Jonathan S. Ellis

Slip and coupling phenomena at the liquid–solid interface

The no-slip boundary condition, as a precept of interfacial fluid dynamics, constitutes a central dogma amongst some physicists and engineers. However, over the past decade, it has become a topic of some controversy because of the proliferation of theoretical and experimental evidence for the existence of slip, especially at micro- and nanoscopic scales. In this review, we consider the models, techniques, and results, both experimental and by simulation, concerning interfacial slip and mechanical coupling at solid–liquid (outer slip), and adsorbate–substrate (inner slip) interfaces. Outer slip is a viscous process, normally described by a planar discontinuity between the upper layer of surface particles and the adjacent liquid layer. A number of factors can lead to slip, including surface–liquid affinity, high shear rates, surface roughness, and the elasticity of any intermediary film layer. Inner slip can be a viscoelastic process, and is related to adhesion and friction. Although it has received little attention, it will be important when dealing with self-assembled monolayers and more complex biosensor applications. Finally, we consider stochastic coupling as an aspect of the concept of slip.

Conformational chemistry of surface-attached calmodulin detected by acoustic shear wave propagation

A thickness shear-mode acoustic wave device, operated in a flow-through format, was used to detect the binding of ions or peptides to surface-attached calmodulin. On-line surface attachment of the protein was achieved by immobilisation of the biotinylated molecule via a neutravidin-biotin linkage onto the surface of the gold electrode of the detector. The interaction between calmodulin, and calcium and magnesium ions induced an increase in resonant frequency and a decrease in motional resistance, which were reversible on washing with buffer. Interestingly, the changes in resonant frequency and motional resistance induced by the binding were opposite to the normal operation of the detector. The response was interpreted as a decrease in surface coupling (partial slip at the liquid/solid interface) instigated by exposure of hydrophobic domains on the protein, and an increase in the thickness, and hence effective wavelength, of the acoustic device, corresponding to an increase in the length of calmodulin by 1.5 A. This result is consistent with the literature value of 4 A. In addition, the interaction of the protein with peptide together with calcium ions was detected successfully, despite the relatively low molecular mass of the 2-kDa peptide. These results confirm the potential of acoustic wave physics for the detection of changes in the conformational chemistry of monolayer of biochemical macromolecules at the solid/liquid interface.

Contact angle-based predictive model for slip at the solid-liquid interface of a transverse-shear mode acoustic wave device

We have revisited the Blake–Tolstoi theory [Coll. Surf. 47, 135 (1990)] for molecular and hydrodynamic slip and applied it to the fundamental description of acoustic wave devices coupled to a liquid of finite thickness. The aim is to provide a framework for a predictive model for slip, based on surface–liquid interactions and contact angle. This theory provides a description of slip that links hydrodynamic boundary slip to a schematic, molecular description involving the wettability of the liquid–solid interface. We redevelop the model, using current acoustic sensors notation, then evaluate its qualitative behavior as a predictive model for slip length in the context of acoustic wave devices. Finally, we discuss the limitations of the model and consider the advantages of a predictive model for boundary slip.

Surface immobilisation and properties of smooth muscle cells monitored by on-line acoustic wave detector

The attachment of rat aortic smooth muscle cells to various surfaces has been monitored by a thickness shear mode acoustic wave device incorporated into an on-line configuration. Using the total injection analysis method, laminin and fibronectin were adsorbed to the device surface, to be followed by introduction of cells into the system. The results of these experiments in terms of frequency and motional resistance measurements were also compared with those for cell attachment to the bare gold electrode of the sensor. The responses of the surface-bound cells to the introduction of various ions, depolarisation events and damage subsequent to exposure to hydrogen peroxide were also observed. Morphological changes in the cells, as confirmed by scanning electron microscopy, are correlated with results of the acoustic wave measurements.

Label-free detection of neuron–drug interactions using acoustic and Kelvin vibrational fields

Kelvin and acoustic fields of high-frequency have been employed in the non-invasive investigation of immortalized hypothalamic neurons, in order to assess their response to different concentrations of specific drugs, toxins, a stress-reducing hormone and neurotrophic factors. In an analytical systems biology approach, this work constitutes a first study of living neuron cultures by scanning Kelvin nanoprobe (SKN) and thickness shear mode (TSM) acoustic wave techniques. N-38 hypothalamic mouse neurons were immobilized on the gold electrode of 9 MHz TSM acoustic wave devices and gold-coated slides for study by SKN. The neurons were exposed to the neurochemicals betaseron, forskolin, TCAP, and cerebrolysin. Signals were collected with the TSM in real-time mode, and with the SKN in scanning and real-time modes, as the drugs were applied at biologically significant concentrations. With the TSM, for all drugs, some frequency and resistance shifts were in the same direction, contrary to normal functioning for this type of instrument. Possible mechanisms are presented to explain this behaviour. An oscillatory signal with periodicity of approximately 2 min was observed for some neuron-coated surfaces, where the amplitude of these oscillations was altered upon application of certain neurotrophic factors. These two new techniques present novel and non-invasive electrodeless methods for detecting changes at the cellular level caused by a variety of neuroactive compounds, without killing or destroying the neurons.

Viscoelastic modeling with interfacial slip of a protein monolayer electrode-adsorbed on an acoustic wave biosensor.

