Epoxy-terminated self-assembled monolayers containing internal urea or amide groups

Article abstract of DOI:10.1021/la5049375

We report the synthesis of new coupling agents with internal amide or urea groups possessing an epoxy-terminal group and trimethoxysilyl-anchoring group. The structural characterizations of the corresponding self-assembled monolayers (SAMs) were performed by polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS). The molecular assembly is mainly based on the intermolecular hydrogen-bonding between adjacent amide or urea groups in the monolayers. Because of the steric hindrance of amide or urea groups, the distance between the alkyl chains is too large to establish van der Waals interactions, inducing their disorder. The reactivity of the epoxy-terminal groups was successfully investigated through reaction with a fluorescent probe. We show that SAMs containing internal urea or amide groups exhibited a higher density of accessible epoxide groups than the corresponding long-chain (C₂₂) glycidyl-terminated SAM.

Full text of DOI:10.1021/la5049375

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Article
Epoxy-Terminated Self-Assembled Monolayers
Containing Internal Urea or Amide Groups
Michaël Ramin, Gwénaëlle Le Bourdon, Karine Heuze, Marie
Degueil, Thierry Buffeteau, Bernard Bennetau, and Luc Vellutini
Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5049375 • Publication Date (Web): 13 Feb 2015
Downloaded from http://pubs.acs.org on February 18, 2015
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EpoxyꢀTerminated SelfꢀAssembled Monolayers
Containing Internal Urea or Amide Groups
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Michaël A. Ramin, Gwénaëlle Le Bourdon, Karine Heuzé, Marie Degueil, Thierry Buffeteau,
Bernard Bennetau, Luc Vellutini*
Université Bordeaux, ISM, UMR 5255, Fꢀ33405 Talence, France
CNRS, ISM, UMR 5255, Fꢀ33405 Talence, France
AUTHOR EMAIL ADDRESS: luc.vellutini@uꢀbordeaux.fr
RECEIVED DATE:
TITLE RUNNING HEAD: Epoxyꢀterminated SAMs
* to whom correspondence should be addressed.
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ABSTRACT
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We report the synthesis of new coupling agents with internal amide or urea groups
possessing an epoxyꢀterminal group and trimethoxysilyl anchoring group. The structural
characterizations of the corresponding selfꢀassembled monolayers (SAMs) were performed by
PMꢀIRRAS. The molecular assembly is mainly based on the intermolecular hydrogenꢀbonding
between adjacent amide or urea groups in the monolayers. Due to the steric hindrance of amide
or urea groups, the distance between the alkyl chains is too large to establish van der Waals
interactions, inducing their disorder. The reactivity of the terminatedꢀepoxy groups was
successfully investigated through reaction with a fluorescent probe. We show that SAMs
containing internal urea or amide groups exhibited a higher density of accessible epoxide groups
than the corresponding longꢀchain (C22) glycidylꢀterminated SAM.
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KEYWORDS: Selfꢀassembled monolayers; Silylated coupling agents; PMꢀIRRAS
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1. INTRODUCTION
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Surface chemistry for immobilization of biomolecules onto solid surfaces plays a crucial
role for many applications such as biosensors/biochips,1,2,3 medical implants,4 and drug/gene
delivery carriers.5 Biofunctional surfaces can be obtained if the immobilization process preserves
the original recognition specificity of biomolecules and simultaneously allows strong binding
with the surface.6 Generally, the covalent way is favored because it leads to an irreversible
bonding with the support, producing uniform surface coverage. In contrast, physical adsorption
via intermolecular forces implies weak attachment of biomolecules, which may be removed by
buffers when performing the assays.
