Phenolic compounds exhibit toxic effects and there are still many challenges to find an efficient way for degradation and removal of phenol especially from food. In the present study, magnetically immobilized laccase was prepared and applied as an efficient heterogeneous biocatalyst for biodegradation of phenol from sesame oil. Laccase was attached covalently to magnetic Fe3O4 nanospheres and nanorods, and the characteristics of the immobilized enzyme were studied. The magnetic supports were analyzed by scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and Fourier-transform infrared spectroscopy. Storage stability analysis of immobilized enzyme showed that more than 70% of initial activity was kept after 15 days at 4 °C. More than 70% phenol degradation was achieved and a 60% decrease in the activity of immobilized laccase was observed after 20 independent cycles. The development of enzyme immobilization techniques on magnetic supports may expand the potential applications of heterogeneous biocatalysts in food industries.
Polyphenolic compounds that exhibit severe toxicity to human health are commonly found in many foods such as seeds, nuts, fruits, and beans . To date, conventional methods for treatment of industrial effluents containing phenolic compounds have been associated with either operational limitations or high costs . Hence, there is urgent need to develop an economical, effective and innovative way for treatment of such waste. Consequently, there is great interest to develop novel technologies for removal and biodegradation of phenolic compounds and to optimize the physical, chemical and biological methods that have already been developed. In particular, novel applications of enzymes for organic waste treatments have been investigated . Enzymes work as so-called biocatalysts. Enzymes are biocatalysts capable of breaking bonds within organic molecules and/or catalyze their transformation into less hazardous or more biodegradable materials .
Sesame is an ancient oil crop that belongs to the Pedaliaceae plant family. It was a treasured oil seed crop in ancient times because of its high oil yield, the easiness of oil extraction, the great stability, tenderness and pleasant taste, and the resistance of the plants to drought. However, there are different phenolic compounds in sesame oil. One of them is sesamol which is a laccase substrate. Laccases are phenol–oxidizing enzymes that can have both degradative and polymerizing effects on phenolic compounds [5,6,7,8].
As immobilized enzymes (i.e. heterogeneous catalysts), laccases may help to overcome some of limitations by improving the heat and pH stability of the enzyme. Magnetic separation of the enzyme from the reaction mixture, on the other hand, is an easy process that allows use of the enzyme several times and development of cost-effective reaction systems . Immobilization of laccase may also produce some other improvements such as resistance to harsh conditions and chemical reagents when compared with free enzyme .
Many studies have been done on enzyme immobilization over the past decade, with a significant proportion of them being on the laccase . One of the most widely used methods in industrial applications is covalent binding. In this technique, functional groups on the surface of the solid support are activated by using a suitable modifier and then react with the protein functionalities .
Magnetic nano-materials bearing functionalized surfaces form an outstanding class of solid supports . In particular, one-dimensional iron oxide nanostructures are currently the focus of research because they are known to have promising physical properties and a great potential for biotechnological applications . Diverse methods have been developed for preparation of magnetic nanorods or nanospheres. The most notable features of iron oxides are their ease of synthesis, their high resistance to unfavorable physical and chemical conditions, and their low toxicity .
Their surface can be modified with thin layers of organic molecules, polymers or biomolecules . The range of inorganic substrates used however, is much wider and may include, but not limited to, silica, metal oxides and metal sulfides were used as supports . Several researchers have added some kind of dendrimers, especially PAMAM to provide more active sites to bind enzymes .
The aim of the present study was to develop an efficient method for degradation of phenols from sesame oil by laccase immobilized on two different iron oxide structures, namely nanorods and nanospheres. We hypothesize that this environmentally friendly catalytic system may initiate more research and progress on the removal of toxic material from food products from a green chemistry point of view.
Materials and Methods
Polyethylene glycol (PEG), tetraethyl orthosilicate (TEOS), methyl acrylate and ethylene diamine were purchased from Merck (Darmstadt, Germany). Laccase from Trametes versicolor (N10 U mg−1), Aminopropyltriethoxysilane (APTES), 2,2′-Azino-bis (3-ethylbenzothiazoline- 6-sulfonic acid) diammonium salt (ABTS) and Folin & Ciocalteus phenol reagent was obtained from Sigma-Aldrich (St. Louis, MO, USA).
