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Short communication Electroactive self-assembled monolayers: A versatile function to t symmetric voltammetric peak Olivier Alévêque, Eric Levillain Université d'Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou, 2 Bd Lavoisier, 49045 Angers Cedex, France abstract article info Article history: Received 3 December 2014 Received in revised form 17 December 2014 Accepted 18 December 2014 Available online 29 December 2014 Keywords: Self-assembled monolayers Cyclic voltammetry Lateral interactions Generalized Gaussian function We propose a versatile function to t adsorption voltammetric peak in order to extract the characteristic parameters such as the full width at half maximum, the peak potential, the peak current and the surface coverage. Based on the generalized Gaussian function and supported by generalized lateral interaction model, this method has been tested on different electroactive self-assembled monolayers (i.e. ferrocene, bithiophene, tetrathiafulvalene and radical aminoxyl units). © 2014 Elsevier B.V. All rights reserved. 1. Introduction Currentvoltage behaviors of electroactive self-assembled mono- layers (SAMs) can be simple or complex with sharp, normal or broad shapes [1,2] and one of the major obstacles facing those who wish to analyze raw data is how to extract characteristic parameters (E p ,i p , and FWHM) from experimental cyclic voltammograms (CVs). The only alternative to CV peak analysis is often to use the graphical powerful analysis tools of the Turnkey instrument control software (i.e. EC-Lab from Biologic, VersaStudio Software from Princeton Applied Research). Previous works [37] have proposed to t unusual CV peaks via usual or unusual functions such as Gaussian, Lorentzian, or Extreme function but these calculations were either very intricate or not based on a theoretical support. Herein, we propose a versatile method to t experimental peak in order to extract characteristic parameters (E p ,i p , FWHM and Γ) of cyclic voltammograms (CVs), especially for non-ideal voltammograms where no algebraic equation exists. This approach is compared to the extended Laviron's interaction model [8,9] and then to electrochemical data obtained from several electroactive SAMs. 2. Generalized lateral interaction model In a previous work [8,9], we have presented a theoretical study to complete the lateral interaction model proposed by E. Laviron [10], by extending this initial model to non-random distributions of electroactive sites adsorbed on surface. This model enables currentvoltage behaviors to be simulated and allows extracting characteristic parameters of cyclic voltammograms (CVs) obtained from any surface distribution of electroactive SAM. To summarize, the generalized lateral interaction model can be de- ned according to the main following hypotheses [912]: The electroactive centers are distributed on substrate with a unimodal statistical distribution of electroactive neighbors. A pa- rameter ϕ(θ), between 0 and 1, dened for a normalized surface coverage θ, quanties the segregation level of the electroactive centers. For a randomly distributed SAM, ϕ(θ)= θ, and when a seg- regation exists on the surface, ϕ(θ) N θ. The sum of normalized surface coverage θ o and θ R of oxidized (O) and reduced (R) species is constant and equal to θ, The surface occupied by one molecule of O is equal to the surface occupied by one molecule of R, The electrochemical rate constant k s is independent of the coverage, a OO ,a RR and a OR are the interaction constants between molecules of O, molecules of R and molecules of O and R, respectively. a ij is positive for an attraction and negative for a repulsion. For a full reversible reaction (k s ), CVs are reversible, and the characteristic parameters such as full width at half maximum (FWHM), peak potential E p and peak current i p are dened as: E p ϕ θ ðÞ ð Þ¼ E 0 0 þ RT nF Sϕ θ ðÞ ð1Þ Electrochemistry Communications 51 (2015) 137143 Corresponding author. Tel.: +33 241735090; fax: +33 241735405. E-mail address: [email protected] (E. Levillain). http://dx.doi.org/10.1016/j.elecom.2014.12.023 1388-2481/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

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Page 1: Electroactive self-assembled monolayers: A versatile ...okina.univ-angers.fr/publications/ua6658/1/...2015.pdf · Fig. 3. Evaluation of GG function peak fitting on experimental CVs

