Electrochemistry study of permselectivity and interfacial electron transfers of a branch-tailed fluorosurfactant self-assembled monolayer on gold

Article abstract of DOI:10.3390/molecules23112998

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

Full text of DOI:10.3390/molecules23112998

molecules
Article
Electrochemistry Study of Permselectivity and
Interfacial Electron Transfers of a Branch-Tailed
Fluorosurfactant Self-Assembled Monolayer on Gold
Shanshan Li 1,
*
, Qingying Luo 1, Zhiqing Zhang 1, Guanghui Shen 1, Hejun Wu 1,
Anjun Chen 1, Xingyan Liu 1, Meiliang Li 1 and Aidong Zhang 2,
*
1
College of Food Science, Sichuan Agricultural University, Ya’an 625014, China;
cherry12112009@163.com (Q.L.); zqzhang721@163.com (Z.Z.); shenghuishen@163.com (G.S.);
hejunwu520@163.com (H.W.); anjunc003@163.com (A.C.); lxy05@126.com (X.L.); liml@sicau.edu.cn (M.L.)
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry,
Central China Normal University, Wuhan 430079, China
2
*
Correspondence: lss@sicau.edu.cn (S.L.); adzhang@mail.ccnu.edu.cn (A.Z.); Tel.: +86-155-2700-9697 (S.L.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Received: 25 October 2018; Accepted: 15 November 2018; Published: 16 November 2018
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: 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
−
3−
3+
large hydrophobic (ClO4 and PF6−) ones. Meanwhile, the redox reaction of the Fe(CN)6
couple was nearly completely blocked by the FS-SAM, whereas the electron transfer of Ru(NH3)6
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.
Keywords: branch-tailed fluorosurfactant; self-assembled monolayer; amphiphilic surface;
permselectivity; electron transfer; “click” reaction
1. Introduction
Fluorosurfactants (FSs) are synthetic organofluorine chemical compounds that possess multiple
fluorine atoms. Their molecular structure displays a fluorinated hydrophobic “tail” and a hydrophilic
“head”, wherein the hydrophobic part consists of a fluorocarbon group and the hydrophilic part
consists of a polyoxyethylene chain [1]. FSs are well known for their unique physicochemical
properties, such as high surface activity, excellent high temperature and chemical resistance, as
well as hydrophobic and oleophobic properties, that provides their high suitability for various field
applications [
applications concerning antifogging [
of electrodes [ ] and sensors [ ]. Consequently, properties regarding FS-modified surfaces have
2
,3]. For example, FSs are widely used to change the hydrophobicity of surfaces in
4
], antimicrobial [ ], oil-water separation [ ], and preparation
5
6
7
8
attracted increasing attention and have been widely investigated in different fields, especially in the
electrochemical field [9–11].
Molecules 2018, 23, 2998; doi:10.3390/molecules23112998
www.mdpi.com/journal/molecules

Molecules 2018, 23, 2998
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Great efforts have been devoted to electrochemical studies for in-depth understanding of
fundamental structure–property relationships of FS-modified surfaces. For instance, Chen et al. [12,13]
investigated applications of FS-modified surfaces in electroanalysis. They found that the electro-oxidation
of L-cysteine and tertiary amines becomes more facile at the FS-modified gold electrode. However, most
of the reports focused on straight-chain FS-modified surfaces. No direct observations of interfacial
phenomena of branch-tailed FS-modified surfaces have been reported. Strengthening the research
on branch-tailed FS-modified surfaces and the correlation between molecular structure and surface
properties, especially in foundational electrochemical fields, is thus needed.
As an attractive way to functionalize solid surfaces, self-assembled monolayer (SAM) technology
offers a unique tool for constructing well-defined, stable structures on surfaces with controlled
physicochemical characteristics for a variety of applications [14,15]. Most of the abovementioned
investigations are based on the SAM technology. The “click” reaction is one of the most versatile
and beneficial chemistry tools in surface modifications [16]. Previous reports showed that this
chemoselective and site-specific modification technique can be implemented in the fabrication
of SAM-modified electrodes via the triazole-forming “click” reaction between azide-terminated
SAM-modified electrodes and alkynyl-terminated molecules [17,18]. In this work, a branch-tailed
FS-SAM has been prepared on a gold electrode surface by using the SAM technology and the “click”
reaction, as shown in Scheme 1, and was used in a comparative study of various supporting electrolyte
ion penetration in the monolayer. Furthermore, the effect of the FS-SAM system on the electrochemical
behavior of various probes with different features was also studied.
Scheme 1. Schematic illustration of the fabrication of the FS-SAM on gold.
For fabricating the FS-SAM on gold surface, we first mixed
ω-tetra (ethylene glycol) hexanethiol
(HS(CH2)6(OCH2CH2)4OCH2-C CH, OEG-CCH) and -propargyl tetra (ethylene glycol) hexanethiol
≡
ω
(HO(CH2CH2O)4(CH2)6SH, OEG-OH). The obtained mixed thiols were then self-assembled onto
the surface of the gold electrode. Subsequently, azide-derivatized p-perfluorononenyloxy benzene
(N3-Ar-O-C9F17) was linked to the propargyl-terminated OEG-CCH by forming a stable 1,2,3-triazole
ring via a copper(I)-catalyzed “click” reaction. A branch-tailed FS-SAM was then formed on the
gold electrode surface. Various techniques, such as X-ray photoelectron spectrometry (XPS), grazing
incidence reflectance (GIR) infrared spectroscopy, and contact angle (CA) measurements, were used
to characterize the prepared surfaces. Cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS) measurements were performed to evaluate the barrier property of the FS-SAM
using Fe(CN)63−, Ru(NH3)63+, and dopamine as redox probes in the presence of different electrolyte
ions. We focus primarily on the effect of film structure on the electrolyte ion penetration and the
interfacial electron transfers of structurally different probes. These studies provide a background for
future application of FSs in electrochemical systems.

