Journal of Colloid and Interface Science 381 (2012) 11–16

Contents lists available at SciVerse ScienceDirect

Journal of Colloid and Interface Science

www.elsevier .com/locate / jc is

Effects of acidity on the size of polyaniline-poly(sodium 4-styrenesulfonate)composite particles and the stability of corresponding colloids in water

Ligui Li a, Linhui Ferng b, Yen Wei a, Catherine Yang c, Hai-Feng Ji a,⇑a Department of Chemistry, Drexel University, Philadelphia, PA 19104, USAb College of Bioresources, National I-Lan University, Taiwanc Department of Chemistry, Rowan University, Glassboro, NJ 08028, USA

a r t i c l e i n f o

Article history:Received 26 November 2011Accepted 7 May 2012Available online 14 May 2012

Keywords:PolyanilinePoly(sodium 4-styrenesulfonate)PANIPSSPANI–PSS nanoparticlesConductive polymer

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.05.004

⇑ Corresponding author.E-mail address: hj56@drexel.edu (H.-F. Ji).

a b s t r a c t

The practical application of polyaniline-poly(sodium 4-styrenesulfonate) (PANI–PSS) composite particleshas been held back by the low stability of their dispersed state in water. In this work, we present a gen-eral oxidation approach to prepare PANI–PSS composite nanoparticles that can form highly stable col-loids in water or buffer over a wide range of pH from 1 to 11. We demonstrate that the size of thePANI–PSS composite particles can be controlled by the acidity of precursor solutions. It is hypothesizedthat the number of negatively charged sites on PSS, which can be affected by the acidity of the precursorsolutions, plays an important role in determining the size of the PANI–PSS composite particles and thestability of corresponding colloids in water.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Conjugated polymers have shown wide applications in photo-voltaic devices, light emitting diodes, field effect transistors, sen-sors and actuators, etc. Polyaniline (PANI) is one of the moststudied conjugated polymers due to its widely tunable electricalconductivity [1–5] and environmental stability [6–12]. PANI canbe readily synthesized from commercially available starting mate-rials under mild conditions by oxidation methods [13–19]. One ofthe drawbacks of PANI is that it is insoluble in most common sol-vents, which limits its practical usage in many applications. Mostpolymer electronics are based on uniform and thin polymer films,which are typically prepared by spin-coating, printing or layer-by-layer assembling [20–24] a homogeneous and stable solution ofpolymers. For this, a polymer must be soluble or dispersible in sol-vents. Furthermore, to prepare a thin polymer film from a roll-to-roll printer, either a stable solution or a colloid containing verysmall particles of polymers is essential in order for the printer noz-zles to remain unclogged.

Due to the cost-effective and environment-friendly nature ofwater as a solvent, a significant amount of work has been focusedon the development of water-soluble or water-dispersible PANI.One of the most successful approaches is to add polyelectrolytesas supporting substrates during the polymerization of PANI.

ll rights reserved.

To date, poly(sodium 4-styrene sulfonate) (PSS) is one of the mostwidely used supporting polyelectrolytes. Besides supporting PANI[25], PSS has also been used to support poly(ethylene dioxythioph-ene) (PEDOT) [26] and polypyrrole (PPY) [27,28] in the preparationof conductive polymers with good water solubility or dispersibility.

Most reported water dispersible PANI–PSS composite particles,however, are large and aggregate in water or buffers when leftalone for minutes or hours. Recently, several ultrafine PANI–PSScomposite particles have been reported. These ultrafine particlescan be well dispersed to form stable colloids in water or buffers.Using interfacial polymerization, Dorey et al. [29,30] synthesizedsmall PANI–PSS composite particles with high molar ratios of PSSin the PANI–PSS composite particles. One drawback of this com-posite is the very low electric conductivity due to the high molarratio of PSS to PANI. Tripathy et al. [31] developed small PANI–PSS composite nanoparticles with the aid of an enzyme-catalyzedtemplated polymerization. The disadvantage of this method is itsrelatively high cost compared with that of other approaches. Thereis still a need to develop a facile approach for synthesizing water-soluble PANI–PSS or highly dispersible and stable PANI–PSS com-posite nanoparticles that can be mass produced at a low cost.

In the oxidative reactions to produce PANI or PANI–PSS com-posites, acids are needed in the polymerizations. A great deal ofwork has been conducted on the effect of acids on the size of PANIparticles and their corresponding electrical conductivity [4,32–35].However, less attention has been paid to the effect of acids on thesize of PANI–PSS composite particles. In this work, we report that

Fig. 1. PANI–PSS composite particles in buffers with different pH after standing for 6 h (PANI–PSS/HCl and PANI–PSS/CSA) and one month (PANI–PSS/CH3COOH).

