Electrospun polyetherimide nanofibers with reduced graphene oxide-zeolitic imidazolate framework for conductivity improvement

Reduced graphene oxide (rGO) combined with zeolitic imidazolate framework (ZIF), i.e., rGO-ZIF incorporated with polyetherimide (PEI) electrospun nanofiber, was produced using the electrospinning technique. The produced nanofibers had high porosity with enhanced conductivity. The ionic conductivity, porosity, morphology, and diameter of rGO-ZIF nanofiber were optimized by manipulating the weight percentage of rGO-ZIF and PEI in the electrospinning solution. Morphology, porosity, and contact angle analyses revealed that the 30 wt% PEI with 0.3 wt% of rGO-ZIF can produce nanofibers without beads with 136.3 ± 35 nm mean diameter and porosity of around 92.3%. Furthermore, electrochemical impedance spectroscopy (EIS) analysis revealed that with the addition of 0.3 wt% of rGO-ZIF, charge transfer resistance decreased, and the ionic conductivity of PEI nanofibers increased to 5.23 × 10–4 S/cm, nearly 200 times higher than the ionic conductivity of pure PEI nanofibers. The excellent ionic conductivity, low charge transfer resistance, and high porosity of electrospun rGO-ZIF/PEI-based composite nanofibers make them suitable for electrochemical sensing electrode applications.


Introduction
Electrospinning is a popular technique whereby electrostatic force is employed to produce polymer nanofibers from a polymer precursor solution with diameters ranging from microns down to several tens of nanometers. In the electrospinning technique, morphology, size, and membrane porosity can be optimized by simply manipulating the process parameters such as flow rate, working distance, applied voltage, concentration, and conductivity of the polymer solution [1,2]. Recent advances in electrospinning applications have expanded to many fields such as sensing [3,4], fuel cells [5,6], filtration [7], surface and electrode modification [8][9][10], enzyme application [2,11], and drug delivery [12]. Electrospun membrane fibers can enhance the active surface area and the adsorption of electrolytes and reactants. Moreover, the high surface area and one-dimensional structure of electrospun nanofibers reduce grain energy and improve the modified surface's conductivity [13,14].
Zeolitic Imidazolate Framework (ZIF) is a metal-organic framework (MOF) that consists of an imidazolate linker and metal ions with porous characteristics and a wide range of capabilities such as thermal and chemical stabilities [15]. These attributes allow ZIF to have a broad range of applications, such as separation [16,17], catalysis [18], and sensing [19], due to its rich porous structure, high surface-tovolume ratio, and high structural tunability. In this regard, a combination of MOFs with reduced graphene oxide (rGO) nanocomposites has been shown to inherit some of the unique properties of graphene, thus leading to materials with enhanced electrical, optical, and adsorption properties [20][21][22]. The excellent electrical properties of rGO-ZIF8 nanocomposite, due to the synergistic effect of graphitic surface and ZIF-8 crystals, highlight its potential as a new platform for applications such as electrochemical detection of dopamine [23]. In other research literature, ZIF-8 was grown on rGO-coated polyurethane (PU) foam that enhances the absorption capacity of oil [24]. However, polyetherimide (PEI) as a polymer matrix to rGO-ZIF is yet to be investigated for electrochemical electrode applications.
Recent advances in one-dimensional nanocomposite MOFs, notably nanofiber-based MOFs composite structures, have attracted considerable attention due to their continuous structures, large surface area, excellent electrical and ionic conductivity, and high thermal and mechanical stability, which are highly desirable for oxygen/CO 2 reduction catalyst coating applications, selective gas filtration coatings, high stable charge transfer electrode coating [25,26], industrial dye absorption, and gas absorption applications. For instance, in a previous study, Zn incorporated Co-ZIF electrospun carbon nanofiber exhibited strong catalytic performance in oxygen reduction reaction with high durability, which is close to the commercial Pt/C catalyst [27]. Similarly, it has been proven that the CNCo 3 O 4 NPs incorporated ZIF-67 NPs electrospun carbon nanofiber results in highly porous active sites for increasing electrochemical reaction with high chemical stability, which can be effectively utilized for preparing anodic electrodes for lithium-ion batteries [25]. Besides, the nanofiber morphology optimization of PEI electrospun fibers using various nonvolatile solvents has been previously studied [28]. Other works demonstrated the effect of temperature and relative humidity on the morphology of the electrospun PEI [29]. Recently, our group has shown that electrospun PEI nanocomposites with graphene oxide as filler have dramatically improved electrical conductivity [13]. Moreover, the improvement is often observed at low loadings of graphitic filler due to the large interfacial area and high aspect ratio of these materials, requiring small amounts of filler to achieve percolation. Therefore, it is timely to introduce a graphitic-MOF structure, namely, rGO-ZIF8, into electrospun nanofibers to construct a novel membrane as an electrode that offers high adsorption capacity and mass transfer rate and high ionic conductivity, which could enhance the electrochemical reaction rate and sensitivity of the electrode surfaces.
This study investigates the fabrication of bead-free rGO-ZIF/PEI composite nanofibers with high porosity and enhanced ionic conductivity via the electrospinning technique. rGO-ZIF/PEI nanofiber membranes were produced using electrospinning technology onto screen-printed carbon electrodes (SPCE). The properties of the electrospun nanofiber membrane were characterized in terms of morphology, porosity, liquid uptake, hydrophobicity, and conductivity. PEI with optimum concentration of rGO-ZIF produces nanofiber with high hierarchical porosity, less hydrophobicity, and a large surface area that leads to low charge transfer resistance and high ionic conductivity. Developed conductive rGO-MOF nanofiber matrices can be addressed for further developing rGO-ZIF/PEI membrane for sensors and other industrial sensors anodic material development.

