Characterization and Antibacterial Properties of Centrifugally Spun Polyvinylpyrrolidone/Copper(II) Acetate Composite Fibers

Abstract

The demand for effective antibacterial materials is growing rapidly in today’s world. Both metallic and metal oxide nanoparticles have been widely used as antibacterial agents against various bacterial species due to their unique mechanisms of destroying bacterial membrane cells. The current study explores the antibacterial activity of centrifugally spun fibers prepared from copper acetate polyvinylpyrrolidone (PVP) ethanol precursor solutions against both Gram-negative and Gram-positive bacteria. During the synthesis of the composite fibers, the physical and chemical conditions were optimized. The structure and morphology of the PVP/Cu-Ac fibers were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and thermogravimetric analysis (TGA). The antibacterial activity of PVP/copper acetate fibers was tested against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. The PVP/Copper acetate fibers demonstrated bactericidal activity against both bacterial strains, making the PVP/copper acetate composite fibers an effective material for biomedical applications.

1. Introduction

Over the last few decades, there has been an increasing interest in new materials with high antibacterial activity, especially for species that are acquiring new antibiotic resistance. New antibacterial materials have been developed to treat bacteria that have developed new resistance to antibiotics, leaving healthcare with limited options [1]. Different materials, such as antibiotics, disinfectants, and antiseptics, have been used as antibacterial agents against Gram-negative and Gram-positive bacteria [2,3]. Common antibiotics have been shown to be less effective against Staphylococcus aureus (S. aureus); therefore, infection management is becoming a significant challenge for patients in hospitals [4]. Methicillin-resistant Staphylococcus aureus (MRSA) is an epidemic that threatens the community and healthcare systems [5]. Furthermore, Escherichia coli (E. coli) is becoming increasingly resistant to common antibiotics around the world [6]. This has led researchers to focus on developing new antibacterial materials that are not antibiotics to reduce bacterial infections in hospital settings. Nanotechnology is becoming increasingly utilized to produce nanostructured fibers for biomedical purposes, including metal-embedded nanofibers with antibacterial properties.

Multiple transition metals, such as Cu, Zn, Cd, PB, and Co, have been found to be effective antibacterial agents against both Gram-positive and Gram-negative bacteria due to their capacity to effectively cause damage to the cell membrane [7,8]. Copper, in particular, is abundant on Earth and both affordable and sustainable [9]. It is also an essential trace element in living organisms because it serves as a cofactor for key enzymes in the body [10]. Although small levels of copper are typically found in the human body, high blood copper concentrations are harmful and may cause health issues [11]. Copper and copper (II) oxide nanoparticles (NPs) have been found to be an effective antibacterial agent against both Gram-positive and Gram-negative bacteria [12]. Additionally, results reported in the literature have shown that copper is also effective against fungi, algae, and viruses in addition to bacteria [13].

However, researchers have not determined the exact antibacterial mechanism of these metals. Contact killing is thought to be caused by their positive charge, which binds to bacterial cell walls with a negative charge, causing membrane damage [14]. Metal ions can form reactive oxygen species (ROS), which induce cellular stress, affecting transcription and causing downregulation of many genes, particularly those involved in bacterial energy production, ultimately resulting in cellular death [7,14]. Yalcinkaya et al. assessed the effectiveness of several oxidative metals’ antibacterial activity and found that CuO > ZnO = ZnO/TiO₂ > AgNO₃ > ZrO₂ > TiO₂ > SrO₂ [15]. According to numerous studies, copper and its precursors exhibit the strongest antibacterial properties [15,16]. Various copper composite nanofibers have been tested against both Gram-positive and Gram-negative bacteria, demonstrating good antibacterial activity. The results showed that copper salts outperformed other copper precursors in terms of antibacterial activity due to their capacity to release Cu²⁺ instantly [17]. Copper (II) acetate, a copper salt, is a greener alternative that can be synthesized by reacting copper (II) oxide with acetic acid [18]. Previous research has proved that Cu is an effective antibacterial agent. The antibacterial activity of copper could be enhanced by embedding Cu NPs in polymer nanofibers. The unique properties of nanofibers, such as their high surface area to volume ratio, high porosity, and mechanical and thermal stability, make them promising candidates for tissue repair, wound dressing, and tissue engineering [19]. Drug delivery for medicinal therapies has been successfully accomplished by the application of nanofibers [19].

