The Promising Antibacterial and Anticancer Activity of Green Synthesized Zinc Nanoparticles in Combination with Silver and Gold Nanoparticles

Zinc nanoparticles (ZnNPs) are showing promising medical applications. However, their cytotoxicity is relatively high. This study aims to use a green synthesis technique based on the natural propolis (honeybees glue) and produce three counterparts of Zinc nanoparticles (ZnO-NPs, AuNPs@ZnO and AgNPs@ZnO). Then, the three would be analyzed for their potential antibacterial activity and cytotoxicity. The study compares the antibacterial activity and cytotoxicity of ZnO-NPs alone to its combination with gold and silver (AuNPs@ZnO and AgNPs@ZnO). The results showed that AgNPs@ZnO had contributed significantly to antibacterial activity against Gram-positive and Gram-negative bacteria. Moreover, AuNPs@ZnO and AgNPs@ZnO showed similar cytotoxicity to ZnO-NPs with higher safety. Furthermore, the Scanning and Transmission Electron Microscopes’ micrographs (SEM and TEM) displayed the biosynthesized ZnNPs that have a spherical shape with sizes ranging from 17 to 70 nm, from 45 to 75 nm, and from 22 to 73 nm for ZnONPs, ZnO-AgNPs, free AgNPs, and ZnO-AuNPs, respectively. In addition, zeta potential values for ZnONPs were 11.4 mV, while ZnO-AgNPs and ZnO-AuNPs have a higher overall charge of 13.6 mV and 23.9 mV, respectively. Furthermore, ZnNPs exhibited antibacterial activity, yet AgNPs@ZnO demonstrated the highest antibacterial action against Gram-negative and Gram-positive bacteria. In addition, cytotoxicity analyses of biosynthesized ZnNPs on three cell lines; breast cancer cell (MCF-7), liver cancer cell (HepG-2) and Human skin fibroblast (HSF) revealed that Zn-NPs in a combination could function as an anticancer agent. Therefore, there are promising medical applications for the biosynthesized ZnNPs in combinations with other metals, such as ZnO-AgNPs, as safe antibacterial and anticancer medicines.


Introduction
Nanotechnology is an exploding field that introduces potential medical, industrial, biotechnological, and chemical applications. Nanoparticles are synthesized with at least one dimension ranging between 1 and 100 nm [1], giving nanoparticles gigantic capability due to the high surface-to-volume ratios [2]. Not only the ratio of surface area to volume affects the particle size of nanoparticles, but the physical, biological and chemical characteristics of the nanoparticles also vary from their conventional material [3]. Nanoparticles can be synthesized by various chemical and physical approaches [4,5]; however, these approaches are expensive and include organic solvents and toxic agents that threaten both environment and living organisms [6]. Consequently, biologically mediated nanoparticle synthesis has been considered an inexpensive, eco-friendly, and safe alternative approach [7]. Therefore, fungi, bacteria, whole plants, and natural extracts have been utilized as a non-toxic strategy to produce and stabilize nanoparticles [8].
Natural and plant extracts have various advantages over other environmentally friendly synthesis processes. They are widely disseminated, easily extensible, readily available, safe to handle, and less costly [9]. Propolis is a natural resinous beehive product that combines wax, pollen, and bee enzyme. Honeybees gather it from leaf buds and exudates of various plants [8]. Propolis reveals biological and pharmacological activities because its complex bioactive components are derived from indigenous plant sources. Based on its broad spectrum of biomolecules, such as alkaloids, flavonoids, phenols, terpenoids, and steroids, it was found that propolis has promising potential in the synthesis of nanoparticles [10,11].
The Zinc oxide nanoparticles (ZnO-NPs) have attracted worldwide attention for their unique features, distinguishing them from their more significant components, such as flexible semi-conductor and piezoelectric capabilities [12]. ZnO-NPs are characterized by high conductivity, lengthy stability, superior thermal and electric ability, and excellent antibacterial potential. It is proposed that the size, crystallinity, band gap, and surface area of ZnO-NPs contribute to their antibacterial activity [13]. In addition, ZnO-NPs have been proposed to cause cancer cell toxicity by dissolving into Zn 2+ ions at low pH, which creates reactive oxygen species (ROS) and causes cytotoxicity in cancer cells [14]. Consequently, drugs, cosmetics, and animal food supplements are just a few of the applications for ZnO-NPs [15]. Besides zinc, silver (Ag) and gold have been observed for their killing activities against bacteria, viruses, fungi, microbes, and cancer [16][17][18][19][20]. As a result, research into silver and gold nanoparticles and their positive effects on medicine and human health has become a potential concern. Several features of Ag and Au nanoparticles have been identified in the medical field, including medical devices, biosensors, and drug delivery systems. Furthermore, these nanoparticles have high hopes for inhibiting multidrug-resistant bacteria and viruses across a broad spectrum of antibacterial, viral, and anticancer agents [21,22]. ZnO-NPs have shown to be a promising anticancer as it has the potential to destruct tumor cells with a minor effect on healthy cells; besides, several studies have proven the effectivity of ZnO-NPs against several cell lines in a dose-dependent manner, such as MCF-7 and HepG-2 [23]. However, the safety of nanoparticles should be taken into consideration, as the passage of those NPs through normal cells, such as skin cells could influence cell apoptosis, oxidative stress and DNA damage [24].
Consequently, merging other metal nanoparticles, such as silver and gold nanoparticles, with ZnO-NPs could be a promising approach for many fields and introduce various beneficial applications. Therefore, this study compares the antibacterial and anticancer activity on MCF-7 and HEPG2 to compare the results with the previous studies on cytotoxicity on HSF of the ZnO-NPs, AuNPs@ZnO, and AgNPs@ ZnO using a green blended approach based on the natural propolis.

