MOF-on-MOF thin film coated optical fiber sensor for rapid humidity sensing

We present a multi-mode optical fiber sensor that integrates a thin MOF-on-MOF coating layer (Co-ZIF-90 on ZIF-8) for humidity sensing. To fabricate the sensor, we first etched the fiber etched using commercial buffered oxide etchant (BOE) containing 40% ammonium fluoride and 49% hydrofluoric acid (HF) in a 6:1 ratio. A thin ZIF-8 layer was then grown on the fiber as the seeding layer using a simple solution method. Afterwards, the Co-ZIF-90 layer was grown on top of the ZIF-8 layer by immersing it in a freshly prepared mixture of equal volumes of “Co” and “HImCA”. This process yielded a Co-ZIF-90-on-ZIF-8 thin layer approximately 400 nm thick. The humidity sensing performance was characterized accordingly. Experimental results show that the developed optical fiber sensor exhibits high sensitivity to water vapor (79.2% transmission changes) with a rapid adsorption time of 1 second and desorption time of 2 seconds, respectively. The MOF-based fiber sensor’s excellent reversibility in the adsorption and desorption of sensing target molecules was also demonstrated. This proof-of-concept study provides an alternative method for easy fabrication of MOF-based optical sensing devices and opens up new application potentials for combination of functional material and optics for selective chemical detection and sensing.