In situ hydrolysis of imine derivatives on Au(111) for the formation of aromatic mixed self-assembled monolayers: Multitechnique analysis of this tunable surface modification

Article abstract of DOI:10.1021/la202696a

This paper presents a novel method for preparing aromatic, mixed self-assembled monolayers (SAMs) with a dilute surface fraction coverage of protonated amine via in situ hydrolysis of C=N double bond on gold surface. Two imine compounds, (4′-(4-(trifluoromethyl)benzylideneamino)biphenyl-4-yl) methanethiol (CF₃-C₆H₄-CH=N-C₆H ₄-C₆H₄-CH₂-SH, TFBABPMT) and (4′-(4-cyanobenzylideneamino)biphenyl-4-yl)methanethiol (CN-C ₆H₄-CH=N-C₆H₄-C₆H ₄-CH₂-SH, CBABPMT), self-assembled on Au(111) to form highly ordered monolayers, which was demonstrated by infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). A nearly upright molecular orientation for CF₃- and CN-terminated SAM was detected by near edge X-ray absorption fine structure (NEXAFS) measurements. Afterward, the acidic catalyzed hydrolysis was carried out in chloroform or an aqueous solution of acetic acid (pH = 3). Systematic studies of this hydrolysis process for CN-terminated SAM in acetic acid at 25 °C were performed by NEXAFS measurements. It was found that about 30% of the imine double bonds gradually cleaved in the first 40 min. Subsequently, a larger hydrolysis rate was observed due to the freer penetration of acetic acid in the SAM and resultant more open molecular packing. Furthermore, the molecular orientation in mixed SAMs did not change during the whole hydrolysis process. This partially hydrolyzed surface contains a controlled amount of free amines/ammonium ions which can be used for further chemical modifications.

Full text of DOI:10.1021/la202696a

ARTICLE
pubs.acs.org/Langmuir
In Situ Hydrolysis of Imine Derivatives on Au(111) for the Formation of
Aromatic Mixed Self-Assembled Monolayers: Multitechnique Analysis
of This Tunable Surface Modification
Ying Luo,† Matthias Bernien,‡ Alex Kr€uger,‡ Christian F. Hermanns,‡ Jorge Miguel,‡,§ Yin-Ming Chang,‡
Simon Jaekel,‡ Wolfgang Kuch,‡ and Rainer Haag*,†
†Institut f€ur Chemie und Biochemie, Freie Universit€at Berlin, Takustr. 3, 14195 Berlin, Germany
‡Institut f€ur Experimentalphysik, Freie Universit€at Berlin, Arnimallee 14, 14195 Berlin, Germany
§Diamond Light Source, Harwell Science & Innovation Campus, OX11 0DE Chilton, Didcot, United Kingdom
S Supporting Information
b
ABSTRACT: This paper presents a novel method for prepar-
ing aromatic, mixed self-assembled monolayers (SAMs) with
a dilute surface fraction coverage of protonated amine via in
situ hydrolysis of CdN double bond on gold surface. Two
imine compounds, (40-(4-(trifluoromethyl)benzylideneamino)
biphenyl-4-yl)methanethiol (CF3ꢀC6H4ꢀCHdNꢀC6H4ꢀ
C6H4ꢀCH2ꢀSH, TFBABPMT) and (40-(4-cyanobenzylide-
neamino)biphenyl-4-yl)methanethiol (CNꢀC6H4ꢀCHdNꢀ
C6H4ꢀC6H4ꢀCH2ꢀSH, CBABPMT), self-assembled on Au-
(111) to form highly ordered monolayers, which was demon-
strated by infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). A nearly upright
molecular orientation for CF3- and CN-terminated SAM was detected by near edge X-ray absorption fine structure (NEXAFS)
measurements. Afterward, the acidic catalyzed hydrolysis was carried out in chloroform or an aqueous solution of acetic acid
(pH = 3). Systematic studies of this hydrolysis process for CN-terminated SAM in acetic acid at 25 °C were performed by NEXAFS
measurements. It was found that about 30% of the imine double bonds gradually cleaved in the first 40 min. Subsequently, a larger
hydrolysis rate was observed due to the freer penetration of acetic acid in the SAM and resultant more open molecular packing.
Furthermore, the molecular orientation in mixed SAMs did not change during the whole hydrolysis process. This partially
hydrolyzed surface contains a controlled amount of free amines/ammonium ions which can be used for further chemical
modifications.
’ INTRODUCTION
The first method, which is coadsorption, was introduced in
1989 by Whitesides and his colleagues.6,13 Several mixed SAMs
on gold were prepared by soaking chips in a mixed solution
composed of two different alkanethiols. Relationships of the
surface wettability to tail groups, solvents, and chain lengths were
surveyed by means of contact angle measurements. Imaging and
manipulating monolayers at nanoscale became possible after the
development of modern experimental technology. The first
systematic atomic force microscopy (AFM) study for n-alka-
nethiol mixed SAMs was performed in Hara’s group.14 The AFM
image visually revealed the monolayer growth and the phase
separation in the mixed SAMs. Apart from these examples of
alkanethiols, Kang et al.15ꢀ17 proved that the mixed SAMs of
40-substituted-4-mercaptobiphenyls could also be established
through coadsorption approach. They suggested that the
Since the 1980s, the research on self-assembled monolayers
(SAMs) on surfaces has attracted much attention. The control-
lable fabrication of organic molecules on metal surfaces, e.g.,
Au(111), alters the interface properties, thus making the surface
applicable for catalysis, biosensing, and electronics.1,2 In these
SAMs, single thiol molecules are densely packed due to the
strong interchain πꢀπ stacking or van der Waals interaction3
and the chemical bond formations between the substrate gold
and sulfur atoms.4,5 Therefore, a highly ordered structure is
normally observed.
