Surface-Enhanced Raman Spectroscopy for Trace Detection of Tetracycline and Dicyandiamide in Milk Using Transparent Substrate of Ag Nanoparticle Arrays

Abstract

Tetracycline (TC) is an antibiotic utilized in dairy livestock, which can leave trace residues in milk and other dairy products. Dicyandiamide (DCD), a grassland fertilizer, but also potentially an adulterant to increase milk apparent protein content, has been found in milk products that were subsequently banned. Both these chemicals can pose human health risks via the consumption of dairy foods, and therefore rapid, simple detection methodologies are necessary. Herein, we fabricated a transparent and homogeneous substrate by fabricating silver nanoparticle (Ag NP) arrays based on the anodic aluminum oxide (AAO) template-assisted electrochemical deposition approach. The periodic nanostructure was optimized to obtain high SERS activity by varying nanopore diameters and Ag NP sputtering duration to have Ag NP nanogaps in the sub 10 nm regime so that the high sensitivity was achieved with detecting R6G analytes at the 1 × 10–10 M level. The substrate was also used to detect TC and DCD in milk at low concentration levels of 1 × 10–9 M and 1 × 10–7 M, respectively, with good linear correlation between concentration and intensity (TC R2 = 0.96; DCD R2 = 0.99). The assignments of Raman bands of tetracycline and dicyandiamide were also given by density-functional-theory (DFT) calculations. The advantage of this transparent SERS substrate is that it is especially useful for measurement of analytes at the interface between Ag NP substrate layer and the opaque liquid containing the analytes without special pretreatment of the sample. Thus, this method may provide an alternative practical and reliable platform for rapid detection of contaminants in foods.