SERS detection of urea and ammonium sulfate adulterants in milk with coffee ring effect

In the current work, surface-enhanced Raman scattering (SERS) based on gold nanoparticles (AuNPs) and silver-coated gold nanoparticles (Au@AgNPs) with coffee ring effect was employed to simultaneously ascertain urea and ammonium sulfate (AmS) in milk. A small drop (2 µL) of milk with adulterants was dried on a gold-coated slide to examine concentrations ranging from 5, 10, 20, 40 and 80 mg/dL based on spectra ranging from 400 to 1500 cm.-1 A uniform distribution of analytes, with enhanced Raman signals was detected in a small region (maximum 1.9 mm) of coffee ring across the centre of coffee ring pattern. Nanoparticles with core (Au) diameter of 26 nm and shell thickness (Ag) of 6.5 nm were confirmed using transmission electron microscopy (TEM) images. A strong Raman peak at 980 cm-1 was assigned to AmS, while that at 1001 cm-1 was ascribed to urea. With AuNPs, coefficients of determination (R2) of 0.9873 and 0.9859 were achieved for urea and AmS, respectively, while for Au@AgNPs values of 0.9827 and 0.9855 were obtained for urea and AmS, respectively. This study revealed that SERS based on coffee ring effect has the potential to be further exploited for detecting other banned and hazardous adulterants in milk and milk products.