Graphene-high rich polycaprolactone nanofibers prepared via alternating current electrospinning as advanced sustainable sorbents for environmental analysis

Graphene-high rich polycaprolactone nanofibers prepared via alternating current electrospinning as advanced sustainable sorbents for environmental analysis Pavel Holec a , Martina H´ akov´ a b , Ivona Kolichov´ a b , Jan Vinter a , Tom´ aˇ s Kalous a , Mark´ eta Hujerov´ a a , Dalibor ˇ Satínský b,* , Jakub Erben a,* a The Technical University of Liberec, Faculty of Textile Engineering, Department of Nonwovens and Nanofibrous Materials, Studentsk´ a 1402/2, Liberec 46001, Czech Republic b Charles University, Faculty of Pharmacy in Hradec Kr´ alov´ e, Department of Analytical Chemistry, Ak. Heyrovsk´ eho 1203, Hradec Kr´ alov´ e 50003, Czech Republic ARTICLE INFO Keywords: Graphene Polycaprolactone Nanofibers Sorbent On-line solid phase extraction Environment ABSTRACT The increasing demand for environmental quality control demands advanced analytical tools for potentially harmful substances monitoring. The monitoring tools for trace level contaminant analyses typically rely on advanced chromatographic methods coupled with sensitive detectors and sustainable sample pre-treatment techniques to remove ballast matrix components. This study presents a straightforward approach for preparing a novel nanofibrous composite extraction sorbent based on biodegradable polyε -caprolactone (PCL) polymer nanofibers and graphene (GR) nanoparticles prepared via alternating current (AC) electrospinning of polymer solution. Using AC electrospinning, it was possible to prepare more voluminous and porous layers with a higher GR particle content compared to the more common direct current (DC) method. The new composite material demonstrates high extraction efficiency of target analytes for chemical analysis of surface water pollutants while offering. The PCL-GR composite, with GR particle content up to 45 wt%, was self-supporting, flexible, and did not require energy-intensive carbonization processes as more common GR materials. The composite was successfully tested for the retention of selected pesticides, insecticides, and residues typical for industrial pollution using on-line extraction coupled to chromatography, demonstrating its potential as solid-phase extraction (SPE) material for a common surface water pollutants. 1. Introduction Discovering new methods and approaches for simplifying sample analysis is one of the key directions in which both analytical and environmental chemistry is heading. Solid-phase extraction (SPE) is a widely used technique for sample pretreatment, an essential step in chemical analysis. It serves to remove contaminants from the analyzed sample while concentrating the analytes. Considering the sources of chromatographic errors, more than one-third originate from sample preparation. Furthermore, this stage accounts for approximately two-thirds of the total time required for the analytical process, making sample pretreatment one of the most critical * Corresponding authors. E-mail addresses: [email protected] (D. ˇ Satínský), [email protected] (J. Erben). Contents lists available at ScienceDirect Environmental Technology & Innovation journal homepage: www.elsevier.com/locate/eti https://doi.org/10.1016/j.eti.2025.104591 Received 6 August 2025; Received in revised form 29 September 2025; Accepted 26 October 2025 Environmental Technology & Innovation 40 (2025) 104591 Available online 28 October 2025 2352-1864/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). steps in the entire analysis workflow. This is particularly true for environmental samples, which often have complex matrices that require specific preparation (Backe and Field., 2012). Automation of process is one way to improve the analytical methods. On-line coupling the SPE step and HPLC belongs to the advanced trends in this field. It provides reproducible experimental conditions, with minimal demands for the operator’s skills and effect of external conditions (H´ akov´ a et al., 2020, 2018a). Among the commercially used sorbents for SPE, porous synthetic polymers are common representatives (Torabi et al., 2024; Fontanals et al., 2019). Their main advantage lies in the great diversity of chemical and physicochemical properties of usable polymers, which can further be modified to achieve a wide range of possible interactions between the sorbent and the analyte (Fontanals et al., 2020). The most used polymers for SPE sorbents include polystyrene (PS) and its copolymer with divinylbenzene (DVB). Due to their high hydrophobicity and low wettability, such materials are not suitable for the retention of more polar analytes from water compartment and can be chemically modified with more polar groups or comonomers (Huang et al., 2020; Ning et al., 2022; Yin et al., 2019). Polymer particles are most commonly used, or networks