# Alkanethiol self-assembling on gold: Influence of high frequency ultrasound on adsorption kinetics and electrochemical blocking

Florian Roy, Abdeslam Et Taouil, Fabrice Lallemand, Olivier Heintz, Virginie Moutarlier, Jean-Yves Hihn

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To cite this version:

Florian Roy, Abdeslam Et Taouil, Fabrice Lallemand, Olivier Heintz, Virginie Moutarlier, et al.. Alkanethiol self-assembling on gold: Influence of high frequency ultrasound on adsorption kinetics and electrochemical blocking. Ultrasonics Sonochemistry, 2018, 40 (Part B), pp.9-16. 10.1016/j.ultsonch.2017.02.009. hal-02129925

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# Alkanethiol self-assembling on gold: Influence of high frequency ultrasound on adsorption kinetics and electrochemical blocking

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Florian Roy<sup>a</sup>, Abdeslam Et Taouil<sup>a</sup>, Fabrice Lallemand<sup>a</sup>, Olivier Heintz<sup>b</sup>, Virginie Moutarlier<sup>a</sup>,
Jean-Yves Hihn<sup>a,*</sup>

## ARTICLE INFO

**Article history:**

Received 30 November 2016

Received in revised form 30 January 2017

Accepted 9 February 2017

Available online 11 February 2017

**Keywords:**

Self-assembling

Monolayer

Alkanethiol

Gold

Cyclic voltammetry

High frequency ultrasound

## ABSTRACT

Self-assembling of undecanthiol (C11SH) on polycrystalline gold was investigated under two different conditions. The kinetics of C11SH grafting was studied without and under high frequency ultrasound irradiation. Two electrochemical experiments were extensively carried out in order to determine electrochemical surface blocking of adsorbed layers as a function of grafting time: chronoamperometry in-situ monitoring and cyclic voltammetry. Interestingly, the grafting process is highly accelerated under sonication, and C11SH modified substrates of good quality are obtained after 3 h' immersion under ultrasound irradiation. This would allow elaboration of high-quality alkanethiol modified samples within much shorter times. Water contact angle measurements and X-ray Photoelectron Spectroscopy (XPS) confirmed the presence of adsorbed undecanthiol on the gold surface. A very close link between electrochemical blocking, surface hydrophobicity and species chemical grafting was established.

© 2017 Elsevier B.V. All rights reserved.

## 1. Introduction

Chemical grafting of self-assembled monolayers (SAMs) by adsorption of organic molecules on metal substrates has received considerable interest over the last three decades. Simple immersion of the substrate in a solution containing active molecules leads to spontaneous formation of covalent bonds between molecules and metals. The triviality of use and the possibility of fabricating interfaces with a well-defined composition, structure and thickness enable numerous applications in different areas, barriers against corrosion [1,2], adhesion [3,4], and lubrication [5]. To permit such spontaneous and strong grafting molecules, various types of SAMs have been developed with different reactive headgroups depending of metal affinity (e.g., phosphonic [6,7], carboxylic [8,9], or sulfonic acids [10] which can be grafted on titanium [7,11], copper [12] or stainless steel [8,9,13]). In the literature, the system alkanethiol on gold is the most commonly studied. The grafting process on gold has been described as an exothermically homolytic cleavage of the S–H bond [14,15] with formation of H₂ [16]. Thanks to inertness of gold, recent years have witnessed the development of [17,18] biosensor thiolate SAM-DNA. Formation of compact and well-organized monolayers requires long-

term modification (i.e. 16–24 h), which makes their industrial application impossible. To overcome this problem, ultrasound can be used as a grafting kinetics accelerator.

