ZnO Surface Acoustic Wave Sensors Built on Zein-Coated Flexible Food Packages

We present an intelligent food packaging platform, which consists of a ZnO surface acoustic wave (SAW) sensor directly built on the protein zein. The zein layer with 2 mu m thickness acting as a biodegradable food-protecting coating is deposited onto the polyimide flexible substrate through spin-coating. The piezoelectric ZnO film is deposited on the zein layer using RF magnetron sputtering technique. The ZnO thin film grown on zein possesses the polycrystalline wurtzite structure with the preferred c-axis orientation. The ZnO surface is tuned to the hydrophilic state using UV exposure to enhance the sensitivity of the SAW devices. The sensor has a 687.38 MHz operating frequency, and the measured SAW properties are in good agreement with the simulation using the multilayer transmission line (MTL) model. The humidity detection is realized through the SAW frequency shifts with a sensitivity of 3151.22 PPMv. The unique combination of a ZnO SAW sensor with anti-microbial zein-coated flexible substrate enables wireless sensing of food freshness and achievement of food protection, which is promising for the intelligent food packages.