A resumable two-photon fluorescent probe for Cu2+ and S2- based on magnetic silica core-shell Fe3O4@SiO2 nanoparticles and its application in bioimaging.

A two-photon fluorescent probe for Cu2+ and S2- has been strategically prepared with naphthalimide derivative platform (NPE) covalently grafted onto the surface of magnetic core-shell Fe3O4@SiO2 nanoparticles. The probe (NPE-Fe3O4@SiO2) exhibits selective response to Cu2+ with enhanced fluorescence and efficient separation of Cu2+ with external magnetic field. The consequent product NPE-Fe3O4@SiO2-Cu of NPE-Fe3O4@SiO2 and Cu2+ can work as an excellent sensor for S2- by removing Cu2+ from the complex with fluorescence decreased, recovering the fluorescence of the probe. Therefore, the constituted Off-On-Off type fluorescence monitoring system means the probe is resumable. Moreover, the probe has been used to quantitatively detect Cu2+ and S2- with low detection limits, which are 0.28 μM and 0.12 μM, respectively. Furthermore, the probe shows low cytotoxicity and excellent membrane permeability, which has been successfully applied for monitoring Cu2+ and S2- in living cells and imaging Cu2+ in deep-tissue with two-photon excited fluorescence.