synthesis and layer-by-layer deposition of spherical poly(3,4-ethylenedioxythiophene) nanoparticles...

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Synthesis and Layer-by-Layer Deposition ofSpherical Poly(3,4-ethylenedioxythiophene)Nanoparticles - Toward Fast Switching Timesbetween Reduced and Oxidized States

Kevin Muller, Mi-Kyoung Park, Markus Klapper, Wolfgang Knoll,Klaus Mullen*

A microemulsion polymerization for the synthesis of poly(3,4-ethylenedioxythiophene)(PEDOT) nanoparticles is described. Spherical and electrically conducting PEDOT particleswith diameters as small as 27� 8 nm were obtained by using decyltrimethylammoniumbromide as a cationic surfactant. PEDOT nanoparticle multi-layers, alternated with poly(styrene sulfonate), were preparedby the layer-by-layer deposition technique. Electrochemicalsurface plasmon resonance experiments revealed a ten-foldincrease in the switching time between redox states as com-pared with Baytron P/poly(ethyleneimine) multilayers. Theenhanced charge transport can be attributed to an improveddiffusion of the charge-balancing counterions into the PEDOTnanoparticles.

Introduction

Due to its high stability, high conductivity in the doped

state, and transparency in thin oxidized films poly(3,4-

ethylenedioxythiophene) (PEDOT) is one of the most

promising examples of conducting polymers.[1,2] Many

applications of PEDOT have been reported ranging from

antistatic and conductive coatings for various devices to

hole injecting layers used in organic light emitting

diodes.[3–5] Today, PEDOT is commercially available as a

water-soluble PEDOT-poly(styrene sulfonate) (PEDOT:PSS,

Baytron P) complex.[6]

K. Muller, M.-K. Park, M. Klapper, W. Knoll, K. MullenMax-Planck Institute for Polymer Research, Ackermannweg 10,D-55128 Mainz, GermanyFax: (þ49) 6131 379 100; E-mail: [email protected]

Macromol. Chem. Phys. 2007, 208, 1394–1401

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

An interesting aspect of Baytron P is the opportunity to

affect the oxidation level reversibly by chemical or electro-

chemical doping/dedoping (switching between the oxi-

dized and reduced state). This feature allows Baytron P to

be utilized as an electroactive material in various sensory

devices, such as glucose biosensors or hydrogen chloride

and ammonia vapor sensor devices.[7–10] The switching

time between the oxidation states is determined by elect-

ron hopping (between neighboring oxidized and reduced

sites of the PEDOT segments) and the ionic diffusion of

charge-compensating counterions through the polymer

film to the electroactive PEDOT sites.[11] In this process,

during oxidation, counterions are incorporated into the

PEDOT film in order to compensate the positive charge.

On the other hand during reduction, as the PEDOT

segments are neutralized, the counterions are released.

It was demonstrated in the literature that the charge-

compensating counterions, as well as the analytes,

DOI: 10.1002/macp.200700142

Synthesis and Layer-by-Layer Deposition of Spherical Poly(3,4-ethylenedioxythiophene) Nanoparticles . . .

penetrate only slowly through the Baytron P film.[12,13]

This can be attributed to the low accessibility of the PEDOT

segments due to high amount of PSS in the Baytron P

complex.[14,15] Due to this fact, the switching time of such

devices is rather slow (about 20 s).[12,13] An approach for

increasing the switching time would be to enhance the

accessibility of the electroactive PEDOT sites for the coun-

terions. One way to increase the accessibility is through

the use of dispersible PEDOT latex particles. In this case, no

PSS is needed to keep the rather insoluble PEDOT segments

in solution and the latex particles can be processed directly

out of dispersion.[16] The majority of PEDOT nanoparticles

have been produced using iron(III) salts and different anio-

nic emulsifiers such as sodium dodecylsulfate (SDS) and

sodium dodecylbenzenesulfonate (SDBS) in oxidative

emulsion polymerization processes.[16–18] Disadvantages

of these nanoparticles are their undefined shapes and

relatively large size with broad size distributions. This is

presumably due to the interaction between the anionic

emulsifier and the iron(III) cations which destabilizes the

emulsion system during the polymerization. As such, the

use of a cationic emulsifier would circumvent the inter-

action between the surfactant and the positively charged

oxidant.