Low frequency ultrasound is frequently used during the SAM grafting process. It can be used for metal cleaning [19] before substrate immersion and to clean the surface after SAM formation to remove physisorbed species or other impurities that may have formed during the immersion process. Few studies have been conducted on the use of ultrasound during SAM grafting [20–22]. An increase in the rate and saturated amount of self-assembled adsorption of aromatic thiolate species (4-pyridinethiolate and phenylthiolate) on polycrystalline gold under low frequency ultrasound irradiation (20 kHz) has been reported by Atobe et al. [21]. Dai et al. showed that low frequency ultrasonic irradiation (40 kHz) formed a very low ionic permeability and almost defect-free structure of dodecanethiol SAMs on polycrystalline gold in a short period of time (15 min) [20]. Even if it is well known that cavitation and mechanical activities are far less aggressive at high frequencies [23–27], no study has ever been reported on formation of SAMs on gold under high frequency ultrasound.

In this paper, grafting of undecanthiol on gold under high frequency ultrasound irradiation was investigated. In the first part, the formation of a well ordered and compact monolayer after 19 h' modification is verified. High frequency ultrasound is then used during grafting, as a tool to decrease modification time. To

<footnote><sup>a</sup> Institut UTINAM, UMR 6213 CNRS, Univ Bourgogne Franche-Comté, 30 Avenue de l'Observatoire, 25009 Besançon Cedex, France</footnote>

<footnote><sup>b</sup> ICB Laboratory, UMR 6303 CNRS, Univ Bourgogne Franche-Comté, 9, Av. Alain Savary, 21078 Dijon Cedex, France</footnote>

<footnote>* Corresponding author.
E-mail address: jean-yves.hihn@univ-fcomte.fr (J.-Y. Hihn).</footnote>
explore this grafting mechanism, the influence of each effect of
ultrasound has been investigated. Characterization of undecanthiol
SAM by water contact angles, X-ray photoelectron spectroscopy
(XPS) and polarization modulation infrared reflection spectroscopy
(PM-IRRAS) was carried out. Cyclic voltammetry and in-situ
chromoamperometry were also used to investigate electron trans-
fers through grafted layers in order to determine covering rates
and study adsorption kinetics.

## 2. Experimental

### 2.1. Chemicals

De-ionized water (Milli-Q, resistivity 18 MΩ·cm) was distilled twice before use. Sulfuric acid (VWR, 98%, 7664-93-9), potassium hexacyanoferrate (II) trihydrate (ALFA AESAR, 98+%, 14456-95-1), sodium sulfate (SIGMA-ALDRICH, 99+, 7757-82-6), ferrocene (ALFA AESAR, 99%, 102-54-5), sodium perchlorate anhydrous (ALFA AESAR, 95%, 7791-03-9), absolute ethanol (ACROS, pure, 64-17-5), 1-undecanthiol, (SIGMA-ALDRICH, 98%, 5332-52-5) were used as received.

#### 2.1.1. SAM preparation

A polycrystalline gold electrode (0.24 cm²) was mechanically polished (grain size 10, 1 and 0.25 µm) and rinsed twice with de-ionized water. The substrate was then cycled in 0.5 M H₂SO₄ solution from 0 to 1.7 V (vs Ag/AgCl) at 0.1 V s⁻¹ for 50 cycles. The real surface area was calculated with the method reported by Dai et al. [20]. The substrate was finally immersed in modification solution (1 mM undecanthiol in absolute ethanol).

### 2.2. Sonochemical cell

High frequency ultrasound irradiation was carried out using
commercial equipment (Meinhardt Multifrequency system
575/856/1135 kHz) at a frequency of 575 kHz. Solution volume
for experiments conducted under ultrasound was 8 mL in a beaker
immersed in 120 ml of water (Fig. 1). Calorimetry measurements
showed a transmitted power of 19 W i.e. 2375 W/dm⁻³ in the reac-
tor inner cell.

