beyond doping and charge balancing: how polymer acid ......conducting polymers, such as pani, proton...

Beyond Doping and Charge Balancing: How Polymer Acid TemplatesImpact the Properties of Conducting Polymer ComplexesMelda Sezen-Edmonds*,† and Yueh-Lin Loo*,†,‡

†Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States‡Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States

ABSTRACT: Polymer acids are increasingly used as dopants/counterions to access andstabilize the electrically conducting states of conducting polymers. Beyond doping and/orcharge balancing, these polymer acids also serve as active components that impact themacroscopic properties of the conducting polymer complexes. Judicious selection ofthe polymer acid at the onset of synthesis or manipulation of the interactions between thepolymer acid and the conducting polymer through processing significantly impacts theelectrical conductivity, piezoresistivity, electrochromism, mechanical properties, andthermoelectric efficiency of conducting polymers. As polyelectrolytes, these polymer acidsenable conducting polymer complexes to transport ions in addition to electrons/holes.Understanding the role of the polymer acid and its interactions with the conductingpolymer generates processing−structure−function relationships for conducting polymer/polymer acid complexes, which can help overcome challenges that were associated withthese materials, such as low electrical conductivity and sensitivity to humidity, and enablethe design of conducting polymer complexes with desired functionalities.

Electrically conducting polymers belong to a class ofmaterials that possess electrical conductivities approachingthose of metals while having plastic-like mechanical properties.Their mechanical compliance with flexible substrates coupledwith their unique optoelectronic properties, such as mixed ionicand electronic conduction, and electrochromism, have enabledtheir use in a broad range of applications, including confor-mable neural recording devices, transparent electrodes, bio-sensors, strain gauges, and smart windows.1−4 Although thediscovery of their synthesis goes back to the 19th century,breakthroughs in conducting polymer research came in the1970s, when Heeger, MacDiarmid, and Shirakawa showed thatpolyacetylene’s conductivity can be increased by more than6 orders of magnitude upon iodine doping.5,6 This transforma-tion of polyacetylene from its electrically insulating form toan electrically conducting form stems from delocalization ofcharges along polyacetylene’s conjugated backbone upon iodineincorporation. One substantive drawback despite high conductiv-ities with these materials is that iodine-doped polyacetylene iseasily oxidized in air and is thus environmentally not stable.7,8

Other conjugated polymers, such as polyaniline (PANI),poly(3,4-ethylenedioxythiophene) (PEDOT), and polypyrrole,

have shown promise with their improved environmental stability.9,10

Accessing the electrically conducting states of these polymersalso requires delocalization of charges along their conjugatedbackbones. In some conducting polymers, such as PEDOT,delocalization of charges is achieved through electron trans-fer by oxidizing or reducing the conducting polymer. In otherconducting polymers, such as PANI, proton doping is neededto access their electrically conducting states. In both systems,incorporation of counterions is needed in order to preservecharge neutrality of the conducting polymer complex.11−13 Inthe early development of these conducting polymers, small-molecule acids were employed as proton dopants and/or thecounterions. The volatility of small-molecule acids, however,limits the ambient stability of these conducting polymers.14

Moreover, these conducting polymers are insoluble in commonsolvents due to the extensive conjugation along the polymerbackbone. This intractability necessitates simultaneous thin-filmdeposition during synthesis like electropolymerization on con-ducting substrates. Though an exciting discovery, the necessityof an underlying conducting substrate has limited the use ofthese early generation conducting polymers in practical applica-tions.15,16

Replacing small-molecule acids with polymer acid dopants/counterions, such as poly(acrylic acid), poly(styrenesulfonate)(PSS), and poly(2-acrylamido-2-methyl-1-propanesulfonic acid)(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA),enhances the ambient stability of conducting polymers due to

Received: July 11, 2017Accepted: August 30, 2017Published: August 30, 2017

Beyond doping and/or chargebalancing, these polymer acidsalso serve as active componentsthat impact the macroscopicproperties of the conducting

polymer complexes.

Perspective

pubs.acs.org/JPCL

© 2017 American Chemical Society 4530 DOI: 10.1021/acs.jpclett.7b01785J. Phys. Chem. Lett. 2017, 8, 4530−4539

pubs.acs.org/JPCLhttp://dx.doi.org/10.1021/acs.jpclett.7b01785

the nonvolatile nature of the polymer acids. Excess acid groupsalso introduce water dispersibility to the resulting conductingpolymer complex.14−17 In addition to ambient electricalstability, the use of polymer acids also enhances the electro-chemical and physiological pH stability of conducting polymersand opens the possibility of their use in electrochemical bio-sensors, artificial muscles, and smart windows.2,18,19 Thisenhanced stability and water dispersibility, however, has tradi-tionally come at the expense of electrical conductivity. Theincorporation of polymer acids generally introduces structuraldisorder that is correlated with reduced electrical conductivity(

forms particles with smaller hydrodynamic diameter and PANIbecomes more crystalline.21,26 These structural differences inturn result in conductivity improvements in PANI−PAAMPSAthin films from 0.4 to 1.1 S/cm as the molecular weight of thePAAMPSA template is reduced from 724 to 45 kg/mol, asshown in Figure 1b (unfilled circles).27 Yoo et al. showed thatby narrowing the size distribution of the PAAMPSA template,PANI crystallinity and the connectivity between conductingdomains can be further improved, and the conductivity ofas-cast PANI−PAAMPSA films can be increased to 2.4 S/cm(filled circles in Figure 1b).25

Given mass transport limitations, the molecular weight ofPAAMPSA necessarily impacts the extent of doping, whichalters the conductivity of PANI−PAAMPSA, albeit to a smallerextent than the structural influence discussed above. That dop-ing is incomplete is evidenced by control experiments inwhich different PANI−PAAMPSA grades are exposed to HClvapor. When PANI−PAAMPSA synthesized with a 255 or724 kg/mol PAAMPSA template is exposed to HCl vapor, therespective conductivities increase from 0.55 and 0.4 S/cm to0.65 and 0.55 S/cm, respectively. This modest increase in con-ductivity indicates that these PANI−PAAMPSA grades werenot fully doped as-synthesized. Conversely, the conductivitiesof PANI−PAAMPSA that is synthesized with 106 or 45 kg/molPAAMPSA templates remain unchanged upon HCl exposure,an observation that indicates these PANI−PAAMPSA gradesto be fully doped as-synthesized. Results from solid-state NMRexperiments suggest that mass transport limitations during PANItemplate synthesis in the presence of high molecular weightPAAMPSA results in the incorporation of a small fraction ofaniline to the polymer chain in its deprotonated quinoid form.27