rGO and rGO-ZIF composite synthesis
Graphene oxide (GO) was synthesized from graphite using a modified Hummer method [30]. In this method, 5 g of graphite powder was added to 200 mL of concentrated sulfuric acid (H 2 SO 4 ) and stirred for 1 h while cooled with an ice bath. 30 g of potassium permanganate (KMnO 4 ) was added slowly into the mixture within 2 h while maintaining a temperature below 15 °C. The mixture was stirred continuously for a further 2 h. Next, the stirring of the mixture was continued at room temperature for another 20 h until the mixture became viscous. The temperature of the mixture was increased again to 70 °C and stirred for 2 h. 100 mL milliliters of distilled water was then slowly added to the mixture and stirred for 10 min before being sonicated for 30 min at 70 °C. The temperature of the mixture was heated to 90 °C and stirred for 1 h, after which 100 mL of H 2 O was slowly added and stirred for a further 1 h. The mixture was then sonicated for 30 min at 70 °C, before adding 30 mL of H 2 O 2 to stop the reaction. The solution was then allowed to cool down to around 25 °C while continuously stirring. Finally, the solution was sonicated at room temperature for 1 h, followed by washing with 1 M HCl and DI water to remove impurities until the pH was neutral. The mixture was centrifuged at 10,000 rpm at 4 °C for 10 min. Finally, the slurry was poured into a petri dish and dried in an oven at 60 °C for 24 h. rGO was produced by reducing the GO using L-ascorbic acid. Typically, 3 g of GO was added to 1 L of DI water and was sonicated for 4 h. 30 g of L-ascorbic acid was then added to the GO solution and stirred with a magnetic stirrer for 1 h. Herein, the pH of the solution was controlled by adding NH 4 OH dropwise to 10. The resulting mixture was stirred at 95-100 °C for 3 h, before the product was filtered and then washed with distilled water. Finally, the resulting rGO was dried using a laboratory oven at 70 °C for 24 h.
The rGO-ZIF composite was prepared according to the previously reported protocol [31]. Briefly, 40 mg of rGO was added to the 1 mM zinc nitrate. Then, the mixture was added to 70 mL of methanol solution and sonicated for 8-10 h. A total of 328 mg of 4.65 mmol of 2-Methylimidazole was then added to the resulting solution and sonicated for 12 h. The resulting powder was separated from the solution using filter paper and washed with methanol before the final product was dried in a laboratory oven at 100 °C for 12 h.