Nanofibers can be prepared by multiple methods, including electrospinning [20], phase separation, template synthesis, self-assembly, dry spinning [21], and centrifugal spinning [22,23,24,25,26,27]. Centrifugal spinning has been widely used to prepare polymer/ceramic fibers from conductive and nonconductive precursor solutions to produce fibers with high production rate/almost 500 faster than electrospinning, which significantly can reduce the production cost of fibers/nanofibers [22,23,24,25,26,27]. However, electrospinning is the most widely used method due to its simplicity and low cost [21]. Nanofibers are known for their flexibility; nanofiber mats facilitate easy loading and attachment, enabling control over the bulk mechanical properties [1]. The morphology of fibers can be controlled by monitoring the electrospinning parameters and the solution concentration [28]. Studies have indicated that thicker nanofibers are produced when the polymer concentration is higher and the relative humidity is low [29]. Thicker nanofibers support sustained release properties, which enhance antibacterial activity [29]. Additionally, Hamdan et al. noted that designing nanofibers with a small diameter (300–1000 nm), positively charged surfaces, rough surfaces with a high surface area, hydrophobic surfaces, and large pore diameters can promote better adhesion of nanofibers, thereby improving their antibacterial activity [19]. Generally, nanofibers are made of polymers using electrostatic force. Polyvinylpyrrolidone (PVP), a water-soluble polymer, has been demonstrated to be safe for use on human skin due to its temperature resistance, low toxicity, biological inertness, and non-irritant and non-sensitizing properties [4]. Due to these features, PVP has been utilized as a polymer in various biomedical applications, most notably in the preparation of nanofibers. Previous research has demonstrated that copper precursor NPs embedded in PVP nanofibers are effective against bacteria [30]. Different polymers/nanoparticle precursor solutions, including PVP, were used by our group to prepare centrifugally spun composite fibers as antibacterial agents against both Gram-positive and Gram-negative bacteria [22,30,31,32]. However, to the best of the author’s knowledge, the antibacterial activity of PVP/copper acetate (CuAc/PVP) composite fibers has not been investigated

The objectives of this research are to process and examine the structure and morphology of centrifugally spun CuAc/PVP fibers and assess their inhibition of Gram-positive and Gram-negative bacteria growth, specifically S. aureus and E. coli, which differ in their cell membrane and cell wall compositions. SEM, TGA, EDS, and XRD were used to characterize and assess the morphology of the fibers. CuAc/PVP composite fibers were successfully synthesized through centrifugal spinning and coated with different concentrations of copper acetate. CuAc/PVP composite fibers could be used as a wound dressing on damaged skin to prevent the growth of bacteria. These fibers mimic the skin’s extracellular matrix and provide a gentle application, even on sensitive skin.

2. Experimental

2.1. Materials

All chemicals were used as received without further purification or treatment. Polyvinylpyrrolidone (PVP) (96.8%) with an average molecular weight of 1,300,000 and copper (II) acetate (CuAc, Cu(CH₃COO)₂ 99.99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Absolute ethanol was used as the solvent for solution preparation.

