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Effect of water-soluble vitamins on the structure and properties of poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate)
Accepted Manuscript Effect of water-soluble vitamins on the structure and properties of poly(3,4ethylenedioxythiopehene):poly(styrenesulfonate) Shupeng Zhang, Yijie Xia, Jianyong Ouyang PII:
S1566-1199(17)30109-X
DOI:
10.1016/j.orgel.2017.03.006
Reference:
ORGELE 4003
To appear in:
Organic Electronics
Received Date: 24 January 2017 Revised Date:
28 February 2017
Accepted Date: 6 March 2017
Please cite this article as: S. Zhang, Y. Xia, J. Ouyang, Effect of water-soluble vitamins on the structure and properties of poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate), Organic Electronics (2017), doi: 10.1016/j.orgel.2017.03.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1400 VB3-treated 1200
VB6-treated
800
400 200 130 140 150 o Treating Temperature ( C)
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120
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600
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Conductivity (S/cm)
VC-treated 1000
Effect of water-soluble ACCEPTED vitamins on the structure and properties of MANUSCRIPT poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate) Shupeng Zhang, 1,2,‡ Yijie Xia1,3,‡ and Jianyong Ouyang1,*
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1. Department of Materials Science and Engineering, National University of Singapore, Singapore 117576. E-mail:[email protected]
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2. School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu, China 210094.
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3. School of Mechanical Engineering, University of Shanghai for Science and Technology, Shanghai China 200093
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S. Zhang and Y. Xia contributed equally to this work.
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‡
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ABSTRACT
Intrinsically conducting polymers can have important application in biology because they can be conductive
and
have
good
biologicacl
compatibility.
Poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) has been the most popular conductive polymer in biological
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application due to its solution processability in water. PEDOT:PSS can be used as electrode materials or active materials of biological devices or circuits. It is important to study the effect of biomaterials on the structure and properties of PEDOT:PSS films. In this work, water-soluble
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vitamins that are biomaterials needed for organisms are used to treat PEDOT:PSS. They can significantly enhance the conductivity of PEDOT:PSS from 0.3 S cm-1 up to higher than 1000 S
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cm-1. The conductivity enhancement depends on the structure of vitamins. The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3. The vitamin-induced changes in the structure and properties of PEDOT:PSS were studied by UV-Vis absorption spectroscopy,
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temperature-dependence of resistance measurements, atomic force microscopy and cyclic voltammetry. The characterizations indicate that vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are
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important to understand the application of PEDOT:PSS in biology and the development of new
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biological application of PEDOT:PSS.
Keywords: vitamin, biocompatibility, conductivity enhancement, phase segregation, conformation
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1. Introduction
Conductive polymers have important application in many areas because of their unique structure and properties[1, 2]. They can have the high electrical conductivity, interesting optical properties and high mechanical flexibility. In addition, they are ionically conductive
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because of the presence of polycations or polyanions. These properties together with their biocompatibility render them important application in biology[1, 3]. Conductive polymers have been exploited in biosensors, tissue-engineering scaffolds, neural probes, drug-delivery
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devices, and bio-actuators [2-18]. A biosensor has a biological sensing element either
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intimately connected to or integrated within a transducer that converts biochemical signals to electronic signals. Conductive polymers with high conductivities have been attracting much interest in enhancing response speed, sensitivity and versatility of biosensors in diagnostics to measure vital analytes, because the high conductivity can facilitate the transport of electric
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charges produced by the biochemical reactions to electronic circuit [2]. Electrically conducting polymers can be used as suitable matrixes of enzymes, antibody and protein molecules[19, 20].
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Among the conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has been
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investigated as the electrode or active material in electrically active tissues such as the nervous system, heart, and skeletal muscle [21, 22]. PEDOT and its composites have been even employed as the next generation of electrodes for bioprosthetics and biosensors [23, 24].
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS,
chemical
structure shown in Fig.1) is the most successful conducting polymer in terms of the practical application because of its advantages in solution processability, mechanical, and optical properties [25]. It has high transparency in the visible range. It can also be dispersed in water 3
and some organic solvents, andACCEPTED thus high-quality PEDOT:PSS films can be readily prepared MANUSCRIPT on various substrates by conventional solution processing techniques, such as coating, spraying and printing [26]. Moreover, it is biocompatible and has excellent thermal stability. These properties enable the important application of PEDOT:PSS in many areas such as light
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emitting diodes (LEDs), chemical and biological sensors, and regenerative medicine. However, the as-prepared PEDOT:PSS films have a quite low conductivity of about 1 S cm-1. The low conductivity is ascribed to the existence of excess PSS that is an insulator and is
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used to stabilize PEDOT chains in solvents. Higher conductivity can improve biosensing
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performance [27]. The conductivity must be increased in order to acquire electrical signals. A couple of methods have been reported to significantly enhance the conductivity of PEDOT:PSS films, including the addition of an organic compound like ethylene glycol, dimethyl sulfoxide [28], ionic liquid, into PEDOT:PSS aqueous solution or post-treatment of
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PEDOT:PSS films with organic molecules, inorganic salts, or inorganic and organic acids [26, 29-40]. However, many chemical agents used for the conductivity enhancement is poisonous and not biologically compatible.
