1 Introduction

Nature has provided ways and insights into the simple and green synthesis of nanoparticles (NP) [1, 2]. Plant extract and microorganism synthesis has gained extensive attention as a green, sustainable, and eco-friendly protocol for synthesizing a wide range of nanomaterials [3, 4]. To date, many successful attempts have been made to biosynthesize metal and metal oxide NPs using plant extracts (leaves, roots, flowers) and microorganisms (fungi, bacteria, and microalgae) [5, 6]. In contrast to synthesis using microbes, plant extracts are the best candidates for green biosynthesis because the extracts contain biomolecules (polyphenols, terpenoids, phenolic acids, and alkaloids) that oxidize or reduce the metal ions and stabilize the dispersible nanoparticles after the synthesis process [7, 8]. The production cost is lower in plant extract–mediated synthesis because the raw materials (especially plants) are readily available and no additional purification process is required compared to microbial-mediated synthesis [9, 10]. Moreover, this strategy is a sustainable and safe strategy for nanoparticle production because the crystal growth and dispersion of nanoparticles can be easily controlled [11, 12]. In several cases, the synthesis mediated by plant extracts has been shown to improve the biological properties (bioavailability, biocompatibility, cell internalization, antioxidant activity) of the metal and metal oxide nanoparticles and reduce toxicity [13,14,15].

Nowadays, several research groups focus on the green synthesis of metal and metal oxide nanoparticles using plant extract for biomedical applications [16,17,18,19,20,21,22,23,24]. The extracts of medicinal plants have been shown to possess acceptable antioxidant activity, which results from free radical degradation [25]. Metal-based nanoparticles functionalized with plant extracts as natural antioxidants were found to provide targeted delivery and antibacterial activity [26, 27]. Among different antioxidant metal oxide nanoparticles, CuO and Cu2O NPs have attracted significant attention due to their low cost, abundant availability of copper salt, and high antioxidant activity of CuO and Cu2O NPs [28]. Antioxidant properties of CuO and Cu2O NPs depend on their nature, polymorphism, crystal structure, chemical composition, surface charge, particle size, surface-to-volume ratio, surface coating, and dispersion state [29]. Aside from antioxidant applications, CuO and Cu2O NPs were reported for the treatment of cancer diseases [30], oxidative stress diseases [31], and cardiovascular diseases [15, 32]. CuO and Cu2O NPs also offer many other applications, including photocatalysis, antibacterial, biosensing, and energy storage [33, 34].

To date, several physical and chemical processes are currently used for CuO and Cu2O NP synthesis, which allows for obtaining particles with preferred properties [35]. However, these production methods have many disadvantages due to hazardous organic solvents, a costly reagent, the difficulty of isolating the nanoparticles, and the longer time required [36]. Herefore, there is an essential need for a clean, reliable, cost-effective, and environmentally friendly process for the synthesis of nanoparticles [37]. The plant extract–mediated synthesis of CuO and Cu2O NPs using plant extracts such as Anthemis nobilis [38], Calotropis gigantea [39], Gloriosa superba [40], Aloe vera [37], Carica papaya [41], and Emblica officinalis [42]. The microbe-mediated synthesis of CuO and Cu2O NPs has also been reported. However, the major drawback of microbe-mediated synthesis is the complex procedures, i.e., microbial growth conditions, and isolation of microorganisms, which require a longer reaction time and thus increase the cost of scaling up [43].

Previous research has shown that both CuO and Cu2O NPs have strong antioxidant activity, but the antioxidant properties of copper mixed oxides (CuO/Cu2O NPs) have not yet been investigated. This work aims to synthesize CuO/Cu2O NPs by an environmentally friendly method using a leaf extract of Phoenix dactylifera L. and investigate the effects of extract/precursor contractions on the size, shape, and crystal structure, and estimate their antioxidant activity. The phytochemical constituents of Phoenix dactylifera L. act as bio-reducing agents and stabilizers for the formation of CuO/Cu2O NPs, which showed good antioxidant and cytotoxic activities. The synthesized CuO/Cu2O NPs were characterized to determine the properties of bioactive constituents (capping agents) in each leaf extract [44]. The method presented here is an alternative to the solvothermal synthesis of CuO and Cu2O NPs in the context of green chemistry. Phoenix dactylifera L is one of the most important plant sources rich in active compounds such as polyphenols, terpenoids, phenolic acids, and alkaloids, all of which have significant antioxidant activity in biological systems [45]. Copper is one of the useful and essential substances for the human body, providing balance and stability in tissues. CuO and Cu2O NPs have been introduced by the US Environmental Protection Agency (EPA) as effective antibacterial agents [45]. Accordingly, the synthesized CuO/Cu2O NPs can be used as drug carriers (chemotherapeutics) for antioxidant and antimicrobial applications.