As opposed to protonic doping, the conductive state ofPEDOT is accessed through redox doping. The polymer acid,in this case, does not play the role of a dopant. Instead, itbalances the charge of the electrically conductive oxidized stateof PEDOT. The choice of counterion similarly affects the mor-phology, the conformation of the PEDOT chain, and the stabil-ity of the oxidized PEDOT and results in differences in theelectrical conductivity of the resulting PEDOT:counterioncomplexes.28−30 The use of hyaluronic acid, for example, resultsin PEDOT:hyaluronic acid complexes with negligible con-ductivities, whereas PEDOT:PSS has conductivities that rangefrom 10−3 to 103 S/cm, depending on the synthesis and pro-cessing conditions.20,30 Collectively, these examples providestrong evidence that implicate the role of the polymer acidbeyond doping and charge balancing.Beyond electrical conductivity, the choice of the polymer

acid template also impacts the piezoresistive properties ofthe conducting polymer complexes. Piezoresistivity definesthe change in electrical resistance of conducting materials withmechanical deformation and is quantified by a unitless parametercalled the gauge factor that relates changes in electrical resis-tance to applied strain. The gauge factor of conducting mate-rials critically impacts their applicability as the active com-ponents in flexible sensors. High gauge factors are needed forstrain sensors, whereas near-zero gauge factors are needed forflexible thermoresistive or chemoresistive sensor applications toprevent mechanical deformation-related drifts in the measuredresistance.4,31,32 The ability to tune the piezoresistivity of con-ducting polymers can therefore augment their deployment inthese different applications. Figure 1c (black data points) showsthe impact that the molecular weight of PAAMPSA has onthe gauge factor of the resulting PANI−PAAMPSA thin films.

The gauge factor decreases linearly and becomes negative withdecreasing PAAMPSA molecular weight.21 The dependence ofthe gauge factor of PANI−PAAMPSA on the molecular weightof the PAAMPSA template is reminiscent of the anticorrelationbetween the electrical conductivity of PANI−PAAMPSA andthe PAAMPSA molecular weight.27 Similarly, this change ingauge factor can be correlated with structural differences betweenthe different PANI−PAAMPSA grades. A positive gauge factoris observed in samples comprising high molecular weightPAAMPSA (255 and 724 kg/mol); these samples have thelargest hydrodynamic radii as-synthesized, and the resultingfilms are largely amorphous (Figure 1a). A negative gaugefactor, on the other hand, is observed in the thin films ofPANI−PAAMPSA that are synthesized with lower molecularweight (45 and 106 kg/mol) PAAMPSA templates; thesePANI−PAAMPSA grades are more crystalline than those synthe-sized with higher molecular weight templates. We believe thenegative gauge factor to be a result of strain-induced alignment ofPANI crystallites that can bridge neighboring PANI−PAAMPSAparticles.21 A lack of PANI crystallites that connect individualPANI−PAAMPSA particles causes the mostly amorphousPANI−PAAMPSA samples to have positive gauge factors; stretch-ing of these films results in an increase in their resistance due toincreased separation between the conducting domains undertensile strain. The ability to tune the piezoresistive response ofPANI−PAAMPSA has enabled the use of this material in abroad range of flexible sensor applications, such as strainsensors that require high gauge factors, and thermoresistive orchemoresistive sensors that require near-zero gauge factorsfor accurate sensing under mechanical deformation.4,21

Although the examples we have given thus far focus onthe electrical properties of PANI that is template synthesizedon PAAMPSA, the choice of polymer acid dopant/counterionimpacts the macroscopic properties of electropolymerizedconducting polymers as well. The polymer acid that isused to dope polypyrrole during its electropolymerization, forexample, determines its hydrophobicity, roughness, Young’smodulus, and the brittleness of the resulting electricallyconducting polypyrrole films. Polypyrrole when doped withhyaluronic acid forms rough films with high moduli (Young’smodulus of 706 MPa) that are not suited for muscle celldifferentiation. In contrast, doping polypyrrole with poly-(2-methoxyaniline-5-sulfonic acid) results in conducting polymerfilms with smoother surfaces and lower moduli (Young’smodulus of 30 MPa) that are capable of electrically stimulatingcell differentiation in tissue engineering applications.33,34

As an alternative to altering the synthetic parameters to affectthe structure of conducting polymers, the incorporation ofcosolvents or postdeposition solvent annealing has been shownto impact the macroscopic properties of these conducting poly-mer complexes.3,35−37 Commercially available PEDOT:PSS, forexample, is synthesized through oxidative polymerization of3,4-ethylenedioxythiophene (EDOT) along PSS, where the PSStemplate acts as a counterion to charge balance the oxida-tively doped PEDOT. This template polymerization results inPEDOT:PSS particles with conducting PEDOT-rich cores andinsulating PSS-rich shells that are dispersible in water. CastingPEDOT:PSS dispersions forms structurally heterogeneous filmswith PEDOT-rich domains separated by PSS-rich shells in amatrix of free-PSS.38,39 The electrical conductivity of PEDOT:PSSis highly dependent on how PEDOT:PSS particles and thefree-PSS matrix are distributed in thin films, with contiguityof PEDOT-rich domains providing conducting pathways for

The Journal of Physical Chemistry Letters Perspective

DOI: 10.1021/acs.jpclett.7b01785J. Phys. Chem. Lett. 2017, 8, 4530−4539

4532

http://dx.doi.org/10.1021/acs.jpclett.7b01785

charge transport. One route to improve the conductivity ofPEDOT:PSS is the addition of cosolvents, such as dimethylsulfoxide (DMSO) and ethylene glycol (EG), which induce struc-tural changes in PEDOT:PSS at both molecular and interparticlelevels. The addition of cosolvents increases the PEDOT crystallitesize and decreases the π−π stacking distance. At the inter-particle level, this cosolvent treatment increases the size and thepurity of PEDOT-rich domains, redistributes PEDOT:PSS par-ticles and free PSS, and results in formation of thin films withmore contiguous conducting pathways.20,35−37 These struc-tural changes have been correlated with an increase in theelectrical conductivity of PEDOT:PSS (Clevios PH1000,Heraeus) from about 10 to 103 S/cm with the addition of5 wt % EG prior to deposition.20,35−37 Similarly, postdepositionsolvent annealing of PEDOT:PSS thin films has also shownto effectively increase the conductivity of PEDOT:PSS. Theexposure of PEDOT:PSS thin films to cosolvents, such asdichloroacetic acid (DCA), for example, removes free PSS andeffectively enhances the connectivity between PEDOT-richdomains. Accordingly, the electrical conductivity increases by2 orders of magnitude.3,35 Structural modification of conduct-ing polymer complexes induced by the addition of cosolvent orby postdeposition solvent annealing go beyond improving theelectrical conductivity; they can similarly enhance their thermo-electric properties. The figure of merit (ZT) of PEDOT:PSSafter EG exposure, for example, has been reported to be com-parable to those of nanostructured and epitaxially growninorganic thermoelectric materials at room temperature.40−42

