Integrated optical read-out for polymeric cantilever-based sensors

This thesis presents a novel read-out method developed for cantilever-based sensors. Cantilevers are thin beams clamped at one end and during the last 10 years they have emerged as an interesting new type of bio/chemical sensor. The specific recognition of a chemical manifests itself as a bending of the cantilever from the generated surface stress. Conventionally the read-out used for this type of sensors is external and thereby very bulky. It is beneficial to fabricate a miniaturised system. Moreover, improved sensitivity is obtained by fabricating the cantilever in a polymeric material that has a low Young’s modulus instead of the conventional materials Si and Si3N4. Here, a novel read-out method is presented where optical waveguides are used to integrate the light into the cantilever. It is an all-polymer device where both the cantilever and the waveguides are fabricated in the negative resist SU-8. Waveguides are structured on either side of the cantilever that is free-hanging in a microfluidic channel. Light is guided into the system and is either transmitted through the cantilever or reflected off the cantilever front-end, depending on the mode of operation. This work shows that waveguides, only supporting the fundamental mode at 1 310 nm and with a propagation loss of only 1.2 dB/cm can be fabricated and integrated with free-hanging cantilevers. A theoretical model is developed to analyse the read-out sensitivity of the two different read-out modes. From calibration experiments the minimum detectable cantilever deflection in the transmission mode is measured as 45 nm, which compares well with the calculated value of 30 nm. Proof-of-principle is shown for the reflection mode as well but no conclusive value can be determined for the read-out sensitivity. It is believed both these novel principles present interesting alternatives for integrated read-out for cantilever based sensors to enable to fabrication of point-of-care analysis systems.