Study on Aggregation Induced Emission of Casein for 1,8-Naphthalimide Bearing Two Acetic Carboxylic Groups by Spectroscopic Method and Analytical Application
A water-soluble 1,8-naphthalimide-based fluorescent probe 2,bearing two acetic carboxylic groups,exhibited specificity emission enhancement for casein. The mechanism is the acetic carboxylic groups of 2 docks into the hydrophobic sub-domains of casein and binds with the Trp and Tyr residues through hydrophobic interaction and hydrogen effect. The un-bound Trp and Tyr residues are buried in hydrophobic cave,which makes casein be more compact. The chromophore of naphthalimide adsorbs on the surface of casein micelle,and the aggregation induced emission enhancement( AIE) occurs. When the excitation wavelength is set at 300 nm,the fluorescence intensity around 480 nm of probe 2 is proportional to the concentration of casein,based on which,an AIE quantitative assay of casein is presented at pH 5. 5. The linear range was 0.1-9.5 mg / L and the detection limit was 2.9 μg / L. Relative standard deviation was less than 3% with different concentration of casein parallel determination for nine times. The melamine,urea,whey, serum album,collagen I and crude ovalbumin did not interfere or only interfered slightly under the permission of ± 5.0% relative error. The results of detecting casein in milk powder by this method were in good agreement with those of Biuret method.