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It is critical to maintain the activity of the molecules, increase stability, and ensure
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accessibility of the analyte to perform biological events [41]. So, biocompatible molecules are desired to enhance the conductivity of PEDOT:PSS. To our best knowledge, there is no report to investigate the effect of biological materials on the structure and properties of PEDOT:PSS. In this work, we found that the conductivity of PEDOT:PSS can be significantly enhanced by some water-soluble vitamins and the conductivity enhancement is related to the vitamins. These discoveries are important for the biological application of PEDOT:PSS and the development of new biological application of PEDOT:PSS. 4
ACCEPTED MANUSCRIPT 2. Material and methods 2.1 Materials PEDOT:PSS aqueous solution (CleviosTM PH 1000) was purchased from Heraeus. The
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concentration of PEDOT:PSS was 1.3% by weight, and the weight ratio of PSS to PEDOT was 2.5 in solution. All other chemicals, including thiamine hydrochloride (vitamin B1), nicotinic acid (vitamin B3), pyridoxine (vitamin B6) and L-ascorbic acid (vitamin C) were
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obtained from Sigma-Aldrich. All the materials were used as received without further
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purification.
2.2 Treatment of PEDOT:PSS films with vitamins
PEDOT:PSS films were prepared by spin coating the CleviosTM PH1000 aqueous solution
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on glass substrates of 1.3×1.3 cm2, which were pre-cleaned sequentially with detergent, de-ionized (DI) water, acetone and isopropanol. They were then dried at 120 ℃ on a hot plate for 15 min. The treatment with a vitamin solution was performed by dropping 100 µL
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aqueous solution of vitamin agents on a PEDOT:PSS film on a hot plate. The films dried
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after about 5 min. They were cooled down to room temperature, and then were rinsed with DI water and dried at the corresponding temperature again. The conductivities were measured after the films cooling down to room temperature.
2.3 Characterization of PEDOT:PSS films The conductivities of the polymer films were measured by the van der Pauw four-point probe technique with a Keithley 2400 source/meter. The electrical contacts were made by 5
pressing indium on the four corners of each PEDOT:PSS film on a glass substrate. The ACCEPTED MANUSCRIPT temperature dependences of the resistivities of the untreated and treated PEDOT:PSS films were tested using a Janis Research VPF-475 dewar with liquid nitrogen as coolant and a Conductus LTC-11 temperature controller. The UV-Vis-NIR absorption spectra of the
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polymer films were taken with a Varian Cary 5000 UV-Vis-NIR spectrometer, and the AFM images of the polymer films were obtained using a VeecoNanoScope IV Multi-Mode AFM with the tapping mode. The X-ray photoelectron spectroscopy (XPS) spectra were collected
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with an Axis Ultra DLD X-ray photoelectron spectrometer equipped with an Al K an X-ray
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source (1486.6 eV). The thicknesses of the polymer films were determined with an Alpha 500 step profiler. Cyclic voltammetric (CV) measurements were carried out with an ECO CHEMIE AUTOLAB PGSTAT 302N + FRA2 system in 0.1 M NaCl solution with a Au disc coated with a PEDOT:PSS film as the working electrode. The PEDOT:PSS films for the CVs
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were prepared by dropping PEDOT:PSS aqueous solution on a Au disc with a diameter of 2 mm and subsequently drying at 120
. A Pt wire and Ag/AgCl (3 MNaCl) were used as the
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counter and reference electrode, respectively. The scan rate was 50 mV s-1.
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3. Results and discussion
Several typical water soluble vitamins including vitamin B1, vitamin B3, vitamin B6 and vitamin C were chosen to treat PEDOT:PSS. Their chemical structures are listed in Figure 1. As-prepared PEDOT:PSS films from the CleviosTM PH1000 aqueous solution had a conductivity of 0.3 S cm-1. As shown in Figure 2, the conductivity of the PEDOT:PSS films was significantly enhanced. The conductivities of the polymer films treated with 0.1 M vitamin B3, B6 and C solutions are 764, 383 and 548 S/cm at 120 oC, respectively. But the 6
conductivity is only slightly enhanced to 3 S/cm for the PEDOT:PSS films treated with 0.1 ACCEPTED MANUSCRIPT M vitamin B1 aqueous solution. The different conductivity enhancements can be understood in terms of the chemical structures of the vitamins. Vitamin B3 has a carboxylic acid group, and there are many hydroxyl groups for vitamin B6 and vitamin C. In contrast, there is only
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one hydroxyl group for vitamin B1. As reported in literature, molecules with multiple hydroxyl group or carboxylic acid group can give rise to significant conductivity enhancement of PEDOT:PSS films, and molecules with carboxylic acid group usually have
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stronger effect on conductivity enhancement than those have hydroxyl groups [30, 33, 37]. It
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is consistent with our results that B3 treated films has higher conductivity than others. We also tried to use other vitamins, such as vitamin A, D, E, K, B2, and B7 to treat PEDOT:PSS films. But they were insoluble or had very low solubility in water. The conductivity enhancement also depends on the temperature during the treatment (Fig.
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2). The conductivity increases with the elevating temperature from 120 to 140 slightly drops when the temperature is further increased to 160 S cm-1 for the PEDOT:PSS films treated at 130-150
, and then
. They are higher than 1000 by vitamin B3. The highest
. This optimal treating temperature coincides with that of the other
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temperature of 140
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conductivity of vitamin B3-treated PEDOT:PSS film is 1285 S cm-1 at the optimal treating
treatment methods [29-37]. Hence, the effect of the temperature during the treatment on the conductivity of the PEDOT:PSS films should be related to the thermal properties of the PEDOT:PSS films. As reported in literature, the polymer conformation changes after the conductivity enhancement [29-37]. A high temperature can facilitate the conformational change. The conductivities are much higher than that of PEDOT:PSS treated with EG or DMSO. The conductivities are 735 S cm-1 and 680±50 S cm-1 for the PEDOT:PSS films 7
prepared from PH1000 aqueousACCEPTED solution added with 6 vol% EG and 5 vol% DMSO at 120 MANUSCRIPT [28,40]. The resistances of the PEDOT:PSS films before and after the B3, B6 and C treatment at 140 ℃ were investigated from room temperature down to 110 K (Fig. 3a). When the
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temperature is below 230 K, the resistances of the treated PEDOT:PSS films decreases rapidly with the elevating temperature. When above a certain temperature, (230 K for vitamin B3 and C, 280 K for vitamin B6), they become insensitive to temperature, suggesting treated
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PEDOT:PSS films behaviors almost like a metal or semimetal.
