Tailoring CdO-CuO-ZnO Mixed Metal Oxide Nanocomposites for Anticancer Activity via Co-Precipitation Method

Introduction

Nanoparticles (NPs) have garnered significant attention over the last decade for both commercial and medical applications due to their diverse range of properties. They are used in the production of numerous consumer goods, including clothing, electronics, computers, food coloring, and food.^1,2 Additionally, they are used in advanced technologies, including photocatalysts, sensors, piezoelectric components, fuel cells, and solar cells.^3–6 Remarkable properties of metal oxide semiconductors include their inherent p-n characteristics, broad light absorption, quick dynamic response, and improved sensitivity to changes in humidity. Basically, the ability to tune bandgap energy by combining two unique materials is a significant benefit in the field of nanotechnology. Additionally, the synthesis of nanocomposite materials with adjustable shapes, sizes, and surface characteristics is crucial for a variety of practical applications.^7,8

Multi-metal oxide (MMO), ie formed by combining two or more metals in an oxide matrix, could result in the production of novel materials with superior chemical and physical characteristics and a high potential for technological advancement.⁹ The synthesis of metal oxide nanocomposites has been accomplished using a variety of techniques, including micro-emulsion synthesis, sol-gel technique, co-precipitation method, hydrothermal processing, microwave-assisted synthesis, wet chemical method, solid-state reaction, etc. The co-precipitation method is the most popular among these due to its effectiveness, low growth temperature requirement, simplicity, quickness, and low cost.¹⁰

Currently, a comprehensive cancer treatment system that combines surgery, chemo, radio, and biologically targeted therapy has been developed. All aforementioned treatments, however, have limitations. Even after four years, patients with tumors still have a very low chance of survival. Clinical experience has demonstrated that, aside from a few specific medications, the majority of current therapies are unable to improve the cure rate of cancer patients. Normal tissue cells may also be harmed by radiotherapy. As a result, researchers and clinicians are exploring alternative cancer treatment approaches to enhance the current system. In recent years, nanomaterials have gained significant popularity in antitumor research. Because of their distinct physicochemical properties, nanoparticles stand out as a prominent example among nanomaterials.^11–15Consider chemotherapy as an example: the majority of current chemotherapeutic drugs are non-specific cytotoxic agents, which can lead to tumor cells developing drug resistance drugs and cause damage to normal tissue cells.

Despite its inherent toxicity, cadmium (Cd) has shown great promise in anticancer applications. Oxidative stress may originate from CdO nanoparticles (NPs) that produce reactive oxygen species (ROS) within cancer cells. Developments in bioremediation methods and material science, such as the synthesis of less hazardous quantum dot structures, are essential for weighing the advantages and disadvantages of cadmium use. CdO nanoparticles (NPs) induce oxidative stress by generating ROS within cancer cells.^16,17 ROS causes damage to cellular components, including DNA, lipids, and proteins, ultimately triggering apoptosis or necrosis.^18,19 Metal oxides exhibit strong anticancer effects of CdO NPs on various cancer cell lines, indicating their potential as a targeted therapeutic agent. Mazhar et al reported the presence of CdO NPs with other metals (such as Ni).²⁰ Compared to standards such as doxorubicin, CdO-containing nanocomposites often show similar or enhanced efficacy in vitro, particularly at optimized doses, with IC50 values in the extreme range for specific cancer types.²¹ Cadmium oxide (CdO) nanoparticles can be utilized safely by both human and animal cells, despite their anticancer effects. The good news is that when used as anticancer agents, CdO NPs are safe for mammalian cells, including human cells. In developed countries, CdO is frequently produced because of its unique properties. Despite acting similarly to other nanoparticles, CdO is more effective and efficient at rupturing cell walls and eliminating cancerous cells.²²

