Fig. 38. Amplified fluorescent sensing mechanism of detecting Hg²⁺ through HCR. In the absence of Hg²⁺, GO absorbs the DNA probes (helper DNA, HP1, and HP2) via noncovalent interactions and quenches the fluorescence of HP1. However, in the presence of Hg²⁺, the helper DNA opens HP1 because of the formation of stable T–Hg²⁺–T structures and consequently induces continuous HP1–HP2 hybridizations, which cannot adsorb on GO, leading to the generation of amplified fluorescence. [Reprinted with permission from ref. 602, J. Huang, X. Gao, J. Jia, J.-K. Kim and Z. Li, Graphene Oxide-Based Amplified Fluorescent Biosensor for Hg²⁺ Detection through Hybridization Chain Reactions, Anal. Chem., 2014, 86, 3209–3215. Copyright© American Chemical Society.].