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. 2023 Aug 8;8(33):30681–30693. doi: 10.1021/acsomega.3c04543

Hydrothermal Synthesis of Fe-Doped Cadmium Oxide Showed Bactericidal Behavior and Highly Efficient Visible Light Photocatalysis

Iram Shahzadi †, Muhammad Aqeel ‡, Ali Haider §, Sadia Naz ∥, Muhammad Imran ⊥, Walid Nabgan #,*, Ali Al-Shanini ¶,*, Anum Shahzadi ∇, Thamraa Alshahrani ○, Muhammad Ikram ⧫,*
PMCID: PMC10448685  PMID: 37636921

Abstract

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Cationic dyes present in industrial effluents significantly reduce the effectiveness of remediation operations. Considering the terrible impact of these pollutants on environment and biodiversity, investigating strategies to remove potentially harmful compounds from water is becoming an increasingly intriguing issue. In this work, we employed a simple hydrothermal technique to synthesize Fe-doped CdO (2, 4, and 6 wt %) nanostructures and assessed their efficacy in degrading methylene blue (MB) dye and inhibiting the growth of Staphylococcus aureus and Escherichia coli, respectively. Structural, morphological, and optical characterization of produced nanomaterials was also performed using X-ray diffraction, TEM, and UV absorption spectra. The photocatalytic decomposition of MB was significantly enhanced (58.8%) by using Fe (6 wt %)-doped CdO catalysts for 80 min under irradiation. In addition, 2.05–5.05 mm inhibitory zones were seen against Gram-positive bacteria (S. aureus), whereas the range for Gram-negative bacteria (E. coli) was 1.65–2.75 mm. These nanostructures were shown to be very effective inhibitors of beta-lactamase, d-alanine-d-alanine ligase B, and fatty acid synthase inhibitor by in silico molecular docking investigations.

1. Introduction

Transparent conducting oxide (TCO) such as indium oxides, tin oxides, zinc oxides, and cadmium oxides exhibit considerable electrical conductivity and beneficial optical transparency, also explored as potentially useful material in optoelectronics owing to moderate impedance and significant optical reflectivity,1−3 in addition, they are capable of producing reactive species like hydroxyl radicals that may disintegrate organic contaminants. Among the several semiconducting metal oxides available, researchers are intrigued by CdO as it is an n-type semiconductor with high binding affinity, with considerable transmission in the visible spectrum, limited resistance ≈10–2 −10–4 Ω cm, and direct bandwidth in ∼2.2–2.7 eV region.4,5 TCO-doped CdO offers multiple optoelectronic applications, including gas sensors, smart windows, photo-transistors, surface acoustic, photovoltaic panels, flat panel displays, translucent conductive filaments, infrared light hot mirrors, low-emissive windows, heat mirrors, narrow resistors, among others,6,7 depending on suitable electrical conductivity and optical properties.8 Doping CdO with metal elements such as manganese (Mn), fluorine (F), tin (Sn), dysprosium (Dy), aluminum (Al), cobalt (Co), nickel (Ni), copper (Cu), and iron (Fe) can modify its physical attributes; these dopants improve n-type conductivity and increase electrical conductivity and optical band gap.9−13

Doped and undoped CdO have been prepared by various processes, notably reactive sputtering,3 chemical bath deposition, microwave irradiation,14,15 activated reactive evaporation,16 hydrothermal,17 spray pyrolysis,18 sonochemical methods,19 chemical vapor deposition, combustion,20 conventional precipitation,21 and sol–gel.22 Among these, the sol–gel method is an attractive approach for preparing metal oxide materials widely used in ceramics, catalysts, electronics, sensors, optics, and thin films. Owing to its distinctive features, this approach is intended to produce homogeneous composites with extremely high purity (99.99% purity) on an industrial scale. The lower temperature required during synthesis provides another potential benefit of this technology over conventional methods, allowing metal and ceramic nanomaterials to be formed at temperatures between 70 and 320 °C. In contrast, the processes mentioned above generate nanomaterials at temperatures between 1400 and 3600 °C.23−25

