a silk fibroin membrane containing an entrapped biocatalyst such as an enzyme or microorganism is provided by forming a mixture of a biocatalyst solution and a silk fibroin solution , forming a membrane by casting the mixture , drying the membrane and mechanically treating the resultant membrane at a temperature and humidity to produce the membrane in β - form and to structurally stabilize the membrane . mechanically treating may be by stretching or compressing . a biocatalyst sensor is formed by coating an electrically conductive substrate with a gas - permeable layer and coating the gas - permeable layer with the silk fibroin membrane containing an entrapped biocatalyst .