In situ electrochemical contact angle study of the biorecognition basedon C60DF monolayer interface

Renyun Zhang a, Sebastian Gutmann c, Jian Zhou a, Xueyan Zhao b, Hongguang Li b,Liqiang Zheng b,*, Xuemei Wang a,*a State Key Lab of Bioelectronics, Southeast University, Nanjing 210096, Chinab Ministry key Laboratory of Colloid and Interface Chemistry, Shandong University, Jinan 250100, Chinac Institute of Pure and Applied Chemistry, University Oldenburg, D-26111 Oldenburg, Germany

Applied Surface Science 255 (2008) 585–588

A R T I C L E I N F O

Article history:

Available online 3 July 2008

PACS:

81.05.Tp

81.16.Dn

Keywords:

Biorecognition

Electrochemistry

C60DF

Atomic force microscopy (AFM)

In situ electrochemical contact angle

measurements (ECAM)

A B S T R A C T

In this study, the specific DNA binding behavior of a water soluble C60 derivative (C60DF) and redox-

controlled hydrophilic-to-hydrophobic transformations of the relative C60DF monolayer on surfaces of

glassy carbon electrode (GCE) have been demonstrated by using in situ electrochemical static contact

angle analysis. The results illustrate that in situ electrochemical contact angle detection based on C60DF

functional film can be utilized to sensitively probe some specific bio-molecular recognition like DNA

interaction, which will provide the new strategy to develop the promising multi-signal responsive

biosensors for relevant biological process.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

Due to the electronic versatility of the fullerenes, theelectrochemical redox and photoinduced energy transfer offullerenes in solution have been commendably studied [1–3]. Itis already known that C60 displays six one-electron reversibleredox couples in non-aqueous solutions and is sensitive to light atwavelength longer than 500 nm [4,5]. Meanwhile, the electro-chemical study of C60 thin solid film has also attracted muchattention [6,7]. Bard and coworkers have reported the electro-chemical properties of C60 film on glassy carbon electrode (GCE)and explored the reduction behavior of C60 on different electrodes[1]. Besides, researchers have investigated the self-assembly of C60

and its derivatives on functionalized ITO electrode and a C60

derivative on Au(1 1 1) surface by using AFM, in situ scanningtunneling microscope (STM) and electrochemical studies [8,9].Recently, DNA cleavage mechanism in the presence of fullereneshas attracted considerable attention [10–12]. However, the poorsolubility of fullerenes precluded the direct study of C60 sensitizedDNA damage under physiological conditions.

* Corresponding authors. Tel.: +86 25 83792177; fax: +86 25 83792177.

E-mail addresses: lqzheng@sdu.edu.cn (L. Zheng), xuewang@seu.edu.cn

(X. Wang).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.06.172

In this study, we have explored the specific DNA bindingbehavior of a water-soluble C60 derivative C60DF [13] (seemolecular structure in Scheme 1), using AFM and in situelectrochemical contact angle measurements (ECAM). It is alreadyknown that in situ electrochemical contact angle measurementscould be very useful to illustrate the relative hydrophilic/hydrophobic features and wetting properties such as redox-controlled hydrophilic-to-hydrophobic transformations on sur-faces. Based on these considerations, the unique hydrophobic/hydrophilic properties of C60DF monolayer has been prepared andthe related DNA binding behavior has been demonstrated in thisreport by using AFM and in situ electrochemical contact anglemeasurements. Our observations illustrate that the sensitivebiorecognition based on C60DF functional film may raise thepossibility for the development of the promising multi-signalresponsive biosensors to detect some specific biological process.

2. Experiment

2.1. Modification of glassy carbon electrode

C60DF was synthesized according to the literature [13]. TheC60DF monolayer was prepared by using the similar method asreported in the literature [14].

Scheme 1. Molecular structure of C60DF and procedure for self-assembly of film on glassy carbon electrode.

Fig. 1. (a) Cyclic voltammogram of the C60DF monolayer-functionalized GCE in

0.1 M phosphate buffer solution (pH 7.1), and (b) in the presence of CT-DNA, under

nitrogen, scan rate: 100 mV s�1.

R. Zhang et al. / Applied Surface Science 255 (2008) 585–588586

2.2. In situ electrochemical static contact angle measurements

ECAMs were performed on the modified GCE using a CAM200optical contact angle analyzer (KSV, Finland) and CHI660 electro-chemical workstation. A platinum wire (u = 0.1 mm) was used as acounter electrode. All potentials are reported here versus the Ag-wire quasi-reference electrode, which has a 0.07 V potentialdifference with SCE [15]. For the C60DF functionalized electrodes,the applied potential was switched between 0.90 V, 1.1 V and�0.80 V. In each case the potential was kept for 15–30 s prior tocontact angle measurements and the images were fitted using theYoung-Laplace equation [15].

2.3. AFM and UV-Vis spectroscopy

AFM studies were carried out on mica by using typing mode onNanoscope IIIa instrument. UV–vis study was performed using UV-4100 instrument (HITACHI) and showed results identical to thatreported in literature [16].

