Functional Polyelectrolytes

Functional Polyelectrolytes

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13698 DOI: 10.1021/la903785g Langmuir 2009, 25(24), 13698–13702Published on Web 1/04/2009 pubs.acs.org/Langmuir ©2009 American Chemical Society

Functional Polyelectrolytes†

Kirk S. Schanze* and Abigail H. Shelton

Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200

Received October 6, 2009

Thisperspective seekstoidentifyanareaofsoft materialsresearch focusedonthestudy offunctionalpolyelectrolytes.

These materials combine the useful properties intrinsic to polyelectrolyte chains, with added functionality provided by specific molecular (or polymeric) functional groups that are present in the polymer backbone or as a pendant functionality. Examples are provided to demonstrate how the combined functionality can be used to create films and assemblies with interesting and useful optical, electro-optical, and electronic properties.

Introduction

Polyelectrolytes are polymers featuring electrolyte groups within the repeat unit structure. Polyelectrolytes can be categorized as natural (e.g., polynucleic acids and polypeptides), modified natural (e.g., chitosan and cellulose), and synthetic (e.g., polystyrene sulfonateandnafion).1Upondissociationoftheelectrolytegroups in solution, polyelectrolyte chains become charged and the charge typesincludecationic,anionic,andzwitterionic(polyampholytes).

Natural and synthetic polyelectrolytes have been the subject of experimental and theoretical study for nearly a century.2 Interest in polyelectrolytes is driven by their properties, including water solubility, ionic conductivity, strong intra- and interchain interactionsthatgiverisetousefulsolutionproperties,interactionwith ions in solution, and surface activity. These properties, which are intrinsic to the polyelectrolyte chain, imbue the materials with properties that are very useful from an application perspective, such as additives to modify solution viscosity and induce gelation or as stabilizers for colloidal suspensions. Because of these properties, polyelectrolytes are important additives in a number of products, including soaps, cosmetics, and foods.

Despite the ongoing use of polyelectrolytes, there remains considerable research interest in these materials.3 One area of considerablerecentresearchinterestintheareaofpolyelectrolytes is the study of polyelectrolyte multilayer (PEM) films. Initially reported by Decher,4,5 the field of PEM films has rapidly expanded, with many fundamental studies of the process as well as the use of the multilayer films in areas such as nanoporous membranes, delivery capsules, and sensors.6,7 The layer-by-layer (LbL) deposition technique, which provides the basis for the construction of PEM films, is based on the intrinsic property of polyelectrolytes to adsorb at charged interfaces and to form complexes with oppositely charged polyelectrolyte chains.

Functional Polyelectrolytes

In this perspective, we wish to point out a novel direction in the area of polyelectrolyte chemistry and materials science that is focused onthe development ofa new class ofpolyelectrolytesthat we define as functional polyelectrolytes. We will define these materials and give some specific examples from the recent literature (including articles in the present Langmuir issue) to highlight research and directions in this growing field. Here we define a functional polyelectrolyte asbeing a synthetic polymeror copolymer that contains ionizable electrolyte groups as well as additional functionality imparted by the molecular (polymer) structure.The most commonexamplesofadded functionality are redox activity imparted by the inclusion of easily oxidized or reduced units and optical properties (light absorption, light emission, and photochromism) imparted by the inclusion of specific chromophores or a π-conjugated polymer backbone. Whereasthesearethemostcommonexamplesoffunction,others can be envisioned, including catalytic function and molecular recognition.Forbrevity,wechoosenottoincludemetal-complexbased coordination polymers or ionic dendrimers in our discussion of functional polyelectrolytes; however, in many cases, these materials could be included in this categorization.

As noted above, the area of polyelectrolyte chemistry and materials science has been very active in Langmuir, and the present special anniversary issue contains a number of articles focused on functional polyelectrolytes.

Redox Polyelectrolytes

Redox polyelectrolytes feature a polyelectrolyte chain that includes a redox-active moiety in the repeat unit structure. Several excellent examples of these materials are provided in the recent literature, and here we highlight work on two specific example systems. The first that we select is the family of poly(viologens) first reported by Schlenoff andco-workers in thelate 1990s.8,9The poly(butanylviologen) polymer was used along with poly(styrene sulfonate) (see Figure 1 for structures) to construct PEM films via the LbL approach. The LbL films were robust and exhibited the electrochemical response of the viologen moieties. The results indicated that the films exhibit good redox conductivity because the viologen units remain electrochemically active, even for films with 10 bilayers. In addition to the redox functionality, the

†Part of the “Langmuir 25th Year: Molecular and macromolecular self- assemblies” special issue. *Corresponding author. E-mail: kschanze@chem.ufl.edu w: http:// w.chem.ufl.edu/∼kschanze. (1) Koetz, J.; Kosmella, S. Polyelectrolytes and Nanoparticles; Springer-Verlag:

Berlin, 2007. (2) Staudinger, H. Die Hochmolekularen Organischen Verbindungen, Kautschuk und Cellulose; Springer: Berlin, 1932. (3) More than 1000 research articles in Langmuir contain “polyelectrolyte” in the title or abstract. (4) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831–835. (5) Decher, G. Science 1997, 277, 1232–1237. (6) Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials;

Decher, G., Schlenoff, J., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (7) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 2319–2340. (8) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552–1557. (9) Stepp, J.; Schlenoff, J. B. J. Electrochem. Soc. 1997, 144, L155–L157.

