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

Abstract

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.

Simultaneous Measurement of 10,000 Protein-Ligand Affinity Constants Using Microarray-Based Kinetic Constant Assays

“…35 Label-free optical biosensing methods, such as surface plasmon resonance reflectometry (SPR), [36][37][38][39][40][41][42] reflection interference spectroscopy, [43][44][45] dielectric wave guide reflectometry, [46][47][48][49] and imaging ellipsometry, 50-54 complement fluorescence-based detection by doing away with labeling and providing both endpoint and kinetic measurements of binding reactions. However, these biosensors only detect a small number of reactions (no more than a few hundred reactions) at a time and often require special (and costly) sensor surfaces.…”

Fundamental limit to the use of effective medium theories in optics

“…DPI is being used by a wide range of bio-scientists and surface scientists to elucidate the thickness and density of ultrathin layers [11,12]. However, it is limited to providing the thickness and refractive index of "equivalent" continuous layers.…”

Optimisation of distributed feedback laser biosensors

“…a chemical solution) flowed over the sensing surface, or for monitoring the formation of a thin film on the sensing surface, for example, the binding of an analyte to immobilised antibodies [6]. The resulting changes in effective index can be observed as a change in phase, amplitude, in-or out-coupling angle or wavelength depending on the type of sensor [3][4][5][6][7][8].…”

“…An easily measurable frequency difference of Df ¼ 100 MHz corresponds to a wavelength shift of 0.8 pm with l ¼ 1550 nm, according to (5). Inserting this value into (1), we obtain Dm eff min ¼ 1.7 Â 10 26 , and from (3), we then obtain Dm t min ¼ 8.6 Â 10 23 and Dt min ¼ 0.13 nm, an improvement of one order of magnitude on the previously calculated value.…”

“…The evanescent field of a guided wave is highly sensitive to refractive index changes at the surface of a waveguide, so can be used to detect very small changes in the refractive index and thickness of a surface layer. Techniques often used in biochemical research include fibre-optic chemical sensing [3], surface plasmon resonance [4] and dual-polarisation interferometry [5]. The latter two may be implemented as a label-free system, that is, not requiring the use of fluorescent dyes, as is the case with many optical biosensing systems.…”