In order to create surfaces with more complex functionalities,
for instance, as biosensors or electronics devices, several different
molecules have been lately employed in mixed SAMs, that
modify not only the chemical but also the structure properties
of the surface. Some methods for constructing the mixed SAMs
have been presented.6ꢀ12 Simultaneously, the surface proper-
ties and the corresponding applications have been studied as
well.13ꢀ20
Received: July 14, 2011
Revised:
November 23, 2011
Published: November 29, 2011
r
2011 American Chemical Society
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fractional surface coverage of the mixed SAMs might be related to
the direction of the molecular dipole moments. Even though
coadsorption is an universal method for mixed SAMs construc-
tion, the composition in a monolayer is often different than in
solution, and phase separation is generally hard to predict.
At the beginning of this century, dissociative adsorption of
asymmetric disulfide7,19,20 was used to prepare mixed photo-
responsive SAMs, in which more free space around the outermost
azobenzene moiety was obtained. This allowed reversible molec-
ular transformation via photoisomerization that is normally diffi-
cult to achieve in single-component SAMs.20 Furthermore, Chen
et al.8 took advantage of the two methods mentioned above and
coadsorbed asymmetrical and symmetrical disulfides on Au(111).
The results revealed that mixed disulfides formed more homo-
geneous SAMs than the mixed thiols.
Figure 1. Idealized mixed SAMs on gold surface prepared by in situ
hydrolysis of imine molecules under the acidic condition.
The third approach is different from the first two methods due
to its two-step deposition process.9 A pre-prepared chip posses-
sing a densely packed SAM of one surfactant is immersed into a
solution of the second surfactant to form a mixed SAM via ligand
exchanges. The disadvantage of this method is that the displace-
ment preferably takes place at the etch pits and the defect sites in
the gold surface, resulting in the formation of molecular domains.
Irradiation promoted exchange reaction (IPER) is an exten-
sion of the third method and a way to prepare strongly hetero-
geneous mixed SAMs that contain components with different
types of molecular chains, e.g., alkanethiol and biphenyl thiol.10,11
In this method, electron irradiation of SAMs is performed,
which leads to partial decomposition of molecules or distor-
tion of the molecular conformation and orientation. These
“destroyed” molecules are then replaced in the resultant ligand
exchange procedure.
Chemical modification is also an effective and convenient
means. For instance, Chapman et al.12 introduced reactions of
amines with a SAM that presented interchain carboxylic anhy-
dride groups on gold to produce mixed SAMs for resisting the
nonspecific absorption or realizing biospecific binding of pro-
teins. However, this method is mainly used to establish a SAM
with a 1:1 coverage ratio of two components. To some extent, the
applications of such mixed SAMs are therefore restricted.
Although the above-described methods have been widely
used, the surface coverage of different components cannot be
well and flexibly controlled.21 To solve these problems, novel
strategies are still needed.
It is well-known that the coupling between aldehyde and
amine can be readily carried out and that the produced imine
compound undergoes hydrolysis under the acidic condition. Due
to the easy manipulation and cleanliness of these two reactions,
they gradually became used for surface modification. To our
knowledge, the first paper about imine formation on gold surface
was published by Horton Jr. et al.22 in 1997, in which a novel
method was presented for attaching enzymes and other receptor
systems on gold via covalent bonding of amine derivatives with
aldehyde-terminated SAMs. Based on this encouraging result,
different biomolecules, e.g., DNA/RNA,23,24 proteins,25,26 and
fluorescent dyes,27 were soon thereafter immobilized on gold or
silicon oxide surfaces for application in biosensor technology.
Moreover, a nitro-substituted aromatic imine was attached on
surfaces via this imine coupling protocol and a new nanopattern-
ing system was established after selective molecular transforma-
tions resulted from X-ray irradiation.28ꢀ30 More recently, Tauk
et al. reported the dynamic covalent chemistry of imine con-
densation that was used as a tool to construct complex mixed
Figure 2. Chemical structures of two imine compounds TFBABPMT
and CBABPMT.
gradients with a broad range of interfacial functionalities.31
Besides the single-step coupling, stepwise synthesis of π-
conjugated imine monolayer was also developed for preparing
molecular electronics.32ꢀ34 Previous works about hydrolysis
of imine on surfaces, spectroscopic characterizations, and corre-
sponding applications, however, were mainly performed in
Reinhoudt’s group.27,35 They not only demonstrated the acid
catalyzed cleavage of CdN double bonds on gold or silicon oxide
surfaces but also achieved a directional movement of poly-
(propyleneimine) dendrimers on gradient glass surface based
on the reversible imine formation.
In this study, we have taken advantage of the hydrolysis of
aromatic imine compounds to form mixed SAMs possessing
protonated amines with different coverage ratios (Figure 1).
