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Cite this: DOI: 10.1039/c2sc21020g

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Organic electrochemical transistors monitoring micelle formation†

Giuseppe Tarabella,a Gaurav Nanda,bMarco Villani,a Nicola Copped�e,aRobertoMosca,aGeorge G. Malliaras,c

Clara Santato,b Salvatore Iannottaa and Fabio Cicoira*d

Received 20th July 2012, Accepted 29th August 2012

DOI: 10.1039/c2sc21020g

Organic electrochemical transistors (OECTs) exploit electrolyte gating to achieve the transduction of

ionic currents. Therefore, they are ideally suitable to sense different chemo/bio species dissolved in the

electrolyte. Current modulation in OECTs relies on doping or dedoping of the OECT channel by

electrolyte ions. Nevertheless the role played by the specific physicochemical properties of an electrolyte

on OECT operation is largely unknown. Here we investigate OECTs, making use of aqueous solutions

of the micelle-forming cationic surfactant cetyltrimethylammonium bromide (CTAB) as the electrolyte.

Micelle-forming salts are remarkable model systems to study the doping and dedoping mechanism of

OECTs, because the aggregation of dissociated ions into micelles at the critical micelle concentration

permits to modify the size and the type of the species that dope or dedope the OECT channel in situ. The

current modulation of OECTs using a CTAB electrolyte shows a marked increase close to the critical

micellar concentration. The measurement of the transistor’s drain current as a function of CTAB

concentration provides a simple, fast method to detect the formation of micelles from dissociated ions.

Introduction

Organic electrochemical transistors (OECTs) have been exten-

sively investigated for applications in bioelectronics and sensing,

because of their ability to operate in aqueous solutions at low

voltages (<1 V).1–6 OECTs consist of an organic channel (typi-

cally a conducting polymer thin film) in ionic contact with a gate

electrode via an electrolyte solution. These devices have been

used as converters between ionic currents in the electrolyte and

electronic currents in the organic channel,7 and as sensors to

monitor the attachment of cancer cells and fibroblasts cultured

directly on the channel.8 OECTs have also been employed as

sensors for hydrogen peroxide,9,10 glucose11,12 and dopamine13 by

exploiting the ability of the conducting polymer channel to detect

electrochemical reactions in the electrolyte. Faradaic operation

in the presence of Ag gate electrodes permits their use as sensors

for chloride ions.14 OECTs with single strand DNA probes

immobilized on a Au gate electrode have been used to detect

complementary DNA targets.15

aCNR-IMEM, Institute of Materials for Electronics and Magnetism,Parco Area delle Scienze 37/A, 43124, Parma, ItalybDepartment of Engineering Physics, �Ecole Polytechnique de Montr�eal,2500 chemin de Polytechnique, Montr�eal, Qu�ebec, H3T 1J4, CanadacDepartment of Bioelectronics, �Ecole Nationale Sup�erieure des Mines,CMP-EMSE, MOC, 13541, Gardanne, FrancedDepartment of Chemical Engineering, �Ecole Polytechnique de Montr�eal,2500 chemin de Polytechnique, Montr�eal, Qu�ebec, H3T 1J4, Canada.E-mail: fabio.cicoira@polymtl.ca

† Electronic supplementary information (ESI) available: experimentaland characterization details, OECT layout and absorption spectra. SeeDOI: 10.1039/c2sc21020g

This journal is ª The Royal Society of Chemistry 2012

It has been demonstrated that current modulation in OECTs

relies on doping or dedoping of the OECT channel by electrolyte

ions, rather than on field-effect doping.16 However, the role

played by the specific physicochemical properties of an electro-

lyte on OECT operation is largely unknown. Here, we report on

OECTs with an aqueous solution of the cationic surfactant

hexadecyltrimethylammonium bromide, also known as cetyl-

trimethylammonium bromide (CTAB), as the electrolyte. CTAB

forms micelles in aqueous solutions (Fig. S1†) above the critical

micellar concentration (CMC, about 1 � 10�3 M at 298 K).17–19

Micelle-forming salts are remarkable model systems for studying

the doping and dedoping mechanism of OECTs, because the

aggregation of dissociated ions into micelles at the CMC permits

the modification of the size and the type of the species that dopes

or dedopes the OECT channel in situ. Micelles are of primary

interest for drug delivery systems,20–22 and a compact sensor that

detects micelle formation would constitute a technological

breakthrough.