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Selfꢀassembled monolayers (SAMs) based on the chemistry of the organosilanes are
widely used to immobilize, in a covalent way, biomolecules such as DNA or proteins onto solid
supports.7,8,9,10,11 The chemical modification of Si/SiO2 surfaces by epoxy groups represents an
interesting approach for the covalent immobilization of biomolecules containing amino or thiol
groups in one step.12 Generally, the commercially available (3ꢀglycidoxypropyl)trimethoxysilane
(GPTS) is used to functionalize the silica surfaces for the chemical immobilization of small
molecules,13,14,15 polymers,16,17,18 or biomolecules.19,20 However, in the case of non atomically
flat substrates, this shortꢀchain arm can adopt various orientations and some of epoxide groups
may not be accessible to the species to immobilize. Thus, in a recent study, we have shown that a
very weak sensor response was measured when GPTS SAM was used to immobilize E. coli
bacteria in liquid medium onto relatively rough silica surfaces (RMS ≈ 5.5 nm).21 In contrast, a
significant sensor response was measured when a very longꢀchain (C22) glycidylꢀterminated
SAM was used to immobilize bacteria on the surface. This improvement was attributed to a
marked surface smoothing leading to a better efficiency of antigenꢀantibody recognition
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processes at the surface. The longꢀchain SAM affords a higher density of accessible epoxide
groups to immobilize antibodies receptors.22 However, it must be noticed that the synthetic
routes leading to longꢀchain alkyl groups are time consuming and give relatively weak yields
with poorly soluble silylated coupling agents. The urea group is widely recognized as an
important building block in supramolecular architectures,23 and the assemblies of
organic/inorganic hybrid materials.24,25 The intermolecular interactions between the organic
fragments with urea groups provide hydrogen bonding and consequently offer an attractive route
to selfꢀdirect the organization of molecular system.
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Recently, we described a readily accessible way to synthesize functionalized ureido
silylated coupling agents26 by using isocyanate chemistry.27 The presence of the urea group in the
linear alkyl chain improves the solubility of coupling agents in organic solvents and urea moiety
is easily generated by a condensation reaction of isocyanate with an amino group.28 These
functionalized ureido silylated coupling agents allow to prepare functionalized hydrogenꢀ
bonding SAMs which can be grafted onto silica substrate. Indeed, the urea groups within the
monolayer selfꢀassemble strongly via hydrogen bonds. Conversely, the distance between the
alkyl chains is too large to establish van der Waals interactions due to the steric hindering effect
of urea groups, inducing the disorder of the alkyl chains.
Functionalized ureido silylated coupling agents have been used by our group for the
formation of vinylꢀterminated monolayer.29 The successful chemical modifications to obtain
vinylꢀterminated SAMs allowed the covalent grafting of biomolecules without any degradation
of the monolayer. In addition, we have shown that functionalized hydrogenꢀbonding SAMs lead
to new platforms for the development of smooth silicaꢀbased surfaces, useful for structural
biological studies.30 Indeed, we have found that an acidꢀterminated ureido silylated compound
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based SAM was more suitable to maintain the structural integrity of viruses after its
immobilization onto a soft organic SAM, rather than onto a mineral surface (mica, for example).
The Atomic Force Microscopy (AFM) measurements, at the atomic level, show clearly that
SAM surfaces allow imaging virus without distortion. Moreover, these SAM surfaces may
prevent nonꢀspecific adsorption as previously reported with poly(ethyleneglycol) PEGꢀbased
monolayers.12,31,32
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In this paper, we report the synthesis of two new coupling agents with internal amide and
urea groups, possessing an epoxyꢀterminal group and a trimethoxysilyl anchoring group. Herein,
we report the formation of epoxyꢀterminated monolayers possessing hydrogenꢀbonding network.
The structural characterizations have been performed using PMꢀIRRAS.
2. EXPERIMENTAL SECTION
2.1. Synthesis of Amido and Ureido Silylated Coupling Agents. The synthesis of
amido and ureido silylated coupling agents and their precursors are reported in Supporting
1
Information (S2 to S8). All the compounds have been fully characterized by H, 13C and 29Si
NMR spectroscopy and by HRMS spectroscopy.
2.2. Materials and substrates. The SiO2/Au substrates were supplied by Optics Balzers
AG. They correspond to Goldflex mirror with SiO2 protection layer (GoldflexPRO, reference
200785). Their absolute reflectance was higher than 98% in the 1.2−12 ꢁm spectral range. The
thickness of the SiO2 layer, measured by ellipsometry, was 215 ± 7 Å, using a refractive index of
1.46 (Iꢀelli2000 NFT ellipsometer, λ = 532 nm). A homogeneous surface was observed by
atomic force microscopy (AFM) with a rootmeanꢀsquare (rms) roughness of 9 Å
(Thermomicroscope Autoprobe CP Research, Park Scientific).
