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Probing photoinduced electron transfer reactions by in situ electrochemical contact angle measurements Xuemei Wang, Shiri Zeevi, Andrei B. Kharitonov, Eugenii Katz and Itamar Willner* Institute of Chemistry and The Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. E-mail: [email protected] Received 11th June 2003, Accepted 7th August 2003 First published as an Advance Article on the web 26th August 2003 In situ electrochemical static contact angle measurements are employed to probe photoinduced electron transfer processes at electrode surfaces. Photosystems consisting of tris(2,2 0 -bipyridyl)ruthenium(II), (Ru(bpy) 3 2+ ), N,N 0 -dimethyl-4,4 0 -bipyridinium (MV 2+ ), Na 2 EDTA or CdS nanoparticles, MV 2+ , triethanolamine (TEOA) were included in aqueous droplets that were coupled with a ferrocene monolayer-functionalized electrode. Upon the oxidation of the interface (0.5 V vs. Ag quasi-reference electrode), and the illumination of the systems, photoinduced electron transfer to the modified electrode proceeds. The electron transfer processes yield photocurrents, and the photoelectrochemical transformations are followed by contact angle measurements. Similarly, a CdS/bipyridinium monolayer is assembled on an Au electrode support. The redox transformations of the bipyridinium units control the hydrophilic/hydrophobic properties of the interface, and are followed by contact angle analyses. Irradiation of the CdS/bipyridinium layer associated with the electrode biased at 0.3 V yields a photocurrent. The changes in the wetting properties of the irradiated interface are investigated by contact angle measurements. 1. Introduction Contact angle measurements are often used to characterize the hydrophilic/hydrophobic features and wetting properties of monolayer- or thin film-functionalized surfaces. 1 The signal- controlled switching of the hydrophilic/hydrophobic proper- ties of chemically modified surfaces attracts research efforts directed to the photochemical patterning of surfaces 2 or the electrochemically induced mechanical movement of solvents. 3 Several studies have addressed the use of contact angle measurements to follow redox-controlled transformations 4 or photochemically activated hydrophobic-to-hydrophilic or hydrophilic-to-hydrophobic transformations on surfaces. 5 The interactions and chemical reactivity of chemically modified surfaces with ingredients composing the droplet coupled with the surface may alter the hydrophilic/hydrophobic properties of the surface, thus allowing the probing of the chemical reac- tivity at the surface/drop interface by contact angle measure- ments. Recently we reported on the use of static contact angle analyses for probing electrocatalytic and bioelectrocatalytic processes at redox-active monolayer- or thin film-functiona- lized electrodes in conjunction with the appropriate substrates or enzyme and substrates contained in the droplets. 6 Photoinduced electron transfer reactions are extensively stu- died within the context of photochemical energy conversion and storage. 7 Photoinduced electron transfer reactions based on organic dyes, 8 transition metal complexes 9 or semiconduc- tor nanoparticles 10 have been studied in detail. Similarly, photoinduced electron transfer reactions between photoactive chromophores 11 or semiconductor nanoparticles 12 and elec- trode surfaces provide a means for light-to-electrical energy conversion, and many photoelectrochemical systems were organized based on these fundamental reactions. Photoin- duced electron transfer reactions occurring in aqueous drops associated with chemically modified electrodes or photoin- duced electron transfer reactions occurring at electrode supports, may alter the hydrophilic/hydrophobic properties of surfaces, thus enabling the characterization of the processes by contact angle analyses. Here we report on the use of contact angle measurements to follow photoinduced electron transfer reactions initiated by the molecular photosensitizer Ru(II)– tris-bipyridine or by CdS-nanoparticles. We demonstrate the use of contact angle measurements to probe photoinduced electron transfer cascades and photocurrent generation. As far as we are aware, the systems described here are the first examples that apply in situ contact angle measurements with coupled electrochemical/photochemical transformations. 2. Experimental 2.1. Chemicals and reagents N,N 0 -Dimethyl-4,4 0 -bipyridinium dichloride (MV 2+ ) (1), triethanolamine (TEOA) and 1-[3-(dimethylaminopropyl]-3- ethylcarbodiimide hydrochloride (EDC) were purchased from Aldrich. Ethylenediaminetetraacetic acid disodium salt (Na 2 EDTA) was purchased from Sigma, and tris(2,2 0 -bipyri- dyl)ruthenium(II) chloride and cystamine dihydrochloride were purchased from Fluka. N-Methyl-N 0 -aminopropyl-4,4 0 -bipyri- dinium dichloride (3), was synthesized as described, 13 and the synthesis of N-2-methylferrocenecaproic acid (2), followed the reported method. 14 The CdS nanoparticles (3–5 nm deter- mined by TEM) modified by mercaptopropionic acid or mer- captopropylamine, were prepared according to the reported methods. 15,16 2.2. Preparation of the ferrocene-functionalized electrodes Gold-coated glass plates (the Au layer is ca. 5 nm thick, Ana- lytical-Polystem, Germany) were cleaned by immersion in hot (60 C) ethanol solution for 5 min followed by thorough rin- sing with distilled water. The clean plates were interacted with a solution of cystamine dihydrochloride (0.1 M) for 2 h. The N-2-methylferrocenecaproic acid (2) (1 mM) was then coupled 4236 Phys. Chem. Chem. Phys., 2003, 5, 4236–4241 DOI: 10.1039/b306661d This journal is # The Owner Societies 2003 PCCP Published on 26 August 2003. Downloaded by Christian Albrechts Universitat zu Kiel on 23/10/2014 02:32:49. View Article Online / Journal Homepage / Table of Contents for this issue

