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Tiopronin monolayer-protected silver nanoparticles with different core sizes (average diameter = 2, 5, 20 nm) were prepared by using different mole ratios of silver nitrate/tiopronin. Ligands on the silver particles were partially displaced by fluorescein-labeled thiolate single-stranded oligonucleotides or their complementary unlabeled oligonucleotides through ligand exchange. The fluorophores on silver particles showed a surface-enhanced fluorescence (SEF) dependent on the size of metallic cores. The particles could be coupled through hybridizations of oligonucleotides bound on the particles. The coupled particles were aggregated due to multiple displacements of oligonucleotides on each particle, resulting in stronger SEF. The dye-labeled oligonucleotides were assembled on the silver islands on the solid substrate, and the complementary oligonucleotide-displaced particles were coupled via oligonucleotide hybridization. The couplings between particles and islands resulted in an obvious fluorescence enhancement.
Zhang et al. (2005) studied this question.