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The temperature dependences of the resistances of conducting polymers are usually described by the 1D variable-range-hopping (VRH) model [42].
T0 1 / 2 R (T ) = R0 exp T
Where T0 = 16/kBN(EF)L//L⊥2 is the energy barrier between localized states, N(EF) is the
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density of the states at the Fermi level, and L//(L⊥) is the localization length in the parallel (perpendicular) direction. As shown in Figure 3b, the lnRvsT-1/2 has a linear relationship at
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the temperature range below a certain temperature (230 K for vitamin B3 and C, 280 K for vitamin B6). The temperature dependence of the resistance of the vitamin B3-treated
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PEDOT:PSS film is saliently different from that of the untreated PEDOT:PSS film. The latter has a linear lnRvsT-1/2 relationship in the whole temperature range, and the estimated T0 value is 1474 K. However, the T0 values of the B3, B6 and C-treated PEDOT:PSS films could be also obtained by analyzing the temperature dependences of the resistances by the 1D VRH model from 110 to 230 K. They are 70 K, 520 K and 121 K, respectively. The decreases in the T0 value demonstrate that vitamin treatment can significantly reduce the energy barrier for the interchain and inter-domain charge hopping. These results indicate that the insulator 8
PSS shell is the dominant factor for the energy barrier for the charge transport through the ACCEPTED MANUSCRIPT as-prepared Clevios PH1000 films [25]. The vitamin treatment can induce the phase segregation between the hydrophobic PEDOT and hydrophilic PSSH and lead to the wash-away of some PSSH chains from the PEDOT:PSS films. The insulator PSSH shell is
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thus significantly reduced. As a result, the PEDOT chains become the dominant factor for the charge transport after the vitamin treatment.
The mechanism for the conductivity enhancements of PEDOT:PSS through the treatment
PEDOT:PSS
films
were
characterized
by
UV-vis-NIR
absorption
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vitamin-treated
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with vitamins was studied by various chemical and physical characterizations. The
spectroscopy and XPS. They could be the strong evidences to support the loss of PSS chains from the PEDOT:PSS film. Figure 4 presents the absorption spectra of the treated and untreated PEDOT:PSS films in the UV range. The characteristic absorption at 225 nm is due
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to the benzene rings of PSS. The decreasing intensity of the band suggest the loss of some PSS chains from the PEDOT:PSS films after the treatment with the B3, B6 and C. The PSS loss is consistent with the conductivity enhancement. The peak at around 300 nm for the
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vitamin B6-treated film is attributed to the vitamin B6. As shown in Fig.4b, the neat B6 film
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also exhibits an absorption peak around 300 nm. It seems that some B6 still remained on the film. While the Vc and B3-treated films did not shown any peaks from Vc and B3, which means that Vc and B3 have been washed away from films during rinsing process. Thus, Vc and B3 treatment can remove more PSS than B6. It is consistent with the UV results. The PSS loss from the PEDOT:PSS films during the vitamin treatment is also confirmed by the S2p XPS of the B3-treated PEDOT:PSS films (Fig. 5). The XPS bands with binding energies between 166 and 172 eV are the S 2p band of the sulfur atoms in PSS, whereas the 9
two XPS bands with the binding energies between 162 and 166 eV are the S 2p bands of the ACCEPTED MANUSCRIPT sulfur atoms in PEDOT. The intensity ratio of these XPS bands are related to the PSS loss from the PEDOT:PSS films. The S 2p XPS intensity ratio of PEDOT to PSS increases after a vitamin B3, B6 or C treatment.
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The loss of some PSS chains and the presence of vitamin in the PEDOT:PSS film can effect the film morphology. This is revealed by the AFM study of the vitamin B3, B6 and C -treated PEDOT:PSS films. As shown in Figure 6, the surface morphology of as-prepared
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and treated PEDOT:PSS films are different. An as-prepared PEDOT:PSS film was quite
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smooth with a roughness of 1.56 nm. The morphologies of the as-prepared PEDOT:PSS films are strongly affected by the presence of excess PSS chains. Generally, the PEDOT chains have to adopt a coil conformation in water because of the Coulombic attraction between PEDOT and PSS. This coil conformation is reserved in the solid PEDOT:PSS films
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[25]. The excess PSS chains can be reduced through a treatment with various compounds as reported in literature [26, 29-40]. In this research work, PSS was reduced by treating the Clevios PH1000 with vitamins. As we analysis before, the vitamin B3, B6 and C treatment
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can remarkably lower the PSS amount and significantly enhance the conductivity of the
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PEDOT:PSS films. The morphologies of the Clevios PH1000 films become different after they are treated with B3, B6 and C as shown in images b-d in Figure 6. It became rough for the treated PEDOT:PSS film. The roughness of the vitamin B3, B6 and C- treated films are 2.19, 1.43 and 1.38 nm, respectively. The increase in the roughness is owing to the increasing grain size after the treatment. The removal of some PSSH chains can give rise to the conformational change of the PEDOT chains.