### 2.3. Characterization methods

Electrochemical experiments were carried out using an AUTO-
LAB PGSTAT30 potentiostat driven by Nova.10 software. A classical
three-electrode setup was used with Ag/AgCl/KCl 3 M as the refer-
ence electrode (210 mV/SHE), platinum wire as the counter-
electrode and modified polycrystalline gold electrode as the work-
ing electrode (0.24 cm²). Cyclic voltammetry in water was carried
out in 10 mM Potassium hexacyanoferrate (II) with 0.2 M in

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Experimental set-up with an inner embedded into the multifrequency sonoreactor.</figcaption></figure>

sodium sulfate (background salt) in pure water. Ethanol's elec-
trolyte was made up of 1 mM in ferrocene with 0.1 M in lithium
perchlorate in absolute ethanol. The process of in-situ following
of steady state current during grafting is described by Kye and al.
[28,29]. All cyclic voltamograms were performed at 100 mV/s.

XPS was used to measure the elemental composition of ele-
ments. Spectra were acquired using monochromatized Al Kα radi-
ation (1486.6 eV). The X-ray radiation source operated in a vacuum
of 3·10⁻⁹ mbar. Binding energies of core levels were calibrated as a
function of the C1s binding energy set at 284.8 eV, a characteristic
energy of alkyl moieties. Deconvolutions were performed using
mixed Gaussian–Lorentzian curves (80% of Gaussian character).

Polarization Modulation Infrared Reflection Absorption Spec-
troscopy data were collected from a Bruker VERTEX 70 PMA 50
equipped with a liquid-nitrogen-cooled mercury-cadmium-
telluride (MCT) detector and a photoelastic modular PMA: ZnSe.
The infrared light, reaching the sample surface at an angle of 82°,
was modulated between s- and p-polarizations at a frequency of
50 kHz. Signals generated from each polarization (Rs and Rp) were
detected simultaneously by a lock-in amplifier and used to calcu-
late the differential surface reflectivity ΔR/R = (Rp − Rs)/(Rp + Rs).
The spectra were taken by collecting 250 scans with a spectral res-
olution of 2 cm⁻¹.

## 3. Results

### 3.1. Characterization of the C11SH monolayer on polycrystalline gold

To graft well-organized monolayers on gold, a long-term mod-
ification is studied. To evaluate coverage and packing in monolayer
structures, a PM-IRRAS spectroscopic measurement was under-
taken. Fig. 2 shows IR spectra of the C–H stretching region for
C11SH grafting over 19 h. The peak positions of symmetric and
asymmetric vibration frequencies of CH₂ and CH₃ groups give
information on analyzed layer organization. As shown in Fig. 2,
bands appearing at 2849 and 2916 cm⁻¹ correspond to CH₂ asym-
metric and symmetric vibration, respectively (νₐ CH₂ and νₛ CH₂),
and bands at 2871 and 2959 cm⁻¹ are assigned to CH₃ symmetric
and asymmetric vibration frequencies (νₛ CH₃ and νₐ CH₃), which
is in agreement with a well-organized adsorbed alkyl chain [30–
34].

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** 2800–3000 cm⁻¹ region from PM-IRRAS spectrum C11SH gold modified for 19 h in silent condition and 3 h under high frequency ultrasound irradiation.</figcaption></figure>
XPS analyses were carried out on bare gold modified with C11SH. 19 h grafting, and spectra shows the presence of S2p, Au4f and C1s signals, confirming the grafting of C11SH layers on the surface. Fig. 3 shows P2p, Au4f and C1s signals for C11SH modified gold for 19 h' grafting. The C1s signal shows one single peak attributed to aliphatic carbons due to the C11SH alkyl chain at 284.8 eV. Two peaks appear at spectra Au4f, one at 84 eV and the other at 87.7 eV, corresponding to Au4f<sub>7/2</sub> and Au4f<sub>5/2</sub>, respectively. The S2p signal was separated into two components: one centered at 162 eV for thiolate groups chemically grafted onto a metal surface (-S-Metal) and the other centered at 163.5 eV for free thiol groups (-S-H) only physically adsorbed on the surface. For 19 h' grafting, the ratio S<sub>chemisorbed</sub>/S<sub>total</sub> was found to be 0.90. As no other peak appears at 169 eV, the presence of oxidized sulfur species at the surface is rejected.