Electrochemistry Communications 51 (2015) 137–143

Contents lists available at ScienceDirect

Electrochemistry Communications

j ourna l homepage: www.e lsev ie r .com/ locate /e lecom

Short communication

Electroactive self-assembled monolayers: A versatile function to fitsymmetric voltammetric peak

Olivier Alévêque, Eric Levillain ⁎Université d'Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou, 2 Bd Lavoisier, 49045 Angers Cedex, France

⁎ Corresponding author. Tel.: +33 241735090; fax: +3E-mail address: [email protected] (E. Levilla

http://dx.doi.org/10.1016/j.elecom.2014.12.0231388-2481/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 December 2014Received in revised form 17 December 2014Accepted 18 December 2014Available online 29 December 2014

Keywords:Self-assembled monolayersCyclic voltammetryLateral interactionsGeneralized Gaussian function

We propose a versatile function to fit adsorption voltammetric peak in order to extract the characteristicparameters such as the fullwidth at halfmaximum, the peak potential, the peak current and the surface coverage.Based on the generalized Gaussian function and supported by generalized lateral interaction model, this methodhas been tested on different electroactive self-assembled monolayers (i.e. ferrocene, bithiophene,tetrathiafulvalene and radical aminoxyl units).

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Current–voltage behaviors of electroactive self-assembledmono-layers (SAMs) can be simple or complex with sharp, normal or broadshapes [1,2] and one of the major obstacles facing those who wish toanalyze raw data is how to extract characteristic parameters (Ep, ip,and FWHM) from experimental cyclic voltammograms (CVs). Theonly alternative to CV peak analysis is often to use the graphicalpowerful analysis tools of the Turnkey instrument control software(i.e. EC-Lab from Biologic, VersaStudio Software from PrincetonApplied Research).

Previous works [3–7] have proposed to fit unusual CV peaks viausual or unusual functions such as Gaussian, Lorentzian, or Extremefunction but these calculations were either very intricate or not basedon a theoretical support.

Herein, we propose a versatile method to fit experimental peak inorder to extract characteristic parameters (Ep, ip, FWHM and Γ) of cyclicvoltammograms (CVs), especially for non-ideal voltammograms whereno algebraic equation exists. This approach is compared to the extendedLaviron's interaction model [8,9] and then to electrochemical dataobtained from several electroactive SAMs.

2. Generalized lateral interaction model

In a previous work [8,9], we have presented a theoretical study tocomplete the lateral interaction model proposed by E. Laviron [10], by

3 241735405.in).

extending this initialmodel to non-randomdistributions of electroactivesites adsorbed on surface. Thismodel enables current–voltage behaviorsto be simulated and allows extracting characteristic parameters ofcyclic voltammograms (CVs) obtained from any surface distribution ofelectroactive SAM.

To summarize, the generalized lateral interaction model can be de-fined according to the main following hypotheses [9–12]:

– The electroactive centers are distributed on substrate with aunimodal statistical distribution of electroactive neighbors. A pa-rameter ϕ(θ), between 0 and 1, defined for a normalized surfacecoverage θ, quantifies the segregation level of the electroactivecenters. For a randomly distributed SAM, ϕ(θ) = θ, and when a seg-regation exists on the surface, ϕ(θ) N θ.

• The sum of normalized surface coverage θo and θR of oxidized (O) andreduced (R) species is constant and equal to θ,

• The surface occupied by one molecule of O is equal to the surfaceoccupied by one molecule of R,

• The electrochemical rate constant ks is independent of the coverage,• aOO, aRR and aOR are the interaction constants betweenmolecules of O,molecules of R andmolecules of O and R, respectively. aij is positive foran attraction and negative for a repulsion.

