Electrochemical Studies of Chemically Modified Nanometer-SizedElectrodes

Jing Guo ,a, b Chu-Ngi Ho,a Peng Sun*a

a Department of Chemistry, East Tennessee State University, Box 70267, Johnson City, TN 37614-0054b Current address: Department of Chemistry, University of Utah*e-mail: sunp@etsu.edu

Received: August 15, 2010;&Accepted: September 20, 2010

AbstractSelf-assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) has been successfully deposited onto nanome-ter-sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4-ATP SAMs modified electrode showpeaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modi-fied macro electrode. The electrochemical behavior and mechanism of outer-sphere electron transfer reaction onthe 4-ATP SAMs modified nanometer-sized electrode has also been studied. The 4-ATP SAMs modified electrodeis further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical commu-nication between Pt nanoparticles and Au metal on the Pt nanoparticles/4-ATP SAMs/Au electrode. However, scan-ning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.

Keywords: Nanoelectrodes, Chemically modified electrodes, Self-assembled monolayers

DOI: 10.1002/elan.201000517

1 Introduction

Electrode is a platform to study electrochemical reac-tions. To study electrochemical reaction in nanoscale, themost direct way is to use a nanometer-sized electrode.Methods to fabricate unpolishable [1–10] and polishable[11–14] nanometer-sized electrodes have been introduced.However, polishable nanometer-sized electrode is muchattractive since it is reliable and reusable. Schumman�sgroups and Mirkin�s group [11–13] have developed amethod to produce polishable nanometer-sized electrodeby using a laser puller. The method has become a routinemethod to prepare nanometer-sized Pt electrode. A laserpuller produced polishable nanometer-sized electrodehave been proved to be a versatile tool in single mole-cules detection, high resolution imaging, single cell studyand fast heterogeneous kinetic detections [12,13, 15].

While a bare nanometer-sized electrode has been usedin most of the aforementioned studies, a modified nano-meter-sized electrode could be very useful. Physically ad-sorbed modification and chemical modification could beused to obtain a modified electrode [16]. Although manymethods have been developed to chemically modify dif-ferent kinds of electrode materials, the SAMs of alkane-thiols modified Au electrode is more attractive since theSAMs is well-ordered and close-packed [17]. This naturemakes the SAMs of alkanethiols to be suitable modelsfor fabricating sensors, investigating electron transferacross molecular films and synthesizing nanometer sizedmaterials [17]. The studies of the electrochemical behav-

iors of a SAMs modified nanometer-sized electrode couldprovide background knowledge on the fabrication of fastresponse and high dense miniaturized sensors or novelsensing strategies. For SAMs modified Au electrodes thathave been prepared by using the same polishing, cleaningand modification procedures, the number of pinholes onthese electrodes should be proportional to their electrodearea. Thus, a SAMs modified nanometer-sized Au elec-trode is more likely to be pinhole free compared with theSAMs on a macro Au electrode. The pinhole free mono-layer modified nanometer-sized electrode could be a veryuseful tool to study the kinetics of outer-sphere electrontransfer reaction.

Pioneering work on a modified nanometer-sized Pt[6, 18] and Au electrodes [10,19] have been done before.Lemay et al. has used enzyme layer modified nanometer-sized electrode to study the catalytic response [10]. Intheir method, a layer of enzyme has been physically ad-sorbed on the electrode surface. Detailed studies of theelectrochemical behaviors of a chemically modified nano-meter-sized electrode, especially a chemically modifiedAu nanometer-sized electrode, have not been done yet.Our experience shows that the difficulties to the studiesof a chemically modified Au nanometer-sized electrode isfrom the small signal to noise ratio since the surface areais too small, rather than from the fabrication of the elec-trode since the fabrication of a nanometer-sized Au elec-trode with a radius bigger than 30 nm is as mature as thatfor a nanometer-sized Pt electrode. In principle, by in-creasing the scan rate relatively big signal to noise ratio

