Chemical Synthesis and Antipseudomonal Activity of Al-Doped NiO Nanoparticles

Synthesis of efficient antibacterial agents has become extremely important due to the emergence of antibiotic resistant bacteria. This is especially true for Pseudomonas aeruginosa, an opportunistic pathogen having ability to rapidly develop resistance to multiple classes of antibiotics thus limiting the efficacy of antibiotics approved for clinical use. Aluminum (Al)-doped NiO nanoparticles are of special interest due to their enhanced antipseudomonal properties at certain Al-doping levels. The composite hydroxide mediated (CHM) approach was opted for the synthesis of pure nickel oxide (NiO) and Al-doped nickel oxide (Ni1–xAlxO; x = 5, 10, 15, 20, 25, and 30 wt.%) nanoparticles. X-ray diffraction (XRD) technique was used for structural analysis of these nanoparticles. Morphology and elemental composition of these nanoparticles were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, respectively. The optical properties were investigated by using UV-visible spectroscopy and Kubelka-Munk Theory and Tauc relation were employed for energy bandgap calculation of these nanoparticles. The antibacterial activity of representative Al-doped NiO nanoparticles was assessed on multidrug-resistant clinical P. aeruginosa strains. The agar well and disc-diffusion methods were used to assess the antibacterial efficacy of (Al)-doped NiO compared to pure NiO nanoparticles. Interestingly, a gradual increase in the antibacterial activity was observed with increasing Al-doping concentration and the highest antibacterial activity was observed at x = 15 wt.% Al-doping concentration. The antipseudomonal efficacy of Ni1–xAlxO nanoparticles was comparable to aztreonam antibiotic, primarily used for Gram-negative bacterial infections. Hence, it is proposed that these nanoparticles can be used for coating surgical devices, bone prostheses, medical implants, antibacterial clothing and in pharmaceutical formulations as burn ointments to produce the antimicrobial effect.