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2. Results
2.1. Characterization of the FS-SAM
The grafting of N3-Ar-O-C9F17 onto the SAMs was characterized by XPS, GIR spectroscopy,
and CA measurement. The wide-scan XPS spectrum, as shown in Figure 1A, of the FS-SAM shows
characteristic peaks from N 1s (399.9 eV) [19] and F 1s (687.9 eV) [20], which indicates the successful
formation of triazole on the SAM. The C 1s core-level spectrum of the FS-SAM can be curve fit into
three peak components with BEs of approximately 284.7, 286.4, and 293.1 eV, as shown in Figure 1B,
attributable to the C–H, C–N/C–O [21], and C–F [20], respectively. The XPS N 1s results in Figure 1C,D
supply additional evidence of triazole formation on FS-SAM. For the FS-SAM, only a single peak was
observed at 399.8 eV, as shown in Figure 1C, whereas the control N 1s spectrum of N3-Ar-O-C9F17
shows two N 1s peaks at 399.8 and 403.6 eV, as shown in Figure 1D, respectively. The splitting of N 1s
peak indicates the presence of two N species in N3-Ar-O-C9F17, which reflects the different charged
state of N atoms in the azide group [22]. The single peak at 399.8 eV for the FS-SAM confirms the
reaction between the azide and propargyl groups.
Figure 1. X-ray photoelectron spectrometry (XPS) wide-scan spectrum (A); C 1s core-level spectrum
(
B
); and N 1s core-level spectrum ( ) of the fluorosurfactant self-assembled monolayer (FS-SAM); and
C
XPS N 1s core-level spectrum of N3-Ar-O-C9F17 (D).
In addition, GIR spectra, as shown in Figure 2, also suggest that “click” chemistry was successfully
applied to the hydroxyl/propargyl-terminated binary OEG-SAM. The GIR spectrum of N3-Ar-O-C9F17
(curve a) clearly reveals the appearance of an absorbance peak at 2107 cm−1, which is characteristic of
the azide groups. The peaks at 1284, 1183, and 1135 cm−1 are attributed to C-F stretching vibrations [23
]
and peaks at 1600 and 1500 cm−1 are the characteristic stretching peaks of the C=C bond in the benzene
ring [24]. The hydroxyl/propargyl-terminated binary OEG-SAM (curve b) shows characteristic peaks
at 3300 and 2130 cm−1
(
≡
C-H and C
C stretching) [25], 3500 cm−1 (-OH stretching), and 1103 cm−1
≡
(C-O-C stretching) [24]. After the triazole formation with N3-Ar-O-C9F17, the peaks from the propargyl
and azide groups disappeared and the characteristic C-F and benzene ring stretching peaks were
observed on FS-SAM (curve c).
Moreover, CA measurements were used to characterize the surfaces. Initially, the water CA of the
hydroxyl/propargyl-terminated SAM was 53
After the N3-Ar-O-C9F17 grafting onto the surface, the CA value increased to 121
FS-SAM, resulting from the highly hydrophobic -CF3 groups. All the above evidence demonstrates the
successful grafting of N3-Ar-O-C9F17 into the hydroxyl/propargyl-terminated SAMs.
±
2◦, indicating a moderate wettability of the surface.
◦
±
2 for the formative
We further optimized the condition for preparing the FS-SAM, and characterized the composition
of the monolayers by CA [26,27], EIS [28], and XPS measurements. The results revealed that the optimal
assembly solution for preparing the binary OEG-SAM is with the OEG-CCH fraction of 25%. Under the
optimization condition, the obtained FS-SAM consists of 56% FS and 44% OEG-OH. The details can be
seen in Section 1 in the Supplementary material. Since the molecular chain of FS is longer than that
of OEG-CCH, and the tailed -C9F17 group has a high occupied surface area, a compact and highly
fluorinated surface can be obtained.