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Cyclic voltammetry (CV) isACCEPTED a technique MANUSCRIPT often used to characterize a material’s redox properties. The B3, B6 and C -induced change in the conformation of the polymer chains were further confirmed by the cyclic voltammetry of the PEDOT:PSS films before and after the treatment. It has been revealed that the electrochemical activity of PEDOT:PSS is quite
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sensitive to the conformation of the PEDOT chains. New electrochemical activity appears at a low electrochemical potential range after the PEDOT chains change from a coil conformation to an extended-coil conformation. As shown in Figure 7, the electrochemical
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activity of the PEDOT:PSS films significantly increases in the electrochemical range of -0.8
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to 0 V versus Ag/AgCl after the treatment with B3. Moreover, there is almost no electrochemical activity at potential lower than -0.4 V vs Ag/AgCl for the pristine PEDOT:PSS film, whereas remarkable redox process can be observed in this range after the treatment.
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These experimental results indicate that vitamins B3, B6, and C can induce the reduction of PSSH amount and the PEDOT chain conformational change. The H+ from vitamin B3 will replace PEDOT+ to associate with PSS- and form PSSH. Then PSSH will be removed during
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the rinsing process. The hydrophilic –OH groups in vitamin B6, and C will interact with the
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hydrophilic PSS chains, which cause separation between PEDOT and PSS chains. Thus, vitamins B3, B6, and C can induce phase separation between the PEDOT and PSS chains. PEDOT usually has a coil conformation to follow the coiled PSS chain. The disappearance of the Coulombic attractions between PEDOT and PSS can induce the PEDOT chain change to extended-coil or linear structure, makes the positive charges on PEDOT chains more delocalized. The removal of PSSH also leads to the reduction of the energy barrier for charge hopping, thus resulting in the conductivity enhancement. 11
ACCEPTED MANUSCRIPT 4. Conclusions We demonstrated that water-soluble vitamins can significantly enhance the conductivity of PEDOT:PSS film. The conductivity can be increased from 0.3 S cm-1 up to higher than 1000
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S cm-1 through vitamin B3 treatment. The vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are important to understand the application of PEDOT:PSS in biology and the development of
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new biological application of PEDOT:PSS.
Acknowledgements
This research work was supported by a research grant from the Ministry of Education, Singapore (R-284-000-136-112). S. Zhang thanks to the China Scholarship Council (CSC)
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for the scholarship (201303070320), Natural Science Foundation of China (51402151), Natural Science Foundation of Jiangsu province (BK20161493) and the Zijin Intelligent Program by the Nanjing University of Science and Technology. Y. Xia thanks to the Young
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Eastern Scholar (QD2016012) of Shanghai Municipal Education Commission.
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Figure Captions
Fig. 1 Chemical structures of PEDOT:PSS, vitamin B1, B3, B6 and C. Fig. 2 (a) Conductivities of PEDOT:PSS films treated with 0.1 M vitamin B1, B3, B6 and C solutions at 120 oC. The conductivity of untreated PEDOT:PSS is also presented for comparison. (b)
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Dependence of the conductivity of PEDOT:PSS films treated with 0.1 M vitamin B3, B6 and C on the treating temperatures.
Fig. 3 Temperature dependences of the normalized resistances of untreated and vitamin treated
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PEDOT:PSS films. (a) Normalized resistances of untreated and vitamin-treated PEDOT:PSS films.
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(b) Analysis of the resistance-temperature relationships of the untreated and vitamin-treated PEDOT:PSS films with the 1D VRH model. The resistances are normalized to that of the corresponding PEDOT:PSS films at 110 K. The treating temperature for PEDOT:PSS films was 140 o
C.
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Fig. 4 UV-vis-NIR absorption spectra of PEDOT:PSS films before and after treatments with 0.1 M vitamin B3, B6 and C at140 oC.
Fig. 5 S2p XPS spectra of untreated and vitamin B3, B6 and C-treated PEDOT:PSS films at
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140 oC.
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Fig. 6 AFM images of PEDOT:PSS films (a) untreated and treated with vitamin B3 (b), B6 (c) and C (d) at 140 oC.The unit for the AFM images is µm. Fig. 7 Cyclic voltammograms of untreated and 0.1 M B3, B6 and C treated PEDOT:PSS films at 140 oC.
16
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Fig. 1
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Fig. 2
800
764
(a)
600
548
500 383
400 300 200 100 0
0.3
3
PH1000
VB1
VB3
VB6
Vitamin
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1400 VB3-treated 1200
(b)
VB6-treated
800 600 400 200
130 140 150 o Treating Temperature ( C)
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120
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Conductivity (S/cm)
VC-treated 1000
VC
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-1
Conductivity (S cm )
700
18
160
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Fig.3
(a)
0.8
0.6
0.4
PEDOT:PSS VB6 VC VB3
0.2
100
150
200
250
300
350
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Temperature (K)
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Normalized Resistance
1.0
0.36788
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Normalized Resistance ln(R)
1
0.13534 0.05
0.06
PEDOT:PSS VB6-treated VC-treated VB3-treated
0.07
-1/2
0.08
(K
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T
(b)
19
-1/2
)
0.09
0.10
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• Water-soluble vitamins are used to treat PEDOT:PSS • The conductivity enhancement depends on the chemical structure of vitamins • The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3 • The mechanism for conductivity enhancement is proposed.
S1566-1199(17)30109-X
DOI:
10.1016/j.orgel.2017.03.006
Reference:
ORGELE 4003
To appear in:
Organic Electronics
Received Date: 24 January 2017 Revised Date:
28 February 2017
Accepted Date: 6 March 2017
Please cite this article as: S. Zhang, Y. Xia, J. Ouyang, Effect of water-soluble vitamins on the structure and properties of poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate), Organic Electronics (2017), doi: 10.1016/j.orgel.2017.03.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Conductivity (S/cm)
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Effect of water-soluble ACCEPTED vitamins on the structure and properties of MANUSCRIPT poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate) Shupeng Zhang, 1,2,‡ Yijie Xia1,3,‡ and Jianyong Ouyang1,*
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1. Department of Materials Science and Engineering, National University of Singapore, Singapore 117576. E-mail:[email protected]
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2. School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu, China 210094.