12 L. Li et al. / Journal of Colloid and Interface Science 381 (2012) 11–16

ultrafine PANI–PSS composite nanoparticles and stable colloids ofPANI–PSS can be prepared in the presence of weak acids or lowconcentration of strong acids. The corresponding mechanism isalso discussed.

1.0PANI:PSS/CH3COOH pH=1

2. Experimental details

2.1. Materials

Aniline (anhydrous, 99%), ammonium peroxydisulfate (98%)were purchased from Alfa Aesar company. Aniline was distilled un-der reduced pressure and stored in a freezer at 0 �C. Poly (sodium4-styrenesulfonate) (PSS) with Mw of �70,000, acetic acid, metha-noic acid, propanoic acid, hydrochloric acid, nitric acid, perchloricacid, methyl sulfonic acid, sulfuric acid, and DL-camphorsulfonicacid (98%) (CSA) were purchased from Sigma–Aldrich Co. Ltd.

300 400 500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

PANI:PSS/HClPANI:PSS/CSA

Fig. 2. UV–Vis absorption spectra of PANI–PSS/HCl, PANI–PSS/CSA, and PANI–PSS/CH3COOH composite particles in a buffer solution at pH = 1. The spectra in pH = 0are close to these spectra, suggesting that the PANI in the composites are fullydoped.

2.2. Sample preparations

2.2.1. SynthesisThe PANI–PSS samples were synthesized by a rapid mixing

polymerization method [36]. Typically, the polymerization wasperformed in a 50-mL test tube. To synthesize the PANI–PSS com-posite, solutions of aniline (32 mM) were prepared by dissolving35.8 mg aniline in 12 mL distilled water with various concentra-tions of doping acids from 40 mM to 1.0 M. Solutions of PSS wereprepared by dissolving 41 mg of PSS in 12 mL of aqueous ammo-nium peroxydisulfate (8 mM) with various concentrations of dop-ing acids from 40 mM to 1.0 M. The two solutions were mixed inthe 50-mL test tube and the mixed solution was shaken vigorouslyfor 5 min while polymerization occurred. The mixture was thenplaced in the dark for 24 h at room temperature until thepolymerization reaction was complete. The molar ratio of aniline

and 4-styrenesulfonate repeating unit in PSS in all the onset poly-merization mixtures was 2:1 unless indicated.

Excess acid and small molecules were removed from the as-pre-pared PANI–PSS dispersions by means of dialysis (dialysis tube,12,000–14,000 g/mol cut-off, Fisher Scientific) in distilled wateruntil neutral dispersion was achieved. After dialysis, 6 mL ofPANI–PSS dispersions were transferred to an aluminum pan anddried in an oven at 45 �C. The solids were weighed to determinethe concentration of the dispersions.

2.2.2. Preparation of the PANI–PSS colloids in buffersA buffer solution [37] containing 0.10 M citric acid, 0.10 M

potassium dihydrogenphosphate, 0.10 M sodium tetraborate,

0 1 2 3 4 5

10

20

30

40

50

60

Part

icle

Siz

e (n

m)

pH

Fig. 4. The particle size of PANI–PSS composite versus pH of the precursor solutionswhich is adjusted by various concentration of HCl.

L. Li et al. / Journal of Colloid and Interface Science 381 (2012) 11–16 13

0.10 M tris(hydroxymethyl) aminomethane, and 0.1 M potassiumchloride, was prepared. 1 mL of this buffer was diluted to 4 mLwith distilled water, and hydrochloric acid or sodium hydroxidewas added to adjust the pH of the solution to the desired valuein the range of 0–11. Finally, 100 ll of the neutral PANI–PSS disper-sions from the dialysis tube was transferred to the 5-ml buffersolutions, and the resultant dispersions were used to study the sta-bility of the dispersion and to make Ultraviolet–visible (UV–Vis)absorption measurements on it.

2.2.3. Preparation of the polymer filmAfter dialysis, the neutral PANI–PSS dispersions were drop-cast

on pre-cleaned glass substrates (2 � 2 cm2), followed by drying invacuum at 40 �C for 5 days until a film was formed for electricalconductivity measurement and atomic force microscopy (AFM)characterization.