With ZT of 0.42, solvent-annealed PEDOT:PSS is promising asan active material in waste-heat recovery devices.41,43 Coupledwith its solution processability and mechanical compliancewith flexible substrates, this improved ZT makes the fabricationof lightweight, flexible, and large-area thermoelectric devicespossible.In the case of PANI−PAAMPSA, conductivity improvements

with postdeposition solvent annealing are more sensitive tosolvent properties. Although both DCA and DMSO exposuresenhance the conductivity of PEDOT:PSS, only treatment withDCA among these two solvents increases the electrical conduc-tivity of PANI−PAAMPSA by more than two orders of mag-nitude (to 40 S/cm).3 In PANI−PAAMPSA thin films, becausethe polymer acid also proton dopes PANI, there are stronginteractions between the aniline repeat units and −SO3− groupsof PAAMPSA. DCA is a good solvent for PAAMPSA and hasan ionization constant, pKa, lower than that of PAAMPSA;exposing PANI−PAAMPSA thin films to DCA thereforeplasticizes PAAMPSA and induces structural rearrangementby disrupting the electrostatic interactions between anilinerepeat units and −SO3− groups of PAAMPSA. This structural

rearrangement results in the elimination of the particulatenature of PANI−PAAMPSA, increases PANI crystallinity, andenhances the connectivity of the conducting domains.3,4

DMSO, with a pKa higher than that of PAAMPSA, on theother hand, cannot induce such structural rearrangementsdespite being a good solvent for PAAMPSA, and no conductivityimprovement is observed in PANI−PAAMPSA thin films uponDMSO exposure. As yet another control experiment, Yoo et al.also tested HCl, a bad solvent, but it has a lower pKa thanPAAMPSA and no structural rearrangement is observed whenHCl is used.3 These examples show that the choice of solventfor manipulating structure in conducting polymer complexesnot only depends on the solubility of the polymer acid in thesolvent of choice but is also affected by the type and relativestrength of intermolecular interactions between the polymeracid, conducting polymer, and solvent.3,44

Seeing the effects of postdeposition DCA annealing on theelectrical conductivity of PANI−PAAMPSA thin films andknowing the sensitivity of the piezoresistive response of PANI−PAAMPSA to its microstructure, in our more recent work wealso tested the piezoresistivity of DCA-treated PANI−PAAMPSAfilms.4,21 Because DCA treatment increases the crystallinity ofPANI and enhances the connectivity of the conducting domains,a negative gauge factor is expected from these samples, aresponse that is similar to the piezoresistive response of theas-cast PANI−PAAMPSA films that are synthesized with lowmolecular weight PAAMPSA templates. Figure 1c (blue datapoints) shows that DCA treatment of PANI−PAAMPSA thinfilms results in a negative gauge factor of comparable mag-nitude for all grades independent of the molecular weight ofPAAMPSA used in the synthesis.4,21 By redefining the inter-molecular interactions between the conducting polymer and thepolymer acid dopant through postdeposition solvent anneal-ing, we are able to significantly impact the morphology, andconsequently, the electrical conductivity and the piezoresistivityof PANI−PAAMPSA.Separately, Tarver et al. showed that morphological control

of PANI−PAAMPSA through postdeposition DCA annealingeliminates hysteresis during electrochromic switching of PANI−PAAMPSA. Moreover, this treatment results in PANI−PAAMPSAadopting its emeraldine salt form that shows significant near-IRabsorption, opening the possibility of using this postdepositionprocessed material in smart windows that can regulate heattransmission in addition to visible light transmission to increasethe energy efficiency of residential and commercial build-ings.2,22 Examples given in this section show that tunability ofthe structure of the conducting polymer/polymer acid com-plexes not only enables optimization of their optoelectronicperformance but also provides access to new functionalitiesinaccessible with conducting polymers doped with small-molecule acids for different applications.Mixed Ionic and Electronic Conductivities in Conducting

Polymer/Polymer Acid Complexes. Materials that can transportions in addition to electrons/holes form an intermediate classof materials between liquid and polymer electrolytes that showonly ionic conductivity and inorganic semiconductors andmetals that only have electronic conductivity (in the form ofelectron or hole transport). This mixed conductivity can beemployed in many different applications in which the ionicallyconducting media interface with electronically conductingmaterials, such organic electrochemical transistors (OECTs),1

actuators,45 supercapacitors,46 controlled drug-delivery devi-ces,47 and neural monitoring devices.48 Conducting polymers

Tunability of the structure of theconducting polymer/polymer acidcomplexes not only enables opti-mization of their optoelectronicperformance but also providesaccess to new functionalities

inaccessible with conducting pol-ymers doped with small-moleculeacids for different applications.



4533


that have polymer acid dopants/counterions are promising can-didates as mixed ionic and electronic conductors.46 Havinga polymer acid dopant/counterion that in itself is a poly-electrolyte enables ion transport in these conducting polymercomplexes while still rendering them electronically conduc-ting, environmentally stable, and solution-processable.46,49−51

Such mixed ionic and electronic conduction differentiates theseconducting polymers from inorganic semiconductors and small-molecule acid-doped conducting polymers that typically onlytransport electrons and/or holes.Figure 2a illustrates mixed conduction in PEDOT:PSS; this

mechanism is contrasted to Figure 2b, which illustrates hole-only conduction in boron-doped silicon when they are incontact with a liquid electrolyte. At the interface betweenPEDOT:PSS and the electrolyte solution, ions can move intoand through PEDOT:PSS with relative ease because PSSsupports ion transport.52 Moreover, the free volume created bythe disordered PSS matrix further facilitates ion transport. Thetight packing of the covalently bound silicon network, on theother hand, limits ion transport. Additionally, the presence of anative oxide layer atop boron-doped silicon prevents theelectrolytes from coming into direct contact with silicon andcreates an additional barrier to ion transport.23 These differ-ences make conducting polymer complexes uniquely capableof mixed conduction. In this case, PSS is responsible fortransporting ions and PEDOT is responsible for transportingholes.This feature of mixed ionic and electronic conductivity

enabled by the presence of the polymer acid template facilitatescommunication between ion-containing biological environ-ments and electronically conducting inorganic semiconductor-or metal-based electronic devices.53 Coupled with their bio-compatibility and mechanical compliance, conducting polymersare ideal candidates to directly and conformably interface withliving tissues. Figure 3a shows a micrograph of a flexible brainmonitoring device based on PEDOT:PSS-coated microelec-trodes. Leveraging the mechanical compliance of PEDOT:PSS,this microelectrode array can conform to the topography of