INTRODUCTION
In medical sciences, the role of nanotechnology has become vital particularly in targeted therapies with better outcomes and fewer side effects (Kirui et al., 2013). Various metal oxide nanoparticles have already been used in different areas i.e., as a catalyst in wastewater treatment to replace the bleaching process (Srihasam et al., 2020), in commercial paints to kill airborne pathogens, as sensors, semiconductors, for controlled drug release and as antimicrobial agents. There is emerging interest in the synthesis of magnetic NPs of Fe, Co and Ni due to their superior properties in magnetically controlled drug delivery, hypothermia cancer cell treatment and magnetic resonance imaging (Imran Din and Rani, 2016). The applications of metal oxide nanoparticles have gained a lot more interest in antibacterial studies due to their significant concentration-dependent antimicrobial properties (Dizaj et al., 2014;Iqbal et al., 2014). The nanoparticles interact with the surfaces of bacteria as they exhibit high specific surface area .
P. aeruginosa is one of the most important pathogenic bacteria of humans, plants and animals (Wang K. et al., 2017). Infections caused by this notorious pathogen are often difficult to eradicate despite intense antibiotic treatments (Lister et al., 2009). The morbidity and mortality rates due to rapidly evolving MDR P. aeruginosa are on the rise (Waters and Smyth, 2015). In the last report by World Health Organization (WHO), this pathogen has been classified as a top priority critical pathogen against which there is a dire need to develop novel antimicrobial therapeutics (Tacconelli et al., 2017). Different metal-oxide nanoparticles such as cobalt oxide, nickel oxide, zinc oxide, cerium oxide, titanium dioxide, copper oxide, magnesium oxide, silicon dioxide, iron oxide, cadmium oxides, and tin dioxide have shown significant antibacterial activities against various bacteria (Parham et al., 2016;Gupta et al., 2020b). The antibacterial activity of pure-NiO nanoparticles against Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli) and Gram-Positive bacteria (Staphylococcus aureus, Streptococcus pneumonia) has been reported (Baek and An, 2011). NiO nanoparticles were found better antibacterial agents than tetracycline and gentamicin antibiotics against Gram-negative and Gram-positive bacteria (Gupta et al., 2020a). The antibacterial activity of NiO nanoparticles using different antimicrobial and biophysical studies revealed that these nanoparticles have stronger antibacterial activity against Gram-positive bacteria compared to Gram-negative bacteria (Behera et al., 2019). In comparison to the previously reported antibacterial activity of pure NiO as well as CuO-NiO composites against different bacteria, the Al-doped NiO nanoparticles have better antibacterial efficacy against Gram-negative bacteria "Pseudomonas aeruginosa". The Al-doped NiO nanoparticles were found effective for curbing P. aeruginosa growth and population similar to other reported materials (Argueta-Figueroa et al., 2014;Paul and Neogi, 2019). NiO-NPs synthesized from stevia plant leaves have also been found effective against Gram −ive E. coli than the Gram +ive bacteria, Bacillus (12 mm) and Streptococcus pneumoniae (14 mm) [2]. Similarly, NiO nanoparticles biosynthesized from fresh leaves of the Neem plant have shown bacterial cell membrane disruption, which resulted in the death of bacterial cells (Helan et al., 2016). Moreover, the inhibition efficiency of bacterial growth was enhanced in the presence of amoxicillin together with Ni nanoparticles (Jeyaraj Pandian et al., 2016).
The antibacterial activity of Al-doped NiO nanoparticles has not been studied yet. Hence, we attempted to work and understand the antibacterial activity of variably Al-doped NiO nanoparticles against MDR P. aeruginosa clinical isolates. Different methods like wet chemical co-precipitation, sol-gel, hydrothermal or composite hydroxide mediated (CHM) were exercised for the synthesis of nanoparticles (Hulteen et al., 1999;Xu et al., 2007). The composite hydroxide mediated (CHM) route can be preferred to other wet chemical routes to achieve uniform particles' size with less impurities. The major benefits of this method are; (1) particle distribution in large scale, (2) fast to work at low temperatures, (3) completion of well dispersed nanoparticles, (4) reduction agents influence on the properties of nanomaterial etc. By using CHM method at a lower temperature, the uniform preparation of nanostructures can only be achieved due to the eutectic point of composite hydroxides (Hu et al., 2009). In the present work, CHM route was used for the synthesis of Ni 1−x Al x O; x = 0, 5, 10, 15, 20, 25 and 30 wt.% nanoparticles. These nanoparticles were structurally analyzed by the XRD technique and optical properties were investigated by using UV-visible spectroscopy. Kubelka-Munk Theory and Tauc relation were employed for the calculation of the energy band gap of these nanoparticles. The morphology and elemental composition of these nanoparticles were investigated by SEM and EDX spectroscopies, respectively. Antibacterial activity of Aldoped and pure NiO samples was tested using agar well and diskdiffusion methods. The zone of inhibition of bacterial growth was measured in millimeters (mm). Kinetic growth curves were recorded to further evaluate the growth dynamics of bacteria in the presence of Al-doped NiO nanoparticles by measuring the optical density of the cultures as a qualitative measure of culture turbidity at 600 nm.

Synthesis of Ni 1−x Al x O Nanoparticles
The CHM method was used for the synthesis of Ni 1−x Al x O; x = 0, 5, 10, 15, 20, 25, and 30 wt.% nanoparticles due to its eco-friendly characteristics. Ni 1−x Al x O nanoparticles were prepared from the reaction of nickel chloride and aluminum in the presence of sodium and potassium hydroxide as given by Eqs 1 and 2 and Table 1; All desired chemicals according to their concentrations as specified in Table 1 were taken in seven Teflon beakers separately and placed in the furnace at 200 • C for 24 h. Usually, CHM

Structural, Morphological, and Optical Characterizations
Stoe Stadi P Combi X-ray diffractometer with CuKα source of radiations of wavelength 1.5405 Å was used to determine the crystal structure of these nanoparticles at room temperature. The JEOL -Model JSM-6390 -Scanning Electron Microscope (SEM) was used to visualize the microstructures and elemental composition of these nanoparticles. The optical properties of these nanoparticles were investigated by using a UV-visible spectrometer. Kubelka-Munk Theory and Tauc relation were used for computation of energy band gap of Ni 1−x Al x O; x = 0, 10, 20 and 30 wt.% nanoparticles.

