Fluorescent cholesterol sensing using enzyme-modified CdSe/ZnS quantum dots

CdSe/ZnS quantum dot nanocrystals with wurtzite structure were synthesized using trioctylphosphine oxide (TOPO) templates. For biological applications, the capping surfactants, TOPO were replaced with mercaptoacetic acid (MAA). The carboxylic groups in MAA were activated by esterification of n-hydroxysulfo-succinimide (sulfo-NHS) catalyzed by water-soluble 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), namely through the EDC/NHS coupling reaction. MAA not only provided water solubility to CdSe/ZnS quantum dots but also acted as a linker between cholesterol oxidase (COx) and the quantum dots due to its carboxyl group. The CdSe/ZnS–COx bioconjugates showed sensitive and linear decrease in the photoluminescence (PL) peak intensity with cholesterol concentration up to 9 mM. The PL intensity variation was elucidated based upon collisional quenching by hydrogen peroxide generated from the enzymatic oxidation reaction between COx and cholesterol. This collisional quenching mechanism was confirmed by monitoring the response of bovine serum albumin-modified CdSe/ZnS bioconjugates to cholesterol molecules. Furthermore, the bioconjugates showed specificity to cholesterol molecules due to selective enzymatic oxidation reaction by COx. A simple quantum dot-based optical biosensor is proposed for precision cholesterol detection.