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2.3. Formation of Self-Assembled Monolayers (SAM-8 and SAM-15). The substrates
were cleaned and activated just before the grafting. They were treated successively with milliꢀQ
water (18 Mꢂ.cm) and hot chloroform (10 min at least). Then, the substrates were exposed to
UVꢀozone (homemade apparatus, λ = 185−254 nm) for 30 min and introduced into the
silanization flask immediately. Trichloroacetic acid (TCA, 0.4 mg, 10 mol %) as catalyst was
added to a solution of epoxyꢀterminated coupling agents (8 or 15) (2.5 × 10−4 mol.L−1) in
anhydrous toluene (100 mL). This solution was introduced into the silanization flask at 18 °C
under inert atmosphere. The substrates were immersed for 12 h. They were sonicated in toluene
(5 min), chloroform (5 min) and milliꢀQ water (18.2 Mꢀcm, 10 min) and then dried under
vacuum for 10 min.
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Contact angles were performed on a Krüss DSA 100 goniometer, at 20°C in static mode.
The results correspond to the mean of least 3 measurements.
2.4. Immobilization of dansylcadaverin onto SAM-8 and SAM-15. The substrates
functionalized by SAM-8 and SAM-15 were dipped into the solution of dansylcadaverin (10ꢀ4
M) in toluene during 3 h at 37°C. The substrates were sonicated in toluene (5 min.) and milliꢀQ
water (10 min.) and then dried under inert gas stream. The upꢀright widefield microscope was a
Leica DMR (Leica Microsystems, Wetzlar, Germany) using objective HCX PL APO 63X oil
(NA 1.32). The resolutive camera used on this system is a CoolSnap HQ (Photometrics, Tucson,
USA). This system is equipped with an excitation (Ex/bp: 360/40) and emission (Em/bp: 528/38)
filter wheels (Sutter instrument, Novato, USA). This system was controlled by MetaMorph
software (Molecular Devices, Sunnyvale, USA).
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2.5. PM-IRRAS experiments. PMꢀIRRAS spectra were recorded on a ThermoNicolet
Nexus 670 FTIR spectrometer at a resolution of 4 cmꢀ1, by coadding several blocks of 1500
scans (30 minutes acquisition time). All spectra were collected in a dryꢀair atmosphere after 30
min of incubation in the chamber. Experiments were performed at an incidence angle of 75°
using an external homemade goniometer reflection attachment.33 The infrared parallel beam was
directed out of the spectrometer with an optional flipper mirror and made slightly convergent
with a first BaF2 lens. The IR beam passed through a BaF2 wire grid polarizer (Specac) to select
the pꢀpolarized radiation and a ZnSe photoelastic modulator (PEM, Hinds Instruments, type III)
which modulates the polarization of the beam at a high fixed frequency (74 KHz) between the
parallel (p) and perpendicular (s) linear states. After reflection on the sample, the double
modulated (in intensity and in polarization) infrared beam was focused with a second ZnSe lens
onto a photovoltaic MCT detector (Kolmar Technologies, Model KV104) cooled at 77 K. In all
experiments, the PEM was adjusted for a maximum efficiency at 2500 cmꢀ1 to cover the midꢀIR
range in only one spectrum. For calibration measurements, a second linear polarizer (oriented
parallel or perpendicular to the first preceding the PEM) was inserted between the sample and
the second ZnSe lens. This procedure was used to calibrate and convert the PMꢀIRRAS signal in
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R d
p( ), where R d and R 0 stand for the pꢀpolarized
terms of the IRRAS signal (i.e. 1−
p( )
p( )
R 0
p( )
reflectance of the film/substrate and bare substrate systems, respectively).34
2.6. ATR experiments, Determination of the optical constants of 8 and 15, Spectral
simulation of the IRRAS spectra of SAM-8 and SAM-15 for a compact monolayer. The
ATR spectra of compound
8 and 15 were recorded with a ThermoNicolet Nexus 670 FTIR
spectrometer equipped with a liquid nitrogen cooled narrowꢀband mercury cadmium telluride
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(MCT) detector using a SilverꢀGate (germanium crystal) ATR accessory (Specac). The electric
field of the infrared beam was polarized either perpendicular (sꢀpolarized) or parallel (pꢀ
polarized) to the plane of incidence with a BaF2 wire grid polarizer (Specac). Each spectrum was
obtained, at a resolution of 4 cmꢀ1, by coadding 500 scans.