The second major factor in the construction of sensory

devices from electroactive polymers is their processability

as well-defined layers. An approach which produces well-

defined films is the layer-by-layer (LbL) ionic self-assembly

method,[19,20] wherein a charged substrate is alternatively

exposed to solutions containing oppositely charged species

to form integrated ultrathin films. It has been used to

create highly tuned, functional thin films with the control

of composition and structure.[21]

Herein, we report the synthesis of positively charged

PEDOT nanoparticles by an oxidative microemulsion

polymerization in the presence of cationic surfactants.

The positively charged surface of PEDOT nanoparticles

permitted the use of the LbL self-assemblymethod offering

a flexible procedure for the preparation of PEDOT devices

with a high surface area. The multilayer growth of

positively charged PEDOT nanoparticles and negatively

charged PSS was monitored by UV-vis spectroscopy. In

addition, for the determination of the switching behavior,

the redox properties of the PEDOT nanoparticle layered

architecture, compared with a Baytron P multilayer, were

evaluated using chronoamperometry and electrochemical

surface plasmon resonance (SPR) measurements.

Experimental Part

Materials

3,4-Ethylenedioxythiophene (EDOT) and Baytron P (PEDOT:PSS)

were provided by H.C. Starck (Leverkusen, Germany). Decyltri-



methylammonium bromide (DETAB), iron(III) p-toluenesulfo-

nate hexahydrate, poly(diallyldimethylammonium chloride)

(PDADMAC; Mw < 200000), poly(sodium 4-styrenesulfonate)

(PSS; Mw 70 000), poly(ethyleneimine) (PEI; Mw 25 000), 3-

aminopropyltriethoxysilane (APS), and sodium 3-mercapto-1-

propanesulfonate (MPS) were obtained by Sigma-Aldrich. Iron(III)

chloride hexahydrate was purchased by Acros Organics. All

materials were used without further purification. A phosphate

buffer solution was prepared with Na2HPO4 and NaH2PO4 (0.05 M,

pH¼ 7.40). Millipore water (18 MV) was used throughout the

experiments.

Characterization

FT-IR spectra were obtained with a Nicolet 730 FT-IR spectrometer

using a Thermo Electron Endurance ATR single-reflection ATR

crystal. UV-vis spectra were obtained using a Perkin-Elmer

Lambda 900 spectrometer. Dynamic light scattering measure-

ments were performed on a Malvern Zetasizer 3000. For the

determination of electrical conductivity, PEDOT films were

prepared using a blade coater (200mmslit). Conductivitymeasure-

ments were carried out using a Jandel microposition four-point

probe connected to a Keithley 2700DMM digital multimeter.

Transmission electron microscopy (TEM) studies were performed

on carbon meshes using a LEO EM 912 microscope. The average

particle diameters were measured directly from each TEM image.

The diameters of a minimum of 100 particles were measured and

the values averaged by using an image analysis software (Soft

Imaging System). For atomic forcemicroscopy (AFM), a Dimension

3100 scanning probe microscope (Digital Instruments, Santa

Barbara, CA) was used in tapping mode employing Olympus

cantilevers with spring constants ranging between 33.2 and

65.7 N �m�1 and resonant frequencies of 277–346 Hz (as specified

by the manufacturer).

Preparation of PEDOT Nanoparticles

DETAB (2.65 g, 9.5mmol)was stirred inwater (45mL) for 1 h. EDOT

(1.00 g, 7.04 mmol) was added dropwise to the surfactant solution

and stirringwas continued for 2 h. The polymerizationwas started

by the addition of the iron(III) salt (molar ratio of iron(III)/EDOT

being 2:1) dissolved in water (5 mL). The microemulsion was

stirred for 3 h at room temperature. After the addition of excess

methanol to the reaction mixture, separation of the PEDOT

nanoparticles was achieved by centrifugation. The particles were

washed with methanol and redispersed in methanol (15 mL) by

ultrasonification for 20 min.

