Cyclodextrin-assisted synthesis of water-dispersible polyaniline nanofibers by controlling secondary growth

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Abstract

Uniform polyaniline (PANI) nanofibers (average diameter about 30 nm) are readily synthesized via a mediated-polymerization method in an aqueous medium by the use of cyclodextrin (CD) to control the secondary growth. Oligomers formed at an early stage act as seeds for the formation of the fully grown PANI nanofibers. The nucleation-growth theory – “birth-and-spread growth” mechanism – is employed to provide insight into the important effect of CD on the formation of PANI nanofibers. This mediated-polymerization method is able to produce high quality nanofibers in bulk.

Highlights

► Bulk quantities of uniform polyaniline nanofibers were synthesized by a green way in one step. ► Oligomers protected by cyclodextrins as seeds is essential. ► The birth-and-spread growth mechanism is employed to explain the role of cyclodextrin.

Introduction

Cyclodextrins (CDs) and their chemically modified derivatives have been the subject of numerous investigations [1], [2], [3] and can be used in organic [4], [5] or in polymer synthesis [6], [7], [8]. The CDs are able to enclose smaller molecules to form host–guest complexes [1], [2], [6], [7], [8] because of their unique shape, which forms a hydrophobic interior where the guest molecule is encapsulated, and a hydrophilic exterior that solubilizes the complex in water. This inclusion phenomenon leads to significant changes of the solution properties and reactivity of the guest molecule which can be manipulated in different reactions. Polyaniline (PANI), on the other hand, is a conducting polymer that has been widely studied due to its simple preparation, good thermal and environmental stability, structure versatility, and potential applications as electrical and optical materials [9], [10]. In sensor applications, PANI nanofibers have greater sensitivity and a faster response time due to a high effective specific surface area and shorter penetration depth for target analyte molecules compared with its conventional bulk materials [11], [12]. Recently, it was reported that PANI nanofibers can join together by a camera flash [13]. The “flash welding” technique has enabled the preparation of asymmetric PANI nanofiber films and the creation of polymer–polymer nanocomposites and photo-patterning polymer nanofiber films. In the past years, nanostructured PANIs have been prepared by chemical and electrochemical oxidative polymerization of aniline within templates [14] or with structure-directing agents [15], [16], [17], [18], [19], [20]. An interfacial polymerization method has also been reported whereby PANI fibers with a diameter of 50 nm are produced at the interface of two immiscible liquids [21]. Recently an extremely simple “nanofiber seeding” method for synthesis of PANI nanofibers was reported [22]. A new PANI–metal composite formed from metal salts and PANI nanofibers as chemical sensor shows an enhanced response as much as four orders of magnitude compared to normal PANI sensors [23]. It is therefore desirable to prepare water-dispersible PANI nanofibers for use in the preparation of PANI–inorganic composite films cast from aqueous solution as sensors. Judicious choice of inorganic nanoparticles will enable analyte specific sensors with high sensitivity to be produced.

More recently, a dilute polymerization method was reported [24], in which PANI nanofibers were synthesized by careful control of the nucleation and growth steps. However, this method is not suitable for the preparation of PANI nanofibers in large quantities. In order to obtain high-quality PANI nanofibers, Li et al. investigated the reaction process from a viewpoint of classic precipitate theory [25]. In the traditional method, after the formation of PANI oligomer, aniline molecules can grow either on previously formed PANI products or continue to precipitate through forming new nuclei via homogenous nucleation (Scheme 1a). Therefore, it is essential to suppress the heterogenous nucleation (second growth). Therefore, high-quality PANI nanofibers can be attained by suppressing heterogenous nucleation or controlling growth sites of aniline molecules. Moreover, in previous work reported by Yoshida [26], CDs can form inclusion complexes with the hydrophobic part of aniline monomers and PANI by including them into the cavities. In addition, CDs also can absorbed on PANI products by H-bondings. The conformation of PANI is confined to a rodlike structure (all trans state) and spread over the whole length. Furthermore, PANIs are isolated from each other by insulated CD molecules. Therefore, CD molecules can be used to protect formed PANI fibers and prevent aniline polymerization on their surfaces.

Alpha-CD has better water-solubility than beta-CD. In addition, the inner diameter of alpha-CD is close to that of aniline monomers while that of gamma-CD is rather big although it also has good water-solubility. Therefore, in this paper, water-dispersible PANI nanofibers were synthesized via a mediated-polymerization method in an aqueous medium by the use of alpha-CDs (CDs) which are used to control nucleation and growth of PANI nanofibers. The complexation of aniline in CD greatly enhances the solubility of aniline in water. This eliminates the use of organic solvents or acids. Furthermore, CDs that remain in solution, after the precipitation of the PANI, can be re-used. At the early stages of polymerization, oligomer chains act as seeds for the formation of the fully grown PANI nanofibers (Scheme 1b). The diameters of the PANI nanofibers are quite uniform (∼30 nm). This is among the smallest diameter reported for PANI nanofibers and nanowires, without the use of any template. This method does not depend on any specific template or dopant and is able to produce high quality PANI nanofibers in one step, up to five times more compared to the diluted polymerization technique.

Section snippets

Materials

Aniline (99.5%) and ammonium persulfate (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Aniline monomer was distilled under reduced pressure. α-Cyclodextrin (99.9%, Fluka) and ammonium persulfate (APS) were used as received without further treatment.

Preparation of PANI nanofibers

A typical polymerization process is as follows. 0.10 mL (1.07 mmol) of aniline was added to 0.107 g of CD in a 20 mL of distilled water. When the solution was cooled to 2.5 °C, an aqueous solution of APS as oxidant (0.25 g in 5 mL of

Morphology

The SEM images (Fig. 1) show that the morphology of the PANI products changed with different CD concentrations (represented by [CD]) at fixed aniline concentration ([An] = 0.04 M). When [CD] was 1 mM, agglomerates of irregularly shaped granular particles and fibers were formed (Fig. 1a). The structure and morphology of the PANI products changed significantly as [CD] was increased. The PANI nanofibers started to appear when [CD] was 2 mM (Fig. 1b) although the PANI granular particles dominated. As

Conclusion

We report the synthesis of water-dispersible PANI nanofibers by using CD to control secondary growth. This synthetic approach is an optimization of the “seed fiber” method and the diluted polymerization method. The CD concentration has an important effect on the formation of PANI nanofibers. It allows us to directly produce high quality pure polymer nanofibers in water, which is important in sensor and biological applications. This modified method has the following advantages: (i) a green way

Acknowledgments

H. Xia thanks the support from Independent Innovation Foundation of Shandong University (2010JQ013), Natural Science Foundation of Shandong Province (ZR2010EM006), Natural Science Foundation of China (Grant Nos. 50990061, 51002086, 51021062 and 51172126) and 973 program (Grant No. 2010CB630702).

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