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Synthetic Metals 160 (2010) 849–854

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

ontrolled fabrication of nanostructured polypyrrole on ion associationemplate: Tubes, rods and networks

in Wei, Tingyang Dai, Yun Lu ∗

epartment of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,anjing University, 22# Hankou Road, Nanjing, Jiangsu Province 210093, PR China

r t i c l e i n f o

rticle history:eceived 23 October 2009eceived in revised form 7 January 2010

a b s t r a c t

Polypyrrole (PPy) with variable nanostructure such as nanotubes, nanorods and networks has been fab-ricated by employing an ion association of heparin–methylene blue as new morphology-directing agent.The ion association exhibited both advantages of hard and soft template in template-induced chemi-

ccepted 22 January 2010vailable online 3 March 2010

eywords:olypyrrole

cal polymerization of pyrrole. By altering the ration of ion association component, PPy could be easilyswitched in morphology between nanotubes and nanorods. With the coordination effect of ion associ-ation and oxidant Fe3+, PPy networks with high conductivity could also be obtained in static condition.The achieved nanostructured polypyrrole were all doped with heparin, a biochemical material, and had

ue in

anostructureemplateeparin–methylene blue

potential exploitation val

. Introduction

Conductive polymers with one-dimensional (1D) nanostructureuch as nanotubes, nanorods have been used as molecular build-ng blocks to construct nanoelectronic devices [1,2], field-effectransistors [3], sensors [4,5] and actuators [6]. The straightforwardpproach toward 1D nanostructure is to confine the chemical orlectrochemical oxidation of monomers in a porous “hard tem-late” such as zeolites [7], alumina [8], and tract-etched polymericembranes [9]. Another popular route is to employ a structure-

irecting molecule such as surfactants [10–12] and organic acids13] as a “soft template”. The traditional “hard” and “soft” tem-late approaches both have their own demerits, such as expensive,reparation complexity (for hard template), instability and low effi-iency (for soft template). Recently, our group has developed aerial of novel “hard template” systems based on dye molecules,uch as methyl orange [14–16], benzyl orange [17] and porphyrinerivate [18], which exhibited both merits of hard and soft tem-late, such as stable, good versatility, cheap, and easy removalf the template, in construction of microstructures of conductiveolymers. In this work, varied one-dimensional nanostructuredolypyrrole (PPy) such as nanotubes and nanorods as well as PPy

etworks directly assembled by PPy microtubules were preparedy using an ion association of heparin–methylene blue as a neworphology-directing agent.

∗ Corresponding author. Tel.: +86 25 83686423; fax: +86 25 83686423.E-mail address: yunlu@nju.edu.cn (Y. Lu).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.01.032

electrochemistry and biochemistry.© 2010 Elsevier B.V. All rights reserved.

Heparin (Hep), a highly-sulfated glycosaminoglycan consistedof repetitive sulfated groups (Fig. 1a), is widely used in clinicalissues as an effective antithrombotic drug [19]. It was reportedthat the PPy doped with heparin showed a significantly increasesin its electrical stability [20] and was well suited to support cellattachment and growth due to its high surface hydrophilicity andproper roughness [21,22]. Methylene blue (MB), a water-solublephenothiazine dye (Fig. 1b), is extensively investigated in photo-dynamic reaction [23,24] and can bind to glycosamino-alglycanssuch as heparin to form Hep–MB ion association [25].

In our study, it was found that by applying the ion associa-tion (IA) of heparin–methylene blue (Hep–MB) as the template,in which Hep also acts as a dopant, conducting PPy with variableone-dimensional nanostructure could be achieved in a facile andefficient way. To the best of our knowledge, such study on the tem-plating fabrication of nanostructured PPy using Hep–MB IA has notbeen reported yet. The major focus of this article is on the regula-tion effect of pyrrole monomer (Py) on the IA in the construction ofnanostructured PPy and the formation mechanism of PPy nanorods,nanotubes and networks with IA template. Moreover, the influenceof the ration of IA component, the stirring or static reaction condi-tion and the kinds of oxidants on the final morphology of achievedPPy was discussed in detail.

