Sodium 4-mercaptophenolate capped CdSe/ZnS quantum dots as a fluorescent probe for pH detection in acidic aqueous media.

Development of the fluorescent pH detection method is promising due to the sensitivity, easy operation, and low-cost, etc. However, traditional organic fluorophores have still some disadvantages such as the tedious preparation and purification as well as low photostability and water solubility, which limits the rapid detection application. Semiconductor quantum dots (QDs) have recently risen to prominence as an alternative for organic fluorophores in fluorescence analysis by virtue of their convenient synthesis and superior optical properties. In this study, we report on sodium 4-mercaptophenolate functionalized CdSe/ZnS QDs (denoted as OPhS-QDs), which can serve as a selective "on-off" fluorescence probe for aqueous media pH. OPhS-QDs exhibit strong fluorescence in near neutral medium. As a Lewis organic base, OPhS- moieties on QDs surface easily binds to proton under acidic conditions to yield 4-mercaptophenol capped QDs (i.e. HOPhS-QDs), which acts as an efficient hole trapper. As a result, the QDs photoluminescence (PL) is switched off. Under optimal conditions, the present probe exhibits a good linear relationship between fluorescence response and pH values in the pH range 3.0-5.2. Furthermore, the present probe exhibits a high selectivity for proton over other common cations and has been successfully used for pH detection in real water samples.