Highly Sensitive Aptamer-Based Colorimetric Detection of Melamine in Raw Milk with Cysteamine-Stabilized Gold Nanoparticles

: In this paper, a rapid and facile visual method for detecting melamine in raw milk was developed by using T-rich single-stranded DNA (ssDNA) aptamers as the recognition element and cysteamine-stabilized gold nanoparticles (CS-AuNPs) as the color indicator. The electrostatic interaction between the negatively-charged melamine-binding aptamers and the positively-charged CS-AuNPs induced the aggregation of the CS-AuNPs and a remarkable change in color from wine red to blue. The specific recognition between melamine and T-rich aptamers could cause the formation of melamine-aptamer complex through hydrogen bonding. Compared with the electrostatic interaction between CS-AuNPs and aptamers, the strongly specific recognition between melamine and aptamers played a dominant role, thus the presence of melamine could prevent the aptamers-induced aggregation and color change of CS-AuNPs. As a result of this, using CS-AuNPs as the colorimetric probe, a simple and sensitive aptasensor for melamine determination is developed. Under the optimum conditions, the absorbance change (ΔA 526) of CS-AuNPs was linearly proportional to melamine concentration in the range of 1–24 nM, with the detection limit of 0.389 nM. The application of the proposed method for the determination of melamine in raw milk samples spiked with different amounts of melamine suggested satisfactory recoveries between 92.8% and 112.2%, which are in full accordance with the results from HPLC. Due to the advantages of low cost, high sensitivity, simple operation procedure, reduced detection time, and practical application, this method would be expected for on-site and real-time screening of melamine in raw milk.