The ferrocene system was used as an electrochemical probe in ethanol solution in order to monitor electron transfers between the modified electrode and the solution. Fig. 4 shows the cyclic voltammetry result of bare and C11SH modified electrode. Due to long insulating alkyl chains, current is much lower for covered gold, leading to the complete disappearance of redox peaks and a relatively flat signal. This confirms the grafting as a well-blocking homogeneous layer.

For a more thorough investigation, an electrochemical in-situ grafting follow-up was carried out using chronoamperometry.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Cyclic voltammetry in (10 mM Ferrocene + 0,1 M LiClO<sub>4</sub>) ethanol solution for bare gold and gold modified with C11SH for 19 h. Scan rate: 100 mV/s.</figcaption></figure>

The gold electrode was immersed in 9 mL of ethanol solution containing 0.1 M of LiClO<sub>4</sub> and 10 mM of ferrocene. A potential of 0.46 V/ECS was applied to enable ferrocene oxidation. The current

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** S2p, Au4f and C1s signals from XPS analysis for gold samples modified with C11SH for 19 h in silent condition and 3 h under ultrasound irradiation.</figcaption></figure>
response decreases until it reaches a stable value (due to diffusion layer stabilization). At t = 0, 1 mL of ethanol solution with 10 mM of C11SH, 10 mM of Ferrocene and 0.1 M of $LiClO_4$ was injected into the cell. Fig. 5 shows the chronoamperometric response all along self-assembling of C11SH. In silent condition (solid line), a peak appears right after injection of the C11SH solution due to the agitation generated. As described by [28,29], the current response decreases with time in two steps: a fast decrease corresponding to rapid adsorption of C11SH on the surface, and a slow decrease, due to the long rearrangement adsorbed layer to obtain a well-organized and packed coating. After about 10 h, the current tends towards stable low values showing the slowing down of the auto-organization process.

## 3.2. Effects of high frequency ultrasound irradiation on C11SH grafting

Ultrasound is known to promote and accelerate chemical reactions [35–38]. Therefore, the effects of high frequency ultrasound during C11SH surface modification were investigated. For this purpose, the same electrochemical methods were used. Modification times of more than 3 h under sonication were not possible due to fast heating of the ultrasonic transducer, which could damage it. For more thorough investigations of the effects of ultrasound irradiation on C11SH grafting on bare gold, water contact angle measurements as well as spectroscopic characterization (especially XPS) were carried out.

Fig. 5 (dashed line) shows chronoamperometry experiments for modifications carried out under high frequency ultrasound irradiation. The description and characterization of the sonochemical cell are given in the experimental section. Before C11SH injection, the current response of bare gold is higher than in silent condition due to agitation generated by ultrasound activity. After C11SH injection, a current peak is observed due to the agitation generated in the solution, as it was the case in silent condition. However, the decrease in current is much faster under ultrasound, thus indicating faster C11SH adsorption on gold. Stabilization of current is observed after about 2 h 30 min, and the current value reached once the monolayer organization has been attained is low and take place as early as 3 h under ultrasound while it took about 8 h in silent condition.

To study the electrochemical blocking rate, cyclic voltammetry was used at different grafting times, with and without ultrasound. C11SH layers were grafted onto gold in an absolute ethanol solution

(1 mM in C11SH) for 9 min, 1, 2 or 3 h under high frequency ultrasound irradiation. For comparison purposes, they were also grafted in silent condition for 15 min, 3, 7 and 19 h. The charge quantity (Q) passed during the experiment can be compared before and after grafting so as to define an electrochemical blocking rate (τ) given by the formula:

$$ \tau(\%) = \frac{Q \text{ before grafting} - Q \text{ after grafting}}{Q \text{ before grafting}} \times 100 \quad (1) $$

