Polymers for Anion Recognition and Sensing

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Rapid Communications

Polymers for Anion Recognition and Sensing

Ali Rostami, Mark S. Taylor*

In biological systems, the selective and high-affi nity recognition of anionic species is accom-plished by macromolecular hosts (anion-binding proteins) that have been “optimized” through evolution. Surprisingly, it is only recently that chemists have systematically attempted to develop anion-responsive synthetic macromolecules for potential applications in medi-cine, national security, or environmental monitoring. Recent results indicating that unique features of polymeric systems such as signal amplifi cation, multivalency, and cooperative behavior may be exploited productively in the context of anion recognition and sensing are documented. The wide variety of interactions—including Lewis acid/base, ion-pairing, and hydrogen bonding—that have been employed for this purpose is refl ected in the structural diversity of anion-responsive macromolecules identifi ed to date.

1. Introduction

Molecules capable of the selective recognition and sensing of anions represent an intriguing class of targets, with implications in medical diagnostics, environmental moni-toring and remediation, and the discovery of biological probes or potential therapeutic agents. [ 1 , 2 ] Anions pose unique challenges for molecular recognition: they are characterized by diverse geometries, charge distributions and sizes, and their high enthalpies of hydration present a signifi cant obstacle to developing receptors able to func-tion in aqueous medium. The range of potential appli-cations and the challenging nature of the problem have motivated intensive research efforts in the area of anion recognition in recent years. [ 3 ]

The exceptional affi nities and selectivities displayed by naturally occurring anion-binding proteins serve as a source of inspiration for the development of synthetic systems. For example, the crystal structure of a sulfate-binding protein indicates that anion binding is achieved without the direct participation of positively charged

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A. Rostami , Prof. M. S. Taylor Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON M5S 3H6, Canada E-mail: [email protected]

moieties, but through precisely positioned hydrogen-bonding interactions between the sulfate anion and the NH groups of the protein backbone, serine OH, or tryp-tophan NH groups. [ 4 ] The principles of complementarity and preorganization underlie the design of synthetic anion receptors and sensors able to interact with ana-lytes through hydrogen bonding, Lewis acid–base com-plexation, Coulombic interactions, and other noncova-lent contacts (e.g., anion–arene interactions and halogen bonding). [ 3 ] Research reported over the past few decades has identifi ed a wide variety of functional groups and molecular architectures for this purpose, and thermody-namic studies have provided insight into the roles played by such factors as conformational rigidity and solvation. In several instances, these discoveries have been exploited in the development of sensors for “real-life” monitoring of analyte concentration in complex environments. [ 5 ] Chal-lenges remaining for the fi eld include the development of receptors capable of selective and high-affi nity anion recognition in water: in this regard, the performance of Nature’s anion-binding proteins remains unparalleled by synthetic systems.

The development of anion-responsive polymeric sys-tems has recently emerged as an exciting direction for research in anion recognition. Macromolecules offer unique opportunities for achieving sensitivity and selectivity by mechanisms that take advantage of their

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Mark S. Taylor received his B.Sc. in chemistry in 2000 from the University of Toronto, where he carried out research in the group of Mark Lautens. Mark pursued graduate studies at Harvard from 2000 to 2005 under the supervi-sion of Prof. Eric Jacobsen, working in the area of enantioselective catalysis. After completing his Ph.D., he took up a postdoctoral fellowship at the Massachusetts Institute of Technology where he developed new types of conjugated polymers in the research group of Prof. Timothy Swager. In July 2007, he returned to the University of Toronto as an Assistant Professor in the Department of Chemistry. His current research interests are at the interface of the areas of catalysis and supramolecular/materials chemistry, and include catalyst-controlled func-tionalization of sugars, fundamental studies of halogen bonding interactions, and new anion-responsive molecules and materials.

Ali Rostami obtained his B.Sc. degree (Honors) in chemistry from University of Mazandaran (Iran) in 2001, and his M.Sc. degree in 2006 from University of Alberta (Canada) working on heterocyclic synthesis utilizing domino Nazarov-Schmidt processes in the group of Prof. F. G. West. In 2007, he started his Ph.D. studies under the supervision of Prof. Mark Taylor at University of Toronto, working on the synthesis, sensory, and self-assembly properties of polysquaramides.

distinct properties and behaviors; [ 6 ] however, anion sen-sory materials have emerged slowly in comparison to those for cation sensing. This article will discuss recent advances in polymer-based anion-responsive materials, with an emphasis on the wide structural diversity of polymers that can be employed for this purpose, and the mechanisms of their anion-induced responses. Following an introduction to the key concepts that form the basis of polymer-based chemical sensing schemes, the applica-tion of these concepts to anion-responsive systems will be discussed; the polymers are grouped according to the nature of their interaction with anionic analytes (Lewis acid–base, Coulombic, hydrogen bonding, etc.). The sensing of polyanionic biomolecules (particularly, DNA), which has been the subject of several recent review arti-cles, will not be covered, [ 7 ] and the focus will instead be placed on the recognition of nonpolymeric anionic ana-lytes. Likewise, anion sensors immobilized in “inert” polymer matrices [ 8 ] will not be discussed, and systems in which the polymeric architecture has a demonstrated effect on the sensitivity and/or selectivity of the system will be emphasized.

