Sensitive, specific, smartphone-based quantitative sensing of glyphosate by integrating analyte-triggered anti-aggregation/anti-autocatalysis of metal nanoparticles with Tyndall-effect colorimetric signaling

This work initially describes a new class of pesticide nanosensor for highly sensitive, equipment-free detection of glyphosate as a model analyte by using Tyndall effect (TE) for colorimetric signaling. In the absence of analyte, o-phenylenediamine (OPD) can be oxidized by bivalent copper (Cu2+) ions into 2, 3-diaminophenazine (OPDox), from which copper nanoparticles (CuNPs) are also formed to further act as nanocatalysts for catalyzing the above redox reactions. The resultant large number of positively-charged OPDox products can make citrate-capped gold NPs (AuNPs) aggregate via N–Au chemistry and electrostatic forces. With a 635-nm laser pointer pen, both the CuNPs and aggregated AuNPs can efficiently scatter its red light beam, leading to a significantly-enhanced TE response (compared with the dispersed AuNPs). For the glyphosate sample, however, only a weak TE signal can be recorded, because the analyte can chelate with the Cu2+ ions to inhibit the OPDox production, the generation/antocatalysis of CuNPs, and the OPDox-induced AuNPs’ aggregation. The glyphosate level is inversely proportional to the TE signal which can be measured by a smartphone for quantitative analysis. The results show that the developed nanosensor enables instrument-free detection of glyphosate with a limit down to 117 nM. To the best of our knowledge, this may be the first report of designing enhanced TE-based colorimetric nanosensors with two types of colloidal nanoparticles for potential on-site detection of pesticide residue in various fields like food safety and environmental monitoring.