For a full reversible reaction (ks → ∞), CVs are reversible, andthe characteristic parameters such as full width at half maximum(FWHM), peak potential Ep and peak current ip are defined as:

Ep ϕ θð Þð Þ ¼ E00 þRTnF

Sϕ θð Þ ð1Þ

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138 O. Alévêque, E. Levillain / Electrochemistry Communications 51 (2015) 137–143

FWHM ϕ θð Þð Þ ¼ 2 RTnF ½ ln 1þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2−Gϕ θð Þ4−Gϕ θð Þ

s

1−ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2−Gϕ θð Þ4−Gϕ θð Þ

s0BBBBB@

1CCCCCA

−G

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2−Gϕ θð Þ4−Gϕ θð Þ

sϕ θð Þ� ≈

Gϕ θð Þb1j jRTnF

2 ln 2ffiffiffi2

pþ 3

� �−3

ffiffiffi2

p

2Gϕ θð Þ

!

ð2Þ

(A)

Ef

f Eb

2Ef

Ef

(B)

f Eb

(C)

2Ef

Fig. 1. A comparison of peak fitting from an ideal CV (n= 1, k = 1000 s−1, E0 = 0.500 V, T =(A) Fit vs. Lorentzian function (3 unknown parameters): E0 = 0.500 V, FWHM= 0.074 V and=0.500V, FWHM=0.095 V and ΓSG=4.85·10−10mol·cm−2. (C) Fit vs. a GG function (4 unknthat E0 = 0.500 V, FWHM = 0.086 V and ΓSG = 5.02·10−10 mol·cm−2 for a GG function at 3

ip ϕ θð Þð Þ ¼ n2 F2vAΓmax

RTθ

2 2−Gϕ θð Þð Þ ð3Þ

with, G ¼ aOO þ aRR−2aOR et S ¼ aRR−aOO Gj jand Sj j≤2ð ÞG and Splay a primordial role and can be defined as “interaction” parametersof O and R, respectively.

The parameter G defines the shape of the peak (FWHM) and thepeak intensity (ip) and the parameter S, the peak potential (Ep) as afunction of θ.

0

2

0

2 .

. 1

SG

EFWHM b

a aihtiwbE Eb abn F S v

2

0

0

2 . . ln 2

e. . 2

E Eb

SG

E

FWHM ba awith i

ba

n F S v

0

0

1

2 . . ln 2

..e

. . 1/2 . . 1/

cc

E Eb

SG

E

FWHM ba c a chtiw ib c b c

an F S v

0

2

0

2 .

. 1

SG

EFWHM b

a aihtiwbE Eb abn F S v

2

0

0

2 . . ln 2

e. . 2

E Eb

SG

E

FWHM ba awith i

ba

n F S v

0

0

1

2 . . ln 2

..e

. . 1/2 . . 1/

cc

E Eb

SG

E

FWHM ba c a chtiw ib c b c

an F S v

293 K, v = 0.1 V·s−1, A = 0.2 cm2, FWHM= 0.089 V and ΓSG = 5.00·10−10 mol·cm−2).ΓSG = 6.15 10−10 mol.cm−2. (B) Fit vs. a Gaussian function (3 unknown parameters): E0ownparameters): E0=0.500 V, FWHM=0.088 V and ΓSG=4.95·10−10mol·cm−2. Noteunknown parameters.

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139O. Alévêque, E. Levillain / Electrochemistry Communications 51 (2015) 137–143

3. The generalized Gaussian function (GG function)

Known as the exponential power distribution, or the generalizedGaussian distribution, this distribution is a parametric family of sym-metric distributions. It includes all normal and Laplace distributions,and in some limiting cases it includes all continuous uniform distribu-tions on bounded intervals of the real line.

Applied to a CV peak, the generalized Gaussian function can beexpressed as:

i E; a;b; cð Þ ¼ a2 � b � Γ 1þ 1=cð Þ e

− E−Epj jb

� �c

with Ep; peakpotentialΓ; theGammafunction

�ð4Þ

Fig. 2. Evaluation of GG function peak fitting on calculated CVs from generalized Lavironmodeltwo phase segregations (G=1 and G=−1; S=−G). (A) (line) calculated CVs for 5.00, 3.95, 2GG function (4 unknown parameters). (B) (line) calculated CVs for 5.00, 3.95, 2.95, 2.20, 1.50, 0unknownparameters). (C) Fitted surface coverage,fitted apparent potential,fitted FWHMandfi

