A Comparative Study of Surfactant Adsorption on Model Surfaces Using the Quartz Crystal Microbalance and the Ellipsometer

Reactive Simulations of Silica Functionalization with Aromatic Hydrocarbons

Reactive molecular dynamics simulations are used to model the covalent functionalization of amorphous silica with aromatic hydrocarbons. Simulations show that the surface density of silanol-terminated phenyl, naphthyl, and anthracenyl molecules is lower than the maximum value calculated based on molecule geometry, and the simulation densities decrease faster with the number of aromatic rings than the geometric densities. The trends are analyzed in terms of the surface-silanol bonding configurations, tilt angles, local conformational ordering, and aggregation of surface-bound molecules under steady-state conditions. Results show that the surface density is affected by both the size and symmetry of the aromatic hydrocarbons. The correlations among bonding, orientation, and surface density identified here may guide the selection or design of molecules for functionalized surfaces.

A Two-Phase Model for Adsorption from Solution Using Quartz Crystal Microbalance with Dissipation

Quartz crystal microbalance with dissipation (QCM-D) conveniently monitors mass and mechanical property changes of thin films on solid substrates with exquisite resolution. QCM-D is frequently used to measure dissolved solute/sol adsorption isotherms and kinetics. Unfortunately, currently available methodologies to interpret QCM-D data treat the adlayer as a homogeneous medium, which does not adequately describe solution-adsorption physics. Tethering of the adsorbate to the solid surface is not explicitly recognized, and the liquid solvent is included in the adsorbate mass, which is especially in error for low coverages. Consequently, the areal mass of adsorbate (i.e., solute adsorption) is overestimated. Further, friction is not considered between the bound adsorbate and the free solvent flowing in the adlayer. To overcome these deficiencies, we develop a two-phase (2P) continuum model that self-consistently determines adsorbate and liquid-solvent contributions and includes friction between the attached adsorbate and flowing liquid solvent. We then compare the proposed 2P model to those of Sauerbrey for a rigid adlayer and Voinova et al. for a viscoelastic-liquid adlayer. Effects of 2P-adlayer properties are examined on QCM-D-measured frequency and dissipation shifts, including adsorbate volume fraction and elasticity, adlayer thickness, and overtone number, thereby guiding data interpretation. We demonstrate that distinguishing between adsorbate adsorption and homogeneous-film adsorption is critical; failing to do so leads to incorrect adlayer mass and physical properties.

Adsorption and Lubrication of α-Gel (α-Form Hydrated Crystal) Dispersion at Solid/Liquid Interfaces

We characterized the adsorption and desorption of α-gel (α-form hydrated crystal) dispersions in aqueous media using a quartz crystal microbalance with dissipation monitoring (QCM-D) technique. The α-gel was formed from a mixture of N-[3-(dimethylamino)propyl]docosanamide (APA-22) L-lactic acid salt, 1-octadecanol (C₁₈OH), and water. The solid substrate employed in this study as a model for hair was silica. The QCM-D measurements revealed that the α-gel dispersions yielded a rigid adsorption film on the negatively charged silica surface. The adsorption mass decreased with decreasing domain size (on the micrometer scale) of the α-gel dispersions. The adsorption film highly restricted the desorption of the α-gel from the silica surface even after rinsing with water. The adsorption film also exhibited excellent lubrication ability in aqueous media both before and after rinsing with water. We expect that the α-gel formed by APA-22 L-lactic acid is a potential ingredient for formulating an environment-friendly hair conditioner owing to its high adsorption, limited desorption, and excellent lubrication abilities on the solid surface.

