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Gedikli, H., Akdogan, A., Karpuz, O., Akmese, O., Kobya, H. N., and Baltaci, C. (2024). “Aflatoxin detoxification by biosynthesized iron oxide nanoparticles using green and black tea extracts,” BioResources 19(1), 380-404.


Researchers have recently been interested in employing nanoparticles (NPs) obtained from herbal extracts through green synthesis for various applications. This study investigated the detoxification of aflatoxins, which are toxic substances produced by molds Aspergillus flavus and Aspergillus parasiticus. The present work examined the levels of aflatoxins in hazelnut and peanut puree. Turkish black tea extract (BTE), Turkish green tea extract (GTE), green synthesized black tea-based iron oxide nanoparticles (BTFeONPs), and green tea-based iron oxide nanoparticles (GTFeONPs) were produced for aflatoxin removal. Characterizations and various antioxidant and antimicrobial activities of the tea extracts and iron oxide nanoparticles (FeONPs) were investigated. The aflatoxin levels of hazelnut puree used for this study were 6.57 ± 0.06 µg/kg for aflatoxin B1 and 13.03 ± 0.16 µg/kg for total aflatoxin, whereas the aflatoxin levels of (AFLB1) peanut puree were 7.79 ± 0.15 µg/kg for AFLB1 and 15.21 ± 0.12 µg/kg for total aflatoxin. Using soluble BTE resulted in a 40 to 50% decrease in aflatoxin levels in hazelnut and peanut purees, while soluble GTE led to a 30 to 45% decrease. Meanwhile, using BTFeONPs and GTFeONPs resulted in a 33 to 48% and 40 to 50% decrease, respectively, in aflatoxin levels in hazelnut and peanut purees.

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Aflatoxin Detoxification by Biosynthesized Iron Oxide Nanoparticles Using Green and Black Tea Extracts

Hasan Gedikli,a Arda Akdogan,a Omer Karpuz,b,* Osman Akmese,c Havva N. Kobya,a and Cemalettin Baltaci a

Researchers have recently been interested in employing nanoparticles (NPs) obtained from herbal extracts through green synthesis for various applications. This study investigated the detoxification of aflatoxins, which are toxic substances produced by molds Aspergillus flavus and Aspergillus parasiticus. The present work examined the levels of aflatoxins in hazelnut and peanut puree. Turkish black tea extract (BTE), Turkish green tea extract (GTE), green synthesized black tea-based iron oxide nanoparticles (BTFeONPs), and green tea-based iron oxide nanoparticles (GTFeONPs) were produced for aflatoxin removal. Characterizations and various antioxidant and antimicrobial activities of the tea extracts and iron oxide nanoparticles (FeONPs) were investigated. The aflatoxin levels of hazelnut puree used for this study were 6.57 ± 0.06 µg/kg for aflatoxin B1 and 13.03 ± 0.16 µg/kg for total aflatoxin, whereas the aflatoxin levels of (AFLB1) peanut puree were 7.79 ± 0.15 µg/kg for AFLB1 and 15.21 ± 0.12 µg/kg for total aflatoxin. Using soluble BTE resulted in a 40 to 50% decrease in aflatoxin levels in hazelnut and peanut purees, while soluble GTE led to a 30 to 45% decrease. Meanwhile, using BTFeONPs and GTFeONPs resulted in a 33 to 48% and 40 to 50% decrease, respectively, in aflatoxin levels in hazelnut and peanut purees.

DOI: 10.15376/biores.19.1.380-404

Keywords: Aflatoxins; Iron oxide nanoparticle; Black tea; Green tea; Green synthesis

Contact information: a: Dept. of Food Engineering, Gumushane University, 29100, Gumushane, Turkey; b: Dept. of Genetics and Bioengineering, Gumushane University, 29100, Gumushane, Turkey; c: Central Research Laboratory, Gumushane University, 29100; *Corresponding author:


Aflatoxins pose a serious concern due to their high cancer-causing potential and ability to remain unaffected by metabolic processes in tissues and masses. According to the Food and Agriculture Organization of the United Nations (FAO), approximately 25% of global food sources are affected by aflatoxin contamination (Wu 2007). Consequently, aflatoxin contamination has consistently posed a global issue in the agricultural sector.

