A pH-responsive assembly based on upconversion nanocrystals and ultrasmall nickel nanoparticles

We report a hybrid assembly based on NaYF4: Yb3+, Er3+/Tm3+@NaYF4 (core@shell) upconversion nanocrystals and ultrasmall Ni nanoparticles (abbreviated as core@shell@Ni) for rapid response towards pH in different PBS solutions. In this approach, Ni nanoparticles located on the surface of the core@shell upconversion nanocrystal serve as an efficient quencher for upconversion luminescence (UCL). A series of dynamic processes of the S-4(3/2) -> I-4(15/2) transition of Er3+ in Er3+-doped core@shell@Ni have been presented in order to discuss the energy transfer mechanisms between UCNPs and Ni nanoparticles. The upconversion luminescence can be recovered by incubating the core@shell@Ni assembly with pH 5.0 PBS that oxidizes Ni to Ni2+, while the UCL could be still quenched when the hybrid assembly was incubated with pH 7.4 PBS. However, the core@shell@Co assembly's response toward pH is not distinguished due to the rapid recovery of emission intensity both in pH 5.0 and 7.4 PBS. The ability of the core@shell@Ni assembly to distinguish different pH values may be a fine platform for the real-time optical monitoring of chemical reactions and many important physiological processes associated with pH.