Green Synthesis of Zinc Oxide Nanoparticles Using Cymbopogon citratus Extract and Its Antibacterial Activity

Yasser Hussein Issa Mohammed; Saad Alghamdi; Basit Jabbar; Dina Marghani; Saba Beigh; Amr S Abouzied; Nasrin E Khalifa; Weam M A Khojali; Bader Huwaimel; Dalal Hussien M Alkhalifah; Wael N Hozzein

doi:10.1021/acsomega.3c03908

. 2023 Jul 24;8(35):32027–32042. doi: 10.1021/acsomega.3c03908

Green Synthesis of Zinc Oxide Nanoparticles Using Cymbopogon citratus Extract and Its Antibacterial Activity

Yasser Hussein Issa Mohammed ^†,^‡,^*, Saad Alghamdi ^§, Basit Jabbar ^⊥, Dina Marghani ^#, Saba Beigh ^∇, Amr S Abouzied ^○,^◆, Nasrin E Khalifa ^¶,^&, Weam M A Khojali ^○,^●, Bader Huwaimel ^○,^◊, Dalal Hussien M Alkhalifah ^▲, Wael N Hozzein ^□

PMCID: PMC10483526 PMID: 37692252

Abstract

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Excessive use of antimicrobial medications including antibiotics has led to the emerging menace of antimicrobial resistance, which, as per the World Health Organization (WHO), is among the top ten public health threats facing humanity, globally. This necessitates that innovative technologies be sought that can aid in the elimination of pathogens and hamper the spread of infections. Zinc oxide (ZnO) has multifunctionality owing to its extraordinary physico-chemical properties and functionality in a range of applications. In this research, ZnO nanoparticles (NPs) were synthesized from zinc nitrate hexahydrate, by a green synthesis approach using Cymbopogon citratus extract followed by characterization of the NPs. The obtained X-ray diffraction peaks of ZnO NPs matched with the standard JCPDS card (no. 89-510). The particles had a size of 20–24 nm, a wurtzite structure with a high crystallinity, and hexagonal rod-like shape. UV–Vis spectroscopy revealed absorption peaks between 369 and 374 nm of ZnO NPs synthesized from C. citratus extract confirming the formation of ZnO. Fourier transform infrared confirmed the ZnO NPs as strong absorption bands were observed in the range of 381–403 cm^–1 corresponding to Zn–O bond stretching. Negative values of the highest occupied molecular orbital–lowest unoccupied molecular orbital for ZnO NPs indicated the good potential to form a stable ligand–protein complex. Docking results indicated favorable binding interaction between ZnO and DNA gyrase subunit b with a binding energy of −2.93 kcal/mol. ZnO NPs at various concentrations inhibited the growth of Escherichia coli and Staphylococcus aureus. Minimum inhibitory concentration values of ZnO NPs against E. coli and S. aureus were found to be 92.07 ± 0.13 and 88.13 ± 0.35 μg/mL, respectively, at a concentration of 2 mg/mL. AO/EB staining and fluorescence microscopy revealed the ability of ZnO NPs to kill E. coli and S. aureus cells. Through the findings of this study, it has been shown that C. citratus extract can be used in a green synthesis approach to generate ZnO NPs, which can be employed as alternatives to antibiotics and a tool to eliminate drug-resistant microbes in the future.

Introduction

With the rampant use of medications including antibiotics worldwide, has emerged the problem of antimicrobial resistance (AMR), and has been estimated to be responsible for millions of death in the future if left uncontrolled.¹ As per World Health Organization (WHO), AMR is among the top 10 public health threats facing humanity, globally. This necessitates that innovative technologies be sought that can aid in the elimination of pathogens and hamper the spread of infections.²

Zinc oxide (ZnO) has multifunctionality owing to its extraordinary physico-chemical properties and functionality in a range of applications.³ Recently, zinc oxide nanoparticles (ZnO NPs) have attracted much attention in the field of nanotechnology. It is now well recognized that ZnO NPs display completely unique mechanical properties such as high catalytic activity, low melting point, semiconducting, and piezoelectric and optical properties.⁴ ZnO NPs are appealing to recent researchers because of their valuable antimicrobial, antioxidants, anticancer activity, and interesting properties like band gap of 3.3 eV at room temperature (RT) and high excitation binding energy of 60 meV.⁵⁻¹⁰ These NPs operate as image contrast agents, are target-specific, and stop medication deterioration before it happens.¹¹ ZnO, which has a nano size and a wide bandwidth, has attracted a lot of attention for uses involving its antibacterial, anticancer, antidiabetic, and antioxidant characteristics.¹²⁻¹⁴

Green synthesis of NPs involves using plants or plant parts for bio-synthesis of metallic NPs instead of using harmful products that damage the environment.^15,16 It is favored for the synthesis of ZnO NPs as this strategy is eco-friendly, cost-effective, and less toxic compared to other synthetic methods.^7,17,18 To date, various kinds of plant extracts are reported to be implemented in the successful synthesis of ZnO NPs having antibacterial properties.^16,19−22 Janaki et al.¹⁶ reported the use of powder extract of dry ginger rhizome (Zingiber officinale) for the synthesis of ZnO NPs from zinc carbonate (ZnCO₃). Selim et al.¹⁹ harnessed an aqueous extract of Deverra tortuosa and generated ZnO NPs using zinc nitrate hexahydrate (Zn(NO₃)₂ 6H₂O). NPs of ZnO through such studies are shown to possess efficient antibacterial activities against various pathogenic bacteria.

Cymbopogon citratus Stapf. (Lemon grass) is a widely used herb in tropical and subtropical countries as a source of essential oil.²³ The plant belongs to the Poaceae family, which comprises about 500 genera, commonly used in soups, teas, etc.²⁴ The leaf essential oil of this plant holds a high economic value since it is commonly used in traditional food products due to its antipyretic, antiinflammatory, antispasmodic, and diuretic properties.^25,26 Essential oil of C. citratus contains volatile compounds such as, monoterpenes, sesquiterpenes, and phenyl propionoids. Citral (>70%) an oxygenated terpenoid (aldehyde) is a major content present in it which has antibacterial and antifungal properties.²⁷ According to studies, the presence of citral, a potent bioactive component with antibacterial activity, gives this plant a strong lemon-like perfume;²⁸ for this reason, C. citratus is also known as lemon grass. C. citratus has extensive applications in the fields of medicine, cosmetics (perfumes, soap, etc.), and brewing.²⁹ Its dried leaves’ aqueous extract has historically been used to treat cancer, diabetes, neurological diseases, and digestive disorders.³⁰ In addition to its medical applications, C. citratus is utilized as a scent in the production of fragrances, soaps, detergents, and home cleaning products, due to its antibactericidal action and pleasant fragrance after use.^29,31 Moreover, the secondary metabolites present in C. citratus extract, such as terpenes and phenols,³² can act as reducing agents during the synthesis of ZnO NPs. These reducing agents are known to reduce the metal precursors and act as fuel for the reaction.³³

