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We present a new optical polymer thin film sensor for high sensitivity and rapid response gas detection. Amphiphilic polymer co-networks (APCN) are used as sensor matrices for fast nitrogen dioxide detection. The sensor consists of an APCN in which the indicator has been embedded. The APCNs were synthesized by free radical induced photo polymerization and result in poly(2-(dimethylamino)ethyl acrylate)-1-polydimethylsiloxane(PDMAEA-l-PDMS). Thin and optical clear films of nanophase-separated co-networks are formed which are well suited for simple impregnation with indicators. Due to the high gas permeability of silicone containing APCNs, accurate gas detection at the sub-ppm level is feasible within seconds. As a result of nanophase separation, there is a spatial separation between areas in which the indicator reagent is well immobilized and areas that advantageously take care of the diffusive transport of the analyte. Thanks to the huge interface between the heterogeneous phases and therefore the good accessibility of the indicator, it is possible to design sensors with high sensitivity. Here, we describe the determination of nitrogen dioxide in the sub ppm-range, which occurs within seconds, using DPPD (N,N'-diphenyl-1,4-phenylendiamine) as indicator. The thin film response is reproducible and irreversible. With our kinetic optical method response times within seconds were achieved.