Schanze and Shelton Perspective

viologen-functionalized PEM films exhibit UV-visible electrochromism associated with the absorption difference between the oxidized(V2þ)andreduced(Vþ•) statesoftheviologenunitaswell as the ability to catalyze the electroreduction of dioxygen.9 Thus, the viologen functionality imparts the polyelectrolyte with optical and catalytic functionality, in addition to redox properties.

Ferrocenyl-functionalized polyelectrolytes have been the subject of a series of investigations by Vancso and co-workers.10-14 The pair of polyelectrolytes shown in Figure 2 has backbones based on poly(ferrocenylsilane), and they feature polyelectrolyte nature imparted by the presence of the ammonium (cationic) and sulfonate (anionic) side groups. The ferrocene (Fc) functionality imbuesthe polyelectrolytes with redox activity, and the combined redox and polyelectrolyte nature of thematerials has beenusedto fabricate PEM films and capsules that exhibit the redox response characteristic of the Fc/Fcþ couple. One recent study provides a veryniceexample ofhow the combinedpolyelectrolyte and redox functionality can be used to fabricate structures that have remarkable stimuli-responsive properties. In particular, Ma and co-workers used the cationic and anionic ferrocene polyelectrolytes to construct polyelectrolyte capsules by using the LbL approach on MnCO3 template particles.12,15 When the ferrocene units were in the reduced state (Fc0), the capsules were imperme- able to a moderate-molecular-weight dextran. However, following the addition of Fe3þ, the ferrocene units are oxidized (Fcþ) and the capsules exhibit remarkable swelling and become permeable to dextran (Figure 2). The swelling and increased permeability are attributed to charge repulsion within the ferrocene polyelectrolytes. In a more recent study, Hempenius et al. reported a hydrogel that is fabricated from a cross-linked polyelectrolyte based onthe poly(ferrocenylsilane) backbone(Figure3).14 The hydrogel undergoes reversible redox-induced swelling/deswelling due to electrostatic interactions between the anionic sulfonate units and the Fcþ sites that are produced by oxidation.

Conjugated Polyelectrolytes

Conjugatedpolyelectrolytes(CPE)representanimportantclassof functionalpolyelectrolytes.CPEsfeature a backboneconsistingofa delocalized,π-conjugatedelectronic system surrounded by the ionic electrolyte groups that give the materials polyelectrolyte character. CPEswerefirstreportedin1987byWudl,Heeger,andco-workersas an approach to synthesize a self-doped conducting polymer,16 and researchintothepropertiesandapplicationsofCPEshasblossomed inthepastdecade.17Thisperiodhaswitnessedtremendousgrowthin interestinCPEs,witha large number of studiesthathave examined the fundamentalproperties of thematerials andhaveexplored their use in a variety of applications including chemo- and biosensors, polymer electronic devices, and solar cells.18

ThepropertiesofCPEsaretheresultofsynergisticeffectsarising from the interplay of their dual functionality. In particular, the πconjugated backbone imparts the materials with a broad function, includingstrongoptical absorptionand fluorescence,conductivity forneutral(exciton)andcharged(polaron)states,andanamplified response to external stimuli owing to the delocalized electronic structure of the backbone. In addition, the polyelectrolytefunctionality imparts the materials with the properties intrinsic to polymer electrolytes, viz., water solubility, ionic conductivity, strong intra- and interchain interactions, interaction with ions in solution,surfaceactivity,and a propensityto adsorbat interfaces. Here we provide a fewexamples of some of the recent research on CPEs to highlight the ways in which these novel functional materials are being used in applications.In addition,we draw the reader’s attention to two research articles in the present Langmuir special issue that provide additional examples of the unique properties and applications of conjugated polyelectrolytes.19,20

An early study by Rubner and Reynolds illustrates how the combined polyelectrolyte and conjugated function can be used to construct novel PEM films for optoelectronic device application.21 In particular, using a pair of sulfonate (anionic) and

Figure 1. (a) Redox cycling between oxidizedand theone-electron-reducedformofpoly(butanylviologen) (PVB).(b) Schematicdiagram of PEM multilayer films of PBV and polystyrene sulfonate showing electron hopping between viologen units in the PEM structure. Reprinted with permission from ref 8. Copyright 1997 American Chemical Society.

(10) Hempenius, M. A.; Peter, M.; Robins, N. S.; Kooij, E. S.; Vancso, G. J.

Langmuir 2002, 18, 7629–7634. (1) Hempenius, M. A.; Brito, F. F.; Vancso, G. J. Macromolecules 2003, 36, 6683–68. (12) Ma, Y. J.; Dong, W. F.; Hempenius, M. A.; Mohwald, H.; Vancso, G. J.

