Neville John Freeman

The metrics of surface adsorbed small molecules on the Young's fringe dual-slab waveguide interferometer

A method for analysing thin films using a dual-waveguide interferometric technique is described. Alternate dual polarization addressing of the interferometer sensor using a ferroelectric liquid crystal polarization switch allowed the opto-geometrical properties (density and thickness) of adsorbed layers at a solid?liquid interface to be determined. Differences in the waveguide mode dispersion between the transverse electric and transverse magnetic modes allowed unique combinations of layer thickness and refractive index to be determined at all stages of the layer formation process. The technique has been verified by comparing the analysis of the surface adsorption of surfactants with data obtained using neutron scattering techniques, observing their behaviour on trimethylsilane coated silicon oxynitride surfaces. The data obtained were found to be in excellent agreement with analogous neutron scattering experiments and the precision of the measurements taken to be of the order of 40?pm with respect to adsorbed layer thicknesses. The study was extended to a series of surfactants whose layer morphology could be correlated with their hydrophilicity/lipophilicity balance. Those in the series with longer alkyl chains were observed to form thinner, denser layers at the hydrophobic solid/aqueous liquid interface and the degree of order attained at sub-critical micelle concentrations to be correlated with molecular fluidity.The technique is expected to find utility with those interested in thin film analysis. An important and growing area of application is within the life sciences, especially in the field of protein structure and function.

Young’s fringes from vertically integrated slab waveguides: Applications to humidity sensing

Using a multiple layer optical waveguide system consisting of two vertically slab waveguides, classical Young’s fringes may be obtained in the far-field diffraction plane. In agreement with the simple theory of diffraction interference the spacing of the far-field fringes is easily observed on mm to cm dimensions without further transformation of the output light. The simple methods of fabrication and means of optical coupling should provide a readily adaptable method for examining the principles of interferometry in an integrated optical format. The structure acts to transform polarized incident plane wave input light into separate slab modes of the device which emerge as two closely spaced and coherent sources at the output. The elements required for a classical Young’s fringe demonstration are therefore all embodied in this approach. The basic concept can be applied to an optical method for sensing. In one example of this we demonstrate measurement of the phase difference induced between the upper and lower propagating modes in structures due to water vapor diffusion into the layers which are formed from hydrophilic polymers. The Young’s fringe patterns exhibit a spatial intensity distribution which is sensitive to water vapor introduced over the surface of the structure. Differences in the effective index between the modes of the two waveguides during the diffusion of the vapor causes phase shifts which result in redistribution in the fringe pattern. The anticipated limit of detection of these devices is lower than 1 ppm for water vapor.

Real time, high resolution studies of protein adsorption and structure at the solid?liquid interface using dual polarization interferometry

A novel method for the analysis of thin biological films, called dual polarization interferometry?(DPI), is described. This high resolution (<1??), laboratory-based technique allows the thickness and refractive index (density) of biological molecules adsorbing or reacting at the solid?liquid interface to be measured in real time (up to 10 measurements per second). Results from the adsorption of bovine serum albumin (BSA) on to a silicon oxynitride chip surface are presented to demonstrate how time dependent molecular behaviour can be examined using DPI. Mechanistic and structural information relating to the adsorption process is obtained as a function of the solution pH.

Interfacial adsorption of fibrinogen and its inhibition by RGD peptide: a combined physical study

The Arg-Gly-Asp (RGD) peptide sequence is known as a cell recognition site for numerous adhesive proteins present in the extracellular matrix (ECM) and in blood. Whilst surface immobilized RGD groups enhance cell attachment, RGD components present in solution can effectively inhibit cell attachment by competing with endogenous ligands for the same recognition site. In contrast to the widely reported binding to cell integrin, this study demonstrates a new RGD feature: its inhibitive effect on fibrinogen adsorption. Through a combined analysis of spectroscopic ellipsometry, neutron reflection and dual polarization interferometry, we show that the kinetic process of fibrinogen adsorption as a model pro-coagulant at the silica/solution interface and in the absence of any cells can be substantially reduced by the addition of RGD in solution and that the extent of the reduction is dependent on the relative concentration of RGD.

Comparison of the performance of an array of nanoband electrodes with a macro electrode with similar overall area.

