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Antimicrobial activities and neuroprotective potential for Alzheimer’s disease of pure, Mn, Co, and Al-doped ZnO ultra-small nanoparticles

  • Imran Ullah , Reinhard B. Neder EMAIL logo , Mushtaq Ahmad , Abdul Qadir Khan , Abdur Rauf EMAIL logo , Abdulrahman Alshammari and Norah Abdullah Albekairi
Published/Copyright: September 12, 2024
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 13 Issue 1

Abstract

The current research aimed to gain insights into the synthesis, characterization, and biomedical applications of ultra-small (US) zinc oxide (ZnO) and manganese (Mn), cobalt (Co), aluminum (Al)-doped ZnO nanoparticles (NPs). These NPs were synthesized using the sol–gel method and treated with various organic ligand molecules, serving as surface modifiers and stabilizers. The influence of ligand molecules on the growth kinetics was observed by monitoring the synthesis time until gel formation, which revealed that the ligand molecules significantly slowed down gelation. Moreover, the shape and final size of NPs were also analyzed. X-ray diffraction (XRD) confirmed single-phase crystallization in all samples. Ultraviolet-visible (UV-Vis) spectroscopy revealed a broad absorbance peak in the range of 347–355 nm. Tauc’s method estimated an optical bandgap of 3.1–3.16 eV. Infrared Fourier transform (FT-IR) spectroscopy corroborated the formation of ZnO NPs decorated with various functional groups. Structural studies were performed using DISCUS software, where all necessary parameters were refined, and suggested a crystallite/NP size in the range of 3–10 nm. The citrate molecule (cit), a capping agent, exhibits the smallest crystallite/NPs. The samples were explored for antimicrobial and anti-acetylcholinesterase enzyme (AChE) activities. Among all samples, only 3–5% Mn-doped ZnO with acetate (ac) molecules as ligands showed antimicrobial activities at different concentrations. Moreover, 3% and 5% Co-doped ZnO with ac, and 3% Co-doped ZnO with dimethyl-l-tartrate (dmlt) and cit, were also active at various concentrations against Gram-positive bacteria, methicillin-resistant Staphylococcus aureus (MRSA), and Bacillus cereus (BC). The highest zone of inhibition of 7.5 ± 0.2 mm against MRSA and 10.0 mm for BC were observed. The lowest zone of inhibition was reported as 3.25 ± 0.25 mm against MRSA and 3.0 mm against BC. A direct relationship between the zone of inhibition and the concentration was observed. ZnO NPs inhibit 87.39 ± 0.002% AChE, while 3% Al-doped, 3 and 5% Co-doped NPs inhibit 78.8 ± 0.017%, 56.2 ± 0.002%, and 62.7 ± 0.051% AChE, respectively. An intermediate response of AChE inhibition was observed: 42.0 ± 0.018% for 3% Mn-doped NPs and 32.6 ± 0.0034% for 5% Mn-doped NPs. Various strategies were employed to further optimize their activities.

Graphical abstract

Keywords: doped ZnO; US-NPs; sol–gel; antimicrobial; anti-AChE

1 Introduction

Richard Feynman, an American scientist, first highlighted the concept of nanotechnology in 1959. Nanomaterials are materials with dimensions reduced to the nanoscale. Their use in electronics, energy storage, medicine, and other fields makes them an exciting area of research [1,2,3,4,5,6]. Research is ongoing in fabricating, characterizing, and exploring the contribution of nanoparticles (NPs) to science and technology [7,8,9]. At the nanoscale, the surface-to-volume ratio increases significantly, resulting in the observation of the quantum confinement effect, specifically when the size of NPs is comparable to Bohr radii [10]. Metal oxide, specifically zinc oxide (ZnO) NPs, have been extensively studied. They are wide and direct bandgap semiconductors, with bandgaps ranging from 3.0 to 4.5 eV, and a large exciton energy of 60 meV. Neder et al. performed structural analysis and local structure studies of ZnO NPs using the atomic pair distribution function (PDF) [11,12,13]. Zobel et al. performed in situ studies for ZnO time-resolved structure analysis [14]. Moreover, Meulenkamp studied their growth mechanisms [15]. Surface modification alters and tunes NP properties [16]. Control over size, shape, and morphology are the key factors in choosing an appropriate synthesis approach, along with cost-effectiveness, environmental friendliness, non-toxicity, and functionality. Two general synthesis approaches are top-down and bottom-up, with more innovative approaches also being introduced [17]. The top-down approach is straightforward, while the bottom-up approach involves more complex steps and is known for better control over size, shape, and morphology [18]. The sol–gel method is a specific choice for obtaining ultra-small (US) sizes, while the precipitation method is fast and preferable for the bulk production of NPs [19]. Particles with dimensions less than 10 nm are categorized as quantum dots, which possess unique quantum mechanical properties. US-NPs, ranging from 3 to 15 nm [20].

NPs are characterized through various techniques, including X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectroscopy, infrared Fourier transform (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Structural analysis, including crystal structure, phase identification, grain size, shape, and morphology, is performed using XRD. Small-angle X-ray scattering (SAXS) is preferred for size distribution and understanding the growth mechanisms. Further details can be found in the study of Mourdikoudis et al. [21].

The non-toxic nature of ZnO NPs has opened new avenues for biomedical applications, including their use as drug carriers, biosensing materials, and photocatalysts. ZnO NPs are effective in removing contaminants from wastewater and exhibit strong antibacterial properties, making them useful in medical applications. They are also valuable in environmental remediation by breaking down harmful chemicals through photocatalysis. Doping ZnO NPs with manganese (Mn), cobalt (Co), and aluminum (Al) enhances their photocatalytic efficiency, stability, and antibacterial properties, making them highly effective in advanced water treatment and pollutant degradation [22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Increasing antibiotic resistance and Alzheimer’s disease (AD) are two significant healthcare challenges. The misuse of antibiotics strengthens bacterial resistance, leading to prolonged antibiotic use and increased mortality rates [36,37,38]. Fifty-five million people are living with AD and other dementia, and this might increase to two billion by 2,050, as reported by the World Health Organization (WHO) and Alzheimer’s Disease International (ADI) [39]. AD is a neurodegenerative disorder characterized by memory loss with no effective treatment to date. In Alzheimer’s patients, AChE activity is abnormally increased, further exacerbating the decline in acetylcholine levels. Biocompatible, environmentally friendly, cost-effective, and non-toxic metal, metal oxide, and metal peroxide NPs have the potential to combat microbes and AD [40,41,42,43,44,45,46].

In this work, ZnO, Mn x , Co x , and Al x -doped Zn1−x O US-NPs stabilized with different organic ligand molecules were synthesized using the sol–gel method. The effects of these ligand molecules on the shape and size were analyzed. For structural analysis, XRD was performed on all samples. Rietveld-type refinement was carried out using the DISCUS package [47]. Lattice parameters “a” (P_lata) and “c” (P_latc), the position of zinc (P_z_zn) in the unit cell, the Debye–Waller factor (P_biso), stacking fault probabilities (P_stack), and diameter along various directions were refined. The results of their structural analysis were found to agree with previous studies [14]. Furthermore, their potential as antimicrobial agents against Gram-positive bacteria, methicillin-resistant Staphylococcus aureus (MRSA, and Bacillus cereus (BC), as well as their anti-AChE activities, was explored.