Preparation of Supports
Synthesis of Fe3O4 Nanospheres
2.5 g (9.2 mmol) FeCl3·6H2O and 3.6 g (43.9 mmol) sodium acetate were mixed in 35 mL of ethylene glycol and stirred magnetically for 15 min. The mixture was then heated in a Teflon-lined autoclave for 10 h at 180 °C. After cooling to ambient temperature, the dark precipitate was filtered off, washed several times with water and ethanol, and finally dried in an oven at 60 °C .
Surface Modification of Fe3O4 Nanospheres
In order to apply a silica coating on the magnetite nanospheres, 0.5 g of the obtained sample was suspended in a mixture of 85 mL of ethanol and 15 mL of deionized water. Then 4.0 mL of tetraethyl orthosilicate (TEOS) was added dropwise under ultrasound irradiation (USC600D at 130 W and 45 kHz) during a period of 25 min. After addition of 1.5 mL of ammonia solution (15 M), the mixture was stirred by means of a mechanical stirrer at 35 °C for 12 h. Finally, the product was separated by an external magnet, washed with deionized water in successive cycles until neutral pH, and oven-dried at 60 °C overnight .
For surface modification of the silica coated magnetite nanospheres by amine groups, 1.0 g of the sample (Fe3O4 NS@SiO2) was refluxed with 4.0 mL of (3-aminopropyl)triethoxysilane (APTES) in 50 mL of dried toluene at 110 °C for 24 h. Then the product was separated and washed with dichloromethane (3 × 10 mL) and dried at room temperature . In the next step, a PAMAM dendrimer structure was extended via Michael reaction. 1.0 g of Fe3O4 NS@SiO2-NH2 was dispersed in 100 mL of a 20% v/v solution of methyl acrylate and methanol, and sonicated for 7 h at RT. The precipitate was then washed with methanol and sonicated with a 50% v/v solution of ethylene diamine and methanol for 3 h. The product was then washed several times with methanol and water and oven-dried at 60 °C to give Fe3O4 NS@SiO2@PAMAM-NH2 .
In the last step of surface modification, 1.0 g of Fe3O4 NS@SiO2@PAMAM-NH2 was stirred with 80 mL of a 5% solution of glutaraldehyde in phosphate buffer (0.1 M Na2HPO4-NaH2PO4, pH 8.0) for 2 h at RT. The product (Fe3O4 NS@SiO2@PAMAM-NH2-GA) was then washed with the same buffer, separated by a magnet, and finally oven-dried at 35 °C .
Synthesis of Fe3O4 Nanorods
Fe3O4 nanorods were synthesized by using PEG (MW = 2000) as a template, according to He et al. , with slight modifications. Briefly, 1.5 g FeSO4·7H2O and 0.5 g Na2S2O3·5H2O were added into a solution of 10 mL of PEG and 5 mL of H2O and transferred to a Teflon-lined 30 mL autoclave. 2.0 g of NaOH was then added and the sealed autoclave was heated in an oven at 150 °C for 24 h. The resulting solid was separated using an external magnet, washed three times with distilled water and then with ethanol, and finally dried at room temperature .
Surface Modification of Fe3O4 Nanorods
Surface modification of Fe3O4 nanorods using the procedure defined in 2.2.2 resulted in the desired Fe3O4 NR@SiO2@PAMAM-NH2-GA support.
Characterization of Fe3O4@SiO2@PAMAM-NH2
Fourier transform infrared spectra (FT-IR) were recorded on a Thermo Nicolet Avatar FT-IR300 spectrometer (KBr disks). Powder XRD patterns of the synthesized magnetic nanoparticles were collected on a X′PERT PRO (PANalytical, Netherlands) instrument using Cu Kα radiation with 2θ angle covering 0.5° to 70°. Particle size and morphology was examined using a scanning electron microscope (SEM, SIGMA VP-500 FESEM from Carl Zeiss Germany). Magnetization measurements were performed at RT using a vibrating sample magnetometer (VSM).
