Horseradish peroxidase-catalyzed oxidative polymerization of aniline in bicontinuous microemulsion stabilized by AOT/SDS
Graphical abstract
The bicontinuous microemulsion plays a template role in the PANI biosynthesis, and the PANI conductivity also depends on the activity and stability of the solubilized HRP, which are determined by the microdomain size of the microemulsions and the reaction temperature.
Introduction
As a conducting polymer, polyaniline (PANI) has good electronic conductivity as well as chemical stability. PANI could be thus used as metal anti-corrosion coating, electromagnetic shielding material, sensing element and so on [[1], [2], [3], [4], [5]]. PANI is usually obtained by oxidative polymerization of aniline monomers. PANIs prepared under different conditions, have different branching degrees, different oxidation states and doping levels, which results in different electronic and optical properties of PANI. The oxidative polymerization of aniline is a complex process. It involves several highly reactive species and leads to various by-products (formed by branching or intramolecular cyclization reaction) [5]. In the presence of a template, however, the reaction pathways to form by-products can be suppressed to some extent [[5], [6], [7]]. Here the so-called template refers to some additives that have a positive and desired effect on the synthesis of conducting PANI. It is shown that the effect of a template is exerted by its direct interaction with the aniline monomers and the growing polyaniline chains or by keeping the reaction in its spatial confinement (forming a microreactor) [8,9].
The chemical/electrochemical polymerization is a conventional approach for the synthesis of conducting polymers [10,11]. This approach usually requires strong acids and strong oxidants/high oxidation potentials, thereby resulting in formation of by-products during the process [9,12,13]. In contrast, the enzymatic polymerization of aniline could be carried out under mild conditions (low acidity, low oxidation potential of the oxidant) with good selectivity [5,9,12]. Therefore, it is of great potentiality to develop an enzymatic approach for the biosynthesis of PANI. Currently, there have been many reports on the enzymatic biosynthesis of PANI [6,8,9,[14], [15], [16], [17]]. It is shown that, for the enzymatic polymerization, conducting PANI could be obtained without template [5,12]; however, in the presence of a template it is easy to obtain PANI with large molecular weight and good electronic conductivity [[5], [6], [7]]. The positive role of templates in the biosynthesis of conducting PANI has been confirmed [[7], [8], [9],15], especially when a heme-containing mimic enzyme (such as hemoglobin) is used as a catalyst for the biosynthesis (it is reported that without template it is difficult to obtain high-quality PANI [5,18,19]).
The templates used for the synthesis of conducting polymers are divided into two categories: one is a hard template and the other is a soft one. Currently, the mostly used templates are soft ones, which are composed mainly of strongly acidic polyelectrolytes or various aggregates formed via self-assembly by strong acid-based surfactants. The major role of these templates in the synthesis of PANI is manifested in the following three aspects [5,7]: one is to regulate the reaction process to obtain the desired structure and properties of PANI, the second is to provide a dopant for PANI, and the third is to improve the water solubility and processability of PANI. Compared with anionic polyelectrolytes, surfactant aggregates as templates are more favored due to their well-defined microstructure. The surfactant aggregates that have been tried so far as templates are micelles [7,17,[20], [21], [22]] and vesicles [14,15,23,24]. These two templates are formed by aggregation of surfactants in a single aqueous medium and thus have fewer microstructure-tuning parameters when compared with microemulsions composed of oil, water, surfactant and cosurfactant. Microemulsions can be divided into two categories, i.e., droplet microemulsions and bicontinuous microemulsions. Compared with droplet microemulsions [25], bicontinuous microemulsions not only have large interfacial area, low interfacial tension, and low viscosity, but also have the unique property of zero average interfacial curvature [[26], [27], [28], [29]]. If bicontinuous microemulsions could serve as a template, it would be beneficial to the linear growth of polyaniline chains and to the spins hops between polyaniline chains, and the conductivity of the PANI thus obtained would be improved. There has been no report in this respect, so it is worthwhile to try to do so. In order to understand the template effect of bicontinuous microemulsion on the biosynthesis of conducting PANI, it is necessary to study the influence of the microemulsion structure on the conductivity of the PANI obtained by enzymatic polymerization of aniline in anionic surfactant stabilized bicontinuous microemulsions and the underlying mechanism.
Section snippets
Materials
Sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT, ≥97%) was obtained from J&K Scientific Ltd. Decane (C10H22, >99%), sodium dodecylsulfate (SDS, AR) and horseradish peroxidase (HRP, pI ~ 7, >300 U/mg) were obtained from Aladdin Co., Ltd. 2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS, ≥98%) was obtained from Sigma-Aldrich. Aniline (ANI, AR) and hydrogen peroxide (H2O2, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents were of
Phase behavior of Na2HPO4/citric acid buffer-decane-AOT/SDS pseudoternary system
AOT is a strong acid-based anionic surfactant, which has been widely used as a building block for constructing an anionic template for enzymatic polymerization of arylamines [5,15]. In the present study, AOT is also used to formulate a bicontinuous microemulsion. According to the literature [33,34], the phase inversion temperature of AOT-stabilized bicontinuous microemulsions is relatively high (40 °C–50 °C), which is not suitable for the enzyme-catalyzed polymerization of aniline (enzymatic
Conclusions
The oxidative polymerization of aniline catalyzed by HRP in the AOT/SDS stabilized bicontinuous microemulsion was studied for the first time. It was observed that there exist significant correlations between the PANI conductivity and the experimental parameters (γ, α and T). A bicontinuous microemulsion with lower γ and α values is better for the enzymatic synthesis of conducting PANI. In order to understand the underlying mechanism, the microstructure of bicontinuous microemulsions and the
CRediT authorship contribution statement
Wei Jin: Investigation, Methodology, Writing - original draft. Rongrong Wang: Validation, Visualization. Xirong Huang: Conceptualization, Writing - review & editing, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful for the financial supports from the National Natural Science Foundation of China (21773143) and the Key Research & Development Plan of Shandong Province(2019GSF107090).
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2022, Colloids and Surfaces B: BiointerfacesCitation Excerpt :Table 4 also lists a study employing a bicontinuous microemulsion involving anionic and zwitterionic surfactants as reaction medium [25]. In this bicontinuous microemulsion, HRP activity increased 4.7-fold in comparison to its activity in AOT/SDS stabilized bicontinuous microemulsion [22]. Betaine-like head group of SB-12 was responsible for such enhancement.