Droplet detection: Simplification and optimization of detecting conditions towards high sensitivity quantitative determination of melamine in milk without any pretreatment

A new droplet configuration was developed to simplify and optimize the SERS detecting condition using silver colloid as SERS substrate. Samples were measured in solution to improve the evenness of SERS response. The influences of the reflectivity of substrates and height of sample solution on Raman intensity have been studied systematically. Aluminum tape was employed as the supporting platform for SERS detection. Based on the high reflectivity and hydrophobicity of aluminum tape, this configuration is both highly efficient and convenient. The detection limit of melamine is 10^-2 ppb in water and the distribution of SERS intensity is regular over the droplet profile with good reproducibility. Under this configuration, melamine in milk can be quantitatively detected without any sample pretreatment in a special order of agent addition. A good linear relationship was obtained at concentrations ranging from 0.05 to 10 ppm (R² = 0.9963). Our scheme has advantages such as simplicity, improved reproducibility, less time and good accuracy over other methods, which offers tremendous potential for the on-site examination of trace melamine in three types of milk (pasteurized milk, skimmed milk, sweet milk).