The cytotoxic properties of nanoparticles (NPs) are significantly influenced by their size and shape, as demonstrated in numerous previous studies.^23–25 Smaller NPs typically have a larger surface area relative to their volume, which can lead to increased interactions with cellular components and potentially higher cytotoxicity. For instance, NPs showed the highest ROS production due to their high dislocation density, indicating that the surface defects effectively generated ROS.²³ ROS damages cellular components, including proteins, DNA, and mitochondria, amplifying cytotoxicity. Moreover, cellular uptake is influenced by the size of NPs, as smaller NPs are more efficiently internalized via endocytosis, enabling deeper penetration into cells and tissues.^23,24 Additionally, the shape of NPs plays a crucial role in determining their cytotoxicity, particularly in terms of membrane interactions, immune evasion, and targeting, as well as intracellular trafficking.²⁵ Anisotropic shapes, such as polyhedral and rod-like structures with sharp edges or high aspect ratios, induce mechanical stress, causing membrane puncture and leakage of cytoplasmic content, thereby enhancing cytotoxicity.²⁴ In contrast, spherical NPs exhibit lower cytotoxicity due to their smoother surfaces and reduced physical damage during cellular uptake.^23,25,26 Moreover, the association of size/shape with cytotoxicity, which leads to mechanisms such as oxidative stress, ion release, protein formation, … etc, plays a crucial role in influencing their cytotoxic properties. The oxidative stress generated by the smaller NPs and defect-rich shapes produces more ROS, overwhelming cellular antioxidant defenses.^23,25 The release of metallic ions from metal oxide NPs depends on the size and shape of the NPs. Smaller particles release ions more rapidly, resulting in acidic environments that lead to enhanced ROS generation and increased cytotoxic effects. Size and shape alter protein adsorption patterns, influencing immune recognition and cellular interactions.²⁴

The surface, size, and structure characteristics of nanomaterials make them effective anticancer agents. CdO, CuO, and ZnO are metal oxide nanoparticles that are excellent anticancer agents that may be utilized to treat a variety of cancer-related malignant tumors, including lung (A549) cancer, human hepatoblastoma (HepG2), breast carcinoma (MCF-7), and colon cancer (Caco-2).^20,27–30 Jana et al investigated α Fe2O3–ZnO nanocomposite prepared via the core-shell method and its antibacterial and anticancer activities.³¹ According to Hanan et al, the ZnO/CdO mixed metal oxide exhibits anticancer activity.³² The photocatalytic and biological activities of the CdO-MgO nanocomposite prepared by microwave synthesis have been described by Karthik et al.³³ Elias et al investigated comparative studies in the UV–Vis spectroscopy, photoluminescence, and cytotoxicity of Cu₂O/CuO, ZnO NPs, and Cu₂O/CuO–ZnO NPs.³⁴ Lefojane et al utilized the green chemistry route to synthesize a CdO/CdCO₃ nanocomposite. CdO/CdCO₃ nanocomposite has a selective cytotoxic effect on breast cancer cells.³⁵ The study by Skheel et al focuses on the environmentally friendly production of CdO NP S by employing turmeric plant extract as a covering and reducing agent. Human colon cancer cells (HT29) were shown to be significantly inhibited by green-synthesized CdO_x NPs nanoparticles.³⁶ Mazhar et al investigated Cd_1-xNi_xO synthesized using the co-precipitate calcination technique. The cytotoxicity effect of the prepared Cd_1-xNi_xO samples was assessed on four cancer cell lines: human hepatoblastoma (HepG2), human lung adenocarcinoma (A549), human breast cancer (MDA-MB231), and breast carcinoma (MCF-7).²⁰

Our knowledge indicates that the mixing of metal nanoparticles has been a significant subject matter of recent research. This work focuses on mixing three metal oxides and characterizing the resulting samples using various physical techniques, including analysis of their structural and optical properties. Mixing these metal oxides can enhance their properties. By using the co-precipitation method, we will produce CdO, CuO, ZnO (NPs), and mixed metal oxides of (CdO-CuO-ZnO). Considering that the anticancer effects of the nanocomposite have been under-researched, our research aims to determine how the nanocomposite affects cancer cells.

Experimental Details

Materials

Distilled water (DW), Cadmium nitrate terthydrate (Cd(NO₃)₂.4H₂O) (99%) (HIMEDIA), Copper nitrate trihydrate (Cu(NO₃)₂.3H₂O) (98%) (HIMEDIA), Zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O) (99%)(HIMEDIA), Sodium hydroxide NaOH (98%) (HIMEDIA) are utilized in this study.