The pollution of freshwater supplies and aquatic ecosystems is a pressing problem threatening human development and prosperity across globe. In recent years, semiconductor nanocomposite materials have been acknowledged as viable replacements for traditional materials for their high stability, unique physical properties, high aspect ratio, and large-surface area, all of which contribute to their superior photocatalytic performance, photovoltaic responsiveness, and potent microbicidal potential against microbes.26 In some investigations, iron oxide nanoparticles were shown to inhibit bacterial growth by inducing reactive oxygen species (ROS), oxidative stress, and cell membrane rupture. Fe3O4 nanoparticle robust magnetic properties, extended endurance, high biocompatibility, low toxicity, etc., make them useful in a variety of magnetic biomedical applications, removal of heavy metal ions, and absorption of electromagnetic waves.27 CdO nanoparticles are a photocatalyst for dye degradation28 because of their high charge carrier mobility and excellent visible light absorption. It was recently discovered that CdO nanopowder has potent antibacterial properties. Because CdO is paramagnetic at ambient temperature, it is rarely researched as a metal oxide semiconductor.29 Its doping with magnetic metal ions or combining it with ferromagnetic semiconductors [iron (Fe)] might boost its magnetic characteristics because Fe2+ ion is a somewhat smaller size than Cd2+ (ionic radii of Fe2+ and Cd2+ are 0.78 and 0.95 Å, respectively).30

According to our knowledge, there are few reports on the fabrication and evaluation of undoped and doped CdO nanopowders. This work demonstrates the hydrothermal synthesis of Fe-doped CdO nanostructures as a novel and effective antibacterial agent against Staphylococcus aureus bacteria. Iron (Fe) was used as a doping agent to reduce the crystallite size of prepared nanostructures, providing the large-surface area that caused to increase the antibacterial potency of CdO. Various techniques, including X-ray diffraction (XRD) patterns and FESEM, were employed for structural characteristics, while Raman and UV–vis. spectroscopies for investigating the optical properties of nanopowder samples. Furthermore, to acquire insights into a putative mechanism determining the bactericidal potential of Fe-doped CdO NPs, molecular docking investigations were conducted toward specified enzymatic targets of cell wall formation and fatty acid biosynthesis processes.

2. Experimental Details

2.1. Materials

Cadmium acetate [Cd(CH3CO2)2·2H2O, 99.0%] was acquired from Sigma-Aldrich (Germany). Iron sulfate [FeSO4.7H2O, [99.0%] and NaOH, 99.99% were obtained from Uni-Chem (India) and PenReac (Germany), respectively. Methylene blue (MB) was utilized as supplied by BDH laboratories (United Kingdom), and the chemical structure is presented in Figure 1. In present work, all chemical products obtained were in pure form and used without further purifying process.

Figure 1.

Figure 1

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Chemical structure of methylene blue.

2.2. Preparation of Fe–CdO Composites

To analyze the behavior of Fe at low concentrations, CdO/Fe (1:0, 1:2, 1:4, and 1:6) nanoparticles were synthesized via the hydrothermal method. Cadmium acetate and iron sulfate (0.5 mol L–1) solutions were prepared separately in double-distilled water. Then, solutions were mixed to the prescribed quantity and agitated continuously at ambient temperatures (25–28 °C), along with adding 2 molL–1 sodium hydroxide dropwise. The preceding solution was poured into a 100 mL autoclave coated with Teflon for 10 h at 160 °C in the oven and cooled at room temperature. White precipitates yielded were extensively rinsed with double-distilled water and ethanol to alleviate contaminants. The resultant was dried at 80 °C, pulverized to a powder and annealed at 500 °C for 3 h, as illustrated in Figure 2.31

Figure 2.

Figure 2

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Schematic representation of the preparation of Fe–CdO samples.

2.3. Photocatalytic Activity

In photocatalysis,32 produced materials were monitored by evaluating MB aqueous-solution photodegradation trend (10 mg L–1). A visible radiant source of 400 W mercury lamp with wavelength 400–700 nm with lamp solution distance of about 20 cm was used. Under vigorous stirring, 50 mL of MB solution was added to 10 mg of sample powder in a beaker of equal volume (200 mL). Before the irradiation operation, the solution was agitated in the dark for 15 min to establish an adsorption–desorption equilibrium between strain and sample. The dye degradation reaction was monitored at intervals of 20 min, whereas all solutions were exposed to radiation for 80 min. UV–vis absorbance was measured using a 5 mL sample of each solution. The final degradation was evaluated by varying the maximum absorption spectrum (λmax ∼ 665 nm) using an UV–vis spectrophotometer, and the mechanism is illustrated in Figure 3.

