Zinc Oxide Sensing Devices for Meat Spoilage Detection

Abstract

Introduction In North America, over 20% of the initial production of meat is lost due to waste. Losses and waste in industrialized regions are highest at the end of the food supply chain due to high per capita meat consumption and large amounts of waste generated by retailers and consumers [1]. Smart materials and devices can reduce this waste by detecting meat spoilage in the early stages. When meat proteins begin to degrade, they release biogenic amines (e.g. putrescine, NH2(CH2)4NH2). Detection of these amines can lead to an early indication of meat spoilage [2-4]. Gas sensing semiconductors such as ZnO can be used for this detection by measuring a change in electrical resistance due to charge transfer caused by the amine molecule on the surface. In this work, a chemiresistive response to amines is tested for by utilizing model test gases, including methylamine (NH2CH3) and dimethylamine (NH(CH3)2), with novel, synthesized ZnO nanomaterials optimized for this application. Sensor Fabrication and Testing ZnO nanostructures are synthesized directly onto interdigitated electrode substrates via a two-step hydrothermal process. ZnO nanomaterial deposition on the substrate surface is first seeded from solution, followed by nanostructure growth from a second solution at elevated temperatures. The morphology of the ZnO nanostructure deposits is controlled by adjusting the pH of the growth solution and by adjusting the point at which the seeded substrates are immersed into the growth solution. Different metal catalyst nanoparticles are deposited onto the ZnO via a wet chemical method to optimize the device response to amines and lower the operating temperature [5]. The ZnO nanostructures are characterized by SEM, TEM, and XPS (Figure 1).The sensor response to amines is tested using a home built apparatus. The amine test gas is diluted in air through mass flow controllers and then flowed over the device under test while the device resistance is monitored. The device temperature is set by a resistive heater and thermocouple located near the device and a closed loop controller. The entire apparatus is controlled by a central computer through a LabView script. Interference from moisture is tested for by flowing water saturated air over the device under test and monitoring for changes in resistance. Results and Conclusions A strong chemiresistive response to methylamine is found using the ZnO nanostructures, with a large increase in sensitivity occurring after decorating the nanostructures with Pd catalyst particles (Figure 1) [6]. The morphology of the ZnO nanostructure is optimized during synthesis as described above and is found to have a strong impact on the sensor response. The optimal response is found from a flower-like nanostructure morphology with a high surface area, large aspect ratios, and a percolated electrical network connectivity, as shown in Figure 1. Optimization of the morphology and the catalyst loading leads to a decrease in the operating temperature of the device and the observance of room temperature sensing. Experiments are underway to further characterize these responses and optimize the devices for high sensitivity, room temperature (or low temperature) operation. Interferences from moisture in the air would lead to issues with adoption of the technology and therefore tests are underway to check for such interferences. The ZnO materials have been successfully doped with Ga and Al, and experiments are planned to optimize the device response via this doping, as well as by UV exposure of the ZnO.