the brain surface. Figure 3b shows that the PEDOT:PSSmicroelectrode array has at least an order of magnitude lowerimpedance than either the gold or silicon electrodes that arestandard electrodes in use today. A low impedance in neuralrecording devices is correlated with a higher signal-to-noiseratio. The PEDOT:PSS microelectrode array is able to providesingle-neuron-resolution monitoring of human brain activityfrom the brain surface without the need to penetrate braintissues.54 Similarly, conducting polymer-based OECTs haveshown promise as noninvasive neural recording devices.Highly conformable arrays of PEDOT:PSS-based OECTs thatare placed on the somatosensory cortex, for example, are shownto record electrophysical signals with little to no invasion, withmore sensitivity than other surface electrodes and comparablesensitivity as electrodes that penetrate brain tissues.1 Becauseconducting polymers can conduct ions through the bulk, theircapacitance is defined by their volume. In contrast, the capac-itance of inorganic semiconductors is limited to their surfacearea. This difference in capacitance can be significant; it hasbeen shown that the volumetric capacitance can be almost2 orders of magnitude higher than its double-layer capacitancefor the same material.55,56 Bulk ion transport thus dramaticallyincreases the sensitivity of the brain monitoring devices com-pared to conventional inorganic brain monitoring devices.The enhanced sensitivity of these PEDOT:PSS-based neuralrecording devices is attributed to the PEDOT:PSS active layer’sability to capture and transport ions generated in the neuralsystem in addition to its ability to detect local field potentialswith higher signal-to-noise ratio than the conventional electro-des due to the low impedance of PEDOT:PSS electrodes.54,57

Having a combination of ion-conducting and electron/hole-conducting regions in these polymer acid templated conductingpolymers can also enable easy fabrication of electronicallycontrolled drug delivery devices. Figure 3c shows the devicearchitecture of an ion pump based on PEDOT:PSS interfac-ing with an electrolyte solution. The ion pump is composed oftwo electronically (hole transport) and ionically conductingPEDOT:PSS electrodes (dark blue) that are connected by an

Figure 2. Scheme of (a) the conducting polymer (PEDOT:PSS) and (b) inorganic semiconductor (boron-doped silicon) interfacing a biologicalenvironment (electrolyte). The hydrated ion sizes are the same in both cases. Reprinted with permission from ref 23. Copyright 2014 AmericanChemical Society.



4534


overoxidized PEDOT:PSS region (pink) that only conductsions. The overoxidized PEDOT loses its electrical conductivitydue to disruption of its conjugation pathway, but the PSSportion of this polymer complex remains ionically conductive.PSS thus facilitates ion transfer and forms a solid-state saltbridge between the two electrodes in this ion pump. When apotential bias is applied between the two electrodes, thePEDOT on the left electrode is oxidized, and its PSS releases a

cation (denoted as M+ in Figure 3c) in order to charge balancethe oxidized PEDOT. The PEDOT on the right electrode issimultaneously reduced. Upon PEDOT oxidation/reduction onthe left/right electrodes, respectively, free M+ cations movefrom the left electrode to the right electrode through theoveroxidized PEDOT:PSS bridge. Ion transfer continues withan applied potential bias until PEDOT is completely oxidizedand reduced on the left and right electrodes, respectively.58

Critical to operation of this ion pump is the ability for PSS totransport ions and for PEDOT to exhibit an electronic (hole)conductivity that is redox-controlled. Figure 3d shows thetransport of ions across a concentration gradient between twoelectrolytes in this ion pump. The electrodes are initiallybrought in contact with the identical electrolyte solutions con-taining potassium ions, K+ (solid black bars). After applying abias for 25 min, a decrease in the K+ concentration is observedin the left electrolyte with a concomitant increase in the K+

concentration in the right electrolyte (cross hatched bars).58

The ion pump has been successfully demonstrated in vivo todeliver neurotransmitters to the auditory system of a guinea pig,and has also been used in combination with PEDOT:PSS-basedneural monitoring devices to monitor epileptiform dischargesinduced in mouse hippocampus and then to deliver aninhibitory neurotransmitter to stop epileptiform activity.47,59

These examples show that beyond enabling electron con-duction in conducting polymers, polymer acid dopants/counterions, with their ion conduction, add critical functionalitythat would otherwise be missing in small-molecule acid-dopedconducting polymers.

Given the applicability of mixed ionic and electronic con-ductivity in various electronic applications, especially as theypertain to human health, it is important to understand whatdetermines the extent of ion and electron/hole transport inconducting polymers. Ion transport is facilitated by the freevolume created by the structurally disordered polymer aciddopants/counterions; yet the same structural disorder limitselectron/hole transport. That this structural disorder aids iontransport but impedes electron/hole transport suggests that therelative contributions of ionic and electronic conductivity canbe controlled by tuning the morphology of these conductingpolymer systems.49,50 In order to understand how structuralchanges affect ion transport in PEDOT:PSS thin films, Rivnayet al. performed one-dimensional moving front experimentsto monitor the movement of K+ ions in PEDOT:PSS thinfilms.37,51 Figure 4a shows the dependence of the electronic(hole) conductivity (blue) and K+ ion mobility in PEDOT:PSSthin films on the amount of EG added to PEDOT:PSS prior todeposition. While the addition of cosolvents, such as EG orDMSO, increases the electronic conductivity of PEDOT:PSS,this cosolvent addition significantly retards ion mobility.

Figure 3. (a) Optical micrograph of a conformable PEDOT:PSS-basedneural interface array. The scale bar is 200 μm. (b) Electrochemicalimpedance of the PEDOT:PSS microelectrode array shown in(a) (filled circles) and conventional Au-based electrodes (emptycircles). The inset shows the impedance of the PEDOT:PSSmicroelectrode array (blue) and of conventional implantable siliconprobes (red) composed of different arrays of electrodes. Reprinted bypermission from Macmillan Publisher Ltd.: Nature Neuroscience(ref 54), copyright (2014). (c) Scheme of an ion pump based onPEDOT:PSS electrodes (dark blue) connected with an overoxidizedPEDOT:PSS bridge (pink). Light blue regions represent the elec-trolytes that are brought into contact with the PEDOT:PSS electrodes.When a potential bias is applied between the two electrodes, ionsfrom one electrode can be carried to the other electrode through theionically conducting but electronically insulating overoxidizedPEDOT:PSS bridge. (d) K+ ion concentration in the electrolytesplaced on the left (AB) and right (CD) electrodes of the ion pumpshown in (c). The black bar represents the K+ concentration in theinitial electrolyte solution, and the horizontal hatched bar representsthe K+ concentration of the electrolyte that is in contact with the left(AB) and right (CD) electrodes, respectively, when the ion pump isoff. The cross hatched bar represents the K+ concentration 25 minafter the pump is turned on. The K+ concentration decreases in the left(AB) electrolyte and increases by the same amount in the right (CD)electrolyte. Reprinted by permission from Macmillan Publisher Ltd.:Nature Materials (ref 58), copyright (2007).

Having a combination ofion-conducting and electron/hole-conducting regions inpolymer acid templated

conducting polymers can alsoenable easy fabrication of

electronically controlled drugdelivery devices.