Friction between soft contacts at nanoscale on uncoated and protein-coated surfaces

The understanding of friction on soft sliding biological surfaces at the nanoscale is poorly understood as hard interfaces are frequently used as model systems. Herein, we studied the influence of elastic modulus on the frictional properties of model surfaces at the nanoscale for the first time. We prepared model silicone-based elastomer surfaces with tuneable modulus ranging from hundreds of kPa to a few MPa, similar to those found in real biological surfaces, and employed atomic force microscopy to characterize their modulus, adhesion, and surface morphology. Consequently, we used friction force microscopy to investigate nanoscale friction in hard-soft and soft-soft contacts using spherical colloidal probes covered by adsorbed protein films. Unprecedented results from this study reveal that modulus of a surface can have a significant impact on the frictional properties of protein-coated surfaces with higher deformability leading to lower contact pressure and, consequently, decreased friction. These important results pave the way forward for designing new functional surfaces for serving as models of appropriate deformability to replicate the mechanical properties of the biological structures and processes for accurate friction measurements at nanoscale.

Universal scaling of adsorption of nonionic surfactants on carbonates using cloud point temperatures

HYPOTHESIS

Nonionic surfactants alter the wettability of oil-wet carbonate surfaces to a water-wet state. The degree of surfactant adsorption is expected to determine the extent of the wettability alteration. Furthermore, the structure of the hydrophobic and hydrophilic units of the surfactant should affect the degree of adsorption and correlate with the wettability alteration.

EXPERIMENTS

The adsorption on Indiana limestone was measured for nonionic surfactants with two different types of hydrophobic units and hydrophilic polyethoxylate units ranging from 15 to 40 mers. Measurements were conducted for several surfactant concentrations and temperatures.

FINDINGS

Adsorption increased with temperature and for surfactants with fewer hydrophilic groups. The adsorption occurs as micelles rather than individual surfactant molecules. An increase in adsorption is observed for the more hydrophobic surfactants at higher temperature and is attributed to the increase in micelle sizes. Adsorption collapses onto a universal curve as a function of the difference between cloud point of the surfactant and system temperature. At the same time wettability alteration was found to have a direct correlation with surfactant adsorption. These findings are critical for judicious selection of nonionic surfactants for analysis and design of wettability alteration for oil reservoirs.

Insight into Kinetics and Mechanisms of AOT Vesicle Adsorption on Silica in Unfavorable Conditions

The structure of adsorbed surfactant layers at the equilibrium state has already been investigated using various experimental techniques. However, the comprehension of the formation of structural intermediates in nonequilibrium states and the resulting adsorption kinetics still remain a challenging task. The temporal characterization of these intermediate structures provides further understanding of the layer structure at equilibrium and of the main interactions involved in the adsorption process. In this article, we studied the adsorption kinetics of AOT vesicles on silica at different pHs at ambient temperature. The AOT vesicles were formed in a brine solution. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to obtain information on the kinetics of surfactant adsorption and on the structure of the adsorbed layer at the equilibrium state. Additionally, neutron reflectivity experiments were performed to provide a detailed description of the mean surfactant concentration profile normal to the surface at equilibrium. Results suggest that vesicles in the bulk influence the adsorption mechanisms. In acidic conditions, after a time-dependent structural rearrangement step, followed by the rupture of initially adsorbed vesicles, the formation of a bilayer was observed. At an intermediate and basic pH, in spite of the electrostatic repulsion between the negatively charged surfactants and silica, results demonstrated the existence of an adsorbed layer composed of AOT vesicles. Vesicles are more or less closely packed depending on the pH of the solution. Results show a non-negligible influence of NaCl addition at pH values where adsorption is initially inhibited. Vesicle adsorption at the intermediate and basic pH is probably due to the combination of attractive van der Waals interactions promoted in high ionic strength systems and the formation of hydrogen bonds. Interpretation of adsorption kinetics gave insight into adsorption mechanisms in an electrostatic repulsion environment.