Preparation of PEI and rGO-ZIF/PEI electrospinning precursor solution preparation
For the preparation of PEI electrospinning precursor solution, 1 g, 2 g, and 3 g weights of PEI polymer were dissolved in 10 mL NMP solvent to form 10 wt%, 20 wt%, and 30 wt% pure PEI electrospinning precursor solutions, respectively. The solutions were stirred vigorously at 70 °C for 4-5 h until transparent viscous liquids without any flakes were obtained. For rGO-ZIF/PEI electrospinning precursor solution preparation, the prepared rGO-ZIF was dispersed into 10 mL of NMP/PEI polymer mixture using a sonicator (Q700, QSonica). The concentration of the rGO-ZIF was managed as 0.1 wt%, 0.3 wt%, and 0.5 wt%.

Electrospinning method
20 mL of the 10 wt% PEI solution was transferred into a disposable syringe and fitted with a blunt needle of 24G. The electrospinning process was set up as in Fig. 1. High voltage was supplied to the setup by a high voltage direct current (DC) power supply (Genvolt 73,030), and the distance between the tip to the grounded collector was set to be 20 cm. The flow rate of the solution was maintained at 0.8 mL/h using a syringe pump (NE-1000), and a voltage of + 20 kV was applied to the nozzle to form a stable Taylor cone. An SPCE wrapped in aluminum foil was attached to the collector to deposit electrospun fibers for the conductivity analysis. A hole was punched in the aluminum foil covering the SPCE to ensure only the working electrode was coated with electrospun fibers. The process was carried out for two and half hours under a consistent temperature of 25 °C and relative humidity of 70%. Finally, electrospun fibers were dried at room temperature and placed in a desiccator. The same steps were repeated using different concentrations of PEI with different concentrations of rGO-ZIF-PEI.

Porosity and liquid uptake test
The n-butanol soak-up method was used to report the variation in bulk porosity of different PEI and rGO-ZIF/PEI nanofiber matrices [32,33]. Briefly, to measure the porosity, the electrospun fiber mats were cut into a square shape with a dimension of 2 cm × 2 cm and then soaked in n-butanol for 2 h at room temperature. Then, the mats were removed from the n-butanol solutions and dried with a tissue before weighing. The bulk porosity of the fiber mats was calculated using Eq. (1).
where m p and m b are the masses of square fiber mats before and after soaking in n-butanol, respectively. Meanwhile, ρ b and ρ p refer to the densities of n-butanol and PEI, respectively. Solvent uptake of electrospun fiber mat can be determined using Eq. (2).
where m b is the weight of the fiber mats after soaking in n-butanol and m p is the weight of the fiber mats before soaking in n-butanol.

Electrochemical impedance spectroscopy (EIS)
The electrospun nanofiber ionic conductivity was determined using frequency response analysis (FRA) in electrochemical impedance spectroscopy (EIS) (Autolab, pgstat204). PEI and rGO-ZIF/PEI nanofibers were electrospun onto the aluminum-wrapped SPCE, which was fixed onto the grounded collector of the electrospinning setup. EIS characterization was performed in acetate buffer solution with pH 4.6, and the frequency was manipulated between 1 MHz and 1 Hz. The charge transfer resistance, R ct , of the fibers was recorded to determine the ionic conductivity of the nanofiber membrane using the following equation [34]: where σ is the ionic conductivity of the nanofiber membrane (S/cm), L is the thickness of the fiber mat in cm (= 0.005 cm), R ct is the charge transfer resistance in ohm (Ω), and A is the contact area of the symmetrical electrode in cm 2 (0.1257 cm 2 ) [35].