The performance of two electrode architectures with broadly similar overall active electrode areas are examined. The first is an electrode comprising a single contiguous area (a disc) and the second is an electrode in which the cumulative electrode area is dispersed over a wide area as a 50 nm thickness platinum nanoband. A direct comparison of the electrochemical performance of these two electrodes has been made. The relatively simple nanoband electrode architecture is shown to have benefits, including two orders of magnitude greater mass transport limited currents, the ability to measure faster electrode kinetics (by a similar factor), a three orders of magnitude lowering of the Limit of Detection and a significantly reduced susceptibility to hydrodynamic perturbations. The consequences and implications of these performance characteristics on the uses of such a nanoband electrode have been considered.

Quantification of the effects of melittin on liposome structure

An optical technique, dual-polarization interferometry, has been used to examine lipid structures at the solid/liquid interface. Changes in the lipid structures, in real time, were examined as a consequence of challenging them with a peptide (melittin) that is known to induce liposome rupture. This work suggests that it should be possible to obtain a better understanding of the detail of the melittin rupture process.

A systematic study of the influence of nanoelectrode dimensions on electrode performance and the implications for electroanalysis and sensing

Micron resolution photolithography has been employed to make microsquare nanoband edge electrode (MNEE) arrays with reproducible and systematic control of the crucial dimensional parameters, including array element size and spacing and nanoelectrode thickness. The response of these arrays, which can be reproducibly fabricated on a commercial scale, is first established. The resulting characteristics (including high signal and signal-to-noise, low limit of detection, insensitivity to external convection and fast, steady-state, reproducible and quantitative response) make such nanoband electrode arrays of real interest as enhanced electroanalytical devices. In particular, the nanoelectrode response is presented and analysed as a function of nanometre scale electrode dimension, to assess the impact and relative contributions of previously postulated nanodimensional effects on the resulting response. This work suggests a significant contribution of migration at the band edges to mass transfer, which affects the resulting electroanalytical response even at ionic strengths as large as 0.7 mol dm(-3) and for electrodes as wide as 50 nm. For 5 nm nanobands, additional nanoeffects, which are thought to arise from the fact that the size of the redox species is comparable to the band width, are also observed to attenuate the observed current. The fundamental insight this gives into electrode performance is discussed along with the consequent impact on using such electrodes of nanometre dimension.

Nanoscale electrode arrays produced with microscale lithographic techniques for use in biomedical sensing applications

A novel technique for the production of nanoscale electrode arrays that uses standard microfabrication processes and micron-scale photolithography is reported here in detail. These microsquare nanoband edge electrode (MNEE) arrays have been fabricated with highly reproducible control of the key array dimensions, including the size and pitch of the individual elements and, most importantly, the width of the nanoband electrodes. The definition of lateral features to nanoscale dimensions typically requires expensive patterning techniques that are complex and low-throughput. However, the fabrication methodology used here relies on the fact that vertical dimensions (i.e. layer thicknesses) have long been manufacturable at the nanoscale using thin film deposition techniques that are well established in mainstream microelectronics. The authors report for the first time two aspects that highlight the particular suitability of these MNEE array systems for probe monolayer biosensing. The first is simulation, which shows the enhanced sensitivity to the redox reaction of the solution redox couple. The second is the enhancement of probe film functionalisation observed for the probe film model molecule, 6-mercapto-1-hexanol compared with microsquare electrodes. Such surface modification for specific probe layer biosensing and detection is of significance for a wide range of biomedical and other sensing and analytical applications.

Quantifying structural changes and stoichiometry of protein interactions using size and density profiling

The use of Dual Polarisation Interferometry, an emerging analytical biophysical technique, is described for the determination of the optogeometrical properties (thickness and density) at high resolution of adsorbed protein layers at the solid–liquid interface. The technique has been used to quantify, in real time and at subatomic resolution, the structural changes occurring in two well-characterised protein interaction systems, an antibody–antigen interaction and the biotin–streptavidin interaction. The real-time data obtained on structural changes during the interactions is in excellent agreement with previously reported X-ray crystallography and neutron reflection data. The precision of the measurements taken was of the order of 0.01 nm with respect to protein size. The dual-parameter approach also allowed the stoichiometry of both of these interactions to be calculated, giving values that confirm the current understanding of the interactions. This approach provides detailed insights into the inherent and subtle link between structural change and function in proteins, to a degree not previously possible through mass change measurements alone. The technique is expected to find utility in the increasingly important study of protein structure and function.