2. Experiment

2.1. Materials

Pyrrole monomer was purchased from Aldrich and distilledunder reduced pressure before use. Heparin (sodium salts)

850 M. Wei et al. / Synthetic Metals 160 (2010) 849–854

(CCLL

2

wvUwwAatvdToap

2

tnvu

2

nweiaKmuoUmsa

Fig. 1. Molecular structures of heparin (a) and methylene blue (b).

biochemical reagent, Shanghai Sanjie biotechnology Co. Ltd.,hina), methylene blue (CP, Tianjing chemical reagent institute,hina), ferric chloride (FeCl3, CP, Sinopharm Chemical Reagent Co.td., China) and ammonium peroxydisulfate (APS, AR, Shanghaiingfeng Chemical Reagent Co. Ltd., China) were used as received.

.2. Preparation of PPy nanotubes and nanorods

In a typical procedure, 5 mg heparin and 10 mg methylene blueere dissolved in 10 ml deionized water respectively, and the pH

alue of the aqueous solutions was adjusted to 1 using 1 M HCl.nder ultrasonic condition, the heparin solution was added drop-isely into the methylene blue solution, and then 1 mmol pyrroleas added and dispersed uniformly by further sonicated for 30 min.t last, an aqueous solution of APS (10 ml, 1 mmol oxidant) wasdded dropwisely into the above mixture solution, and the reac-ion was allowed to proceed for 12 h at room temperature withigorous stirring. The resulting product was filtered, washed witheionized water and ethanol, and dried by infrared dehydration.o obtain PPy samples with different nanostructures, the amountsf heparin and methylene blue were changed to 10/5 mg, 20/5 mgnd 5/20 mg, respectively, and the above-mentioned preparationrocedure was repeated.

.3. Preparation of PPy networks

The formation of PPy networks were completed by repeatinghe above-mentioned preparation procedure for PPy nanotubes andanorods, except that a static condition was employed instead ofigorous stirring and FeCl3 (1 mmol, in 10 ml deionized water) wassed as the oxidant to replace APS in the reaction process.

.4. Characterizations

The morphologies of the samples were observed on a scan-ing electron microscope (SEM, JEOL Highlighted SEM-5610,ith energy dispersive spectroscopy (EDS)) and a transmission

lectron microscope (TEM, JEOL JEM-200CX). Fourier-transformnfrared (FT-IR) spectroscopy measurements were performed on

Bruker Fourier transform spectrometer model VECTOR22 usingBr pressed discs. Conductivity was measured using a four-probeethod on a WR-2B digital multimeter at room temperature

sing compressed pellets of powders. UV–vis absorption spectra

f methylene blue in acidic aqueous solution were recorded on aV-240 spectrometer (Shimadzu, Japan). A SDT 2960 thermogravi-etric analyzer was used to investigate the thermal stability of the

ample with nitrogen as pure gas at a flow rate of 100 ml/min andheating rate of 15 ◦C/min.

Fig. 2. UV spectra of MB, MB–Hep and MB–Hep–Py solutions.

3. Results and discussion

The UV–vis absorption spectra of MB, Hep–MB and Hep–MB–Pywere shown in Fig. 2. It was found that when Hep was added intoMB solution, the intensity of the peaks located at 614 and 664 nmdecreased, indicating the formation of IA [26]. A further decreasecould be observed when Py monomer was dissolved in Hep–MBsystem, suggesting that Py monomer may take part in the formationof the Hep–MB–Py assembly or have regulation effect on IA.

In the TEM image (Fig. 3), one can clearly find the morphologychange before and after the addition of Py monomer to the IA sys-tem. A complex of needle-like and granular IA was restructuredto more uniformed strip-like structure, even if the solution wasquite dilute (0.1 mg/ml heparin and 0.1 mg/ml methylene blue).Considering the effect of IA as template in inducing PPy nanos-tructures, solutions of IA with the concentration of 0.25–1 mg/mlHep and 0.25–1 mg/ml MB were applied respectively in thepolymerization.

When using APS as an oxidant, we could obtain PPy nan-otube and PPy nanorods respectively with different IA componentrations. PPy nanotubes with external diameter ranging from 250to 500 nm, inner diameter ranging from 50 to 250 nm and lengthranging from 2000 to 6000 nm could be achieved in a 0.25 mg/mlheparin–0.5 mg/ml methylene blue solution (Fig. 4c and d). AndPPy nanorods with diameter of 100 nm, length ranging from 200to 1000 nm could be achieved in a 0.5 mg/ml heparin–0.25 mg/mlmethylene blue solution (Fig. 4a and b).

