DNA-templated assembly and electropolymerization of anilineon gold surface

Yong Shao, Yongdong Jin, Shaojun Dong *

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun 130022, China

Received 26 July 2002; received in revised form 14 August 2002; accepted 14 August 2002

Abstract

Biomolecule template gives new opportunities for the fabrication of novel materials with special features. Here we report a route

to the formation of DNA–polyaniline (PAn) complex, using immobilized DNA as a template. A gold electrode was first modified

with monolayer of 2-aminoethanethiol by self-assembly. Thereafter, by simply immersing the gold electrode into DNA solution,

DNA molecules can be attached onto the gold surface, followed by the DNA-templated assembly and electropolymerization of

protonated aniline. The electrostatic interactions between DNA and aniline can keep the aniline monomers aligning along the DNA

strands. Investigations by surface plasmon resonance (SPR), electrochemistry and reflection–absorption UV/Vis–Near IR spec-

troscopy substantially convince that PAn can be electrochemically grown around DNA template on gold surface. This work may be

provides fundamental aspects for building PAn nanowires with DNA as template on solid surface if DNA molecules can be in-

dividually separated and stretched. � 2002 Elsevier Science B.V. All rights reserved.

Keywords: DNA; Template; Self-assembly; Polyaniline; Electrochemistry

1. Introduction

The electrostatic and topographic properties of bio-logical macromolecules such as DNA complexes can beexploited for the templated generation and assembly ofsupramolecular aggregates of organic and inorganicbuilding blocks [1,2]. The power of DNA as a moleculartemplate is enhanced by our ability to synthesize virtu-ally any DNA sequence by automated methods, and toamplify any DNA sequence from microscopic to mac-roscopic quantities by means of polymerase chain re-action (PCR). Therefore, DNA is particularly suitableto serve as a construction component in nanosciences[3]. The use of DNA as template for spontaneous as-semblies of cationic cyanine dye [4], fullerene derivatives[5], CdS semiconductor nanoparticles [6], silver nano-wires [7], gold nanowires [8], and Pd clusters [9] to formsupramolecular structures has been extensively reported.In all cases, the negatively charged phosphate backbone

of the DNA double helix has been employed to interactelectrostatically with inversely charged species presentedin solution. Because metal nanoparticles are alwayswrapped by charged organic layer, the metal nanowiresformed by such metal nanoparticles for electronic in-terconnections of nanometer-scale electronics devicesshould be resistive [7]. The conductive polymers may beprovide another option for this problem.

Conductive polymers have been extensively studiedbecause of their highly electrical conductivity and me-chanical flexibility, their ability to be electrochemicallyswitched between electronically insulting and conduct-ing states. Polyaniline (PAn) has been one of the mostextensively investigated conducting polymers because ofits excellent stability and promising electronic proper-ties. PAn film [10] and PAn/poly(anion) composite films[11,12] have been widely studied. Specially patternedconductive polymers such as PAn nanoparticles andPAn/nanocrystalline composite particles have beensynthesized by electrochemistry [13] and ultrasonic ir-radiation [14]. PAn nanowires have also been commonlyprepared by chemical and electrochemical oxidation ofthe monomers with anodized alumina [15], track-etched

Electrochemistry Communications 4 (2002) 773–779

www.elsevier.com/locate/elecom

*Corresponding author. Tel.: +86-431-5262101; fax: +86-431-

5689711.

E-mail address: dongsj@ns.ciac.jl.cn (S. Dong).

1388-2481/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S1388-2481 (02 )00442-3

