Electrochemistry Study of Permselectivity and Interfacial Electron Transfers of a Branch-Tailed Fluorosurfactant Self-Assembled Monolayer on Gold

We investigated the permselectivity and interfacial electron transfers of an amphiphilic branch-tailed fluorosurfactant self-assembled monolayer (FS-SAM) on a gold electrode by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The FS-SAM was prepared by a self-assembly technique and a “click” reaction. The barrier property and interfacial electron transfers of the FS-SAM were also evaluated using various probes with different features. The FS-SAM allowed a higher degree of permeation by small hydrophilic (Cl− and F−) electrolyte ions than large hydrophobic (ClO4− and PF6−) ones. Meanwhile, the redox reaction of the Fe(CN)63− couple was nearly completely blocked by the FS-SAM, whereas the electron transfer of Ru(NH3)63+ was easier than that of Fe(CN)63−, which may be due to the underlying tunneling mechanism. For hydrophobic dopamine, the hydrophobic bonding between the FS-SAM exterior fluoroalkyl moieties and the hydrophobic probes, as well as the hydration resistance from the interior hydration shell around the oligo (ethylene glycol) moieties, hindered the transport of hydrophobic probes into the FS-SAM. These results may have profound implications for understanding the permselectivity and electron transfers of amphiphilic surfaces consisting of molecules containing aromatic groups and branch-tailed fluorosurfactants in their structures.

In addition to the water CA data analysis, the EIS data were recorded and are summarized in Figure S1 (B). By referring to Xing's method [3], the composition of the binary OEG-SAMs are analyzed and compared to the fractional surface coverage calculated on the basis of the Cassie and Israelachvili equations, in Figure S2. It can be seen in all cases the OEG-CCH contents are higher in monolayers than in the solutions, indicating a stronger tendency for OEG-CCH to assemble on gold. It is assumed the desolvation effect may play a major contribution to this discrepancy due to the higher solvation of OEG-OH in polar ethanol solvent as compared to OEG-CCH. In general, we observed that the Israelachvili equation describes the compositional data obtained from the analysis of the ESI data better than the Cassie equation. This may indicate that the binary SAMs are mixed on very small length scales [4]. The OEG-CCH fraction in the binary OEG-SAMs is found to be ca. 56%, when the assembly solution with OEG-CCH fractions of 25% was used. When N3-Ar-O-C9F17 molecules were graft onto the binary OEG-SAMs with OEG-CCH fraction of 56%, the optimum FS-SAM can be obtained, with the highest At % of F at 13.69 %.

Analysis of the composition of the FS-SAMs
Assuming that all the -CCH crosslinked with the N3-Ar-O-C9F17, the average composition of the prepared FS-SAM can be described as C24.08H32.06F9.52N1.68O5S, as shown in Scheme S1. According to that, the theoretical calculated At % of F in the FS -SAM is 12.93%, which is slightly below the measured value 13.69 % by XPS. This may be due to that the -C9F17 groups protruded from the monolayer surface, thus the photoelectron signals of F atoms are more easily detected in XPS measurements than other atoms in the inner layer. Generally, we think that the "click"reaction efficiency is nearly 100%, and the composition of the obtained FS-SAM is 56% FS and 44% OEG-OH.
Scheme S1. Schematic illustration of the composition of the prepared FS-SAM.
In the absence of electroactive species, the electrochemical impedance Z of a well-organized SAM covered electrode can be interpreted by a simple equivalent circuit (as shown in Figure S3) of the capacitance in series with the solution resistance (Rs) [5,6] . The total impedance (Z) measured in the EIS experiment is Rw, Warburg impedance.
Using the Rct values obtained from the impedance plots, the rate constant value of Fe(CN)6 3-for the binary OEG SAM and FS-SAM modified electrodes were determined as follows [6,7], where θ is the surface coverage of the monolayers on the gold electrodes, Rct 0 is the charge transfer resistance of bare gold electrode, Rct is the charge transfer resistance of the corresponding monolayer modified electrodes, R is the gas constant, T is the temperature, F is the Faraday's constant, n is the number of electrons, c is the concentration of the redox couple, kapp and k0 is the apparent and the real rate constants, respectively.

Synthesis of ω-tetra (ethylene glycol)hexanethiol (OEG-OH)
The OEG-OH 4 was synthesized according to the method reported in the literature [8] and the synthesis of PEG-thiol is outlined in Scheme S2. The synthesis was typically performed as follows: (i) A mixture of 51 mL of tetraethylene glycol 1 (100 mmol), 30 mL THF and 6 mL of 50% NaOH (38 mmol) was heated to 100 o C and stirred for 45 min. Then 9.6 g of 6-bromo-1-hexene (20 mmol) were added to the mixture and refluxed for 16h. The reaction mixture was cooled to room temperature and then 10% HCl solution was added under stirring. The crude product was extracted by ethyl acetate and dried with anhydrous sodium sulfate. Column chromatography (petroleum ether: ethyle acetate, 2:1) afforded pale yellow oily product 2 (31.19 g, yield 88%).

Synthesis of ω-propargyltetra(ethylene glycol) hexanethiol (OEG-CCH)
The OEG-CCH 7 was synthesized according to method reported in literature [9]: (i) The initial synthesis step was coupling of tetraethylene glycol and 6-bromo-1-hexene similar to synthesis of OEG-OH, and the synthesis is outlined in Scheme S3. The next step was the formation of alkyne terminal group. Typically, The synthesized compound 2 (1.38 g, 2.5 mmol) was dissolved in 20 mL of dry CH2Cl2. In an ice place 0.30g of NaH (6.25 mmol) was added to compound 2 and 3-bromo-1-propargyl (0.66 g, 2.75 mmol) was slowly added and the mixture was stirred for 30 min. Stirring was continued for 12 hrs at room temperature. The crude product was extracted with ethyl acetate and dried with anhydrous Na2SO4. The product