formed by their additional crosslinking. A promising innovative alternative to sorbent particles could be polymeric nanofibers (An et al., 2021; Qi et al., 2008; Ifegwu et al., 2014), whose specific surface area is comparable to that of commonly used polymeric sorbent particles (Fontanals et al., 2020). These materials are also compact and mechanically self-supporting, simplifying their processing into extraction sorbents. Similar to particle-based sorbents, fiber-based counterparts can also be further functionalized by additional materials (nanoparticles, nanotubes, metal organic frameworks, crown-ethers, and surface coatings) can be incorporated during or after their fabrication, or the fiber surface can be chemically modified to further alter their final properties and expand the portfolio of their usability for sorption purposes (Chigome et al., 2011). Although nanofibers can be prepared using various methods (Alghoraibi and Alomari, 2018), electrospinning of polymer solutions is one of the most commonly used methods for their production. Its advantages include the variability of the spinning process, and thus the resulting fiber structures, the ability to spin a wide range of polymers, the possibility of incorporating soluble and insoluble substances into the resulting fibers, as well as relatively low costs for equipment and the production process (Xue et al., 2019). By far the most frequently used technology is direct current (DC) electrospinning, which typically results in formation of only thin planar sheets with a limited degree of overall porosity. These factors greatly limit the possibilities of its use as an on-line high-pressure SPE method. When compared to DC electrospinning, a less common method is alternating current (AC) electrospinning, which offers advantages for sorbent preparation, such as the preparation of fibrous materials with higher porosity and voluminousness (Pokorny et al., 2014). At the same time, it enables to incorporate higher number of additives into the electrospun solution than the standard DC method. When comparing the application potential of DC and AC nanofibrous layers for extraction purpose, AC materials provide superior performance due to their higher porosity and mechanical durability, leading to improved extraction efficiency and repeatability of analyses (Erben et al., 2022). Polyε -caprolactone (PCL) is one of the widely used polymers for electrospinning fabrication of nanofibers (Cipitria et al., 2011). PCL is a semicrystalline linear aliphatic polyester with a low glass transition temperature (-60 ◦C) and melting temperature (50–70 ◦C) (Ainhoa et al., 2023). This polymer is biodegradable, but its degradation is not so rapid as to compromise its functionality in extraction applications, making biodegradability an additional advantage of this material. PCL is widely used in medicine and bioengineering (Mochane et al., 2019), as well as a sorbent material for SPE (Topsoy et al., 2022; Tahmasebi, 2018). The possibility of incorporating modifying substances into fibers during electrospinning can be utilized for the preparation of composite materials combining a carrier polymer and functional particles, such as graphene particles (Torabi et al., 2024). Graphene, as a planar, nonpolar, and rigid moiety could affect the extraction properties of the resulting material, via hydrophobic effect and π - π interactions (Cipitria et al., 2011). Due to its electron-rich, double-sided polyaromatic structure, graphene exhibits strong affinity for aromatic compounds, which is particularly advantageous for interacting with common water pollutants that are typically aromatic in nature (H´ akov´ a et al., 2018a). However, using graphene in on-line extraction flow techniques could be very problematic, due to very small particle size. This problem could be overcome by using nanofibers as a scaffold where the graphene is incorporated. Then, extraction properties of graphene-based composite results from properties of both parts – polymer and graphene. In the available literature, two basic approaches to prepare nanofiber-graphene composites are commonly described. The first involves electrospinning a polymer solution with the addition of graphene, where the resulting nanofibers typically contain only a few percent of graphene (Matsumoto et al., 2013; Bao et al., 2010; Abolhasani et al., 2017; Wang et al., 2013; Song et al., 2015). For materials requiring higher graphene concentrations, a method involving the carbonization of a carbon-based material (e.g., nanofibers) is used, producing a material that can contain over 90 % graphene (Kuzmenko et al., 2017; Song et al., 2014). However, this method results in the loss of mechanical properties of the product, rendering it brittle. This drawback is addressed by incorporating a fibrous or planar carrier, which, however, reduces the relative graphene content in the material. We