Electrochemical ZnO-Based Impedimetric Sensor for Aqueous Ammonia Detection for Precision Animal Agriculture

Abstract

Introduction Meat consumption has been steadily increasing since the 1960s 1 . As living standards and income levels rise, more people include meat into their diet. Although an important source of nutrients, livestock production has a negative influence on greenhouse gas emissions, water consumption, water pollution, and more, with cattle being the main culprits 2 . However, while farming practices have improved over the last 60 years resulting in improved efficiencies in water, land, and crop use, studies suggest that cattle farmers can do more to reduce their overall environmental impact towards more sustainable production of animal protein 3 . The primary value of ruminants in the food chain is their ability to convert non-protein nitrogen, cellulose-rich carbohydrates, and other precursors to a digestible form of protein for human consumption. This is achieved through bacterial fermentation in the rumen that converts nitrogen (ammonia) to bacterial protein, which provides the animal with an essential protein supply to support growth and thereby meat and milk production for human consumption. Too little ammonia in the rumen limits bacterial protein synthesis. However, excess ammonia is converted to urea, which is excreted urine, and is an environmental burden. Excess nitrogen in manure is released to the atmosphere and is part of the issue with nitrogen runoff 4 . Our group is currently developing robotic technologies, termed RUMENS ( Rumen Understanding through Millipede-Engineered Navigation and Sensing ), to enable live, in vivo mapping of the ruminal environment, based on a suite of commercial and custom built sensors, of various compounds present in rumen 5 . Here, we propose a miniature, rapid update zinc oxide (ZnO) film-based sensor for real-time liquid ammonia detection in the rumen. Method Zinc acetate (ZnAc) and 2-methoxyethanol (2ME) were purchased from VWR, ethanolamine and ammonium hydroxide solution (28%) were purchased from Sigma Aldrich. All chemicals were used as received without any purification. Planar electrodes (inset of Figure 1 ), with dimensions of 1-mm-long and 2.54-mm-wide, were obtained from thermally evaporated Cr and Au on cleaned glass substrate, with the thicknesses of 3 nm and 50 nm, respectively. For ZnO layer deposition, 140 mg of ZnAc was dissolved in 1 mL of 2ME and 45 μL of ethanolamine at 60 °C for about 8 hours. The solution was spin cast at 600 rpm for 60 seconds. After manually patterning between two adjacent devices (to avoid crosstalk), the sample was then placed on a hot plate set to 300 °C in air for 10 minutes. After removal and cooling, the samples were rinsed consecutively in deionized H 2 O, acetone, ethanol, and IPA for 10 seconds each and then dried on a hotplate in air for 10 minutes at 200 °C 6 . An impedance measurement, using RLC meter (NF ZM 2372), was used to test the resistance with different ammonium concentrations. Ammonia of 10 -1 , 10 -2 , 10 -3 and 10 -4 M were prepared by diluting a known amount of ammonium hydroxide solution in DI water. A 3 μL of analyte was dropped onto the ZnO layer, directly between two planar gold electrodes. Devices were thoroughly (5 minutes) washed in DI water between measurements. Results and Conclusions Figure 1 shows the relationship between channel impedance (Z’) and concentration of ammonia at different scan frequencies, ranging from 1 to 100 kHz. The measurements reveal that at a fixed frequency, the impedance decreases with increased ammonia concentration. Also, at a single concentration, at low frequencies (<100 Hz), increasing the frequency results in decreasing the impedance, until a minimum impedance is reach without further decreases. For lower concentrations, the transition occurs at higher frequencies. A possible sensing mechanism is the nitrification of NH 4 + into nitrate ions, NO 3 - , with nitrification reaction: NH 4 + + 2O 2 → NO 3 - + H 2 O + 2H + . The energy released during oxidation should be sufficient for