2 Crystal structures

Zinc oxide can be found in three different crystal structures: zincblende, rock salt, and wurtzite. ZnO rarely crystallizes in the zincblende structure, where the lattice parameters are a = b = c and α = β = γ = 90 ° and belong to the space group F 4 ̅ 3 m (No.216), as shown in Figure 1a. In the rocksalt structure, a = b = c and α = β = γ = 90 ° and it belongs to the space group Fm 3 ̅ m (No.225), as shown in Figure 1b. ZnO usually crystallizes in the wurtzite structure and belongs to a hexagonal crystal system with the space group P 6 3 mc (No.186), where a = b = 3.24920 and c = 5.2700 Å and α = β = 90 ° and γ = 120 ° , with a c / a ratio of ∼1.66 for the ideal structure. Every Zn2+ ion is surrounded by four equivalent O 2 − ions, resulting in ZnO4 corner-sharing tetrahedra. The bond length in tetrahedra ranges from 1.8 to 2.0 Å, as shown in Figure 1c.

Figure 1 Three different crystal structures of ZnO. (a) zincblende, (b) rock salt, and (c) wurtzite.
Figure 1

Three different crystal structures of ZnO. (a) zincblende, (b) rock salt, and (c) wurtzite.

3 Materials and methods

3.1 Chemicals

A detailed list of chemicals (analytical grade) used during the synthesis of Mn x , Co x , and Al x -doped Zn1−x O US-NPs is provided in Table S1 and was used without further purification.

3.2 Stabilizer selection

Post-synthesis stability of NPs is crucial due to size-dependent properties, and the ligand molecules have the potential to prevent aggregation or agglomeration. Moreover, slowing the growth kinetics in different directions can result in the formation of faceted NPs, which can interact uniquely with other materials [48,49]. For the present work, three different organic ligand molecules, citric acid (cit), 1, 5-diphenyl-1,3,5-pentanetrione (pent), and dimethyl-l-tartrate (dmlt), were used without further purification.

3.3 Synthesis of ZnO and doped ZnO NPs

Synthesis approaches play a vital role in obtaining NPs of the desired size, shape, and morphology [50]. The two utmost approaches are the top-down [51] and bottom-up approach [52]. For the current work, the synthesis recipe introduced by Spanhel and Anderson, modified by Meulenkamp [15], Wood et al. and Chory et al. [53,54,55] was adopted. A mixture of 0.6104295 g of zinc acetate dihydrate and an optimized amount of ligand molecules were first dissolved in 100 mL of ethanol using magnetic stirring (300 rpm) at room temperature. Once the solution became transparent, 3 mL of tetra-methyl ammonium hydroxide (TMAH, 25% in methanol) was added gradually to obtain the desired pH value of 9–12, favorable for nucleation. The addition of TMAH made the solution cloudy and usually took 2–6 h depending on the ligand used. A mixture of 12 mL of hexane and 8 mL of acetone was used as a precipitator. The cloudy solution was centrifuged three times at a speed of 4,000 rpm followed by washing with acetone. The final product was placed in a desiccator overnight to dry completely.

For Mn x , Co x , and Al x -doped synthesis, an optimized amount of dopant precursors (cobalt acetate tetrahydrate, manganese acetate tetrahydrate, and aluminum acetate basic) was added to the mixture at the first stage, and the same steps stated above were followed. The complete flow chart is shown in Scheme 1.

Scheme 1 A schematic chart for the synthesis of pure and doped ZnO US-NPs.
Scheme 1

A schematic chart for the synthesis of pure and doped ZnO US-NPs.

3.4 Optimal conditions

The chemical reaction is carried out at room temperature by tweaking the amount of base (TMAH) through a trial-and-error method to determine the right conditions for producing US-NPs. Interestingly, the reaction takes place under regular atmospheric pressure.

3.5 Characterization

XRD scans are performed using X’pert (Philips) and smart-lab (Rigaku) diffractometers, employing copper radiation with wavelengths K α 1 = 1.54056 Å and K α 2 = 1.54439 Å and K α 1 = 1.540593 Å and K α 2 = 1.544414 Å, from 2 θ = 20 ° to 120 ° (corresponding to 1.4 to 7.01 Å−1 on the Q-scale) with a step size of 0.00836, respectively. The instrumental parameters are determined using lanthanum hexaboride (LaB6). The tube currents are set to 35 mA for X’pert and 160 mA for smart-lab. The source voltages are set to 40 kV for X’pert and 45 kV for smart-lab. An opening slit of 10 mm, incident slits of 1/2 and 1 ° , and a soller slit of 2.5 ° are used for X’pert and smartlab diffractometer, respectively. UV-Vis spectroscopy is performed using a UH5300 double-beam spectrometer from Hitachi to gain insight into optical and electronic properties. A 1 mg·mL−1 solution of synthesized samples in dimethyl sulfoxide (DMSO) is measured several times in a 1 cm long quartz crystal cuvette. The solution is sonicated for several hours before measurement. To gain an understanding of the binding vibrations in ZnO US-NPs and to identify the presence of functional groups, FT-IR spectroscopy was carried out using a Nicolet iS50 FT-IR spectrometer (Thermo Scientific), employing an attenuated total reflection (ATR) method. The scans were acquired over the spectral range of 500–4,000 cm − 1 .

3.6 Structural studies

DISCUS software, a tool for refining NP models against experimental data (XRD), was employed for structural studies. DISCUS uses the DIFFEV program, which is a part of the DISCUS package and relies on a differential evolutionary algorithm [56]. DISCUS reads asymmetric unit cells and extends them in various directions. A model is developed several times, incorporating various types of defects, and is trimmed to the appropriate size. The corresponding powder XRD (PXRD) is calculated using the Debye scattering equation (Eq. 1) each time and is averaged before comparing it to the experimental data. Throughout the refinement process, the goodness of fit (R-value) is assessed at each generation or iteration. The resulting set of parameters is iteratively transmitted internally to DIFFEV to facilitate ongoing refinement [47]:

(1) I ( Q ) = ∑ i ∑ j f i f j sin ( Q r ij ) Q r ij

I ( Q ) is the scattering intensity, and f i , f j , and r ij = r i − r j denote the atomic form factors of the i th and j th scatterers and the distance between them.

