taurine-induced fabrication of nano-architectured conducting polypyrrole on biomedical titanium

CommunicationMacromolecularRapid Communications

wileyonlinelibrary.com574 DOI: 10.1002/marc.201300843

1 . Introduction

Biomolecule-induced fabrication of nano-architectured materials has played special role in nanoscience and nano-technology. [ 1 ] Most biomolecules are prone to interacting

each other and even self-assembling, and include pep-tides and proteins, which forms well-defi ned structure and space acting as templates for scaffolding nano-block units into unmatched nano-architectures. [ 2 ] Notably, some specifi c biomolecules are capable of providing active sites in ordered pattern along the molecular chains, containing DNA and heparin, which can control the nucleation, organ-ization, and binding of the building block units (typically, inorganic nanoparticles) in the fabrication of nano-archi-tectures. [ 3 ] Over the last decade, the biomolecule-induced fabrication of nano-architectured materials used in nano-devices has been studied systematically and proven to be a promising research subject. [ 4 ] Glutathione, a tripeptide, has been utilized to synthesize the highly ordered snow-fl ake-like bismuth sulfi de nano-architectures. [ 5 ] L -Cysteine, a common amino acid in the body, has been reported in the synthesis of metal sulfi de nano-architectures. [ 4a ] However, biomolecules, in particular small biomolecules, in the induction of the fabrication of the nano-architec-tured polymer materials, has received less attentions and remained more elusive than the aforementioned nano-architectured inorganic matters.

The nano-architectured conducting polymers (CPs) are a class of nano-architectured functional polymers with

In this article, taurine, one of the small biomolecules associated with bone metabolism, is fi rstly utilized to induce the fabrication of nano-architectured conducting polypyrrole (NCPPy) on biomedical titanium in diverse pH values of phosphate buffer solution (PBS). Accordingly, the possible mechanism for the fabrication of NCPPy is proposed, which is dependent on the states of polytaurine from the polymerization of taurine, i.e., the inability of forming polytaurine and unordered restricted space results in taurine-incorporated and polytaurine-incorporated tightly packed nanoparticles (pH 6.2 and 8.0), respectively, and however, ordered restricted space constructed by polytaurine chains induces the fabrication of polytaurine-incorporated nanopillars (pH 6.8) and poly-taurine-incorporated nanowire networks (pH 7.4).

Taurine-Induced Fabrication of Nano-Architectured Conducting Polypyrrole on Biomedical Titanium

Jingwen Liao , Haobo Pan , Chengyun Ning ,* Guoxin Tan , Zhengnan Zhou , Junqi Chen , Shishu Huang *

J. Liao, Prof. C. Ning, Z. Zhou, J. Chen School of Materials Science and Engineering , South China University of Technology , Guangzhou 510641 , China E-mail: [email protected] Dr. H. Pan Center for Human Tissues and Organs Degeneration, Shenzhen Institute ofAdvanced Technology, Chinese Academy of Science , Shenzhen 518055 , China Dr. G. Tan Institute of Chemical Engineering and Light Industry, Guangdong University of Technology , Guangzhou 510006 , China Dr. S. Huang State Key Laboratory of Oral Diseases , West China Hospital of Stomatology, Sichuan University , Chengdu 610041 , China E-mail: [email protected] Dr. S. Huang Department of Orthopedics and Traumatology , The University of Hong Kong , Hong Kong SAR , 999077 , China

Macromol. Rapid Commun. 2014, 35, 574−578

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

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2.2 . Fabrication of Nano-Architectured Conducting PPy

The small electrochemical cell included biomedical titanium sheet (effective area of 15 mm × 15 mm) as a working electrode, copper sheet as a counter electrode, saturated calomel electrode (SCE) as a reference electrode, a 0.2 M KCl solution as electrolyte containing 0.1 M Py. A prenucleation fi lm (PNF) was obtained at 0.8 V (vs SCE) for 20 s at room temperature under the control of electrochemical station (Zennium Zahner, Germany), and dried in a vacuum atmosphere. Typically, in PBS (0.5 M ) as electrolyte containing 0.2 M Py and 0.1 M taurine, nano-architectured con-ducting PPy on PNF/biomedical titanium (as a working electrode) was obtained by cyclic voltammetry (CV) of −1.8 to 1.8 V (scan-ning range), 100 mV S −1 (scanning rate) and 10 scanning cycles. The as-obtained products were rinsed for several times in deion-ized water, and dried under vacuum.