Covalent Immobilization of Laccase
For covalent immobilization of enzyme onto the GA-modified Fe3O4@SiO2@PAMAM-NH2 support, an enzyme solution was prepared by dissolving 1.0 mg of laccase (10 U) in 10 mL of a citrate buffer solution (100 mM, pH 4.5), and then 5.0 mg of the GA-modified Fe3O4@SiO2@PAMAM-NH2 particles were dispersed in the enzyme solution by ultra-sonication. Then the reaction mixture was stirred at room temperature for 2 h and the enzyme-bound magnetic nanomaterial was collected using an external magnet. The enzyme-bound particles were rinsed with citrate buffer to remove unbound laccase. The laccase-bound particles were tested as described below to determine the laccase loading and activity. Incubation times, pH, and temperatures were independently varied to optimize the bounding process for maximal laccase immobilization .
Laccase Activity Assay and Protein Determination
Enzyme activity was measured by spectrophotometry (Pharmacia LKB ultraspect III, Sweden), with ABTS (0.1 mM in citrate buffer; 100 mM, pH 4.5) as indicator substrate. The absorbance was measured at 420 nm after 15 min of incubation at 40 °C for determining the activity of Laccase. 5.0 mg of the laccase-immobilized specimen was aged in a micro tube according to the standard method and then the solid was separated by using an external magnet of 1.7 T. The transparent liquid after centrifugation at 3000 rpm was used for spectrophotometry. The activity is reported here as international units (IU) with 1 IU being the amount of enzyme which can produce 1 μmol of product within one minute under standard assay conditions . The protein was also measured according to our previous report .
Study of Immobilized Ezyme Stability
pH and Thermal Stability
For investigating thermal and pH stability of free and immobilized laccase, solutions with different pH in the range of 2.5 to 10.5 at temperatures of 25, 35, 45 and 55 °C were prepared, and immobilized laccase was incubated for 5 h therein. 0.1 M citrate buffer (pH 2.5–6.5), 0.1 M phosphate buffer (pH 7.5–8.5) and 0.1 M Tris–HCl buffer (pH 9.5–10.5) were used to adjust the pH. Typical oxidizing reaction of ABTS as described in Sect. 2.5 was used to determine the related activity. All laccase activities were assayed 3 times.
Study of Reusability and Storage Stability
For determination of the stability of immobilized laccase, several consecutive cycles of ABTS oxidation reactions were run in acetate buffer (100 mM, pH 4.5).
After every cycle, the immobilized laccase was separated from the assay medium with an external magnet and was washed with buffer before the next cycle was performed under the same reaction conditions. The residual activity was assessed after each cycle. The activity after each catalytic cycle was defined as the residual enzyme activity, while the initial activity of the immobilized enzyme was assumed to be 100%. All assays were performed three times. For testing the thermal long-term stability, solutions of free and immobilized enzyme in sodium citrate buffer (0.1 M, pH 4.5) were stored for one month at three different temperatures (4, 25, and 40 °C). The remaining enzyme activity was measured under standard conditions .
The Michaelis–Menten constant (Km) and the maximum reaction rate (Vmax) of free and immobilized laccases were determined by measuring the initial rates of the reactions in dependence on a wide range of substrate (ABTS) concentrations (0.01 to 1.0 mM) in 100 mM citrate buffer (pH 4.5). The reactions were stirred for 5 min using (10 U) of free laccase (0.1 mg) and immobilized laccase (1 mg).