Green Synthesis of Nanoparticles
Three preparations of ZnO-NPs were biosynthesized as described before [25] with some modifications. Briefly, in different containers, 4 g of Zn(NO 3 ) 2 with 1 g of NaOH were added to 100 mL of the propolis extract (3% v/v), where they were added on hot plates at 85 °C for 15 min at 1000 rpm. For the formulation of ZnO-AgNPs and ZnO-AuNPs, 1 mM of AgNO 3 and 1 mM of AuCl 4 were added separately into the formed ZnO-NPs. Then, they were left on the hot plate for two h. All the formations were conducted in the presence of light. After the synthesis, the color change was used to indicate the formation of the nanoparticles.

The Nanoparticle Physicochemical Characterization
The UV-visible spectrum absorption of the three formulated Zn-NPs was measured using a spectrophotometer (Jenway 7200, Staffordshire, UK) in the range of 340-800 nm [26]. Next, the charge of the three ZnO-NPs was calculated via Zetasizer Nano ZS (Malvern, UK) in the field of − 100 to 100 mV. Moreover, the bonds were analyzed using Fouriertransform Infrared Spectroscopy (FT-IR; Perkin Elmer, USA) in the range of 4000-400 cm −1 . Finally, thermogravimetric analysis (TGA, SDT Q600 V20.9 Build 20, USA) was used to measure the thermal stability of the prepared nanoparticles in the temperature range from 42 to 982 °C with a heating rate of 15 °C per min.

The Microscopic Characterization
The surface structure of the formulated ZnO-NPs was studied using SEM, as in [26]. For the sample preparations, gold coats were used in a sputter-coating unit (JFC-1100 E) to be imaged using SEM (JEOL JSM-5300, Tokyo, Japan) at 20 kV. Then the percentage of each compound in the three ZnO-NPs was determined using Energy Dispersive X-ray Spectroscopy (EDX) (JEOL JSM-5300, Tokyo, Japan) at 20 kV for a 10 mm working distance. Finally, TEM was used to study the size and shape of the formulations. First, the nanoparticles were added to carbon-coated copper grids (Pelco International) and visualized using TEM (1230 JEOL Tokyo, Japan). Then, the size of the nanoparticles was analyzed using ImageJ software [27].