2.2. Preparation of CuAc/PVP Composite Fibers

Stock solution of 15% (w/w) PVP was prepared by dissolving PVP in absolute ethanol under stirring. Four precursor solutions with different concentrations of copper (II) acetate (CuAc) 5%, 10%, 25% and 50% relative to the PVP content, were prepared by adding Copper (II) acetate to the PVP solution. The precursor solutions were magnetically stirred overnight at 650 rpm using a Thermo Scientific Cimarec+4 × 4 HP120 stirrer (Thermo Scientific, Waltham, MA, USA) to ensure complete dispersion and homogeneity. The homogeneous CuAc/PVP precursor solutions were then processed by centrifugal spinning using a Cyclone L-1000 M (Fiberio Technology Corporation, McAllen, TX, USA) to prepare the composite fibers. The centrifugal spinning setup system has a spinneret equipped with 30-gauge, half-inch, regular bevel needle (Beckett-Dickerson, Franklin Lakes, NJ, USA). The Cyclone L1000 M can operate at a spinneret rotational speed range from 500 to 22,000 RPM with 1 to 2000 s per spinning run. The rotational speed of the spinneret was checked by using a digital tachometer(Cole-Parmer, Vernon Hills, IL, USA) to ensure the rotational speed was correct. The CuAc/PVP solution was injected into the spinneret and sufficient centrifugal forces were applied to overcome the surface tension of the polymer droplets. The CuAc/PVP composite fibers were then collected, and the process was repeated 5 times to form a fibrous mat. Centrifugal spinning was performed at 8500 rpm for 2 min at room temperature with relative humidity maintained at 55% ± 5%. The spinneret was placed inside the center hole of the Cyclone L1000 M and tightened with a wrench. The spin axis of this spinneret extends centrally and vertically through the hole, perpendicular to the top plate. The fibers were collected on 8 vertical collectors with 13 cm between the columns, which were positioned at a distance of 16 cm from the center of the spinneret. Multiple spinning trials were performed to optimize the fiber formation. The collected fibers were wrapped in aluminum foil and dried in a vacuum oven(MTI Corporation, Richmond, CA, USA) for 24 h.

2.3. Characterization

The morphology of CuAc/PVP composite fibers was analyzed using a field-emission scanning electron microscope (FE-SEM; Sigma VP Carl Zeiss, White Planes, NY, USA). The average fiber diameter was calculated from SEM images taken at 500× magnification by averaging the measurement of 110 fibers using the image analysis software JMicroVision V.1.2.7, (University of Geneva, Geneva, Switzerland) and Origin Pro^® 2020 software (OriginLab Corporation, Northampton, MA, USA). Energy dispersive spectroscopy (EDS) (EDAX, Pleasanton, CA, USA) was performed to evaluate the elemental composition of the composite fibers and to confirm the presence of CuAc NPs at varying concentrations. Thermogravimetric analysis (TGA) of the composite fibers was performed using a Perkin Elmer TGA 7 (Perkin Elmer, Shelton CT, USA) under an air atmosphere. Fiber samples of 10 mg were placed into crucibles and heated from 23 to 700 °C at a heating rate of 5 °C/min. Fourier-transformed infrared spectroscopy (FTIR) data were collected using a Perkin Elmer Frontier FTIR spectrometer (Perkin Elmer, Shelton, CT, USA) equipped with a UATR accessory. Spectra were recorded at a resolution of 2 cm⁻¹ over a range of 650–4000 cm⁻¹. X-ray diffraction (XRD) data was collected using a Rigaku Miniflex 600 diffractometer (Rigaku Americas Corporation, Woodlands, TX, USA) on both fiber and powder samples. The XRD measurements were performed with a Cu source (Kα 1.540 Å). Data were collected over a 2θ range from 5 to 60° with a step of 0.05. The fitting of the XRD patterns was performed using the LeBail procedure in the Fullprof Suite (V 5.20, Grenoble, France) [33].

