Preparation and physicochemical studies on polymeric nanocomposites containing copper oxide nanoparticles

ABSTRACT The current work aims to modify carboxymethyl cellulose (CMC) and polyvinylpyrrolidone (PVP) with copper oxide nanoparticles (CuO NPs) to obtain new nanocomposites of CMC, PVP, and CuO NPs (CMC/PVP/CuO NPs) with distinguished properties. The interaction between the components of the nanocomposites was suggested and supported by using Gaussian 09W 07 Software and the average particle size was manually determined from TEM images using ImageJ software developed at the National Institutes of Health (NIH). The preparation methods were optimized, and the obtained nanocomposites were characterized with suitable techniques to explore their characteristics and to help expect or predict the suitable fields of applications.


Introduction
Polymeric nanocomposites can be considered as materials with different amounts of nanoparticles as fillers in the target polymers [1,2]. As the distribution homogeneity of such nano-materials is high within the nanocomposite, the interaction of the nano-materials with the polymer matrix would be high too [3,4]. In addition, nanocomposites show remarkable characteristics in many directions, such as electrical, optical, biocompatibility, and biodegradability, which are beneficial in industrial, medical, drug release, packaging, and agricultural applications [5,6]. Carboxymethyl cellulose (CMC) showed high viscosity in water and great compatibility with other water-soluble materials like glues, softeners, and resin, therefore, it attracts much attention in numerous industrial applications such as paper, pharmaceuticals, food, agriculture, barriers, and other [7][8][9]. Polyvinyl pyrrolidone (PVP) is a semi-crystalline vinyl polymer with a high glass transition temperature (Tg) due to the considerable closeness in pyrrolidone groups along the polymeric chains [10,11].
Copper oxides are known to be useful in several ways. They are a p-type semiconductor material with limited band gap due to their ease of preparation, nontoxic nature, and reasonably great electrical and optical properties [12][13][14][15][16][17]. Copper oxide nanoparticles are of extraordinary interest due to their potential applications in a wide variety of areas including electronic and optoelectronic gadgets, such as field impact transistors, electrochemical cells, gas sensors, and nano-devices for catalysis. Copper oxide nanoparticles are doped in different polymeric materials and investigated for applications such as gas-detecting materials, solar cells, and optical switches [18,19]. Thus, the present work aims to prepare nanocomposites of copper oxide nanoparticles with CMC and PVP polymers followed by characterization and investigation of the impact of copper oxide nanoparticles on the physical and chemical properties of the obtained nanocomposites.

Materials and techniques
Carboxymethylcellulose (CMC) (MW 41 kDa) was purchased from Alpha Chemika, India. Polyvinyl pyrrolidone (PVP) and other conventional chemicals were purchased from Sigma-Aldrich Co. and used without further purification. Laser ablation to prepare CuO NPs was performed by using PRII 8000 Continuum laser, Electro-optics, Inc. FTIR spectra were recorded on a ThermoScientific Nicolet Spectrometer iS10 FT-IR Spectrometer. UV/Vis Spectroscopy was recorded with the JASCO 630 UV/Vis Spectrometer, Japan. These spectra cover the range from 200 to 800 nm. X-Ray powder diffraction spectroscopy (XRPD) Philips PW150 was employed to record the powder diffraction patterns in the presence of Ni clarified using Cu Kα radiation (λ = 1.540 A°) at 40 kV, 30 mA, and a scanning range of 2θ = 18-80. Scanning electron microscopy (SEM) was used to study the surface and morphology of the nanomaterial. For this purpose, JEOL JSM-6510 LV SEM was used after coating the nanoparticles with a gold thin film before the SEM processing. Transmission electron microscopy (TEM) was employed to observe images and particle sizes of the obtained samples by using a JEOL JEM-2100 transmission electron microscope operating at 120 kV. The samples were prepared by depositing a suspension of fine sample powder onto a copper grid coated with a holey carbon foil and dried at ambient temperature. The average particle size of the generated CuO-NPs was determined manually by analyzing the TEM micrograph using Image Software developed at the National Institutes of Health (NIH) [20].

Preparation of copper oxide nanoparticles (CuO NPs) by laser ablation technique
Copper oxide nanoparticles (CuO NPs) were synthesized successfully by laser ablation technique in aqueous media [21,22]. A high purity copper target of dimension 4 × 4 × 2 (mm) was polished and sonicated before subjecting to a Nd:YAG nanosecond pulsed laser adopting a 1064 nm beam of width 6 nm and power 4 Watt with pulse frequency 10 Hz. Under the experimental conditions of laser ablation, Cu was allowed to be oxidized into CuO, and the copper target was immersed in a double-distilled water vessel, and the obtained CuO NPs were characterized.

