Introduction

Nano- and microtechnology enable the encapsulation of antioxidant compounds in polymeric or nonpolymeric systems [1]. Electrospinning technique came forward with the growing interest in nanotechnology. It is a useful, simple, versatile, and cost-effective technique which facilitates production of a continuous nonwoven nano- or microfibers [2]. Electrospinning, initially a single-fluid blended process [3, 4], is moving forward to the coaxial [5, 6], side-by-side [7], tri-axial [8, 9], tri-fluid side-by-side [10], and their combinations [11], and also needleless and surface-free electrospinning [12]. All these progresses are inevitably based on the innovations of spinneret [13]. However, the reasonable selection of the encapsulated active ingredients represents the most popular and common strategy for developing functional nanofibers [14].

In recent years, bioactive electrospun mats have attracted special interest that can be used in various applications such as food (e.g., active packaging) and biomedical applications (e.g., drug delivery systems, facial mask, tissue engineering scaffolds, and wound dressings). The electrospinning process exhibits great potential for effective incorporation of various active agents including antioxidants, antibacterials, food additives, flavors/fragrances, vitamins, therapeutic agents, etc. [2, 15]. The incorporation of active agents to an electrospun mat enables protection and prevents their degradation due to external factors (e.g., temperature, oxygen, and moisture) [15].

Essential oils are natural bioactive liquids that consist of complex mixtures of aromatic and volatile organic compounds obtained through physical processes from plants [16,17,18]. Essential oils exhibit an extensive range of biological activities such as antimicrobial, antioxidant, and anti-inflammatory. Due to the strong demand for pure natural ingredients in various fields, the essential oil market is growing, especially for aromatherapy, perfumery, food flavoring, edible coating, phytomedicine, and cosmetic products [19, 20]. Among essential oils, lavender essential oil has attracted much attention for esthetic and therapeutic applications [21]. It has a wide range of applications due to showing antibacterial, anti-depressive, analgesic, antifungal, anti-thrombotic, antioxidant, carminative, and sedative features [22, 23].

The low water solubility, high volatility, low stability, and thermal and oxygen sensitivity of essential oils influence their biological activity [24]. In this context, nanolevel entrapment of essential oils can be achieved using an electrospinning technique to overcome their use limitations. PLA has been proven to be a promising polymer matrix to obtain essential oil-loaded functional electrospun mats due to its hydrophobicity and high loading capacity [25,26,27,28]. PLA is one of the most commercial, naturally originated polymers which is a biocompatible and biodegradable linear aliphatic polyester [27, 29].

The objective of this work is to develop and characterize lavender essential oil-incorporated PLA-based nanofibrous mats with various concentrations of essential oil. Kolliphor was used as a biocompatible nonionic surfactant. Characteristics of lavender essential oil-incorporated PLA electrospun nanofibers including morphology and diameters, structural change, thermal properties, wettability behavior, and antioxidant activities of electrospun fibers were evaluated. In order to estimate the antioxidant activities of electrospun fibers, DPPH free radical scavenging, ABTS+ radical scavenging, and cupric reducing antioxidant capacity (CUPRAC) experiments were performed.

Materials and method

Materials

PLA filament (density: 1.25 g/cm3, MFI: 19.6 g/10 min measured at 190 °C, and a load of 2.16 kg) was purchased from ABG plastics (İstanbul, Turkey), and Kolliphor RH40 was bought out from Sigma-Aldrich Chemical Company (St. Louis, MO, USA). N,N-Dimethylformamide (DMF) (~ 99.8% purity) and dichloromethane (DCM) (~ 99% purity) were purchased from Sigma-Aldrich (St. Louis, MO, USA), respectively.

Experimental procedure

Extraction of lavender essential oil

Lavandula angustifolia was collected from Marmaris region of Muğla Province of Turkey in 2022. Essential oils were obtained by hydrodistillation technique of the lavender plant as soon as it was collected. After hydrodistillation of the plant (500 g) in distilled water for 2 h, the essential oil was collected, extracted via the liquid–liquid extraction technique using hexane, and finally organic phase was dried with anhydrous MgSO4. Hexane solvent evaporated under vacuum. The essential oil was placed in sealed bottles and stored in the refrigerator (4 °C) until experiments.