Probing photoinduced electron transfer reactions by in situ electrochemical contact angle measurements

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Probing photoinduced electron transfer reactions by in situelectrochemical contact angle measurements

Xuemei Wang, Shiri Zeevi, Andrei B. Kharitonov, Eugenii Katz and Itamar Willner*

Institute of Chemistry and The Farkas Center for Light-Induced Processes, The HebrewUniversity of Jerusalem, Jerusalem 91904, Israel. E-mail: [email protected]

Received 11th June 2003, Accepted 7th August 2003First published as an Advance Article on the web 26th August 2003

In situ electrochemical static contact angle measurements are employed to probe photoinduced electron transferprocesses at electrode surfaces. Photosystems consisting of tris(2,20-bipyridyl)ruthenium(II), (Ru(bpy)3

2+),N,N0-dimethyl-4,40-bipyridinium (MV2+), Na2EDTA or CdS nanoparticles, MV2+, triethanolamine (TEOA)were included in aqueous droplets that were coupled with a ferrocene monolayer-functionalized electrode.Upon the oxidation of the interface (0.5 V vs. Ag quasi-reference electrode), and the illumination of the systems,photoinduced electron transfer to the modified electrode proceeds. The electron transfer processes yieldphotocurrents, and the photoelectrochemical transformations are followed by contact angle measurements.Similarly, a CdS/bipyridinium monolayer is assembled on an Au electrode support. The redox transformationsof the bipyridinium units control the hydrophilic/hydrophobic properties of the interface, and are followed bycontact angle analyses. Irradiation of the CdS/bipyridinium layer associated with the electrode biased at �0.3 Vyields a photocurrent. The changes in the wetting properties of the irradiated interface are investigated bycontact angle measurements.

1. Introduction

Contact angle measurements are often used to characterize thehydrophilic/hydrophobic features and wetting properties ofmonolayer- or thin film-functionalized surfaces.1 The signal-controlled switching of the hydrophilic/hydrophobic proper-ties of chemically modified surfaces attracts research effortsdirected to the photochemical patterning of surfaces2 or theelectrochemically induced mechanical movement of solvents.3

Several studies have addressed the use of contact anglemeasurements to follow redox-controlled transformations4

or photochemically activated hydrophobic-to-hydrophilic orhydrophilic-to-hydrophobic transformations on surfaces.5 Theinteractions and chemical reactivity of chemically modifiedsurfaces with ingredients composing the droplet coupled withthe surface may alter the hydrophilic/hydrophobic propertiesof the surface, thus allowing the probing of the chemical reac-tivity at the surface/drop interface by contact angle measure-ments. Recently we reported on the use of static contact angleanalyses for probing electrocatalytic and bioelectrocatalyticprocesses at redox-active monolayer- or thin film-functiona-lized electrodes in conjunction with the appropriate substratesor enzyme and substrates contained in the droplets.6

Photoinduced electron transfer reactions are extensively stu-died within the context of photochemical energy conversionand storage.7 Photoinduced electron transfer reactions basedon organic dyes,8 transition metal complexes9 or semiconduc-tor nanoparticles10 have been studied in detail. Similarly,photoinduced electron transfer reactions between photoactivechromophores11 or semiconductor nanoparticles12 and elec-trode surfaces provide a means for light-to-electrical energyconversion, and many photoelectrochemical systems wereorganized based on these fundamental reactions. Photoin-duced electron transfer reactions occurring in aqueous dropsassociated with chemically modified electrodes or photoin-duced electron transfer reactions occurring at electrodesupports, may alter the hydrophilic/hydrophobic properties

of surfaces, thus enabling the characterization of the processesby contact angle analyses. Here we report on the use of contactangle measurements to follow photoinduced electron transferreactions initiated by the molecular photosensitizer Ru(II)–tris-bipyridine or by CdS-nanoparticles. We demonstrate theuse of contact angle measurements to probe photoinducedelectron transfer cascades and photocurrent generation. Asfar as we are aware, the systems described here are the firstexamples that apply in situ contact angle measurements withcoupled electrochemical/photochemical transformations.