Water contact angle measurement
The hydrophobicity of the electrospun nanofiber mat was tested using a water contact angle analyzer (goniometer AST/VCA -3000S). The samples were placed on the sample stage, and a charge-coupled device (CCD) camera was used to capture the water droplet profile. Three contact angle measurements were captured at different positions of the sample tangentially, and the average result was reported.

Field emission scanning electron microscopy (FESEM)
The morphology of the electrospun fiber was investigated using a field emission scanning electron microscope (FESEM, Variable Pressure Zeiss Supra 35) at an accelerating voltage of 5 kV. The samples were sputtered-coated with a thin layer of 15-nm gold before the FESEM measurement. ImageJ software (version 1.8.0_172) was used to determine the fiber size diameter. The mean and standard deviation values were obtained by selecting 150 fibers. Figure 2 shows the FESEM micrographs and size distribution histograms of electrospun nanofibers produced from PEI polymer solution of varying concentrations. The mean diameters of 1000.5 ± 537 nm, 1105.7 ± 433 nm, and 1170 ± 322 nm were produced from 10 wt%, 20 wt%, and 30 wt% of PEI precursor solution, respectively. These results indicate that the mean diameter of electrospun PEI fiber is within the micro-size range, which increases with the increasing concentration of PEI solution. However, only microfibers produced from 30 wt% of PEI solution have narrow size distribution. Microfibers produced from lower concentrations of PEI solutions are agglomerated and polydisperse. A similar observation has been made [36,37], where lower concentrations of precursor solution, surface tension, and applied electric force were found to interrupt the polymer filaments and result in polymer beads and droplets [36,37]. Furthermore, lower viscoelastic forces of low concentration of polymer solutions may interrupt the polymer entanglement resulting in non-uniform and agglomerated nanofibers [36,38]. Table 1 shows the porosity and liquid uptake of the pure PEI electrospun nanofibers. Here, it is shown that the liquid uptake of 10 wt%, 20 wt%, and 30 wt% of electrospun PEI nanofibers was 518.18%, 532.78%, and 1337.84%, respectively. These results indicate that the liquid uptake of PEI nanofiber increases with the increasing concentration of precursor solution. Generally, a high percentage of porosity leads to a high amount of liquid adsorption, whereby the percentage of liquid uptake increases [39]. Therefore, based on FESEM results and porosity analysis, nanofibers derived from 30 wt% of PEI polymer were selected as a matrix for rGO-ZIF/PEI composite in the next step. Figure 3 shows FESEM micrographs and mean diameter size distribution histograms of PEI nanofibres with the addition of 0.1 wt%, 0.3 wt%, and 0.5 wt% of rGO-ZIF particles into 30 wt% PEI. According to Fig. 3, the fiber diameter decreases tremendously from 1170 ± 322 (Fig. 2(c), i.e., without rGO-ZIF) to 212.1 ± 64 nm, 136.3 ± 35, and 115.6 ± 45 nm by the addition of 0.1 wt%, 0.3 wt%, and 0.5 wt% of rGO-ZIF, respectively. Although the fiber diameter decreases with the increasing concentration of the rGO-ZIF, bead formation also increases with the increasing concentration of rGO-ZIF. As seen, 0.1% concentration of rGO-ZIF produced nanofibers with the lowest number of beads, whereas 0.5 wt% of rGO-ZIF produced the highest number of beads, which may be attributed to the high electric force applied to the electrospinning jet. With the increasing concentration of rGO-ZIF, the conductivity of the precursor solution increases, whereby applied electric force and electrospinning jet are increased. This high electric force interrupts the stability and continues polymer filament formation, leading to bead formation [40]. As the PEI precursor with 0.3 wt% of rGO-ZIF produced nanofibers with the lowest number of beads, this concentration was chosen as

