Quaternary ammonium salt–modified polyacrylonitrile/polycaprolactone electrospun nanofibers with enhanced antibacterial properties

Abstract

In this experiment, octadecyltrimethylammonium chloride (STAC), a cationic antibacterial agent, was designed to modify hydrolyzed polyacrylonitrile (PAN) through tight electrostatic attraction. Then, the modified PAN was successfully electrospun with polycaprolactone (PCL) to obtain PCL/PAN-STAC nanofibrous membranes with enhanced mechanical properties. The modified PAN was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis and elemental analysis. The morphological, mechanical and antibacterial properties of nanofibrous membranes were investigated. The blended nanofibrous membrane presented a uniform and stable structure with small pore size. Tensile tests indicated that the mechanical property of PCL/PAN-STAC nanofibrous membrane was obviously enhanced by blending. Disk diffusion tests showed that the inhibition zones of PCL/PAN-STAC against Escherichia coli and Staphylococcus aureus were 7.56 ± 0.05 mm and 15.37 ± 0.34mm,, respectively. Shaking method indicated that the antibacterial activity against E. coli was as high as 96.20 ± 0.89% when the use of PCL/PAN-STAC reached 9 mg. Therefore, this antibacterial nanofibrous membrane is very favorable for applications such as protective filtration masks and wound dressing.

Keywords

Antibacterial electrospinning polyacrylonitrile modification quaternary ammonium salt

Increasing the quality of people’s life is an eternal topic. As a threat to human beings, the bacterial infection has brought serious concerns related to human health in daily life and medical hygiene, such as bacteria spreading in the air, wound healing and chronic disease.^1–7 Recently, antibacterial nanofibrous membranes have become the focus of researchers for their extremely high surface area, nanoporous features and design flexibility, which are desirable for applications like air filtration and wound dressing.^8–10

Electrospinning technology is an emerging technology for preparing fibers with submicron scale diameters.¹¹ It is a versatile, simple, fast and low-cost process for the controlled preparation of continuous polymer fiber mats.¹² Polyacrylonitrile (PAN) is often chosen to prepare nanofibers for medical and filtration applications owing to its thermal stability and tolerance to most solvents.¹³ Moreover, the abundant –CN of PAN allows chemical modification to obtain specific functions.^14,15 For instance, Akkaya and Erdogan Ozseker prepared antibacterial nanofibrous membranes by fixing the tetracycline to the --COOH after the hydrolysis of PAN nanofibrous membranes.¹⁶ Ng et al. modified PAN nanofibrous membranes through alkaline hydrolysis and grafting with chitosan molecules, which were further immobilized with poly(hexamethylene biguanide) to enhance antibacterial activity.¹⁷

Quaternary ammonium salt (QAS) is a type of synthetic antibacterial agent commonly chosen for its outstanding antibacterial activity and technical advantages, whose antibacterial mechanism can include three stages:¹⁸ (a) contact stage: there is electrostatic attraction between the positively charged QAS and the negatively charged bacterial surface, and the alkyl chain will interact with the membrane proteins of bacteria due to its lipophilicity; (b) destruction stage: the two forces cause the alkyl group of QAS to penetrate into the bacteria, thereby damaging the cell wall and the cytoplasmic membrane, leading to the leakage of cytoplasmic constituents; and (c) death stage: with the leakage of cytoplasmic constituents, bacteria will die for the abnormal metabolism, which is called “contact death.”¹⁹ QAS has been widely used to physically or chemically modify PAN nanofibers to obtain antibacterial membranes. Gliscinska et al. prepared antibacterial PAN nanofibrous membranes through physically adding a QAS-based bioactive agent to spinning solution.¹³ Cheah et al. prepared chitosan-modified PAN nanofibrous membranes by grafting chitosan on the hydrolyzed PAN nanofibrous membranes, which were further quaternized for better antibacterial activity.²⁰ However, surface-based anchoring and treatments may reduce antibacterial durability and enhance shielding effectiveness.²¹