2. Experimental

2.1. Chemicals and reagents

N,N0-Dimethyl-4,40-bipyridinium dichloride (MV2+) (1),triethanolamine (TEOA) and 1-[3-(dimethylaminopropyl]-3-ethylcarbodiimide hydrochloride (EDC) were purchased fromAldrich. Ethylenediaminetetraacetic acid disodium salt(Na2EDTA) was purchased from Sigma, and tris(2,20-bipyri-dyl)ruthenium(II) chloride and cystamine dihydrochloride werepurchased from Fluka. N-Methyl-N0-aminopropyl-4,40-bipyri-dinium dichloride (3), was synthesized as described,13 and thesynthesis of N-2-methylferrocenecaproic acid (2), followed thereported method.14 The CdS nanoparticles (3–5 nm deter-mined by TEM) modified by mercaptopropionic acid or mer-captopropylamine, were prepared according to the reportedmethods.15,16

2.2. Preparation of the ferrocene-functionalized electrodes

Gold-coated glass plates (the Au layer is ca. 5 nm thick, Ana-lytical-Polystem, Germany) were cleaned by immersion in hot(60 �C) ethanol solution for 5 min followed by thorough rin-sing with distilled water. The clean plates were interacted witha solution of cystamine dihydrochloride (0.1 M) for 2 h. TheN-2-methylferrocenecaproic acid (2) (1 mM) was then coupled

4236 Phys. Chem. Chem. Phys., 2003, 5, 4236–4241 DOI: 10.1039/b306661d

This journal is # The Owner Societies 2003

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to the cystamine monolayer using EDC (5 mM) solution in0.1 M HEPES buffer solution (pH 7.1) at room temperaturefor 2 h (Scheme 1). After the modification, the electrodes wererinsed with distilled water and dried under a flow of nitrogen.

2.3. Preparation of the CdS/bipyridinium-functionalizedelectrodes

The cystamine-modified gold-coated glass plates were preparedas described above. The mercaptopropionic acid-functiona-lized CdS nanoparticles (1 mg mL�1) were then covalentlylinked to the cystamine monolayer17 using EDC (5 mM) solu-tion in 0.1 M HEPES buffer (pH 7.1) at room temperature,reaction time 2 h. The resulting electrodes were rinsed with dis-tilled water. The assembly was further reacted with N-methyl-N0-aminopropyl-4,40-bipyridinium dichloride (3) using EDC(5 mM) solution in 0.1 M HEPES buffer solution (pH 7.1)at room temperature, reaction time 2 h, to yield the bipyridi-nium/CdS-functionalized gold interface (Scheme 2).The modified electrodes were characterized by cyclic voltam-

metry in a three-electrode electrochemical cell configurationusing a potentiostat (EG&G, Model 283) connected to a com-puter (EG&G Software PowerSuite 1.03). The exposed area ofthe electrode in the electrolyte solution was ca. 0.9 cm2.

2.4. Contact angle measurements

In situ photoelectrochemical static contact angle measurementswere performed on the respective modified gold-coated glassplates using a CAM2000 optical contact angle analyzer(KSV Instruments, Finland), and an EG&G, Model 283potentiostat. The light source was a blue LED (StanleyUB5306X) with a maximum light output at 465 nm and a band

width of ca. 40 nm. The intensity of the LED is ca. 2800 mcd.The LED was set at 5 cm from the drop.An aqueous droplet composed of 0.1 M phosphate buffer