Corrosion inhibition of iron surfaces with phosphatidic acid

Preventing the corrosion of iron in inaccessible structures requires a coating method that reaches all surface areas and creates a uniform protective layer. An ages old practice to protect iron artefacts is to coat them with animal fat, that is, a mixture of lipids. This “method” is accidentally ingenious: some natural phospholipids found in animal fat have the potential to form a tightly packed self-assembled monolayer on metal oxide surfaces, similar to the surfactant monolayers that have attracted increasing attention lately. Thus, the most primitive corrosion prevention method may point at a way to coat complex iron structures in an industrial environment. Here the ability of phosphatidic acid, a natural lipid, to coat and protect iron surfaces was examined. Iron coated quartz crystal microbalance (QCM) sensors were used for the experiments, to monitor the deposition of the lipid as well as the acidic corrosion (dissolution) of iron in situ, in real time. The sensors were coated by self-assembled monolayers of di-myristoyl phosphatidic acid using the liposome deposition method. In this process, 50-100 nm vesicles formed by the lipid are delivered in an aqueous solution and spontaneously coat the iron surfaces upon contact. QCM and ellipsometry measurements confirmed that continuous bilayer and monolayer surface coatings can be achieved by this method. QCM measurements also confirmed that the layers were corrosion resistant in 0.01M acetic acid solution that would dissolve the thin iron layer in minutes in the absence of the protective coating. XPS results suggested a chemisorption-based mechanism of phosphatidic acid attachment to the iron surface. Hence, liposome deposition of phosphatidic acid offers a suitable solution to coat iron surfaces in inaccessible structures in situ.

Atomic Force Microscopy Force Mapping Analysis of an Adsorbed Surfactant above and below the Critical Micelle Concentration

Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.

Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes

We report investigations on the salt sensitivity of the thermoresponsive behavior of PNIPAAm brushes applying the quartz crystal microbalance coupled with spectroscopic ellipsometry technique. This approach enables a detailed study of the optical and mechanical behavior of the polymer coatings. Additional conclusions can be drawn from the difference between both techniques due to a difference in the contrast mechanism of both methods. A linear shift of the phase-transition temperature to lower temperatures with the addition of sodium chloride was found, similar to the behavior of free polymer chains in solution. The thermal hysteresis was found to be decreased by the addition of sodium chloride to the solution, hinting to the interaction of the ions with the amide groups of the polymer, whereby the formation of hydrogen bonds is hindered. The results of this study are of relevance to the application of PNIPAAm brushes in biological fluids and demonstrate the additional potential of the ion sensitivity besides the better known thermosensitivity.

Competitive surfactant adsorption of AOT and Tween 20 on gold measured using a quartz crystal microbalance with dissipation

Competitive surfactant adsorption of anionic surfactant AOT and nonionic surfactant Tween 20 on gold was investigated by using a quartz crystal microbalance with dissipation (QCM-D) at 25 °C. The adsorption isotherm of pure AOT did not reach a plateau at the CMC, but rather adsorption continued to increase gradually at concentrations higher than the CMC before reaching a plateau. This behavior is evidence of competitive adsorption between AOT and impurities. The adsorbed layer of AOT on gold became more viscoelastic as the concentration of AOT increased. Tween 20 reached the plateau adsorption on gold before its concentration reached the CMC, suggesting that the attraction between Tween 20 and gold is very strong. The Tween 20 adsorbed layer was rigid when compared to the AOT adsorbed layer, as indicated by low dissipation. The addition of Tween 20 to a surface covered by AOT resulted in an increase in adsorbed mass, suggestive of the insertion of Tween 20 into the AOT adsorbed layer as expected because Tween 20 is able to separate the repulsive headgroups of AOT. When AOT was added to a preformed Tween 20 layer, a drop in the adsorbed amount was found between 0 and 0.1 CMC, and then no change was observed until the CMC of AOT was reached; the adsorbed amount then increased, reaching a final adsorption greater than that of pure AOT. All data support the formation of mixed surfactant layers on the surface. Although a two-step model fit both AOT and Tween 20 adsorption kinetic data well, AOT was found to adsorb much more slowly than Tween 20.