Device fabrication

Planar OECTs based on the conducting polymer poly(3,4-eth-

ylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) were

fabricated using a lithographic patterning process described

elsewhere.9 Two parallel stripes of PEDOT:PSS acted as the

transistor channel and the gate electrode. The electrolyte solution

was confined over the channel and the gate electrode using a

PDMS well (Fig. 1).

As the electrolyte, we employed CTAB aqueous solutions at

concentrations ranging from 1 � 10�6 M to 1 � 10�2 M. The

Chem. Sci.

Fig. 1 OECT device layout and electrical contacts. The distance

between the parallel PEDOT:PSS stripes (dark blue) is 100 mm. The

overlapping of the electrolyte well reservoir (gray) with the PEDOT:PSS

polymer defines the channel of the OECT.

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diameter of the CTAB micelles, estimated by dynamic light

scattering measurements at a CTAB concentration of 1 � 10�2

M, was determined to be about 4 nm, in agreement with values

reported in the literature.23–26

The mechanism of operation of OECTs based on PEDOT:PSS

has been described elsewhere.14 To detect changes due to the

formation of micelles, the OECT modulation |(I� I0)/I0|, where I

is the off source-drain current (for gate voltages,Vgs 0 V) and I0is the on source-drain current (for Vg ¼ 0 V), was measured as a

function of the CTAB concentration, varyingVg between 0 and 1

V and keeping the drain-source voltage (Vd) constant at �0.4 V.

Results and discussions

For each salt concentration, the values of the drain current were

obtained from transient current measurement (current versus

time) in steady state conditions (Fig. 2). Source-drain current

versus time plots of OECT employing CTAB as electrolyte, for

concentrations of 10�5 M (Fig. 2a) and 10�2 M (Fig. 2b), clearly

show that the OECT off current is lower at higher CTAB

concentration (i.e. in the presence of micelles).

A different behaviour is found for OECTs employing NaCl as

electrolyte, whose off current does not depend on the electrolyte

concentration. Therefore, in addition to the typical dependence

on Vg, the OECT modulation showed a clear dependency on

electrolyte concentration. Close to the CMC (about 5 � 10�4 M)

the CTAB OECT response at Vg ¼ 1 V increases to achieve its

maximum value above the CMC at 10�3 M and 10�2 M (Fig. 3a).

Fig. 2 Source-drain current versus time plots (transient current

measurements) acquired at Vg ¼ 0.8 V and Vd ¼ �0.4 V for CTAB

concentrations of 10�2 M (a) and 10�5 M (b).

Chem. Sci.

Below 10�4 M, no dependence on the concentration is observed

and the behavior is similar to other electrolyte systems, such as

NaCl (Fig. 3b).

