Functionalised conjugated materials as building blocks of electronic nanostructures

Functionalised conjugated materials as building blocks

of electronic nanostructures

Diego F. Acevedo, Juan Balach, Claudia R. Rivarola, Marıa C. Miras and

Cesar A. Barbero*

Received 20th April 2005, Accepted 30th June 2005First published as an Advance Article on the web 21st September 2005DOI: 10.1039/b505585g

Two different approaches towards conjugated material (carbon nanotubes, conjugated

polymers) functionalisation are presented: covalent bonding of functional groups and non

covalent interaction with soluble polymers. Covalent functionalisation of carbon

nanotubes is made by reaction of the aromatic ring with aryl radicals, produced by

reduction of diazonium ions. In the case of conducting polymers, covalent

functionalisation is brought about by reaction of polyanilines with diazotized aromatic

amines (including amino terminated azo dyes). The non covalent functionalisation of

carbon nanotubes is made by wrapping the nanotubes with soluble conducting

polyanilines. The functionalised materials are characterised by FTIR spectroscopy, X-ray

diffraction, dynamic light scattering, ultraviolet-visible absorption and emission

spectroscopy, transmission electron microscopy, cyclic voltammetry, differential

electrochemical mass spectroscopy and conductivity measurements. The materials are used

to build ionic self assembled multilayers using a layer-by-layer deposition process. The

charge transport and electrocatalytic behaviour of the assemblies, relevant to the

application of the assemblies in nanostructured electrochemical biosensors, are evaluated

using different redox molecules and/or its intrinsic electroactivity as probes.

1. Introduction

Conjugated carbon based materials, such as carbon nanotubes and conducting polymers could beused to build electronic devices. While it is possible that some carbon nanotube devices couldcompete with silicon based devices (e.g. in power transistors), it is more likely that wet processing ofcarbon nanotubes will be used to produce simple devices of technological interest. In that sense,they are closer to conjugated polymers which have been applied in low cost electronic devices(‘‘plastic electronics’’).1 An impediment to do that is the low solubility of pristine carbon nanotubes.The low solubility is due to weak interaction with solvent molecules and strong p–p and dispersiveinteraction between individual nanotubes. A quite similar effect exists in conjugated polymers dueto similar interactions. Material processability could be achieved by attachment of functionalgroups which interact efficiently with solvents and, due to steric effects, decrease the interactionbetween neighbouring chains or nanotubes. Even more, carbon nanotubes and conductingpolymers could be combined to create materials with new properties.Self assembly of multilayers is a way to create nanostructures where at least one dimension,

perpendicular to the multilayer assembly, is in the nanometric range. The self assembly fromsolution requires overcoming the entropy decrease, due to complex formation, involved in theadsorption process. This could be performed allowing strong interaction (electrostatic, hydrogenbonding, etc.) between layers and by depositing species with low translational (macromolecules,nanostructures) and/or configurational (rigid structures) entropy. A less understood factor is the so

Departamento de Quımica, Universidad Nacional de Rio Cuarto, Agencia Postal No 3, 5800 Rio Cuarto,Argentina. E-mail: [email protected]

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This journal is �c The Royal Society of Chemistry 2005

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called ‘‘hydrophobic effect’’ which is related with the change of entropy due to water and/or smallion displacement upon bilayer formation.2

The formation of ionic self-assembled multilayers by a layer-by-layer procedure was introducedby Decher in 1991.3 The technique is based on the alternating adsorption of multiply chargedcationic and anionic species from solutions (usually aqueous) of each species. The popularity of themethod is due to the ability to create highly tailored polymer thin films with a nearly unlimitedrange of functional groups incorporated within the structure of the film. This flexibility is achievedthrough the use of a simple adsorption process that is readily accessible. The area has progressed toinclude the incorporation of electroactive polymers, organic dyes, semiconductor quantum dots,electrochemically active species, and a wide variety of proteins and other biosystems. Ionic selfassembled multilayers could be built using conjugated materials, like carbon nanotubes andconducting polymers. Both are rigid and have high molar mass but they are insoluble in commonsolvents and carbon nanotubes bear no charge. Carbon nanotubes4 are relatively new nanomater-ials that display attractive structural, mechanical, and electronic properties, including improvedelectrochemical activity.5 The inherent insolubility of pristine carbon nanotubes in solvents, inparticular in water, has complicated their handling as chemical reagents and has limited theirintegration into nanostructured systems. The challenge of solubilizing carbon nanotubes in waterhas been addressed through non-covalent modification with surfactants6 or polymers7 or theircovalent modification with hydrophilic functionalities.8 Since the goal of the work is to useconjugated materials as building blocks of electrostatic polyelectrolyte multilayers, it is desirableto solubilize the carbon nanotubes by adding charged functionalities. To do that, both covalentmodification and polyelectrolyte based solubilization could be used. The number of polymers thathas been used to render carbon nanotubes soluble by non covalent interaction, has been ratherlimited.6,9 On the other hand, carbon nanotube modified electrodes have been used in electro-chemical sensors.10 Different groups have built ionic self-assembled multilayers using carbonnanotubes which are shortened and oxidized by treatment with chemical oxidants (e.g. HNO3).