2. Polymer-Based Chemical Sensors: General Principles

Polymers possess unique properties that can offer special advantages for the development of analyte-responsive systems. Several classes of conjugated polymers display optoelectronic properties that can be harnessed for signal transduction by electrochemical, colorimetric, or fl uores-cence techniques; mechanisms for signal transduction include analyte-induced changes in polymer conforma-tion that result in modulation of the effective conjugation length, or the introduction or destruction of local traps, giving rise to fl uorescence quenching or unquenching events. The ability to process such materials into thin fi lms or membranes facilitates their integration into func-tional devices, and the repeating structures of polymers may give rise to effects such as multivalency or coop-erativity in the context of supramolecular interactions. However, the most striking distinctions between small molecule and polymeric chemical sensors often arise from the ability of the latter to achieve signal amplifi cation , giving rise to high levels of sensitivity for the analyte of interest. The ability of fl uorescent conjugated polymers to function as amplifying chemical sensors was elucidated more than 15 years ago [ 9 ] and the concept forms the basis of several high-performance sensory materials for use in trace detection and biosensing. Although a brief descrip-tion of this phenomenon is provided below, the reader is referred to several authoritative review articles and lead references for more comprehensive discussions. [ 6 ]

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Studies by Zhou and Swager [ 9 ] of the fl uorescence quenching-based response of cyclophane-based poly-(phenyleneethynylenes) to bis(pyridinium) ions demon-strated that the incorporation of analyte-binding motifs gave rise to signal gain due to the “molecular wire effect ” (Figure 1 ). Control experiments using nonpolymeric cyclo-phane receptor 1 indicated that polymer 2 exhibited a 65-fold enhancement of sensitivity (as measured by the Stern–Volmer constants K SV for static quenching), resulting from the ability of excitons to diffuse along the polymer chain and thus to “sample” the occupancy of mul-tiple analyte -binding sites. Later studies (particularly in the context of the detection of ultratrace concentrations of trinitrotoluene and related nitroaromatics) revealed that harnessing inter- in addition to intrapolymer exciton diffusion (e.g., in thin fi lms) provides a mechanism for increased levels of signal amplifi cation. [ 10 ] The concepts emerging from this pioneering work have proved to be remarkably general, and underlie the application of con-jugated polymers for the detection of diverse analytes in a wide variety of media.

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Figure 1 . Signal “gain” in fl uorescence quenching of conjugated polymers by a paraquat derivative.

3. Anion-Responsive Polymers

Several of the pioneering studies illustrating the ability of analytes to effect changes in the conformation, bandgap, or aggregation behavior of polymers, and the results of these changes on photophysical properties or conductivity were conducted in the context of cation detection. [ 11 ] None-theless, an expanding body of research illustrates that ani-onic analytes also represent interesting targets for these approaches. A remarkable feature of this work is the wide structural diversity of the polymers that have been discov-ered for anion recognition. This diversity is highlighted in

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Figure 2 . (a) Anion-induced desilylation/cyclization as the basis for fl u(c) cyanide detection by conjugate addition to a dicyanovinyl-substit

the discussion that follows: anion-responsive polymers are grouped by structure and according to the nature of the functional group that is proposed to interact with the anionic species. The principle of signal amplifi cation out-lined in the previous section has played a key role in these efforts: several interesting polymer backbones that enable the transduction of anion-binding events into measur-able signals have been devised. In addition, polymer-based phenomena other than the “molecular wire ” effect—including cooperativity and multivalency—have been brought to bear on the problem of anion sensing. The increased sensitivity or affi nity resulting from these effects has enabled applications of polymers to solve chal-lenging problems in anion recognition, including devel-oping sensors that function in aqueous environments.

4. Anion-Responsive Chemodosimeters

Although the majority of the anion-responsive polymers developed to date have been designed to interact revers-ibly with their analytes (see below), anion-induced, irre-versible covalent bond formation, or cleavage represents another viable detection method. An implementation of this concept in the context of a polymeric anion sensor was reported by Kim and Swager [ 12 ] who exploited the reactivity of fl uoride toward Si–O bonds as a basis for selective anion detection (Figure 2 ). Reaction of polymer 3a with fl uoride [ n -Bu 4 N + F − , 1.6 × 10 − 7 M in tetrahydrofuran (THF)] triggered cleavage of the silyl ether group and cyclization to the cor-responding coumarin (Figure 2 a), accompanied by a sig-nifi cant redshift of the polymer emission spectrum. Control experiments with an alkylidenemalonate not appended to a poly(phenyleneethynylene) indicated that the degree of

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oride detection; (b) a fl uoride-responsive polymeric chemodosimeter; uted fl uorescent polymer.


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amplifi cation resulting from exciton migration to the newly generated fl uorophore was approximately 100 fold.

A fl uorescent, polymeric chemodosimeter for the selec-tive detection of cyanide anion has been developed, taking advantage of the reactivity of cyanovinyl groups as elec-trophiles in conjugate addition processes. [ 13 ] Addition of Bu 4 N + CN − to a solution of cross-conjugated polymer 3b in N,N -dimethylformamide (DMF) was accompanied by a blueshift of the absorption spectrum and quenching of the polymer fl uorescence at 597 nm. The polymer displayed a high level of selectivity for cyanide over other anions (hal-ides, azide, phosphate, sulfate, acetate, phenolate) or poten-tial nucleophiles (amines and benzenethiol). Comparison with a nonpolymeric model compound indicated that both the magnitude of the cyanide-induced fl uorescence quench (31.6 fold vs 10.3 fold) and the detection limit (14 ppb vs 70 ppb) in DMF were improved by incorporation of the reactive functional group into the polymeric system.