Figure 3.

Figure 3

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Photocatalysis by harvesting mechanism visible light.

2.4. Antibacterial Activity

Pure and doped CdO nanomaterial’s in vitro antibacterial performance was tested at Escherichia coli and S. aureus collected through mastitis fluid specimens. A well-diffusing test for E. coli and S. aureus in mannitol salt and MacConkey agar was conducted for in vitro microbicidal evaluation. Media plates were wiped with typical 0.5 Mc-Farland microbial cultures, and 6 mm diameter wells were formed with a sanitized borer. Each well occupied varying concentrations of pure and Fe-doped CdO (500 and 1000 μg/0.05 mL), equated with enrofloxacin as positive (5 μg/0.05 mL) and deionized water (DIW) (50 μL) as the negative control in a sterile atmosphere.

2.5. Molecular Docking Studies

The core enzymes for cell wall assembly and fatty acid biosynthesis systems have been revealed as prospective targets for the development of antibiotics. β-Lactamase, for instance, is a prominent objective of β-lactam antibiotics because its inhibition disrupts cell wall synthesis.33,34 In this, β-lactamase and d-alanine-d-alanine ligase B (ddlB) from cell wall formation35 along with fatty acid synthase inhibitor (FabI) of fatty acid synthesis pathway36 were specified as promising targets for in silico docking analyses to ascertain the potential mechanism of Fe-doped CdO NPs for S. aureus.

The 3D structure of chosen enzyme targets was retrieved from the protein data bank using PDB IDs: 1MWU for β-lactamaseS. aureus (resolution: 2.6 Å),37 5TW8 for ddlBS. aureus (resolution: 1.7 Å)38 and 4CV1 for FabIS. aureus (resolution: 1.9 Å).39 The programme ICM Molsoft was employed in docking investigations.40 Using ICM receptor preparation tool and energy minimization tool, crystal structures for docking investigations were generated and optimized separately. Water molecules in crystal structures and native ligands [i.e., (2R,4S)-2-[(1R)-1-{[(2,6-dimethoxyphenyl)carbonyl]amino}-2-oxoethyl]-5,5-dimethyl-1,3-thiazolidine-4-carboxylic acid for β-lactamase, ceftaroline for ddlB and 1-(3-amino-2-methylbenzyl)-4-[2-(thiophen-2-yl)ethoxy]pyridin-2(1H)-one for FabI] were eliminated. The active site was designated via a grid box encompassing the co-crystallized ligand. In each, the top 10 docked conformations were determined. Each produced conformation was evaluated with the optimal binding affinity, and the least binding score was chosen for further study. Discovery studio visualizer41 and PyMOL were employed for docked complex 3D interaction study. The structure of Fe-doped CdO NPs was fabricated by modifying the CdO crystal structure acquired from PubChem by applying the ligEdit tool of ICM.

2.6. Statistical Analysis

The statistical study of the antimicrobial potency, measured using inhibition regions (mm), was performed through one-way variance analysis by SPSS 20. All experiments were performed in triplicate.

3. Results and Discussion

The XRD spectra of pristine and doped CdO samples, as depicted in Figure 4a, were performed to examine Fe influence on structural and phase composition analysis of host lattice CdO. Peaks situated at 2θ = 33.25, 39.02, 56.17, 65.96, and 68.92 were indexed to (111), (200), (220), (311), and (222) planes consecutively and match JCPDS# 73-2245 well.42 The XRD spectra presented cubic and polycrystalline structures of CdO and Cd2+, which may be replaced by Fe2+ ions at octahedral coordination.15 The addition of Fe into CdO caused peaks to move slightly toward lower θ values, which may be attributed to a slight change in the cadmium crystal lattice constant that could be owing to the difference in atomic radii or some intrinsic crystal defects. So, this confirms the successful incorporation of Fe into the crystal lattice.43 The Debye Scherrer formula computed the crystallite diameter: D = kλ/ßCosθ, where k is the shape factor (0.9), θ is the peak Bragg’s angle, λ is the X-ray radiation wavelength (1.54 Å), and β is the broadening of diffraction line measured at peak full width of half maximum (fwhm). Calculated crystallite size decreased (45–31 nm), and the broadness of peaks enhanced with an increasing amount of dopant material attributed to lower radii of Fe2+ ions than Cd2+, leading to a decline in the lattice gap. It can cause modification in crystallite behavior.44Figure 4b,c represents selected-area electron diffraction (SAED) images for pure and 4% Fe–CdO nanomaterials with four concentric well-defined spotty rings attributed to the (hkl) values of (111), (200), (220), and (311) planes which define high crystalline nature and polycrystalline behavior of nanomaterials and are well consistent with XRD results.