Microbiology, Culture Conditions, and Antibacterial Activity
The antibacterial activity of Ni 1−x Al x O; x = 5, 10, 15, 20, 25 and 30 wt.% nanoparticles was checked against MDR P. aeruginosa. Pure cultures of the clinically isolated P. aeruginosa were obtained and sub-cultured on Pseudomonas cetrimide agar (a selective media for P. aeruginosa growth) and incubated for 24 h at 35 • C. For bacterial growth, a 0.5 McFarland suspension of pure bacterial colonies in normal saline was prepared and 100 µL of the suspension was spread onto Muller Hinton agar (MHA) plates. The plates were left for 10 min to let the culture absorb into the agar. Ni 1−x Al x O nanoparticles suspension of all samples (x = 5∼30 wt.%) ranging from 20,000 to 1.95 µg/ml concentrations were freshly prepared before use. The antibacterial activity was performed by agar well-diffusion and disk-diffusion methods (Kourmouli et al., 2018). The culture plates were seeded with tested organisms and allowed to solidify thereafter punched with a sterile cork borer (5.0 mm diameter) to cut uniform wells. For the agar well diffusion method the Petri plates seeded with culture were punched with a sterile cork borer (6.0 mm diameter) and 100 µl of the nanoparticles suspension was poured into each well with the help of a micropipette. Filter paper discs (6 mm) were saturated with 10 µl of respective nanoparticles suspension and placed on agar plates and incubated at 35 • C ± 2 • C for 18∼24 h. The zones of inhibition (ZOI) of bacterial growth around wells and discs were measured in millimeters (mm). Nuclease-free water was used as a blank or negative control while aztreonam antibiotic disks were used as a positive control (Luangtongkum et al., 2007). We further check the effects of the doped and pure NiO nanoparticles on the growth dynamics of P. aeruginosa. The bacterial culture was prepared in Luria-Bertani (LB) broth in the presence of colloidal suspension (1 mg/ml) of nanoparticles in 5 ml test tubes. Nanoparticle-free microbial culture under the same growth conditions was used as a control. The bacterial growth was monitored by UV-vis Spectrophotometer at a wavelength of 600 nm for every 2 h.

Structure and Morphology
The crystal structure and phase purity of Ni 1−x Al x O; x = 0, 5, 10, 15, 20, 25 and 30 wt.% nanoparticles were determined by X-ray diffraction (XRD). The XRD spectra of Ni 1−x Al x O nanoparticles from 2θ = 20 • to 80 • are shown in Figure 1. The diffraction peaks for Ni 1−x Al x O nanoparticles were properly indexed according to FCC crystal structure using X'pert High Score software and the peaks were found to exactly match with JCPDS card # 01-073-1523. The diffraction planes (111), (200),  Figure 1. Moreover, no impurity peak was observed in XRD spectra and the characteristics peak at 43.2 • corresponds to (200) plane of FCC crystal structure of Ni 1−x Al x O nanoparticles. The intensity of the diffraction peaks was found to increase with increasing contents of Al in Ni 1−x Al x O nanoparticles. It was also observed that the characteristic peaks were slightly shifted toward a higher angle with increasing contents of Al as shown in the inset of Figure 1. The change in the intensity and position of diffraction peaks confirmed Al-doping at Ni sites in Ni 1−x Al x O nanoparticles (Stuart, 2015). This may also indicate the changing in crystal structure due to vacancies and defects creation at the lattice sites or the charge imbalance through Al-doping at Ni lattice sites (Thermo Nicolet Corporation, 2001  Ni 1−x Al x O nanoparticles' size from 30∼85 nm with Al contents (x) is shown in Figure 2H. Some nanoparticles were observed to be agglomerated and clustered due to magnetic interaction among these magnetic nanoparticles (Muhammed Shafi and Chandra Bose, 2015). The average particle sizes of Ni 1−x Al x O nanoparticles were further calculated through histograms and standard deviation as shown in the insets of respective SEM images in Figures 2A-G. The mean diameter of Ni 1−x Al x O nanoparticles was found to be 34, 39, 46, 52, 61, 69, 82 nm for x = 0, 5, 10, 15, 20, 25 and 30 wt.%, respectively. The morphology of the nanostructures can be controlled by varying the synthesis parameters. The basic parameters that can affect the grain-growth or the particle size are; (i) Stirring, (ii) Concentration of dopants, (iii) Reaction temperature, (iv) Reaction time, (v) pH-scale value, and (vi) Reagents rate etc. In current study, these nanoparticles have been synthesized under the same synthesis conditions, but the different sizes of ionic radii of Al and Ni caused the strain field that can disturb the grain growth process. Thus, the interactions between the dopants and surface/grain boundaries may vary the surface energy, thus leading the stabilization of the surfaces/grain boundaries, as a result, the particle size of Ni 1−x Al x O nanoparticles has been increased with increasing contents of Al. Moreover, Al doping has decreased the crystallite size as evident from XRD and large number of crystallites have merged together during nucleation process to form nanoparticles of relatively large in size as clear from Figure 2F. The EDX spectroscopy was used to determine the quantitative elemental composition of Ni 1−x Al x O nanoparticles. The EDX spectra of Ni 1−x Al x O; x = 0, 5, 10, 15, 20, 25 and 30 wt.% nanoparticles are shown in Figures 3A-G