focused on the preparation of high-content graphene-polycaprolactone composite nanofibers via AC electrospinning of polymer solution without the use of a carbonization step and their application as extraction sorbent for on-line extraction high performance liquid chromatography (SPE-HPLC) application in environmental analysis. Different ration of the graphene content in polymer solution was tested during fabrication process and the effect on the extraction yield of contaminants was evaluated. Model contaminants were selected from the group of aromatic compounds with various substituents (bisphenols, chlorphenols, insecticides) due to the expected influence of the graphene content on enhanced extraction yield. Graphene-free polycaprolactone was also used to compare the effect of native polymer on extraction, which in previous studies represented a very interesting alternative to commercial C18 or polymeric MIP/RAM sorbents (Martina et al., 2018; Kholov´ a et al., 2023b, 2023a). Thus, we showed a new method for fabrication of carbon-rich sorbent that exhibits good affinity for aromatic pollutants, attributed to the presence of high content of graphene. P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 2 2. Experimental part 2.1. Materials Polyε -caprolactone pellets (M w 80,000 g/mol, CAS: 83259–71–6) were purchased from Polysciences, Inc. (Warrington, Pennsylvania, USA). Formic acid (p.a., CAS: 64–18–6), acetic acid (p.a., CAS: 64–19–7), and acetone (p.a., CAS: 67–64–1) were obtained from Penta (Prague, Czech Republic). Graphene nanoplatelets (surface area 750 m 2 /g, CAS: 7782–42–5) with particle size below 2 µm were, bisphenol A (≥99 %, CAS: 80–05–7), bisphenol C (≥99 %, 79–97–0), bisphenol S (≥98 %, CAS: 80–09–1), bisphenol Z (≥99 %, CAS: 843–55–0), bisphenol AF (≥99 %, CAS: 1478–61–1), bisphenol AP (≥99 %, CAS: 1571–75–1), 3-chlorophenol (98.0 %, CAS: 108–43–0), fenoxycarb (99.5 %, CAS: 72490–01–8), hydroxypyren (98.0 %, CAS: 5315–79–7), deltamethrin (99.6 %, CAS: 52918–63–5) and kadethrin (90.8 %, CAS: 58769–20–3), acetonitrile (HPLC grade, CAS: 75–05–08) and methanol (HPLC grade, CAS: 67–56–1) were purchased from Sigma-Aldrich (Darmstadt, Germany). Ultra-pure water was prepared by a Milli-Q system (Millipore, St. Luis, Missouri, USA). 2.2. Spinning solution 2.2.1. Preparation The PCL solution was prepared by dissolving PCL granules in a mixture of formic acid and acetic acid (w/w 1:1) to achieve a final concentration of 14.3 %. The dissolution process took 12 h at room temperature using a magnetic stirrer. Once the PCL was completely dissolved, the resulting solution was diluted with acetone under constant stirring to obtain a polymer concentration of 10 %. This resulted in a final solvent ratio of 1:1:1. Graphene was added to the stirring solutions in mass ratios relative to the polymer (PCL:GR) of 10:0, 10:1, 10:2, 10:3, 10:4, 10:5, 10:6, 10:7, 10:8, 10:9, and 10:10. This theoretical composition was also used to label the individual fiber material samples. Prior to electrospinning, the solutions were sonicated using a probe sonicator three times for 10 s each to avoid excessive heating of the solution and sedimentation of graphene particles. 2.2.2. Characterization Viscosity, specific electrical conductivity, and surface tension were determined for each of the prepared PCL solutions. Viscosity measurements were performed using a Haake Rotovisco (Thermo Fisher Scientific, Prague, Czech Republic) with a cone-plate geometry in a continuous mode. The cone used was C35/1◦Ti L, with a gap size of 0.2 mm, and the measurement was conducted over 30 s with a linear increase in shear rate from 100 to 3000 s⁻¹ . Each measurement was repeated at least three times. Specific electrical conductivity was measured using an Eutech Instruments CON 510 conductometer (Eutech Instruments, Landsmeer, The Netherlands) with an acid-resistant probe K10/6MM8. Each sample was measured five times. Surface tension of the solutions was determined using the maximum bubble pressure method, utilizing a PocketDyne device (Krüss, Hamburg, Germany), with a bubble lifetime of approximately 210 ms. Each solution was measured twelve times. All measurements were carried out at a temperature of 22 ◦C. Fig. 1. Electrospinning setup (a) with a rotating drum collector (1), collected fibrous material (2), nanofibrous plume (3), overflow electrode (4), and magnetic clutch (5). Cross-sectional detail of the electrode (b) with an overflow disk electrode (6), screw pump (7), power supply (8), and reservoir with circulating polymer solution (9). P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 3 2.3. Electrospinning set-up The prepared 10 % PCL solution and the PCL+GR solutions were electrospun using an overflow electrode with a screw pump via the AC electrospinning method (Fig. 1). The polymer solution was placed in a reservoir, from which it was pumped to the top of the