Portable glyphosate detection using metal-catechol dye colorimetric assays combined with a new smartphone detection platform

Simple and cost-effective glyphosate assays were developed based on competitive demetallation using the dyes pyrocatechol violet (PV) and zincon (ZCN). The dyes served as colorimetric probes in the form of metal-catechol dye complexes: [Cu 2 II PV] at pH 6.5 and [Zn II ZCN] at pH 7.4 or pH 9.0. UV–Vis spectroscopy measurements were used to monitor complex formation and demetallation (taken to determine optimal reaction media), and to assess and optimize assay stability (with cooling shown to retard probe decomposition by up to a factor of 17). Subsequently, assay selectivity was evaluated based on complex stability constants to assess interference from complexing agents that may be present in real samples. This approach can be used to judge the applicability of the assays to real samples. Finally, calibration curves from large-scale UV–Vis measurements yielded low detection limits of 0.80–1.91 μM (0.14–0.32 mg/L). Most importantly, a novel smartphone-based colorimetric detection system was developed. It is the first of its kind to combine all features necessary for a convenient and user-friendly smartphone detection platform. Key novelties include the field-ready controlled lighting enclosure (3D-printed cuvette holder within a photo box), robust image calibration, automated color extraction based on markers, simultaneous evaluation of a large number of images with an automated output of annotated images, and a user-friendly color analysis software. The Blue proportion = B/(R + G + B) parameter was identified as optimal, with detection limits of 0.87–1.68 μM (0.15–0.28 mg/L) using [Zn II ZCN] at pH 7.4, being comparable to results wit a commercial UV–Vis spectrometer.