Perspective pubs.acs.org/ac
Perspectives on Moving Ionic Liquid Chemistry into the Solid Phase Isiah M. Warner,*,† Bilal El-Zahab,‡ and Noureen Siraj† †
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, United States
‡
ABSTRACT: Ionic liquid (IL) chemistry has evolved over the past century, such that these organic salts have impacted virtually every area of science and engineering. In the area of chemistry, initial applications of these salts were primarily the domain of chemists or chemical engineers who desired to manipulate the properties of IL solvents for a variety of applications including tuning various chemical processes. Since then, the chemistry of these organic salts has progressed such that changing an important property of a solvent (e.g., melting point or hydrophobicity) often involves simply altering the counterion of the organic salt. It is with this simplicity in mind that we have recently embarked upon the use of such chemistry to manipulate important properties of solid-phase ionic organic materials. To differentiate this chemistry from ionic liquid chemistry, we have coined the acronym GUMBOS (group of uniform materials based on organic salts). In this perspective article, we describe and demonstrate how ionic liquid chemistry can provide distinct and sometimes unique chemistry for solid-phase applications. Solid phase properties which can be manipulated via this chemistry include, but are not limited to, magnetism, melting point, hydrophobicity, fluorescence quantum yields, nanoformulations, material aggregation, viscosity, viscoelasticity, and cytotoxicity. In addition, we discuss a few examples to demonstrate how GUMBOS chemistry, until now, has been beneficial to the general area of materials chemistry and, more broadly, to the field of analytical chemistry. We also project future applications of this technology. t has been 100 years since the first discovery of ionic liquids (ILs) in 1914,1 which are defined as organic salts with melting points below 100 °C.2,3 However, organic salts with melting points between 25 °C and 100 °C are actually solids, not liquids, and are sometimes referred to as “frozen” ILs. Thus, the “liquid” part of this definition is quite arbitrary and more reflective of the desire of many IL researchers to use these compounds as solvents at or near room temperature. To achieve this goal, various ILs have been synthesized using an organic ion and an inorganic or organic counterion to provide incompatibility in size and shape, which has been predicted to lead to asymmetric packing and thus to liquids rather than solids as is usually observed for salts.2 Rogers and Seddon have estimated that more than 1018 ternary ILs,2 many of which are solids, can be synthesized by combining various cations and anions. This is an astronomical number for a small subset of ILs. Thus, this incredible latitude for possible ILs adds inconceivable flexibility to the preparation of enormous numbers of compounds with tunable or task-specific properties.2 Reflecting on the versatility of IL chemistry, we have recently embarked on development of tunable solid-phase materials from organic salts.4 To underscore the differences between these materials and ILs, we have defined them as a group of uniform materials based on organic salts (GUMBOS)5−9 and have suggested a defined melting point range of 25−250 °C (Figure 1). In defining this acronym, we have used “GUMBOS”
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© 2014 American Chemical Society
Figure 1. Melting point difference between GUMBOS and ionic liquids (ILs).
as both singular and plural and the word “uniform” to refer to the properties of the salts. Tunable properties of ILs, and thus also GUMBOS include, but are not limited to (1) melting point, (2) solubility, (3) hydrophobicity, (4) polarity, (5) thermal stability, (6) toxicity, and (7) viscosity. Such versatile chemistry has generated a vast number of applications in a variety of fields, a few of which are outlined in Figure 2. The Received: April 25, 2014 Accepted: July 12, 2014 Published: July 13, 2014 7184
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broadening the tunable ranges for task-specific applications. In addition, by increasing hydrophobicity, water-insoluble nanomaterials (i.e., nanoGUMBOS) can be fabricated from GUMBOS.4−7,10−13 Such a strategy would allow development of tunable nanomaterials for analytical and biomedical applications, but likely without some of the inherent toxicity problems associated with some nanomaterials.14 There are many ongoing discussions with regard to the toxicity of ILs.15,16 Most of these discussions center on prior proclamations of ILs as “green solvents”. This designation has arisen because of two major factors. The first is that ILs are largely nonvolatile4 and, therefore, will not likely contribute to volatile organic compounds (VOCs) in the atmosphere, as do organic solvents. Destruction of the ozone layer by VOCs has been widely accepted as a major environmental problem. The second reason is that, when employing ILs as solvents, these materials can be easily reclaimed after use.17 Therefore, in the context of using ILs as solvents, these two properties have allowed proclamations of ILs as “green solvents”. However, it would not be rational to proclaim that all ILs can be consumed or released into the environment without comprehensive toxicity and environmental safety studies. Nevertheless, based on decades of data in the literature, it is clearly established that the toxicities of ILs are easily modulated.18−22 It is also wellrecognized that, for a given functional ion, the toxicity of compounds of that ion can be tuned using various counterions.23−27 In fact, many ILs have been prepared as possible drug formulations or as sweetener/antimicrobial combinations,28,29 such that one can indeed ingest some ILs without harm. Thus, tunable toxicity is possible in much the same manner, since many other properties of ILs are tunable. In this perspective article, a few properties of solid phase organic salts (GUMBOS) are delineated that can be gleaned from IL chemistry in order to develop materials for analytical and other materials applications. In addition, a few examples of GUMBOS materials which have thus far proved extremely useful for analytical and biomedical applications are provided.
Figure 2. Applications of ILs in various fields of science and engineering.
flexibility of IL chemistry has also allowed simple and creative approaches to development of novel tunable solid-phase materials (GUMBOS) through exploitation of these interactions in the solid phase. Simply stated, this approach has allowed solid-phase materials to be designed for selected applications (task-specific) rather than adapted for such purposes. An added advantage is that preparation of novel task-specific GUMBOS from ionic compounds is quite simple, sometimes requiring only a single-step ion-exchange reaction.5 It was not totally unexpected that the GUMBOS approach was not initially accepted by much of the IL community, since the applications of ILs are almost exclusively as liquids (solvents or lubricants). However, we were convinced that the development of GUMBOS materials would allow easy design and delivery of desired properties to solid-phase ionic materials and thus to new applications of such materials. For example, variations in the constituent ions could produce materials, which are extremely hydrophobic or, alternatively, extremely hydrophilic. Although GUMBOS share properties similar to those of ILs, these solid-phase compounds would be more amenable to materials and biomedical applications, thus
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SYNTHESES OF GUMBOS AND nanoGUMBOS If the desired molecule is already a salt, then the procedure for conversion to GUMBOS is relatively simple. Depending on which charged species of the salt is designated as essential, a simple ion exchange for the counterion can be achieved using a series of tabulated ions (see examples in Figure 3) previously used for IL chemistry. Counterions not in such lists can also be
Figure 3. Examples of typical cations and anions used for ILs. 7185
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Figure 4. Schematic illustration of the synthetic route for production of nanoGUMBOS from small and neutral organic molecules. In this example, an imidazolium salt was produced from a neutral imidazole, followed by an anion exchange to produce a GUMBOS, which was finally converted to a nanoparticle using a nanoGUMBOS synthesis procedure.
Figure 5. Schematic of various methods for synthesis of nanoGUMBOS.
Following the synthesis of hydrophobic GUMBOS, nanoGUMBOS can be produced using various approaches, depending on the desired size, shape, and degree of uniformity. The three primary methods that we have employed for preparation of nanoGUMBOS are summarized in Figure 5. The simplest of these is a reprecipitation method23,30 which, as indicated by its name, involves precipitation of the GUMBOS into nanosize clusters after mixing dilute drops of the GUMBOS solution with a miscible solvent, which acts as a nonsolvent for the GUMBOS, while sonicating. Particles produced using this method are generally spherical. However, one-dimensional (1D) structures have been observed as well.10 In addition, if this approach is template-assisted (e.g., by use of micelles), then the size uniformity of the nanoparticles is significantly improved. The melt−emulsion−quench method4 requires melting the GUMBOS, followed by emulsification and then refreezing. The advantage of this approach is the possibility of additive-free preparations, making it suitable for high-purity uses such as medical applications. Use of porous templates such as track-etched polycarbonate or anodic aluminum oxide membranes with cylindrical pores have also allowed production of 1D nanoGUMBOS such as rods, wires, and tubes.7 However, this approach is limited by throughput, size, and composition of the templating material. Finally, we have recently submitted a manuscript which focuses on
used. However, the most remarkable and well-defined changes in properties (melting points, hydrophobicity, etc.) are usually achieved by using counterions that have been developed and tested over the years for IL chemistry. Despite past efforts to focus on liquid forms of ILs, many of these cation/anion combinations are solids. Production of solids, rather than liquids, has often been viewed as a failure within the IL community. Since our focus is on solid-phase materials, such production would be deemed a success. If the molecule is not a salt, then a charge must be added to the molecule of interest. For example, if a cation is desired, then the most direct approach for synthesis of the cationic part of GUMBOS may involve addition and quaternization of an amine group into the compound (Figure 4). This step is followed by anion exchange in which the counterion is often replaced with an anion, depending on your level of desired hydrophobicity and application, typically from a list of anions previously developed for IL chemistry. Similar procedures exist if you desire to start with an anion, followed by replacement of the counterion using cations from a list of cations previously developed for IL chemistry. It is quite likely that as GUMBOS chemistry progresses, more counterions will be added to the repertoire of possible counterions since solid-phase applications will likely evolve to facilitate applications quite different from liquid-phase applications. 7186
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development and evaluation of various methods and associated parameters for production of nanoGUMBOS. The literature can be consulted for this information or E-mail one of the authors to obtain a reprint.