To explore the influence of component ratio on the PPy morphol-ogy, a serial of experiments with varied IA component ratios werecarried out. The results in Table 1 showed that in a solution withlow concentration of heparin and high concentration of methyleneblue, PPy tended to form nanotube structure and per contra PPynanorods could be observed. Meanwhile, we found that the com-ponent ratio did not influence the morphology of IA, indicating thatthe form of different PPy nanostructures may be achieved in theprocess of polymerization.

It is well known that heparin is a long chain linear polysac-charide with many negative charges [27]. Generally, such longchain molecule contains mainly extended conformation due to thestrong repulsion of the negative charges [28]. Therefore, as shown

in Scheme 1, after the positive charged MB molecules are boundto the heparin chain by electrostatic forces [29], the generated IAshould also possess an extended conformation. Such a structureis beneficial for the adsorption of the protonated Py monomers,via the electrostatic forces between Py and the heparin chain

M. Wei et al. / Synthetic Metals 160 (2010) 849–854 851

Fig. 3. TEM image of template without (left) and with Py (right).

anoro

a[(

ctbaeI

TT

Fig. 4. TEM and SEM images of PPy n

nd/or the �–� interactions between Py and the MB molecules30], and promotes the formation of the straight strip-like templateScheme 1).

Based on the above assumption, in a solution with low con-entration of heparin and high concentration of methylene blue,

he negative charges on heparin are surrounded and counteractedy positive charged methylene blue. The weak electrostatic inter-ction between Py and heparin makes the Py monomers couldffectively correlate only with MB and assemble at the exterior ofA template. Hence, after the polymerization and post-rinse proce-

able 1he obtained PPy nanostructures at different Hep–MB concentration.

Hep = 0.25 mg/ml Hep = 0.5

MB = 0.25 mg/ml Granule NanorodMB = 0.5 mg/ml Nanotubes NanotubMB = 1 mg/ml Nanotubes NanotubMB > 1 mg/ml Nanotubes Nanotub

ds (a and b) and nanotubes (c and d).

dure, only hollow PPy nanotubes remained due to decomposition ofIA template. On the contrary, a solution with high concentration ofheparin and low concentration of methylene blue led to strong elec-trostatic interaction and the polymerization of Py proceed insidethe template, resulting in a nanorod structure (Scheme 2).

It is interesting that when FeCl3 was applied as the oxidant, nomatter how the component ratio changes, only separated PPy gran-ule and strip-like template could be obtained (Fig. 5). It indicatedthat the IA did not serve as an effective template at the presenceof Fe3+ and there may be different formation mechanisms for the

mg/ml Hep = 1 mg/ml Hep > 1 mg/ml

s Nanorods Nanorodses Nanorods Nanorodses Nanotubes Nanorodses Nanotubes

852 M. Wei et al. / Synthetic Meta

vF

anb

owning to the formation of IA, negative charge density of heparin

Scheme 1. Schematic diagram for the formation of strip-like template.

ariable nanostructures of the PPy products initiated by APS andeCl (will be discussed later).

3

To our surprise, however, when static condition was appliedt the presence of Fe3+, PPy networks composed of connectedanorods with a diameter of 100 nm could be observed (Fig. 6a and). To investigate the formation process of the PPy networks, exper-

Scheme 2. Schematic diagram for the form

ls 160 (2010) 849–854

iments of reaction-terminating were processed. One and threeminutes after the dripping of FeCl3, the reaction solution was takenout respectively and dried on copper grid to monitor the reactionprocess by TEM. It was clear that in the beginning of the reaction,the oligomers of Py were around the strip-like IA template, imply-ing that the polymerization proceeded on the IA template (Fig. 6c).At the same time, the formed nanorods showed a trend to connecteach other. Three minutes after dripping of FeCl3, the network hadformed (Fig. 6d) and its yield collected at 5 min has been a half ofthat of PPy granules obtained from the system that oxidation of Pyby FeCl3 without IA had carried through 12 h. Such obvious acceler-ation suggested that the growth of PPy may obey a new mechanismwith FeCl3 acting as the oxidant.