Given its substantial threat to human and animal health and the economy, numerous studies have been conducted to explore more efficient and environmentally friendly methods for detoxifying aflatoxins (Peng et al. 2018). Physical, chemical, and biological methods are the primary detoxification methods for degrading aflatoxins in humans, animals, food, and feed. Although each approach has limitations, they have effectively protected us from aflatoxins and mitigated substantial economic losses worldwide (Karlovsky et al. 2016).

Chemical methods used for aflatoxin detoxification include chlorine, hydrogen peroxide, ozone, bisulfite, ammonia, alkali, and various chemical applications. However, these chemical methods have limitations when applied to food due to potential issues related to chemical residues. Despite these concerns, chemical degradation remains a practical method for aflatoxin-decontaminating foods (Stoloff and Trager 1965). Sodium bisulfite, a food additive widely used in the industry, is particularly effective at inactivating AFLB1 in maize, even more so than ammonia and sodium hydroxide at low concentrations (Moerck et al. 1980). Ammonia has also been extensively studied for detoxifying aflatoxins in animal feeds, achieving over 95% degradation of aflatoxins in both gaseous and aqueous phases (Brekke et al. 1978).

In recent years, natural phytochemicals have emerged as promising solutions for aflatoxin inactivation, offering safety advantages over traditional techniques. For instance, the aqueous extract of ajowan (Trachyspermum ammi) seeds has demonstrated an 80% reduction in aflatoxin content (Hajare et al. 2005). Dialysed extracts of Trachyspermum ammi seeds effectively degraded aflatoxin G1 (AFLG1) by 90% (Velazhahan et al. 2010). Vasaka (Adhatoda vasica Nees) leaf extract has shown potential in reducing aflatoxins in both liquid media and animals, with cafestol and coffeol as the main components. Vasaka leaf extract was reported to detoxify 98% of AFLB1 (Vijayanandraj et al. 2014). Barleria lupulina leaf extract has also demonstrated the ability to degrade aflatoxins, with detoxification rates of 61.1% for AFLB1, 71.4% for aflatoxin B2 (AFLB2), 94.4% for AFLG1, and 58.8% for aflatoxin G2 (AFLG2) (Kannan and Velazhahan 2014). Ocimum basilicum leaf extract achieved a 90.4% and 88.6% reduction in AFLB1 and AFLB2, respectively (Iram et al. 2016). Additionally, various flavonoids found in plants, such as tea leaves, have been shown to inhibit aflatoxin production (Tian et al. 2023).

Mo et al. (2013) investigated the inhibitory effects of tea extracts, including pu’er tea, black tea, and jasmine tea, on aflatoxin production by Aspergillus flavus. Most tea extracts were found to inhibit AFLB1 production. Choudhary and Verma (2004) studied the protective effects of black tea against aflatoxin-induced lipid peroxidation in mouse liver. They observed that the decrease in antioxidant enzyme activities may contribute to increased lipid peroxidation during aflatoxicosis. Furthermore, coumarin in green tea extracts inhibited P450 enzyme activity in the liver and decreased AFLB1-DNA insertion in vitro (Peng et al. 2018).

Moreover, developing metal NPs, through green synthesis, has gained attention as a potential solution to aflatoxin contamination. NPs, due to their unique size distribution and morphology, offer various possibilities for combating aflatoxins (Vaseeharan et al. 2010; Chinen et al. 2015). Green synthesis methods utilizing plant extracts as reducing agents have been proposed to produce metal NPs (Senthilkumar and Sivakumar 2014; Nakhjavani et al. 2017). Plants contain a wide range of secondary metabolites with redox capacity, making them ideal for nanoparticle biosynthesis (Sun et al. 2014; Abdelghany et al. 2023). Tea leaves, rich in polyphenols, alkaloids, essential oils, polysaccharides, inorganic elements, and vitamins, are well-suited for the green synthesis of NPs. Green tea contains various catechins and flavonoids, while black tea has oxidized compounds with antioxidant properties (Sharangi 2009). Green synthesis using plants offer advantages such as safety, affordability, simplicity, rapid synthesis, environmental friendliness, control over nanoparticle size and shape, suitability for large-scale production, and avoidance of cell culture maintenance (Yeltekin 2020).