2.3. Characterizations

AFM measurements were conducted in tapping mode with aVeeco diNanoScope 3D instrument. Images of 512 � 512 pointswere acquired at a scanning rate of �0.5 Hz per line. UV–Vis spec-tra were recorded on a Lambda 2 spectrometer (Perkin–Elmer,Wellesley, MA). The electrical conductivity experiments were con-ducted with a Keithley 2636A source meter via the four-probemethod.

Particle-size determination was conducted on a HORIBA SZ-100Dynamic Light Scattering Particle Size Analyzer at a scattering an-gle of 90� at 25 �C. All the PANI–PSS dispersions obtained afterthorough dialysis (roughly 1 mg/mL) were diluted with a 10 mMNaCl aqueous solution to a concentration of 0.0001 wt.% or lowerfor DLS measurement. In the dilutions, the particles are wellisolated. Each measurement was repeated three times for an aver-aged particle size.

Since the PANI–PSS composite nanoparticles are greater than2 nm and less than 1000 nm, we use terms ‘colloid’ and ‘unstablecolloid’ in this paper. We use the general term ‘dispersion’, butnot ‘solution’ to describe the mixture of these particles with wateror buffers.

3. Results and discussion

We used three acids in the polymerization, and investigated theeffect of the acids on the size of PANI–PSS composite particles andthe stability of the dispersions. PANI–PSS/HCl, PANI–PSS/CSA, andPANI–PSS/CH3COOH, stand for PANI–PSS composite particles pre-pared with 1 M hydrochloric acid, 1 M DL-camphorsulfonic acid,and 1 M acetic acid, respectively, as dopant during the polymeriza-tion process. Fig. 1 shows pictures of PANI–PSS/HCl, PANI–PSS/CSA,and PANI–PSS/CH3COOH particles in various buffer solutions withpH of 1, 7, and 11. After standing for several hours, precipitates

Fig. 3. AFM images of PANI–PSS/HCl, PANI–PSS/CSA, a

were observed from PANI–PSS/HCl and PANI–PSS/CSA systems inall buffer solutions, suggesting low stability of these dispersionsat all pH, most notably at high pH. The worst case was PANI–PSS/CSA composite particles in the buffer solution at a pH of 11,as evidenced from complete precipitation. The PANI–PSS/CH3COOH composite particles, on the other hand, showed remark-ably stable dispersions in buffers over a wide range of pH, evenafter standing for months as evidenced from Fig. 1. They showedtremendous stability even when the concentration was as high as1 mg/mL in distilled water.

Fig. 2 shows UV–Vis absorption spectra of PANI–PSS/HCl, PANI–PSS/CSA, and PANI–PSS/CH3COOH particles. All the PANI–PSS com-posite particles have a broad absorption peak between 310 nm and450 nm, attributed to the p–p� transition in PANI, and a mainabsorption peak centered at 750 nm, attributed to the polaronstructure of the main chains of PANI [38,39]. The main absorptionpeaks of PANI–PSS/HCl and PANI–PSS/CSA particles are centeredaround 760 nm, indicating that PANI components have comparableconjugation lengths. In contrast, the PANI–PSS/CH3COOH particlesshow a narrower, blue-shifted peak at 740 nm, indicating that thepolaron structure is more localized and that the conjugation lengthof PANI is shorter.

The sizes of these PANI–PSS composite particles were studiedby means of AFM. The diameters of PANI–PSS/HCl and PANI–PSS/CSA particles are approximately 50 nm (Fig. 3a and b), and thatof PANI–PSS/CH3COOH particles is less than 10 nm (Fig. 3c). Be-sides, films from the PANI–PSS/HCl and PANI–PSS/CSA compositesshow relatively rough surface (with RMS roughness of ca. 22 nm)while that from PANI–PSS/CH3COOH composite shows a rathersmooth surface (RMS roughness = 1.2 nm), which indicates thatthe particles in PANI–PSS/HCl and PANI–PSS/CSA composites aremuch larger than that in PANI–PSS/CH3COOH composite.

nd PANI–PSS/CH3COOH composite nanoparticles.

HCl CSA

H2SO4 CH3SO3H HClO4

Fig. 5. PANI–PSS/acids composite particles in buffers with a pH of 11 after storing for months. All the PANI–PSS/acids composite particles were prepared in the presence of40 mM acids (as indicated in each image) in the precursor solutions.

Fig. 6. The color of as-prepared PANI–PSS composites in distilled water withdifferent pH (adjusted by HCl) of the precursor solutions.