2 Experimental

2.1 Chemicals, reagents, and plant materials

Phoenix dactylifera L. leaves of Ghars quality were collected from El Oued, Southeast of Algeria (latitude: 31°–34° N, longitude: 6°–8° E). Copper sulfate (CuSO4·5H2O, 98%, Sigma-Aldrich), ethanol absolute (C2H5OH, 99%, Sigma-Aldrich), methanol (CH3OH, 96%, Sigma-Aldrich), sodium phosphate monobasic (NaH2PO4, 99%, Biochem Chemopharma), sulfuric acid (H2SO4, 96%, Biochem Chemopharma), hydrochloric acid (HCl, 35%, Biochem Chemopharma), ascorbic acid (C6H8O6, 99%, Biochem Chemopharma), acetic acid (CH3COOH, 99.5%, Biochem Chemopharma), sodium acetate (CH3COONa, 99%, Biochem Chemopharma), ammonium molybdate ((NH4)6Mo7O24, 99%, Sigma-Aldrich), 2,2-diphenyl-1-picrylhydrazyl (DPPH, C18H12N5O6, 95%, Alfa Aesar), iron(III) chloride (FeCl3, 99%, Biochem Chemopharma), iron(II) sulfate (FeSO4·7H2O, 99%, Biochem Chemopharma), and 2,4,6-tripyridyl-s-triazine (TPTZ, C18H12N6, 98%, Alfa Aesar).

2.2 Preparation of plant extract

The leaves of Phoenix Dactylifera L. were rinsed several times with distilled water and then dried at room temperature and protected from sunlight for 5 days to preserve the chemical composition as much as possible [46]. Briefly, 10 g of powder of these leaves was added to 60 ml of ethanol/water mixture (70%/30%). This preparation was shaken at room temperature for 24 h. The extracts were then filtered using Whatman No. 1 filter paper and stored at 4 °C.

3 Biosynthesis of copper mixed oxide nanoparticles

For the synthesis of CuO/Cu2O NPs, four different samples were prepared with different ratios of CuSO4·5H2O solution to Phoenix Dactylifera L. (v/v). Specifically, 1 ml of the extract of Phoenix dactylifera L. was added to 30 ml of aqueous CuSO4·5H2O solution (4, 6, 8, and 10 mM). For each sample, the mixture was stirred for 2 h at 70 °C with a magnetic stirrer until the color turned to deep brown and a precipitate was observed. The precipitate of each sample was centrifuged at 1000 rpm for 30 min and washed with deionized water to remove impurities and centrifuged again for 20 min. The brown precipitate obtained was dried at 80 °C for one night and then annealed in air at 400 °C for 2 h [46].

4 Characterization of copper mixed oxide nanoparticles

The crystalline structure of CuO/Cu2O NPs was examined by using X-ray diffraction (XRD, Rigaku Miniflex 600) using CuKα radiation (40 kV and 30 mA) with a wavelength of 1.5418 A and scanning speed of 0.5° [47]. The particle size and shape were analyzed using a scanning electron microscope (SEM, TESCAN VEGA 3). The bonding characteristic of CuO/Cu2O NPs was analyzed using a Fourier transform infrared spectrometer (FTIR, Nicolet iS5, Thermo Fisher Scientific) in a spectral range of 4000–500 cm−1. The light absorbance and bandgap energy of CuO/Cu2O NPs were determined by the UV–vis absorption spectrum (Shimadzu UV-1800s) in the wavelength range of 200–800 nm.