NPs composed of platinum, palladium, gold, and silver exhibit remarkable catalytic activity. However, these metals are expensive. As an alternative, metal oxide NPs can be synthesized, including zinc oxide, copper oxides, nickel oxides, manganese oxides, titanium oxides, cobalt oxides, and iron oxides (Kamran et al. 2019). Iron, in particular, exists in nature mainly in the form of oxides such as magnetite (Fe3O4), hematite (α-Fe2O3), maghemite (γ-Fe2O3), and hydroxides (Goethite and lepidocrocite, α- and γ-FeOOH), silicates, and carbonates. Iron oxide NPs are easy to synthesize, can be surface-modified, and some of them are magnetically recyclable. Thus, some can be manipulated magnetically and have specific magnetic, optical, and chemical features. As a result, these NPs have been widely explored for a variety of applications, including drug delivery, catalysis, photonics, dye removal, and environmental purposes (Laurent et al. 2008; Gawande et al. 2013; Datta et al. 2016). Elemental iron in the zerovalent state exists mainly in a bcc crystal structure as α-Fe and is highly prized for environmental applications (Nemecek et al. 2016), particularly in its nanoscale form. Magnetite is a popular choice due to its biocompatibility (Schwertmann and Cornell 2008). Maghemite (γ- Fe2O3), which has a spinel structure with a face-centered cubic (FCC) close-packed organization is another important class of magnetic transition metal oxide materials (O’Handley 2000). Magnetite (Fe3O4) is a common magnetic iron oxide with a cubic inverse spinel structure. Oxygen in this compound forms a FCC close packing, while Fe cations occupy interstitial tetrahedral and octahedral sites (Klein and Dutrow 2007). The hopping of electrons between Fe2+ and Fe3+ ions in magnetite’s octahedral sites at room temperature makes it a major material type that is classified as half-metallic.

There is an increasing amount of research on using green-synthesized NPs and effective phytochemicals against aflatoxins. Asghar et al. (2018) examined the impact of NPs produced from green tea and black tea on aflatoxin adsorption and found that the adsorption decreased as the initial concentration of AFLB1 increased. They also successfully made iron, copper, and silver NPs using the leaf extract of Syzygium cumini (Asghar et al. 2020). Silver NPs exhibited considerable antimicrobial activity, and iron NPs had a higher adsorption capacity for AFLB1 than copper and silver NPs. Raesi et al. (2022) studied the effect of NPs, including FeONPs, and reached the result of complete removal of AFLB1 under UV irradiation. Zahoor and Khan (2018) showed that nearly 90% removal of AFLB1 was accomplished for magnetic carbon composites which have the presence of FeONPs on adsorbent surfaces.

Mycotoxins are likely to be present in various plant and animal-based food products consumed by humans, including rice, corn, soybeans, wheat, barley, walnuts, nuts, peanuts, spices, dried fruits, and processed items susceptible to mycotoxin-producing organisms. Aflatoxins are particularly problematic in the chocolate, pastry, and dessert industries, as well as in peanut and hazelnut puree used as raw materials in many food products.

Comparing the published reports on the properties of NPs produced by the green synthesis method and NPs synthesized by physical and chemical methods, there needs to be more information on the antibacterial and antifungal properties of metal NPs, including using aflatoxin adsorbents. In this study, the authors synthesized FeONPs using extracts from locally produced Turkish teas from the Black Sea Region through a green synthesis method. The antioxidant and antibacterial activities of the tea extracts, as well as the synthesized NPs and their ability to adsorb aflatoxins in hazelnut and pistachio puree, were investigated.