11

Such treatments could alter the electronic properties of the carbon nanotubes because the chemicalstructure is changed. Conducting polymers are well known conducting materials12 which have beenused for various applications.13 Among others, those materials have been used to build sensors14

and biosensors15 based on the effect of analytes on the electronic properties (e.g. conductivity).Conducting polymers have been previously used to build ionic self-assembled multilayers.16 Rubnerand coworkers17 prepared polyaniline (emeraldine base) solutions in water/dimethylacetamide atpH 2.5 (protonated form ¼ polycation) and self assembled it with poly(styrenesulfonate) (poly-anion). Since polyaniline is insoluble in water, a cosolvent is required which interacts strongly withthe polymer chains. Mattoso and coworkers used poly(alkoxyanilines), which are soluble in water indoped state to build multilayers.18 An alternative is to covalently link charged functional groups topromote interaction with other polyelectrolytes in the multilayer. The presence of charges could alsorender the polymer soluble in aqueous media due to ion–dipole interaction with the solvent.Sulfonated polyanilines,19 suitable to build ionic self-assembled multilayers, were produced byelectrophilic sulfonation,20 copolymerization21 or nucleophilic addition.22 The resulting polyanionwas self-assembled with polycations.23 Altogether, the variety of groups which have been linked topolyaniline to build ionic self-assembled multilayers has been somewhat limited.In the present communication we describe the synthesis of soluble charged conjugated materials

based on conducting polymers and carbon nanotubes. The materials are used to build electrostaticself assembled multilayers. The electronic properties of the ionic self-assembled multilayers, relevantto their applications in electrochemical biosensors, are tested using electrochemical probes.

2. Experimental

Aniline (Merck) was purified by distillation at reduced pressure. Multiwall carbon nanotubes (SunNatotech, China) were produced by CVD, using Ni as catalyst. The nanotubes are 10–30 nm indiameter and 200 nm long (TEM). Poly(diallyldimethylammonium chloride) (PDAMAC, Aldrich)has a MW ¼ 400.000 g mol�1. All aqueous solutions were prepared with ultrapure water. All otherreagents were of analytical quality.

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2.1 Polyaniline synthesis

Polyaniline (emeraldine form) was prepared, as described in the literature,24 by oxidation of aniline(0.1 mol L�1) in 1 mol L�1 HCl with ammonium persulfate (equimolar) at temperatures below 5 1C.The temperature was monitored during the polymerization. When the reaction was completed, afterthe maximum of the temperature has occurred, the polymer was filtered out under suction andwashed with 1 mol L�1 HCl solution (0.5 L) and water (1 L). The polymer was then converted intoits base form by stirring for 24 h in a 0.1 mol L�1 ammonia solution. The emeraldine base form(50% oxidation) used in all reactions had an intrinsic viscosity [Z] of 1.12.

2.2 Diazonium ion synthesis and coupling to polyaniline (PANI)

The amines were diazotized with sodium nitrite and concentrated HCl in an ice bath.25 PANI wassuspended in TRISs buffer (pH ¼ 8) and mixed with the diazonium salt solution in an ice bath. Thesolid was filtered under vacuum and washed first with 1 L of 1 mol L�1 HCl solution and then with1 L distilled water. The products were filtered out of the mixture under vacuum and dried (dynamicvacuum for 48 h).

2.3 Covalent modification of carbon nanotubes

Diazotized amines were mixed with carbon nanotubes in an ultrasonic bath. Then, the temperaturewas raised to 60 1C and heated for 1 h. The reaction mixture was acidified with concentrated HCland centrifuged (10,000 rpm) to separate the modified nanotubes. The precipitate was washedseveral times with water and dried under vacuum. The powder was dissolved in 1 M NaOH formultilayer assembly.

2.5 Non covalent solubilization of carbon nanotubes

Carbon nanotubes were mixed with 0.5% (w/v) solution of the soluble polyaniline in 1 M NaOH.The solution was stirred overnight and any precipitate present filtered out.