Molecules 2018, 23, 2998
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Figure 2. Grazing incidence reflectance (GIR) spectra of the N3-Ar-O-C9F17 (curve a), the binary
OEG-SAM (curve b), and the FS-SAM (curve c).
2.2. Electrolyte Ion Permeability of the FS-SAM
The electrochemical response of a thin film is strongly influenced by electrolyte properties, such as
ion charge, size, and hydrophilicity/hydrophobicity. It is well known that SAMs formed on substrates
are not perfectly uniform but may have some defects and allow some degree of penetration by solvent
molecules and electrolyte ions. To obtain insight into the ion permeability of the FS-SAM, we used CV
to study the film capacitance (C) (C gives rise to current during the charging of the electrode double
layer and exhibits a direct correlation with supporting electrolyte) in different inert electrolytes without
any redox species. Figure 3 shows the CV data of the FS-SAM-modified gold electrode between
and 200 mV in 0.1 M KCl (curve a), 0.1 M KF (curve b), 0.1 M LiClO4 (curve c), and KPF6 (curve d) at
50 mV/s. Table 1 exhibits the calculated film capacitances from the corresponding CV data according
−
200
to the formula C = ic/2vA, where ic is the charging current (µA), v is the scan rate (mV/s), and A is the
electrode area (cm2) [29].
Figure 3. Cyclic voltammetry (CV) on the FS-SAM-modified gold electrode between
−
200 and 200 mV
in different electrolyte solutions: 0.1 M KCl (curve
a), 0.1 M KF (curve
b
), 0.1 M LiClO4 (curve ), and
c
KPF6 (curve d). Scan rate, 50 mV/s. pH: 7.0.

Molecules 2018, 23, 2998
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As shown in Table 1, the capacitanc−e of the FS-SAM is larger in hydrophilic Cl−
−
[30] solution. The largest and the least film
[ [31]
30] and F−
solution than that in hydrophobic ClO4
[
32]−and PF6
capacitance was obtained in the Cl− and PF6 solutions, respectively. The results indicate that the
FS-SAM permits the solvent and the hydrophilic ions, but not the hydrophobic ions, to permeate into
it. On one hand, this phenomenon might be ascribed to the hydrophobic bond formation between
the hydrophobic ions and the p-perfluorononenyloxy benzene moieties at the exterior of the FS-SAM.
On the other hand, amphiphilic SAMs that consists of OEG and hydrophobic components have been
reported to possess a hydration shell around the OEG moieties [33]. Transporting in−to the hydration
−
interior of the FS-SAM is thermodynamically unfavorable for the hydrophobic ClO4 and PF6 ions.
Consequently, the capacitance of the −FS-SAM decreased in hydrophobic electrolyte soluti−ons.
−
−
Moreover, the ionic sizes of ClO4 and PF6 ions are larger than the sizes of Cl and F . The large
sizes of the former will certainly defy their chances of permeation. Meanwhile, we observed that
the capacitance of−the FS-SAM is smaller in KF than that in KC1 solutions. This finding means that
−
FS-SAM allows C1 to permeate into the interior more easily compared with F , which exhibits good
hydrophilicity. This result may also be ascribed to the large size of the hydrated F , thus hindering its
−
transport in the layer. A similar phenomenon about the film capacitance of the alkanethiol SAMs in
C1− and F− solutions has been observed by Porter and coworkers [34].
Table 1. Capacitance (C) results of the FS-SAM-modified gold electrodes in different electrolytes.
Electrolyte
0.1 M KCl
0.1 M KF
0.1 M LiClO4
0.1 M KPF6
C (µF/cm2)
0.73 ± 0.03
0.55 ± 0.04
0.25 ± 0.01
0.22 ± 0.06
2.3. Electrochemical Response of Hydrophilic Probes on the FS-SAM
CV and EIS are powerful tools to investigate the electrochemical properties of SAM-modified
3−
electrodes by using redox probes. In this study, two hydrophilic redox probes, namely, Fe(CN)6 and
Ru(NH3)6
3+
[35], were used to investigate the blocking property and interfacial electron transfers of
the FS-SAM. The control experiments on the binary OEG-SAM-modified gold electrode and bare gold
are also performed for comparison.
3−
2.3.1. Fe(CN)6 Redox Reaction
Figure 4A shows the CV data and Figure 4B shows the EIS data of the SAM-modified gold
electrodes in 10 mM K3Fe(CN)6 with different supporting electrolytes. Each of the EIS spectra consists
of a semicircle portion and a linear line portion, referring to the electron transfer reaction and diffusion,
respectively. The semicircle diameter represents the charge transfer resistance (Rct) at the electrode
surface. Using the corresponding Rct values from the EIS spectra, the rate constant (k0) values of
3−
Fe(CN)6 for the binary OEG-SAM and FS-SAM-modified electrodes were determined (details can be
seen in Section 2 in the Supplementary Information). The obtained results of CV and EIS analysis are
summarized in Table 2.