Using complementary acoustic and optical techniques for quantitative monitoring of biomolecular adsorption at interfaces

The great wealth of different surface sensitive techniques used in biosensing, most of which claim to measure adsorbed mass, can at first glance look unnecessary. However, with each technique relying on a different transducer principle there is something to be gained from a comparison. In this tutorial review, different optical and acoustic evanescent techniques are used to illustrate how an understanding of the transducer principle of each technique can be exploited for further interpretation of hydrated and extended polymer and biological films. Some of the most commonly used surface sensitive biosensor techniques (quartz crystal microbalance, optical waveguide spectroscopy and surface plasmon resonance) are briefly described and five case studies are presented to illustrate how different biosensing techniques can and often should be combined. The case studies deal with representative examples of adsorption of protein films, polymer brushes and lipid membranes, and describe e.g., how to deal with strongly vs. weakly hydrated films, large conformational changes and ordered layers of biomolecules. The presented systems and methods are compared to other representative examples from the increasing literature on the subject.

Combined optical and acoustical method for determination of thickness and porosity of transparent organic layers below the ultra-thin film limit

Analysis techniques are needed to determine the quantity and structure of materials composing an organic layer that is below an ultra-thin film limit and in a liquid environment. Neither optical nor acoustical techniques can independently distinguish between thickness and porosity of ultra-thin films due to parameter correlation. A combined optical and acoustical approach yields sufficient information to determine both thickness and porosity. We describe application of the combinatorial approach to measure single or multiple organic layers when the total layer thickness is small compared to the wavelength of the probing light. The instrumental setup allows for simultaneous in situ spectroscopic ellipsometry and quartz crystal microbalance dynamic measurements, and it is combined with a multiple-inlet fluid control system for different liquid solutions to be introduced during experiments. A virtual separation approach is implemented into our analysis scheme, differentiated by whether or not the organic adsorbate and liquid ambient densities are equal. The analysis scheme requires that the film be assumed transparent and rigid (non-viscoelastic). We present and discuss applications of our approach to studies of organic surfactant adsorption, self-assembled monolayer chemisorption, and multiple-layer target DNA sensor preparation and performance testing.

Adsorption of a nonionic symmetric triblock copolymer on surfaces with different hydrophobicity

This study investigates the adsorption of a symmetric triblock nonionic polymer comprising ethylene oxide (EO) and propylene oxide (PO) blocks (Pluronic P-105, EO(37)PO(56)EO(37)) on a range of substrates including hydrophobic, i.e., polypropylene (PP), poly(ethylene terephthalate) (PET), nylon, and graphite, and hydrophilic, i.e., cellulose and silica. The adsorption process and the structure of the hydrated adsorbed layers are followed by quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR), and atomic force microscopy. The unhydrated surfaces are characterized by ellipsometry and contact angle techniques. The adsorption kinetics and the extent of adsorption are determined by monitoring the changes in resonance frequency and refractive index of sensors coated with ultrathin films of the various substrates. Langmuirian-type adsorption kinetics is observed in all cases studied. The amount of adsorbed Pluronic on hydrophobic polymer surfaces (PP, PET, and nylon) exceeds that on the hydrophilic cellulose. The hydrophobic (graphite) mineral surface adsorbs relatively low polymer mass, typical of a monolayer, while micellar structures are observed on the hydrophilic silica surface. The amount of water coupled to the adsorbed polymer layers is quantified by combining data from QCM, and SPR are found to increase with increasing polarity of the substrate. On the basis of contact angle data, the nonhydrated adsorbed structures produce modest increases in hydrophilicity of all the substrates investigated. Overall, insights are provided into the structure and stability of both hydrated and nonhydrated adsorbed triblock copolymer.

C12E6 and SDS surfactants simulated at the vacuum-water interface

The effect of surface coverage on the aggregate structure for the nonionic hexaethylene glycol monododecyl ether (C(12)E(6)) and anionic sodium dodecyl sulfate (SDS) surfactants at vacuum-water interface has been studied using molecular dynamics simulations. We report the aggregate morphologies and various structural details of both surfactants as a function of surface coverage. Our results indicate that C(12)E(6) tail groups orient less perpendicularly to the vacuum-water interface compared to SDS ones. Interfacial C(12)E(6) shows a transition from gaslike to liquidlike phases as the surface density increases. However, even at the largest coverage considered, interfacial C(12)E(6) aggregates show more disordered structures compared to SDS ones. Both surfactants exhibit a non-monotonic change in planar mobility as the available surface area per molecule varies. The results are interpreted on the basis of the molecular features of both surfactants, with particular emphasis on the properties of the surfactant heads, which are nonionic, long, and flexible for C(12)E(6), as opposed to ionic, compact, and rigid for SDS.

