Fabrication of reverse symmetry polymer waveguide sensor chips on nanoporous substrates using dip-floating

Reverse symmetry waveguide biosensors employ substrates with a refractive index less than the analyzed aqueous cover sample (1.33). This design offers higher sensitivities for detecting micron scale biological objects such as bacteria and living cells. In the present paper, the fabrication details of reverse symmetry polymer waveguide sensors using nanoporous silica substrates with refractive index 1.2 are presented. Using nanoporous substrates the direct spin-coating deposition of polymer films is not feasible, since the solvent used to dissolve the polymer fills up the nanopores leading to a substrate RI more than 1.33. Instead, a technique—referred to as dip-floating—was applied to create freely floating 150-nm thick polymer films on the surface of water. The films were transferred to the nanoporous substrates simply by pressing the substrates through the air-floating film–water interface. A heat molding technique using a PDMS grating was used to create integrated gratings on the polymer films for light coupling.