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The optical constants (refractive index
and extinction coefficient
) of
k ν
n ν
( )
( )
compounds
8 and 15 have been determined from polarized ATR spectra, using the procedure
decribed by Dignam et al.35 The computer program used to calculate the IRRAS spectra for a
compact monolayer of SAM-8 and SAM-15 deposited onto SiO2/Au substrates is based on the
Abeles’ matrix formalism,36, 37 which has been generalized for anisotropic layers.38 Several
parameters must be fixed in the program such as the the thickness of the SAMs (set to 33 and 35
Å for SAM-8 and SAM-15, respectively), the angle of incidence (set to 75°) and the polarization
of the infrared beam. The pꢀpolarized reflectance of the covered R d and bare R 0
p( )
p( )
substrates have been calculated using the spectral dependence of the optical constants of
and 15 of gold.39
compounds
8
3. RESULTS AND DISCUSSION
3.1. Synthesis of Amido and Ureido Silylated Coupling Agents.
The synthesis of a novel trimethoxysilane
8 with an amide group in the alkyl chain is
reported in Scheme 1. It is performed in 8 steps from commercially available starting
compounds.
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Scheme 1. Synthesis of amido silylated coupling agent (8). a) Pyridine, Ac2O, 60 °C, 3 h. b)
OsO4 (0.01 equiv), Oxone® (4 equiv), DMF, 3 h, TA. c) NaN3 (1.2 equiv), DMF. d) PPh3 (1.5
equiv), H2O, THF, 40°C, 3 h. e) DCC (1.1 equiv), NHS (1.1 equiv),
4 (1.4 equiv), CH2Cl2. f)
K2CO3, MeOH, H2O. g) NaH (1.3 equiv), epichlorhydrine (5.5 equiv), THF. h) HSi(OMe)3 (5
equiv), cat. Karstedt (0.025 equiv Pt), 65 °C, 20 h.
The unsaturated alcohol has been acetylated by treatment with acetic anhydride in
pyridine to avoid the intramolecular lactonization and the esterification of compound
1 in the
following steps. The oxidative cleavage of the terminal alkene to carboxylic acid (compound
2
)
has been performed one pot, in high yield, with Osmium(VIII) oxide (called also tetroxide
osmium) in catalytic amount using Oxone® (KHSO5. ½ KHSO4. ½ K2SO4) as coꢀoxidant.40
Nucleophilic displacement of the bromide from the bromoꢀalkene with azide afforded
intermediate
3
, which was easily converted to the primary amine
4
using the Staudinger
reduction.41 The coupling reaction between the acid
2
and amine 4 derivatives to generate the
amido compound
5 was performed by using the carbodiimide chemistry to activate the
carboxylic acid group into an active ester.42 Then, the acetyl group of compound
5
was
deprotected using K2CO3 in methanol to obtain the compound
6 in high yield (70%). The
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intermediate alcohol
6
was reacted with epichlorhydrine and NaH under anhydrous conditions in
5
6
one step procedure, yielding
7 8, the
(28%).22 To obtain the amido silylated coupling agent
7
8
9
addition of the Si(OMe)3 group was achieved by hydrosilylation reaction of the unsaturated
with a large excess of HSi(OMe)3 in the presence of Karstedt’s catalyst.29
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precursor
7
The synthesis of novel epoxy alkoxyorganosilane with an urea group in the alkyl chain
required 7 steps (Scheme 2).
Br
OH
11
Br
OSiPh2tBu
11
OSiPh2tBu
11
H2N
OSiPh2tBu
11
N3
a
b
c
9 (72%)
10 (74%)
11 (84%)
H
H
H
H
N
N
OSiPh2tBu
N
N
OH
11
d
e
8
11
8
O
O
12 (80%)
13 (80%)
O
O
H
H
N
H
N
H
N
O
(MeO)3Si
N
O
f
g
8
11
10
11
O
O
14 (21%)
15 (100%)
Scheme 2. Synthesis of ureido silylated coupling agent (15). a) TBDPSCl (1.5 equiv), imidazole
(1.2 equiv), CH2Cl2. b) NaN3 (1.2 equiv), DMF. c) H2, Pd/C, AcOEt. d) 10ꢀisocyanatodecene
(0.9 equiv), CH2Cl2. e) TBAF (1.8 equiv), THF. f) NaH (1.3 equiv), epichlorhydrine (5.5 equiv),
THF. g) HSi(OMe)3 (5 equiv), cat. Karstedt (0.025 equiv en Pt), 65 °C, 20 h.
