Ionic strength mediated hydrophobic force switching of CF 3-terminated ethylene glycol self-assembled monolayers (SAMs) on gold

Article abstract of DOI:10.1039/b712968h

We have synthesised novel oligo(ethylene glycol), CF₃-terminated switching self-assembled monolayers, which allow the force experienced by a hydrophobic object to be controlled via the ionic strength of the environment. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712968h

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Ionic strength mediated hydrophobic force switching of CF3-terminated
ethylene glycol self-assembled monolayers (SAMs) on gold{
Nelly Bonnet, David O’Hagan{* and Georg Ha¨hner*
Received (in Cambridge, UK) 22nd August 2007, Accepted 20th September 2007
First published as an Advance Article on the web 1st October 2007
DOI: 10.1039/b712968h
colloidal chemistry. Similar surfactants might become important in
bio-related fields and in connection with biosensors since they have
the potential to adsorb/immobilise proteins reversibly if the switch
can be tailored to occur at an ionic strength that is compatible with
proteins.
We have synthesised novel oligo(ethylene glycol), CF3-termi-
nated switching self-assembled monolayers, which allow the
force experienced by a hydrophobic object to be controlled via
the ionic strength of the environment.
Alkane-thiol based self-assembled monolayers (SAMs) have seen
an ever increasing level of interest since they were first introduced
more than two decades ago. Part of their attraction is due to the
ease of their preparation in combination with the great flexibility
they offer to create concentrated planes of functionality by
modifying the surfactant molecules. One prominent example are
oligo(ethylene glycol) (OEG)-terminated alkanethiol SAMs. They
have been shown to resist the non-specific adsorption of some
proteins1,2 and hence are of significant interest in the life sciences
and related areas. Inspired by their protein repelling properties
several studies were carried out to determine the underlying
mechanisms and the interactions involved, e.g. refs. 3–9. It had
been observed that the protein adsorption increases with the
hydrophobicity of the substrate.1,2 Mixed films of hydrophobically
and OEG terminated surfactants were prepared to study the
influence of these functionalities on protein adsorption proper-
ties.10 Other studies reported on the force between charged and
hydrophobic probes and the monolayer films.6,7 The forces
measured with a hydrophobic probe (attraction or repulsion) were
found to correlate with the protein adsorption properties of the
films (adsorption or resistance to adsorption).5,7 The forces
indicated the existence of an electrical double layer,5–7,11 suggesting
that an electrostatic component is involved. This has been
confirmed by theoretical calculations.12,13
The surfactants we have synthesized are OEG-modified
alkanethiols with three or four ethylene glycol units, and with a
hydrophobic trifluoropropyl or trifluorobutyl group on top, here
referred to as molecules 1, 2 and 3, and shown in Fig. 1. These
surfactants allow the systematic study of the influence of increasing
hydrophobicity (1 vs. 2) or additional OEG groups (1 vs. 3) on the
force experienced by an object approaching the surfaces.
The surfactants were synthesised following the synthetic route
illustrated in Fig. 2 and similar to the one reported in ref. 18 (see
ESI{ for details on the synthesis).
The required SAMs were prepared by immersing gold substrates
(50 nm thick evaporated gold films on Si wafers primed with 5 nm
Cr as an adhesion promoter) into 1 mM solutions of 1–3 in
dimethylformamide for 12–16 h. The films were subsequently
characterized by photoelectron spectroscopy (XPS), ellipsometry,
contact angle measurements, and atomic force spectroscopy. All
measurements were performed at room temperature.