3.7 Antimicrobial assay

A homogeneous solution (1 mg·mL−1) of the synthesized sample was obtained by dispersing 1 mg of the sample in 1 mL of DMSO. The stock solution was diluted to obtain various concentrations (25%, 50%, 75%, 100%) using the following relation:

A solution = x 100 × 1 , 000 μ g / mL

To investigate the antimicrobial assay of the synthesized samples, the agar well diffusion method [57] was preferred. To avoid any contamination, the equipment was subjected to autoclaving at 120°C for 50 min at a pressure of 15 psi . A 0.028 g·mL−1 agar was prepared in deionized water and left on glass plates to solidify under ultraviolet (UV) light. MRSA and BC were swabbed onto the agar surface and incubated for 24 h at 35°C, ensuring uniform colony growth. A pipette tip was used to make five small wells ranging in size from 3 to 5 mm, and were filled using different concentrations (250, 500, 750, and 1,000 µg·mL−1) of the synthesized sample and standard (Ciprofloxacin). The plates were then again incubated at room temperature for 24 h to observe the zone of inhibition (in mm).

3.8 Anti-acetylcholinesterase enzyme (anti-AChE) assay

In the present work, AChE inhibition was determined using a slightly modified method inspired by Rocha et al. [58] and Ahmed et al. [59], utilizing a double-beam spectrophotometer. Upon substrate addition, the enzyme–substrate reaction occurred promptly, and the development of yellow color was used to track the hydrolysis process. Hydrolysis rates (V) were determined at 1 mmol acetylthiocholine (S) concentration in a 1 mL assay mixture containing 50 mmol phosphate buffer at pH 7.4, 10 mmol DTNB, and different concentrations (75, 100, and 125 µg·mL−1) of ZnO + pent, ZnO + dmlt, and 3%, 5% Mn, Co-doped ZnO NPs stabilized with different organic ligand molecules. The mixtures were incubated at 25°C for 5 min, and absorbance readings were taken at 421 nm.

3.9 Statistical analysis

All results are presented as mean ± SE.

4 Results and discussion

4.1 Reaction mechanism

In the present work, the reaction is carried out in ethanol, where the metal salts dissociate into metal and acetate ions, followed by the release of water molecules, as depicted in (2):

(2) Zn ( CH 3 COO ) 2 · 2 H 2 O → Zn 2 + + 2 ( CH 3 COO ) − + 2 H 2 O

Acetic acid is formed by capturing H + from water molecules and producing OH − , as shown in (3):

(3) CH 3 COO − + H 2 O → CH 3 COOH + OH −

The pH of the solution is approximately 6–7, depending on the ligand molecules used. The nucleation of NPs is triggered only if the pH is in the range of 10–12. The addition of a base provides an excessive number of OH − and ammonium ( NH 4 + ). Zinc ions react to form Zn ( OH ) 2 and the complex of tetra-ammine zinc ions ( Zn ( NH 3 ) 4 2 + ), which further reacts with OH − to form ZnO , as shown in (4)–(6):

(4) Zn 2 + + 2 OH − → Zn ( OH ) 2

(5) Zn ( OH ) 2 + OH − → ZnO + H 2 O + OH −

(6) ( Zn ( NH 3 ) 4 2 + ) + OH − → ZnO + 4 NH 3 + H 2 O

4.2 XRD

All synthesized samples were measured in the 2 θ range of 20 – 120 ° at room temperature, as shown in Figure 2. No additional peaks corresponding to impurities were identified. The high intensities of the peaks are clear evidence of high crystallinity, and the broadness of the peaks indicates the formation of US-NPs. Being a layered structure, ZnO has the probability of stacking faults along the c-axis, which is determined from the ratio of the FWHM of the (102) and (110) peaks.

Figure 2 XRD of pure ZnO (US-NPs).
Figure 2

XRD of pure ZnO (US-NPs).

The (102), (110), and (103) peaks were chosen to provide insight into the effect of ligand molecules on the size of the grain/particles. A pseudo-Voigt function (Eq. 4), with no instrumental contribution, was fitted to these well-resolved peaks. The corresponding FWHM (Eq. 5) values are shown in Figure 3. The results clearly show that cit and dmlt as capping agents/stabilizers result in the formation of the smallest grains/particles. The powder X-ray diffraction (PXRD) of doped samples is depicted in Figure 4. The effect of Mn, Co, and Al incorporation into the lattice is shown in Table 1. A slight shift in the peak position is observed upon 3% Mn, Co, and Al-doping to lower 2 θ values. No significant differences in crystallite/NPs size were observed after 3% doping, as shown in Figure 5. Growth in size is visible by increasing the dopant percentage, possibly because metal ions with larger ionic radii than the host are incorporated into the lattice, resulting in lattice expansion. Moreover, the strain induced by dopant ions in the crystal lattice can be attributed to lattice expansion. The Ostwald ripening process is another possibility, usually influenced by the doping material, resulting in size growth:

(7) p − V ( x ; f ) = η · L ( x , f ) + ( 1 − η ) · G ( x , f )

Figure 3 FWHM of peaks (102), (110), (103), and (112).
Figure 3

FWHM of peaks (102), (110), (103), and (112).

Figure 4 XRD of doped ZnO (US-NPs).
Figure 4

XRD of doped ZnO (US-NPs).

Table 1

Peak position with and without doping

Peak Position (ZnO-ac) Position (3% Mn-doped ZnO-ac) Position (3% Co-doped ZnO-ac) Position (3% Al-doped ZnO-ac)
(100) 31.878 31.886 31.885 31.852
(002) 34.535 34.526 34.505 34.526
(101) 36.298 36.272 36.245 36.306
Figure 5 FWHM of peaks (102), (110), (103), and (112) for doped ZnO NPs.
Figure 5

FWHM of peaks (102), (110), (103), and (112) for doped ZnO NPs.

p − V ( x ; f ) , L ( x , f ) , and G ( x , f ) are pseudo-Voigt, Lorentzian, and Gaussian functions, respectively. x is an independent variable, and η is the peak shape parameter whose value ranges from 0 to 1.

(8) FWHM = u θ 2 + v θ + w

In the context of θ -dependent FWHM, u , v , and w are Caglioti parameters and need to be determined, and θ is half of the Bragg’s angle.

4.3 Structural analysis of ZnO and doped ZnO NPs

Refinement of pure ZnO NPs and various metal-doped ZnO NPs was carried out using the DISCUS package for in-depth investigation (Figures 6–8). The most crucial parameters were refined, including lattice parameters, zinc position, Debye–Waller factor, stacking fault probabilities, and diameters in various directions. Each parameter is provided with initial (minimum and maximum) and allowed values, respectively. The program does not accept any value below or above the allowed range. To simulate the model of the crystallite/NPs, an asymmetric unit cell of the corresponding material is provided. Using an initial guess, the program simulates the crystallite/NPs 50 times, introducing stacking faults in the process. The model is decorated with the corresponding ligand molecules, and PXRD is calculated, averaged, and compared with experimental data. The program refined all parameters effectively. The largest and smallest refined diameters are 10.86 and 2.25 nm, suggesting that the size of crystallite/NPs is in the range of 3–10 nm. The corresponding refined values and goodness of fit are shown in Table 2 and Figure 8 (with the lattice parameter and diameter measured in A ̇ ). The refinement signifies a bipyramid shape where two resembling pyramids are connected at their bases with various facet formations, as shown in Figure 6.