5. Lewis Acid–Base Interactions: Organoboron, Organosilicon, and Metal Cation-Containing Anion-Responsive Polymers

Anions often participate in strong Lewis acid–base inter-actions, and a number of receptors have been identifi ed


Figure 3 . (a) Representative anion-responsive organoboron polymertral changes.

that employ Lewis acidic main group or metal-based functional groups to achieve high anion affi nity. [ 14 ] Like-wise, diverse strategies for incorporating Lewis acidic sites into stimulus-responsive polymers have been devel-oped. The macromolecules employed include boron- and silicon-based polymers, as well as materials that incorpo-rate metal cations.

Organoboron-based polymers display interesting opto-electronic properties when the boron-based p orbitals contribute to the conjugated π -system of the polymer backbone. [ 15 ] The coordination environment of the boron centre (tricoordinate vs tetracoordinate) may have a sig-nifi cant infl uence on the effective conjugation length, providing a mechanism for transducing anion-binding events into measurable signals. A pioneering applica-tion of anion sensing using organoboron-based conju-gated polymers was reported by Miyata and Chujo: [ 16 ] poly(vinylenephenylenevinylene borane) 4a (Figure 3 ) displayed a fl uorescence quenching response to fl uoride anion in chloroform solvent. The response was selective for fl uoride over the other halide anions, and the observation of complete quenching upon addition of 0.5 equivalents of Bu 4 N + F − ( ≈ 10 − 6 M in CHCl 3 ) indicated some degree of signal amplifi cation relative to a nonpolymeric borane species.

In recent years, several novel anion-responsive poly-mers bearing Lewis acidic borane functional groups have

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s; (b) two-step binding process of 4c inferred from anion-induced spec-


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Figure 4 . Structure of an anion-responsive, fl uorescent poly(silane).

been developed. [ 17 ] Polyfl uorene 4b containing dibenzobo-role groups displayed a fl uorescence quenching-based response to F − , CN − , and I − (Bu 4 N + counterion, THF sol-vent): fl uoride concentrations as low as 0.1 mM could be detected using a fi lm of 4b prepared by spin coating. [ 17b ] Li and Jäkle [ 17 c] developed effi cient transmetallation-based protocols for the synthesis of fl uorene-based organobo-rane polymers, in which bulky mesityl or tris(isopropyl)phenyl (Tip) groups provide steric protection from air oxi-dation. Polymer 4c showed selective colorimetric and fl u-orescence responses to tetrabutylammonium fl uoride and cyanide in THF solvent, but was nonresponsive to larger, less basic anions (Cl − , Br − ). A two-step binding process was inferred from the nature of the spectral changes: initial binding (up to ≈ 0.5 equivalents of anion) was proposed to occur at alternating boron sites, resulting in a redshifted emission maximum consistent with charge transfer between adjacent tetracoordinate (electron-rich) and tri-coordinate (electron-defi cient) moieties. Further addition of anion resulted in occupation of the remaining Lewis acidic sites.

Although incorporation of the boron-based p orbital into the main chain of a π -conjugated system has been identifi ed as a useful design principle, a number of anion-responsive boron-containing materials that do not benefi t from this type of conjugation have been devel-oped. [ 18 ] For example, functionalization of polystyrene with thiophene-substituted arylboranes was achieved through a series of organometallic transformations of poly(4-trimethylsilylstyrene). [ 17a ] Changes in the absorb-ance and emission spectra of the resulting materials sig-naled the presence of Bu 4 N + F − and CN − in THF solvent. Analysis of Stern–Volmer quenching constants indicated roughly eightfold signal amplifi cation for polymer 4d relative to a nonpolymeric model compound: intrap-olymer exciton migration was proposed as a mechanism for this effect. The borasiloxane-based polythiophenes synthesized by Lee and co-workers [ 17 d] are another structurally distinct class of organoboron polymers that have been employed for chemical sensing. Redshifts of the absorption and emission spectra of polymer 4e relative to model compounds were consistent with some degree of through-space electronic communication through the borasiloxane cages. Although detection of amine vapors was pursued as the major sensory application of 4e , a fi lm of the polymer on an indium tin oxide-coated glass electrode displayed a green-to-orange colorimetric response to Bu 4 N + F − in THF solvent.

Polysilanes display interesting optoelectronic proper-ties that result from the delocalization of σ -conjugated electrons along the polymer backbone. This feature, com-bined with the high fl uoride affi nity of many organosil-icon compounds, has been exploited in anion recognition by Fujiki and co-workers. [ 19 ] Fluoroalkylated polysilane 5

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(Figure 4 ) displayed a sensitive fl uorescence quenching-based response to nanomolar concentrations of fl uoride anion (Bu 4 N + counterion, THF solvent), with a Stern–Volmer quenching constant of 1.35 × 10 7 M − 1 indicating signifi cant levels of amplifi cation due to exciton migra-tion in this system. The polymer displayed a high level of selectivity for fl uoride, being unresponsive to other anions such as bromide and chloride. The structure of 5 was tuned to enhance the fl uoride affi nity of the polysi-lane backbone: replacement of the electron-withdrawing fl uoroalkyl groups by nonfl uorinated moieties resulted in a loss of sensitivity of more than three orders of magni-tude, while replacing the methyl substituents with more sterically encumbered alkyl groups led to a loss of the fl u-orescence response.