The porosity and liquid uptake of the varying percentage of rGO-ZIF into electrospun PEI
The porosity and liquid uptake of 30 wt% of PEI electrospun nanofiber after adding rGO-ZIF are given in Table 2. Accordingly, 0.1 wt%, 0.3 wt%, and 0.5 wt% of rGO-ZIF/PEI electrospun nanofibers exhibit porosities of 84.39%, 92.30%, and 88.25%, respectively. The liquid uptake of 0.1 wt%, 0.3 wt%, and 0.5wt% rGO-ZIF/PEI nanofibers was 781.48%, 1976%, and 1241.03%, respectively. The decrease in porosity of nanofibers produced with 0.5wt% rGO-ZIF/PEI compared to 0.3wt% suggests the presence of entangled beads and agglomeration may greatly reduce the surface area of nanofibers in 0.5 wt% of rGO-ZIF nanofibers [41]. Hence, it can be concluded that increasing the concentration of rGO-ZIF reduces the fiber diameter and increases the porosity up to a limit of 0.5 wt% rGO-ZIF, where the high viscosity of the precursor solution results in the production of entangled beads after electrospinning and reduces the porosity and liquid uptake. Figure 4 shows the contact angle analysis results of the electrospun rGO-ZIF/PEI nanofibers. It can be seen that the  contact angle of 0, 0.1, 0.3, and 0.5 wt% of rGO-ZIF in 30 wt% of PEI nanofibers was found to be 119.03 ± 4.16°, 121.13 ± 2.0°, 111.28 ± 9.74°, and 125.45 ± 5.57°, respectively. The contact angle of electrospun PEI nanofiber increased slightly when 0.1 wt% and 0.5 wt% of rGO-ZIF were added. Fiber diameter plays a significant role in the wettability and conduct angle of the nanofiber matrix. Smaller diameter nanofibers have an improved hydrophobic surface, whereas a larger diameter of fibers can increase the roughness of the fiber and decrease the contact angle [42]. However, the contact angle of 0.3 wt% rGO-ZIF/PEI nanofiber was lower than other fibrous samples. These counter results may be attributed to the high porosity of the 0.3 wt% rGO-ZIF/PEI composite (according to Table 2) that enhances the water adsorbance of the fibers.