2.4. Antibacterial Testing

The antibacterial testing of the PVP and the CuAc/PVP composite fibers was performed against Gram-positive S. aureus and Gram-negative E. coli. The Kirby-Bauer disk diffusion method was used to evaluate the antibacterial properties of the composite fibers. Agar plates were prepared by inoculating 100 µL of bacterial suspension, which was uniformly spread using a sterile L-shaped glass rod. The bacterial solutions transferred to each Petri dish contained approximately 2 × 10⁷ CFU/mL. The different fibrous mats were cut into circular disks of 12 mm diameter using a punch to allow for consistent and clear visualization of antibacterial performance. The fiber disks were then placed on the agar plates using sterile tweezers. Finally, the plates were incubated at 36° C for 48 h. The antibacterial testing of the CuAc/PVP composite fibers was conducted against Gram-positive S. aureus and Gram-negative E. coli.

3. Results and Discussion

Centrifugally spun PVP and CuAc/PVP composite fibers were produced after several optimization trials involving variations in the process parameters such as rotational speed of the spinneret, spinning time, solution concentration, and relative humidity. For optimal fiber formation, a 15% (w/w) PVP solution was required; lower or higher polymer concentrations did not result in homogeneous fibrous mats. A spinneret rotational speed of 8500 rpm was determined to be optimal for generating fine fibers, considering a relative humidity of 55 ± 5%. Higher rotational speeds led to the formation of beaded fibers.

The SEM images shown in Figure 1 illustrate the morphology and average fiber diameters of composite fibers produced with varying concentrations of CuAc, ranging from 2.266 to 2.663 µm. The corresponding fiber diameter distributions, presented in histograms, Figure 1C,F,I,L,O, indicate that the average fiber diameter does not increase proportionally with the CuAc NPs concentration. The fibers showed an average diameter of 2.380 ± 0.80, 2.656 ± 0.66, 2.663 ± 0.53, 2.659 ± 0.58, and 2.266 ± 0.59 μm, for the PVP, 5% CuAc, 10% CuAc, 25% CuAc, and 50% CuAc, respectively. Notably, fibers containing 10% (w/w) showed the largest average diameter, while those with 50% (w/w) displayed the smallest. This trend may be influenced by variations in spinning parameters, such as rotational speed and relative humidity, which require further investigation.

Figure 2 and Figure 3 present the EDS mapping of the CuAc/PVP composite fibers containing 25 and 50% (w/w) CuAc, respectively. The presence of carbon, oxygen, nitrogen, and copper confirms that the polymer matrix and active material were homogenously distributed.

Table 1. Elemental composition based on EDS analysis collected from the SEM analysis.

	Sample
Element	5%	SE	10%	SE	25%	SE	50%	SE
C	64.07	0.57	62.44	3.51	65.01	1.52	67.90	2.51
N	18.26	0.54	17.74	1.09	15.41	2.23	23.67	1.59
O	15.64	0.19	17.70	2.43	15.57	0.69	14.32	1.37
Cu	1.91	0.15	2.10	0.29	5.25	1.29	5.10	0.34

(note: SE is the standard error based on 5 independent samples).

presents the elemental composition based on EDS analysis collected during SEM characterization. Table 1 shows that the sample with 25% (w/w) CuAc NPs had a measured copper content of 5.25%, while the 50% (w/w) CuAc NPs sample showed a slightly lower copper content of 5.10%. This slight discrepancy is probably due to surface effects and the distribution of the material. Since EDS is a surface-sensitive technique, copper may appear less abundant if it is more deeply embedded or dispersed unevenly, which can occur at higher concentrations as a result of aggregation or phase separation. Additionally, variations in fiber thickness and the inherent limitations of EDS quantification may contribute to this unexpected result. It can be determined from the EDS mapping that the CuAc NPs were very well distributed in the sample.

Table 2. Weight percentage composition of CuAc/PVP fibers.

