bio-inspired dopamine functionalization of polypyrrole for improved adhesion and conductivity

CommunicationMacromolecularRapid Communications

wileyonlinelibrary.com350 DOI: 10.1002/marc.201300761

1 . Introduction

Biomaterials have been attracting considerable attentions in the frontier of material engineering and life science because of their unique mechanical properties and bioac-tivities. [ 1–3 ] Nowadays, there is a growing interest in eclec-tically conductive biomaterials that can carry electrical stimuli to activate cell functions or trigger cell response. [ 4–6 ] Among various conductive biopolymers, polypyrrole (PPy) has been extensively investigated due to its economical preparation, long-term stability, and good biocompati-bility. [ 7,8 ] However, PPy is in a form of black precipitate that is insoluble in water and in most organic solvents, making

it diffi cult to be further functionalized and processed. [ 9–11 ] Moreover, PPy has poor mechanical and adhesion proper-ties, which are major obstacles for industrial usages. For example, Pyo et al. reported the self-delamination of the PPy fi lm deposited on actuators. [ 12 ] Faverolle et al. found that the adhesion between PPy coatings and bare glasses was practically zero. [ 13 ] Although researchers have been modifying PPy properties to address its adhesion problem, most of the modifi cations so far involved harsh post-treat-ment conditions (e.g., low pH, toxic additives, and non-environmental friendly solvents). These harsh treatments reduce the biocompatibility of PPy and limit its bio-related applications. [ 13–15 ] Thus, it is desired to have a benign approach to modify PPy with good adhesion and addi-tional chemical and physical properties for its biomedical applications.

More recently, inspired by the marine mussel adhesive chemistry, dopamine (DA), a small biomolecule having a similar structure to the essential adhesive component of mussel protein, has been identifi ed to self-polymerize to form a nanoscale polydopamine (PDA) fi lm on almost all support surfaces under alkaline conditions. [ 16–18 ] Since then, PDA has been intensively studied for func-tional coating and adhesion improvement of numerous bio- and nano-related materials as recently reviewed by Lee et al. [ 18 ] and Dreyer et al. [ 19 ] With this regard, here we

We report the functionalization of polypyrrole (PPy) with a “sticky” biomolecule dopamine (DA), which mimics the essential component of mussel adhesive protein. PPy is one of the most promising electrically conductive polymers with good biocompatibility. The research fi ndings reveal that the DA functionalization enhances the dispersibility and stability of PPy in water and its fi lm adhesion to substrate surface signifi cantly. The electrical conductivity of PPy increases to a maximum value and then decreases with the increasing DA concentration. An optimal DA to pyrrole (Py) mole ratio is found to be between 0.1 and 0.2, at which both conductivity and adhesion of DA-functionalized PPy has been improved.

Bio-Inspired Dopamine Functionalization of Polypyrrole for Improved Adhesion and Conductivity

Wei Zhang , Fut K. Yang , Zihe Pan , Jian Zhang , Boxin Zhao*

W. Zhang, F. K. Yang, Z. Pan, B. Zhao Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1 E-mail: [email protected] W. Zhang, F. K. Yang, Z. Pan, J. Zhang, B. Zhao Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1 J. Zhang Department of Electrical and Computer Engineering , University of Waterloo , 200 University Avenue West , Waterloo, Ontario , Canada , N2L 3G1

Macromol. Rapid Commun. 2014, 35, 350−354

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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0.5 g of the sample was compressed into a pellet by a 13-mm pellet die with 15 000 pounds of pressure.

The powder structures were characterized by scan-ning electron microscope (SEM) and shown in Figure 1 . From Figure 1 a, we can see that pure PPy has globular shapes with a diameter of about 300 nm. It is inter-esting to note that the PDA-modifi ed PPy has a fi brous morphology (Figure 1 b). Although the mechanism of DA polymerization and the structures of the resulting PDA are still under debate at present, it is clear that the polymerization of DA played a vital role in the forma-tion of PPy fi brous nanostructure. As the DA/Py mole ratio increases, the fi bers became more twisted and tan-gled, and their aggregations became more compact as shown in Figure 1 c. Additional experiments were per-formed to synthesize PPy fi rst and then add DA into PPy to form PDA. We did not observe fi brillar morphology of the resulted PPy. As a result, we believe that PPy and PDA are strongly bonded either through chemical bonds or hydrogen bonding and π – π interactions between the DA and Py during syntheses to form a completely new mor-phology. The TEM image Figure 1 d shows the morphology of a single core–shell PDA–PPy fi ber at 2 DA/Py mole ratio. The diameter and length of this nanofi ber is about 50 nm and 1 μ m, which are in agreement with the SEM results.