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3. School of Mechanical Engineering, University of Shanghai for Science and Technology, Shanghai China 200093
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S. Zhang and Y. Xia contributed equally to this work.
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‡
1
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ABSTRACT
Intrinsically conducting polymers can have important application in biology because they can be conductive
and
have
good
biologicacl
compatibility.
Poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) has been the most popular conductive polymer in biological
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application due to its solution processability in water. PEDOT:PSS can be used as electrode materials or active materials of biological devices or circuits. It is important to study the effect of biomaterials on the structure and properties of PEDOT:PSS films. In this work, water-soluble
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vitamins that are biomaterials needed for organisms are used to treat PEDOT:PSS. They can significantly enhance the conductivity of PEDOT:PSS from 0.3 S cm-1 up to higher than 1000 S
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cm-1. The conductivity enhancement depends on the structure of vitamins. The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3. The vitamin-induced changes in the structure and properties of PEDOT:PSS were studied by UV-Vis absorption spectroscopy,
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temperature-dependence of resistance measurements, atomic force microscopy and cyclic voltammetry. The characterizations indicate that vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are
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important to understand the application of PEDOT:PSS in biology and the development of new
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biological application of PEDOT:PSS.
Keywords: vitamin, biocompatibility, conductivity enhancement, phase segregation, conformation
2
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1. Introduction
Conductive polymers have important application in many areas because of their unique structure and properties[1, 2]. They can have the high electrical conductivity, interesting optical properties and high mechanical flexibility. In addition, they are ionically conductive
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because of the presence of polycations or polyanions. These properties together with their biocompatibility render them important application in biology[1, 3]. Conductive polymers have been exploited in biosensors, tissue-engineering scaffolds, neural probes, drug-delivery
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devices, and bio-actuators [2-18]. A biosensor has a biological sensing element either
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intimately connected to or integrated within a transducer that converts biochemical signals to electronic signals. Conductive polymers with high conductivities have been attracting much interest in enhancing response speed, sensitivity and versatility of biosensors in diagnostics to measure vital analytes, because the high conductivity can facilitate the transport of electric
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charges produced by the biochemical reactions to electronic circuit [2]. Electrically conducting polymers can be used as suitable matrixes of enzymes, antibody and protein molecules[19, 20].
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Among the conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has been
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investigated as the electrode or active material in electrically active tissues such as the nervous system, heart, and skeletal muscle [21, 22]. PEDOT and its composites have been even employed as the next generation of electrodes for bioprosthetics and biosensors [23, 24].
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS,
chemical
structure shown in Fig.1) is the most successful conducting polymer in terms of the practical application because of its advantages in solution processability, mechanical, and optical properties [25]. It has high transparency in the visible range. It can also be dispersed in water 3
and some organic solvents, andACCEPTED thus high-quality PEDOT:PSS films can be readily prepared MANUSCRIPT on various substrates by conventional solution processing techniques, such as coating, spraying and printing [26]. Moreover, it is biocompatible and has excellent thermal stability. These properties enable the important application of PEDOT:PSS in many areas such as light
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emitting diodes (LEDs), chemical and biological sensors, and regenerative medicine. However, the as-prepared PEDOT:PSS films have a quite low conductivity of about 1 S cm-1. The low conductivity is ascribed to the existence of excess PSS that is an insulator and is
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used to stabilize PEDOT chains in solvents. Higher conductivity can improve biosensing
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performance [27]. The conductivity must be increased in order to acquire electrical signals. A couple of methods have been reported to significantly enhance the conductivity of PEDOT:PSS films, including the addition of an organic compound like ethylene glycol, dimethyl sulfoxide [28], ionic liquid, into PEDOT:PSS aqueous solution or post-treatment of
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PEDOT:PSS films with organic molecules, inorganic salts, or inorganic and organic acids [26, 29-40]. However, many chemical agents used for the conductivity enhancement is poisonous and not biologically compatible.
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It is critical to maintain the activity of the molecules, increase stability, and ensure
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accessibility of the analyte to perform biological events [41]. So, biocompatible molecules are desired to enhance the conductivity of PEDOT:PSS. To our best knowledge, there is no report to investigate the effect of biological materials on the structure and properties of PEDOT:PSS. In this work, we found that the conductivity of PEDOT:PSS can be significantly enhanced by some water-soluble vitamins and the conductivity enhancement is related to the vitamins. These discoveries are important for the biological application of PEDOT:PSS and the development of new biological application of PEDOT:PSS. 4
ACCEPTED MANUSCRIPT 2. Material and methods 2.1 Materials PEDOT:PSS aqueous solution (CleviosTM PH 1000) was purchased from Heraeus. The
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concentration of PEDOT:PSS was 1.3% by weight, and the weight ratio of PSS to PEDOT was 2.5 in solution. All other chemicals, including thiamine hydrochloride (vitamin B1), nicotinic acid (vitamin B3), pyridoxine (vitamin B6) and L-ascorbic acid (vitamin C) were
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obtained from Sigma-Aldrich. All the materials were used as received without further
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purification.
