Evaluation of the antioxidant content and the antioxidant capacity
Quantitative analysis of compounds in hawthorn extracts is presented in Table 1. Water extract of hawthorn berries compared to ethanol extract pointed out higher total phenolic content and anthocyanins content. The contents of polyphenols and anthocyanins are directly related to the antioxidant properties. Polyphenols, ascorbic acid and anthocyanins are responsible for the reduction and stabilisation of metal ions in the process of obtaining metal nanoparticles (Della Pelle et al. 2015). Alirezalu et al. (2018) investigated antioxidant activity in flowers and leaves of hawthorn species. The antioxidant activity widely varied in species and in different organs of each individual plant, ranging from 0.9 to 4.65 mmol Fe3+/g in dry weight of the plant, calculated through the FRAP method. The total amount of phenolics ranges from 7.21 to 87.73 mg GAE/g in dry weight of the plant, which is consistent with the obtained results. Concentration of flavonoids in water extraction is about 20% higher compared to ethanol extract. Similarly, ascorbic acid concentration is 1.5 times higher in water extract than in alcohol extract, which can be associated with a lower solubility of ascorbic acid in ethanol solution. Tadić et al. (2008) using HPLC analysis determined free radical scavenging and antimicrobial activities of hawthorn berries ethanol extract. Their analysis showed that the major flavanol components in the extract were hyperoside, vitexin, rutin, isoquercitrin and quercetin. Total flavonoids content was equal to 35.4 ± 2.48. Depending on the extract concentration, its DPPH free radical scavenging activity varied approximately from 90 to 40%. Salmanian et al. (2014) investigated phenolic content and antioxidant properties of hawthorn seed and pulp extracts. Results showed strong antioxidant properties both in pulp and seed extracts. High antioxidant properties were correlated with high level of polyphenols. Using HPLC method, they established content of gallic acid, chlorogenic acid and caffeic acid. In pulp extraction, the concentration of the compounds was equal: 0.022, 0.509 and 0.012 mg/g dried plant. Urbinaviciŭte et al. (2006) determined the content of total flavonoids in hawthorn ethanolic extracts. Extract with different concentration of aqueous ethanol (40–96%v/v) was compared. Results of concentration of three flavonoids: rutin, vitexin-2″-O-rhamnoside, hyperoside and chlorogenic acid in the hawthorn extracts confirmed significant impact of their contents in ethanol solutions Miao et al. (2016) investigated chemical constituents of 80% ethanol, 80% methanol, 80% acetone and pure deionised water extracts of hawthorn fruits. The highest antioxidant activities (established by the DPPH scavenge capacity and ferric reducing power) was obtained in water extract. The researchers confirmed that, flavonoids, polyphenols, vanillic acid, gallic acid, catechin and chlorogenic acid, all contributed to the antioxidant activity.
Impacts of parameters on synthesis of metal nanoparticles
Analysis of the statistical significance of the parameters showed a meaningful impact of the type of extract, temperature and pH of the solution. In the case of process time, the parameter turned out to be important only when creating copper nanoparticles. Table 2 shows the input factors taken into account and the extent of their variation. The mean size of nanoparticles was assumed as the output factor.
Effect of type of extract and type of metal ions precursor
In Fig. 2, UV–Vis absorption spectra of synthesised silver and copper nanoparticles using water and ethanol extracts of hawthorn are presented. The pH value of the solutions affect the localised surface plasmon resonance (LSPR) peak of silver nanoparticles achieved at 390–423 nm (Shrivas et al. 2016). Absorption spectra of AgNPs obtained by alcohol extract were characterised by maximum peak shift towards the longer wavelength as well as higher absorbance compared to AgNPs based on water extract. Similar dependences were observed from the DLS analysis. Statistical analysis showed a significant effect of the silver nanoparticle precursor on the size of the obtained nanoparticles. In the formation of copper nanoparticles, the water-based extract proved better, while for the formation of silver nanoparticles the ethanol extract exhibited better properties. Using the given substances, nanoparticles of a smaller diameter and better homogeneity were obtained. Copper nanoparticles, in a small extent, showed the localised surface plasmon resonance. Despite the preparation of stable nanoparticles, a maximum peak of CuNPs can be detected only in appropriate conditions. The maximum absorbance for CuNPs, depending on the particle properties, is around 600 nm of wavelength (Young-Tae et al. 2019). Figure 2b shows the obtained UV–Vis spectra for CuNPs. The presence of polyphenol compounds affects the colour of the solution. Maximum absorbance at 300 nm can cover LSPR formed from CuNPs. Dang et al. (2011) investigated the formation of copper nanoparticles using various reducing agents (ascorbic acid solution and NaBH4 solution). Only using strong reducing agent when the solution was red, the plasmon absorbance (562 nm) appeared. In the case of obtaining solutions in a different colour (yellow, brown, green), LSPR does not occur.