Characterization of lavender essential oil

Varian Saturn-2100 T ion trap GC–MS device was used for the characterization of the essential oil components. The essential oil sample was diluted with n-hexane at a ratio of 1:10, and 0.2 µL of the solution was injected into the GC–MS device. Injection temperature was 250 °C. The measurement conditions used for GC–MS analysis are as follows; (i) The oven temperature was set at 60 °C and waited for 5 min and then 3 °C/min to 225 °C waited for 10 min. It was increased to 300 °C at a rate of 25 °C/min and kept for 3 min. An Rtx®-5 capillary column (30 m × 0.25 mm, 0.25 µm) was used, and the carrier gas was He. Split ratio is 1:20. The ion trap temperature was determined as 150 °C, the transferline as 180 °C, and the manifold temperature as 120 °C.

In order to determine the components of essential oils obtained by hydrodistillation method, each component separated by gas chromatography was ionized, and mass spectra were taken. The obtained results were verified by comparing them with the Rt (retention time) values of the standard substances in gas chromatography. Four different diagnostic methods were used regarding GC–MS charts: gas chromatography substance control, mass spectrum, the literature comparison, and RI (retention index). Components present in trace amounts are represented by t.

In the integration results of the chromatograms of essential oils obtained by hydrodistillation from the Lavandula angustifolia plant, 41 components were identified together with their percentages. “NIST 2005” library data were used to determine the components in the essential oil. Relative concentrations (%) of essential oil are given in Table 1.

Table 1 Composition of Lavandula angustifolia essential oil obtained by GC–MS analysis
Full size table

Fabrication of the electrospun mats

PLA filaments were dissolved in a binary-solvent system of DMF and DCM (1:1 v/v) to produce a 10% (w/v) PLA solution by stirring at ambient temperature overnight. Afterward, kolliphor (3%w/v) was added into the PLA solution in proportion to the total mass of PLA. The homogeneous PLA solution (10 mL) was taken into a plastic syringe. PLA electrospun mats were obtained by an electrospinning device (INOVENSO NE300 Multinozzle Electrospinning Machine, Turkey). The electrospinning process parameters were fixed at a voltage of 25 kV and a flow rate of 2 mL h−1 with a rotating collector spinning at 200 rpm. The collector was covered with aluminum foil, with a 100 mm distance between the tip and the rotating collector. The humidity was lower than 50%. The residual solvent was evaporated overnight by placing the nanofiber webs in a fume hood.

To fabricate lavender essential oil-loaded PLA electrospun mats, the amount of lavender essential oil was determined to be 0%, 5%, 7.5%, 10%, and 12.5% of the total polymer weight. According to the increased amounts of lavender essential oil, the electrospun mats were named PL1, PL2, PL3, PL4, and PL5, respectively. Further, the process conditions were the same as the neat PLA mat. Figure 1 illustrates the fabrication of lavender essential oil-loaded PLA nanofiber mats.

Fig. 1
figure 1

Schematic illustration of lavender essential oil-loaded nanofiber mat production

Full size image

Characterization

Morphological analysis of electrospun mats

A Carl Zeiss/Gemini 300 Scanning Electron Microscope (SEM) with a 10 kV voltage (ZEISS Ltd., Germany) was used to examine the surface morphologies of all electrospun mats. The samples were coated with gold for about 20 min prior to analysis. The diameters of the fibers were counted using ImageJ (version 1.520 software), with 100 fibers chosen at random for each sample.

Spectroscopic analysis

The chemical structures of PLA mats and lavender oil, and also the physical attraction between mats and lavender essential oil were verified using Fourier transform infrared (FTIR) spectroscopy. The data were obtained with a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific, USA) and an ATR adapter (Smart Orbit Diamond, USA) in the wavelength range of 4000–500 cm−1, with 16 scans at 4 cm−1 resolution.

Thermal analysis of electrospun mats

Thermal analysis of electrospun fibers was carried out by a TA/SDT 650 TGA (USA). The TGA analysis was performed in a nitrogen atmosphere with a heating rate of 20 °C min−1 over a temperature range of 30–600 °C and in an oxygen atmosphere with a heating rate of 20 °C min−1 over a temperature range of 600–900 °C.

Contact analysis of electrospun mats

The contact angle of samples was measured using Attention Theta Lite optical tensiometer (Biolin Scientific, Gothenburg, Sweden), which captured 15–20 records per second in conventional mode for a single drop (3 µL pure water). The contact angle results were obtained by dropping distilled water onto 2 × 2 cm mat samples. The pictures were captured 10 s after the deposition. Each mat sample yielded at least seven acquisitions. Additionally, each sample was taken at three different sites to provide an average contact angle value.