Figure 6 Final suggested Model of ZnO NPs without (a) and with citrate-stabilized (b) based on XRD data refinement.
Figure 6

Final suggested Model of ZnO NPs without (a) and with citrate-stabilized (b) based on XRD data refinement.

Figure 7 Schematic of citrate molecule attachment.
Figure 7

Schematic of citrate molecule attachment.

Figure 8 The goodness of fit.
Figure 8

The goodness of fit.

Table 2

Refined parameters and their corresponding values

Sample name P_lata P_latc P_z_zn P_biso P_stack P_aa_dia P_cc_dia P_hoh_dia
ZnO ac 3.23901 5.18958 0.39145 1.92100 0.09531 74.9115 108.6188 63.2912
ZnO cit 3.24675 5.20649 0.38745 1.98060 0.07732 58.6703 42.4064 40.9344
ZnO pent 3.23646 5.19013 0.38751 1.91560 0.05601 78.11280 81.97060 48.48093
ZnO dmlt 3.24213 5.21317 0.38512 1.93238 0.02796 48.55360 31.87900 95.79885
3% Mn – ZnO ac 3.23633 5.19039 0.38243 1.95899 0.09668 79.62217 90.25679 55.51118
3% Mn – ZnO cit 3.24522 5.20665 0.38619 2.07671 0.07085 58.65078 34.67142 43.14034
3% Mn – ZnO pent 3.23755 5.20122 0.37949 2.28365 0.10982 77.60022 58.57095 58.78221
3% Mn – ZnO dmlt 3.23574 5.21303 0.38528 2.23463 0.07324 74.84885 35.39911 44.84623
5% Mn – ZnO ac 3.23836 5.19497 0.38394 2.43065 0.12340 79.93556 79.93556 62.96924
5% Mn – ZnO cit 3.24620 5.21888 0.38980 2.47807 0.11236 63.98719 53.62492 98.93822
5% Mn – ZnO pent 3.25694 5.25632 0.36266 1.26744 0.12537 33.11816 22.55922 91.44874
5% Mn – ZnO dmlt 3.24515 5.22968 0.37724 2.25117 0.08474 51.83550 46.80184 90.93870
3% Co – ZnO ac 3.23069 5.19738 0.37330 1.72624 0.08643 55.50845 46.19934 98.32239
3% Co – ZnO cit 3.25014 5.22283 0.39096 2.37353 0.13687 79.63631 51.60688 48.60547
3% Co – ZnO pent 3.24981 5.22323 0.39145 2.22731 0.11652 77.44628 63.52468 55.00147
3% Co – ZnO dmlt 3.23633 5.19007 0.38244 2.29995 0.14030 78.24615 89.88307 66.20509
5% Co – ZnO ac 3.24928 5.20869 0.39376 2.12630 0.06407 79.21581 76.23872 75.75505
5% Co – ZnO cit 3.23681 5.18996 0.38002 2.34351 0.17937 79.63058 63.77353 99.96952
5% Co – ZnO pent 3.23832 5.20974 0.37958 2.32586 0.11586 62.30255 48.26083 91.07546
5% Co – ZnO dmlt 3.23522 5.19062 0.37836 2.42685 0.12657 79.76083 60.81242 99.80237
3% Al – ZnO ac 3.24240 5.20577 0.38377 2.10126 0.06709 58.47890 52.21315 98.65446

Ligand molecules can bind either via physisorption (through van der Waals interactions) or chemisorption (proper bonding scheme) to the active site at the surface and act as stabilizers. Specifically, cit molecules (tricarboxylic acid) can bind via various binding schemes. In a monodentate binding mode, a single carboxylate group from the citrate molecule connects with a zinc atom on the NP’s surface. In contrast, bidentate binding involves a carboxylate group forming two bonds with the surface. Additionally, there is the chelating scheme, where multiple atoms from the citrate molecule coordinate with one or several zinc atoms on the surface of the NPs. Among these, chelation is the most preferred due to its ability to greatly enhance the binding strength and stability of the NPs. Figure 7 illustrates all three binding schemes. They can slow down Oswald repining, as well as aggregation and agglomeration.

4.4 TEM

TEM was performed on ZnO NPs stabilized with citric acid to analyze their size and size distribution in detail. The TEM images indicate a range of particle sizes from 1.5 to 6 nm. To quantify the distribution, a histogram of the measured NP sizes was plotted. A log-normal distribution was fitted to the data, providing a precise characterization of the size variation within the sample. The analysis revealed that the mean size of the particles is 3.06 nm, with a standard deviation of 0.253 nm. This suggests a relatively narrow size distribution, indicating good control over the synthesis process. The resulting size distribution and log-normal fit are visually represented in Figure 9. This highlights the uniformity and consistency of the NP sizes within the sample. These results are in close agreement with the findings obtained from XRD data refinement.

Figure 9 TEM images of the ZnO NPs stabilized with cit at different scales: (a) at 20 nm, (b) at 5 nm, and the corresponding histogram (c).
Figure 9

TEM images of the ZnO NPs stabilized with cit at different scales: (a) at 20 nm, (b) at 5 nm, and the corresponding histogram (c).

4.5 UV-Vis spectroscopy

The optical and electronic properties of ZnO NPs stabilized with cit are explored using UV-Vis spectroscopy. A broad absorption peak in the range of 347–355 nm was observed, corresponding to hexagonal wurtzite ZnO [60]. The results are in close agreement with the findings by Davis et al. [61], as depicted in Figure 10. A blue shift compared to the absorption peak of bulk ZnO is observed, attributed to quantum confinement and the Burstein–Moss effect, which typically emerges at the nanoscale. The broadening of the absorption peak might be due to the size distribution and the presence of citrate molecules on the surface. A direct bandgap (E g) (n = 2) ranging from 3.1 to 3.16 eV is obtained using Tauc’s (Eq. 6) and inverse logarithmic derivative method (Eq. 7) [62], as shown in Figure 10:

(9) ( α h ν ) = A · ( h ν − E g ) n

(10) ∆ ( h ν ) ∆ ln ⁡ ( α h ν ) = 1 n ( h ν − E g )

Figure 10 UV-Vis absorption spectrum of (a) pure ZnO nanoparticles (NPs) stabilized with citrate and (c) Mn-doped ZnO NPs stabilized with citrate; corresponding optical bandgap (b) for pure ZnO and (d) for Mn-doped ZnO NPs using the Tauc’s relation; and (e) bandgap of ZnO NPs stabilized with citrate using the inverse logarithmic derivative method.
Figure 10

UV-Vis absorption spectrum of (a) pure ZnO nanoparticles (NPs) stabilized with citrate and (c) Mn-doped ZnO NPs stabilized with citrate; corresponding optical bandgap (b) for pure ZnO and (d) for Mn-doped ZnO NPs using the Tauc’s relation; and (e) bandgap of ZnO NPs stabilized with citrate using the inverse logarithmic derivative method.