Molecules 2018, 23, 2998
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3−
A) and electrochemical impedance spectroscopy (EIS) (B) of 10 mM Fe (CN)6 in
Figure 4. CV (
different electrolyte solutions on the binary OEG-SAM and the FS-SAM-modified gold electrodes: the
binary OEG-SAM-modified gold electrodes in 0.1 M KCl (curve ) and 0.1 M KPF6 (curve ) and the
). CV scan rate,
a
b
FS-SAM-modified gold electrodes in 0.1 M KCl (curve
100 mV/s; pH: 7.0.
c) and 0.1 M KPF6 (curve d
3−
Table 2. CV and EIS results for 10 mM Fe (CN)6 on different electrodes.
−1
Probe
Electrode
Electrolyte
∆Ep/mV
Rct/Ω cm2
k0/cm·s
0.1M KCl
0.1 M KPF6
391
602
6359
9374
0.42
0.28
Binary OEG-SAM
3−
Fe(CN)6
0.1M KCl
0.1 M KPF6
—
—
15681
58139
0.17
0.05
FS-SAM
The peak potential difference (
on the SAM-modified electrodes, with larger
∆
EP) and Rct values are used as a measure of electron transfer rate
EP and Rct indicating a slower electron transfer, and we
∆
interpret any increase in this value to be caused by surface imperfections or barriers. On the bare gold
3−
electrode (data not shown), Fe(CN)6 displays a well-defined response in 0.1 M KCl solution, with a
CV
ct values (curve a) of the binary OEG-SAM-modified electrodes in 0.1 M KCl solution, as expected, are
high owing to the inhibited electron transfer by the OEG-SAM on the electrode surface. Additionally,
∆Ep value of 72 mV and an Rct value of 0.96 Ω ∆Ep and
cm2. Figure 4A,B and Table 2 show that the
R
the increase in
∆
Ep and Rct was great when KPF6 (curve b) was used as an electrolyte instead of KCl
3−
(curve a). Th−is result indicates that the electron transfer rate between the electrode and Fe(CN)6 is
−
higher for Cl compared with PF6 , with a k0 value of 0.42 and 0.28 cm
·
s
−1, respectively. The variation
in the electron transfer rate with a change in the anion can be explained by the hydrophobicity of the
counterion. Given that the chloride ion is less hydrophobic, its migration in and out of the hydrophilic
−
OEG monolayer is more favorable compared with that of PF6
.
3−
After the FS-SAM formation, CV analysis revealed that the redox reaction of Fe (CN)6 is further
inhibited by the FS monolayer, as shown in Figure 4A, curves c and d. Figure 4B shows the comparison
of the impedance plots of the binary OEG-SAM and FS-SAM-modified electrodes in 10 mM K3Fe(CN)6
with KCl or KPF6 as the supporting electrolyte. The impedance plots of the FS-SAM, as shown in
Figure 4B, curves c and d, show large semicircles in the high frequency region, suggesting a higher
Rct and a stronger blocking ability of the FS-SAM compared with the binary OEG-SAM. Apparently,
3−
the electron transfer kinetics of Fe(CN)6 decreased in all cases for the FS-SAM systems compared
with the binary OEG-SAM systems. Meanwhile, the electron transfer kinetics was lower when KPF6

