Production of pumice-containing nanofibers by electrospinning technique

Gelatin powder EMPROVE^® exp (Catalog No. 1.04078.1000) was received from Merck KGaA (Darmstadt, Germany). Acetic acid was purchased from EMBOY Chemicals & Trade Co. Ltd (Istanbul, Turkey). Distilled water was used throughout all experiments. Whatman™ filter paper (in the 8-μm particle size range) was used for filtering the pumice. Raw pumice samples were obtained from Van, Turkey. For obtaining nanofibers, a Fytronix ESP-900 electrospinning system (Elazığ, Turkey), consisting of a high voltage unit, syringe pump unit, and a flat collector, was used.

The electrospinning technique is a process utilized to obtain ultrathin materials with different diameters [13]. To achieve this aim, different kinds of materials have been used in recently published studies. Besides the use of zein, bioactive compounds, poly(vinyl alcohol), probiotic bacteria, and poly(caprolactone) in electrospinning techniques [7, 9, 14,15,16,17], different kinds of elements such as silver and copper are used to fabricate nanostructures [17, 18]. In the industry, the obtaining of novel materials is gaining importance, and in this respect, alternative production techniques with new materials, providing a larger contact area on the surface of the materials, have become the object of immense attention. In the present study, preprocesses supported with the electrospinning technique provided the means for obtaining a nanoscale material. Therefore, the results indicated below reflect that the electrospinning techniques, together with potential preprocessing conditions, could be evaluated as a promising technique for other potential geological-based materials.

SEM images of the electrospun nanomats are given in Figure 1 (SEM images of JP6 nanomaterials), Figure 2 (SEM images of JP12 nanomaterials), Figure 3 (SEM images of pumice before electrospinning), and Figure 4 (EDX-SEM of electro-spun pumice nanomaterials).

Fig. 1

Fig. 2

Fig. 3

Fig. 4

According to the images, the average diameters of the nanomaterials indicate the efficiency of the electrospinning process. In the present study, the average diameter of nanoencapsulated pumice fabricated at 6 kV (JP6) was defined as 98.6 ± 92 nm within gelatin nanomats (GN6) having 31.8 ± 9 nm (p < 0.05). In this nanoprocess, the minimum and maximum values of NP6 were found to be 25.4 nm and 482.5 nm, while those of GN6 were determined as 9.0 nm and 50.6 nm, respectively. Among nanotechnology studies available from the literature, Ceylan et al. [7] defined that the average diameter of probiotic bacteria-loaded nanostructures was 381.1 ± 131 nm. On the other hand, the average diameter of non–nanoencapsulated materials was lower in the same study. As can be seen from the previous studies forming part of the literature, the use of different materials in nanoscience can affect the morphological structures. However, loading of different materials to the nanostructures increased the average diameters of the obtained nanostructures. Ceylan et al. [19] reported that Lactobacillus rhamnosus-loaded poly (vinyl) alcohol and sodium alginate-based nanofibers (LR) and also poly(vinyl) alcohol and sodium alginate-based ranging within 60.1 nm < diameter of nanofibers < 522.1 nm could be produced. Meral et al. [8] revealed that nisin and curcumin-loaded nanomats having 172 nm average diameters were obtained. In the present study, electrospun nano-materials obtained at 6 kV had 31.8 nm ≤ x ≤ 98.6 nm as average diameters. These obtained nanomaterials including pumice might provide a larger contact area on the surface of the potentially used materials.

Electrospinning parameters may directly affect the properties of the nanomaterials. For example, the average diameters of the nanoencapsulated pumice (JP12) were 85.8, while the average diameter of the nanomats (non–nanoencapsulated: GN12) was found to be 35.2 nm (p < 0.05). Also, when the supplied voltage was increased (12 kV), the average diameter of nanoencapsulated materials decreased (from 98.6 nm to 85.8 nm), while that of nanomats increased (from 31.8 nm to 35.2 nm). Minimum and maximum values of the nanoencapsulated materials obtained from pumice were decreased as well. As could be seen from the values, these changes in NP12 were in the range of 2.68% and 13.21%. On the other hand, the changes in minimum and maximum values excluding pumice (GN12) were 75.96% and 36.30%, respectively. It is revealed that nanoencapsulated materials, especially, possessed a higher average diameter stability as compared to the nanomats obtained using the electrospinning technique. In the production of nanoencapsulated pumice, the electrospinning parameters and the applied voltage can affect the diameter of electrospun nanomaterials. As stated by Ojha et al. [20], the electrospinning parameters, including distance, voltage, and flow rate, can play a role in obtaining electrospun nanomaterials. The nanomats, including nanoencapsulated pumice, which could be used in the production of various materials, were successfully developed by electrospinning technique using two different applied voltages.

