Synthesis, Characterization, and Antimicrobial Activity of CoO Nanoparticles from a Co (II) Complex Derived from Polyvinyl Alcohol and Aminobenzoic Acid Derivative

Cobalt oxide nanoparticles (CoO NPs) were synthesized by the calcination method from the Co (II) complex which has the formula [Co(PVA)(P-ABA)(H2O)3], PVA = polyvinyl alcohol, and P-ABA = para-aminobenzoic acid. The calcination temperature was 550°C, and the products were characterized by element analysis, thermal analyses (TGA and DTA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), UV-Vis spectra, and scanning electron microscopy (SEM) techniques. The kinetic and thermodynamic parameters (∆H ∗, ∆G ∗, and ∆S ∗) for the cobalt (II) complex are calculated. The charges been carried by the atoms cause dipole moment 10.53 and 3.84 debye and total energy 11.04 × 102 and 24.80 × 102k Cal mol−1 for the Co (II) complex and cobalt oxide, respectively. X-ray diffraction confirmed that the resulting oxide was pure single-crystalline CoO nanoparticles. Scanning electron microscopy indicating that the crystallite size of cobalt oxide nanocrystals was in the range of 36–54 nm. Finally, the antimicrobial activity of cobalt oxide nanoparticles was evaluated using four bacterial strains and one fungal strain. Two strains of Gram-positive cocci (Staphylococcus aureus and Enterococcus faecalis), two strains of Gram-negative bacilli (Escherichia coli and Pseudomonas aeruginosa), and one strain of yeast such as fungi (Candida albicans) were used in this study.


Introduction
Recently, metal oxide nanoparticles (MONPs) have been playing a very marked role in many fields of physics, chemistry, and other material science [1,2]. e transition metals are able to shape a large variety of oxide complexes. Metal oxide nanoparticles (MONPs) exhibit special chemical and physical properties due to their fixed size and more density of the corner. e particle size is likely to be influenced by three important groups of basic properties in more material. e first one involves the structural properties, specifically the lattice symmetry and cell parameter [3][4][5]. Cobalt oxide nanoparticles have been considerably used in various applications; pigments, rechargeable batteries, catalysts, magnets, sensors, solar energy absorbers, electrochromic devices, and biological activity [6][7][8][9][10][11]. Usage of cobalt oxide NPs in these applications is based on the physical and chemical properties of this oxide such as crystallinity, composition, crystal size, morphology, and surface area [12]. e physicochemical properties of metal oxide nanoparticles can be planned by synthesis using different methods [13,14]. e different methods used for synthesis of metal oxides are shown in Figure 1. e calcination method produced very fine, homogeneous crystalline, high-product-purity, and well-dispersed nanoparticles [15][16][17]. Polyvinyl alcohol is one of the more important biodegradable, hydrophilic, and semicrystalline polymers, with excellent physical, chemical, and thermal stability. Additionally, polyvinyl alcohol (PVA) is a nontoxic, ecofriendly vinyl polymer soluble in water and highly biocompatible [18][19][20][21]. ese properties for PVA led to a wide range of applications in pharmaceutical, packaging, medical, cosmetic, and food industries [22].
ere are groups that have investigated binary or mixed complexes between polyvinyl alcohol and metal ions [23,24]. Metal compounds with 4-aminobenzoic acid are among the most explored types of complexes in coordination chemistry and play an important role in catalysis, other materials science, and biochemistry [25,26]. In this article, preparation and characterization of the cobalt (II) complex containing polyvinyl alcohol and p-aminobenzoic acid ligands are described and the resulting complex was used as a raw material for synthesis of nanoparticles of CoO.

Materials and Methods.
Polyvinyl alcohol, paraaminobenzoic acid, and cobalt (II) chloride hexahydrates supplied by Sigma-Aldrich were used without further purification. 3 ]. Polyvinyl alcohol ligand (2 g) was dissolved in 50 mL distilled water by stirring at 50°C for 60 min; after cooling, a CoCl 2 .6H 2 O solution (6.42 g in 35 mL distilled water) was added drop wise to the polyvinyl alcohol solution under quickly stirring, and then, the P-aminobenzoic acid solution (3.70 g in 30 mL ethanol) was added to the mixture. Finally, the mixture was refluxed for 3 h and then cooled. e light pink complex was separated by filtration, washed with EtOH, and dried over

