Glyphosate Detection Through Piezoelectric and Fiber Optic Sensors Based on Molecular Imprinted Polymers
Glyphosate (N-(phosphonomethyl)glycine) is a popular, controversial, nonselective, widely used herbicide in agriculture, aquatic applications, and forestry to kill weeds. It has been detected in drinking water, soils, and groundwater, often above the maximum threshold limit. Studies investigating Glyphosate have found its harmful and adverse effects on human and animal health and the environment. Therefore, its detection, quantification, and monitoring are crucial.
Usually, glyphosate detection and quantification in water is performed using analytical methods, which allow high sensitivities and low limits of detection (LOD). However, they require sample preparation, complex procedures, expensive benchmark laboratory equipment and are very time-consuming.
Chemical sensors allow the combination of a selective layer, which is specific to the target analyte, and a transducer, which will be responsible for the translation of the properties of the target analyte (i.e., concentration) in a measurable variation (i.e., frequency, light intensity, fluorescence, etc.).
The researchers developed the proof of concept for an in-situ operating system for glyphosate detection by combining a fluorescent MIP (molecularly imprinted polymer) with optical fiber sensing platforms. The optical fiber chemical sensors add the advantages of optical fiber systems, namely immunity to electromagnetic interferences, the possibility to design low-cost sensing systems, and portability potential.
Firstly, Two MIPs were developed and optimized for glyphosate detection using a quartz microbalance (QMB) with respect to their binding ability to the target analyte and selectivity to several tested interferents. The non-imprinted polymer (NIP) was also evaluated. As QMBs directly measure the mass adsorbed on the crystal surface, the interactions between the imprinted polymers and non-imprinted polymers with the target analyte (glyphosate) and interferents were evaluated.
The MIPs showed higher sensitivity to glyphosate when compared to the respective NIPs, and higher sensitivity to glyphosate compared to interferents, including the glyphosate degradation product, AMPA ((aminomethyl)phosphonic acid).
The selected fluorescent MIP was combined with optical fiber sensor platforms and tested with solutions of glyphosate with known concentration: (i) the tip of an optical fiber was coated with the fluorescence MIP and integrated with a home-made closed cell; (ii) the fluorescent MIP was deposited on a 3-D printed fiber support, combined with a dip optical fiber bundle, which can be dived directly in the solutions to analyze. Using this fluorescent MIP as a selective layer allowed a signal-off strategy, as the fluorescence of the MIP was quenched by the analyte glyphosate.
The proof of concept for a portable and in situ sensor for glyphosate detection has been demonstrated in this paper, allowing fast measurements directly into the solution. The obtained limit of detection (LOD) is still high for water analysis (0.54 and 2 mg/L for the fiber tip and dip probe, respectively). Therefore, further developments are needed before practical deployment.
The uniqueness of this work is a "glyphosate sensor pen" for fast measurements, in-field applications, and real-time detection for practical purposes. However, further research is required to optimize its performance in a real-world scenario and miniaturized, remote, and real-time monitoring by integrating the sensors into an IoT platform.