polycarbonate [16], mesoporous silica [17], and one-di-mensional aqueous channels of reverse liquid crystallinephase [18] served as templates. But these porously in-side-grown templates are sometimes tedious to fabricateand the severe chemical conditions involved in the syn-thesis limit their further applications. DNA-templatedpolymerization paves the way for easy control over theshapes and sizes of the hybrid biomaterials. Recently,Nagarajan et al. [19,20] experimentally demonstratedthe feasibility of complex of PAn and special polyelec-trolyte templates such as poly(vinylphosphonic acid) orDNA synthesized in pH 4.3 aqueous solution by usingan enzyme, horseradish peroxidase (HRP), as biocata-lyst. They suggested that the phosphate-based templatescould react electrostatically with protonated anilinemonomers. By this strategy, the pH and charge densitynear the template molecule is different form that of thebulk solution, and provides the requisite local environ-ments to facilitate the para-directed coupling of theaniline molecules catalyzed by the enzyme and thesubstrate H2O2 [21]. Additionally, The mild and envi-ronmentally compatible reaction conditions involved inthe synthesis have provided opportunities for the use ofsynthetic oligonucleotide as template and polyethyleneglycol modified hematin as biomimetic catalyst [22]. Onthe other hand, DNA-templated photopolymerizationof a water-soluble DNA–PAn complex has been re-ported by the use of tris(2,20-bipyridyl)ruthenium½RuðbpyÞ2þ3 � as a photocatalyst [23]. This complex canbe fabricated as an organic red-emitting diode with afast turn-on response, in which PAn should be a p-typeconductor, however, DNA can work as an n-type con-ductor [24]. Thus it is expected that electrons and holescan be injected from electrode through DNA and PAn ifthe DNA–PAn complex is attached to electrode, al-though the electronic transport properties of DNA itselfare still being debated [25,26]. Here we first report theDNA-templated assembly and electropolymerization ofaniline on gold surface. The strategy gives an easy wayfor the formation of DNA–PAn complex and may besuggests fundamental aspects for PAn nanowires basedon DNA template and electrochemistry. All methodsbased on literatures [20–23] are carried out in solutionfor the formation of DNA–PAn complex and specialcatalysts are needed. Our strategy provides another al-ternation for the surface formation of the DNA–PAncomplex and no any catalyst is needed on the conditionof electropolymerization.

2. Experimental

2.1. Reagents and materials

Aniline was purified by redistillation and stored in thedark. All other reagents were used as received.

100 lg ml�1 calf thymus DNA (Sigma) and 20 mM 2-aminoethanethiol hydrochloride (AET, Aldrich) weredirectly dissolved in pure water for use. 5 mM anilinesolution was prepared in 10 mM acetic acid–sodiumacetate buffer (pH 4.2). Thin (47–50 nm) Au films wereprepared by thermal evaporation of Au onto a glassslide substrate (refractive index¼ 1.61) for SPR mea-surements. Milli-Q water was used throughout experi-ments.

2.2. Stepwise self-assembly of AET, DNA, and anilinemonomer on gold surface

The gold substrates were cleaned with freshly madepiranha solution (caution: piranha solution should behandled with extreme care), and rinsed with water, andfinally rinsed with ethanol. Then the substrates wereimmersed into an aqueous solution of 20 mM AET 10h, followed by thoroughly washing with ethanol andwater. The AET covered Au substrates were exposedto 100 lg ml�1 DNA aqueous solution 3 h, and thor-oughly rinsed with water. The as-prepared Au sub-strates were then immersed into 5 mM aniline solutionfor special time scale, and carefully washed with10 mM acetic acid–sodium acetate buffer solution(pH 4.2).

2.3. Electropolymerization of the assembled anilinemonomers

Electrochemical polymerization of the assembledaniline was carried out in a teflon cuvette with goldfilm as the working electrode, a platinum wire ascounter electrode, and Ag/(AgCl, saturated KCl) asreference electrode. All potentials were recorded withrespect to this reference electrode. 50 mM acetic acid–sodium acetate buffer solution (pH 4.2) was used aselectrolyte. The complex of DNA and PAn wereformed by cyclic potential scan from 0 to 0.7 V severaltimes with scan rate 50 mV s�1 until the current re-sponse stabilized. After polymerization, the gold filmwas rinsed with water thoroughly for further experi-ments. The process is also illustrated in Scheme 1. Notethat the electropolymerization is performed in an ani-line-free solution so the only source of monomer on themodified electrode is the aniline accumulated on theDNA strands at the early assembly stage. For the

Scheme 1. The formation of DNA–PAn complex on Au surface.

774 Y. Shao et al. / Electrochemistry Communications 4 (2002) 773–779

control experiments, the similar procedures were re-peated for the exact duration only without the self-assembly of DNA. In the two cases, after elctropoly-merization in buffer solution without the monomers,the resulted Au electrode was again immersed into 5mM aniline solution for another assembly and so on.

