1. Introduction
Electrospun nanofibrous membranes (ENMs) are an important category of fibrous structures that can be produced via electrospinning. Due to their ease of production, interconnected porosities, and high surface area, ENMs have found a wide range of applications. Some of these applications, most notably protective clothing (PC) and wound dressing, require smart ENMs with sensing properties, such as detecting pH changes in the environment. Using pH-sensitive nanofibrous membranes in the PC sector helps the wearer be more aware of chemical pollution in the surrounding atmosphere, leading to better protection. This function can be very useful in clothing designed to protect against chemical agents and industrial chemical hazards. In the wound treatment sector, creating dressings with an indicator function that can sense the status of the skin and convey the information to the healthcare professional can reduce the frequency of dressing changes [
1].
Studies have been carried out to fabricate pH-sensitive ENMs for smart beverage packaging [
2], textiles [
3,
4], tissue engineering [
5,
6,
7], detecting the microenvironment of various biological processes [
8] and microscopic observations of microorganisms [
9]. Schueren et al. [
10] used polyspinning to develop pH-sensitive polyamide 6.6 nanofibers for a limited range of pH values. Agarwal et al. [
11] produced nylon ENMs containing five different pH indicators in specific proportions to cover a wide range of pH values. These smart ENMs detect changes in solution pH (from pH 1 to 10) based on characteristic colors for each pH. Schuern et al. [
12] developed pH-sensitive polycaprolactone (PCL) and PCL/chitosan nanofibrous structures, noting that inclusion of 20% chitosan reduced pH response time from more than 3 h to around 5 min. Rostami et al. [
4] also reported successful fabrication of polyamide (PA) 6 and PCL ENMs equipped with pH sensitive dyes that detect variations in solution pH. Although they demonstrated that fibrous structure influences halochromic properties, the same sensing behavior was reported for both PA and PCL. These membranes are only useful only for detecting pH changes in a solution medium; however, detecting pH changes in vapor and at low concentrations of agent molecules is also important.
In addition, previous works have used hydrophobic polymers with a uniform nanofiber diameter for the production of sensor nanofibers, and some reports allude to the potential of super-hydrophilic polymers in hydrogel form for use in pH-sensitive applications [
13,
14]. Mounting evidence also indicates that fiber membranes with a bimodal diameter distribution are more efficient at trapping water vapor molecules [
15] and aerosol particles [
16]. Bimodal fiber diameter ENMs from hydrophilic-hydrophobic polymers have great potential for increasing the pH sensitivity of the resulting membranes.
In the present study, polyurethane (PU) and poly (2-acryloylamido-2-methylpropanesulphonic acid) (PAMPS) were used as polymers with hydrophobic and hydrophilic properties, respectively. PAMPS is a water-soluble and ionic polymer with a high degree of hydrophilicity, and is often touted as a superabsorbent material for PC [
17,
18] and wound dressing [
19,
20,
21] applications. The sulfonate group of PAMPS gives the polymer a degree of hydrophobicity and an anionic quality. The hydrophilic nature of PAMPS leads to a final structure with enhanced water absorption and transport properties. Fine nanofibers in the range below 100 nm have been electrospun from PAMPS because of its electrolytic nature [
20,
21]. Despite the good water absorption and fine diameter of PAMPS nanofiber membranes, the mechanical and dimensional stability of these structures is not sufficient to warrant their use in end products such as pH-sensitive membranes. This problem can be solved by blending PAMPS with PU. PU is a popular polymer that is used extensively in electrospinning processes [
22,
23,
24]. This polymer has unique properties such as excellent hydrolytic stability, good mechanical properties, and resistance to chemicals, microorganisms and abrasion [
25,
26,
27]. PU has hydrophobic properties and lends well to nanofibrous structures with diameters ranging from 200 to 600 nm. The effect of graphene oxide (GO) on the sensing behavior of final structures has also been investigated. Due to its excellent mechanical, thermal, and electrical properties, and the presence of hydrophilic groups (e.g., hydroxyl, carbonyl) on the surface, GO can improve the water vapor permeability and adsorption of water of nanocomposites [
17,
18]. Hua and coworkers showed that polyvinyl alcohol (PVA) combined with GO exhibited higher pH sensitivity, and in hydrogel form can be used for loading and selectively releasing drugs at physiological pH [
19].
Against this backdrop, the present study aims to fabricate and characterize bimodal fiber nanofibrous membranes by electrospinning one hydrophobic polymer (PU) and one hydrophilic polymer (PAMPS). GO was also used to improve the response time of nanofibrous membranes with respect to detecting pH. The membranes were fabricated by simultaneous spinning of two different polymers from two opposing nozzle setups. The diameter distribution was measured by scanning electron microscopy (SEM). The pH-sensing behavior of the bimodal PU/PAMPS/GO nanofibrous membranes was investigated in both solution and vapor tests.
2. Materials and Methods
2.1. Materials
All chemicals were purchased from commercial sources and used as received unless noted otherwise. N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, monomer, ≥99%) were obtained from Daejong Co. (Shiheung, South Korea). 2,2-azobisisobutyronitrile (AIBN) and four pH-indicator dyes (bromophenol blue, bromocresol green, methyl red, bromophenol red) were supplied by Sigma-Aldrich (St. Louis, MO, USA). PU was obtained from Byer (Leverkusen, Germany). Graphite was obtained from Daejong Co. (Shiheung, South Korea). Other materials including concentrated sulfuric acid, hydrochloric acid, hydrogen peroxide, sodium nitrate and potassium permanganate (KMnO4) were purchased from Merck (Darmstadt, Germany).