The hydroxyl group of the 11ꢀbromoꢀ1ꢀundecanol was protected as tertꢀ
butyldiphenylsilyl (TBDPS) ether
9
to avoid undesired further reactions. The azide derivative 10
. The azide compound 10
was obtained from an azide substitution of halide compound
9
constitutes an useful precursor of amine since it can be easily reduced in the corresponding
amino compound 11 in high yield (84%) by a catalytic hydrogenation over 10 % Pd/C in AcOEt.
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It is interesting to note that the silyl protective group TBDPS was stable during hydrogenation
reaction if a nonꢀprotic solvent was used.43 Then, this amine compound 11 has been coupled with
10ꢀisocyanatodecene to form the urea derivative 12 in high yield (80 %). The cleavage of the
TBDPS protective group of 12 was performed easily to provide alcohol 13 by treatment with
tetraꢀbutylammonium fluoride (TBAF) in THF.44 The intermediate alcohol 13 was reacted with
epichlorhydrine and NaH under anhydrous conditions, yielding 14 (21%). Finally, the ureido
silylated coupling agent 15 was obtained by hydrosilylation reaction of the unsaturated precursor
14 with a large excess of HSi(OMe)3 in the presence of Karstedt’s catalyst.
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FTIR spectra of compounds 7, 8, 14, 15 are reported in Figures 1A and 1B. These spectra
are presented in the region of the amide I (νC=O) and amide II (δNH+νCꢀN) modes, at around 1650
cmꢀ1 and 1550 cmꢀ1, respectively. The presence of hydrogenꢀbonding between the molecules is
revealed by the wavenumber difference (ꢁν) between the amide 1 and 2 modes. Indeed, the
smaller is the wavenumber difference, the stronger are the hydrogen bonds between adjacent
amide or urea groups. A ꢁν value of 176 cmꢀ1 has been measured for a diluted solution of
7 in
CCl4, showing that any intermolecular hydrogenꢀbonding is possible. This value decreases up to
89 and 99 cmꢀ1 in the solid state for the alkene precursor
7
and the silylated amido coupling agent
8
, respectively. The higher value measured for compound 8 is certainly due to the presence of the
ꢀSi(OMe)3 group which induces steric hindrance and consequently increases the distance
between amide groups. For the ureido derivative, a ꢁν value of 131 cmꢀ1 has been observed for a
diluted solution of 14 in CCl4, associated to weak intermolecular hydrogenꢀbonding. This ꢁν
value decreases up to 37 and 44 cmꢀ1 in the solid state for the alkene precursor 14 and the
silylated ureido coupling agent 15, respectively. As with compound 8, the addition of bulky silyl
group to obtain the ureido coupling agent 15 induces weaker hydrogen bonds. The ability to selfꢀ
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assemble through intermolecular hydrogenꢀbonding seems to be better for the silylated ureido
5
6
coupling agent 15 (44 cmꢀ1) than for the amide analog compound
8
(99 cmꢀ1).
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0,04
7 liquid
7 solid
8 solid
1645
A
0,06
0,04
0,02
0,00
14 liquid
14 solid
15 solid
1578
B
1615
1665
1682
1635
0,03
0,02
0,01
0,00
1617
1573
1534
1546
1506
1700
1650
1600
1550
1500
1700
1650
1600
1550
1500
Wavenumbers, cm-1
Wavenumbers, cm-1
Figure 1. A) IR spectra in the 1800ꢀ1450 cmꢀ1 spectral range of
concentration (5.10ꢀ3 M) and in the solid state for
and 8 compounds. B) IR spectra in the 1800ꢀ
7 in CCl4 solution at low
7
1500 cmꢀ1 spectral range of 14 in CCl4 solution at low concentration (5.10ꢀ3 M) and in the solid
state for 14 and 15 compounds.