The kinetic parameters (Km; Vmax) were calculated using Michaelis–Menten equation as mentioned below :
Phenol Degradation Potential of the Immobilized Laccase
Phenolic compounds were extracted from sesame oil by using the modified method reported by Zeb et al.  to measure total phenol content. Briefly, 1 g of sesame oil was stirred with 10 mL of n-hexane for 20 min and centrifuged at 3000 g for 5 min. The solvent was decanted and then 10 mL of methanol–water (6:4) mixture was added to the aqueous fraction and was homogenized for 10 min and centrifuged at 3000 g for 5 min and methanol fraction was collected by three time replacement and separated using the above method. The volume of the collected methanol part that contains phenolic compounds, was reduced to 2 mL by air drying and filtered. 400 μL of this solution was mixed with 10.0 mg of the immobilized laccase in a 1 mL capped-vial and aged for 8 h at room temperature. The same procedure was used for the free enzyme with 10 μL of a laccase solution (10 mg in 1.0 mL of citrate buffer). After this time, 100 μL of sodium carbonate 7% was used to stop the reaction and after 2 min, the samples were centrifuged and 200 μL of the supernatant was separated. The total poly phenol content determination was based on a modified Folin–Ciocalteu method . In brief, 800 μL FCR 10% was added to 200 μL sample equilibrated at room temperature for 5 min. After adding 2 mL NaHCO3 7.5%, the mixture was diluted by doubly distilled water and incubated at 45 °C for 15 min while shaking at 150 rpm. Finally, the mixture was kept in the dark at room temperature for 2 h and its absorbance was measured at 765 nm against an appropriate blank.
Results and Discussion
In order to immobilize the enzyme on the magnetic support, first a silica coating process was applied. A schematic representation of the chemical steps of laccase immobilization on silica coated magnetic nanoparticles is shown in Scheme 1.
Characterization of the Support
Scanning electron microscopy (SEM) was used to illustrate the surface morphology of the obtained iron oxide nanospheres (Fig. 1a) and nanorods (Fig. 1b). Both samples appeared as aggregates due to their intrinsic magnetic nature. The length and diameter of the obtained nanorods are around 2000 and 50 nm  and Fe3O4 nanospheres have an average diameter of about 300 nm .
In the FT-IR spectrum of GA-modified Fe3O4@SiO2@PAMAM-NH2 support (regardless of morphology of the magnetic core, Fig. 1.c.x), the Fe–O stretching band was observed at 558 cm−1, and the characteristic Si–O–Si symmetric stretching, asymmetric stretching and bending vibrations occurred at 1076, 793 and 459 cm−1, related to the coated silica layer. In addition, the C–H stretching vibrations of propyl chains were observed at 2926 cm−1 and the N–H stretching mode of the –NH2 functional groups appeared at 3413 cm−1, which was expected due to extension of –aminopropyl and PAMAM dendrimer shells. Modification by using glutaraldehyde at the last stage resulted in the characteristic bands of aldehyde at 1727 cm−1 attributable to C=O stretching, and 2854 cm−1 attributable to the aldehyde C–H stretching. The FT-IR spectrum of bare Fe3O4 (regardless of morphology) is also shown in Fig. 1c-y for comparison.
In the XRD pattern of Fe3O4@SiO2@PAMAM-NH2 nanospheres and nanorods (Fig. 1d), the reflections are relatively broad most probably caused by small coherently scattering domains. All reflections can be assigned to pure magnetite according to the standard values in the card (JCPDC 19-0629), and are in good agreement with the data published in the literature .
Magnetic properties of the prepared Fe3O4 nanospheres and nanorods were investigated before and after surface modification at room temperature. It was found that the Fe3O4 nanospheres have a saturation magnetization of 28.1 emu/g before (Fig. 2a), and 25.4 emu/g after surface modification (Fig. 2b). For the Fe3O4 nanorods the corresponding values were 18.2 emu/g before (Fig. 2c), and 16.7 emu/g (Fig. 2d) after surface modification, respectively. This degree of magnetization saturation ensures rapid and easy separation of the material from the reaction mixture in a magnetic field. These data also reveal that surface modification of both Fe3O4 nanospheres and rods does not reduce the magnetic properties drastically. In addition, both samples exhibited a negligible hysteresis loop in the magnetization with low coercivity, suggesting that these materials have typical superparamagnetic behavior. The latter indicates that the material can easily be re-dispersed and reused after a magnetic separation process [30, 31].
Also, the magnetic saturation value of the Fe3O4 nanospheres is higher than that of Fe3O4 nanorods, which could be attributed to the larger size and almost one-dimensional structure of the nanorods. On the contrary, the highly oriented self-assembled structure of the Fe3O4 nanospheres can justify their observed magnetic properties. May be the source of low saturation magnetization is due to the high shape anisotropy of Fe3O4 nanorods that prevented magnetizing in other directions than along their easy magnetic axis .