4535


The decrease in ionic conductivity upon EG addition is attributedto a loss in free volume with increased connectivity of PEDOT-rich domains.37 This postdeposition processing thus impartsan ability to tailor the relative extent of ionic and electronic

conductivities per application needs. When building OECTsbased on conducting polymers in which high ionic conduc-tivity is needed, for example, the anticorrelation betweenthe electronic and ionic conductivities in PEDOT:PSS with EGaddition must be kept in mind in optimizing the device per-formance.

Ef fect of Relative Humidity on the Electrical Properties ofConducting Polymer/Polymer Acid Complexes. Polymer acids arehygroscopic; one disadvantage of using polymer acids to dopeand/or charge balance conducting polymers is how waterabsorption impacts macroscopic properties. Humidity affectsionic conductivity and the extent of solvation of the polymeracid and can also change the morphology of the conductingpolymer due to plasticization of the polymer acid template.Figure 4b shows changes in the electronic and ionic con-ductivities of PEDOT:PSS thin films with humidity.24 As rela-tive humidity increases, ionic conductivity increases becausewater facilitates ion transport.60 Interestingly, the electronicconductivity decreases with humidity up to 40% relativehumidity and then recovers and even exceeds the electronic

Figure 4. (a) Change in the electronic conductivity (blue) and K+ ionmobility (red) in PEDOT:PSS thin films with the addition of EG tothe PEDOT:PSS dispersion prior to deposition. Reprinted withpermission from ref 37. (b) Conductivity of PEDOT:PSS thin filmsmeasured by impedance spectroscopy (blue) as a function of %relative humidity. Electronic (black) and ionic (red) conductivities arecalculated from the impedance spectra by equivalent circuit modelfittings. Reprinted with permission from ref 24.

The change in the piezoresistivepolarity of PEDOT:PSS thin filmsupon drying indicates that waterabsorption induced morphologi-cal changes not only impact thestatic electrical properties of

PEDOT:PSS but also affect howthe conducting PEDOT domains

rearrange under dynamicalmechanical deformation.

Figure 5. Change in the relative resistance (black) of as-cast PEDOT:PSS films under cyclic tensile strain (blue) (a) in air and (b) afterencapsulation in a N2-filled glovebox. Change in the relative resistance (black) of as-cast PANI−PAAMPSA-724 films under cyclic tensile strain(blue) (c) in air and (d) after encapsulation in a N2-filled glovebox.



4536


conductivity of the dry film as the relative humidity continuesto increase. This nonmonotonic change in the electronic con-ductivity with relative humidity suggests that the presence ofwater has competing effects on the structure of PEDOT:PSSthin films. Water uptake by PSS is manifested as an increasein the PEDOT:PSS film thickness by more than 10% withina few minutes of exposing dry PEDOT:PSS films to 70%relative humidity.61 The initial decrease in electronic con-ductivity up to 40% relative humidity is attributed to swell-ing of the electrically insulating PSS-rich shell and PSS matrix,which in turn increases the separation between PEDOT-rich conducting domains. As the relative humidity furtherincreases from 40%, increased solvation of the counterionsreduces the electrostatic interactions between PEDOT andPSS, and results in an increase in the hole mobility.24,44,62

This increase in hole mobility counterbalances the decreasein the conductivity due to increased separation betweenconducting domains, and the electronic conductivity recoversand exceeds what is reported for the dried PEDOT:PSSfilms.24

Inspired by this work showing how morphological changesinduced by water uptake of the polymer acid affect the elec-tronic conductivity and knowing that the piezoresistiveresponse of conducting polymers is also highly sensitive totheir thin-film morphology, we conducted experiments tostudy the effect of humidity on the piezoresistive response ofPEDOT:PSS (Clevios PH 1000, Heraeus) thin films. Figure 5ashows the change in the electrical resistance of as-castPEDOT:PSS (black) under cyclic applied strain (blue) in air.The resistance increases with increasing tensile strain andrecovers when the strain is removed (positive gauge factor).When the film is kept under vacuum for a few hours andthen encapsulated in a N2-filled glovebox to eliminate fur-ther absorption of water, we instead observe a negativegauge factor. The resistance of the encapsulated-PEDOT:PSSfilm decreases with increasing tensile strain (Figure 5b).This change in the piezoresistive polarity of PEDOT:PSS thinfilms upon drying indicates that water absorption inducedmorphological changes not only impact the static electricalproperties of PEDOT:PSS but also affect how the conduct-ing PEDOT domains rearrange under dynamical mechanicaldeformation.The positive gauge factor observed in PEDOT:PSS in air is

correlated with the separation between conducting domainswith stretching. When PEDOT:PSS films are kept in air,water absorption causes swelling of PSS-rich domains andthis phenomenon in turn disrupts the connectivity betweenPEDOT-rich conducting domains. Due to a lack of connectivitybetween individual conducting domains, PEDOT:PSS particlesbehave like separate entities.63 Upon stretching, they can slideby each other, and the conducting domains become furtherseparated, which results in an increase in resistance with tensiledeformation. When water is removed from the PEDOT:PSSfilms, on the other hand, PEDOT:PSS particles come closer,and PEDOT chains from one particle can interact withthe neighboring PEDOT-rich domains.63,64 The interactionsbetween neighboring PEDOT:PSS particles at low relativehumidity have been reported to be very strong, so thateven upon fracture, the crack propagates through particlesas opposed to along interparticle boundaries.63 These stronginteractions connect conducting domains so they no longerbehave as individual particles. Although the exact mechanism ofthe reversible decrease in the resistance of the encapsulated

PEDOT:PSS films with tensile strain is unknown, stretchingmust cause rearrangement or alignment of this already-con-nected conducting PEDOT domains in a way to promotecharge transport.

As a control experiment, we also tested the effect of humidityon the piezoresistive response of PANI−PAAMPSA. UnlikePEDOT:PSS, conducting PANI is preferentially located on theexterior of PANI−PAAMPSA particles.3 We therefore hypo-thesized that water uptake from air is not the reason for thepositive gauge factor observed in high molecular weight PANI−PAAMPSA samples (shown in Figure 1b).26 Swelling of thepolymer acid should instead bring the conducting domainscloser, as opposed to separating them. Electrical conductivitymeasurements support our hypothesis. Unlike PEDOT:PSSthin films, PANI−PAAMPSA films show increased electricalconductivity in air in comparison to films tested under vacuumdue to increased ionic conductivity and increased solvation ofthe polymer acid with increased humidity.3 In PEDOT:PSSthin films, on the other hand, water uptake from air disrupts theconduction pathway and results in a decrease in the elec-trical conductivity.24 In order to understand the effect ofhumidity on the piezoresistive response of PANI−PAAMPSA,we performed cyclic strain tests on PANI synthesized on724 kg/mol PAAMPSA, PANI−PAAMPSA-724, because itforms the largest particles and the least crystalline films(Figure 1a).27 In order to understand if this positive gaugefactor is a result of water uptake, we compared the piezo-resistive response of PANI−PAAMPSA-724 films tested inair and after keeping the same film under vacuum andencapsulating in a N2-filled glovebox. Figure 5c,d shows therelative change in the resistance of PANI−PAAMPSA-724films with applied cyclic strain in air and after encapsula-tion, respectively. In both films, the resistance increases withapplied tensile strain and recovers when the strain is removed.We do not observe a change in the polarity of the piezo-resistive response of PANI−PAAMPSA-724 upon drying thefilms. These experiments show that PANI−PAAMPSA is morerobust in its electrical properties in response to humidity com-pared to PEDOT:PSS. We believe this robustness is cor-related with the morphology of PANI−PAAMPSA with theconducting PANI on the exterior of the hygroscopic polymeracid. These findings can also implicate the possibility of usingprocessing routes, as opposed to using different conductingpolymer complexes, that determine the distribution of the con-ducting polymer and the polymer acid dopant/counterion in