Preparation of Cobalt Oxide Nanoparticles by
Calcinations. Direct calcination of the prepared [Co(PVA)(P-ABA)(H 2 O) 3 ] complex at 550°C with a calcination time of 3 h afforded CoO (NPs). e morphology and size of the as-prepared CoO sample was further investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Physical Measurements and Instrumentation.
e C, H, and N contents of the solid Co (II) complex were determined by using an Elemental Analyser system GmbhVario El analyzer. FT-IR spectra of the compound and cobalt (II) oxide were obtained by the KBr disc technique; in the wavenumber range of 4000-400 cm −1 , using a ermo Nicolet (6700) FT-IR spectrophotometer. e electronic absorption spectral measurements in the ultraviolet and visible regions were carried out in dimethyl sulfoxide on a UV-2102 PC Shimadzu spectrophotometer using a 1 cm matched quartz cell in the wavelength range 200 to 900 nanometer. e magnetic susceptibility of the Co (II) complex was measured at room temperature using a magnetic susceptibility balance of type MSB-Auto. TGA and DTA analyses were completed employing a Shimadzu DTG-60 instrument using a heating rate of 10°C/min in the air atmosphere. e XRD patterns of the CoO NPs were registered using an XRD diffractometer Model (PW 1710) control unit (Philips). e anode material was copper (II) Kα (λ �1.54180Å), 40 KV 30 mA; optics: automatic, divergence slit. e scanning electron microscope (SEM) used was JEOL JFC-1100E ION SPUTTERING, DEVICE, JEOL JSM (5400-LV) SEM.  2 e Scientific World Journal complexes derived from the experimental structure data using the Amsterdam Density Functional (ADF) program [27].

Antimicrobial Activity
(1) Bacterial and Fungal Strains. e antimicrobial activity of cobalt oxide NPs was evaluated using four bacterial strains and one fungal strain. Two strains of Gram-positive cocci (Staphylococcus aureus and Enterococcus faecalis), two strains of Gam-negative bacilli (Escherichia coli, and Pseudomonas aeruginosa), and one stain of yeast such as fungi (Candida albicans) were used in this study.
(2) Inoculum and Cobalt (II) Oxide Nanoparticles Preparation. Bacterial and fungal inocula were prepared from fresh pure cultures in Muller Hinton broth. Each bacterial and fungal suspension were compared with 0.5 McFarland standard. e CoO nanoparticles solution for antimicrobial activity was prepared by diluting powdered cobalt oxide nanoparticles with 1% DMSO.
(3) Agar Well Diffusion Assay. e antimicrobial activity of cobalt oxide nanoparticles against the selected microorganisms was evaluated using the agar well diffusion assay. e bacterial inoculum was spread on the Muller-Hinton agar using a sterile cotton swab by the lawn culture technique. After inoculation, we made 2 wells in the agar plate with the help of the backside blue micropipette tips. en, cobalt oxide nanoparticle extracts (100 μL) were added to one well and 1% DMSO to another well. Erythromycin 15 , vancomycin 30 , ciprofloxacin 5 , amikacin 30 , and ketoconazole 50 were used as a positive control. e plates were then incubated for 18-24 hrs at 37°C. After incubation, the zone of inhibition around the wells was observed, and the zone diameter was measured in millimeter (mm) by using a ruler.

Results and Discussion
e Co (II) complex was prepared by the reaction of polyvinyl alcohol and p-aminobenzoic acid in stoichiometric proportions to yield the identical compound according to Scheme 1.
is Co (II) compound was prepared and used as a precursor for cobalt oxide nanoparticles (CoO NPs) by the calcination method. e data correspond to the metal: L1 : L2 ratio of the 1 : 1:1. Also, the complex is air stable, insoluble in common organic solvents, but partially soluble in dimethylsulphoxide (DMSO). e molar conductivity was determined in DMSO with value in the 45 ohm −1 cm 2 mol −1 for the Co (II) compound.

