Design and Synthesis of Fluorescent Silica Nanoparticle Conjugates for Metal Ion Sensing Applications

A novel series of fluorescent nanoparticle conjugates were designed and synthesized for the selective turn-off sensing of low concentrations of Cu2+ ion (nM-μM) in 2:1 ethanol:10 mM HEPES (pH 7). Silica nanoparticles (~250 nm) were modified with heterobifunctional polyethylene glycol (PEG) linkers and a third generation PAMAM dendron to function as chelator for Cu2+ ions. The organic dye fluorescein isothiocyanate (FITC) was subsequently conjugated to the dendron to act as a fluorescent sensitizer for the nanoparticle that is quenched upon the binding of Cu2+. Fluorescent nanoparticle conjugates using third and fourth generation PAMAM dendrimers (SNP-G3-FITC and SNP-G4-FITC) were synthesized for comparison. The dendron conjugate (SNP-PEG8-G3S-FITC) was determined to have a higher dynamic range (0.10-1.99 μM Cu2+) than the dendrimer counterparts (0.02 -0.30 μM Cu2+). A follow- up series of sensors were designed and synthesized using FITC-conjugated silica nanoparticles for use in characterization of the surface using Fluorescence Resonance Energy Transfer (FRET). These conjugates provide examples of an attractive modular framework for other fluorescent nanoparticles to be synthesized and tailored to analytes of interest. Silica nanoparticle conjugates can be readily synthesized using known reactions, easily isolated from reaction solutions, and they remain undetectable in fluorescent experiments