Spider dragline silk-based humidity alarm sensor with ultra-high sensitivity

The sensitivity of Fabry-Perot (FP) sensor is determined by the change of cavity length. To achieve high sensitivity, we employ supercontraction property of spider dragline silk (SDS) to fabricate a FP cavity, and the length of the cavity can change up to 99.9% of the initial cavity length. The cavity consists of two reflecting surfaces which are the fiber end and the glass plane coated with gold. These two reflecting surfaces are connected by 1 cm long SDS, and the initial length of the FP cavity is 2 mm. Supercontraction property enables SDS shrink axially in length under high relative humidity (RH) environment, shrinking SDS pulls two reflecting surfaces closer to each other, so that the cavity length decreases and the free spectral range (FSR) of interference spectrum increases when environmental RH is more than 68%. Thus the proposed FP cavity length is adjustable with the change of RH, and it can change dramatically in the high humidity. FSR of the obtained spectrum can change from 0.600 nm to 466.538 nm when the RH increases from 68%RH to 95%RH. The experimental results indicate that the average sensitivity of the proposed SDS-based sensor is 27.355 nm/%RH in the range of 78%RH-95%RH, and the maximum sensitivity is 266.825 nm/%RH when RH increases from 94%RH to 95%RH. Supercontraction property makes SDS have the potential for RH sensing in high humidity range with high sensitivity. The proposed humidity sensor works only when the environmental RH increases to more than 68%RH, and its sensitivity increases with the increase of RH, thus it can work as a humidity alarm to protect storage from wetting.