Abstract
The high cost, potential toxicity, and possible enzyme inhibition ability of artificial mediators have limited the large-scale application of laccase (Lac)/mediator systems. Here, sinapic acid (SA), a natural mediator, was covalently attached to amino-functionalized magnetic nanoparticles (MNPs) via amide bond formation. The as-prepared SA@MNPs were characterized by Fourier-transform infrared spectroscopy, scanning electron microscopy, cyclic voltammetry, and thermogravimetric analysis. The SA@MNPs were then applied to evaluate the activity of the immobilized mediator for Lac-catalyzed dye decolorization using indigo carmine (IC) as a model dye. When SA and SA@MNPs were used as Lac mediators, IC decolorization yields of ∼93% and 96%, respectively, were obtained after 60 min. Moreover, SA@MNPs exhibited an IC decolorization yield of ∼90% after being reused for 8 cycles. The Lac/SA@MNP system was shown to degrade IC by breaking down the chromophoric group. The easy recyclability, good reusability, nontoxicity, and relatively low cost of SA@MNPs make this immobilized natural mediator a promising tool for dye treatment.
Highlights
Laccases (Lacs; p-diphenol:oxygen oxidoreductase EC 1.10.3.2) are multicopper oxidases that can use molecular oxygen to oxidize a great variety of aromatic substrates, typically phenolic compounds [1]
A strong peak was observed at ∼575 cm− 1 corresponding to the Fe–O–Fe vibration. ese results indicated that the magnetic nanoparticles (MNPs) were successfully coated with APTES to give amino-functionalized MNPs
In the FT-IR spectrum of sinapic acid (SA)@MNPs, peaks were observed at ∼3352 and 1680 cm− 1 corresponding to the –NH stretching vibration and C O stretching vibration, respectively, of an amide bond [21]
Summary
Laccases (Lacs; p-diphenol:oxygen oxidoreductase EC 1.10.3.2) are multicopper oxidases that can use molecular oxygen to oxidize a great variety of aromatic substrates, typically phenolic compounds [1]. The direct oxidation of nonphenolic compounds by Lacs is limited because the redox potentials of these enzymes (typically 0.5–0.8 V) are lower than those of such substances [2]. Lacs are able to oxidize nonphenolic substrates with the help of redox mediators, which are small molecules that facilitate electron transfer between enzymes and substrates [3]. Lac mediators are divided into two types: artificial and natural. Artificial mediators are synthetic small molecules such as 2,2-azino-bis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS), 1-hydroxybenzotriazole (HBT), and 2,2,6,6tetramethylpiperidine 1-oxyl (TEMPO). Such mediators have been applied in Lac-mediator-based catalytic reactions, and the oxidation mechanisms have been elucidated [3, 5,6,7].
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