The devices used are given in the analysis methods. All chemicals were purchased from Merck and Sigma Aldrich companies, Darmstadt, Germany.

Preparation of BTE and GTE and Soluble Powders of Extracts

For tea extracts, 1.5 kg black tea was mixed with 10 L water. The extracts obtained by boiling the mixtures for 2.5 hours were filtered through sieves of different sizes. The dry matter content of the filtrate was determined by a digital refractometer (Hanna HI96801, Hanna Instruments Inc., Woonsocket, RI, USA). The dry matter content was increased to 10% using a rotary evaporator at 60 °C under 150 mbar pressure. The concentrated tea extracts were subjected to drying with a spray dryer (SD-06 spray, Labplant UK (Ltd), Filey, England) at 200 °C for 4 h with a liquid flow rate of 5 mL/min. Water-soluble tea powder was obtained at the end of the spray drying process.

Preparation of BTFeONP and GTFeONP

A total of 250 mL of 0.2 M Fe+3 solution was prepared from FeCl3.6H2O, and 0.1 M Fe+2 250 mL solution was prepared from FeSO4. Equal proportions of both solutions were taken, and 250 mL solution was prepared. The pH value of 250 mL tea extracts with 10% dry matter content was adjusted to 11 using 1.0 M NaOH. The extract solutions were continuously stirred, and Fe+2/Fe+3 solution was added 1 drop/s. The solution was stirred at 500 rpm for another 1 h. The solution turned black as Fe3O4 was formed. The resulting FeONP solution was centrifuged (Nuve NF 800R, Ankara, Turkey) at 7000 rpm. The precipitates were dried at 60 °C using a vacuum glass desiccator and an oven (Yusefi et al. 2020).

Characterization of FeONPs

Fourier-transform infrared (FTIR) spectrophotometer analysis

The presence of functional groups or the identification of chemical bonds in NPs was evaluated using FTIR analysis (Perkin-Elmer Inc., Waltham, MA, USA). The spectral study was performed in the range 400 to 4300 cm-1.

Scanning electron microscopy (SEM) analysis

The morphology and size of the synthesized metal NPs were studied using SEM (SEM-Quanta FEG 250 ThermoFisher Scientific, Waltham, MA, USA).

Energy dispersive X-ray analysis (EDAX)

The elemental composition of the NPs was characterized by EDAX.

Treatment of Hazelnut and Peanut Puree with Tea Extracts and FeONPs

A total of 2.5 kg roasted hazelnuts were mixed with 1 L hazelnut oil and treated with a robot coupe R30 brand vertical type shredder and grinder at 3600 rpm for 10 min to obtain hazelnut puree. Homogenization control and aflatoxin analysis were performed to determine the amount of toxin in the obtained purees (n = 3).

Approximately 1000 g of hazelnut and pistachio puree was mixed in an Arcelik brand blender for 25 min. A total of 200 g of puree sample, 2 g BTE or GTE, and 2 g BTFeONP or GTFeONP were mixed and homogenized with a Waring brand blender for 2 min. Then, 50 mL of each sample was placed in non-sterile flat-bottomed falcon/centrifuge tubes (n = 3). The same procedure was repeated for 1.0% and 2.0% additions of BTE, GTE, BTFeONPs and GTFeONPs. According to the variable parameters specified in Table 1, the control sample was detoxified at 25, 45, and 75 °C for 2 and 4 h. At all temperature and time parameters, the samples were stirred for 2 min at every 30 min interval.

Table 1. Numbers and Abbreviations of Aflatoxin Analyses

Treatment of Aflatoxin Standards with Tea Extracts and FeONPs

The solution containing AFLB1, AFLB2, AFLG1, and AFLG2 was treated with 0.25% and 0.50% soluble black and green tea extracts and FeONPs obtained at room temperature. The solutions obtained were treated for 2 h at room temperature with occasional stirring. Afterward, aflatoxin-containing solution samples were analyzed by immunoaffinity chromatography (IAC, VICAM Aflatest, Vicam Company, Watertown, MA, USA) and high-performance liquid chromatography with fluorescence detector (HPLC-FLD; Agilent 1200, Agilent Technology, Santa Clara, CA, USA) techniques.

