Ratiometric fluorescence determination of phoxim in traditional Chinese medicine matrices using Astragalus residue-derived carbon dots and rhodamine B.

Abstract

An internally referenced fluorescence method was developed for the determination of phoxim in traditional Chinese medicine matrices. Astragalus residue-derived carbon quantum dots (AR-CQDs) were prepared from Astragalus residue and urea by one-step hydrothermal treatment and combined with Rhodamine B (RhB) to form a dual-emission probe. The AR-CQDs were quasi-spherical, with an average particle size of 4.02 nm, a fluorescence quantum yield of 10.33%, and abundant oxygen- and nitrogen-containing surface groups. Under 320 nm excitation, the AR-CQDs emission at 413 nm served as the response signal, whereas the RhB emission at 580 nm served as the reference signal. Phoxim selectively attenuated the 413 nm emission, and F413/F580 was linearly related to phoxim concentration from 5 to 100 μM (R2 = 0.9952), with a detection limit of 0.106 μM after a 1 min incubation. UV-Vis absorption, fluorescence lifetime, inner-filter-effect correction, zeta-potential, and dynamic light-scattering results suggested that phoxim-induced surface interactions accompanied by aggregation-related effects contributed to the fluorescence attenuation, while the exact microscopic mechanism was not uniquely resolved.In Astragalus, red date, goji berry, and tap-water samples, recoveries were 96.90%-99.60% with RSDs of 1.72%-3.32%, and the results showed acceptable agreement with LC-MS/MS. The method provides a simple ratiometric approach for phoxim screening within the tested matrix and concentration ranges while enabling the analytical valorization of Astragalus residue.