Optical Properties
The optical properties of Ni 1−x Al x O nanoparticles were investigated by UV-visible spectroscopy in the ranges of 250 to 1,000 nm. The Kubelka-Munk model was employed to determine the energy bandgap "E g " of Ni 1−x Al x O nanoparticles. The Kubelka-Munk F(R) 2 and Tauc's relation are given in Eqs 3 and 4 below (Shahid et al., 2018). where "n" represents an exponent which is "2" for indirect and " 1 /2" for direct bandgap transition, "h" is Plank's constant, "hυ" is the energy associated with photon in eV and "R" calculated by Kubulka-Munk theory and Tauc's relation of Ni 1−x Al x O nanoparticles were found to be 2.3, 3.2, 4.5 and 3.3 eV for x = 0, 10, 20 and 30 wt.%, respectively. The value of E g was increased overall with increasing contents of Al in Ni 1−x Al x O nanoparticles, which could be attributed to the increased particle size. Generally, semiconductor nanomaterials absorb light due to which valence electrons jump from valence band to conduction band which creates a hole behind if the nanoparticles have size comparable to that of de-Broglie wavelength. As there are different energies for conduction band and free electrons that cause quantization of their energy levels (Shahid et al., 2016) thus, the E g of Ni 1−x Al x O nanoparticles was increased due to an increase in particle size as compared to bulk NiO.

Disc-Diffusion and Agar Well Diffusion Method
The antibacterial activity of Ni 1−x Al x O; x = 0, 5, 10, 15, 20, 25 and 30 wt.% nanoparticles was assessed by agar well and disc-diffusion method as given in Figures 5A-D. No significant antibacterial activity was observed for pure NiO nanoparticles. Relatively significant antibacterial activities were observed for Ni 1−x Al x O nanoparticles with x = 5 and 10 wt.% as shown in Figures 5B,C. Surprisingly, a remarkable increase in anti-Pseudomonal activity was observed for Ni 1−x Al x O nanoparticles with x = 15 wt.% as shown in Figure 5D. The ZOI of bacterial growth for Ni 1−x Al x O nanoparticles with x = 15 wt.% was comparable with ZOI of aztreonam antibiotic used in the form of a disk as a positive control. Aztreonam is used primarily to treat the infections caused by most of the Gram-negative bacteria i.e., P. aeruginosa (Lemke et al., 2017). For Al-doped NiO nanoparticles area/size of ZOI ranging from 10 to 22 mm were observed at concentrations of 3.90 to 1,000 µg/ml. A remarkable increase in ZOI is suggestive of enhanced antibacterial activity of Ni 1−x Al x O nanoparticles with x = 15 wt.% ( Table 3). NiO NPs have been found to have bactericidal activity depending on the bacterial species and NPs concentrations (Nikolova and Chavali, 2020). Promising antibacterial activity of NiO against Gram-negative bacteria has already been reported in which they have suggested that the release of metal ions from metal oxide nanoparticles could be one of the key factors behind their antibacterial properties (Wang et al., 2010). One of the advantages of doping metal oxides with metal dopants is that they  decrease VB to VC bandgap and help to convert low efficient UV-visible light-absorbing materials into highly efficient light-absorbing materials (Djerdj et al., 2010) and also affect the overall surface area and morphology of inactive metal oxides (Lin et al., 2005). A decrease in the bandgap effectively generates photo-induced charge carriers and eventually helps to generate •OH, O2-•, HOO•, which are highly oxidized species. These highly oxidized species directly attack the cell membrane leading to plasmolysis and cell death (Guillard et al., 2008). The antimicrobial characteristics of Fe-doped TiO 2 have been observed to increase in the fluorescent light more efficiently than un-doped TiO 2 (Yadav et al., 2016). Enhanced antibacterial properties of Co-doped in ZnO nanoparticles against Klebsiella pneumoniae, Salmonella typhi, P. aeruginosa, Staphylococcus aureus, Bacillus subtilis and Shigella dysenteriae bacterial strains has been reported (Nair et al., 2011). Similarly, surface functionalization of NiO NPs with 5-amino-2-mercaptobenzimidazole enhanced both the bactericidal (against P. aeruginosa and S. aureus) and antifungal activity compared with the non-functionalized NiO NPs (Nikolova and Chavali, 2020).