Amine functionalized mixed SAMs are particularly interesting for
the studies of biomaterials due to their selective reactions with
other complex functional groups.36ꢀ39 However, no article has
been reported on the hydrolysis process of imine bonds on
surfaces to date. Most kinetics studies in SAMs were performed
for hydrolysis of ester,40ꢀ43 metal complexation,44 and imine
formation.45,46 Additionally, unlike most of the application-
related previously reported mixed SAMs,47ꢀ49 in which mol-
ecules bearing active tail groups protrude over the “background”
molecules, the mixed SAMs produced with our method expose
ammonium ions in the depressed region due to the stiff back-
bones. This might be of interest to scientists for applications in
the future.
The synthesis of two target aromatic imine compounds
TFBABPMT and CBABPMT is introduced first in this paper
(Figure 2). Then, we will discuss the preparation of their SAMs
on gold surface and the characterizations using infrared reflection
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absorption spectroscopy (IRRAS), contact angle measurement,
X-ray photoelectron spectroscopy (XPS), and near edge X-ray
absorption fine structure (NEXAFS) for these monolayers. Acid-
catalyzed hydrolysis of these imine functionalized SAMs by two
methods as well as intensive investigation about the process of
this two-dimensional reaction and the quality of the correspond-
ing mixed SAMs by NEXAFS will be subsequently presented.
In the final section, we will give a summary and conclusion for
this work.
argon stream. The pretreated substrates were immersed in ethanolic
solution of imine compound TFBABPMT or CBABPMT for 22 h under
argon atmosphere at room temperature followed by annealing at 40 °C in
abs. ethanol for two periods of 0.5 h each. Then, the sample was rinsed
with p.a. acetonitrile, abs. ethanol, and dried with an argon stream.
In Situ Hydrolysis of Imine SAMs on Gold. The hydrolysis of
imine compounds was carried out by immersion of gold chips presenting
imine monolayer in p.a. chloroform (3 mL) at 30 °C (method a) or in
an aqueous solution of acetic acid (pH = 3.0, 4.5 mL, method b) at 40 or
25 °C. After the reaction, the samples were rinsed with p.a. tetrahy-
drofuran (method a) or Millipore water (method b), abs. ethanol, and
dried with argon.
’ EXPERIMENTAL SECTION
Infrared Reflection Absorption Spectroscopy (IRRAS). In-
frared reflection absorption spectra were measured using a nitrogen-
purged NICOLET 8700 FT-IR spectrometer equipped with a liquid
nitrogen cooled MCT detector. A molecular sieve filter device and a
hydrophobic PEFT membrane made from active carbon were used to
purify the nitrogen stream. All spectra were measured with a wavenum-
ber resolution of 4 cmꢀ1 and averaged over 2048 scans at an angle of
incidence of 85° relative to the surface normal. The spectra are reported
in absorbance units Log(1/R) after linear baseline correction. The
spectrum of gold-coated glass washed with piranha solution for 5 min
was used as reference.
Contact Angle Measurement. Static contact angles were mea-
sured on freshly prepared samples with a contact angle system OCA 20
from Dataphysics Instruments GmbH. Millipore water was used as the
liquid for the drop formation. At least three drops at different locations
on each sample were used.
General Methods. NMR spectra were recorded using a Delta
400 MHz instrument. High resolution mass measurements were run with
Agilent 6210 ESI-TOF spectrometer. IR spectra for bulk materials were
recorded on a Nicolet Avator 320 FT-IR spectrometer.
Reagents. All organic solvents, except for abs. ethanol from Merck,
were purchased from Acros and used as received. The substrate 40-
aminobiphenyl-4-methanethiol for synthesizing imine compounds
TFBABPMT and CBABPMT was prepared according to our previous
work.50 Two aldehydes 4-(trifluoromethyl) benzaldehyde and 4-
cyanobenzaldehyde were obtained from Acros. The former was purified
by distillation before every reaction, while the latter was directly used
without any further purification.
Synthesis. (a). TFBABPMT. 40-Aminobiphenyl-4-methanethiol (25.5 mg,
0.118 mmol) was dissolved in a solvent mixture of p.a. methanol
(1.20 mL) and p.a. tetrahydrofuran (0.360 mL). Then, a solution of
4-(trifluoromethyl) benzaldehyde (21.0 mg, 0.118 mmol, 98%) in p.a.
tetrahydrofuran (0.300 mL) was slowly added. The mixture was stirred at
room temperature overnight under Ar. The solvents were removed under
reduced pressure and additional silica gel column chromatography
(chloroform/isopropanol 30:1 + 3% vol triethyl amine) gave a yellow
powder with 34% yield. 1H NMR (400 MHz, CD2Cl2): δ (ppm) 1.87 (t, 3J
(H, H) = 8 Hz, 1H; SH), 3.80 (d, 3J (H, H) = 8 Hz, 2H; CH2), 7.35 (d, 3J
(H, H) =8Hz, 2H;CH), 7.42(d,3J(H, H) =8Hz, 2H;CH),7.60(d,3J(H,
H) = 8 Hz, 2H; CH), 7.67 (d, 3J(H,H) =8Hz,2H;CH), 7.76(d,3J(H, H) =
8 Hz, 2H; CH), 8.07 (d, 3J (H, H) = 8 Hz, 2H; CH), 8.61 (s, 1H; CH); MS
(ESI-TOF): m/z = 372.1024 ([M+H]+, calcd. for C21H17F3NS+: 372.1028);
IR (powder, cmꢀ1): ν = 1123, 1162, 1322, 1624, 2547.
