Multilayer coatings for Bloch surface wave optical biosensors

Abstract

Sensors using surface plasmon resonance (SPR) are established as the method of choice in label-free optical biosensing. Their sensitivity for small refractive index changes at the surface originates from the enhanced evanescent field at the surface of a thin metal layer. However, the small number of well-suited metals (Ag, Au) with fixed optical constants limits a further refinement of the SPR performance in terms of dispersion and resonance width. An alternative can be found in Bloch Surface Waves (BSW) sustained at specially designed dielectric multilayer stacks with low absorption losses. Due to the low losses an enormous narrowing of the resonance is obtained, promising the reduction of the detection limit for such a label-free sensor. In order to deposit these multilayers on plastic sensor chips, plasma ion assisted vacuum evaporation (PIAD) was applied as deposition method. SiO2, TiO2, and Ta2O5 single layer properties were balanced in terms of absorption losses, stability in aqueous environment and film stress. Dielectric multilayer stacks could be designed in a way, that resonance performance is optimal and the total stack thickness as low as possible. Optimized Bloch stacks were successfully coated on a large number of polymer chips. The application could be demonstrated by the detection of cancer biomarkers using an analytical instrument that was developed with the BSW chips as core element.