The XPS survey spectra showed the presence of carbon, oxygen,
fluorine and gold as expected. Elemental ratios were determined
from single region spectra. All ratios (C/O, C/F, O/F,
Cether/Caliphatic) were close to the expected values with no indication
for the presence of a significant amount of contamination. The
results compare well to those published for similar molecules in the
literature.9 Detailed results are given in the ESI.{ The resulting
densities of molecules on the surface were extracted from the
measured C/Au signal ratios using an octadecanethiol and a
dodecanethiol film as a reference.9 The values obtained are 77, 81
and 73% for 1, 2 and 3, respectively. They are in agreement with
results reported for similar methyl terminated films.9
Here we report on several surfactant films based on oligo(ethy-
lene glycol) modified alkanethiols that switch between an attractive
and a repulsive force on approach with a hydrophobic probe
depending on the ionic conditions of the environment. Recently, a
significant level of interest has emerged in SAMs with dynamic
properties, in particular those with switching properties. External
switching parameters such as an electric field,14,15 pH,16 or
temperature have been explored in connection with SAMs.17
The ionic strength is an external parameter that can be easily
controlled. The films presented here are interesting for the
reversible immobilisation of hydrophobic (nano)particles and in
The ellipsometry measurements of the SAMs gave thickness
values of 1.8 nm for 1, 1.9 nm for 2, and 2.0 nm for 3. A value of
EaStCHEM School of Chemistry, University of St Andrews, North
Haugh St Andrews, UK KY16 9ST. E-mail: gh23@st-andrews.ac.uk;
Fax: +44 1334 463808; Tel: +44 1334 463889
{ Electronic supplementary information (ESI) available: Details of the
synthesis of the surfactants, surface analysis (XPS, ellipsometry, contact
angle measurements, atomic force spectroscopy) and calculation of
molecular density. See DOI: 10.1039/b712968h
Fig. 1 Schematic of the three surfactants synthesised: (1-mercaptoundec-
11-yl)tri(ethylene glycol) 3,3,3-trifluoropropyl ether (1), (1-mercaptoundec-
11-yl)tri(ethylene glycol) 4,4,4-trifluoro-n-butyl ether (2) and (1-mercap-
toundec-11-yl)tetra(ethylene glycol) 3,3,3-trifluoropropyl ether (3).
{ Author for correspondence on the synthesis only. do1@st-andrews.ac.uk
5066 | Chem. Commun., 2007, 5066–5068
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Fig. 2 General synthetic pathway for the (mercaptoundec-11-yl)-
poly(ethylene glycol) trifluoro alkyl ether surfactants.
1.45 was used for the refractive index of the organic films.10
Compared to the theoretical lengths of the molecules of 2.64, 2.76
and 2.92 nm, and taking into account that the alkane chains are
tilted similar to alkanethiol films,19 the resulting packing density
for the films relative to a dodecanethiol SAM can be determined to
be around 81, 82 and 82%, respectively. These values are consistent
with those obtained from XPS.
The average advancing water contact angle values obtained
were 93, 96 and 94u for 1, 2 and 3, respectively. Values for the
CH3-terminated counterparts of 1 and 2 are 94 and 107u,
respectively.9 Our results indicate similarly densely packed films
with a significant fraction of hydrophobic groups exposed. The
addition of one CH2 group (1 vs. 2), however, appears to have a
smaller effect on the hydrophobicity in the case of our CF3-
terminated surfactant, where only a moderate increase from 93 to
96u was found.
Fig. 3 Force–distance curves recorded in KNO3 solutions of different
concentrations with a hydrophobic probe on approach of the SAMs of the
three surfactants synthesised. Top: surfactant 1; middle: 2, bottom: 3. 1/k
indicates the experimental Debye lengths of the repulsive forces.
was observed. This switch occurred, however, at different
concentrations for the different films. It was between 1 and
10 mM for 1, below 1 mM for 2, and between 10 and 100 mM
for 3. The decay lengths of the forces were determined from
exponential fits to the data and found to be in accordance with the
expected Debye lengths based on the ionic strengths of the solution
for concentrations of 1 mM and above.21 Note that for lower than
1 mM concentrations a deviation from the theoretical Debye
length has been observed before6 and is likely due to dissolved
CO2. The actual ion concentration will therefore be higher.