In order to determine the amino functional group density, CHN analyses were performed and the results are presented in Table 1. Clearly, after surface treatment, N is present in both PAPAM-modified magnetic nanospheres and nanorods, and there in no significant difference in their nitrogen loading capacity.
Enzyme Immobilization and Protein Measurement
The prepared composites were used as supports for laccase immobilization. Enzyme attachment was observed to complete within 30 min of incubation (Fig. 3). However, different raw materials, different sources of enzymes, and various immobilization treatments can lead to different results. The amount of protein in the laccase solution and in the supernatant after immobilization was determined by measuring the activity of free protein and bounded protein on the supports according to Amin et al. . The amount of laccase immobilized per gram of solid material was found to be 28.9 mg/g.
pH and Thermal Stability of the Immobilized and Free Laccase
The stability of the free (Fig. 4a) and immobilized laccase on both nanorods (Fig. 4b) and nanospheres (Fig. 4c) of Fe3O4@SiO2@PAMAM-NH2 was investigated at different pH values (varying from 2.5 to 10.5) and temperatures (25–55 °C) by using ABTS as standard substrate. The data revealed maximal activity in the pH range of 4.5–5.5 at room temperature for both free and immobilized laccase. The covalently immobilized laccase in comparison to the free enzyme however, showed a much broader stability profile. This observation allows concluding that laccase immobilization results in a significant stabilizing effect towards heat and extended pH values. By increasing the temperature toward 55 °C, an expected decrease in activity was observed which has been shown to be due to enzyme denaturation . At elevated temperature, loss of activity of the enzyme was less pronounced for the immobilized enzyme in comparison to the free laccase. This achievement should depend on the immobilization method, as well as on the carrier charge and structure . As a catalyst, the majority of fungal laccases have optimal activity in mild acidic conditions (pH 4.0–6.0) and the temperature range of 35–45 °C [34, 35]. For practical purposes, thermal and pH stability of a biocatalysts are the most important factors and this improvement can be justified by the reduced conformational mobility of the enzyme after its covalent binding to an aldehyde group . Covalent immobilization of an enzyme on a support normally limits its free movements and conformational changes and results in increased stability towards thermal denaturation [37,38,39].
Reuse and Storage Stability of Immobilized Laccase
For industrial applications and processes, the total costs are the most important factor which needs to be minimized. For biocatalysts, reusability and storage life time are the two most important factors which impact on process costs . Therefore, several cycles of usage, separation and resuspension were performed to reveal loss of protein and activity. In addition, the decay of activity was monitored during storage under diverse temperature regimes.
The data obtained from the reusability experiments (Fig. 5) show the decrease in activity of immobilized laccase. But even after 20 cycles of magnetic separation and resuspension, still 40% of initial activity could be measured. At larger scales, reusability of immobilized enzymes is an important parameter for industrial applications that can drastically reduce the final cost of production. Denaturation of the immobilized enzyme and leakage from the solid support always happens. Thus long-term and repeated usage is always combined with a more or less decrease in the enzyme activity. Product inhibition may be a third reason for loss of enzyme activity.
Normally, enzymes lose their activity over time in solution and are not stable; therefore the storage of free enzymes is particularly difficult. Immobilization is one of the best ways to solve this problem. Figure 6 shows the storage stability results for free and immobilized laccase at 4, 25 and 40 °C. There are obvious differences between the activity decay of free and immobilized laccase over storage time. The activity of free laccase is completely lost within a day at 40 °C, while immobilized laccase retains activity to a longer extent at the same temperature. At room temperature, on the other hand, the free laccase lost most of its activity within a week, whereas immobilized enzyme retained about 80% of the initial activity. Finally, at 4 °C, immobilized laccase can keep more than half of its activity for around four weeks, whereas free laccase has almost no activity after this period. The covalent bonding between enzyme and the solid support, and prevention of denaturation as a consequence, may be the reason for prolonged storage times and increased stability [36, 39, 41].