The high impact of the solvent on the size of the particles being formed was confirmed. The characteristics of both extracts showed a higher concentration of flavonoids for aqueous solutions. CuNPs require strongly stabilising compounds; hence, smaller CuNPs particles were obtained. Silver nanoparticles do not require compounds with strong reducing properties. The high concentration of polyphenols and ascorbic acid can block the formation of large amounts of AgNPs nucleons, hence a significant proportion of larger particles. Fernández-Agulló et al. (2013) analysed the effect of the solvent (water, methanol, ethanol and mixtures of water and alcohol) on the antioxidant and antimicrobial properties of walnut green husk extracts. The highest extraction yield was achieved with water. The optimization of reducing agent’s concentration has also been reported in the literature. The reducing agent affects the size of copper nanoparticle as high concentrations may decrease the size while maintaining the concentration of the precursor. The reducing agent should be at least five times more concentrated compared to the precursor. The nucleation rate is also related to the concentration of the reducing agent. The stabilising agent or the surfactant also affects the size of nanoparticles (Din and Rehan 2017). An important compound found in plants, and, thus, in extracts is carbohydrates. Glucose and others greatly contribute to the reduction of metal ions forming nanoparticles. It has been confirmed that carbohydrates dissolve in water well; however, they have limited solubility in alcohols, including ethanol (Alves et al. 2007). The content of sugars can significantly affect the formation of nanoparticles, including their stability and size (Shakeel et al. 2016). Hence, the smaller size of copper nanoparticles using aqueous extract can be obtained.
Effect of pH
The effect of the pH of the solution was investigated in the range of 7–11. Figure 2 shows the UV–Vis spectra at different pH of silver nanoparticles for water and ethanol extracts. The colour of reaction mixture was pH dependent. Specially, influence of the solution pH was best in the case of silver nanoparticles synthesis from ethanol extract. At neutral condition, AgNPs had the smallest size and were characterised by the highest absorbance on the UV–Vis spectrum. At pH 9 and 11, the particles were characterised by dark brown colour with lower intensity and wider size distribution (Table 2). Ibrahim (2015) obtained the highest absorbance intensity at pH 4.5 for silver nanoparticles synthesised using banana peel extract. On the other hand, the DLS analysis revealed a significant effect of the alkaline environment on the reduction of particle size, regardless of the type of extract and the type of nanoparticles to be formed. In Fig. 3, impact of the pH of solutions on diameter of metal nanoparticles is presented.
Effect of temperature
The temperature also affected the process of reduction, both for silver and copper ions. Reaction mixtures prepared at 20 °C showed light brown colour. At higher reaction temperature, dark reddish-brown colour and more intense SPR peaks were revealed. Size distribution analysis (Fig. 4) presented that the size of metal nanoparticles decreased with the temperature increase, but only when using water extract. At 60 °C, all systems of metal nanoparticles, regardless of the type of salt used, revealed the smallest size. Similarly, results were achieved by Baghizadeh et al. (2015), who synthesised silver nanoparticles using seed extract of Calendula. They confirmed also that the rate of silver nanoparticles formation increased with increasing temperature. Both for silver and copper suspensions, ethanol extract in the lower temperature allowed to achieve smaller particles. Higher temperature could deactivate alcohol extract and decrease stabilising properties of the reagent.
Effect of reduction time
The maximum reduction of metal ions was obtained after 60 min. The rapid generation of nanoparticles resulted from the excellent reducing potential of the active components of hawthorn extracts and their stabilising properties (Fig. 5).
Table 3 presents systems of silver and copper nanoparticles with the smallest particle size distribution and parameters of the processes. From the analysis of zeta potential, solutions with high stability were as follows: Cu(CH3COO)2—water extract, CuSO4—water extract as well as Ag(CH3COO)—ethanol extract and AgNO3—ethanol extract. Presented mixtures were also characterised by the highest homogeneity (Fig. 6).
