1. Probing the Hydration of Ultrathin Antifouling Organosilane
Adlayers using Neutron Reflectometry
Natalia M. Pawlowska,†
Helmut Fritzsche,‡
Christophe Blaszykowski,§
Sonia Sheikh,†
Mansoor Vezvaie,‡
and Michael Thompson*,†,§
†
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
‡
Canadian Neutron Beam Centre, Chalk River Laboratories, Bldg. 459, Chalk River, Ontario K0J 1J0, Canada
§
Econous Systems Inc., 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
*S Supporting Information
ABSTRACT: Neutron reflectometry data and modeling support the existence of a relatively thick,
continuous phase of water stemming from within an antifouling monoethylene glycol silane adlayer
prepared on oxidized silicon wafers. In contrast, this physically distinct (from bulk) interphase is much
thinner and only interfacial in nature for the less effective adlayer lacking internal ether oxygen atoms. These
results provide further insight into the link between antifouling and surface hydration.
■ INTRODUCTION
Contact of artificial materials with biological fluids (e.g., blood,
serum, and urine) is very common in many aspects of modern
medicine. Whether applied in vitro or in vivo, the range of
equipment/devices is immense and includes, for instance, the
circuitry used in renal hemodialysis and coronary bypass,
catheters, stents, and pacemakers as well as biosensors for
clinical diagnostics and, more recently, lab-on-a-chip technol-
ogy. Medical applications, which involve surfaces made of
titanium, stainless steel, ceramics, or plastic polymers, have
spawned considerable research into the molecular and cellular
interactions that occur at the material/biological fluid inter-
face.1
There are two key factors that are responsible for this
activity: first and foremost are the surface-induced deleterious
effects that may be instigated as a result of material
bioincompatibility, and second is biosensor platform fouling
that radically interferes with the detection process.2
The
common ground that connects these issues is the spontaneous
surface adsorption of proteins.2
Accordingly, it is not surprising
that extensive literature documenting the development of
antifouling coatings has appeared over the years.2
One of the most studied antifouling surface chemistries is the
family of poly- and oligoethylene glycol-based constructs (PEG
and OEG).3
Although the origin of the “PEG/OEG-effect” has
long been debated, there is now a general consensus that
hydration plays a key role, especially for coatings made of short
EG building block molecules where the role of entropy linked
to chain flexibility/compression is less significant.3
One
explanation for surface antifouling connected to hydration is
the concept of a “water barrier”, where embedded and
interfacial water molecules are tightly bound and organized
into permeated structures that have an energy cost in terms of
disturbance.3
Such barriers have been shown to be physically
distinct, solute-free exclusion zones projecting up to several
hundred micrometers into the contiguous aqueous phase.4
An
alternative suggestion hypothesizes that there is simply no
energy gain provided through adsorption of proteins (solvated
in the aqueous medium) to a stably hydrated surface.5
Recently, we showed that surface hydration is indeed
intimately involved in the mechanism of antifouling, for thiol
and silane monoethylene glycol (MEG) adlayers prepared on
gold6
or quartz.7
In the latter work, another key feature was the
observation that the internal ether oxygen atom is necessary to
dramatically alter the dynamics of full serum adsorption, the
synergy being strongest for silane adlayers possessing terminal
hydroxyl moieties (MEG−OH, Scheme 1, top).7
In contrast,
the effect was considerably less pronounced for OTS−OH
adlayers lacking internal atoms of oxygen (Scheme 1, bottom).7
It was then hypothesized that the unique antifouling properties
of the MEG−OH system are rooted in a special intrafilm zone
of hydration from which a continuous, physically distinct,
interfacial phase of water stems.6,7
Herein, we examine this theory and present the results of a
follow-up study investigating the hydration of both MEG−OH
Received: October 7, 2013
Revised: December 18, 2013
Published: January 28, 2014
Letter
pubs.acs.org/Langmuir
© 2014 American Chemical Society 1199 dx.doi.org/10.1021/la4038233 | Langmuir 2014, 30, 1199−1203

2. and OTS−OH silane adlayers using neutron reflectometry
(NR). For the purpose of NR, we prepared the adlayers on
oxidized silicon wafers (Si/SiO2), rather than monolithic
quartz, in order to exploit the oxide layer to enhance the
detection of our ultrathin films and their interfacial region. For
the antifouling MEG−OH system, modeling of NR data is
compatible with the presence of a continuous, relatively thick
transition zone of water toward bulk. Conversely, this physically
distinct phase is thinner for the OTS−OH system and confined
to the adlayer | bulk water interface. These observations
support the critical role hypothesized for the internal ether
oxygen atom with respect to surface hydration and antifouling.