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ADVANTAGES OF IL CHEMISTRY While the literature abounds with examples of the many benefits of using ILs as solvents, it is important to summarize the distinct advantages of this chemistry in the context of this manuscript. We note that details regarding syntheses and applications of ionic liquid chemistry in analytical chemistry can be found in a recent review.3 Recent published work from our laboratory is used in this manuscript as an example of employing IL chemistry for sensor development.31 Much of the basic premise of that research was to develop a simple strategy for fabrication of optoelectronic tongues or colorimetric-taste sensor arrays as cited in a book by Marks.32 Since aqueous samples are quite common, most of these sensors are required to operate in aqueous environments. However, this approach is inherently challenging, because of interferences from water since the dyes employed in these sensor arrays should be very hydrophobic in order to avoid dissolution in aqueous medium. A more restrictive problem is that the numbers of dyes that fulfill this criterion are limited. In addition, the ability to use the same dyes in both optoelectronic noses and tongues31 is also limited. Another problem is that matrices on which these dyes are printed must be very hydrophobic, often requiring specialized hydrophobic surfaces. Finally, data derived from these sensor arrays are frequently digitized and expressed as difference maps (i.e., a colorimetric representation of a difference graphical representation derived by subtracting the color of the sensor dye matrix exposed to the sample from the color of the original sensor dye matrix). Clearly, such subtraction requires extremely reproducible data. Despite the inherent limitations cited above, very creative research in this area has come from the laboratories of Professor Kenneth Suslick of the University of Illinois33−35 and others.36−39 In fact, it is the creativity of that recent research that has inspired us to investigate whether ILs may be suitable alternatives to the hydrophobic dyes commonly used for such studies. In order to demonstrate the utility of IL chemistry for producing very hydrophobic compounds for such use, we first assembled 12 commonly used hydrophilic and anionic pH indicator dyes with considerable aqueous solubility. We then prepared hydrophobic ILs from these dyes by ion exchange of their counterions (mostly sodium) using the chloride salt of the trihexyl-(tetradecyl)phosphonium cation, also known as P66614, as described in our recent publication.31 All hydrophobic ILs produced from this reaction showed excellent characteristics, including a profound lack of solubility in aqueous media. Moreover, common matrices such as alumina, silica, filter paper, and fabric could be used to produce the desired sensor arrays. Here, we present one example using an array produced using IL-dyed threads sewn onto cotton fabric.31 Only 8 of the 12 ILs were used to produce the cotton threads (Figure 6) used in this example. While preparing these threads, another distinct advantage was observed for this approach: no expensive dewaxing of the thread40 was required, since the ILs dissolved the wax while simultaneously coating the threads. The highly reproducible difference image in Figure 6 is representative of exposing the IL-coated cotton threads to vapors of trifluoroacetic acid (TFA), followed by digitization of the colored thread and then acquisition of a difference map by subtracting
Figure 6. Photonic IL sensor array fabricated from IL-stained cotton threads. (A) Cotton thread spools stained with chemosensory ILs (P refers to the P66614 ion and other initials refer to the dyes).30 (B) (i) sensor array fabricated using a common sewing machine (panel i); digital images of the cotton-thread-based-IL sensor array before exposure (panel ii) and after exposure (panel iii) to trifluoroacetic acid (TFA); and the difference map generated by calculating the red-greenblue (RGB) color differences after exposure to TFA (panel iv).
the TFA exposed image from the original threads image. Suslick33,41 and others37,42 have demonstrated that this approach can be highly reproducible and allows rapid identification of unknowns. Such is also true for our work. However, we have shown that IL chemistry can be used to more easily produce very hydrophobic dyes from very common water-soluble pH dyes and that these dyes are also compatible with commonly used matrices, e.g., cloth, filter paper, silica, and alumina. IL Chemistry for Hydrophobicity in the Solid Phase. In traditional organic chemistry courses, students are taught that increasing the hydrophobicity of a molecule is most easily achieved by adding hydrophobic appendages, e.g., addition of alkyl groups to that molecule. In fact, this approach is among the most common for increasing the hydrophobicity of drug molecules, and many examples in this regard are found in the literature.30,43,44 However, it is also well-established that synthesis of organic molecules involving multiple step reactions can lead to poor product yields and thus to more-expensive final products. Moreover, one can easily argue that the final product, although often more hydrophobic, is a very different molecule than the original and, consequently, may have very different medicinal properties for this reason alone. Thus, the question arises as to whether it is accurate to compare the medicinal properties of the newly synthesized compound to its parent compound. As shown in the example from the previous section, the world of IL chemistry allows simple conversion of a hydrophilic compound to a more hydrophobic compound if the molecule is already a salt. This conversion process often involves an ionexchange reaction where a more hydrophilic counterion is replaced with a more hydrophobic one. Many ions are already ranked as hydrophobic or hydrophilic based on decades of chemistry developed for production of hydrophobic and 7187
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Table 1. Structure of Anions Used with Rhodamine 6G (R6G) and log Ko/w Values of the Resulting GUMBOS
number of GUMBOS were synthesized by using a cationic near-infrared cyanine dye [1,1′,3,3,3′,3′- hexamethylindotricarbo-cyanine (HMT)] and various anions. These GUMBOS displayed a wide range of solubilities and melting points. The solubility in water was largely dependent on the anion employed. Anions such as tetrafluoroborate showed enhanced aqueous solubility, which was postulated to result from hydrogen bonding, accompanied by dipole-induced dipole interactions with the surrounding water molecules. These variations also produced new spectral properties as a result of differences in aggregation of dye molecules in nanoGUMBOS. It was also predicted that anion exchange could lead to control of these aggregate assemblies to produce predominantly H- or J-type aggregates. Particle-size-dependent spectral variations were not observed for particles sizes in the range of 50−300 nm in diameter.12 Anisotropic fluorescence emission studies revealed that soluble GUMBOS had little or no effect on anisotropy in dilute solutions, independent of the anion. For nanoGUMBOS, higher anisotropies were observed with a strong dependence on the particular anion used. These results support an anionic influence model and the diminishing effects of ions dissociated in the soluble state.12 In support of experimental observations, molecular dynamics (MD) simulations reinforced the contention that a more hydrophobic anion induced H-aggregation and low fluorescence yields, while anions with lower hydrophobicity induced J-aggregates and higher fluorescence yields. Further analyses of the stacking angle of the cations based on data from MD simulations showed a strong correlation between experimental and theoretical data, and provided an effective and predictive model for the observed aggregation and fluorescence quantum yields.12 On the basis of the above results, we have also recently studied carbazoleimidazolium-based GUMBOS in DCM solution in order to further examine the effects of anion variations.9 While the iodide form of this GUMBOS compound in DCM has a quantum yield of 0.28, the other anions (OTf, NTf2, and BETI) showed considerably higher respective quantum yields of 0.94, 0.73, and 0.99. These variations are attributed in part to differences in aggregation and also to larger Stokes shifts for the more hydrophobic anions. While quantum yield improvements have been previously observed with polymeric derivatives and other super complexes of carbazole,48,49 GUMBOS anion-pairing are shown to yield comparable improvements with smaller molecules and through the use of much simpler chemistry. Electrosprayed solid films of these GUMBOS showed 7−14 nm red shifts in fluorescence maxima, as compared to methanolic solutions of these same compounds.