Analysis of Iron Content of FeONPs

A total of 0.2500 g of FeONP samples were placed in a microwave incinerator. Next, 7 mL of 65% HNO3 and 1 mL of 37% H2O2 were added, and the containers were sealed and placed in the CEM Mars 5 microwave (CEM Mars 5 Mars, CEM Corp., Matthews, NC, USA) combustion unit. After the digestion process was completed, the volumes of the solutions were made up to 50 mL with distilled pure water. For the calibration curve, a 50.0 mg/L intermediate stock solution was prepared from Fe 1000 mg/L master stock solution, and standard Fe solutions at concentrations of 1.0, 2.0, 4.0, 7.5, and 10.0 mg/L were designed from this solution to prepare the calibration curve. The microwave plasma atomic emission spectrometry (MP-AES, 4200 MP-AES System, Agilent Technology, Santa Clara, CA, USA) readings were taken for calibration, and the samples were filtered through a 0.45-micron filter before reading (NMKL 170 and NMKL 161).

Aflatoxin Analysis

The method of aflatoxin analysis in nutshells includes the steps of extraction with methanol/water, clean-up with immuno-affinity column (IAC) containing monoclonal antibodies specific for AFLB1, AFLB2, AFLG1, AFLG2, elution from the column with methanol, post-column photochemical derivatization, and HPLC-FLD analysis. Working solutions of aflatoxins (AFLB1, AFLB2, AFLG1, and AFLG2) for HPLC-FLD were prepared using the master stock aflatoxin standard solution (Merck, Aflatoxin Mix (0.20 µg/mL for AFLB1/ AFLG1 to 0.05 µg/mL for AFLB2/ AFLG2) 34036-1ML-R, catalog no. 34036, Merck KGaA, Darmstadt, Germany).

Sample preparation and HPLC-FLD analysis

The 25.0 g of sample, 5 g of NaCl, and 125 mL of 70% methanol were blended at high speed for 2 min, and the extracts were filtered through the Whatman filter paper. 5 mL of the filtrate was placed in a 20-mL syringe containing 10 mL of purified water and fitted with an immunoaffinity column (IAC) (VICAM brand, Vicam Science Technology, Milford, MA, USA). The solution was passed through the IAC at a 3 mL/min flow rate. The immunoaffinity column was washed with 20 mL of deionized water at a flow rate of 5 mL/min, followed by three air passes through the immunoaffinity column. Aflatoxins bound to the column were eluted with 1.0 mL pure methanol at a 0.5 mL/min flow rate. Then, 1.0 mL of distilled water was passed through; the aflatoxin solution was collected and transferred to an amber glass vial with a volume of 1.8 mL.

The HPLC conditions comprised the isocratic mobile phase, water, methanol, and acetonitrile (6:3:2). A total of 350 µL of 4 M HNO3 and 0.120 g KBr were added to 1.0 L of the mobile phase. The flow rate was 1.0 mL/min, and the column temperature was set to 22 °C. A 100 µL sample volume was injected. The fluorescence detector (FLD) was set to an excitation wavelength of 360 nm and an emission wavelength of 430 nm. Post-column derivatization with electrochemically generated bromine was used to increase the fluorescence intensity of aflatoxin derivatives. A 100 μA electrochemical cell was placed between the column and the fluorescence detector using a PTFE tube (30 cm) (Baltaci et al. 2013).

Antioxidant Activity Analyses

The total flavonoid (TFC) and phenolic contents (TPC) were determined using established methods, as described in the literature (Yuksel et al. 2022). The ferric-reducing antioxidant power assay (FRAP) analysis was conducted following the study by Benzie and Strain (1996). The antioxidant capacities were determined using the ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assay, as per the method implemented by Baltaci et al. (2022). The total antioxidant capacity (TAC) was measured using the phosphomolybdate assay technique described by Umamaheswary et al. (2007).

