Letter pubs.acs.org/Langmuir
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.
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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
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 technology. 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 interface.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 © 2014 American Chemical Society
Received: October 7, 2013 Revised: December 18, 2013 Published: January 28, 2014 1199
dx.doi.org/10.1021/la4038233 | Langmuir 2014, 30, 1199−1203
Langmuir
Letter
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.
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
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-
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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 MEGTFA 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/ 1200
dx.doi.org/10.1021/la4038233 | Langmuir 2014, 30, 1199−1203
Langmuir
Letter
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. 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.
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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 onelayer 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 (
