2.1. Preparation of the Sample

Sigma-Aldrich provided analytical-grade zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), strontium nitrate (Sr(NO₃)₂), and NaOH. All of the compounds utilized in this study were of analytical grade and were not purified further. The ZnO : Sr nanoparticles were made using a simple soft chemical approach described in a previous paper [20]. In 200 mL of deionized water, 0.2 M zinc acetate dihydrate was dissolved. The solution was continually agitated with a magnetic stirrer until it became homogenous. As a dopant of precursor, strontium nitrate was served at concentrations of 0, 3, 5, and 7 at.%, respectively. The 0.97 mol of zinc acetate and 0.3 mol of strontium nitrate were taken in a separate beaker. 1 mol of sodium hydroxide was taken in another beaker and dissolved. To synthesize Sr-doped ZnO materials, the dissolved solutions were added in a common beaker under magnetic stirring with rpm 400 per min. All the chemical solutions were taken as per the targeted stoichiometry ratio.

To maintain the pH of the starting solution at 8, the mixed solution was agitated at 85°C for 2 hours and then maintained at room temperature for roughly 24 hours without being disturbed. The residual ions were eliminated when the white solid precipitate was filtered out. The end-product was washed with DI water and ethanol. Before characterization, the produced samples were calcined at 570°C for 3 hours using a tubular furnace (Make & Model: Compact).

2.2. Material Identification

The crystal structure was determined using a PANalytical-PW 340/60 X’pert PRO X-ray diffractometer with Cu-K radiation (1.5406) and a scanning rate of 0.02°/sec. Transmission Electron Microscopy (TEM, Hitachi H-7100), Field Emission Scanning Electron Microscopy (FESEM) (Hitachi SU8000), and energy dispersive X-ray (EDX) analysis were used to examine the sample’s morphology and elemental composition (model: JEOL-JSM 6390 with attachment INCA-Penta FETX3 OXFORD). The spectrofluorometer (Jobin Yvon FLUROLOG-FL3-11) was used to examine photoluminescence (PL) spectra.

2.3. Measurement of Antibacterial Activity

The antibacterial behaviour of the synthesized materials was examined using the Mueller-Hinton agar well diffusion method against one gram-positive bacteria (Staphylococcus aureus) and one gram-negative bacteria (Escherichia coli). To distribute the bacterial cultures (fresh) over the plates, the spread plate technique was utilized whereas the prepared medium was placed into petri plates. Discs with a diameter of 6 mm and a weight of 20 g were distributed onto the petri plates and gently pressed to make sure that the discs have adhered with agar. A disc consisting of gentamicin, a typical antibiotic, was also inserted on every plate for comparison. The prepared plates were kept for incubation for 24 hours at 35 degrees Celsius. The spread diameter of the inhibition zone around the paper discs was estimated and denoted in millimeters after the incubation period.

3.1. Structural Research

The XRD of Sr-doped ZnO nanoparticles with different strontium atomic ratios is shown in Figure 1 (0, 3, 5, and 7 at.%). The hexagonal wurtzite structure is represented by all of the diffraction peaks at (100), (002), (101), (102), (110), (103), and (112). Furthermore, the JCPDS card no. 36-1451 is sufficiently indexed for these diffraction peaks. There are no peaks of metallic ZnO₂ or any other phases found. Furthermore, the powerful and crisp diffraction peaks indicate that the as-made nanoparticles are highly crystalline.

Table 1 shows the computed “D” values for the wavelength of the X-ray used (1.5406), the full width at half maximum (FWHM) of the 101 plane, and the angle of diffraction. The crystallite size measured ranges from 39 to 46 nm. The increasing incorporation of Sr at the ZnO sites, respectively, may be causing the steady declines in crystallinity of produced nanoparticles.

The computed lattice constants ‘a’ and ‘c’ are remarkably close to the conventional values of strontium-doped ZnO nanoparticles up to 7 at.%. The substitution of strontium increased the particle size because Sr²⁺ (1.12 Å) has a larger ionic radius than Zn²⁺ (0.74 Å). The further increase in doping concentration slightly reduced the overall particle size. This increase in the particle size indicated that the dopant has been entered into the host lattice effectively. The C was decreased and the particle size increased for Sr (3 at.%). For Sr (5 at.% and 7 at.%), the C was increased and the particle size reduced. Ouhaibi et al. observed in their research that the particle size was increased up to Sr of 3 at.% and reduced for further incorporation of Sr ions [23].

3.2. Analysis of Morphology and Elements

The surface morphology of ZnO and Sr-doped ZnO nanoparticles is shown in Figure 2. The FESEM pictures reveal a uniform and highly crystalline ZnO nanostructure. The top surface of undoped ZnO nanoparticles had a typical hexagonal cross section (Figure 2(a)). The addition of different concentrations of strontium (3, 5, and 7 at.%) resulted in the development of material on the nanoparticles. These generated nanoparticles are consistently dispersed over the surface and have high communication between them, as seen in the TEM image (Figure 2(e)). As a result, doping leads to a remarkable impact on the grain size of the ZnO nanoparticles, resulting in changes in morphology.

The surface morphology was changed due to Sr doping in the ZnO lattice. The ZnO and Sr particles were highly accumulated at the surface and formed a cluster near the surface, and the size was increased. With further addition of Sr, the cluster size was decreased. The increase in crystallite size indicates that the dopant Sr has been substituted in Zn²⁺ sites. The incorporation of Sr produces some distortion on the lattice and leads to some supplementary defects. These are responsible for the change in morphology.