(b). CBABPMT. Solvent abs. ethanol was degassed with Ar for 30 min. To
a solution of 40-aminobiphenyl-4-methanethiol (43.1 mg, 0.200 mmol) in
pre-degassed abs. ethanol (9.00 mL), 4-cyanobenzaldehyde (27.0 mg, 0.200
mmol) in pre-degassed abs. ethanol (3.80 mL) was slowly added. The
mixture was stirred at room temperature for 22 h under Ar. During the
reaction, light yellow precipitate resulted, and after the reaction, these
precipitates were collected via centrifugation and washed with abs. ethanol
two times. Further removal of the solvents under vacuum gave a light yellow
powder with 50% yield. 1H NMR (400 MHz, CD2Cl2): δ (ppm) 1.87 (t,
3J (H, H) = 8 Hz, 1H; SH), 3.80 (d, 3J (H, H) = 8 Hz, 2H; CH2), 7.35 (d,
3J (H, H) = 8 Hz, 2H; CH), 7.42 (d, 3J (H, H) = 8 Hz, 2H; CH), 7.60
(d, 3J (H, H) = 8 Hz, 2H; CH), 7.67 (d, 3J (H, H) = 8 Hz, 2H; CH), 7.79
(d, 3J (H, H) = 8 Hz, 2H; CH), 8.05 (d, 3J (H, H) = 8 Hz, 2H; CH), 8.59
(s, 1H; CH); MS (ESI-TOF): m/z = 329.1133 ([M+H]+, calcd. for
C21H17N2S+: 329.1107); IR (powder, cmꢀ1): ν = 1620, 2239, 2559.
Monolayer Preparation. Gold-coated glass (for IRRAS and
contact angle measurements) and gold-coated mica (for XPS and
NEXAFS) wafers were purchased from Arrandee and Georg Albert
Physical Vapor Deposition, respectively. Au films on glass substrates were
treated by piranha solution (3:1 mixture of concentrated sulfuric acid and
30% hydrogen peroxide) for 5 min at room temperature, subsequently
rinsed with Millipore water and abs. ethanol for 1 min, and finally dried in
an argon stream. On the other hand, Au films on mica substrates were
purified by immersion in abs. ethanol for 10 min at room temperature,
subsequently washed with abs. ethanol for 1 min, and then dried in an
Near Edge X-ray Absorption Fine Structure (NEXAFS).
NEXAFS measurements were performed using synchrotron radiation
from the bending magnet beamline PM3 at the storage ring BESSY II.
Absorption spectra were acquired by total electron yield detection,
recording the sample drain current as a function of photon energy. The
spectra were normalized to the total electron yield of a freshly
evaporated gold grid upstream to the experiment. Angle dependent
measurements were carried out using p-polarized X-rays with 95%
degree of polarization and angles of 20° (or 30°), 55°, and 90° between
the incoming X-ray wavevector and the surface. Energy resolutions were
set to 100 and 150 meV at the C and N-K edges, respectively. Calibration
of the photon energy was carried out by NEXAFS measurements
of gaseous N2 and setting the position of the first N π* resonance to
400.88 eV.51 Typical photon flux densities of about 1013 ꢀ1 cmꢀ2 were
s
used to prevent radiation damage. No change in the spectra with increasing
illumination was observed for these flux densities. Test measurements with
flux densities of about 1015 sꢀ1 cmꢀ2 showed a conversion of CF3 groups
to CF2 deduced from the chemical shift of the C 1s XPS resonance after
illumination for 1 h.
X-ray Photoelectron Spectroscopy (XPS). XPS measurements
were performed using a Phoibos 100 electron energy analyzer from
SPECS GmbH and X-rays from the synchrotron radiation of the
bending magnet beamline PM3 at the storage ring BESSY II with
photon energies of 385 and 260 eV for C 1s and S 2p XPS spectra,
respectively. The electron takeoff angle was set to 0°, corresponding to
normal emission. The binding energy is calibrated by referencing to the
Au 4f7/2 resonance of the substrate at 84.0 eV measured for each photon
excitation energy. Components in the XPS spectra were fitted by linear
and Shirley backgrounds and pseudo Voigt functions. S 2p XPS spectra
were fitted using S 2p3/2ꢀS 2p1/2 doublets separated by 1.2 eV and an
area ratio of 2:1.
’ RESULTS AND DISCUSSION
SAMs Preparation. Ethanol which was chosen in this experi-
ment is a standard solvent applied for SAMs preparation.
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Table 1. Assignment of the Vibrational Modes Probed by
IRRAS in Units of cmꢀ1a
TFBABPMT
Bulk SAM
3027 3026 3032, 3057, 3072 3024, 3060, 3071
CBABPMT
Bulk
SAM
ν(CH), ring
νas(CH2)
νs(CH2)
ν(SH)
2923
2855
2547
/
2928 2878
2852
2879
---
/
2559
2239
---
ν(CtN)
2231
1490
/
ν(CdC), ring ip 1499
1492 1490
νs(CF3)
νas(CF3)
1322
1336
/
/
1123, 1162 ---
/
δ(CH), ring ip 1002
1003 1003
1003
---
Figure 3. IR spectra of imine compound TFBABPMT in bulk (bottom)
and in SAM on gold (top). Inset: amplification for high frequency region.