Sample	CuAc, %	PVP, %
PVP	0.00	100.00
5% (w/w) CuAc/PVP	4.38	95.62
10% (w/w) CuAc/PVP	7.94	92.06
25% (w/w) CuAc/PVP	16.41	83.59
50% (w/w) CuAc/PVP	16.44	83.56

presents the results of the TGA analysis, which was performed under an air atmosphere, to determine the amount of copper acetate present in the sample. Based on the results, the 5% (w/w) CuAc sample contained approximately 4.4% (w/w) of CuAc, while the 10% (w/w) CuAc sample showed a content of approximately 8% (w/w) of CuAc. Interestingly, the 25 and 50% (w/w) CuAc samples both exhibit a similar copper content of 16.4% (w/w). This observation suggests that there may be a solubility or a loading limit for CuAc in the PVP matrix.

The thermal degradation of PVP fibers and PVP/CuAc composite fibers, containing CuAc (5, 10, 25, and 50 wt.%), was investigated using thermogravimetric analysis (TGA) under an air atmosphere. As shown in Figure 4A, the pristine PVP fibers exhibited a constant degradation during heating up to 600 °C, with almost a negligible residual mass. The TGA results also showed that the first initial weight loss occurred between 50 and 75 °C which was attributed to moisture/water evaporation. The thermal degradation of the composite fibers with different CuAc concentrations exhibited an additional weight loss around 200–320 °C which was attributed to the dehydration of copper acetate [31,32]. A significant mass loss was also observed between 320 and 460 °C for all composite fibers. Notably, samples with 25% and 50 wt.% of CuAc showed higher residual mass, indicating improved thermal stability due to inorganic residue retention [34]. The three peaks with most contribution to mass loss of PVP/CuAc composite fibers are shown in Figure 4B. whereas dehydration, decomposition of the anhydrous acetate, oxidation, and stabilization took place. During the dehydration process, the copper acetate loses its water to form anhydrous acetate in the range of 50–75 °C. Secondly, the decomposition of the anhydrous acetate involves the reduction in the copper (II) to metallic copper (Cu), deriving in volatile products such as carbon dioxide, acetone, and acetic acid. During oxidation (Figure 4A), the copper binds to oxygen resulting in Copper Oxide (CuO) and at the end of this process, the samples were stabilized with a constant residual mass.

Derivative thermogravimetric (DTG) analysis provided further insight into the decomposition kinetics of the PVP and PVP/CuAC fibers (Figure 4B). The PVP fibers exhibited a sharp, single degradation peak between 300 and 350 °C, characteristic of polymer matrix breakdown [32]. In contrast, the incorporation of CuAc introduced additional peaks between ~200–460 °C, corresponding to copper acetate decomposition and its interaction with the polymer [34]. At higher CuAc loadings (25% and 50 wt.% with respect to PVP concentration in the precursor solution), the main degradation peak of CuAc was observed at 200 °C [34], which was increased in intensity at increasing CuAc concentration, suggesting altered degradation pathways [35]. The increased residual mass above 475 °C in these samples confirmed the formation of thermally stable inorganic components, reflecting enhanced structural stability at elevated temperatures [36].

Figure 5 presents the FTIR spectra of pure PVP, pure CuAc, and CuAc/PVP composites.

Table 3. Main FTIR peak location and corresponding assignments for pure PVP, pure CuAc, and various CuAc/PVP composite fibers [37,38,39,40,41,42,43,44,45,46].