The presence of Zn, O, and Sr atoms in produced nanoparticles is indicated by the EDX spectra (Figures 3(a)–3(d)). The highly intense Zn and O peaks were found, indicating that the nanoparticles were mostly ZnO with a few impurities. The at.% value of each element present in the table samples was listed as a table in each figure. All the elements’ presence ratios are matched well with the targeted stoichiometry ratio. From the EDX spectra, the presence of dopant ions in the host material is proven.

3.3. Photoluminescence of ZnO : Sr Nanoparticles

ZnO and Sr-doped ZnO nanoparticles had photoluminescence spectra that were excited at 325 nm (Figure 4). At 408 nm (2.922 eV), the spectrum reveals a strong peak violet emission. The product’s emission peak positions are nearly unchanged, as can be shown. Shown is an intrinsic characteristic of Sr-doped ZnO nanoparticles themselves when excited at 325 nm. Excitonic recombination is responsible for the UV emission at 361 nm, which indicates the near band edge emission of ZnO [24, 25]. The purple-blue emission around 436 nm is most likely due to a neutral oxygen vacancy defect’s triplet to ground transition [26]. The presence of impurity (Sr) atoms in the host ZnO matrix, as well as zinc and oxygen vacancies, is most likely responsible for the violet emission at 408 nm. Furthermore, doping Sr ions can result in larger active defects in the ZnO lattice, which can enhance the photocatalytic activity of the Sr-doped ZnO crystals in the visible region [27].

A series of visible emissions have also been reported in the wavelength range of 400 to 600 nm. Various types of intrinsic defects, including defect states, interstitials, and oxygen vacancies, can all be blamed for visual emission. The incorporation of Sr ions can create more active defect sites in the ZnO host lattice, which absorbs more visible light. This produces better photocatalytic activity of the Sr-doped ZnO crystallites in the visible range. Surface imperfections caused blue emission at 436 nm and green-blue emission at 492 nm. As a result, the increased PL emission implies that the flaws that exist in Sr-doped ZnO improve photocatalytic activity.

3.4. Antibacterial Activity

Sr-doped ZnO nanoparticles were tested for antibacterial activity against one gram-positive pathogen (E. coli) and one gram-negative pathogen (S. aureus), which are the most common bacteria that cause dihedral illness. The antibacterial activity of these nanoparticles is stronger in S. aureus and E. coli (Figure 5). Bacterial growth is inhibited by atomic percent in the presence of nanoparticles at specific concentrations. Surprisingly, this amount varies greatly depending on the size of the nanoparticles. Figure 6 shows the measured inhibitory zone diameters of the samples. Gram-positive bacteria possess a thicker cell wall and a single cytoplasmic membrane with many layers of peptidoglycan polymers [28] (20-80 nm). The gram-negative bacterial wall, on the other hand, is made up of two cell membranes and a plasma membrane with a thin layer of peptidoglycan [29] that is 7-8 nm thick. Nanoparticles with sizes in this range can easily pass through the peptidoglycan, making them vulnerable to destruction. The bacterial concentration was found to decrease as the concentration of Sr-doped ZnO was increased. The growth of E. coli and S. aureus was suppressed more at a concentration of 7% Sr-doped ZnO.

These ZnO nanoparticles have a biocidal impact and are effective at delaying bacterial development. These findings could lead to significant technologies in the future, such as antibacterial systems and medical equipment. The antibacterial behaviour of the nanostructure may be ascertained to two possible mechanisms: (i) the generation of increased levels of Reactive Oxygen Species (ROS), such as superoxide anion (O₂^-), hydroxyl radical (^-OH), and hydrogen peroxide (H₂O₂), and (ii) the deposition of nanoparticles on the surface of bacteria in the cytoplasm region, causing cellular function disruption [30].

UV and visible light can both activate ZnO with a crystal flaw. When an appropriate photon strikes ZnO, an electron is stimulated from the valance band to the conduction band, leaving a hole in the valance band. Superoxide anion radicals are formed when an electron in the conduction band combines with dissolved oxygen molecules (O^-₂). The valence band hole separated the water molecule into OH- and H⁺ ions. HO₂ radicals are formed when superoxide anion radicals (O₂^-) combine with H⁺. H⁺ reacts with the HO₂ radicals to form the H₂O₂ molecule. The H₂O₂ can easily enter the cell membrane, killing the bacteria.

The presence of Sr in ZnO structure increased the antibacterial activity against both gram-positive Staphylococcus aureus and gram-negative Escherichia coli. The increment order is the function of the doping ratio of Sr. The maximum zone inhibition was received for the highest doping concentration of Sr (7%).

AL-Jawad et al. observed a similar enhancement of antibacterial activity against S. aureus and E. coli in Ag-doped ZnO thin films [31]. Fe-doped ZnO thin films also exhibited a better antibacterial response under UV light irradiation against S. aureus and E. coli [32].

The Sr-doped ZnO particles are allowed by the cell wall of E. coli for an electrostatic interaction; this might cause a breakdown of the membrane barrier, and ultimately its cell got damaged. This electrostatic interaction is more at gram-positive bacteria than at gram negative-bacteria. A similar effect is produced in S. aureus bacteria also. According to XRD particle size calculation, higher doping percentage of Sr reduced the overall size of the particles. These small-sized particles can enter more numbers, and the cell damage is also proportionately increased [33].