Preparation of PEDOT Nanoparticle Multilayers

The gold substrates were prepared by thermal evaporation of gold

(450 nm) on top of a chromium (1.5 nm) layer onto LaSFN9 glass

substrates. The substrates were then immersed in an ethanolic

MPS solution (1�10�3M) overnight, rinsed with ethanol, and

dried under a stream of nitrogen in order to obtain a negatively

charged surface. The MPS functionalized gold substrates were

alternatively dipped in PDADMAC [1 mg �mL�1 in 0.2 M NaCl (aq)]

www.mcp-journal.de 1395

K. Muller, M.-K. Park, M. Klapper, W. Knoll, K. Mullen

Figure 1. Preparation of PEDOT nanoparticles in microemulsion.

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and PSS [1 mg �mL�1 in 0.2 M NaCl (aq)] for 15 min each. The

substrates were thoroughly rinsed with water between dippings.

Two bilayers of PDADMAC and PSSwere deposited to give a higher

charge density and a smooth surface. For UV-vis spectroscopy

measurements, a quartz slide was used as the substrate. The

quartz slide was functionalized with APS (1� 10�3M in toluene) to

give a positively charged surface. Before the deposition of PEDOT

nanoparticles on the (PSS/PDADMAC)2-gold substrate, methanol

was exchanged for water by centrifugation, removal of the

supernatant, and redispersion of the positively charged PEDOT

nanoparticles in ethanol and then in water. The PEDOT dispersion

was sonificated for 5 min prior to the deposition. The substrates

were dipped in the PEDOT dispersion for 2 h and then rinsed with

water. For the assembly of Baytron P multilayer, PEI (1 mg �mL�1)

was used as a polycation. A (PSS/PDADMAC)2-gold substrate was

alternatively dipped into PEI solution and diluted Baytron P

solution (20 times diluted with MilliQ water from the original

solution).

Electrochemical Measurement and Electrochemical

Surface Plasmon Spectroscopy

Chronoamperometry was performed with a mAutolab type-III

potentiostat (Eco Chemie B. V., Netherlands) in a three-electrode

Teflon cell with the gold substrate as the working electrode

(0.502 cm2), a platinum wire as a counterelectrode, and an Ag/

AgCl reference electrode. The attenuated total reflection (ATR)

setup in the Kretschmann configuration, combined with an

electrochemical cell, was used for the in situ electrochemical SPR

measurements. Details about this setup are published else-

where.[22,23] The Au/glass substrates were clamped against the

Teflon cell with an O-ring providing a liquid-tight seal. Surface

plasmonswere excited at themetal-dielectric interface, upon total

internal reflection of a polarized He-Ne laser (632.8 nm) beam. The

optical/electrochemical processes on the gold were detected by

monitoring the reflectivity as a function of the incident angle or at

a fixed angle as a function of time while a constant voltage was

applied to the multilayers.

Figure 2. FT-IR spectrum of PEDOT nanoparticles (sample 2).

Results and Discussion

PEDOT Nanoparticle Synthesis

For the preparation of the PEDOT nanoparticles, a

microemulsion procedure was chosen. Due to high surfac-

tant concentrations, microemulsions are thermodynami-

cally stable and the monomer droplets are sufficiently

small that they become the locus of polymerization.[24]

Emulsification of the EDOT monomer in an aqueous phase

using different emulsifiers, such as octyltrimethylammo-

nium bromide (OTAB), DETAB, and tetradecyltrimethyl-

ammonium bromide (TETAB) was investigated. OTAB did

not appear to be suitable for the stabilization of the

microemulsion system since it gave undefined PEDOT

nanoparticles with mean diameters as large as 1 mm. The

octyl chain of the OTAB detergent may be too short to



stabilize the monomer droplets. Analogous findings were

made for the TETAB emulsifier, which formed aqueous

solutions with high viscosities. In contrast, DETAB

was found to be well suited for the stabilization of the

monomer-filled micelles. Polymerization was then per-

formed upon the addition of iron(III) chloride or iron(III)

p-toluenesulfonate to the EDOT microemulsion (Figure 1).

After polymerization, the nanoparticles were separated

from the residual iron salts and redispersed inmethanol or

water, respectively.