0 1 2 3 4 51E-4

1E-3

0.01

0.1

σ (S

/cm

)

pH

Fig. 7. Electric conductivity of PANI–PSS/HCl composite versus pH of the precursorsolutions, which is adjusted by various concentration of HCl.

14 L. Li et al. / Journal of Colloid and Interface Science 381 (2012) 11–16

To understand the effect of acids in the precursor solutions onthe size the PANI–PSS composite nanoparticles and whether thesize of nanoparticles is associated with the stability of the disper-sions, we further investigated the PANI–PSS composite nanoparti-cles prepared in the presence of 1 M strong acids, including sulfuricacid, nitric acid, perchloric acid, methyl sulfonic acid, and weakacids, including methanoic acid and propanoic acid. We observedlarger size of PANI–PSS composite particles when prepared in thepresence of all the strong acids and small particles when preparedin the presence of all the weak acids. The stability of dispersionsshowed similar results to those of PANI–PSS/HCl, PANI–PSS/CSA,and PANI–PSS/CH3COOH, i.e. the large particles precipitate fromthe dispersion and the small particles formed stable colloids inwater or buffer. These results showed a connection between theparticle size and the stability of the dispersions.

To determine whether the size of the particles is affected by thestrengths and/or the concentration of acids, we prepared a series ofPANI–PSS composite particles in a range of acids, each spanning arange of concentrations. Dynamic light scattering (DLS) (Fig. 4)showed that the particle size decreased from approximately50 nm to approximately 10 nm, when the pH of the precursor solu-tions was changed from 0 to 3. The particle size remained un-changed when the pH of the precursor solutions went from 3 to5. The pH range of 3–5 represented the pH of 1 M weak acids (suchas acetic acid) we used, and the size of the particles prepared in HClat pH value in the range of 3–5 are equivalent to those prepared in1 M of weak acids. Similar effects of pH on the particle size havealso been observed in the presence of other strong acids, includingCSA, HClO4, H2SO4, and CH3SO3H (Fig. 5).

Previously, Loo et al. showed that molecular weight and poly-dispersity index (PDI) of the polymer electrolyte templates can sig-nificantly affect the particle size and size distribution of the finalcomposites [40]. Based on their observation, we used one type ofPSS in all our experiment for polymerization to avoid the complex-ity. Therefore, the above results demonstrate that the size ofPANI–PSS composite nanoparticles is controlled by the acidity orthe proton concentration of the precursor solutions, but not thetypes of PSS and the nature of acids as we thought previously. Itis noteworthy that the PANI–PSS composite nanoparticlesprepared in HCl at concentrations giving pH > 2 display the same

stability as PANI–PSS/CH3COOH in the previously mentioned buffersolutions when stored for months (Fig. 5).

It is well known [41–44] that protonated aniline forms a com-plex with the negatively charged PSS due to electrostatic attrac-tion. Without the negatively charged PSS, polymerization ofaniline in the presence of acids yields PANI, which is not water dis-

Low pH

High pH

polymerization

polymerization

Protonated aniline styrenesulfonic anion on PSS

PSS main chain Protonated PANI aggregate PANI:PSS particle

Scheme 1. Effect of pH and number of negatively charged groups in PSS on the particle size of PANI–PSS composites.

L. Li et al. / Journal of Colloid and Interface Science 381 (2012) 11–16 15

persible. These facts indicate that the number of negative chargeson PSS, which can be affected by pH, should contribute to the sizeof PANI–PSS composites particles. The pKa of poly (4-styrenesulf-onic acid) is approximately 1.0 [45–47]. When strong acids at1 M concentration are used to initiate the polymerization, morethan 90% of the sodium 4-styrenesulfonate groups in PSS are con-verted to electrically-neutral 4-styrenesulfonic acid groups. Due tothe small number of negatively charged sites on PSS, polymeriza-tion of aniline resulted in PANI polymers that are loosely anchoredon the PSS template, which results in larger PANI–PSS compositeparticles (Scheme 1a). The PANI in these particles has longerchains, as suggested by the UV–Vis results. When the pH of theprecursor solutions is increased, more styrenesulfonate groups onPSS remain in their salt form. Consequently, polymerization of ani-line results in shorter PANI polymer chains that stick tighter onPSS, producing smaller nanoparticles of PANI–PSS composite(Scheme 1b). When the pH is higher than 3.0, more than 99.9% ofstyrenesulfonic groups on PSS are in the salt form, so the size ofthe nanoparticles is no longer affected by the pH. In addition, itshould be noted that when the pH is higher than 5, the polymeri-zation is slow due to the lack of H+ to initiate the reaction (Fig. 6)[43].