2.4. SPR measurements

The glass slide covered with gold film was pressedonto the base of a half-cylindrical lens (n ¼ 1:61) via anindex-matching oil. Linearly p-polarized light having awavelength of 670 nm from a diode laser was directedthrough the prism onto the gold film in the Kretshmannconfiguration by using SPR apparatus (SPR 2000, theInstitute of electronics, Chinese Academy of Sciences,China). The intensity of the reflected light was measuredas a function of the angle of incidence, h, using a pho-todiode with a chopper/lock-in amplifier technique. ForSPR combined with electrochemical detection, the as-prepared Au/glass substrates were mounted against theteflon cuvette with 1 ml volume using a Kalrez O-ring,which provided a liquid-tight seal. The teflon cuvetteallowed for the simultaneous recording of the electro-chemical data and the application of a voltage biased tothe sample.

2.5. Electrochemical measurements

After electropolymerization of the assembled anilinemonomers, electrochemical measurements (CHI 800,Shanghai Chenhua Instruments, China) were carried outin potential range from 0 to 0.7 V with 50 mM aceticacid–sodium acetate buffer solution (pH 4.2) as elec-trolyte. The gold film on the glass slide used for theexcitation of surface plasmon modes was also served asthe working electrode, simultaneously with a platinumwire as counter electrode, and Ag/(AgCl, saturated KCl)as reference electrode.

2.6. Measurements of reflection–absorption UV/Vis–NIRspectroscopy

The as-prepared Au film was thoroughly rinsedwith water, and then dried with pure nitrogen forspectroscopic characterization. The reflection–absorp-tion UV/Vis–NIR spectra were obtained by Cary 500Scan UV/Vis–NIR Spectrophotometer (Varian, USA)with a similar bare Au film as reference. The recordedspectra at all incident angles of light from 40� to 70�were extracted from results of the control experimentsat the identically corresponding angle. The controlexperiments were first carried out with the same as-sembly time for aniline in the case of only AETmonolayer present, and secondly followed by DNAassembly.

3. Results and discussion

3.1. SPR responses to the layer-by-layer assemblies andthe formation of DNA–PAn complex

It is well know that functional group-terminatedalkanethiol monolayer formed by self-assembly canprovide platforms to further build novel two- and three-dimensional molecular architectures on solid surfaces.But normally, densely packed, well-ordered monolayerfrom long carbon-chained alkanethiol gives a barrier toredox reaction of electroactive species presented on toplayer or in solution [27]. In this text, the selected AETmolecule has short carbon chain and can form mono-layer with some pinholes or defects in the same way asdisordered long carbon-chained alkanethiol monolayer[28], which may be served as electron-transport channels[29] for oxidation of aniline monomers assembled ontoits surface. Additionally, one AET molecule can bestrongly bound by its amino ends through electrostaticinteraction to two consecutive phosphate groups alongthe same DNA strands [30] in aqueous solution [31] oron solid surfaces [32,33], however not to DNA bases[30,34]. Fig. 1 demonstrates results of the assemblies bySPR measurements. Due to the small amount of theassembled molecules and their small effect on underlyingAu film as in the case of AET [35], the correspondingSPR responses are small but even enough to justify theadsorption of the individual species of interest. Thedifferential SPR responses in the inset of Fig. 1 againmake these layer-by-layer assemblies very practicallyvisible. The SPR responses experience SPR angle shift-ing to higher values upon the self-assembly of AET. Butthe adsorption of DNA onto preformed AET mono-layer and later the binding of aniline to the previouslyassembled DNA induce clearer changes of the SPRshapes than SPR angle shifts. It is expected that theprotonated amine-terminated alkanethiol monolayeradsorbed on Au film can closely attach the DNAthrough its phosphate groups to Au surface due toelectrostatic attraction. And as convinced by SPR, theprotonated aniline monomers can be assembled onto Aufilm surface with the assembled DNA as a bridge-likelayer or a template as suggested by Nagarajan et al. [20].