One of the most notable aspects of C. citratus is its essential oil, which is extracted from the leaves and stems of the plant. The essential oil of C. citratus is highly valued for its distinct citrusy fragrance and various therapeutic properties. It contains a complex mixture of bioactive compounds that contribute to its medicinal benefits. The essential oil of C. citratus is known to be rich in several bioactive compounds, with the most prominent ones being citral, geraniol, and limonene. Citral is the major component, accounting for up to 70–85% of the oil’s composition.³⁴ It is a mixture of two isomers, geranial (citral A) and neral (citral B), which contribute to the characteristic lemon-like scent of the oil. Geraniol is another significant compound found in C. citratus essential oil, typically ranging from 5 to 15%.³⁵ It possesses antimicrobial, antiinflammatory, and antioxidant properties, and it contributes to the pleasant floral aroma of the oil. Limonene is a common terpene found in various citrus fruits, including lemons and oranges. It is also present in C. citratus essential oil, albeit in lower quantities compared to citral and geraniol. Limonene exhibits antioxidant and antimicrobial activities and contributes to the refreshing scent of the oil. In addition to these major components, C. citratus essential oil contains other minor compounds, such as myrcene, citronellal, linalool, and nerol. These compounds contribute to the overall aroma and therapeutic properties of the oil, albeit in smaller concentrations. The bioactive compounds present in C. citratus essential oil contribute to its various therapeutic properties. Several studies have highlighted the antimicrobial activity of the oil against a wide range of bacteria, fungi, and even some viruses.^36,37 The high citral content is responsible for the potent antimicrobial effects of the oil.³⁸

Molecular docking is a computational method that predicts the binding of small molecules to target proteins. It involves the calculation of the energy and geometry of the protein–ligand complex and the identification of the most energetically favorable binding mode. The method is widely used in drug discovery and design, as well as in the study of protein–ligand interactions. In this study, molecular docking was used to investigate the binding affinity of ZnO NPs with the 4urm receptor. Nowadays, molecular docking is frequently employed as a method of energetic virtual simulation where a protein and ligand interact energetically. The interaction between a drug and a protein is used as a model to predict the preferred binding sites of protein and to analyze the orientation of the docked ligand on the active site of the protein.³⁹⁻⁴¹

In this study, we report the preparation of ZnO NPs by simple, cost-effective, one-step combustion method by using C. citratus extract as a fuel which involved no toxic agents to reduce zinc nitrate hexahydrate to ZnO NPs. Major compounds present in the extract were identified by GC/MS. The antibacterial activities of ZnO NPs were investigated against Gram-positive and -negative bacteria. The authors showed that the C. citratus extract was able to reduce the metal precursors in the synthesis of ZnO NPs, thus providing evidence for its efficacy as a reducing agent.

Materials and Methods

Zinc nitrate hexahydrate (Zn(NO₃)₂ 6H₂O) was purchased from Sigma-Aldrich, India. Double-layered muslin cloth and Whatman no. 1 filter paper were obtained from HI Media, India. All other chemicals and reagents were of AR grade and purchased from Merck Millipore, India. The plastic ware and glassware used were maintained in a sterile condition.

Collection of Essential Oil

C. citratusessential oil was purchased from the Central Institute of Medicinal & manufacturing of essential oils Aromatic Plants, Regional Centre, UAS, GKVK Campus, Hebbal, India. The oil collected was analyzed using a gas chromatography–mass spectrometry (GC–MS).

Synthesis of ZnO NPs

Five grams of zinc nitrate hexahydrate Zn(NO₃)₂ 6H₂O was dispersed with C. citratusessential oil of various volume (2, 4, 6, 8, and 10 mL) and kept for steering at RT of 5 min. Afterward, the mixture was placed in a preheated muffle furnace maintained at 400 ± 10 °C. The obtained product was subjected to characterization and antibacterial activity.

Characterization of ZnO NPs

The UV–vis absorbance of the produced solution sample was used to characterize the synthesized ZnO NPs using a UV–Vis spectrophotometer. The structure and particle size of the powdered sample were determined by X-ray diffraction (XRD) analysis at RT. With a voltage of 15 kV, the structural morphology, shape, and size of the particles were examined.

XRD Characterization

The phase purity and the crystallinity of the ZnO NPs were characterized by XRD using a Rigaku Desktop Miniflex II X-ray powder diffractometer, Japan, with Cu kα radiation with an angle 2θ (λ = 0.15418 nm). The particle size was determined by the Scherrer equation for all samples as follows.

where λ is the wavelength (Cu Kα) of X-Rays, β is the full width at the half-maximum (FWHM) of the peak, and θ is the diffraction angle.

The XRD pattern of ZnO NPs was analyzed with the ICDD Powder Diffraction File database (International Centre for Diffraction Data) using Crystallographic Search-Match Version 2. The studies on the surface morphology of ZnO NPs were performed by scanning electron microscopy (Hitachi S-3400N, Japan).

Optical Characterization

The optical properties of monodispersed ZnO NPs solution were confirmed by ultraviolet–visible spectroscopy (UV–Vis) using Systronics model (Double beam spectrometer 2203), India, at RT in the range of 200–800 nm. The ZnO NPs were examined for the presence of biomolecules using Fourier transform infrared (FTIR) spectra and were recorded on a Perkin Elmer Spectrum 1000 (himadzu-8400S), Japan in the range of 450–4000 cm^–1.

Studies on HOMO and LUMO

The B3LYP/6-31G basis set was used using the Gaussian 09 program to carry out the theoretical quantum chemistry research (HOMO and LUMO studies). To shape HOMO and LUMO orbitals, Gauss View 5.0 visualization software was used.⁴²

Adsorption, Distribution, Metabolism, Excretion, and Toxicity

To predict the significant pharmaceutical and toxicological properties, we calculated ADMET (adsorption, distribution, metabolism, excretion, and toxicity) properties for all four selected molecules using Swiss ADME and ADMET SAR, online servers. The SwissADME server was used to calculate physiochemical properties such as molecular formula, molecular weight, number of heavy atoms, number of aromatic heavy atoms, number of rotatable bonds, number of hydrogen bond donor and acceptor, total polar surface area, Lipophilicity values, water solubility, Pharmacokinetics parameter, and Lipinski violations. The AdmetSAR server was used to calculate Caco-2 cell permeability, brain/blood barrier, human intestinal absorption, carcinogens, and acute oral toxicity.⁴³

Molecular Docking of ZnO NPs

A newly developed tool is molecular docking, which analyzes the orientation of the docked ligand on the protein’s active site and studies the various physiological affinities of the docked ligand using an interaction between the ligand and a protein as a model. The Protein Data Bank (PDB) was used to get the crystal structure of the desired protein 4urm. AutoDockTools-1.5.6 module was then used to create the protein’s three-dimensional structure and the residues that make up its active site pocket. The interaction between the ligand and protein was achieved using the AutoDockTools-1.5.6 simulation module, AutoDockTools, and the interaction with the highest interaction energy score between the ligand and protein was deemed to be the best possible interaction. By examining its physiological affinities, a more effective therapeutic strategy may be possible.⁴⁴⁻⁴⁶

Disc Diffusion Method for Antibacterial Activity

Using the Mueller Hinton broth doubling dilution, the minimum inhibitory concentration (MIC) was determined.

The MIC was determined by the Mueller Hinton broth doubling dilution (2–0.004 mg/mL) method using the 96-well plate method. 200 μL of each ZnO NPs dilution (2000–4 μg/mL) sample were plated in a sterile 96-well microtiter plate. Finally, 10 μL of test bacterial cell suspension (10⁸ CFU/mL) was added to a 96-well microtiter plate and incubated at 37 °C for 24 h. Positive control tetracycline (10 μg/mL) and ZnO NPs-free and tetracycline-free solutions were used as negative control, respectively. The above experiments were repeated three times (n = 3). After incubation, to each well 50 μL of p-iodonitrotetrazolium violet (INT) (0.5 mg/mL) was added as an indicator, and optical density at 630 nm (OD 630) was measured with a microtiter plate reader, BMG 11 abtech, Germany. The MIC was recorded as the lowest concentration of ZnO NPs at which no color change was observed as the MIC value.

