack of fit for encapsulation efficiency was 0.06, indicating that the obtained model’s lack of fit was insignificant. The encapsulation efficiency model was judged as fair for explaining the experiments’ results based on the analysis of variance and the lack of fit of the results. Furthermore, the coefficient of determination and the coefficient of determination-adjusted values (R2 = 0.99 and Rajd2 = 0.99) were high, demonstrating a good relationship among the observed and predicted values of the adopted model.
The observed value of the output response according to the predicted value was illustrated in Fig. 3. As illustrated, most points formed a straight line, demonstrating good agreement between the experiments’ values for encapsulation efficiency and the predicted ones. This finding is consistent with the results shown in Table 5. When R2 was close to 1, the predicted points formed one line as a function of observed values61,62.
Observed vs predicted values for encapsulation efficiency.
The significance of the coefficients in the second-order multiple linear regression models with a 95% confidence interval was shown in Table 6, which was calculated by a Student’s t-test. The least squares method was used to estimate the regression coefficients for linear, quadratic, and interaction (Table 6). All the coefficients had a highly significant effect, as the p-value was less than 0.05. Furthermore, it has been shown that the lower the p-value, the higher significant the coefficients are in the regression model63,64.
The results also indicated that the concentration of the ratio of MEO/GA had the most significant impact on the EE%, followed by the interaction of MEO/GA and MEO/ethanol and the second-order interaction of MEO/GA. Thus, MEO/GA positively impacted the EE%, with a high positive t-ratio (62.81). However, the values of MEO/GA*MEO/ethanol, (MEO/GA)2, and MEO/ethanol had a significant negative impact on the EE%, with t ratios of − 20.39, − 14.86, and − 12.28, respectively.
The mathematical models of the response surfaces of the adopted model were presented as second-order polynomials. The equations displayed the significant terms of the model for encapsulation efficiency (EE%).
After validating the models obtained from the RSM, the optimal fit of the two parameters that enabled high encapsulation efficiency was the focus of this study. The effects of the ratios MEO/GA and MEO/ethanol on encapsulation efficiency were illustrated in the response surface shown in Fig. 4. The optimum zone for the two factors exists on the area of the cube formed by the area where the ratio of MEO/ethanol is minimal and the ratio of MEO/GA is maximal.
2D (a) and 3D (b) representation of the optimized parameters leading to the desired encapsulation efficiency.
The desirability plots corresponding to the responses for encapsulation efficiency were displayed in Fig. 5. Apparently, the encapsulation efficiency reached its maximal value (86.58%) with an MEO/GA ratio of 1/10 and an MEO/ethanol percentage of 10%, with a desirability of 99%.
Profile for predicting optimal conditions for encapsulation efficiency.
The results indicated that the optimized conditions corresponding to the experimental design corner (run 2) had an MEO/ethanol volume ratio of 1:10 and an MEO/GA percentage of 10%.
This finding confirmed previous research, which indicated that encapsulation efficiency was reduced by enhancing the co-solvent ratios of ethanol65,66,67,68. This reduction could be explained by the high quantities of ethanol disturbing the non-covalent bonding required for essential oil (EO) incorporation in the GA. The introduction of GA improved the encapsulating ability of the MEOs. When the weighted EO-to-GA ratio was 1:10, the maximum encapsulation efficiency was 87%. The MOE emulsion droplets likely moved more freely in the medium69. Due to the increased amount of GA, the number of essential oil molecules diffused into the combined solution likely increased. This increase might also allow for better protection of the oil from coalescence, which eventually integrated into the GA.
Characterization of nanocapsules
Morphology and particles size analysis
A scanning electron microscope (SEM) was used to study GA and MEO-GA morphology at 500×. According to Fig. 6, the pure sample of GA occurs in amorphous, irregular crystals of various sizes with no defined form. The MEO-GA decreased in particle size and changed in morphology compared to GA. The SEM micrographs of GA showed continuous tissues, and those of MEO-GA exhibited porous surfaces with amorphous shapes. Their porous surfaces were caused by the amount of essential oil presented in the GA.
The SEM micrographs of GA (a) and MEO-GA (b) at × 500.
The mean hydrodynamic diameter of the nanoparticles was measured using dynamic light scattering (DLS). As shown in Fig. 7, the MEO-GA exhibited a mean diameter of 124.98 nm, which was consistent with the results attained by Matche and Adeogun70. These results seem to confirm previous research by Barre et al.15, showing that adding EO-to-GA decreased the particle size of the nanoencapsulated essential oil mixed with the GA. Hasheminejad et al.71 contended that this decrease may have been caused by the enhanced protonation of amino groups, leading to the completion of ionic cross-linking. Likewise, the nanoelaboration method’s reduction in nanocapsule particle size may be attributed to a greater packing of polymer chains because of the large number of amino groups in GA that induced EO–GA association71.
Particle size (µm) of MEO-GA nanoparticles distribution.
