Ultrasensitive colorimetric assay melamine based on in situ reduction to formation of CQDs-silver nanocomposite

In this work, the in situ reduction of Ag+ by carbon quantum dots (CQDs) to the formation of CQDs-silver nanocomposite (CQDs-silver NPs) was exploited to develop a facile and ultrasensitive colorimetric sensor platform. Due to the high affinity between Ag and melamine, melamine could significantly induce the well-dispersed CQDs-silver NPs aggregation. Consequently, the solution color of CQDs-silver NPs changed from bright yellow to red. Based on this finding, we proposed a new colorimetric method to qualitatively and quantitatively detect melamine by the naked eyes and UV-vis spectroscopy, respectively. Under the optimum conditions, the linearity of melamine with the solution absorbance (ΔR) was plotted in the concentration range of 1 × 10−10-1 × 10−8 g/mL (y = 0.55 + 0.049x, R = 0.9596) and 2.5 × 10−8-5 × 10−7 g/mL (y = 3.43 + 0.42x, R = 0.9961). The developed method showed excellent detection sensitivity for melamine with the detection limit as low as 7.9 × 10−12 g/mL (eq. 65.3 pmol L−1). Finally, the developed method was employed to detect the amount of melamine in cow milk and milk powderand a recovery of 87.6-116%, and RSD of 1.6-3.9% was achieved.