The current literature is dominated by two main cation-families, ammonium and phosphonium based ILs. The quaternization of phosphines has been described in the literature by Bradaric et al. [27] and Ermolaev et al. [28], whilst the corresponding reaction for amines is described by Busi et al. [29]. The reaction schemes are somewhat similar in that both electron rich starting materials undergo a S_n2 addition reaction in the presence of a haloalkane at elevated temperatures over time. A general reaction scheme for the quaternization of a trialkylphosphine with 1-chloropropane to form the phosphonium salt is presented in Figure 1 (top); whilst the quaternization of 1-substituted imidazole with 1-chloropropane to form the imidazolium salt is shown in Figure 1 (bottom). Both reactions can be undertaken without solvent, as the reactants are liquid in both cases. Once the reaction is complete, the new IL formed is easily isolated by vacuum removal of the volatile halo-alkanes.

Variants of the halide ILs can be prepared via their ion-exchange metathesis reaction with group I organic anions [30,31]. Key to the success of this reaction is the choice of solvent that it is undertaken in, it must serve to solvate the new IL formed and preferably promote the precipitation of the by-product (a classical alkali-halide salt). As the IL begins to form over time, so too do the ionic interactions of alkali and halide atoms, respectively, which transfer out of the reaction solvent. The new IL can then be isolated by filtration of the by-products and drying over time.

A list of the of the more popular ions produced via quaternization and ion-exchange reactions can be seen in Figure 2; beginning with the tetra-alkylated phosphoniums and ammoniums [(i) and (ii)], N-heterocyclic amines [(iii) and (iv)], the amides [(v) and (vi)], hexafluorinated phosphates [(vii) and (viii)], benzenesulfonates [(ix) and (x)] and finally some group III tetrahalides [(xi) and (xii)].

The cis-trans isomerization of azo compounds and stilbene has also been used to produce photo-responsive solutes for switching the viscosity of the resulting solution [40,41]. Azobenzenes are particularly good candidates for use as chemical sensors as the photoisomoerization event not only yields a change in color [42], but also a significant change in polarity [43,44]. The use of organogelators (a molecule which exhibits significant electrostatic interactions leading to the formation of an interconnected network [45,46]) has proved a worthy route for the development and incorporation of the photo-responsive chemistries of azo compounds into the solid state. Murata et al. [47] developed a cholesterol-based gelator containing azobenzene; irradiation of the system at 330–380 nm led to isomerization of the trans azo linkage to its cis conformation and thereby disrupting gelation. Irradiation of the sol at wavelengths longer than 460 nm resulted in reversal of the isomerization and re-formation of the gel. Pozzo and co-workers [48] employed the photochromic equilibrium of 3,3-diphenyl-3H-naphthopyran, which brings about a large conformational change. Two forms of the naphthopyran can be distinguished: a colored open form and a colorless closed form [48]. In the closed form the naphthopyran unit does not influence the stacking process and the molecule acts an efficient gelator. Irradiation with UV light converts the naphthopyran unit into the open form and prevents the carbamate group from stacking. The isomerization is accompanied by a color change, and while the gel liquefies it becomes yellow. Heating converts the pyran unit back into the closed form, and upon cooling a colorless gel is again obtained (Figure 3).

Of particular interest to many are the photo-responsive molecule Spiropyran (SP) and its zwitterionic isomer Merocyanine (MC) [49,50]. The two isomers exist in a photo-dynamic equilibrium controlled by the application of UV and visible light, respectively [51,52]. Furthermore, MC can act as a Lewis base in the presence of an acid or metal ion solution, forming protonated forms and ion-complexes. Particularly interesting is that the ion/proton binding can be reversed using white light, releasing the bound species and reforming the inactive SP isomer. In addition to the SP and MC isomers, the protonated and metal-ion chelated forms exhibit unique optical properties and the entire system is therefore inherently self-indicating of their status (Figure 4).