3. Results and discussion

Initially, the surface of the GCE was functionalized withhydroxyl groups (see Scheme 1). This procedure could covalentlyassembled C60DF on GCE surface by the reaction of the carboxyl ofC60DF with hydroxyl on the GCE surface. Our cyclic voltammetricstudy indicates that the exposed surface area of the functionalizedelectrode is ca. 0.20 cm2. Coulometric assay of the oxidation of theC60DF monolayer illustrates an apparent surface coverage of4.0 � 10–10 mol cm�2, which is consistent with the literature [17].

Fig. 1(a) illustrates a typical cyclic voltammetry of C60DF withsome redox peaks at ca. 0.92 V, 0.20 V,�0.25 V, and�0.59 V, whichhave demonstrated the comparable characterization as that in theliterature [18]. Upon addition of CT-DNA, significant changesoccurred with considerable decrease of the peaks of C60DFmonolayer (Fig. 1(b)), indicating the strong interaction of C60DFwith DNA. The redox properties of the C60DF monolayer transformthe interface between the hydrophobic C60DF interface and themore hydrophilic C60DF. - monolayer. Since an aqueous droplet ofthe phosphate buffer is placed on the C60DF functionalized

electrode, it could provide the platform for controlling thehydrophilic/hydrophobic properties of the relative surface anddemonstrating the specific DNA binding behavior of C60DF.

As shown in Fig. 2, upon application of the positive potential onthe electrode E = 0.90 V or 1.10 V, the contact angle corresponds tou = 458, whereas upon the reductive potential of E = �0.80 V, themonolayer is in the reduced C60DF configuration and the contactangle changes to 358. The changes of the contact angle areremarkable upon changing the applied potentials of the monolayerbetween the C60DF and C60DF. - states. Because of the short chain ofC60DF and the densely modified monolayer, the fullerene headgroup is not easy to completely bend to the electrode surface, thusthe contact angle changes are mainly due to the redoxtransformation in the interface of the monolayer. It should benoted that control experiments revealed that no significantchanges were observed on a bare GCE electrode, indicating thatthe contact angle changes on the modified GCE electrode mainlyoriginate from the redox transformations occurring on themonolayer.

Fig. 2. ECAM images of C60DF monolayer on GCE. (A) Images of the aqueous droplet that in the absence and (B) the presence of CT-DNA in 0.1 M phosphate buffer solution (pH

7.1), under nitrogen, on a modified GCE surface, where (a) applied potential E = 0.90 V; (b) applied potential E = �0.80 V.

Fig. 3. AFM images of C60 DF on mica in the absence and (b) presence of CT-DNA.

R. Zhang et al. / Applied Surface Science 255 (2008) 585–588 587

Based on those observations, we have further explored thespecific binding behavior of C60DF to DNA. AFM studies providedthe direct evidence for the fresh DNA binding information of C60DF.Fig. 3 illustrates AFM images of the C60DF in the absence andpresence of CT-DNA, which demonstrates some large particlesappear on DNA chains, and may lead to the bend/or aggregation onthe binding sites of DNA chains so that big particles appear onsome specific part of DNA. As reported in the literature [11,12],light irradiation on C60 molecules could cause DNA cleavage, butthe DNA cutting ability of C60 derivatives containing carboxylicacid group is lower than that of other derivatives [18]. In our AFMstudies, almost no short DNA fragment has been detected,indicating that the binding of C60DF to DNA does not lead to thecleavage of DNA. The binding of C60DF to DNA may also facilitatethe remarkable changes of the monolayer propertyor structure,which was demonstrated by our ECAM studies of C60DF, where theC60DF surface became more hydrophilic after interacting withDNA. As shown in Fig. 2, upon addition of DNA and application ofthe positive potential on the electrode, E = 0.90 V, the electro-oxidized monolayer in the form of oxidative C60DF binding to DNA,the contact angle corresponds to u = 308, whereas upon thereductive potential of E = �0.80 V, the monolayer is in the reducedconfiguration and the contact angle changes to below 158.Therefore, the observations that the much bigger contact anglechanges in the presence of DNA than that in the absence of DNAoccur under the identical experimental conditions are consistentwith the results of our AFM and cyclic voltammetric studies(Scheme 1).

4. Conclusion

In summary, we have demonstrated the specific DNA bindingbehavior of a water soluble C60DF and the hydrophilic/hydro-phobic features of the relative C60DF monolayer as well as the

redox-controlled hydrophilic-to-hydrophobic transformations onsurfaces of GCE by using AFM and ECAM analysis. Our observationsillustrate that the relevant biorecognition based on C60DFfunctional film raises the possibility for the development of thepromising multi-signal responsive biosensors to detect somespecific biological process.

In summary, we have demonstrated the specific DNA bindingbehavior of a water soluble C60DF and the hydrophilic/hydro-phobic features of the relative C60DF monolayer as well as theredox-controlled hydrophilic-to-hydrophobic transformations onsurfaces of GCE by using AFM and ECAM analysis. Our observationsillustrate that the relevant biorecognition based on C60DFfunctional film raises the possibility for the development of thepromising multi-signal responsive biosensors to detect somespecific biological process.

Acknowledgements

This work was supported by NSFC (20675014, 90713023) andMinistry of Science & Technology of China. Mr. Sebastian Gutmannspecially thanks the support from DAAD foundation of Germany.

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