The FTIR spectra of pure PPy, PDA-PPy at 2 DA/Py mole ratio, and pure PDA are presented in Figure 2 . In the pure PPy spectrum (Figure 2 a), typical absorbance associated with asymmetric and symmetric ring stretching and C–N stretching are found at 1550 and 1428 cm −1 . The peaks at 987 and 1023 cm −1 region corresponds to the C–H in-plane bending vibration. [ 20 ] The band of N–H stretching in the Py ring is located at around 1670 cm −1 . [ 21 ] The characteristic signal of pure PDA (Figure 2 c) appears at 1495 and 2946 cm −1 , which are due to the C=C stretching

report the synergetic combination of PDA with PPy, where the additional functional groups from PDA improves the adhesion properties of PPy without limiting its biocom-patibility. We found that the combination makes PPy more hydrophilic, which allows PPy to disperse in water more uniformly, and unexpectedly, more electrically conductive, with a remarkable improvement of almost two orders of magnitude. To the best knowledge of the authors, this is the fi rst time PDA is used to functionalize PPy, and the new features of PDA-PPy make the material stand out as a promising candidate for many practical applications. For instance, PDA–PPy can be potentially utilized for tissue engineering since PDA can serve as suitable templates for cell adhesion and growth and PPy can serve as conductive platforms for tissue “communica-tions.” Key advantages of PDA–PPy include its enhanced conductivity and adhesion to substrates, making it suit-able to be applied at the electrical–biological interface.

2 . Results and Discussion

To make the polydopamine-functionalized polypyrrole PDA-PPy, we added 0.05 g pyrrole monomer and different amount of dopamine hydrochloride in 25 mL Tris solu-tion (pH 8.5, 10 × 10 −3 M ) with a constant stirring to make a homogeneous solution. The solution was cooled down to 8 °C on a cooling plate. Ammonium persulfate (APS)/Tris solution (0.5 g/5 mL) was then added to the DA/Py solution drop wisely under vigorous stirring for 18 h while the low reaction temperature was maintained. Afterward, the pre-cipitates formed from the reaction, the PDA–PPy particles, were collected by centrifugation and subsequent washed with deionized water. Finally, the precipitates were freeze-dried for 24 h into powder. To measure bulk conductivity,

Figure 1. SEM images of a) PPy with globular shape; b) fi brous PPy morphology resulted from 0.5 DA/Py mole ratio; c) more compacted fi brous PPy morphology resulted from 2 DA/Py mole ratio; and d) TEM topography image of a single PDA-PPy fi ber resulted from 2 DA/Py reacting mole ratio.



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dispersion of the PDA–PPy powders in deionized water after 6 h. It can be seen that a higher DA/Py mole ratio leads to better dispersion, while phase separation occurs at lower DA/Py mole ratio. Dynamic light scattering (DLS) experi-ments were performed to examine the change of particle size with respect to the DA/Py mole ratio. Note that PDA–PPy at low mole ratio of 0–0.2 cannot be measured by DLS due to the aggregation of particles. Figure 3 b showed that the particle size of PDA–PPy decreased from 430 to 110 nm with increasing DA/Py mole ratio from 0.5 to 2. The result suggests that PPy become more hydrophilic with DA modifi cations.

Lack of good adhesion is one of the main problems of PPy fi lms, which signifi cantly limits the fi lms’ applications. To overcome this problem, we combined mussel-adhesive-inspired molecule DA with Py. The adhesion properties of PDA-modifi ed PPy to glass substrate were characterized by 180° peeling tests with the tape/PPy/glass system. [ 24 ] In a typical experiment, 1 mL of PDA–PPy reacted solution was directly cast into a fi lm with a thickness of 20 ± 3.6 μ m by drying the solution on a 2 × 4 cm 2 glass slide at 80 °C for 5 h. The samples were tested within 30 min after drying to avoid moisture accumulation on the fi lm. To test bonding strength of the fi lm on the glass, the PDA–PPy fi lm was taped by pressing a piece of Scotch tape with a pressure of 20 kPa. The pressure helps remove air bubbles trapped and establish good contact between the tape and the fi lm. The bottom of the tape was gently folded back at 180°, and the tape was peeled from the bottom upwards at a speed of 15 mm s −1 . The delamination occurred either at the tape/PPy interface or PPy/glass interface. In our system, the peak force F p , which is defi ned as the force required to initiate peeling, has been utilized to compare the adhesion between glasses and PDA–PPy fi lms.

Figure 4 a plotted the peak force versus DA/Py mole ratio at a constant peeling rate of 15 mm s −1 ; the inserts are the digital images of corresponding tape surfaces after peeling. We observed a clear trend that F p increased with the DA percentage. At 0 and 0.1 mole ratios, inter-facial failure mostly occurred at the fi lm/glass interface where most of the fi lm stuck to the tape after peeling. At 0.2–0.5 mole ratios, the failure occurred at both tape/fi lm and fi lm/glass interfaces. At 1 and 2 mole ratios, the failure occurred only at the tape/fi lm interface since the tape after peeling appeared to be clean. It is important to note that pure PPy fi lm can easily delaminate from glass, while at 2 DA/Py mole ratio, the bond between PDA–PPy fi lm and glass was so strong that the bond remained intact even at a peeling force of 350 N m −1 ; this signifi -cant adhesion improvement can be attributed to the di-catechol functional groups in PDA, which are known for good adhesion. [ 25 ] Enhanced adhesion and particle disper-sion of PDA–PPy would make the material more process-able and applicable in practical engineering.

in the aromatic ring and C–H stretching of the aromatic side chain, respectively. [ 22 ] The broad band in the range of 3040−3400 cm −1 can be assigned to the peak of hydroxyl functional group. [ 23 ] By comparing the three spectra, it can be concluded that PDA–PPy (Figure 2 b) contains the characteristics of both pure PPy and PDA, supporting the observation that PDA has been successfully incorporated into the PPy network.