The force experienced by the hydrophobic tip is a superposition
of the hydrophobic attraction, which is independent of the ionic
strength, and a repulsive electrostatic component due to the OEGs
and the electrolyte that depends on the ionic strength.7 The latter is
screened in high ionic concentrations, i.e., it is of short range. As a
result, the long range hydrophobic force dominates in highly
concentrated solutions. The relative strengths are influenced by the
relative amounts of hydrophobic and OEG groups in the film. The
switching is therefore a consequence of the resulting electrostatic
interaction between the hydrophobic probe and the OEG units in
Force measurements on these films were performed with AFM
tips coated by thermally evaporating a nickel/chromium mixture
(5 nm) as an adhesion promoter followed by 50 nm of gold. Probes
were made hydrophobic by deposition of a dodecanethiol self-
assembled monolayer from solution. The hydrophobicity of the
tips was checked by recording force–distance curves on an
dodecanethiol SAM under electrolyte solution at different
concentrations. The results indicated hydrophobic and stable
probes (see ESI{). Force–distance curves on the films prepared
from the synthesised molecules were measured in KNO3 solutions
of different concentrations. Fig. 3 shows the forces experienced by
a probe upon approaching the different surfaces and depending on
the distance between the probe and surface. These curves are
obtained by converting the measured cantilever deflection vs. piezo
displacement curves as previously described.20 Zero distance
corresponds to a hard-wall potential. Fig. 3 displays typical results
obtained after averaging 10 curves.
At low KNO3 concentrations the force experienced by the
hydrophobic tip was repulsive for all prepared SAMs. When the
ionic strength was increased a switch from repulsion to attraction
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the electrolyte7 in combination with the interaction with the
hydrophobic end group.
Notes and references
1 C. Pale-Grosdemange, E. S. Simon, K. L. Prime and G. M. Whitesides,
J. Am. Chem. Soc., 1991, 113, 12–20.
It is likely that the trifluoromethyl group at the top causes
slightly more disorder in the films than a methyl group22 and also
induces a stronger dipole moment. This is supported by the
contact angle values measured for 1 and 2. The CF3-terminated
SAMs show generally slightly lower water contact angle values
than their methyl terminated counterparts which could be due to a
disordering at the interface.22 The fact that the contact angles we
measure for our CF3-terminated surfactant films are lower than
those of the corresponding CH3-terminated films therefore suggest
a higher degree of disorder at the top of the molecules. This
disorder will expose more of the ethylene glycol units such that the
resulting films resemble more closely a ‘mixed film’ than their
methyl terminated counterparts. Other groups have reported that
discrete mixtures of hydrophobic and resistant groups coadsorbed
show different amounts of protein adsorption10 and can reversibly
adsorb proteins depending on the temperature.17 The species
presented here combine both hydrophobic and hydrophilic
structural elements and could obviate the need for adsorption
from multicomponent solutions making the preparation of such
films easier.
2 K. L. Prime and G. M. Whitesides, J. Am. Chem. Soc., 1993, 115,
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21 J. Israelachvili, Intermolecular & Surface Forces, Academic Press,
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In summary, the present study demonstrates that the interaction
between a hydrophobic probe and the prepared SAMs can be
reversibly switched by changing the ionic strength of the
environment. For the films presented the switch occurred at
concentrations between 1 and 100 mM depending on the
composition of the surfactant. The addition of a hydrophobic
methylene group to the surfactant molecule shifted the switch to
lower ionic strengths, while adding an ethylene glycol group
moved the switch to higher concentrations. This indicates that the
switching property can be tailored depending on the composition
of the surfactant and suggests that a suitable ratio of hydrophobic
and OEG groups might produce a switch at concentrations
compatible with the natural environment of proteins.
Financial support from the University of St Andrews is
gratefully acknowledged.
22 D. Barriet and T. R. Lee, Curr. Opin. Colloid Interface Sci., 2003, 8,
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5068 | Chem. Commun., 2007, 5066–5068
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