Voltammetry of Ferrocenated Gold Nanoparticles on a Nanometer-Sized Electrode

DOI: 10.1002/elan.201400024

Voltammetry of Ferrocenated Gold Nanoparticles ona Nanometer-Sized ElectrodePeng Sun,*[a] Tong Sun,[a, b] Samir Anz,[a] Jason Corey,[a] and Yunshi Tan[a]

1 Introduction

A nanometer-sized electrode is a very useful tool for elec-trochemists to study nanoscale reactions [1–7]. Electro-chemical studies which cannot otherwise be accomplishedusing a conventional electrode, such as determining thekinetics of fast electron transfer reactions [1,2], detectingelectrochemical reactions within a single biological cell[3], probing the effect of electric double layer (EDL) onelectrochemical reactions [4–5], studying the electro-chemical behaviors of a single nanoparticle [6], or elec-trochemically sensing a single molecule [7] etc., can beaccomplished with a nanometer-sized electrode.

To date, nearly all electrochemical measurements havebeen conducted using an electrode with a radius largerthan that of the electroactive species being studied. Withthe development of the techniques in fabricating nanome-ter-sized electrodes [2], it is possible to fabricate a nano-meter-sized electrode with a radius that is comparable toor smaller than that of an electroactive supermolecule,such as ferrocenated gold nanoparticles [8–11]. The diam-eter of such a supermolecule can range from 1 nm to10 nm [8–11] and can be regarded as a freely diffusionmolecule[8–10]. The study of electroactive supermole-cules by using an electrode whose radius is comparable orsmaller than electoactive supermolecules is fundamentallyand practically important. First, the study is useful in de-tecting conventional electrochemical theories. The diffu-sion equation (Nernst�Plank equation or Fick�s law),which describes the statistical behaviors of BrownianMotion (BM) for a large amount of electroactive mole-cules, is based on an assumption that the size of an elec-troactive species is negligible [12]. This assumption is trueif the size of the electroactive molecule is much smallerthan that of the electrode. However, when the size of theelectrode is comparable to or smaller than that of theelectroactive molecules, this assumption cannot be made.Thus, the diffusion equation may not be suitable to de-

scribe the electrochemical behaviors of electroactive su-permolecules on a very small electrode. Second, it maybe possible to directly observe stochastic collision currentresulting from the BM of a few amounts of electroactivesupermolecules since these molecules can have multipleelectroactive centers [13,14]. The stochastic collision cur-rent is almost impossible to be observed on a macro- ormicroelectrode [14].

In addition, the study of the electrochemical behaviorsof electroactive supermolecules using a nanometer-sizedelectrode can provide fundamental knowledge for minia-turized sensors. This is because functionalized nanoparti-cles can usually be adsorbed on a nanometer-sized elec-trode under a certain condition [6]. The formed function-alized nanometer-sized electrode can be a good platformfor highly sensitive and selective, and fast responsivesensing interface. This is especially useful since our expe-rience shows that it is difficult to chemically modifya nanometer-sized Pt or Au electrode to form a function-alized nanoscale interface. Indeed, there are very few re-ports on the study of chemically modified nanometer-sized electrodes [15,16]. This is because nanometer-sizedPt and Au electrodes can be dissolved in aqueous solutionwhen their potentials are scanned [6].

We studied ferrocenated gold nanoparticles by usinga nanometer-sized electrode. We found the electrochemi-cal behaviors of ferrocenated gold nanoparticles are relat-ed to the size of the electrode. Electrochemical phenom-enon which cannot be explained by conventional theories

Abstract : So far, almost all electrochemistry theories arebased on an assumption that the size of an electroactivemolecule is negligible. However, this assumption will notbe true if the size of an electroactive molecule is compa-rable to or smaller than the size of an electrode beingused to study the electroactive molecule. In this paper,the electrochemical behaviors of ferrocenated gold nano-

particles have been studied using an electrode witha radius that is smaller than the radius of the particle.This allows for the observation of phenomena whichcannot be explained by conventional electrochemical the-ories. Also, stochastic collision current, which cannot beobserved on a macro- or microelectrode, can be directlyobserved.