2.6 Procedure for self-assembly of the multilayers

The electrodes were immersed (15 min each) successively in solutions of the conjugated material(0.5% w/v in 1 M NaOH) and PDAMAC (2% w/v in water). Between immersions, the plates werewashed by immersing for 15 min in ultra pure water, without stirring. Since we used differentelectrode materials, we did not want to use different covalent modification procedures of theelectrode surface for each material. It is possible to adsorb spontaneously PDAMAC on theelectrode surface to produce a first charged layer on GC, ITO and Au. Therefore, no covalentmodification of the electrode surface was necessary. All operations were performed at roomtemperature.

2.7 Ultraviolet-visible spectroscopy

To obtain the ultraviolet-visible spectra, clear solutions were prepared by dissolving the polymers inaqueous solutions. The spectra were taken in 1 cm path length quartz cells using a diode arrayUV-visible spectrophotometer (HP 8452A). The fluorescence measurements of solutions, in NaOH1 M, were made in a Spex Fluoromax fluorometer.

2.8 FTIR spectroscopy

Fourier Transform Infrared Spectroscopy (FTIR) measurements of chemically prepared polymerswere performed on the powder dried under vacuum at ambient temperature for 48 h. The polymerswere mixed with KBr and pellets for transmission measurements were prepared. The spectra weretaken in an Impact 400 FTIR spectrometer (Nicolet) with a resolution of 4 cm�1, using 200 scans.

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2.9 Conductivity

Conductivity measurements were carried out using the Van der Pauw method on polymer filmsdeposited onto LDPE. The samples were first doped by a 1 h immersion in aqueous 1 M HCl andthen dried in vacuo over 24 h. Four contacts were painted on the films with silver paint. The currentwas applied with a galvanostat (Amel 2049) while the voltage was measured with a 6 1/2 digitmultimeter (HP). The thickness of the films, necessary to calculate the conductivity was estimatedusing the relationship proposed previously.26

2.10 Electrochemistry

Electrochemical measurements were carried out in a conventional three electrode cell, controlled bya computerised potentiostat (GAMRY PC4). Glassy carbon electrodes are 3 mm thick plates of V10(Carbone Lorraine). The plates are polished with a diamond paste of diminishing particle size(10–0.3 mm). The GC plates are tested running GC of the couples of interest before self-assembling.ITO electrodes (Blazers) are thin films of indium doped tin oxide on glass with a surface resistivityof 10 O. All potentials are quoted against the saturated calomel electrode (SCE).

2.11 X-Ray diffraction

X-Ray diffractograms were recorded on an automatic Rigaku DII X-ray diffractometer using CuKaradiation of wavelength, l ¼ 1.540 60 and l ¼ 1.544 34 nm. The diffractometer was operated at40 kV and 20 mA.

2.12 Dynamic light scattering (DLS)

Dynamic light scattering measurements were performed in aMalvern 4700 DLS. The measurementswere made with an angle of 901, at 25 1C.

2.13 Transmission electron microscopy

TEM samples were obtained by evaporation of a nanotube dispersion onto a gold grid. The imageswere taken with a Philips EM 301 microscope.

3. Results and discussion

3.1 Synthesis of functionalized polyanilines

Since the goal is to be able to build ionic self-assembled multilayers using conjugated polymers, it isnecessary to synthesize charged soluble conducting polymers. It has been shown27–29 that couplingof diazonium ions (with polyaniline) could render a variety of functionalised polyanilines using thesame modification reaction (Scheme 1).In that way, it is also possible to covalently link charged groups such as sulfonate (–SO3

�) andcarboxylate (–COO�), to the polyaniline backbone. The FTIR spectrum of a typical modifiedpolyaniline (Fig. 1), is compared with that of PANI, and reveals the presence of new bands whichcan be seen at 1040 and 997 cm�1. Those are assigned to the anti-symmetric and symmetricstretching of the SQO group.30 The results confirm that the reaction allows the synthesis ofpolyanilines with linked charged groups.The conductivity of polyaniline decreases upon modification due to a reduction in p-conjugation

of the chain by steric effects from the attached groups.31 However, the conductivity decreases uponmodification (Table 1) is of less than an order of magnitude. This is quite acceptable forelectrochemical applications, where thin films are used. The small decrease is true only ifconductivity of materials in the fully protonated state, which were equilibrated with 1 M HCl,are compared. In Table 1 are also described the conductivity values obtained with films lessprotonated, equilibrated with pH 4 solutions. It is apparent that deprotonation has a clear effect onconductivity.The conducting polyelectrolytes are all soluble ion aqueous media (pH 4 9), giving clear

solutions, because ion–dipole interactions are strong enough to overcome the chain to chain

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interactions. Indeed, dynamic light scattering measurements of solutions do not show particlepresence (above 1 nm). In the case of conducting polyelectrolytes bearing –COO� groups, at lowerpHs, the –COO� groups become protonated (to form –COOH) and the polyelectrolyte precipitates

Fig. 1 FTIR spectrum of polyaniline (dashed line) and a polyaniline modified by reaction with p-sulfoben-zenediazonium (full line).