( ) and 4 ( ) unknownparameters, then compared (line) to generalized Lavironmodel (see Eqand fitted peak intensity vs. modeled surface coverage from a GG function peak fit in two cases:see Eqs. (1)–(3) for G = −1.

and the characteristic parameters are defined as:

FWHM ¼ 2 � b � ln 2ð Þ½ �1c ð5Þ

ip ¼ a2 � b � Γ 1þ 1=cð Þ ð6Þ

ΓSG ¼ an FAv

with A; theelectrodeareav; thescanrate

�ð7Þ

In order to avoid confusionwith theGamma function (Γ), the symbolof the surface coverage is named ΓSG (i.e. ΓSG ¼ Γmax � θ).

(n= 1, k= 1000 s−1, E0= 0.500 V, T= 293 K, v= 0.1 V·s−1, A= 0.2 cm2,) in the case of.95, 2.20, 1.50, 0.85, 0.52, 0.25 and 0.05·10−10mol·cm−2 [9] for G=+1. (dotted) Fit vs. a.85, 0.52, 0.25 and 0.05·10−10 mol·cm−2 [9] for G=−1. (dotted) Fit vs. a GG function (4ttedpeak intensity vs.modeled surface coverage fromaGG function peakfit in two cases: 3s. (1)–(3) for G=+1. (D) Fitted surface coverage, fitted apparent potential, fitted FWHM3 ( ) and 4 ( ) unknown parameters, then compared (line) to generalized Lavironmodel

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140 O. Alévêque, E. Levillain / Electrochemistry Communications 51 (2015) 137–143

4. Discussions

4.1. Starting from an ideal case

Let us take the ideal but simplistic case (i.e. Langmuir model andG = S = 0): all adsorption sites are equivalent and there is no interac-tion between immobilized electroactive centers, leading to a CV shapeindependent of the surface coverage. In this particular case, an algebraicequationwas formulated by Laviron [11] to describe the CVs, and allowsus to verify the robustness of fits generally used.

f Eð Þ ¼ n2 F2AvΓmaxθRT

enFRT E−E0ð Þð Þ

1þ enFRT E−E0ð Þð Þ� �2 ð8Þ

Gaussian, Lorentzian and Extreme functions do not match the idealcase (Fig. 1A and B) and astonishingly, some previous works haveused these empirical functions to estimate Ep, ip, FWHM and ΓSGparameters.

Fig. 3. Evaluation of GG function peak fitting on experimental CVs of different electroactive SAchemical parameters deduced of peak fitting: E0 = 0.515 V, FWHM = 0.118 V, ΓSG = 5.33·293 K [13]. Electrochemical parameters deduced of peak fitting: E0 = 0.375 V, FWHM =0.1 V·s−1, at 0.1 V·s−1 and 293 K [15]. Electrochemical parameters deduced of two peak3.12·10−10 mol·cm−2. (D) CV of bithiophene SAM in 0.1 M Bu4NPF6/CH2Cl2 at 0.1 V·s−1, aE01 = 0.700 V, FWHM1 = 0.100 V, E02 = 0.996 V, FWHM2 = 0.039 V, ΓSG = 0.92·10−10 mo(i.e. a1 = a2).

Conversely, the GG function provides a relevant fit (Fig. 1C). None-theless it is important to note that the GG function requires 4 parame-ters and not 3, as Gaussian and Lorentzian functions, and that multipleminima can occur during the refinement because b and c seem to be de-pendent variables.

4.2. Dependence of b and c

The dependence of b and c can be solved by finding a correlation be-tween the generalized lateral interaction model and the GG function.Mathematically, Eqs. (2) and (5) lead to:

FWHM ¼G�ϕ θð Þj jb1 RTnF

2 ln 2ffiffiffi2

pþ 3

� �−3

ffiffiffi2

p

2G � ϕ θð Þ

!¼ 2 � b � ln 2ð Þ½ �1c

ð9Þand Eqs. (3) and (6) to:

ip ¼ n2 F2vAΓmRT

θ2 2−Gϕ θð Þð Þ ¼

a2 � b � Γ 1þ 1=cð Þ ð10Þ

Ms. (A) CV of Tempo SAM in 0.1 M nBu4NPF6/CH2Cl2, at 0.1 V·s−1 and 293 K [8]. Electro-10−10 mol·cm−2. (B) CV of ferrocene SAM in 0.1 M nBu4NPF6/CH3CN, at 0.1 V·s−1 and0.079 V, ΓSG = 3.60·10−10 mol·cm−2. (C) CV of TTF SAM in 0.1 M Bu4NPF6/CH2Cl2 atfitting: E01 = 0.232 V, FWHM1 = 0.123 V, E02 = 0.547 V, FWHM2 = 0.057 V, ΓSG =t 0.1 V·s−1 and 293 K [14]. Electrochemical parameters deduced of two peaks fitting:l·cm−2. Note that the two CV peaks were fitted with two GG functions of the same area

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141O. Alévêque, E. Levillain / Electrochemistry Communications 51 (2015) 137–143

From Eqs. (9) and (10), the parameter b can be expressed as a func-tion of the parameter c via a simple relation and, remarkably, indepen-dent of G·ϕ(θ):

b ¼ 2RTnF

3ffiffiffi2

p−2 ln 2

ffiffiffi2

pþ 3

� �3ffiffiffi2

p� Γ 1þ 1=cð Þ−4 � ln 2ð Þ½ �1c

24

35: ð11Þ

Fig. 4. Comparison of GG function peak fitting and experimental CVs of mixed Tempo SAMs, prNote that 3 and 4 unknown peak fitting parameters were symbolized by ( ) and ( ), respectiExperimental CVs of Tempo SAM in 0.1 M nBu4NPF6/CH2Cl2, prepared from different Tempo:d[8]. Note that these surface coverages were deduced by integration of the voltammetric signalfitted surface coverage. Estimation of S parameter from Eq. (1). (C) Fitted FWHM vs. fitted sursurface coverage. Estimation of ip parameter from Eq. (3). (E) Fitted b and calculated b see Eq. (dispersion of estimated c values in the case of 4 unknown peak fitting parameters.

From Eqs. (9) and (10), a relationship between c and G·ϕ(θ) allowsto estimate the boundary values ( G � ϕ θð Þj jb1) of c, according to:

4−2Gð Þ � ln 2ð Þ½ �1c ¼ 2 ln 2ffiffiffi2

pþ 3

� �−3

ffiffiffi2

p

2Gϕ θð Þ

!� Γ 1þ 1=cð Þ: ð12Þ

A numerical solving of Eq. (12) shows that the parameter c is strictlygreater than 1 (i.e. close to 1.0166, 1.5691 and 2.0182 for G·ϕ(θ) equalto +1, 0 and −1, respectively).

epared with random distributed electroactive centers on Au substrate (see reference [8]).vely and were then compared to results (represented by★) from reference [8]. (A) (line)ecanethiol ratios, leading to 4.7 (i.e. θ = 100%), 3.7, 2.8, 2.1, 1.4 and 0.8·10−10 mol·cm−2

. (dotted) Fit vs. a GG function (4 unknown parameters). (B) Fitted apparent potential vs.face coverage. Estimation of G parameter from Eq. (2). (D) Fitted peak intensity vs. fitted11) vs. fitted surface coverage. (F) Fitted c vs. fitted surface coverage, displaying a greater

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142 O. Alévêque, E. Levillain / Electrochemistry Communications 51 (2015) 137–143

To close this section, theGG function supported by lateral interactionmodel must be expressed as:

i E; a; cð Þ ¼ a2 � b � Γ 1þ 1=cð Þ e

− E−E0j jb

� �c

with

cN1

b ¼ 2RTnF

3ffiffiffi2

p−2 ln 2

ffiffiffi2

pþ 3

� �3ffiffiffi2

p� Γ 1þ 1=cð Þ−4 � ln 2ð Þ½ �1c

24

35:

8><>:

ð13Þ

Note that, for 3 N c N 1, b varies quasi linearly with c and can approx-imated to:

b ≈3NcN1 RT

nF

ffiffiffiffiffiffi2π

p� c−5

3

� : ð14Þ

5. GG function vs. generalized lateral interaction model

First of all, we simulated some CVs from generalized lateral interac-tion model with relevant G and S parameters in extreme cases (i.e.G = −S = +1 and G = −S = −1) with ϕ θð ÞNθ (Fig. 2A and B).Then, each CV shape was fitted with GG function (i.e. 3 and 4 unknownparameters) (Fig. 2C and D). Then, the characteristic parameters (Ep, ip,FWHMand ΓSG) of each CVwere extracted and, finally, plotted against θ(Fig. 2C and D) for comparison to expected values from generalizedLaviron model.

The right agreement betweenmathematical shapes andmodeled CVpeaks in two extreme cases of phase segregation provides evidence ofthe versatility of GG function, for both 3 and 4 unknown parameters.The characteristic parameters are consistent with Laviron's model in awide range of surface coverage and we can notice that the values ofFWHM for a peak fit with 3 parameters are less likely to agree withthe Laviron's model. This slight difference is due to the relationship be-tween b and c (Eq. (11)).

5.1. GG function vs. experimental CVs

It is well-known that the baseline can disturb the extraction ofunknown parameters by peak fitting from experimental data and,

herein, a monotonic polynomial function (i.e. g xð Þ ¼ �∑n

i¼0ai � xi ) was

selected for the baseline approximation in order to circumvent thisproblem.

5.1.1. Different electroactive SAMsFig. 3 shows that GG function peak fitting is successfully extracting

Ep, ip, FWHM and ΓSG of experimental electroactive SAMs (i.e. ferrocene[13], bithiophene [14], tetrathiafulvalene [15] and radical aminoxyl [10]units), equally to one CV peak as to two. In the latter case, the two CVpeaks were fitted with two GG functions of the same area (i.e. a1 =a2 — Fig. 3C and D).

As expected, the usual slight asymmetry of the CV peak of ferroceneSAM leads to a correct but no more result of the peak fitting (Fig. 3B).

5.1.2. Mixed Tempo SAMsThe robustness of the GG function peak fitting was tested through

mixed SAMs (Fig. 4). We opted for mixed Tempo:decanethiol SAMs,preparedby successive dilutions [8]. Such conditions lead to the randomdistributed (i.e. ϕ θð Þ ¼ θ) electroactive centers on Au substrate andpredict a linear dependence of peak potential see Eq. (1) and FWHMsee Eq. (2) with the surface coverage [8].

A right agreement between GG peak and experiment shape wasobserved in the full range of surface coverage (Fig. 4A). The electro-chemical parameters (Ep, ip, FWHM and ΓSG) extracted from GG func-tion peak fitting agree with those obtained in reference [8] via theBiologic EC-Lab Software, although with slight differences (Fig. 4B, Cand D) and the surface coverage dependence of Ep, FWHM and ip pro-vides direct estimations of interaction parameters G and S. We cannotice that 3 or 4 unknown peak fitting parameters produced verysimilar results, although the surface coverages from 4 parameters seemundervalued.

So, what is better for GG function peak fitting of experimental data?It is a GG functionwith 3 unknownparameters because the variations ofc and b with surface coverage from 3 or 4 peak fitting parameters (Fig.4E and F) display a greater dispersion of estimated c values in the caseof 4 unknown parameters.

6. Conclusion

We propose a versatile function to fit adsorption voltammetric peakin order to extract the characteristic parameters such as the full width athalf maximum, the peak potential, the peak current and the surfacecoverage.

This work suggests to fit adsorption voltammetric peak with a 3parameter GG function in order to finely estimate electrochemical pa-rameters. It should be borne in mind that such calculations requireexperimental data with the respect of drastic experimental conditions(pure solvent, supporting electrolyte and compounds, substrate withvery low roughness, ohmic drop compensation …).

Finally, this approach is dedicated to SAMs but it should also apply toa few electroactive layers.

Conflict of interest

The author declare that there is no conflict of interest.