Ionic conductivity analysis of electrospun rGO-ZIF/PEI nanofibers
The EIS Nyquist plot from Fig. 5 implicit the charge transfer resistance (R ct ) of the 30 wt% of PEI electrospun nanofiber with the addition of 0.1 wt%, 0.3 wt%, and 0.5 wt% of rGO-ZIF. The subplot data on the top right of Fig. 5 shows the Nyquist plot for 0 wt% rGO-ZIF (pure PEI). The calculated R ct values of 0.1 wt% to 0.5 wt% rGO-ZIF/ PEI electrospun nanofibers from the Nyquist plot data are tabulated in Table 3. The electrospun nanofibers of 30 wt% of PEI only (0 wt% rGO-ZIF) were found to have higher R ct than rGO-ZIF/PEI electrospun nanofibers. However, furthermore, results show a drop in R ct from 8.82 × 10 1 Ω to 6.61 × 10 1 Ω when the concentration of rGO-ZIF is increased from 0.1 to 0.3 wt% of rGO-ZIF. Generally, the charge transfer resistance of the MOF-coated electrode system falls with increasing ion and electron transfer reactions, typically electron-hole recombination via catalysis by metal oxide surface sites. Therefore, this decreasing R ct implicates that the internal electron transfer rate of rGO-ZIF/PEI composite nanofiber increases with the increasing amount of rGO-ZIF. This increasing electron transfer trend is attributed to the synergistic effects of the high electric conductivity of rGO and the host high active site of ZIF-8 that increase anodic electrochemical reactions [20,23]. Furthermore, the high surface area offered by the nanometric scale and one-dimensional structure of electrospun nanofibers enhances the charge and mass transfer rate that leads to high ionic conductivity [13,14]. In addition, higher porosity obtained from the 0.3 wt% rGO-ZIF/ PEI nanofibers adsorbs a high amount of electrolyte and offers a larger surface area for anodic reaction and electron diffusion, which results in lower R ct and higher ionic conductivity [43]. However, 0.5 wt% of rGO-ZIF composite electrospun nanofiber exhibits higher R ct (7.18 × 10 1 Ω) than 0.3 wt% of the rGO-ZIF composite electrospun nanofiber. This consequence ascribes to the low porosity and surface area resulting from the high number of beads in the nanofiber. The ionic conductivity of the 0.1 wt% to 0.5 wt% rGO-ZIF loaded in 30 wt% of PEI nanofiber was calculated based on charge transfer resistance (R ct ) values using Eq. 3, and the results are shown in Table 3. The ionic conductivity of electrospun 30 wt% PEI (without the addition of rGO-ZIF) and with the addition of rGO-ZIF of 0.1 wt%, 0.3 wt%, and 0.5 wt% was calculated to be 2.71 × 10 −6 S/cm, 4.46 × 10 −4 S/cm, 5.23 × 10 −4 S/cm, and 4.83 × 10 −4 S/cm, respectively. Electrospun 30 wt% PEI (without rGO-ZIF) has lower ionic conductivity than the electrospun rGO-ZIF/PEI ionic conductivity. The addition of rGO-ZIF increased the ionic conductivity of electrospun rGO-ZIF/PEI drastically by two orders of magnitude. As mentioned earlier, this enhanced ionic conductivity may be attributed to the high conductivity of rGO and the high adsorption capacity of ZIF-8 [20,23]. However, when the concentration further increases to 0.5 wt% of rGO-ZIF, the conductivity decreases, which may be attributed to the high R ct resulting from the formation of the beads in electrospun nanofiber, which reduce the surface area. Consequently, 0.3 wt% of rGO-ZIF composite nanofibers exhibits the lowest R ct and the highest conductivity. In addition, it is worth highlighting that no intercepts appear at the high-frequency limit (at zero reactance) in the rGO-ZIF/ PEI nanofiber-coated electrode Nyquist plot. We hypothesize that this consequence might be attributed to the degradation of rGO-ZIF coating at a high frequency (1 MHz).
Furthermore, to validate the impact of rGO and ZIF on the ionic conductivity property of 0.3 wt% of rGO-ZIF composite, the corresponding weight percentage of only rGO and ZIF (components of rGO-ZIF composite) added PEI

Conclusions
This study has proposed a facile technique for producing rGO-ZIF-based composite PEI nanofiber with excellent conductivity and porosity. The results of this study reveal that the mean diameter of the 30 wt% PEI electrospun nanofiber significantly decreased from 1170 ± 322 to 136.3 ± 35 nm when 0.3 wt% of rGO-ZIF was added. These results are attributed to the high conductivity of rGO-ZIF that assists in thinner fiber formation during the electrospinning process. Liquid uptake and porosity analysis implied that the porosity of PEI electrospun nanofiber rises with increasing concentration rGO-ZIF, with 0.3 wt% found as the optimum concentration of rGO-ZIF for producing beadless electrospun rGO-ZIF/PEI nanofiber with high porosity. Significantly, the ionic conductivity of rGO-ZIF/PEI electrospun nanofibers was enhanced with increasing concentration of rGO-ZIF, with 0.3 wt% of added rGO-ZIF resulting in a measured ionic conductivity two orders of magnitude greater than that of pure PEI nanofibers. Therefore, this study has proved that the rGO-ZIF/PEI composite electrospun nanofibers with 0.3 wt% of rGO-ZIF particles exhibit lower charge transfer resistance, high ionic conductivity, high porosity, and desirable surface properties, with excellent suitability for use as a sensor electrode matrix for various electrochemical sensing applications such as gas sensor, biosensors, and heavy metal ion detectable sensor.
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