Annealing-Controlled Nanogap Distribution Modulates Plasmonic Field in 2D-Pack Chiral Nanoparticles for Ultrafast PCR

Plasmonic photothermal polymerase chain reaction (PCR) has emerged as a promising molecular diagnostic platform because it enables ultrafast thermal cycling through efficient light-to-heat conversion. To date, most studies have focused on maximizing optical absorption and heating efficiency, whereas the mechanistic link between nanoscale plasmonic field modulation, photothermal behavior, and actual PCR amplification performance remains insufficiently understood. Herein, we developed a lithography-free chiral plasmonic photothermal PCR platform based on the two-dimensional packing of bi-functionalized Helicoid-III (H3) gold nanoparticles (NPs), in which annealing-time-dependent statistical nanogap distributions were used as structural design variables. Scanning transmission electron microscopy–electron energy loss spectroscopy (STEM-EELS) and three-dimensional finite element method (3D FEM) simulations revealed that the nanogap evolution reconfigured the local plasmonic modes, electromagnetic field localization, and optical absorption pathways within the assembled film. Rather than monotonically increasing with gap narrowing, the photothermal response was optimized at an intermediate coupling state, whereas excessively narrowed gaps suppressed the local plasmonic response at chiral optical resonances. These nanogap-dependent optical changes were correlated with distinct photothermal heating behaviors, thermal cycling kinetics and PCR amplification efficiencies. Notably, the AT20 min substrate, which exhibited the highest chiroptical response, completed 30 photothermal PCR cycles within approximately 3 min and produced the strongest target amplification. This study establishes a structure–field–thermal–amplification relationship in 2D chiral plasmonic assemblies and provides design criteria for high-speed plasmonic photothermal PCR platforms based on nanogap-dependent chiral near-field engineering.