Melamine dependent fluorescence of glutathione protected gold nanoclusters and ratiometric quantification of melamine in commercial cow milk and infant formula

Companies processing the milk for the further production of powdered infant formulation normally check the protein level through a test measuring nitrogen content. The addition of melamine which is a nitrogen-rich organic chemical in milk increases the nitrogen content and therefore enhances its apparent protein content. However, the melamine causes kidney failure and death owing to the formation of kidney stone. Thus the determination of melamine in humans and milk products have gained great significance in recent years. The gold nanoclusters (AuNCs) have attracting features due to its unique electronic and optical properties like fluorescence nature. Therefore one can use AuNCs in the field of biosensor, bio-imaging, nanobiotechnology, drug delivery, diagnosis etc. We report, a new ratiometric nanosensor established for the selective and sensitive detection of melamine based optical sensing using glutathione stabilized AuNCs. The AuNCs were characterized by high-resolution transmission electron microscopy (HR-TEM), UV-visible and Photoluminescence (PL) spectroscopic techniques. In the presence of melamine, the PL intensity at 430 nm increases owing to the (turn-on) enhancement in fluorescence, whereas PL intensity at 610 nm decreases due to the melamine-induced aggregation and subsequent aggregation-enhanced emission quenching. The observed changes were ascribed to the hydrogen bonding interaction between melamine and AuNCs, which led to the aggregation of the nanoclusters. This was confirmed by dynamic light scattering and HR-TEM measurements. The present probe showed an extreme selectivity towards the determination of 28.2 mu M melamine in the presence of 100-fold excess of common interfering molecules such as Alanine, Glycine, Glucose, Cystine etc. The proposed method was successfully applied to determine melamine in cow milk. (C) 2017 Elsevier B.V. All rights reserved.