X-ray powder diffraction analysis
X-ray diffraction (XRD) was used to identify the differences in peaks’ positions and intensity in GA and MEO-GA. As shown in Fig. 8, the two samples were characterized by two large peaks at ~ 8° and 18° (2θ), with little modifications in position and significant variations in intensity. Other studies have also found that essential oil can successfully be incorporated into GA72,73.
XRD patterns of GA and MEO-GA.
Fourier transform infrared spectroscopy analysis
Fourier transform infrared spectroscopy (FTIR) was performed to investigate the encapsulation of MOE in GA. The FTIR spectra of the MEO, GA, and MEO-GA samples are illustrated in Fig. 9. Differences in band position, shape, and intensity are indicators of interactions between the invited molecules and GA74,75. The MEO spectrum indicated that the characteristic broad band was at 3465 cm−1 relative to O–H bond stretching, and those of CH2 asymmetric stretching were at 2929 cm−1. The sharp band at 1748 cm−1 was assigned to the bending vibration of C=O groups, whereas the sharp bands at 1442 cm−1 and 1374 cm−1 were associated with CH3, CH2 and O–H bending, respectively. The bands at 1,218 cm−1, 1,078 cm−1, and 980 cm−1 were assigned to C–C and C–O bond stretching and to out-of-the-plane C–H and O–H stretching, respectively. These observations were consistent with the presence of 1,8-cineole, camphor, α-pinene, myrtenal, β-himachalene, and borneol, which were the main components of MEO (Table 3). Several authors have already studied the compounds presented in this essential oil59,76.
FTIR spectra of MEO, GA and MEO-GA.
The spectra of GA was characterized by prominent vibrational peaks at 993 cm−1 (stretching vibrations of C-O), 1367 cm−1 (CH3 and C–H bending bands), 1639 cm−1 (C=O bond stretching), and 2929 cm−1 (CH2 asymmetric stretching). The band at 3302 cm−1 resulted from the hydrogen-bonded OH group77. The typical absorption band for the amino group of GA in the area of 3400–3500 cm−1 was likely obscured by the broad O–H group absorption band33.
The FTIR spectra of the MEO-GA formulations were a combination of previously characterized spectra. In addition, the C–H and O–H absorption bands were altered in position, intensity, and shape, thus indicating success in combining MEO with GA. The interaction of the polysaccharide with the essential oil may have influenced structural composition alteration78. According to Singh et al.79, the alterations generated by adding EO to GA indicated that chemical modifications occurred due to the incorporation of essential oils into GA. Consequently, the essential oil’s interaction with the functional groups contained in the polymer matrix led to identified modifications in peaks and displacements caused by stretching, vibrations, and bending. Furthermore, Vali et al.80 found that the peak shift indicated that the EO interacted with the GA due to the edible coatings and was effectively encapsulated.
Thermogravimetric analysis
Thermogravimetric analysis (TGA) was demonstrated the thermal stability of essential oil after the encapsulation process81. The thermal behavior of EO was investigated in both forms (free and encapsulated) (Fig. 10). The MEO began to evaporate at room temperature, and the weight loss was 90% at 217 °C, illustrating MEO’s unstable nature. In the case of GA, the first one caused the dehydration process (112 °C), and the decomposition process (250 °C) caused the second one. In addition to GA’s dehydration and decomposition, the TGA of MEO-GA was characterized by additional weight loss caused by the volatilization of MEO until 440 °C. These findings provided further justification for the incorporation of MEO into GA and indicated that, during the encapsulation process, the MEO became more stable. The addition of MEO reduced the melting temperature of the nanoparticles, which could be related to the interaction of the EO with the polymer82. This illustrates the successful encapsulation of S. rosmarinus and C. atlantica (MEO) essential oil combined with GA and the bioactive compound’s encapsulation. This finding was comparable to that of Hadidi et al.83 study, which found that the incorporation formed between eucalyptus oil and GA led to the effective encapsulation of eucalyptus essential oil inside a carrier polymer (GA). Azadmanesh et al.84 determined that including Eucalyptus globulus oil in nanomaterials, such as GA nanocapsules, enhanced the nanocapsules’ thermal stability. Lian et al.85 established that contact between EO and polysaccharides could enhance the polysaccharide–EO complex’s thermal stability and that polysaccharides could improve heat resistance through intermolecular synergy, including hydrophobic synergy.
The TGA thermogram of MEO, GA and MEO-GA.
Antifungal activity
The antifungal activity of the essential oil mixture, both in free and encapsulated forms, was among the most active forms examined. These samples were tested against two brown rot fungi (P. placenta and G. trabeum).