solution, pH 7.1, (5� 0.5 mL) that included as the photochemi-cal system: (i) N,N0-dimethyl-4,40-bipyridinium, 1� 10�3 M,Ru(II)–tris-bipyridine, 5� 10�5 M, Na2EDTA, 5� 10�2 M,or (ii) N,N0-dimethyl-4,40-bipyridinium, 1� 10�3 M, CdSnanoparticles, 0.5 mg mL�1, triethanolamine, 5� 10�2 M,was placed on a ferrocene monolayer-functionalized Au elec-trode. Alternatively, an aqueous droplet composed of 0.1 Mphosphate buffer solution, pH 7.1, (5� 0.5 mL) containingtriethanolamine, 5� 10�2 M, was placed on a CdS/bipyridi-nium-functionalized Au electrode. A thin silver wire (f ¼ 0.1mm) and a platinum wire (f ¼ 0.1 mm) were used as aquasi-reference electrode and a counter electrode, respectively,and were introduced into the droplet. Such thin wires werechosen to prevent the distortion of the drops. All potentialsare reported here versus the Ag-wire quasi-reference electrode.We found that the reference potential of a SCE and the Agwire in 0.1 M phosphate buffer, pH 7.1, have the relationVSCE ¼ VAg wire + 0.07 V.6 For the ferrocene (2)-functiona-lized electrodes, the applied potential was switched between0.5 and 0.1 V. For the bipyridinium/CdS-functionalized elec-trodes, the applied potential was switched between �0.3 and�0.8 V. In each case the potential was kept for 15–30 s priorto contact angle measurement to allow the drop to adjust itsshape. For all systems, the images of the drops were recordedin the respective oxidized and reduced states of the interfaces.To extract the precise contact angle values, the drop imageswere fitted using the Young–Laplace equation.18 The contactangle values were determined with the precision of �0.5�.The droplet was under an inert atmosphere generated by a flowof Ar gas through a funnel placed at a distance of 3 cm abovethe droplet. The experiments were performed within shorttime-intervals (less than 2 min) in order to prevent the dropletheating and evaporation upon the illumination.

3. Results and discussion

The photosensitized reduction of the electron acceptor N,N0-dimethyl-4,40-bipyridinium, MV2+ (1), by transition metalcomplexes19 such as Ru(II)–tris-bipyridine, Ru(bpy)3

2+, or bysemiconductor particles20 such as TiO2 or CdS is well estab-lished. The system outlined in Scheme 1 represents one config-uration for the photochemical control of the hydrophilic/hydrophobic properties of a chemically functionalized surface,

Scheme 1 Assembly of the ferrocene-functionalized monolayer on agold-coated glass plate for the photochemically induced electron trans-fer between photosystems solubilized in aqueous droplets and theelectrode for photocurrent generation.

Scheme 2 Assembly of CdS nanoparticles modified with N-methyl-N0-aminopropyl-4,40-bipyridinium on a gold-coated glass plate.

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and the method to follow a photocurrent generation processby means of contact angle measurements. A glass surfacecoated with an Au-layer was modified by the covalent linkageof the N-2-methylferrocenecaproic acid (2) to the cystaminemonolayer associated with the electrode. The reversible redoxproperties of the ferrocene monolayer transform the interfacebetween the hydrophobic ferrocene interface and the morehydrophilic ferrocenylium cation monolayer. An aqueousdroplet that includes the photosensitizer Ru(bpy)3

2+, MV2+

as an electron acceptor, and Na2EDTA as a sacrificial electrondonor is placed on the functionalized electrode and providesthe photosystem for the generation of the photocurrent inthe system and for controlling the hydrophilic/hydrophobicproperties of the surface. Excitation of the Ru(bpy)3

2+ photo-sensitizer results in its oxidative quenching by MV2+ to yieldMV+�. The oxidized photosensitizer is then reduced byNa2EDTA, a process that leads to the accumulation of theN,N0-dimethyl-4,40-bipyridinium radical cation in the droplet.Upon application of the positive potential on the electrode,E ¼ 0.5 V, the electrochemically oxidized monolayer in theform of the ferrocenylium cation is reduced by the photogen-erated MV+� (E� ¼ �0.70 V). Thus, under steady-state irra-diation, a photocurrent is generated. As the reduction of theferrocenylium cation by MV+� is fast, a steady-state concentra-tion of ferrocene/ferrocenyl cation in the monolayer structureis formed, even though the electrode is subjected to the oxida-tive potential. As a result, the interface turns hydrophobicalthough the oxidative potential that corresponds to E ¼ 0.5V is applied on the electrode. Switching the applied potentialto E ¼ 0.1 V reduces the ferrocenylium cation monolayer tothe hydrophobic ferrocene interface. This blocks the electrontransfer cascade, and the photocurrent is inhibited.Fig. 1(A) shows the cyclic voltammogram of the ferrocene-