Disk Diffusion Test
The antibacterial activity of ZnO-NPs, AuNPs@ZnO and AgNPs@ZnO preparations were evaluated at different concentrations which are 10 mg/mL, 5 mg/mL, 2.5 mg/mL, and 1.25 mg/mL. All concentrations were tested against Grampositive Staphylococcus sciuri (S. sciuri) (MW415886) and Gram-negative Pseudomonas aeruginosa (P. aeruginosa) (OL375153) and Salmonella enterica (NCTC 13,348) using agar disk diffusion method [27,28]. Briefly, bacterial cultures from each bacterium separately were spread on Tryptic Soya Agar (TSA) plates. Then sterile discs with a diameter of 6 mm were loaded onto the agar and saturated with 10 µL of 10 mg/mL of different nanoparticles. Antibiotics were used for each bacterium as a positive control; the negative control was discs without treatment. Finally, the plates were left at 37 °C for 16-18 h, and the clear zones were measured.

MIC and MBC Tests
The minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of the formulated nanoparticles were measured as described in previous studies [27,29]. Briefly, the bacterial cultures in tryptic soya broth (TSB) with mid-log-phase (10 5 ~ 10 6 CFU/mL) were incubated with various concentrations of ZnO-NPs, AuNPs@ZnO and AgNPs@ZnO (5000, 2500, 1250, 625, 310, 155, 75, 37 µg/mL) at 37 °C for 18 h in a 96-well plate. The clear well indicates the inhibition ability of the nanoparticles; thus, 10 µL of each clear culture was spotted on TSB for evaluating the MBC.

Bacterial Cell Membrane Integrity
The cell membrane integrity of S. sciuri and P. aeruginosa was evaluated using SEM [30]. Briefly, P. aeruginosa was incubated with 650 µg/mL of each nanoparticle and S. sciuri with 75 µg/mL of ZnO-NPs and AuNPs@ZnO, and 40 µg/ mL of AgNPs@ZnO separately overnight at 25 °C. Then, the bacteria were coated with gold with the sputter-coating unit (JFC-1100 E, Japan) and imaged using SEM (Jeol JSM-5300, Japan).

Cytotoxicity Assay
Two cancer cell lines (MCF-7, HepG-2) and a normal cell line (HSF) were purchased from Nawah Scientific Inc., (Mokatam, Cairo, Egypt) and maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 100 mg/mL of streptomycin, and 100 units/mL of penicillin as an antibacterial and antifungal agent. Cells were grown to the confluence at 37 °C and 5%CO2 atmosphere. Once confluence is reached, the cells detached by trypsin-EDTA, centrifuged and then resuspended in the growth media for further studies [31].

Anticancer Activity
To measure the anticancer activity of the nanoparticles, MCF-7 and HepG-2 cells were seeded in a 96-well plate with a density of 5000 cells/well for 24 h at 37 °C and 5% CO2 atmosphere. Then, the cells were incubated with various concentrations of NPs (10, 20, 50, 100, 200, 500 and 1000 μg/mL) and incubated for 72 h at 37 °C and 5% CO2 atmosphere. MTT assay was used to assess cell viability. First, 100μL (10 μL of MTT reagent + 90 μL of DMEM) was added to each well and incubated at 37 °C for four h to form formazan crystals by metabolically active cells. After that, the medium was removed, 100 μL of solubilized agent (DMSO) was added, then OD was measured at 570 nm.

Cytotoxicity on HSF
To assess the cytotoxicity of the nanoparticles, HSF cell lines were used. First, 5000 cells (passage 8) were seeded in a 96-well plate in each well and incubated for 24 h at 37 °C and 5% CO 2 atmosphere. Then nanoparticles were added with the same concentrations on cancer cells (10, 20, 50, 100, 200, 500 and 1000 μg/mL) and incubated for 24 h at 37 °C and 5% CO 2 atmosphere. Following, cell viability was assessed by MTT assay.

Cytotoxicity of Cisplatin
Cisplatin, a commercial cancer drug, was used to assess the cytotoxicity against MCF-7 and HepG-2 compared with HSF. At first, cells were seeded in a 96-well plate, with a seeding density of 5000 cells/well for 24 h. Then, broad concentrations of cisplatin were added (2.5, 5, 10, 20, 40, 80 and 100 μM) and incubated for 24 h. Following, cell viability was assessed by MTT assay.