Fig. 6 shows the cyclic voltammetry of C11SH modified bare gold samples in an aqueous solution of (5 mM $Fe(CN)_6^{4-}$ + 0.2 M $Na_2SO_4$) for different modification times. Table 1 reports electrochemical surface blocking rates for the different samples. Before grafting, a reversible redox signal on bare gold is observed, as expected, due to oxidation/reduction of the $Fe(CN)_6^{4/3-}$ complex. In silent condition (Fig. 6A), after surface modification, changes in the $Fe(CN)_6^{4/3-}$ electrochemical signal are observed as a consequence of electron transfer hindering. After only 15 min of immersion in the C11SH solution, the disappearance of the reduction peak is observed. Although the oxidation peak is still present, an increase in oxidation potential as well as a more sluggish signal suggests slower kinetics compared to bare gold. This trend is amplified for 3 h' grafting, giving an electrochemical blocking rate of 46%. When immersion time is increased to 7 h, surface blocking increases to about 67% (Table 1). The electrochemical signal of the sample shows the disappearance of oxidation and reduction peaks within the electrochemical window, due to the insulating properties of the deposited C11SH monolayer. As seen in chronoamperometry experiments, the electron transfer process is attenuated with grafting time until it reaches its maximum after 19 h' modification. The electrochemical blocking rate of 97%, read after this time, proves the grafting of compact and well-ordered monolayers.

<figure><img src="image_8.png" /><figcaption>Fig. 5. Chronoamperometry at 0.46 V SCE in 9 ml of ethanol with 10 mM of Ferrocene 0.1 M $LiClO_4$. One ml of 10 mM of undecanthiol, 10 mM of Ferrocene and 0.1 M of $LiClO_4$ have been injected at t = 0: in silent condition and under ultrasound irradiation.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Cyclic voltammetry in (10 mM $Fe(CN)_6^{4-}$ + 0.2 M $Na_2SO_4$) aqueous solution for bare gold and gold modified with C11SH in silent condition (A) for 15 min, 3 h, 7 h, 19 h and under high frequency ultrasound irradiation (B) for 9 min, 1 h, 2 h, 3 h. Scan rate: 100 mV/s.</figcaption></figure>
<caption>**Table 1**</caption>

Electrochemical blocking rates of gold modified with C11SH for 15 min, 3 h, 7 h and 19 h in silent condition and for 9 min, 1 h, 2 h and 3 h under high frequency ultrasound irradiation (evaluated from charge quantity delivered during cyclic voltammetry experiments before and after modification (Eq. (1)).

<table><thead><tr><th colspan="4">Electrochemical blocking rates (±3%)</th></tr><tr><th>Modification times</th><th>Silent condition</th><th>Modification times</th><th>Sonication</th></tr></thead><tbody><tr><td>15 min</td><td>16%</td><td>9 min</td><td>20%</td></tr><tr><td>3 h</td><td>46%</td><td>1 h</td><td>34%</td></tr><tr><td>7 h</td><td>88%</td><td>2 h</td><td>55%</td></tr><tr><td>19 h</td><td>97%</td><td>3 h</td><td>97%</td></tr></tbody></table>

Fig. 6B shows the same experiments for grafting carried out under high frequency ultrasound irradiation. Atobe et al. as well as Dai et al. have demonstrated that the adsorption rate of thiol species is increased under low frequency ultrasound irradiation [20,21]. Indeed, in Atobe's work, adsorption of carboxy ethane thiolate reaches a maximum adsorption amount of about 85.10<sup>-11</sup> mol/cm<sup>2</sup> independently from ultrasound use. However, under sonication this amount is reached much faster than in silent conditions (0.1 h vs 2 h). The authors attributed this behavior to promotion of mass transport by high-speed micro-jet stream generated by ultrasonic cavitation.

Our work shows that grafting of C11SH on polycrystalline gold is also possible under high frequency sonication, as shown by cyclic voltammetry experiments. Indeed, all modified samples show a marked decrease in electroactivity, as a result of the C11SH layers grafted on the surface. Just as for modification in silent condition, coating under sonication shows a decrease in electrochemical response over time. When comparing results obtained from samples modified 9 min under ultrasound irradiation with samples immersed 15 min in silent condition, the sonicated layers are observed to be more covering. The electrochemical blocking rate is 16% after 15 min' modification in classical conditions, while it already reaches 20% for 9 min' immersion under ultrasound irradiation. After 1 h modification under sonication, the electrochemical blocking rate has increased to 34% and keeps increasing to 55% after 2 h. After 3 h' grafting, results are very different for classical monolayers and irradiated ones. Indeed, in silent condition, the redox signal is still great with an electrochemical blocking rate of 46%, while it reaches 97% for sonicated monolayers. In silent condition, a modification time of 19 h is necessary to reach such a blocking rate.

