(Invited) Impedimetric Detection of Histamine Using PEDOT:PSS-Based Organic Electrochemical Sensor for Precision Animal Agriculture

Abstract

Introduction As human world population is expected to reach 9.8 billion by the year 2050 [1], the consumption of animal products is also expected to increase by a large margin, including dairy and meat. Therefore, improving the feed nutrients convergence to final products in a sustainable way is highly needed. One approach is diagnosing animal diseases in early stage with the use of biosensors, which can monitor various biomarkers, chemical molecules, present inside rumen. This will allow improvement in animal diet, with subsequent increase of energy convergence from diet to animal protein [2].Our group is currently developing a set of technologies to accomplishing such early diagnosis. One of the components is a miniature 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 [3]. Histamine is a biomarker present inside rumen, of which high levels are closely linked to subacute ruminal acidosis (SARA). SARA is a common disease in high-producing lactating cows, leading to decrease in production of animal proteins [4]. Here, we report on an electrochemical impedimetric histamine sensor. Method Polymeric semiconductor poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, or PEDOT:PSS, (Clevios PH 1000) was purchased from Heraeus (Holding GmbH Hanau, Germany). Histamine, ethylene glycol (EG), isopropyl alcohol (IPA), 3-glycidoxypropyltrimethoxysilane (GOPS), dodecylbenzenesulfonic acid solution (DBSA), acetone, and phosphate-buffered saline powder (pH=7.4) were all purchased from Sigma Aldrich (St. Louis, MO, USA). All chemicals were used without any purification. PEDOT:PSS solution was prepared by mixing 95 Vt.% PEDOT:PSS aqueous suspension with 1 Vt.% of GOPS, 3.98 Vt.% of EG, and 0.02 Vt.% DBSA. The electrolytes were histamine dissolved in phosphate buffer saline (PBS, pH=7.4). The glass substrates were cleaned by sonicating in DI water, IPA, and acetone. Cr and Au were thermally evaporated (Lesker, Nano 36) on clean substrates, with a thicknesses of 4 nm and 30 nm, respectively. Subsequently, a plasma processing in high mode for 3 minutes was carried out, using reactive ion etching (Glow Research) to activate the substrate surface. Next, PEDOT:PSS film was spin cast at 2000 rpm for 60 seconds, followed by annealing at 140 °C for 70 minutes in air, resulting in a thickness of 150 nm. Finally, PEDOT:PSS was patterned using dry plasma etching through a shadow mask.An impedance measurement was carried out to test the impedance with different histamine concentrations (from 0.1 µM to 1 mM). 3 μL electrolyte was dropped on the PEDOT:PSS layer (at the right-top of Figure1). Devices were cleaned by immersion in DI water for 10 min between measurements to remove remaining histamine. Results and Conclusions As shown in Figure 1, the real part (Z’) of impedance was increasing with the increased histamine concentration: at lower frequencies, the increase in Z’ was monotonically correlated with the concentration (right side of the graph), which did not hold at higher frequencies (left side of the graph). More specifically, we can find that from the left-top inset, the real part of the impedance has a significant positive relationship with the histamine concentration, with R2 as high as 0.95. Since PEDOT:PSS is a ionic semiconductor, which means the ions could diffuse into PEDOT and PSS moieties. The active aliphatic amino group on the ethylamine part in histamine is protonated in neutral electrolyte. Thus, histamine plays the role of cations, which can reversibly penetrate into PEDOT:PSS layer then block the holes transport by a dedoping process. Consequently, the diamine will be oxidized, while the partially doped PEDOT segment will be reduced to the dedoped state. Higher histamine concentration represents higher degree of dedoping process. Therefore, the conductivity of PEDOT:PSS will decrease, leading to increase in impedance [5].