Here, α , h ν , and E g represent the absorption coefficient, incident photon energy, and optical bandgap, respectively.

Mn doping introduces a red shift in the absorption peak, indicating a decrease in optical bandgap to about 2.79 eV, as shown in Figure 10. Mn doping introduces new electronic states. Moreover, Mn ion incorporation into the ZnO lattice induces lattice strain, further influencing the optical properties.

4.6 FT-IR spectroscopy

Various functional groups attached to the surface of ZnO NPs stabilized with citrate molecules were confirmed through FT-IR spectroscopy, as shown in Figure 11. Many absorption bands were observed at 838.05, 948.08, 1,102.76, 1,436.04, 1,559.62, 1,999.73, 2,240.52, and 3,340.81 cm − 1 . A few bands were observed below 948.08 cm − 1 , indicating the formation of ZnO NPs. The band at 838.05 cm − 1 is associated with the stretching vibrations of Zn – O bonds in the ZnO lattice. The band at 948.08 cm − 1 corresponding to the stretching vibration of C − N and 1,102.76 cm − 1 is characteristic of C − O stretching vibrations in citrate molecules. The band at 1,436.04 cm − 1 may correspond to asymmetric stretching vibrations in the citrate molecules. The band at 1,559.62 cm − 1 is associated with symmetric stretching vibrations of carboxylate groups in citrate molecules, the band at 1,999.73 cm − 1 belongs to C − O stretching vibrations in citrate molecules and bands at 2,240.52 and 3,340.81 cm − 1 indicate the presence of water molecules or the OH group.

Figure 11 FT-IR spectroscopy of ZnO NPs.
Figure 11

FT-IR spectroscopy of ZnO NPs.

4.7 Antimicrobial activity

The antibacterial activities of all synthesized samples, including the ZnO NPs, 3% and 5% Mn, Co, and 3% Al-doped ZnO NPs, as well as the standard drug ciprofloxacin, were explored against Gram-positive bacteria MRSA and BC. ZnO NPs stabilized with various ligand molecules showed no activity. The 3% Mn-doped ZnO NPs with ac molecules show a 3.5 ± 0.5 mm zone of inhibition at 750 μg·mL−1 concentration and increases to 6 ± 0.0 mm at 1,000 μg·mL−1 in a dose-dependent mode against BC and remains inactive against MRSA. Upon increasing the doping concentration from 3 to 5%, the sample becomes active against both strains even at the lowest concentration of 250 μg·mL−1, producing zones of inhibition of 5.5 ± 0.5 mm and 5 ± 0 mm against MRSA and BC, respectively. Interestingly, the zone of inhibition (7.5 ± 0.5 mm) did not grow after 500 μg·mL−1 concentration against MRSA. Dose-dependent activities are observed against BC. At 500, 750, and 1,000 μg·mL−1 concentrations, the observed zones of inhibition are 6 ± 0, 7 ± 0, and 7 ± 0 mm against BC. The same trend was observed for the Co-doped ZnO NPs. 3% Co-doped ZnO NPs with ac and cit molecules as stabilizers were active against BC even at the lowest concentration of 250 μg·mL−1. The results are shown in Figure 12. 3% Co-doped ZnO stabilized with dmlt shows activity against BC at 750 μg·mL−1 and MRSA at 1,000 μg·mL−1 concentration. 5% Co-doped ZnO with ac molecule as stabilizers were active against BC for all concentrations. This might be due to the generation of metal ions, Zn2+, Mn2+, and Co2+. Interaction of metal ions with various functional groups within bacterial cells can lead to disturbance in normal metabolic processes and disruption of biomolecule functions. Moreover, the generation of reactive oxygen species (ROS) can contribute to oxidative stress, eventually leading to bacteriolysis. Research is ongoing to understand the exact mechanism.

Figure 12 Zone of inhibition of gram-positive bacteria when treated with doped ZnO NPs (a)–(f), and Zone of inhibition for the standard drug ciprofloxacin (g).
Figure 12

Zone of inhibition of gram-positive bacteria when treated with doped ZnO NPs (a)–(f), and Zone of inhibition for the standard drug ciprofloxacin (g).

4.8 Anti-AChE activity

Many people worldwide are suffering from AD. AD leads to various consequences, including a decline in cognitive abilities. Several approaches have been developed to combat AD. Research is ongoing to find a suitable remedy. Cholinesterase enzyme inhibitors, such as Donepezil, Galantamine, and Rivastigmine, are available. We explore the anti-AChE assay of the synthesized samples. The percentage inhibition was measured at various concentrations (75, 100, 125 μg·mL−1). Pure ZnO stabilized with pent and dmlt showed the highest zones of inhibition of 87.39 ± 0.002% and 71.5 ± 0.02% at 125 μg·mL−1 concentration. Among doped samples, 3% Al-doped ZnO stabilized with acetate molecules showed a 78.8 ± 0.017% zone of inhibition at the highest concentration, and 5% Co-doped ZnO with citrate molecules as stabilizers showed 62.7 ± 0.051% inhibition at the highest concentration. All other doped samples showed intermediate zones of inhibition, as depicted in Figure 13. All samples follow the dose-dependent activities. The results confirmed that the direct interaction of ligand molecules with enzymes or the provision of an unfavorable environment for enzyme activity contributes to the observed inhibition.

Figure 13 Zone of inhibition against acetylcholinesterase enzyme (AChE) of pure and doped ZnO stabilized with different ligand molecules (a)–(k).
Figure 13 Zone of inhibition against acetylcholinesterase enzyme (AChE) of pure and doped ZnO stabilized with different ligand molecules (a)–(k).
Figure 13

Zone of inhibition against acetylcholinesterase enzyme (AChE) of pure and doped ZnO stabilized with different ligand molecules (a)–(k).

Metal-doped samples provide additional metal ions, which might affect enzyme activity, while Al-doping significantly reduces AChE activity. The exact mechanism is still unknown and a fruitful debate. Many possibilities can arise; one possibility is the direct interaction, where the adsorption of NPs onto the enzyme surface due to electrostatic interactions or the metal ion binding to the specific residues on the AChE leads to disruption of the AChE activities [63]. In indirect interaction, exposure of NPs to biological environments can generate ROS, resulting in oxidative damage to the enzyme [64]. The utilization of ligand molecules as surfactants, specifically at the nanoscale, alters surface properties [65]. They can directly affect enzymatic activity; for instance, the interaction of citrate molecules can result in the formation of complexes with certain amino acid residues on the enzyme, affecting AChE activity.

3% Mn-doped ZnO NPs stabilized with ac showed significant antimicrobial activity against BC and MRSA in a dose-dependent mode. However, for MRSA, higher concentrations did not show proportional increases in activity, likely due to NPs settling. ZnO stabilized with pent and 3% Al-doped ZnO stabilized with ac exhibit the highest activities against AChE, as listed in Tables 3 and 4.

