Green Synthesis of Silver Nanoparticles Using Waste Products (Apricot and Black Currant Pomace) Aqueous Extracts and Their Characterization

An eco-friendly and low-cost method for synthesizing and capping silver nanoparticles with aqueous extracts of apricot and currant wastes is reported. *e chemical profiles of the extracts were analysed using liquid chromatography-mass spectrometry (LC-MS). Total phenolic content and total flavonoid content of extracts were determined. *e antioxidant activity of the synthesized nanoparticles was evaluated by DPPH (1,1-diphenyl-2-picrylhydrazyl) power assays. Cyclic voltammetry study was performed to determine the reducing ability of the aqueous extract of the black currant and apricot pomaces. Characterization of AgNPs was carried out using energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and UV-visible spectroscopy. Zeta potential of obtained colloidal solutions varies from −33.41 to −24.23mV indicating the moderate stability of synthesized nanoparticles. *e synthesized nanoparticles efficiently demonstrated a bactericide effect on Escherichia coli.


Introduction
One of the key trends in the development of the modern field of nanotechnology is the concept of environmental safety and the introduction of "green" technologies [1][2][3][4]. A fundamental key to the strategy of the "green" synthesis of metal nanoparticles is the use of low-cost, environmentally friendly, and renewable materials. "Green"/biogenic/phytochemical synthesis or biosynthesis of metallic nanoparticles is known to be a simple, ecological, cost-effective, cheap technique and is much safer for humans as compared to the conventional physical, chemical, and electrochemical methods [5][6][7][8][9]. Notably, plants in agricultural and timber industry are found to be rich in "green" organic compounds such as aldehydes, terpene alcohols, polysaccharides, flavonoids, alkaloids, phenolic acids, and polyphenolic compounds and may act as powerful reducing and capping agents in a single-step and one-pot reaction for the production of nanoparticles (NPs) such as silver, gold, or copper [10][11][12][13][14][15].
From the reported literature, extracts of various parts of plants such as root, seeds, leaves, stem, and flower have been used for the synthesis of metallic nanoparticles. For example, Lakshmanan used the aqueous extract of Cleome viscosa to produce silver particles with the mean sizes between 410 and 430 nm [16]. e aqueous solution of Origanum vulgare plant extract was also used to synthesize AgNPs [17]. Plants extracts from biomass of Ocimum tenuiflorum, Solanum trilobatum, Syzygium cumini, Centella asiatica, and Citrus sinensis were used for the synthesis of silver nanoparticles (AgNPs) with antibacterial properties [18]. However, the "green" reducing compounds extracted from agricultural wastes or agro-food byproducts without harm to plants and environment was seldom reported. is has prompted researchers to search for some cheap and effective "green" reducing agents extracted from fruit and vegetable waste that can be used to produce nanoparticles which is cost-effective for large scale production.
Apricot and black currant are promising sources of "green" organic compounds. e fruit pomaces represent a large portion of waste generated during juice processing (36% of fruit weight). As fruit pressing is an incomplete extraction process, the pomaces contain a large amount of valuable "green" compounds. Based on this observation, it is appropriate to assess the prospects of the use of plant extracts as a reducing agent to produce nanoparticles, namely, silver (AgNPs). e synthesis of silver nanoparticles using different natural extracts has been also reported because of the great interest in the "green" synthesis of AgNPs used in nanomedicine and microbiological applications.
However, analysis of component composition and preliminary assessment of the reducing ability of plant extracts as indirect comparative parameters affecting the efficiency of nanoparticle synthesis have not been considered elsewhere. In this study, the reducing capacity was characterized using the cyclic voltammetry method, and total phenolic and polyphenolic content were determined to analyse the potential use of apricot and black currant pomaces as a source of reducing agents in the synthesis of nanomaterials. e structural, antimicrobial, electrochemical, and antioxidation properties of produced AgNPs were characterized.