Synthesis and Characterization of Zn-NPs Using FTIR and EDX
Our results showed that using color formation confirmed the synthesis of different ZnO-NPs ( Fig. 1), by adding different metals, the NPs color changed. To ensure the effective formulation of the ZnNPs, FTIR analysis was done on the three samples: ZnO-NPs, AgNPs@ZnO and AuNPs@ ZnO. The three curves displayed many characteristic peaks in all samples (Fig. 2). The familiar peaks are at 3437 cm −1 , 1634 cm −1 and1384cm −1 , which attribute to the O-H bond, the double bonds of the free stretching carboxylic acids (COOH), C = O amides and zinc carboxylate bond vibration or bend, respectively [32][33][34]. In addition, at least one peak presents in the range of 600 to 400 cm −1 , corresponding to the Zn-O bond vibrations, whereas the peak near 2093 cm −1 is for the C-N bond [30][31][32][33].
For further elucidation of the purity and composition of the three synthesized Zn-NPs, analysis using EDX was done. The results approved the composition of ZnO in the three samples, while Ag and Au only appeared for ZnO-AgNPs Fig. 1 The formation of ZnO-NP, AuNPs@ZnO and AgNPs@ZnO and ZnO-AuNPs, respectively (Fig. 3). For instance, ZnO-NPs contain almost 70% zinc and 29% oxygen atoms, similar to the reported ratio [35]. For ZnO-AgNPs, zinc, oxygen and silver atoms represent 49%, 33% and 0.64%, respectively, but for ZnO-AuNPs, zinc, oxygen, and gold atoms represent 39%, 37%, and 1%, respectively, as reported before [36]. This technic aims to confirm nanoparticle preparation by estimating the zinc ratio to oxygen, gold and silver. The sodium atoms and other organic molecules that appeared in the FTIR and EDX are expected to be residues from NaOH used in the preparation, or they may be originated during the nanoparticle biosynthesis using propolis extract [33,35], as the chemical composition of the used propolis was previously discussed [37].

Characterization of Zn-NPs Using Zeta-Potential
The zeta potential results showed that the three samples have a positive overall charge. While ZnO-NPs have an overall 11.4 mV, ZnO-AgNPs and ZnO-AuNPs presented a higher overall charge of 13.6 mV and 23.9 mV, respectively (Fig. 4). The zeta potential results showed the stability of the green synthesized nanoparticles; the high value in positive or negative ensures continuous repulsion, and the low value indicates the possibility of aggregations and ± 30 mV was recommended as a stable nanoparticle [38,39]. Accordingly, these results indicate the instability of the formulation and forming aggregates as a part of the limitation. However, the values are similar to or higher than the previous formulations [28,40].

Microscopic Characterization of the Formulated Zn-NPs
SEM and TEM were used to analyze the shape, size and morphology of the three biosynthesized ZnNPs. ZnO-NPs come in different morphologies and sizes and have various properties. Herein, the TEM micrographs depicted the spherical shape of the formulated ZnONPs ranging from 17 to 70 nm (Fig. 5a-c). For the ZnO-AgNPs, the size ranges from 45 nm for free AgNPs to 75 nm, while ZnO-AuNPs range from 22 to 73 nm (Fig. 5e-i). On the other hand, SEM micrographs display the outer surface structure of the formulated nanoparticles (Fig. 6).

TGA Characterization
In ZnO-NPs, the thermogravimetric analysis showed that the weight loss occurs sequentially with increasing temperature. The first step revealed a minor weight loss through the heating procedure in the range of (40-300 °C). This little weight loss was assigned to the elimination of physically adsorbed water. In the second step, a major weight loss was also noticed in the range of (380-484 °C) which was assigned to the combustion of organic materials. Finally, no more weight loss was recorded up to 800 °C, indicating the formation of nanocrystalline ZnO as the decomposition was accomplished at 480 °C. However, the thermogravimetric analysis for AgNPs@ ZnO and AuNPs@ZnO showed a minor weight loss (2%) through the heating procedure in the range of (43-197 °C) and (43-149 °C), respectively, due to the loss of water molecules from the sample. Furthermore, another weight loss (6% for AgNPs@ZnO and 25% for AuNPs@ZnO) was noticed in the range of (288-970 °C) and (185-981 °C), respectively. The lower weight loss in the AgNPs@ZnO indicates higher thermal stability of the nanomaterial (Fig. 7).