Molecules 2018, 23, 2998
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was−used as an electrolyte compared with KCl, k0 values are 0.17 and 0.05 cm
·
s
−1 in the presence of
PF6 and Cl− ions, respectively. The results are consistent with our previous hypothesis based on
−
the capacitance of the FS-SAMs. The hydrophobic binding of negatively charged PF6 ions to the
hydrophobic p-perfluorononenyloxy benzene moieties creates an additional energetic barrier for the
3−
transport of negative Fe(CN)6 in the monolayer and thus hinders the faradaic reactions.
3+
2.3.2. Ru(NH3)6 Redox Reaction
Figure 5A shows the CV plots and Figure 5B shows the impedance plots of the SAM-modified
gold electrodes in 10 mM Ru(NH3)6Cl3 with different supporting electrolytes. Table 3 summarizes the
3+
obtained results of CV and EIS analysis. On the bare gold electrode (data not shown), for Ru (NH3)6
in the presence of 0.1 M KCl,
∆
Ep (83 mV) and Rct (1.24
Ω
cm2) values are comparable with those for
3−
Fe(CN)6 obtained under the same condition, indicating that fast kinetics can be obtained for both
the probes on the bare gold electrode.
Unexpectedly, on the binary OEG-SAM-modified electrode, the CV of Ru(NH3)63+ in the presence
of 0.1 M KCl, as shown in Figure 5A, shows that the Ru(III) redox reaction is facile. The small
∆
Ep (114 mV) implies that the reaction is quasireversible. Such a behavior indicates that the binary
OEG-SAM shows a poor blocking ability to the redox reaction of Ru(NH3)63+, which is in contrast to
that for Fe(CN)63−. When KPF6 was used as an electrolyte, both
conformity with our observations using Fe(CN)63 discussed previously. The hydrophobicity of PF6
may hinder its migration in and out of the hydrophilic OEG monolayer.
∆
Ep and Rct increased, which are i−n
3+
Figure 5. CV (
A
) and EIS (
B
) of 10 mM Ru(NH3)6 in different electrolyte solutions on the binary
OEG-SAM and the FS-SAM-modified gold electrodes: the binary OEG-SAM-modified gold electrodes
in 0.1 M KCl (curve ) and 0.1 M KPF6 (curve ), and the FS-SAM-modified gold electrodes in 0.1 M
a
b
KCl (curve c) and 0.1 M KPF6 (curve d). CV scan rate, 100 mV/s. pH: 7.0.
3+
Table 3. CV and EIS results for 10 mM Ru(NH3)6 on different electrodes.
−1
Probe
Electrode
Electrolyte
∆Ep/mV
Rct/Ω cm2
k0/cm·s
0.1 M KCl
0.1 M KPF6
114
131
1497
2064
1.78
1.29
Binary OEG-SAM
3+
Ru(NH3)6
0.1 M KCl
0.1 M KPF6
126
138
1524
2196
1.75
1.22
FS-SAM