Extract Preparation.
e black currant and apricot extracts, obtained from the pomaces that remained after mechanical pressing of the fresh fruits to produce juices, were assayed for the composition of the main compounds, redox, and antioxidant activity. Black currant or apricot pomaces were mixed with the distilled water in the 1 : 10 (w/v) ratio at 25°C. e mixture was placed in the ultrasound bath. e ultrasound of 27 kHz frequency and 6 W/cm 2 intensity was being applied for 2 hours. During the sonication, the air was continuously bubbled through the solution. Due to the high intensity of sonication, the ultrasound cavitation occurs in the solution intensifying the extraction process. In the end of the process, the temperature of the solution reached 40°C due to the influence of ultrasound. After the extraction, the solution was filtered through paper filter and used to synthesize AgNPs.

Characterization of the Pomace Extracts Composition.
e obtained extracts were analysed with LC-MC technique. A Finnigan MAT (San Jose, CA, USA) Spectra System P4000 pump was used in with a UV6000LP diode array detector and a Finnigan AQA mass spectrometer. Analyses were carried out on a Superspher RP-18, 125 × 2 mm, 4 μm column (Macherey-Nagel, Germany), protected by a guard column packed with the same material and maintained at 40°C. Analyses were performed using electrospray ionization (ESI) in the positive ion mode, with acquisition set at 12 and 50 eV, capillary voltage at 4 kV, source voltage at 4.9 kV, detector voltage at 650 V, and probe temperature at 400°C. Eluent A and eluent B were 2.5% acetic acid and methanol, respectively. e flow rate was 0.33 mL min −1 , and the elution program used was as follows: 0-5 min, 0% B; 5-30 min, 100% B; 30-35 min, 100% B.

Synthesis of AgNPs by Using Pomace Extracts.
e 10-30 mL portion of each extract was used for the synthesis of AgNPs. e equivalent volumes of the extract solution and 0.01 M AgNO 3 were heated separately in the glass beakers in the water bath to the temperature of 80°C. en, the extract solution was added to the AgNO 3 and left on a water bath for 30 min. After this, the heating was turned off and the solution was cooled down to the room temperature. In order to monitor the changes in Ag + concentration during synthesis, an electrochemical technique of chronopotentiometry, consisting of the silver (Ag + |Ag) working electrode and saturated silver chloride (Ag|AgCl|Cl -) reference electrode, was used. e potential of the Ag + |Ag electrode depends on Ag + concentration according to the following Nernst equation: e potential of saturated silver chloride depends on Clconcentration and thus remained unchanged during the synthesis. e calibration plot was built to find the correlation between the Ag + |Ag electrode potential and Ag + concentration in the range 0.001 M, . . ., 0.1 M. During the synthesis, the Ag + |Ag electrode was immersed into the reaction solution and its potential vs. time with a digital multimeter UNI-T UT61B with a 1 mV accuracy was recorded. e change in the potential was used to find the change in Ag + concentration after the synthesis.

UV-Vis Spectrophotometry.
e synthesized silver nanoparticles were investigated at regular interval using an UV-Vis spectrophotometer (Shimadzu-2450 UV-vis spectrophotometer) in the range between 200 and 800 nm. e samples were diluted with deionized water and UV-visible spectra were recorded using 1 cm Quartz cuvette at 25°C.

Dynamic Light Scattering (DLS).
e zeta potential was measured through electrophoretic light scattering at 25°C, 150 V, in triplicate for the sample (Malvern Zetasizer Nano-ZS, Malvern Instruments, UK). e sample was prepared by diluting the 10 ml of the AgNPs solution in 10 ml distilled water. After that, the 10 ml of sample solution was put in the cuvette. e cuvette was placed in the cell holder and scanned in range of 1-100 nm using a fixed angle of 173°.

Scanning Electron Microscopy.
A scanning electron microscope (SEM) was used to examine the morphology of synthesized AgNPs. A drop of colloidal system of BCPE-AgNPs or APE-AgNPs was applied on the carbon-coated copper plate and an image was taken at an acceleration voltage of 10 kV. EDX analysis was conducted with the same instrument to confirm the elemental composition of the sample.

X-Ray Diffraction (XRD).
Crystalline metallic silver was examined by x-ray diffraction (XRD). e XRD pattern was recorded using Bruker AXS D8 Advance diffractometer (Shimadzu, Japan) with CuKα radiation monochromatic filter. e diffracted intensities were recorded from 30 to 80°2θ angles.