Table 3

Comparison of antimicrobial activities of doped ZnO NPs

Sample Pathogen type Concentration Zone of inhibition (mm) Reference
3% Mn-doped ZnO ac BC 750 μg·mL−1 3.5 ± 0.5 This work
3% Mn-doped ZnO ac BC 1,000 μg·mL−1 6 ± 0.0 This work
5% Mn-doped ZnO ac MRSA 250 μg·mL−1 5.5 ± 0.5 This work
5% Mn-doped ZnO ac BC 250 μg·mL−1 5 ± 0.0 This work
5% Mn-doped ZnO ac MRSA 500 μg·mL−1 7.5 ± 0.5 This work
2.5% Mn-doped ZnO BC 2 mM 2.2 [66]
2.5% Mn-doped ZnO BC 2 mM 3 [66]

MRSA = methicillin-resistant Staphylococcus aureus and BC = Bacillus cereus.

Table 4

Comparison of enzyme inhibition activity of ZnO, Al-doped, and AgNO3 HP-Ag NPs

Sample Enzyme Concentration Zone of inhibition (%) Reference
ZnO pent AChE 75 μg·mL−1 82.8 ± 0.01 This work
ZnO dmlt AChE 75 μg·mL−1 34.7 ± 0.005 This work
3% Al-doped ZnO AChE 75 μg·mL−1 72.9 ± 0.027 This work
AgNO3 AChE 75 μg·mL−1 24.57 ± 1.48 [67]
HP-Ag NPs AChE 74 mM 60.10 ± 2.60 [67]

AChE = acetylcholinesterase enzyme, HP = Hypecoum pendulum (plant), Ag = silver, NPs = nanoparticles. Mn = manganese, pent = 1, 5-diphenyl-1,3,5-pentanetrione, and dmlt = dimethyl-l-tartrate.

5 Conclusions

The potential of nanomaterials has been vastly explored. We synthesized ZnO and doped ZnO ultra-small NPs decorated with different organic ligand molecules as stabilizers via the sol–gel method and characterized them using various characterization techniques. XRD reveals the impact of ligand molecules on the size, shape, and morphology of the synthesized NPs. UV-Vis spectroscopy shows a broad absorption peak in the range of 347–355 nm, corresponding to a bandgap of 3.1–3.11 eV. The presence of various functional groups is verified through FT-IR. Various doped samples show dose-dependent activities against MRSA and BC. Furthermore, the synthesized US-NPs were tested against AChE activities. The findings provide compelling insights with important implications for the treatment of AD. The inhibitory effects observed during analysis highlight the potential of these NPs in targeting AChE. The results of the study rank these NPs as candidates of considerable therapeutic value. The unique properties introduced by doping elements enhance their ability to interact with AChE. These findings encourage further research in the field of nanomedicine.

Acknowledgments

The authors are thankful to the Researchers Supporting Project number (RSPD2024R1035), King Saud University, Riyadh, Saudi Arabia.

  1. Funding information: Authors state no funding involved.

  2. Author contributions: Imran Ullah (IU): experimental work, characterization, writing original draft. Reinhard B. Neder (RBN): conceptualization, investigation, and supervision. Abdul Qadir Khan (AQK): pharmacological screening. Mushtaq Ahmad (MA): experimental work. Abdur Rauf (AR): analysis and data curation. Abdulrahman Alshammari (AA): analysis and investigation. Norah Abdullah Albekairi (NAA): biological studies.

  3. Conflict of interest: The contact author, Dr. Abdur Rauf, is the associated editor of GPS. The other authors state no conflict of interest.

  4. Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2024-05-01
Accepted: 2024-08-08
Published Online: 2024-09-12