UV Results Analysis
The formation and synthesis of the formulated nanoparticles were confirmed by measuring the UV-Vis spectrum. Figure. 8 shows the OD of ZnO-NPs, AuNPs@ZnO, and AgNPs@ZnO in the 340-800 nm range. For ZnO-NP the results showed maximum absorption at 355 nm, while for AgNPs@ZnO and AuNPs@ZnO, a significate enhancement has been observed with maximum absorption at 378 and 377 nm, respectively. These results are due to a Schottky barrier at the interface of the Metals such as Au and Au with ZnO [41].

Antibacterial Assays
Zones of inhibition (ZOI) evaluated the antibacterial potentials of ZnO-NPs, AgNPs@ZnO, and AuNPs@ZnO in the form of MIC and MBC as indicators of bacterial susceptibility to the formulated nanoparticles. The results of the disc diffusion method, MIC, and MBC showed that the antibacterial activity of the prepared zinc nanoparticles depended on the type of nanoparticles, concentration, and bacterial species (Table 1). For instance, AgNPs@ZnO showed significant antibacterial activity over the other formulas only against S. sciuri, with a maximum ZOI equal to 13.0 ± 8.5 mm. Subsequently, AuNPs@ZnO significantly affects ZOI (mm) on S. sciuri with 11 ± 7.0, P. aeruginosa with 11.0 ± 8.0 and S. enterica with 9.0 ± 6.5. These findings revealed that S. sciuri was more vulnerable to AgNPs@ZnO than P. aeruginosa and S. enterica by their cell membrane architecture (Fig. 9). Moreover, the inhibition and killing activity of the biosynthesized nanoparticles were screened by MIC and MBC tests, as indicated in Table 2. The MIC and MBC results also agreed with the disk diffusion assay since AgNPs@ZnO was the most effective nanoparticle against all the bacterial strains. One limitation of using nanoparticles in studying their antibacterial activity is using the Zno-NPs or their combination with gold and silver from the stock solution that may containtraces of precursors that did not enter the formation process. The previous investigation proposed that ZnO-NPs interfere with the metabolic pathway in the bacteria [23], in addition to the antibacterial activity on the Gram-positive and Gram-negative bacteria and fungus [24]. It is clear that, adding silver or gold to ZnO-NPs enhances the activity, but its concentration still too high to be used in an animal model or clinical trials. Thus, there is a

Antibacterial Action Analysis by SEM
The antibacterial activity of nanoparticles against bacterial cells was also confirmed based on the morphology of the treated bacteria. The bacterial outer membrane integrity was evaluated using SEM analysis. The bacterial surface of the untreated S. sciuri (Gram-positive) has a dense, spherical-shaped membrane with an intact cellular structure with secretions to form a biofilm (Fig. 10). Simultaneously, the Zn-NPs' treatment seems to inhibit the bacterial secretions for biofilm formation and cause morphological alteration and membrane damage.
Moreover, the biosynthesized AgNPs@ZnO caused the release of cytosolic components through membrane damage of P. aeruginosa (Gram-negative) bacteria (Fig. 11). However, AuNPs@ZnO and ZnO-NPs had a minor effect on P. aeruginosa bacteria.

