Preparation and application of molecular imprinted quartz microbalance chips for detection of melamine

Melamine (2,4,6-triamino-1,3,5-triazine), a nitrogen-rich chemical, was intentionally added by some companies to dairy products (liquid milk or milk powder) and provender for domestic livestock to increase the apparent protein content in 2007. Serious disorders such as renal failure were observed in many infants fed with those food products in 2008. In late 2008, trace amounts of melamine were detected in US-made infant formula products. Today, infant formula products and pet food contaminated with melamine pose serious threat to food safety. Current analytical methods for detection of melamine are mainly based chromatographic methods (e.g., HPLC, GC–MS), which are usually time consuming, expensive, and require complex procedures of sample preparation. The detection of melamine with molecular imprinted polymers (MIP) has been the subject of intense research. When the polymer matrix, upon removing the template molecule, is added into a solution containing the target and the competitive molecules, it will exhibit a high selectivity towards the template molecule due to size of the binding site and its three dimensional geometry. In this study, molecular imprinted polymeric films specific to melamine were prepared on Quartz Crystal Microbalance (QCM) chips. Firstly, following the pre-organization by secondary forces, the template molecule (melamine) and functional monomer (methacrylic acid) were polymerized with UV initiated polymerization in the presence of the cross-linking agent ethylene glycol dimethacrylate. After removal of template molecule from the polymer, non-covalently molecular imprinted, efficient, reusable sites were created on the films for specific detection of melamine. Some of the findings of the study are as follows. Optimal pre-organization of melamine-methacrylic acid was achieved at pH 6.0. But, the films prepared at pH 6.0 were stripped off the QCM chip surface. DMSO was determined to be the best solvent for production of melamine imprinted films. The maximum melamine adsorption on melamine imprinted QCM films was observed at pH 6.0. Studies on melamine imprinted QCM chips were conducted at initial melamine concentrations of 1-1000 ng/mL; results were obtained at concentrations of 50-1000, 1-10 and 1-1000 ng/mL with a linearity nearly of 100%, 81% and 98%. It was observed that melamine adsorption onto the imprinted QCM chips exhibited better fit to Freundlich isotherm model at low concentrations and to Langmuir model at high concentrations.