A new nanosensor composed of laminated samarium borate and immobilized laccase for phenol determination
© Hu et al.; licensee Springer. 2014
Received: 30 November 2013
Accepted: 4 February 2014
Published: 15 February 2014
A new nanosensor composed of laminated samarium borate and immobilized laccase was developed for phenol determination. The laminated samarium borate was synthesized by a mild solid-state-hydrothermal (S-S-H) method without any surfactant or Template. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were used to characterize the samples. The morphology of the as-prepared materials was characterized by SEM, which shows that laminated samarium borate are uniform nanosheets with a layer-by-layer self-assembled single-crystal structure. These laminated samarium borate have typical diameters of 3 ~ 5 μm and the thickness of each layer is in the range of 10 ~ 80 nm. And then, these SmBO3 multilayers were used to immobilize the laccase. The proposed nanosensor composed of laminated samarium borate and immobilized laccase was successfully developed for phenol determination. Cyclic voltammetry were used to study the nanosensor. The proposed nanosensor displayed high sensitivity toward phenolic compounds. The linearity of the nanosensor for the detection of hydroquinone was obtained from 1 to 50 μM with a detection limit of 3 × 10-7 M (based on the S/N = 3).
With the development of the economy, more and more pollutants are eroding the human survival environment. Then the detection and treatment of environmental pollutions have aroused great attentions of scientists. Belonging to multicopper proteins, laccases are widely existed in nature especially fungi [1, 2]. It is a phenol oxidase that can catalyze oxidation of many organic pollutants in water . Wan and his group  had elaborated the progress on the research of laccases, namely the active center of copper ions, the three-dimensional structure of protein, and its catalytic mechanism. Substrate specificity of laccases was exploited to remove pollutants from the environment without creating the negative effects associated with many other methods [5, 6]. It is well known that the enzyme is often easily inactivated in practical applications due to complex environment conditions, which limit its further industrial application [7, 8]. Consequently, immobilized laccases have received much attention from researchers in recent years because of its substantial advantages over free laccases such as continuous reuse, easy separation of the product from the reaction media, easy recovery of the enzyme, and improvement in enzyme stability. Nowadays, many different types of methods have been employed in the immobilization of enzymes, such as adsorption, entrapment, cross-link, and covalent attachment. Recently, it is reported that laccase has been successfully immobilized [9–11] on many different types of supports, such as activated carbon , magnetic chitosan , alginate chitosan , porous glass , chitosan/poly(vinyl alcohol) composite nanofibrous membranes , cellulose-polyamine composite , alginate, kaolinite, polymer beads and membranes polystyrene microspheres, short-range ordered aluminum hydroxide, and so on [17–20]. However, leakage, desorption, and the loss of enzyme activity were major problems in laccase immobilization, which was related to many factors involving the enzyme itself, polymer matrix, reaction reagents, and process conditions . Therefore, it is of great interest in developing novel technologies on laccase immobilization to improve catalytic activity of laccase and increase its industrial application.
Among those laccase supports, inorganic materials are more attractive because of their regular structure, good mechanical, chemical, and thermal stabilities [21–23]. Nanomaterials have attracted increasing attention for their novel properties and potential applications with small dimensions [24, 25]. Inorganic nanomaterials of rare-earth borate compounds show high vacuum ultraviolet (VUV) transparency and exceptional optical damage thresholds. Acentric lanthanide borate crystals are useful in a wide variety of photonic devices for unique optical, nonlinear optical, laser, electronic, and other physical properties [24, 25]. In the past decades, the rare-earth borates are widely used in many fields [26–30] and a number of synthetic methods have been employed to fabricate them. However, many routes suffer from the use of high temperature, tedious processes, and environmental pollution. Therefore, it is still an attractive and necessary topic for the development of environmentally friendly, facile, and reproducible methods to fabricate rare-earth borate nanometer materials.
In this paper, we choose a novel laminated SmBO3 multilayer as support for the immobilization of laccase. The SmBO3 multilayer samples were synthesized via the solid-state-hydrothermal (S-S-H) method, which exhibits many advantages, such as no side products, facile operation, and low cost. Then laccase was immobilized in SmBO3 nanosheets for the fabrication of the nanosensor. The performance of the proposed nanosensor composed of the laminated samarium borate and immobilized laccase in the catalytic determination of phenolic compounds has been investigated in detail.
Reagents and apparatus
All reagents were analytical grade in the synthesis system. H3BO3 (>99.0%), Sm2O3 (>99.99%), Na2HPO4 · 12H2O (>99.0%), C6H8O7 · H2O (>99.8%), hydroquinone (>99.99%), and 2, 6-dimethoxyphenol (>99.99%) were purchased from Shanghai Chemical Reagent Co, Ltd. (Shanghai, China) and used without any purification. Laccase was provided by Shanghai Daidi Industrial Development Co, Ltd. (Shanghai, China) and stored at 4°C before using.