Preparation of CdO, CuO, and ZnO Nanoparticles

(Cd(NO₃)₂·4H₂O) was dissolved in 100 mL of distilled water while being continuously stirred for ten minutes. The solution is obtained by adding 0.1 M NaOH dropwise to the solution to neutralize the acidity and adjust the pH to 7. This adjustment was performed while the solution was continuously stirred for 1 hour at room temperature. The finished solutions are stored for the night in an airtight container. After repeatedly cleaning with distilled water and drying at 100°C for one hour, a mortar and pestle are used to grind the precipitate into a fine powder. To synthesize the nanoparticles, the powder is finally annealed for 2 h at 500 °C. For the other two oxides (CuO and ZnO), these procedures were repeated using Cu(NO₃)₂·3H₂O and Zn(NO₃)₂·6H₂O as starting materials.

Synthesis of CdO-CuO-ZnO Nanocomposite

The CdO-CuO-ZnO nanocomposite was prepared by dissolving 0.03 M copper nitrate, zinc nitrate, and cobalt nitrate in 300 mL of distilled water, with continuous stirring for 10 minutes. The exact process was utilized for producing CdO, CuO, and ZnO, which were then used for producing the nanocomposite (Figure 1).

Figure 1 Flowchart of preparation the CdO, CuO, ZnO (NPs) and CdO-CuO-ZnO nanocomposite.

Characterization

The structural properties of the prepared CdO, CuO, ZnO (NPs), and CdO–CuO–ZnO nanocomposite were investigated using the XRD technique (XD-2 X-ray diffractometer using CuK_α (λ = 1.54 Å) at 36 kV and 20 mA, China). The concentrations of each element in the samples were measured using a TXRF (xrf,s8 tiger, German), in the Yemeni Geological Survey and Minerals Resources Board. TEM (JEM-2100, Japan) was used to calculate the nanoparticle size. ImageJ software was used to evaluate the size of TEM images. The diffuse reflectance (DR) spectrum of the as-prepared CdO, CuO, ZnO, and CdO-CuO-ZnO nanocomposites was measured using (UV–Vis–NIR) spectrophotometer (Cary 5000-model DRA-2500) over the wavelength range of 400–700 nm (Model: JASCO, (V-750, Japan)). The photoconductivity studies were carried out using the Keithley source meter [Model: Keithley 6517B electrometer/high resistance meter, USA]. The magnetization versus coercivity of the calcined samples was measured using a VSM (Model: Lake 7410, USA) at the Egypt National Research Centre. Cell Lines cancer cells were obtained from the American Type Culture Collection; cells were cultured using DMEM (Gibco, USA) supplemented with 10% FBS (Gibco, USA) and 1% penicillin-streptomycin mixture (100 IU/mL penicillin and 0.1 mg/mL streptomycin). All other chemicals and reagents were from Sigma or Invitrogen.

Anticancer Activity

Cells were seeded at a density of 2–2.5×10⁴ cells/well at a 96-well tissue culture plate containing a volume of 180µL complete growth medium per well for 24 hours before being treated with 20 µL different dilutions of the tested compound. Drug stock solutions were prepared in DMSO. Eight concentrations (300, 100, 30, 10, 3, 1, 0.3, and 0.1 µg/mL) were prepared for each compound in the growth media. Cells were then treated for 72 h.

Then, each well received a dose of freshly prepared MTT salt (5 mg/mL; Sigma). The MTT solution was carefully removed, and each well received an equal amount of (200 L) DMSO before being incubated for 60 minutes with shaking. The Multiskan^® EX (Thermo Scientific, USA) MicroPlate Reader was utilized to measure the absorbance of each well at 590 nm in order to identify cell proliferation. The experiment was run in triplicate three times. The non-linear fit of the dose-response curve in GraphPad Prism was used to calculate the IC50 values.

