In situ detection of thiram in fruits and vegetables by colorimetry/surface-enhanced Raman spectroscopy

In this study, a rapid, stable, and sensitive colorimetric/surface-enhanced Raman spectroscopy (SERS) method was developed based on a polyvinyl alcohol-decorated Au nanoparticle (PVA-Au) colloid for thein situ detection of thiram residues in fruits and vegetables. The PVA-Au colloid acted as both a colorimetric and SERS sensor. The colorimetric detection of thiram is based on the color change and absorption band shift caused by aggregation of the PVA-Au colloid. The stable and controllable colorimetric process has been verified. The influence of the media pH and reaction time on the aggregation of the PVA-Au colloid were also investigated. The reflectance and SERS spectra of the thiram samples were acquired and analyzed. The limit of the colorimetric detection of thiram in apples, pears, and vegetables was 4.62, 7.83, and 10.74 ppb (mu g kg(-1)), respectively, which is well below the maximum residue level of 5 ppm (mg kg(-1)) set by the National Food Safety Standard of China and the US Environmental Protection Agency. Meanwhile, rapid, in situ, and non-invasive SERS detection was used to acquire SERS spectra of thiram residues based on the localized surface plasmonic resonance effect of the PVA-Au colloid. The naked-eye visible and stable colorimetric detection is suitable for the rapid detection of pesticide residues in a large number of plant samples, while the positive results could be further confirmed and verified by SERS. The results demonstrate that this colorimetric/SERS method has great potential to detect thiram residues in real foods and improve the detection efficiency.