functionalized electrode. The observed anodic and cathodicpeaks, E� ¼ 0.33 V, demonstrate the peak-to-peak separationDEp ¼ 64 mV (potential scan rate of 100 mV s�1) that is largerthan expected from Laviron theory21 for the fully reversibleredox processes of surface-confined species, but is typical fora quasi-reversible redox process.22 The peak currents increaselinearly with the increased potential scan rate (with a slightdeviation at a very high potential scan rate originating fromthe non-compensated iR-potential drop), Fig. 1(A), inset,which confirms the surface-confined structure of the ferroceneunits.21 Coulometric assay of the oxidation (or reduction)wave of the ferrocene monolayer indicates a surface coveragethat corresponds to 1.8� 10�10 mol cm�2. This surface cover-age corresponds to ca. 40% of the densely packed monolayerbased on treating the ferrocene group as a sphere with a 0.66nm diameter.23

Fig. 1(B) shows the images of the water droplet that includesthe photosystem Ru(bpy)3

2+/MV2+/Na2EDTA when the sys-tem is in the dark and the applied potential on the electrode isE ¼ 0.1 V, image (a), and when the applied potential isE ¼ 0.5 V, image (b), generating the ferrocenylium monolayeron the electrode. When the monolayer is in the reduced ferro-cene configuration, the contact angle corresponds to y ¼ 73�,whereas upon oxidation of the monolayer to the ferrocenyliumcation, the contact angle changes to 63�. The changes of thecontact angle are reversible, and upon cycling the monolayerbetween the ferrocene and ferrocenylium cation states, thecontact angles change reversibly between high and low values,respectively. This is consistent with the transformation of themonolayer interface between the hydrophobic and hydrophilicstates, respectively. It should be noted that control experimentsreveal that upon the switching of the applied potentials on abare Au-electrode between the values E ¼ 0.1 to 0.5 V, nonoticeable changes in the contact angle of the aqueous dropletare observed, indicating that the contact angle changes on themodified Au-electrode originate from the redox transforma-tions occurring on the monolayer. Fig. 1(B), image (c) shows

the image of the droplet under conditions where the potentialapplied on the electrode is E ¼ 0.5 V and the droplet is irra-diated for 30 s. Although the applied potential is adequate tooxidize the monolayer to the ferrocenylium cation state, thecontact angle under irradiation increases to the value of 71�,very similar to the value characteristic of the hydrophobicferrocene monolayer state.In a control experiment, where the droplet is irradiated for

30 s and the monolayer is kept in the ferrocene monolayer con-figuration (E ¼ 0.1 V) no noticeable changes in the contactangle are observed between the ‘‘dark’’ and illuminated sys-tems. This control experiment clearly indicates that the photo-induced reduction of MV2+ to MV+� has no effect on thecontact angle, but the interaction and coupling of the photo-product, MV+�, with the ferrocenylium cation monolayerchanges the contact angle. These results may be explained bythe electron transfer cascade occurring in the system asdepicted in Scheme 1. The reduced electron acceptor, MV+�,generated in the droplet by the photochemical process, reducesthe ferrocenylium cation monolayer. Since, however, thepotential applied on the electrode is E ¼ 0.5 V, the monolayeris reoxidized to the cationic state, leading, under illumination,to the constant electron flow from the excited photosensitizerto the electrode. This electron transfer yields a steady-stateratio between the reduced ferrocene monolayer and the ferro-cenylium cation state, and leads to the hydrophobic propertiesof the interface. The fact that the observed contact angle isclose to the value characteristic of the hydrophobic ferro-cene-monolayer state suggests that the reduction of the mono-layer by MV+� accumulated in the droplet is faster than theoxidation of the ferrocene sites by the electrode. The direc-tional electron transfer occurring between the photosystem

Fig. 1 (A) Cyclic voltammogram of the ferrocene monolayer-functio-nalized gold electrode, recorded in 0.1 M phosphate buffer solution,pH 7.1, under Ar, scan rate 100 mV s�1. Inset: Peak current value asa function of the potential scan rate. (B) Images of the aqueous dropletthat include Ru(bpy)3

2+ (5� 10�5 M), MV2+ (1) (1� 10�3 M) andNa2EDTA (5� 10�2 M) in 0.1 M phosphate buffer solution, pH 7.1,under Ar, on the ferrocene-functionalized Au surface, where (a) systemis in the dark, applied potential E ¼ 0.1 V; (b) system is in the dark,applied potential E ¼ 0.5 V, and (c) droplet is irradiated for 30 s,applied potential E ¼ 0.5 V.