Keywords: Nanometer-sized Electrode · Gold nanoparticles · Stochastic collision current

[a] P. Sun, T. Sun, S. Anz, J. Corey, Y. TanDepartment of Chemistry and Biochemistry, California StatePolytechnic UniversityPomona, California, 91768*e-mail: [email protected]

[b] T. SunCurrent address: Department of Chemistry, Queens Collegeat City University of New York

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can be observed if the size of the electrode is smallerthan the size of the electroactive molecules.

2 Experimental

2.1 Chemicals

Ferrocenemethanol (FcCH2OH, 97%) , HAuCl4 ·3H2O,and 6-(mercaptohexyl)ferrocene were obtained from Al-drich (Milwaukee, WI). 1-decanethiol and tetraoctylam-monium bromide and tetrabutylammonium perchlorate(TBAP) were obtained from Alfa Aesar. Trisodium cit-rate dehydrate and NaBH4 were purchased from FisherScientific. Aqueous solutions were prepared from deion-ized water (Milli-Q, Millipore Co.).

2.2 Preparation of Nanometer-Sized Pt Disk Electrodesand Ferrocenated Gold Nanoparticles

Polished nanometer-sized Pt electrodes were prepared asdescribed in [2]. The diffusion-limited current on a nano-meter-sized Pt electrode in the solution of 1 mM Ferro-cenmethanol (FcCH2OH) and 0.1 M KNO3 was used tocalculate the effective radius of the electrode.

1-decanethiol monolayer protected gold nanoparticleswere synthesized by using the method described in [17].To prepare ferrocenated gold nanoparticles, 0.0424 g of 1-decanethiol protected gold nanoparticles and approxi-mately 0.20 g of 6-(mercaptohexyl)ferrocene were dis-solved in 5 ml of toluene, respectively. The two solutionswere stirred vigorously for 24 hours. And then, it isevaporated to 2 mL. Next, 50 mL of absolute ethanol wasadded to the solution and refrigerated for 10 hours toallow ferrocenated gold nanoparticles to precipitate. Thesolution was then filtered to separate ferrocenated goldnanoparticles, and finally, the ferrocenated gold nanopar-ticles were dried in a vacuum at room temperature.

The diameter of gold nanoparticles measured fromtransmission electron microscopy (Philips Tecnai 10, FEICompany, Holland) were 8–12 nm. The method used tocharacterize ferrocenated gold nanoparticles was similarto [11]. There were approximately 126 Fc attached ona nanoparticle with an average molecular weight of ap-proximately 6.5� 104 g mol�1. 10 mM Ferrocenated goldnanoparticles solution (the concentration is not accurate)was prepared by dissolving dry ferrocenated gold nano-particles in CH2Cl2 with TBAP as electrolyte.

2.3 Instruments and Procedure

Axonpatch 200B which is computer controlled via anAD/DA board (USB-6341) was used as the potentiostat.The low pass filter of the potentiostat was set to 1 kHz.The sampling rate was set to 2000 sample/second forcyclic voltammetry and 5000 sample/second for chro-noampermetry. A two electrodes system was used. Thereference electrode was a 1 milimeter in diameter silverwire (AgQRE).

3 Results and Discussion

We studied the electrochemical behavior of Ferrocenatedgold nanoparticles using nanometer-sized electrodes.Both phenomena, which agree and disagree with conven-tional electrochemical theories, were observed.

3.1 Phenomenon which can be Explained byConventional Electrochemical Theories

Figure 1A shows a cyclic voltammogram of ferrocenatedgold nanoparticles on a 6 nm in radius nanometer-sizedelectrode. The voltammogram was obtained soon afterthe electrode was immersed in the solution. As one cansee, the shape is sigmoidal, and the forward and backwardscans nearly overlap. This means there is no adsorption atthe beginning. Fitting the theoretical curve to the experi-mental one shows that the rate constant of the oxidationof ferrocene on ferrocenated gold nanoparticle is around0.6 cm s�1, which is smaller than that for the oxidation offerrocene. The small rate constant can be ascribed to themolecular barrier between the electroactive center, ferro-cene, and gold nanoparticle.