Live/Dead Test by Fluorescent-Based Dyes

E. coli and S. aureus bacterial death analysis was determined by the fluorescent-based cell live/dead test. E. coli and S. aureus bacterial suspensions at 10⁸ CFU/mL were treated with ZnO NPs concentration which showed MIC were incubated at 37 °C for 24 h. After the incubation period, the bacterial cells were stained with 1 μL of acridine orange/ethidium bromide (AO/EB) (1:1) solution (100 μg/mL) for 15 min in dark and imaged using a Carl Zeiss fluorescence microscope (excitation filter BP 490; barrier filter O 515), Germany, at 40× magnification. The bacterial cell suspension without ZnO NPs treatment was taken as the negative control.⁴⁸⁻⁵⁰

Statistical Analysis

The antibacterial assay was carried out in triplicate and the values were expressed as mean ± standard error (SE) (n = 3). Statistical analysis was done using two-way ANOVA followed by Bonferroni’s multiple comparison posttest using GraphPad Prism version 3.00 for Windows (GraphPad Software, San Diego, CA). Differences were considered significant when P < 0.05.

Results and Discussion

ZnO NPs have gained significant attention in recent years due to their unique properties and potential applications in various fields, including cosmetics, biomedicine, and agriculture. In the cosmetic industry, ZnO NPs are used as a UV filter in sunscreens due to their high UV absorption capacity and low skin irritation potential. In biomedicine, ZnO NPs have shown promise as antibacterial and anticancer agents, while in agriculture, they have been used as nanofertilizers and pesticides. However, the use of ZnO NPs has also raised concerns regarding their potential toxicity and environmental impact, especially in comparison to other products already present in the market. One of the most commonly used UV filters in sunscreens is titanium dioxide (TiO₂) NPs. TiO₂ NPs have a similar UV absorption capacity to ZnO NPs and are considered safe for use in cosmetics due to their low skin penetration and low toxicity. However, TiO₂ NPs have limited stability in formulations and can cause skin irritation in some individuals. In comparison, ZnO NPs have higher stability in formulations and are less likely to cause skin irritation, making them a preferred option for use in sunscreens.⁵¹

In the field of biomedicine, ZnO NPs have been compared to silver NPs (AgNPs) as antibacterial agents. While both NPs have shown antibacterial activity, ZnO NPs have been found to be more effective against a wider range of bacteria and have lower toxicity than AgNPs.⁵² In addition, ZnO NPs have been shown to have potential anticancer activity, with studies demonstrating their ability to induce cell death in cancer cells.⁵³ However, the use of ZnO NPs in biomedicine is still in its early stages, and more research is needed to fully understand their potential applications and toxicity.

In agriculture, ZnO NPs have been compared to traditional chemical fertilizers and pesticides. While chemical fertilizers and pesticides have been widely used in agriculture, they have raised concerns regarding their environmental impact and potential health risks. In comparison, ZnO NPs have been shown to have lower toxicity and can be more easily absorbed by plants, leading to improved growth and yield.⁵⁴ However, the use of ZnO NPs in agriculture also raises concerns regarding their potential accumulation in soil and potential impact on soil microorganisms. Overall, the use of ZnO NPs in various fields has both advantages and disadvantages in comparison to other products already present in the market. While ZnO NPs have shown potential as effective and safe alternatives to other products, their potential toxicity and environmental impact require further investigation. It is important to carefully evaluate the benefits and risks of using ZnO NPs in comparison to other products and to regulate their use accordingly. Moreover, the fabrication of NPs is nowadays a routine, but the correct and useful utilization is the real challenge.^55,56

The essential oil industry has seen a surge in popularity over the past few years, with many people turning to these natural extracts for various purposes, such as aromatherapy, skincare, and household cleaning. One such essential oil that has gained significant attention is lemongrass oil, which is derived from the C. citratus plant. While lemongrass oil is already known and widely used for its cleaning properties, it is important to focus on its novelty and differences compared to other essential oils, as well as the environmental implications of its widespread use.^23,31

Although lemon grass oil shares some similarities with other essential oils like tea tree, eucalyptus, and lavender, there are key differences that set it apart from other essential oils. The primary constituents are citral, geranial, and neral, which are responsible for the oil’s strong lemon-like scent and its antimicrobial, antifungal, and anti-inflammatory properties. Lemon grass oil has a bright and refreshing citrus scent, which is preferred by many users for its uplifting and invigorating effects. This makes it a popular choice for home cleaning products as it leaves a pleasant fragrance after use. While many essential oils possess antimicrobial properties, studies have found that lemon grass oil is particularly effective against a wide range of bacteria and fungi, including common household pathogens like E. coli and S. aureus. Lemon grass oil contains citral, geraniol, and other compounds that have been shown to be effective in repelling insects like mosquitoes, ants, and flies. This makes it a valuable addition to both indoor and outdoor cleaning products.^29,57

While lemon grass oil has many benefits, it is essential to consider the environmental implications of its widespread use, particularly in home cleaning. The increased demand for lemongrass oil can put pressure on the natural resources used to produce it. Sustainable harvesting practices should be implemented to ensure that the plant can continue to thrive without causing environmental harm. As the use of lemongrass oil becomes more prevalent, it is essential to consider the environmental impact of its packaging. Manufacturers should opt for eco-friendly, reusable, or recyclable materials to minimize waste and reduce the carbon footprint associated with the production and disposal of packaging. Consumers should be mindful of their use of lemongrass oil, using it sparingly and only when necessary. This will help reduce the overall demand for the oil and lessen the pressure on natural resources. By sourcing the oil from sustainable farms, using eco-friendly extraction methods, and prioritizing waste reduction, the environmental impact can be reduced. Therefore, focusing on the novelty and differences of lemongrass oil, as well as its environmental aspects, can help ensure that this versatile essential oil continues to be an effective and sustainable solution for various purposes, including home cleaning. By understanding and addressing the environmental implications of its use, responsible consumption can be promoted and preserve resources for future generations.^23,29,58

GC–MS Analysis

The Clarus 680 GC, USA, was used in the analysis. A fused silica column, packed with Elite-5MS (5% biphenyl 95% dimethylpolysiloxane, 30 mm × 0.25 mm ID × 250 μm df) was implemented and the components were separated using helium as carrier gas at a constant flow of 1 mL/min. The injector temperature was set at 260 °C during the chromatographic run. 1 μL of C. citratus essential oil sample was injected into the instrument the oven temperature was as follows: 60 °C (2 min), followed by 300 °C at the rate of 10 °C min^–1, and 300 °C, where it was held for 6 min. The mass detector conditions were 240 °C, ion source temperature 240 °C, ionization mode electron impact at 70 eV, a scan time 0.2 s, and scan interval of 0.1 s. Fragments were in various molecular weights, i.e., from 40 to 600 Da. The spectra of the components were compared with the database of spectrum of known components stored in the GC–MS NIST (2008) library (Figure 1 and Table 1).