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in a voltammetric response could be obtained on a verysmall modified electrode since the peak height of the vol-tammograms on a chemically modified electrode is pro-portional to the scan rate [6,16, 17, 19]. However, it couldbe difficult to perform cyclic voltammetry on a nanome-ter-sized electrode at high scan rate due to the followingreasons: 1) the amplitude of the noise could dramaticallyincrease at high scan rate, 2) currently the typical band-width of a resistor feedback based pA current to voltagetransducer is less than 100 Hz. Here, we concentrated onthe studies of the electrochemical behaviors on a chemi-cally modified Au nanometer-sized electrode whoseradius is bigger than 50nm but smaller than 300 nm. Theelectrodes are chemically modified with SAMs of aromat-ic thiols (4-ATP) and then further modified with Pt nano-particles. We have used cyclic voltammetry and scanningelectron microscopy to characterize the SAMs and Ptnanoparticles on the SAMs. For 4-ATP SAMs modifiedelectrode, peaks can be observed on the cyclic voltammo-grams obtained on the electrode, and the peak height isproportional to the scan rate. We also studied outer-sphere and inner-sphere electron transfer reactions on 4-ATP SAMs modified electrode. The 4-ATP SAMs modi-fied electrode is further modified with Pt nanoparticles,our results showed that the Pt nanoparticles did notevenly cover the electrode. Electrocatalysis of the reduc-tion of H+ on the Pt nanoparticles could be observed.

2 Experimental

2.1 Chemicals

Ruthenium hexamine (Ru(NH3)6Cl3, 97%) is purchasedfrom Strem Chemicals (Newburyport, MA). Ferrocene-methanol (FcCH2OH, 97 %) is obtained from Aldrich(Milwaukee, WI). 4-aminothiophenol (98 %) is from AlfaAesar. Aqueous solutions are prepared from deionizedwater (Milli-Q, Millipore Co.). Buffer solution is pre-pared by titrating 0.1 M K2HPO4 solution with H3PO4 toa desired pH value.

2.2 Electrodes

Polished nanometer-sized Au electrodes are prepared asthat in reference 20. Briefly, a 25-mm annealed Au wire issealed in a borosilicate glass capillary (1.0 mm o.d.,0.58 mm i.d.), then, a P-2000 laser pipette puller (SutterInstrument Co., Novato, CA) is used to break the capilla-ry. After pulling, a manipulator is used to move the pip-ette vertically towards the slowly rotating disk of the mi-cropipet beveller (model BV-10, Sutter Instrument Co.)to expose and polish the electrode surface.

2.3 Electrodes Cleaning Procedures

After a polished nanometer-sized electrode is ready, itsradius is checked in 1mM Ru(NH3)6Cl3 buffer solution.Only the electrode on which the cyclic voltammetric re-

sponse is stable and almost has no hysteresis is used to ac-complish further experiments. Then, the electrode isrinsed with deionized water and immersed in hot 1 :3H2O2 (30 %) and concentrated H2SO4 solution (CAU-TION! HANDLE WITH CARE) for 10 minutes. Andthen, the electrode is electrochemically cleaned by cyclingthe electrode potential between 1.2 and �0.3 V (vs. Ag/AgCl) in 0.5 M H2SO4 until the cyclic voltammogramcharacteristic for a clean Au electrode is obtained (seeFigure 1). Finally, the electrode is rinsed with deionizedwater and is dried with flow nitrogen.

2.4 Formation of 4-ATP SAMs on an Au Nanometer-Sized Electrode and Anchoring Pt Nanoparticles ona 4-ATP SAMs Modified Au Nanometer-SizedElectrode

4-ATP SAMs modified nanometer-sized Au electrode isobtained by immersing a clean nanometer-sized Au elec-trode in 10 mM 4-ATP ethanol solution for 8 hours.Then, the electrode is thoroughly cleaned with ethanoland deionized water.

