Biomolecular recognition by label free interferometric methods

Ageing population in Europe will increasingly burden the health care system, e.g. chronic diseases are becoming more and more common and are influencing the quality of life of rapidly increasing number of people. New solutions for biomolecular recognition measurements are required to reduce the number of costly laboratory tests, hospital controls and idle visits to general practices. Optical interferometric measurement methods are extremely sensitive and thus they are very suitable for biomolecular recognition. Usual sensitivity range for interferometric methods is from 10-5 to 10-8 refractive index units. This large dynamic range covers well most biomolecular applications. In an integrated Young interferometer sensor, which we have focused on in our studies, the incident sensing light is divided into two parallel light paths, reference and sample path. Sample path has an antibody coating on its surface, where the measured analyte (antigen) binds specifically and the other light path acts as a reference of the measurement to compensate, e.g., temperature fluctuations. This compensation explains the extremely good sensitivity obtained with most Young interferometers. It also makes the sensor more cost efficient since there is no need for very precise stabilisation of humidity and temperature around the sensor chip. In this paper we present the theoretical and empirical functioning and sensitivity of two integrated Young interferometers. We have studied the sensitivity of a channel waveguide based interferometer with integrated Y-junction as a beam splitter and a vertically integrated dual-slab waveguide interferometer. The sensitivity of the channel waveguide based Young interferometer was found to be in the order of 10-8 effective refractive index units (i.e., in the order of 10-6 refractive index units). Compared to this conventional channel waveguide based Young’s interferometer; we found that the dual-slab waveguide interferometer (Farfield Inc.) has the advantages of easy fabrication and large tolerance to the input coupling beam. The sensor performance was estimated by using glucose solutions with various concentrations. Glucose concentrations within physiological range 30mg/dl ~ 500mg/dl are of the most interest. As a result the dual-slab waveguide interferometer yields an average phase resolution of 0.002 rad which corresponds to an effective refractive index change of 4×10-8 (tai to a refractive index change of 5×10-6)