The choice of the polymer acidand/or manipulation of the inter-molecular interactions betweenpolymer acid, conducting poly-mer, and solvent through pro-cessing can significantly impactoptoelectronic properties, relativecontributions of ionic and elec-tronic conductivity of conducting

polymer complexes, and thesensitivity of the electrical prop-

erties to varying humidity.



4537


thin films to control how humidity affects the morphology andconsequently the electrical properties of conducting polymer/polymer acid complexes.Beyond a template, a dopant, and/or a charge balancing

agent, polymer acids are active components that bring addi-tional functionalities to conducting polymer complexes. Themacroscopic properties of these conducting polymer complexesare not only determined by the conducting polymer itselfbut also depend on the molecular properties of the polymeracid. The choice of the polymer acid and/or manipulation ofthe intermolecular interactions between polymer acid, con-ducting polymer, and solvent through processing can sig-nificantly impact optoelectronic properties, relative contribu-tions of ionic and electronic conductivity of conducting poly-mer complexes, and the sensitivity of the electrical prop-erties to varying humidity. With this understanding of therole of polymer acids, we can begin to think about tailoringmacroscopic properties by properly designing conductingpolymer/polymer acid complexes.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected] (M.S.-E.).*E-mail: [email protected] (Y.-L.L.).ORCIDMelda Sezen-Edmonds: 0000-0003-0476-6815Yueh-Lin Loo: 0000-0002-4284-0847NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful for funding from the Princeton Centerfor Complex Materials that is supported by NSF-MRSEC underNSF Award DMR-1420541.

■ REFERENCES(1) Khodagholy, D.; Doublet, T.; Quilichini, P.; Gurfinkel, M.;Leleux, P.; Ghestem, A.; Ismailova, E.; Herve,́ T.; Sanaur, S.; Bernard,C.; et al. In Vivo Recordings of Brain Activity Using OrganicTransistors. Nat. Commun. 2013, 4, 1575.(2) Davy, N. C.; Sezen-Edmonds, M.; Gao, J.; Lin, X.; Liu, A.; Yao,N.; Kahn, A.; Loo, Y.-L. Pairing of near-Ultraviolet Solar Cells withElectrochromic Windows for Smart Management of the SolarSpectrum. Nat. Energy 2017, 2, 17104.(3) Yoo, J. E.; Lee, K. S.; Garcia, A.; Tarver, J.; Gomez, E. D.; Baldwin,K.; Sun, Y.; Meng, H.; Nguyen, T.-Q.; Loo, Y.-L. Directly Patternable,Highly Conducting Polymers for Broad Applications in OrganicElectronics. Proc. Natl. Acad. Sci. U. S. A. 2010, 107 (13), 5712−5717.(4) Sezen, M.; Register, J. T.; Yao, Y.; Glisic, B.; Loo, Y.-L.Eliminating Piezoresistivity in Flexible Conducting Polymers forAccurate Temperature Sensing under Dynamic Mechanical Deforma-tions. Small 2016, 12 (21), 2832−2838.(5) Chiang, C. K.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E.J.; MacDiarmid, A. G. Conducting Polymers: Halogen DopedPolyacetylene. J. Chem. Phys. 1978, 69 (11), 5098−5104.(6) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.;Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. ElectricalConductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39 (17),1098−1101.(7) Münstedt, H. Ageing of Electrically Conducting OrganicMaterials. Polymer 1988, 29 (2), 296−302.(8) Pochan, J. M.; Pochan, D. F.; Rommelmann, H.; Gibson, H. W.Kinetics of Doping and Degradation of Polyacetylene by Oxygen.Macromolecules 1981, 14 (1), 110−114.