Water contact angle measurements were carried out to appraise surface hydrophobicity. A more marked hydrophobic behavior is expected after surface modification by C11SH grafting due to presence of the alkyl chain. Table 2 shows results for C11SH modified gold samples for different modification times with and without ultrasound irradiation. Bare gold led to an angle of 68°, while adsorbed C11SH caused it to rise to more hydrophobic values. Water contact angle measurements confirm the presence of grafted C11SH layers in all cases, whether or not ultrasound is used. Moreover, it is interesting to note that contact angle measurements follow the same trends as electrochemistry results.

Indeed, in silent condition, contact angles increase quickly during the first 7 h' modification as they move from 96° to 101°. After this period, the contact angle still increases but slowly to reach 102° at 19 h. Under high frequency ultrasound irradiation, contact angles progressively increase during the 3 h' modification from 93° to 103°. These results show that the same surface hydrophobicity behavior is obtained after 19 h grafting in silent condition and after 3 h grafting under ultrasound irradiation.

PM-IRRAS analyses were also conducted. Fig. 1 (dashed line) shows PM-IRRAS spectra for C11SH SAM grafted for 3 h under sonication. The stretching band positions of methylene symmetric and antisymmetric at 2850 cm<sup>-1</sup> and 2918 cm<sup>-1</sup>, respectively, indicate a trans zig-zag extended conformation associated with solid crystalline-phase C11SH (corresponding to a well-organized monolayer). Symmetric vibration frequencies (v<sub>a</sub> CH<sub>3</sub> and v<sub>s</sub> CH<sub>3</sub>) must tend to 2955 and 2871 cm<sup>-1</sup>, respectively, for well-organized monolayers. Obtained spectra show vibration frequencies of v<sub>a</sub> CH<sub>2</sub> = 2917 cm<sup>-1</sup>; v<sub>a</sub> CH<sub>3</sub> = 2955 cm<sup>-1</sup>; v<sub>s</sub> CH<sub>2</sub> = 2849 cm<sup>-1</sup>; v<sub>s</sub> CH<sub>3</sub> = 2870 cm<sup>-1</sup>. These values are consistent with a high degree of organization.

Fig. 2 shows a comparison of S2p, O1s, Au4f and C1s signals from XPS analysis for gold samples modified with C11SH for 19 h in silent condition and 3 h under ultrasound irradiation. For both grafting methods, C1s and Au4f spectra show a similar peak appearance with a similar intensity. O1s spectra grafting of 3 h in sonication shows a weak peak at 532.6 eV corresponding to the presence of some oxygen traces at the gold surface. However, only peaks present in S2p spectra after 19 h' grafting in silent condition appear in S2p spectra after 3 h' grafting under ultrasonic irradiation. Therefore, no oxidized sulfur species are created during grafting under sonication and the O1s peak is certainly due to the few oxygen residues present at the surface. Table 3 shows the role played by chemisorbed species for 19 h' grafting in silent condition and 3 h' grafting under high frequency ultrasound irradiation by displaying the ratio of chemisorbed—S—Metal groups on the total S2p signal (S<sub>chemisorbed</sub>/S<sub>total</sub>). For samples elaborated in sonication, this ratio reaches 0.88, which is close to that found for 19 h in silent condition. Table 3 also shows the ratios of normalized area, separately for C1s and S2p, versus that for Au4f. The C1s/Au4f ratio provides information on the packing densities of the monolayer. For grafting under sonication, this ratio was found to be 0.96, while it drops to 0.87 for grafting in silent condition, suggesting that compactness is lower in the second case. However, both grafting methods show the same S2p/Au4f ratio, which indicates similar packing densities. XPS results confirm that similar covered monolayers are obtained for grafting for 19 h in silent condition or 3 h under ultrasonic irradiation.