2.2 Treatment of PEDOT:PSS films with vitamins
PEDOT:PSS films were prepared by spin coating the CleviosTM PH1000 aqueous solution
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on glass substrates of 1.3×1.3 cm2, which were pre-cleaned sequentially with detergent, de-ionized (DI) water, acetone and isopropanol. They were then dried at 120 ℃ on a hot plate for 15 min. The treatment with a vitamin solution was performed by dropping 100 µL
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aqueous solution of vitamin agents on a PEDOT:PSS film on a hot plate. The films dried
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after about 5 min. They were cooled down to room temperature, and then were rinsed with DI water and dried at the corresponding temperature again. The conductivities were measured after the films cooling down to room temperature.
2.3 Characterization of PEDOT:PSS films The conductivities of the polymer films were measured by the van der Pauw four-point probe technique with a Keithley 2400 source/meter. The electrical contacts were made by 5
pressing indium on the four corners of each PEDOT:PSS film on a glass substrate. The ACCEPTED MANUSCRIPT temperature dependences of the resistivities of the untreated and treated PEDOT:PSS films were tested using a Janis Research VPF-475 dewar with liquid nitrogen as coolant and a Conductus LTC-11 temperature controller. The UV-Vis-NIR absorption spectra of the
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polymer films were taken with a Varian Cary 5000 UV-Vis-NIR spectrometer, and the AFM images of the polymer films were obtained using a VeecoNanoScope IV Multi-Mode AFM with the tapping mode. The X-ray photoelectron spectroscopy (XPS) spectra were collected
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with an Axis Ultra DLD X-ray photoelectron spectrometer equipped with an Al K an X-ray
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source (1486.6 eV). The thicknesses of the polymer films were determined with an Alpha 500 step profiler. Cyclic voltammetric (CV) measurements were carried out with an ECO CHEMIE AUTOLAB PGSTAT 302N + FRA2 system in 0.1 M NaCl solution with a Au disc coated with a PEDOT:PSS film as the working electrode. The PEDOT:PSS films for the CVs
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were prepared by dropping PEDOT:PSS aqueous solution on a Au disc with a diameter of 2 mm and subsequently drying at 120
. A Pt wire and Ag/AgCl (3 MNaCl) were used as the
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counter and reference electrode, respectively. The scan rate was 50 mV s-1.
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3. Results and discussion
Several typical water soluble vitamins including vitamin B1, vitamin B3, vitamin B6 and vitamin C were chosen to treat PEDOT:PSS. Their chemical structures are listed in Figure 1. As-prepared PEDOT:PSS films from the CleviosTM PH1000 aqueous solution had a conductivity of 0.3 S cm-1. As shown in Figure 2, the conductivity of the PEDOT:PSS films was significantly enhanced. The conductivities of the polymer films treated with 0.1 M vitamin B3, B6 and C solutions are 764, 383 and 548 S/cm at 120 oC, respectively. But the 6
conductivity is only slightly enhanced to 3 S/cm for the PEDOT:PSS films treated with 0.1 ACCEPTED MANUSCRIPT M vitamin B1 aqueous solution. The different conductivity enhancements can be understood in terms of the chemical structures of the vitamins. Vitamin B3 has a carboxylic acid group, and there are many hydroxyl groups for vitamin B6 and vitamin C. In contrast, there is only
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one hydroxyl group for vitamin B1. As reported in literature, molecules with multiple hydroxyl group or carboxylic acid group can give rise to significant conductivity enhancement of PEDOT:PSS films, and molecules with carboxylic acid group usually have
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stronger effect on conductivity enhancement than those have hydroxyl groups [30, 33, 37]. It
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is consistent with our results that B3 treated films has higher conductivity than others. We also tried to use other vitamins, such as vitamin A, D, E, K, B2, and B7 to treat PEDOT:PSS films. But they were insoluble or had very low solubility in water. The conductivity enhancement also depends on the temperature during the treatment (Fig.
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2). The conductivity increases with the elevating temperature from 120 to 140 slightly drops when the temperature is further increased to 160 S cm-1 for the PEDOT:PSS films treated at 130-150
, and then
. They are higher than 1000 by vitamin B3. The highest
. This optimal treating temperature coincides with that of the other
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temperature of 140
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conductivity of vitamin B3-treated PEDOT:PSS film is 1285 S cm-1 at the optimal treating
treatment methods [29-37]. Hence, the effect of the temperature during the treatment on the conductivity of the PEDOT:PSS films should be related to the thermal properties of the PEDOT:PSS films. As reported in literature, the polymer conformation changes after the conductivity enhancement [29-37]. A high temperature can facilitate the conformational change. The conductivities are much higher than that of PEDOT:PSS treated with EG or DMSO. The conductivities are 735 S cm-1 and 680±50 S cm-1 for the PEDOT:PSS films 7
prepared from PH1000 aqueousACCEPTED solution added with 6 vol% EG and 5 vol% DMSO at 120 MANUSCRIPT [28,40]. The resistances of the PEDOT:PSS films before and after the B3, B6 and C treatment at 140 ℃ were investigated from room temperature down to 110 K (Fig. 3a). When the
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temperature is below 230 K, the resistances of the treated PEDOT:PSS films decreases rapidly with the elevating temperature. When above a certain temperature, (230 K for vitamin B3 and C, 280 K for vitamin B6), they become insensitive to temperature, suggesting treated
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PEDOT:PSS films behaviors almost like a metal or semimetal.
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The temperature dependences of the resistances of conducting polymers are usually described by the 1D variable-range-hopping (VRH) model [42].