The percentage of mycelial growth inhibition in the essential oil mixture, both in free and encapsulated forms, against P. placenta and G. trabeum at different concentrations, was illustrated in Fig. 11. According to the results shown in Fig. 11, the percentage of mycelial growth inhibition increases with concentration, this response is dose-dependent, which all three graphs indicated. Figures 11 and 12 show significant differences between the samples in the average mycelial growth inhibition of P. placenta (Fig. 11a) and G. trabeum (Fig. 11b). A significant difference (p < 0.05) was identified in the average mycelial growth inhibition between (a) GA and MEO, (b) GA and MEO-GA, and (c) MEO and MEO-GA. The MEO-GA samples, followed by the MEO sample, showed the highest average mycelial growth inhibition for both brown rot fungi. The lowest average mycelial growth inhibition for both brown rot fungi was found in the GA samples. Based on two-way ANOVA tests, the applied treatment significantly affected mycelial growth inhibition for both brown rot fungi.
The inhibition percentages of GA, MEO, and MEO-GA against wood decay fungi growth of (a) Poria placenta and (b) Gloeophyllum trabeum.
Antifungal activity of; essential oil-gum arabic (MEO-GA), mixture essential oil (MEO) and gum arabic (GA) against wood decay fungi growth of (a) Poria placenta and (b) Gloeophyllum trabeum. All in the concentration of 62.5 µg/mL. The negative control of two rot fungi on malt-agar medium was carried out by substituting the MEO-GA and GA with a 0.2% agar solution. After 7 days of incubation at 25 °C, the fungus fully covered the plates in the Petri dishes.
The effective concentration for 50% of mycelium growth (EC50) of the samples and chemical fungicide (Nystatin) are listed in Table 7, with a lower EC50 reflecting high antifungal activity. Table 7 indicates that MEO-GA provided the lowest EC50 values of all the fungi investigated (5.15 and 12.63 µg/mL for P. placenta and G. trabeum, respectively), followed by MEO (15.69 and 26.42 µg/mL for P. placenta and G. trabeum, respectively), and GA (144.88 and 327.36 µg/mL for P. placenta and G. trabeum, respectively).
Several researchers have investigated the application of GA in the nanoencapsulation of essential oils as food preservative57. Nevertheless, few studies have considered the application of encapsulated essential oils as wood protection against decay fungi86, and no studies have investigated the nanoencapsulation of a mixture of essential oils for application in wood protection. The strong antifungal effect of MEO could be attributed to the synergistic effect of the major compounds of S. rosmarinus (monoterpene) on those of C. atlantica (sesquiterpene), as Annemer et al.22 study indicated. MEO-GA, as an edible coating, showed more marked results for wood preservation than MEO against P. placenta and G. trabeum. This significant potential can be explained by synergistic actions in a mixture of essential oils with complex combinations of several components and GA. The authors Izadi et al.87, Ali et al.21, Cheong and Zahid88, Maqbool et al.86, and Valiathan and Athmaselvi89 observed that edible coatings, for instance, GA, could increase essential oil antifungal activity by attaching to the fungal surface and liberating their contents close to the cell membrane or transferring the fungal components immediately. In contrast, Maqbool et al.86 determined that GA molecules had no fungicidal effects. Lili Cai et al.90 indicated that four essential oils encapsulated with methyl-β-cyclodextrin (MβCD) substantially slowed the fungal growth of both fungi P. placenta and G. trabeum.
Conclusion
In the present study, the nanoencapsulation of MEO in GA was optimized using a response surface design, and the effect of nanoencapsulation on antifungal activity was evaluated against brown rot fungi. As determined by characterization evaluation, the oil mixture was successfully nanoencapsulated into GA. The RSM results indicated that encapsulation efficiency could achieve a value of 87% in optimal conditions: 10% of MEO/ethanol and a 1:10 ratio of MEO/GA. The DLS data confirmed the nanoencapsulation process by measuring the average diameter. The SEM, FTIR and XRD analysis revealed that GA and MEO complexes exhibited different properties before and after the inclusion procedure, which enabled the conclusion of the incorporation process. The TGA results also indicated that the presented nanoparticles showed higher thermal stability than the MOE. Regarding antifungal activity, the nanoencapsulation of MEO with GA significantly improved the activity against G. trabeum and P. placenta. Therefore, the MEO-GA formulated in this research can be used as a green alternative for controlling brown rot fungi and, therefore, can potentially be applied in the wood industry.
Data availability
The data used to support the findings of this study are available from the corresponding author upon request.
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Author would like to thank Researchers supporting Project number (RSP2023R110) at King Saud University, Riyadh, Saudi Arabia for financial support.
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This project was supported by the Researchers Supporting Project number (RSP2023R110), King Saud University, Riyadh, Saudi Arabia.
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S.A., Y.E.z., A. E.z. and A.F. Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Visualization, Writing-original draft, Writing—review & editing. B.S., H.S., T.B., A.A., R.U. and N.F. Validation. All authors have read and agreed to the published version of the manuscript.
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Annemer, S., Ez-zoubi, A., Ez zoubi, Y. et al. Optimization and antifungal efficacy against brown rot fungi of combined Salvia rosmarinus and Cedrus atlantica essential oils encapsulated in Gum Arabic. Sci Rep 13, 19548 (2023). https://doi.org/10.1038/s41598-023-46858-7
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DOI: https://doi.org/10.1038/s41598-023-46858-7
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