Results and Discussion

Structural Analysis

The phase identification and crystal structure were studied utilizing XRD data. The (NPs) XRD pattern confirms the presence of the phases cubic CdO, monoclinic CuO, and hexagonal ZnO. The diffraction peaks match the typical JCPDS Card No. well. CdO (01–1049), CuO (048–1548), and ZnO (01–1136), likewise, CdO-CuO-ZnO nanocomposite (Figure 2).

Figure 2 XRD patterns of the CdO, CuO, ZnO (NPs) and CdO-CuO-ZnO nanocomposite.

The diffraction peaks of CdO, CuO, and ZnO can be observed in the multi-metal oxide composite pattern, as shown in Figure 2. Due to the close diffraction angles, some of the diffraction peaks of metal oxides in the XRD pattern of the composite were shifted. The peaks of CdO in the CdO-CuO-ZnO nanocomposites have the highest intensities, and CuO has the lowest in the mixture.

The average crystallite sizes of each phase were calculated using full width at half maximum calculations and whole powder pattern fitting with Scherrer’s equation^37–41 (Eq. 1).

(1)

Where λ is the wavelength of the X-ray used, β is the full width at half-maximum intensity (FWHM) (in radians), and θ is the diffraction angle.

To determine the cubic CdO lattice parameters^42,43 (Eq.2):

(2)

For the monoclinic CuO structure, the unit cell parameters a, b, and c were calculated using the relation shown below^42,44 (Eq.3):

(3)

To determine the hexagonal ZnO structure, the unit parameter a and c lattice parameters (Eq.4):⁴⁵

(4)

There is a slight increase in the lattice parameters of CuO and ZnO resulting from the nanocomposite mix, as shown in Table 1.

Table 1 XRD Analysis and Particles Size Were Utilized to Determine the Physical Characteristics of the CdO, CuO, ZnO NPs, and CdO-CuO-ZnO Nanocomposite

It is clear that mixing the studied nanocomposites raised the dislocation density while lowering the crystallite size. The reduction in crystallite size often corresponds to a greater density of dislocations and defects in the crystal lattice. By upsetting the regular atomic arrangement, these defects result in microstrain. Reducing the crystal size improves the efficiency of both photolysis and biological processes. This outcome was in line with Munawar et al⁴⁶ findings regarding the impact of grain size on the overall stress of the nanocomposite.

Elemental Analysis

TXRF spectra (Table 2) and histograms Figure 3 show the results of the elemental chemical analysis of prepared samples carried out by TXRF for CdO-CuO-ZnO nanocomposite. CdO, CuO, and ZnO make up the majority of the nanocomposite material. All other elements, including P, S, Ca, Fe, etc, are present in trace amounts. This outcome is consistent with previous studies.^47,48

Table 2 TXRF Analysis of CdO-CuO-ZnO Nanocomposite

Figure 3 TXRF of CdO-CuO-ZnO nanocomposite.

TEM Analysis

CdO, CuO, ZnO (NPs), and CdO-CuO-ZnO nanocomposite are shown in the TEM images and SAED patterns, displayed in Figure 4a and b respectively. For pure CdO, spherical particles with an average size of can be seen. For pure CuO, spherical particles with an average size of have been observed. For pure ZnO, clumped spherical particles (of about ) are seen. The composite shows a mixture of spherical particles. The composite’s average particle size was determined to be 41.5 nm. All of the samples had nanosized particles, according to TEM images. The ImageJ program was used to measure particle sizes, and a histogram is presented in Figure 4c. The presence of well-defined concentric rings in the SAED patterns confirms the nanocrystalline nature of all samples. The circular edges in the SAED patterns correspond to the XRD d-spacing of CdO, CuO, ZnO (NPs) and CdO- CuO-ZnO nanocomposite.

Figure 4 Continued.

Figure 4 (a) TEM image of the CdO, CuO, ZnO (NPs) and CdO-CuO-ZnO nanocomposite. (b) SAED patterns the CdO, CuO, ZnO (NPs) and CdO-CuO-ZnO nanocomposite. (c) Histograms calculated from TEM images the CdO, CuO, ZnO (NPs) and CdO-CuO-ZnO nanocomposite.