3.2. Characterization of Epoxy-Terminated Monolayers. Epoxyꢀterminated amido
(SAM-8) and ureido (SAM-15) monolayers (Scheme 3) were characterized by wettability
measurements. Contact angle values of 72° 1° and 70° 2° (with water) were measured for
±
±
SAM-8 and SAM-15. These values are similar to those found in the literature for an
organosilicon coupling agent containing a long hydrocarbon chain (C22) and a glycidyl endꢀ
group (72°± ±10°) for a GPTS
1°).22 They are much higher than that reported in the literature (55°
monolayer.45 These results show that the glycidylꢀterminated longꢀchain SAM possesses a
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relatively high hydrophobic character, whereas GPTS monolayers exhibit a poor barrier effect to
water.
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Scheme 3. Schematic representation of the epoxyꢀterminated amido (SAM-8) and epoxyꢀ
terminated ureido (SAM-15) selfꢀassembled monolayers.
The PMꢀIRRAS spectra of epoxyꢀterminated monolayers are presented in Figure 2 in the
3200ꢀ2600 cmꢀ1 region, characteristic to the methylene stretching vibrations, and in the 1700ꢀ
1400 cmꢀ1 region, characteristic to the amide I and II modes. The methylene stretching region
provides specific information about the conformation and van der Waals interactions of the alkyl
chains. No significant difference appears in the wavenumbers of the antisymmetric (
νaCH2) and
symmetric ( sCH2) stretching vibrations of the methylene groups observed respectively at 2928
ν
and 2855 cmꢀ1, revealing a disorder of the alkyl chains in the monolayers for the two epoxyꢀ
terminated monolayers.46 In previous papers concerning functionalized hydrogenꢀbonding
SAMs, we have shown that the distance between the alkyl chains is too large (4.5−5 Å) to
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establish van der Waals interactions due to the presence of urea groups.26,29 This study shows
that the presence of amide groups is also not favorable to form well ordered SAMs.
3
4
5
6
7
8
9
0.010
0.008
0.006
0.004
0.002
0.000
0.010
0.008
0.006
0.004
0.002
0.000
2928
A
B
SAM-15
SAM-8
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2928
2855
2855
1638
1571
1466
1466
1649
1550
3150 3000 2850 2700 1800 1700 1600 1500 1400
3150 3000 2850 2700 1800 1700 1600 1500 1400
Wavenumbers, cm-1
Wavenumbers, cm-1
Figure 2. PMꢀIRRAS spectra of the silylated amido (A) and ureido (B) coupling agents grafted
onto SiO2/Au substrate.
The two broad bands observed at 1649 (1638) cmꢀ1 and at 1550 (1571) cmꢀ1, attributed to
the amide I (νC=O) and amide II (δNH+νCꢀN) modes of amide (urea) group, respectively, confirm
that the molecules are grafted onto the surface. The width of these bands is relatively broad,
indicating a distribution of the hydrogen bonds in the SAMs. The ꢃν values of 99 cmꢀ1 and 67
cmꢀ1 are indicative of strong association of the amide and urea groups by hydrogen bonds in the
epoxyꢀterminated monolayers, respectively. These values appeared significantly lower than those
measured for the free amide (176 cmꢀ1) and urea (131 cmꢀ1) groups in diluted CCl4 solution,
respectively. From this result and considering the disorder of the alkyl chains, the major driving
forces operating in the molecular assembly are mainly based on the strong intermolecular
hydrogenꢀbonding between the amide or urea groups in the monolayers. It is noteworthy that the
monolayers containing internal urea or amide groups are formed only after 12 hours of
immersion in the silanization solution whereas the grafting takes at least 20 hours for monolayers
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possessing long alkyl chains.22 This feature is certainly due to the fact that using hidden polar
functionalities increased the solubility in organic solvent and also the affinity with hydrophilic
surfaces.
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To estimate the surface coverage of SAM-8 and SAM-15 onto SiO2/Au surface, the
IRRAS spectra expected for a compact monolayer of the molecules
(Supporting information, S9 to S12). This calculation has been performed using the isotropic
optical constants of and 15 determined from polarized ATR spectra (Supporting information,
8 and 15 were calculated
8
S9) and considering a thickness of 33 and 35 Å for SAM-8 and SAM-15, respectively.47 The
simulated IRRAS spectra of SAM-8 and SAM-15 are reported in Supporting information (S12)
to be compared with the experimental ones. The intensities of the experimental and calculated
spectra were in the same order of magnitude, revealing a complete surface coverage of SAM-8
and SAM-15 onto SiO2/Au surface. The relative intensity of the amide 1 and amide 2 bands
calculated for the ureido silylated coupling agent didn’t reproduce that observed in the
experimental IRRAS spectrum due to the preferential orientation of the C=O groups parallel to
the surface to form intermolecular hydrogenꢀbonding between adjacent urea groups. In contrast,
for amido silylated coupling agent, the C=O groups seem to be randomly oriented.