Neutron reflectometry is one of various techniques8
available
to investigate the properties of water at surfaces. In practice,
NR probes the specular reflection of neutrons off atomic nuclei
and, being sensitive to scattering by light isotopes (i.e., H and
D), indeed allows for the assessment of interfacial water
density9
as well as film water absorption.10,11
Reflectivity is
collected as a function of the neutron scattering vector (q),
which depends on the neutron wavelength and scattering angle.
Most modeling programs use some variation of a recursion
formula simulating specular reflectivity of neutrons from a
stratified medium and calculate a reflectivity curve based on a
model of the system under study. The program used herein,
MOTOFIT, relies on the Abeles matrix method that provides
identical results to the traditional Parratt formalism.12,13
The
generated model curve is then compared to measured
reflectivity data and assessed for fitting quality using the least-
squares method (χ2
). Data fitting for a selected layered model is
optimized by adjusting the thickness (d), interfacial roughness
(σ), and scattering length density (SLD) of the various layers
under investigation. More details on NR can be found
elsewhere.14,15
■ EXPERIMENTAL SECTION
Silicon wafers [(111), 100 mm in diameter, 5000 μm thick] were
heated at 850 °C to prepare oxide films ∼160−180 Å thick, which
were subsequently rinsed with various solvents and then hydrated
overnight in a humidity chamber. A 1/1000 (v/v) solution of MEG-
TFA or OTS-TFA surface modifiers were prepared by diluting 75 μL
of neat silane in 75 mL of anhydrous toluene. The oxidized Si/SiO2
wafers were placed in the silane solution for 60 min followed by
rinsing and sonication in various solvents. The silanized wafers were
then soaked in a 1/1 (v/v) solution of MeOH/H2O overnight to
cleave the terminal trifluoroacetyl (TFA) headgroup (Scheme 1). A
detailed description of Si wafer oxidation, cleaning, and silanization is
described in the Supporting Information.
NR measurements were performed on the D3 neutron
reflectometer at the NRU reactor in Chalk River, Ontario, Canada.
NR measurements on the prepared samples started with both the
incident and reflected neutron beams on the side of the air/film
interface (see Figure 1 of the Supporting Information). For
measurements in liquid, the samples (now part of a cell) were
mounted in the so-called “flipped” sample configuration, where the
neutrons traverse the Si disc, as depicted in Figure 1. For these
measurements, the technique of contrast variation was used, where the
SLD of the bulk solution was matched to that of the SiO2 layer
(SLDSiO2 = 3.48 × 10−6
Å−2
) to enhance the scattering contrast of the
adlayer and interfacial region. The solution was prepared from a 175/
Scheme 1. Two-Step Formation of the MEG−OH Adlayer on Si/SiO2 Substrate (top) and of Its OTS−OH Alkylated Homolog
Lacking Internal Ether Oxygen Atoms (bottom).7a
a
Note: This schematic representation merely depicts the performed surface chemistry, not the actual surface coverage of the adlayers or the degree
of order and packing of the surface-modifying residues within.
Langmuir Letter
dx.doi.org/10.1021/la4038233 | Langmuir 2014, 30, 1199−12031200

3. 125 (v/v) mixture of D2O/H2O (SLDB = 3.54 × 10−6
Å−2
) and herein
referred to as “contrast-matched water” (CMW). For measurements
performed in air, neutrons hit samples from the ‘air’ side.