5 Total antioxidant capacity

The total antioxidant capability (TAC) of the samples was estimated by the phosphomolybdenum method [35]. The TAC assay is based on the reduction of molybdate ions MoO42− (Mo6+) into green MoO2+ (Mo5+) in the presence of antioxidants (CuO/Cu2O NPs) in an acid milieu [36]. TAC assay usually detects antioxidants such as some phenolics, ascorbic acid, α-tocopherol, and carotenoids. An amount of 0.2 ml of each concentration of CuO/Cu2O NPs has been mixed with 2 ml of reagent solution consisting of 0.6 M H2SO4, 4 mM ammonium molybdate, and 28 mM NaH2PO4. These were incubated at a temperature of 95 °C for 1.5 h; after cooling, it was measured for absorbance at 695 nm. The total antioxidant capacity of the samples was obtained from an ascorbic acid calibration curve. The latter was traced using various ascorbic acid concentrations from 0.01 to 0.1 mg/ml. The total antioxidant capability is represented as milligrams of ascorbic acid equivalence per gram of nanoparticles (mg EAA/g NPs).

6 Ferric reducing antioxidant power

The reducing power of ferric ion was determined by the ferric reducing antioxidant power (FRAP) method prescribed by Benzie and Strain [48]. This method is based on the reduction of Fe3+ ions into Fe2+ ions through the antioxidant (CuO/Cu2O NPs); the reaction is detected by the transfer from the yellow color of Fe3+ ions to the blue Fe2+ ions [48]. The increase in UV–Vis absorbance indicates the elevation of the reducing power of the tested samples. The FRAP solution was prepared by admixing 2.5 ml of 10 mM TPTZ prepared in 40 mM HCl, 2.5 ml of 20 mM FeCl3, 25 ml of acetate buffer (pH ~ 3.6), and 3 ml of distilled water [49]. About 30 µl of the sample (CuO/Cu2O NPs) was mixed with 970 µl of FRAP solution and incubated for 30 min at 37 °C. The reaction was monitored using UV–vis spectroscopy by measuring the absorbance at 593 nm. In this test, iron sulfate (FeSO4) was used as the standard. The results of the reductive power of the samples are presented in milligrams of FeSO4 equivalent per 100 mg of CuO/Cu2O NPs (mg E FeSO4/100 mg NPs) [48].

7 Free radical-scavenging activity (DPPH)

The DPPH assay is a rapid and the most widely used assay for characterizing the antiradical activity of plant extracts [50]. It is expressed as IC50 which denotes the concentration of each sample required to scavenge 50% of DPPH free radicals. The DPPH assay is based on measuring the capacity of antioxidants (CuO/Cu2O NPs) to scavenge the DPPH radical [40]. Briefly, 2 ml of a methanolic solution of 0.1 mM DPPH was mixed with 1 ml with different CuO/Cu2O NP concentrations. The obtained mixture was incubated in the dark for 15 min at room temperature [51]. The absorbance is measured at 517 nm against a control consisting of 1 ml methanol and 2 ml DPPH solution.

The percentage of inhibition is calculated using the following equation:

$$\mathrm{Inhibition}\left(\mathrm{\%}\right)=[( \frac{{\mathrm{Abs}}_{\mathrm{control}} -{\mathrm{ Abs}}_{\mathrm{sample}}}{{\mathrm{Abs}}_{\mathrm{control}}})\times 100 ]$$
(1)

where.

Abscontrol is the absorbance of the control (containing no antioxidants).

Abssample is the absorbance of the sample after 15 min.

The antiradical activity is then expressed by the IC50 value, where IC50 is the sample concentration necessary to obtain 50% of the reduced form of the DPPH radical.