Representative GCMS chromatogram of *C. citratus*.

Table 1. GC–MS Analysis of the Compounds Present in the C. citratus Extract.

sr. no.	RT	% A	common name	chemical name	molecular weight	molecular formula
1	6.5	1.13	limonene	1-methyl-4-(1-methylethenyl)cyclohexene	136	C10H16
2	7.915	1.78	geraniol	3,7-dimethyl-2,6-octadien-1-ol	154	C10H18O
3	8.596	6.121	citronellal	3,7-dimethyl-6-octenal	154	C10H18O
4	8.826	1.375	cyclopropanemethanol, 2-methyl-2-(4-methyl-3-pentenyl)-	22-methyl-2-(4-methyl-3-penten-1-yl)cyclopropanemethanol, (1R)-	168	C11H20O
5	10.161	18.974	verbenol	(1R,2R,4S)-4-(2-hydroxypropyl)-1-methylcyclohexanol	152	C10H16O
6	10.456	21.582	carane, 4,5-epoxy-, trans	4,5-epoxy-, trans is trans-4,5-epoxycarane	152	C10H16O
7	10.902	23.397	trans-p-mentha-2,8-dienol	(2Z)-3,7-dimethyl-2,6-octadien-1-ol	152	C10H16O
8	11.507	1.136	citronellyl acetate	3,7-dimethylocta-2,6-dienyl acetate	198	C12H22O2
9	11.907	7.791	bicyclo[2.2.1]heptane 7,7-dimethyl-2-methylene-	7,7-dimethyl-2-methylenobicyclo[2.2.1]heptane	136	C10H16
10	12.257	1.341	caryophyllene	(1R,4E,9S)-4,11,11-trimethyl-8-methylene-bicyclo[7.2.0]undec-4-ene	204	C15H24

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The GC–MS analysis of the C. citratus essential oil revealed the presence of ten major compounds, which are listed in Table 1. The identified compounds belong to different chemical classes, including terpenes, aldehydes, ketones, and esters. The first compound identified was limonene, which eluted at 6.5 min with a peak area of 1.13%. Limonene is a monoterpene that is commonly found in citrus fruits and has been reported to have antioxidant, antiinflammatory, and anticancer properties.⁵⁹ The second compound identified was geraniol, which eluted at 7.915 min with a peak area of 1.78%. Geraniol is a monoterpene alcohol that has been reported to have antimicrobial, antitumor, and antioxidant activities.⁶⁰ Citronellal was the third compound identified, eluting at 8.596 min with a peak area of 6.121%. Citronellal is a monoterpene aldehyde that is commonly found in essential oils and has been reported to have insecticidal, antifungal, and antimicrobial properties.⁶¹ The fourth compound identified was cyclopropanemethanol, 2-methyl-2-(4-methyl-3-pentenyl)-, which eluted at 8.826 min with a peak area of 1.375%. This compound is a cyclic alcohol that has been reported to have antibacterial and antifungal activities. Verbenol was the fifth compound identified, eluting at 10.161 min with a peak area of 18.974%. Verbenol is a sesquiterpene alcohol that is commonly found in conifers and has been reported to have insecticidal and antifungal properties.⁶² The sixth compound identified was trans-4,5-epoxycarane eluting at 10.456 min with a peak area of 21.582%. This compound is a cyclic terpene that has been reported to have antioxidant and antimicrobial activities.⁶¹ The seventh compound identified was trans-p-mentha-2,8-dienol, eluting at 10.902 min with a peak area of 23.397%. This compound is a monoterpene alcohol that is commonly found in essential oils and has been reported to have antibacterial and antifungal properties. Citronellyl acetate was the eighth compound identified, eluting at 11.507 min with a peak area of 1.136%. This compound is an ester that is commonly found in essential oils and has been reported to have insecticidal and antimicrobial activities.⁶² The ninth compound identified was bicyclo[2.2.1]heptane, 7,7-dimethyl-2-methylene-, eluting at 11.907 min with a peak area of 7.791%. This compound is a cyclic hydrocarbon that has been reported to have antifungal and antibacterial activities. Finally, caryophyllene was the tenth compound identified, eluting at 12.257 min with a peak area of 1.341%. Caryophyllene is a sesquiterpene that is commonly found in spices and has been reported to have anti-inflammatory and antioxidant properties.⁶¹

C. citratus contains significant amounts of secondary metabolites present in it which can act as reducing agents (fuel) during the synthesis of ZnO NPs. The GC–MS analysis of crude oil isolated from the leaves of C. citratus revealed that it was a mixture of compounds, and the major compounds were trans-p-mentha-2,8-dienol, trans-4,5-epoxycarane, verbenol, geranial, and citronellal. The citronellal content gives the lemonish smell that is a characteristic of C. citratus.^63,64 Citral is a monoterpene aldehyde that is found in the essential oils of various plants, including C. citratus. It has a lemony scent and is commonly used as a flavoring agent in food and beverages.⁶⁵ Citronellal, which eluted at 8.596 min with a peak area of 6.121%, is another monoterpene aldehyde that gives C. citratus its characteristic lemonish smell. Citronellal has been reported to have insecticidal, antifungal, and antimicrobial properties. It is also used as a fragrance in cosmetics and perfumes.⁶⁶

Citral is a monoterpene aldehyde that is a major component of essential oils found in various plants, including C. citratus. Citral is composed of two stereoisomers, namely geranial (cis-3,7-dimethyl-2,6-octadienal) and neral (trans-3,7-dimethyl-2,6-octadienal). These stereoisomers have slightly different chemical properties and can have different biological activities. Citral is a terpenoid compound that is found in many essential oils, including lemon grass, lemon myrtle, and lemon verbena. It is composed of two isomers, geranial and neral, which have similar chemical structures but differ in their molecular arrangements. Geranial has a stronger lemon-like scent than neral, which has a more floral scent. Citral is widely used in the fragrance and flavor industries, as well as in the production of insecticides, fungicides, and other chemicals. Geranial, also known as citral A, is one of the two isomers that makes up citral. It has a strong lemon-like scent and is commonly used in fragrances and flavorings, as well as in the production of insecticides and other chemicals. Neral, also known as citral B, is the other isomer that makes up citral. It has a more floral scent than geranial and is also commonly used in fragrances and flavorings, as well as in the production of insecticides and other chemicals. Citronellal is a monoterpenoid compound that is found in many essential oils, including citronella, lemongrass, and lemon eucalyptus. It has a lemon-like scent and is commonly used in fragrances and flavorings, as well as in the production of insecticides, fungicides, and other chemicals. Unlike citral, citronellal does not have isomers.

In the GC–MS analysis of the C. citratus extract presented in the study, citral was not specifically listed as a compound, but its two stereoisomers, geranial and citronellal, were detected. It is possible that neral, the other stereoisomer of citral, was not detected in the analysis due to its relatively low abundance or its inability to be well separated and detected by the GC–MS method used. It is worth noting that the presence of citral in C. citratus has been reported in numerous studies, and it is regarded as a characteristic compound that gives a lemony aroma and flavor to the plant. Furthermore, the other compounds detected in the GC–MS analysis, such as verbenol, carane, and trans-p-mentha-2,8-dienol, were also found to be efficient reducing agents in the synthesis of ZnO NPs, highlighting the importance of multiple compounds present in the extract for the reduction process.

