Ratiometric Fluorescence Biosensor of Allosteric Ag Nanoclusters by G-Quadruplex-Aided Displacement Amplification

Conventional displacement reactions often suffer from drawbacks such as high background signal and low reaction rates. To overcome this limitation, we propose a G-quadruplex (G4) folding-aided displacement amplification (G4DA) strategy to illuminate the ratiometric fluorescence of allosteric Ag nanoclusters as bicolor signaling reporters (aAg554 and aAg610) for the rapid and sensitive detection of a specific targeting trigger (tT). The recognizable element is totally blocked in the stem of a modular hairpin for minimizing nonspecific background responses, while it can be exclusively unlocked by a short-stranded key effector via disturbing the sticky toehold. Preferably, red aAg610 emitters are developed only in two proximal template splits through directional complementary hybridization. Upon the effector invasion and affinity binding, the strand-exchange events are executed to drive progressive G4DA operation, which is kinetically sped up by the rapid intramolecular folding of rigid G4 structures with more stable geometry, thereby displacing tT for repetitive recycling amplification. During this process, the disassembly of a duplex complex switches red aAg610 into green aAg554 to produce reversely changed fluorescence for conformation-dependent ratiometric signaling. Without enzyme participation and tedious chemical modification, this G4DA-based approach is achievable with simplified operation, reaction dynamics, productive yield, and assay sensitivity, further suggesting a new methodological paradigm for potential biosensor, bioanalysis, and therapeutic applications.