Specifying the Origin of Chiral Sensitivity through Conformal Nanogap Engineering in a Single Helicoid Gold Nanoparticle

Chiral plasmonic nanoparticles (NPs) amplify chiro-optical signals, yet practical enantioselective sensing often stalls because a large structural dissymmetry (g-value) does not by itself ensure that analytes reach the strongest chiral near-fields. Here, we show that the decisive design variable is not dielectric tuning alone, but dielectric tuning implemented in a conformal, accessibility-preserving manner within the nanogap of a single helicoid gold nanoparticle. An ultrathin (∼1.5 nm) polystyrene thiol (PS-SH) layer tunes the local index contrast (Δn up to ∼0.45) while leaving the chiral nanogap open to molecular infiltration. This accessible conformal nanogap concentrates optical helicity and electric dipole (ED)-magnetic dipole (MD) coupling in the analyte-relevant volume, thereby amplifying handedness-dependent molecular back-action rather than merely shifting the resonance through a scalar refractive-index change. Transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS) and 3D-finite element method (FEM) show selective enhancement of gap-localized modes and optical helicity, while fixed-total-concentration L/D-ratio measurements, opposite-handed and achiral controls, and even/odd decomposition of the resonance shifts separate scalar dielectric contributions from κ-dependent chiral interactions. As a result, surface modified Helicoid-III (M-H3) delivers larger enantiomer-specific spectral shifts and up to 66% higher sensing sensitivity than bare H3 in colloidal suspension. These results identify accessible chiral nanogap engineering, rather than g-value enhancement alone, as a governing design principle for chiral nanophotonic enantioselective sensing.