The Application Of PH Detection In Aqueous Media By CdSe Nanocrystals

Since controlling hydrogen ion concentration (i.e. pH) is fairly important in various fields, developing a facile pH detection method is highly desired and significant.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 paper, on the basis of relative literatures, we designed two types of fluorescent probes based CdSe/ZnS QDs, and the performance and sensing mechanism were investigated in detail. The specific research includes the following two parts:In the first part, with oil-soluble CdSe/ZnS core-shell quantum dots (Xem= 610 nm) as the fluorophore and p-aminothiophenol (pATP) as the functionalized ligand of hydrogen ion sensitive, the pATP-QDs probe was easily obtained via ligand exchange. The pATP-QDs were characterized by various analytic techniques including ultraviolet-visible, ultrafourier transform infrared, fluorescence, time-resolved fluorescence spectroscopies, and transmission electron microscope etc. pATP-QDs exhibits weak fluorescence since pATP acts as an effective hole trapper. As an organic base, pATP easily binds to proton and the formation of pATP-H+ complex alters the energy level of Highest Occupied Molecular Orbital (HOMO)for pATP, which enables pATP-QDs unfavorable for the effective hole transfer. Thus, the QDs photoluminescence (PL) was switched on. PL "off-on" mode,high PL quantum yield of QDs as well as extremely high stability of pATP-H+ complex (logKs = 7.47) enables pATP-QDs to exhibit an excellent pH detection performance. Under optimal conditions, the present probe exhibits a good linear relationship between fluorescence response and pH values in pH range 3.2～6.0. 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.In the second part, we report a 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 exhibits 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 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.