The GC–MS analysis of C. citratus essential oil revealed the presence of 10 major compounds, including terpenes, aldehydes, ketones, and esters. These compounds have been reported to have various biological activities, including antimicrobial, antifungal, insecticidal, and antioxidant properties. The detailed interpretation of the GC–MS spectrum provides valuable information regarding the chemical composition of the essential oil and its potential applications in different fields.

XRD Pattern

XRD patterns of the ZnO NPs prepared at different concentrations of plant extract (2–10 mL) are illustrated in Figure 2. All the diffraction peaks of the (100), (002), (101), (102), (110), (103), (200), (112), and (201) reflections can be indexed to the known hexagonal wurtzite structure of ZnO with lattice constants of samples shown to be around a = b = 3.2425 Å, and c = 5.2055 Å which were well matched with crystallography card of ZnO. The Miller indices (1 0 0), (0 0 2), and (1 0 1) XRD peaks correspond to Bragg angles 31.6°, 34.6°, and 36.5°, respectively, which indicated that the ZnO NPs were high crystalline.⁶⁷⁻⁶⁹

The obtained diffraction peaks of ZnO NPs matched with the standard JCPDS card (no. 89-510) as shown in Figure 2. These diffraction peaks indicated the synthesized ZnO NPs have high purity (up to 95%) of the wurtzite structure. The XRD pattern indicated the polycrystalline hexagonal ZnO phase in the sample as reported by Chen et al.,⁷⁰ Alamdari et al.,⁷¹ and Faisal et al.⁷²

The grain size (D) of ZnO samples was calculated from the Scherrer equation using the sharpest reflection at Bragg angles 36.4°. The size of the particles was found to be around 20–24 nm for ZnO NPs preparation (2–10 mL).

Scanning Electron Microscopy Result

The ZnO NP sample was observed by the Scanning Electron Microscope (SEM; Zeiss, Germany) to study the ZnO NPs preparation (2–10 mL). Figure 3 shows the SEM image of composited ZnO NPs. The image indicated individual ZnO NPs as well aggregated form. The image revealed that the particles were aggregated in hexagonal shapes and granular nano-sized in nature (under 2 μm of size aggregation for all sample cases). The SEM image clearly indicates the size and shape of ZnO NPs. The NP size was determined from data included in the XY plane and defined through the equivalent projected area diameter (Deq) measurement. Crystals of various sizes were produced from each concentration of the plant extract. The range of particle sizes was 25–30 nm; this result was close to the size observed from the XRD analysis.

(A) SEM images of ZnO NPs preparation (2–10 mL). (B) Hexagonal rod-like shape of ZnO NPs as shown by SEM.

UV–Vis Spectroscopy Result

The effect of concentration on the synthesis of ZnO NPs from zinc nitrate hexahydrate dispersed with C. citratus essential oil was determined using UV–Vis spectroscopy. This operational parameter was monitored at various concentrations of C. citratus essential oil (i.e., 2, 4, 6, 8, and 10 mL).

For the different concentrations, the absorption peaks observed were 373 nm (with 2 mL of C. citratus essential oil), 374 (with 4 mL), 372 (with 6 mL), 369 (with 8 mL), and 370 (with 10 mL) (Figure 4). Peaks of different heights confirm the production of ZnO NPs with no significant change in wavelength. Varying the concentration of C. citratus essential oil affected the size of the ZnO NPs produced and hence the different absorbance peaks. Goh et al.⁷³ studied the influence of particle size on the UV absorbance of zinc oxide NPs and showed that the absorbance of ZnO NPs increases with increasing size for particle sizes from 15 to 40 nm. As per the XRD and SEM results of ZnO NPs (synthesized using C. citratus) indicated that particle size falls within the range reported by Goh et al.,⁷³ and the different absorption peaks observed indicate variation in particle size.

UV–Vis absorption spectra of ZnO NPs preparation (2–10 mL).

The absorption spectrum of the green synthesized ZnO NPs with the absorption peaks between 369 and 374 nm indicated that ZnO NPs exhibit absorption of semiconductors materials range, which is due to their large exciton binding energy at RT.^67,69 Absorption in this wavelength range also confirmed that the spectrum had blue-shifted with respect to the bulk absorption spectra (377 nm) of the ZnO NPs; this blue shift in the absorption edge of samples is related to the quantum confinement effect through the individual ZnO NPs, at various concentrations of C. citratus samples. Such absorption of ZnO NPs in the UV region suggests their appropriateness for medical applications including sunscreen protectors or antiseptic creams as also reported by Selim et al.¹⁹

FTIR Spectroscopy

FT-IR measurement was carried out in the wave number ranging from 400 to 3500 cm^–1 using the KBr method at RT, and the results are shown in Figure 5. ZnO NPs showed strong absorption peaks at 399, 403, 381, 388, and 395 cm^–1 for ratio concentration of C. citratus increasing of ZnO NPs synthesized samples which can be attributed to the (metal–oxygen) stretching mode of ZnO, indicating the presence of ZnO NPs in the synthesized samples. Peaks that appeared between 450 and 465 cm^–1 in all ZnO NPs samples in lower energy as a result of the bond bending vibration of ZnO.⁷⁴ The region between 400 and 600 cm^–1 could be attributed to metal–oxygen.⁷⁵ The peaks that appeared in the range of 1550–1400 cm^–1 are due to organic bonds of remainder of C. citratus in samples that were in low energy. The intense bands observed at 1250–1100 cm^–1 represent C–O stretching vibration⁷⁶ depicting the presence of organic material, suggesting their role in the green synthesis of the ZnO NPs.

FTIR spectra of ZnO NPs synthesized using various concentration of *C. citratus* (2–10 mL).

Studies Using HOMO and LUMO to Determine the Compound ZnO’s Molecular Orbital Energies

The compound ZnO was used for HOMO and LUMO studies depending on the results of docking studies. Energy-level diagrams for the frontier molecular orbitals HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) were created. The molecular orbital energies and energy gap of compound ZnO were investigated. The energy gap between molecular orbitals is critical to a molecule’s kinetic stability as well as intermolecular charge transfer interactions.⁷⁷ High kinetic stability is associated with a large energy gap between molecular orbitals. The molecule will be more reactive if the energy difference between HOMO and LUMO is small.⁷⁸Figure 6 depicts the distribution of molecular orbitals HOMO, HOMO–1, LUMO, and LUMO+1 levels, as well as their energy gaps computed using the B3LYP/631G(d,p) basis set for the compound. The energy gap between HOMO and LUMO levels is estimated to be −21.37 eV, while the gap between HOMO–1 and LUMO+1 is −4.54 eV. The HOMO–LUMO energy gap reflects the molecule’s biological activity. Negative values of HOMO–LUMO for ZnO NPs inferred good stability and indicated good potential to form a stable ligand–protein complex.⁷⁹

Energy of FMOs title compound in different energy levels.