Preparation of CMC/PVP/CuO NPs nanocomposites
A solution of CMC was prepared by dissolving 3 g CMC in 300 ml of distilled water, and a solution of PVP was prepared by dissolving 3 g PVP in 60 ml of distilled water. A polymer blend solution of CMC and PVP was prepared by mixing the two previously prepared solutions of polymers at a 1:1 ratio (CMC: PVP) while continuously stirring until a homogeneous viscous liquid was formed. Samples of the nanocomposite were prepared by adding different amounts of CuO NPs to the polymer mixture as listed in Table (1), cast on plastic Petri dishes, and dried in an oven at 50 °C for 24 h.

Characterization of the obtained CMC/PVP/ CuO NPs nanocomposites
The prepared samples of polymer nanocomposites were characterized by using XRD, FT-TR, and UV/Vis. Spectroscopy as well as Scanning and Transmittance Electron Microscope (SEM & TEM).

The anti-bacterial activity measurements
The antibacterial activity of the investigated nanocomposites was tested against Gram Positive bacteria (Staphylococcus aureus) and Gram-Negative bacteria (Escherichia coli). Each of the nanocomposites was dissolved in DMSO, and a solution of 1 mg/mL concentration was prepared separately. Paper discs of Whatman filter paper were prepared with standard size (5 cm) were cut and sterilized in an autoclave. The paper discs soaked in the tested solutions were placed aseptically in Petri dishes containing nutrient agar media (Agar 20 g + beef extract 3 g + peptone 5 g) seeded with Staphylococcus aureus and E. coli. Petri dishes were incubated at 36 °C and inhibition zones were recorded after 24 h of incubation. Each treatment was replicated three times. The antibacterial activity of a common standard antibiotic Amoxycillin was also recorded under the same conditions. The % activity index was calculated by Eq. 1 [23,24].
The XRD patterns of the investigated CMC/PVP/CuO NPs nanocomposites (NC2-NC6) show nearly parallel curves with a small deviation in both intensity (I) and full width at half maximum (FWHM) as summarized in Table 2. This indicates that the change in crystallinity of the nanocomposites is correlated with the amount of CuO-NPs included. The obtained data point to an increase in the amorphous nature of the nanocomposites with increasing CuO-NPs content. This may be attributed to the interstitial position of CuO-NPs within the polymeric matrix at random and/or specific positions that result from the distribution of the nanoparticles within the polymeric matrix. This will be reflected, of course, on most of the physical characteristics of the nanocomposites.

Transmission electron microscopy (TEM)
Images and particle sizes were observed by TEM using a JEOL JEM-2100 Transmission Electron Microscope operating at 120 kV. The samples were prepared by depositing a suspension of the fine sample powder onto a copper grid coated with a holey carbon foil and dried at ambient temperature. TEM image of the prepared CuO NPs shown in Figure (2) indicated more or less distorted spherical shape domains with a slight deviation in shape and particle size. The average particle size of the generated CuO NPs was calculated by averaging approx. 121 particles from TEM images using ImageJ software developed at the National Institutes of Health [30]. The data derived from the image analysis is summarized in Table 3. The average particle diameter (D) can be calculated from the average particle size after the approximation that the particles are quasi-spherical in shape according to Eq. 2.

Scanning electron microscopy (SEM)
The surface of the nanomaterials was observed by SEM (JEOL JSM-6510 LV) after coating the nanoparticles with a gold thin film before the SEM processing. SEM images of the selected nanocomposite samples as well as the parent blend were used to calculate the surface roughness parameters of the samples, which are summarized in Table 4. Three-dimensional images can be used to estimate the roughness parameters including average roughness (R a ), root mean square roughness (R q ), the maximum height of the roughness (R t ), maximum roughness valley depth (R v ), maximum roughness peak height (R p ), the average maximum height of the roughness (R tm ). Such measured parameters specify and support the suitability of the studied sample for specific applications Three samples of the nanocomposites of different concentrations of CuO NPs, namely NC1, NC2 and NC4, were selected to be micro-graphed as shown in Figure 3. SEM micrographs of the selected samples of different content of CuO showed that CuO NPs were more or less  higher in roughness for NC4 than that for NC2 and NC1, which is expected based on the content of CuO NPs in the samples.

FT-IR spectroscopy
Figure (4) shows the FTIR spectra of the PVP, CMC, and CMC/PVP blend. Figure (4a) shows PVP characteristic absorption bands of C=O stretching vibration at 1660 cm −1 [31][32][33]. Stretching and scissoring vibrations of the CH 2 -CH 2 group at 2956 and 1448 cm −1 and a broad absorption band of OH stretching vibrations at 3446 cm −1 are recognized [34,35]. Figure (4b) shows CMC characteristic bands for OH stretching and a small band for C-H stretching at 3430 cm −1 and 2909 cm −1 , respectively, while the strong band at 1605 cm −1 is related to the COO-group. Bands at 1420 and 1320 cm −1 are related to CH 2 scissoring and OH bending vibrations, respectively. The band at 1060 cm −1 is assigned to CH -O -CH 2 stretching [36,37]. FTIR spectra of the CMC/PVP blend (NC1) shown in Figure (

