Abstract.
We present a new optical biosensor technique based on registration of dual optical s-polarized modes on a photonic crystal surface. The simultaneous registration of two optical surface waves with different evanescent depths from the same surface spot permits the segregation of the volume and the surface contributions from an analyte, while the absence of metal damping permits an increase in the propagation length of the optical surface waves and the sensitivity of the biosensor. Our technique was tested with the binding of biotin molecules to a streptavidin monolayer that has been detected with signal/noise ratio of ~15 at 1-s signal accumulation time. The detection limit is ~20 fg of the analyte on the probed spot of the surface.

Fig.1 The calculated dispersion of the 7-layers PC structure in water and measured experimental points (black diamonds) at l = 532 nm and l = 442 nm laser wavelengths. The two optical surface modes are clearly seen as red curves (with an enhancement about 1000) inside the band gap (with an enhancement much less than 1).

Fig.2 The biosensor scheme (a) and a typical raw experimental signal from the diode array (b).

Fig.3 Immobilization of streptavidin on a biotinylated surface (a) and free biotin binding to the streptavidin monolayer: changes of the layer thickness (b) and RI of the buffer (c). The measurement time is 1 second per point (no posterior data averaging and smoothing). In color inserts the corresponding processes are illustrated.