Antioxidant activity assays

Antioxidant activities were performed by DPPH free radical scavenging assay, ABTS+ radical scavenging, and CUPRAC [30]. The antioxidant activities of electrospun fibers were performed according to the method reported in Reference [2]. Electrospun fibers’ solutions were prepared at different concentrations (25–12.5–6.25–3.125 ppm). Ultra-pure water was used as a control. The α-tocopherol (TOC) and butylhydroxytoluene (BHT) were used as standards to compare the antioxidant activities. Results of CUPRAC were expressed as absorbance, while the results were given as IC50 values for DPPH and ABTS radical scavenging activities that were calculated from the graph plotted of sample concentration against antioxidant activity percentage. Bioactivity measurements were determined by a 96-well microplate reader (SpectraMax 340PC384, Molecular Devices, Sunnyvale, CA, USA).

Results and discussion

SEM analysis of the electrospun mats

The surface morphology of the electrospun mats is indicated in Fig. 2. All mat samples have smooth and beadless morphology. They are also uniform without any agglomeration. However, the PL1 sample has some bending and a little entanglement. Actually, it has been reported that as the amount of additives (essential oils) added to polymer solutions increases, it is expected that the fiber diameters will increase [31, 32]. In this study, with the addition of lavender essential oil into the PLA solutions, the fiber diameter is decreased. This can be related to the gradual decrement in polymer viscosity [33]. Lavender essential oil could be acted as a plasticizer between oil molecules and PLA polymer chains, and solution viscosity decreased due to its oily nature. However, this phenomenon provides an advantage for nanofiber mats, such as faster wound healing due to the increased surface area.

Fig. 2
figure 2

SEM micrographs and average fiber diameter sizes of nanofibers (A1–A2) PL1, (B1–B2) PL2, (C1–C2) PL3, (D1–D2) PL4, and (E1–E2) PL5, respectively

Full size image

The electrospun PLA mats have an average diameter of 431.4 ± 100 nm. When essential oil was added into polymer solutions, the average diameter decreased to 228.2 ± 53 nm. Similar findings have been found for lowering the fiber diameter of PVA/psyllium husk nanofibers containing D-limonene [34]. Moreover, essential oils are highly volatile compounds, and they can quickly evaporate. During the process, it is inevitable that there will be some essential oils losses [35].

Overall, all samples have a fiber diameter of lower than 500 nm. More specifically, three major dimension ranges can be identified: small nanoscaled fibers in the range of 0.1–0.3 μm, medium fibers in the range of 0.4–1.0 μm, and the fibers in the range of 1.0–1.7 µm are considered larger fibers [36]. In this study, all nanofiber mats can be classified between small and medium groups.

ATR-FTIR Spectroscopy of the electrospun mats

The FTIR analysis evaluates to confirm the state of unwanted bond formation [37]. Figure 3 shows comparative FTIR spectra of neat PLA mat and lavender oil-loaded PLA mats. The FTIR spectra investigation has not revealed a chemical bond peak between the lavender essential oil and the PLA polymer. When the FTIR spectra are investigated, specific adsorption bands of PLA are observed. In this regard, electrospun PLA mats indicate asymmetric and symmetric C-H stretching peaks at 2994 cm−1 and 2943 cm−1 as well as –CH3 bending at 1452 cm−1 [38, 39]. Moreover, the strong characteristic peaks of –C=O at 1748 cm−1 are due to the carbonyl stretching [40]. The peaks at 1381 cm−1 and 1359 cm−1 are related to asymmetric and symmetric –CH3 stretching, respectively [41]. The existence of –C–O bending, –C–O–C stretching, and –C–O stretching resulted in the peak at 1266 cm−1 and strong peaks at 1181 cm−1, 1083 cm−1, and 1043 cm−1, respectively. Lastly, the neat PLA mat has C–C stretching at 956 cm−1 [42, 43].