The resulting dispersion was stable at room tempera-

ture for several weeks at various solid concentrations

(2–10% by weight). FT-IR spectroscopy of the precipitated

particles showed the characteristic ring vibration of the

thiophene ring at 1 460 cm�1 and of PEDOT at 1 380 cm�1,

which can be attributed to the quinoidal C–C and C––C

structure (Figure 2). Further vibrations at 1 170 and

DOI: 10.1002/macp.200700142


Figure 3. TEM image of PEDOT nanoparticles (sample 2).

Figure 4. Particle size distribution (by number) of sample 1,obtained from TEM images.

1 000 cm�1 were assigned to the C–O–C bond stretching,

whereby the C–S bond vibrations in the thiophene ring

were found at 974 and 850 cm�1, respectively.[25] Morpho-

logy studies of the PEDOT nanoparticles were performed

by applying TEM. The obtained nanoparticles showed a

spherical shape and an average particle diameter of 27 nm

(�8 nm) (Figure 3 and 4). This was measured directly from

TEM images by counting the diameters of more than 100

particles using an image analysis software.

It was possible to tune the size of the particles by vary-

ing the surfactant concentration. As expected for micro-

emulsion polymerization, the sizes of the PEDOT particles

decreased with increase in emulsifier concentration

(Table 1: samples 3 and 4). Oxidative polymerization

using iron(III) p-toluenesulfonate proceeded faster than the

polyreaction using iron(III) chloride and was accomplished

after 2 h. This is consistent with the higher solubility of

iron(III) p-toluenesulfonate in the EDOT monomer leading

to a better diffusion into the monomer-filled micelles.

To reveal that the obtained PEDOT nanoparticles are

electrically conductive, conductivity measurements were

Table 1. Oxidative Polymerization of EDOT in microemulsion polyme

Sample Oxidant [DETAB] [EDOT]

mol � LS1 mmol � LS1

1 FeCl3 0.40 141

2 FeCl3 0.38 149

3 Fe(OTs)3 0.38 144

4 Fe(OTs)3 0.30 153

a)Optained By TEM.



performed by the four-point probe on blade coated PEDOT

nanoparticle films. Measurements show the conductivity

to be 0.30 S � cm�1 (�0.08 S � cm�1) at room temperature. In

comparison, the commercially available Baytron P exhi-

bits, depending on the way of processing, a conductivity

ranging from 1� 10�5 S � cm�1 (used for passive matrix

displays) to 1 S � cm�1 (used for antistatic devices).[5] The

similar conductivities of the PEDOT nanoparticles and the

Baytron P demonstrates that conductivities in the metallic

range could be obtained and sufficient percolation path-

ways through the prepared nanoparticle film were

present. This might be attributed to a migration of the

low molecular weight surfactant from the nanoparticle to

the surface of the film, a behavior which has been reported

for various traditional latex particles.[26,27]

It was verified that well defined, spherical, and electri-

cally conductive PEDOT nanoparticles were obtained by

facile, oxidative microemulsion polymerization, using

cationic emulsifiers. As such, the applicability of these

colloids for the construction of PEDOT multilayer devices

by LbL ionic self-assembly was investigated using their

positively charged surface due to residual cationic emulsi-

fiers. For a possible application as sensors, the PEDOT

rization.

Oxidant/EDOT Yield Particle sizea)

% nm

202:100 42 27 (W8)

195:100 37 29 (W7)

200:100 50 34 (W8)

350:100 50 45 (W5)



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nanoparticle multilayers were electrochemically charac-

terized. Thereby, the switching time between the oxidized

and the reduced PEDOT state was investigated as this is a

key parameter for these applications. The results were

subsequently compared to the switching time of Baytron

P multilayers, prepared and characterized under similar

conditions.

Figure 6. UV-vis spectra of the LbL films of (PEDOT/PSS)n (n¼ 1–5)on (PDADMAC/PSS)2-coated glass slide (inset: Absorbance at 1 170nm at a function of the number of bilayers).