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solubilized in the droplet and the monolayer interface shouldyield a photocurrent.Fig. 2 shows the in situ redox transformations of the ferro-

cene monolayer and the generated photocurrent in the system.Applying a potential step from E ¼ 0.5 to 0.1 V on the elec-trode in the dark, point (k), results in a transient current cor-responding to the electrochemical reduction of the ferrocenemonolayer. At point (l) the light is switched on preservingthe biasing potential E ¼ 0.1 V. The photochemical processin the system, however, does not result in photocurrent forma-tion. The ferrocene monolayer is retained in the reduced con-figuration, thus no electron transfer to the electrode occurs.At point (m) the light is switched off and the potential stepfrom E ¼ 0.1 to 0.5 V is applied on the electrode. Theobserved current transient corresponds to the electrochemicaloxidation of the ferrocene units to the ferrocenylium cations.At point (n) the light is switched on and the potentialE ¼ 0.5 V is applied on the electrode. This results in the anodicphotocurrent formation mediated by the oxidized ferroceny-lium cations. At point (o) the light is switched off and thepotential E ¼ 0.5 V is still applied on the electrode, and thephotocurrent decreases. Note, however, that the photocurrentdoes not decay immediately to zero. Since MV+� was accumu-lated in the droplet and the potential applied on the electroderegenerates the ferrocenylium cation, the photocurrent pro-ceeds even in the dark until all MV+� is consumed. It is,however, difficult to monitor the complete decay of thephotocurrent since the droplet evaporates. The contact anglesfor the systems generated at points (k), (l), (m), (n) and (o)correspond to 73, 73, 63, 71, and 64�, respectively, consistentwith the hydrophilic/hydrophobic properties of the monolayerin the different states. Thus, the system enabled the analysis ofa directional photoinduced electron transfer process at achemically modified electrode using in situ electrochemicalcontact angle measurements. It should be noted that in theabsence of MV2+ in the droplet, no photocurrent could beobserved upon the irradiation of the droplet that includesRu(bpy)3

2+/Na2EDTA and the simultaneous biasing of theelectrode at E ¼ 0.5 V. Also, no photocurrent and no contactangle changes were observed upon illumination of the systemin the absence of Ru(bpy)3

2+. Thus, the direct electron transferquenching of the photoexcited Ru(bpy)3

2+ by the ferro-cenylium cation does not occur or is extremely inefficient.

Consequently, the contact angle of the droplet corresponds to64�, consistent with the formation of the hydrophilic interface.Similar results are observed when the CdS nanoparticles

are incorporated as the light-active component in the aqueousdroplet, Scheme 1. In this system, N,N0-dimethyl-4,40-bi-pyridinium, MV2+ is used as electron acceptor, the CdSnanoparticles act as the photoactive component and triethanol-amine, TEOA, functions as a sacrificial electron donor.Photoexcitation of the CdS nanoparticle yields the electron-hole pair. The reduction of MV2+ by the conduction bandelectrons and the oxidation of TEOA by the valence bandholes leads to the accumulation of MV+� in the droplet.Fig. 3 shows the in situ redox transformations of the ferro-

cene monolayer and the photocurrent generated upon thecoupling of the droplet that includes the MV2+/CdS/TEOAphotosystem with the ferrocene monolayer-modified electrode.Application of a potential that corresponds to E ¼ 0.1 Von the electrode, Fig. 3, point (p), that preserves the reducedferrocene monolayer state, does not yield any photocurrent.Also, nearly no current change is observed upon irradiationof the system, point (q). Switching the light off and steppingthe potential of the electrode to E ¼ 0.5 V, point (r), resultsin the transient current corresponding to the oxidation of theferrocene units and formation of the ferrocenylium cations.At point (s) light is switched on while the potential E ¼ 0.5V is applied on the electrode that results in the anodic photo-current formation. Switching off the light results in a decay inthe photocurrent, point (t). The contact angle of the illumi-nated droplet associated with the electrode biased at E ¼ 0.5V is 72�, very similar to the contact angle of the droplet asso-ciated with the electrode biased at E ¼ 0.1 V (under dark orlight conditions), points (p) and (q), respectively. While thecontact angle of the MV2+/CdS/TEOA containing dropletassociated with the electrode biased at E ¼ 0.5 V in the dark,corresponds to 63.5�, it increases to 72� upon irradiation,points (r) and (s), respectively. This is consistent with the factthat illumination of the system yields MV+� that reduces theferrocenylium cation monolayer. This process leads to theformation of the photocurrent, and the conversion of theelectrode surface into a hydrophobic interface as a result ofthe formation of a ferrocene-rich monolayer.It should be noted that in the absence of MV2+ irradiation

of the CdS/TEOA-containing droplets associated with the

Fig. 2 Chronoamperometric curve recorded for the N-2-methylferro-cenecaproic acid (2)-functionalized Au-coated glass electrode in con-tact with an aqueous droplet, 5 mL, consisting of Ru(bpy)3