As alluded to above, metal cations have been pursued for more than a decade as targets of polymer-based chemical sensors. [ 11 ] The polymers obtained upon cation binding may be viewed as potential anion-responsive materials, in which interactions between the analyte and polymer-bound metal ion result in alterations of the optoelectronic properties of the polymer. A sensory scheme of this type was implemented by Schanze and co-workers, [ 20 a] using carboxylate-functionalized poly(phenyleneethynylene) 6a as an amplifying fl uorescent indicator (Figure 5 ). The conjugated polyelectrolyte was shown to interact strongly with Cu 2 + ions, resulting in fl uorescence quenching with a K SV of approximately 10 6 M − 1 in aqueous HEPES buffer. Titration of the polymer–Cu 2 + adduct with pyrophosphate (PPi) anion resulted in a recovery of fl uorescence (30-fold enhancement in the presence of 20 mM PPi): an analytical detection limit of 80 nM was estimated. The system was shown to be selective for PPi over other monovalent and divalent anions (F − , Cl − , Br − , I − , HSO 4 − , NO 3 − , HCO 3 − , H 2 PO 4 − , CH 3 CO 2 − , SO 4 2 − , CO 3 2 − , HPO 4 2 − ), and a control experiment with the metal chelator ethylenediaminetetraacetic acid provided support for a mechanistic hypothesis involving sequestration of Cu 2 + by the PPi anion. The system was employed in a real-time assay of the activity of an alkaline phosphatase enzyme. Bunz and co-workers [ 20 b] described another effi cient sensor for pyrophosphate based on a carboxylate-functionalized poly(phenylene ethynylene) (PPE): in their study, interaction of the anionic PPE with

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Figure 5 . (a) Turn-on fl uorescence response of an anionic PPE–Cu 2 + adduct to pyrophosphate; (b) representative metal-bound polymers for applications in anion detection.

10 nm cobalt–iron spinel nanoparticles resulted in quenching of the polymer fl uorescence. Addition of PPi to the polymer–nanoparticle adduct effected a fl uorescence recovery that enabled the detection of high nanomolar


concentrations of the analyte, even in the presence of rel-atively high concentrations (0.1 mM) of phosphate anion.

A number of related systems involving anion-induced “unquenching” of metal-bound fl uorescent polymers

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have been developed. Figure 5 illustrates some of the structurally distinct classes of macromolecules that have been employed for this purpose. [ 21 ] Li and co-workers [ 21 a,b] have developed imidazole-functionalized polyacetylene 6b and polyfl uorene 6c . Both polymers have been applied for the detection of cyanide (Bu 4 N + counterion, ethanol, and THF solvent, respectively) by a “turn-on” fl uorescence mechanism in which polymer–Cu 2 + interactions are dis-rupted upon addition of anion. Polymer 6c has also been employed in combination with gold nanoparticles (Au NPs) as a system for cyanide detection: the Au NPs were effi cient quenchers of the polymer fl uorescence (presum-ably through interactions between the metal surface and

Figure 6 . Anion-responsive cationic polymers.

the imidazole functional groups), but addition of cyanide led to displacement of the polymer from the nanoparticles, accompanied by recovery of the poly-mer-based emission. [ 21c ] Interactions between Fe 3 + and the benzimidazole moieties of polymer 6d form the basis of a method for the detection of phos-phates (H 2 PO 4 − , HPO 4 2 − ) reported by the group of Iyer. [ 21d,f ] The system was highly selective for phosphate, with pyrophosphate and polyphosphate being the only interfering anions iden-tifi ed, and was sensitive to concentra-tions as low as 6.6 × 10 − 6 M . In each of the systems described above, the revers-ibility of the anion-binding process has not been demonstrated, and it is pos-sible that the polymer–metal anion adducts act as dosimeters rather than sensors. Nonetheless, the versatility of this general approach, in which both the functional group interacting with the metal ion and the identity of the metal itself may be tuned, is apparent from the range of anionic analytes that have been successfully targeted in the applications reported to date.

6. Ion-Pairing Interactions: Anion Recognition by Cationic Polyelectrolytes

Taking advantage of ion-pairing inter-actions through the use of cationic receptors has been a key design prin-ciple for achieving anion recognition in competitive (aqueous) medium. Indeed, the majority of “small-molecule” anion receptors that function in water are



positively charged species. [ 22 ] Likewise, polymers bearing cationic substituents represent interesting candidates for use as anion-responsive macromolecules. Although sensory applications of cationic polyelectrolyes have gen-erally been targeted toward DNA, a number of examples illustrate the utility of such macromolecules in high-affi nity detection of mono- or divalent anions in aqueous medium (Figure 6 ).

One of the earliest reports of an anion-responsive organic polymer is the work of Tour and Brockmann, [ 23 ] who observed that zwitterionic polymer 7a displayed a near-IR colorimetric response to alkali metal iodide salts in methanol solvent. Control experiments indicated

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that cation binding to the triethylene glycol moieties was required for the iodochromic response, and that the nucleophilicity of the halide anion also played an impor-tant role. The results were interpreted in terms of a cat-ion-assisted nucleophilic attack of iodide on the carbonyl ylide functional groups. More recently, the group of Leclerc has developed a cationic, water-soluble imidazolium-bearing polythiophene derivative ( 7b ) that displays selec-tive colorimetric and fl uorescence responses to iodide anion in deionized water. [ 24 ] The absorption spectrum of the polymer in the presence of aqueous NaI was similar to that of the polymer alone in the solid state, suggesting an anion-induced aggregation mechanism. The redshift of the absorption spectrum was accompanied by a sig-nifi cant decrease in emission intensity upon addition of iodide (concentrations as low as 2 × 10 − 6 M ). Noteworthy aspects of this system include its selectivity for iodide, [ 25 ] its ability to function in pure aqueous medium, and the dependence of the sensory properties on subtle structural features of the polymer (e.g., polythiophene 7c was con-siderably less sensitive toward iodide than 7b ).

