Fluorescence quenching upon binding of copper ions in dye-doped and ligand-capped polymer nanoparticles:: A simple way to probe the dye accessibility in nano-sized templates

The synthesis and properties of well-defined core-shell type fluorescent metal-chelating polymer nanoparticles NP, in the 15 nm diameter range, with a fluorophore (9,10-diphenylanthracene: DPA) entrapped in the particle core and a selective ligand (1,4,8,11-tetraazacyclotetradecane: Cyclam), grafted onto the surface are presented. NPs with different number of dye-per-particle are readily obtained by entrapment of the fluorophore within the polymer core. The ligand-coated NPs exhibit a high affinity for Cu2+ ions in aqueous solution and quenching of the DPA fluorescence is observed upon binding of copper. The quenching of fluorescence arises through energy transfer (FRET) from the dye to the copper-cyclam complexes that form at the NP surface with an operating distance (d) in the 2 nm range. A simple core-shell model accounts for the steady-state and time-resolved fluorescence titration experiments: dye molecules located in the outer sphere (thickness d) of the NPs are quenched while the fluorescence of dyes embedded more deeply is not affected by the binding of copper ions. The observed high quenching efficiency (60-65 %), which is tightly correlated to the volumic and microstructural features of the NPs, shed light on the enhanced accessibility inherent in nano-sized templates. The response towards different metal ions was investigated and this confirmed the selectivity of the nanoparticle template-assembled sensor for cupric ions.