FTIR Peaks (cm−1)
CuAc	50% (w/w) CuAc/PVP	25% (w/w) CuAc/PVP	10% (w/w) CuAc/PVP	5% (w/w) CuAc/PVP	PVP	Assignment
3462						O-H
3358	3400	3389	3389	3389	3389	OH
3265	3265	3265	3266	3266		OH
2986	2959	2955	2955	2955	2955	Asymmetric CH2
2918	2918	2918	2918	2918	2918	Sym CH2
1650	1650	1643	1642	1643	1637	C=O/OH (from H2O)
1594						sym C-O
	1546	1550	1552	1556		Cu2+ = O and Cu2+-N pyridine ring
	1494	1492	1491	1494	1494	C-H, C-N
	1458	1461	1465	1463	1462	C-N
1435	1438	1438	1438	1438	1430	sym CH2
	1423	1423	1423			C-O sym
1417						Sym C-O
1354						CH3 sym
	1374	1373	1373	1371	1372	C-H
	1318	1317	1317	1313	1318	C-H
	1287	1287	1286	1282	1287	CH2 Wag, C-N
	1274	1274				CH2 Wag, C-N
	1223	1220	1225	1224		CH2 Wag, C-N
	1169	1169	1169	1165		CH2 twist of the pyrrole
1047						CH rocking
1031						CH3 Rocking
	1018	1016	1012	1012	1012	C-C, CH2 rock
	926	926	930	929	937	C-C
	896	891	891	890	888	Pyrrolidone ring breathing
	842	842	841	842	847	CH2
	732	732	734	734	736	CH2 rock
683						COO-Cu

shows the peak locations and their corresponding assignments of the polymer-based samples. All major peaks identified in the spectra are listed in Table 3. All samples exhibit the presence of water, as indicated by a broad, weak peak around 3400–3600 cm⁻¹. The fiber samples generally exhibit similar FTIR spectra to those of pure PVP, which is expected due to the high concentration of PVP in the samples. These peaks are located at 1637, 1494, 1462, 1430, 1372, 1318, 1287 cm⁻¹. In the presence of CuAc, some shifts in peak positions are observed. The peak at 1637 cm⁻¹ shifts slightly higher by approximately 8 cm⁻¹ in the CuAc/PVP samples. The peak located at 1430 cm⁻¹ is shifted to 1438 cm⁻¹, indicating a structural change due to the presence of copper. Additionally, a distinct peak appears at approximately 1550 cm⁻¹ in the CuAc/PVP samples, which is absent in the spectrum of pure PVP. This peak is attributed to Cu²⁺ interacting with O in the pyridine ring or to Cu-O vibration modes, as it is also present as a main peak in the spectrum of the CuAc dihydrate sample.

The diffraction patterns for CuAc dihydrate and the samples are shown in Figure 6 and Figure 7, and the LeBail fitting data are presented in

Table 4. Crystal structure parameters from LeBail fitting for pure CuAc and 50% (w/w) CuAc/PVP.

Sample	Space Group	a(Å)	b(Å)	c(Å)	α(°)	β(°)	γ(°)	χ2	Rp/Rwp
CuAclit	C 2/c	13.168	8.654	13.858	90.0	117.02	90.0	N/A [42]	N/A
CuAc	C/2c	13.151	9.595	13.852	90.0	116.9	90.0	1.3	8.0/6.8
Cu3(OH)4(CH3COO)2 lit	Pbca	20.974	7.207	13.122	90.0	90.0	90.0	N/A [39]	N/A
50% (w/w) CuAc/PVP	Pbca	20.984	7.214	12.914	90.0	90.0	90.0	3.26	6.5/8.6

. The diffraction pattern for PVP typically exhibits weak, diffuse peaks located around 11.25° and 21°, as summarized in

Table 5. Position of key diffraction peaks from various CuAc/PVP samples.

Sample	Peak 1 Position	Peak 2 Position	Peak Position 3
PVPlit	N/A	10.50	21.0 [33]
PVPlit	N/A	11.25	21.21 [38]
5% (w/w) CuAc/PVP	N/A	10.15	21.32
10% (w/w) CuAc/PVP	8.52	10.28	21.80
25% (w/w) CuAc/PVP	8.46	10.38	21.70
50% (w/w) CuAc/PVP	8.46	10.55	21.75