T0 1 / 2 R (T ) = R0 exp T
Where T0 = 16/kBN(EF)L//L⊥2 is the energy barrier between localized states, N(EF) is the
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density of the states at the Fermi level, and L//(L⊥) is the localization length in the parallel (perpendicular) direction. As shown in Figure 3b, the lnRvsT-1/2 has a linear relationship at
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the temperature range below a certain temperature (230 K for vitamin B3 and C, 280 K for vitamin B6). The temperature dependence of the resistance of the vitamin B3-treated
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PEDOT:PSS film is saliently different from that of the untreated PEDOT:PSS film. The latter has a linear lnRvsT-1/2 relationship in the whole temperature range, and the estimated T0 value is 1474 K. However, the T0 values of the B3, B6 and C-treated PEDOT:PSS films could be also obtained by analyzing the temperature dependences of the resistances by the 1D VRH model from 110 to 230 K. They are 70 K, 520 K and 121 K, respectively. The decreases in the T0 value demonstrate that vitamin treatment can significantly reduce the energy barrier for the interchain and inter-domain charge hopping. These results indicate that the insulator 8
PSS shell is the dominant factor for the energy barrier for the charge transport through the ACCEPTED MANUSCRIPT as-prepared Clevios PH1000 films [25]. The vitamin treatment can induce the phase segregation between the hydrophobic PEDOT and hydrophilic PSSH and lead to the wash-away of some PSSH chains from the PEDOT:PSS films. The insulator PSSH shell is
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thus significantly reduced. As a result, the PEDOT chains become the dominant factor for the charge transport after the vitamin treatment.
The mechanism for the conductivity enhancements of PEDOT:PSS through the treatment
PEDOT:PSS
films
were
characterized
by
UV-vis-NIR
absorption
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vitamin-treated
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with vitamins was studied by various chemical and physical characterizations. The
spectroscopy and XPS. They could be the strong evidences to support the loss of PSS chains from the PEDOT:PSS film. Figure 4 presents the absorption spectra of the treated and untreated PEDOT:PSS films in the UV range. The characteristic absorption at 225 nm is due
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to the benzene rings of PSS. The decreasing intensity of the band suggest the loss of some PSS chains from the PEDOT:PSS films after the treatment with the B3, B6 and C. The PSS loss is consistent with the conductivity enhancement. The peak at around 300 nm for the
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vitamin B6-treated film is attributed to the vitamin B6. As shown in Fig.4b, the neat B6 film
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also exhibits an absorption peak around 300 nm. It seems that some B6 still remained on the film. While the Vc and B3-treated films did not shown any peaks from Vc and B3, which means that Vc and B3 have been washed away from films during rinsing process. Thus, Vc and B3 treatment can remove more PSS than B6. It is consistent with the UV results. The PSS loss from the PEDOT:PSS films during the vitamin treatment is also confirmed by the S2p XPS of the B3-treated PEDOT:PSS films (Fig. 5). The XPS bands with binding energies between 166 and 172 eV are the S 2p band of the sulfur atoms in PSS, whereas the 9
two XPS bands with the binding energies between 162 and 166 eV are the S 2p bands of the ACCEPTED MANUSCRIPT sulfur atoms in PEDOT. The intensity ratio of these XPS bands are related to the PSS loss from the PEDOT:PSS films. The S 2p XPS intensity ratio of PEDOT to PSS increases after a vitamin B3, B6 or C treatment.
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The loss of some PSS chains and the presence of vitamin in the PEDOT:PSS film can effect the film morphology. This is revealed by the AFM study of the vitamin B3, B6 and C -treated PEDOT:PSS films. As shown in Figure 6, the surface morphology of as-prepared
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and treated PEDOT:PSS films are different. An as-prepared PEDOT:PSS film was quite
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smooth with a roughness of 1.56 nm. The morphologies of the as-prepared PEDOT:PSS films are strongly affected by the presence of excess PSS chains. Generally, the PEDOT chains have to adopt a coil conformation in water because of the Coulombic attraction between PEDOT and PSS. This coil conformation is reserved in the solid PEDOT:PSS films
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[25]. The excess PSS chains can be reduced through a treatment with various compounds as reported in literature [26, 29-40]. In this research work, PSS was reduced by treating the Clevios PH1000 with vitamins. As we analysis before, the vitamin B3, B6 and C treatment
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can remarkably lower the PSS amount and significantly enhance the conductivity of the
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PEDOT:PSS films. The morphologies of the Clevios PH1000 films become different after they are treated with B3, B6 and C as shown in images b-d in Figure 6. It became rough for the treated PEDOT:PSS film. The roughness of the vitamin B3, B6 and C- treated films are 2.19, 1.43 and 1.38 nm, respectively. The increase in the roughness is owing to the increasing grain size after the treatment. The removal of some PSSH chains can give rise to the conformational change of the PEDOT chains.
10
Cyclic voltammetry (CV) isACCEPTED a technique MANUSCRIPT often used to characterize a material’s redox properties. The B3, B6 and C -induced change in the conformation of the polymer chains were further confirmed by the cyclic voltammetry of the PEDOT:PSS films before and after the treatment. It has been revealed that the electrochemical activity of PEDOT:PSS is quite
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sensitive to the conformation of the PEDOT chains. New electrochemical activity appears at a low electrochemical potential range after the PEDOT chains change from a coil conformation to an extended-coil conformation. As shown in Figure 7, the electrochemical
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activity of the PEDOT:PSS films significantly increases in the electrochemical range of -0.8
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to 0 V versus Ag/AgCl after the treatment with B3. Moreover, there is almost no electrochemical activity at potential lower than -0.4 V vs Ag/AgCl for the pristine PEDOT:PSS film, whereas remarkable redox process can be observed in this range after the treatment.