Optical Studies

Utilizing diffuse reflectance spectroscopy, the optical properties of synthetic CdO, CuO, ZnO (NPs), and CdO-CuO-ZnO nanocomposite materials were assessed. Figure 5 shows the sample reflectance spectrum as a function of wavelength in the 400–700 spectral range (a). According to the results, the reflectance spectrum increased for CdO NPs and the CdO-CuO-ZnO nanocomposite. In contrast, for CuO NPs, the reflectance decreased with increasing wavelength. For ZnO NPs, the reflection initially increased to a low value, then decreased with increasing wavelength. The Kubelka-Munk equation was applied to convert the measured reflectance into absorbance.^49,50

(5)

Figure 5 Diffuse reflectance spectrum (R%) of the CdO, CuO, ZnO (NPs) and CdO-CuO-ZnO nanocomposites.

Where R is the diffuse reflectance (%) and F(R) is the Kubelka-Munk function corresponding to the absorbance (Figure 5). Utilizing the modified K-M equation, the material’s bandgap energy () and the type of optical transition between the valence band (VB) and conduction band (CB) were identified:^51,52

(6)

The type of optical transition is related to the exponent factor n, and the band tailing and material disorder are measured by the constant A, which is dependent on the probability of the optical transition. The energy of the photon is (). The values of a direct allowed band gap, an indirect allowed band gap, a direct forbidden band gap 1/2, and an indirect forbidden band gap are 2 and 3, respectively. To calculate the material’s , plot versus E and extrapolate the linear portion of the plot up to .^51,53

The type of transition is determined by the best linear fit utilizing various values of n, and this kind of representation is referred to as a Tauc model. The Tauc plot for the CdO-CuO-ZnO nanocomposite and the as-prepared CdO, CuO, and ZnO (NPs) are shown in Figure 6. We obtained the values shown in Table 3.

Table 3 Optical Bandgap Energy Calculators for the Prepared Sample

Figure 6 The plot of (F(R)hν) 2 vs E (eV) for direct band gap.

In comparison to the bandgap energies of CdO, CuO, and ZnO (NPs), the extrapolated bandgap value of the CdO-CuO-ZnO nanocomposite is close to that of CuO. Decreasing crystalline size and particle size can both lead to a decrease in the energy gap of synthesis materials through mechanisms such as the influence of surface states. These effects are fundamental to understanding and engineering the electronic properties of nanomaterials and have important implications for various applications in electronics, photonics, and optoelectronics.⁵⁴ The UV-Vis results suggest that the combination of three semiconductors may affect the band structure, bactericidal effects, and light-absorbing capacities of the CdO-CuO-ZnO nanocomposite photocatalyst. This result is consistent with Muhammad Ishfaq et al.⁵⁵

Zeta Potential and Size Distribution

Determining the zeta potential is an essential method for estimating the surface charge of nanoparticles; it aids in assessing the colloidal stability of NPs. The size of nanoparticles in a colloidal solution is a crucial consideration for their application in various domains, particularly those in biomedical fields. One well-liked and effective technique for figuring out the size of particles in a colloidal solution is dynamic light scattering or DLS.^56,57 The Zeta potential values between −30 mV and +30 mV indicate poor colloidal stability and are likely to experience flocculation and agglomeration.⁵⁸

The zeta potential and size distribution were used to analyze the stability behavior of synthesized CdO, CuO, ZnO (NPs), and CdO-CuO-ZnO nanocomposite, as shown in Table 4 and Figure 7. The mean size distribution of the CdO, CuO, ZnO (NPs), and the CdO-CuO-ZnO nanocomposite ranged from 634 to 3242 nm in diameter. Consistent with our analysis, the average particle size reported by DLS was larger than the one determined by TEM. The discrepancy between TEM and DLS is consistent with many other studies that reported synthesizing a variety of NPs.⁵⁸

Table 4 Zeta Potential and Particle Size for the CdO, CuO, ZnO (NPs), and CdO-CuO-ZnO Nanocomposite

Figure 7 Zeta Potential Distribution the CdO, CuO, ZnO (NPs), and CdO-CuO-ZnO nanocomposite.