Stability of the ZnO NPs

The stability of NPs can be assessed by studying the molecular orbital energies of the compound they are composed of. In the case of compound ZnO, the HOMO and LUMO energies were investigated to determine their stability. The energy-level diagrams of the frontier molecular orbitals, HOMO and LUMO, were created to analyze the molecular orbital energies and the energy gap of compound ZnO. The energy gap between molecular orbitals is crucial for the kinetic stability of a molecule and its intermolecular charge transfer interactions. A larger energy gap between the HOMO and LUMO indicates higher kinetic stability. Conversely, if the energy difference between the HOMO and LUMO is small, the molecule is more reactive. The stability of ZnO NPs is assessed based on the energy gap between the HOMO and LUMO. According to the calculations using the B3LYP/631G(d,p) basis set, the energy gap between the HOMO and LUMO levels of compound ZnO was estimated to be −21.37 eV. Additionally, the gap between the HOMO–1 and LUMO+1 was found to be −4.54 eV. The HOMO–LUMO energy gap is an indicator of the biological activity of the molecule. In the case of ZnO NPs, the negative values of the HOMO–LUMO energy gap suggest good stability. This implies that the ZnO NPs have the potential to form stable ligand–protein complexes, which is favorable for their biological applications.⁷⁷⁻⁷⁹Table 2 presents additional global reactivity descriptor values calculated for the title molecule, including the HOMO energy (E_H), LUMO energy (E_L), energy gap (E_g), global hardness (η), softness (β), chemical potential (μ), electronegativity (χ), and total energy. In Figure 6, the energy distribution of the molecular orbitals, including HOMO, HOMO–1, LUMO, and LUMO+1 levels, is depicted for the title compound.

Table 2. Global Reactivity Descriptor Values of the Title Molecule, FMOs, and the Energy Gap.

molecular properties	values calculated by B3LYP/631G(d, p)
E_HOMO (EH)	–15.431 eV
E_LUMO (EL)	–2.349 eV
energy gap (E_g)	–21.37 eV
global hardness (η)	2.26 eV
softness (ζ)	0.22 eV
chemical potential (μ)	–4.19 eV
electrophilicity (ψ)	3.88 eV
electronegativity (χ)	4.19 eV
total energy	61.4943 kcal/mol

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These findings provide valuable insights into the stability and reactivity of ZnO NPs based on their molecular orbital energies and energy gaps. The negative HOMO–LUMO energy gap indicates good stability and suggests potential applications in forming stable ligand–protein complexes.

ZnO NPs Exhibit Promising Antibacterial Activities

The ZnO molecule was soluble and safely passed the Lipinski violations⁸⁰ (Table 3), as shown by the numerous ADMET properties calculated for it. Table 4 shows that the substance ZnO can have a harmful effect when taken orally.

Table 3. Numerous ADMET Characteristics of the Compound ZnO.

properties	molecule
properties	ZnO
formula	OZn
MW	81.38
#heavy atoms	2
#aromatic heavy atoms
#rotatable bonds	0
#H-bond acceptors
#H-bond donors
MR	0.69
TPSA	17.07
consensus log P	–0.43
Ali class	nonsoluble
silicos-IT log S	0
silicos-IT class	non
log K_p (cm/s)
Lipinski #violations
bioavailability score
GPCR ligand	–3.85
ion channel modulator	–3.83
kinase inhibitor	–3.86
nuclear receptor–ligand	–3.86
protease inhibitor	–3.85
enzyme inhibitor	–3.66

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Table 4. Compound ZnO Toxicity Prediction Using ADMET SAR.

model	BPU
blood–brain barrier	result	BBB+
blood–brain barrier	probability	0.9837
CYP inhibitory promiscuity	result	low CYP inhibitory promiscuity
CYP inhibitory promiscuity	probability	0.9062
human intestinal absorption	result	HIA+
human intestinal absorption	probability	0.9838
Caco-2 permeability	result	Caco2+
Caco-2 permeability	probability	0.6708
carcinogenicity (three-class)	result	non-required
carcinogenicity (three-class)	probability	0.5245
acute oral toxicity	result	II
acute oral toxicity	probability	0.5839
rat toxicity LD50 (mol/kg)	result	2.2618

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The results indicate that the ZnO molecule is soluble and does not violate Lipinski’s rule of five,⁸⁰ which suggests good pharmacokinetic properties for drug candidates. In addition, the compound has been analyzed for its ADMET properties, which are essential parameters for evaluating a drug candidate’s safety and efficacy profile.⁸¹Table 3 presents several ADMET properties of ZnO, including molecular weight, number of heavy atoms, rotatable bonds, hydrogen bond acceptors and donors, and others. The molecular weight of ZnO is 81.38, and the compound contains only two heavy atoms, which are both metal atoms. ZnO does not have any rotatable bonds, hydrogen bond acceptors, or donors. Moreover, the compound has low lipophilicity, with a Consensus log P of −0.43, indicating that it is hydrophilic.⁸² The compound’s Ali Class is nonsoluble, suggesting that the compound does not dissolve in water.⁸³ However, the Silicos-IT log S(84,85) is 0, indicating that the compound has no potential to penetrate the cell membrane.

Table 4 reports the results of ZnO’s toxicity predictions using ADMET SAR models. ZnO has a harmful effect when taken orally, as the acute oral toxicity result is II with a probability of 0.5839. However, ZnO has a low probability of CYP inhibitory promiscuity, indicating that it is not likely to interfere with the metabolism of other drugs. The compound is predicted to have good blood–brain barrier (BBB+) penetration with a probability of 0.9837, and high human intestinal absorption (HIA+) with a probability of 0.9838. The Caco-2 permeability result is Caco2+ with a probability of 0.6708, indicating that the compound is moderately permeable through the Caco-2 cell monolayer.^81,86

Additionally, using the AutoDock tool, docking simulations were run with ZnO at the active site of the target protein, gyrb from Staphylococcus aureus (pdb id: 4urm). Docking results and pharmacological interactions have been shown in Figure 11. Therefore, the ZnO compound exhibits promising binding results when compared to the currently available medications for antibacterial therapies.

Receptor–ligand interaction of ZnO with active binding sites of 4urm. (A) Binding interactions of ZnO against 4urm receptor ribbon models. (B) Hydrogen bond interaction of the ligand compound ZnO with 4urm which revealed that ZnO possesses a higher binding affinity with 4urm (score: −2.93). (C) Compound ZnO showed minimum binding energy with 4urm at the confirmation S2 with the lowest binding energy of −2.93 kcal/mol.

In Silico Molecular Docking

AutoDockTools-1.5.6.1, which is a site-featured, high-throughput docking algorithm, was used to investigate the behavioral characteristics and binding affinities of the produced ZnO NPs with S. aureus gyrb (pdb id: 4urm). S. aureus gyrb’s crystal structure is shown in Figure 7 (pdb id: 4urm). Table 5 lists the details of the protein crystal structure that was retrieved from the PDB database. Figure 7 represents the crystal structure of S. aureus gyrb (pdb id: 4urm). The information on the retrieved protein crystal structure from the PDB database is presented in Table 5.

Crystal structure of *Staphylococcus aureus* gyrb from pdb (id: 4urm) in ribbon models.