The results discussed above are better visualized in a modu-

lation versus concentration plot (Fig. 4). For CTAB concentra-

tions between 1 � 10�6 and 1 � 10�4 M, the OECT modulation

was weakly affected by concentration changes, whereas between

1 � 10�4 M and 1 � 10�3 M it clearly increased with increasing

concentration. Indeed, at Vg ¼ 1 V, the OECTmodulation varies

from about 0.45 for concentrations below 1 � 10�4 M up to

about 0.9 at 1 � 10�3 M and 1 � 10�2 M. Such behaviour

(reproduced on four different OECTs) clearly differs from that of

an analogous OECT employing an aqueous solution of NaCl as

the electrolyte, which does not form micelles. The modulation of

OECTs using NaCl as the electrolyte is practically independent

of electrolyte concentration and remains as low as about 0.3

between 1 � 10�6 M and 1 � 10�2 M. Such behaviour is in

agreement with previous results,9 which demonstrated that the

current modulation in OECTs mostly depends on the double

layer capacitances at the gate/electrolyte and electrolyte/channel

interfaces, which depend weakly on the electrolyte concentra-

tion.27 As indicated above, the increase of modulation for

OECTs using CTAB electrolytes (hereafter indicated as CTAB

OECTs) occurs near the CMC. The gate-source current,

constantly monitored during OECT operation, was several

orders of magnitude lower than the drain-source current

(Fig. S2†). This excludes the possibility of Faradaic processes

between the gate electrode and the electrolyte solution.14 The

increase of the modulation is therefore a consequence of the

formation of CTA+ micelles, which dedope PEDOT:PSS more

effectively than dissociated CTA+ ions. This result, also

confirmed by gate voltage shift data (Fig. S3†), is rather

surprising. The large size of the micelles, compared to dissociated

ions, was expected to lead to a hindered incorporation into the

PEDOT:PSS film and hence to a weaker OECT modulation.

To confirm that the modulation of CTAB OECTs relies on

electrochemical dedoping of PEDOT:PSS by CTA+ micelles,

rather than on other effects such as capacitive coupling,28 we

exploited the electrochromic properties of PEDOT:PSS (i.e. its

property of changing colour upon change of its oxidation

state).29,30 In PEDOT:PSS OECTs, the oxidation state is

controlled by Vg, which, by inducing dedoping by positive elec-

trolyte ions, leads to the reduction–oxidation (redox) reaction:

PEDOT+:PSS� + M+ + e� 4 PEDOT0 + M+:PSS� (1)

Fig. 3 OECT response |(I� I0)/I0| as a function ofVg, withVd¼�0.4 V,

for different CTAB (a) and NaCl (b) concentrations.

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 OECTmodulation (|(I� I0)/I0|, where I is the off current (Vgs 0)

and the I0 is the on current (Vg ¼ 0)) versusmolar concentration of CTAB

and NaCl aqueous solutions at Vg ¼ 1 V, with the drain-source voltage

(Vd) kept constant at�0.4 V. The error bars correspond to the maximum

spread between the minimum and maximum value of the modulation

obtained on 5 repeated measurements. The lines are guides to the eye.

Fig. 5 Electrochromic switching of the PEDOT:PSS OECT channel.

Ultraviolet–Visible optical absorption spectra (in arbitrary units, a.u.) of

the PEDOT:PSS channel of a working OECT, with a 1 � 10�2 M CTAB

aqueous electrolyte solution. The blue dotted line corresponds to the

spectrum at Vg ¼ 0 V whereas the red continuous line corresponds to the

spectrum at Vg ¼ 1 V, with Vd ¼ �0.4 V. The wavelength steps are 1 nm.

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Upon application of a sufficiently high gate voltage, the colour

of the PEDOT:PSS films changes reversibly from light to dark

blue. This change corresponds to an electrochemical switching

between the oxidized (PEDOT+) and the reduced (PEDOT0)

states of PEDOT, accompanied by the reversible incorporation

of positively charged species (M+).31 Therefore, the electro-

chromic switching is proof of the electrochemical dedoping.32–34

Indeed, we visually observed a colour change during operation

of CTABOECTs, at concentration above the CMC. However, to

further prove that CTA+ micelles are incorporated into the

PEDOT:PSS film during OECT operation, we performed an in

situUV/Vis spectroscopy study on the PEDOT:PSS channel of a

working OECT using a 1 � 10�2 M aqueous CTAB solution as

the electrolyte (at Vd ¼ �0.4 V, varying Vg from 0 to 1 V). For

this measurement we used a specially fabricated OECT, which

could be mounted and operated in the location of the sample

cuvette of a UV-Vis spectrophotometer (Fig. S4†).