Bacterial Growth Curve Analysis
The growth curve kinetics of P. aeruginosa in the presence of pure and Al-doped NiO nanoparticles is shown in Figure 6. These results indicate that the antibacterial activity of Al-doped NiO nanoparticles is effectively improved by increasing Al-doping concentrations. The maximum bactericidal activity has been observed at 15 wt.% of Al-doping in NiO nanoparticles. The hypothetical antibacterial mechanism of the NiO-NPs is shown in Figure 7. The electrostatic forces of attraction between nanoparticles and cell membrane cause membrane disruption and cell apoptosis leading to leakage of intracellular components from the cell (Alzahrani et al., 2018). Another possible mechanism could be the release of metal ions from nanoparticles which neutralize the surface charge of Gram-negative bacteria, slowing down their growth rate thus, ultimately leading to malfunction of core proteins for survival resulting in cell death (Paul and Neogi, 2019). NiO NPs released Ni ions which might interfere with the intracellular metabolism of Ca 2+ ions leading to cellular damage. NiO NPs have also been shown to interact with phosphorous and sulfur of bacterial DNA and other functional groups of proteins leading to protein leakage and bacterial death (Ezhilarasi et al., 2018).
Moreover, the penetration of nanoparticles inside the bacterial cell interacts and inhibits the electron transport chain, damaging the DNA by breaking hydrogen and phosphate bonds, denaturing tertiary structures of proteins and damaging the powerhouse (mitochondria) of the bacterial cell by oxidative stress due to generation of ROS by inorganic metallic oxides (Srihasam et al., 2020). The potential antibacterial properties mainly depend on the size, shape, concentration, stability, morphology and exposure/treatment time (Jesudoss et al., 2016).

CONCLUSION
The Ni 1−x Al x O; x = 0, 5, 10, 15, 20, 25 and 30 wt.% nanoparticles were successfully synthesized by CHM method. The FFC crystal structure, phase purity and Al-doping at Ni sites in Ni 1−x Al x O nanoparticles were probed by the XRD technique. The slight shift in the characteristic diffraction peaks toward a higher angle indicated the stresses produced with Al-doing in Ni 1−x Al x O nanoparticles. The particle size of Ni 1−x Al x O nanoparticles was found to be 30∼85 nm. The value of E g was observed to be 2.3∼4.5 eV with increasing Al-doping in Ni 1−x Al x O nanoparticles, which was due to the increased particle size of these nanoparticles. A gradual increase in antibacterial activity was observed with increasing Al-doping up to x = 15 wt.% in Ni 1−x Al x O nanoparticles. Maximum bacterial growth inhibition was observed at doping level x = 15 wt.% while exceeding this doping has negatively impacted the antipseudomonal activity. These measurements and analysis suggest that Al is a suitable candidate for doping in NiO nanoparticles to enhance antipseudomonal properties.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
SI and MM designed the study and wrote the draft. SS, ZM, and GG helped in performing experiments. Mubasher and MAb helped in the characterization of synthesized nanoparticles. MAr provided bacterial strains for experiments. SA reviewed and approved the final draft. All authors contributed to the article and approved the submitted version.