Evidence of the presence of reactive epoxide groups on the surface was provided in
epifluorescence microscopy using a fluorescent probe. The dansylcadaverin (λex = 335 nm, λem =
508 nm in methanol) was grafted onto the surface via a nucleophilic attack of the epoxide groups
by the amino group and ringꢀopening condensation.22 For both surfaces, the fluorescence
intensity is homogenously distributed on the whole surfaces, showing that the fluorophore was
homogeneously distributed on the surfaces. As expected, blank experiment, consisting of the
direct treatment of the silica surface by the fluorescent probe, only led to a very weak
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fluorescence signal (lower than 100 RFU) that could be assimilated to the background. As shown
in Figure 3, the signal intensity was slightly higher for SAM-15 (close to 800 RFU) than for
SAM-8 (680 RFU), indicating that ureaꢀSAM exhibited a higher density of accessible surface
epoxide groups. This result is consistent with the higher intensity of the alkyl and amide bands
observed in the PMꢀIRRAS spectra for SAM-15 than for SAM-8. Interestingly, the epoxyꢀ
terminated long chain (C22) SAM exhibits a low reactivity (600 RFU) which may be due to the
high degree of ordering of molecules inducing a steric hindrance between the epoxy groups.22
The presence of the amide or urea groups in the middle of the alkyl chains does not affect the
reactivity of terminalꢀepoxy groups. The better reactivity of SAM-15 could be related to the
stronger intermolecular hydrogenꢀbonding allowing a better cohesion of molecules and limiting
locally the conformational constraints.
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900
800
700
600
500
400
300
200
100
0
Background
SAM-8
SAM-15
Figure 3. Diagram of mean fluorescence intensities for epoxyꢀterminated monolayers grafted
with dansylcadaverin as a fluorescent probe (each point corresponds to the mean value of four
replicates and the errors bars represent
± SD).
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4. CONCLUSION
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In this paper, we described the successful synthesis of silylated coupling agents with
internal amide or urea moieties possessing epoxyꢀterminal groups. These amido and ureido
silylated coupling agents have shown their ability to autoassociate through hydrogen bonds. The
PMꢀIRRAS characterization confirms the grafting of these two silylated coupling agents to
afford the corresponding epoxyꢀterminated monolayers. The molecular assembly is mainly based
on the intermolecular hydrogenꢀbonding between the adjacent amide or urea groups in the
monolayers which favors the kinetic of the formation of the SAMs. The reactivity of the
terminatedꢀepoxy groups was successfully investigated through reaction with a fluorescent
probe, giving evidence that SAMs containing internal urea or amide groups exhibited a higher
density of accessible epoxide groups than for the corresponding longꢀchain (C22) glycidylꢀ
terminated SAM. These functionalized hydrogenꢀbonding SAMs show promise as a novel
platform for the biofunctionalization of surfaces in one step.
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AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: luc.vellutini@uꢀbordeaux.fr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors gratefully acknowledge the financial supports of the “Ministère de la Recherche”
and the CNRS. The microscopy was done in the Bordeaux Imaging Center a service unit of the
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CNRSꢀINSERM and Bordeaux University, member of the national infrastructure France
BioImaging. The help of Christel Poujol is acknowledged.
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SUPPORTING INFORMATION AVAILABLE
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Synthesis of amido and ureido silylated coupling agents and their precursors. Pꢀ and sꢀpolarized
ATR (germanium crystal) spectra of compounds
compounds and 15. Isotropic optical constants of compounds
experimental IRRAS spectra of SAM-8 and SAM-15 with those calculated for compact
monolayers of and 15, respectively. This material is available free of charge via the Internet at
8
and 15. Anisotropic optical constants of
8
8
and 15. Comparison of
8
http://pubs.acs.org.
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(47) These thicknesses correspond to the full length of 8 and 15, considering an allꢀtrans
conformation of the alkyl chains.
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