Molecules 2018, 23, 2998
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3+
3−
on the binary
∆Ep and Rct values of
The significantly different apparent kinetics of Ru(NH3)6 and Fe(CN)6
OEG-SAM needs to be clarified. Compared with Fe(CN)63−, the small
3+
Ru(NH3)6 indicate that the latter displays much faster kinetics than the former. The electrostatic
interaction at the monolayer/solution interface may play an important role in the observed response.
The binary OEG-SAMs can be assumed to be negatively charged in aqueous solution due to tightly
bound hydroxide ions with a hydration shell, similar to the results of OEG- and methyl-terminated
SAMs [36,37]. The negative hydration shell may be favorable for the positively charged probe
3+
Ru(NH3)6 to be accessed.
Interestingly, the expected blocking behavior for Fe(CN)6 on the FS-SAM was not observed for
Ru(NH3)63+. The response of Ru(NH3)63+ on the FS-SAM, in fact, is less suppressed compared with
that on the binary OEG-SAM. Fe(CN)6 (6 Å) and Ru(NH3)6 (6.4 Å) have comparable sizes, we
conclude, if the redox reaction of Ru(NH3)63+ is assumed to occur by access to the electrode surface
through the pinhole defects present in the SAMs, the redox reaction of Fe(CN)6 should also exhibit
3−
3−
3+
3−
facile kinetics. Given that this phenomenon was not observed, the Ru(NH3)63+ redox reaction does
not occur by access of the redox species to the electrode surface but by electron tunneling through
the bonds, as shown in Scheme 2. The FS molecule with an aliphatic chain length of 2.4 nm of the
–CH2– and –CH2CH2O– units, provided through bond tunneling pathway [38]; and the delocalized
π
electrons, which are present in the aromatic ring, triazole, and alkene units, can bridge the redox
species and the electrode surface effectively to facilitate the electron transfer. The redox reaction of
Ru(NH3)63+ has been reported to follow the outer-sphere electron transfer mechanism [39
40]. In this
,
case, physical contact between the redox species and the electrode surface is not essential for the
electron transfer process since it is favored by the tunneling mechanism. In the case of the FS-SAM in
this study, the aromatic and triazole rings with delocalized electrons facilitate the electron transfer of
Ru(NH3)63+, and the aliphatic chains provided the bond tunneling pathway. In contrast, the redox
3−
reaction of Fe(CN)6 follows an inner-sphere electron transfer mechanism, where the access of redox
species to the electrode surface is necessary for the electron transfer reaction to occur [40,41].
3−
3+
Scheme 2. Proposed electron transfer mechanism for Fe(CN)6
and Ru(NH3)6
on the
FS-SAM-modified electrode.
2.4. Electrochemical Response of Hydrophobic Dopamine+ on the FS-SAM
Figure 6 shows CV results for dopamine+ (DA+) (pKa = 8.92) on the binary OEG-SAM and the
FS-SAM-modified electrodes. The results are summarized in Table 4. With 0.1 M KCl as a supporting
electrolyte, the anodic peak potential (Ep,a) of DA+ becomes significantly more positive on the binary
OEG-SAM-modified electrode compared with that on the bare gold electrode, as shown in Table 4, and
becomes even more positive when we used KPF6 instead of KCl, as shown in Table 3 and Figure 4.
On the FS monolayer, the hydrophobic, singly charged DA+ response nearly disappears.

Molecules 2018, 23, 2998
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Table 4. CV results for 10 mM dopamine on different monolayer modified gold electrodes.
Electrode
Electrolyte
Ep,a/mV
Bare Au
0.1 M KCl
174
0.1 M KCl
0.1 KPF6
347
539
Binary OEG-SAM
FS-SAM
0.1 M KCl
0.1 KPF6
—
—
Figure 6. CV of 10 mM DA+ on the binary OEG-SAM-modified gold electrodes in 0.1 M KCl (curve
and 0.1 M KPF6 (curve ), and on the FS-SAM-modified gold electrodes in 0.1 M KCl (curve ) and
0.1 M KPF6 (curve d). pH: 7.0. Scan rate, 100 mV/s. pH: 7.0.
a)
b
c
The slow kinetics and poor responses of DA+ on the binary OEG-SAM may result from a large
distance of access approach to the monolayer surface, which may be due to unfavorable molecular
orientation and interactions of DA+ with the monolayer, as well as possible complications in the
electron transfer mechanism of DA+. The 2e−, 2H+ redox reaction of DA+ shown in Equation (1) may,
for example, alter the local pH, affecting surface charge density. Meanwhile, the results reveal that
the positive charge of DA+ does not significantly facilitate its access to the negative binary OEG-SAM.
This phenomenon is different from that of Malem and Mandler’s results [42], which reported improved
kinetics of DA+ on the negative HS(CH2)2COO− SAM compared with that of the bare gold electrode.
This outcome may result from the short alkyl chain monolayer used in their study.
(1)
In previous studies, Chen et al. [12,13] studied the oxidation of different electroactive species
at a straight-chain FS-modified gold electrode. The results show that the straight-chain FS-SAM
showed an inhibiting effect on the oxidation of glucose, uric acid, ascorbic acid, and oxalate, but
facilitated L-cysteine oxidation. They think the low coverage of the adsorbed thiol-containing
L-cysteine species might account for the more facile electron-transfer kinetics. They also found
that the straight-chain FS-SAM facilitated the electron transfers of tertiary amines oxidation, which
may be attributable to the FS adsorption layer being more compact. The amines studied include
tri-n-ethylamine, tri-n-propylamine, and tri-n-butylamine. Different from Chen’s results, in our work,
the branch-tailed FS-SAM suppressed the electron transfer of dopamine redox. On one hand, it may
partly be due to the structural differences between the branch-tailed FS-SAM and the straight-chain