To evaluate the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging capacity, 0.1 mL of the filtered sample was mixed with 3 mL of DPPH solution (10 mM) in a test tube. The mixture was vigorously vortexed and then incubated in the dark at room temperature for 30 min. The absorbance of the solution was measured at 517 nm, and the DPPH scavenging capacity was expressed as a percentage of inhibition. Trolox and ascorbic acid were used as standards (Baltaci et al. 2022).

Antimicrobial Activity

Microorganisms used in this study were obtained from the Gumushane University Food Engineering Department laboratories. The antimicrobial activities of tea extracts and FeONPs were determined using the disc diffusion method against ten microorganisms, including Aeromonas hydrophila ATCC 35654, Escherichia coli ATCC 25922, Bacillus subtilis ATCC 6633, Shigella flexneri ATCC 12022, Listeria monocytogenes ATCC 7644, Enterococcus faecalis ATCC 29212, Bacillus cereus ATCC 9634, Salmonella typhimurium ATCC 23566, Aspergillus flavus ATCC 46283, Staphylococcus aureus ATCC 25923, Saccharomyces cerevisiae S288C, and Candida albicans ATCC 10231 (Matuschek et al. 2014).

The antimicrobial activity assessment was conducted in two stages: activation and cultivation of microorganisms. Bacteria were activated for 24 h at 36 °C, while yeasts were activated for 48 h at 25 °C and adjusted to 0.5 McFarlan. Sterile solid media containing 1% of the microorganisms were prepared and poured into Petri dishes, where wells of 5 mm were opened. Different concentrations of prepared samples were transferred into the wells, with dimethyl sulfoxide as a control. Petri dishes containing bacteria were then incubated for 24 h at 36 ºC, and those containing yeast and mold were incubated for 48 h at 27 ºC. The results were determined by measuring the zones around the discs after incubation (Matuschek et al. 2014).

Statistical Analysis

Statistical analyses were performed using Microsoft Excel software with XLSTAT (Addinsoft, Version 2020 New York, NY, USA).


Aflatoxin Quantities of Natural Samples

Hazelnut and pistachio samples were homogenously pureed, and at this stage, the amount of aflatoxin in the samples (n = 3) was analyzed (Table 2). When the analysis results of the samples obtained naturally from hazelnut and pistachio purees for the study are evaluated according to the Turkish Food Codex (TGK) contaminants regulation, it was observed that the results were not appropriate.

Table 2. Analysis of Naturally Contaminated Hazelnut and Pistachio Purees

FeONPs and Their Characterization

The most common biocompatible magnetic nanomaterials are pure iron oxides such as maghemite (γ-Fe2O3) and magnetite (Fe3O4). In this study, Fe3O4 NPs were obtained from Turkish black and green teas.

The color change in the reaction mixtures revealed the formation of FeONPs from both black tea and green tea. The color in the solutions of FeCl3 or FeSO4 with tea leaf extract changed rapidly from yellow to black. This color change indicates that FeONPs were formed, as has been shown by surface plasma resonance, indicating the reduction of metal ions by tea leaf extract (Asghar et al. 2018; Gebremedhn et al. 2019).

The formation of FeONP was confirmed by the change in the pH of the solution. The pH of the FeONP solution before reduction was set to 11.00, and the pH value after the reaction was measured as 6.30. The pH decreased during reduction in all samples and shifted to the acidic range. The studies conducted in the literature stated that the pH of the plant extract decreased with the formation of NPs after reduction (Madhavi et al. 2013; Asghar et al. 2020). Bioactive molecules, including catechins, alkaloids (such as caffeine), gallic acid, and flavonoids, in the biological matrix of black and green tea extracts were combined with Fe2+ and Fe3+ to form ferric and ferrous compounds. The addition of NaOH allows OH to interfere with the reaction mechanisms. Competition between ferric and ferrous compounds and OH bond formation with iron ions results in ferrous hydroxide and ferric hydroxide.