Adsorption isotherms of aqueous C12E6 and cetyltrimethylammonium bromide surfactants on solid surfaces in the presence of low molecular weight coadsorbents

In this work, we evaluate the effects of the low molecular weight compounds toluene, phenol, and 1-hexanol on the adsorption of two surfactants on one solid surface. The surfactants are cetyltrimethylammonium bromide (CTAB, cationic) and hexaethylene glycol monododecyl ether (C12E6, nonionic). The surface is gold, although X-ray photoelectron spectroscopic analysis reveals the presence of a large number of oxygenated sites that render the surface hydrophilic (contact angle 10 degrees). Adsorption isotherms are measured using a quartz crystal microbalance with dissipation monitoring (QCM-D). Although our measurements do not allow the determination of the morphology of the aggregates directly, we rationalize our results by referring to AFM images from the literature. On the basis of primarily the dissipative signal and on AFM studies done by others, our results are consistent with CTAB forming a patchy cylindrical structure and C12E6 likely yielding a monolayer structure. The presence of cosolutes almost doubles the mass of surface aggregates and increases the rigidness of the aggregates for CTAB, consistent with a morphological change from cylinders to flat bilayers. Part of the increase in adsorbed mass is likely due to increased surface area covered by admicelles. For C12E6, cosolutes cause small changes in the mass adsorption and essentially no change in the flexibility of surface aggregates.

Complex adsorption behavior of rodlike polyelectrolyte-surfactant aggregates

A quartz crystal microbalance (QCM) and an optical reflectometer have been used to quantify the long-term adsorption behavior of polyelectrolyte-surfactant aggregates of alkyltrimethylammonium and poly(4-vinylbenzoate) or pCnTVB at the silica-water interface. In solution, these polyelectrolyte-surfactant aggregates exist as weakly anionic semiflexible rodlike structures of several nanometers in radius and hundreds of nanometers in length. The optical reflectivity (OR) data confirmed our earlier proposed model of a two-stage adsorption process (Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir, 2004, 20 (4), 1085-1094) where free CTA+ ions initially adsorb and charge reverse the silica surface, thus allowing the weakly anionic aggregates to adsorb. Combining data from the two techniques allows a distinction to be made between contributions to the measured signal from the bulk and the interface. The isotherm determined by OR showed a clear plateau at higher concentrations, whereas the isotherm obtained by QCM continues to increase across all concentrations tested. This indicates a significant influence of the bulk fluid on the measured signals from the QCM as the concentration is increased. Slow changes in the apparent adsorbed mass observed with the QCM were not reproduced in the OR data, suggesting that these effects were also caused by the bulk and were not a densification of the adsorbed layer. The combination of techniques clarifies the adsorption kinetics and mechanism of adsorption in polyelectrolyte-surfactant aggregate systems.

Thiol Functionalized Silica-Mixed Matrix Membranes for Silver Capture from Aqueous Solutions: Experimental Results and Modeling

The study deals with an aqueous phase application of Mixed Matrix Membranes (MMMs) for silver ion (Ag(+)) capture. Silica particles were functionalized with 3-mercaptopropyltrimethoxy silane (MPTMS) to introduce free thiol (-SH) groups on the surface. The particles were used as the dispersed phase in the polysulfone or cellulose acetate polymer matrix. The membranes were prepared by the phase inversion method to create more open and interconnected porous structures suitable for liquid phase applications. The effects of the silica properties such as particle size, specific surface area, and porous/nonporous morphology on the silver ion capture capacity were studied. It was demonstrated that the membranes are capable of selectively capturing silver from a solution containing significant concentrations of other metal ions like Ca(2+). The membranes were studied to quantify the dynamic capacity for silver ion capture and its dependence on residence time through the adjustment of transmembrane pressure. The thiol-Ag(+) interaction was quantified with Quartz Crystal Microbalance in a continuous flow mode experiment and the observations were compared with the membrane results. One dimensional unsteady state model with overall volumetric mass transfer coefficient was developed and solved to predict the silver concentration in the liquid phase and the solid silica phase along the membrane thickness at varying time. The breakthrough data predicted using the model is comparable with the experimental observations. The study demonstrates successful application of the functionalized silica-mixed matrix membranes for selective aqueous phase Ag(+) capture with high capacity at low transmembrane pressures. The technique can be easily extended to other applications by altering the functionalized groups on the silica particles.