2+

(5� 10�5 M), MV2+ (1) (1� 10�3 M) and Na2EDTA (5� 10�2 M)in 0.1 M phosphate buffer solution, pH 7.1. At point: (k) the systemis in the dark, E ¼ 0.1 V; (l) the light is switched on, E ¼ 0.1 V; (m)the light is switched off, E ¼ 0.5 V; (n) the light is switched on,E ¼ 0.5 V; and (o) the light is switched off, E ¼ 0.5 V. The experimen-tal contact angle values for the different states are presented. Theexperiments were conducted under Ar.

Fig. 3 Chronoamperometric curve recorded for the N-2-methylferro-cenecaproic acid (2)-functionalized Au-coated glass electrode in con-tact with an aqueous electrolyte droplet, 5 mL, consisting of CdSnanoparticles (1 mg mL�1), MV2+ (1) (1� 10�3 M) and triethanol-amine (1� 10�2 M) in 0.1 M phosphate buffer solution, pH 7.1. Atpoint: (p) the system is in the dark, E ¼ 0.1 V; (q) the light is switchedon, E ¼ 0.1 V; (r) the light is switched off, E ¼ 0.5 V; (s) the light isswitched on, E ¼ 0.5 V; and (t) the light is switched off, E ¼ 0.5 V.The experimental contact angle values for the different states are alsopresented. All the experiments were conducted under Ar.

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electrode biased at E ¼ 0.5 V does not lead to the generationof a detectable photocurrent. Also, irradiation of the aqueousdroplet without the CdS nanoparticles did not result in aphotocurrent and any measureable changes of the contactangle. Thus, the direct transfer of conduction-band electronsto the ferrocenylium cation monolayer is inefficient. Accord-ingly, the contact angle of the droplet is 64� consistent withthe formation of a hydrophilic interface.The photosystem that includes the CdS nanoparticles and

the bipyridinium electron acceptor can be assembled on theelectrode surface itself.17 Photoinduced electron transfer onthe electrode surface may then control the hydrophilic/hydro-phobic properties of the interface. The method to assemble thephotoactive interface on an electrode is shown in Scheme 3.CdS nanoparticles capped with mercaptopropionic acid werecovalently linked to a cystamine monolayer associated withthe Au-surface. N-Methyl-N0-(3-aminopropyl)-4,40-bipyridi-nium (3) was then covalently linked to the CdS nanoparticles.The parallel assembly of the CdS/bipyridinium layer on apiezoelectric Au-quartz crystal indicates that the surface cover-age of the CdS nanoparticles is ca. 6.7� 1012 particles cm�2

and of the bipyridinium units 7.9� 10�10 mol cm�2. TheCdS/bipyridinium layer associated with the Au-electrode wasrecently studied as a functional photoelectrochemical elec-trode.17 Illumination of the functionalized electrode in the pre-sence of triethanolamine, TEOA, as a sacrificial electrondonor, leads to the formation of a photocurrent.The bipyridinium units associated with the CdS exhibit a

redox potential that corresponds to E ¼ �0.70 V. Accord-ingly, the contact angles of an aqueous droplet that consistsof 0.1 M phosphate buffer, pH 7.1, and TEOA as electrondonor on the bipyridinium-functionalized CdS nanoparticlelayer associated with the electrode, were measured in the darkat two different potentials: E ¼ �0.3 and�0.8 V, Scheme 3(A).

Upon biasing the potential of the system at E ¼ �0.3 V, thebipyridinium units exist in their oxidized state, whereas atE ¼ �0.8 V the bipyridinium units are in the reduced radical-cation configuration. While the oxidized bipyridinium unitsrender the interface hydrophilic, the reduction of the bipyridi-nium components at E ¼ �0.8 V to the radical cation struc-ture, makes the surface less hydrophilic. This is, indeed,reflected by the respective contact angle values. At an appliedpotential of E ¼ �0.3 V the contact angle corresponds to 66�

while switching the potential to E ¼ �0.8 V changes the con-tact angle to 75�, indicating that the formation of an interfaceis less hydrophilic. By the cyclic switching the potential appliedon the electrode between E ¼ �0.3 and �0.8 V, the contactangles are reversibly switched between low and high values,respectively, Fig. 4. Control experiments reveal that the appli-cation of the two potentials on a CdS nanoparticle-modifiedelectrode that lacks the bipyridinium units results in similarcontact angles for the two potential values. This implies thatthe changes in the contact angles originate from the reversibleredox transformations associated with the bipyridiniumcomponents linked to the CdS nanoparticles.The previous system has indicated that the bipyridinium

units associated with the CdS nanoparticles may electricallyinteract with the bulk electrode and undergo reduction or oxi-dation upon the application of the respective potentials. Thepossibility, however, of stimulating photoinduced electrontransfer between the photoexcited CdS nanoparticles and thebipyridinium electron acceptor units, allows the designingof a photoelectrochemical cycle where the photoinduced elec-tron transfer is coupled to a secondary electron transfer tothe electrode, as outlined in Scheme 3(B). In the dark, at anapplied potential of E ¼ �0.3 V the bipyridinium units existin their oxidized state. The contact angle of the droplet corres-ponds to 66�, consistent with the hydrophilic properties of theinterface. Photochemical excitation of the CdS nanoparticlesleads to the formation of the electron-hole pair. Reductionof the bipyridinium units to the respective radical cations,followed by scavenging of the holes by TEOA, leads to theaccumulation of the bipyridinium radical cations on the inter-face. Since, however, the electrode is biased at E ¼ �0.3 V, theradical cations are oxidized by the electrode, leading to the for-mation of a photocurrent. As the oxidation of the bipyridi-nium radical cation units is slow, the stepwise photoinducedelectron transfer in the system and the formation of the photo-current yield an interface that is enriched with the hydrophobicbipyridinium radical cation. Accordingly, the contact angle ofthe illuminated droplet associated with the electrode (biased at

Scheme 3 (A) Electroswitchable control of the hydrophilic/hydro-phobic properties of the CdS/bipyridinium-functionalized Au-surface.(B) Light-induced control of the hydrophilic/hydrophobic propertiesof the CdS/bipyridinium-functionalized surface.

Fig. 4 Changes in the contact angles of an aqueous drop composedof triethanolamine (5� 10�2 M) in 0.1 M phosphate buffer solution,pH 7.1, on a gold-coated glass electrode functionalized with theCdS/bipyridinium nanoparticles upon switching the potential from�0.3 V, point (i), to �0.8 V point (ii), in the dark.

4240 Phys. Chem. Chem. Phys., 2003, 5, 4236–4241

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E ¼ �0.3 V) is 74�, very similar to the contact angle of thesame interface in the dark biased at E ¼ �0.8 V (where thebipyridinium radical cation is generated electrochemically).Fig. 5 shows the photocurrent that is generated in the sys-

tem. At point (x) the electrode is biased under the light atE ¼ �0.8 V. This results in the reduction of the bipyridiniumunits to the less hydrophilic radical cation form. This yieldsa contact angle that corresponds to 76�. At point (y) the elec-trode is biased under the light at E ¼ �0.3 V. This results inthe formation of a photocurrent. The contact angle of theinterface change corresponds to 74�, consistent with the photo-induced formation of the bipyridinium radical cation at theelectrode interface. At point (z) the light is switched off, whilebiasing the electrode at E ¼ �0.3 V. The photocurrent decayssince the bipyridinium radical cations are oxidized by theelectrode, and the contact angle changes accordingly to 66�,consistent with the formation of a hydrophilic interface.

4. Conclusions

The present study has demonstrated the use of contact anglemeasurements for probing photoinduced electron transferand photocurrent generation. As far as we are aware, the pre-sent study represents the first effort to combine photoinducedelectron transfer reactions with in situ electrochemical contactangle measurements. The photochemical control of the hydro-phobicity/hydrophilicity of surfaces could be used in thefuture to transport liquids between illuminated and darkdomains, or to move liquids in capillaries.

Acknowledgements

The support of this research by The Israel Science Foundation(Research No. 101/00) is gratefully acknowledged.

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Fig. 5 Chronoamperometric curve recorded for the Au-coated glasselectrode functionalized with the CdS/bipyridinium nanoparticles incontact with an aqueous droplet, 5 mL, consisting of triethanolamine(5� 10�2 M) in 0.1 M phosphate buffer solution, pH 7.1. At point(x) the electrode is biased at E ¼ �0.8 V; (y) the electrode is biasedunder the light at E ¼ �0.3 V, and at point (z) the light is switchedoff, E ¼ �0.3 V. The experimental contact angle values are also pre-sented. All the experiments were conducted under Ar.

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