δ(CH), ring op 816, 836
---
819, 834
a The abbreviations “ip” and “op” indicate in-plane and out-of-plane,
respectively.
CꢀC stretching at ∼1610 cmꢀ1 cannot be easily identified on the
spectra of SAMs, probably due to its weak absorbance and the
influences of other peaks nearby, such as CdN stretching at
∼1620 cmꢀ1 52
.
All these results are consistent with the IR data
reported for biphenyl-based thiols, p-terphenylthiol, and p-terphe-
nylmethanethiol on gold surface.53,54 Moreover, the observation of
characteristic peaks for symmetric CF3 stretching mode (1336 cmꢀ1
,
,
TFBABPMT) and CꢀN stretching mode (2231 cmꢀ1
CBABPMT) in the spectra of SAMs further indicated that SAMs
formation of these two thiol compounds on gold surface was
successful. Additional evidence for this argument is the signal
disappearance of the SꢀH vibration in thiol group (TFBABPMT:
2547 cmꢀ1, CBABPMT: 2559 cmꢀ1) due to the dissociation of
SꢀH single bond during the self-assembly process.1 In addition,
oxidized sulfur species (e.g., sulfone) that generally give rise to
strong IR vibrations in the region range from 1130 to 1170 cmꢀ1
were not observed, suggesting that no oxidation product appeared
in SAMs.55
Figure 4. IR spectra of imine compound CBABPMT in bulk (bottom)
and in SAM on gold (top). Inset: amplification for high frequency
region.
There are several striking changes in the spectra of SAMs
when compared to bulk samples. First, in the bulk spectrum of
TFBABPMT (Figure 3), the bands at 1123 and 1162 cmꢀ1 are
associated with the asymmetric CF3 stretching mode, whose
dipole moment is perpendicular to the molecular axis, with
components both in and out of the aromatic ring plane.56 The
dramatic reduction of these two bands while in the spectrum of
SAMs causes them to be buried by other peaks or even more
likely to completely disappear. Second, in both cases (Figures 3
and 4), the bands at 816, 836, 819, and 834 cmꢀ1 assigned as out-
of-plane CꢀH wagging mode (Table 1) with a dipole moment
orientated perpendicularly to the aromatic ring plane disappear
as well.57 The dramatic band intensity diminution and the
absence of the bands imply that the transition dipole moments
of νas(CF3) and δ(CH) modes probably orientate parallel to the
gold surface according to the surface selection rule. Therefore, it
can be speculated that the molecules in SAMs are upright on the
surface or slightly tilt to the surface normal.
However, the solubility of these two imine compounds in ethanol
is not satisfactory. Therefore, their corresponding supersaturated
ethanolic solutions were ultrasonicated for 10 min at room
temperature, and then centrifuged at 4000 rpm for 10 min
followed by filtration. UVꢀvis measurements were performed prior
to the fabrication each time to confirm the reproducible concentra-
tion of the filtrate (TFBABPMT: ca. 0.05 mM, CBABPMT: ca.
0.1 mM, after dilution of the saturated solution). Subsequently, gold
grafting was achieved as the procedure introduced in the Experi-
mental Section.
IR Spectroscopy. IRRAS is a common method to acquire rough
information about molecular structure and orientation in SAMs.
Figure 3 and Figure 4 present the spectra in bulk and in SAM of
imine compound TFBABPMT and CBABPMT, respectively. In
general, the spectra in two different states are comparable, which
initially suggests the presence of these two organic molecules on
surfaces. For example, the peaks in the high frequency region from
2800 to 3100 cmꢀ1 (Figures 3 and 4, inset) are assigned to CꢀH
stretching of methylene group and phenyl ring. The other two
peaks at ∼1490 cmꢀ1 and ∼1003 cmꢀ1 are associated with in-plane
aromatic CdC vibrational mode and in-plane CꢀH bending in
phenyl ring (Table 1). However, the vibration of 1,4-disubstituted
In previous studies, we found that hydrolysis of imine derivatives
in solution could be achieved without extra acidic reagents when
chloroform was applied as the solventdue to the existence of a trace
of hydrochloric acid inside it. So, the hydrolysis investigations of
SAMs presenting CF3 terminal groups were carried out first in
chloroform at 30 °C. The IR spectra were recorded over time and
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Figure 6. Static water contact angle during the hydrolysis of CF3-
terminated SAMs at 40 °C (a) and of CN-terminated SAMs at 25 °C (b)
in acetic acid aqueous solutions with a pH value of 3.