3.2. Electropolymerization of the assembled aniline

Normally the electropolymerization of the aniline onelectrode surface should be initiated by strong protonacid conditions [36]. Based on this, the occurrence ofelectropolymerization of the aniline seems to be difficultin our condition, pH 4.2 aqueous solutions. However,Nagarajan et al. [20] argued that close association of theprotonated aniline with the DNA, resulting from elec-trostatic interaction between the protonated aniline andthe phosphate groups in the DNA, provides high proton

Y. Shao et al. / Electrochemistry Communications 4 (2002) 773–779 775

concentration around the phosphate groups. Thereforea unique local environment is maintained for polymer-ization of aniline around DNA strands in solution withhigher pH than that needed for the conventional syn-thetic methods. This microenvironment facilitates apredominantly para-directed coupling and deters para-sitic branching during the polymerization of aniline.Even in pH 6.5 the DNA–PAn complex can be formedby photochemistry [23]. In order to avoid the interfer-ence of the redox reaction from Au film itself and elec-trochemical desorption of the assembled alkanethiolaway from Au surface at extremely positive potentials,potential for the electropolymerization was controlled

over the range from 0 to 0.7 V. In addition, note thatafter the self-assembly of AET, the Au film should bethoroughly washed with water several times. If not,small amount of AET molecules simultaneously as-sembled by the terminated amino group [37] to Au filmcan be oxidized (data not shown). As shown in Fig. 2,two differences can be observed from the correspondingcyclic voltammograms (CVs) with and without DNA for30 min assembly time of aniline. The first, the moreobvious redox pairs appear between 0.2 and 0.4 V in thepresence of DNA and the peak currents increase withincreasing of CV scans, which can be seen in the inset ofFig. 2. The second, however, the redox currents after

Fig. 1. SPR responses to layer-by-layer assemblies of 2-aminoethanethiol (curve b), DNA (curve c), polyaniline electropolymerized around DNA

strands for assembly time 30 min (curve d) and 60 min (curve e), respectively, onto bare gold-film surface (curve a). Inset shows the responses of

differential SPR to the identical assemblies with each extracted from that to bare gold film.

Fig. 2. Cyclic voltammetry responses to electropolymerization processes of aniline electrostatically assembled around DNA strands 30 min (curve a).

Curve b are control experiments without DNA and other conditions are identical with curve a. The range from 0.15 to 0.40 V is magnified in the

inset, showing increasing of peak current with the increasing of CV scans. The first scan is separately determined from other scans in curve a. 50 mM

acetic acid–sodium acetate buffer (pH 4.2) with aniline free is used as electrolyte.

776 Y. Shao et al. / Electrochemistry Communications 4 (2002) 773–779

0.4 V obtained in the presence of DNA are much higherthan that in the absence of DNA and the decrease of thecurrents with increasing of CV scans is remarkable inthe presence of DNA in the same potential range. Basedon literatures [38,39], PAn film in acidic solution has tworedox pairs between 0 and 1.0 V. In our conditions, thefirst redox peaks can be observed, but the potential isnot positive enough to observe the second ones and onlytheir initial parts of the anodic process should be found.But the currents for electropolymerization of the as-sembled aniline should be overlapped by the secondredox pairs resulting from oxidation of the previouslyformed PAn on DNA strands. In addition, at the initialstage of electropolymerization the former should behigher that the later. With the proceeding of electropo-lymerization, the amount of PAn formed on Au surfaceincreases but that of the assembled monomers decreasegradually. Therefore, we observed the gradual increaseof peak currents for the redox pairs between 0.2 and0.4 V but gradual decrease of currents after 0.4 V. Whenthe redox currents are stabilized after some CV scans,which are indicative of complete polymerization of theassembled aniline, the CVs of the resulted Au film areshown in Fig. 3. The areas of the anodic peaks in thepotential range 0.2 to 0.4 V increase with increasing ofthe assembly time for aniline monomers from 30 to 120min, showing the assembly continuousness of aniline orthe growth of PAn chains. However, the decrease of theareas of the anodic peaks for 240 min assembly time canbe observed and may be caused by desorption of theassembled DNA due to the long-term soaking of the Au