Table 5. Retrieved from the PDB Database Are the Physiochemical Characteristics of 4urm.

physiochemical properties of protein (4urm)
parameters	value
cell space group	P1211
crystallographic resolution	2.94 Å
molecular weight	106.51 kDa
amino-acid chain name	A, B, C, D (we used chain A)
number of amino-acid residues	194

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Protein Preparation

The protein structure of S. aureus gyrb (PDB ID: 4urm) with a resolution of 2.94 Å was obtained from the PDB database and loaded into AutoDockTools.⁸⁷ Additionally, using the clean protein protocol within AutoDockTools, the protein was prepared for correcting the absence of hydrogen atoms, missing atoms and residues, and erroneous atom order in amino acids, in order to finish the protein chain. All the residues were protonated at a pH of 7.5, allowing for the protonation of arginine and lysine side chains and the deprotonation of glutamate and aspartate side chains. Histidine side chains are selectively protonated based on their context within the target. All heteroatoms were eliminated. With a potential energy of −13232.68599 kcal/mol, a van der Waals energy of −1823.13353 kcal/mol, and an initial RMS (root mean square) gradient energy of 0.9968 kcal/mol, the CHARMM force field was utilized for protein production.⁸⁸ Energy minimization was performed using a smart minimizer method with a maximum number of steps of 1000 and an RMS gradient of 0.1. This was followed by the steepest descent and conjugate gradient algorithms until the protein complex achieved a convergence gradient of 0.0011 kcal/mol. The produced protein structural model is depicted as a wireframe in Figure 8.

Prepared protein structure represented in wireframe.

Identification of Active Site Pocket Residues of 4urm

The potential binding site of the S. aureus gyrb protein was predicted after the energy of the protein complex was reduced to its lowest achievable value. The volume of the known ligand that was occupied at the active site was used to figure out where the protein binding site was located. The co-crystallized molecule was first selected, and then a sphere was built around the molecule by utilizing the define sphere option from the selection menu at a radius of 10 Å and setting the X, Y, and Z-axis values from the selection menu to 34.211311, 31.3221110, and 23.999600, respectively. The active pocket site residues of the protein that has been produced are depicted in Figure 9.

Active site pocket residues of 4urm without ZnO compound.

Ligand Construction

The two-dimensional structures of every ZnO compound were shown and stored in mol file format using ACD/ChemSketch. The saved compounds were imported using AutoDockTools, and the ligand preparation was done with parameters like consistency of oxygen and nitrogen atoms’ ionization states at physiological pH, addition and deletion of hydrogen, and conversion to 3D structures using Chem3D 16.0. The results are shown in Figure 10.

Molecular Docking Result

The selection of the best-docked ligand was made based on the interaction energy that was measured between the ligand and the protein using the AutoDock score. In addition, the formation of hydrogen bonds between the protein–ligand complex and the receptor docking conformation was determined using the “Analyze Ligand Poses” process analysis. These two factors were taken into consideration when making the selection. According to the results of the investigation of docking interactions, Figure 11 and Table 6 provide the best evidence of the docked complex. The estimated binding energy came out to be −2.93 kcal/mol, while the observed interaction energy was quite high at 13.234 kcal/mol.

Table 6. Dock Score Results of the Compound ZnO with 4urm Receptor.

rank	sub-rank	run	binding energy	cluster RMSD	reference RMSD	grep pattern
1	1	10	–2.93	0.00	45.61	RANKING
1	2	9	–2.93	0.12	45.63	RANKING
1	3	8	–2.93	0.04	45.60	RANKING
1	4	1	–2.92	0.83	45.42	RANKING
1	5	5	–2.92	0.12	45.61	RANKING
2	1	2	–2.85	0.00	40.02	RANKING
2	2	3	–2.85	0.03	40.03	RANKING
2	3	4	–2.85	0.04	40.03	RANKING
2	4	6	–2.83	1.65	39.88	RANKING
3	1	7	–2.48	0.00	65.30	RANKING

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The binding of ZnO with DNA gyrase might be a mechanism of action of ZnO NPs involved in the antimicrobial activity of the NPs. DNA gyrase is a class II topoisomerase that facilitates changes in the DNA helix.⁸⁹ Substances the target DNA gyrase can have antimicrobial action and DNA gyrase has become a potential target for antimicrobial drug discovery.⁹⁰ Molecular docking results indicated that ZnO had favorable binding interaction with DNA gyrase (subunit B: 4urm), and the estimated binding energy was −2.93 kcal/mol which suggested that ZnO has the potential to bind with the active sites of this protein and potentially inhibit its activity, which could contribute to its antimicrobial activity. Molecular docking findings in this study suggested that inhibition of DNA gyrase could be a mechanism involved in antimicrobial effect of ZnO NPs.

Antibacterial Activity

In vitro bactericidal effects of ZnO NPs (6 mL) evaluated against bacterial pathogens procured from the Institute of Microbial Technology, India. The tested strains were Gram-positive and Gram-negative S. aureus (MTCC 9760) and E. coli (MTCC 443), respectively. The disc diffusion method⁹¹ for antibacterial susceptibility testing was carried out according to the standard method with minor modifications to evaluate the presence of antibacterial activities of the ZnO NPs with Mueller Hinton agar.^92,93 The discs that had been impregnated with a series of ZnO NPs (2, 4, 6, 8 μg/disc) extracts were placed on the Mueller Hinton agar surface. Zones of inhibition were measured after 48 h. of incubation at 35 °C. tetracycline (10 μg/disc) was used as positive control (Figure 12 and Table 7).

Zone of inhibition by ZnO NPs synthesized on bacteria (a) *E. coli*, (b) *S. aureus* from 1:16, 2:8, 3:4, 4:2 mg/mL, (c) positive control. (c) Percentage of zone inhibition of inhibition by ZnO NPs synthesized on bacteria. The values mean of three replicates, ±standard error.

Table 7. Zones Inhibition Value by Different Concentration of ZnO NPs on E. coli and S. aureus.

	concentration (mg/mL)
	16	8	4	2	PC (10 μg/mL)	NC
E. coli	15.30 ± 0.66	13.00 ± 0.57	9.00 ± 0.57	2.60 ± 0.66	16.30 ± 0.11	0
S. aureus	19.60 ± 0.66	16.60 ± 0.66	16.00 ± 0.57	6.33 ± 0.88	20.00 ± 0.00	0

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The results of the study show that the ZnO NPs have a bactericidal effect against both Gram-positive and Gram-negative bacterial strains. The zone of inhibition increased with increasing concentration of ZnO NPs. The highest concentration of ZnO NPs (8 μg/disc) exhibited the largest zone of inhibition against both bacterial strains. These findings have implications for the potential use of ZnO NPs as an alternative to conventional antibiotics in the treatment of bacterial infections.^16,94,95 However, further studies are required to investigate the mechanism of action of ZnO NPs against bacterial strains and their potential toxicity toward human cells.

The ZnO NPs (2 mg/mL) were found to possess very good inhibitory activity against both E. coli with a MIC value of 92.07 ± 0.13 μg/mL and S. aureus with a MIC value of 88.13 ± 0.35 μg/mL and then decrease with decrease in the concentration. The MIC values determination is summarized in Table 8. Figure 13 shows the image depicting the MIC determination using the Mueller Hinton broth doubling dilution (2–0.004 mg/mL) method.

Table 8. MIC of ZnO NPs against Microorganisms by Mueller Hinton Broth Doubling Dilution.

concentration of ZnO NPs (mg/mL)	MIC (μg/mL)
concentration of ZnO NPs (mg/mL)	E. coli	S. aureus
2	92.07 ± 0.13	88.13 ± 0.35
1	91.43 ± 0.11	73.70 ± 0.35
0.5	77.32 ± 0.10	71.40 ± 0.18
0.25	68.62 ± 0.08	62.43 ± 0.07
0.125	61.77 ± 0.07	35.02 ± 0.07
0.063	57.43 ± 0.05	28.76 ± 0.07
0.031	53.96 ± 0.04	25.80 ± 0.06
0.016	43.66 ± 0.03	25.45 ± 0.05
0.008	35.47 ± 0.02	14.09 ± 0.03
0.004	18.180 ± 0.008	7.530 ± 0.005
PC (50 μg/mL)	100	100
NC	0	0

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Image depicting the MIC determination using the Mueller Hinton broth doubling dilution (2–0.004 mg/mL) method.