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  55. Effect of ex situ, eco-friendly ZnONPs incorporating green synthesised Moringa oleifera leaf extract in enhancing biochemical and molecular aspects of Vicia faba L. under salt stress
  56. Biosynthesis and characterization of selenium and silver nanoparticles using Trichoderma viride filtrate and their impact on Culex pipiens
  57. Photocatalytic degradation of organic dyes and biological potentials of biogenic zinc oxide nanoparticles synthesized using the polar extract of Cyperus scariosus R.Br. (Cyperaceae)
  58. Assessment of antiproliferative activity of green-synthesized nickel oxide nanoparticles against glioblastoma cells using Terminalia chebula
  59. Chlorine-free synthesis of phosphinic derivatives by change in the P-function
  60. Anticancer, antioxidant, and antimicrobial activities of nanoemulsions based on water-in-olive oil and loaded on biogenic silver nanoparticles
  61. Study and mechanism of formation of phosphorus production waste in Kazakhstan
  62. Synthesis and stabilization of anatase form of biomimetic TiO2 nanoparticles for enhancing anti-tumor potential
  63. Microwave-supported one-pot reaction for the synthesis of 5-alkyl/arylidene-2-(morpholin/thiomorpholin-4-yl)-1,3-thiazol-4(5H)-one derivatives over MgO solid base
  64. Screening the phytochemicals in Perilla leaves and phytosynthesis of bioactive silver nanoparticles for potential antioxidant and wound-healing application
  65. Graphene oxide/chitosan/manganese/folic acid-brucine functionalized nanocomposites show anticancer activity against liver cancer cells
  66. Nature of serpentinite interactions with low-concentration sulfuric acid solutions
  67. Multi-objective statistical optimisation utilising response surface methodology to predict engine performance using biofuels from waste plastic oil in CRDi engines
  68. Microwave-assisted extraction of acetosolv lignin from sugarcane bagasse and electrospinning of lignin/PEO nanofibres for carbon fibre production
  69. Biosynthesis, characterization, and investigation of cytotoxic activities of selenium nanoparticles utilizing Limosilactobacillus fermentum
  70. Highly photocatalytic materials based on the decoration of poly(O-chloroaniline) with molybdenum trichalcogenide oxide for green hydrogen generation from Red Sea water
  71. Highly efficient oil–water separation using superhydrophobic cellulose aerogels derived from corn straw
  72. Beta-cyclodextrin–Phyllanthus emblica emulsion for zinc oxide nanoparticles: Characteristics and photocatalysis
  73. Assessment of antimicrobial activity and methyl orange dye removal by Klebsiella pneumoniae-mediated silver nanoparticles
  74. Influential eradication of resistant Salmonella Typhimurium using bioactive nanocomposites from chitosan and radish seed-synthesized nanoselenium
  75. Antimicrobial activities and neuroprotective potential for Alzheimer’s disease of pure, Mn, Co, and Al-doped ZnO ultra-small nanoparticles
  76. Green synthesis of silver nanoparticles from Bauhinia variegata and their biological applications
  77. Synthesis and optimization of long-chain fatty acids via the oxidation of long-chain fatty alcohols
  78. Eminent Red Sea water hydrogen generation via a Pb(ii)-iodide/poly(1H-pyrrole) nanocomposite photocathode
  79. Green synthesis and effective genistein production by fungal β-glucosidase immobilized on Al2O3 nanocrystals synthesized in Cajanus cajan L. (Millsp.) leaf extracts
  80. Green stability-indicating RP-HPTLC technique for determining croconazole hydrochloride
  81. Green synthesis of La2O3–LaPO4 nanocomposites using Charybdis natator for DNA binding, cytotoxic, catalytic, and luminescence applications
  82. Eco-friendly drugs induce cellular changes in colistin-resistant bacteria
  83. Tangerine fruit peel extract mediated biogenic synthesized silver nanoparticles and their potential antimicrobial, antioxidant, and cytotoxic assessments
  84. Green synthesis on performance characteristics of a direct injection diesel engine using sandbox seed oil
  85. A highly sensitive β-AKBA-Ag-based fluorescent “turn off” chemosensor for rapid detection of abamectin in tomatoes
  86. Green synthesis and physical characterization of zinc oxide nanoparticles (ZnO NPs) derived from the methanol extract of Euphorbia dracunculoides Lam. (Euphorbiaceae) with enhanced biosafe applications
  87. Detection of morphine and data processing using surface plasmon resonance imaging sensor
  88. Effects of nanoparticles on the anaerobic digestion properties of sulfamethoxazole-containing chicken manure and analysis of bio-enzymes
  89. Bromic acid-thiourea synergistic leaching of sulfide gold ore
  90. Green chemistry approach to synthesize titanium dioxide nanoparticles using Fagonia Cretica extract, novel strategy for developing antimicrobial and antidiabetic therapies
  91. Green synthesis and effective utilization of biogenic Al2O3-nanocoupled fungal lipase in the resolution of active homochiral 2-octanol and its immobilization via aluminium oxide nanoparticles
  92. Eco-friendly RP-HPLC approach for simultaneously estimating the promising combination of pentoxifylline and simvastatin in therapeutic potential for breast cancer: Appraisal of greenness, whiteness, and Box–Behnken design
  93. Use of a humidity adsorbent derived from cockleshell waste in Thai fried fish crackers (Keropok)
  94. One-pot green synthesis, biological evaluation, and in silico study of pyrazole derivatives obtained from chalcones
  95. Bio-sorption of methylene blue and production of biofuel by brown alga Cystoseira sp. collected from Neom region, Kingdom of Saudi Arabia
  96. Synthesis of motexafin gadolinium: A promising radiosensitizer and imaging agent for cancer therapy
  97. The impact of varying sizes of silver nanoparticles on the induction of cellular damage in Klebsiella pneumoniae involving diverse mechanisms
  98. Microwave-assisted green synthesis, characterization, and in vitro antibacterial activity of NiO nanoparticles obtained from lemon peel extract
  99. Rhus microphylla-mediated biosynthesis of copper oxide nanoparticles for enhanced antibacterial and antibiofilm efficacy
  100. Harnessing trichalcogenide–molybdenum(vi) sulfide and molybdenum(vi) oxide within poly(1-amino-2-mercaptobenzene) frameworks as a photocathode for sustainable green hydrogen production from seawater without sacrificial agents
  101. Magnetically recyclable Fe3O4@SiO2 supported phosphonium ionic liquids for efficient and sustainable transformation of CO2 into oxazolidinones
  102. A comparative study of Fagonia arabica fabricated silver sulfide nanoparticles (Ag2S) and silver nanoparticles (AgNPs) with distinct antimicrobial, anticancer, and antioxidant properties
  103. Visible light photocatalytic degradation and biological activities of Aegle marmelos-mediated cerium oxide nanoparticles
  104. Physical intrinsic characteristics of spheroidal particles in coal gasification fine slag
  105. Exploring the effect of tea dust magnetic biochar on agricultural crops grown in polycyclic aromatic hydrocarbon contaminated soil
  106. Crosslinked chitosan-modified ultrafiltration membranes for efficient surface water treatment and enhanced anti-fouling performances
  107. Study on adsorption characteristics of biochars and their modified biochars for removal of organic dyes from aqueous solution
  108. Zein polymer nanocarrier for Ocimum basilicum var. purpurascens extract: Potential biomedical use
  109. Green synthesis, characterization, and in vitro and in vivo biological screening of iron oxide nanoparticles (Fe3O4) generated with hydroalcoholic extract of aerial parts of Euphorbia milii
  110. Novel microwave-based green approach for the synthesis of dual-loaded cyclodextrin nanosponges: Characterization, pharmacodynamics, and pharmacokinetics evaluation
  111. Bi2O3–BiOCl/poly-m-methyl aniline nanocomposite thin film for broad-spectrum light-sensing
  112. Green synthesis and characterization of CuO/ZnO nanocomposite using Musa acuminata leaf extract for cytotoxic studies on colorectal cancer cells (HCC2998)
  113. Review Articles
  114. Materials-based drug delivery approaches: Recent advances and future perspectives
  115. A review of thermal treatment for bamboo and its composites
  116. An overview of the role of nanoherbicides in tackling challenges of weed management in wheat: A novel approach
  117. An updated review on carbon nanomaterials: Types, synthesis, functionalization and applications, degradation and toxicity
  118. Special Issue: Emerging green nanomaterials for sustainable waste management and biomedical applications
  119. Green synthesis of silver nanoparticles using mature-pseudostem extracts of Alpinia nigra and their bioactivities
  120. Special Issue: New insights into nanopythotechnology: current trends and future prospects
  121. Green synthesis of FeO nanoparticles from coffee and its application for antibacterial, antifungal, and anti-oxidation activity
  122. Dye degradation activity of biogenically synthesized Cu/Fe/Ag trimetallic nanoparticles
  123. Special Issue: Composites and green composites
  124. Recent trends and advancements in the utilization of green composites and polymeric nanocarriers for enhancing food quality and sustainable processing
  125. Retraction
  126. Retraction of “Biosynthesis and characterization of silver nanoparticles from Cedrela toona leaf extracts: An exploration into their antibacterial, anticancer, and antioxidant potential”
  127. Retraction of “Photocatalytic degradation of organic dyes and biological potentials of biogenic zinc oxide nanoparticles synthesized using the polar extract of Cyperus scariosus R.Br. (Cyperaceae)”
  128. Retraction to “Green synthesis on performance characteristics of a direct injection diesel engine using sandbox seed oil”
https://doi.org/10.1515/gps-2024-0096
Keywords for this article
doped ZnO; US-NPs; sol–gel; antimicrobial; anti-AChE
Creative Commons
BY 4.0