Previous research has explored the behavior of the SP and MC isomers in systems capable of user controlled transition metal ion uptake and release [39,53], as effective solvatochromic probes [54,55] and as hybrid materials that exhibit user controlled multi-switchable optical properties [56]. Combining the photo-chemistry of SP and MC with the responsive chemistries of a hydrogel was first investigated by Szilagyi et al. [57]. We have expanded the area slightly to combine the properties of photo-responsive hydrogels with ILs producing ionogels [18,58], which will now form the basis of the next discussion.

One of the issues for microfluidic or “lab on a chip” devices is the controlled movement of liquid throughout the device. The practical solution has been to employ high power consuming pumps and/or actuating valves, which means that most prototype devices are anything but a “lab on a chip”. One novel alternative to this approach, is to control fluid movement using stimulus-responsive polymer valves integrated into the fluidic system that can be controlled using light [18]. The valve was based on an ionogel of which there were two distinct components: (a) The polymer gel, a co-polymer based on (poly(N-isopropylacrylamide) (pNIPAAM) and SP, and (b) phosphonium ILs.

pNIPAAM is a well known hydrogel that can swell and contract thereby drawing in and expulsing aqueous liquids as a function of pH and temperature [59,60]. In this work, the authors initially soaked the ionogels in acidic solution, which caused them to swell, and protonated the SP as part of the polymer backbone (Figure 4 (b),(c)). Figure 5 is a brief summary of the work, in which the ionogels were incorporated into a microfluidic device based on poly(methylmethacrylate) (PMMA). Four ionogel devices were prepared based on phosphonium cations with amide and benzensulfonate anions.

Irradiation of the ionogels with white light caused the protonated MC isomer to revert back to its SP closed form. This induced a change in the polarity of the ionogel (hydrophilic, MC-H⁺ to hydrophobic, SP), and the subsequent expulsion of water from the gel, causing it to contract. The role of the IL proved crucial in controlling the rate of contraction (and hence the fluid movement) as the thermal relaxation of the MC-H⁺ to SP is significantly affected by its physico-chemical interactions with the IL [54,55]. Through variation of the ionogel composition; the micro-valves were tuned to open at different rates when illuminated under the same white light source (Figure 2 (c)–(f), and Figure 6.)

A particular advantage of using ionogels occurs in applications where the intrinsic ionic conductivity of the ionogel can be exploited, for example in electrochemical sensors and devices. Ion-selective electrodes (ISEs) are a particular breed of electrochemical sensor, which convert the activity of a given analyte in solution into a voltage potential. ISEs employ polymer gels based mainly on poly(vinylchloride) (PVC) and PMMA [61,62], for the selective detection of important environmental and biological analytes at levels as low as nanomolar concentrations in some cases [63,64]. As PVC and PMMA exhibit high glass transitions, organic plasticizers are used to produce flexible transparent polymer membranes. They are prepared by co-dissolution with a suitable molecular solvent, producing the membrane as the solvent evaporates [65]. The sensing agents are often incorporated into the polymer membranes via their co-dissolution with the polymer and the plasticizer solution.

A prerequisite for a plasticizer therefore is that it must exhibit a markedly lower glass transition itself. Phosphonium ILs have been shown to exhibit glass transitions as low as −77 °C [66], and have been shown to plasticize both PMMA and PVC, producing Тflexible films suitable for use in ISEs [67,68].

A particular advantage of using ILs as plasticizers for PVC/PMMA gels is that they can further replace some of components needed for a functioning ISE. For example, an ion-exchanging salt is typically used in ISE membranes to facilitate the movement of the analyte between the aqueous and polymeric phases [69]. Chernyshov et al. have reported the selective detection of dopamine and adrenaline at concentrations as low as 1 × 10⁻⁷ M, using an IL as plasticizer without the need for an ion-exchanging salt [70].

Ionogels have also recently found use in amperometric sensors. Nádherná et al. described the use of an ionogel based on poly(ethyleneglycol) and 1-butyl-3-methylimidazolium hexafluorophosphate [C₄mim][PF₆] as an amperometric sensor for nitrogen dioxide. Using the ionogel as the electrolyte, the authors reported a detection limit as low as 0.3 ppm [71].