[47,48,49]. The diffraction patterns for the CuAc/PVP composites are shown in Figure 6A, and an expanded view focusing on the PVP peaks is provided in Figure 6B to facilitate comparison. In the present study, the lower-angle diffraction peak of PVP located at approximately 11.2° was observed to shift to lower angles and split into two features at approximately 8.5° and a second one at 10.5° in 2θ. In contrast, the weak diffraction peak at approximately 21° in 2θ was observed to shift to a slightly higher angle as the CuAc content increased. Teng et al. reported a similar change in the relative positions of the PVP diffraction patterns, where the peak at approximately 11.2° shifted to a lower diffraction angle in the presence of water. They observed that the lower peaks shifted to lower scattering angles, while the higher-angle halo shifted to higher scattering angles [47]. Similarly, Timaeva et al. observed a shift and splitting of the diffuse peak in the PVP located at approximately 11°, which was observed to shift down to 9.5° in 2θ, attributed to the presence of half-bound water molecules [48]. Kahn et al. reported diffraction peaks in PVP powder and fibers at 5 and 12°, which were suggested to reflect the degree of crystallinity in PVP [49].

Additional peaks observed in the diffraction patterns located at 6.5, 15.10, 16.0, 18.54, 27.23, and 34.85° correspond to the 200, 002, 400, 113/020, and 421/322 planes, respectively, of the Cu₃(OH)₄(CH₃COO)₂ phase [50]. Among all samples, only the 50% (w/w) CuAc composite sample showed additional peaks beyond the 200-reflection associated with the copper-containing phase. In addition, all the CuAc-PVP composite samples showed a diffraction peak at approximately 6.5° in 2θ, which has been shown to be present in the Cu₃(OH)₄(CH₃COO)₂ material. The LeBail fitting of the diffraction pattern showed a reduced χ² of 3.26; in general, a χ² value below 5 is considered a good fit [51,52]. In addition, the Rp/Rwp factors are below 10 which also indicates a good fitting for both the CuAc and the 50% CuAC-PVP composite sample. The CuAc in the samples was identified as having a C2/c crystal lattice, which is representative of the dehydrated form of CuAc hydrate [50]. A summary of the XRD fitting results is shown in Table 4.

Antibacterial Analysis

The antibacterial activity of pure PVP fibers and CuAc/PVP composite fibers containing 5, 10, 25, and 50% (w/w) CuAc were tested against S. aureus and E. coli. For the test, membranes with a diameter of 12 mm were prepared for each sample and used in the Kirby-Bauer disk diffusion assay. The fiber mats had an average thickness of 430 ± 90 μm, 5% 400 ± 20 μm, 10% 280 ± 40 μm, 25% 295 ± 20 μm, and 50% 260 ± 70 μm, for the PVP, 5% CuAc/PVP, 10% CuAc/PVP, 25% CuAc/PVP, and 50% CuAc/PVP, respectively. Figure 8 and 9 show the inhibition zones formed by each sample against E. coli and S. aureus after incubation at 36 °C for 48 h.

As shown in Figure 8 and Figure 9, inhibition growth zones were observed in all CuAc/PVP composite fibers tested, including those with 5, 10, 15, 25, and 50% (w/w) concentrations against E. coli and S. aureus. The pristine PVP fibers did not show any antibacteria effect against E. coli and S. aureus. The average diameter of the inhibition zone for each sample tested against E. coli was measured using Image-J. The CuAc/PVP fiber membranes demonstrated a maximum inhibition zone of 15 mm out of a total membrane diameter of 12 mm, indicating that minimal bacterial growth occurred within the fibrous region, as can be observed in the images. A trend was observed in which higher CuAc concentrations correspond to larger inhibition zones. The inhibition zone diameters for 5, 10, 25, and 50% (w/w) CuAc samples against E. coli were 12.9, 10.6, 14.1, and 15.5 mm, respectively. The average maximum inhibition zone diameter against S. aureus was observed to be 15.9 mm compared to the 12 mm composite disk used. The corresponding individual measurements were 10.3, 11.6, 14.5, and 15.9 mm for 5, 10, 25, and 50% (w/w) CuAc samples, respectively.

