Sensitive Detection of Glyphosate by a Water-Gated Organic Transistor

Abstract

Introduction Glyphosate (GlyP), a widely used herbicide in the agricultural field, was classified as a probable carcinogenic compound (Group 2A) by the International Agency for Research of Cancer (IARS) in 2015. Since then, the importance of the GlyP detection in aqueous media has increased in the field of environmental chemistry and clinical research. The concentration of GlyP in ground and tap water is strictly regulated. For example, the permissible limit for the GlyP concentration in drinking water set at 0.7 ppm in the United States. To-this-date, liquid chromatography (LC) is a common method to sensitively detect GlyP. However, the apparatuses of LC are expensive and require trained personnel, requesting the development of affordable, simple and sensitive sensors for the on-site GlyP detection. Thus, our research has focused on organic field-effect transistors (OFETs) as chemical sensing platforms, because of their low-cost and solution-processability, compact size, and mechanical flexibility[1]. Here, we report an OFET-based chemical sensor for the detection of GlyP in water. Design of the OFET sensor Polythiophene derivatives (PTs) have been widely studied and applied as organic semiconductors in OFET devices. Moreover, PTs has been employed for materials in chemical sensors because their optical and electrical properties can be tuned by introducing artificial receptors into their 3- and/or 4-positions as side chains. However, such OFET-based sensors based on PT attached with artificial receptors are still rare due to the typical high applied voltage (>|20V|) of the OFET which causes materials degradation. Thus we designed a water-gated OFET (WG-OFET) for chemical sensing applications, which could be operated at an ultra-low voltage (<|0.3| V) derived from an electrical double-layer capacitor. For the detection of GlyP, we employed a carboxy side-chain attached to PT (poly{3-(5-carboxypentyl)thiophene-2,5-diyl}, P3CPT) [2] as the semiconductor layer. Due to the molecular wire effect of the π conjugated polymer [3], the sensitive detection of GlyP is expected based on the competitive binding among the carboxylate terminals of P3CPT, copper(II) ions and GlyP in an aqueous solution. Fabrication Method For the fabrication of the WG-OFET sensor device, we applied the side-gate structure (Figure 1). Compared to the conventional multi-stacked OFETs, the planar structure enables easy fabrication process. First, Au electrodes were selected as gate, source and drain, and were deposited on the same substrate. Then, an amorphous perfluoropolymer (Cytop) was used as the hydrophobic bank and patterned with oxygen plasma etching. Finally, a semiconducting polymer P3CPT was deposited onto the channel region by a drop-cast method. In order to prepare the dielectric layer made of aqueous media, 50 µL of 100 mM HEPES (4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid) buffer solution containing 100 mM NaCl was dropped onto the channel region and the gate terminal. After dropping the solution, 40 μM of a copper (II) ion was added into the HEPES buffer solution. The fabricated device was then allowed to stand for 10 minutes to stabilize the transistor characteristics. Results and Conclusions Figure 2 shows the transfer characteristics of the fabricated WG-OFET with the addition of the increasing amount of GlyP from 10 µM to 300 µM in a HEPES buffer solution at pH 7.4 at r.t. As expected, the changes of transfer characteristics were observed by increasing the concentration of GlyP, indicating that the GlyP removed the copper ion from the side-chain of P3CPT. From the low concentration part range of the titration isotherm, the limit of detection (LOD) was estimated to be 0.26 ppm. Notably, the obtained LOD value meets the criteria for the detection of GlyP in drinking water. Furthermore, the selectivity study as shown in Figure 3 demonstrated the highest response of the WG-OFET device to GlyP among the structurally similar carboxylates and phosphates, because of the strong binding affinity of GlyP for copper(Ⅱ) ions. We expect that this study is one step forward in the development of the OFET-based GlyP sensors for environmental and clinical assessment.