Fig. 3
figure 3

FTIR spectra of nanofiber mats

Full size image

The FTIR spectra of lavender essential oil reveal the anticipated terpenoid component O–H stretch (3447 cm−1), symmetric C–H stretch (2967 cm−1), anti-symmetric C–H stretch (2925 cm−1), C=O stretch (1737 cm−1), and wide and C–O stretch (1100 cm−1) characteristics [44]. These characteristic peaks were not seen in the lavender essential oil-loaded nanofiber mats (PL2, PL3, PL4, and PL5) due to the characteristic peaks of PLA hindering the characteristic peaks of lavender essential oil.

In summary, the characteristic peaks of PLA nanofiber mats were found in the lavender essential oil-loaded nanofiber mats. As a result, there was no difference in the FTIR spectra of PL2, PL3, PL4, and PL5 and the FTIR spectrum of PL1.

Thermal properties of the electrospun mats

The thermal stability of neat PLA fibers and essential oil-incorporated nanofibers was evaluated by TGA analysis. All nanofibers followed a similar thermal decomposition behavior (Fig. 4). The TGA curves of neat PLA and PLA/lavender essential oil nanofibers exposed the weight loss in two stages. The evaporation of moisture and residual solvents was achieved below 200 °C. In addition, the remarkable weight loss in the next stage ranged between 300 and 395 °C was due to the thermal degradation of PLA chains. Additionally, the last stage above 395 °C can be owed to the carbonization of remaining polymeric material. Similar observations were reported by [26] for guar gum and thyme essential oil-loaded PLA-based electrospun nanofibers .

Fig. 4
figure 4

TGA thermograms of nanofibers

Full size image

Contact analysis of the electrospun mats

The hydrophilicity–hydrophobicity character of electrospun mats was examined using an optical tensiometer, and the contact angle values are shown in Fig. 5. The wettability of a material surface is influenced by its chemical and geometrical structure [45, 46]. Sufficient wettability is a desired biobased material characteristic when evaluating electrospun mats for food packaging and/or wound healing applications since it helps assist absorption of both moisture and exudates resulting from physicochemical changes in foods and wounds [36]. If the contact angle value is more than 65°, the surface is considered hydrophobic [47]. PLA nanofibers have highly hydrophobic polymer with more than 110° contact angle degree [48, 49]. The neat electrospun mat has shown WCA as 129° ± 6.4°. A difference was found in the WCA value between neat PL1 mat and PL2 mats. The incorporation of kolliphor and 5% lavender essential oil decreased the WCA value from 129° ± 6.4° to 103° ± 3°. This can be ascribed to the chemical structure of the nonionic solubilizer Kolliphor RH40, which contains both hydrophilic polyethylene glycols and glycerol ethoxylate chain and hydrophobic glycerol polyglycol ester groups. The existence of numerous oxyethylene groups, as well as –C=O and =C–O groups, in Kolliphor RH40 molecules contributes to its high solubility. But, hydrocarbon chains with one -OH group are also present in RH40 molecules. Therefore, this group reduces the hydrophobicity of these chains to some amount [50]. Thus, surfaces made from hydrophobic polymers become hydrophilic (especially when PL3, PL4, and PL5 samples are compared). Although the amount of oil increases, a decrease in hydrophobicity is associated with this condition. While there is no correlated increase in contact angle value with increasing lavender oil concentration, all samples have a higher WCA value when compared to the pure sample. The existence of lavender essential oil is anticipated to enhance the water-repellent effect because essential oils have a highly hydrophobic nature [25, 51]. These results are consistent with the literature. Furthermore, the decrement of WCA values following the mats containing 7.5% lavender oil (PL3) is assumed to be due to an excessive interaction between kolliphor and lavender essential oil.

Fig. 5
figure 5

Water contact angles of nanofibers

Full size image

On the other hand, the movement ability of liquid in a nonwoven fabric is determined by liquid characteristics, interactions between the liquid and the nonwoven, and fiber geometry (porosity and fiber diameter). The affinity of the liquid to a nonwoven and the space between the fibers impact liquid flow in the porous media [52]. In this regard, the PL5 sample, which is the material with the highest amount of oil, has the highest porosity (Fig. 6), and this sample is expected to be more hydrophilic, especially compared to the PL3 and PL4 samples. At the same time, it is expected that the contact angle will increase even more with the addition of oil to pure PLA nanofibers (PL1); however, the porosity of the PL2 sample with 5% oil additive is greater than that of PL1 sample. Decreasing the diameter of nanofibers leads to increased porosity, which increases the surface-to-volume ratio and reduces the contact angle.

