Rapid detection of vegetable cooking oils adulterated with inedible used oil using fluorescence quenching method with aqueous CTAB-coated quantum dots

Abstract

Vegetable cooking oils adulterated with inedible used oils or impure oils have posed a severe food safety problem. Current detection methods cannot meet the needs for rapid detection of adulterated oils on line or in field. Therefore, the objective of this research was to develop a rapid optical sensing method based on fluorescence quenching of CdSe/ZnS quantum dots (QDs) for identification of adulterated vegetable cooking oils. High quality hydrophilic photoluminescent nanoparticles were synthesized by encapsulating hydrophobic QDs into the micellar structure of an amphiphilic surfactant cetyltrimethyl ammonium bromide (CTAB) via phase transfer method. TEM, AFM, and fluorescence spectroscopy were used to characterize the prepared CTAB-coated QDs. Oil samples were first captured into the core of the two-layer-structural micelle and then the fluorescence of CTAB-coated QDs, working as fluorescence probes, was selectively quenched by components of the adulterated oils. Heavy metal ions and free radicals were presumed to be main quenchers. After quenching for 1min, fluorescence intensity was measured and converted to quenching percentage to determine the adulteration concentration. The results showed that in comparison with oil-soluble QDs, water-soluble CTAB-coated QDs had a greater ability to identify oil adulteration. A good quantitative relationship between quenching percentage and adulteration concentration (y=5.96x+14.99; R2=0.94) was obtained. The sensitive, simple and low-cost sensing method did not require sample pretreatment and could detect refined used oils at 0.4% or higher concentrations in soybean oil within 2min, showing great potentials for rapid screening of used oils and quantitative analysis of used oil adulteration in field.