charged electrode using a screw pump. The advantage of using a screw pump with closed solution circulation was the continuous mixing, which prevented the agglomeration of graphene particles. The alternating current applied on the electrode was of frequency 50 Hz with a sinusoidal waveform, and the effective voltage 40 kV. Fibers were formed from the tip of the electrode, with the resulting fiber bundle being carried by the electric wind onto a grounded rotating drum collector covered with antistatic spunbond-type nonwoven polypropylene fabric (surface weight 18 g/m²). The distance between the collector and the electrode was 250 mm, and the collector’s peripheral velocity was 30 m/s. Electrospinning was conducted at room temperature with a relative humidity of 45 %. 2.4. Nanofibrous material characterization 2.4.1. Surface morphology The surface morphology of the prepared nanofibrous material samples was analyzed using a scanning electron microscope (SEM) Tescan Vega3 (Tescan, Brno, Czech Republic) at an acceleration voltage of 10 kV. Prior to imaging, the samples were coated with a 7 nm layer of gold using a rotary vacuum sputter coater Q150R (Quorum, Lewes, UK). Manual measurements of fiber diameters from the obtained images were conducted using the ImageJ software (version 1.54 g, Bethesda, Maryland, USA), with 300 fiber diameters measured for each sample. 2.4.2. Total porosity A gravimetric method was used to determine the total porosity of the fibrous material. For each sample, a layer of material with a total area of 100 cm² was taken, and its thickness was measured using a digital micrometer 49–63 (Testing Machines, Delaware, USA) at a pressure of 400 Pa according to the standard test procedure EDANA – NWSP 120.1.R0. The weight was measured on analytical scales KERN ADJ 200–4 (KERN, Balingen, Germany) with an accuracy of four decimal places. The specific densities of PCL (1.145 g/ cm³ (Rosa et al., 2004)) and GR (2.2 g/cm³ (Hwang et al., 2013)) were used for calculating the total porosity. 2.4.3. Water contact angle The contact angle was measured using a See System goniometer (Advex Instruments, Brno, Czech Republic). A 5 µl water droplet was applied to each nanofiber sample. Each droplet was then photographed using the See System software (version 6.2, Brno, Czech Republic). The evaluation was conducted using the three-point method, where three points were marked on the picture (both ends where the droplet touched the nanofiber layer, and the highest point of the droplet). The contact angle was then automatically calculated by the software. 2.4.4. Specific surface area The specific surface area was determined using the gas adsorption isotherm method based on the Brunauer–Emmett–Teller (BET) equation, employing an Autosorb iQ device (Anton Paar, Graz, Austria) in standard mode. Fiber samples (1000 mg) were placed in glass cells and degassed for 24 h at 50 ◦C before measurement. Krypton was used for the analysis, and the data were processed using ASiQwin software (version 4.0). 2.4.5. Graphene content determination A thermogravimetric analysis (TGA) was used to determine the amount of GR in the PCL fibers. This exploited the possibility of complete thermal and Thermo oxidative degradation of PCL and GR in distinct non-overlapping temperature ranges and under different atmospheres. The measurements were performed on a thermogravimetric analyzer Q500 (TA Instruments, Delaware, USA). The analysis was conducted in the temperature range of 25–800 ◦C. The thermodegradation of PCL was carried out in an inert nitrogen atmosphere within the temperature range of 25–630 ◦C, after which the nitrogen was replaced by reactive synthetic oxygen in the range of 630–800 ◦C to burn off the graphene and the carbonaceous residues of PCL. The flow rate of both gases used was 60 mL/min. The mass fraction of GR in the fibrous material was then determined from the mass loss curve in the corresponding temperature interval. 2.4.6. Mechanical properties To determine Young’s modulus, ultimate tensile strength, and elongation, a LabTest 2.050 universal testing machine (Labortech, Opava, Czech Republic) with mechanical grips equipped with a load cell of nominal capacity 100 N was used. The tensile test was conducted in accordance with ISO 13 934–1, utilizing test specimens with working dimensions of 50 ×100 mm, subjected to a loading speed of 100 mm/min. Young’s modulus and tensile strength values were calculated using the porosity values for each set of samples, determined based on the respective density (taking into account the PCL to GR ratio), volume, and mass. For each set, five specimens were tested. 