Electrochemical Activity.
e electrochemical activity of the pomace extracts as well as synthesized nanoparticles was studied using cyclic voltammetry. e three-electrode system was composed of Glassy Carbon Electrode (GCE) as the working electrode, a saturated silver chloride electrode (E � +0.2 V/NHE) as the reference electrode, and a platinum plate as an auxiliary one. e potential scan rate was 100 mV/s in the potential range 0, . . ., +1.0 V/SSCE. e portion of the extract or AgNPs solution was mixed with acetate buffer 0.1 M (pH 4) and NaClO 4 (70 : 28 : 2). Solution temperature was kept at 25°C.

Fourier-Transform Infrared Spectroscopy (FT-IR).
FTIR analysis was performed to investigate the various functional groups of synthesized AgNPs. FT-IR spectra of extracts, BCPE-AgNPs, and APE-AgNPs were measured by Bruker Tensor 27 FTIR spectrometer with a diamond crystal accessory in spectral range of 4000-600 cm −1 with a resolution of 4 cm −1 .
e radical scavenging efficacy of BCPE-AgNPs and APE-AgNPs was determined using DPPH assay [20]. e BCPE-AgNPs and APE-AgNPs were mixed with ethanol at various concentrations. Each dilution (0.5 mL) was mixed with 3 mL of an ethanol solution of DPPH (0.1 mmol). e mixture was incubated in the darkness at room temperature and the absorbance of the DPPH solution was measured at λ � 517 nm to (A control ) 30 minutes after adding the extract. In the blank experiment, ethanol was used instead of the sample; COT (butylated hydroxyl toluene) was used as a positive control. e ability to scavenge the DPPH free radical was calculated using the following equation: 2.4.8. Antibacterial Properties. Escherichia coli strain UKM C-906 was used as object for estimation of antimicrobial effect of silver nanoparticles disperse system. Active concentrations estimation in liquid medium of meat-peptone broth was conducted by the method of standard evaluation of the antibiotics action [19]. In the set of tubes that contained 2 cm 3 of nutrient medium, 2 cm 3 of AgNPs disperse system with different concentration was added as well as 2 cm 3 of E. coli inoculum. In control tube, 2 cm 3 of medium, 2 cm 3 of aseptic distilled water, and 2 cm 3 of inoculum were put. All tubes were incubated at 37°C for 24 hours. e results were evaluated as the difference in the optical density of the suspensions by photoelectric colorimeter at λ � 540-670 nm. Visual analysis of the bacteria growth on the selective medium Endo agar was used as an additional control.
Minimal bactericidal concentration (MBC) was determined as the lowest concentration of AgNPs solution in the tube in which there were not any significant changes of turbidity and no colony in Endo agar was detected. Minimal inhibitory concentrations (MIC) were estimated as the concentration in tube in which turbidity (optical density) was doubly decreased compared to control.

Extract Characterization.
e water extracts from black currant and apricot pomaces were analysed by LC-MS. e relative retention times (RT) and mass spectra of the extract's components were compared with those of authentic samples and with mass spectra from a data library. e results of LC-MS of the apricot and black currant pomace extracts are shown in Table 1. Black currant pomaces extract mainly consists of phenolic compounds, predominantly chlorogenic acid (5.4%), protocatechuic acid (15.6%), and caffeic acid (10.1%) and minor quantities of aldehydes and flavonoids.
A complex mixture of organic compounds was already identified in apricot pomace extract, as flavonoids, phenolic acid derivatives (1-caffeoylquinic acid). Chlorogenic acid (4.6%), catechin (5.8%), and caffeic acid (8.2%) were found in large amounts in the sample of apricot pomace extract. is extract is characterized by the presence of phenolic compounds such as quinic acid (7.1%), chlorogenic acid (6.8%), and kaempferol (5.6%). e rich source of phenolic acid and flavonoids in a fruit wastes extract might be Advances in Materials Science and Engineering 3 responsible for reduction of metal ions and efficient stabilization of synthesized nanoparticles. Table 2 illustrates the chemical structures of the components found in both extracts. us, the analysis of the composition of the aqueous black currant and apricot pomaces extracts indicated that they contain a wide range of organic substances and therefore are a potential raw material for the use as a reducing and capping agent especially promising for producing nanoscale materials. e reducing power of extracts is a significant indicator of their potential for producing nanoscale materials. e reducing properties of the extracts have connection with their redox behaviour and the electrochemical activity [26]. Cyclic voltammetry is one of the most common techniques to determine reducing power of the extracts [27].
us, in this work, the electrochemical oxidation of the black currant and apricot pomace extracts was tested. e cyclic voltammograms for the black currant and apricot pomace extracts showed no clear reversible peaks but only gave a shoulder and a monotonously increasing peak (Figure 1). Several parameters can be extracted from the cyclic voltammetry curves to characterize the apricot pomace and black currant pomace extracts as reducing agents for producing nanoscale materials. e top scan represents the oxidation of the compounds that are contained in the extracts, generating a positive (anodic) current I a � 3 μA, peaking at an electrode potential E a1 � 0.52 V and E a1 � 0.7 V for black currant and apricot pomace extracts, respectively. Literature survey reveals that the lower the potential of oxidation is, the stronger the reducing potential of the plant's extract is. us, the reducing power is higher for aqueous apricot pomace extract than that for black currant pomace extract. On the reverse scan, a negative (cathodic) peak is not produced; this indicates that the oxidized form of the compounds contained in the extract cannot be reduced back to its original form [28,29].