When the cyclic voltammogram was collected after theelectrode had been soaked in a ferrocenated gold nano-particles solution for almost 5 minutes while the electrodepotential kept scanning, we found that a small peak ap-peared in the backward segment (scan from 0.8 V to0.0 V) of the cyclic voltammogram (see Figure 1B). Thepeak became larger when the electrode potential wasscanned for a longer time (see Figure 1C). This meansthat the oxidized ferrocenated gold nanoparticles can beadsorbed on a nanometer-sized electrode. In the forwardsegment (scan from 0.0 V to 0.8 V) of the cyclic voltam-mograms in Figures 1A–C, we did not observe a peak.However, we can observe both oxidation and reductionadsorption peaks if the potential is scanned at a high rate(see Figure 1D), though the oxidation peak is obviouslysmaller than the reduction peak, and there was a suddendecrease in the reduction peak (see Figure 1D). Thismeans ferrocenated gold nanoparticles can only be veryweakly adsorbed on a nanometer-sized electrode. A pos-sible explanation is that the electrode is negativelycharged at lower potential, thus, the positively chargedoxidation form of ferrocenated gold nanoparticles can beadsorbed on the electrode surface. When the positivelycharged oxidized form of ferrocenated gold nanoparticleis reduced, there will be no electric attraction betweenferrocenenated gold nanoparticle and the electrode. Thus,the reduced form of ferrocenated gold nanoparticle canonly weakly be adsorbed on its surface. The adsorption offerrocenated gold nanoparticles is related to the size ofthe electrode. Generally, the smaller the electrode, thelonger the time needed for adsorption to occur.

Since the ferrocenated gold nanoparticles are weaklyadsorbed on a nanometer-sized electrode, a gentle washof the electrode can remove most of the adsorbed fer-rocenated gold nanoparticles. The remaining ferrocenated


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gold nanoparticles on the electrode shows cyclic voltam-mograms which are similar to that on a chemically modi-fied electrode (see Figure 2). The peak height is propor-tional to the scan rate, and the peak position is almost in-dependent of the scan rate (Figures are not shown). The

peak-peak separation is around 20 mV, which is an indica-tion that the adsorption is not a strong chemical adsorp-tion.

3.2 Phenomenon which Cannot be Explained byConventional Electrochemical Theories

The limiting current of the oxidation of ferrocenated goldnanoparticles on electrodes with a radius much largerthan the radius of the nanoparticles is decreased if theconcentration of the electrolyte is increased. This phe-nomenon is the same as that observed on a macroelec-trode [11]. However, when the size of the electrode iscomparable to the size of the nanoparticles, the limitingcurrent on the electrode is almost independent of theconcentration of supporting electrolyte (see Figure 3).

Since the thickness of the electric double layer is typi-cally less than 1 nm and the ferrocenated gold nanoparti-cles is neutral, the decrease in current with the increasein supporting electrolyte cannot be ascribed to the effectof electric double layer or to the migration. The increasein the concentration of supporting electrolyte can in-crease the ionic strength which can enhance the aggrega-tion of ferrocenated gold nanoparticles [18]. This can in-crease the radius of the ferrocenated gold nanoparticle

Fig. 1. (A–D): Cyclic voltammograms obtained on a 6 nm radius Pt electrode in 10 mM ferrocenated gold nanoparticles and 0.01 MTBAP CH2Cl2 solution. Scan rates: 0.05 V/s (for A-C), and 2 V/s (for D). Curves were recorded after the electrode potential wasscanned in the solution for less than 5 minutes (A), or 10 minutes (B), or 15 minutes (C) or more than 15 minutes (D). The gray linein A is the theoretical voltammogram for the oxidation of ferrocene with a rate constant of 0.6 cm/s.

Fig. 2. Cyclic voltammograms were obtained in 0.01 M TBAPCH2Cl2 solution on the electrode with (black curves) and without(gray curves) adsorbed ferrocenated gold nanoparticles. Scanrates (from top to bottom): 20 V/s, 10 V/s, and 5 V/s.


