Aggregation-induced emission of 1,8-naphthalimide-casein micelle: investigation by synchronous spectrographic method

A novel 1,8-naphthalimide probe 1, bearing two acetic-acid moieties was synthesized. The acetic-acid groups, docked into the sub-domains of casein micelle and bound with tryptophan residues, and the 1,8-naphthalimide chromophore adsorbed on the surface of casein micelle, forming a supermolecule, 1-casein micelle, which exhibited the aggregation-induced synchronous emission (AISE) characters. The effect of pH on the intensity of supermolecule was investigated, and the result indicated that the emission enhancement was mainly due to the 1,8-naphthalimide chromophore aggregated onto the casein micelle. Based on AISE, a novel casein quantification method was developed, which exhibited a good linear range of 0.05-10.0 μg ml(-1) and 0.07-9.5 μg ml(-1) with the detection limits of 2.8 and 3.0 ng ml(-1) . The effects of metal ions and pH on the system of 1-casein micelle were investigated. The proposed method was applied to determine casein in milk samples, and the results were in good agreement with the result of the Biuret method.