A shell-core structure forms where the core consists of ferrous and ferric hydroxides. Dehydration of ferric and ferrous hydroxides allows the formation of FeONPs in the form of a Fe3O4-magnetite structure. Fe3O4 nanoparticles were capped and stabilized by green and black tea leaf extracts originating organic active compounds (Awwad and Salem 2012; Ganapathe et al. 2020). After drying under vacuum, 29.87 g BTFeONP and 22.46 g GTFeONP were obtained. The yields were 2.99% and 2.25% for GTEs and BTEs with 10% dry matter content, respectively. The higher yield of GTFeONPs is attributed to the fact that they are richer in polyphenolic compounds than black tea.

FTIR analysis

The FTIR was used to identify possible phenolic compounds responsible for the conjugation and reduction of BTFeONPs and GTFeONPs.

The FTIR spectrum of the BTE solution is given in Fig. 1. Here, at 3242 cm–1, are the O-H and N-H stretching vibrations of polyphenols. This broad absorption band is due to the hydroxyl (OH) functional groups in alcohols and phenolic compounds. C=O bond stretching in polyphenols and C=C bond stretching in aromatic ring is seen in a strong band at 1628 cm–1. C-H stretching and O-H stretching in alkane and carboxylic acid are peaks at 2923 cm–1, respectively. C-O stretching in an amino acid is observed in the 1031 cm–1 band.

Fig. 1. FTIR spectrum of BTE and BTFeONP

Fig. 2. FTIR spectrum of GTE and GTFeONP

In a previous study of identical FTIR bands of various tea types, such as black, oolong, and green tea, it was concluded that the FTIR bands of the tea extract solution consisted of polyphenols appearing at 3388, 1636, and 1039 cm–1 and were related to O-H/N-H, C=C, and C-O-C stretching, respectively (Chai et al. 2015). Therefore, the FTIR spectrum shows that the main functional groups in tea are polyphenols, carboxylic acid, and amino acid compounds (Senthilkumar and Sivakumar 2014).

In the IR spectrum of green tea, the band at 3218 cm–1 is due to stretching vibrations of O-H groups in water, alcohols, and phenols and N-H stretching in amines. The peaks at 2926 and 2864 cm–1 are C-H stretching in alkanes and O-H stretching in carboxylic acid; the strong band at 1605 cm–1 is due to C=C stretching in aromatic rings and C=O stretching in polyphenols. The band at 1343 cm–1 shows C-N stretching of the amide in the protein, a band at 1741 cm–1 shows C-O-C stretching in polysaccharides, the band at 1032 cm–1 shows C-O stretching in the amino acid, and finally, the weak band at 824 cm–1 shows out-of-plane bending of C-H.

The FTIR measurements were performed to characterize BTFEONPs and GTFeONPs and to observe the presence of polyphenols and caffeine (Fig. 2). The bands at 3600 and 2900 cm-1 are characterized by the O-H stretching vibration assigned to the -OH polyol group, such as catechins, C-H, and -CH2– vibration of aliphatic hydrocarbons. The band at 1582 cm−1 is characterized by the presence of conjugated ketones, quinones. The bands at 1449 cm−1 and 1027 cm-1 are associated with the C-N stretching vibration and C=O stretching vibration of aromatic amines in the structure of caffeine, respectively. These results are in accordance with previous studies and indicate the presence of tea polyphenols as capping agents on the FeONP surface (Asghar et al. 2018).