2.3 . Characterization

Field-emission scanning electron microscopy (FE-SEM; ZEISS Ultra 55, Germany) with acceleration voltage of 5 kV was employed to examine the architectures of NCPPy coated with gold nanolayer. Attenuated total refl ection Fourier-transform infrared (ATR-FTIR; Bruker Vector 33, Germany) was utilized to analyze the chemical composition of as-obtained products. The surface potentials of specimen obtained in PBS containing 0.1 M taurine but free of Py were characterized using Kelvin probe force microscopy (KFM, Shimadzu SPM-9600, Japan). The size of Py nanodroplets in elec-trolyte was measured by dynamic light scatter zetasizer nano-analyzer (Malvern Nano-ZS, Britain).

3 . Results and Discussion

In the attempt to study the effect of the pH values of PBS on the fabrication of NCPPy, considering the effective buffering range of PBS, we chosen pH values at 6.2, 6.8, 7.4 (physiological value), and 8.0. It was found that, as depicted in Figure 1 , the pH value nonlinearly affected the obtained architectures of NCPPy. More specifi cally, tightly packed nanoparticles (Figure 1 a, 236.3 ± 22.7 nm in dia-meter) were formed in PBS of pH 6.2. With the increase of the pH value to 6.8, gourd-shaped nanopillars (Figure 1 b, 217.1 ± 29.6 nm in diameter of the top) with opening were obtained. In the PBS of pH 7.4 close to physiological envi-ronments, the obtained nano-architecture was nanowire (Figure 1 c, 210.4 ± 10.1 nm in diameter) with approxi-mately 200 nm in diameter, which randomly weaved to be 3D networks. While tightly packed nanoparticles (Figure 1 d, 228.8 ± 21.3 nm in diameter) were unexpect-edly formed again in PBS of pH 8.0. Notably, the diameter values of the aforementioned nanoparticles were close to that of the nanowires as well as the top of nanopillars (i.e., various nano-architectures with 200 nm in at least one dimension), suggesting that there was at least one stage in common in fabricating NCPPy in various pH values of PBS.

electroactivity, [ 6 ] which has become an attractive hot research subject in the fi eld of nanoscience and nano-technology. [ 7 ] When being in conjunction with satisfac-tory tissue compatibility, [ 8 ] the nano-architectured CPs possess promising application potentials in biological sensing, drug delivery, neural probes, and even in tissue engineering, etc. [ 9 ] The nano-architectured CPs, CPs nanowires and nanotubes in particular, possess high aspect ratio and high surface–volume ratio as well as target-binding properties, which are mainly fabricated through templated approaches, including hard templates (e.g., organic or inorganic porous membranes) and soft templates (e.g., surfactants). [ 10 ] However, toxic additive or post-treatment organic reagents are inevitably involved in the templated approaches, [ 11 ] causing tissue infl amma-tion and even cancer in the clinical application of nano-architectured CPs. [ 12 ] Additionally, specimens fabricated by the biomolecule-induced approach are of biomimeti-cally chemical and structural compatibility with optimal biological response to autologous molecules. [ 3 ] Therefore, the biomolecule-induced fabrication of the nano-architec-tured CPs may be attractive and suitable in corresponding biomedical fi elds.

