A method of making an optochemical sensor, the method comprising: providing a reflective substrate having a major surface; affixing a detection layer comprising at least one intrinsically microporous polymer to at least a portion of the major surface; depositing a substantially continuous semi-reflective metallic layer on at least a portion of the detection layer, the semi-reflective metallic layer comprising palladium and having a network of fine irregular cracks therein; and heating the detection layer and semi-reflective metallic layer in the presence of molecular oxygen at a temperature sufficient to cause the cracks to widen. Sensors prepared according to method are also disclosed.