Molecules 2018, 23, 2998
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ones; on the other hand, it may be caused by the individual differences in the studied species. As we
have just investigated the effects of the branch-tailed FS-SAM on only one kind of electroactive organic
specie, dopamine, and the obtained data is still very limited. We will conduct further studies on the
behavior of more electroactive organic species on the branch-tailed FS-SAMs and compare with the
straight-chain ones.
The important feature of surfactant molecules is their amphiphilicity. The specific hydrophobic
and hydrophilic portions in the FS-SAM may play a very important role to the response of hydrophobic
and hydrophilic probes on the monolayer surface, although other factors may also be important, for
example, the electron transfer pathways and electrostatic force. The poor responses of DA+ and
3−
Fe(CN)6 on the FS monolayer demonstrated that the FS-SAM will suppress the response of both
hydrophobic and hydrophilic probes. On one hand, for the hydrophobic DA, the hydrophobic bonding
at the hydrophobic FS-SAM exterior and the hydration resistance in the interior may hinder the probe to
transport into the film. On the other hand, approaching the hydrophobic exterior is thermodynamically
unfavorable for the hydrophilic probes, such as Fe(CN)63−. However, as the FS molecule displays a
branch structure of the tail group, the exterior is defective thus allowing some degrees of permeation
by solvent and the small size hydrophilic electrolyte ions, similar to Cl−. Therefore, compared with
that in hydrophobic electrolyte solution, the FS-SAM is less resistive in hydrophilic electrolyte solution,
displays higher film capacitance, and contributes to faster electrochemical kinetics of electroactive
species present in the electrolyte solution.
3. Materials and Methods
3.1. Materials
Perfluorononene (C9F18) was obtained from Juhua Group Corporation, Quzhou, China.
Azide-derivatized p-perfluorononenyloxy benzene (N3-Ar-O-C9F17),
ω-tetra (ethylene glycol) hexanethiol
(OEG-OH), and -propargyl tetra (ethylene glycol) hexanethiol (OEG-CCH) were synthesized in our
ω
laboratory (details can be seen in Section 3 in the Supplementary material). Dopamine, KCl, KF, KPF6,
LiClO4, K3Fe(CN)6, Ru(NH3)6C13, CuBr, and tris(benzyltriazolylmethyl)amine were purchased from
Sigma-Aldrich. The other reagents were commercially available as the highest purity. Doubly distilled
deionized water was used for the preparation of the aqueous solutions.
3.2. Fabrication of the FS-SAM
The polycrystalline gold electrode (2 mm in diameter, model CHI101, CH Instruments, Inc.,
Austin, TX, USA) was first polished with chamois leather containing 0.05 µm alumina powders
for 10 min, rinsed thoroughly with deionized water, and then was electrochemically cleaned in a
0.5 M H2SO4 solution by cycling from 0 V to 1.65 V until a stable current profile was observed.
After rinsing with deionized water and ethanol, the electrode was dried in a stream of nitrogen,
followed by cleaning with H2 (3 mbar) plasma in a Harrick Plasma Cleaner (Sterilizer PDG-32G,
Ithaca, NY, USA) for 10 min. To form thiols SAM, the cleaned gold electrode was incubated in a mixed
3:1 (molar ratio) OEG-OH/OEG-CCH ethanol solution (total thiol concentration: 0.4 mM) for 24 h,
followed by rinsing with ethanol and drying under nitrogen flow. The click reaction was carried out
by immersing the propargyl-exposed SAMs in a solution containing 0.1 mM N3-Ar-O-C9F17, 1 mM
CuBr, 1 mM L-ascorbic acid sodium salt, and 1 mM tris(benzyltriazolylmethyl)amine in 3:1 dimethyl
sulfoxide/water for 30 min. Then, the electrode was taken out and rinsed with ethanol and doubly
distilled deionized water.
3.3. Surface Characterization
XPS measurements were carried out on a VG Multilab 2000 XPS spectrometer (Thermo Electron
Corp., Waltham, MA, USA) using monochromated Al Kα radiation. GIR spectra were recorded on a
Bruker Optics Tensor 27 Fourier-transform infrared spectrometer (Ettlingen, Germany) coupled to an