(9) Meng, H.; Perepichka, D. F.; Bendikov, M.; Wudl, F.; Pan, G. Z.;Yu, W.; Dong, W.; Brown, S. Solid-State Synthesis of a ConductingPolythiophene via an Unprecedented Heterocyclic Coupling Reaction.J. Am. Chem. Soc. 2003, 125 (49), 15151−15162.(10) Kaplan, S.; Conwell, E. M.; Richter, A. F.; MacDiarmid, A. G.Solid-State 13C NMR Characterization of Polyanilines. J. Am. Chem.Soc. 1988, 110 (23), 7647−7651.(11) MacDiarmid, A. G.; Mammone, R. J.; Kaner, R. B.; Porter, S. J.;et al. The Concept of D̀oping’ of Conducting Polymers: The Role ofReduction Potentials. Philos. Trans. R. Soc., A 1985, 314 (1528), 3−15.(12) Cao, Y.; Smith, P.; Heeger, A. J. Counter-Ion InducedProcessibility of Conducting Polyaniline and of ConductingPolyblends of Polyaniline in Bulk Polymers. Synth. Met. 1992, 48(1), 91−97.(13) Epstein, A.; Ginder, J. M.; Zuo, F.; Woo, H.-S.; Tanner, D. B.;Richter, A. F.; Angelopoulos, M.; Huang, W.-S.; MacDiarmid, A. G.Insulator-to-Metal Transition in Polyaniline: Effect of Protonation inEmeraldine. Synth. Met. 1987, 21 (1−3), 63−70.(14) Chen, S.-A.; Lee, H.-T. Structure and Properties of Poly-(acrylicAcid)-Doped Polyaniline. Macromolecules 1995, 28 (8), 2858−2866.(15) Moon, H.; Park, J. Structural Effect of Polymeric Acid Dopantson the Characteristics of Doped Polyaniline Composites. Synth. Met.1998, 92 (3), 223−228.(16) Hwang, J. H.; Yang, S. C. Morphological Modification ofPolyaniline Using Polyelectrolyte Template Molecules. Synth. Met.1989, 29 (1), 271−276.(17) Lapkowski, M. Electrochemical Synthesis of Polyaniline/Poly-(2-Acryl-Amido-2-Methyl-1-Propane-Sulfonic Acid) Composite.Synth. Met. 1993, 55 (2−3), 1558−1563.(18) Wallace, G.; Spinks, G. Conducting Polymers - Bridging theBionic Interface. Soft Matter 2007, 3 (6), 665.(19) Tarver, J.; Yoo, J. E.; Dennes, T. J.; Schwartz, J.; Loo, Y.-L.Polymer Acid Doped Polyaniline Is Electrochemically Stable beyondpH 9. Chem. Mater. 2009, 21 (2), 280−286.(20) Wei, Q.; Mukaida, M.; Naitoh, Y.; Ishida, T. MorphologicalChange and Mobility Enhancement in PEDOT:PSS by Adding Co-Solvents. Adv. Mater. 2013, 25 (20), 2831−2836.(21) Sezen-Edmonds, M.; Khlyabich, P. P.; Loo, Y.-L. Tuning theMagnitude and the Polarity of the Piezoresistive Response ofPolyaniline through Structural Control. ACS Appl. Mater. Interfaces2017, 9 (14), 12766−12772.(22) Tarver, J.; Yoo, J. E.; Loo, Y.-L. Polyaniline Exhibiting Stableand Reversible Switching in the Visible Extending into the near-IR inAqueous Media. Chem. Mater. 2010, 22 (7), 2333−2340.(23) Rivnay, J.; Owens, R. M.; Malliaras, G. G. The Rise of OrganicBioelectronics. Chem. Mater. 2014, 26 (1), 679−685.(24) Wang, H.; Ail, U.; Gabrielsson, R.; Berggren, M.; Crispin, X.Ionic Seebeck Effect in Conducting Polymers. Adv. Energy Mater.2015, 5 (11), 1500044.(25) Yoo, J. E.; Bucholz, T. L.; Jung, S.; Loo, Y.-L. Narrowing theSize Distribution of the Polymer Acid Improves PANI Conductivity. J.Mater. Chem. 2008, 18 (26), 3129.(26) Yoo, J. E.; Krekelberg, W. P.; Sun, Y.; Tarver, J. D.; Truskett, T.M.; Loo, Y.-L. Polymer Conductivity through Particle Connectivity.Chem. Mater. 2009, 21 (9), 1948−1954.(27) Yoo, J. E.; Cross, J. L.; Bucholz, T. L.; Lee, K. S.; Espe, M. P.;Loo, Y.-L. Improving the Electrical Conductivity of Polymer Acid-Doped Polyaniline by Controlling the Template Molecular Weight. J.Mater. Chem. 2007, 17 (13), 1268.(28) Culebras, M.; Gomez, C.; Cantarero, A. Enhanced Thermo-electric Performance of PEDOT with Different Counter-IonsOptimized by Chemical Reduction. J. Mater. Chem. A 2014, 2 (26),10109−10115.(29) King, Z. A.; Shaw, C. M.; Spanninga, S. A.; Martin, D. C.Structural, Chemical and Electrochemical Characterization ofpoly-(3,4-Ethylenedioxythiophene) (PEDOT) Prepared with VariousCounter-Ions and Heat Treatments. Polymer 2011, 52 (5), 1302−1308.



4538

mailto:[email protected]:[email protected]://orcid.org/0000-0003-0476-6815http://orcid.org/0000-0002-4284-0847http://dx.doi.org/10.1021/acs.jpclett.7b01785

(30) Hofmann, A. I.; Katsigiannopoulos, D.; Mumtaz, M.;Petsagkourakis, I.; Pecastaings, G.; Fleury, G.; Schatz, C.;Pavlopoulou, E.; Brochon, C.; Hadziioannou, G.; et al. How toChoose Polyelectrolytes for Aqueous Dispersions of ConductingPEDOT Complexes. Macromolecules 2017, 50 (5), 1959−1969.(31) Hattori, Y.; Falgout, L.; Lee, W.; Jung, S.-Y.; Poon, E.; Lee, J.W.; Na, I.; Geisler, A.; Sadhwani, D.; Zhang, Y.; et al. MultifunctionalSkin-Like Electronics for Quantitative, Clinical Monitoring ofCutaneous Wound Healing. Adv. Healthcare Mater. 2014, 3 (10),1597−1607.(32) Liu, N.; Fang, G.; Wan, J.; Zhou, H.; Long, H.; Zhao, X.Electrospun PEDOT:PSS-PVA Nanofiber Based Ultrahigh-StrainSensors with Controllable Electrical Conductivity. J. Mater. Chem.2011, 21 (47), 18962.(33) Gelmi, A.; Higgins, M. J.; Wallace, G. G. Physical Surface andElectromechanical Properties of Doped Polypyrrole Biomaterials.Biomaterials 2010, 31 (8), 1974−1983.(34) Gilmore, K. J.; Kita, M.; Han, Y.; Gelmi, A.; Higgins, M. J.;Moulton, S. E.; Clark, G. M.; Kapsa, R.; Wallace, G. G. Skeletal MuscleCell Proliferation and Differentiation on Polypyrrole Substrates Dopedwith Extracellular Matrix Components. Biomaterials 2009, 30 (29),5292−5304.(35) Kim, Y. H.; Sachse, C.; MacHala, M. L.; May, C.; Müller-Meskamp, L.; Leo, K. Highly Conductive PEDOT:PSS Electrode withOptimized Solvent and Thermal Post-Treatment for ITO-FreeOrganic Solar Cells. Adv. Funct. Mater. 2011, 21 (6), 1076−1081.(36) Palumbiny, C. M.; Heller, C.; Schaffer, C. J.; Körstgens, V.;Santoro, G.; Roth, S. V.; Müller-Buschbaum, P. MolecularReorientation and Structural Changes in Cosolvent-Treated HighlyConductive PEDOT:PSS Electrodes for Flexible Indium Tin Oxide-Free Organic Electronics. J. Phys. Chem. C 2014, 118 (25), 13598−13606.(37) Rivnay, J.; Inal, S.; Collins, B. A.; Sessolo, M.; Stavrinidou, E.;Strakosas, X.; Tassone, C.; Delongchamp, D. M.; Malliaras, G. G.Structural Control of Mixed Ionic and Electronic Transport inConducting Polymers. Nat. Commun. 2016, 7, 11287.(38) Zhou, J.; Anjum, D. H.; Chen, L.; Xu, X.; Ventura, I. A.; Jiang,L.; Lubineau, G. The Temperature-Dependent Microstructure ofPEDOT/PSS Films: Insights from Morphological, Mechanical andElectrical Analyses. J. Mater. Chem. C 2014, 2 (46), 9903−9910.(39) DeLongchamp, D. M.; Vogt, B. D.; Brooks, C. M.; Kano, K.;Obrzut, J.; Richter, C. A.; Kirillov, O. A.; Lin, E. K. Influence of aWater Rinse on the Structure and Properties of poly-(3,4-Ethylenedioxythiophene):Poly-(styrene Sulfonate) Films. Langmuir 2005, 21(24), 11480−11483.(40) Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.;Fabretto, M.; Hojati-Talemi, P.; Dagnelund, D.; Arlin, J.; Geerts, Y. H.;et al. Semi-Metallic Polymers. Nat. Mater. 2013, 13 (2), 190−194.(41) Kim, G.-H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Dopingof Organic Semiconductors for Enhanced Thermoelectric Efficiency.Nat. Mater. 2013, 12 (8), 719−723.(42) Liu, S.; Deng, H.; Zhao, Y.; Ren, S.; Fu, Q. The Optimization ofThermoelectric Properties in a PEDOT:PSS Thin Film through Post-Treatment. RSC Adv. 2015, 5 (3), 1910−1917.(43) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.;Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figureof Merit in the Conducting Polymer poly-(3,4-ethylenedioxythio-phene). Nat. Mater. 2011, 10 (6), 429−433.(44) Yu, Z.; Xia, Y.; Du, D.; Ouyang, J. PEDOT:PSS Films withMetallic Conductivity through a Treatment with Common OrganicSolutions of Organic Salts and Their Application as a TransparentElectrode of Polymer Solar Cells. ACS Appl. Mater. Interfaces 2016, 8(18), 11629−11638.(45) Smela, E. Conjugated Polymer Actuators for BiomedicalApplications. Adv. Mater. 2003, 15 (6), 481−494.(46) Malti, A.; Edberg, J.; Granberg, H.; Khan, Z. U.; Andreasen, J.W.; Liu, X.; Zhao, D.; Zhang, H.; Yao, Y.; Brill, J. W.; et al. An OrganicMixed Ion-Electron Conductor for Power Electronics. Adv. Sci. 2016, 3(2), 1500305.

