Revisiting the determination of full steady-state coverage of redox centers on self-assembled monolayers

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Revisiting the determination of full steady-state coverage of redox centers on self-assembled monolayers

Olivier Alévêque, Pierre-Yves Blanchard, Tony Breton, Marylène Dias, Christelle Gautier, Eric Levillain ⁎

Laboratoire MOLTECH-Anjou, Université d'Angers - CNRS, 2 boulevard Lavoisier 49045, Angers Cedex, France

a b s t r a c t a r t i c l e i n f o

Article history:

Received 25 November 2011

Received in revised form 14 December 2011 Accepted 15 December 2011

Available online xxxx Keywords:

Full steady-state coverage Self-assembled monolayers Cyclic voltammetry Langmuir isotherm

This work establishes an experimental estimation of the full steady-state coverage of redox centers on self- assembled monolayers. The results demonstrate that the assessment of the full surface coverage and the adsorption kinetic from a Langmuir model impose drastic experimental conditions.

1. Introduction

Since the pioneering work by Nuzzo and Allara in 1983[1], self- assembled monolayers (SAMs) of alkanethiols have gained much attention in the interfacial electrochemistry and other researchﬁelds [2].

In 1994, Blanchard et al.[3]have shown that the full steady-state coverage (Γmax) and the adsorption kinetic of alkanethiolate monolayers onto a microcrystalline gold surface could be experimentally estimated on the assumption that the Langmuir isotherm can be used to describe the adsorption reaction. Despite some works based on Langmuir approach[4-6], the experimental determination of the full steady-state coverage of electroactive sites on SAMs stays difﬁcult and is often estimated from a CPK model (Corey–Pauling–Koltun space-ﬁlling model), considering that redox molecules are spheres of a given diameter and assuming a hexagonal closest-packing[7].

Here, we propose to revisit the estimation of the full steady-state coverage of redox centers on SAMs via cyclic voltammetry (CV) and to discuss the experimental difﬁculties to access this value.

2. Experimental

2.1. Compounds and materials

We used redox-responsive TEMPO SAMs, known to be active, stable and providing electrocatalytic activities in both aqueous and

non-aqueous solvents. As previously observed[8–10], CVs of 1, 2 or 3 SAMs exhibit a reversible one-electron process at 0.49 V (vs. Ag/AgNO3) in 0.1 M Bu4NPF6/CH2Cl2 and the shape of CVs depends on surface coverage.

The synthesis and electrochemical characterizations of nitroxyl radical (TEMPO = 2,2,6,6-Tetramethylpiperidine-1-oxyl) derivatives 1,2and3(Scheme 1) were described in Reference[11].

Electrochemical experiments were carried out with a Biologic SP- 300 potentiostat at 293 K. Cyclic voltammetry was performed in a three-electrode cell equipped with a platinum-plate counter electrode. Reference electrodes were Ag/AgNO3 (0.01 M CH3CN). CVs were recorded in dry HPLC-grade methylene chloride (CH2Cl2). Sup- porting electrolyte was Bu4NPF6. Based on repetitive measurements, absolute errors on potentials were found to be approximately ~ 3 mV.

Au substrates were prepared by deposition of ca. 5 nm of chromi- um followed by ca. 50 or 100 nm of gold onto a glass substrate through a shadow mask (MECACHIMIQUE/France) using physical vapor deposition system (PVD ME300 PLASSYS/France) and were made immediately before use (No electrochemical post-treatment was undergone after completion). This Protocol, commonly used in the literature[12], provides, via X-ray experiments, reproducible Au (111) surfaces with high crystallographic quality, low roughness (Ra less than 2 nm, estimated by AFM from lateral scales close to 1.0μm) and with a deﬁned geometry.

Frequency measurements were recorded simultaneously to the cyclic voltammetry with a Quartz Crystal Analyser QCA922 connected to the potentiostat. Considering that no viscoelastic change occurs at the electrode interface, a mass sensitivity of 5 ng·cm⁻²·Hz⁻¹, calculated by integration of the Cu-stripping voltammetric curve of a CuSO4aqueous solution[13], was used to calculate the mass variation

⁎Corresponding author. Tel.: + 33 241735095; fax: + 33 241735405.

E-mail address:[email protected](E. Levillain).

doi:10.1016/j.elecom.2011.12.014

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from the frequency variation. From Reference [3], the time dependence of the frequency shift (ΔF) deduced from a quartz crystal mi- crobalance (QCM) can be estimated to a direct result of the formation of the monolayer. Therefore, the surface coverage (Γ) of the QCM electrode surface is expressed as a unitless quantityθ (i.e.Γ/Γmax) according to

θð Þ ¼t −ΔF tð Þ

ΔF_max: ð1Þ

2.2. Langmuir adsorption isotherm

From Reference [3], the overall adsorption reaction of thiol deriv- ative (R–SH) on Au surface follows