In this article, we synthesized an antibacterial polyelectrolyte through PAN hydrolysis and electrostatic interaction with cationic QAS-octadecyltrimethylammonium chloride (STAC), which enables the antibacterial agent STAC to incorporate comprehensively the polymer chain, resulting in the interspersion of STAC in the entire nanofibrous membranes from the surface to the inside after electrospinning. The polyelectrolyte PAN-STAC was further electrospun by blending with polycaprolactone (PCL) for enhanced mechanical performance; PCL is a commonly used polymer for electrospinning, widely used in the biomedical field for its good biocompatibility and biodegradability and excellent mechanical strength,^22–30 so it is often chosen to blend with other materials to provide a stable structure.^31,32 The chemical composition, thermal stability and hydrolysis degree of the modified PAN were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and elemental analysis. Release tests of PAN-STAC were carried out to confirm the combination fastness of electrostatic interaction. The morphology, pore size and mechanical properties of the nanofibrous membranes were investigated. Finally, antibacterial activity of the blended nanofibers against Gram-negative bacteria Escherichia coli and Gram-positive bacteria Staphylococcus aureus were evaluated qualitatively and quantitatively.

Experimental section

Materials

PAN (Mn = 85,000 g/mol) was purchased from Shanghai Chemical Fibers Institute. PCL (Mn =80,000 g/mol) was provided by Sigma Co., Ltd. STAC (Mn = 348.05 g/mol) was obtained from Aladdin Reagent Co., Ltd. N,N-dimethylformamide (DMF), trichloromethane and isopropanol were purchased from Sinopharm Chemical Reagent Company, Ltd. E. coli (ATCC 8099) and S. aureus (ATCC 6538) were purchased from Nanjing Clinical Biological Technology Co., Ltd. Nutrient agar medium and nutrient broth medium were provided by Hangzhou Microbial Reagent Co., Ltd.

Preparation of PAN-H

The PAN hydrolysis adopted oil bath heating. Briefly, PAN (10 g) was added into the magnetically stirred NaOH solution (10 g NaOH, 6%) at 105°C for 3 h. After hydrolysis, the solution was adjusted to pH 3 with HCl solution at 40°C. Finally, the product was washed with deionized water, then it was dried and ground into powder named as PAN-H.

Synthesis of ion exchange PAN-STAC

PAN-H was pretreated in the stove at 140°C for 1 h. Then it was immersed in 30% NaOH solution in a 60°C water bath for 1 h, and finally washed with distilled water and dried, which was recorded as PAN-Na.

PAN-Na (1 g) was added to the STAC solution (0.03 M) in a neutral environment, and the ion exchange reaction was carried out for 8 h in the shaker. The product was obtained by vacuum filtration, washed by deionized water for three times and dried in the stove at 30°C, which was recorded as PAN-STAC.

Electrospinning of antibacterial nanofibrous membranes

In order to obtain stable nanofibers, PAN, PAN-STAC, PCL, PCL/PAN-STAC were electrospun by an electrospinner for comparison. PAN solution (10 wt%) was dissolved by DMF, and the electrospinning was carried out with a 9.9 kV voltage and a flow rate of 0.5 mL/h. PAN-STAC, PCL, PCL/PAN-STAC (weight ratio of 1:1) were all dissolved by isopropanol/chloroform (weight ratio of 1:1) to obtain 10 wt% concentration. The PAN-STAC nanofibers were obtained with a 6.6 kV voltage and a flow rate of 0.6 mL/h. The electrospinning of PCL solution was under an 11.1 kV voltage and a flow rate of 0.6 mL/h, and the conditions of PCL/PAN-STAC were an 8.9 kV voltage and a flow rate of 0.6 mL/h. The syringes were all equipped with an 18-gauge needle, the distance between the end of the needle and the receiving plate was set as 15 cm, and the receiving plate was covered with a piece of aluminum foil to collect nanofibers. During the electrospinning process, the room temperature was set as 25°C and the air relative humidity was 40%. The prepared nanofibrous membranes were dried under vacuum at 30°C for 24 h to remove the residual solvents. The schematic preparation is displayed in Figure 1.