Cytotoxicity Assays
The anticancer activity of the three formulated nanoparticles was evaluated on MCF-7 and HepG-2 cell lines. The cytotoxicity test was determined by MTT assay, which was done for seven different concentrations of ZnO-NP, AuNPs@ZnO and AgNPs@ZnO (10, 20, 50, 100, 200, 500 and 1000 µg/ mL) and the cell growth without treatment as a negative control. The findings presented no significant difference in both types of cell lines, MCF-7 and HepG-2 (Fig. 12a  200 µg/mL of AuNPs@ZnO, AgNPs@ZnO and ZnO-NP on HEPG2 cell line, respectively). However, the cell viability was decreased at higher concentrations (500 µg/mL and 1000 µg/mL for AuNPs@ZnO, AgNPs@ZnO and ZnO-NP). For example, the viability of the cells at a concentration of 500 µg/mL reached 35.80%, 38.20% and 36.31% for AuNPs@ZnO, AgNPs@ZnO and ZnO-NP on the MCF-7 cell line, respectively. In addition, the cell viability was 51.58%, 45.14% and 42.18 for the same NPs and concentrations on the HepG-2 cell line, respectively (Fig. 12a, b). Those results have been confirmed with MTT images for the cells (Fig. 13, 14 and 15), representing the cells' viability at law concentrations compared with necrosis and cell death at high concentrations of NPs.
These results indicated that the combination of metals in the preparation of nanoparticles, whether with silver or gold ions, formulate less toxic nanoparticles to MCF-7 cells and HepG-2 cells, compared to using Zn nanoparticles alone.
The HSF cell line was used to measure the safety and toxicity of those nanoparticles. It was shown that AuNPs@ ZnO and AgNPs@ZnO could not induce cell death until the highest concentration was used, 1000 µg/mL (Fig. 12c). However, treating the HSF cell line with ZnO-NP at higher concentrations decreased the cell viability to 50.60%. Thus, Our results indicated that combining two metals was less toxic than using Zn alone on normal cell lines such as HSF.
In addition to measuring the cytotoxicity of our prepared nanoparticles, we figured out the effect of commercial cancer drugs, such as cisplatin, on cancer cell lines, MCF-7 and HepG-2, and normal cell lines, HSF. We measured the cytotoxicity of cisplatin using various concentrations (2.5, 5, 10, 20, 40, 80 and 100 µM) after incubation for 24 h. Our results showed that increasing the concentration of cisplatin will eventually increase cell death in all types of cells, whether it is cancerous such as MCF-7 and HepG-2 or a normal cell line, such as HSF (Fig. 16). This indicated the high toxicity of cisplatin on normal cells, although it highly affects cancerous ones. ZnO-NPs are now being researched for their anticancer potential in vitro and in vivo trials. Zn-NPs  could induce cell death through increasing cellular ROS generation and oxidative stress, which lead to cytotoxicity [36,42]. This study aimed to study the potential of ZnO-NP and its combination with gold and silver nanoparticles on MCF-7 and HepG-2 cell lines. Combining Zn-NP with other metals decreased the cell viability but did not show a promising result. However, it succeeded in Zadeh's work, combining Copper/Zinc nanoparticles on MCF-7 cancer cells [43]. This indicates that further studies are needed to explore the potential of combining metal nanoparticles to formulate particles with high medical value yet lower cytotoxicity [44][45][46].

Conclusion
This study produced three zinc nanoparticles (ZnNPs) formulas (ZnO-NPs, AuNPs@ZnO, and AgNPs@ZnO) using a green synthesis technique based on honeybees propolis. The physicochemical properties of the three formulas were analyzed, presenting a spherical shape and a high positive zeta potential value. In addition, the three formulas demonstrated significant antibacterial activity, particularly AgNPs@ZnO, which exhibited the most potent antibacterial effect against both Gram-negative and Grampositive bacteria. Moreover, cytotoxicity tests of the three biosynthesized ZnNPs on cell lines (MCF-7 and HepG-2) revealed that ZnO-NPs, AuNPs@ZnO and AgNPs@ ZnO have anticancer activity. However, The toxicity of ZnO-NPs is higher than AuNPs@ZnO and AgNPs@ZnO Fig. 9 The antibacterial activity among various Zn-NPs formulations against S. sciuri, P. aeruginosa and S. enterica bacteria through disc diffusion. AB stands for antibiotics as a positive control, and disc-free stands for disc without drugs as a negative control against the HSF cell line. Therefore, the biosynthesized AgNPs@ZnO can be employed in promising applications as antibacterial and anticancer drugs with a higher safety range. Further research is needed to explore the potential of combining different metal nanoparticles for medical applications with the lowest cytotoxicity levels.