Another example of anion detection by an imidazolium-functionalized conjugated polymer was reported recently by Lee and co-workers, [ 26 ] who described applications of polydiacetylene 7d for the detection of anionic surfactants in aqueous medium. Surfactants are applied exten-sively in various industrial settings and are of concern as environmental pollutants. The imidazolium-bearing poly(diacetylene) displayed a colorimetric response, accompanied by an increase in fl uorescence emission intensity, upon addition of anionic surfactants such as sodium dodecyl sulfate (SDS, at concentrations as low as 2 × 10 − 7 M ), sodium dodecyl carboxylate (SDX), sodium dodecyl phosphate (SDP), and sodium dodecylbenzenesulfonic acid (SDBS) in aqueous HEPES buffer. The system was selec-tive for anionic surfactants, showing no response to other anionic species or other (neutral or cationic) surfactants; furthermore, the four anionic surfactants elicited three distinct color changes that enabled the differentiation of SDS, SDC/SDP, and SDBS. The polymer–surfactant com-plexation was formulated as involving ion-pairing inter-actions between the surfactant head groups and the imidazolium moieties, as well as contacts between the hydrocarbon regions of the surfactant and polymer, driven by the hydrophobic effect. Computational studies indi-cated that binding of this type could result in distortion of the polydiacetylene π -system in a manner consistent with the observed spectroscopic changes.

Adenosine triphosphate (ATP) is an analyte of much interest due to its central role in intracellular energy transfer and cell signaling. Synthetic systems for ATP binding or detection include synthetic “small mole-cule” receptors and biomacromolecular hosts (oligopep-tides [ 27 ] or RNA aptamers [ 28 ] ). Shinkai and co-workers [ 29 ]

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studied the ATP-responsive behavior of ammonium-functionalized polythiophene 7e . A key design feature (one that is shared with polymer 7b discussed above) is the sterically encumbered 3,4-disubstituted thiophene backbone, which was proposed to increase the sensitivity of 7e toward analyte-induced conformational changes. The polymer displayed a redshift in absorbance that was manifested in a visible color change from yellow to pink upon addition of ATP in water or aqueous HEPES buffer. Whereas 7e was not responsive toward other anionic species (chloride, bicarbonate, inorganic phosphates), other nucleoside triphosphates (UTP and GTP) also gave rise to a colorimetric response; it is noteworthy that ATP and GTP gave rise to opposite induced circular dichroism effects and could thus be distinguished by this chiroptical technique. The spectroscopic effects were interpreted in terms of the formation of polymer–ATP supramolecular aggregates through a combination of ion-pairing, hydro-phobic, and π – π interactions. Polymer 7e also underwent fl uorescence quenching in the presence of ATP, with an estimated detection limit of 10 − 8 M .

Ammonium-functionalized conjugated polyelectrolyte 7f was employed as a selective, ratiometric fl uorescence-based sensor for pyrophosphate anion in 2-( N -morpolino)-ethanesulfonic acid (MES) buffered aqueous solution by Schanze and Zhao. [ 30 ] Addition of PPi at pH 6.5 triggered a redshift in the absorbance spectrum of the polymer, along with the decrease of a fl uorescence emission band at 435 nm and simultaneous increase of emission intensity at 530 nm; both effects were suitable for determination of PPi concentration (with a detection limit of ≈ 340 nM) using a convenient and robust ratiometric format. The spectral changes were consistent with backbone planarization and enhanced interpolymer exciton coupling arising from anion-induced polymer aggregation. The selectivity of this system toward PPi over other inorganic anions such as phosphate was interpreted as arising from the unique ability of pyrophosphate to crosslink polymer chains, giving rise to polymer aggregation. In this regard, similarities may be drawn between this system and the behavior of certain pyridinium-based polymers that have been reported to undergo selective, anion-induced self-assembly into micellar aggregates (without an associated fl uorescence response). [ 31 ]

7. Polymers Bearing Hydrogen Bond-Donor Groups

The strength and directionality of hydrogen bonding interactions, combined with the relative ease with which hydrogen bond-donor groups may be incorporated into a range of preorganized scaffolds, have contributed to the extensive development of anion receptors based on these

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Figure 7 . Fluorescent, polymeric anion sensors based on acidic OH groups.

interactions. Likewise, diverse approaches for incorpo-rating hydrogen bond donor groups into polymeric scaf-folds have been investigated for the purpose of anion recognition. The polymeric nature of these receptors gives rise to unique effects, including signal amplifi ca-tion, cooperative binding, and enhanced selectivity. (We note that, consistent with the idea that hydrogen bonding may be viewed as an incipient proton transfer reaction,

Figure 8 . Polymeric anion sensors based on acidic NH groups.

receptors bearing acidic functional groups are able to interact with anions through either noncovalent interactions or full-fl edged Brønsted acid/base equi-libria, depending on the p K a values of the species involved and the nature of the solvent in which the interactions are studied. [ 32 ] Likewise, the polymers described in this section may either undergo deprotonation or hydrogen bonding in the presence of anions: in cases where data have been provided in support of one of the two possible modes of interaction, they are discussed below.)

