Figure 7.

Common gold nanoparticle-based hybridization probes. (A) In the off state of a NanoFlare, a fluorophore-labeled flare strand is hybridized to a gold-bound recognition strand, allowing for gold to quench fluorescence. Target binding to the recognition strand displaces the flare sequence, separates it from gold, and turns on fluorescence.^[44] (B) In the StickyFlare construct, the fluorophore-labeled strand is designed to be complementary to the target. Therefore, upon probe-target binding, the target can be monitored, providing valuable spatiotemporal information about its dynamics.^[134] (C) Gold MBs consist of fluorophore-labeled hairpin ONTs conjugated to the surface of gold nanoparticles. The principle of action is the same as that of MBs, except the central gold nanoparticle acts as a quencher.^[98] (D) As opposed to chemical conjugation, fluorophore-labeled ONTs may also be adsorbed onto surface-coated gold nanoparticles. For example, as depicted in the figure, fluorophore-tagged hairpin ONTs can be adsorbed onto polydopamine (colored black)-coated gold nanoparticles.^[167,168] Polydopamine acts as a further source of quenching and can be utilized in photothermal therapy due its ability to absorb NIR light. (E) Anisotropic gold nanoparticles have also been used as cores. For example, a gold nanorod (functionalized with the recognition strand) that acts as a quencher can be attached to a gold nanocross (functionalized with a complementary fluorophore-labeled strand). Upon target binding, the fluorophore-labeled strand attached to the nanocross is released, inducing fluorescence turn on. The presence of the nanocross further enhances the fluorescence due to surface enhanced resonance.^[169]