The UV/Vis absorption of the PEDOT:PSS OECT channel

showed significant changes upon application of Vg (Fig. 5). The

most striking feature is the appearance of a broad absorption

peak located between 500 and 650 nm at Vg ¼ 1 V, which is

absent at Vg ¼ 0 V. This peak corresponds to the optical

absorption of the reduced form of PEDOT0, which is dark blue,

whereas its pristine, oxidized (PEDOT+) state is light blue.35–37

The presence of this electrochromic effect proves that CTA+

micelles, despite the fact that they are bulkier than the dissociated

CTA+ ions, dedope the PEDOT:PSS channel upon application

of a positive Vg.

Understanding the detailed mechanism of PEDOT:PSS

doping/dedoping by CTA+ micelles is rather challenging and is

beyond the scope of this Article. In particular, further investi-

gations are required to understand whether CTA+ micelles

change structure when they reach the PEDOT:PSS surface.

However, a few hypotheses can be proposed. During the

dedoping process, the electrolyte cations neutralize the PSS� sites

in the PEDOT:PSS film, according to eqn (1). The overall effect

is a decrease in the number of positive charge carriers (holes)

injected into the polymer from the metal contacts and hence a

decrease of the transistor (drain-source) current. We expect that

This journal is ª The Royal Society of Chemistry 2012

positively charged CTA+ micelles, due to their large size,

neutralize a larger number of PSS� sites with respect to CTA+

dissociated ions. Furthermore the high efficiency of CTA+

micelles in dedoping PEDOT:PSS can tentatively be attributed to

their high surface charge, which depends on CTAB concentra-

tion: at 10�5 M, i.e. below the CMC, the z-potential is close to

zero, as expected for fully dissociated CTAB, whereas increasing

the concentration to 10�4 M and 10�2 M leads to higher surface

charge.38,39 We expect that the higher local charge carried by

micelles would neutralize a larger number of PSS sites with

respect to dissociated ions. The insertion of micelles into the

polymer might be favoured by the internal structure of

PEDOT:PSS, which consists of PEDOT rich clusters, separated

by lamellae of PSS� (Fig. S5†).40 Such a structure might facilitate

the intercalation of the CTA+ micelles within the PEDOT:PSS

layer, in proximity of the PSS sites.

Conclusions

In conclusion, we have demonstrated for the first time the ability

of OECTs to detect and monitor, in real-time, micelle formation.

We used aqueous solutions of the cationic surfactant CTAB,

whose concentration was varied between 1 � 10�6 and 1 � 10�2

M, as electrolytes for OECTs. The OECT modulation increased

above the CMC of CTAB, revealing that positively charged

CTA+ micelles dedope PEDOT:PSS more efficiently than CTA+

dissociated ions. These results indicate that monitoring

PEDOT:PSS OECT modulation as a function of the concen-

tration of a micelle-forming cationic surfactant provides a simple

and fast method to detect the formation of micelles. On the other

hand, the modulation achieved with the commonly used elec-

trolyte NaCl was independent of the concentration, confirming

the key role of micelles in OECT operation. UV-Vis absorption

spectroscopy measurements, performed on a working OECT

with a CTAB concentration above the CMC, showed that the

optical absorption of PEDOT:PSS between 400 and 700 nm

increases upon application of a positive Vg, corresponding to the

electrochromic switching between oxidized (light blue) and

reduced (dark blue) PEDOT forms. This result proves that CTA+

Chem. Sci.

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micelles dedope the PEDOT:PSS layer, thus resulting in OECT

current modulation. Our results, besides opening new opportu-

nities for studying the operating mechanism of OECTs, pave the

way to new exciting applications since micelles play a primary

role in biological processes and drug-delivery systems, in

particular as potential nanovectors where drugs can be enclosed

and released in a controlled manner by OECTs.

Acknowledgements

This research was supported by NSERC discovery grants to FC

and CS. FC acknowledges the Department of Chemical Engi-

neering of �Ecole Polytechnique de Montr�eal for a starting grant.

We are grateful to Arthur Yelon, Antonella Badia and Jill A.

Miwa for fruitful discussions. Marco Pola and Gianfranco Galli

are acknowledged for technical assistance.

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