Taking advantage of the poly(dimethyl siloxane) (PDMS) optical and structural properties, it is possible to develop a highly sensitive PDMS-based photonic lab on a chip, which has been named Propagating Multiple Internal Reflection (PMIR) system. For the application in this system, the surface of PDMS was chemically modified and then Horseradish Peroxidase (HRP) was immobilized, having then a bioreactor with photonic transduction. Absorbance measurements in 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) ABTS solutions with increasing hydrogen peroxide (H₂O₂) concentrations allowed to determine the peroxide with a sensitivity of 0.025±0.004 (a.u.)/μM and a limit of detection (LOD) of 2.8±0.4 μM.