Au_ðSÞþR−S−H_ðSolvÞ⇄Au−S−R_ðSÞþ1

2H2 ðSolvÞ: ð2Þ The surface coverageΓis expressed as the unitless quantityθ, the fraction of available sites that have reacted or, equivalently, the fraction of a compact monolayer (0≤θ≤1). The Langmuir isotherm dic- tates the rate of surface reaction and is given by

dθð Þt

dt ¼k_að1θð ÞtÞCk_dθð Þt ð3Þ whereθis the fraction of surface covered, (1−θ) is the fraction of surface exposed, C is the alkanethiol concentration, and kaand kd

are the association and dissociation constants, respectively. Integra- tion of Eq.(3)yields the time course of the monolayer formation θð Þ ¼t C

Cþ^kk^d_a

1expððk_aCþk_dÞtÞ

½ : ð4Þ

Eq.(4)can be simpliﬁed according to

θð Þ ¼t K⁰½1expðk_ObstÞ with

k_Obs¼k_aCþk_d K⁰¼ C

Cþk_d k_a

: 8>

><

>>

: ð5Þ

At a given concentration, the steady-state coverage is given by θmaxð Þ ¼C θðt→∞Þ ¼ 1

1þCK¹_Eq

with K_Eq¼k_a

k_d: ð6Þ

The free energy of adsorption of the monolayer is found from the equilibrium constant KEq

ΔG_Ads¼ RT Ln K _Eq

¼ RT Ln k_a

k_d : ð7Þ

3. Results and discussion 3.1. Steady-state coverage

To determine the concentration dependence for the steady-state surface coverage, Au/glass substrates were immersed up to 480 h (according to Eq.(5), the lower the concentration is, the more the immersing time is important) in1,2or3dichloromethane solution with a concentration varying from 10⁻¹⁰to 10⁻³M. For each concentration, the electrochemical stability of 1, 2 or 3 SAMs were tested in 0.1 M Bu4NPF6/CH2Cl2 by CV under repetitive cycles and the surface coverages were deduced by integration of the voltammetric signals.

Fig. 1 displays the concentration dependence on the surface coverage for1,2or3SAMs. Assuming that the immersion times agree with steady-state surface coverages (kinetic constants were estimated in Section 3.2), ﬁtting experimental data with the Eq.(6)gives the full steady-state coverage (Γmax) and the equilibrium constant K_Eq (Table 1). The full surface coverage and the equilibrium constant are quasi chain-length (n) independent of the linker. This result suggest that, to minimize the free energy of the organic layer, the molecules adopt conformations that allow high degrees of van der Waals interactions [14] with the neighboring electroactive moieties; these arrangements yield a secondary level of organization in the monolayer, independent of the chain length.

To compare the experimental estimation of the full steady-state coverage to a CPK approach, we have used two procedures

1. Considering that radical nitroxyl molecules are spheres of diameter 6.7 Å, the calculated maximum coverage of1,2and 3SAMs has been estimated to be around 4.6 × 10⁻¹⁰mol·cm⁻², assuming a hexagonal closest-packing [7a],

2. According to the procedure described by Enoki et al. [7b], we ten- tatively assume the shapes of molecules1,2and3to be rectangu- lar blocks, 4.82 × 6.70 Å2 in size, with orthogonal close-packing.

From this assumption, the calculated maximum coverage of1,2 and3SAMs is estimated to be 5.0 × 10⁻¹⁰mol·cm⁻².

Scheme 1.Nitroxyl radical derivatives1,2and3.

Fig. 1.(●) Concentration dependence for steady-state surface coverage, deduced by integration of the voltammetric signal, for SAMs prepared from3in 0.1 M Bu4NPF6/ CH2Cl2, at 293 K. (dash line) Fit of Langmuir adsorption isotherm (Eq.(6)) to raw experimental data.

O. Alévêque et al. / Electrochemistry Communications 16 (2012) 6–9

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The latter is in good agreement with the experimentally obtained maximum coverage, which includes uncertainties caused by the surface area used and the presence of disorder in the monolayer.

3.2. Adsorption kinetics

QCM experiments were carried out to estimate the kinetic constants of the adsorption reaction of1,2and3on Au substrates. At C = 1 mM, the adsorption reaction of1,2or3in CH2Cl2takes up to several minutes to achieve the maximum adsorption rate. The raw data wereﬁtted with Eqs.(5) and (6), and the agreement between theﬁt and data is shown inFig. 2for a representative scan. From Eq.(5)and the values of KEqpreviously obtained (Table 1), the association and dissociation constants, k_a, and k_drespectively, were estimated at 293 K (Table 1). As previously observed [2,15,16], the higher the chain-length (n) is, the more the association constant is high. Indeed, short-chain n-alkanethiols have lower reactive sticking probabilities on gold surfaces than do long chains[17].

After a period of one time constant (τ= 1/kobs), the time course of the monolayer formation (θ(t)/θmax(C)) reaches 1−e⁻¹, i.e. approximately 63% of itsﬁnal (asymptotic) value. From this value, we can estimate the time course of the monolayer formation for very low concentrations of 1,2 or3 solution. From Table 1and for a 1/kd

time constant (for C→0), about 175, 93 and 69 h are required to achieve 63%ﬁnal value ofθmaxfor1,2and3, respectively. According- ly, 480 h experimental immersing time (ca. θ–94% for 1) reaches

agreement with a quasi-steady-state coverage at very low concentration (i.e. 10⁻¹⁰M).

