Dual polarization interferometry [1] is a relatively new technique, which explores the molecular layers adsorbed to the surface of a waveguide by means of an electromagnetic evanescent wave of a laser beam; it measures the refractive index and the thickness of very thin films, and is also used to study the protein adsorption at solid/water interface. Furthermore, if the refractive index of the studied film is fixed, the birefringence can be observed quantitatively so that the anisotropy of the film can be analyzed. DPI is a quantitative and real-time technique with the dimensional resolution of the order of angstroms [2].
A laser light is focused into two waveguides of the DPI. One of the waveguides is the sensing waveguide whereas the second one maintains the reference beam. When the light passes through the waveguides, their combination in the far field creates a two dimensional interference pattern. The polarization of the laser is rotated, in order to excite alternately the polarization modes of the waveguides. The refractive index and the thickness of the adsorbed layer can be subsequently calculated by measuring the interferogram for both polarizations. The real time measurements are possible because the DPI switches rapidly the polarization, in this way the conformational information of the molecular interactions can be observed, as well as the reaction rates, affinities and thermodynamics.
Fig. 1 Schematic of dual polarisation interferometer working principle [4]
One of DPI applications of growing interest is that involving membrane protein and lipid studies. It allows to study the formation of lipid bilayer, the understanding of lipid structures [3], protein adsorption at solid/water interface, affinity and kinetics of lipid – lipid and protein – lipid interactions, structural changes taking place during interactions, and the structural nature of protein – lipid complexes.
References:
[1] Cross, G et al., A new quantitative optical biosensor for protein characterisation, Biosensors and Bioelectronics 19 (4): 383, 2003.
[2] Swann, MJ et al., Dual Polarization Interferometry: A Real-Time Optical Technique for Measuring (Bio)Molecular Orientation, Structure and Function at the Solid/Liquid Interface, Handbook of Biosensors and Biochips. Vol. 1. Wiley. Pt. 4, Ch. 33, pp. 549–568, 2007.
[3] Sanghera, N et al., Insight into early events in the aggregation of the prion protein on lipid membranes. Biochimica et Biophysica Acta 1788 (10): 2245–51, 2009.
[4] Dual polarisation interferometer, http://www.farfield-group.com/technology.asp
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