SEM, EDAX, and MP-AES analyses of FEONPs

The elemental composition of the NPs and the relative abundance of the synthesized FeONPs were determined by EDAX, the results of which are shown in Figs. 3 and 4. The percentage of Fe metal was 7.56% in green tea NPs and 8.62% in black tea NPs. In Fe analysis with MP-AES device, it was 10.73 ± 1.51% in GTFeONP and 9.91 ± 1.67% in BTFeONPs. The results obtained from EDAX and MP-AES analyses confirm each other. Following the incineration of the sample in the acid solution by means of a microwave, the solid residue was subsequently dissolved in a nitric acid solution for MP-AES analysis. During this process, a small amount of precipitation was observed. Despite the minimal amount of precipitation, the quantity of possible missing iron content of NPs was negligible, as evidenced by the closer EDAX and MP-AES results.

Due to surface plasmon resonance, reduced FeONPs were revealed in EDAX analysis with a characteristic optical absorbance peak at 3 keV. An intense signal was observed in EDAX, indicating the reduction of ferrous ions to elemental iron. GTFeONP and BTFeONP had a composition of 49.31 and 46.85% oxygen (O), 41.97 and 40.12% carbon (C), respectively. The EDAX shows the presence of Fe along with species, such as carbon, oxygen, potassium, and small amounts of phosphorus, sulfur, and chlorine, from the tea extract, indicating that tea extracts are rich in minerals. Both BTFeONPs and GTFeONPs exhibited a negative zeta potential of -15.3 mV, suggesting that FeONPs have a sufficient surface charge for electrostatic stability to prevent excessive aggregation. The spectrum results showed that FeONPs were successfully formed.

Fig. 3. EDAX analyses of BTFeONP

Fig. 4. EDAX analysis of GTFeONP

The SEM technique was used to visualize the size and shape of the synthesized FeONPs (Figs. 5 and 6). The SEM images confirmed that GTFeONP had a spherical shape with particle size ranging from 18 to 97 nm. BTFeONP also had a spherical shape with particle size ranging from 40 to 97 nm. Images presented the agglomeration presence among NPs, which were similar to those of previous studies in the literature as well (Asghar et al. 2018). The variation in FeONP size distribution is due to the various reducing properties of different naturally occurring compounds present and prevalent in GTE and BTE.

Fig. 5. SEM images of GTFeONPs at different scales a. 500 nm b. 2 μm

Fig. 6. SEM images of BTFeONP at different scales a. 400 nm b. 2 μm

Aflatoxin Analyses

For aflatoxin analyses, two different concentrations, three different temperatures, and two different time variables were used for each product (Table 1). Purees and solutions containing naturally occurring aflatoxins were treated with soluble tea extracts and FeONPs. Detoxification was observed to appear in all aflatoxin species. It was observed that the concentrations of tea extracts and FeONPs, temperature, and time were effective parameters in the detoxification of all aflatoxin species.

The peanut and hazelnut purees treated with powdered tea extracts

In the study of aflatoxin analysis in peanut purees treated with BTE, AFLG2, AFLG1, AFLB2, and AFLB1, the total aflatoxin detoxification percentages were 41.9, 48.6, 50.0, 41.2, and 43.7%, respectively, in the sample that had the highest temperature, time, and concentration (SFS10) (Fig. 7). The increase in percent adsorption of aflatoxins with increasing temperature increases the sorptive effect between active sites on soluble black and green tea extracts, aflatoxin species, and adjacent aflatoxin molecules in the adsorbed phase. The sorptive forces are caused by stretching with temperature (Figs. 7, 8, 9, and 10). A study found that AFLB1 absorption increases with increasing temperature (Thieu and Pettersson 2008). The results also showed that the adsorption process is endothermic in nature.

Fig. 7. Graph of % detoxification of aflatoxins in peanut puree by soluble BTE

Fig. 8. Graph of % detoxification of aflatoxins in natural hazelnut puree with soluble BTE

Fig. 9. Graph of % detoxification of aflatoxins in peanut puree with soluble GTE

Fig. 10. Graph of % detoxification of aflatoxins in hazelnut puree with soluble GTE

Fig. 11. Graph of % detoxification of aflatoxins in peanut puree by BTFeONP

Fig. 12. Graph of % detoxification of aflatoxins in hazelnut puree by BTFeONP