Surface modified and functionalized polyaniline and polypyrrole films

ELSEVIER

Synthetic

Surface modified

Metals

and functionalized

84 (1997)

59-60

polyaniline

and polypyrrole

films

E.T.Kang’, K.G.Neoh’, K.L.Tar? and F.C.Loh’ Department of Chemical Engineering’ and Department of Physics2 National University of Singapore, Kent Ridge, SINGAPORE 119260

Abstract Surface modified and ftmctionalized polyaniline (PANi) and polypyrrole(PPY) films from ozone treatment, Ar plasma treatment, near-UV-light induced graft copolymerization with acrylic acid(AAc), and covalent immobilization of the enzyme trypsin were characterized by angle-resolved X-ray photoelectron spectroscopy(XPS). Both ozone and Ar plasma treatment result in the oxidation of carbon atoms and cause a significant decrease in the intrinsic oxidation states ([=N-]/[-NH-] ratio) of the base polymers. Surface graft copolymerization with AAc readily gives rise to self-protonated PANi and PPY surface structures. The concentration of surface grafted AAc polymer increases with increasing monomer concentration and near-UV irradiation time used during grafting, but decreases with Ar plasma pretreatment time. Covalent immobilization of trypsin on the AAc polymer grafted PANi and PPY films is facilitated by the presence of water-soluble carbodiimide (WSC). The activity of the immobilized enzyme increases initially with increasing concentration of surface grafted AAc polymer, but becomes saturated at moderate to high AAc polymer concentration. Keywords:

Polyaniline;

Polypyrrole;

Interface preparation; Photoelectron

spectroscopy.

1. Introduction Surface modification of polymer via molecular design is a convenient and efficient method for introducing specific functionalities, such as hydrophilicity/hydrophobicity, adhesion, lubrication and biocompatibility, into the existing polymers[l]. Among the many techniques available, surface modification via graft copolymerization appears to be the most versatile in tailoring surface structure and composition for specific applications, such as protein/enzyme immobilization[2,3]. In this study, we investigated the surface modification of electroactive polyaniline (PANi) and polypyrrole (PPY) films via graft copolymerization with acrylic acid @AC). The surface modified polymer films were subsequently functionalized via covalent immobilization of a model enzyme, trypsin. These functionalized electroactive polymers may have potential applications as biomaterials, biocompatible materials and biosensors. 2. Experimental Emeraldine (EM) base film of about 10 pm in thickness was prepared according to the procedures reported in the literature[4]. The 75% intrinsically oxidized nigraniline (NA) film ( [=N-]/[-NH-] -, 0.75 ) was obtained by subjecting the EM film to one cycle of acid-base treatment[5]. Polypyrrole (PPY) via oxidative electrochemical film was synthesized polymerization[5]. Ar plasma treatments were carried out on a Shimadzu bell-jar-type glow discharge cell ( Model LCVD 12), at an applied frequency of 5 KHz, a plasma power of 28W, and an Ar pressure of 0.04 Tom Both pristine and plasma pretreated subjected to near-UV-light induced graft films were copolymerization with acrylic acid @AC), according to the 03796779/97/%17.00 PII SO379-6779(96)038354

0 1997

Ekevier

Scimce

S.A

All

rights

reserved

method reported earlier[6]. The techniques of trypsin immobilization and activity assay used in the present work are similar to those reported for conventional polymer substrates[3]. Surface characterization by X-ray photoelectron spectroscopy (XPS) was carried out on a VG ESCALAB MKII spectrometer with a Mg Ka X-ray source (1253.6 eV photons)[5].

3. Results and Discussion XPS has been shown to be an ideal tool for the study of intrinsic structure and charge transfer (CT) interactions in Ncontaining electroactive polymers[7]. For PPY and PANi, the quinonoid imine (=N-), benzenoid amine (-NH-), and positively charged nitrogens associated with a particular intrinsic redox state and protonation level of the polymer can be quantified in the Nls curve fitted spectrum. Ar plasma treatment of the EM and NA base films, followed by atmospheric exposure, results in the oxidation of some carbon atoms, first to C-O species, and then to C=O and COOH species for samples with extended plasma treatment time. The plasma treatment does not result in the direct oxidation of the nitrogen heteroatoms. Instead, plasma treatment and the accompanied carbon oxidation cause a steady decrease in the intrinsic oxidation state ( [=N-]/[-NH-] ratio) of both polymers. Similar decrease in the intrinsic oxidation states, but with a substantially enhanced extent of carbon oxidation, has also been observed in EM, NA and PPY films when subjected to ozone treatment[5]. Due to the reactive nature of the conjugated polymer surface, the pristine EM and PPY films are readily susceptible to near-UV-light induced surface graft copolymerization with AAc.