hydrophilic ILs. There are many cations classified as hydrophobic and hydrophilic, as well as many anions classified as hydrophobic and hydrophilic. Therefore, these ions can be respectively used with counteranions or countercations to produce desired hydrophobic/hydrophilic properties for a given salt. Recent studies from our laboratory have demonstrated that this IL chemistry is directly applicable to the solid phase, i.e., to GUMBOS.23 In this example, we explore cytotoxicity of the cationic dye, Rhodamine 6G (R6G), which has been studied for decades with one resounding conclusion. The R6G dye is toxic to both normal and cancer cells. However, such studies were almost always done with R6G salts using the most common counteranion of chloride (Cl−). A recent study from our laboratory has examined the change in cytotoxicity of R6G as a result of changing this counteranion.23 In this study, a simple ion-exchange reaction was used for conversion of R6G chloride to different compounds with a variety of counterions whose hydrophobicity is known from IL chemistry. This conversion was followed by measurement of the relative hydrophobicity of each compound using octanol/water partition coefficients (Ko/w). The relative hydrophobicities of the various R6G compounds are displayed in Table 1. The values at the bottom of the figure are the logarithms of the octanol−water partition coefficients for these various compounds of R6G using the counteranions noted in the table. The R6G compounds produced from the anions displayed in Table 1 are all solid-phase compounds and clearly show a trend in hydrophobicity, which is consistent with relative hydrophobicities observed for ILs using these particular counteranions. Moreover, in this GUMBOS approach, development of water-insoluble compounds allowed production of nanoGUMBOS using a facile reprecipitation method.6 By using nanoGUMBOS and in vitro studies, it was determined that, while the hydrophilic forms (e.g., Asc−) of R6G were toxic to both normal and cancer cells, the hydrophobic nanoGUMBOS showed toxicity only to cancer cells.23 In addition, as the doubling rate of various cancer cells increased, the toxicity toward these cells also increased, suggesting greater toxicity toward more-aggressive cancer cells. Fluorescence Properties and Quantum Yields. The literature abounds with examples of near-infrared dyes that are nonfluorescent at high solution concentration due to dye aggregation.45−47 Therefore, there was considerable doubt when we proposed that highly fluorescent nanoparticles could be derived from aggregates of such compounds. Our first report on such fluorescence behavior in the solid state was in 2009,11 followed by a detailed report of the spectral properties of similar compounds in the following year.12 In this latter study, a 7188
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Viscosity and Viscoelasticity of ILs and GUMBOS. By now, it should be apparent that ILs (and therefore GUMBOS) have many unusual properties, i.e., properties that do not necessarily follow traditional chemical logic and intuition that chemists have always used to formulate strategies for developing chemicals with certain physical properties. Thus, it also follows that the associated chemical reactions or physical interactions which produce such properties are not always apparent. Another example of this kind of anomalous behavior of ILs can be found in the recent chemical literature.50 In that study, the authors examined the effects of various molecular solvents on the physical properties of a given IL and noted a very interesting phenomenon. The viscosity of a binary mixture of different molecular solvents with a given IL was dependent primarily on the mole fraction of the molecular solvent and not on its chemical identity. This was a remarkable finding since the molecular solvents included a series of many dissimilar materials such as water, various benzene derivatives, and various nitrile derivatives. In our GUMBOS and IL research, we have observed a phenomenon that correlates with the viscosity effect observed in the research cited above.51 In evaluating various composites (binary blends of GUMBOS or IL and polymer) as absorbents by using a quartz crystal microbalance (QCM) sensor, we noted that, for low vapor concentrations of organic vapors, a plot of the ratio of the change in frequency (Δf) to the change in motional resistance (ΔR) is directly proportional to the molecular weight of absorbed vapor. Since Δf is known from the Sauerbrey equation52 to be directly proportional to the change in mass as a result of absorption of gas, it follows from our observation that ΔR must be proportional to moles of gas, as represented in Figure 7. Despite the obvious proportionality
frequency and dissipation factor at different harmonics were measured during vapor absorption, and analyses of the data revealed that the composite film behaved as a Maxwellian viscoelastic material. In addition, our data and calculations showed that the motional resistance change (or dissipation change) is primarily due to viscosity changes in the coating material. Since the viscosity of ILs or GUMBOS are dependent on the mole fraction of molecular solvent as noted above,50 variation of motional resistance with moles of vapor absorbed is consistent with this observation. Therefore, it is clear that this discovery adds another dimension (molecular weight) to QCM measurements, which have traditionally been simply used for mass detection. Multifunctional Properties. We have also recently demonstrated that multiple properties can be incorporated into single GUMBOS. In one example, we desired to produce a GUMBOS that was tumor-targeting, fluorescent, and magnetic.8 To demonstrate this concept, we synthesized a GUMBOS compound that incorporated a phosphonium derivative (tumor targeting) and a dysprosium ion (fluorescent and paramagnetic). This compound was shown to possess all three properties, as well as toxicity toward cancer cells, which was demonstrated by using in vitro studies and cellular imaging. In other studies, nanoGUMBOS suspensions have been shown to retain the parent material’s properties such as fluorescence, magnetism, and chirality at very low concentrations. This is part of the driving force behind nanoGUMBOS research, which allows unique functional properties at minute concentrations of GUMBOS which can reduce cost, viscosity, and potential toxicity.
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CONCLUSIONS AND FUTURE DIRECTIONS Since our first report on group of uniform materials based on organic salts (GUMBOS) a few years ago,4 we have demonstrated, via several publications already in the literature,4−12,23,51,53−63 that these materials have remarkable utility in a wide range of fields. However, we believe that the applications that we have demonstrated thus far are only a small sampling of possibilities in materials, biomedical, and analytical chemistries. We believe that the literature on GUMBOS (or equivalent chemistry) will continue to increase as more examples of this type of chemistry are published. In fact, there are already a few examples of this type of chemistry which has not been categorized as solid-phase ionic liquid (IL) chemistry (GUMBOS chemistry). For example, after years of searching, we have recently found examples of this type of chemistry buried in the literature. Examples of such GUMBOS chemistry include varying types of applications.64−67 While it is not apparent that all of these references are related to GUMBOS, close examination of these manuscripts will indeed show relevance. In fact, some of these are not even classified as related to IL chemistry. In addition, although not focused in a comprehensive manner as our studies, we believe that other such studies exist in the literature. Finally, we believe that the convenience and simplicity of GUMBOS chemistry will enable many applications in far reaching fields such as solid-state ionics and photonics. For example, polymerization of ILs has served as a common practice to produce solid-state ILs,68,69 even though the process is complicated and time-consuming. Other related studies for developing ionic nanoparticle networks have also been reported.70 We also note that various formulations using counterions have been shown to greatly influence properties of
Figure 7. Variation of ΔR with number of moles of vapors absorbed. The coating material used is a binary blend of 1-butyl-3methylpyridinium hexafluorophosphate (∼90%) and cellulose acetate (∼10%). The amount of coating material, as calculated by using the Sauerbrey equation, is 65 μg/cm2.
shown in this plot, it was difficult for reviewers to believe the theoretical basis for this observation. This is because these results are substantially different from other observations in the literature with other absorbents. While we have only observed this phenomenon for composites of GUMBOS or very viscous ILs, it is the use of GUMBOS which allowed this initial discovery. In order to fully understand the theoretical basis of this observation, we have conducted detailed studies using a quartz crystal microbalance with dissipation monitoring (QCM-D), and the results are outlined in a recent publication.53 The 7189
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(15) Jastorff, B.; Stormann, R.; Ranke, J.; Molter, K.; Stock, F.; Oberheitmann, B.; Hoffmann, W.; Hoffmann, J.; Nuchter, M.; Ondruschka, B.; Filser, J. Green Chem. 2003, 5, 136−142. (16) Frade, R. F.; Afonso, C. A. Hum. Exp. Toxicol. 2010, 29, 1038− 1054. (17) Mohammad Fauzi, A. H.; Amin, N. A. S. Renew. Sustainable Energy Rev. 2012, 16, 5770−5786. (18) Zhao, D.; Liao, Y.; Zhang, Z. CLEAN−Soil, Air, Water 2007, 35, 42−48. (19) Hough, W. L.; Smiglak, M.; Rodriguez, H.; Swatloski, R. P.; Spear, S. K.; Daly, D. T.; Pernak, J.; Grisel, J. E.; Carliss, R. D.; Soutullo, M. D.; Davis, J. J. H.; Rogers, R. D. New J. Chem. 2007, 31, 1429−1436. (20) Rogers, R. D.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1077−1078. (21) Rogers, R. D. Nature 2007, 447, 917−918. (22) Sowmiah, S.; Srinivasadesikan, V.; Tseng, M.-C.; Chu, Y.-H. Molecules 2009, 14, 3780−3813. (23) Magut, P. K. S.; Das, S.; Fernand, V. E.; Losso, J.; McDonough, K.; Naylor, B. M.; Aggarwal, S.; Warner, I. M. J. Am. Chem. Soc. 2013, 135, 15873−15879. (24) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351−356. (25) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391− 1398. (26) Weingärtner, H. Angew. Chem., Int. Ed. 2008, 47, 654−670. (27) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123− 150. (28) Carter, E. B.; Culver, S. L.; Fox, P. A.; Goode, R. D.; Ntai, I.; Tickell, M. D.; Traylor, R. K.; Hoffman, N. W.; Davis, J. J. H. Chem. Commun. 2004, 630−631. (29) Bica, K.; Rodriguez, H.; Gurau, G.; Andreea Cojocaru, O.; Riisager, A.; Fehrmann, R.; Rogers, R. D. Chem. Commun. 2012, 48, 5422−5424. (30) Kasai, H.; Murakami, T.; Ikuta, Y.; Koseki, Y.; Baba, K.; Oikawa, H.; Nakanishi, H.; Okada, M.; Shoji, M.; Ueda, M.; Imahori, H.; Hashida, M. Angew. Chem., Int. Ed. 2012, 51, 10315−10318. (31) Galpothdeniya, W. I. S.; McCarter, K. S.; De Rooy, S. L.; Regmi, B. P.; Das, S.; Hasan, F.; Tagge, A.; Warner, I. M. RSC Adv. 2014, 4, 7225−7234. (32) Marks, R. Handbook of Biosensors and Biochips; John Wiley & Sons: Chichester, U.K., 2007. (33) Suslick, K. S. MRS Bull. 2004, 29, 720−725. (34) Feng, L.; Musto, C. J.; Kemling, J. W.; Lim, S. H.; Zhong, W.; Suslick, K. S. Anal. Chem. 2010, 82, 9433−9440. (35) Suslick, B. A.; Feng, L.; Suslick, K. S. Anal. Chem. 2010, 82, 2067−2073. (36) Salinas, Y.; Ros-Lis, J. V.; Vivancos, J.-L.; Martinez-Manez, R.; Marcos, M. D.; Aucejo, S.; Herranz, N.; Lorente, I. Analyst 2012, 137, 3635−3643. (37) Huang, X.; Xin, J.; Zhao, J. J. Food Eng. 2011, 105, 632−637. (38) Huo, D.-Q.; Zhang, G.-P.; Hou, C.-J.; Dong, J.-L.; Zhang, Y.-C.; Liu, Z.; Luo, X.-G.; Fa, H.-B.; Zhang, S.-Y. Chin. J. Anal. Chem. 2010, 38, 1115−1120. (39) Gouma, P.; Sberveglieri, G. MRS Bull. 2004, 29, 697−702. (40) Reches, M.; Mirica, K. A.; Dasgupta, R.; Dickey, M. D.; Butte, M. J.; Whitesides, G. M. ACS Appl. Mater. Interface 2010, 2, 1722− 1728. (41) Zhang, C.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 11548− 11549. (42) Sen, A.; Albarella, J. D.; Carey, J. R.; Kim, P.; McNamara, W. B., III. Sens. Actuators B 2008, 134, 234−237. (43) Breton, M.; Prevel, G.; Audibert, J.-F.; Pansu, R.; Tauc, P.; Pioufle, B. L.; Francais, O.; Fresnais, J.; Berret, J.-F.; Ishow, E. Phys. Chem. Chem. Phys. 2011, 13, 13268−13276. (44) Kandela, I.; Lee, W.; Indig, G. Biotechnol. Histochem. 2003, 78, 157−169. (45) Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. Rev. 1992, 92, 1197−1226. (46) Kim, J. S.; Kodagahally, R.; Strekowski, L.; Patonay, G. Talanta 2005, 67, 947−954.