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and thus decrease the diffusion coefficient of the nano-particles since the diffusion coefficient, D, equalskBT/(6phrH), where kB is the Boltzmann constant, T is theabsolute temperature, h is solvent viscosity and rH is theradius of the ferrocenated gold nanoparticle. If the size ofelectroactive particle is much smaller than that of theelectrode, the limiting current on the electrode can be ex-pressed by the following equation:

i ¼ 4nFDcr

where F is the Faraday constant, n is the number of elec-trons transferred per one particle, c is the concentrationof the particles and r is the radius of the electrode. Afteraggregation, the product of n and c should not change,thus, the decrease in D should result in a decrease in thelimiting current on a relatively large nanometer-sizedelectrode. When studying ferrocenated gold nanoparticlesby using an electrode whose radius is smaller than theparticles, there will be a big edge effect on the faradiccurrent. This is because any collision of ferrocenated goldnanoparticles on the electrode surface area withina radius of r+ rH will make a faradic current if the elec-

trode potential is positive enough. Since rH is increasedafter aggregation, the edge effect on the limiting currentwill offset the decrease in the limiting current which re-sulted from a decrease in diffusion coefficient. Thus, lim-iting current on a very small electrode is almost notchanged before and after aggregation.

Figure 4 shows chronoamperometry on a 3nm in radiuselectrode, there should be no adsorption since the mea-surement is accomplished in a short time lapse. The diam-eter of the electrode is smaller than the diameter of theferrocenated gold nanoparticles which is around 8–12 nm(see Figure 4D). One can find in Figure 4B that in fer-rocenated gold nanoparticle solution the interval of thedistribution of the limiting current at excitation voltage of0.8 V is wider compared with that in the blank solution atthe same excitation voltage on the same electrode (seeFigure 4B). Such a difference can be seen in the histo-gram of the distribution of current (see Figure 4C). Thisdifference cannot be ascribed to the instability of the in-strument or systematic error. Because (1) the interval ofthe distribution of the limiting current at 0.1 V (wherethere is no faradic reaction) on the same electrode in dif-ferent solution (blank solution and solution containingferrocenated gold nanoparticles) is the same (figure is notshown); (2) the interval of the distribution of the limitingcurrent at different excitation voltage, such as 0.8 V(where there is faradic reaction) and 0.1 V (where thereis no faradic reaction), on the same electrode in the samesolution also shows a difference; (3) this phenomenon isnot accidentally observed because the same phenomenoncan also be observed at different excitation voltage, suchas 0.7 V, 0.6 V (where there is faradic reaction). Since fer-rocenated gold nanoparticle has multiple electroactivecenters, the spike in current could be attributed to sto-chastic collision of ferrocenated gold nanoparticles ona very small electrode.

4 Conclusions

Our results show that the adsorption of ferrocenated goldnanoparticles on a nanometer-sized electrode is related toits size and the time lapse the electrode potential isscanned in ferrocenated gold nanoparticles solution. Gen-erally, there is no adsorption if the scanning time is lessthan several minutes, and more time is needed until ad-sorption appears if the electrode is very small. The ad-sorption of ferrocenated gold nanoparticles on a nanome-ter-sized electrode is similar to that observed on a macroe-lectrode, and can be explained by conventional electro-chemical theories. However, the electrochemical behav-iors of ferrocenated gold nanoparticles on an electrodewhose radius is smaller than the radius of the particleswill differ from that predicted by conventional electro-chemical theories. There is evidence which shows that sto-chastic collision of ferrocenated gold nanoparticles ona very small electrode can be observed.

Fig. 3. Voltammograms obtained on a 97 nm (A) and 4 nm (B)in radius Pt electrode in 10 mM Ferrocenated gold nanoparticlesand x M TBAP CH2Cl2 solution, x is 0.01 (black curve) or 0.05(gray curve). Scan rate 0.01 V/s. For clearness, only the segmentof the positive scan was shown.