Taurine (2-aminoethane sulfuric acid) is a well-known dissociated sulfur-containing amino acid. It serves numerous physiological and pharmacological functions, such as affecting protein phosphorylation, infl uencing membrane ion channels, eliminating oxide-free radicals, regulating cellular Ca 2+ fl ux. [ 13 ] Taurine has been reported in a high concentration in bone cells, and involves in bone metabolism, which enhances bone tissue forma-tion and inhibits bone loss. [ 14 ] In this paper, we employed taurine, as a small biomolecule, to induce the fabrication of NCPPy (with approximately 200 nm in at least one dimension) on biomedical titanium in diverse pH values of PBS. In the possible mechanism we proposed, the fabri-cation of NCPPy was determined by the ability of taurine being electropolymerized to polytaurine and the order of restricted space constructed by polytaurine chains.

2 . Experimental Section

2.1 . Chemicals

All chemicals of analytical grade were purchased from Aladdin Chem Co., Ltd. and used without further treatment if not speci-fi ed otherwise. Titanium (Ti) sheet for biomedical application (0.2 mm in thickness) was obtained according to standard ASTM (American Society for Testing & Materials) F67–2002 from Baoji Qichen New Material Technology Co., Ltd. Biomedical titanium sheet prior to use was rinsed and degreased ultrasonically in deionized water, ethanol, and acetone, respectively, and polished chemically in hydrofl uoric acid (HF, 0.55 M ) and nitric acid (HNO 3 , 0.25 M ) solution of 1:1 in volume, and followed by multiple rinse with deionized water.



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the esterifi cation between –HSO 3 of taurine and –NH– of Py (as H from –NH– of Py is stable). As such, it was indi-cated that –HSO 3 (675 cm −1 above) was present in taurine (Figure 2 a), and as end group in polytaurine (Figure 2 b–d). Therefore, it was reasonable for judging that NCPPy obtained was incorporated with polytaurine in PBS of pH 6.8, 7.4, and 8.0, while being incorporated with taurine in PBS of pH 6.2.

To further verify the judgment above, the cyclic voltam-mograms (Figure 3 ) of synthesis of NCPPy were recorded. According to Wang et al.’s report, taurine was electropoly-merized to polytaurine under specifi c CV condition (close to electrochemical parameter in this paper), which was utilized to modify the detection electrode. [ 18 ] As demon-strated in the cyclic voltammograms, the oxidation peak at −0.3 to 0.3 V in Figure 3 b–d, and the reduction peaks at −1.0 to −1.5 V in all cyclic voltammograms, indicated that polytaurine was generated by the electropolymeriza-tion of taurine in PBS of pH 6.8 (Figure 3 b), 7.4 (Figure 3 c), and 8.0 (Figure 3 d). However, it was not prone to being harvested in PBS of pH 6.2 (Figure 3 a) possibly due to the enhanced energy barrier of esterifi cation. It was found that, except for the cyclic voltammogram in Figure 3 a, the oxidation peaks and reduction peaks in other three cyclic voltammograms positively shifted with the increase of cycle number. It was implied that the resulting NCPPy exerted somehow catalytic effect on the formation of polytaurine, which deserved more studies although anal-ogous performance of microstructured-conducting PPy had been reported in Shi’s studies. [ 19 ] Moreover, enhanced peak heights, particularly for the reduction peak, were observed upon continuous scanning, suggesting that the polytaurine was continuously grown. Focusing on the reduction peak recorded in PBS of pH 8.0 (Figure 3 d),

The chemical structures of various NCPPy fabricated on biomedical titanium were characterized by ATR-FTIR spectra. As shown in Figure 2 , peaks at 1528, 1459, and 780 cm −1 appeared in four spectra were assigned to the C = C stretching vibration, C–N stretching vibration, C–H out-plane ring deformation of PPy, respectively. [ 15 ] The peaks at 1154 and 1029 cm −1 were attributed to the asym-metric and symmetric O = S = O stretching vibration, [ 16 ] and the peak at 675 cm −1 corresponded to –HSO 3 stretching vibration as well, [ 17 ] indicating that taurine or taurine-based substrate was incorporated or doped in NCPPy matrix obtained in PBS of various pH value. Furthermore, the band at 1366 cm −1 associated with –SO 2 NH– (sulfa-mide group) appeared in Figure 2 b–d but not in Figure 2 a, which could be ascribed to condensation polymerization (forming polytaurine) of taurine molecules, rather than

Figure 1. FE-SEM images of NCPPy fabricated by taurine-induc-tion in PBS of various pH values. a) 6.2, b) 6.8, c) 7.4, and d) 8.0.

