Antibacterial Properties of PMMA Functionalized with CuFe 2 O 4 /Cu 2 O/CuO Nanoparticles

: We have prepared a composite thin coating by incorporation of CuFe 2 O 4 /Cu 2 O/CuO nanoparticles in polymethyl methacrylate (PMMA) matrix by using the solution casting method. The electrical explosion of two twisted wires (EETW) was used to obtain multicomponent CuFe 2 O 4 /Cu 2 O/CuO nanoparticles with an average particle size of 20–70 nm. The microscopic studies showed that the nanoparticles in the composite coatings are evenly distributed. However, nanoparticles are strongly agglomerated as the powder concentration in the coating increases to 5 wt.% and 10 wt.%, as the size of particle agglomerates increases to 50 and 100 µ m, respectively. Therefore, nanoparticles were pre-treated with ultrasound when introduced into the PMMA matrix. The thermal stability of the composite coating does not change with the introduction of CuFe 2 O 4 /Cu 2 O/CuO nanoparticles in the amount of 5 wt.%. The inclusion of nanoparticles in the PMMA matrix signiﬁcantly enhances its antibacterial activity. The addition of 5 wt.% nanoparticles inhibited the growth of E. coli by 100% and the growth of MRSA by 99.94% compared to pure PMMA already after 3 h of exposure of bacteria on the surface of the composites. This research provides an easy-to-manufacture and cost-efﬁcient method for producing a CuFe 2 O 4 /Cu 2 O/CuO/PMMA composite coating with a broad application as an antibacterial material.


Introduction
Currently, polymethyl methacrylate (PMMA) is one of the most common polymer materials for biomedical applications [1]. The use of PMMA in medicine is primarily due to its good biocompatibility, physical and chemical stability, and affordability [2]. However, the polymer exhibits some drawbacks, such as a tendency to fatigue failure, low bending strength, low thermal stability, and the lack of the activity against microbial biofilm formation [3]. The biofouling tendency is related to the internal porosity of the PMMA base, which contributes to biofilm formation [4]. Although PMMA products have already shown promise in animal toxicity experiments [5], long-term use of such products is limited due to the occurrence of bacterial contamination [6]. In this regard, great attention is paid to the antibacterial and antifouling activity of PMMA products.
Several basic approaches (strategies) are commonly adopted to impart antibacterial properties to PMMA products, such as: the use of copolymers [7,8], the inclusion of antibiotics [6,9] or nanoparticles of both metals [10][11][12], as well as synthetic [13] and natural [14] polymers or ceramics [15,16]. Thus, the introduction of cationic polyethyleneimine led to a chemical interaction with the polar acid groups of the bacterial wall [3] but caused the destruction of the polymeric matrix. The approach based on the introduction of antibacterial nanoparticles into the monomer prior to synthesis has a number of implications, ranging from changes in the nanoparticle sedimentation behavior and disperse composition to the influence on the polymer properties. Thus, antibacterial PMMA/silver nanoparticle (37-47 nm) nanocomposites have been obtained using an in situ radical polymerization technique [16]. However, the presence of Ag nanoparticles led to a decrease in the polymerization reaction rate and an increase in the average molecular weight of the resulting polymer. In this regard, the polymer surface modification is most preferable [17]. Currently, the search for non-toxic polymer modifiers is under way. The most promising direction is the use of photocatalysts, as they do not cause adverse biological and environmental effects [18]. However, due to the wide bandgap, the effective photocatalysts such as TiO 2 and ZnO show activity only in the UV region of the light spectrum [19].