CN-terminated SAMs on Au on mica were carried out to
monitor the composition of the molecular layer. C 1s and S 2p
XPS signals are plotted in Figure 7 over the binding energy for
both SAMs. The binding energy of the individual C atoms within
the molecule depends on their chemical environment, which
leads to characteristic chemical shifts of the corresponding
resonances in the XPS spectrum. In Figure 7a, the resonance at
291.9 eV can be attributed to the C atom of the CF3 group. Since
it terminates the SAM, its signal is not damped by overlying
molecules. From the intensity ratio of the CF3 resonance to the
ones originating from the rest of the molecule of 0.24 ( 0.02 and
the occurrence of the corresponding atoms in the molecule of
1:20, the damping constant a of the C signal can be evaluated
according to
Figure 5. (a) IR spectra of CF3-terminated SAMs in the range from
1200 cmꢀ1 to 1400 cmꢀ1 after hydrolysis in chloroform of 1 d, 5 d, 6 d,
and 7 d. (b) Peak area ratio of ν(CArN) to νs(CF3) during hydrolysis.
are shown in Figure 5. Obviously, νs(CF3) peak moderately reduced
with an increase of the reaction time, and a concentration-
dependent slight red shift from 1336 cmꢀ1 to 1334 cmꢀ1 was
observed as well, which is supported by the research of Kang.56
Furthermore, a new peak at 1265 cmꢀ1 appeared and rapidly
increased, which may be associated with the CAr-N stretching
vibration from the aromatic primary amine produced. Similar
results, the appearance of CAr-N stretching peak and an intensity
decrease of ν(CtN) peak, were also obtained in CN-terminated
SAMs when the same hydrolysis condition was applied (IR spectra
are not shown). The hydrolysis was not complete, however, even
after one week owing to not enough acid in the chloroform.
Therefore, acetic acid aqueous solution (pH = 3) was used instead.
The hydrolysis reaction was performed at 40 °C and was usually
completed within a few hours. The difference from the first method
is that the peak of CꢀN stretching at ∼1265 cmꢀ1 was hardly
observable probably because protonation of the amino group
occurred in a relatively strong acidic condition and resulted in
too few amino groups on the surface to be detected. Furthermore, it
is encouraging that no bands of δ(CH) mode occurred after the
hydrolysis process in both cases indicating that the molecular
orientation in SAMs remained constant.
Contact Angle Measurements. Contact angle measurements
were performed for monitoring the hydrolysis of SAMs in the
aqueous solution of acetic acid at pH 3. Apparently, CF3 group is
much more hydrophobic compared to CN group so that a slower
hydrolysis rate of the corresponding SAMs can be expected.
Experiments were first carried out at 40 °C, and after 5 h, the
contact angle of CF3-terminated SAMs dropped from 100° to
22° (Figure 6a). In the case of SAMs presenting CN groups,
however, the reaction rate was too fast to be recorded well under
this condition. Therefore, the lower reaction temperature of
25 °C was used. The contact angle of this monolayer was reduced
from 40° to 25° within 90 min (Figure 6b). Since the static water
contact angle of amine-terminated SAMs is 62 ( 1° as reported
by Pandey et al.,58 protonated amine was therefore suggested as
the product after the hydrolysis reaction in such cases.
IcðCF3Þ
IcðtotalÞ
1 ꢀ eꢀa
¼
eꢀað1 ꢀ eꢀ20a
Þ
assuming a homogeneous distribution of C atoms in the SAM.59
This leads to a = 0.21 ( 0.02 per C atom along the molecule. The
C 1s XPS signal of the cyano group (Figure 7b) displays a binding
energy of 286.2 eV.60 Its intensity ratio is 0.19 ( 0.02, which is
similar to that of the CF3-terminated SAM. The remaining
intensity is characterized by two main resonances at 284.3 and
285.1 eV that can be attributed to aromatic C atoms in the phenyl
rings. Since the cyano group is part of the conjugated system of
the topmost phenyl ring, the signal of this carbon atom is shifted
toward higher binding energies. The resonance positions of
ꢀCHdNꢀ and ꢀSꢀCH2ꢀ cannot be unambiguously identi-
fied due to their small contribution to the C 1s XPS signal. For
the CF3-terminated SAM the C 1s resonance around 284.5 eV
can be assigned to aromatic C. The N 1s XPS spectra (not
shown) of the CF3- and CN-terminated SAMs both display a
single resonance at 398.5 and 399.2 eV binding energy, respec-
tively. The difference in binding energy may be attributed to
electrostatic effects in the photoemission process.60 The con-
tributions of the N atoms in the cyano and imine groups cannot
be separated due to their almost identical chemical shifts.61,62 In
Figure 7c and d, S 2p XPS spectra are shown for the two SAMs.63
In both cases, the spectrum is dominated by a S 2p doublet with
the S 2p3/2 resonance at 162.1 eV. These doublets which
correspond to sulfur groups bound to the gold surface prove
the successful tethering of the molecules to the surface. A small
contribution of sulfur with a 2p3/2 binding energy of 161.2 eV
X-ray Photoelectron Spectroscopy. High-resolution X-ray
photoelectron spectroscopy (XPS) measurements of CF3- and
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Figure 7. C 1s (left) and S 2p (right) XPS spectra of CF3- (top) and CN- (bottom) terminated SAMs on Au/mica acquired with 385 and 260 eV photon
energy.
spectra, the same damping constant can be used
IsðSHÞ
NðSHÞ
¼
IsðSAuÞ
NðSAuÞ eꢀ21a
3
where N(SH) and N(SAu) are the number of unbound and
bound molecules, respectively. The resulting percentage of un-
bound molecules is below 1% in both cases. The XPS results
show that for both types of molecules a well-defined SAM was
formed with a negligible percentage of unbound molecules.