Figure 2. ATR-FTIR spectra of taurine-doped NCPPy obtained in PBS of various pH values. a) 6.2; b) 6.8; c) 7.4; d) 8.0.

Figure 3. Cyclic voltammograms recorded in fabrication of NCPPy in PBS of various pH values. a) 6.2, b) 6.8, c) 7.4, and d) 8.0.



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between the Py nanodroplets and the polytaurine chains, as illustrated in Figure 5 c,d, Py nanodroplets are induced into restricted space and perform electrochemical poly-merization, and the resulting nanograins (Figure S1, Sup-porting Information) can be observed at fi rst cycle stage. As CV proceeds (Figure 5 e–g), polytaurine chains are grown longer, accompanied with the self-assembly of Py nanodroplets in restricted space and subsequent electro-chemical polymerization. Several differences between PBS of pH 6.8 and 7.4, such as the concentration of Py nano-droplets (the solubility of Py is sensitive to the pH level of solvent), the construction rate of ordered restricted space, and the subsequent self-assembling rate of Py nano-droplets, lead fi nally to the fabrication of diverse nano-architectures, i.e., nanopillars (Figure 1 b) and nanowire networks (Figure 1 c). Regarding to nano-architecture obtained in PBS of pH 6.2 and 8.0, the failures for the formation of polytaurine and construction of ordered restricted space, both result in the random self-assembly of Py nanodroplets, which subsequently harvest the same tightly packed nanoparticles (Figure 1 a,d) in the two pH values of PBS. Put it together, the inability of forming poly-taurine results in tightly packed nanoparticles (pH 6.2) incorporated with taurine, and unordered restricted space

the corresponding current which was larger than that in other three cyclic voltammograms, was negatively increased to from about −5.8 mA to over −8.0 mA at a high rate. Now, it may seem that the higher growth rate and the resulting unordered restricted space (constructed by polytaurine chains) result in formation of tightly packed nanoparticles. Only if growth state of polytaurine is appropriate will ordered restricted space be constructed.

In numerous studies, KFM has been considered an unique supplementary technique to indirectly verify the presence of specifi c molecule or substance by the surface potential mapping. [ 20 ] Here, we indirectly con-fi rmed whether the polytaurine was grown in PBS free of Py on biomedical titanium using KFM, as represented in Figure 4 . The pristine biomedical titanium surface (Figure 4 a) possessed 10-point average potential ( V zjis, absolute value) of 0.06 V, which was close to that of the biomedical titanium (Figure 4 b) after undergoing CV in PBS (pH 6.2) containing 0.1 M taurine. By contrast, the neg-atively higher V zjis of 1.46 V could be read from Figure 4 c recorded on biomedical titanium surface after under-going CV in PBS (pH 7.4) containing 0.1 M taurine. Com-bined information from both Figures 2 and 3 , we there-fore concluded that the negatively high V zjis (Figure 4 c above was attributed to the polytaurine grown on bio-medical titanium in PBS of pH 7.4, and no polytaurine was formed on biomedical titanium in PBS of pH 6.2.

Given all above, we propose a possible mechanism for the fabrication process of the NCPPy, as illustrated in Figure 5 . Dissolved Py monomers and Py nanodroplets or micelles are in dynamic equilibrium depending on ionic strength, pH value of electrolyte, and system tempera-ture, etc. In this electrochemical system, the Py nanodro-plets (Figure 5 a) of 190.5 nm (Figure 5 a#) in average dia-meter are dispersed in PBS (pH 6.8 and 7.4) as electrolyte with dissolved taurine. After CV being triggered, taurine monomers are electropolymerized to polytaurine chains standing on biomedical titanium by the condensation polymerization (Figure 5 h, between sulfonic acid group and amino group, but not intramolecular esterifi cation), which construct an ordered restricted space (Figure 5 b). Because of the CV potential, and the hydrogen bonding

Figure 4. KFM surface potentials of pristine biomedical titanium (a), and biomedical titanium after undergoing CV in PBS of pH 6.2 (b) and pH 7.4 (c) containing 0.1 M taurine (free of Py).