Electrochromic materials undergo a reversible change in their optical properties in response to an applied voltage [72]. They are good candidates for sensing templates as the color can vary by the choice of the electrochrome (of which there are many [73,74]) and its concentration. The color generated can therefore act as a sensor signal in response to an electrical potential [75]. Ionogels are good candidates for electrochromic devices as their intrinsic ionic conductivity can facilitate the charge required to produce the optical response [17].

A publication by Ahmad et al. describes the development of a complementary electrochromic device based on Tungsten oxide and Prussian blue [76]. The authors prepared on ionogel using sol-gel hydrolysis chemistry of Tetraethyl orthosilicate (TEOS) in the presence of the IL 1-ethyl-3-methylimidazolium bis(trifluoromethane)sulfonamide [C₂mIm][NTf₂], which formed a solid-state device upon solidification between two electrodes. In this case the responsive chemistry of the ionogel facilitated the current flow toward the electrodes (to which the chromophores were deposited), which in turn caused a change in the optical properties of the device. The authors reported an ionic conductivity of 1.2 × 10⁻² S/cm for the ionogel, which allowed for fast switching times whilst the coloration efficiency of the device was calculated to be 64 cm²/C.

Recent publications have detailed the use of ionogels as the solid-state electrolyte for electrochromic devices based on 4,4’-disubstituted bypyridinium salts (viologens) [17,77], poly(aniline) [78], poly(pyrrole) [79] and Prussian Blue [80].

Polymer optodes are similar to ISEs in terms of their composition, and how the analyte transfer is facilitated from the sample into the membrane phase [81]. Ionogels have been employed within optodes, mainly as the plasticizer to produce the film. Zhu et al. used phosphonium based ionogels with a conventional chromoionophore for the detection of important inorganic acids such as HCl and H₂SO₄ [82]. The authors detailed enhanced selectivity toward hydrophilic anions for the ionogels versus conventional plasticizers, which they have attributed to the increased dielectric constant of the ionogel. A publication based on a similar concept by Topal et al. detailed the increased selectivity of zinc phthalocyanines to pH when incorporated into imidazolium ionogels [83]. One particular publication on ionogels as optodes detailed significant template simplification (as a result of the particular IL used), whilst the ionogel/optode produced three distinct colors in the presence of Cu²⁺ and Co²⁺ ions [15] as shown in Figure 7.

Lunstroot et al. have pioneered the concept of ionogels as solid-state luminescence devices in two separate publications [84,85]. In both cases the ionogels were based on a siloxane support, whilst the IL was used to bind to lanthanide element that exhibited photoluminescence upon UV irradiation. In fact Cheminet et al. recently expanded the concept of optically responsive ionogels to fluorescent materials [86]. A phenylene–ethynylene oligomer was synthesized and chemically tethered to an imidazolium cation; which exhibited fluorescence in the solid state as part of a siloxane based ionogel.

Along with the electrochemical applications of ILs (as discussed), ILs have gained momentum in bio applications. Recent work in the area include ILs as biocatalytic reactions [87,88], biosensors [89], protein stabilization [90] and biopreservation [91]. They have been proposed as unique solvents for biomolecules such as proteins/enzymes because of their unusual solvation characteristics. Work by Fujita et al. [89,92] and others [90,93] have shown that some proteins are, in fact, soluble, stable and remain active in some ILs. As a case study Cytochrome c (cyt. c) was found to have enhanced solubility and stability in a biocompatible IL solution based on the dihydrogen phosphate anion [92]. This is an important observation since proteins are sometimes unstable when handled in vitro, and stabilizing agents are a necessary component to ensure their long-term stability.

This is especially true of proteins that have pharmaceutical potential since lack of stability is a limitation to widespread use of some protein therapeutics. It has been well documented that enzyme performance in an IL is affected by several parameters including water activity, pH and impurities [94]. Other important factors that play a role in enzyme stability/activity include IL polarity, hydrogen bond basicity and nucleophillicity of anions, ion kosmotropocity and viscosity.

