A ratiometric fluorescence strategy based on polyethyleneimine surface-modified carbon dots and Eosin Y for the ultrasensitive determination of protamine and trypsin

In this work, we report a novel and ultrasensitive dual-signal fluorescence emission detection system for protamine and trypsin based on the electrostatic interaction between polyethyleneimine (PEI) surface-modified positively charged carbon quantum dots (CDs-PEI) and the anionic fluorescent dye Eosin Y. The fluorescence system exhibited yellow-green fluorescence from Eosin Y and blue fluorescence from CDs-PEI. As a cationic peptide, protamine quenched the yellow-green fluorescence of Eosin Y at 542 nm through electrostatic interaction. In the presence of trypsin, protamine was specifically hydrolyzed by trypsin, which led to the subsequent recovery of the fluorescence of Eosin Y. Simultaneously, the blue fluorescence emission of CDs-PEI at 452 nm remained constant during the whole process. Hence, a ratiometric fluorescent nanoprobe for protamine and trypsin detection with high sensitivity was successfully constructed based on CDs-PEI and Eosin Y. For protamine detection, the ratiometric fluorescence intensity (I542/I452) exhibited an excellent linear relationship in the range of 0.1–5.2 μg mL−1 with a limit of detection (LOD) of 0.03 μg mL−1. And the linear relationship between I542/I452 and trypsin concentration ranged from 0.4 to 56 ng mL−1 with an LOD of 0.21 ng mL−1. Upon evaluating the performance of this method for the detection of trypsin in actual human urine samples, satisfactory results were finally obtained.