Several fl uorescent polymers func-tionalized with acidic OH groups have been found to act as sensitive materials for anion detection. For example, poly-quinoline 8a (Figure 7 ) was studied by Wang and co-workers [ 33 a] as a fl uores-cent chemosensor for Bu 4 N + F − in DMSO solvent. The development of a redshifted absorption peak at 500 nm, along with a new feature in the emission spectrum at 620 nm, was proposed to arise from deprotonation of the polymer OH groups, giving rise to intramolecular charge transfer (ICT) between the phenolate and quinoline moieties. The response

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was selective for F − over less basic anions (Cl − , Br − ), and only a minimal response toward H 2 PO 4 − was observed. Compar-ison with a nonpolymeric hydroxyquino-line revealed that the polymer displayed enhanced sensitivity to 100 equivalents of fl uoride (147-fold enhancement, compared to 17 fold for the model com-pound in DMSO), as well as an improved F − /H 2 PO 4 − selectivity in comparison to its nonpolymeric analog. Two other fl uorescent polymers that interact with anions through OH functional groups are depicted in Figure 7 . [ 33b,c ] Both showed selective responses to basic anions, and deprotonation of the acidic OH groups by

analyte was proposed to take place in each case. In addition to OH groups, acidic NH groups have been

incorporated into conjugated polymers to give rise to anion-sensory materials; among the functional groups that have been employed are pyrrole ( 9a – 9c ), [ 34 ] amide ( 9d ), [ 35 ] and urea ( 9e ) [ 36 ] groups (Figure 8 ). Important prec-edent for these efforts is the application of polypyrrole fi lms in anion-selective electrodes for detection of

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chloride, bromide, nitrate, and perchlorate: a combination of hydrogen bonding interactions involving the pyrrole NH groups, along with ion-pairing interactions accompa-nying oxidation of the polymer, are likely responsible for the polypyrrole–analyte interactions. [ 37 ] Pioneering work in the development of new classes of hydrogen-bonding poly mers for anion recognition was carried out by Aldakov and Anzenbacher, who incorporated anion-binding dipyr-rolylquinoxaline (DPQ) moieties into thiophene-based polymers by electrochemical polymerization. [ 34 a] In analogy to the polypyrrole sensors mentioned above, a key feature of this design is the prospect for modulating the level of positive charge in the polythiophene—and thus the extent of Coulombic interaction with anionic analytes and/or the p K a of acidic NH groups—by applying an electric potential. Polymers 9a and 9b displayed colori-metric responses to tetrabutylammonium fl uoride, pyro-phosphate and phosphate in DMSO/water mixed solvent at a constant potential of –0.12 V (potentials are refer-enced to the ferrocene/ferrocenium redox couple), with tetrabutylammonium perchlorate as the supporting electrolyte. Apparent association constants for 9a with these anions ranged from 1.1 × 10 4 M − 1 for pyrophos-phate to 9.0 × 10 4 M − 1 for dihydrogenphosphate. Electro-chemical oxidation of polymer 9a resulted in an increase of the apparent affi nity constant for PPi from 1.1 × 10 4 M − 1 at –0.12 V to > 10 6 M − 1 at + 0.58 V, and in situ con-ductivity measurements using interdigitated micro-electrodes revealed a concentration-dependent decrease in the conductivity of polymer 9b upon addition of tetrabutylammonium pyrophosphate. The ability to use a single material for colorimetric and/or conductivity-based anion sensing is a noteworthy aspect of this system from the standpoint of developing robust and versatile sensory devices. Another approach for the incorporation of DPQ hydrogen-bond donors into an anion-responsive polymer was reported by Sun and co-workers, [ 34 b] who prepared poly(phenyleneethynylene) 9c bearing this functional group. The polymer underwent fl uorescence quenching by fl uoride ( K SV = 5.2 × 10 5 M − 1 ) and pyrophosphate ( K SV = 3.8 × 10 5 M − 1 ) in dichloromethane solvent, accompa-nied by a color change from yellow to red; these responses were ascribed to deprotonation of a NH group by the basic analytes. A degree of amplifi cation of 34 fold was esti-mated based on experiments with a nonpolymeric model compound.

Both amide and urea functional groups have been applied extensively in anion recognition, and their instal-lation into several classes of fl uorescent polymers has been explored. Poly(phenylacetylene) 9d , functionalized with naphthalimide substituents bearing acidic amido groups, functions as a fl uorescent chemosensor for fl uoride anion in acetonitrile solvent. [ 35 ] Addition of Bu 4 N + F − (but not Cl − , Br − , or I − ) to a solution of 9d resulted in an attenuation of


the polymer fl uorescence at 460 nm, accompanied by the development of a redshifted emission band at 580 nm. These spectral changes enabled the detection of fl uoride at concentrations as low as 1 × 10 − 5 M in acetonitrile. Given that fl uoride has been demonstrated to effect deprotona-tion of simple naphthalimides, [ 38 ] it may be that such a processes underlies the behavior of polymer 9d . Yang and co-workers [ 36 ] synthesized poly(fl uorenes) substituted with diarylurea groups, and demonstrated that these macro-molecules are responsive to basic anions in THF solvent. The blue emission of polymer 9e was quenched upon addi-tion of such analytes: association constants determined by fi tting changes in emission intensity to 1:1 binding iso-therms were 1.0 × 10 6 M − 1 (AcO − ), 2.7 × 10 5 M − 1 (F − ) and 5.5 × 10 4 M − 1 (H 2 PO 4 − ). An anion-modulated photoinduced elec-tron transfer process was proposed as a mechanism for the quenching behavior, and fl uorescence lifetime meas-urements in the presence and absence of anions were con-sistent with static (rather than dynamic) quenching.