LbL Deposition of PEDOT Nanoparticles

Multilayer assemblies were prepared through the LbL

deposition of positively charged PEDOT nanoparticles with

negatively charged PSS. The first layer of PEDOT nano-

particles was formed on an LbL self-assembled layer of

(PSS/PDADMAC)2, where the outermost layer was nega-

tively charged. Precoating of substrates with a (PSS/

PDADMAC)n multilayer was performed as this system is

known to promote the association of a higher charge

density and a uniform surface, improving adsorption of

the nanoparticles on the negatively charged surface.[28]

The straightforward formation of the nanoparticle LbL

films is attributed to the combination of two factors: (i)

electrostatic interactions of the positively charged PEDOT

nanoparticles with the PSS layer and (ii) van der Waals

attractive forces.[29,30] First, the morphology of nanopar-

Figure 5. AFM image of PEDOT nanoparticles (sample 4) on (PSS/PDADMAC)2 on a gold substrate (5� 5 mm2). A cross-section alongthe horizontal line indicated by the arrow is shown beneath theimage.



ticles on a precoated layer was studied by AFM (Figure 5).

The majority of the nanoparticles were observed as iso-

lated objects in a 5� 5 mm2 area. The cross-section of the

AFM image shows that the nanoparticles have a height of

30–45 nm, which is comparable to their lateral size

measured by TEM.

Incomplete surface coverage with nanoparticles was

also observed, presumably due to the electrostatic repul-

sion. The density of surface-charged particles often can be

increased by the addition of salt which screens the electro-

static repulsions.[31] However, the addition of NaCl (0.2 M)

to the nanoparticle dispersion induced coagulation of

the nanoparticles. Nonetheless, a well dispersed layer of

PEDOT nanoparticles was deposited on the (PSS/PDAD-

MAC)2-coated gold substrate. Ultrasonication of the

nanoparticle solution prior to deposition significantly

reduced the aggregation of the nanoparticles on the

substrate.

The LbL deposition of PEDOT nanoparticles and PSS up to

five bilayers on a quartz substrate was monitored by

UV-vis spectroscopy (Figure 6). The broad peaks near 750

and 1 150 nm correspond to the oxidized PEDOT.[32]

Absorbance at 1 170 nm (Figure 6, inset) of the films was

directly proportional to the number of layers. Similar

results were observed by SPR for the linear growth of the

nanoparticle/PSS multilayers on gold electrodes. Thus, LbL

films of PEDOT nanoparticles and PSS were successfully

deposited on both quartz substrates and gold electrodes.

Electrochemical SPR

The changes in optical properties and the swelling/

shrinking process of PEDOT nanoparticles upon applica-

tions of oxidative and reductive potentials were studied by

DOI: 10.1002/macp.200700142


Figure 7. SPR scans of (PEDOT nanoparticles/PSS)4 multilayer on agold substrate while applying a constant potential of 0.6 and�0.8 V (versus Ag/AgCl) in 50� 10�3 M PB (pH¼ 7.40) aqueoussolution.

Figure 8. Time-dependent SPR reflectivity changes of (a) (BaytronP/PEI)3 multilayer on gold electrodes at a fixed incidence angle(61 8) and (b) (PEDOT nanoparticles/PSS)4 multilayer at a fixedincidence angle (58 8) upon application of potential square wavesbetween �0.8 and 0.6 V (versus Ag/AgCl) with a time step of 20 s.All experiments were measured in 50� 10�3 M (pH¼ 7.40) aqu-eous solution.

the electrochemical SPR measurements.[22,23,33] The SPR

spectra of the redox states of the PEDOT multilayer were

obtained while applying potentials corresponding to �0.8

and 0.6 V, respectively (Figure 7). The resonance angle

(minimum angle) and the shape of the curve were

significantly altered during the oxidation process of the

PEDOTmultilayer. It was observed that the resonance peak

shifted to the lower angle and the curve became both

deeper and sharper. During oxidation, the complex di-

electric constant (erþ iei, real and imaginary, respectively)