Figure 1.

Schematic illustration of the preparation process of modified PAN and electrospinning of antibacterial nanofibrous membranes.

Characterization

The chemical composition of the modified PAN was analyzed by FTIR (Spectrum Two, Shanghai Perkin Elmer Instrument Co., China). The samples to be tested were prepared by KBr compression method, and the infrared spectrum was obtained by scanning 32 times in the range from 4000 cm⁻¹ to 400 cm⁻¹ with air as the background. The thermal stability of the modified PAN was characterized by a thermogravimetric analyzer (TGA 4000, Perkin Elmer Co., Ltd). The samples were heated from 50°C to 800°C at a heating rate of 10°C/min under a nitrogen atmosphere of 0.15 MPa. To confirm the conversion of the nitrile group into the carboxylic group, the weight percent of each element was analyzed by an elemental analyzer (Vario EL III, Elementar Ltd, Germany). The morphologies of the nanofibers were observed by scanning electron microscopy (SEM; TM 3000, Hitachi Ltd, Japan). The samples were cut into a size about 8 mm × 8 mm, which were then stuck to the sample stage with conductive glue and treated with gold spraying. The average pore size and pore size distribution were observed by using a capillary flow porometer (Porolux100, Porometer Ltd, Germany) with sample size around 10 mm × 10 mm. Mechanical performances of nanofibers were tested by a strength tensile tester of fiber (XQ-1C, Shanghai New Fiber Instrument Co., Ltd, China). The fiber mats were cut into size of 5 mm × 50 mm, which were fixed by two specimen clamps between which the distance was set to 4 cm. The pre-tension was set to 0.2 cN, and the tensile speed was 10 mm/min.

Release of STAC

The STAC release study was done in 200 mL ultrapure water, where 100 mg PAN-STAC powder and mixture of PAN/STAC (weight ratio of 8:2) were respectively tested, incubated over 37°C in a shaker at 120 r/h; 1 mL of the solution was collected and replaced with 1 mL ultrapure water at 2, 4, 6, 9, 12, 24, 48 and 72 h. The concentration of STAC was qualitatively confirmed by ultraviolet-spectrometric analysis with eosin Y as biological color reagent, and the optical density values were procured at 518 nm.

Antibacterial tests

The agar diffusion plate methods for E. coli and S. aureus were carried out to test the antibacterial activity of PCL/PAN-STAC nanofibers visually.³³ The cold-stored bacteria were cultured on the agar solid medium by drawing “131” with an inoculating loop, and placed in the incubator at 37°C. After 24 h, the colony at the corner of “131” was sampled with an inoculating loop and cultured into 20 mL nutrient broth; then the nutrient broth containing the bacteria was placed in a constant temperature shaker at 37°C for 24 h. The cultured bacteria was diluted to 1:100 in deionized water (concentration of ∼ 10⁷ CFU/mL), 0.25 mL of which was evenly spread on the nutrient agar. The nanofibers to be tested (diameter: 7 mm) were placed on top, and then the culture dish was placed in the incubator at 37°C for 24 h.

The shake flask method for E. coli was used as a quantitative test referring to the standard GB/T 20944.3-2008.³⁴ The culture of bacteria was the same as that for the disk diffusion tests. The nutrient broth containing bacteria were diluted to 1:100 in new nutrient broth and continued to be diluted to 1:100 in phosphate-buffered saline (PBS) buffer (5 × 10⁵ CFU/mL). PCL/PAN-STAC nanofibers with nine different mass gradients were placed in sealed breathable bottles containing 5 mL bacterial broth, and then the bottles were placed in the shaker at 37°C for 18 h. After the shaking, 0.1 mL bacterial broth diluted 1:100 in PBS buffer was evenly spread on the nutrient agar, and incubated at 37°C for 24 h. The antibacterial activity was calculated with the following equation

A ntibacterial activity = \frac{N_{0} - N_{1}}{N_{0}} \times 100 %

(1)

where N₀ and N₁ are the numbers of colonies that have grown in the control and test nutrient agar, respectively.