2.4.7. Solvent resistance Considering the intended final use of the prepared nanofibrous layers as extraction materials for dynamic liquid environments, the fibers were tested for their resistance to swelling or dissolving in standard HPLC mobile phases based on organic solvents (acetonitrile P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 4 and methanol). To determine the solubility in common HPLC solvents, the nanofibrous layers with PCL:graphene ratios of 10:0, 10:1, 10:5, and 10:10 were tested. These samples were immersed in acetonitrile (ACN), methanol (MeOH) and mixtures of ACN:MeOH, ACN: H 2 O, and MeOH:H 2 O (1:1 wt ratios). Samples of approximately 0.10 g were placed in glass vials and submerged in 4 mL of the respective solvent. They were stirred for 24 h. After removal, the samples were dried in a desiccator for 48 h. Weight loss was subsequently calculated from the mass before and after testing. SEM morphology of the samples after exposure to the solvents was performed following the methodology described above. 2.5. SPE extraction 2.5.1. Preparation of standard solutions and samples Each standard was dissolved in methanol at a concentration 1 mg/mL to prepare the stock solution. The mixed standard stock solution was prepared by mixing 50 µL of each standard and diluting with water to the concentration 50 mg/L. All standard sock solutions were stored at −20 ◦C at dark. Working solutions were prepared at the day of measurement via diluting with distilled water. For testing the extracting abilities of the materials, the concentration 10 mg/L was used. 2.5.2. Preparation of extraction pre-columns Extraction pre-columns were manually assembled by packing an appropriate amount of nanofibrous layer into an empty column cartridge (5 ×4.6 mm i.d.; see Supplementary Material – S1). Cartridge was connected to the on-line SPE HPLC system using the guard pre-column holder. Initially, each pre-column was washed/activated by 100 % acetonitrile, then several on-line SPE HPLC analysis cycles were performed without sample injection. Not-retained GR and other impurities and ballasts were washed out by this procedure. Column suitability for use was verified by UV detection (decreasing signal of detector). 2.5.3. The on-line SPE HPLC An on-line SPE-HPLC system Shimadzu Nexera X2 was used for the simultaneous pre-concentration and determination of model analytes. SPE was carried out using nanofibrous pre-columns. Methanol or acetonitrile (0–40 %) was used as organic modifier in aqueous washing mobile phase at a flow rate 1.0 mL⋅min −1 . Chromatographic separations were carried out using a Kinetex® Biphenyl 150 ×4.6 mm, 5 µm particle size analytical column from Phenomenex (Torrance, California, USA) at a gradient of the mobile phase consisting of water (solvent A) and methanol (solvent B) at a flow rate of 1.0 mL/min at temperature of 20 ◦C. 10 µL of sample was injected in the extraction pre-column filled with composite nanofibrous sorbent and washing mobile phase preconcentrated the analytes. At the same time, the analytical column was equilibrated to the initial conditions of the gradient (40 % A). After 1.0 min the valve was switched and the elution of the retained analytes started. The gradient program started after switching the valve (detailed information on the gradient elution program is provided in the Supplementary Material – S2). The detection of analytes was achieved using UV detection at 220 nm and 240 nm (for hydroxypyrene and bisphenol S). The total run time was 12.5 min. 3. Results and discussion 3.1. Composite nanofibrous sorbent preparation and characterization 3.1.1. Spinning solutions properties The dynamic viscosity, electrical conductivity, and surface tension values of the selected PCL spinning solutions with added GR are presented in Fig. 2. The addition of graphene nanoparticles resulted in a gradual reduction in the dynamic viscosity of the solutions (at a constant shear rate of 500/s), decreasing from 0.139 ±0.04 Pa⋅s for the PCL solution without GR (labelled 10:0) to 0.106 ±0.009 for the 10:10 solution (Fig. 2a). In contrast, the electrical conductivity of the solutions significantly increased with the addition of GR, rising from an initial value of 12.5 ±2–176 ±14 mN/m. Unlike viscosity, conductivity did not increase linearly; a notable rise occurred after the addition of even a small amount of GR (10:1 solution), followed by a near-exponential increase, as shown in Fig. 2b. Fig. 2. Spinning solutions properties dependence on PCL:GR ratios – dynamic viscosity at shear rate of 500/s (a), electrical conductivity (b) and surface tension (c). P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 5 This can be attributed to graphene’s inherent high electrical conductivity, though possible impurities introduced alongside the GR may also have had an effect. The surface tension of the solutions exhibited only minor variations with the addition of GR, fluctuating around 32 mN/m (see Fig. 2c). Therefore, the primary factors influencing the subsequent electrospinning of the PCL solutions were the increase in electrical conductivity and, to a lesser degree, the reduction in viscosity. Surface tension had a minimal effect. 