Fig. 6
figure 6

Porosities of the nanofibers (a) PL1, (b) PL2, (c) PL3, (d) PL4, and (e) PL5, respectively

Full size image

The contact angle of PL3, PL4, and PL5 nanofibers was much larger than that of PL1 nanofibers due to the hydrophobicity of the lavender essential oil. This also suggests the successful incorporation of essential oil into the nanofibers. Essential oil-incorporated nanofibers have improved hydrophobic properties, making them suitable for food packaging as a barrier against moisture transfer between food and the environment [53]. Consequently, we demonstrated that because essential oils are hydrophobic, their absorption at higher concentrations increases the water contact angle and, as a result, the hydrophobicity of nanofibers.

Antioxidant activity

Lavender essential oil contains bioactive compounds such as alpha-terpineol, caryophyllene, linalool, linalyl acetate, lavandulyl acetate, geranyl acetate, and terpinen-4-ol which was stated as a natural antioxidant [54]. It can be predicted that the incorporation of lavender essential oil into the PLA nanofibers will provide bioactivity. Hence, in this work, three different antioxidant activity assays were used for the evaluation of the bioactivities of electrospun fibers (Table 2). The α-TOC and BHT were used as the antioxidant positive control.

Table 2 Antioxidant properties of electrospun nanofibers
Full size table

As shown in Table 2, the incorporation of essential oil into the PLA matrix enhanced the antioxidant activity of the resulting nanofibers. The antioxidant activity of electrospun fibers increased with increasing the lavender essential oil concentration in all assays, as expected. This was also evidence of successful lavender essential oil encapsulation by electrospinning [55]. The related studies also showed that the antioxidant activity enhanced with the increase in essential oil amount in nanofibers [26, 56,57,58].

In the DPPH free scavenging activity, all electrospun fibers showed good antioxidant activity which exceeded BHT. Especially, the PL5 sample (12.5 wt% essential oil) exhibited approximately two times better activity than BHT. The presence of the highest amount of essential oil in the nanofiber mats enhanced the antioxidant activity, and this result is in good accordance with contact angle results. It can be considered that the nanofibrous mats that become more hydrophilic show better antioxidant activity by releasing better oil.

The ABTS+ radical scavenging activity results showed that, among the essential oil-incorporated fibers, the PL6 sample was found to be more active (IC50 = 14.66 ± 0.21 µg/mL), followed by PL5 and PL4 samples.

The result of the CUPRAC assay demonstrated that all nanofibers exhibited better activity (A0.5 = 22.13–30.16 µg/mL) than α-TOC (A0.5 = 40.55 ± 0.04 µg/mL).

Notably, the neat PLA mat also demonstrated antioxidant activity in all assays. The previous literature has also reported that PLA has antioxidant activity [59,60,61]. In addition, the comparison between the antioxidant activities of the lavender essential oil and electrospun fibers showed that the bioactivities of the fibers were higher than the essential oil.

Conclusion

In this study, antioxidant nanofibrous mats based on PLA were successfully fabricated using the electrospinning technique. To determine the effect of lavender essential oil concentration on the properties of mats, five different concentrations (0–12.5 wt%) of oil in the presence of kolliphor (3% w/v) were investigated. The results obtained in SEM analysis showed that all nanofiber mats have smooth and bead-free morphology with lower than 500 nm. The concentration of lavender essential oil affected the fiber diameter sizes. In addition to that, when the essential oil concentration was increased in the PLA mats containing essential oils, the thermal stability increased slightly compared to the pure PLA mats. The 7.5 wt% essential oil-incorporated nanofiber (PL3) has the highest contact angle value at 157.6° ± 7.8°, while the 5 wt% essential oil-incorporated nanofiber (PL2) has the lowest value at 103° ± 3°. It is thought that the interaction between kolliphor and oil may have increased with the high amount of essential oil. By increasing the essential oil amount in nanofibers, antioxidant activities were significantly enhanced. The 12.5 wt% essential oil-incorporated nanofiber had the highest antioxidant activity in all assays. These promising results confirm that PLA/lavender essential oil functional nanofibers ensure a good potential for various fields such as the fresh-keeping packaging, biomedical, and pharmaceutical applications. However, further research should be executed to determine the essential oil release behavior and cytotoxicity of the mats. The further efforts can beath a path to better applying them for drug delivery and other applications.