A Cascade FRET-Mediated Ratiometric Sensor for Cu2+Ions Based on Dual Fluorescent Ligand-Coated Polymer Nanoparticles

Core-shell type dual fluorescent nanoparticles (NPs) in the 1.6 nut diameter range with a selective ligand (cyclam) attached to the surface and two fluorophores-9,10-diplienyl-anthracene (donor, D) and pyrromethene PM 567 (acceptor, A)-embedded within the polymer core were synthesized and their fluorescent and coppersensing properties were studied and compared to single D-doped and Adoped NPs. The acceptor (A) and donor (D) dyes were chosen to allow two sequential F6rster resonance energy transfer (FRET) processes from D to A and from the encapsulated dyes to copper complexes that form at the surface and act as quenchers. NPs with different D/A loads were readily obtained by two consecutive entrapments of the dyes. Dual NPs present tunable fluorescence emission that is dependent on the doping ratio. FRET from D to A results in sensitized emission frorn A upon excitation of D, with FRET efficiencies reaching 80% at high acceptor loads. A 9-fold amplification of the signal of A is observed at high D-to-A ratios. Single- and dualdye-doped NPs were used to detect the presence of cupric ions in water by using the quenching of fluorescence as a transduction signal. In accordance with the spectral overlaps and the values of the critical distance (R-0), of D- and A-copper complex pairs, the acceptor is much more sensitive than the donor. In dual fluorescent NPs, the sensitized emission of A is efficiently attenuated whereas the remaining emission of D is much less affected, allowing the detection of copper in a ratiometric manner upon excitation at a single (D) wavelength. Dual-dye-doped NPs with the highest acceptor loads (23 A-per-NP) were found to be the most sensitive for the detection of copper over a wide range of concentrations (20 nm to 8.5 mu m). Owing to its great convenience and modularity, the cascade FRET strategy based on dual fluorescent NPs holds great promise for the design of various sensing nanodevices.