3.1.2. Fibrous material The process of producing fiber layers containing GR using AC electrospinning consistently resulted in the formation of a nanofiber plume with a typical tubular shape determined by the geometry of the working electrode. The diameter of this plume increased as the GR content in the solution rose. This was likely due to the increasing electrical conductivity of the solutions, which caused greater repulsion between the identically charged material during each half-wave of the applied voltage. A visually observable decrease in the volume of transferred material was also noted with the increasing GR content, as the rising proportion of GR progressively impaired the quality of the electrospinning process. The fiber plume became less homogeneous as its diameter increased, and at higher GR concentrations, it also lost coherence, making it more difficult to collect the resulting fibrous material on the rotating collector. Fig. 3 shows the nanofiber layers produced with increasing GR content, spun using AC electrospinning. All GR concentrations in the solutions were successfully converted into nanofiber layers without visible macroscopic defects using this method. As can be seen, the layers darkened with increasing GR content, indicating a higher actual GR concentration within the layers themselves. The observed decrease in the homogeneity of the fiber plume with increasing GR content in the electrospun solutions also affected the productivity of AC electrospinning (see Fig. 4a). Productivity dropped from the initial value of 10.7 g/h to 1.9 g/h for the solution with the highest GR concentration. The decline in productivity between the 10:1 and 10:6 ratios was more gradual, but it then decreased sharply again after reaching PCL:GR ratio of 10:7. The 10:7 ratio was therefore considered a critical point, beyond which the spinning productivity significantly declined. This decrease was caused by the reduced cohesion of the fiber plume, leading to its breakage during collection, with portions of the spun material depositing on the collector’s edges or entirely missing the collector. A similar trend was observed in the areal density of the layers produced. While the productivity of the solution with the highest GR concentration (10:10) dropped to about one-fifth of that for the GR-free solution (10:0), the areal density decreased to approximately one-twelfth (from 65 to 5.5 g/m²). This reflected the aforementioned increase in the fiber plume diameter with rising GR content, along with the reduction in the productivity of the AC electrospinning process. SEM images of the selected fibrous layers are demonstrated in Fig. 5. The native PCL material from the initial solution without GR (10:0) provided a mixture of smooth surface nanofibers. Increasing the GR content in the polymer solutions resulted in wrinkled structure of the fibers that can be attributed mainly to GR particles content. At low GR concentrations, these manifested as surface irregularities on the originally smooth fibers, followed by irregular changes in fiber diameters. At higher GR concentration, this morphological change of the fibers, especially wrinkling, is more pronounced. Table 1 presents the resulting average fiber diameters along with their standard deviations. The fiber diameters progressively decreased with the increasing GR content in the electrospun solutions, as did the standard deviations. The higher fiber diameters and their deviations are typical for PCL prepared by AC electrospinning and do not adversely affect the material’s reproducibility (Sivan et al., 2022). In the case of the 10:5 sample, batch-to-batch reproducibility was performed, which showed that the RSD between individual batches (n=6) of fiber diameters was 8 % and of graphene content was 6 %. However, the standard deviations were often so high that differences between adjacent samples could not be distinctly identified. Despite this, the overall trend of decreasing diameters was clear. This phenomenon can be explained by two interrelated factors – higher electrical conductivity of the electrospun solutions and the decreasing concentration of PCL, which acted as the binding element of GR in the final product. The measured contact angle values of water on the resulting layers are presented in Fig. 6a. Due to the hydrophobic nature of the base material (PCL) and the additive (GR), no significant change in the contact angle was observed with increasing GR content, which remained in the range of 130–140◦. A similar observation applies to the relative volumetric porosity of the samples, which remained consistently at 90–95 % regardless of the amount of GR present (see Fig. 6b). However, a significant change was observed in the specific surface area. This parameter increased approximately exponentially with the concentration of GR in the electrospun solution (see Fig. 6c). This behavior can likely be attributed to the decreasing diameters of the prepared fibers and the roughening of their surfaces due to the presence of the GR particle aggregates, and the occurrence of numerous globular GR structures. Fig. 7 presents the TGA curves of the individual fiber materials used to determine the actual GR content. The determination was based on the use of distinct and clearly separated zones of thermal decomposition, representing the mass fractions of PCL and GR, taking advantage of their different thermal degradation behaviours. The decomposition occurred at distinct temperature ranges (PCL between 240 and 440 ◦C and GR between 630 and 720 ◦C), and the analysis was conducted under different atmospheric conditions Fig. 3. The macroscopic appearance of the resulting fiber layers with increasing PCL:GR ratios (in the corresponding spun solution). P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 6 (inert nitrogen for PCL interval and reactive synthetic oxygen for GR interval). The mass losses observed with increasing temperature correspond to the percentage weight fractions of each component, along with the incombustible residue representing impurities from both components. The results of the TGA analyses of fiber layers are summarized in Fig. 8 and Table 2, which shows that the actual concentrations of GR in the final product were always slightly lower than the values corresponding to the composition of the respective spinning solutions. The gap between the two curves enhanced with higher concentrations of GR in the solution and was approximately 10 % of the theoretical value. Thus, the transfer efficiency of GR from the polymer solution was around 90 %, demonstrating that the designed spinning system was suitable for the effective preparation of polymer fiber layers with an ultra-high GR content. Fig. 4. The productivity of AC electrospinning depending on the PCL:GR ratio in the solution (a), and the surface density of the resulting layers after one hour of electrospinning (b). Fig. 5. SEM images of the electrospun material from solutions 10:0, 10:1, 10:5 and 10:10. The scales shown on the images to the left apply to all other images in the corresponding series. Table 1 Fiber diameters (d) and the corresponding standard deviation ( σ d ) as a function of the electrospun solution. sample 10:0 10:1 10:2 10:3 10:4 10:5 10:6 10:7 10:8 10:9 10:10 d [nm] 1170 1160 960 1070 850 850 800 860 670 650 530 σ d [nm] 570 410 280 390 210 270 250 290 200 180 230 P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 7 The results of the TGA analysis were further supported by complementary attenuated total reflectance Fourier-transform infrared spectroscopy (ATR–FTIR) using a germanium crystal, which clearly revealed the characteristic C–O–H bond peak of PCL at 1246 cm⁻¹ (Supplementary Material – S4). The presence of graphene, i.e., a specific carbon form, was reflected by an overall increase in the slope of the spectral curve compared to the control sample (10:0). The ratio of the distances between plateau regions at 930 cm⁻¹ strongly correlated with the actual graphene concentration determined by TGA, suggesting that, after further calibration, this method could serve as a rapid and cost-effective qualitative assessment of graphene content in the fibers. Finally, a four-point probe method was used to measure electrical conductivity. Conductivity was confirmed, particularly in the 10:10 sample, where it increased with compression of the layer without loss of fibrous structure, reaching a value of 8.33 kΩ. This suggests the material’s potential applicability in electrochemical systems or as a sensing element, benefiting from its flexibility and non-brittle nature (Supplementary Material – S5). Fig. 6. Contact angles (a), relative porosity (b), and specific surface area (c) of selected fiber layers. Fig. 7. The TGA curves of selected fiber samples: The first significant weight loss corresponded to the thermal degradation of PCL, the second to the GR. The TGA curves of all prepared fiber samples are shown in Supplementary material – S3. P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 8 Fig. 8. The curves of mass concentrations of GR in the dry matter of the prepared fiber layers – the theoretical value corresponding to the composition of the spinning solution (w theoretical ) and the value determinated by TGA analysis (w real ). Table 2 Graphene wt. content of prepared PCL:GR fiber materials determined by TGA analysis. sample 10:0 10:1 10:2 10:3 10:4 10:5 10:6 10:7 10:8 10:9 10:10 GR [%] 0.0 7.7 15.1 20.2 26.0 30.7 31.5 38.9 39.2 43.6 45.5 σ [%] - 0.8 0.7 1.9 1.0 1.9 1.6 0.6 0.2 0.6 0.7 Fig. 9. Results of the tensile strength tests for selected fibrous materials (a) and the corresponding values of Young’s modulus and maximum elongation (b). P. Holec et al. Environmental Technology & Innovation 40 (2025) 104591 9