To check the dispersion of PDA–PPy particles, 0.005 g washed powder was added into 3 mL of deionized water and mixed in an ultrasonic bath for 5 min. It was found that pure PPy destabilizes and precipitates immedi-ately after mixing, while PDA-modifi ed PPy tends to disperse uniformly. Figure 3 a shows a digital photo of the

Figure 3. a) Typical plot of particle size verses DA/Py mole ratio of DLS experiment; and b) digital photo of vials with 3 mL water-dispersed PDA–PPy solution after 6 h.

Figure 2. FTIR spectra of a) pure PPy, b) 2 DA/Py mole ratio PDA-PPy, and c) pure PDA, and their characteristics were assigned accordingly.



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PPy dropped to 1.5 × 10 −3 S cm −1 for the compressed pel-lets and 1.1 × 10 −3 S cm −1 for the fi lm at 1 DA/Py mole ratio, and become almost non-conductive at the ratio of 2. It is important to note that PDA has been shown to be non-conductive by four-point probe measurements of its compressed pellet and therefore, the phenomenon might be explained by the fact that there are more DA at higher DA/Py mole ratio, leading to thicker depositions of PDA on PPy and insulation. The conductivity of PPy com-pressed pellets has been found to be slightly higher than that of the fi lms. This can be explained that by the fact that the fi lm was directly casted from the product solu-tion without purifi cations that remove unreacted Py, PPy with short chains, and salt, all of which are detrimental to conductivity. A comparison of Figure 4 a,b suggests an optimal DA/Py mole ratio to be between 0.1 and 0.2, where the resulting PDA-functionalized PPy exhibited both improved conductivity and adhesion properties (Figure 4 ), which are important when designing biocom-patible conducting polymer-based materials.

It is also worthwhile to mention that the PDA–PPy with higher conductivity can be obtained by combining PPy with other highly conductive materials, such as silver nanoparticles, graphene, and iron oxides; [ 26–28 ] the appli-cations of these materials in biotechnology are limited due to poor processability, high cost, and hash reaction conditions. To achieve better biocompatibility, conductive PPy/cellulose-derivatives composites with a conductivity ranged from 10 −4 to 10 0 S cm −1 were prepared and investi-gated intensively nowadays. [ 29 ] In comparison to the PPy/cellulose composition, our PDA-modifi ed PPy materials have similar conductivities but are much easier to pre-pare and process.

3 . Conclusions

PPy was functionalized with a “sticky” biomolecule DA. We found that the incorporation of PDA can: 1) change morphology, PPy structure has been dramatically shifted from globular to fi brous; 2) enhance adhesion, the adhe-sion strength of PPy was proportional to DA/Py mole ratio; 3) alter electrical properties, the conductivity of PPy increased to a maximum value and then decreased with the increasing PDA concentration; and 4) regulate par-ticle size, smaller PPy nanoparticle can be obtained by incorporating more PDA to PPy. All of these new features, combined with the biocompatible nature of these two molecules, signifi cantly improved the processability of PPy and broadened their potential applications. Moreover, this work may open the possibility of using DA to func-tionalizing more biocompatible conductive polymers; such materials could fi nd broad applications in artifi cial muscles, scaffold, and biosensors.

The electrical conductivity of both PDA–PPy powder compressed pellet and solution casted fi lms with various DA/Py mole ratios were characterized by a standard four-point probe setup and the results were shown in Figure 4 b. It was found that the mole ratio of DA/Py affected con-ductivity. A maximum conductivity of 3.8 S cm −1 for the compressed pellets and 0.2 S cm −1 for the fi lm were detected at the 0.1 mole ratio, which are about two orders of magnitude higher than that of the pure PPy. We attrib-uted the improvement in the PPy conductivity to the fact that PDA is negatively charged with a zeta poten-tial of −34.7 ev in water, and therefore may counterbal-ance the positive charge of PPy, and act as a “dopant” to further improve the conductivity. The conductivities of

Figure 4. a) Plots of peak force verses DA/Py mole ratio at a constant peeling speed rate of 15 mm s −1 ; the inserts are the digital photo of corresponding tape surfaces after peeling. b) Electrical conductivity of the PDA–PPy compressed pellets and fi lm with various DA/Py mole ratios. The shaded area indicates the optimal DA/Py mole ratio having both excellent conductivity and adhesion.



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Acknowledgements : The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ministry of Research and Innovation of Ontario for the fi nancial support.

Received: October 7, 2013; Revised: November 6, 2013; Published online: December 13, 2013; DOI: 10.1002/marc.201300761

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Keywords: adhesion ; dopamine ; electrical conductivity ; functionalization ; polypyrrole

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bio-inspired dopamine functionalization of polypyrrole for improved adhesion and conductivity

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