Figure 4.

Figure 4

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(a) XRD of synthesized pure and Fe-doped CdO (2, 4, and 6 wt %) products, (b,c) SAED images of undoped and 4% Fe–CdO samples and (d) FT-IR observed graphs.

To analyze the chemical bond structure, the FTIR technique is the most common, which recorded spectra of pristine and doped samples in the 4000 to 500 cm–1 range, as shown in Figure 4d. The band nearby 680 cm–1 is ascribed to the Cd–O stretching vibration.42 The signal at 3420 cm–1 is related to the OH stretching vibration of atmospherically absorbed water molecules, while at 2355 and 1626 cm–1, it attributes to the adsorbed CO2 and C–O vibrations, respectively.45 A modest band around 1419 cm–1 corresponds to the bending mode of −CH3, in which CO2 and water molecules may have been formed in the air during its synthesis.46 The band range from 700 to 500 cm–1 in pure and doped samples correspond to metal–oxygen stretching vibrations.47

Dynamic studies of the lattice structure provide key phonon mode information about high-quality nano-products. Raman scattering is a vital characterization technique to investigate semiconductor products and structures. Figure 5a shows Raman spectra in the 200–1200 cm-1 range of CdO and Fe–CdO nanostructures at room temperature. As reported in available articles, CdO presents Raman modes at 273, 880, and 933 cm–1.49 In principle, according to structural rocksalt selection principles, both longitudinal optical (LO) and transverse optical (TO) modes are dipoles forbidden. In the CdO spectrum, all features could be assigned to second-order Raman scattering.48 Recently, the calculated two-phonon density of states has a discrete band of 273 cm–1 and 2 TA mode at the L point of Brillouin zone (BZ) of CdO. A large curve at 931 cm –1 is observed as a 2LO mode overtone at BZ’s C- or L-point. The 2LO (L)-band changes to a reduced energy level and becomes broad. The possible reason for this shift may be a decrease in nanostructure size upon doping.50 According to Thema et al., the above transverse and longitudinal modes of CdO with internal compressive stress may be manifested in the induced surface effect.51

Figure 5.

Figure 5

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(a) Raman spectra of CdO and Fe-doped CdO products and (b) PL spectra.

Figure 5b represents the PL spectra of CdO and Fe-doped CdO nanomaterials, which depict distinct emission peaks at 411 and 428 nm and a broad band at 458 nm. The position and shape of detected bands at 411, 428, and 458 nm of pure and doped nanomaterials are almost the same. The 428 nm emission peak may be manifested in an e– and h+ combination.52 The emission peak originates from electron transition from shallow donor level formed by CdO/Fe interstitials to shallow acceptor level formed by CdO/Fe vacancies. It can be attributed to the radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy on the surface of nanostructures.53 In contrast, the 408 nm band is corresponded to near band edge or band edge emission. In contrast, depending on the particles’ size the 408 nm band corresponds to near band edge or band edge emission.54 The 458 nm shoulder peak is assigned to deep trap or surface-state emission.55 Pure CdO possessed lower PL intensity, suggesting a low recombination rate that ascribed to phosphorescence phenomena. Upon Fe doping, PL intensity was increased, indicating the high recombination rate or low separation capacity of the electron hole pair.56 It has been observed that doped samples’ PL intensity is increased compared to pure CdO, which may be attributed to the fact that as Fe-doped CdO samples are illuminated, and more electron–hole pairs are generated. The immediate recombination of produced pairs provides more photons, resulting in high PL intensity.

Morphological analysis of CdO products was investigated via FESEM, as depicted in Figure 6a. The FESEM image of (pure) CdO represents nearly spherical agglomeration of particles. Figure 6b–d depicts the increase in particle agglomeration and non-uniform distribution upon doping. The van-der Waals forces produce the common agglomeration phenomenon.56,57

Figure 6.

Figure 6

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FESEM of (a) CdO and (b–d) Fe (2, 4, and 6) wt % induced CdO.