A promising but insufficiently studied class of photocatalytic antibacterial agents are narrow-gap semiconductors-ferrite oxide compounds containing Fe 3+ and another metal cation. Ferrites are characterized by absorption in the visible region, tunable optoelectronic properties, and high chemical and photochemical stability [20]. Recently, photocatalysts based on ferrite heterojunctions have been described. The ZrO 2 /ZnFe 2 O 4 nanocomposite provided a high degree of wastewater treatment under the sun irradiation due to the effective suppression of the charge carriers' recombination at the heterojunction [21]. The same mechanism explained the high photocatalytic activity of the BiVO 4 /ZnFe 2 O 4 composite [22]. Cobalt and zinc ferrites exhibited antibacterial activity under visible light irradiation due to photocatalytic generation of electron-hole pairs and reactive oxygen species (ROS), with the two-phase system with CoFe 2 O 4 /ZnFe 2 O 4 contact being the most active [23]. Ce 3+ -doped spinel CuFe 2 O 4 nanoparticles exhibited antibacterial activity against Klebsiella pneumoniae and Staphylococcus aureus [24].
A non-toxic and promising reagent to generate ROS is copper ferrite CuFe 2 O 4 , which has attracted considerable research attention due to its thermal resistance and unique magnetic, electrical, magneto-optical, gas-sensitive, electrochemical, and catalytic properties. Compared with other catalysts, copper ferrites are the most environmentally friendly [25] and can find applications in medicine for the treatment of breast cancer [26]. The use of composite CuO/CuFe 2 O 4 nanoparticles with a p-n heterojunction can improve the efficiency of the photocatalytic water decomposition reaction. The synthesis and studies of such systems have been actively pursued in recent years. CuO/CuFe 2 O 4 nanocomposites exhibit conducting or semiconducting properties depending on the temperature, which has potential applications in various electronic devices, including sensors [27]. Thus, CuO/CuFe 2 O 4 core-shell heterostructures have been used as sensors for hydrogen sulfide detection [28], but CuO/CuFe 2 O 4 thin films as sensors for CO 2 [29], H 2 , and C 2 H 5 OH detection [30]. The basic methods of ferrite synthesis are the sol-gel method, the co-precipitation method, the hydrothermal method, and solid-state reactions. Additionally, one can mention the combustion synthesis method used, for example, to produce copper ferrites, which consists in burning the solutions with L-arginine [31] and citric acid [32] as fuel. In this case, the formation of nanoparticles with a p-n heterojunction occurs at high temperatures in the vapor phase but is complicated by a number of chemical reactions with the formation of byproducts that contaminate the target product.
Considering the above-mentioned developments and their limitations, in the present work the electrical explosion of two twisted wires (EETW) was used to obtain multicomponent CuFe 2 O 4 /Cu 2 O/CuO nanoparticles. During EETW in an oxygen-containing atmosphere of iron wire together with copper, one of the phases formed is the corresponding ferrite with a spinel structure. The excess of the second metal forms the copper oxide phase (I, II). The ferrite and oxide phases are evenly distributed in the particle volume and separated by interphase boundaries, forming a p-n heterojunction. Moreover, the complex microstructure of CuFe 2 O 4 /Cu 2 O/CuO nanoparticles has several advantages, such as: (1) CuFe 2 O 4 electrostatic interaction and complexing with phosphate in the cell wall [33], (2) copper ions, Cu 2+ , released from the samples would change the permeability of the bacterial cell wall, prevent the cell from absorbing nutrients, and ultimately affect the cell growth and vitality [34], and (3) iron (III) and copper ions can play a synergistic role in the antibacterial activity of nanoparticles [35]. Furthermore, the obtained CuFe 2 O 4 /Cu 2 O/CuO nanoparticles were encapsulated in PMMA film to afford it high antibacterial activity. To our knowledge, there have been no works on the application of CuFe 2 O 4 /Cu 2 O/CuO nanoparticles as antibacterial agents for PMMA modification.