Pt nanoparticles with an average diameter of 5 nmwere prepared as described by Jiang et al. [21]. The 4-ATP SAMs modified Au electrode is immersed in the Ptcolloid solution for 30 min. Finally, the electrode is thor-oughly rinsed with large amount of deionized water.

2.5 Instrumentation and Procedures

Scanning electron microscopy (SEM) is operated on aQuanta FEG-450. The electrode is not coated with con-ductive materials before SEM characterization. Electro-chemical experiments were carried out using an Epsilonpotentiostate with a Low current module (BSAi, WestLafayette, IN). Two-electrode system is used in the wholeexperiment, a 0.25-mm-diameter Ag wire coated withAgCl serves as a reference electrode.

Fig. 1. Cyclic voltammograms obtained on a 47 nm in radiusbare Au nanometer-sized electrode. Electrochemical cell: Ag/AgCl/100 mM NaCl//0.5 M H2SO4 aqueous solution/Au elec-trode; scan rate:100 mV/s

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3 Results and Discussion

3.1 The SAMs of 4-ATP SAMs Modified Nanometer-Sized Au Electrode

3.1.1 Cyclic Voltammetry on 4-ATP Modified Nanometer-Sized Au Electrode

The electrochemical behavior of 4-ATP modified Auelectrode is complicated, basically, three groups of waves(peak A, B and C, we adopt here the system of labelingthe peaks used in References [22,23]) could be observedin the voltammograms depends on the pH of the solutionand numbers of cycling. The height of peak A and B de-creases and the height of peak C increases with the pHvalue of the solution increase [22]. Contradictory infor-mation could be found in References, for example [22]mentioned peak B and C could disappear at solutionpH>6 while [24] said both Peak B and C could be ob-served even when solution pH>9. In our experiment, wecould only observe peak C (see Figure 2). Although thereis no oxidation counterpart of the reduction peak C andthe mechanism for the peak C is not clear [22], peak C isa characteristic peak in the voltammograms obtained ona 4-ATP/Au electrode and is stable at solution pH>6.Since the electrochemical transformation of the 4-ATPSAMs on a Au electrode and the follow-up chemical re-actions are not our concern, peak C could be used to dis-tinguish if there is a layer of 4-ATP SAMs on Au elec-trode or not.

Figure 2 shows the cyclic voltammograms obtained ona 4-ATP monolayer modified nanometer-sized Au elec-trode. The effective radius of the electrode is 85 nm. Thepeak height is proportional to the scan rate (see the insetin Figure 2a), which is the main characteristic expectedfor electron transfer between immobilized redox speciesand electrode surface. However, the oxidation counter-part of peak C is missing, this is consistent with the phe-nomenon observed in References [22,23]. The peak posi-tion of peak C is 0.062 V for solution pH 8, 0.087 V forsolution pH 7 and 0.116 V for solution pH 6. This meansthe peak position negatively shifts by almost 28 mV as thesolution pH is increased by 1 unit (see Figure 2). Thismeans proton involves in the reaction and the ratio ofnumber of electrons transferred in the reaction to thenumber of protons involved in the reaction is 2 :1. Theshift of peak position with respect to the solution pH isalso evidence that the peak we observed is from 4-ATPSAMs rather than from noise. From the integration of thepeak C area in Curve 1 in Figure 2a, one can approxi-mately evaluate that 9.75 �10�14 C charges involves in thereaction. In principle, 3.6 �10�14 C charges is needed toreduce molecules of the SAMs on a perfect 85 nm inradius Au electrode provided that the number of elec-trons transferred in the reaction is 2 and the cross sectionarea of the thiol molecule is 0.2 nm2 [25]. The roughness,which is defined by the ratio of the true to the geometricsurface area, is around 2.7. This value is similar to theone for a macroelectrode [26].