Basing on the fact that ferric ion could coordinate with heparin[31], we considered that FeCl3 could be enriched in the IA tem-plate, which makes the polymerization happens only inside of IAtemplate in static condition, no matter how the IA component ratiochanges. Also, the enrichment of oxidant could also explain theacceleration in the formation of PPy networks. On the other hand,

decreased and the coordination became weak and fragile [32]. In astirring condition, the coordination may be easily broken and theoxidant was dispersed in solution, which led to a mixed productsof the separated PPy granule and the strip-like templates. Anyway,

ation of PPy nanorods and nanotubes.

M. Wei et al. / Synthetic Metals 160 (2010) 849–854 853

granu

satciPi

namvrtaotat

Fig. 5. TEM images of PPy

uch mesoscopic scaled structure were usually constructed easilyt the static condition due to the mutual attraction between nanos-ructures via van der Waals forces and chemical bonds (includingoordination bonds) [33]. On the other hand, without stirring, thensufficient mixing of reactants may result in the deposition ofPy granules in the reaction system. These granules may play anmportant role in acting as linking-point in the networks [34].

Fig. 7 showed the FT-IR spectra of the PPy nanorod, nanotubes,etworks and granules. The PPy with nanostructure displayed somebsorption in common with PPy granule. The characteristic asym-etry and symmetry stretching of pyrrole ring, stretching of C–N,

ibration of C–H were located at 1545, 1475, 1100 and 780 cm−1,espectively. The strong peaks near 1175 and 909 cm−1 impliedhe doping state of PPy [35]. For nanostructured PPy, the uniquebsorptions such as the band at 3445 cm−1 assignable to hydroxyl

n heparin and the bands at 1634 and 1384 cm−1 attributed respec-ively stretching of C O and N–O on heparin, confirmed the dopingction of heparin. No sign of methylene blue could be found in spec-ra, suggesting that the dye can be removed by rinse easily. The

Fig. 6. TEM and SEM images of PPy networks (a and b) and the T

le and strip-like template.

ratio of integral intensity of absorption at 1548 and 1486 cm−1 areusually considered to be the criteria of effective conjugate chainlength of PPy [36,37]. The FT-IR spectra indicated that nanostruc-tured PPy had longer effective conjugate chain, which suggested ahigher conductivity [38].

It has been reported that the conductivity of one-dimensionalconductive polymer was directly dependent on its diameter[39,40]. The ration of layers with ordered PPy chain confined bytemplate in the whole structure was low if the diameter was wide.This theory indicated that as the diameter decreases, the conduc-tivity increases due to more ordered chains. In this experiment,the conductivities of achieved PPy consist with the theory. PPynanorods with a diameter of 100 nm showed a higher conductiv-ity (6.9 S/cm) than nanotubes with external diameter ranging from250 to 500 nm (3.5 S/cm). The conductivity of PPy networks was

34.5 S/cm, which increased dozens of times compared with thatof PPy granules (1.5 S/cm). We believed that it is the connectedstructure of PPy networks that counteract the influence of insulatedcavities, and led to a much higher conductivity.

EM images of PPy 1 min and 3 min after reaction (c and d).

854 M. Wei et al. / Synthetic Metals 160 (2010) 849–854

ods; (

4

thnibathtaPi

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Fig. 7. FT-IR spectra of conductive PPy with different morphologies (a) nanor

. Conclusion

In summary, we discussed the formation of Hep–MB IA andhe regulation effect of Py on IA in acidic aqueous solution, andence developed IA as a new template for the formation of PPyanotubes, nanorods and networks in an easy chemical polymer-

zation method. This template showed considerable merits fromoth soft and hard template such as stable, good versatility, cheapnd easy removal in the fabrication of nanostructured PPy. In par-icular, we found that with Fe3+ as an oxidant and without stirring,ighly conductive PPy networks could be achieved. We attributedhe formation of such networks to the coordination bond of Fe3+

nd heparin and the static reaction condition. The nanostructuredPy were all doped with heparin and are with potential applicationn electrochemistry and biochemistry.

cknowledgements

This work was supported by the National Natural Scienceoundation of China (No. 50773030), Open project of State Keyaboratory of Supramolecular Structure and Materials, Jilin Uni-ersity (No. SKLSSM200903) and the Testing Foundation of Nanjingniversity.

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