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Abstract
Capillary-based backscattering interferometry has been used extensively as a tool to measure molecular binding via interferometric refractive index sensing. Previous studies have analysed the fringe patterns created in the backscatter direction. However, polarisation effects, spatial chirps in the fringe pattern and the practical impact of various approximations, and assumptions in existing models are yet to be fully explored. Here, two independent ray tracing approaches are applied, analysed, contrasted, compared to experimental data, and improved upon by introducing explicit polarisation dependence. In doing so, the significance of the inner diameter, outer diameter, and material of the capillary to the resulting fringe pattern and subsequent analysis are elucidated for the first time. The inner diameter is shown to dictate the fringe pattern seen, and therefore, the effectiveness of any dechirping algorithm, demonstrating that current dechirping methods are only valid for a subset of capillary dimensions. Potential improvements are suggested in order to guide further research, increase sensitivity, and promote wider applicability.
Highlights
Backscattering interferometry (BSI) has been widely adopted as a tool to measure the binding kinetics of receptor–guest systems in many different modalities [1–4]
Backscattering interferometry is an optical technique where the interference of reflected and refracted light from a glass capillary is utilised to remotely sense the refractive index of a solution held within the capillary
The custom aluminium stage is held at a constant temperature by a Peltier thermoelectic module (Laird annular SH-10 controlled by Wavelength Electronics WTC3243HB) and holds the capillary which contains a liquid of refractive index n2 that can be exchanged using a fluid pump (Cole-Parmer Masterflex)
Summary
Backscattering interferometry (BSI) has been widely adopted as a tool to measure the binding kinetics of receptor–guest systems in many different modalities [1–4]. BSI has found extensive use as a refractive index sensor [5–8] with limits of detection in some modalities down to 10−9 refractive index units (RIU) [3]. The simplicity of BSI coupled with the ability to measure free-solution binding makes the technique attractive in many analytical scenarios [9]. Whilst finding great success in this research field, BSI does not have a unified model that is independent of commercial software [10,11]. To further improve the technology, every aspect of BSI must be analysed to leverage all the information available. The effect of the polarisation state of the incident light on the final fringe pattern for all capillary dimensions has never been fully described [11]
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