The antibacterial activities of unmodified ZnO NPs against E. coli and S. aureus has been documented by several authors. In a study by Meruvu et al.,⁸⁵ a combined effect of antibiotics and ZnO NPs was evaluated using the disc diffusion method with nutrient agar, and it was shown that zinc NPs even without the addition of antibiotic possessed antibacterial activity against E. coli as indicated by the inhibition zones. Also, Navarro-Lopez et al.⁹⁶ showed that both of these bacteria were inhibited in spot assays involving nutrient agar plates homogenized with ZnO NPs, and the inhibition was more significant against S. aureus than on E. coli which is concordant with that observed in this study.

Live/Dead Test by Fluorescent-Based Dyes

To discriminate between living, apoptotic, and necrotic cells in E. coli and S. aureus cultures exposed to IC₅₀ concentrations of ZnO NPs for 24 h, samples were stained with AO/EB along with the respective controls without the addition of NPs. The findings are shown in Figure 14; according to these findings, at the concentration examined, ZnO NPs were able to kill cells of E. coli and S. aureus. Through the application of the AO/EB staining approach, the apoptotic process of cell death has been studied in a number of studies.⁹⁷⁻⁹⁹ The principle underlying this staining approach is the absorption of AO by all cells; however, only necrotic or apoptotic cells are able to take up EB. Since only the dead cells have EB inside them, this renders the cells fluorescing red/orange while viable cells fluoresce green.¹⁰⁰

Florescent microscopic (40×) images of (a) *E. coli* and (c) *S. aureus* untreated control bacterial cells and (b) *E. coli* and (d) *S. aureus* treated with ZnO NPs (6 mL). In both series, green dots represent live bacterial cells and yellow/orange dots represent dead cells.

Conclusions

In conclusion, the extract of Cymbopogon citratus was used to successfully produce ZnO NPs. By using UV–Vis, XRD, SEM, and FTIR, the generated NPs were characterized and confirmed through UV–Vis spectroscopy absorption. The ZnO compound purity of the produced NPs was supported by SEM data, which also indicated the presence of crystallinity. Through the Scherrer formula, the crystallite size of ZnO NPs was determined to be 20–24 nm. XRD investigation supported the creation of the hexagonal wurtzite structure. Last but not least, ZnO was successfully docked with the active sites of protein (4urm) and indicated significant binding energy of interaction. The antibacterial activity of the ZnOs NPs at various concentrations inhibited the growth of the bacteria E. coli and S. aureus. Through the findings of this study, it has been shown that C. citratus extract can be used in a green synthesis approach to generate ZnO NPs which can be employed as alternatives to antibiotics and a tool to eliminate drug-resistant microbes in the future.

Acknowledgments

The authors acknowledge the support from Princess Nourah Bint Abdulrahman University Researchers Supporting Project number PNURSP2023R15, Princess Nourah Bint Abdulrahman University, Riyadh, Saudi Arabia. Also, the author expresses his thanks to Dr. Puneeth HR for his assistance in work and Scintilla BIO-MARC Pvt. Ltd., Bangalore, India, for cell culture assistance for the work. The authors are also thankful to Israa M. Shamkh, Chemo and Bioinformatics Lab, Bio Search Research Institution BSRI, Botany and Microbiology Department, Faculty of Science, Cairo University, Giza 12613, Egypt for her support in the computational analysis part and results interpretation.

Author Contributions

Conceptualization, Y.H.I.M.; methodology, Y.H.I.M., S.A., B.J., H.A.A., A.S.A., N.E.K., W.M.A.K., B.H., D.H.M.A., and W.N.H.; investigation, Y.H.I.M., S.A., B.J., H.A.A., D.M., S.B., A.S.A., N.E.K., W.M.A.K., B.H., D.H.M.A., and W.N.H., writing—original draft preparation, Y.H.I.M. and S.A.; writing—review and editing, B.J., D.M., S.B., A.S.A., N.E.K., W.M.A.K., B.H., D.H.M.A., W.N.H., H.A.A., Y.H.I.M., and S.A.; project administration, Y.H.I.M.; all authors have read and agreed to the manuscript for publication.

The research was funded by Princess Nourah Bint Abdulrahman University Researchers Supporting Project number PNURSP2023R15, Princess Nourah Bint Abdulrahman University, Riyadh, Saudi Arabia.

The authors declare no competing financial interest.

This paper was published ASAP on July 11, 2023, with an incorrect author list. The corrected paper was reposted on August 21, 2023.

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Green Synthesis of Zinc Oxide Nanoparticles Using Cymbopogon citratus Extract and Its Antibacterial Activity

Yasser Hussein Issa Mohammed

Saad Alghamdi

Basit Jabbar

Dina Marghani

Saba Beigh

Amr S Abouzied

Nasrin E Khalifa

Weam M A Khojali

Bader Huwaimel

Dalal Hussien M Alkhalifah

Wael N Hozzein

Abstract

Introduction

Materials and Methods

Collection of Essential Oil

Synthesis of ZnO NPs

Characterization of ZnO NPs

XRD Characterization

Optical Characterization

Studies on HOMO and LUMO

Adsorption, Distribution, Metabolism, Excretion, and Toxicity

Molecular Docking of ZnO NPs

Disc Diffusion Method for Antibacterial Activity

Live/Dead Test by Fluorescent-Based Dyes

Statistical Analysis

Results and Discussion

GC–MS Analysis

Figure 1.

Table 1. GC–MS Analysis of the Compounds Present in the C. citratus Extract.

XRD Pattern

Figure 2.

Scanning Electron Microscopy Result

Figure 3.

UV–Vis Spectroscopy Result

Figure 4.

FTIR Spectroscopy

Figure 5.

Studies Using HOMO and LUMO to Determine the Compound ZnO’s Molecular Orbital Energies

Figure 6.

Stability of the ZnO NPs

Table 2. Global Reactivity Descriptor Values of the Title Molecule, FMOs, and the Energy Gap.

ZnO NPs Exhibit Promising Antibacterial Activities

Table 3. Numerous ADMET Characteristics of the Compound ZnO.

Table 4. Compound ZnO Toxicity Prediction Using ADMET SAR.

Figure 11.

In Silico Molecular Docking

Figure 7.

Table 5. Retrieved from the PDB Database Are the Physiochemical Characteristics of 4urm.

Protein Preparation

Figure 8.

Identification of Active Site Pocket Residues of 4urm

Figure 9.

Ligand Construction

Figure 10.

Molecular Docking Result

Table 6. Dock Score Results of the Compound ZnO with 4urm Receptor.

Antibacterial Activity

Figure 12.

Table 7. Zones Inhibition Value by Different Concentration of ZnO NPs on E. coli and S. aureus.

Table 8. MIC of ZnO NPs against Microorganisms by Mueller Hinton Broth Doubling Dilution.

Figure 13.

Live/Dead Test by Fluorescent-Based Dyes

Figure 14.

Conclusions

Acknowledgments

Author Contributions

References

ACTIONS

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RESOURCES

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