8 Results and discussion

The Phoenix dactylifera L.–mediated synthesis of CuO/Cu2O NPs is more advantageous than chemical and physical synthesis as it is a clean, non-toxic, cost-effective, and environmentally friendly approach. Moreover, Phoenix dactylifera L. is easily available in nature, and this makes it a preferable plant material for scaling up at the industrial level [20, 24]. Biosynthesis of CuO/Cu2O NPs depends on secondary metabolites (polyphenols, terpenoids, phenolic acids, and alkaloids) contained in the extract, which are also responsible for reducing the metal ions [4]. During synthesis, plant extracts are used as bio-reducing agents and capping agents. The reduction process consists of returning Cu2+ ions to Cu0. The Cu0 represented by the Cu NPs is converted into CuO/Cu2O NPs after annealing in the incinerator at 400 °C for 2 h [52].

$$\mathrm{Cu}^{2+}+2e^-\xrightarrow{\mathrm{Reduction}\;\mathrm{with}\;\mathrm{plant}\;\mathrm{extract}}\mathrm{Cu}\left(0\right)$$
(2)
$$\mathrm{Cu}\left(0\right)\xrightarrow{\mathrm{Annealing}\;\mathrm{at}\;400^\circ\mathrm C\;2\mathrm h}C\mathrm{uO}/{\mathrm{Cu}}_2O\;\mathrm{NPs}$$
(3)

The phytochemical analysis of the extract from the leaves of Phoenix dactylifera L. revealed that they contain flavonoids, condensed tannins, and saponins [21]. The most important visual observation during the reaction is the change of the color of the solution from green to brown within 30 min. The brown color is a clear indication of the formation of CuO/Cu2O NPs. Based on this evidence, a possible mechanism for the reduction of Cu2+ and the formation of CuO/Cu2O NPs was proposed using Phoenix dactylifera L. extract (Scheme 1). The antioxidant activity of CuO/Cu2O NPs reflects their ability to scavenge free radicals in the organism. In vitro methods were used to investigate the antioxidant potential of CuO/Cu2O NPs. The dispersion of the nanoparticles was injected into a free radical producing system, and the inhibitory effect on free radicals was measured by three assays, i.e., reducing antioxidant power of iron (FRAP), total antioxidant capacity (TAC), and radical scavenging activity (RSA), for DPPH.

Scheme 1
scheme 1

Protocol of plant extraction and biosynthesis mechanism of CuO/Cu2O NPs using Phoenix dactylifera L. leaf extract

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9 Crystal structure and composition

XRD results show variation in crystallite size and composition (phase CuO/Cu2O ratio) related to the change in the concentrations of CuSO4·5H2O. Figure 1 shows the XRD pattern of the CuO/Cu2O NPs prepared using the Phoenix dactylifera L. extract and different concentrations of CuSO4·5H2O. This diffractogram affirms the existence of two crystalline phases, cuprous oxides (Cu2O) and cupric oxide (CuO). The peaks position with 2θ values of 32.5°, 35.5°, 38.8°, 48.8°, 58.3°, 61.7°, 66.3°, and 68.4°, corresponding to the crystalline planes of (110), (002), (111), (202), (202), (113), (311), and (220) which confirm the formation of the monoclinic crystal structure for CuO (JCPDS-01–089-5899) [53]. The other five characteristic peaks at 2θ values of 29.5°, 36.3°, 42.4°, 61.3°, and 73.4° are attributed to the crystal planes of (110), (111), (200), (220), and (311), which correspond to the cubic phase Cu2O (JCPDS-00–005-0667). The crystallite size was calculated from the full width at half maximum intensity (FWHM) measured on the corrected diffraction profile using the D = Kλ/(β cos θ), using the Scherrer formula [54], as shown in Table 1. For the cubic crystal structure, K = 0.94, λ wavelength of X-ray; d = the full width at FWHM of the peak [55].

Fig. 1
figure 1

XRD pattern of CuO/Cu2O NPs prepared using Phoenix dactylifera L. extract and different concentrations of CuSO4·5H2O

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Table 1 Average crystallite size, crystal shape, phase ratio CuO/Cu2O, particle size, and particle shape of CuO/Cu2O NPs prepared using Phoenix dactylifera L. extract and different concentrations of CuSO4·5H2O
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XRD results show that the crystallite sizes increase for the CuO (from 15.7 to 23.0) and Cu2O (from 18.3 to 35.4 nm) phases which slightly increase as the CuSO4·5H2O concentration increases from 4 to 10 mM. Interestingly, the ratio of the CuO/Cu2O phase increases from 58/42 to 98/2 with increasing the CuSO4·5H2O concentration from 4 to 10 mM (Table 1). Typically, increasing the CuSO4·5H2O concentration (reactant concentration) accelerates the nucleation rate and shortens the reaction time; thus, nanocrystals with larger crystallite sizes are formed. Similarly, increasing the CuSO4·5H2O concentration (reactant concentration) provides more opportunity to produce the CuO phase rather than the Cu2O phase. This can explain the increase in the CuO/Cu2O phase (from 58/42 to 98/2) by increasing the CuSO4·5H2O concentration (from 4 to 10 mM). Table 1 shows the average crystallite size, crystal shape, and phase ratio CuO/Cu2O of the prepared CuO/Cu2O NPs.