Acknowledgments

All calculations were performed with SigmaPlot™ 12.5.The authors thank Flavy Alévêque for her critical reading of the

manuscript.

References

[1] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Self-assembledmonolayers of thiolates on metals as a form of nanotechnology, Chem. Rev. 105(2005) 1103–1169.

[2] A.L. Eckermann, D.J. Feld, J.A. Shaw, T.J. Meade, Electrochemistry of redox-activeself-assembled monolayers, Coord. Chem. Rev. 254 (2010) 1769–1802.

[3] W. Huang, T.L.E. Henderson, A.M. Bond, K.B. Oldham, Curve-fitting to resolveoverlapping voltammetric peaks — model and examples, Anal. Chim. Acta. 304(1995) 1–15.

[4] M. Palys, T. Korba, M. Bos, W.E. Vanderlinden, The separation of overlapping peaksin cyclic voltammetry by means of semi-differential transformation, Talanta 38(1991) 723–733.

[5] L.Y.S. Lee, R.B. Lennox, Electrochemical desorption of n-alkylthiol SAMs on polycrys-talline gold: studies using a ferrocenylalkylthiol probe, Langmuir 23 (2007)292–296.

[6] L.Y.S. Lee, T.C. Sutherland, S. Rucareanu, R.B. Lennox, Ferrocenylalkylthiolates as aprobe of heterogeneity in binary self-assembled monolayers on gold, Langmuir 22(2006) 4438–4444.

[7] H.H. Tian, Y. Dai, H.B. Shao, H.Z. Yu, Modulated intermolecular interactions inferrocenylalkanethiolate self-assembled monolayers on gold, J. Phys. Chem. C 117(2013) 1006–1012.

[8] O. Aleveque, P.Y. Blanchard, T. Breton, M. Dias, C. Gautier, E. Levillain, F. Seladji,Nitroxyl radical self-assembled monolayers on gold: experimental data vs Laviron'sinteraction model, Electrochem. Commun. 11 (2009) 1776–1780.

[9] O. Aleveque, P.Y. Blanchard, C. Gautier, M. Dias, T. Breton, E. Levillain,Electroactive self-assembled monolayers: Laviron's interaction model extendedto non-random distribution of redox centers, Electrochem. Commun. 12 (2010)1462–1466.

[10] E. Laviron, Surface linear potential sweep voltammetry: equation of the peaksfor a reversible reaction when interactions between the adsorbed molecules

Page 7: Electroactive self-assembled monolayers: A versatile ...okina.univ-angers.fr/publications/ua6658/1/...2015.pdf · Fig. 3. Evaluation of GG function peak fitting on experimental CVs

143O. Alévêque, E. Levillain / Electrochemistry Communications 51 (2015) 137–143

are taken into account, J. Electroanal. Chem. Interfacial Electrochem. 52 (1974)395–402.

[11] E. Laviron, General expression of the linear potential sweep voltammogram in thecase of diffusionless electrochemical systems, J. Electroanal. Chem. InterfacialElectrochem. 101 (1979) 19–28.

[12] E. Laviron, L. Roullier, General expression of the linear potential sweep voltammo-gram for a surface redox reaction with interactions between the adsorbed mole-cules: applications to modified electrodes, J. Electroanal. Chem. InterfacialElectrochem. 115 (1980) 65–74.

[13] P.Y. Blanchard, S. Boisard, M. Dias, T. Breton, C. Gautier, E. Levillain, Electrochemicaltransduction on self-assembled monolayers: are covalent links essential? Langmuir28 (2012) 12067–12070.

[14] O. Aleveque, L. Sanguinet, E. Levillain, Spectroelectrochemistry on electroactive self-assembled monolayers: cyclic voltammetry coupled to spectrophotometry, Electro-chemistry Communications 51 (2015) 108–112.

[15] P.Y. Blanchard, O. Aleveque, S. Boisard, C. Gautier, A. El-Ghayoury, F. Le Derf, T.Breton, E. Levillain, Intermolecular interactions in self-assembled monolayers oftetrathiafulvalene derivatives, Phys. Chem. Chem. Phys. 13 (2011) 2118–2120.