SEM and STEM analysis of silver and copper nanoparticles
The surface morphology of prepared silver and copper nanoparticles were analysed using SEM. Figure 7 presents silver nanoparticles and copper nanoparticles obtained with the use of water and ethanol extracts of hawthorn. A greater degree of agglomeration was observed in the suspensions of nanoparticles derived from acetate salts, which indicates lower stability of the system. In the case of nanoparticles derived from AgNO3 and CuSO4 salts, smaller particles were obtained. However, particles with a higher dispersion and more regular shape were obtained for the nanoparticles synthesised in the alcohol extract, which may indicate that ethanol improves the system stability and creates symmetrical nanoparticles. STEM micrographs confirmed agglomeration of AgNPs and CuNPs, which may also affect their antimicrobial properties (Fig. 8). In both the cases, nanoparticles with high homogeneity were obtained. Cui et al. (2018) applied hawthorn fruit extract to green synthesis of selenium nanoparticles. They obtained monodispersed and stable SeNPs with an average size of 113 nm.
FTIR analysis of silver and copper nanoparticles
Fourier transform infrared spectroscopy (FTIR) was used to identify the possible biomolecules which are responsible for the reduction and stabilisation of AgNPs and CuNPs. Figure 9 presents the FTIR spectra of AgNPs and CuNPs nanoparticles synthesised using water and ethanol extracts of hawthorn. Metal nanoparticles were synthesised using AgNO3 and CuSO4. The major spectra show bands at 3300, 2914, 2850, 1740, 1600, 1435, 1400, 1295, 1055, 1030 and 875 cm−1 (Fig. 9). The strong infrared band near 3300 cm−1 was observed for the O–H stretching vibration of hydroxyl group. The absorption peak at 2914 cm−1 might be induced by C–H stretching vibration of the CH2 and CH3 groups (Pasandide et al. 2017). The absorption at 1740 cm−1 was caused by C=O stretching vibration, while the absorption at about 1625–1600 cm−1 was due to the C=O asymmetrical stretching vibration of free carboxyl groups typical for the structure of flavonoids. The group of peaks in the range of 1600–1400 cm−1 corresponds to C=C bonds, whereas the most intense bands between 1100 and 1000 cm−1 correspond to C–O bonds, which indicated there were alcohols and phenols in the samples.
FTIR spectrum confirmed the presence of bioactive compounds in hawthorn berries extracts. These bioactive compounds were presumed to act as reducing and capping agents for AgNPs and CuNPs. The presence of functional groups such as C–O, C=C and C=O derived from alkaloids, flavones and anthracenes helped in the synthesis of metal nanoparticles (Neagu et al. 2013).
Antimicrobial activity of synthesised metal nanoparticles
The growth of the microorganisms treated with the prepared nanoparticles suspension was assessed. A suspension of eight nanoparticles, differing in salts of silver or copper ions and a type of hawthorn extract, was chosen for the analysis. Table 4 presents the survival results of microorganisms. The results were presented as percent of survival compared to the reference samples (using water or ethanol instead of nanoparticle suspensions).
Based on the results of the carried out research, the increased microbiological activity of silver nanoparticles compared to copper nanoparticles was found. There was no significant effect of the type of extract on microbial activity of nanoparticles. All suspensions showed a high degree of inhibition of A. niger fungus after 24 h (Figs. 10, 11). In all systems, a decrease in the growth of microorganisms was observed compared to control samples by 2–34% on average for copper nanoparticles and by 9–79% for silver nanoparticles after 48 h. Both silver and copper nanoparticles showed the lowest activity against E. coli bacteria. The highest activity for both copper and silver nanoparticles was observed against S. cerevisiae, achieving a growth inhibition rate from 34 to 14% for copper and from 79 to 32% for silver compared to controls.
The basis of the mechanism of toxic action of metal nanoparticles is the high activity to release metal ions suitable for the nanoparticle (Cui et al. 2018). The particle size contained in the nanoscale allows them to migrate through membranes and cell walls, affecting cellular homeostasis (Shakeel et al. 2016). The limited microbiological activity of the obtained metal nanoparticles may result from the agglomeration of particles during and after the preparation process, which was confirmed by the SEM and STEM photomicrographs. Despite the narrow distribution of nanoparticles, the total particle size could not limit the growth of microorganisms. In the A. niger samples, delayed cell growth was observed. One of the main mechanisms of toxicity of nanoparticles is the formation of ROS, which can damage cell organelles and initiate the production of an increasing number of free radicals. ROS are able to oxidise double bonds of fatty acids in cell membranes, which results in increased permeability of membranes, contributing to osmotic stress (Thakur et al. 2018; Zain et al. 2014). It could cause inhibition of cell development. After 24 h, further agglomeration of nanoparticles occurred, which limited the negative effects of metal nanoparticles, causing an increase in the number of cells.