Determination of the activity of γ-glutamyl transpeptidase and of its inhibitors by using the inner filter effect on the fluorescence of nitrogen-doped carbon dots

A fluorescence (FL) probe for determination of γ-glutamyl transpeptidase (GGT) activity and evaluation of inhibitors was developed based on the inner filter effect (IFE) of nitrogen-doped carbon dots (N-CDs). Bright green emissive N-CDs were synthesized by one-step hydrothermal technique with catechol and ethylenediamine. The excitation and emission wavelengths for N-CDs were 408 and 510 nm, respectively. γ-L-Glutamyl-4-nitroanilide (γ-G4NA) was employed as the substrate of GGT. The absorption spectrum of GGT catalytic product (4-nitroaniline, 4-NA) overlapped greatly with the excitation spectrum of N-CDs. 4-NA acted as the absorber in IFE to quench the FL of N-CDs. Thus, the FL quenching of N-CDs was closely related to GGT activity. The established FL method offered good linear relationship within 2.0–10.0 U L⁻¹ (R², 0.982) and 10.0–110.0 U L⁻¹ (R², 0.998) with a low detection limit of 0.6 U L⁻¹. The method was successfully applied to investigate GGT activity in human serum samples with acceptable recoveries (99.1–105.0%). The approach was also employed for screening GGT inhibitors from different polar extracts of Schisandra chinensis. Results indicated that this strategy presents superior characteristics for GGT sensing. This method has great potential as a candidate for diagnosis of GGT-related diseases and high-throughput drug discovery.