Cationic ester-containing gemini surfactants: adsorption at tailor-made surfaces monitored by SPR and QCM

Adsorption of a series of ester-containing cationic surfactants at a surface containing 90% methyl groups and 10% carboxyl groups was studied by two surface analysis techniques, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM). Such a surface, which is at the same time hydrophobic and negatively charged, is of interest as a model for many polymeric surfaces. Two different types of ester gemini surfactants and their monomeric counterparts were included together with nonester containing surfactants of similar structure. The results show that the gemini surfactants give the same adsorbed amount at the surface as the monomeric surfactants when compared at the same bulk concentration normalized to the critical micelle concentration (cmc) in bulk. Since the cmc of the geminis is around 20 times lower than the cmc of the corresponding monomeric surfactants, the gemini surfactants are much more effective in covering the surface. The two techniques gave similar relative values but the QCM values were always higher than those from SPR, which is due to the former method taking also adsorbed water into account. The adsorption, as measured by both methods, was found to follow closely the Langmuir adsorption model.

pH-responsive behavior of selectively quaternized diblock copolymers adsorbed at the silica/aqueous solution interface

The desorption and subsequent pH-responsive behavior of selectively quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) films at the silica/aqueous solution interface has been characterized. The copolymer films were prepared at pH 9, where micelle-like surface aggregates are spontaneously formed on silica. The subsequent rinse with a copolymer-free electrolyte solution adjusted to pH 9 causes partial desorption of the weakly or non-quaternized copolymers, but negligible desorption for the highly quaternized copolymers. Further rinsing with a pH 4 electrolyte solution results in additional desorption and extension (swelling) of the remaining adsorbed copolymer film normal to the interface. This pH-responsive behavior is reversible for two pH cycles (9-4-9-4) as monitored by both quartz crystal microbalance with dissipation monitoring (QCM-D) and also zeta potential measurements. The magnitude of the pH-responsive behavior depends on the mean degree of quaternization of the PDMA block. Moreover, a combination of contact angle data, zeta potential measurements and in situ atomic force microscopy (AFM) studies indicates that the pH-responsive behavior is influenced not only by the number of cationic binding sites on the adsorbed copolymer chains but also by the adsorbed layer structure.

A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions

The widespread exploitation of biosensors in the analysis of molecular recognition has its origins in the mid-1990s following the release of commercial systems based on surface plasmon resonance (SPR). More recently, platforms based on piezoelectric acoustic sensors (principally 'bulk acoustic wave' (BAW), 'thickness shear mode' (TSM) sensors or 'quartz crystal microbalances' (QCM)), have been released that are driving the publication of a large number of papers analysing binding specificities, affinities, kinetics and conformational changes associated with a molecular recognition event. This article highlights salient theoretical and practical aspects of the technologies that underpin acoustic analysis, then reviews exemplary papers in key application areas involving small molecular weight ligands, carbohydrates, proteins, nucleic acids, viruses, bacteria, cells and lipidic and polymeric interfaces. Key differentiators between optical and acoustic sensing modalities are also reviewed.

