Designing fluoroprobes through Forster resonance energy transfer: surface modification of nanoparticles through click chemistry

Aqueous-phase 83 nm poly(propargyl acrylate) (PA) nanoparticles were surface-functionalized with sparingly water soluble fluorescent moieties through a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) (i.e., click transformation) to produce fluoroprobes with a large Stokes shift. For moieties which could not achieve extensive surface coverage on the particles utilizing a standard click transformation procedure, the presence of beta-cyclodextrin (beta-CD) during the transformation enhanced the grafting density onto the particles. Moieties containing oxadiazolyl groups exhibited an 84% increase in grafting density when the transformation was performed in the presence of the oligosaccharide, going from 1.04 oxadiazolyl groups/nm(2) to 1.91 oxadiazolyl groups/nm(2). Similarly, an azide-modified coumarin 6 (AD1) underwent a 17% enhancement in grafting density from 1.56 AD1 groups/nm(2) to 1.82 AD1 groups/nm(2) when the transformation was done in the presence of b-CD. A polyethylene glycol modified naphthalimide-based emitter (AD2) was less sensitive to the presence of b-CD due to its elevated water solubility and exhibited a 5% increase in grafting density. In contrast, a carbazolyl-containing moiety which could achieve 100% surface coverage of the particles without the use of b-CD exhibited a slight retardation in the incorporation rate (and final grafting density) onto the particle when the oligosaccharide was employed. Photoluminescence studies of the particles modified singly or with multiple moieties indicated that when oxadiazolyl and carbazolyl groups were attached to the particles, an exciplex was formed that had a peak emission at ca. 400 nm. The absorption of the surface attached naphthalimide-based dye exhibited a complete spectral overlap with the carbazole/oxadiazole exciplex emission and photoluminescence excitation studies indicated an efficient energy transfer process from the carbazolyl and oxadiazolyl groups to the dye, resulting in an emission maxima at 510 nm for the modified particles and a total Stokes shift of 180 nm. This large Stokes shift is an important determinant of the ultimate sensitivity of a fluoroprobe, where scattering and background fluorescence can interfere with the detection of low concentrations of an analyte and the ability to manipulate the separation between the excitation and emission wavelengths is a critical parameter for optimal detection.