Green-emission nitrogen-doped carbon quantum dots from alkaline N-methyl-2-pyrrolidinone for determination of β-galactosidase and its inhibitors.

A new fluorescence method was established for sensitive detection of β-galactosidase (β-gal) activity in spiked human serum and screening of inhibitor. Nitrogen-doped carbon quantum dots (N-CQDs) were prepared by solvothermal polymerization of N-methyl-2-pyrrolidinone in an alkaline condition. The colloidal N-CQDs exhibit good water solubility, stability, and emit bright green fluorescence with a maximum emission peak at 528 nm upon excitation at 420 nm. β-gal specifically catalyzes the decomposition of its substrate P-nitrophenyl-β-D-galactopyranoside into 4-nitrophenol, whose absorption spectrum overlaps well with the excitation spectrum of the N-CQDs. As a result, the fluorescence of the N-CQDs is remarkably quenched by 4-nitrophenol via an inner filter effect. The sensing platform presents a linear response range for β-gal activity from 0.05 to 3.0 U·L-1 with a low limit of detection of 0.023 U·L-1. An acceptable precision is obtained with a relative standard deviation (RSD) of 3.1% for 1.0 U·L-1 β-gal (n = 11). The method was applied to determine β-gal in spiked human serums with recoveries in the range  96.3-104.7%. The method was employed to evaluate inhibitor screening with D-galactal and chloroquine diphosphate as models.