Fabrication of CdS quantum dots with egg white and application in the assay of hypochlorous acid and myeloperoxidase activity and inhibition

Abstract

The myeloperoxidase (MPO)/H₂O₂–Cl⁻ enzymatic reaction system and its product hypochlorous acid (HOCl) are closely related to many disease processes, and new methods to detect the levels of HOCl and MPO are being focused on. MPO is the only known enzyme for the catalytic production of HOCl in biological systems; therefore, monitoring the HOCl levels is a selective and direct readout of MPO activity. This study reported a simple and efficient fluorescence assay of HOCl and MPO activity and inhibition. Highly fluorescent CdS quantum dots (CdS QDs) were prepared in one pot where NaOH-pretreated egg white served as a stabilizer. These CdS QDs exhibit strong green emission centered at ca. 550 nm and enable rapid and selective fluorescence response to HOCl with a linear detection range of 8.0–250 μM and a limit of detection (LOD) of 2.5 μM. Moreover, the CdS QDs were further applied for sensing MPO based on the fluorescence quenching exerted by its reaction product HOCl. Detection of MPO is accomplished with a linear range from 0.1 to 40 mU mL⁻¹ (1 U is the MPO concentration for catalysis of 1 micromolar substrate per minute) and a LOD of 0.06 mU mL⁻¹. The developed synthesis method can be applied to large-scale synthesis of CdS QDs, and the strategy to sense HOCl and MPO activity and inhibition has potential biomedical applications such as clinical diagnosis and drug screening.