### 3.3. Discussion on the effects of high frequency ultrasound

Ultrasound generates numerous effects when propagated in solution. The aim of this section is to determine which effects are the main reasons for such acceleration of the SAM grafting process. First, propagation of transductor vibrations to the solution leads to heating, which can accelerate the monolayer grafting process. Nevertheless, high frequency ultrasound transmits little caloric energy

<caption>**Table 2**</caption>

Water contact angles of gold modified with C11SH for 15 min, 3 h, 7 h and 19 h in silent condition and for 9 min, 1 h, 2 h and 3 h under high frequency ultrasound irradiation.

<table><caption>**Table 3**</caption><thead><tr><th colspan="4">Contact angles (±1°)</th></tr><tr><th>Modification times</th><th>Silent condition</th><th>Modification times</th><th>Sonication</th></tr></thead><tbody><tr><td>15 min</td><td>96°</td><td>9 min</td><td>93°</td></tr><tr><td>3 h</td><td>99°</td><td>1 h</td><td>97°</td></tr><tr><td>7 h</td><td>101°</td><td>2 h</td><td>100°</td></tr><tr><td>19 h</td><td>102°</td><td>3 h</td><td>103°</td></tr></tbody></table>

XPS signals ratio of S—Metal groups on total S2p signal, ratio of C1s/Au4f and ratio of S2p/Au4f for C11SH samples obtained for 19 h in silent condition and 3 h with ultrasound irradiation.

<table><thead><tr><th>Grafting times</th><th>S<sub>chemisorbed</sub>/S<sub>total</sub></th><th>C1s/Au4f</th><th>S2p/Au4f</th></tr></thead><tbody><tr><td>19 h</td><td>0,9</td><td>0,87</td><td>0,058</td></tr><tr><td>3 h with US</td><td>0,88</td><td>0,96</td><td>0,058</td></tr></tbody></table>
(in comparison to low frequency), and the air flow used in all experiments is sufficient to stop temperature increase. Another important phenomenon induced by ultrasound is acoustic streaming, which promotes agitation within the solution and thereby increases mass transfer. In a general manner, high frequency waves are strongly attenuated (proportional to $1/f^2$) [39]. Therefore, high frequency ultrasound generates greater convective flow than low frequency [40]. In the same time, the distribution of the acoustic energy is known to produce more chemical effects, easily highlighted by luminol experiments and dosimetry [41,42]. On the contrary, low frequency ultrasound generates a more violent cavitation, as quoted by previous works [43,44].

First, it is interesting to see whether rapid formation of C11SH monolayers under ultrasound is due to the simple agitation effect. Mass transfer experiments (Fig. 7) were carried out through polarization curves in an ethanol solution of 10 mM of ferrocene with 0.1 M of lithium perchlorate at 1 mV/s under ultrasound and at different rotation speeds. An equivalent level of mass transfer compared to that under sonication was found for 300 rotations per minute (rpm). To compare grafting, in silent condition, under sonication and in ultrasound similar agitation condition (300 rpm), a cyclic voltammetry study was carried out. Fig. 8 shows the cyclic voltammetry of C11SH modified bare gold samples in an aqueous solution of (5 mM Fe(CN)₃⁻ + 0.2 M Na₂SO₄) for 3 h in silent condition, under mechanical agitation (300 rpm) and under high frequency ultrasound irradiation. Table 4 reports the electrochemical surface blocking rates for the different samples. It is the lowest for grafting under mechanical stirring (13%). It seems that agitation slows down self-assembling of C11SH. To verify this, the same experiment was carried out for an immersion time of 7 h in agitated and silent condition. An electrochemical blocking rate of 52% was found for grafting under mechanical stirring, while the same rate was 88% in silent condition. This confirms that agitation only decreases and disfavors SAM grafting process. Therefore, comparison between high frequency ultrasound and mechanical stirring at the same convective conditions seems to point out that chemical effects may be the most important contribution from high frequency irradiation. Indeed, organothiols grafting on gold happens through homolytic fission of SH bonds to form sulfur radicals that will interact and bond to the metal surface. Thus, production of radical species by ultrasound activity can very likely enhance this process through different radical chemistry mechanisms.