The electrical conductivities of films of PANI–PSS/HCl compos-ites were measured with a four-point probe method. As shown inFig. 7, the electrical conductivity of the films of PANI–PSS/HCl com-posites decreases when the pH of the precursor solution increases,which may be due to smaller size of the PANI–PSS/HCl particlesThis observation is opposite to what was previously observed byLoo et al. in PANI composite particles templated by other polymerelectrolytes during polymerization [40,48,49]. The reasons for thisis not clear and will be investigated in the future and reported indue course. Taking both the conductivity and stability of the dis-persions into consideration, pH at approximately 2 in the precursorsolution is the optimal pH at which to prepare the PANI–PSS com-posites for practical applications. It should be pointed out thatalthough the electrical conductivity of the PANI–PSS compositesprepared at pH of 2.0 is lower than those prepared at lower pH,the conductivity can be dramatically enhanced to practical levelvia secondary doping and post treatments [39,50–52], which willbe discussed in due course.

4. Conclusion

In summary, we have demonstrated that PANI–PSS compositeparticles that are prepared from weak acids or low concentrationof strong acids can form stable colloids in water or buffers over arange of pH from 1 to 11. We hypothesized that the number of neg-ative charges on PSS plays an important role in controlling the sizeof PANI–PSS composite particles during the polymerization pro-cess. This facile synthetic approach, combined with small size ofthe PANI–PSS composites particles and the great stability of thecolloids of these particles in water and buffers, is promising forpreparing PANI–PSS composites for practical applications in poly-mer electronics.

Acknowledgment

We gratefully acknowledge the support of Drexel University.

References

[1] Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91.[2] A.J. Epstein, J. Joo, R.S. Kohlman, G. Du, A.G. MacDiarmid, E.J. Oh, Y. Min, J.

Tsukamoto, H. Kaneko, J.P. Pouget, Synth. Met. 65 (1994) 149.[3] Y. Cao, J. Qiu, P. Smith, Synth. Met. 69 (1995) 187.[4] S. Bhadra, D. Khastgir, N.K. Singha, J.H. Lee, Prog. Polym. Sci. 34 (2009) 783.[5] Y. Liao, C. Zhang, Y. Zhang, V. Strong, J. Tang, X.-G. Li, K. Kalantar-zadeh, E.M.V.

Hoek, K.L. Wang, R.B. Kaner, Nano Lett. 11 (2011) 954.[6] V.G. Kulkarni, L.D. Campbell, W.R. Mathew, Synth. Met. 30 (1989) 321.[7] Y. Wang, M.F. Rubner, Synth. Met. 47 (1992) 255.[8] P. Rannou, M. Nechtschein, Synth. Met. 84 (1997) 755.[9] J. Tarver, J.E. Yoo, T. Dennes, J. Schwartz, Y.-L. Loo, Chem. Mater. 21 (2009) 280.

[10] J. Tarver, J.E. Yoo, Y.-L. Loo, Chem. Mater. 22 (2010) 2333.[11] D. DeLongchamp, P.T. Hammond, Adv. Mater. 13 (2001) 1455.[12] X. Wang, M. Shao, G. Shao, Z. Wu, S. Wang, J. Colloid Interface Sci. 332 (2009)

74.[13] A. Ray, G.E. Asturias, D.L. Kershner, A.F. Richter, A.G. MacDiarmid, A.J. Epstein,

Synth. Met. 29 (1989) 141.[14] S.K. Manohar, A.G. Macdiarmid, A.J. Epstein, Synth. Met. 41 (1991) 711.[15] B. Wang, J. Tang, F. Wang, Synth. Met. 18 (1987) 323.[16] Y. Wei, X. Tang, Y. Sun, W.W. Focke, J. Polym. Sci. Part A: Polym. Chem. 27

(1989) 2385.[17] D. Li, J. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135.[18] H.D. Tran, J.M. D’Arcy, Y. Wang, P.J. Beltramo, V.A. Strong, R.B. Kaner, J. Mater.