3.1.2 Outer-Sphere Electron Transfer Reaction on 4-ATPSAMs Modified Nanometer-Sized Au Electrode

Jiang et al. [21]and Wang et al. [27] showed that the 4-ATP monolayer on a macro-Au electrode does not oralmost does not affect the reduction of Fe(CN)6

3�. Thismeans the electron could tunnel through the 4-ATP mon-olayer in an outer-sphere electron transfer reaction sinceit is believed that the reduction of Fe(CN)6

3� is an outer-sphere electron transfer reaction [28,29]. If so, the elec-trochemical behaviors of other outer-sphere electrontransfer reactions, like the reduction of Ru(NH3)6

3+ orthe oxidation of FcCH2OH, on a 4-ATP SAMs/Au elec-trode should be the same as that on the Au electrodebefore modification. However, we found the voltammo-

Fig. 2. Curve 1–3 in (a) and curves in (b) and (c) are obtainedin the following electrochemical cell : Ag/AgCl/ buffer solution /4-ATP SAMs modified Au electrode. Curve 4 in (a) is obtainedon the electrode without the 4-ATP SAMs. pH value of thebuffer is 8 (a); 7 (b) and 6 (c). Electrode radius: 85 nm. Scanrates for curves 1 to 4 in (a) are 200 mV/s; 150 mV/s; 100 mV/s;100 mV/s, and is 150 mV/s for curves in (b) and (c). Inset (a):Height of peak C of curves 1–3 vs. their scan rate.

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Electrochemical Studies of Chemically Modified Nanometer-Sized Electrodes

grams of the reduction of Ru(NH3)63+ or the oxidation of

FcCH2OH on a 4-ATP SAMs modified nanometer-sizedAu electrode are eventually stabilized and the limitingcurrent is only a third of that on the bare electrode (seeFigure3 a and b). Also, there is a positive shift of halfwave potentials for the oxidation of FcCH2OH (see Fig-ure 3b). Although the extent of decrease in limiting cur-rent is different for different 4-ATP SAMs modifiednanometer-sized Au electrode, the phenomenon is repro-ducible. According to References [22–24], 4-ATP mole-cules on the electrode are electrochemically dimerized toform 4’-mercapto-N-phenylquinone diimine (NPQD). Itseems that the entire electrode is covered by NPQD mol-ecules because the current of the oxidation of hydrazineon the modified electrode, which is believed to be aninner-sphere electron transfer reaction [28], is very smallcompared with the same reaction on the electrode beforemodification (see Figure 3c).

The SAMs can reduce the rate constant for the reduc-tion of Ru(NH3)6

3+[30], however, it seems that the reduc-tion of Ru(NH3)6

3+ on a 4-ATP SAMs/nanometer-sizedAu electrode is still reversible because the E1/2 of the re-action is the same as that on a bare Au electrode (seecurve 1 and curve 2 in Figure 3a). So, the decrease in cur-rent cannot be totally ascribed to the decrease in rateconstant. From the structure and length of NPQD and 4-ATP[22–24], one can anticipate that 4-ATP moleculesorient normal to the electrode and NPQD moleculeseither orient at an angle from the electrode or just disor-derly cover the electrode. Since electron tunneling canonly accomplished from the head group of 4-ATP orNPQD (=NH2

+) rather than from the side of the mole-cules, this means that the effective electrode area is de-creased after the 4-ATP SAMs has been transformed intoNPQDs. This can also explain why the current of the re-duction of Ru(NH3)6

3+ keeps decreasing and finally is sta-bilized. For a macroelectrode, the 4-ATP monolayers isless compact and pinhole is predominant, so, even NPQDis formed the effective electrode area does not decreasemuch. Thus, the decrease in current on a macroelectrodeis not significant.

The decrease in current for the oxidation of FcCH2OHon a 4-ATP SAMs/nanometer-sized Au electrode can beascribed to the decrease in effective electrode area andthe decrease in reaction rate constant because the E1/2 ofthe reaction has a positive shift compare to that on a bareAu electrode [29] (see the curve 1 and 2 in Figure 3b).