Transverse-shear mode acoustic wave devices have been used as real-time, label-free detectors of conformational shifts in biomolecules on surfaces. However, material changes in the biochemical monolayers and coupling between the substrate and the surrounding liquid make it difficult to isolate the desired signal, so an understanding of these phenomena is required. An important step in this understanding is knowledge of the material properties of the linker layer that attaches a biochemically selective molecule to the gold surface, in our case, neutravidin. With the goal of obtaining material properties for a neutravidin monolayer, for use in future studies, neutravidin adsorption to the gold surface of an acoustic wave biosensor is described as a viscoelastic monolayer using one-dimensional modeling. Neutravidin is described as forming hydrated, viscoelastic monolayers, and slip is allowed at all interfaces. An impedance model is numerically fit to experimental values using a two-parameter minimization algorithm and values for the shear modulus of the neutravidin monolayer, in agreement with literature values for similar proteins, are obtained. Slip is found on the electrode surface prior to neutravidin adsorption. These results will be used for future modeling studies involving this protein as a linker protein.

Acoustic coupling at multiple interfaces and the liquid phase response of the thickness shear-mode acoustic wave sensor

Interfacial slip at two interfaces is included in an acoustic wave model for a transverse-shear mode (TSM) device, coated with a thin viscoelastic film in contact with a liquid, producing series resonant frequency (fs) and motional resistance (Rm) shifts in the same direction.

Interaction of surface-attached haemoglobin with hydrophobic anions monitored by on-line acoustic wave detector

The behaviour of proteins on surfaces and at interfaces is an important field with applications in drug development, clinical diagnostics and studies of device biocompatibility. A key factor is the conformation of surface-bound proteins, which can affect chemical signalling and drug binding. A recent study of the interactions of haemoglobin with hydrophobic anions at a liquid-liquid interface has shown that a pH- and orientation-dependent conformational change occurs in the haemoglobin molecule upon interaction with these anions. To corroborate these results, we use an acoustic wave detector to study binding of solution-phase hydrophobic anions to surface-adhered haemoglobin. The orientation of protein is controlled by thiol chemistry, which generates hydrophilic and hydrophobic surfaces. Tetraphenylborate-based anions are introduced to the haemoglobin coated surface via an on-line flow-injection system to monitor the signal in real-time. Changes in the acoustic properties of the surface, measured piezoelectrically, are related to interactions between the protein and the anions. Signal strength is proportional to the degree of interaction between the salts and the haemoglobin, which in turn, is influenced by its conformation.

Finite-element simulations of the influence of pore wall adsorption on cyclic voltammetry of ion transfer across a liquid–liquid interface formed at a micropore

Adsorption onto the walls of micropores was explored by computational simulations involving cyclic voltammetry of ion transfer across an interface between aqueous and organic phases located at the micropore. Micro-interfaces between two immiscible electrolyte solutions (micro-ITIES) have been of particular research interest in recent years and show promise for biosensor and biomedical applications. The simulation model combines diffusion to and within the micropore, Butler-Volmer kinetics for ion transfer at the liquid-liquid interface, and Langmuir-style adsorption on the pore wall. Effects due to pore radius, adsorption and desorption rates, surface adsorption site density, and scan rates were examined. It was found that the magnitude of the reverse peak current decreased due to adsorption of the transferring ion on the pore wall; this decrease was more marked as the scan rate was increased. There was also a shift in the half-wave potential to lower values following adsorption, consistent with a wall adsorption process which provides a further driving force to transfer ions across the ITIES. Of particular interest was the disappearance of the reverse peak from the cyclic voltammogram at higher scan rates, compared to the increase in the reverse peak size in the absence of wall adsorption. This occurred for scan rates of 50 mV s(-1) and above and may be useful in biosensor applications using micropore-based ITIES.

Conformational states of nucleic acid–peptide complexes monitored by acoustic wave propagation and molecular dynamics simulation

Material properties, interfacial slip, and conformational changes are modelled for the immobilisation of HIV-1 TAR RNA and subsequent binding of tatpeptide fragments to the surface of a transverse-shear mode acoustic wave device. The modelling is based on previously-reported experimental results [N. Tassew, Ph.D. Thesis, 2003, University of Toronto, Toronto, Canada] and follows on from a previous modelling paper [J. S. Ellis and M. Thompson, Langmuir, 2010, 26, 11558]. A three-layer shear acoustic model is used to represent the system, where the layers describe the biomolecular monolayers and the contacting bulk buffer solution. Each layer is described by geometric, viscoelastic, and interfacial parameters, which are numerically fit to experimental values using a two-parameter minimisation algorithm. The neutravidin and TAR are described as distinct viscoelastic monolayers. Binding of tatpeptide fragment to the TAR monolayer is modelled using a complex slip parameter and a change in length, corresponding to a straightening of the molecule. Molecular dynamics (MD) simulations of the TAR-tat fragment system are performed to corroborate the modelling results. Starting structures are computed by molecular docking, and MD simulations of TAR complexed with various length tat fragments are described. The simulations are in general agreement with the modelling results and literature values from similar molecular dynamics experiments. A new parameter is introduced to describe biomolecule–solvent affinity, and is compared to interfacial coupling values obtained from modelling. This research demonstrates that acoustic wave devices can be used to detect conformational shifts in surface-attached biomolecules, provided molecular details about such shifts are known.