(47) Jonsson, A.; Inal, S.; Uguz, I.; Williamson, A. J.; Kergoat, L.;Rivnay, J.; Khodagholy, D.; Berggren, M.; Bernard, C.; Malliaras, G.G.; et al. Bioelectronic Neural Pixel: Chemical Stimulation andElectrical Sensing at the Same Site. Proc. Natl. Acad. Sci. U. S. A. 2016,113 (34), 9440−9445.(48) Abidian, M. R.; Ludwig, K. A.; Marzullo, T. C.; Martin, D. C.;Kipke, D. R. Interfacing Conducting Polymer Nanotubes with theCentral Nervous System: Chronic Neural Recording Using poly-(3,4-ethylenedioxythiophene) Nanotubes. Adv. Mater. 2009, 21 (37),3764−3770.(49) Armand, M. Polymers with Ionic Conductivity. Adv. Mater.1990, 2 (6−7), 278−286.(50) Riess, I. Polymeric Mixed Ionic Electronic Conductors. SolidState Ionics 2000, 136−137 (1−2), 1119−1130.(51) Stavrinidou, E.; Leleux, P.; Rajaona, H.; Khodagholy, D.; Rivnay,J.; Lindau, M.; Sanaur, S.; Malliaras, G. G. Direct Measurement of IonMobility in a Conducting Polymer. Adv. Mater. 2013, 25 (32), 4488−4493.(52) Larsson, K. C.; Kjal̈l, P.; Richter-Dahlfors, A. OrganicBioelectronics for Electronic-to-Chemical Translation in Modulationof Neuronal Signaling and Machine-to-Brain Interfacing. Biochim.Biophys. Acta, Gen. Subj. 2013, 1830 (9), 4334−4344.(53) Martin, D. C. Polymers Manipulate Cells. Nat. Mater. 2007, 6(9), 626−627.(54) Khodagholy, D.; Gelinas, J. N.; Thesen, T.; Doyle, W.;Devinsky, O.; Malliaras, G. G.; Buzsaki, G. NeuroGrid: RecordingAction Potentials from the Surface of the Brain. Nat. Neurosci. 2014,18 (2), 310−315.(55) Rivnay, J.; Leleux, P.; Ferro, M.; Sessolo, M.; Williamson, A.;Koutsouras, D. a.; Khodagholy, D.; Ramuz, M.; Strakosas, X.; Owens,R. M.; et al. High-Performance Transistors for Bioelectronics throughTuning of Channel Thickness. Sci. Adv. 2015, 1 (4), e1400251−e1400251.(56) Proctor, C. M.; Rivnay, J.; Malliaras, G. G. UnderstandingVolumetric Capacitance in Conducting Polymers. J. Polym. Sci., Part B:Polym. Phys. 2016, 54 (15), 1433−1436.(57) Buzsaḱi, G.; Anastassiou, C. a; Koch, C. The Origin ofExtracellular Fields and Currents–EEG, ECoG, LFP and Spikes. Nat.Rev. Neurosci. 2012, 13 (6), 407−420.(58) Isaksson, J.; Kjal̈l, P.; Nilsson, D.; Robinson, N. D.; Berggren,M.; Richter-Dahlfors, A. Electronic Control of Ca2+ Signalling inNeuronal Cells Using an Organic Electronic Ion Pump. Nat. Mater.2007, 6 (9), 673−679.(59) Simon, D. T.; Kurup, S.; Larsson, K. C.; Hori, R.; Tybrandt, K.;Goiny, M.; Jager, E. W. H.; Berggren, M.; Canlon, B.; Richter-Dahlfors, A. Organic Electronics for Precise Delivery of Neuro-transmitters to Modulate Mammalian Sensory Function. Nat. Mater.2009, 8 (9), 742−746.(60) Doan, T. C. D.; Ramaneti, R.; Baggerman, J.; Tong, H. D.;Marcelis, A. T. M.; Van Rijn, C. J. M. Intrinsic and Ionic Conductionin Humidity-Sensitive Sulfonated Polyaniline. Electrochim. Acta 2014,127, 106−114.(61) Elschner, A.; Kirchmeyer, S.; Lovenich, W.; Merker, U.; Reuter,K. PEDOT: Principles and Applications of an Intrinsically ConductivePolymer; CRC Press: Boca Raton, FL, 2011.(62) Xia, Y.; Ouyang, J. PEDOT:PSS Films with SignificantlyEnhanced Conductivities Induced by Preferential Solvation withCosolvents and Their Application in Polymer Photovoltaic Cells. J.Mater. Chem. 2011, 21 (13), 4927.(63) Lang, U.; Naujoks, N.; Dual, J. Mechanical Characterization ofPEDOT:PSS Thin Films. Synth. Met. 2009, 159 (5−6), 473−479.(64) Zhou, J.; Anjum, D. H.; Lubineau, G.; Li, E. Q.; Thoroddsen, S.T. Unraveling the Order and Disorder in Poly-(3,4-ethylenediox-ythiophene)/Poly-(styrenesulfonate) Nanofilms. Macromolecules 2015,48 (16), 5688−5696.



4539

beyond doping and charge balancing: how polymer acid ......conducting polymers, such as pani, proton...

Documents