Results and discussion

FTIR and TGA

Figure 2 shows the FTIR spectra of PAN, PAN-H, STAC and PAN-STAC, which proved the hydrolysis of PAN and ion exchange with STAC. PAN contained a large amount of nitrile groups (–C≡N) whose stretching vibration was shown as a sharp peak around 2244 cm⁻¹. After hydrolysis, –C≡N reacted a lot and the peak disappeared in the FTIR spectrum of PAN-H. The occurrence of peaks at 3330, 1652, 1552 and 1407 cm⁻¹ confirmed the existence of –OH, –C=O, –NH₂ and C–O functional groups, respectively in the PAN-H powder, indicating the transformation of nitrile groups into amide (–CONH₂) and carboxylic groups (–COOH), as literatures has recorded.^35–37 The vibrational peaks around 2914 and 2843 cm⁻¹ in the four samples were associated with the stretching vibrations of CH₃ and CH_2, which are most intensive in STAC and PAN-STAC, qualitatively suggesting the combination of PAN-H and STAC. Meanwhile, the band at 1457 cm⁻¹ (C–N stretching from STAC) was also observed in the spectrum of PAN-STAC, which proved that STAC was successfully immobilized in the PAN-H.

Figure 2.

Fourier transform infrared spectroscopy spectra of PAN, PAN-H, STAC and PAN-STAC.

The weight loss curves and residual carbon rates of PAN, PAN-H, STAC and PAN-STAC are shown in Figure 3, which were measured by TGA. The curves well verified the modification process. Thermal decomposition of PAN belonged to two-step weight loss, as well as its hydrolysis product PAN-H. PAN had almost no weight loss before 310°C because only cyclization reaction occurred, after which it started to decompose as random scissions of the molecular chain occurred rapidly, whereas the hydrolysis caused PAN-H to decompose around 165°C mainly due to dehydrogenation.^37,38 The TGA curve of STAC was one-step weight loss, and the rapid loss mainly happened from 228°C to 296°C, during which PAN-STAC also accelerated thermal decomposition, reflecting the incorporation of STAC. The residual carbon rate intuitively shows the change in the thermal stability of the product. After hydrolysis, the residual carbon rate dropped from 52.37% to 32.35%, which further dropped to 3.95% after combination with STAC, indicating the weakening of thermal stability but successful incorporation of STAC. Meanwhile, without doubt, the thermal stability meets the requirements of applications such as wound dressing under normal temperature.

Figure 3.

Thermal properties of PAN, PAN-H, STAC and PAN-STAC: (a) thermogravimetric analysis curves; (b) residual carbon rate.

Hydrolysis of PAN

The elemental analysis of PAN and PAN-H powder are presented in Table 1 to quantify the degree of hydrolysis. According to the conservation of carbon atom mass, we could calculate that the mass of PAN-H was 16.17 g. There was release of 0.0968 mol NH₃ during the hydrolysis of PAN based on the reduction of N element mass, which equaled to the molar amount of the carboxyl groups. If all nitrile groups had hydrolyzed to carboxyl groups, the theoretical yield of carboxyl groups was 0.1887 mol. Hence, the degree of hydrolysis was 51.30%, indicating that a considerable number of nitrile groups had hydrolyzed to carboxylic groups as confirmed by FTIR spectrum presented subsequently.

Table 1.

Elemental composition of PAN and PAN-H powder

Material	Elemental composition (wt%)
PAN	C: 66.42
	H: 5.77
	N: 24.58
PAN-H	C: 41.08
	H: 5.79
	N: 6.82