Determination of the Synthesis Efficiency.
e potential of Ag + |Ag electrode variation with time is given in Figure 2. e electrode potential corresponds to Ag + concentration in the solution; thus, synthesis process can be monitored. A sharp decrease of the potential is caused by AgNO 3 mixing with the extract and the beginning of the synthesis.
Cyclic voltammograms correspond to the Ag + concentration variation during synthesis. Calculated degree of conversion of Ag in the "green" synthesis with black currant pomaces extract is 57.4% and with apricot pomace extract is 68.7%. e higher abundances of phenolic acids in the apricot pomace aqueous extract may be responsible for the improved AgNP synthesis with it. e concentration of AgNPs in the solution after synthesis with black currant pomaces extract was 45 mg/L and after synthesis with apricot pomace extract was 59 mg/L.

AgNP Characterization.
e typical surface plasmon resonance (SPR) absorption band was observed at 430 and 390 nm for the AgNPs obtained with black currant and apricot pomace extracts, respectively ( Figure 3). All samples produced a λ max of 420 nm and were of deep amber colour, indicating the presence of AgNPs. e higher abundances of many phenolic compounds in the black currant pomace extract may be responsible for the improved stability of AgNPs with this extract. e main characteristics of silver nanoparticles in the dispersed system were investigated, namely, the average size, morphology, and stability of nanoparticles (Table 3). e spherical AgNPs synthesized with black currant pomace extract displayed a particle size of 40-60 nm with moderate stability (Zeta potential of −33.41 mV). e hydrodynamic diameter of AgNPs synthesized using apricot extract was found to be 50-82 nm. e Zeta potential analysis of the synthesized APE-AgNPs showed a peak at value of -24.3 mV. e negative charge of particles has a tendency to provide more stable particles inhibiting the aggregation process [25,30]. e negatively charged surface of the nanoparticles indicates that anionic capping agents such as polyphenols and flavonoids from the black currant and apricot pomace extracts are coordinated to the surface of silver nanoparticles [24,29,31]. To confirm the presence of the crystalline silver in the synthesized AgNPs with aqueous black currant and apricot pomace extracts, EDX patterns were obtained ( Figure 4). It is known that AgNPs display typical optical absorption peaks in the range 2.7-3.4 keV.
e Ag peak    e presence of the other peaks in the spectra suggests that the BCPE-AgNPs and APE-AgNPs were surrounded by organic constituents. e morphology of the AgNPs was examined with SEM ( Figure 5). e SEM images revealed the formation of individual silver nanoparticles as well as several aggregates. A greater degree of agglomeration was observed in the suspensions of nanoparticles derived with apricot pomace extract as the reductant, which indicates lower stability of the system. In the meantime, the AgNPs obtained with BCPE do not tend to agglomerate even when solution is diluted before SEM. is confirms higher stabilization properties of BCPE compared to APE. e FT-IR analysis was used to identify the various functional groups of synthesized AgNPs which were responsible for the reduction of Ag + to BCPE-AgNPs and APE-AgNPs ( Figure 6). e FT-IR spectrum of the black currant and apricot pomace extracts shows peaks at around 3400 cm −1 which can be assigned to ] (OH) vibrations of polyols such