Due to the swelling of CdO, CuO, and ZnO nanoparticles, as well as the CdO-CuO-ZnO nanocomposite in an aqueous medium, the sizes of the nanoparticles, as determined by DLS and TEM, differ. For the CdO, CuO, ZnO (NPs), and CdO-CuO-ZnO nanocomposite, the corresponding zeta potentials were –20.5, –21.7, 2.78, 12, and 5.77 mV. However, ZnO NPs and the CdO-CuO-ZnO nanocomposite were less stable than CdO and CuO NPs. This outcome aligns with the findings of Hussein.⁵⁹

Anticancer Activity

Mitochondria are the primary intracellular ROS source. Nearly all organelle species are known to be oxidized by the most reactive ROS, called OH radicals. ROS produced in the cell generates O^2- via electron capture on the surfaces of the nanocomposite and nanoparticles. Free radicals contribute to an imbalance in the oxidant/antioxidant phenomena of the cell by oxidizing lipids, denaturing proteins and nucleic acids, and activating an antioxidant defense system.^60,61

In lysosomes, reactive oxygen species (ROS) can damage cellular redox disequilibria, lead to DNA point mutations, and impair mitochondrial respiration and apoptosis. ROS can also peroxide the lipids in cell membranes. H₂O₂ is produced in the cytoplasm, where it undergoes the Fenton reaction to generate OH, which damages DNA, causes cellular death, and readily diffuses across mitochondrial membranes. High intracellular ROS levels support the expression of redox-sensitive signaling pathways and death receptor nanoparticles under conditions of typical oxidative stress.^62,63 The toxicity of the nanocomposite to cells is shown in Figure 8.

Figure 8 The mechanism underlying the cytotoxicity activity of CdO-CuO-ZnO nanocomposite is depicted schematically.

The information is displayed as the average of the IC50 values from three distinct experiments. Drug stock solutions were prepared in DMSO. Eight concentrations (300, 100, 30, 10, 3, 1, 0.3, and 0.1 µg/mL) were tested, with MTT at a final concentration of 0.5 mg/mL, using the Multiskan^® EX Microplate Reader (Thermo Scientific, USA) to measure each well’s absorbance at 590 nm. Utilizing GraphPad Prism 8’s dose-response curves, the experiment was run three times in triplicate to determine the IC50 values. The cell viability data (obtained by the MTT experiment) was plotted against the logarithm of the concentration and IC50 values for the prepared samples and doxorubicin Figure 9. Where IC50 (mg):1–10 (Very strong), 11–20, (strong), 21–50 (moderate), 51–100 (weak) and above 100 (non-cytotoxic).²¹

Figure 9 Cell viability% versus logarithm of Concentration IC50 values obtained for samples.

According to the previous reference,²¹ CuO (NP) has been shown to be strong for cancer, while CdO NPs are moderate. ZnO NPs are strong for A549 and moderate for Caco-2. The CdO-CuO-ZnO nanocomposite is effective against both types of cancer. The mixed oxides are better as anti-cancer substances compared to the single oxides, as shown in Figure 10 and Table 5.

Table 5 In vitro Anti-Proliferative Activities of the Tested Compounds Against Human Colon (Caco-2) and Lung (A549) Cancer Cell Lines Treated for 72 h

Figure 10 Percentage cytotoxicity of Caco-2 and A549 cells after incubation with CdO, CuO, and ZnO (NPs), and CdO-CuO-ZnO nanocomposite.

Although metal oxide nanocomposites exhibit varying toxicity profiles depending on their physicochemical properties (size, surface area, shape, aspect ratio, and surface functionalization), cell types, cytotoxicity mechanisms, and exposure conditions, these nanocomposites exhibit an influence on cell viability by causing the death of cancer cells while having a negligible effect on normal cells. Sabour et al demonstrated that the Ag and Se dual-doped ZnO-CaO-CuO nanocomposite exhibits stronger toxicity against cancerous Huh-7 cells compared to normal L929 cells. These nanocomposites can inhibit cancer lines in a dose-dependent procedure, while there is no inhibitory influence against normal cells.⁶⁴ In another study, the mixed FeO-MnO nanostructure showed significant anticancer activity against lung cancer cells while having low cytotoxicity against normal cells.⁶⁵ Elsayed et al have investigated the anticancer efficacy of ZnO-Ag nanocomposites against the cancer cell lines (HCT-116 and HELA) without having an impact on the normal cells.⁶⁶ In these studies, as well as numerous published literature reports, metal oxide nanocomposites have been explored for their high anticancer activity against cancerous cells, with minimal effect on normal cells. This characteristic distinguishes them as having a selective affinity for cancerous cells, making them promising anticancer materials.

