Stimuli responsive ion gels based on polysaccharides and other polymers prepared using ionic liquids and deep eutectic solvents

Highlights

• Stimuli responsive ion gels in ionic liquids/deep eutectic solvents are discussed.

• Seaweed polysaccharides, gum based polysaccharides and DNA are covered.

• The application potential of polysaccharide based ion gels is discussed.

Abstract

Ion gels and self-healing gels prepared using ionic liquids (ILs) and deep eutectic solvents (DESs) have been largely investigated in the past years due to their remarkable applications in different research areas. Herewith we provide an overview on the ILs and DESs used for the preparation of ion gels, highlight the preparation and physicochemical characteristics of stimuli responsive gel materials based on co-polymers and biopolymers, with special emphasis on polysaccharides and discuss their applications. Overall, this review summarizes the fundamentals and advances in ion gels with switchable properties prepared using ILs or DESs, as well as their potential applications in electrochemistry, in sensing devices and as drug delivery vehicles.

Introduction

Stimuli-responsive polymeric materials, especially gel like materials, are receiving considerable attention from the scientific community due to their potential applications for drug delivery, as biosensors, shape memory materials, coating and textiles (Stuart et al., 2010, Yang and Urban, 2013). Almost all polymeric gels, irrespective of their synthetic or bio-based origin, show both liquid like flow and elastic behaviour. These characteristics make them good candidates for the fabrication of functional materials. Self-repairing is one of the most important functionalities of gelling materials, being investigated by many researchers. Among the numerous approaches to introduce the self-repairing ability in gels, one of the most popular is based on the addition of cross-linkers (Urban, 2012). There are several reports of self-healing polymeric gel systems fabricated mainly by cross-linking with groups containing the NHCO moiety capable of forming reversible covalent bonds (Hager, Greil, Leyens, van der Zwaag, & Schubert, 2010; Kushner, Vossler, Williams, & Guan, 2009; Phadke et al., 2012). The formation of these self-healing materials involves the sharing of electrons among the host and the guest molecules leading to the formation of covalent bonds. However, in biological living systems, reversible noncovalent molecular interactions play an important role, for instance in the replication of DNA, in the folding of proteins into intricate three-dimensional forms, and in the detection of molecular signals (Berg, Tymoczko, & Stryer, 2002).

The design of stimuli responsive-healing polymeric systems involves the encapsulation of effective healing agents in desired polymeric systems followed by activation employing external stimuli, such as heating, which is a popular way to regain the structural integrity in structurally damaged polymers (Cho, Kim, Oh, & Chung, 2010). After heating, the healing agent moves to the damaged part and promotes the self-healing by initiating suitable polymeric entanglements. In such physically induced healing processes, heating promotes the movement of polymeric chains and diffusion. In such cases the polymeric material is usually heated above its glass transition temperature (Tg) (Prager et al., 1981). The heating above T_g promotes surface rearrangement, followed by wetting, diffusion and re-entanglement of the polymer chains (Kim & Wool, 1983).

Smart polymeric materials are materials that are responsive to different stimuli, such as pH, temperature, light, solvent, magnetic field, redox and mechanical stress. Such materials have potential interest for various fields of applications (Klaikherd, Nagamani, & Thayumanavan, 2009; Balu et al., 2014; Zhuang, Gordon, Ventura, Li, & Thayumanavan, 2013), such as in drug delivery (Li et al., 2014; Liu, Zhou, Guan, Su, & Dong, 2014; Lee, Lee, & Park, 2014), imaging (Sundaresan, Menon, Rahimi, Nguyen, & Wadajkar, 2014), catalysis (Deng et al., 2014), coatings (Zhao et al., 2014), and sensors (Kavanagh, Byrne, Diamond, & Fraser, 2012; Zhang, Jin, Zheng, & Duan, 2014; Li, Xiao, Lin, & Wang, 2012; Yan et al., 2012). Several authors have reviewed stimuli responsive polymeric gels functionalized with metals (Weng, Fang, Zhang, Peng, & Lin, 2013), carbon nanotubes, graphene and multi-responsive complexes (Tang, Kang, Wei, Guo, & Zhang, 2013).

