Single and Multiple Internal Reflection poly(dimethylsiloxane) absorbance-based biosensors

In this paper it is shown how the inclusion of the air mirrors allows both the enhancement of the performance of the Single Internal Reflection (SIR) systems and the implementation of a new type of highly sensitive absorbance-based biosensor, based on Multiple Internal Reflection (MIR). Both configurations are fabricated in a single-mask step and replicated using Poly(dimethylsiloxane) (PDMS) using soft lithographic techniques. Experimental results confirm the ray-tracing predictions: if the air mirror is accurately defined, the propagating light matches the conditions of Total Internal Reflection (TIR) at the PDMS-air interface, which results in a complete reflection of the light towards the flow cell. Hence, the inclusion of such microoptical components reduces the integration time more than 30 times while increasing the SNR and the sensitivity, reaching limits of detection (LOD) close to the mu M level. On the other hand, in the MIR configuration, the shape of the air mirrors has been modified so as to allow the partial focusing of the light at adequate places of the flow cell. With the recursive positioning of air mirrors, the optical path length has been meaningfully lengthened without a dramatic increase of the volume. The experimental results also match with the ray-tracing predictions and have allowed obtaining LOD between 110 nM (PMIR-II) and 41 nM (RMIR) with sensitivities up to three times higher than the SIR configuration. The obtained results are auspicious for the implementation of the absorbance-based photonic lab-on-a-chip systems. (C) 2008 Elsevier B.V. All rights reserved.