UV/Vis spectroscopy
The spectra associated with molecules that absorb energy within UV and/or visible regions to excite nelectrons from bonding or non-bonding states to higher anti-bonding orbitals can be used to estimate information about the electronic transitions of the studied material in different states. Other physical parameters can also be drawn from such spectral data, including the difference between higher and lower occupied molecular states (HUMO-LOMO) and the optical energy gap [40][41][42]. The absorption spectra of the blend sample shown in Figure (6-NC1) showed UV/Vis absorption maxima at 296 nm are related to the moieties in the polymer. Such absorption maxima is reduced to the range of 236-246 nm when CuO NPs are included within the blend matrix as shown in Figure (6) and reflect an interaction between CuO NPs and the blend matrix. The optical bandgap, E g , is determined from the absorbance spectra using Eq. 3.
where B is a constant related to the effective masses of charge carriers associated with valence and conduction bands, E g the bandgap energy, E = hν the photon energy, and n = 2 or 1/2 for direct and indirect transition, respectively. The intersection of the slope of (αhν) 2 vs. hν curve on the x-axis provides the bandgap energy of the sample. The Tauc plots of the system are displayed in Figure 6b. E value at α = 0for CMC/PVP blend showed intercepts at 3.89, and 4.38 eV for indirect and direct optical bandgaps, respectively. These values increased to the range of 4.85-5.08 eV and 5.23-5.40 eV for indirect and direct optical bandgaps, respectively, for NC2-NC6 nanocomposites. These findings are summarized in Table 5 and correlated with the conclusions derived from the UV/Vis spectra. In addition, the optical energy gap can be calculated using the wavelength λ edge in the intersection of the fundamental absorption edge, with the x-axis using the formula; E g = h c/λ edge . Such information can be approved using the data obtained from optimized 2D and 3D structural units in combination with HUMO-LUMO data shown in Figure (7). Values of the optical energy gap (E gap ) were calculated from the UV/Vis spectral data using the Mott and Davis formula [43,44] describing the photon energy (hυ) in terms of absorption coefficient (α). The HUMO and LUMO energy values were also calculated for the optimized structures and compared with those obtained from energy gap calculations.
The interaction between CMC and PVP in NC1 was investigated theoretically by Gaussian 09W 7.0 Software that is used for molecular calculations and to predict the most probable mode of interaction between two molecules or more. In the next context, there are different probable ways of interaction between the blend components through hydrogen bonding in different conformations. Comparing E gap calculated from UV spectroscopic analysis of the prepared blend with those predicted by the Gaussian Software, one can predict the most conformation obtained from hydrogen bonding. A suggested interaction was mentioned early for only one mode of interaction [45][46][47]. Currently, many possible interaction modes can be suggested, but only a few can be of predominant probability supported by the Gaussian Software calculations.

Biological activity
Copper oxide nanoparticles play a key role in preventing fungal, bacterial, and microbial attacks on yeast and molds. Copper oxide nanoparticles effectively participate against the growth of bacteria, such as Bacillus subtilis, Staphylococcus aureus, and Escherichia coli by using the diffusion method. E.coli and B. subtilis show susceptibility against copper and silver nanoparticles. Copper oxide nanoparticles also participate in this type of study by using MRI. In the current study, nanocomposites were tested against the previously mentioned microorganisms using amoxycillin as a reference standard; hence, the results represent the relative activity of the investigated materials. The results are summarized in Table 6.

Conclusion
New nanocomposites of carboxymethyl cellulose, polyvinylpyrrolidone, and copper oxide nanoparticles (CMC/PVP/ CuO NPs) with good properties were prepared, and their characteristics were explored to help predict suitable fields of applications. XRD indicated a change in crystallinity with an increase in the amorphous nature of the nanocomposites correlated with the doped amount of CuO-NPs and reflected on their physical characteristics. TEM imaging showed almost spherical shape domains with a slight deviation in shape and particle diameter that has been determined as 7.17 nm based on the averaging of the TEM image by using ImageJ Software developed at the National Institutes of Health (NIH). SEM imaging proved that the inclusion of CuO NPs induced surface roughness of the nanocomposites. The FTIR absorption spectra reflected interaction between CuO NPs and the polymeric matrix through interaction with C=O and other groups. The calculated optical bandgaps for the nanocomposites are in correlation with the conclusions derived from the UV/Vis spectra and supported by a theoretical prediction by using Gaussian 09W 7.0 Software. Finally, CuO NPs were proven to play an important role in preventing fungal, bacterial, and microbial attacks on yeast and molds, hence, CuO NPs effectively contribute against the growth of bacteria using amoxycillin as reference material.