Morphology of the nanofibrous membranes

The surface morphology was observed to characterize the physical performance of the nanofibrous membranes, and the SEM images are shown in Figure 4. According to the SEM photograph of PAN-STAC membranes, it was found that the modified PAN-STAC was able to be electrospun into nanofibers. However, the fiber diameter was uneven and it tended to swell, which was unstable. The phenomenon was because most --CN in PAN had hydrolyzed to --COOH after hydrolysis modification, causing the final product to be hygroscopic to some extent, for which further improvement is indispensable. It can be seen from the SEM photograph of PCL that the overall morphology was uniform and stable. In addition, PCL was an excellent biodegradable material; it is hydrophobic,^39,40 so it is sometimes designed with hydrophilic materials.⁴¹ That is why we have blended the two materials together. The average diameter of PCL nanofibers in Figure 4(b) was 1.094 ± 0.030 µm, which was consistent with the diameter of PCL nanofibers (1.249 ±0.329 µm) in Figure 4(c) after blending. From the SEM photograph in Figure 4(c), blending has brought significant stability to the PAN-STAC nanofibers. The diameter of PAN-STAC nanofibers was 0.279 ± 0.052 µm; both PCL and PAN-STAC nanofibers were relatively uniform in diameter, and they were uniformly distributed, which made the PCL a perfect support of PAN-STAC.

Figure 4.

Scanning electron microscopy photographs of electrospun nanofibrous membranes: (a) PAN-STAC, (b) PCL, (c) PCL/PAN-STAC.

The porous structure of the nanofibrous membranes is critical, especially for wound protection and air filtration. The pore size distribution of PCL and PCL/PAN-STAC nanofibers is shown in the Figure 5. The average pore diameter of PCL membrane was about 2.743 µm with minimum pore size close to 1 µm, which is consistent with the average diameter of PCL nanofibers. Meanwhile, the average pore diameter of PCL/PAN-STAC membrane is relatively smaller (around 2.407 µm) because of the contribution of PAN-STAC nanofibers, which is prospective to provide a stronger filtration protection in practical applications.

Figure 5.

Pore size of PCL and PCL/PAN-STAC nanofibrous membranes.

Mechanical analysis

Mechanical property is also an important factor which needs to be considered in practical applications. The tensile curves of the three related nanofibers are shown in Figure 6. There is no doubt that hydrolysis modification will weaken the original mechanical property because the morphology of PAN-STAC nanofibers was slightly swollen and unstable. In this study, we tested the tensile property of the PAN nanofibers as a relative reference. From the curve of PAN nanofibers, let alone the PAN-STAC nanofibers, we can see that the tensile property is not outstanding. It was found from the curve of PCL nanofibers that the tensile strength was 3.43 MPa and the elongation at break reached 216.15%, which shows that PCL is flexible and has excellent mechanical properties. When PCL and the PAN-STAC were electrospun with weight ratio of 1:1, the blended nanofibers reached a tensile strength of 3.04 MPa and an elongation at break of 162.0%, enhancing the strength and flexibility of the prospective antibacterial nanofibers, which is favorable for applications such as filtration masks and wound dressing.

Figure 6.

Mechanical properties of PAN, PCL and PCL/PAN-STAC nanofibrous membranes.

Release properties of STAC

The electrostatic interaction is of great importance for durable antibacterial properties. To confirm the combination fastness of hydrolyzed PAN and STAC, the release of STAC was investigated, and is shown in Figure 7. From the release curve of the contrast sample (mixture of PAN and STAC with weight ratio of 8:2), we can see that STAC had rapidly released from the mixture during the first 4 h, and the release speed was close to 2.21 mg/h). The release curve of STAC from PAN-STAC was flat and slow with the release speed around 0.10 mg/h during the first 4 h, and the final release content of STAC only approached 1.68 mg after 3 days. Hence, few parts of STAC could release from PAN-STAC, which means that STAC had been successfully incorporated into the hydrolyzed PAN through tight electrostatic interaction.

Figure 7.

Concentration of released STAC from PAN/STAC mixture as contrast and PAN-STAC powder.