Furthermore, compositional analysis and quality of synthesized nanoparticles were determined using the EDS spectrum. As shown in Figure 7, CdO and Fe (2, 4, and 6 wt %) embedded CdO nanoparticles composed of Cd, O, and dopant element Fe along with the small content of Cu. Zn and Na were owing to contaminants. The Fe peaks in EDS images of samples endorse Fe doping into the CdO lattice structure. For FESEM analysis, Cu coating of fabricated products was found responsible for the impurity peak of Cu in the present study.32

Figure 7.

Figure 7

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EDX spectra of (a) CdO and (b–d) Fe (2, 4, and 6) wt %-induced CdO.

The HR-TEM micrographs reveal nanomaterial size, morphology, and effect of Fe on CdO nanoparticles. Figure 8a,b and insets depict nanosized grains and tiny pores in CdO samples. In pure CdO HR-TEM image, particles are agglomerated and mixed; upon doping of Fe to CdO image (Figure 8b) showed nanosized nearly spherical agglomerated grains can be observed manifested to magneto–static interactions among nanoparticles by Fe2+ content incorporation, which is in agreement with SEM results. In Figure 8c,d, nanoparticle lattice fringes represent interplanar distances 0.231 and 0.267 nm, which correspond to (111) and (200) crystal planes of CdO nanoparticles, respectively,58,59 indicating that Fe content is well mixed into CdO host lattice and agrees with XRD analysis.

Figure 8.

Figure 8

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(a,b) HR-TEM images and (c,d) lattice fringes of pure and (4%) Fe-incorporated CdO nanomaterials, respectively.

UV–vis spectra of CdO and doped CdO samples are presented in Figure 9a. To observe absorbance analysis for the spectral range 250–900 nm at room temperature, an aqueous solution of nanoparticles was obtained under ultrasonication. The absorption spectrum of CdO depicts a sharp peak at ∼297 nm attributable to excitonic absorption behavior for the host lattice (CdO).60 The absorption spectra of doped nanomaterials revealed a slight increase in light absorbance compared to pure CdO. Study samples’ nature and band gap energy values were calculated using correlation among absorption-coefficient (denoted by α). Incident photon energy (presented by hν): (αhν)1/n = A(hν – Eg), where Eg represents the material optical band gap energy and α represents the absorption coefficient. It stands for constant, and n contains a 1/2 (direct band gap) or 2 (indirect optical-band gap) value.61 Direct band-energy of undoped and doped-CdO nanomaterials was observed from energy graph ((αhν)2 vs E) by extrapolation on the x-axis. CdO band gap value is 3.28 ± 0.02 eV;62,63 the increase in the band gap value for CdO samples indicates that the interfaces of Cd and O are effectively incorporated, along with particle size and lattice or surface defects. CdO exhibits lattice defects such as oxygen vacancies and quantum confinement effect. In contrast, doped products range from 3.14 to 2.96 eV with 0.04 to 0.76% error (Figure 9b). Thus, the doping process improves the optical transparency of host material CdO and decreases the band gap of doped samples, and further Fe content in CdO may decrease in the band gap value [45,55]. The decrease in optical band gap may be related to the Burstein-Moss effect, which states that the CdO absorption edge is shifted toward lower band gap values. This effect arises when carrier content exceeds the conduction band edge density of states via Fe doping in host material.44 The other reason for the band gap energy decrease was the semiconductor nature of CdO samples, and oxygen deficiency in CdO structure may be fulfilled by Fe ions, making it more conducting. This phenomenon is common in nanoscale materials such as pure and doped CdO.64

Figure 9.

Figure 9

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(a) UV–vis spectra and (b) direct band gap of pristine and Fe-doped CdO products.