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Fig. 4. (A) cyclic voltammograms obtained on a 3 nm in radius Pt electrode in x M ferrocenated gold nanoparticles and 0.01 MTBAP CH2Cl2 solution, x is 10 mM (gray curve) and 0 M (black curve). (B) Chronoamperometry on the 3nm in radius Pt electrode atdifferent excitation voltage in 0.01 M TBAP CH2Cl2 solution containing 10 (for gray diamond symbols black star symbols), or 0 (forblack diamond symbols) mM Ferrocenated gold nanoparticles. The excitation voltage is 0.8 V (vs. AgQRE) for diamond symbols, or0.1 V (vs. AgQRE) for black star symbols. To ensure that the current is absolutely steady-state current, only current measured aftert=0.6 s is shown. For comparison reasons, diagrams were horizontally moved so that the center of all these diagrams is the same. Allcyclic voltammetry and chronoamperometry were accomplished within 5 minutes after the electrode was immersed in solution. (C)Histogram for the distribution of current for the black and gray diamond symbols in B. (D) Transmission electron microscopy of fer-rocenated gold nanoparticles. The inset shows the histogram of the distribution of diameter of nanoparticles.


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Acknowledgements

Support of this work by an ACS Petroleum ResearchFund for Undergraduate New Investigator (50258-UNI5)and a start up grant from the College of Science at Cali-fornia State Polytechnic University-Pomona is gratefullyacknowledged. The authors thank Judy Whittimore fromCollege of Medicine, East Tennessee State University forobtaining the TEM images.

References

[1] R. M. Penner, M. J. Heben, T. L. Longin, N. S. Lewis, Sci-ence 1990, 250, 1118.

[2] P. Sun, M. V. Mirkin, Anal. Chem. 2006, 78, 6526 –6534.[3] P. Sun, F. O. Laforge, T. P. Abeyweera, S. A. Rotenberg, J.

Carpino, M. V. Mirkin, PNAS, 2008, 105, 443[4] Y. Liu, R. He, Q. Zhang, S. Chen, J. Phys. Chem. C 2010,

114, 10812.[5] D. Krapf, B. M. Quinn, M. Wu, H. Zandbergen, C. Dekker,

Nano Lett. 2006, 6, 2531.[6] P. Sun, F. Li, C. Yang, T. Sun, I. Kady, B. Hunt, J. Zhuang, J.

Phys. Chem. C 2013, 117, 6120[7] P. Sun, M. V. Mirkin, J. Am. Chem. Soc. 2008, 130, 8241 –

8250.

[8] M. J. Hostetler, S. J. Green, J. J. Stokes, R. W. Murray, J.Am. Chem. Soc. 1996, 118, 4212.

[9] S. J. Green, J. Stokes, M. J. Hostetler, J. Pietron, R. W.Murray, J. Phys. Chem. B 1997, 101, 2663.

[10] S. J. Green, J. Pietron, M. Stokes, M. J. Hostetler, R. W.Murray, Langmuir 1998, 14, 5612.

[11] A. Labande, J. Ruiz, D. Astruc, J. Am. Chem. Soc. 2002,124, 1782.

[12] A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fun-damentals and Applications, 2nd ed., Wiley, Chichester2001.

[13] R. J. White, H. S. White, Anal. Chem. , 2005, 77, 214A.[14] I. Cutress, E. J. Dikinson, R. G. Compton, J. Electroanal.

Chem. 2011, 655, 1[15] J. J. Watkins, J. Chen, H. S. White, H. D. Abruna, E. Mai-

sonhaute, C. Amatore, Anal. Chem. 2003, 75, 3962.[16] F. Hoeben, F. S. Meijer, C. Dekker, S. P. J. Albracht, H. A.

Heering, S. G. Lemay, ACS Nano 2008, 2, 2497.[17] M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R.

Whyman, J. Chem. Soc. Chem. Commun. 1994, 801[18] B. D. Smith, J. W. Liu, J. Am. Chem. Soc. 2010, 132, 6300.

Received: March 5, 2014Accepted: March 10, 2014

Published online: April 15, 2014


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Voltammetry of Ferrocenated Gold Nanoparticles on a Nanometer-Sized Electrode