In this work, a system was developed for the extraction and colorimetric assay of malathion as a model of organophosphorus pesticide in the environmental sample matrixes. In this regard, graphene oxide (GO) was synthesized from sugar beet bagasse and applied to produce Fe3O4/GO adsorbent. Subsequently, the prepared adsorbent was employed for magnetic solid phase extraction (MSPE) of malathion. MSPE was performed by the investigation of the influence of some key independent variables including sorbent dosage, extraction time and pesticide concentration. The extraction experiments were designed by Central Composite Design (CCD) under Response Surface Methodology (RSM) by Design-Expert software. Then, the extracted malathion was quantified by colorimetric detection method using Cu@Ag nanoparticles in optimum colorimetric conditions (including reaction time: 45 min, pH of sensor: 7 and NaCl concentration: 0.1 M). Results revealed that the preconcentration process was successfully achieved in a sorbent dosage of 0.2 g/L, extraction time of 6.85 min and pesticide concentration of 250 μg/L. The colorimetric response of A570/A400 was linear in the range of 0.001–5 mg/L (R2 = 0.9925). The sensing mechanism of malathion was based on the aggregation of Cu@Ag nanoparticles in the presence of malathion. Moreover, the naked-eye detection was shown by obvious color change of Cu@Ag core-shell from brassy yellow to purple-grayish, which leads to design naked-eye recognition card for sensing malathion. The detection limit of this method was 0.014 mg/L. The method was successfully applied for the detection of malathion in the environmental samples.