as catechins [30][31][32]. e C-O stretching vibration of the phenolic groups appears as a peak at 1264 cm −1 [30]. e peak at 1652 cm −1 corresponds to the C�C vibrations or aromatic ring ] (C�C) [26]. e band at 1050 cm −1 is associated with C-O-C stretching vibration. e C-O stretching vibration of the phenolic groups appears as a peak at 1251 cm −1 . Peak at 500 cm −1 is attributed to aromatic groups. e peak between 1620 and 1660 cm −1 may encompass the C�O vibration of bonded carboxylic acids, and the C�C vibration of aromatic components.
Comparison of the FTIR spectra of the BCPE and APE and BCPE-AgNPs and APE-AgNPs showed significant difference between the type and intensity of observed peaks. e FTIR spectrum of the synthesized BCPE-AgNPs and APE-AgNPs showed absorption peaks at 1645 and 3440 cm −1 . e strong band at 1645 cm −1 is attributed to carbonyl groups involved in the nanoparticle formation [30][31][32].
e shifts of peaks were observed upon BCPE-AgNPs and APE-AgNPs formation such as at 1637 to 1717 cm −1 , 1465 to 1452 cm −1 , 1293 to 1327 cm −1 , 1044 cm −1 to 1105 cm −1 , and 1713 to 1653 cm −1 , indicating the oxidation of corresponding functional groups. e absence of peaks at 1264 cm −1 , 1289 cm −1 , and 1347 cm −1 may be due to the capping action of C-O group in the synthesis of AgNPs.
e obtained results suggest that the phenolic compounds of the black currant and apricot pomace extracts show vibration stretches in BCPE-AgNPs and APE-AgNPs which might be involved in its formation by acting as capping and stabilizing agents.
e XRD pattern of the synthesized BCPE-AgNPs and APE-AgNPs is provided in Figure 7. Since the XRD technique delivers the information about the geometric arrangements and the distance among the atoms, it is an advantageous technique to support the formation of the nanoparticles. e monoclinic and cubic crystalline planes of the nanoparticles can also be identified by the XRD analysis. e 2θ peaks observed at 37.90°, 64.53°, and 77.38°c orrespond to (111), (200), and (311) crystalline planes, respectively. Figure 7 shows the diffraction peaks corresponding to Bragg's from (111), (200), and (311) planes that are relevant to 2θ � 38°, 43°, and 77°, respectively. XRD result is in good agreement with the various reported values associated with the cubic structure of phytosynthesized AgNPs, thereby confirming the formation of the silver nanocrystals.

Electrochemical Behaviour of AgNPs.
Cyclic voltammetry is one of the most common techniques used in electrochemistry to determine electrochemical activity. Cyclic voltammograms of AgNPs synthesized by black currant and apricot pomace extracts were carried out at scan rate of 100 mVs −1 .
It was observed that there was a significant difference between oxidation peaks values for AgNPs synthesized using black currant extract as compared to apricot pomace extract. e parameters in Table 4 were obtained from the cyclic voltammograms ( Figure 8). e cyclic voltammograms of AgNPs sample show the oxidation peak at 0.42 V/SSCE and 0.52 V/SSCE prepared using black currant and apricot pomace extract as the reductant, respectively (Figure 8). e oxidation of Ag (0) into Ag + is confirmed by the presence of a peak in the range at 0.05 to 0.45 V [33]. e difference in the position of oxidation peak potential of the BCPE-AgNPs (0.53 mV) with a diameter of 40-60 nm and APE-AgNPs   (0.44 mV) with an average diameter of 50-82 nm could be due to the difference in the sizes of AgNPs.