E.T. Kang et al. /SyntheticMetals 84 (1997) 59-60

60

The extent of surface grafting in each case can be readily determinedfrom the corrected area ratios of the Cls peak componentat 288.7eVattributableto the carboxyl groupof the AAc polymer, and the total Nls area, and expressedas the [COOH]/[N] ratio. For both PANi and PPY films, the surface concentration of the grafted AAc polymer increaseswith increasingmonomerconcentrationand near-UV illumination time during grafting, but decreases with increasingAr plasma pretreatmenttime. Figure 1 showsthe variation in surface concentrationof the graftedAAc polymeron EM andPPY films, as a function of the AAc monomerconcentrationusedin the graft copolymerization.A higher [COOH]/[N] ratio is always observedat the more surfaceglancingphotoelectrontake-off angle(a) of 20” in both films, suggesting that the graftedAAc polymeris presentmainlyin the top surfacelayer.

z s g

8

enzymemolecules in the sub-surface layer probablybecomesless accessible. Thus,in spiteof the increasein the amountof enzyme immobilizedat the high concentrationof graftedAAc polymer, the enzyme at top surfacemust have completelydigestedthe TAME molecules.Finally, the lower curve in Figure 2 readily suggeststhat in the absenceof WSC, only a trace amountof enzymeis physicallyabsorbed in theAAc polymerlayer or on the pristinepolymersurface.

O”f

1

PANi ---PPY 0 0 a=75' q cl a=20° near-UV

Time=0.5h

A

PPY

0

PANi PANi

0

6-

0.0 CF0-p-p 0

8 00 1

2

Concentration

4

7

IO

Fig. 2

Concentration of AAc (wt. %) Fig. 1 The effect of AAc monomerconcentrationon the concentrationof surfacegraftedAAc polymer. Covalent immobilization of trypsin on the AAc polymergraftedEM andPPY films is facilitatedby the presence of water-solublecarbodiimide(WSC). The WSC binds and activatesthe COOH groupof the AAc polymerprior to the amide linkageformationbetweenthe COOHgroupandthe -NH, group of the enzyme. The activity of the immobilized enzyme, expressed asthe activity observedin anequivalentamountof the free enzyme,canbe assayedin N-a-tosyl-L-argininemethylester hydrochloride (TAME) solution. Figure 2 showsthe surface enzyme activity as a function of the surfaceconcentrationof grafted AAc polymer for both EM and PPY. At low concentrationsof surfacegrafted AAc polymer, the observed enzymeactivities increaselinearly with increasingAAc polymer concentrations.Similar activitiesare observedfor both typesof electroactivepolymerfilms.This increasein the observedactivity can be associatedwith an increasein the amountof surface immobilizedenzyme. The surfaceenzyme activities, however, becomesaturatedupon further increasingthe concentrationsof surfacegraftedAAc polymer.The saturationin surfaceenzyme activity at high AAc polymerconcentrationsuggests that in the presenceof diffusion limitation, a considerablefraction,of the

4

, 6

of Surface

I 6

Graft

I IO

(NOW.%)

I 12

I 14

([COOHJ/[N,,,,,,,,,J)

The surfaceenzymeactivity asa function of the surfaceconcentrationof graftedAAc polymer.

4. Conclusion Surface modificationsof EM and PPY films were achievedvia Ar plasmatreatmentand/ornear-UV-lightinduced graft copolymer&ion with AAc polymer. The AAc polymer modified films were further functionalized by covalent immobilizationof a modelenzyme,trypsin. 5. References

1. L.S.PennandH.Wang,Polym. Adv. Technol., 5 (1994)809 2. Y.Ikada, Biomterials, I5 (1994)72.5 3. E.A.Kulik, K.Kato, M.I.Ivanchenko and Y.Ikada, Biomatetials, 14 (1993)763 4. A.Ray, G.E.Asturies, D.L.Kershner, A.F.Ritcher, A.G. MacDiarmidandA.J.Epstein,SynthMet.,29 (1989)El41 5. E.T.Kang, K.G.Neoh, K.L.Tan and D.J.Liaw, Surf: Interf: Anal., 24 (1996)51 6. E.T.Kang, K.G.Neoh, K.L.Tan, Y.Uyama and Y.Ikada, Macromolecules, 25 (1992)1959 I. E.T.Kang,K.G.Neoh,K.L.Tan, Adv.Polym.Sci., jO6 (1993) 135