Using ABTS (0.01–1 mM) as a substrate, the Michaelis–Menten parameters (Km and Vmax) were determined for free and immobilized laccase (Fig. 7). The obtained Vmax of free laccase was about 40 μmol mg−1 min−1, while the corresponding value for the immobilized laccase was about 120 μmol mg−1 min−1 (Table 2).
The increasing Km trend can be caused by inactivation of the enzyme resulting from its structural changes, which in turn reduces accessibility of the substrate to its active sites. As a consequence, production of intermediate molecules during substrate degradation could be reduced . The apparent Km value of the immobilized laccase (37 μM) was 1.48 times higher than that of free laccase (25 μM), which represents the lower affinity of the immobilized laccase towards the substrate. Loss of the enzyme flexibility necessary for substrate binding, steric hindrance of the support on the active site, and diffusional resistance to substrate and product molecules, are among the factors that adversely affect the affinity of the immobilized enzymes towards the substrates . These results are in line with previous reports on significant reduction in affinity of immobilized catalysts [43, 44]. Study of immobilization of laccase on Amberlite IR-120 H by Spinelli et al. , showed that the Km value increased 92-fold and the Vmax value decreased fivefold in their study. Çetinus and Öztop  reported that the Km values were increased after immobilization of the enzyme on glyoxal cross-linked chitosan beads, whereas Vmax value was decreased.
Phenol Degradation Studies
The total polyphenol content of samples was measured by using FCR . The results of the FCR method (Table 3) show 74.5% reduction in total produced phenol, indicating enzymatic degradation of the phenols. Both immobilized laccase on Fe3O4 nanorods and nanospheres showed similar results because essentially the surface treatment method for laccase immobilization was the same. Also, in comparison to free laccase which indicated 87.2% reduction in total phenol, the achievement of immobilized laccase would be rationalized if better storage stability and reusability is considered. In a similar study, Alver et al.  reported 80% reduction in total polyphenol content in a longer time. In comparison with other studies , the results obtained in the present research represent a considerable improvement in the activity of immobilized laccases.
Fe3O4 nanospheres and nanorods were prepared as support for immobilization of laccase. Prior to immobilization, a SiO2@PAMAM-NH2 shell, however, was extended. The obtained nano composites were successfully used for immobilization of the enzyme. After immobilization, the enzyme achieved high storage stability even at elevated temperatures and extended pH values. Also, the catalytic degradation of phenolic compounds in sesame oil and the reusability of laccase was improved after its immobilization on Fe3O4@SiO2@PAMAM-NH2. In summary, the results demonstrate that immobilization on magnetic supports can be effective and therefore displays extraordinary promise for practical applications on industrial scale.
2,2′-Azino-bis 3-ethylbenzthiazoline-6-sulfonic acid is a peroxidase substrate suitable for enzyme activity measurement.
3-Triethoxysilylpropylamine was used for amination
The enzyme international unit, One U is defined as the amount of the enzyme that transforms one µmole of substrate / minute.
Poly(amidoamine) is a type of dendrimer containing repetitively branched subunits of amide and amine moieties that is commonly used for surface modification.
Vibrating sample magnetometer, was used to determine magnetization properties of nanostructured iron oxide supports.
X-ray diffraction, was used to study the structure, composition, and physical properties of synthesized MNPs.
Scanning electron microscope was used as a sample imaging tool
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Support of this study by the research council of University of Guilan is gratefully acknowledged. Also many thanks for so much help to Professor Wolfgang Bensch and his working group at the Institute for Inorganic Chemistry (CAU, Kiel), to PD Dr. Christoph Plieth of Centre of Biochemistry and Molecular Biology (BiMo; CAU, Kiel), and to Professor Mohammad Ali Faramarzi and his working group, especially M. Mogharabi, at Tehran University of Medical Sciences.
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Amin, R., Khorshidi, A., Bensch, W. et al. Degradation of Sesame Oil Phenolics Using Magnetic Immobilized Laccase. Catal Lett 150, 3086–3095 (2020). https://doi.org/10.1007/s10562-020-03226-8
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