Synthesis of CuFe 2 O 4 /Cu 2 O/CuO Nanoparticles
CuFe 2 O 4 /Cu 2 O/CuO nanoparticles (NPs) were obtained by electrical explosion of twisted copper and iron wires. A schematic diagram of the setup for producing NPs has been previously reported in [36]. Electrical explosion occurs when a high-current pulse generated by the discharge of an electric capacitor passes through the metal wire. Electrical explosion of Cu and Fe wires was carried out in a gas mixture containing 80 vol.% of argon and 20 vol.% of oxygen, with the Fe wire diameter being 0.30 mm, a Cu wire diameter of 0.20 mm, and a twisted wires' length of 90 mm. The electrical capacitance of the capacitor bank was 3.2 µF and the charge voltage was 28 kV. Under these conditions, the largest amount of copper ferrite is formed in the particles. The Cu:Fe atomic ratio in NPs was 50:50.

Synthesis of CuFe 2 O 4 /Cu 2 O/CuO/PMMA Composite Coating
Nanocomposite films with different contents of NPs (0.5 wt.%, 2 wt.%, 5 wt.%, and 10 wt.%) in the PMMA matrix were prepared by a solution casting method. In a typical procedure, 2.00 g of PMMA was dissolved in 10 mL of acetone at room temperature while stirring until a clear solution was obtained. Then, 0.01, 0.04, 0.1, or 0.2 g of CuFe 2 O 4 /Cu 2 O/CuO powder was added to the PMMA solution while stirring. The NPs-PMMA mixture was sonicated for 20 min to disperse and distribute NPs uniformly in the polymer solution. The final suspension was placed on a slide and left to dry at room temperature for 1 day for the physicochemical measurements and antimicrobial properties' determination.

Characterization
The morphology of NPs was characterized using scanning (SEM; LEO EVO 50, Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM; JEM 2100, JEOL, Tokyo, Japan). The elemental mapping in the NPs was carried out using an energy dispersive spectrometer (X-Max, Oxford Instruments, Abingdon, UK) integrated with a microscope. The phase composition of the NPs was determined using an XRD-6000 X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) using CuKα radiation (λ = 1.514 Å). Quantitative phase analysis of the NPs was carried out using the XPowder software package and the PDF2 database of crystallographic structures. CuFe 2 O 4 /Cu 2 O/CuO/PMMA composite coatings were characterized using the AXIOVERT-200MAT optical microscope (Zeiss, Oberkochen, Germany). FT-IR absorption spectra were obtained using a Nicolet 5700 spectrometer (Thermo Electron, Waltham, MA, USA) in the 4000-400 cm −1 wavelength range. The thermal properties of coatings were studied in argon atmosphere using a TG-DSC instrument (STA449F3 Jupiter, Netzsch, Selb, Germany), a temperature range of 25-600 • C, and a heating rate of 10 • C/min. CuFe 2 O 4 /Cu 2 O/CuO/PMMA film surface wetting was measured at room temperature using a DSA100 optical goniometer (Krüss, Hamburg, Germany). A 5 µL droplet of deionized water was deposited on the surface of the film and the average value of the droplet contact angles in 5 different positions was obtained.

Antibacterial Activity of CuFe 2 O 4 /Cu 2 O/CuO/PMMA Composite Coatings
The antibacterial activities of nanocomposites were estimated using MRSA ATCC 43300 and E. coli ATCC 25922 bacterial strains. The antibacterial activity of the samples was evaluated by the "drop contamination" method according to ISO 22196:2011, "Measurement of antibacterial activity on plastics and other non-porous surfaces". For this purpose, 5 × 5 cm composite plates were prepared. A volume of 400 µL of the bacterial suspension with a concentration of 10 5 CFU/mL was deposited on the surface of the samples and spread evenly over the surface using a spatula. The samples were covered with sterile polyethylene film and incubated for 1, 3, and 6 h at 37 • C and 100% relative humidity. After incubation, the samples were placed in containers with a neutralizer, a 0.9 wt.% NaCl solution, and thoroughly stirred. The solution was plated on Müller-Hinton agar (Thermo Fisher Scientific, Waltham, MA, USA) for MRSA and Endo agar (NICF, St. Petersburg, Russia). The number of grown colonies was evaluated after 24 h of incubation at 37 • C. For each sample, an antibacterial assay was performed with three independent replicates (n = 3).