GUMBOS including hydrophobicity, stability, melt properties, and optical properties. While our current efforts have been mostly focused on exploring the tunability of GUMBOS, we are also interested in coupling functional cations with functional anions. This latter approach could lead to many applications, e.g., functional GUMBOS that carry magnetic receptivity and chemotherapeutic properties for use as active drugs that can be guided for targeted delivery. We foresee that suitable formulations for pure GUMBOS or composites of GUMBOS will enable competitive performances such as high ionic conductivity, better optoelectronics, or impressive fluorescence quantum yields for use in imaging, optoelectronic, and theranostic applications. In summary, we have only scratched the surface of possibilities for this type of chemistry in the solid phase. We hope that this manuscript will inspire others to join us in our quest for tunable solid-phase materials using GUMBOS technology.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 225-578-2829. Fax: 225-578-3971. E-mail: iwarner@lsu. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge useful suggestions and discussions from Dr. Bishnu P. Regmi, Mr. Waduge Indika S. Galpothdeniya, and Dr. Paul K. S. Magut during preparation of this manuscript. Some of the materials described in this manuscript are based upon work supported by the National Science Foundation (NSF) under Grant Nos. CHE-1307611 and CHE-1243916.
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REFERENCES
(1) Walden, P. Bull. Russ. Acad. Sci. 1914, 405−422. (2) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792−793. (3) Anderson, J. L.; Armstrong, D. W.; Wei, G.-T. Anal. Chem. 2006, 78, 2892−2902. (4) Tesfai, A.; El-Zahab, B.; Bwambok, D. K.; Baker, G. A.; Fakayode, S. O.; Lowry, M.; Warner, I. M. Nano Lett. 2008, 8, 897−901. (5) Tesfai, A.; El-Zahab, B.; Kelley, A. T.; Li, M.; Garno, J. C.; Baker, G. A.; Warner, I. M. ACS Nano 2009, 3, 3244−3250. (6) Warner, I. M.; Tesfai, A.; El-Zahab, B. M.; Bwambok, D.; Baker, G. A.; Fakayode, S. O.; Lowry, M.; Tolocka, M. P.; De Rooy, S. PCT/ US2008/086065, 2011. (7) de Rooy, S. L.; El-Zahab, B.; Li, M.; Das, S.; Broering, E.; Chandler, L.; Warner, I. M. Chem. Commun. 2011, 47, 8916−8918. (8) Li, M.; Ganea, G. M.; Lu, C.; De Rooy, S. L.; El-Zahab, B.; Fernand, V. E.; Jin, R.; Aggarwal, S.; Warner, I. M. J. Inorg. Biochem. 2012, 107, 40−46. (9) Siraj, N.; Hasan, F.; Das, S.; Kiruri, L. W.; Steege Gall, K. E.; Baker, G. A.; Warner, I. M. J. Phys. Chem. C 2014, 118, 2312−2320. (10) de Rooy, S. L.; Das, S.; Li, M.; El-Zahab, B.; Jordan, A.; Lodes, R.; Weber, A.; Chandler, L.; Baker, G. A.; Warner, I. M. J. Phys. Chem. C 2012, 116, 8251−8260. (11) Bwambok, D. K.; El-Zahab, B.; Challa, S. K.; Li, M.; Chandler, L.; Baker, G. A.; Warner, I. M. ACS Nano 2009, 3, 3854−3860. (12) Das, S.; Bwambok, D.; El-Zahab, B.; Monk, J.; de Rooy, S. L.; Challa, S.; Li, M.; Hung, F. R.; Baker, G. A.; Warner, I. M. Langmuir 2010, 26, 12867−12876. (13) Jordan, A. N.; Das, S.; Siraj, N.; de Rooy, S. L.; Li, M.; El-Zahab, B.; Chandler, L.; Baker, G. A.; Warner, I. M. Nanoscale 2012, 4, 5031− 5038. (14) De Jong, W. H.; Borm, P. J. Int. J. Nanomed. 2008, 3, 133. 7190
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(47) Otsuka, A.; Funabiki, K.; Sugiyama, N.; Mase, H.; Yoshida, T.; Minoura, H.; Matsui, M. Chem. Lett. 2008, 37, 176−177. (48) Kotchapradist, P.; Prachumrak, N.; Tarsang, R.; Jungsuttiwong, S.; Keawin, T.; Sudyoadsuk, T.; Promarak, V. J. Mater. Chem. C 2013, 1, 4916−4924. (49) Linton, K. E.; Fisher, A. L.; Pearson, C.; Fox, M. A.; Palsson, L.O.; Bryce, M. R.; Petty, M. C. J. Mater. Chem. 2012, 22, 11816−11825. (50) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275−2287. (51) Regmi, B. P.; Monk, J.; El-Zahab, B.; Das, S.; Hung, F. R.; Hayes, D. J.; Warner, I. M. J. Mater. Chem. 2012, 22, 13732−13741. (52) Sauerbrey, G. J. Phys. 1959, 155, 206−212. (53) Regmi, B. P.; Speller, N. C.; Anderson, M. J.; Brutus, J. O.; Merid, Y.; Das, S.; El-Zahab, B.; Hayes, D.; Murray, K. K.; Warner, I. M. J. Mater. Chem. C 2014, 2, 4867−4878. (54) Dumke, J. C.; El-Zahab, B.; Challa, S.; Das, S.; Chandler, L.; Tolocka, M.; Hayes, D. J.; Warner, I. M. Langmuir 2010, 26, 15599− 15603. (55) Cole, M. R.; Li, M.; El-Zahab, B.; Janes, M. E.; Hayes, D.; Warner, I. M. Chem. Biol. Drug Des. 2011, 78, 33−41. (56) Das, S.; de Rooy, S. L.; Jordan, A. N.; Chandler, L.; Negulescu, I. I.; El-Zahab, B.; Warner, I. M. Langmuir 2011, 28, 757−765. (57) Lu, C.; Das, S.; Magut, P. K. S.; Li, M.; El-Zahab, B.; Warner, I. M. Langmuir 2012, 28, 14415−14423. (58) Cole, M. R.; Li, M.; Jadeja, R.; El-Zahab, B.; Hayes, D.; Hobden, J. A.; Janes, M. E.; Warner, I. M. J. Antimicrob. Chemother. 2013, 68, 1312−1318. (59) Sarkar, A.; Kanakamedala, K.; Rajathadripura, M. D.; Jagadish, N. N.; Magut, P. K.; de Rooy, S.; Das, S.; El-Zahab, B.; Warner, I. M.; Daniels-Race, T. Electron. Mater. Lett. 2014, DOI: 10.1007/s13391014-3347-8. (60) Dumke, J. C.; Qureshi, A.; Hamdan, S.; Rupnik, K.; El-Zahab, B.; Hayes, D.; Warner, I. M. Photochem. Photobiol. Sci. 2014, DOI: 10.1039/C4PP00030G. (61) Jordan, A. N.; Siraj, N.; Das, S.; Warner, I. M. RSC Adv. 2014, 4, 28471−28480. (62) Galpothdeniya, W. I. S.; Das, S.; De Rooy, S. L.; Regmi, B. P.; Hamdan, S.; Warner, I. M. RSC Adv. 2014, 4, 17533−17540. (63) Berton, P.; Regmi, B. P.; Spivak, D. A.; Warner, I. M. Microchim. Acta 2014, DOI: 10.1007/s00604-014-1261-2. (64) Shigeyasu, M.; Murayama, H.; Tanaka, H. Chem. Phys. Lett. 2008, 463, 373−377. (65) Ku, B. K.; Fernandez de la Mora, J. J. Phys. Chem. B 2004, 108, 14915−14923. (66) Armitage, B.; Retterer, J.; O’Brien, D. F. J. Am. Chem. Soc. 1993, 115, 10786−10790. (67) Ou, Z.-m.; Yao, H.; Kimura, K. J. Photochem. Photobiol. A: Chem. 2007, 189, 7−14. (68) Yoshizawa, M.; Ogihara, W.; Ohno, H. Polymer. Adv. Technol. 2002, 13, 589−594. (69) Yoshizawa, M.; Ohno, H. Electrochim. Acta 2001, 46, 1723− 1728. (70) Marie-Alexandra, G.; Neouze, G.; Kronstein, M.; Tielens, F. Chem. Commun., 2014, DOI: 10.1039/C4CC02419B
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Perspectives on Moving Ionic Liquid Chemistry into the Solid Phase Isiah M. Warner,*,† Bilal El-Zahab,‡ and Noureen Siraj† †
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, United States
‡
ABSTRACT: Ionic liquid (IL) chemistry has evolved over the past century, such that these organic salts have impacted virtually every area of science and engineering. In the area of chemistry, initial applications of these salts were primarily the domain of chemists or chemical engineers who desired to manipulate the properties of IL solvents for a variety of applications including tuning various chemical processes. Since then, the chemistry of these organic salts has progressed such that changing an important property of a solvent (e.g., melting point or hydrophobicity) often involves simply altering the counterion of the organic salt. It is with this simplicity in mind that we have recently embarked upon the use of such chemistry to manipulate important properties of solid-phase ionic organic materials. To differentiate this chemistry from ionic liquid chemistry, we have coined the acronym GUMBOS (group of uniform materials based on organic salts). In this perspective article, we describe and demonstrate how ionic liquid chemistry can provide distinct and sometimes unique chemistry for solid-phase applications. Solid phase properties which can be manipulated via this chemistry include, but are not limited to, magnetism, melting point, hydrophobicity, fluorescence quantum yields, nanoformulations, material aggregation, viscosity, viscoelasticity, and cytotoxicity. In addition, we discuss a few examples to demonstrate how GUMBOS chemistry, until now, has been beneficial to the general area of materials chemistry and, more broadly, to the field of analytical chemistry. We also project future applications of this technology. t has been 100 years since the first discovery of ionic liquids (ILs) in 1914,1 which are defined as organic salts with melting points below 100 °C.2,3 However, organic salts with melting points between 25 °C and 100 °C are actually solids, not liquids, and are sometimes referred to as “frozen” ILs. Thus, the “liquid” part of this definition is quite arbitrary and more reflective of the desire of many IL researchers to use these compounds as solvents at or near room temperature. To achieve this goal, various ILs have been synthesized using an organic ion and an inorganic or organic counterion to provide incompatibility in size and shape, which has been predicted to lead to asymmetric packing and thus to liquids rather than solids as is usually observed for salts.2 Rogers and Seddon have estimated that more than 1018 ternary ILs,2 many of which are solids, can be synthesized by combining various cations and anions. This is an astronomical number for a small subset of ILs. Thus, this incredible latitude for possible ILs adds inconceivable flexibility to the preparation of enormous numbers of compounds with tunable or task-specific properties.2 Reflecting on the versatility of IL chemistry, we have recently embarked on development of tunable solid-phase materials from organic salts.4 To underscore the differences between these materials and ILs, we have defined them as a group of uniform materials based on organic salts (GUMBOS)5−9 and have suggested a defined melting point range of 25−250 °C (Figure 1). In defining this acronym, we have used “GUMBOS”