In recent years, ionic liquids (ILs) emerged as unique platforms for materials design (Noro, Matsushita, & Lodge, 2008; Smiglak et al., 2014) and as suitable solvent media for the dissolution of several polysaccharides, including cellulose, starch, chitin and DNA (Swatloski, Spear, Holbrey, & Rogers, 2002; Isik, Gracia et al., 2014; Isik, Sardon, & Mecerreyes, 2014; Payal, Bejagam, Mondal, & Balasubramanian, 2015; Wilpiszewska & Spychaj 2011; Prasad, Izawa, Kaneko, & Kadokawa, 2009; Prasad, Murakami et al., 2009; Mondal, Sharma, Mukesh, Gupta, & Prasad, 2013; Sharma, Mondal, Mukesh, & Prasad, 2013a; Sharma, Mondal, Mukesh, & Prasad, 2013b). Among these, DNA draws special attention due to its molecular recognition, biocompatibility, biodegradability and mechanical flexibility features, making it suitable for the design of functional materials useful in molecular sensing in the form of DNA nanomachines and intelligent drug delivery and programmable chemical syntheses (Bath & Turberfield, 2007). DNA based hybrid materials and hydrogels were proposed for applications in targeted drug delivery, tissue engineering bioanalysis, biomedicine and as stimuli-responsive materials (Lee et al., 2012, Um et al., 2006, Peng et al., 2013). Further, due to the responsiveness of DNA towards pH and salt addition, it is also considered as a suitable precursor for designing bio-artificial muscles (Costa, Miguel, & Lindman, 2007; Besteman, Eijk, & van Lemay, 2007). However, from the DNA based ion gel point of view, there is only one report on the preparation of gelatin and DNA based gel polymer electrolytes by treating them with acetic acid and LiClO_4, leading to the formation of ion gels with high ionic conductivity (Pawlicka et al., 2009). There are also limited reports on the preparation of ion gels by the combination of DNA and ionic liquids (ILs) or deep eutectic solvents (DESs). However, ILs are extensively used as suitable substrates for the preparation of other ion gels (Isik, Gracia et al., 2014, Isik, Sardon et al., 2014, Seki et al., 2005, Ueki and Watanabe, 2008).

The preparation of thermo-responsive synthetic polymers, such as polymethyl methacrylates and poly (N-isopropylacrylamide) in [C₂mim][N(Tf₂)] (Fig. 1) was first described by Ueki in 2014. Multi stimuli responsive polymers with switchable wettability functionalized with imidazolium based ILs were synthesised and proposed by Döbbelin et al. in 2009. Apart from the preparation of stimuli-responsive materials comprising polymers in ILs, polymerizable ILs (PILs) were also proposed as suitable substrates for the preparation of functional stimuli-responsive materials and ion gels suitable for gas separations (Cowan, Gin, & Noble, 2016; Kausar, 2017). Well-defined triblock copolymers, namely polystyrene-block-poly(methyl methacrylate)-block-polystyrene (SMS), were used to prepare viscoelastic ion gel with [C₂mim][NTf₂]) (Fig. 1) with tetrahydrofuran, displaying high ionic conductivity (Imaizumi, Kokubo, & Watanabe, 2012).

Up to date the research on functional materials has mainly focused on synthetic thermosetting, thermoplastic and elastomeric polymers based on reversible Diels-Alder reactions. Very less attention has however been given to the preparation of “smart” materials based on renewable/natural polymers, which are of particular significance in line with the principles of ‘green chemistry’ and sustainability (Höfer & Bigorra, 2008).

An overview of the current smart ion gels materials prepared using co-polymers, biopolymers or polysaccharides and ILs or DES, as well as their potential applications, are described and discussed below. The following section is presented according to the response of the ion gels to external stimuli, namely pH, heat, light and magnetism. The chemical structure of different ILs, DESs, and polymers, copolymers, biopolymers and polysaccharides investigated for the preparation of such materials are shown in Fig. 1, Fig. 2.