■ RESULTS AND DISCUSSION
In order to be able to fit the measurements performed in liquid,
the samples were first scanned in air to determine the thickness
of the thermally grown oxide layer, dSiO2, as well as interfacial
roughness values, σSi−SiO2
and σSiO2−1 (see the Supporting
Information). For NR measurements in liquid, the simplest
approach to assess the presence of a transition zone of water
physically distinct from the bulk was to fit the data with a one-
layer model (Figure 1). Within this system, the silane adlayer
and transitional water are treated as one single medium (‘layer
1’) since the adlayers under investigation are very thin (<1
nm)16
and not easily resolved in the scanned q range (0.005−
0.10 Å−1
). Modeling the reflectivity curve in liquid involved a 2-
phase fitting procedure: a general fit of SLD1, d1, and σ1−B
followed by the refinement of all parameters, including those
obtained in air and known substrate parameters (see the
Supporting Information for a detailed description of the fitting
procedure). Reflectivity and resulting SLD profiles in CMW are
gathered in Figure 2, panels a−b and c, respectively. Also added
in Figure 2 (panels a and b) is the reflectivity curve for bare
(unmodified) Si/SiO2, which is noticeably featureless (i.e.,
absent Kiessig fringes) demonstrating that (1) there is no
distinguishable “medium” between bulk water and SiO2 layer
and (2) the prepared D2O/H2O solution is indeed contrast-
matched. The observed critical edge at q ≈ 0.008 Å−1
is in
agreement with the expected value for the Si/CMW interface in
our geometry (Figure 1), with the neutrons going through the
Si wafer.
Upon addition of the MEG−OH adlayer, fringes appeared
on the reflectivity curve (Figure 2a). As opposed to bare SiO2,
these fringes reveal/confirm the presence of a distinguishable
layer sandwiched between the two contrast-matched media
(i.e., SiO2 and bulk CMW). The low amplitude of these fringes
and smooth minima indicate that the intercalated medium is
not well-defined (possibly due to low contrast/high roughness
between adjacent media). The corresponding SLD profile in
Figure 2c reveals this medium to be ∼40 Å thick with a
minimum SLD of ∼3.2 × 10−6
Å−2
encompassing “layer 1”
(∼20 Å) and an additional water gradient spanning over ∼20 Å
until bulk water resumes (χ2
= 2.32 × 10−2
). With respect to
layer 1, a higher than expected SLD at the SiO2 interface
compared to other organic films,9
and close to that of CMW,
suggests that the MEG−OH film absorbed water.17
This would
provide a strong basis for the existence of a special intrafilm
zone of hydration involving the internal atoms of oxygen.7
This
hypothesis is also supported by a recent account on water
penetrating into EG films.11
Equally as interesting is the long-
range effect of this ultrathin MEG−OH adlayer, which is
Figure 1. One-layer model schematically depicting (from bottom to
top) the Si/SiO2 substrate, the silane adlayer as well as interfacial and
bulk water. Also shown are the thickness (d), roughness (σ), and
scattering length density (SLD) values of the various layers. In the
one-layer model, silane adlayer and interfacial water are treated as one
single medium. The arrow represents the direction of the incident
neutron beam.
Figure 2. Reflectivity data and corresponding fits for (a) bare (red ○)
and MEG−OH modified (black ○) Si/SiO2 and (b) bare (red ○) and
OTS−OH modified (black ○) Si/SiO2 measured in CMW. (c)
Corresponding SLD profiles for MEG−OH (dashed red line) and
OTS−OH (solid blue line) systems generated using a one-layer model
(Si substrate not shown). Note: for comparison and clarity purposes,
the SLD baseline for MEG−OH was shifted upward by 10−7
Å−2
, and
both SiO2/layer 1 interfaces were aligned at “0 Å”.
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4. demonstrated by the existence of the additional, physically
distinct interphase/transition zone of water (following the SLD
minimum) that spans over a ∼20 Å gradient until bulk water
resumes (Figure 2c).