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© 2014 American Chemical Society
Figure 1. Melting point difference between GUMBOS and ionic liquids (ILs).
as both singular and plural and the word “uniform” to refer to the properties of the salts. Tunable properties of ILs, and thus also GUMBOS include, but are not limited to (1) melting point, (2) solubility, (3) hydrophobicity, (4) polarity, (5) thermal stability, (6) toxicity, and (7) viscosity. Such versatile chemistry has generated a vast number of applications in a variety of fields, a few of which are outlined in Figure 2. The Received: April 25, 2014 Accepted: July 12, 2014 Published: July 13, 2014 7184
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broadening the tunable ranges for task-specific applications. In addition, by increasing hydrophobicity, water-insoluble nanomaterials (i.e., nanoGUMBOS) can be fabricated from GUMBOS.4−7,10−13 Such a strategy would allow development of tunable nanomaterials for analytical and biomedical applications, but likely without some of the inherent toxicity problems associated with some nanomaterials.14 There are many ongoing discussions with regard to the toxicity of ILs.15,16 Most of these discussions center on prior proclamations of ILs as “green solvents”. This designation has arisen because of two major factors. The first is that ILs are largely nonvolatile4 and, therefore, will not likely contribute to volatile organic compounds (VOCs) in the atmosphere, as do organic solvents. Destruction of the ozone layer by VOCs has been widely accepted as a major environmental problem. The second reason is that, when employing ILs as solvents, these materials can be easily reclaimed after use.17 Therefore, in the context of using ILs as solvents, these two properties have allowed proclamations of ILs as “green solvents”. However, it would not be rational to proclaim that all ILs can be consumed or released into the environment without comprehensive toxicity and environmental safety studies. Nevertheless, based on decades of data in the literature, it is clearly established that the toxicities of ILs are easily modulated.18−22 It is also wellrecognized that, for a given functional ion, the toxicity of compounds of that ion can be tuned using various counterions.23−27 In fact, many ILs have been prepared as possible drug formulations or as sweetener/antimicrobial combinations,28,29 such that one can indeed ingest some ILs without harm. Thus, tunable toxicity is possible in much the same manner, since many other properties of ILs are tunable. In this perspective article, a few properties of solid phase organic salts (GUMBOS) are delineated that can be gleaned from IL chemistry in order to develop materials for analytical and other materials applications. In addition, a few examples of GUMBOS materials which have thus far proved extremely useful for analytical and biomedical applications are provided.
Figure 2. Applications of ILs in various fields of science and engineering.
flexibility of IL chemistry has also allowed simple and creative approaches to development of novel tunable solid-phase materials (GUMBOS) through exploitation of these interactions in the solid phase. Simply stated, this approach has allowed solid-phase materials to be designed for selected applications (task-specific) rather than adapted for such purposes. An added advantage is that preparation of novel task-specific GUMBOS from ionic compounds is quite simple, sometimes requiring only a single-step ion-exchange reaction.5 It was not totally unexpected that the GUMBOS approach was not initially accepted by much of the IL community, since the applications of ILs are almost exclusively as liquids (solvents or lubricants). However, we were convinced that the development of GUMBOS materials would allow easy design and delivery of desired properties to solid-phase ionic materials and thus to new applications of such materials. For example, variations in the constituent ions could produce materials, which are extremely hydrophobic or, alternatively, extremely hydrophilic. Although GUMBOS share properties similar to those of ILs, these solid-phase compounds would be more amenable to materials and biomedical applications, thus
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SYNTHESES OF GUMBOS AND nanoGUMBOS If the desired molecule is already a salt, then the procedure for conversion to GUMBOS is relatively simple. Depending on which charged species of the salt is designated as essential, a simple ion exchange for the counterion can be achieved using a series of tabulated ions (see examples in Figure 3) previously used for IL chemistry. Counterions not in such lists can also be
Figure 3. Examples of typical cations and anions used for ILs. 7185
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Figure 4. Schematic illustration of the synthetic route for production of nanoGUMBOS from small and neutral organic molecules. In this example, an imidazolium salt was produced from a neutral imidazole, followed by an anion exchange to produce a GUMBOS, which was finally converted to a nanoparticle using a nanoGUMBOS synthesis procedure.
Figure 5. Schematic of various methods for synthesis of nanoGUMBOS.
Following the synthesis of hydrophobic GUMBOS, nanoGUMBOS can be produced using various approaches, depending on the desired size, shape, and degree of uniformity. The three primary methods that we have employed for preparation of nanoGUMBOS are summarized in Figure 5. The simplest of these is a reprecipitation method23,30 which, as indicated by its name, involves precipitation of the GUMBOS into nanosize clusters after mixing dilute drops of the GUMBOS solution with a miscible solvent, which acts as a nonsolvent for the GUMBOS, while sonicating. Particles produced using this method are generally spherical. However, one-dimensional (1D) structures have been observed as well.10 In addition, if this approach is template-assisted (e.g., by use of micelles), then the size uniformity of the nanoparticles is significantly improved. The melt−emulsion−quench method4 requires melting the GUMBOS, followed by emulsification and then refreezing. The advantage of this approach is the possibility of additive-free preparations, making it suitable for high-purity uses such as medical applications. Use of porous templates such as track-etched polycarbonate or anodic aluminum oxide membranes with cylindrical pores have also allowed production of 1D nanoGUMBOS such as rods, wires, and tubes.7 However, this approach is limited by throughput, size, and composition of the templating material. Finally, we have recently submitted a manuscript which focuses on
used. However, the most remarkable and well-defined changes in properties (melting points, hydrophobicity, etc.) are usually achieved by using counterions that have been developed and tested over the years for IL chemistry. Despite past efforts to focus on liquid forms of ILs, many of these cation/anion combinations are solids. Production of solids, rather than liquids, has often been viewed as a failure within the IL community. Since our focus is on solid-phase materials, such production would be deemed a success. If the molecule is not a salt, then a charge must be added to the molecule of interest. For example, if a cation is desired, then the most direct approach for synthesis of the cationic part of GUMBOS may involve addition and quaternization of an amine group into the compound (Figure 4). This step is followed by anion exchange in which the counterion is often replaced with an anion, depending on your level of desired hydrophobicity and application, typically from a list of anions previously developed for IL chemistry. Similar procedures exist if you desire to start with an anion, followed by replacement of the counterion using cations from a list of cations previously developed for IL chemistry. It is quite likely that as GUMBOS chemistry progresses, more counterions will be added to the repertoire of possible counterions since solid-phase applications will likely evolve to facilitate applications quite different from liquid-phase applications. 7186
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development and evaluation of various methods and associated parameters for production of nanoGUMBOS. The literature can be consulted for this information or E-mail one of the authors to obtain a reprint.