Preparation and Physicochemical Characteristics of CuFe 2 O 4 /Cu 2 O/CuO NPs
The following phases can form: CuO, Cu 2 O, delafossite (CuFeO 2 ), spinel (CuFe 2 O 4 ), during EETW of the two twisted iron and copper wires, based on the Cu-Fe-O phase diagram [37].  TEM and SEM images of NPs are presented in Figure 2. It was clearly seen that NPs are mostly spherical in shape.
According to SEM data (Figure 2a), large agglomerates up to 1 µm in size are found in all samples. The NPs have a close to spherical shape (Figure 2b), and the particle size ranges from 10 to 100 nm. The smaller NPs are somewhat faceted. Detailed elemental mapping studies showed that Cu, Fe, and O are evenly distributed in the NPs. One of the main problems in the introduction of nanoparticles into polymeric matrices is their agglomeration [40]. The high surface area of nanoparticles and the strong attractive van der Waals interaction between them result in the agglomeration. In various processes, nanoparticle agglomerates behave as a separate large particle [41]. This is determined by the fact that nanoparticle agglomerates have a much smaller contact surface area with the external environment relative to individual nanoparticles, which reduces the advantages of nanoparticle applications. The full potential of nanoparticles can be achieved only by breaking down nanoparticle aggregates, preferably to primary particles. Silver nanoparticles used in the present study formed agglomerates, which was confirmed by the sedimentation analysis data. The particle size study by the density gradient sedimentation method showed a bimodal distribution in size regardless of the phase composition, with the peak maxima being at 30-50 and 100-200 nm. Considering the results of the TEM analysis, the first peak appears to show the distribution of NPs, while the second results from both nanoparticles and NP agglomerates. The peak in the distribution is shifted toward particles of 20-70 nm in size. The specific surface area of the sample was 12.9 ± 1.2 m 2 /g. The agglomeration of nanoparticles leads to a decrease in antibacterial activity, as shown in [42]. However, it was not possible to fully break down the nanoparticle agglomerates by ultrasonic treatment. A further increase in the dispersion time leads to local heating and sintering of nanoparticles.
The microscopic studies showed that the CuFe 2 O 4 /Cu 2 O/CuO NPs in the composite coatings are evenly distributed. However, they are strongly agglomerated, and the size of NP agglomerates increases. Therefore, NP suspension was treated with ultrasound when introduced into the PMMA matrix. The 0.5 wt.% and 2 wt.% NP composite samples included both particles and particle agglomerates up to 20 µm in size, uniformly distributed in the PMMA matrix. The 5 and 10 wt.% NP composite samples also show a uniform distribution of the particle agglomerates in the PMMA matrix, but the size of the agglomerates increases significantly to 50 µm and over 100 µm, respectively. Figure 4 displays the FT-IR spectra of NPs and composites doped with 5 wt.% of NPs.   Figure 4a shows the FT-IR spectrum of NPs. The band at 590 cm −1 is related to Fe 3+ -O 2− stretching of tetrahedral complexes [43]. The bands at 3430 and 1640 cm −1 correspond to O-H stretching of adsorbed H 2 O. The FT-IR spectrum of PMMA (Figure 4b) contains bands located at 2750-3100 cm −1 , which are attributed to asymmetric and symmetric stretching of the CH 3 group. The band at 1730 cm −1 is related to the C=O stretching of carboxyl PMMA [44]. The bands at 1485, 1447, and 1380 cm −1 are attributed to CH 3 deformation. Bends located at 1280-1150 cm −1 are assigned to the stretching or the frequency of the ester group (C-O-C). Absorption peaks at 985 and 960 cm −1 correspond to C-C symmetric stretching. The bands at 831 and 750 cm −1 are related to -CH 2 asymmetric rocking and -CH 2 rocking [45][46][47]. The FT-IR spectrum of the composite (Figure 4c) contains bands attributed to PMMA and NPs and did not show any new peaks. This indicates that PMMA and NPs are only physically blended.