Scheme 1 Reaction scheme of diazonium ion synthesis and reaction with polyaniline.

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Table 1 Conductivity values of polyanilines modified with charged groups

Polymer Structure ra/S cm�1

PANI 3.78

PANI–NQN–C6H5COOH 0.96 (0.15)

PANI–NQN–C6H3(OCH3)–NQN–C6H5–COOH

0.14 (0.05)

PANI–NQN–C6H3(OCH3)–NQN–C6H5–SO3H

0.15 (0.028)

PANI–NQN–C6H4–NQN–C6H5–SO3H

0.20 (0.07)

PANI–NQN–C12H6–NQN–C6H5–SO3H

0.57

PANI–NQN–C6H4–NQN–C6H5–COOH

0.59 (0.10)

PANI–NQN–C6H5–SO3H 2.58 (0.43)

a Conductivity measured on films equilibrated with 1 M HCl. The conductivity of films equilibrated with a buffer solution

(pH ¼ 4) is stated between brackets.

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because the ion–dipole interactions are strongly decreased. On the other hand, the polyelectrolytescontaining –SO3

� groups are more difficult to precipitate by decreasing the pH because the group isnot protonated up to very high proton activities.In Fig. 2 is shown the UV-visible absorption spectrum of a soluble polyaniline (Fig. 2). While it is

not possible to obtain a similar spectrum of PANI, due to its insolubility in aqueous solution, it isnoteworthy that a similar one could be obtained for the base form of polyaniline (emeraldine base)in solvents like N-methylpyrrolidone.While semiconducting polymers present clear fluorescence, conductive polymers do not usually

present luminescence. This is due to an internal redox quenching of the excited units by the oxidisedunits. Accordingly, polyaniline presents luminescence only in the reduced state (leucoemeraldine).32

However, it has been reported that the emeraldine form of polyaniline could present luminescencewhen polymer films are segregated into fully reduced and fully oxidized domains.33 In solution, ithas been shown that polyaniline block copolymers also present luminescence.34 It was suggestedthat different comonomer blocks isolate the fluorophores (aromatic amine units) from thequenchers (quinoneimine units). The most soluble polyanilines present fluorescence (Fig. 3),suggesting that quinonimine and amine units are isolated by the steric effect of the moietiesattached to the backbone. The fluorescence could be used to study non covalent interactions ofconducting polyelectrolytes with other species (e.g. carbon nanotubes, see below).The modification reaction is quite versatile because it could also link groups possessing other

properties (fluorescence, optical absorption, complex formation, magnetism, etc.) to the polymerbackbone, producing conducting polyelectrolytes with special properties. Those could then be usedto build multilayers with more complex properties. It is noteworthy to mention that the modifica-tion reaction could be used to attach more than one group in sequence to the polymer backbone. Inthat way, conducting polyanilines with charged groups and other functionalities in the same chaincould be synthesized.Thin films could also be deposited by drop or spin coating of soluble polyaniline solutions in

aqueous ammonia. The solvent is evaporated and the ammonium salt of the polyelectrolytedecomposed by heating with an infrared lamp. The films are stable in acid media (up to pH 5–6)

Fig. 2 UV-visible spectra of a polyaniline modified by reaction with p-carboxybenzenediazonium salt. Thespectra are obtained with a 0.5% solution in 1 M NaOH.

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and show two redox processes, in a similar way to the parent polyaniline polymer, albeit with thepeak separation between both processes reduced due to steric and inductive effects. The fact thatmodified polyanilines are electroactive furnishes a way to detect the incorporation of the conductingpolymer in the multilayers.

3.2 Synthesis of functionalised carbon nanotubes

Two methods of adding functionalities to carbon nanotubes are described: covalent reaction withdiazonium ions and non covalent interactions with soluble polyanilines.

3.2.1 Covalent reaction with diazonium ions. The covalent reaction chosen is based on themethod of Tour and coworkers.35 It involves the formation of highly reactive aryl radicals by chargetransfer from diazonium ion to the nanotube, with loss of N2. The highly reactive species attach tocarbon nanotubes (Scheme 2).The method has its drawbacks. The most important is that it produces a highly reactive species

which could react not only with the nanotubes but with other molecules, such other diazonium ion

Fig. 3 Fluorescence spectrum of –COOH modified PANI (solution in 1 M NaOH/H2O). Excitation wave-length ¼ 315 nm. Emission range ¼ 340 to 600 nm.