FT-IR Spectra.
e FT-IR spectrum of the Co (II) complex prepared show an absorption peak was confirmed at ] (1178 cm −1 ), and this band has been employed as an estimate polyvinyl alcohol structure because it is a semicrystalline synthetic polymer able to form domains depending on different approach parameters [28]. Additionally, the occurrence of hydrogen-bonded oxygenhydrogen stretching [29,30] and antisymmetric stretching vibrations of both CH 2 and COC at 2921 cm −1 and 1138 cm −1 for the Co (II) complex, respectively, is found. On the other hand, for polyvinyl alcohol ligand cross linked by the cobalt (II) compound, the band maximum corresponding to the bonded OH group at 3420 cm −1 , was shifted to lower frequencies, 3292 cm −1 . e wavenumbers of bands responsible for the amine group in the FT-IR of Co (II) also change in comparison to free P-aminobenzoic acid ligand. e (P-ABA) ligand displaying a band at 1640 cm −1 for the imine ] (C � N) group, which results from the Schiff base condensation of P-ABA, was shifted to a lower frequency of 1626 cm −1 after complexation [29]. Finally, the appearance of bands at 548 cm −1 and 444 cm −1 corresponds to ] (metal-O) and ] (metal-N), respectively [31] (Figure 2).

Electronic Spectra and Magnetic Moments.
e electronic spectra of the Co (II) complex and CoO NPs have been recorded in the DMSO solvent. e spectra show two distinct bands in the 32,258 and 26,455 cm −1 which corresponds to π⟶π * and n⟶π * transitions within polyvinyl alcohol and P-aminobenzoic acid moieties [32,33], respectively. ere are characteristic bands assigned to the d-d transitions in the Co (II) complex typical of an octahedral structure (Figures 3 and 4). Additionally, the cobalt ((II) compound exhibits a d-d band in 19,230 cm −1 , and the magnetic moments value was 4.36 BM typical for the octahedral Co (II) complex [34]. e energy of the cobalt (II) complex elucidates that the difference in energy between high occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO) is called the (HOMO-LUMO) gap; also, this difference in energy between these two boundary orbitals indicated to form the strength and  16.09%, found: 15.92%) with a corresponding DTG peak at 120°C accompanied with a broad endothermic peak in the D.T.A curve at 122°C. e second, third, and fourth steps related to decomposition of the rest of the organic ligands. ese steps are marked on the DTG curve at 158, 250, and 548°C with correlating exothermic peaks in the D.T.A curve at 160, 252, and 550°C, respectively. e residue corresponds to a cobalt oxide (calc. 22.29%, found: 20.84%).

Kinetic Analysis and ermodynamic Parameters for the Co (II) Complex.
Nonisothermal kinetic studies of the Co (II) complex were accomplished applying two different e Scientific World Journal steps: the Coats-Redfern and the Horowitz-Metzger [35,36] methods (Figures 8 and 9). e kinetic parameters were calculated given to two equations and are listed in Table 1. e activation variables ∆H * , ∆S * , and ∆G * for the decomposition stages of this compound are recorded in Table 2. Negative ∆S * numbers of the both steps of decomposition of the Co(II) complex propose that the stimulated compound is highly ordered than the reactants, and the reactions are lower than normal. e positive numbers of (∆G * ) denote the decomposition reaction is not spontaneous. Figure 10 illustrates the relation between α and temperature for the Co (II) compound in dynamic air. e figure indicates that the cobalt (II) compound is stable.

X-Ray Powder Diffraction (XRD), SEM, and TEM
Analysis. XRD patterns of the CoO nanoparticles calcined at 550°C are shown in Figure 11, which indicates cobalt oxide has a cubic phase structure (Table 3). e average grain size of CoO is determined using the Scherrer relation, and it was found to be around 54 nm. Figure 12 shows the scanning electron microscopy image where the size of all mesoporous cobalt oxide nanocrystals is distorted growth to near-spherical. On the other hand (Figure 13), as shown in the TEM image, cobalt oxide nanoparticles (with a higher electron density) appear as darker spots. Moreover, the TEM picture shows clearly that the product is entirely composed of crystals with a relatively uniform, separate small spherical and cluster morphology.        e Scientific World Journal 7

Conclusions
In conclusion, we have demonstrated the facile synthesis of the spherical formed aggregates of cubic-CoO (NPs) by the calcination method of the Co (II) complex. e phase and morphology of the cobalt (II) oxide nanocrystals are characterised by XRD, SEM, and TEM. Results of X-ray sample display that all peaks can be well listed to the phase of CoO. SEM micrographs offer that there are many micropores through the nanocrystals for the sample calcined at 550°C for 3 hours. Also, the antimicrobial activity is tested against standard fungal and bacterial species, and the results present that the calcination method increased antibacterial and antifungal activity than other preparation methods due to small particle size, production of reactive oxygen species (ROS), and large surface area.

Data Availability
e data used to support the findings of this study are available within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.