Near Edge X-ray Absorption Fine Structure. Near edge
X-ray absorption fine structure (NEXAFS) spectroscopy probes
the unoccupied molecular orbitals (MOs) in the vicinity of a
certain element in the sample. It is therefore sensitive to the type
of chemical bond of an element and its orientation. NEXAFS
spectra at the NꢀK edge measured with p-polarized X-rays and
an angle of 55° between the k-vector of the X-rays and the surface
are presented for the CF3- (dashed) and CN- (solid) terminated
SAM (Figure 8). At this so-called magic angle the dependency of
the NEXAFS signal on the orientations of the MOs cancels out.
For the CF3-terminated SAM, the only nitrogen contained in the
molecules is the one within their imine bonds resulting in two π*
resonances at 398.4 and 401.4 eV photon energy. In addition
to these resonances, two π* resonances show up at 398.9 and
400.2 eV for the CN-terminated SAM. These resonances corre-
spond to excitations into the cyano π* MO oriented perpendi-
cular and parallel to the plane of the phenyl ring, respectively.65ꢀ67
The former orbital is delocalized over the phenyl ring, whereas
the latter is localized at the cyano group. The orientations of these
orbitals were determined by angle-dependent NEXAFS measure-
ments at the NꢀK edge for the CN-terminated SAM (Figure 9).
The corresponding spectra at the CꢀK edge and for the CF3-
terminated SAM are shown in Figures S1 and S2 of the Support-
ing Information. All π* resonances of the CN-terminated SAM in
the range of 398 to 402 eV photon energy display a pronounced
dependence on the incidence angle of the X-rays. The average tilt
Figure 8. NꢀK NEXAFS spectra of CF3- (dashed) and CN- (solid)
terminated SAMs on Au/mica measured with p-polarized X-rays at an
angle of incidence of 55° with respect to the surface (magic angle).
frequently assigned to atomic sulfur or highly coordinated
sulfur64 is observed for the CF3-terminated SAM. Contributions
of unbound thiol groups to the S 2p XPS signal can be identified
at 163.3 eV. The intensity ratio of the unbound to bound sulfur
groups is 0.14 and 0.17 for the CF3- and CN-terminated SAM,
respectively. Unbound molecules can either be incorporated in
the SAM presumably by πꢀπ stacking or be loosely adsorbed on
top of the SAM. The first case is unlikely because thiol-terminated
SAMs show advancing and reducing contact angles of 67° and
62°,56 which are very different from what we observed in the
case of CF3-terminated SAM (100°) and CN-terminated SAM
(40°). For loosely adsorbed molecules, the S 2p XPS signal of
the bound molecules is significantly damped. Since the kinetic
energy of the photoelectrons is nearly the same as in the C 1s
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Figure 9. Angle-dependent NꢀK NEXAFS spectra of CN-terminated
Figure 10. NꢀK edge NEXAFS spectra of the initial (black) CN-
terminated SAM on Au/mica and after inlay into acetic acid of pH = 3 for
increasing time measured with p-polarized X-rays at the magic angle.
SAM on Au/mica measured with p-polarized X-rays.
Table 2. Average Tilt Angles of π Orbitals with Respect to the
Surface Normal for the CF3- and CN-Terminated SAM
can be assumed with an average tilt of the molecular backbone
of about 34° ( 7°. Assuming a twist angle of 48° for the
CF3-terminated SAM leads to an average tilt of the molecules
of 17° (7°. This furthermore demonstrates the high degree of molec-
ular order within the two SAMs.
In Situ Hydrolysis of Imine on Gold. Figure 10 shows the
NꢀK edge NEXAFS spectra of CN-terminated SAMs after inlay
of the SAM into acetic acid aqueous solution (pH = 3) for
increasing time. All spectra were measured at the magic angle and
normalized to the intensity of the X-rays by means of a reference
signal from a gold grid upstream to the experiment. No normal-
ization to the signal of the substrate (pre edge) was carried out.
The resulting spectra can thus be compared to each other directly
independent of the damping of the substrate signal and the
orientation of the molecules.