In dye degradation, photocatalyst quantity, morphology, surface zone, and irradiation time are key elements that might impact specific dye degradation efficacy.65 Photoactivity for purified CdO and Fe (2, 4, and 6 wt %)-doped CdO were investigated through MB decomposition under a specific light source upon adsorption/desorption achieved in darkness for 15 min. Removal of dye was examined by the graph among illumination time vs MB concentration (C/C0), as depicted by Figure 10a. Following irradiation, both CdO (pure and doped) decomposed MB within 80 min. At the same time, undoped CdO had the lowest decomposition efficiency (47.5%), as indicated in Figure 10b. Intriguingly, the Fe (6 wt %)-doped CdO sample acquired the highest degradation percentage % (58.8%). Result revealed 1:6 specimens as potential luminous photocatalysts manifested by a reduced bandwidth and charge-carrier recombination. The other reason for high degradation of doped CdO may be the transition metal Fe, which provides better confinement of electrons upon irradiation that slow down the recombination of excitons.64 In Fe-doped CdO samples, Fe exists in the Fe3+ or Fe2+ form, and these ions further increase the probability of capturing electrons. At the beginning of the process, rapid desorption was observed owing to an abundance of binding sites on a large surface of products and slowed down when MB dye residual degradation took place.66,67 Furthermore, the linearity of Ln (C0/C) and irradiation time (Figure 10c) was employed to improve a first-order kinetic model for degrading kinetics, as detailed below

3. 1

where k is the apparent rate constant and C0 and C are concentrations for pollutant (mg L–1) in DIW at t0 and t times, respectively. The calculated values of k of pure and doped CdO during irradiation source are 0.1414, 0.1492, 0.1776, and 0.1923 (min)−1. Rate constants and MB photodegradation values of Fe-doped CdO obtained considerable improvements. The improvement in the value of k is due to a decrease in the band gap of nanomaterials, and lower band gap materials can harvest more light for photocatalysis. As depicted in Figure 10a, CdO nanoparticles degraded a large amount of pollutant during the first 20 min, and then, the degradation process occurred slowly. Due to the (nonlinear degradation behavior of CdO with respect to the time) dependence of degrading kinetics on Figure 10a data, the linear fit line graph deviated from the initial points.

Figure 10.

Figure 10

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(a) Plot between C/C0 versus irradiation time, (b) degradation % graph, and (c) ln(C0/C) versus illumination time.

To verify durability and recyclability, the 1:6 nanomaterial was subjected to light source exposure for up to four cycles for photo-decomposition of MB from contaminated water. After four sequential cycling photo activities (Figure 11a), nanoparticles turn off contaminant colorant removal under identical conditions. Figure 11b illustrates that the degradation % of the product decreased from 58.8 to 51.3% after four cycles prescribed for photocatalyst loss via rinsing/centrifugation in recycling trials. According to recycling test results, photocatalysts were relatively robust and exhibited substantial potential for polluted water treatment. The degree of mineralization was estimated by measuring total organic carbon (TOC) in treated water. The investigation was conducted on Fe-doped CdO nanostructures (2, 4, and 6 wt %) at various time intervals of up to 150 min. This investigation (Figure 11c) indicated that the TOC of produced compounds under visible light irradiation decreased steadily with reaction time, and a considerable quantity of mineralization of dye was noticed after 150 min in Fe (6 wt %)-doped CdO. Moreover, results suggest that the dye may undergo a series of intermediate transformations before its deterioration ultimately leads to its complete mineralization.68,69 The comparison of the current work with the literature, in terms of photocatalytic activity and reaction time is shown in Table 1.

Figure 11.

Figure 11

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(a) Fe (6 wt %)-doped CdO nanostructures reusability and (b) degradation % bar graph. (c) TOC of synthesized nanostructures.

Table 1. Comparison of Synthesized Nanostructures Degradation Efficacy with the Literature.

photocatalyst degradation efficiency (%) reaction time (min) dyes ref
Bi2CrO6 56 120 MB (70)
CeO2 34.24 120 MB (71)
SnO2 79 840 MB (72)
Ag/TiO2 51.18 180 MB (73)
TiO2 100 300 MB (74)
WO3 80 180 Methyl orange (75)
ZnO 85 120 Phenol (76)
CdO–Fe3O4 92.85 150 MB (42)
γ-Fe3/Fe3O4/SiO2 87.5 120 MB (77)
CdO–NiO 89.44 180 Methyl orange (78)
Fe–CdO 58.8 80 MB present study
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If photocatalytic efficacy of prepared Fe–CdO nanostructures was considered in terms of duration of the degradation rate, then Fe–CdO showed significant photocatalytic activity as compared to TiO2, WO3, and ZnO (Figure 12).73−76 The degradation rate of the prepared nanostructures can be calculated by eq 2.

3. 2

Figure 12.

Figure 12

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Photocatalytic activity of TiO2, WO3, ZnO, and Fe–CdO nanostructures in terms of the percentage degradation rate.