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ADVANTAGES OF IL CHEMISTRY While the literature abounds with examples of the many benefits of using ILs as solvents, it is important to summarize the distinct advantages of this chemistry in the context of this manuscript. We note that details regarding syntheses and applications of ionic liquid chemistry in analytical chemistry can be found in a recent review.3 Recent published work from our laboratory is used in this manuscript as an example of employing IL chemistry for sensor development.31 Much of the basic premise of that research was to develop a simple strategy for fabrication of optoelectronic tongues or colorimetric-taste sensor arrays as cited in a book by Marks.32 Since aqueous samples are quite common, most of these sensors are required to operate in aqueous environments. However, this approach is inherently challenging, because of interferences from water since the dyes employed in these sensor arrays should be very hydrophobic in order to avoid dissolution in aqueous medium. A more restrictive problem is that the numbers of dyes that fulfill this criterion are limited. In addition, the ability to use the same dyes in both optoelectronic noses and tongues31 is also limited. Another problem is that matrices on which these dyes are printed must be very hydrophobic, often requiring specialized hydrophobic surfaces. Finally, data derived from these sensor arrays are frequently digitized and expressed as difference maps (i.e., a colorimetric representation of a difference graphical representation derived by subtracting the color of the sensor dye matrix exposed to the sample from the color of the original sensor dye matrix). Clearly, such subtraction requires extremely reproducible data. Despite the inherent limitations cited above, very creative research in this area has come from the laboratories of Professor Kenneth Suslick of the University of Illinois33−35 and others.36−39 In fact, it is the creativity of that recent research that has inspired us to investigate whether ILs may be suitable alternatives to the hydrophobic dyes commonly used for such studies. In order to demonstrate the utility of IL chemistry for producing very hydrophobic compounds for such use, we first assembled 12 commonly used hydrophilic and anionic pH indicator dyes with considerable aqueous solubility. We then prepared hydrophobic ILs from these dyes by ion exchange of their counterions (mostly sodium) using the chloride salt of the trihexyl-(tetradecyl)phosphonium cation, also known as P66614, as described in our recent publication.31 All hydrophobic ILs produced from this reaction showed excellent characteristics, including a profound lack of solubility in aqueous media. Moreover, common matrices such as alumina, silica, filter paper, and fabric could be used to produce the desired sensor arrays. Here, we present one example using an array produced using IL-dyed threads sewn onto cotton fabric.31 Only 8 of the 12 ILs were used to produce the cotton threads (Figure 6) used in this example. While preparing these threads, another distinct advantage was observed for this approach: no expensive dewaxing of the thread40 was required, since the ILs dissolved the wax while simultaneously coating the threads. The highly reproducible difference image in Figure 6 is representative of exposing the IL-coated cotton threads to vapors of trifluoroacetic acid (TFA), followed by digitization of the colored thread and then acquisition of a difference map by subtracting
Figure 6. Photonic IL sensor array fabricated from IL-stained cotton threads. (A) Cotton thread spools stained with chemosensory ILs (P refers to the P66614 ion and other initials refer to the dyes).30 (B) (i) sensor array fabricated using a common sewing machine (panel i); digital images of the cotton-thread-based-IL sensor array before exposure (panel ii) and after exposure (panel iii) to trifluoroacetic acid (TFA); and the difference map generated by calculating the red-greenblue (RGB) color differences after exposure to TFA (panel iv).
the TFA exposed image from the original threads image. Suslick33,41 and others37,42 have demonstrated that this approach can be highly reproducible and allows rapid identification of unknowns. Such is also true for our work. However, we have shown that IL chemistry can be used to more easily produce very hydrophobic dyes from very common water-soluble pH dyes and that these dyes are also compatible with commonly used matrices, e.g., cloth, filter paper, silica, and alumina. IL Chemistry for Hydrophobicity in the Solid Phase. In traditional organic chemistry courses, students are taught that increasing the hydrophobicity of a molecule is most easily achieved by adding hydrophobic appendages, e.g., addition of alkyl groups to that molecule. In fact, this approach is among the most common for increasing the hydrophobicity of drug molecules, and many examples in this regard are found in the literature.30,43,44 However, it is also well-established that synthesis of organic molecules involving multiple step reactions can lead to poor product yields and thus to more-expensive final products. Moreover, one can easily argue that the final product, although often more hydrophobic, is a very different molecule than the original and, consequently, may have very different medicinal properties for this reason alone. Thus, the question arises as to whether it is accurate to compare the medicinal properties of the newly synthesized compound to its parent compound. As shown in the example from the previous section, the world of IL chemistry allows simple conversion of a hydrophilic compound to a more hydrophobic compound if the molecule is already a salt. This conversion process often involves an ionexchange reaction where a more hydrophilic counterion is replaced with a more hydrophobic one. Many ions are already ranked as hydrophobic or hydrophilic based on decades of chemistry developed for production of hydrophobic and 7187
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Table 1. Structure of Anions Used with Rhodamine 6G (R6G) and log Ko/w Values of the Resulting GUMBOS
number of GUMBOS were synthesized by using a cationic near-infrared cyanine dye [1,1′,3,3,3′,3′- hexamethylindotricarbo-cyanine (HMT)] and various anions. These GUMBOS displayed a wide range of solubilities and melting points. The solubility in water was largely dependent on the anion employed. Anions such as tetrafluoroborate showed enhanced aqueous solubility, which was postulated to result from hydrogen bonding, accompanied by dipole-induced dipole interactions with the surrounding water molecules. These variations also produced new spectral properties as a result of differences in aggregation of dye molecules in nanoGUMBOS. It was also predicted that anion exchange could lead to control of these aggregate assemblies to produce predominantly H- or J-type aggregates. Particle-size-dependent spectral variations were not observed for particles sizes in the range of 50−300 nm in diameter.12 Anisotropic fluorescence emission studies revealed that soluble GUMBOS had little or no effect on anisotropy in dilute solutions, independent of the anion. For nanoGUMBOS, higher anisotropies were observed with a strong dependence on the particular anion used. These results support an anionic influence model and the diminishing effects of ions dissociated in the soluble state.12 In support of experimental observations, molecular dynamics (MD) simulations reinforced the contention that a more hydrophobic anion induced H-aggregation and low fluorescence yields, while anions with lower hydrophobicity induced J-aggregates and higher fluorescence yields. Further analyses of the stacking angle of the cations based on data from MD simulations showed a strong correlation between experimental and theoretical data, and provided an effective and predictive model for the observed aggregation and fluorescence quantum yields.12 On the basis of the above results, we have also recently studied carbazoleimidazolium-based GUMBOS in DCM solution in order to further examine the effects of anion variations.9 While the iodide form of this GUMBOS compound in DCM has a quantum yield of 0.28, the other anions (OTf, NTf2, and BETI) showed considerably higher respective quantum yields of 0.94, 0.73, and 0.99. These variations are attributed in part to differences in aggregation and also to larger Stokes shifts for the more hydrophobic anions. While quantum yield improvements have been previously observed with polymeric derivatives and other super complexes of carbazole,48,49 GUMBOS anion-pairing are shown to yield comparable improvements with smaller molecules and through the use of much simpler chemistry. Electrosprayed solid films of these GUMBOS showed 7−14 nm red shifts in fluorescence maxima, as compared to methanolic solutions of these same compounds.