FTIR analysis was carried out to confirm the formation of CuO/Cu2O NPs using the Phoenix dactylifera L. extract as a reducing and stabilizing agent. Figure 2 brings together the FT-IR spectrum of Phoenix dactylifera L. leaf extract with different spectra of CuO/Cu2O NPs prepared at different ratios before thermal treatment at 400 °C [56]. FTIR spectra (Fig. 2) of the Phoenix dactylifera L. extract exhibited several absorption bands (at 3264, 1605, 1442, 1283, 1049, and 671 cm−1) corresponding to the functional groups of the biomolecules existing in the plant extract. The broad and strong band at 3264 cm−1 is attributed to hydrogen-bonded OH groups of alcohols and phenols as well as the presence of amide N–H amines [56]. The bands at 1605 and 1442 cm−1 are attributed to the (C = O) stretching of the amide carbonyl and the C–N stretching vibration of the aromatic amine [41]. A weak band at 1283 and 1049 cm−1 is attributed to C–O stretching and C–O–C stretching asymmetric vibration, respectively [57]. The 671-cm−1 band corresponds to the aromatic (C–H) group [58].

Fig. 2
figure 2

FT-IR spectrum of the Phoenix dactylifera L. leaf extract and the as-synthesized nanoparticles using different CuSO4·5H2O concentrations

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The results of FTIR analysis of the prepared CuO/Cu2O NPs at the different concentrations are shown in Fig. 2. The bands situated at 512 cm−1 and 618 cm−1 correspond to CuO vibrations, which confirm the formation of CuO/Cu2O NPs [59], which is in good agreement with literature values. Previous results showed that three characteristic peaks of the vibrations of Cu–O were observed at 421 cm−1, 472 cm−1, and 618 cm−1. However, the FTIR spectrum of Cu2O NPs shows only one peak at about 533.6 cm−1 attributed to Cu–O vibration [58]. The other absorption band at 1200 cm−1 can confirm the existence of the carboxylic acid group [58]. The peak absorption at 1730 cm−1 may be due to C = C stretching vibrations around the C = O amide–conjugated C = O of the proteins involved in the reduction and stabilization process [41]. The peak at 1596 cm−1 represented the C = O stretching of the ketone group. FTIR spectra show that the synthesized CuO/Cu2O NPs might be stabilized through the interactions of –OH and C = O groups in the carbohydrates, flavonoids, tannins, and phenolic acids present in Phoenix dactylifera L.

10 Morphology and particle size

The morphology and particle size of the CuO/Cu2O NPs may be affected by several factors, CuO/Cu2O NPs including pH of the solution, temperature, concentration of the Phoenix dactylifera L. extract used, and concentrations of CuSO4·5H2O used [60]. In this work, all these parameters were kept constant and only the CuSO4·5H2O used was varied. The SEM images (Fig. 3) indicated that they were used to study the formation of CuO/Cu2O NPs and their morphological size. Figure 3 shows the SEM images showing concentrations of CuSO4·5H2O used to have a significant effect on the particle size, and size distribution of the CuO/Cu2O NPs produced. Table 1 shows the particle size and particle shape of the prepared CuO/Cu2O NPs after annealing.