Scheme 2 Reaction scheme of diazonium ion synthesis and carbon nanotube modification.

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molecules, giving undesirable side products. Indeed, in our hands it works well only when thearomatic ring is strongly deactivated towards electrophilic substitution, due to the presence of anelectron withdrawing group (–NO2, –COOH, –SO3H). This limitation makes it difficult to link morecomplex azo dyes to the carbon nanotubes. Therefore, the diversity of the functional groups islimited. As we will see, such a limitation is not present when noncovalent interactions are used.While the original method uses organic solvents (or no solvent)36 and organic nitrites, diazoniumions, produced in aqueous solution by the well known method of aromatic amine diazotization withnitrite ion, could also be used to modify carbon nanotubes.37 Evidence of the attachment of thegroups to the carbon nanotubes is obtained by FTIR spectroscopy (Fig. 4). The presence of bandsassigned to characteristic vibrations of the attached functionalities (e.g. –SO3

�), confirm the linkingof the functional groups to the nanotube wall.Additional information is obtained from X-ray diffraction measurements of the functionalized

carbon nanotubes (Fig. 5). The diffraction pattern of carbon nanotubes shows peaks characteristicof the graphitic planes. Those peaks are still present in the modified carbon nanotubes suggestingthat the modification reactions do not significantly disrupt the walls of the nanotubes. However,new peaks appear indicating that the reaction produce new covalent bonds in the structure.The covalent modification of carbon nanotubes could increase their solubility not only by

incorporation of groups (e.g. –COO�) possessing strong interactions (e.g. ion–dipole) with thesolvent but also by decreasing the interaction between neighbouring nanotubes. Transmissionelectron microscopy results confirm that pristine carbon nanotubes, which are bundled together(Fig. 6a), could be separated into isolated nanotubes by modification (Fig. 6b).Dynamic light scattering measurements of carbon nanotube dispersions reveal a clear decrease in

the size of nanotube aggregates after modification (Fig. 7). The mean size of the smaller particlesdecreases from 866 to 181 nm. The size of bigger aggregates also decreases upon modification. It isnoteworthy to mention that modified nanotube dispersions are stable for months while thedispersion of pristine nanotubes show precipitates in less than 1 h. Higher aggregates, however,remain in the dispersions. It has been recently suggested that the reaction in organic solvents isunable to dislodge the bundles and only by using oleum (H2SO4/20% SO3) as solvent is it possibleto produce free modified nanotubes.38

Fig. 4 FTIR spectroscopy of a modified multiwall carbon nanotube.

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Since electrochemical methods are going to be used to study the electronic properties of ionic self-assembled multilayers, it is necessary to know the electrochemical properties of functionalisedcarbon nanotubes. To do that, glassy carbon (GC) electrodes were modified with functionalisedcarbon nanotubes films by drop coating. The DEp obtained (at 50 mV s�1) is 130 mV, revealing aquasi-reversible behaviour of the redox couple at the modified electrode. The results contrast withthe peak separation (DEp ¼ 549 mV at 50 mV s�1), obtained at the polished GC electrode, whichhas a more sluggish charge transfer. The results show less reversibility than that observed whennanotube bundles grown on a surface were used as the electrode,39 suggesting that chemicalfunctionalisation and/or contact between nanotubes and the base electrode affects somewhat thecharge transfer. However, the improvement in the charge transfer is clear. As it was pointed out byCompton and coworkers,40 such effect does not mean that carbon nanotubes have unique catalyticproperties other than those of graphite edge planes. For our purposes, however, it means that wecould evaluate the presence of functionalised carbon nanotubes on less active electrodes measuring

Fig. 5 X-Ray diffraction of pristine (grey line) and modified carbon nanotubes (black line).

Fig. 6 Transmission electron micrographs of: (a) pristine multiwall carbon nanotubes and (b) modified carbonnanotubes (covalent bonding of –C6H5COO�).

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the electrochemical current of ferrocyanide oxidation. A redox couple more relevant to theapplication of those nanostructures in biosensors is hydrogen peroxide. It is known that multiwallcarbon nanotubes could electrocatalyze the oxidation and reduction of H2O2.

41 Similar results areobtained using functionalised carbon nanotubes. However, while on oxidation only the currentincreases, a clear electrocatalytic effect is observed for H2O2 reduction. Therefore, functionalisedcarbon nanotubes could be used to detect hydrogen peroxide. As the electrode potentials are quiteanodic, closer to those of water oxidation, it is important to check if significant degradation of thenanotubes occurs. Differential electrochemical mass spectroscopy measurements of the sameelectrochemical system reveals that O2 is formed (m/z ¼ 32) during hydrogen peroxide oxidationbut no CO2 (m/z ¼ 44) is detected. This result suggests that no significant nanotube oxidation isproduced during H2O2 oxidation.