<figure><img src="image_10.png" /><figcaption>Fig. 7. Mass transfer in an ethanol solution of 10 mM of ferrocene with 0.1 M of lithium perchlorate for bare gold at 1 mV/s under ultrasound and at different rotation speeds.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Cyclic voltammetry in (10 mM Fe(CN)₄⁻ + 0.2 M Na₂SO₄) aqueous solution for gold modified with C11SH for 3 h in silent condition under mechanical agitation (300 rpm) and under high frequency ultrasound irradiation. Scan rate: 100 mV/s.</figcaption></figure>

<caption>Table 4</caption>

Electrochemical blocking rates of gold modified with C11SH in silent condition, under mechanical agitation (300 rpm) and under high frequency ultrasound irradiation (3 h to 7 h, evaluated from charge quantity delivered during cyclic voltammetry experiments before and after modification (Eq. (1)).

<table><thead><tr><th colspan="4">Electrochemical blocking rates (±3%)</th></tr><tr><th>Modification times</th><th>Silent condition</th><th>Mechanical agitation</th><th>Sonication</th></tr></thead><tbody><tr><td>3 h</td><td>46%</td><td>13%</td><td>97%</td></tr><tr><td>7 h</td><td>88%</td><td>52%</td><td></td></tr></tbody></table>

As a whole, ultrasound irradiation improves SAM elaboration process and permits grafting of high quality layers much faster than in silent conditions. Low frequency ultrasound enables very rapid formation of dense and covering monolayers. High frequency irradiation also leads to good SAM formation but within longer times, compared to low frequency. This was attributed to higher convection effects slowing down the process and to important chemical effects accelerating monolayer grafting through radical mechanisms. Moreover, to our opinion “collateral effects” present at low frequency (i.e. high temperature increase and electrode surface damaging) justifies the study and use of high frequency irradiation in such applications.

## 4. Conclusion

This work aimed to investigate different aspects of self-assembling of 1-undecanthiol on polycrystalline gold. An original grafting technique based on the use of high frequency ultrasound was proposed. Electrochemistry was the main technique used for characterization of modified substrates. Fe(CN)₆⁻³⁻ was used as a redox probe in order to study electronic exchanges between the modified gold and the solution. This technique was found to be very powerful for determining electrochemical surface blocking rates of adsorbed layers. Water contact measurements and XPS results followed exactly the same trends as the results obtained from electrochemical methods. The main conclusion is that surface modification under ultrasound irradiation is judicious as the grafting process is accelerated and well-covered functionalized substrates are obtained after only 3 h' modification, whereas 19 h are necessary in silent condition. Cavitation activity was found to be the trustworthy mechanism in grafting processes activation, in opposition to temperature increase and acoustic streaming. Future works will focus more thoroughly on grafting mechanisms occurring under sonication. In order to do so, two paths should
be explored. Firstly, the influence of binding energies of SAMs faced to ultrasound exposure would be followed to investigate the importance of SAM/substrate interactions during ultrasound irradiation. This could be reached by varying the nature of organic layer and substrate to sweep a high range of binding energies. Secondly, orientation and structuration of the organic tail (e.g. carbon chain) should be determined by combining various techniques such as PM-IRRAS and grazing X-ray diffraction. This should allow validation of mechanistic adsorption sequences already proposed in literature for certain kinds of molecules and its extension to other larger molecules and systems.

## Acknowledgments

The authors would like to thank Franche-Comté region for supporting a PhD grand. We thank UTINAM technical platform, Besançon France and ICB Laboratory, Dijon France, for the access to equipment for XPS analyses.

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