The morphology and structure of the samples were inspected by using a field emission scanning electron microscope (FE-SEM, Hitachi S4800, Tokyo, Japan) at an accelerating voltage of 5 KV. The phase purity and crystallinity of the samples were characterized by X-ray powder diffraction (XRD) performed on a D8 FOCUS diffractometer (Bruker, Madison, WI, USA) with CuKα radiation (λ = 0.154056 nm), employing a scanning rate of 0.02° · s-1, in the 2θ ranges from 10° to 70°. Infrared spectra (4,000 to 400 cm-1) are recorded by Nicolet 5DX Fourier transform infrared spectroscopy (FTIR; Thermo Fisher Scientific, Waltham, MA, USA) equipped with a TGS/PE detector and a silicon beam splitter with 1 cm-1 resolution.
Electrochemical experiments were carried out with a CHI-660B electrochemical workstation (Shanghai, China). Measurements were performed at least three times on a glassy carbon electrode (GCE). A conventional three-electrode system was employed, comprising a GCE (3-mm diameter) as the working electrode, a platinum wire as the auxiliary electrode, and an Ag/AgCl (saturated KCl) as the reference electrode. Voltammetric responses were recorded in 50 ml of substrate solutions prepared in PBS buffer solution. First, the modified electrode was activated by several successive voltammetric cycles from -0.20 to 0.80 V. Second, cycle voltammograms (CVs) at the rate of 50 mV · s-1 were carried out from -0.20 to 0.80 V after subtracting the background. Finally, the GCE was regenerated by 10 successive cyclic voltammetric sweeps in the blank solution. After several measurements, the GCE should be repolished. All the electrochemical measurements were carried out at room temperature.
Preparation of SmBO3 nanocrystals
Immobilization of laccase on SmBO3 nanocrystals
The SmBO3 multilayers were employed as carriers for the immobilization of laccase, and the laccase was immobilized on these materials by the physical adsorption method. In a typical procedure, 100 mg of SmBO3 support was suspended in 10 ml of phosphate buffer (pH = 7.0) containing a certain amount of laccase (about 20 mg). The mixture of the supports and laccase solution was slowly stirred at room temperature for 12 h. Subsequently, the laccase immobilized on SmBO3 was separated by a centrifuge. Then the samples were washed with 10 ml of buffer solution by shaking for 5 min and separated quickly using a centrifuge. The washing procedure was repeated several times until no protein was detected in the supernatant. Finally, the laccase immobilized by SmBO3 were stored at 4°C before using. The percentage of the immobilized laccase on the SmBO3 samples is in the range of 10.7% ~ 15.2%.
Preparation of the glassy carbon electrode
Ultrasonic agitation was used to disperse 1-mg SmBO3-immobilized laccase into 1-ml Nafion to give a suspension (1 mg · ml-1). Before an experiment, the GCE was polished successively with 0.1-μm γ-Al2O3 powder, and then on a polishing cloth. Residual polishing material was removed from the electrode surface by ultrasonic agitation in concentrated HNO3, distilled water, and absolute ethanol. Then, the GCE was coated with 10 μl of laccase immobilized by SmBO3-Nafion suspension (1 mg · ml-1) and the solvent evaporated under room temperature for 1 h. The modified electrode was cleaned with distilled water before use.
Results and discussion
The XRD pattern analysis of as-prepared SmBO3 samples
FTIR spectra analysis
Figure 4b,c shows the FTIR spectra laccase and SmBO3-immobilized lacasse. Compared to the typical absorption peaks of lacasse at 3,401, 2,923, and 1,649 cm-1 and the main absorption peaks of SmBO3 at 1,110, 960, 894, and 827 cm-1, the absorption of SmBO3-immobilized lacasse include all of the above peaks. So it is evident that the laccase was successfully immobilized on SmBO3 nanosheets. Moreover, it can be seen from Figure 4 that the positions of lacasse and those immobilized in SmBO3 are nearly at the same place, suggesting that the lacasse retains its native structure in SmBO3-immobilized lacasse.
In summary, we have demonstrated a nanosensor composed of laminated samarium borate and immobilized laccase for phenol determination. These SmBO3 nanosheets have been successfully prepared via a mild solid-state-hydrothermal method without any surfactant or template, and laccase was successfully immobilized on these multilayers through physical adsorption method. The uniform multilayer-intersected structure could play an important role in the adsorption of laccase. This novel laccase immobilization method based on SmBO3 improved the performance of the laccase for phenol determination. The linear range and bioactivity of laccase-modified electrode can also satisfy the practical application. The present study has enlarged the family of support for laccase immobilization and may provide an efficient approach for phenol determination.
This work is supported by the National Natural Science Foundation of China (No. 91122025, 21103127, 21101118), the State Major Research Plan (973) of China (No. 2011CB932404), the Nano-Foundation of Shanghai in China (No. 11nm0501300), and the Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials (No.2012MCIMKF03).
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