Antimicrobial Activity of AgNPs.
Low values of optical density ("0" or less than "0.075") and absolute absence of bacteria growth on Endo agar allow us to determinate the minimal bactericidal concentration (MBC) of each sample of AgNPs solutions. To estimate and compare the minimal inhibitory concentrations of different AgNPs solutions, a graphical method has been used: agent concentration, which corresponds to the half value of optical density in relation of control tube in each sample, was determined ( Figure 9).
As experimental result demonstrated, the AgNPs obtained with black currant pomace extract have MBC of 10% and MIC of 4.2%, but for apricot pomace extract MBC is 33% and MIC 7.5%. erefore, silver nanosystem, obtained with black currant pomace extract, has stronger bactericidal properties than the other one, obtained with apricot pomace extract.

Antioxidant Properties of AgNPs.
e evaluation of the antioxidant properties of "green" synthesized silver nanoparticles is useful to establish their potential applications in the food, pharmaceutical, and cosmetic industries [34,35]. It  is clearly observed that both the BCPE-AgNPs and APE-AgNPs are potential free radical scavengers with effective inhibition activity in a dose-dependent manner. e BCPE-AgNPs and APE-AgNPs showed higher antioxidant activity than the extracts used in the synthesis. e DPPH scavenging activity exhibited potent inhibition of BCPE-AgNPs and APE-AgNPs activity when compared with standard, while BHT ( Figure 10) was utilized as a positive control. e negative control effect of antioxidant screening obtained with ethanol was 2.1%. e efficient free radical scavenging properties of the BCPE-AgNPs and APE-AgNPs could be due to the combined effect of both AgNPs as well as of the organic compounds present in the black currant and apricot pomaces extracts. e BCPE-AgNPs and APE-AgNPs nanoparticles could show two types of radical scavenging reactions such as rapid H-atom transfer and secondary/slow radical-radical coupling reactions [34]. e obtained result agreed with the other studies, which showed that the antioxidant activity of "green" synthesized AgNPs was more than an extract [35].

Discussion
us, it can be concluded that the higher reducing potential of the apricot pomace extract leads to the more efficient synthesis of silver nanoparticles. However, it seems that apricot pomace extract does not have the capping agent in the necessary quantity to ensure colloidal stability of the system. On the other hand, a low reducing potential, combined with a good capping activity of the main compounds of the black currant pomace extract, leads to the synthesis of nanoparticles with a smaller size and higher antioxidant and antibacterial properties. Several flavonoids, which were found in the black currant pomaces extract, are presented by the flavonol glycosides. e glucosides in the structure of the flavonol glycosides are present in the extract and may be responsible for the more efficient stabilization of synthesized nanoparticles solutions. e data obtained are consistent with the results of the authors that established that the -COOH groups of phenolics and flavonoids in the leaf extract were found to be responsible for the reduction of Ag + ions, while C�O groups of the glycosides contributed to the stabilization of NPs.

Conclusions
Silver nanoparticles were synthesized by the reduction of silver ions with the black currant and apricot pomace aqueous extracts. Detailed gas chromatography-mass spectrometry (LC-MS) results established that polyphenols were present in the extracts (34-60%) and could serve as reducing agents in synthesis. Probably, after reduction of AgNO 3 the subsequent capping of AgNPs occurred through the action of flavonol glycosides that were found in the black currant pomaces extract (∼28%). In the meantime, the use of   apricot pomace extract appeared to be more efficient at the stage of nanoparticle synthesis; however, for colloidal stabilization of APE-AgNPs system it was not as reliable. Electrochemical analysis of aqueous black currant and apricot pomace extracts provides significant information about the reducing ability of the extracts. e apricot pomace extract showed higher reducing power measured by electrochemical methods, as well as higher efficiency for the "green" synthesis of silver nanoparticles. e synthesized BCPE-AgNPs and APE-AgNPs exhibit a spherical shape with small size (40 to 60 nm), uniform dispersion, and high stability. FT-IR spectra showed the presence of organic constituents in BCPE-AgNPs and APE-AgNPs colloids, indicating the engagement of organic components of BCPE and APE as reducing and capping agents in the synthesis process of AgNPs. e inhibitory efficacy (MIC) of Gramnegative microorganisms Escherichia coli was observed for 4.2% and 7.5% solutions of BCPE-AgNPs and APE-AgNPs, respectively. e antioxidant results indicate that the silver nanoparticles showed excellent scavenging rate against DPPH free radicals (40-99 ± 1.2%).

Data Availability
All data generated or analysed during this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.