Although outside the scope of this discussion, these areas have been discussed in an excellent review by Zhao [95]. Abe et al. [96] recently synthesized a number of phosphonium salts that have an alkyl ether group present. The phosphonium salts moiety is commonly found in living creatures, and it was hypothesized that this family of ILs have good affinity with enzyme proteins and may provide a good environment for enzymes. Some examples are based on the immobilization of enzyme-IL systems in chitosan [97] or Nafions [98]. Thus, catalytically active proteins and enzymes may also be confined for biosensors applications in order to achieve direct electron transfer within ionogels.

It is therefore proposed that incorporating these biocompatible ILs into ionogels is a particularly attractive strategy in the field of biosensing. These materials, in theory, will inherit all of the favorable IL properties whilst being in a solid, gel like structure [99].

As the explosion of interest in ILs as diverse, functional agents for multidisciplinary research continues, so too has the advent of ILs exhibiting stimuli responsive chemistries. This represents a subsection topic for both respective fields of research (ILs and SRM’s). Previously solid, lattice structured and anionic SRMs have been shown to form liquids when their respective counter ions are exchanged for those used in IL studies (Figure 2 (i)–(iv)). The new materials therefore exhibit their previous responsive chemistries in the liquid phase, combined with properties inherent to ILs (negligible vapor pressure, ionic conductivity, etc.). Recently, two individual communications described ILs based on the photo and pH responsive chemistries of azobenzene.

Pina et al. first described the synthesis of ILs based on the azobenzene dye Methyl Orange (MO) [121]. ILs were prepared by allowing the sodium salt of MO to undergo ion-exchange metathesis with halide ILs of imidazolium, phosphonium, sulfonium and guanidinium respectively, yielding intrinsically photochromic ionic liquids. The paper details how the behaviour of photochromic ILs can be tuned by choice of the cation, as measured by the individual thermal relaxation rates from the cis back to trans structure of four different ILs dissolved in water and ethanol.

Zhang et al. further developed on the concept of azobenzene functionalized ILs in a separate publication. In this paper, ILs based on imidazolium and pyrrolidinium cations and methyl red (MR) were prepared for use as acido-responsive sensors in aqueous and non-aqueous solutions [122]. MR differs slightly to MO in that a carboxylate group replaces the sulfonate in position R₁. The authors detail the advantages in preparing ILs with MR as shown in Figure 11.

The sodium salt of MR was shown to be sparingly soluble in water, meaning its pH responsive chemistry is void in this solvent. By exchanging sodium for [C₄mIm]⁺, the resultant IL was completely miscible with water and the pH responsiveness of the chromophore allowed a selective response to be observed in pH’s ranging from 1–7 (Figure 11 (top)).

Furthermore, an increased selectivity for the same IL over MR for trifluoromethanesulfonic acid (HOTf) was observed in solutions of ethanol (Figure 11 (bottom)). The authors postulate a differing response mechanism for the ILs response over MR and attribute it to the increased response to HOTf at low concentrations.

This particular publication highlights the novelty and advantages of preparing ILs with stimuli responsive chromophores. The unique ionic nature of these liquids allows well-known chemistries to be performed in previously inhospitable solvent environments.

A recent communication by Pina et al. described the syntheses of intrinsically electrochromic ILs based on the transition metal complexes of ethylenediaminetetraacetic acid (EDTA) [123]. Again the ILs were prepared via ion-exchange of its Na⁺ salt with the relevant halide salt of cations based on phosphonium, imidazolium and ammonium derivatives. The spectroelectrochemical response obtained for the phosphonium IL is shown below in Figure 12.

In this case the electrochromic functionality of the IL is a result of the coordinated central atom switching between two distinct redox states (Co^2+/3+). A secondary electrolyte was not needed as the intrinsic conductivity of the IL facilitated the current between two electrodes.