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  66. Nature of serpentinite interactions with low-concentration sulfuric acid solutions
  67. Multi-objective statistical optimisation utilising response surface methodology to predict engine performance using biofuels from waste plastic oil in CRDi engines
  68. Microwave-assisted extraction of acetosolv lignin from sugarcane bagasse and electrospinning of lignin/PEO nanofibres for carbon fibre production
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  73. Assessment of antimicrobial activity and methyl orange dye removal by Klebsiella pneumoniae-mediated silver nanoparticles
  74. Influential eradication of resistant Salmonella Typhimurium using bioactive nanocomposites from chitosan and radish seed-synthesized nanoselenium
  75. Antimicrobial activities and neuroprotective potential for Alzheimer’s disease of pure, Mn, Co, and Al-doped ZnO ultra-small nanoparticles
  76. Green synthesis of silver nanoparticles from Bauhinia variegata and their biological applications
  77. Synthesis and optimization of long-chain fatty acids via the oxidation of long-chain fatty alcohols
  78. Eminent Red Sea water hydrogen generation via a Pb(ii)-iodide/poly(1H-pyrrole) nanocomposite photocathode
  79. Green synthesis and effective genistein production by fungal β-glucosidase immobilized on Al2O3 nanocrystals synthesized in Cajanus cajan L. (Millsp.) leaf extracts
  80. Green stability-indicating RP-HPTLC technique for determining croconazole hydrochloride
  81. Green synthesis of La2O3–LaPO4 nanocomposites using Charybdis natator for DNA binding, cytotoxic, catalytic, and luminescence applications
  82. Eco-friendly drugs induce cellular changes in colistin-resistant bacteria
  83. Tangerine fruit peel extract mediated biogenic synthesized silver nanoparticles and their potential antimicrobial, antioxidant, and cytotoxic assessments
  84. Green synthesis on performance characteristics of a direct injection diesel engine using sandbox seed oil
  85. A highly sensitive β-AKBA-Ag-based fluorescent “turn off” chemosensor for rapid detection of abamectin in tomatoes
  86. Green synthesis and physical characterization of zinc oxide nanoparticles (ZnO NPs) derived from the methanol extract of Euphorbia dracunculoides Lam. (Euphorbiaceae) with enhanced biosafe applications
  87. Detection of morphine and data processing using surface plasmon resonance imaging sensor
  88. Effects of nanoparticles on the anaerobic digestion properties of sulfamethoxazole-containing chicken manure and analysis of bio-enzymes
  89. Bromic acid-thiourea synergistic leaching of sulfide gold ore
  90. Green chemistry approach to synthesize titanium dioxide nanoparticles using Fagonia Cretica extract, novel strategy for developing antimicrobial and antidiabetic therapies
  91. Green synthesis and effective utilization of biogenic Al2O3-nanocoupled fungal lipase in the resolution of active homochiral 2-octanol and its immobilization via aluminium oxide nanoparticles
  92. Eco-friendly RP-HPLC approach for simultaneously estimating the promising combination of pentoxifylline and simvastatin in therapeutic potential for breast cancer: Appraisal of greenness, whiteness, and Box–Behnken design
  93. Use of a humidity adsorbent derived from cockleshell waste in Thai fried fish crackers (Keropok)
  94. One-pot green synthesis, biological evaluation, and in silico study of pyrazole derivatives obtained from chalcones
  95. Bio-sorption of methylene blue and production of biofuel by brown alga Cystoseira sp. collected from Neom region, Kingdom of Saudi Arabia
  96. Synthesis of motexafin gadolinium: A promising radiosensitizer and imaging agent for cancer therapy
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  98. Microwave-assisted green synthesis, characterization, and in vitro antibacterial activity of NiO nanoparticles obtained from lemon peel extract
  99. Rhus microphylla-mediated biosynthesis of copper oxide nanoparticles for enhanced antibacterial and antibiofilm efficacy
  100. Harnessing trichalcogenide–molybdenum(vi) sulfide and molybdenum(vi) oxide within poly(1-amino-2-mercaptobenzene) frameworks as a photocathode for sustainable green hydrogen production from seawater without sacrificial agents
  101. Magnetically recyclable Fe3O4@SiO2 supported phosphonium ionic liquids for efficient and sustainable transformation of CO2 into oxazolidinones
  102. A comparative study of Fagonia arabica fabricated silver sulfide nanoparticles (Ag2S) and silver nanoparticles (AgNPs) with distinct antimicrobial, anticancer, and antioxidant properties
  103. Visible light photocatalytic degradation and biological activities of Aegle marmelos-mediated cerium oxide nanoparticles
  104. Physical intrinsic characteristics of spheroidal particles in coal gasification fine slag
  105. Exploring the effect of tea dust magnetic biochar on agricultural crops grown in polycyclic aromatic hydrocarbon contaminated soil
  106. Crosslinked chitosan-modified ultrafiltration membranes for efficient surface water treatment and enhanced anti-fouling performances
  107. Study on adsorption characteristics of biochars and their modified biochars for removal of organic dyes from aqueous solution
  108. Zein polymer nanocarrier for Ocimum basilicum var. purpurascens extract: Potential biomedical use
  109. Green synthesis, characterization, and in vitro and in vivo biological screening of iron oxide nanoparticles (Fe3O4) generated with hydroalcoholic extract of aerial parts of Euphorbia milii
  110. Novel microwave-based green approach for the synthesis of dual-loaded cyclodextrin nanosponges: Characterization, pharmacodynamics, and pharmacokinetics evaluation
  111. Bi2O3–BiOCl/poly-m-methyl aniline nanocomposite thin film for broad-spectrum light-sensing
  112. Green synthesis and characterization of CuO/ZnO nanocomposite using Musa acuminata leaf extract for cytotoxic studies on colorectal cancer cells (HCC2998)
  113. Review Articles
  114. Materials-based drug delivery approaches: Recent advances and future perspectives
  115. A review of thermal treatment for bamboo and its composites
  116. An overview of the role of nanoherbicides in tackling challenges of weed management in wheat: A novel approach
  117. An updated review on carbon nanomaterials: Types, synthesis, functionalization and applications, degradation and toxicity
  118. Special Issue: Emerging green nanomaterials for sustainable waste management and biomedical applications
  119. Green synthesis of silver nanoparticles using mature-pseudostem extracts of Alpinia nigra and their bioactivities
  120. Special Issue: New insights into nanopythotechnology: current trends and future prospects
  121. Green synthesis of FeO nanoparticles from coffee and its application for antibacterial, antifungal, and anti-oxidation activity
  122. Dye degradation activity of biogenically synthesized Cu/Fe/Ag trimetallic nanoparticles
  123. Special Issue: Composites and green composites
  124. Recent trends and advancements in the utilization of green composites and polymeric nanocarriers for enhancing food quality and sustainable processing
  125. Retraction
  126. Retraction of “Biosynthesis and characterization of silver nanoparticles from Cedrela toona leaf extracts: An exploration into their antibacterial, anticancer, and antioxidant potential”
  127. Retraction of “Photocatalytic degradation of organic dyes and biological potentials of biogenic zinc oxide nanoparticles synthesized using the polar extract of Cyperus scariosus R.Br. (Cyperaceae)”
  128. Retraction to “Green synthesis on performance characteristics of a direct injection diesel engine using sandbox seed oil”
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