Proceedings of the ASME 2010 International Mechanical Engineering Congress & ExpositionIMECE 2010

November 12-18, 2010, Vancouver, Canada

IMECE2010-38755

DEPOSITION OF POLYPYRROLE ONTO A NOVEL POLYMER ELECTRODE FORUSE IN A BIOMIMETIC ARTIFICIAL EXCITABLE CELL MEMBRANE

Christina V. Haden∗

Soft Materials LaboratoryDept. of Mech. and Aero. Eng.

University of VirginiaCharlottesville, Virginia 22904Email: cvh2k@virginia.edu

Donald A. Jordan

Dept. of Mech. and Aero. Eng.University of Virginia

Charlottesville, Virginia, 22904Email: dj8n@virginia.edu

Pamela M. Norris

Dept. of Mech. and Aero. Eng.University of Virginia

Charlottesville, Virginia, 22904Email: pmn3d@virginia.edu

ABSTRACTA novel and inexpensive bucky gel electrode has been in-

vestigated for use as the electrode substrate for deposition ofpolypyrrole. The electroactive polymer membrane was success-fully deposited and the surface morphology studied using scan-ning electron microscopy. Given the properties of the bucky gelelectrode and its ability to conduct ions, this work establishesthe first step towards a semi-solid ion-gating system to be usedin further applications.

INTRODUCTIONInvestigation into a novel electrode for the deposition of

polypyrole was motivated by interest in developing an artificialexcitable cell membrane, capable of gating ion movement acrossit. Indeed, polypyrrole (PPy) membranes respond in a mannersimilar to voltage-gated ion channels (VGICs) found in cardiaccell membranes [1] to the application of a voltage (±1 V) byopening and closing to ions in a reversible process of oxidationand reduction. As a voltage is supplied to the PPy membranethrough the electrode onto which it is electrochemically poly-merized, an ionically permeable electrode was sought for use inan artificial excitable cell membrane application. Since the arti-ficial cell membrane design consists of PPy embedded within anionic liquid polymer gel for the purpose of controlling ion fluxeswithin the gel, a bucky gel electrode was hypothesized as a po-tential alternative to metal mesh or sputtered electrodes. Indeed,

∗Address all correspondence to this author.

ILPG can be meshed seamlessly to BG using layer-by-layer cast-ing [2], providing a low ionic resistance boundary between thesetwo materials and allowing for the migration of ions through thePPy membrane in a semi-solid (gelatinous) environment.

The development of carbon nanotubes in 1991 by Iijima [3]has since stimulated a flurry of research and developments span-ning several fields. For this work in particular, it has allowedfor the creation of bucky gels (BG) which are ionically and elec-trically conductive polymers produced by the suspension of anionic liquid and multi-walled carbon nanotubes (MWNTs) in apolymer network of PVDF(HFP) [4]. Fukushima et al. [2] sug-gested BGs could be used as an electrode as they demonstratedgood electrical conductivity. Bucky gels have since been usedas electrodes in actuators [5], as double layer capacitors [6], andsensors [7]. They are a versatile material with many desirablequalities, among which is a high ionic conductivity.

Polypyrrole (PPy, Fig. 1) is a thin electroactive polymer gat-ing membrane which has been used as an ionic separator [8],potassium selective electrode [9], actuator [10], and supercapac-itor [11]. Of particular interest is its ability to act as a gatingmembrane capable of selective ion permeability, opening andclosing in response to an applied voltage as it switches betweenoxidized and reduced states. Polypyrrole is produced throughelectrochemical deposition onto an electrode, typically made of anoble metal such as platinum or gold. Metal mesh electrodes andelectro-sputtered metals have also been used to grow PPy mem-branes to maintain the polymer’s access to ions in solution [12].To date, however, no literature exists reporting a metal electrode

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alternative for the electrodeposition of PPy such as the bucky gelelectrode.

FIGURE 1. CHEMICAL STRUCTURE OF PYRROLE, THEMONOMER WHICH FORMS POLYPYRROLE CHAIN AFTER DE-POSITION.

The scope of the work in the present paper is as follows. Abucky gel electrode was created by embedding carbon nanotubesin an ionic fluid polymer gel as described by Fukushima [4]. PPywas then electrochemically deposited onto this electrode using a3-electrode electrochemical cell. Successful deposition of PPyonto the bucky gel electrode was then determined by observationof the chronoamperometric response recorded during depositionand by analyzing the morphology of the membrane in SEM im-ages at 10000X magnification.

EXPERIMENTALMaterials and Chemicals

Multi-walled carbon nanotubes (MWNTs) and theionic liquid 1-Butyl-3-methylimidazolium Tetrafluorobo-rate ([BMIm]BF4) were used as received from Sigma-Aldrich.The polymer poly(vinylidene fluoride-hexafluoropropylene)(PVdF-HFP) (Kynar flex 2801, Arkema) was stored and usedas received. Pyrrole solution from Sigma-Aldrich was dis-tilled prior to every deposition, and deaerated for 30 minutesusing nitrogen gas. Counterion for polypyrrole depositiontetrabutylammonium-tetrafluoroborate (TBA-TFB) (FisherScientific), acetonitrile (Sigma-Aldrich) and Methyl Pentanone(MP) (Sigma-Aldrich) were used as received.

Preparation of Bucky Gel ElectrodesBucky gel electrodes were made by combining 1 % wt multi-

walled carbon nanotubes with 33 % wt [BMIm]BF4, 66 % wtPVDF(HFP), in 2.5 ml MP. The mixture was heated to 80◦C for20 minutes while stirring. The mixture was then promptly placedinto a mold with embedded gold wires for electrical connectivity.The formed bucky gel electrode material was removed from themold after drying for 48 hours.