The maximum ZS of JP6 and JP12 were found as 237.2 nm and 1,038.6 nm, respectively (Figure 5). The small size obtained in the present study can provide resistance to gravitational separation, flocculation, and coalescence. Also, the PDI values of the JP6 and JP12 nanomaterials were determined to be 0.165 and 0.566. In case of PDI, the obtained values were found to be higher than 0.2, which defined a higher heterogeneity as compared to the samples with lower diameters. The obtained result supported this knowledge because of the fact that the maximum size was increased when the applied voltage was increased. In this respect, as could be seen from the results of the present study, depending on the increase of applied voltage and PDI values, the ZS of nanostructures obtained from the electrospinning technique was increased. According to the results of the study as described by Meral et al. [8], a relationship between ZS and PDI values of the nanomaterials was detected. The present study reveals that nanomats with lower diameters produced from pumice were affected by the applied voltage. Ceylan [12] reported that the ZS of chitosan nanoparticles integrated in poly(vinyl alcohol) nanofibers were <600 nm. Also, a previous study noted that homogenization could decrease droplet size, using a sonicator equipped with a probe [21]. Moreover, the initial material properties such as diameter and the type of the used material in nanoscience, combination processes, homogenization, and electro-spinning could affect the fabricated nanomaterials’ ZS and PDI in the present study.

Fig. 5

FTIR can be used to monitor changes in the chemical structure of electrospun nanomaterials. In addition, it is defined as a useful tool to efficiently and quickly characterize the encapsulated molecules [6]. In this respect, Figure 6 indicates the infrared spectra of JPN. In the nanomaterial obtained from pumice, the peak values related to Si–O and Al–O were represented between 600 cm⁻¹ and 720 cm⁻¹. The strong peak at wavenumbers between 3,280 cm⁻¹ and 3,320 cm⁻¹ reflected hydroxyl groups that could be defined based on their moisture content. In the present FTIR spectra, peaks about the 1,450 cm⁻¹ band presented O–C–O anti-asymmetric stretching vibration related to sodium and carbonate forms, while 1,040–860 cm⁻¹ belonged to the symmetric stretching vibration of Si–O–Si as well. Al–OH stretching was determined at around 3,600 cm⁻¹, as asymmetric and symmetric stretching vibrations of C–H bonds at 2,920 cm⁻¹ were revealed. In the literature, according to Ersoy et al. [22], the peaks at 400 cm⁻¹ and 800 cm⁻¹ of the pumice samples might have been associated with the Si–O, while the strong peak at around 1,100 cm⁻¹ was related to Si–O stretching vibrations. Also, as stated by Prajaputra et al. [23], the peaks at ~720, 1,000, 1,650, and 3,475 can be described as the bending vibration of the Si–O–Si bond. Similar peaks [(SiO₄)⁻² group: Si–O–Si, H–O, H–O–H, etc.) related to pumice in the present study were also revealed in the literature [3, 24]. The present study results revealed that electrospun pumice synthesis was successfully obtained in the nanomaterials, as shown in Figure 6 using SEM images and zeta values. There was no significant difference between FTIR JP6 and JP12. Hence, the results of JP12 are presented here.

Fig. 6

DSC thermograms of nanomaterial (JP) are given in Figure 7. There was no remarkable difference between the JPN samples depending on the applied voltages to obtain nanomaterials from pumice. Also, DSC analysis was applied to reveal the alterations in the physical properties of both JP6 and JP12. Endothermic processes, such as the evaporation levels of both JP samples, were determined at around 100°C. The melting point in JP was obtained between 200°C and 300°C. This result may be observed due to the interfacial interaction between the gelatin chair and pumice filler nanostructures. A study related to pumice particles in the thermal, electrical, and mechanical properties of poly(vinyl alcohol)/poly(vinyl pyrrolidone) composites reported that glass transition temperature was observed at around 104°C and 100°C. Also, when the amount of pumice was increased in the given combination, rapid changes were observed from exothermic to endothermic [25]. On the other hand, adding pumice powder did not significantly affect melting temperature as stated by Sahin et al. [26]. In the present nanoformulation, significant differences in DSC results were not detected in JP6 and JP12 nanosamples depending on the increase in applied voltage.

Fig. 7