Molecules 2018, 23, 2998
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external microscope Hyperion 2000 with grazing angle objective (Ettlingen, Germany). The spectral
resolution was set at 4 cm−1 with 1000 co-addition scans. Water CA experiments were measured on a
Dataphysics OCA 20 system (Fielderstadt, Germany) at 25 ◦C.
3.4. Electrochemical Measurements
The electrochemical measurements were performed on an Autolab PGSTAT30 system (Eco Chemie
BV, Utrecht, The Netherlands). The conventional three-electrode system used consisted of a saturated
calomel reference electrode (SCE), a platinum wire auxiliary electrode, and a gold working electrode.
Capacitance (C) measurements were conducted by CV scanning in the potential range between 200 and
−
200 mV vs. SCE at a scan rate of 50 mV/s. The summed cathodic and anodic charging current (ic) at
0 mV was then divided twice the scan rate and normalized by the electrode area [29]. The EIS data were
recorded in the frequency range from 100 kHz to 0.1 Hz, with a root-mean-square amplitude of 10 mV.
The Autolab software FRA 4.9.007 was used to fit the impedance data to an appropriate equivalent
circuit. From the Rct values obtained from the impedance plots, the rate constant (k0) of the electron
transfer reactions on the monolayer-modified electrodes were calculated according to the protocol
described in References [43–45] and the details can be seen in Section 2 in the Supplementary material.
4. Conclusions
In this work, we have prepared an amphiphilic branch-tailed FS-SAM on a gold electrode
surface via thiol self-assembly followed by a copper (I)-catalyzed “click” reaction modification, and
comparatively studied the effect of the FS-SAM system on the electrolyte ion penetration and the
electrochemical behavior of redox probes with various hydrophilic/hydrophobic features.
We found that the hydrophilicity/hydrophobicity of the electrolytes exhibit a significant effect
on the monola−yer cap−acitance. The FS-SAM displays higher va−lue of the−monolayer capacitance in
hydrophilic Cl and F solutions than that in hydrophobic ClO4 and PF6 solutions, indicating that
the FS-SAM allows higher degree of permeation by the small size hydrophilic electrolyte ions than the
large size hydrophobic ones. The high degree of permeation by the electrolytes into the monolayer
contributes to fast electrochemical kinetics of electroactive species present in the system.
Moreover, the FS-SAM slowed down the response of both hydrophilic and hydrophobic redox
probes, even when the probes carry a charge opposite to that of the SAM surface. For hydrophilic
3−
electroactive probes, the redox reaction of the Fe(CN)6 couple is nearly completely blocked by the
3+
FS-SAM. By contrast, the electron transfer of Ru(NH3)6 is easier than that of Fe(CN)63−, which
may be partly due to the outer-sphere electron transfer mechanism of Ru(NH3)63+. The delocalized
electrons in the aromatic ring and triazole can favor the electron transfer of Ru(NH3)63+, and the
aliphatic chains can provide a bond tunneling pathway. For hydrophobic DA+, the hydrophobic
exterior of the FS-SAM does not help its electrochemical response. The hydrophobic bonding between
the exterior fluoroalkyl moieties and the hydrophobic probes, as well as the hydration resistance from
the interior hydration shell around the OEG moieties, hinders the hydrophobic probes to transport
into the FS-SAM. The 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.
Supplementary Materials: The following are available online, Section 1: Analysis of the composition of the
SAMs, Figure S1: Contact angle data and EIS data of the binary OEG-SAMs prepared in the assembly solutions
with different OEG-CCH molar fraction, Figure S2: Fractional surface coverage of the SAMs prepared in the
assembly solutions with different OEG-CCH molar fraction, Scheme S1: Schematic illustration of the composition
of the prepared FS-SAM, Section 2: Detailed computational protocols, Figure S3: Equivalent circuit used for the
analysis of EIS data of the electrochemical interface, Section 3: Detailed experimental protocols for the synthesis
of compounds, Scheme S2: Synthesis of OEG-OH, Scheme S3: Synthesis of OEG-CCH, Scheme S4: Synthesis of
1
N3-Ar-O-C9F17, Figure S4: H NMR spectra of the N3-Ar-O-C9F17
.

Molecules 2018, 23, 2998
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Author Contributions: A.Z. conceived and designed the experiments; S.L. and Q.L. performed the experiments;
G.S., H.W., M.L. synthetized the compounds; Z.Z., A.C., and X.L. contributed reagents/materials/analysis tools;
S.L. wrote the paper.
Funding: This work was supported by the Science and Technology Plan Projects of Sichuan (2018JY0275), the
Scientific Research Funded Projects of the Education Department of Sichuan (18ZB0458) and the Young Teachers
Funded Projects of Sichuan Agricultural University (03120310).
Conflicts of Interest: The authors declare no conflict of interest.
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