As shown in Figure 10, the fibrous PVP membranes incorporated with CuAc exhibited a slightly higher pronounced antibacterial effect against S. aureus compared to E. coli at lower concentration. However, at higher concentration, the toxicity effects were observed to be very similar. Whereas the PVP was not observed to be toxic to either the S. aureus or E. coli. Among the samples tested, the membrane containing CuAc demonstrated a small disparity in the inhibition zone diameters between the Gram-negative and Gram-positive bacterial strains at lower concentrations. This observation suggests the Gram-positive bacteria is more susceptible to disruption by CuAc. The antimicrobial mechanism is attributed to the ability of CuAc to interfere with bacterial metabolic processes, potentially through the inactivation or destruction of cellular components through the generation of reactive oxygen species, ultimately leading to cell death.

Table 6. Comparison of data from literature with the results reported in the present work.

Fibers/Bacteria	Loading (wt%)	Fiber Dia. (μm)	Inhibition Zone Diameter (cm)	Refs.
PEO/Cu in E. coli	0–35	0.2–0.25	1.64 out of 2.26	[32]
PEO/Cu in B. cereus	0–35	0.2–0.25	1.68 out of 2.26	[32]
PVP/Cu in E. coli	0–35	4.9–5.5	1.56 out of 2.26	[32]
PVP/Cu in B. cereus	0–35	4.9–5.5	1.53 out of 2.26	[32]
PVB/CuO in E. coli	0–10	>200	100% inhibition after 1–4 h	[53]
PU/CuO in E. coli	0–10	>200	70% inhibition after 1–4 h	[53]
PAN/CuO in E. coli	0–100	0.14–0.18	inhibition not observed	[54]
PAN/CuO in B. subtilis	0–100	0.14–0.18	inhibition not observed)	[54]
PVP/CuAc in E. coli	0–50	2.26–2.38	1.327 out of 1.2	This work
PVP/CuAc in B. cereus	0–50	2.26–2.38	1.3 out of 1.2	This work

shows a comparison by the results reported in the current manuscript with those already published on similar polymer/composite fiber systems studied as antibacterial agents against Gram-negative and Gram-positive bacteria. The current results show similar trends to those previously reported in the literature using Cu and CuO/polymer composite fibers [32,53,54].

4. Conclusions

CuAc/PVP composite fibers were successfully synthesized using centrifugal spinning of a precursor solution. The synthesized composite fibers were characterized using SEM, EDS, and XRD, which confirmed the presence of both CuAc and PVP in the CuAc/PVP composite fibers. The fibers showed an average diameter of 2.380 ± 0.80, 2.656 ± 0.66, 2.663 ± 0.53, 2.659 ± 0.58, and 2.266 ± 0.59 μm for the PVP, 5% CuAc, 10% CuAc, 25% CuAc, and 50% CuAc, respectively. The FTIR results showed the presence of both materials (CuAc and PVP) in the composite fibers. In addition, the XRD analysis results showed that the CuAc/PVP composite fiber were a mixture of CuAc and PVP. The CuAc was present as in the form of Cu₃(OH)₄(CH₃COO)₂, and the PVP showed expansion of the lattice as the diffraction peak shifted to lower angles more than likely due to the presence of water. The PVP fibers by themselves did not show any significant antibacterial activity against S. aureus and E. coli bacteria. However, the addition of CuAc to the PVP matrix showed antibacterial activity against both S. aureus and E. coli. The results indicated that CuAc/PVP fibers became more effective with increasing concentration of CuAc. Furthermore, stronger bactericidal effects against E. coli were observed in comparison with the S. aureus. However, at a CuAc concentration in the Fibers, 50% the inhibition zone was the same. These fibers may serve as a potential alternative to conventional antibacterial materials such as wound dressing and skin treatment. PVP/CuAc composite fibers could be a potential treatment based on results from studies using the current of the bacterial strains.