p-Aminothiophenol-coated CdSe/ZnS quantum dots as a turn-on fluorescent probe for pH detection in aqueous media

Since controlling hydrogen ion concentration (i.e. pH) is fairly important in various fields, developing a facile pH detection method is highly desired. Owing to the superior optical properties, semiconductor quantum dots (QDs) have been developed as a promising alternative for organic fluorophores in fluorescence analysis. In this study, a hydrophilic p-aminothiophenol (pATP) capped CdSe/ZnS QDs (denoted as pATP-QDs) as a selective "turn-on" fluorescence probe for pH in aqueous media is reported. The pATP-QDs probe easily obtained via ligand exchange are characterized by various analytic techniques including ultraviolet-visible, fourier 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 energetic position of 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 (logK_s =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.