The authors of [48] noted hydrophobicity to be a dominant factor in influencing adhesion on surfaces. Bacteria with hydrophilic properties (E. coli, MRSA) prefer to adhere to hydrophilic surfaces. For example, S. aureus (including MRSA) has a hydrophilic character, which favors the adhesion of this bacterium to metal alloys over ultra-high molecular weight polyethylene that present a more hydrophobic surface. The hydrophilicity of the coatings was evaluated by measuring the water contact angle ( Figure 5). The water contact angle of the PMMA coating was 69.66 • ± 1.1 • . Adding NPs at 0.5, 1, and 5 wt.% increased the contact angle, indicating reduced PMMA surface wettability. As the content of the NPs in the composite increased to 5 wt.%, the contact angles of the deionized water increased to 86.33 • + 5.75 • . This can be attributed to an increase in surface tension by the hydrophobic CuFe 2 O 4 /Cu 2 O/CuO nanoparticles. When 10 wt.% of NPs was added in PMMA, the contact angle for the composite was 66.5 • ± 2.8 • , significantly lower than that for pure PMMA. This decrease in the contact angle reflects a reduction in surface hydrophobicity, which could be caused by a reduction in the total surface area of the nanoparticles due to their agglomeration.
To evaluate the thermal stability of the CuFe 2 O 4 /Cu 2 O/CuO/PMMA composite coating, the thermogravimetric measurements of PMMA and the composite coatings containing 0.5 wt.% to 5 wt.% NPs were performed.
Respective plots of the samples are shown in Figure 6. As seen in Figure 6, the weight loss occurs in two stages. The first stage is observed in the temperature range of 150-190 • C, with a weight loss of 3.3%-7.3%. This is due to the destruction of part of the polymer. The second stage of weight loss occurs between 360 and 390 • C and amounts to 85.4%-96.8%, which is associated with the thermal degradation of the polymer. The kinetics of the second stage of mass loss is almost the same for all samples, which indicates that the destruction mechanism and thermal stability of the composites is not changed significantly as compared to the PMMA polymer. TG curves can be represented as a set of sigmoid curves, each describing a different stage of the chemical process. To evaluate TG thermal parameters, a fitting model of the sample mass as a function of temperature can be represented in the form [49]: where M(T) is the sample mass at temperature T, N is the number of sigmoid components, i.e., the number of process stages, T is the temperature, M i is the magnitude of the mass sample reacted at each stage, b i is the rate parameter of mass change, θ i is the temperature of the maximum rate of mass change at each stage, and τ i is the asymmetry parameter of each stage. As the value of τ increases, the slope of the curve decreases, which means that the rate of the process decreases. Fitted thermal parameters are listed in Table 1. As can be concluded from the presented data, the destruction of PMMA occurs in two stages. The effect of the introduced ferrite nanoparticles on the process of PMMA destruction in the first stage is evident only at a ferrite nanoparticle content of more than 2 wt.%, which results from the increase in the temperature of the maximum destruction rate and the increase in the mass fraction of the polymer subjected to destruction. Additionally, with the increasing ferrite nanoparticle content in the composite, the asymmetry parameter slightly increases, which may be related to the deceleration of PMMA nanoparticles at a higher temperature with the formation of volatile products. In the second stage of the PMMA destruction process, the effect of injected ferrite nanoparticles is less noticeable. The temperature of the maximum destruction rate is almost constant, but the mass fraction of the polymer subjected to destruction is lower. The overall rate of the degradation process decreases with the increasing ferrite nanoparticle content in the composite, and from the increase in the asymmetry parameter it can be concluded that the process of PMMA destruction with the formation of nonvolatile products (carbonization) dominates over the process of PMMA destruction with the formation of volatile products. Thus, it can be concluded that the first stage of composite destruction is dominated by PMMA destruction with the formation of volatile products, while the second stage of the destruction is dominated by the process with the formation of nonvolatile products. The residual amount of degradation products of composites increases from 0.4% to 7% with the increasing nanoparticle content in the composite. The increased content of degradation products for the 2 wt. % and 5 wt.