3.2.2 Non-covalent interaction of soluble polyanilines with carbon nanotubes. As it was pointedout, covalent modification of carbon nanotubes seems to be only possible with some groups. Analternative procedure consists of dispersing the carbon nanotubes using non-covalent interactionswith conducting polymers for dispersion stabilization.42 While it is possible to use non conductingsoluble polymers43 for that goal, the fact that both conducting polymers and carbon nanotubes hasextended electronic clouds allows the creation of p–p interactions between them. We have usedsoluble polyanilines to interact with multiwall carbon nanotubes. The X-ray diffraction pattern ofpristine multiwall carbon nanotubes is not altered by solubilization with conducting polymers,showing only a superposition of the broad bands due to the amorphous polyaniline and the peaksdue to the carbon nanotubes. This result is reasonable, since no covalent linkages are formed duringsolubilization. The DLS measurements of a typical aqueous dispersion of multiwall carbonnanotubes using functionalized polyanilines (Fig. 8) show that the polydisperse multiwall carbonnanotubes sample (peaks at 800 nm, 7800 nm and 20 mm) changes to particles with a mean size of210 nm. Such dispersions of multiwall carbon nanotubes using functionalized polyanilines arestable for months.While it is clear that the presence of a conducting polyelectrolyte improves the dispersion of the

carbon nanotubes, the nature of the polymer–carbon nanotube interaction is less clear. To try tofind out if there are specific p–p interactions, we use fluorescence quenching measurements. As can

Fig. 7 Dynamic light scattering measurement of dispersions in water: (grey line) pristine multiwall carbonnanotubes; (black line) multiwall carbon nanotubes modified with –C6H5COO� functionalities.

Faraday Discuss., 2006, 131, 235–252 | 245


be seen (Fig. 9), the conducting polymer fluorescence decreases by successive additions of carbonnanotubes to the conducting polymer solution.The existence of fluorescence quenching strongly suggests that electron or charge transfer is

possible between conducting polymer chains and carbon nanotubes. Therefore, it is likely that p–pinteractions are present. Whether it is electron or charge transfer, is a question under current study.

Fig. 8 DLS measurement of: (black line) pristine multiwall carbon nanotubes dispersed in water; (grey line)multiwall carbon nanotubes dispersed using 1% of a soluble conducting polyelectrolyte (PANI–NQN–C6H5SO3

�).

Fig. 9 Quenching of the fluorescent emission of a modified polyaniline (PANI–NQN–C6H5COO�) byincreasing amounts (see text) of multiwall carbon nanotubes.

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Thin films could be deposited by drop or spin coating of multiwall carbon nanotubes dispersedusing functionalized polyanilines dispersions in aqueous ammonia. The solvent evaporates and theammonium salt of the polyelectrolyte decomposes, by heating using an infrared lamp. The films arestable in acid and base and show the same redox processes as the soluble polyaniline used tostabilize the dispersions. Therefore, electrochemical techniques could be used to detect theincorporation of the multiwall carbon nanotubes, dispersed using functionalized polyanilines, intothe multilayers.

3.3 Ionic self-assembly of multilayer nanostructures

3.3.1 Ionic self-assembled nanostructures built using functionalised carbon nanotubes. There havebeen reports of using layer by layer self assembly to build ionic self-assembly of multilayernanostructures with carbon nanotubes using heavily oxidized and shortened multiwall carbonnanotubes.44 We have used the functionalised multiwall carbon nanotubes, described above, tobuild the multilayers. As it has been shown above, some electrochemical reactions show fasterelectron transfer on carbon nanotube modified electrodes than on glassy carbon electrodes.Therefore, the current due to a redox reaction, such as Fe(CN)6

4� oxidation, could be used tomeasure the amount of functionalised multiwall carbon nanotubes deposited at each deposition stepduring the layer by layer process. The cyclic voltammograms (Fig. 10) show increasing currentmaxima with each bilayer deposited. Additionally, the peak separation decreases slightly with thenumber of layers. This indicates that charge transfer seems to be at least as fast in the outer layers asin the inner layers. Otherwise, if a significant resistance would be added by each layer, the peakseparation should increase.The fact that the current increases evenly with the number of layers suggest that: (i) all

functionalised multiwall carbon nanotubes layers are connected electrically between them; (ii) theredox probe (Fe(CN)6

4�) is able to penetrate the whole multilayer assembly. The combination ofboth conditions implies that a three-dimensional electrode, with nanometric definition, has beenassembled. The first condition seems difficult to achieve if ionic self-assembled multilayers areconstituted by rigid layers. Since the polyelectrolyte used to build the multilayer (PDAMAC) is not

Fig. 10 Cyclic voltammograms of potassium ferrocyanide (2.13 mM) on self-assembled multilayer offunctionalised multiwall carbon nanotubes and a polyelectrolyte (PDAMAC). Electrolyte ¼ 1 M KCl. Scanrate ¼ 50 mV s�1. The multilayers are deposited on ITO/glass as base electrode.