Effects of copolymer concentration and chain length on the pH-responsive behavior of diblock copolymer micellar films

The pH-responsive behavior of adsorbed diblock copolymer films of PDMA-PDEA (poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate)) on silica has been characterized using a quartz crystal microbalance with dissipation monitoring (QCM-D), an optical reflectometer (OR) and an atomic force microscope (AFM). The copolymer was adsorbed at pH 9 from various copolymer concentrations; QCM-D measurements indicate that the level of desorption when rinsed at pH 9 depends on the initial copolymer concentration. The adsorbed films produced at pH 9 generally have low charge densities; adjusting the solution pH to 4 results in a significant protonation of the constituent copolymers and a related interfacial structural change for the copolymer film. OR studies show no significant change during pH cycling, while QCM-D measurements indicate that the adsorbed mass and dissipation alter dramatically in response to the solution pH. The difference between the QCM-D adsorbed masses and dissipation values at pH 4 and 9 were found to be dependent on the initial copolymer concentration. This is due to differences in the initial conformations within the adsorbed copolymer layers at pH 9. The effect of the PDMA chain length on the pH-responsive behavior has also been studied; both the QCM-D adsorbed mass and dissipation of PDMA54-PDEA24 (shorter PDMA block) at pH 4 and 9 were observed to be greater than those of PDMA9X-PDEA2Y (longer PDMA block). This suggests that the normal extension of the adsorbed PDMA54-PDEA24 copolymer films is more significant than that of the PDMA9X-PDEA2Y films on silica.

Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface

The pH-responsive behavior of cationic diblock poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) copolymer micelles adsorbed at the silica/aqueous solution interface has been characterized. The micellar morphology of this copolymer, initially adsorbed at pH 9, can be dramatically altered by lowering the solution pH. The original micelle-like morphology of the adsorbed copolymer chains at pH 9 completely disappears as the pH is decreased to 4, and a brush-like layer structure is produced. This change results from protonation of the copolymer chains: the subsequent electrostatic repulsions within the film drive the copolymer chains to expand into the aqueous phase. Returning the solution pH from 4 to 9 causes this brush-like layer to collapse, with atomic force microscopy images suggesting degradation of the film. Hence, the pH-responsive behavior of the copolymer film exhibits irreversible morphological changes. Measurements of the adsorbed/desorbed amounts of the copolymer film were conducted using both a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). After an initial rinse at both pH values, the OR adsorbed mass becomes almost constant during subsequent pH cycling, whereas the corresponding QCM-D adsorbed mass changes significantly but reversibly in response to the solution pH. Since the QCM-D measures a bound mass that moves in tandem with the surface, the discrepancy with the OR data is due to changes in the amount of bound water in the copolymer film as a result of the pH-induced changes in surface morphology. The larger effective mass observed at pH 4 suggests that the brush-like layer contains much more entrapped water than the micellar films at pH 9. The pH dependence of the contact angle of the adsorbed film is consistent with the changes observed using the other techniques, regardless of whether the solution pH is altered in situ or the aqueous solution is completely replaced. In fact, comparison of these two approaches provides direct evidence of the exposure of adsorbed micelle core blocks to the solution during pH cycling and the concomitant impact upon all the other measurements.

pH-Responsive Diblock Copolymer Micelles at the Silica/Aqueous Solution Interface: Adsorption Kinetics and Equilibrium Studies

The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer.

Structural characterization of (3-mercaptopropyl)sulfonate monolayer on gold surfaces

We have investigated the structure of (3-mercaptopropyl)sulfonate (MPS) monolayer self-assembled onto gold surfaces by quartz crystal microbalance with energy dissipation monitoring (QCM-D) and various electrochemical methods. QCM-D experiments show that the MPS monolayer behaves as a thin rigid film with a surfacic mass of 166 ng cm(-2). Interfacial capacitance measurements demonstrate that the MPS monolayer is a rather open structure that can be penetrated by the ionic species of the phosphate buffer electrolyte. From MPS reductive desorption experiments, MPS surface concentration corresponds to 4.6 x 10(-10) mol cm(-2), which represents 60% of the coverage reported for a densely packed thiol monolayer. Despite this low packing density, oxidation of catechol is strongly inhibited leading to voltammograms that are free ofdiffusional contribution. This unique behavior has been exploited to show that the MPS monolayer covers the entire gold surface with a surface coverage at least equal to theta = 0.9981, which means a very low number of MPS-free pinholes and/or defects. Kinetics of electron transfer toward soluble redox species has been studied using catechol as a neutral hydrophilic probe, but also ferrocyanide as hydrophilic anion and ferrocenemethanol as neutral hydrophobic molecule. It is proposed that the MPS monolayer provides a high kinetic barrier toward permeation of these species and that electron transfer mainly occurs by electron tunneling through the MPS monolayer.