To determine whether the internal atoms of oxygen play a
role in the formation of the continuous interphase of water, we
next performed NR measurements with the OTS−OH adlayer
that lacks such a feature (Scheme 1). When comparing the
reflectivity profiles of both systems (Figure 2, panels a and b), it
is immediately evident that the interaction of water with OTS−
OH is measurably different. On the basis of initial observations,
the sharp, high-amplitude fringes suggest the existence of a
better-defined intercalated layer with a refractive index now
deviating substantially from the surrounding, contrast-matched
media. The effect is more evident in the SLD profiles in Figure
2c, where the sandwiched medium’s SLD is significantly lower
than that of bulk water. For the OTS−OH system, fitting the
data with a 1-layer model revealed that a transition zone of
water also exists but is much thinner than that of MEG−OH
(confined to ∼20 Å) with a minimum SLD of ∼1.15 × 10−6
Å−2
(χ2
= 6.73 × 10−3
). This SLD value is more representative
of an organic coating,9
suggesting that the OTS−OH adlayer
may be impenetrable by water18
and that interfacial water
organization is different than MEG−OH’s. Moreover, unlike
this latter system, the thickness of “layer 1” modeled for OTS−
OH makes up for the entire thickness of the sandwiched
medium, and the transition toward bulk water is sharper. For
the OTS−OH system that lacks internal ether atoms of oxygen,
water organization appears to be interfacial in nature, with
shorter-range but higher structuration up to the bulk interface.
A final comment must be made regarding adlayer surface
coverage, a parameter that surely plays a role in surface
hydration, hence on the final SLD values. Whether or not
MEG−OH and OTS−OH adlayers present similar surface
coverage, it is reasonable to assume that water would infiltrate
patchy adlayers (an empirical fact) at defected/damaged sites.19
This argument can very well contribute in explaining the water-
like SLD value observed for the MEG−OH organic film
(Figure 2c), and the water absorption properties attributed to
this system. This behavior is to be contrasted with that of the
impermeable OTS−OH adlayer, which as mentioned earlier
displays at the SiO2 interface an SLD value substantially
different from that of CMW. In light of this, it is likely to be the
case that the internal atoms of oxygen in MEG−OH, which
OTS−OH lacks, are involved with maintaining embedded
water molecules within the assembly through H-bonding,
irrespective of whether the interactions occur in interstitial,
uncovered areas or “intermolecularly” in coated ones. There-
fore, the issue of surface coverage does not alter our
conclusions with respect to the distinct interaction pattern of
water with MEG−OH versus OTS−OH adlayers.
■ CONCLUSION
Collectively, neutron reflectometry (NR) data and modeling
showed distinct hydration behavior between the MEG−OH
and OTS−OH systems, confirming the significance of the
internal oxygen atom in determining the adlayers’ interaction
with water. We have proposed herein that a relatively thick
transition zone of water may exist continuously with an
ultrathin (<1 nm) monoethylene glycol silane adlayer prepared
on Si/SiO2 (MEG−OH). Conversely, this physically distinct
phase is thinner and only interfacial in nature for the adlayer
lacking internal ether oxygen atoms (OTS−OH). NR data
support the hypothesis according to which the pronounced
antifouling properties of the MEG−OH system (which the
OTS−OH one lacks) stem from a special intrafilm zone of
hydration that involves the key participation of the internal
atoms of oxygen and from which the diffuse interphase of water
originates. Another key difference between both systems is their
water uptake properties: while OTS−OH adlayers seem to be
impenetrable by water, MEG−OH ones present hydrogel-like
characteristics. Although the systems investigated in the present
study are unique and comparison against other antifouling
coatings is limited, our findings support the link between
antifouling behavior and the manner surfaces are hydrated. To
our knowledge, this work constitutes one of the first examples
studying the hydration of subnanometric EG coatings.
■ ASSOCIATED CONTENT
*S Supporting Information
Experimental details (Si/SiO2 substrate preparation and adlayer
formation), NR scan parameters, and data fitting. This material
is available free of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: mikethom@chem.utoronto.ca.
Author Contributions
All authors contributed equally.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC). The
research was made possible by a reflectometer jointly funded by
Canada Foundation for Innovation (CFI), Ontario Innovation
Trust (OIT), Ontario Research Fund (ORF), and the National
Research Council Canada (NRC). S.S. also gratefully thanks
the Ontario Graduate Scholarship (OGS) program for financial
support.
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