3.2 Attaching Pt Nanoparticles on Nanometer-Sized AuElectrode

The molecules which form 4-ATP SAMs on a Au elec-trode are positively charged [21,22]. Citrate ions protect-ed Pt nanoparticles are negatively charged. So, the Ptnanoparticles could be anchored on a 4-ATP SAMs modi-fied nanometer-sized Au electrode [21]. Since it has beenproved that there is good electronic communication be-tween Pt nanoparticles film and Au metal on a Pt nano-

particles/4-ATP SAMs/ Au electrode,21 there should be anincrease in effective electrode surface area after Pt nano-particles have been attached onto the 4-ATP SAMs modi-fied Au electrode. Thus, the limiting current of the reduc-tion of Ru(NH3)6

3+ on Pt nanoparticles/4-ATP SAMs/ Auelectrode should be larger than that on the electrodebefore anchoring Pt nanoparticles. This can be verified bythe cyclic voltammograms (curve 3 in Figure 3a) which isobtained after the Pt nanoparticles have been attachedonto the 4-ATP SAMs modified nanometer-sized elec-

Fig. 3. Cyclic voltammograms obtained on a 85nm in radius Auelectrode before (curve 1) and after (curve 2) modification with4-ATP SAMs in pH 7 buffer solution with different mediators.Mediators: a) 1 mM Ru(NH3)6

3+ ; b) 1 mM FcCH2OH; c) 5 mMhydrazine . Scan rate 100 mV/s. The curve 3 in (a) is obtainedafter the electrode is further modified with Pt nanoparticles.

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trode. The limiting current of curve 3 in Figure 3a isalmost 1.5 times larger than that on the 4-ATP SAMsmodified nanometer-sized Au electrode, but is muchsmaller than that on the bare nanometer-sized electrode.This phenomenon also means that the electron transferrate on Pt nanoparticles/4-ATP SAMs/Au electrode ismuch smaller than that on the bare Au electrode. It ishard to calculate how many particles are there on theelectrode surface even if we assume that the particles areuniform and 5 nm in diameter, because scanning electronmicroscopic image shows that nanoparticles do not evenlycover the electrode. For example, one can find lumps ofPt nanoparticles in Figure 4, this means that the Pt nano-particles piles up on the electrode. On this point of view,the electrochemical behavior observed on the Pt nanopar-ticles modified electrode cannot represent that on asingle Pt nanoparticle. Even when the radius of the elec-trode is very close or smaller than the diameter of asingle particle, one must be prudent to demonstrate theobservation of single particles attached on a nanometer-sized electrode.

The reductive adsorption and oxidative desorption ofH is one of the characteristics on a Pt electrode. If Ptnanoparticles have been attached, we must observe theelectrochemical catalysis of the reduction of H+ on Ptnanoparticles. This can be verified by the curve 2 inFigure 5, in which there are two small peaks which arethe adsorption and desorption of H (see peak A and A1in the curve 2 in Figure 5) followed by an abrupt currentincrease which is the reduction of H+ to form H2. On a 4-ATP modified nanometer-sized Au electrode, there is nopeaks of the reductive adsorption and oxidative desorp-tion of H.

4 Conclusions

Our results show that nanometer sized Au electrodeswith radii down to 60nm could be chemically modified.The electrochemical behavior at a modified nanometersized electrode is similar to that at a macro electrode.The chemically modified nanometer-sized Au electrodescould be a very useful tool to study the mechanism andkinetics of outer-sphere electron transfer reactions. Sincethe surface area of a nanometer-sized electrode is verysmall, it could be good platform to fabricate very smallsensors or to study the electrochemical behavior of smallamount of metal particles.

Acknowledgements

Support of this work by a New Faculty Start Up Grantfrom East Tennessee State University and an ACS Petrole-um Research Found Grant for Undergraduate New In-vestigator is gratefully acknowledged. The authors thankDave Calvert from Eastman Co. for obtaining the SEMimages. Also, we appreciate Professor M. V. Mirkin fromQueens College for helpful discussions.

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