Fig. 3
figure 3

SEM image of CuO/Cu2O NPs prepared using Phoenix dactylifera L. extract and different concentrations of CuSO4·5H2O and their corresponding particle size distribution: a, b 4 mM, c, d 6 mM, e, f 8 mM, and g, h 10 mM

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As shown in Fig. 3, the average particle size of the CuO/Cu2O NPs gradually increases (from 25 to 100 nm) when CuSO4·5H2O is increased from 4 to 10 mM. The particle morphology of CuO/Cu2O NPs was slightly changed by increasing the CuSO4·5H2O concentration from 4 to 10 mM. At concentrations of 4, 6, and 8 mM (Fig. 3a–c), the morphology of CuO/Cu2O NPs is mainly spherical with different sizes and less agglomeration. On the other hand, at a concentration of 10 mM (Fig. 3d), spherical and rhombohedral shapes with particle size distribution abroad (80–150 nm) were observed. The sample prepared with 4 mM CuSO4·5H2O shows a narrow particle size distribution, which broadens with the increase in CuSO4·5H2O concentration from 4 to 10 mM. As shown in Table 1, the particle size of the CuO/Cu2O NPs with a concentration of 4 mM closely matches the crystallite size calculated from the XRD pattern. This indicates that the 4-mM samples are monocrystalline. The large difference between the crystallite size and the particle size of the 10-mM sample indicates that the CuO/Cu2O NPs prepared at this concentration are polycrystalline.

Flavonoids, condensed tannins, and saponins in the Phoenix dactylifera L. extract are well bound with CuO/Cu2O NPs. These compounds are promising candidates for the reducing and stabilizing of CuO/Cu2O NPs. Stabilization of the CuO/Cu2O NPs is probably due to the binding of the flavonoids, condensed tannins, and saponin molecules to the surface of the CuO/Cu2O NPs. Previous studies reported similar results for the biosynthesis of CuO and Cu2O NPs from other plant extracts. However, these studies did not include the antioxidant activity in the synthesis of CuO and Cu2O. Chinnaiah et al. [61] biosynthesized Cu2O NPs rather than CuO using Datura metel L. and found that the average crystallite size of Cu2O NPs is about 19.56 nm. Kumar and coworkers [62] used Andean sacha inchi (Plukenetia volubilis L.) leaves to prepare monodispersed semicrystalline Cu2O NPs under heating. However, the observed crystallite size of the Cu2O NPs is ~ 46 nm. Ananda Murthy et al. [63] synthesized CuO NPs with a monoclinic structure and a particle size of 19.7 nm using Vernonia amygdalina Del. extract. However, the synthesis of mixed Cu2O/CuO NPs has not been extensively investigated yet. Xolile Fuku [64] biosynthesized Cu/Cu2O/CuO NPs using pomegranate peel extract. The average crystallite size of Cu/Cu2O/CuO NPs was about 20–25 nm which is comparable with our results.

11 UV–visible absorbance and bandgap energy

Cupric oxide (CuO) is a transition metal oxide with a monoclinic structure and a narrow bandgap of 1.3 to 1.7 eV, while the Cu2O NPs have a direct bandgap of 2.0 to 2.5 eV. Generally, several factors may affect the bandgaps of the prepared Cu2O NPs and CuO NPs, i.e., crystallinity, crystallite size, particle size, particle shape, and composition [65]. The small bandgap energies allow Cu2O and CuO NPs to absorb the vast majority of the solar spectrum, and the direct bandgaps endow the CuO/Cu2O NPs with a large absorption coefficient. Figure 4 shows the UV–Vis spectrum of CuO/Cu2O NPs using the Phoenix dactylifera L. leaf extracts at different CuSO4·5H2O concentrations. The UV–vis spectra of all the samples show a strong absorption peak at 275 nm, attributed to the surface plasmon resonance of CuO/Cu2O NPs. This latter is caused by the collective oscillation of the electrons in the free conduction band which is excited by the incident electromagnetic radiation. Also, as observed, the absorption intensity of the samples increased with an increase in copper concentration. This suggests an increase in the number of nanoparticles formed as a result of the reduction of copper ions [40, 62]. The bandgap energy of the prepared samples was determined according to relation (4) [66]:

Fig. 4
figure 4

Optical properties of the CuO/Cu2O NPs prepared using Phoenix dactylifera L. extract and different CuSO4·5H2O concentrations: a UV–vis spectra; b optical energy bandgap for direct transitions; c optical energy gap for indirect transitions; d Urbach energy