The density and refractive index of adsorbing protein layers

The structure of the adsorbing layers of native and denatured proteins (fibrinogen, gamma-immunoglobulin, albumin, and lysozyme) was studied on hydrophilic TiO(2) and hydrophobic Teflon-AF surfaces using the quartz crystal microbalance with dissipation and optical waveguide lightmode spectroscopy techniques. The density and the refractive index of the adsorbing protein layers could be determined from the complementary information provided by the two in situ instruments. The observed density and refractive index changes during the protein-adsorption process indicated the presence of conformational changes (e.g., partial unfolding) in general, especially upon contact with the hydrophobic surface. The structure of the formed layers was found to depend on the size of the proteins and on the experimental conditions. On the TiO(2) surface smaller proteins formed a denser layer than larger ones and the layer of unfolded proteins was less dense than that adsorbed from the native conformation. The hydrophobic surface induced denaturation and resulted in the formation of thin compact protein films of albumin and lysozyme. A linear correlation was found between the quartz crystal microbalance measured dissipation factor and the total water content of the layer, suggesting the existence of a dissipative process that is related to the solvent molecules present inside the adsorbed protein layer. Our measurements indicated that water and solvent molecules not only influence the 3D structure of proteins in solution but also play a crucial role in their adsorption onto surfaces.

Electrochemical quartz crystal nanobalance study of the adsorption/displacement phenomena of proteins and lipids on Pt

The electrochemical quartz crystal nanobalance (EQCN) was used to measure the adsorption behavior of a series of lipids (stearate, oleate, linoleate, and gamma-linolenate) on a Pt surface from a phosphate buffer pH 7.0 solution at 295 K and to investigate their adsorption/displacement behavior with the proteins, beta-lactoglobulin and alpha-lactalbumin, which are known to cause fouling during milk processing. The EQCN technique and the complementary technique of cyclic voltammetry measured simultaneously provided information on the efficiency of solubilization of the proteins by these lipids. Excellent agreement was obtained for the surface concentration of adsorbed lipid from the surface charge density from cyclic voltammetry measurements and the change in mass from the EQCN frequency measurements. The Gibbs energy of adsorption showed the lipids to have a strong affinity for the platinum surface. Addition of protein to a preadsorbed lipid layer showed alpha-lactalbumin to be able to coadsorb with the lipids, while beta-lactoglobulin was able to desorb some of the unsaturated lipids but appeared to coadsorb with the saturated lipid, stearate. Addition of lipid to a preadsorbed protein layer showed the unsaturated lipids to be able to displace some of the protein. A comparison of the desorption ability of the lipids showed stearate to be very inefficient at removing protein, while the other three lipids were able to remove each of the proteins, with the order of efficiency for protein desorption being oleate > linoleate > gamma-linolenate.

A Model for the Quartz Crystal Microbalance Frequency Response to Wetting Characteristics of Corrugated Surfaces

We consider the effect of surface roughness, and its unique wetting behavior, on the response of a quartz crystal microbalance (QCM) resonator operating in contact with a fluid. The rough surface is modeled as sinusoidally corrugated particular to the case of a fixed relationship between amplitude and periodicity, as would arise from polishing with monodisperse spherical particles. The penetration of fluid into the troughs of the corrugations and the resulting meniscus are determined as a competition between surface tension and compression of the trapped gas. Liquid contained below the corrugation peaks, but above the gas/liquid meniscus, is trapped and behaves as an ideal mass layer, contributing a frequency shift that adds to that arising from liquid entrainment. This model allows QCM responses on rough surfaces to be described as a function of liquid properties and contact angle. This permits responses on hydrophobic surfaces to be understood in terms of incomplete surface wetting.