Antibacterial results

Antibacterial tests were first performed qualitatively by the disk diffusion method presented in Figure 8. According to the inhibition zone, the four samples did not show significant antibacterial effect against E. coli, and PCL/PAN-STAC nanofibrous membrane indicated slight antibacterial activity with an inhibition diameter of 7.56 ± 0.05 mm (samples: 7.00 ± 0.04 mm). When it comes to S. aureus, there was almost no inhibition zone around the Al foil, PAN and PCL samples, whereas an apparent inhibition zone with a diameter of 15.37 ± 0.74 mm (samples: 7.00 ± 0.02 mm) appeared around the PCL/PAN-STAC sample, suggesting that the antibacterial agent --STAC could be released from the nanofibrous membranes and inactivate the bacteria because PAN-STAC nanofibers were the solid state of the polyelectrolyte. Hence, besides “contact killing,” we have also established that “release killing” of STAC also functions. The reason why the antibacterial effect against S. aureus was significant is because S. aureus does not have an outer membrane to prevent the penetration of antibacterial molecules.

Figure 8.

Results of antibacterial tests: (a) disk diffusion tests of Al foil, PAN, PCL, PCL/PAN-STAC against Escherichia coli and Staphylococcus aureus; (b) shaking tests of PCL/PAN-STAC with different mass gradients against E. coli; (c) bar graph of antibacterial rate under different dosages of PCL/PAN-STAC nanofibrous membrane.

The antibacterial tests of PCL/PAN-STAC nanofibrous membrane against E. coli were carried out further by the shaking method. Figure 8(b) shows that as the amount of the sample increased, the amount of colony gradually reduced. When the amount of the sample was 9 mg or more, there was almost no bacterial colony that could be observed in the agar medium, see Figure 8(c). The antibacterial rate was stabilized at about 96.20 ± 0.89% when the mass reached 9 mg, accurately reflecting effective antibacterial activity of PCL/PAN-STAC membrane. The antibacterial rate is a bit lower than that of the work of Gliscinska et al. (99.84%) who adopted the physical addition of QAS to the spinning solution,¹³ because both the hydrolysis degree of PAN and space obstruction limited the amount of the antibacterial QAS-STAC to react with PAN-H. However, these results were satisfactory because the hygroscopic PAN-STAC nanofiber component could quickly capture bacteria-carried moisture and inactivate the bacteria by contact and release killing, which is suitable for applications such as antibacterial air filtration membranes.

Conclusions

A polyelectrolyte with durable antibacterial property was prepared by the hydrolysis of PAN and electrostatic interactions with cationic QAS-STAC, comprehensively introducing STAC to the polymer chain, which is superior to physical mixing. FTIR, TGA and elemental analysis of the modified PAN-STAC confirmed the hydrolysis degree and incorporation of STAC. Release tests indicated that the electrostatic interaction between PAN-H and STAC was tight. For enhanced mechanical properties, PAN-STAC was successfully electrospun into a nanofibrous membrane by blending with PCL. Morphology characteristics showed that the PCL/PAN-STAC nanofibers had a smooth and uniform structure with small pore size. Tensile tests exhibited that the mechanical properties of PCL/PAN-STAC nanofibers were greatly strengthened. It was found from the disk diffusion tests that PCL/PAN-STAC had strong antibacterial activity against S. aureus. Furthermore, shake flask tests showed that the antibacterial rate against E. coli had reached 96.20 ± 0.89% stably after 9 mg. The results suggest that PCL/PAN-STAC nanofibrous membrane is a promising candidate for desirable applications such as protective filtration masks and wound dressing.

Footnotes

Data availability statement

Raw data for this study cannot be shared as this data forms part of an ongoing study.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partly supported by the Fundamental Research Funds for the Central Universities (grants 2232020A-08, 2232020D-14, 2232020D-15, 2232020G-01 and 2232019D3-11) and by the National Natural Science Foundation of China (grants 51773037, 51973027, 51803023, 52003044 and 61771123). This work was also supported by the Chang Jiang Youth Scholars Program of China and the Innovation Program of Shanghai Municipal Education Commission (both to Prof. Xiaohong Qin) and the Shanghai Sailing Program (Grants 18YF 1400400 and 19YF 1400700) -- the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201906SIC), Young Elite Scientists Sponsorship Program by CAST and the DHU Distinguished Young Professor Program.

ORCID iDs

Hongnan Zhang

Xiaohong Qin

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