The in vitro microbicidal effectiveness in pure and doped bacteria samples was examined using a well diffusion analysis for measuring inhibition areas, with respective (half minimum inhibitory concentration) IC50 values shown in Table 2 (Figure 13a–d) and Table 3. Data collected from both samples revealed antibacterial activity. Findings demonstrated a synergistic impact between specimens produced and inhibition areas. Statistically, substantial inhibition regions (p < 0.05) for small and large concentrations against E. coli ranged (0.9 ± 0.01 to 1.4 ± 0.06 mm) and (1.65 ± 0.02 to 2.75 ± 0.07 mm), whereas (0 ± 0.0 to 2.65 ± 0.05 mm) and (2.05 ± 0.02 to 4.9 ± 0.06 mm) for S. aureus were attained in case of doped CdO nanostructures, correspondingly (Table 2). All outcomes were equated with enrofloxacin (4.30 ± 0.03 mm) and (7.0 ± 0.04 mm) for E. coli and S. aureus, correspondingly, and with DIW (0 ± 0.0 mm) (Figure 13a,b). Overall, the efficacy % age of doped samples increased from 25.5 to 32.5% and 50 to 63.9% for E. coli and 27.5–37.8% and 60.7–70% for S. aureus at least and maximum concentrations, correspondingly. Broadly, Fe (6 wt %)-doped CdO sample exhibited superior bactericidal efficacy against S. aureus in contrast with E. coli at both concentrations. Fe-doped CdO exhibited marvelous antibacterial efficacy for Gram-positive as compared to Gram-negative bacteria (Figure 13c,d). The antibacterial strength of nanoparticles is mediated by the scale and concentration of manufactured nanostructures.79,80 The bacteria are enclosed externally by ROS, and bacterial death is carried out by the extrusion of cell organelles.81,82 Micro pathogens often collapse as cations associate strongly with negative microbial cell components. Differences in bacterial cell wall architecture may account for the improved bactericidal activity. G −ve microbes have a thinner peptidoglycan coating in their cell wall and an exterior membrane made of proteins and phospholipids, whereas G +ve bacteria have a robust layer of peptidoglycan made of lipoteichoic and teichoic acids in the outer wall.83 Micro pathogenic failure occurs when cations induce dysfunction and enzyme degradation in bacterial ribosomal activity.84 The production of ROS and the release of Cd2+ ions are two variables that contribute toward the antibacterial efficacy of both pure and doped CdO samples. Since bacteria have a negatively charged cell membrane, they have a substantial electrostatic interaction with them and allowing for piercing the membrane and causing harm. When CdO nanoparticles come into touch with bacteria, Cd2+ ions are liberated from the particle’s surfaces and are taken up by the bacteria through their transport mechanisms.85

Table 2. Antibacterial Action of Fe–CdO.

samples inhibition zone (mm)a
 inhibition zone (mm)b
  500 μg/50 μL 1000 μg/50 μL 500 μg/50 μL 1000 μg/50 μL
1:0 0.9 ± 0.01 1.65 ± 0.02 0 ± 0.0 2.05 ± 0.02
1:2 1.1 ± 0.05 2.15 ± 0.04 1.95 ± 0.03 4.25 ± 0.03
1:4 1.25 ± 0.08 2.3 ± 0.05 2.4 ± 0.04 4.45 ± 0.05
1:6 1.4 ± 0.06 2.75 ± 0.07 2.65 ± 0.05 4.9 ± 0.06
enrofloxacin 4.30 ± 0.03 4.30 ± 0.03 7.0 ± 0.04 7.0 ± 0.04
DIW 0 ± 0.0 0 ± 0.0 0 ± 0.0 0 ± 0.0
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a

Inhibition areas of Fe-doped CdO for Gram-negative bacteria.

b

Measurements of zones of inhibition (mm) against Gram-positive bacteria.

Figure 13.

Figure 13

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(a) In vitro antibacterial action of Fe–CdO for E. coli and (b) S. aureus. (c) Efficacy % age for Gram-negative (d) and Gram-positive bacteria, respectively.