A conceptually distinct and versatile method for transducing hydrogen-bonding events into measurable signals using a polymeric architecture is described in a series of publications by Kakuchi and co-workers: [ 39 ] incorporation of acidic NH groups into the side chains of a poly(phenylacetylene) results in a system in which hydrogen bonding interactions with anions infl uence the helical conformation of the polymer. For example, polymer 9f bearing urea-linked L -leucine substituents did not show evidence of a defi ned helical conforma-tion in THF solvent, but addition of tetrabutylammo-nium chloride, bromide, or acetate resulted in dramatic induction of Cotton effects in the circular dichroism (CD) spectrum, along with a redshift of the UV–vis absorp-tion spectrum. [ 39a ] These spectral changes implied that anion binding was accompanied by an alteration in the polymer conformation, leading to a biased helical con-formation along with a change in the effective conjuga-tion length of the π -system. The polymer response was dependent upon the ionic radius (and not, apparently, on the basicity) of the anions, indicating that the spacing of the urea groups along the polymer backbone gives rise to a signifi cant level of selectivity based on anion size. Modi-fi cations of the hydrogen bond-donor group (amides [ 39b ] and sulfonamides [ 39c ] have also been employed) and the appended amino acid have been investigated: for example, the anion response displayed a sensitive dependence on the identity of the amino acid in the urea-linked poly-mers, leading to a family of polymers that were employed in array-based anion sensing. [ 39d ] Recently, this group has reported detailed studies of electron-defi cient urea- functionalized poly(phenylacetylene) 9g that reveal an interesting degree of positive coooperativity for the anion-binding process in certain instances. [ 39e ] The colorimetric response to Bu 4 N + AcO − displayed the sigmoidal shape

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Figure 9 . Synthesis of polymer 10a ; structures of polymer 10b and nonpolymeric model compound 11 . Adapted with permission. [ 43 ]

characteristic of a cooperative response, and analysis using a Hill plot indicated a degree of positive cooperativity ( n ) of 8.0. [ 40 ] A change in polymer confor-mation upon anion binding represents a plausible mechanism for the observed allosteric effect, [ 40a,b ] in which initial binding events facilitate additional interactions with acetate. Both the asso-ciation constant and degree of coopera-tivity were dependent on the identity of the anionic analyte. This type of cooper-ative behavior is a hallmark of binding events in biological systems, and poly-meric systems clearly provide unique opportunities to study and/or exploit such effects in the context of synthetic sensors and receptors.

8. Anion-Responsive Poly(squaramides)

Unlike fl uorescent polymers based on conjugated repeat units such as arylenes, arylenevinylenes, and aryleneethy-nylenes, polyamides represent a largely unexplored class of materials from the perspective of anion sensing. Given the rich anion recognition chemistry of oligoamides [ 41 ] and oligopeptides, [ 27 , 42 ] the documented role of polypeptides as naturally occurring anion-binding macromolecules, [ 4 ] and an extensive literature describing the synthesis and properties of polyamides, it is somewhat surprising that these materials have been ignored as potential anion-responsive materials. Recently, we reported the synthesis of a new class of polyamides composed of 3,4-diaminocy-clobutene-1,2-dione (squaramide) functional groups, and found that a polymer of this type exhibited interesting and complex anion sensory behavior. [ 43 ]

The squaramide group was explored as the basis for an anion-sensitive polyamide because of the superior anion-binding properties of this functional group, which have been ascribed to the high acidities of squaramide NH groups in comparison to those of related compounds such as ureas. [ 44 ] The envisioned synthesis of this class of materials involved a polycondensation between diamines and squarate esters, both of which are readily available. Attempts to prepare polymers by such a pathway more than 25 years ago encountered problems associated with the low solubility of the poly(squaramide) targets and the formation of isomeric squaraine-type linkages during the polycondensation with arylenediamines. [ 45 ] To address the latter problem, we developed an effi cient and high-yielding synthesis of N , N ′ -diarylsquaramides by Lewis acid-catalyzed condensation of anilines with squarate esters. [ 44c ] Under these conditions, polycondensation of diethyl squarate



and 9,9-dioctylfl uorenediamine resulted in polymer 10a (Figure 9 ), which displayed good solubility in amide sol-vents (NMP, DMF). Gel permeation chromatography (GPC) in NMP (with 0.2 wt% LiCl as an additive), using poly(methyl methacrylate standards), provided an estimated molecular weight Mn = 1.8 × 10 4 g mol − 1 , with polydispersity Mw / Mn = 1.7; NMR and GPC analysis of Mn were in good agreement for polymer 10b bearing defi ned end groups.

Poly(squaramide) 10a displayed interesting anion-responsive properties: addition of Bu 4 N + H 2 PO 4 − to a solution of 10a in a competitive organic/aqueous solvent mixture (10% water in NMP) resulted in a “turn-on” fl uorescence response, with a half-maximal effect observed at an anion concentration of 50 μ M. Control experiments indicated that the polymeric system displayed signifi cantly enhanced sensitivity and selectivity in comparison to both the lower-molecular-weight polymer 10b and the nonpolymeric model compound 11 , as illustrated in Figure 10 . These effects are noteworthy given that polymer 10a , unlike the majority of the conjugated fl uorescent polymers discussed above, is unlikely to benefi t from long-range exciton transport as a mechanism for signal amplifi cation. Instead, experiments suggested that anion-modulated polymer aggregation plays a role in the unusual behavior of the poly(squaramide): the addition of Bu 4 N + H 2 PO 4 − triggered the formation of spherical aggregates with a mean hydrodynamic radius of 120 nm, as assessed by dynamic light scattering meas-urements (Figure 11 ). Transmission electron microscopy (TEM) images provided additional support for a model involving anion-induced polymer aggregation. Consistent with a model of this type, the response of polymer 10a (but not that of the nonpolymeric model compound) to H 2 PO 4 − displayed a sigmoidal shape, indicating a cooperative binding process (see above); similar sigmoidal binding curves have been observed for fl uorescent polymers upon cation-induced aggregation. [ 11c , 46 ]

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Macromol. Rapid Comm© 2012 WILEY-VCH Verlag Gm32

