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

Abstract 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.