Table 3. IC50 of Fe–CdO High Concentrations.

samples E. coli S. aureus
  μg/mL μg/mL
1:0 4 2
1:2 2.84 1.44
1:4 1.35 1.15
1:6 0.62 0.43
enrofloxacin 0.27 0.15
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Metal-doped nanoparticles have been documented as good microbicidal agents. Their bactericidal potential is attributed to either cell wall disintegration or suppression of critical biochemical processes necessary for bacterial survival and development.

3.1. Molecular Docking Analysis

Enzymes have been reported as the main virulence factor, and inhibiting their activity will disrupt essential cell processes and ultimately cause bacterial death. Keeping in view the good antibacterial activity of Fe-doped CdO NPs in the current study, we performed in silico docking investigations to evaluate their inhibition tendency against enzymes (β-lactamase and ddlB) from the cell wall biosynthetic route along with FabI from the fatty acid biogenesis. Exploring the mechanism governing the bactericidal activity of NPs is essential and will open new horizons for doped nanoparticles as antibiotic agents.

First enzyme β-lactamase that is required in cell wall biosynthesis pathway showed an excellent binding score of −7.571 kcal/mol for Fe doped CdO NPs and key amino acids of binding pocket implicated in H-bonding interactions were Gln521 (3.0 Å), Tyr519 (3.2 Å), and Asn464 (2.1 Å), as shown in Figure 14 a and b. Similarly, good binding interactions, i.e., H-bonding with Asp264 (3.2 Å and 3.5 Å), Glu183 (2.5 Å), and Arg186 (2.1 Å), were observed inside the active pocket of ddlB enzyme (binding score −5.881 kcal/mol), as depicted in Figure 14c and d.

Figure 14.

Figure 14

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Binding interaction pattern of Fe-doped CdO NPs against selected cell wall synthesis enzymes (a,b). β-Lactamase and (c,d). ddlB from S. aureus.

The optimally docked complex of Fe-doped CdO NPs against FabI exhibited H-bonding interactions with Gly104 (2.2 Å), Arg103 (3.1 Å), and Asn158 (2.0 Å) and a binding score of −5.981 kcal/mol (Figure 15a,b). Fe-doped CdO interactions inside specified enzymes of cell wall biosynthesis and fatty acid biogenesis are predicted as potential inhibitors that are further suggested to be confirmed through enzyme inhibition studies.

Figure 15.

Figure 15

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Binding interaction pattern of Fe-doped CdO NPs against Enoyl-[acyl-carrier-protein] reductase (FabI) from S. aureus.

Using computational tools to explore new drug candidates and novel antibiotics has replaced the traditional method of drug discovery. As molecular docking studies enabled the in-depth analysis of inhibitor–enzyme complexes here, we identified these synthesized NPs as potential inhibitors for beta-lactamase and ddlB enzyme, as evidenced by their tendency to bind inside active pockets.

4. Conclusions

A cost-effective and environmentally favorable hydrothermal method was adopted to effectively prepare Fe-doped CdO nanomaterials with varying Fe concentrations (2, 4, and 6 wt %) to evaluate their microbicidal and dye degradation potential. XRD analysis revealed the cubic structure of CdO and iron (Fe) used as a doping agent reduces the crystallite size of the prepared nanostructures from 45 to 31 nm. The FTIR spectra exhibited metal–oxygen stretching vibrations between 700 and 500 cm–1, while elemental composition confirmed the presence of Fe, Cd, and O. However, doped nanomaterials displayed a significant decrease in band gap from 3.28 eV (CdO) to 2.82 eV (0.06), evidenced by the UV spectra. The formation of electron–hole pairs by Fe-doped CdO nanoparticles allowed them to play a crucial role in removing MB dye, with maximum photodegradation efficiency attained 58.8% after 80 min in MB aqueous solution when subjected to a specific irradiation source. The degradation % of 0.06 sample reduced from 58.8 to 51.3% after four cycles, demonstrating these nanostructures a stable photocatalyst. In addition, Fe-doped CdO exhibited efficient antibacterial efficacy for Gram-positive bacteria, and in silico molecular docking studies predicted inhibition of β-lactamase, d-alanine-d-alanine ligase B (ddlB), and FabI enzymes as a possible mechanism behind the antibacterial activity of Fe-doped CdO NPs.

Acknowledgments

The authors thank Higher Education Commission Pakistan through NRPU # 20-17615 for financial support. This research was funded by the Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R1) and Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c04543.

  • BET analysis (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c04543_si_001.pdf (176.9KB, pdf)

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