and the thickness of the multilayer change due to the

electrochromic properties of PEDOT as well as the swelling

of the PEDOT particles. This swelling is attributed to the

uptake of counterions, accompanied by the hydration of

the polymer.[34,35] If the neutral PEDOT is oxidized, its color

changes from dark blue to transparent sky blue.[36] UV-vis

spectra of the PEDOT film show that the reduced form

exhibits an absorption band centered at 600 nm, ascribed

to the p-p� electronic transition. Upon oxidation, the

intensity of this band decreases and the absorbance at the

longer-wavelength region increases.[37–39] Since the ima-

ginary part of the dielectric constant, ei, relates to the

extinction coefficient at a particular wavelength (in this

case, l¼ 632. 8 nm), it decreases upon the oxidation of the

PEDOT. The real part of the dielectric constant, er, alsodecreases due to the swelling of the PEDOT nanoparticles

upon oxidation, and results in a shifting of the SPR curve to

lower angle. The coupling of electrochemical methods and

SPR spectroscopy gives rise to a high sensitivity for

changes in the refractive index such that very few layers of

the PEDOT nanoparticle multilayer are required for

applications.

A square-wave potential step method coupled with SPS

was used to monitor the changes in the reflectivity of SPR



curves with time during the repeated switching between

the redox states of PEDOT in order to obtain insights into

the swelling-shrinking behavior and the stability of the

film. Baytron P/PEI LbL multilayers, traditionally used as

electrochromic materials in optical devices and biosen-

sors,[3,4,40] were prepared and the resulting electrochemical

properties were compared with those of PEDOT nanopar-

ticle multilayers. The Baytron P complex has a PEDOT to

PSS ratio of 1:2.5 (by weight) and due to excess PSS, it is

negatively charged.[5] Hence, positively charged PEI was

used as the counterpolycation for building multilayers.

Both samples were prepared with similar average thick-

nesses, ca. 23 nm, as measured with SPR spectroscopy. The

SPR reflectivity at a fixed angle was measured while the

potential was stepped between �0.8 and 0.6 V with

time intervals of 20 s. For the (Baytron P/PEI)3 multilayers,

the oxidation of the polymer film (from �0.8 to 0.6 V)

resulted in a fast decrease in the reflectivity followed by a



Figure 9. Chronoamperometric transients measured upon appli-cations of (a) a reductive potential step from 0.8 to �0.8 V ongold electrodes modified with LbL films of (PEDOT nanoparticle/PSS)4 (solid line) and (Baytron P/PEI)3 (dotted line) multilayers.(Inset: the Cottrell plots (current versus t�1/2) of PEDOT nanopar-ticle (closed square) and Baytron P (open triangle) multilayers.The data were recorded in 50� 10�3 M phosphate buffer, pH¼ 7.4.

1400

slow decreasewhich did not reach a plateau even after 20 s

[Figure 8(a)]. Upon the reduction (from 0.6 to �0.8 V), the

reflectivity showed an initial rapid increase and continued

with a slow decrease over 20 s. [Figure 8(a)]. The immediate

changes in reflectivity upon switching potentials are

attributed to the changes in the complex dielectric cons-

tant of Baytron P as a result of its oxidation and reduction.

The subsequent slow changes result from swelling and

shrinking of the film. The swelling is attributed to the

uptake of counterions in order to compensate the charges

of the oxidized film and solution molecules. On the other

hand, the film releases the counterions upon the reduction

resulting in the shrinking of the film. Similar switching

behavior of redox polymer films has been reported.[12,13]

For the (PEDOT nanoparticle/PSS)4 multilayer film, the

reflectivity switched between its maximum and mini-

mumvalues upon the application of reductive (�0.8 V) and

oxidative (0.6 V) potentials in less than 2 s [Figure 8(b)].

This was more than ten times faster than that of the

Baytron P multilayer and indicates that the shrinking and

swelling of the nanoparticles took place immediately after

the application of the potentials. Furthermore, the mini-

mum and maximum values of the reflectivity remained

constant after five cycles of switching indicating the

stability of the nanoparticle films as compared to the

Baytron P films. Faster switching of the PEDOT nanopar-

ticle film, presented in this study, can be attributed to a

better diffusion of the charge-balancing counterions into

the electroactive PEDOT particle layer. A potential ex-

planation for the enhanced diffusion can be that the

PEDOT particles in the outermost layer of the film are not

covered by PSS, whereas in the case of Baytron P the PSS is

located, due to its high hydrophilicity, at the multilayer/

solution interface. Thus, the diffusion of the counterions to

the PEDOT is hindered.