Faraday Discuss., 2006, 131, 235–252 | 247


conductive, it should inhibit electronic conduction between the conductive layers. However, it hasbeen shown45 that the conductivity of PANI/polyelectrolyte assemblies increase with the number oflayers, until a constant value is reached. It is thought that the layers in the ionic self-assembledmultilayers interpenetrate among them, allowing for conduction paths to be built. In the case of thecarbon nanotubes, one could expect less interpenetration owing to the rigid nature of thenanotubes, but some seems to be still present. The second condition depends on the porosity ofthe layer and could be quantitatively tested using a model of mediated electrocatalysis. The modelshould take into account the reaction of the redox couple on the outer layer, the whole multilayerand/or the base electrode. Such mathematical models have already been proposed by Saveant andcoworkers46 and Albery and coworkers,47 to explain redox catalysis mediated by redox polymers. Itis apparent that ionic self-assembled multilayers could provide an excellent test for the theory due tothe possibility of varying the thickness of the layer in well known steps.48

The cyclic voltammograms of H2O2 on the carbon nanotube containing ionic self-assembledmultilayers (Fig. 11) also show oxidation currents which scale up with the number of bilayers in theassembly (insert in Fig. 11). At the functionalised multiwall carbon nanotubes based multilayers,peroxide could be also reduced. The current (measured at �0.4 V SCE) increases in absolutemagnitude with the number of bilayers.Therefore, the assembly could be used as a three-dimensional amperometric sensor of H2O2, to

assemble peroxide based biosensors.

3.3.2 Ionic self-assembled multilayers nanostructures built using carbon nanotubes solubilized by

mean of soluble polyanilines. One special feature of this kind of soluble nanotubes is the presence ofan intrinsically conducting, and electroactive, polymer matrix. With the techniques used up to nowby us (cyclic voltammetry and chronoamperometry), we were not able to detect any improvementon the overall multilayer electronic conduction by the presence of the conductive polymer. It is quitereasonable to think that intermixing of carbon nanotubes from neighbouring layers gives enoughconductivity. On the other hand, the electrocatalytic properties of the multiwall carbon nanotubesshould be present. Indeed, the ionic self-assembled multilayers built using carbon nanotubes

Fig. 11 Cyclic voltammograms of hydrogen peroxide (10 mM) on self-assembled multilayer of functionalisedmultiwall carbon nanotubes and a polyelectrolyte (PDAMAC). Electrolyte ¼ Phosphate buffer (0.1 M, pH ¼ 7).Scan rate ¼ 1 mV s�1. The multilayers are deposited on polished glassy carbon as base electrode.

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solubilized by non-covalent interactions with soluble polyanilines show similar electrocatalyticbehaviour towards ferrocyanide oxidation than functionalised carbon nanotube based ionic self-assembled multilayers. Using a less anodic extreme of the potential scan, to avoid degrading thepolyaniline matrix, it is also observed that hydrogen peroxide could be reduced (Fig. 12). Thecurrent (measured at a given potential) increases with the number of bilayers (see insert in Fig. 12).Recently it has been shown that the wiring of redox enzymes by redox hydrogels is improved by

linking carbon nanotubes to the enzyme.49 Therefore, multiwall carbon nanotubes dispersed usingfunctionalised polyanilines could be useful as building blocks of biosensors based on enzyme/redoxhydrogel multilayers50

3.3.3 Ionic self-assembled multilayers nanostructures built using soluble polyanilines. In the caseof ionic self-assembled multilayers built only with soluble polyanilines as electroactive component,the probe used to test the multilayer is the intrinsic electroactivity of the soluble polyaniline. In Fig.13 are shown the cyclic voltammetric responses of a typical functionalised PANI based ionic self-assembled multilayers, during the build up of the multilayer. As it can be seen the redox chargeincreases with the addition of bilayers. The peak current scales linearly with the number of bilayers.It is noteworthy that the same multilayer built on GC shows a different behaviour. The voltam-metric current signal increases abruptly after the first immersion step. Afterwards, it changes onlyca. 10% for each layer. It seems that strong adsorption of the functionalised PANI on the GCsurface creates a relatively thick polymer layer during the first immersion. Afterwards, the layer bylayer procedure built up the multilayer in a normal way.The cyclic voltammograms are quite similar to those observed when polyaniline is produced by

electrochemical polymerization.51 The peak potential does not shift significantly when the numberof layers is increased, suggesting that the resistance through the multilayer is small. It is noteworthythat the electrochemistry is sensitive enough to detect a few layers of electroactive material.