film in electrolyte solution or the shielding of charge onthe phosphate groups of the DNA by the wrapped PAn[20]. But a relatively small redox pairs can be also ob-served in the absence of the DNA. However, the area ofits anodic peak gets to maximum upon 60 min assemblytime and is always smaller than that in the presence ofDNA indicative of the template features of DNA foraniline assembly. This phenomenon should result fromdefects or pinholes in the monolayer specially formed byshort-chained alkanethiol. The presence of defects orpinholes in the AET monolayer is very possibly causedby disordering in some domains of short-chained alka-nethiol monolayer or AET molecules assembled byterminated amino group [37] that will detach away fromAu surface when electrode potential scans. The anilinemolecules can penetrate into these defects and attainelectrode surface for further electropolymerization. Butthe effect of these defects on the DNA-templated poly-merization of aniline is limited to some extent becausethe peak area of the small redox pairs for the controlexperiments becomes unchangeable upon 60 min as-sembly time (See the inset in Fig. 3). This indicates thefurther growth of PAn in these defects is inhibited.Despite of this, the template features of the assembledDNA for association of aniline is very convincingcompared with the control experiments.

3.3. Characterization of reflection–absorption UV/Vis–NIR spectroscopy

UV/Vis–NIR spectroscopy was adopted for furtherdetailed information involving the formation of theDNA–PAn complex. Because a small amount of PAn isalso present even in the absence of DNA, the controlexperiments were first carried out with the same as-sembly time for aniline in the case of only AET mono-layer present, and secondly followed by DNA assembly.The spectra for assembled DNA–PAn complex wereextracted from those obtained in the control experi-ments in order to avoid the absorptions of Au film itself,AET and DNA, assuming that polymerization of anilineon DNA strands have no effect on the DNA structure.Namely, the differential spectra were recorded. Asshown in Fig. 4, the spectra possess very strong depen-dence on the incident angle of light source. When theangle was adjusted to 70�, characteristic absorptionsappear clearly. The absorptions at 240 and 310 nm areassigned to the p–p transition of the benzenoid rings inPAn. Simultaneously, the emergence of absorption inthe 600 nm region is observed and attributed to theexciton transition of the quinoid rings. But the polaronband at 420 nm in the case of DNA–PAn complexsynthesized by enzyme [20] was not observed in the text.Interestingly, another absorption band in the 500 nmregions, which has not been found for PAn obtained bymost of the normally presented polymerization proce-

Fig. 3. Cyclic voltammetry responses to the resulted DNA–PAn

complex with electrostatic assembly of aniline around DNA strands 30

min (curve a) and 60 min (curve c). Curves b and d are control ex-

periments without DNA assembly and other conditions are identical

with curves a and c, respectively. Inset shows the dependences of the

obtained coulombic charges for anodic peaks at 0.30 V with DNA

(curve e) and without DNA (curve f) on assembly time of aniline. In all

cases, 50 mM acetic acid-sodium acetate buffer (pH 4.2) is used as

electrolyte.

Y. Shao et al. / Electrochemistry Communications 4 (2002) 773–779 777

dures, can be clearly observed in all cases. However,absorption spectra obtained from the electropolymer-ization of aniline have a dependence on the potentialused. It has been proposed that at low oxidation po-tential the PAn results from the polymerization of themonomer radical cations [40]. Neoh et al. [41] haveshown that this absorption band at 500 nm appears atthe same time when oxidant was added to solutioncontaining monomers and that whether it appears or notis dependent on the dopant used. In addition, Armerand Miller [42] suggested that the absorption is attrib-uted to the radical cations of aniline. Based on these, it isvery possible that the absorption of the polaron shifts tothe 500 nm regions in our conditions.

4. Conclusions

We have first demonstrated a simple method byelectrochemistry for the formation of DNA–PAn com-plex on gold surface at a higher pH solution than thatneeded for conventional synthesis procedures. At thiscondition, the target biomolecules could maintain theirnatural structure and bioactivity. The strategy providesan easy and feasible way to fabricate diverse conductivepolymer structures with various functions. However,covalent immobilization of DNA onto such surface [43]may be one of the best ways to obtain high-density layerof DNA and can endure long-term managements. Butthe non-covalent method adopted in our text is a simpleand easy procedure for fundamental studies involvingthe formation of DNA–PAn complex. If two electrodes

or conductors can be bridged by individual DNAstrands as that constructed for metal nanowires [7], theDNA–PAn complex can be similarly formed and the as-prepared nanowires of conductive polymers can be usedto interconnect the two conductors. If so, nanometer-scale circuits can be easily constructed by such con-ductive polymers.

Acknowledgements

This work was supported by the National NaturalScience Foundation of China (Nos. 29835120 and29875028).

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