Chronoamperometry

In order to confirm the fast ion transport through the

nanoparticle multilayer, the electrochemical properties of

the (PEDOT nanoparticle/PSS)4 and (Baytron P/PEI)3 multi-

layers were investigated using chronoamperometric mea-

surements. The current transients of the multilayers on

gold electrodes were recorded as a function of time upon

the application of potential steps. First, current transients

were measured upon the application of a reductive

potential step from 0.6 to �0.8 V (Figure 9).

For both multilayers, the current transients did not fall

to zero even after 120 s. The residual currents are due to the

reduction of the electrolyte on the electrode surface,

resulting in the migration of charged species toward the

surrounding solution, which follows the electrical field.

Counterions and solvent molecules in the film are dragged



along due to an electro-osmotic effect, promoting a conti-

nuous compacting of the polymeric structure.[41] The

chronoamperometric transients of the multilayers were

analyzed by replotting the current transients as a function

of t�1/2 (Figure 9, inset). The linear portion of each curve

obeys the Cottrell equation,[42]

IðtÞ ¼ nFAD1=2 C

p1=2

� �t�1=2

where I(t) is the current transient that decays in the course

of time t(s), n is the number of electrons per unit of the

redox polymer, F is the Faraday constant, A is the electrode

area, D is the diffusion coefficient for the charge propa-

gation in the polymer film, and C is the concentration of

the redox units in the polymer film which is calculated

from the electrochemically active volume and the total

amount of charge passed following a potential step. The

total charge was obtained by integration of the current I(t)

from t¼ 0 to infinity upon the application of an oxidative

potential, which was found to give similar values for both

multilayers. The current transients versus t�1/2 of the first

5 s upon the application of �0.8 V showed a linear depen-

dence, whereby the slope of the linear decay for the PEDOT

nanoparticle multilayer was steeper than that for Baytron

multilayer, indicating a faster decay of the current

transient. In the reduction of the PEDOT nanoparticle

multilayer, large capacitive charges were delivered in the

early stage (in 5 s) due to the high surface area.[43] Upon the

application of the Cottrell equation to the linear portion of

the current versus t�1/2, the ratio of the charge diffusion

coefficients for PEDOT nanoparticles/PSS to Baytron/PEI

multilayer films was calculated to be ca. 3. This enhanced

DOI: 10.1002/macp.200700142


rate of charge transport of the PEDOT nanoparticles is

attributed to an increase in the rate of ion transport from

solution to the redox polymer due to high surface area of

the nanoparticles, which resulted in fast switching

behavior of the nanoparticle multilayers.

Conclusion

A versatile microemulsion polymerization for the fabrica-

tion of electrically conducting PEDOT nanoparticles was

presented. By applying cationic surfactants as stabilizers,

i.e. DETAB, electrostatic interactions between the oxidant,

i.e. iron(III), and the emulsifier could be circumvented. Thus,

spherical PEDOT colloids with average diameters as low as

27 nm were obtained.

A PEDOT nanoparticle device was prepared by the LbL

ionic self-assembly method due to the positively charged

nature of the nanoparticles. Electrochemical SPR measure-

ments demonstrated a switching time of less than 2 s for

the PEDOT nanoparticle/PSS multilayers. This is more than

ten times faster than the switching time of traditional

Baytron P/PEI multilayers. The improved switching time

can be attributed to an enhanced diffusion of the

counterions into the PEDOT nanoparticles. Fast switching

between the oxidized and reduced PEDOT states is ofmajor

importance, as this is a prerequisite for sensor applica-

tions. As such, further directions of this work will include

the application of PEDOT particle multilayers for enzy-

matic biosensors.

Acknowledgements: The authors acknowledge Walter Scholdeifor IR-measurements and the German Science Foundation (SFB625) as well as the Bayer AG for financial support. M.-K. Parkacknowledges the Alexander von Humboldt Foundation for aresearch fellowship.

Received: March 13, 2007; Accepted: March 21, 2007; DOI:10.1002/macp.200700142

Keywords: conducting polymers; chronoamperometry; electro-chemical surface plasmon resonance spectroscopy; layer-by-layerdeposition; PEDOT nanoparticles; TEM

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