Graphene oxide decorated with melamine-imprinted nanobeads for SERS detection of melamine in milk

We present a novel, cost-effective SERS substrate for melamine detection in milk. Melamine is a nitrogen-rich compound illegally used to indicate high protein content. We developed a hybrid substrate based on molecularly imprinted polymer nanobeads functionalized with graphene oxide (MIP nanobeads@fGO). MIPs act as a door for selective detection and provide chemical enhancement. fGO achieves further chemical enhancement of the Raman signal by bond-making through functionalized moieties and electrostatic interaction with the ring moiety of fGO with melamine in SERS. Prior to polymerization, the molecular interaction between vinyl imidazole (VIM) as a functional monomer and melamine as a target molecule was modeled using AutoDockTools GUI (Graphical User Interface) and AutoGrid for molecular modelling simulations and grid calculations. From the clustering histogram, melamine and VIM molecules had the lowest binding energy of -0.77 kcal/mol. Also, the free energy of Melamine-VIM interaction at 298.15 K was -2729.21 kcal/mol, which is evidence that the interaction was energetically favorable. The fGO and MIP nanobeads@fGO were characterized by various techniques including FTIR and RAMAN spectroscopy, DSL and SEM. The SERS performance of MIP nanobeads@fGO was analyzed and showed excellent performance towards melamine with an EF of 1.3 × 10⁶ and good reproducibility with an RSD of 8.3 %. A good correlation was observed between the -log concentration of melamine (μM) and the Raman intensity (a.u.) in a broad linear range from 7.9 E-5 μM to 7.9 E2 μM, with LOD and LOQ of 1.2 μM (0.15 ppm) and 3.6 μM (0.45 ppm), respectively. The USFDA and WHO introduced the tolerable level of melamine in milk and dairy products at no >2.5 ppm. Our LOD is below the tolerable limit, indicating that the MIP nanobeads@fGO substrate can be effectively used for food safety analysis in milk samples.