CF3-terminated SAM
π* phenyl
π* imine
78° ( 5°
79° ( 3°
CN-terminated SAM
π* phenyl
π* imine
67° ( 3°
70° ( 5°
68° ( 5°
65° ( 5°
cyano delocalized
cyano localized
angle of the MOs is obtained from the relative change in intensity
of its corresponding resonance according to
Since the pKa value of aromatic ammonium salt is about 3ꢀ5, a
small amount of free amine might have been produced besides
I µ 0:5ð1 ꢀ PÞsin2 α þ Pðcos2 α cos2 θ þ 0:5 sin2 α sin2 θÞ
+
the protonated amino species (ꢀNH3 ) under the acidic condi-
tion (pH = 3) applied for this in situ hydrolysis. All imine double
bonds in the SAM were cleaved after an inlay time of 1 d. The low
intensity of the π* resonance at 399.1 eV (dark green, Figure 10)
may be attributed to excitations into the electronic system of the
biphenyl rings which only have a small overlap with the core
electrons of the nitrogen. This spectrum is similar to the one for
monolayers self-assembled by the amine derivatives on gold
surfaces, where acetic acid was also used in the preparation
procedure.69 The two resonances of the cyano group completely
disappeared after 1 d, which proves that the cyano functionalized
phenyl rings of the molecules were completely removed by
the cleavage process. The resulting NꢀK spectrum is consistent
with -NH3+ or -NH2 termination of the cleaved SAM.70
The NꢀK edge spectra for intermediate time steps can be
described by a superposition of the initial one and the one of the
fully cleaved SAMs. The ratio of cleaved to complete molecules
in the SAM is given by the relative weights of the two contribu-
tions to the spectra. The percentage of complete imine molecules
in the SAM is plotted over the inlay time evaluated by the
integration of the intensity of the localized cyano resonance
between 399.2 and 401.3 eV photon energy (Figure 11). A
gradual decrease of complete imine molecules was observed up
to 40 min. Between 40 and 80 min, a sudden decrease in coverage
became evident. Such behavior can be explained by the closely
where θ is the angle between the k-vector of the X-rays and the
surface, α the angle between the axis of a vector-type orbital and
the surface normal, and P the degree of linear polarization of the
X-rays.68 The obtained values for the two SAMs are given in
Table 2. To infer the orientation of the molecules, the rotational
freedom around their symmetry axis has to be considered. For
orbitals perpendicular to this axis, the tilt angle β of the molecular
axis with respect to the surface normal is connected to the tilt
angle α of the orbital according to
cosðαÞ ¼ sinðβÞcosðγÞ
where γ is the twist angle of the orbital with respect to the plane
spanned by the surface normal and the symmetry axis of the
molecule. The localized and delocalized π orbitals of the cyano
group are oriented perpendicular to each other. This enables
determination of the average twist and tilt angle of the upmost
phenyl rings of the CN-terminated SAM from the two tilt angles
of these orbitals66 yielding an average tilt and twist angle of β =
34° ( 7° and γ = 48° ( 7°. Since the complete molecule is not
rigid, an overall molecular twist and tilt angle cannot be assigned
unambiguously. However, because the π orbitals of the phenyl
rings and the one of the imine bond display average tilt angles
similar to the cyano group, a straight arrangement of the molecule
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former case, the same reaction was completed in just a few
minutes to several hours when acetic acid was applied. NEXAFS
studies for CN-terminated SAM in acetic acid showed that the
cleavage rate of the reaction in the monolayer was correlated to
the monolayer structure. In the first 40 min, the reaction was
gradually carried out in the highly ordered, densely packed SAM
and produced the mixed SAMs of protonated amine with a
surface coverage about 0ꢀ30% based on the reaction time. After-
ward, acid could readily penetrate the “decomposed” SAM,
which gave a larger hydrolysis rate. Furthermore, NEXAFS
measurements for mixed SAMs indicated that the molecular
orientation remained constant and no free molecules were
observed to lie on the top of the monolayer. Even though this
surface modification is not a simple nth-order reaction and
the rate constant is not given, in situ hydrolysis of imine bonds
has been shown to be an alternate approach for preparing an
aromatic mixed SAM with a dilute surface concentration of
protonated amino group, which could have potential applica-
tions for partially positively charged surfaces or further chemical
modification.
Figure 11. Relative coverage of the sample surface with CN-terminated
imine molecules over inlay time in acidic solution obtained at 25 °C by
integration of the NEXAFS intensity of the localized cyano resonance in
the range of 399.2 to 401.3 eV photon energy.
’ ASSOCIATED CONTENT
packed molecules in the SAM which protect the imine bonds
from the acidic solution. This results in a low initial cleavage rate
taking place either stochastically over the whole SAM with a low
probability or at domain boundaries or defect sites in the SAM
exhibiting a more open molecular packing. Above a certain
threshold, the acid could more freely penetrate the SAM and
resulted in a sudden increase in cleavage rate. This situation may
differ from SAMs grown on Au-covered glass substrates since
their molecular arrangement is likely to be more open because of
the higher surface roughness after cleaning with piranha solution.
In this case, the faster hydrolysis rate might be due to the better
accessibility of acid into the monolayer. The impact of the surface
roughness at the atomic and the molecular scale on the ability of
the imine SAMs to be hydrolyzed has not been studied, nor is it
discussed in this paper. AFM measurements might solve this
question in the future.
The same average tilting angles within the error bars are
determined for the mixed SAMs obtained by hydrolysis from the
angular dependence of the π* resonances as for the initial SAM.
The orientation of the biphenyl groups after an inlay time of one
day could be obtained from the angle dependence of the C π*
resonance at 285.4 eV (SI Figure S3) and resulted in an average
tilting angle of the molecular axis of 34° ( 7° if a twist angle of
48° is assumed. This shows that the high degree of orientational
order of the initial SAMs was preserved during the whole
hydrolysis process.
S
Supporting Information. Additional angle dependent
b
NEXAFS spectra for CF3-terminated SAM and for the CN-
terminated SAM after an inlay time of 1 day in acetic acid solution
included. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: haag@chemie.fu-berlin.de. Tel: +49 (0)30 83852633.
Fax: +49 (0)30 83853357.
’ ACKNOWLEDGMENT
Torsten Kachel is acknowledged for his technical support during
the NEXAFS and XPS measurements. Kenji Yamamoto and
Henryk Kalbe are acknowledged for preparatory experiments.
Financial support by the DFG (Sfb 658) is gratefully acknowledged.
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