Preparation of Polypyrrole MembranesA solution of 0.1 M distilled pyrrole, 0.1 M TBA-TFB, and

1 % wt deionized water in acetonitrile was prepared, chilled on

ice and deaerated using nitrogen gas for 30 minutes prior to de-position. Still under nitrogen atmosphere, potentiostatic depo-sition of PPy was then carried out in a 3-electrode single-cellelectrochemical bath using a Pot/Gal 273A (Princeton AppliedResearch). A platinum mesh is used as the counter-electrode, asaturated calomel electrode (SCE) is used as the reference elec-trode, and a gold or bucky gel as described above is used as aworking electrode. The potentiostatic deposition was performedat +1.2 V vs. SCE for one hour.

Gold electrodes were connected directly to the working elec-trode lead of the Pot/Gal 273A. Bucky gel electrodes, however,were connected using the embedded gold wires. One gold wirefrom the BG electrode was connected to the working electrodelead of the Pot/Gal 273A, and the other to ground (Fig. 2).

FIGURE 2. PPY MEMBRANE DEPOSITION IN A 3-ELECTRODE ELECTROCHEMICAL CELL. GOLD WIRESCONNECT THE BUCKY GEL TO THE POT/GAL 273A ATTHE WORKING ELECTRODE AND GROUND INTERFACES. APLATINUM MESH IS USED AS COUNTER-ELECTRODE ANDSATURATED CALOMEL ELECTRODE FOR REFERENCE.

SEM Imaging

As bucky gel electrodes and PPy membranes are both blackin color and thus indistinguishable to the naked eye, successfuldeposition of the polymer was confirmed by scanning electronmicroscopy (SEM) to observe its characteristic features. Imageswere taken using a JEOL JSM-6700F cold field-emission gunmicroscopy system before and after the deposition of the PPymembrane on the bucky gel and gold electrodes.

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RESULTSElectrochemical Deposition of Polypyrrole

In this study, a working electrode of either gold or buckygel was used to electrochemically polymerize PPy. Depositionon both substrates was carried out in the electrochemical cell at+1.2 V vs. SCE. The chronoamperometric responses are shownin Figure 3. The positive rate of change of current at the on-set of deposition, as seen for both the bucky gel and gold elec-trodes, is indicative of a successful electrochemical deposition.The larger current drawn by the bucky gel electrode is likely dueto it’s larger thickness compared to the gold electrode, requiringlarger currents to supply the same voltage. As PPy accumulateson the electrode surface, currents grow larger due to the addedmaterial thickness. The duration of deposition therefore dictatesthe PPy membrane thickness on both substrates.

FIGURE 3. CHRONOAMPEROMETRIC RESPONSE OF BUCKYGEL AND GOLD ELECTRODES DURING THE DEPOSITION OFPOLYPYRROLE.

Morphology of PolypyrroleSEM images of the bucky gel and PPy grown on both elec-

trodes were obtained (Fig. 4). The bucky gel (Fig. 4, Left) ex-hibits a smooth morphology which is easily distinguished fromthe cauliflower-like features of a PPy membrane (Fig. 4, Cen-ter). After a 1 hour deposition of PPy, however, the BG mor-phology has changed significantly (Fig. 4, Right) and now alsoexhibits the features typical of a PPy membrane. A comparisonof PPy deposited on BG (Fig. 4, Right) with PPy deposited ongold (Fig. 4, Center) demonstrates that both have similar mor-phology, confirming the presense of PPy deposited onto this newelectrode.

FIGURE 4. SCANNING ELECTRON MICROSCOPY (SEM) IM-AGES, ALL AT 10,000X MAGNIFICATION, OF BUCKY GEL(LEFT), PPY MEMBRANE ON GOLD (CENTER), AND BUCKYGEL COATED WITH PPY MEMBRANE (RIGHT).

CONCLUSIONSThis work has demonstrated the use of bucky gel as a vi-

able electrode for the electrochemical deposition of PPy mem-branes. Deposition was successfully carried out at +1.2 V vs.saturated calomel electrode (SCE) on the BG electrode as theworking electrode in a 3-electrode electrochemical cell, in a so-lution of 1 M pyrrole, 1 M TBA-TFB counterion, and 1 wt %distilled water in acetonitrile. A chronoamperometric responseduring membrane growth was observed and indicated a success-ful PPy membrane deposition onto BG electrode as indicated bythe positive rate of change of current.

Deposition of PPy onto BG was also demonstrated qualita-tively through scanning electron microscopy (SEM), using im-ages taken of the BG surface before and after PPy growth to con-firm the presence of the membrane. SEM images at 10,000Xmagnification for the bucky gel material show a characteristi-cally smooth morphology with features on the order of severalmicrons. In SEM images of BG onto which PPy had been de-posited, also at 10,000X magnification, the typical morphologyof PPy membranes is apparent: a cauliflower-like appearancewith features on the submicron scale.

The successful deposition of these materials onto BG indi-cates the ability to create a gel-based ion gating system for usein more complex ion separation systems in a semi-solid envi-ronment, which will eventually lead to the development of anartificial excitable cell membrane.

ACKNOWLEDGMENTThe authors would like to thank Dr. John Scully, Dr. Flo-

rent Bocher and Eric Rouya for their advice and direction on thisproject. We would also like to thank Dr. George Chen for his sci-entific correspondence which provided us with much insight. Inaddition, we wish to thank the Virginia Space Grant Consortiumfor their financial support.

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