Chem. 21 (2011) 3534.[19] N. Carrillo, U. León-Silva, T. Avalos, M.E. Nicho, S. Serna, F. Castillon, M. Farias,

R. Cruz-Silva, J. Colloid Interface Sci. 369 (2012) 103.

16 L. Li et al. / Journal of Colloid and Interface Science 381 (2012) 11–16

[20] J.H. Cheung, W.B. Stockton, M.F. Rubner, Macromolecules 30 (1997) 2712.[21] M.N. Hyder, S.W. Lee, ACS Nano 5 (2011) 8552.[22] D.M. DeLongchamp, P.T. Hammond, Chem. Mater. 16 (2004) 4799.[23] P. Jia, A.A. Argun, J. Xu, S. Xiong, J. Ma, P.T. Hammond, X. Lu, Chem. Mater. 22

(2010) 6085.[24] Q. Tang, J. Wu, X. Sun, Q. Li, J. Lin, J. Colloid Interface Sci. 337 (2009) 155.[25] W. Liu, J. Kumar, S. Tripathy, K.J. Senecal, L. Samuelson, J. Am. Chem. Soc. 121

(1999) 71.[26] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12

(2000) 481.[27] C.K. Baker, Y.J. Qiu, J.R. Reynolds, J. Phys. Chem. 95 (1991) 4446.[28] L.-X. Wang, X.-G. Li, Y.-L. Yang, React. Funct. Polym. 47 (2001) 125.[29] S. Dorey, C. Vasilev, L. Vidal, C. Labbe, N. Gospodinov, Polymer 46 (2005) 1309.[30] C.-W. Kuo, T.-C. Wen, Eur. Polym. J. 44 (2008) 3393.[31] R. Nagarajan, S. Tripathy, J. Kumar, F.F. Bruno, L. Samuelson, Macromolecules

33 (2000) 9542.[32] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 2305.[33] J. Huang, R.B. Kaner, J. Am. Chem. Soc. 126 (2004) 851.[34] Y. Long, Z. Chen, N. Wang, Z. Zhang, M. Wan, Phys. B: Condens. Mater. 325

(2003) 208.[35] J. Huang, R.B. Kaner, Chem. Commun. (2006) 367.[36] J. Huang, R.B. Kaner, Angew. Chem. Int. Ed. 43 (2004) 5817.[37] E. Pringsheim, E. Terpetschnig, O.S. Wolfbeis, Anal. Chim. Acta 357 (1997) 247.[38] Y. Xia, A.G. MacDiarmid, A.J. Epstein, Macromolecules 27 (1994) 7212.

[39] J.E. Yoo, K.S. Lee, A. Garcia, J. Tarver, E.D. Gomez, K. Baldwin, Y. Sun, H. Meng,T.-Q. Nguyen, Y.-L. Loo, Proc. Natl. Acad. Sci. 107 (2010) 5712.

[40] J.E. Yoo, W.P. Krekelberg, Y. Sun, J.D. Tarver, T.M. Truskett, Y.-L. Loo, Chem.Mater. 21 (2009) 1948.

[41] W. Liu, A.L. Cholli, R. Nagarajan, J. Kumar, S. Tripathy, F.F. Bruno, L. Samuelson,J. Am. Chem. Soc. 121 (1999) 11345.

[42] S. Roy, J.M. Fortier, R. Nagarajan, S. Tripathy, J. Kumar, L.A. Samuelson, F.F.Bruno, Biomacromolecules 3 (2002) 937.

[43] L.A. Samuelson, A. Anagnostopoulos, K.S. Alva, J. Kumar, S.K. Tripathy,Macromolecules 31 (1998) 4376.

[44] W. Liu, J. Kumar, S. Tripathy, L.A. Samuelson, Langmuir 18 (2002) 9696.[45] F. Hua, J. Shi, Y. Lvov, T. Cui, Nanotechnology 14 (2003) 453.[46] E. Kharlampieva, T. Tsukruk, J.M. Slocik, H. Ko, N. Poulsen, R.R. Naik, N. Kröger,

V.V. Tsukruk, Adv. Mater. 20 (2008) 3274.[47] Z. Mao, L. Ma, C. Gao, J. Shen, J. Control. Release 104 (2005) 193.[48] J.E. Yoo, J.L. Cross, T.L. Bucholz, K.S. Lee, M.P. Espeb, Y.-L. Loo, J. Mater. Chem. 17

(2007) 1268.[49] J.E. Yoo, T.L. Bucholz, S. Jung, Y.-L. Loo, J. Mater. Chem. 18 (2008) 3129.[50] A.G. MacDiarmid, A.J. Epstein, Synth. Met. 69 (1995) 85.[51] J. Ouyang, C.-W. Chu, F.-C. Chen, Q. Xu, Y. Yang, Adv. Funct. Mater. 15 (2005)

203.[52] Y.H. Kim, C. Sachse, M.L. Machala, C. May, L. Müller-Meskamp, K. Leo, Adv.

Funct. Mater. 21 (2011) 1076.