% NP composite samples, respectively, 6% and 7%, is probably due to the fact that polymer carbonization occurs in the presence of NPs. Thus, the thermal stability of the composite coating does not change with the introduction of NPs in the amount from 0.5 wt.% to 5 wt.%.   As can be seen, the inclusion of nanoparticles in the PMMA matrix significantly enhances its antibacterial activity, especially with the addition of 5 wt.% NPs or more ( Table 2). The addition of 5 wt.% NPs inhibited the growth of E. coli by 100% and the growth of MRSA by 99.94% already after 3 h of exposure of bacteria on the surface of the composites. 10 wt.% 5.17 × 10 2 ± 0.12 × 10 2 0 ± 0 0 ± 0 99.5 100 100 * The antibacterial activity (R %) was expressed as the % reduction of the number of cells, which was calculated using the formula: R = (C 0 − C)/C 0 , where C 0 is the number of viable bacteria recovered from the control (10 5 CFU/mL) and C is the number of viable bacteria in CFU/mL recovered from experimental samples.

Antibacterial Activity of Composites
PMMA used as a control had practically no effect on the viability of the microorganisms studied. These results are consistent with earlier experiments on the modification of PMMA with silver nanoparticles [46], copper oxide [50], and titania nanoparticles [51]. As shown in [52], the modification of cationic polyethyleneimine/anionic pectin hydrogel with polydopamine Cu nanoparticles reduced survival rates for S. aureus from 80.2% to 34.6%, and for E. coli from 71.1% to 22.4%. Antibacterial activity of CuFe 2 O 4 /Cu 2 O/CuO/PMMA composites against E. coli is higher than that against S. aureus (Figure 7b). It can be ascribed to the differences in the membrane structure and composition of these bacteria. E. coli has a complicated cell wall with internal and thin external membranes composed of peptidoglycan, lipopolysaccharides, lipoproteins, and phospholipids molecules. The S. aureus cell wall has a single thick membrane of peptidoglycan. Therefore, E. coli is more impressible for penetrating antibacterial agents [53]. The antibacterial effect of the CuFe 2 O 4 /Cu 2 O/CuO/PMMA composite may be described using three mechanisms: (1) effective interaction of the nanoparticle with the bacterial cell wall due to its high surface-to-volume ratio, (2) release of copper ions, which had a destructive effect on the structures of bacteria cells' membranes, and (3) their capability to produce reactive oxygen species (ROS). According to [54], the main mechanism of the ferrite nanoparticle action is based on ROS generation. ROS generation causes lipid peroxidation, destruction or modification of proteins, violation of the integrity of the bacterial membrane, destruction of enzymes, and RNA damage, leading to cell death.

Conclusions
In this study, we developed novel CuFe 2 O 4 /Cu 2 O/CuO/polymethyl methacrylate PMMA coatings and analyzed their antibacterial performance. The electrical explosion of two twisted wires was used to obtain multicomponent CuFe 2 O 4 /Cu 2 O/CuO nanoparticles. The microscopic studies showed that the nanoparticles in the composite coatings were evenly distributed. The inclusion of nanoparticles in the PMMA matrix significantly enhanced its antibacterial activity. The addition of 5 wt.% nanoparticles inhibited the growth of E. coli by 100% and the growth of MRSA by 99.94% already after 3 h of exposure of bacteria on the surface of the composites. Thus, the active antibacterial composite thin coatings can control bacterial biofilm formation, which could resist the attachment and growth of fouling microorganisms.