A novel fluorescence resonance energy transfer (FRET)-based ratiometric sensor is reported for detecting Hg2+ in water.The polymeric nanoparticles were prepared by one-step polymerization of methyl methacrylate (MMA) and polyethyleneimine (PEI) using tert -butyl hydroperoxide (TBHP) as the initiator.For the nanoparticles

the hydrophilic polyethyleneimine (PEI) chain segments served as the shell and the hydrophobic copolymer of methyl methacrylate (MMA) and the cross-linker constituted the core of the nanoparticles.A hydrophobic fluorescent dye nitrobenzoxadiazolyl derivative (NBD) was embedded in the nanoparticles during the polymerization and used as the donor of the energy transfer process.A spirolactam rhodamine derivative SRHB was synthesized

and then introduced into the particles and used as an ion-recognition element.The presence of Hg2+ in the water dispersion of nanoparticles induced the ring-opening reaction of the spirolactam rhodamine moieties and led to the occurrence of the FRET process

affording the nanoparticle system a ratiometric sensor for Hg2+.The nanoparticle sensor can selectively detect the Hg2+ in water with the detection limit of 1?10-6 mol/L.Additionally

the FRET-based system exhibited large wavelength shift (ca.

150 nm) between the donor excitation peak (430 nm) and the acceptor emission peak (579 nm)

which eliminates any influence of excitation backscattering effects on the fluorescence assay.It has been found that the FRET-based system with smaller nanoparticle as the scaffold exhibited higher energy transfer efficiency and was more preferred for the accurate ratiometric detection.This approach may provide a new strategy for ratiometric detection of analytes in environmental and biological applications.