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These experimental results indicate that vitamins B3, B6, and C can induce the reduction of PSSH amount and the PEDOT chain conformational change. The H+ from vitamin B3 will replace PEDOT+ to associate with PSS- and form PSSH. Then PSSH will be removed during
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the rinsing process. The hydrophilic –OH groups in vitamin B6, and C will interact with the
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hydrophilic PSS chains, which cause separation between PEDOT and PSS chains. Thus, vitamins B3, B6, and C can induce phase separation between the PEDOT and PSS chains. PEDOT usually has a coil conformation to follow the coiled PSS chain. The disappearance of the Coulombic attractions between PEDOT and PSS can induce the PEDOT chain change to extended-coil or linear structure, makes the positive charges on PEDOT chains more delocalized. The removal of PSSH also leads to the reduction of the energy barrier for charge hopping, thus resulting in the conductivity enhancement. 11
ACCEPTED MANUSCRIPT 4. Conclusions We demonstrated that water-soluble vitamins can significantly enhance the conductivity of PEDOT:PSS film. The conductivity can be increased from 0.3 S cm-1 up to higher than 1000
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S cm-1 through vitamin B3 treatment. The vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are important to understand the application of PEDOT:PSS in biology and the development of
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new biological application of PEDOT:PSS.
Acknowledgements
This research work was supported by a research grant from the Ministry of Education, Singapore (R-284-000-136-112). S. Zhang thanks to the China Scholarship Council (CSC)
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for the scholarship (201303070320), Natural Science Foundation of China (51402151), Natural Science Foundation of Jiangsu province (BK20161493) and the Zijin Intelligent Program by the Nanjing University of Science and Technology. Y. Xia thanks to the Young
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Eastern Scholar (QD2016012) of Shanghai Municipal Education Commission.
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Figure Captions
Fig. 1 Chemical structures of PEDOT:PSS, vitamin B1, B3, B6 and C. Fig. 2 (a) Conductivities of PEDOT:PSS films treated with 0.1 M vitamin B1, B3, B6 and C solutions at 120 oC. The conductivity of untreated PEDOT:PSS is also presented for comparison. (b)
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Dependence of the conductivity of PEDOT:PSS films treated with 0.1 M vitamin B3, B6 and C on the treating temperatures.
Fig. 3 Temperature dependences of the normalized resistances of untreated and vitamin treated
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PEDOT:PSS films. (a) Normalized resistances of untreated and vitamin-treated PEDOT:PSS films.
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(b) Analysis of the resistance-temperature relationships of the untreated and vitamin-treated PEDOT:PSS films with the 1D VRH model. The resistances are normalized to that of the corresponding PEDOT:PSS films at 110 K. The treating temperature for PEDOT:PSS films was 140 o
C.
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Fig. 4 UV-vis-NIR absorption spectra of PEDOT:PSS films before and after treatments with 0.1 M vitamin B3, B6 and C at140 oC.
Fig. 5 S2p XPS spectra of untreated and vitamin B3, B6 and C-treated PEDOT:PSS films at
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140 oC.
AC C
Fig. 6 AFM images of PEDOT:PSS films (a) untreated and treated with vitamin B3 (b), B6 (c) and C (d) at 140 oC.The unit for the AFM images is µm. Fig. 7 Cyclic voltammograms of untreated and 0.1 M B3, B6 and C treated PEDOT:PSS films at 140 oC.
16
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig. 1
17
ACCEPTED MANUSCRIPT
Fig. 2
800
764
(a)
600
548
500 383
400 300 200 100 0
0.3
3
PH1000
VB1
VB3
VB6
Vitamin
SC
1400 VB3-treated 1200
(b)
VB6-treated
800 600 400 200
130 140 150 o Treating Temperature ( C)
AC C
EP
TE D
120
M AN U
Conductivity (S/cm)
VC-treated 1000
VC
RI PT
-1
Conductivity (S cm )
700
18
160
ACCEPTED MANUSCRIPT
Fig.3
(a)
0.8
0.6
0.4
PEDOT:PSS VB6 VC VB3
0.2
100
150
200
250
300
350
M AN U
SC
Temperature (K)
RI PT
Normalized Resistance
1.0
0.36788
TE D
Normalized Resistance ln(R)
1
0.13534 0.05
0.06
PEDOT:PSS VB6-treated VC-treated VB3-treated
0.07
-1/2
0.08
(K
AC C
EP
T
(b)
19
-1/2
)
0.09
0.10
ACCEPTED MANUSCRIPT Fig. 4 1.0
(a)
PEDOT:PSS VB6-treated VC-treated VB3-treated
0.6 0.4 0.2
200
250
300
350
400
SC
0.0
RI PT
Absorbance
0.8
Wavelength(nm) (b)
VB6-treated VB6
0.6 0.4 0.2
TE D
Absorbance (a.u.)
0.8
M AN U
1.0
0.0
200
250
300
AC C
EP
Wavelength(nm)
20
350
400
ACCEPTED MANUSCRIPT Fig. 5
2.5
PEDOT 1.0
0.5
0.0 170
RI PT
1.5
SC
Normalized Intensity
2.0
S2p
untreated PSS VB3 treated VB6 treated VC treated
165
AC C
EP
TE D
M AN U
Binding Energy (ev)
21
ACCEPTED MANUSCRIPT (b)
(c)
(d)
RI PT
(a)
AC C
EP
TE D
M AN U
SC
Fig. 6
22
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4.0x10
-5
2.0x10
-5
-2.0x10
-5
-4.0x10
-5
PEDOT:PSS VB3-treated
-1.0
VB6-treated VC-treated -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
EP
TE D
M AN U
SC
Potential (V vs Ag/AgCl)
RI PT
0.0
AC C
Current (A)
Fig. 7
23
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
• Water-soluble vitamins are used to treat PEDOT:PSS • The conductivity enhancement depends on the chemical structure of vitamins • The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3 • The mechanism for conductivity enhancement is proposed.