hydrophilic ILs. There are many cations classified as hydrophobic and hydrophilic, as well as many anions classified as hydrophobic and hydrophilic. Therefore, these ions can be respectively used with counteranions or countercations to produce desired hydrophobic/hydrophilic properties for a given salt. Recent studies from our laboratory have demonstrated that this IL chemistry is directly applicable to the solid phase, i.e., to GUMBOS.23 In this example, we explore cytotoxicity of the cationic dye, Rhodamine 6G (R6G), which has been studied for decades with one resounding conclusion. The R6G dye is toxic to both normal and cancer cells. However, such studies were almost always done with R6G salts using the most common counteranion of chloride (Cl−). A recent study from our laboratory has examined the change in cytotoxicity of R6G as a result of changing this counteranion.23 In this study, a simple ion-exchange reaction was used for conversion of R6G chloride to different compounds with a variety of counterions whose hydrophobicity is known from IL chemistry. This conversion was followed by measurement of the relative hydrophobicity of each compound using octanol/water partition coefficients (Ko/w). The relative hydrophobicities of the various R6G compounds are displayed in Table 1. The values at the bottom of the figure are the logarithms of the octanol−water partition coefficients for these various compounds of R6G using the counteranions noted in the table. The R6G compounds produced from the anions displayed in Table 1 are all solid-phase compounds and clearly show a trend in hydrophobicity, which is consistent with relative hydrophobicities observed for ILs using these particular counteranions. Moreover, in this GUMBOS approach, development of water-insoluble compounds allowed production of nanoGUMBOS using a facile reprecipitation method.6 By using nanoGUMBOS and in vitro studies, it was determined that, while the hydrophilic forms (e.g., Asc−) of R6G were toxic to both normal and cancer cells, the hydrophobic nanoGUMBOS showed toxicity only to cancer cells.23 In addition, as the doubling rate of various cancer cells increased, the toxicity toward these cells also increased, suggesting greater toxicity toward more-aggressive cancer cells. Fluorescence Properties and Quantum Yields. The literature abounds with examples of near-infrared dyes that are nonfluorescent at high solution concentration due to dye aggregation.45−47 Therefore, there was considerable doubt when we proposed that highly fluorescent nanoparticles could be derived from aggregates of such compounds. Our first report on such fluorescence behavior in the solid state was in 2009,11 followed by a detailed report of the spectral properties of similar compounds in the following year.12 In this latter study, a 7188
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Viscosity and Viscoelasticity of ILs and GUMBOS. By now, it should be apparent that ILs (and therefore GUMBOS) have many unusual properties, i.e., properties that do not necessarily follow traditional chemical logic and intuition that chemists have always used to formulate strategies for developing chemicals with certain physical properties. Thus, it also follows that the associated chemical reactions or physical interactions which produce such properties are not always apparent. Another example of this kind of anomalous behavior of ILs can be found in the recent chemical literature.50 In that study, the authors examined the effects of various molecular solvents on the physical properties of a given IL and noted a very interesting phenomenon. The viscosity of a binary mixture of different molecular solvents with a given IL was dependent primarily on the mole fraction of the molecular solvent and not on its chemical identity. This was a remarkable finding since the molecular solvents included a series of many dissimilar materials such as water, various benzene derivatives, and various nitrile derivatives. In our GUMBOS and IL research, we have observed a phenomenon that correlates with the viscosity effect observed in the research cited above.51 In evaluating various composites (binary blends of GUMBOS or IL and polymer) as absorbents by using a quartz crystal microbalance (QCM) sensor, we noted that, for low vapor concentrations of organic vapors, a plot of the ratio of the change in frequency (Δf) to the change in motional resistance (ΔR) is directly proportional to the molecular weight of absorbed vapor. Since Δf is known from the Sauerbrey equation52 to be directly proportional to the change in mass as a result of absorption of gas, it follows from our observation that ΔR must be proportional to moles of gas, as represented in Figure 7. Despite the obvious proportionality
frequency and dissipation factor at different harmonics were measured during vapor absorption, and analyses of the data revealed that the composite film behaved as a Maxwellian viscoelastic material. In addition, our data and calculations showed that the motional resistance change (or dissipation change) is primarily due to viscosity changes in the coating material. Since the viscosity of ILs or GUMBOS are dependent on the mole fraction of molecular solvent as noted above,50 variation of motional resistance with moles of vapor absorbed is consistent with this observation. Therefore, it is clear that this discovery adds another dimension (molecular weight) to QCM measurements, which have traditionally been simply used for mass detection. Multifunctional Properties. We have also recently demonstrated that multiple properties can be incorporated into single GUMBOS. In one example, we desired to produce a GUMBOS that was tumor-targeting, fluorescent, and magnetic.8 To demonstrate this concept, we synthesized a GUMBOS compound that incorporated a phosphonium derivative (tumor targeting) and a dysprosium ion (fluorescent and paramagnetic). This compound was shown to possess all three properties, as well as toxicity toward cancer cells, which was demonstrated by using in vitro studies and cellular imaging. In other studies, nanoGUMBOS suspensions have been shown to retain the parent material’s properties such as fluorescence, magnetism, and chirality at very low concentrations. This is part of the driving force behind nanoGUMBOS research, which allows unique functional properties at minute concentrations of GUMBOS which can reduce cost, viscosity, and potential toxicity.
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CONCLUSIONS AND FUTURE DIRECTIONS Since our first report on group of uniform materials based on organic salts (GUMBOS) a few years ago,4 we have demonstrated, via several publications already in the literature,4−12,23,51,53−63 that these materials have remarkable utility in a wide range of fields. However, we believe that the applications that we have demonstrated thus far are only a small sampling of possibilities in materials, biomedical, and analytical chemistries. We believe that the literature on GUMBOS (or equivalent chemistry) will continue to increase as more examples of this type of chemistry are published. In fact, there are already a few examples of this type of chemistry which has not been categorized as solid-phase ionic liquid (IL) chemistry (GUMBOS chemistry). For example, after years of searching, we have recently found examples of this type of chemistry buried in the literature. Examples of such GUMBOS chemistry include varying types of applications.64−67 While it is not apparent that all of these references are related to GUMBOS, close examination of these manuscripts will indeed show relevance. In fact, some of these are not even classified as related to IL chemistry. In addition, although not focused in a comprehensive manner as our studies, we believe that other such studies exist in the literature. Finally, we believe that the convenience and simplicity of GUMBOS chemistry will enable many applications in far reaching fields such as solid-state ionics and photonics. For example, polymerization of ILs has served as a common practice to produce solid-state ILs,68,69 even though the process is complicated and time-consuming. Other related studies for developing ionic nanoparticle networks have also been reported.70 We also note that various formulations using counterions have been shown to greatly influence properties of
Figure 7. Variation of ΔR with number of moles of vapors absorbed. The coating material used is a binary blend of 1-butyl-3methylpyridinium hexafluorophosphate (∼90%) and cellulose acetate (∼10%). The amount of coating material, as calculated by using the Sauerbrey equation, is 65 μg/cm2.
shown in this plot, it was difficult for reviewers to believe the theoretical basis for this observation. This is because these results are substantially different from other observations in the literature with other absorbents. While we have only observed this phenomenon for composites of GUMBOS or very viscous ILs, it is the use of GUMBOS which allowed this initial discovery. In order to fully understand the theoretical basis of this observation, we have conducted detailed studies using a quartz crystal microbalance with dissipation monitoring (QCM-D), and the results are outlined in a recent publication.53 The 7189
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(15) Jastorff, B.; Stormann, R.; Ranke, J.; Molter, K.; Stock, F.; Oberheitmann, B.; Hoffmann, W.; Hoffmann, J.; Nuchter, M.; Ondruschka, B.; Filser, J. Green Chem. 2003, 5, 136−142. (16) Frade, R. F.; Afonso, C. A. Hum. Exp. Toxicol. 2010, 29, 1038− 1054. (17) Mohammad Fauzi, A. H.; Amin, N. A. S. Renew. Sustainable Energy Rev. 2012, 16, 5770−5786. (18) Zhao, D.; Liao, Y.; Zhang, Z. CLEAN−Soil, Air, Water 2007, 35, 42−48. (19) Hough, W. L.; Smiglak, M.; Rodriguez, H.; Swatloski, R. P.; Spear, S. K.; Daly, D. T.; Pernak, J.; Grisel, J. E.; Carliss, R. D.; Soutullo, M. D.; Davis, J. J. H.; Rogers, R. D. New J. Chem. 2007, 31, 1429−1436. (20) Rogers, R. D.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1077−1078. (21) Rogers, R. D. Nature 2007, 447, 917−918. (22) Sowmiah, S.; Srinivasadesikan, V.; Tseng, M.-C.; Chu, Y.-H. Molecules 2009, 14, 3780−3813. (23) Magut, P. K. S.; Das, S.; Fernand, V. E.; Losso, J.; McDonough, K.; Naylor, B. M.; Aggarwal, S.; Warner, I. M. J. Am. Chem. Soc. 2013, 135, 15873−15879. (24) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351−356. (25) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391− 1398. (26) Weingärtner, H. Angew. Chem., Int. Ed. 2008, 47, 654−670. (27) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123− 150. (28) Carter, E. B.; Culver, S. L.; Fox, P. A.; Goode, R. D.; Ntai, I.; Tickell, M. D.; Traylor, R. K.; Hoffman, N. W.; Davis, J. J. H. Chem. Commun. 2004, 630−631. (29) Bica, K.; Rodriguez, H.; Gurau, G.; Andreea Cojocaru, O.; Riisager, A.; Fehrmann, R.; Rogers, R. D. Chem. Commun. 2012, 48, 5422−5424. (30) Kasai, H.; Murakami, T.; Ikuta, Y.; Koseki, Y.; Baba, K.; Oikawa, H.; Nakanishi, H.; Okada, M.; Shoji, M.; Ueda, M.; Imahori, H.; Hashida, M. Angew. Chem., Int. Ed. 2012, 51, 10315−10318. (31) Galpothdeniya, W. I. S.; McCarter, K. S.; De Rooy, S. L.; Regmi, B. P.; Das, S.; Hasan, F.; Tagge, A.; Warner, I. M. RSC Adv. 2014, 4, 7225−7234. (32) Marks, R. Handbook of Biosensors and Biochips; John Wiley & Sons: Chichester, U.K., 2007. (33) Suslick, K. S. MRS Bull. 2004, 29, 720−725. (34) Feng, L.; Musto, C. J.; Kemling, J. W.; Lim, S. H.; Zhong, W.; Suslick, K. S. Anal. Chem. 2010, 82, 9433−9440. (35) Suslick, B. A.; Feng, L.; Suslick, K. S. Anal. Chem. 2010, 82, 2067−2073. (36) Salinas, Y.; Ros-Lis, J. V.; Vivancos, J.-L.; Martinez-Manez, R.; Marcos, M. D.; Aucejo, S.; Herranz, N.; Lorente, I. Analyst 2012, 137, 3635−3643. (37) Huang, X.; Xin, J.; Zhao, J. J. Food Eng. 2011, 105, 632−637. (38) Huo, D.-Q.; Zhang, G.-P.; Hou, C.-J.; Dong, J.-L.; Zhang, Y.-C.; Liu, Z.; Luo, X.-G.; Fa, H.-B.; Zhang, S.-Y. Chin. J. Anal. Chem. 2010, 38, 1115−1120. (39) Gouma, P.; Sberveglieri, G. MRS Bull. 2004, 29, 697−702. (40) Reches, M.; Mirica, K. A.; Dasgupta, R.; Dickey, M. D.; Butte, M. J.; Whitesides, G. M. ACS Appl. Mater. Interface 2010, 2, 1722− 1728. (41) Zhang, C.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 11548− 11549. (42) Sen, A.; Albarella, J. D.; Carey, J. R.; Kim, P.; McNamara, W. B., III. Sens. Actuators B 2008, 134, 234−237. (43) Breton, M.; Prevel, G.; Audibert, J.-F.; Pansu, R.; Tauc, P.; Pioufle, B. L.; Francais, O.; Fresnais, J.; Berret, J.-F.; Ishow, E. Phys. Chem. Chem. Phys. 2011, 13, 13268−13276. (44) Kandela, I.; Lee, W.; Indig, G. Biotechnol. Histochem. 2003, 78, 157−169. (45) Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. Rev. 1992, 92, 1197−1226. (46) Kim, J. S.; Kodagahally, R.; Strekowski, L.; Patonay, G. Talanta 2005, 67, 947−954.