2.2. Preparation of Electrospinning Solutions
The four pH-indicator dyes were selected to cover a range in pH values, indicated by color changes (
Table 1). Two different polymer solutions were prepared: PU solution containing the four dyes and PAMPS solution containing GO nanolayers. For the PU solution, the dye mixture (PU/dye = 99:1 w/w %) was dispersed in 5 mL of THF/DMF (60:40) with 1 h ultrasonication. PU was dissolved in THF/DMF at 30 °C for 24 h, and each dye was then added to the PU/DMF/THF solution to give an overall 6 wt% PU in THF/DMF. GO nanosheets were prepared according to the modified Hummer method. Briefly, graphite powder (2 g) and NaNO
3 (1 g) were added into H
2SO
4 (46 mL) and stirred in a three-neck flask placed in an ice bath for 2 h. Then, KMnO
4 (6 g) was added gradually under stirring and the temperature of the mixture was kept below 20 °C. The solution was heated to 35 °C and kept for 2 h. After that, distilled water (92 mL) was slowly added and the temperature was kept below 100 °C. After 15 min, an adequate amount of distilled water and 5 mL of 30% H
2O
2 solution were added to wash the residual KMnO
4. Finally, the mixture was filtered and washed with 5% HCl aqueous solution and water until the pH value of the upper layer suspension reached 7. Furthermore, sonication with a cylindrical tip was used for exfoliation of graphite oxide to GO. The sonication process was carried out in room temperature (25 °C) for 20 min. Detailed information with respect to GO and PAMPS/GO formation is reported in our previous study [
28].
2.3. Electrospinning
The simultaneous electrospinning of different precursor solutions from two syringes produced PU and PAMPS nanofibers. For this two-nozzle electrospinning, two syringes, two pumps and two high-voltage power supplies were used. One syringe contained 5 mL of PU solution containing dye while the other syringe contained 5 mL of PAMPS/GO solution. Each needle (22G; L = 34 mm and D = 0.4 mm) was connected to the positive electrode of a high-voltage power supply. The distance between the needle tips and the collector was 14 cm for each nozzle. For the first nozzle, PU solution was pumped at a rate of 0.5 mL/h and the voltage was set at 12 kV. For the second nozzle, PAMPS/GO solution was pumped at a rate of 0.1 mL/h and the voltage was set at 14 kV. Samples with different PAPMS/PU/GO ratios were obtained by changing the electrospinning time of each polymer according to
Table 2. A take-up speed of 32 m/min was used for the collection of electrospun nanofibers. The electrospinning process was carried out at room temperature (22 ± 2 °C) and relative humidity (40% ± 5%). The electrospinning process is shown schematically in
Figure 1.
2.4. Membrane Characterization
To obtain the colors of the dye mixture at each pH, 0.01 g of dye was dissolved in 10 mL of DMF with 1 h ultrasonication. A total of 1 mL of the prepared solution was added to a set of buffer solutions with different pH values (pH = 1, 2, 3, 4, 5, 6, 7 and 8). To determine halochromic behavior of the electrospun membranes, the solution tests were performed by dipping the membranes in each of these same buffer solutions (pH 1–8) for 2 s. Vapor tests were conducted using a homemade apparatus at three pH values (
Figure 2). The desired buffer (pH = 1, 4, 8) was injected by a syringe pump and diluted by air flow provided by an air compressor (20 L/min). The injection rate was regulated to achieve a final flow with a concentration of 1000 ppm. The buffer flow was heated by an oil bath to a temperature of 90 °C and evaporated. The evaporated buffer was then conducted to a glass chamber and put in contact with the sample until a color change was detected. Solution and vapor tests were repeated three times for each sample.
The morphology of the electrospun nanofibers was characterized using transmission electron microscopy (TEM, Zeiss-EM10C Zeiss, Zaventem, Belgium) with an accelerating voltage of 80 kV, and field emission scanning electron microscopy (FE-SEM; Mira3-XMU, TESCAN, Kohoutovice, Czech Republic). The mean fiber diameter and diameter distribution were measured from SEM images using an image-processing program (ImageJ, Bethesda, Maryland, USA).
Images of reference dye solutions and electrospun nanofibers for each pH were obtained with a digital camera (Canon EOS 400D, Brampton, ON, Canada) under the identical lighting environment and camera setting. The RGB and CIELAB values (International Commission of Illumination (CIE) color space coordinates) were measured using Adobe
® Photoshop
® CS5 (San Jose, CA, USA) by taking the average of 100 pixels from 10 points in each image. The total color difference (Δ
E) was calculated to compare the samples in different pH buffer solutions. Δ
E was determined using according to:
where Δ
L represents the brightness difference between samples, Δ
a is the redness difference between samples, and Δ
b is the yellowness difference between samples [
2,
11]. The UV-Vis spectra of all samples were recorded in a wavelength range from 380 to 780 nm with a spectrophotometer (Perkin-Elmer, Waltham, MA, USA; Model Lamba 950 or Model Lamba 45).