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$$\left(\alpha hv\right)=A{\left(hv-{E}_{g}^{opt}\right)}^{n}$$
(4)

where α is the absorption coefficient, \(h\) is the Planck’s constant, \(v\) is the frequency of vibration, A is the proportional constant, \({E}_{g}^{\mathrm{opt}}\) is the optical bandgap energy, and n is a constant that denotes the nature of the electron transition, i.e., n = 2 for the direct transmission, and n = 1/2 for the indirect transmission as shown in Fig. 4b, c. By plotting \({(\alpha h\nu )}^{2}\) and \({(\alpha h\nu )}^{1/2}\) versus photon energy (\(hv\)), the optical energy bandgap for the direct \({E}_{g1}^{\mathrm{opt}}\) and indirect \({E}_{g2}^{\mathrm{opt}}\) transition can be determined, respectively [67]. The value of \({E}_{g}^{\mathrm{opt}}\) are obtained by extrapolating to \({(h\nu \alpha )}^{2}=0\) for direct transition and \({(\alpha h\nu )}^{1/2}=0\) for the indirect transition as shown in Fig. 4b, c, respectively, and also shown in Table 3. As the CuSO4·5H2O concentration increases from 4 to 10 mM, the direct bandgap increases from 1.92 to 2.38 eV, and the indirect bandgap also increases from 1.74 to 1.87 eV. Both direct and indirect energy gap values for the as-synthesized CuO/Cu2O NP samples exhibit a bandgap of 1.3–2.5 eV, as per previous studies [64]. The results are consistent with the literature review that the bandgap increases with a decrease in particle size [68].

Urbach energy is sometimes known as Urbach’s tail and can be detected by UV–vis spectra. The higher value of Urbach energy shows lower crystallinity and disorder in the CuO/Cu2O NPs. The Urbach energy \({E}_{u}\) is determined by taking the reciprocal values of the slopes of the linear part of the \(\mathrm{ln}(a)\) versus photon energy (Fig. 4d) [46]. The estimated Urbach energy values for the samples are shown in Table 2.

Table 2 The effect of concentrations of CuO/Cu2O NPs prepared using Phoenix dactylifera L. extract and different CuSO4·5H2O concentrations
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$$\mathrm{ln}a= \frac{hv}{{E}_{u}}+\mathrm{ constant }(\mathrm{ln}{a}_{0})$$
(5)

The plot of \(\mathrm{ln}(a)\) versus hν of the CuO/Cu2O NPs samples is shown in Fig. 4d. The Urbach energy \({E}_{u}\) was calculated by reciprocating the slope of the linear portion in the photon energy of the curve. The latter is determined as the difference in energy between the ends of the tails of the valence and conduction bands: with the decrease in this energy, being disordered, the disorder can also change depending on the addition of modifying oxides. Table 2 shows that the Urbach energy of the CuO/Cu2O NPs slightly decreases from 0.533 to 0.369 eV with the increase in the particle size (from 25 to 100 nm) and the increase in CuSO4·5H2O concentrations from 4 to 10 mM. These results were explained for Urbach energy due to the effect of structural and thermal perturbation.

12 Evaluation of the antioxidant activity

Table 3 shows the TAC, FRAP, and IC50 DPPH (mg/ml) results of the prepared CuO/Cu2O NPs. The TAC measurements showed that all the synthesized CuO/Cu2O NPs exhibit a significant antioxidant capacity, and the best sample was synthesized at a concentration of 6 mM CuSO4·5H2O with a value of 65.1 ± 3.1 mg EAA/g NPs (see Fig. 5). Cu deficiency affects the antioxidant function of the body and leads to a variety of diseases [69]. The prepared CuO/Cu2O NPs can increase the Cu content in the blood of Cu-deficient patients, and provide improved antioxidant activity. The CuO/Cu2O NPs affect on the cell membrane composition in a pathway to further protect the integrity of the cell membrane structure and function, as well as the tissues and organs. Min and coworkers [69] studied the effects of copper oxide (Cu2O NPs) on the antioxidant function of Cu-deficient Kazakh sheep, where the Cu content in the blood, wool, and liver of Cu-deficient Kazakh sheep was significantly lower than that of healthy animals. The authors supplemented the Kazakh sheep with Cu2O NPs or CuSO4, and as result, the blood Cu concentration increased significantly. Interestingly, from the 5th day, the Cu content of the Cu2O groups was significantly higher than that of the CuSO4 group [69].