Nat. Mater. 2006, 5, 724–729. (13) Ma, Y. J.; Dong, W. F.; Kooij, E. S.; Hempenius, M. A.; Mohwald, H.;

Vancso, G. J. Soft Matter 2007, 3, 889–895. (14) Hempenius, M. A.; Cirmi, C.; Song, J.; Vancso, G. J. Macromolecules 2009, 42, 2324–2326. (15) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202–2205.

(16) Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 1858–1859. (17) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 1293–1309. (18) Jiang, H.; Taranekar, P.; Reynolds,J. R.; Schanze, K. S. Angew. Chem., Int.

Ed. 2009, 48, 4300–4316. (19) Ding, L.; Chi, E. Y.; Chemburu, S.; Ji, E.; Schanze, K. S.; Lopez, G. P.;

Whitten, D. G. Langmuir, 2009, 25, DOI 10.1021/la901457t. (20) Feng, X.; Xu, Q.; Liu, L.; Wang, S. Langmuir 2009, 25, DOI 10.1021/ la901444c. (21) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Adv. Mater. 1998, 10, 1452–1455.

13700 DOI: 10.1021/la903785g Langmuir 2009, 25(24), 13698–13702

Perspective Schanze and Shelton ammonium (cationic) polyelectrolytes with a poly(paraphenylene) backbone (Figure 4), the active layer for a polymer lightemittingdiode(PLED)wasconstructedusingtheLbLdeposition method. In particular, 35 bilayer films consisting of alternating layers of the cationic and anionic CPEs were deposited onto a transparent electrode. The assembly process takes advantage of the polyelectrolyte functionality of the polymers. The optoelectronic device was completed by the evaporation of an aluminum film, giving rise to PLED device architecture in which the lightemitting and semiconducting CPE film is sandwiched between two electrodes. As shown in Figure 4, the device emits blue light under an applied electrical bias. Several years later, working in collaboration with Reynolds, we used a sulfonate-type (anionic) polyelectrolyte with a poly(phenylene ethynylene) conjugated backbone in conjunction with a cationic C60 derivative to construct PEM films that served as the active layer in a polymer solar cell (Figure 5).2 Solar cells that contained 50 bilayers of the CPE and C60 derivative exhibited a relatively good photoresponse, with a peak absorbed photon-to-electron quantum efficiency of ca. 10% and a short circuit current of 0.5 mA under simulated solar illumination conditions (100 mW cm-2).

Several recent studies highlight how the combined polyelectrolyte and conjugated functionality of CPEs can be used in unusual ways. Specifically, Whitten and co-workers have

Figure 3. (Left) Chemical structure of cross-linked polyelectrolyte gel containing ferrocene functional groups. (Right) Photograph of a hydrogel. Reprinted with permission from ref 14. Copyright 2009 American Chemical Society.

Figure 4. (Top) Chemical structures of anionic and cationic poly- (paraphenylene) conjugated polyelectrolytes. (Bottom) Currentvoltage (0) and light-voltage (O) plots for polymer light-emitting diodesconstructedusing35bilayerfilmsofthecationicandanionic conjugated polyelectrolytes. Reprinted with permission from ref 21. Copyright 1998 Wiley-VCH.

Figure 2. (a)Functionalpolyelectrolytescontainingferroceneunits used to prepare polyelectrolyte capsules. (b) Confocal fluorescence microscopeimageof ferrocenepolyelectrolytecapsulessurrounded by solution containing TRITC-labeled dextran (4.4 kD). Capsules are dark because they are impermeable to dextran. (c) Confocal imageofthesamesolutionaftertheadditionofFe3þ,whichservesto oxidize the ferroceneunits. Capsules are fluorescent because oxidation makes them permeable to dextran. Reprinted with permission from ref 12. Copyright 2006 Nature Publishing Group.

Schanze and Shelton Perspective

demonstrated that suspending a microbial sample in an aqueous solution containing a cationic CPEs results in the microbe particlesbecomingcoatedwiththeCPE.23Forexample,asshown in Figure 6, a suspension of E. coli bacteria exhibits bright fluorescence after being exposed to a solution of a cationic poly(phenylene ethynylene) CPE. The fluorescencearises because the bacteria are coated with a thin (monolayer) film; the coating process likely involves the adsorption of the CPE chains due to electrostatic interactions between the outer membrane of the microbial particles and the electrolyte groups on the polyelectrolyte chains. In a separate investigation, Ingan€as and co-workers described the use of amyloid fibrils to template the assembly of “nanowires” consisting of a conjugated polyelectrolyte consisting of a poly(3,4-ethylenedioxythiophene) backbone that is substituted with anionic sulfonate pendant groups (PEDOT-S, Figure 7).24 The self-assembly process takes advantage of the polyelectrolyte nature of PEDOT-S, which allows the CPE to formapolyelectrolytecomplexwiththeamyloidfibrils.Asshown in the transmission electron microscope image in Figure 7, the PEDOT-S modified fibrils form a nanoscale network structure.

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