Fig. 12 Cyclic voltammograms of hydrogen peroxide (10 mM) on self-assembled multilayers of multiwallcarbon nanotubes, dispersed with functionalised polyanilines, and a polyelectrolyte (PDAMAC). Electrolyte ¼phosphate buffer (0.1 M, pH ¼ 7). Scan rate ¼ 1 mV s�1. The multilayers are deposited on polished glassycarbon electrodes.

Faraday Discuss., 2006, 131, 235–252 | 249


4. Conclusions

Functionalised conjugated materials are produced using covalent modification and non covalentinteractions. Soluble polyanilines, bearing charged groups, are produced by reaction of diazoniumions with polyaniline. The polymers have lower conductivity than the parent polymer and showclear fluorescent emission in solution. Covalent modification of carbon nanotubes is achieved byreaction of diazonium ions with pristine carbon nanotubes. In that way, charged functionalities(–COO–, SO3

�) are linked to multiwall carbon nanotubes, rendering them soluble. The modifica-tion does not alter significantly the nanotube graphitic structure. Non covalent solubilization ofcarbon nanotubes is made by interaction of soluble polyanilines with pristine multiwalled carbonnanotubes. The electrochemical reaction of a redox couple: Fe(CN)6

4�/Fe(CN)63� is more

reversible at ionic self-assembled multilayers, built using functionalised carbon nanotubes orcarbon nanotubes dispersed with functionalised PANI, than on unmodified GC electrodes. Theionic self-assembled multilayers, containing functionalised carbon nanotubes, also electrocatalyzehydrogen peroxide oxidation/reduction. The amperometric signal scales with the number of layers,in both cases, suggesting that all layers are connected and there is three-dimensional access to themultilayer by the redox probes. When the multilayers are built using functionalised polyanilines orcarbon nanotubes dispersed using functionalised polyanilines, the assemblies show intrinsicelectroactivity. The peak current of the cyclic voltammograms depends on the number of layers.The nanostructures described here only present nanometric scaling in one dimension. However, theconductive building blocks described here could be easily used to build three-dimensional ionic self-assembled multilayers by depositing the layers onto solid nanoparticles (e.g. polystyrene latex), as ithas been carried out with other materials.52 In any case, these kind of nanostructures make a bridgebetween subnanometric species (chemical compounds such as polymers) and nano-objects likenanoparticles, nanotubes, etc. Both conducting polymers chains and carbon nanotubes are effectivenanowires. Self-assembly of those materials into multilayers are an easy way to produce practicaldevices. Moreover, the synthesis and functionalisation of the conjugated materials as well as the selfassembly of the devices are bottom-up procedures which could be extended to practical applica-tions. In that sense, conjugated materials could be used as building blocks of assembled electronic

Fig. 13 Cyclic voltammograms of self-assembled multilayers made of functionalised polyanilines and apolyelectrolyte (PDAMAC). Electrolyte ¼ 1 M HCl. Scan rate ¼ 50 mV s�1. The multilayers are depositedon ITO/glass plates as base electrodes.

250 | Faraday Discuss., 2006, 131, 235–252


nanostructures. The materials furnish electronic conductivity and/or other properties to theassemblies, playing the role of a ‘‘molecular ink’’ to wire nanostructures. While this could beconsidered a simple role, there are few alternative materials which could be used. Moreover, aneffective wiring of the assemblies at the molecular level is usually a critical factor on deviceperformance.

Acknowledgements

M. Tagliazuchi, G. A. Planes and R. Echevarria are thanked for measuring TEM images, DEMSprofiles and the X-ray diffractograms, respectively. Fruitful discussions with E. J. Calvo and P. N.Bartlett are gratefully acknowledged. D. F. Acevedo and C. Rivarola thank CONICET forgraduate and postgraduate research fellowships, respectively. J. Balach thanks EDUMAT for anundergraduate research fellowship. This work was funded by FONCYT, CONICET, FundacionAntorchas, Agencia Cordoba Ciencia, TENARIS and SECYT-UNRC. C. Barbero is a permanentresearch fellow of CONICET.

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Functionalised conjugated materials as building blocks of electronic nanostructures

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