Table 3 Results for total antioxidant capacity (TAC), ferric reducing antioxidant power (FRAP), and IC50 DPPH assays of CuO/Cu2O NPs prepared using Phoenix dactylifera L. extract with different CuSO4·5H2O concentrations
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Fig. 5
figure 5

The total antioxidant capacity of the CuO/Cu2O nanoparticles prepared using Phoenix dactylifera L. extract with different concentrations of CuSO4·5H2O: a TAC (mg EAA/g NPs); b FRAP (mg E FeSO4/100 mg NPs); c IC50 DPPH (mg/ml); d percentage of DPPH inhibition

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Figure 5 b shows the FRAP results of the prepared CuO/Cu2O NPs. The FRAP assay measures the antioxidant activity by reducing Fe3+ ions to Fe2+ ions by the CuO/Cu2O NPs. Following the reduction of the ferric iron, a blue color develops that can be monitored colorimetrically at 594 nm. The reducing power (FRAP) for different samples varied from 354 to 432 E FeSO4/100 mg NPs, while the concentration at 6 mM gave the most important reducing activity (432 mg E FeSO4/100 mg NPs) (see Fig. 5). In a similar study, Ijaz and coworkers [70] synthesized CuO NPs using Abutilon indicum leaf extract. The authors found that the maximum antioxidant activity value of 9.10 TE/ml was observed by 1000 µg CuO NPs while the minimum value (0.65 ± 0.01 TE/ml) was obtained for 60 µg CuO NPs [70].

Figure 5 c shows the antioxidant activity of the CuO/Cu2O NPs using a DPPH assay. The DPPH assay is used to predict antioxidant activities via a mechanism in which antioxidants inhibit lipid oxidation. The lower IC50 value indicates a stronger ability of CuO/Cu2O NPs to act as DPPH scavengers, whereas the higher IC50 value indicates a lower scavenging activity of CuO/Cu2O NPs. The effect of the different CuO/Cu2O NPs on the antioxidant activity of DPPH radicals is shown in Table 2. As shown in Fig. 5d, the DPPH activity of the CuO/Cu2O NPs was found to increase in a dose-dependent manner. Based on the IC50 results, the CuO/Cu2O NPs synthesized from 6 mM CuSO4·5H2O showed the lowest IC50 value in the order of (0.386 mg/ml), which is evidence that this concentration exhibits anti-free radical activity.

By comparing these results with those obtained in the TAC, FRAP, and IC50 DPPH assays, we can conclude that the CuO/Cu2O NPs prepared from 6 mM CuSO4·5H2O provides a strong antioxidant activity, which confirms that grain size has a significant effect on antioxidant potential. In similar studies [70,71,72], researchers evaluated the antioxidant activity of CuO NPs, and by comparing the results obtained, we can say that we got wonderful results. Atoussi and coworkers [73] synthesized CuO NPs by aqueous leaf extract of Portulaca oleracea (L). The authors found that the CuO NPs possessed the reducing capacity when the IC50 value was 68.3 μg/ml and 79.8 μg/ml, respectively, and the anti-inflammatory ability while the IC50 value was 77.5 μg/ml and 60.7 μg/ml, respectively [73].

13 Conclusion

Plant extract–mediated synthesis is a green, simple, and low-cost technique for producing antioxidant nanoparticles. In this work, antioxidant copper mixed oxide nanoparticles (CuO/Cu2O NPs) were prepared from alcoholic extracts of Phoenix dactylifera L. (source of phenolic compounds) and different aqueous concentrations of CuSO4·5H2O. The composition, crystallinity, morphology, and particle size of CuO/Cu2O NPs were adjusted by changing the CuSO4·5H2O concentration (from 4 to 10 mM). The XRD analysis confirmed the existence of two copper oxide phases, monoclinic (CuO) and cubic (Cu2O). The best antioxidant CuO/Cu2O NPs have a particle size of 55 nm and a CuO/Cu2O phase ratio of about 1:1. The antioxidant activity results indicate that the CuO/Cu2O NPs synthesized with the extract of Phoenix dactylifera L. are potent antioxidants and can protect humans against various oxidative stresses.