Figure 11 . (a) Autocorrelation functions and (b) normalized CONTmeasurements at 90 ° on polymer 10a (2.4 × 10 − 5 M ) in the absencthe presence (gray •, — ) of Bu 4 N + H 2 PO 4 − (2.4 × 10 − 3 M ) in DMF; (c) TEMfi lm of 10a and Bu 4 N + H 2 PO 4 − [sample was cast from a DMF solution and Bu 4 N + H 2 PO 4 − (2.4 × 10 − 3 M )]. Spherical polymer aggregates arepattern is the copper framework of the carbon-coated copper TEM (B3LYP-6-311 + G**) geometry of the 2:1 complex of N,N ′ -dimethylsquarAdapted with permission. [ 43 ]

Figure 10 . (a) Fluorescence “turn-on” response of polymer 10a (�), 10b ( � ) and model compound 11 ( � ) upon addition of Bu 4 N + H 2 PO 4 − in 10% H 2 O/NMP; (b) normalized response of poly mer 10a (black bars) and model compound 11 (gray bars) to anions X–(Bu 4 N + X − , 10% H 2 O/NMP; anion concentration 140 μ M for 10a and 1200 μ M for 11 ). Adapted with permission. [ 43 ]

Calculations suggested a plausible pathway for dihy-drogenphosphate-induced aggregation, in which the anion serves to crosslink polymer chains by noncovalent interactions with two squaramide groups. Similarities may be drawn between this proposal and the postulated mechanism for the pyrophosphate-induced response of the ammonium-functionalized polyelectrolyte 7e devel-oped by Zhao and Schanze. [ 30 ] Poly(aramides) are known to form hydrogen-bonded, β -sheet-type aggregates, [ 46 ] and in analogy it is likely that the squaramide groups play dual roles in mediating interpolymer and polymer–analyte interactions. Although work is needed to understand the structural features of 10a that give rise to its unusual anion-responsive properties, these results suggest that poly-amides represent an interesting and underexploited class of materials from the perspective of chemical sensing.

9. Conclusions

The diverse polymer structures that have been employed for anion recognition, as well as the diverse mechanisms by which polymer–anion interactions may be relayed into measurable signals, indicate that the fi eld of anion-responsive polymers presents substantial opportunities for creativity in the design of new materials. The work

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bH & Co. KGaA, Weinh

IN plots from DLS e (black •, — ) and

image of a dried of 10a (2.4 × 10 − 5 M ) evident. The lacy

grid; (d) calculated amide and H 2 PO 4 − .

that has been carried out to date points toward a number of key features of poly-meric systems that may be of utility in solving outstanding challenges in anion recognition. Two such features are discussed in detail below.

10. Anion Recognition/Detection in Aqueous Solution

A number of the polymeric systems that have been discussed herein are remark-able in their ability to function in pure water or mixed organic/aqueous sol-vent systems: as noted previously, these environments present a particular chal-lenge for anion recognition. The ability of certain polymers to function as anion sensors in aqueous solution may be attributed to several distinct proper-ties. Signal amplifi cation represents a mechanism for overcoming relatively low-affi nity binding, and many of the fl uorescent polymers described above have been shown to benefi t from this effect. In addition, polyelectrolytes participate in particularly favorable

eim www.MaterialsViews.com


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ion-pairing interactions, a phenomenon that underlies the numerous examples of poly(cationic) anion- responsive macro molecules that have been developed to date. Poly-mers also offer unique settings in which to exploit the hydrophobic effect to drive molecular recognition events in water: in addition to promoting binding of amphiphilic analytes (e.g., the binding of anionic surfactants to poly(diacetylene) 7d , [ 26 ] or of ATP by polythiophene 7e [ 29 ] ), the hydrophobic effect likely provides a signifi cant con-tribution to the thermodynamics of systems in which polymer aggregation accompanies analyte binding.

11. Promoting Anion Binding With Secondary Inter- and Intrapolymer Interactions

Detailed studies of binding events in biological systems have uncovered numerous instances in which secondary, intrare-ceptor interactions play crucial roles in the overall thermo-dynamics of host–guest association: the “tightening” of the streptavidin structure that accompanies binding of biotin is a clear example in which an effect of this type helps to drive ultrahigh-affi nity molecular recognition. Experimental evi-dence for such effects, and implications for understanding molecular recognition in biological systems, are discussed in detail in a seminal review article by Williams et al. [ 40c ] In recent years, chemists have begun to take advantage of such secondary attractive interactions to achieve molecular recognition with synthetic receptors in aqueous solution, including examples of anion binding. [ 48 ] Synthetic macro-molecules would appear to be ideally suited for exploiting such effects, given the prospects for extended intra- or interpolymer contacts; indeed, as discussed above, the cooperative binding processes observed for several of the systems described above have been rationalized according to mechanisms in which anion binding is accompanied by reorganization of isolated polymer chains or polymer aggre-gates. Detailed thermodynamic data for these systems (e.g., from isothermal titration calorimetry experiments) are lacking at present, and would be of signifi cant fundamental interest. [ 49 ] The insight gained through such studies may ultimately help to drive the discovery of synthetic materials able to display the selectivity and affi nity that are charac-teristic of Nature's anion-binding macromolecules.

Acknowledgements : Funding from the NSERC (Discovery Grants Program) and the University of Toronto (Connaught Fund) is gratefully acknowledged.

Received: August 10, 2011 ; Revised: September 12, 2011; Published online: October 31, 2011; DOI: 10.1002/marc.201100528

Keywords: anions; chemical sensors; fl uorescence; molecular recognition; polymers; supramolecular chemistry



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