Organic Electrochemical, PEDOT:PSS-Based Impedimetric Histamine Sensor

Abstract

Introduction With human world population expected to reach 9 billion by the year 2050, the USDA predicts the need to double the world food production [1]. Much of this growth will be concentrated in the developing world. The rising standards of living are associated with increased consumption of animal products, including meat and dairy, the growth of which is projected to exceed growth in the global population. As animal agriculture is associated with great environmental costs, including greenhouse gas emissions and high water usage, there is tremendous opportunity for positive impacts on global health and sustainability. Monitoring the chemical content inside the cow rumen (stomach) can aid in early disease detection for a safer food supply, effect remediation of methane emissions through nutritional supplements to reduce greenhouse gases, and tailor feeding to the individual cow metabolism for efficiency.Our group is currently involved in developing a suite of technologies to achieve these lofty goals. One of the components is a robotic agent, termed RUMENS (Rumen Understanding through Millipede-Engineered Navigation and Sensing), an active sensing platform hosting an array of commercial and purpose-built sensors [2]. Histamine is a chemical, of which high concentrations are associated with subacute ruminal acidosis (SARA), a common disease in high-producing lactating cows [3]. Early detection of high levels of histamine could result in more efficient animal farming and the development of the described customizable sensor substrate may pave the way for a variety of future biological sensors. Method Clevios P VP CH 8000 formulation of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was purchased from Heraeus. Ethylene glycol, glycidyloxypropyl trimethoxysilane, isopropyl alcohol (IPA) and acetone were purchased from Sigma Aldrich. All chemicals were used as received without any purification. PEDOT:PSS solution was prepared from 95 Vt.% PEDOT:PSS, 4 Vt.% ethylene glycol and 1 Vt.% glycidyloxypropyl trimethoxysilane [4]. The glass substrates were washed 3 times with DI water, IPA and acetone, respectively. Planar electrodes, with dimensions of 1 mm (W) and 10 mm (L), were obtained from Cr and Au evaporated on cleaned substrate, with the thicknesses of 3 nm and 30 nm, respectively. Then, a plasma processing was carried out using Harrick's Plasma Cleaner PDC-32G in high mode for 10 mins to activate the substrate surface and aid in PEDOT:PSS adhesion and cross linking. The PEDOT:PSS film was deposited by a spin coating method at a rate of 2000 rpm for 60 s. After manually patterning PEDOT:PSS between two adjacent devices using IPA, the final product was annealed at 140 °C for 70 min.An impedance measurement, using RLC meter (NF ZM 2372), was used to test the resistance with different histamine concentrations. The sample concentrations were prepared by dissolving a known amount of histamine in phosphate buffer saline (PBS, pH of 5.5 and 7.4). A 3 μL droplet of PBS electrolyte was dropped onto the PEDOT:PSS layer, directly between two planar electrodes (inset of Figure 1). Devices were washed in DI water between measurements. Results and Conclusions Figure 1 shows the relationship between channel resistance and concentration of histamine in PBS in our PEDOT:PSS-based sensor. It can be seen that the magnitude of impedance is increasing with the increased histamine concentration. The figure indicates minimum, maximum, and mean values calculated on five separate measurements, with very little difference between the extremes (<1%). A sharp increase of resistance between 0.56 and 1.11 µg/mL is likely an artifact of the measurement. Since the molecular size of histamine is much larger than the ions in the PBS, with the increased histamine concentration, more histamine will penetrate into the PEDOT:PSS layer, which blocks the internal charge transport. Therefore, the conductivity of the whole device decreases (or its resistivity increases) with the increased histamine concentration. The results shown correspond to PBS with pH of 7.4, corresponding to an upper limit of histamine inside cow rumen, with pH of 5.5 (lower limit) demonstrating a similar trend. This, we believe, indicates the device as suitable for histamine detection.