GUMBOS including hydrophobicity, stability, melt properties, and optical properties. While our current efforts have been mostly focused on exploring the tunability of GUMBOS, we are also interested in coupling functional cations with functional anions. This latter approach could lead to many applications, e.g., functional GUMBOS that carry magnetic receptivity and chemotherapeutic properties for use as active drugs that can be guided for targeted delivery. We foresee that suitable formulations for pure GUMBOS or composites of GUMBOS will enable competitive performances such as high ionic conductivity, better optoelectronics, or impressive fluorescence quantum yields for use in imaging, optoelectronic, and theranostic applications. In summary, we have only scratched the surface of possibilities for this type of chemistry in the solid phase. We hope that this manuscript will inspire others to join us in our quest for tunable solid-phase materials using GUMBOS technology.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 225-578-2829. Fax: 225-578-3971. E-mail: iwarner@lsu. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge useful suggestions and discussions from Dr. Bishnu P. Regmi, Mr. Waduge Indika S. Galpothdeniya, and Dr. Paul K. S. Magut during preparation of this manuscript. Some of the materials described in this manuscript are based upon work supported by the National Science Foundation (NSF) under Grant Nos. CHE-1307611 and CHE-1243916.
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REFERENCES
(1) Walden, P. Bull. Russ. Acad. Sci. 1914, 405−422. (2) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792−793. (3) Anderson, J. L.; Armstrong, D. W.; Wei, G.-T. Anal. Chem. 2006, 78, 2892−2902. (4) Tesfai, A.; El-Zahab, B.; Bwambok, D. K.; Baker, G. A.; Fakayode, S. O.; Lowry, M.; Warner, I. M. Nano Lett. 2008, 8, 897−901. (5) Tesfai, A.; El-Zahab, B.; Kelley, A. T.; Li, M.; Garno, J. C.; Baker, G. A.; Warner, I. M. ACS Nano 2009, 3, 3244−3250. (6) Warner, I. M.; Tesfai, A.; El-Zahab, B. M.; Bwambok, D.; Baker, G. A.; Fakayode, S. O.; Lowry, M.; Tolocka, M. P.; De Rooy, S. PCT/ US2008/086065, 2011. (7) de Rooy, S. L.; El-Zahab, B.; Li, M.; Das, S.; Broering, E.; Chandler, L.; Warner, I. M. Chem. Commun. 2011, 47, 8916−8918. (8) Li, M.; Ganea, G. M.; Lu, C.; De Rooy, S. L.; El-Zahab, B.; Fernand, V. E.; Jin, R.; Aggarwal, S.; Warner, I. M. J. Inorg. Biochem. 2012, 107, 40−46. (9) Siraj, N.; Hasan, F.; Das, S.; Kiruri, L. W.; Steege Gall, K. E.; Baker, G. A.; Warner, I. M. J. Phys. Chem. C 2014, 118, 2312−2320. (10) de Rooy, S. L.; Das, S.; Li, M.; El-Zahab, B.; Jordan, A.; Lodes, R.; Weber, A.; Chandler, L.; Baker, G. A.; Warner, I. M. J. Phys. Chem. C 2012, 116, 8251−8260. (11) Bwambok, D. K.; El-Zahab, B.; Challa, S. K.; Li, M.; Chandler, L.; Baker, G. A.; Warner, I. M. ACS Nano 2009, 3, 3854−3860. (12) Das, S.; Bwambok, D.; El-Zahab, B.; Monk, J.; de Rooy, S. L.; Challa, S.; Li, M.; Hung, F. R.; Baker, G. A.; Warner, I. M. Langmuir 2010, 26, 12867−12876. (13) Jordan, A. N.; Das, S.; Siraj, N.; de Rooy, S. L.; Li, M.; El-Zahab, B.; Chandler, L.; Baker, G. A.; Warner, I. M. Nanoscale 2012, 4, 5031− 5038. (14) De Jong, W. H.; Borm, P. J. Int. J. Nanomed. 2008, 3, 133. 7190
dx.doi.org/10.1021/ac501529m | Anal. Chem. 2014, 86, 7184−7191
Analytical Chemistry
Perspective
(47) Otsuka, A.; Funabiki, K.; Sugiyama, N.; Mase, H.; Yoshida, T.; Minoura, H.; Matsui, M. Chem. Lett. 2008, 37, 176−177. (48) Kotchapradist, P.; Prachumrak, N.; Tarsang, R.; Jungsuttiwong, S.; Keawin, T.; Sudyoadsuk, T.; Promarak, V. J. Mater. Chem. C 2013, 1, 4916−4924. (49) Linton, K. E.; Fisher, A. L.; Pearson, C.; Fox, M. A.; Palsson, L.O.; Bryce, M. R.; Petty, M. C. J. Mater. Chem. 2012, 22, 11816−11825. (50) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275−2287. (51) Regmi, B. P.; Monk, J.; El-Zahab, B.; Das, S.; Hung, F. R.; Hayes, D. J.; Warner, I. M. J. Mater. Chem. 2012, 22, 13732−13741. (52) Sauerbrey, G. J. Phys. 1959, 155, 206−212. (53) Regmi, B. P.; Speller, N. C.; Anderson, M. J.; Brutus, J. O.; Merid, Y.; Das, S.; El-Zahab, B.; Hayes, D.; Murray, K. K.; Warner, I. M. J. Mater. Chem. C 2014, 2, 4867−4878. (54) Dumke, J. C.; El-Zahab, B.; Challa, S.; Das, S.; Chandler, L.; Tolocka, M.; Hayes, D. J.; Warner, I. M. Langmuir 2010, 26, 15599− 15603. (55) Cole, M. R.; Li, M.; El-Zahab, B.; Janes, M. E.; Hayes, D.; Warner, I. M. Chem. Biol. Drug Des. 2011, 78, 33−41. (56) Das, S.; de Rooy, S. L.; Jordan, A. N.; Chandler, L.; Negulescu, I. I.; El-Zahab, B.; Warner, I. M. Langmuir 2011, 28, 757−765. (57) Lu, C.; Das, S.; Magut, P. K. S.; Li, M.; El-Zahab, B.; Warner, I. M. Langmuir 2012, 28, 14415−14423. (58) Cole, M. R.; Li, M.; Jadeja, R.; El-Zahab, B.; Hayes, D.; Hobden, J. A.; Janes, M. E.; Warner, I. M. J. Antimicrob. Chemother. 2013, 68, 1312−1318. (59) Sarkar, A.; Kanakamedala, K.; Rajathadripura, M. D.; Jagadish, N. N.; Magut, P. K.; de Rooy, S.; Das, S.; El-Zahab, B.; Warner, I. M.; Daniels-Race, T. Electron. Mater. Lett. 2014, DOI: 10.1007/s13391014-3347-8. (60) Dumke, J. C.; Qureshi, A.; Hamdan, S.; Rupnik, K.; El-Zahab, B.; Hayes, D.; Warner, I. M. Photochem. Photobiol. Sci. 2014, DOI: 10.1039/C4PP00030G. (61) Jordan, A. N.; Siraj, N.; Das, S.; Warner, I. M. RSC Adv. 2014, 4, 28471−28480. (62) Galpothdeniya, W. I. S.; Das, S.; De Rooy, S. L.; Regmi, B. P.; Hamdan, S.; Warner, I. M. RSC Adv. 2014, 4, 17533−17540. (63) Berton, P.; Regmi, B. P.; Spivak, D. A.; Warner, I. M. Microchim. Acta 2014, DOI: 10.1007/s00604-014-1261-2. (64) Shigeyasu, M.; Murayama, H.; Tanaka, H. Chem. Phys. Lett. 2008, 463, 373−377. (65) Ku, B. K.; Fernandez de la Mora, J. J. Phys. Chem. B 2004, 108, 14915−14923. (66) Armitage, B.; Retterer, J.; O’Brien, D. F. J. Am. Chem. Soc. 1993, 115, 10786−10790. (67) Ou, Z.-m.; Yao, H.; Kimura, K. J. Photochem. Photobiol. A: Chem. 2007, 189, 7−14. (68) Yoshizawa, M.; Ogihara, W.; Ohno, H. Polymer. Adv. Technol. 2002, 13, 589−594. (69) Yoshizawa, M.; Ohno, H. Electrochim. Acta 2001, 46, 1723− 1728. (70) Marie-Alexandra, G.; Neouze, G.; Kronstein, M.; Tielens, F. Chem. Commun., 2014, DOI: 10.1039/C4CC02419B
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