Abstract
The successful encapsulation of 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), a well-known laccase mediator, within a mesoporous metal-organic framework sample (i.e., MIL-100(Fe)) was achieved using a one-pot hydrothermal synthetic method. The as-prepared ABTS@MIL-100(Fe) was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, nitrogen sorption, and cyclic voltammetry (CV). Our ABTS@MIL-100(Fe)-based electrode exhibited an excellent electrochemical response, indicating that MIL-100(Fe) provides an appropriate microenvironment for the immobilization and electroactivity of ABTS molecules. ABTS@MIL-100(Fe) was then evaluated as an immobilized laccase mediator for dye removal using indigo carmine (IC) as a model dye. Through the application of laccase in combination with a free (ABTS) or immobilized (ABTS@MIL-100(Fe)) mediator, decolorization yields of 95% and 94%, respectively, were obtained for IC after 50 min. In addition, following seven reuse cycles of ABTS@MIL-100(Fe) for dye treatment, a decolorization yield of 74% was obtained. Dye decolorization occurred through the breakdown of the chromophoric group by the Laccase/ABTS@MIL-100(Fe) system, and a catalytic mechanism was proposed. We therefore expect that the stability, reusability, and validity of ABTS@MIL-100(Fe) as a laccase mediator potentially render it a promising tool for dye removal, in addition to reducing the high running costs and potential toxicity associated with synthetic mediators.
Highlights
IntroductionLaccase (benzenediol oxygen oxidoreductase, EC 1.10.3.2) is an enzyme produced by various organisms, including fungi, bacteria, and plants [1]
Laccase is an enzyme produced by various organisms, including fungi, bacteria, and plants [1]
We did observe a slight shift in the optimum reaction pH, likely due to a pH of 5.5 favoring electron transfer between the enzyme, the metal-organic frameworks (MOFs), and the dye molecules. These results indicate that ABTS was immobilized in MIL-100(Fe), it maintained its activity in the mediation of dye decolorization by laccase
Summary
Laccase (benzenediol oxygen oxidoreductase, EC 1.10.3.2) is an enzyme produced by various organisms, including fungi, bacteria, and plants [1] It is an efficient and environmentally friendly catalyst for bioremediation, as it catalyzes the oxidation of phenolic compounds using molecular oxygen as an electron acceptor, producing only water as a byproduct [2]. Molecules 2017, 22, 920; doi:10.3390/molecules22060920 www.mdpi.com/journal/molecules (ABTS), 1-hydroxybenzotriazole (HBT), and TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical) have been employed to enhance the oxidation capabilities of laccase towards nonphenolic compounds with high redox potentials [5,6]. In such cases, the mediator acts as an electron acceptor during the oxidation-reduction reaction, and participates in substrate oxidation. To interpret the mediator-substrate oxidation of laccase, a range of mechanisms have been proposed, including an ionic mechanism, two radical mechanisms, and electron transfer or hydrogen atom abstraction [7,8]
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