Figure 5. Schematic illustration for fabrication of NCPPy. (a#) Size distribution of Py micells. (a-g) Self-assembly process in ordered restricted space. (h) Chemical structure of the polymerization of taurine.



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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : The authors gratefully acknowledge the fi nancial support of National Basic Research Program of China (Grant No. 2012CB619100) and the National Natural Science Foundation of China (Grant Nos. 51372087, 51072057).

Received: November 13, 2013; Revised: November 28, 2013; Published online: December 17, 2013; DOI: 10.1002/marc.201300843

also results in tightly packed nanoparticles (pH 8.0) but incorporated with polytaurine, while ordered restricted space induces the fabrication of nanopillars (pH 6.8) and nanowire networks (pH 7.4) both incorporated with poly-taurine. It should be pointed out that, for the self-assembly fabrication of CNPPy, the size of ordered restricted space must be capable to accommodate the Py nanodroplets, and the diameter of nanowire or nanopillar is dependent on the size of Py nanodroplets rather than the restricted space. It has been confi rmed by the fact that, nanowire networks (Figure S2a, Supporting Information) obtained in PBS (pH 7.4) containing 0.2 M taurine, which is the same nano-architecture (Figure 1 c) as that obtained in PBS (pH 7.4) containing 0.1 M taurine, while being short nanowires of relatively low density (Figure S2b, Supporting Informa-tion) and a smooth PPy fi lm (Figure S2c, Supporting Infor-mation) in PBS containing 0.3 and 0.4 M taurine, respec-tively. In addition, it can be observed that nanowires of Figures 1 c, S2a, and S2b (Supporting Information) possess almost the same diameter. Compared with 0.1 and 0.2 M taurine, the polymerization of 0.4 M taurine produces a narrower ordered restricted space, which is incapable to accommodate the Py nanodroplets and only allow the electropolymerization of dissolved Py (forming smooth PPy fi lm). Regarding the 0.3 M taurine, ordered restricted space may be in a critical state, which limits the smooth movement of Py nanodroplets to biomedical titanium and subsequently obtain short nanowires of low density. To be simple, the states of polytaurine steming from taurine determines the inducing growth orientation of NCPPy.

4 . Conclusions

Taurine as a bone-metabolism-associated small biomol-ecule, was fi rstly reported in inducing the fabrication of NCPPy on the biomedical titanium. The possible mecha-nism for the fabrication of NCPPy was proposed, which was dependent on the ability of taurine being electropoly-merized to polytaurine, as well as on the order of restricted space constructed by polytaurine chains, i.e., the inability of forming polytaurine and unordered restricted space results in taurine-incorporated and polytaurine-incorpo-rated tightly packed nanoparticles (pH 6.2 and 8.0), respec-tively, and however, ordered restricted space constructed by polytaurine chains induces the fabrication of polytau-rine-incorporated nanopillars (pH 6.8) and polytaurine-incorporated nanowire networks (pH 7.4). Here, NCPPy fabricated by the induction of taurine is expected as a potential biomimetic interface on biomedical titanium. Furthermore, this study sheds some light on the fabrica-tion of nano-architectured CPs (and even other specifi c functional materials) induced by the electropolymeriza-tion, self-polymerization, and self-assembly of small bio-molecules (e.g., citric acid and dopamine).

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Keywords: biomedical titanium ; biomolecules ; conducting polymers ; nano-architecture ; taurine



taurine-induced fabrication of nano-architectured conducting polypyrrole on biomedical titanium

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