Through oxidative stress mechanisms, the cytotoxic effects of metal oxide nanoparticles on cancer cell lines are strongly influenced by their optical bandgap energy. Reactive oxygen species (ROS) are produced more easily when the bandgap is smaller, which increases oxidative stress and the cytotoxicity of cancer cells. For example, a study on zinc oxide (ZnO) nanoparticles showed that lowering the band gap energy increased the production of ROS, which in turn caused oxidative stress and autophagy-mediated apoptosis in cancer cells.⁶⁷ Similarly, studies on aluminum-doped ZnO nanoparticles have demonstrated that, in human breast cancer cells (MCF-7), doping increases the bandgap energy of the nanoparticles and enhances their cytotoxicity and oxidative stress response. According to the study, ZnO nanoparticles’ band gap energy increased with Al-doping from 3.51 eV for pure ZnO to 3.87 eV for Al-doped ZnO, improving their selective cytotoxicity toward MCF-7 cells while having no effect on normal cells.⁶⁸ In their investigation of silver-doped titanium dioxide (Ag-TiO₂) nanoparticles, Ahamed et al discovered that doping with silver decreased the band gap energy, which in turn increased the generation of reactive oxygen species (ROS); human lung (A549) and breast (MCF-7) cancer cells experienced increased oxidative stress and cytotoxicity as a result of this rise in ROS.⁶⁹ According to research conducted by Abbas et al, Sn-doped CeO₂ nanostructures greatly adjusted the band gap energies, laying the groundwork for massive cellular ROS production that contributes to the death of cancer cells (Neuroblastoma cell lines).⁷⁰

According to these results, the cytotoxic effects of metal oxide nanoparticles on cancer cells may be influenced by adjusting their bandgap energy, which also affects the particles’ ability to induce oxidative stress. When synthesizing nanoparticles for targeted cancer treatments, this characteristic can be strategically used. The present cytotoxicity of IC50 is compared with that of some NPs (Table 6).

Table 6 Comparison of Cytotoxicity IC50 Values for Some NPs and Nanocomposites That Have Been Published

Conclusions

A novel CdO-CuO-ZnO nanocomposite was synthesized via the co-precipitation method. XRD analysis verified the unique combination of cubic CdO, monoclinic CuO, and hexagonal ZnO structures. Efficient integration of these metal oxides was demonstrated by the nanocomposite’s well-defined crystalline structure, in which CdO contributed the highest intensity peaks and CuO the lowest. The composite’s nanoscale dimensions were validated by Transmission Electron Microscopy (TEM) analysis, indicating that it has potential for use in biomedical applications. With a measured optical bandgap of 1.5 eV, the nanocomposite shows promise for use in photothermal and photodynamic therapies. Furthermore, the results of the zeta potential analysis revealed that the colloidal behavior of ZnO and CdO-CuO-ZnO nanocomposite was less stable than that of CdO and CuO nanoparticles. The CdO-CuO-ZnO nanocomposite’s strong anticancer activity against A549 and Caco-2 cells was one of the study’s key findings. The results of the MTT assay showed a significant cytotoxic effect, with IC50 values of 6.61 (µg/mL) for A549 cells and 10.57 (µg/mL) for Caco-2 cells, highlighting the nanocomposite’s greater effectiveness compared to individual metal oxide nanoparticles. The observed rise in intracellular ROS levels suggests that apoptosis is a crucial mechanism of action that may be used to target cancer treatments. Overall, this study highlights the superiority of this novel CdO-CuO-ZnO nanocomposite in terms of cytotoxicity and biocompatibility, positioning it as a promising candidate for advanced nanomedicine applications in cancer therapy, particularly with targeted delivery approaches.