3.3. Experimental difﬁculties

Drastic experimental conditions were required to obtain full steady-state coverages. First, the PVD procedure to elaborate Au substrates must be reproducible in roughness and in area. Note that all attempts to obtain rational results with Au electrodes extensively polished to a smooth, mirror-likefinish were failed. Second, the elab- oration and electrochemical characterizations of SAMs were performed under thermostated conditions because the Langmuir equation relates the coverage of molecules on surface at afixed tem- perature. Third,1,2and3TEMPO thiol derivatives were pure with only trace amounts (b5%) of disulfides in order to do not impede the formation or alter the structure of the SAM[18]. Note that all the compounds were purified using column chromatography to re- move impurities and trace of disulfide. Without this protocol, CVs of SAMs are not reproducible in charge and in shape.

3.4. Distribution of redox centers on surface vs. Langmuir isotherm

In CH2Cl2, the full widths at half maximum (FWHM) for SAMs prepared from several concentrations of 1, 2 or3 solution in 0.1 M Bu4NPF6/CH2Cl2 (Fig. 3) deviates from the expected value (i.e.

~89 mV at 293 K) of an“ideal system”(i.e. all adsorption sites are equivalent and there are no interactions between immobilized electroactive centers) and from a random distribution of redox centers on surface (i.e. Laviron's interaction model—References [11] and Table 1

Kinetic and thermodynamic constants of SAMs prepared from1,2and3in 0.1 M TBAPF6/CH2Cl2at 293 K.

Compound KEq ΔGads Γmax kobs ka kd

(10^{+ 7}M⁻¹) (kcal·mol⁻¹) (10⁻¹⁰mol·cm⁻²) (s⁻¹) (M⁻¹·s⁻¹) (10⁻⁶s⁻¹)

1 (n = 7)

2.0 +/−0.3 −9.79 +/−0.09 5.06 +/−0.07 0.027 +/−0.002 27 +/−4 1.6 +/−0.4

2 (n = 11)

1.7 +/−0.1 −9.70 +/−0.03 4.94 +/−0.05 0.049 +/−0.004 49 +/−7 3.0 +/−0.7

3 (n = 15)

2.0 +/−0.2 −9.79 +/−0.09 5.03 +/−0.05 0.071 +/−0.005 71 +/−11 4.0 +/−1.0

Fig. 3.(●) Normalized steady-state coverage (θ) for anodic full width (dash line = trend curve) at half maximum (FWHM) for SAMs prepared from3in 0.1 M Bu4NPF6/ CH2Cl2, at 293 K. (solid line) Normalized steady-state coverage (θ) for theoretical FWHM for random distribution.

Fig. 2.(●) Time dependence for QCM frequency shift for SAM prepared from 1 mM of1 in 0.1 M Bu4NPF6/CH2Cl2, at 293 K. (solid line) Fit of Langmuir adsorption isotherm (Eqs.(4) and (5)) to raw experimental data. Inset: (●) Concentration dependence for steady-state surface coverage, deduced by integration of the voltammetric signal, for SAMs prepared from1in 0.1 M Bu4NPF6/CH2Cl2, at 293 K. (dash line) Fit of Lang- muir adsorption isotherm (Eq.(6)) to raw experimental data. Note that the drift, quasi linear dependent of time, was mathematically removed.

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[8]). From References[9] and [10], the non-linearity of FWHM vs.θ provides evidence of a segregation phase (based on Frumkin isotherm—see Eq. (8)) of electroactive centers on surface.

At a given concentration, the steady-state coverage from Frumkin isotherm is given by

C K_Eq¼ θðt→∞Þ

1−θðt→∞Þexp −2β

kTΓmax θðt→∞Þ

with k and T have their usual meanings

β¼interaction parameter between adsorbed molecules

ð8Þ

Accordingly, this result suggests the Langmuir isotherm is not the best model to describe the adsorption reaction of1,2or3on gold surface. However, we cannot determine a relevant value of the third parameterβof the Frumkin model byﬁtting experimental data to Eq.(8)because the Langmuir model with only two parametersΓmax

and K_Eq(Eq.(6)) already provides goodﬁts of raw data[19]. Adding a third parameterβwill increase the r-square but it does not mean that the overall curveﬁt will be improved.

4. Conclusion

With the respect of drastic experimental conditions, the adsorption reaction of redox-responsive TEMPO SAMs can be described by the Langmuir isotherm, even if the Frumkin model is probably better for a segregated distribution of redox centers on surface. Results presented here show that the adsorption free energy is clearly independent of the chain length and the rate of adsorption increases mo- notonously with chain length. To the best of our knowledge, it is the ﬁrst work where the full steady-state coverage and the adsorption kinetics are only estimated fromﬁtting electrochemical data.

Acknowledgments

The authors thank the Contrats de Projets Etat Région (CPER 2007- 2013) and the Plateforme d'Ingénierie et d'Analyse Moléculaire (Angers).

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