Orientationally Coherent Chiral Nanoparticle Superlattices With Emergent and Tunable Collective Chiroptical Activity.

Assembling chiral nanoparticles into superstructures that combine positional order with global orientational coherence remains challenging, as geometrically complex chiral building blocks often become kinetically arrested before orientational alignment is established. Here, we report liquid-air interfacial assembly of polymer-grafted chiral gold nanoparticles into macroscopic two-dimensional superlattice films with long-range positional order and orientational coherence. The orientational coherence, interparticle spacing, and stacking dimensionality can be finely modulated, enabling systematic control over their collective chiroptical response. Combined experimental and simulation studies reveal that orientationally coherent superlattices support a long-wavelength collective chiroptical mode with sign inversion relative to the intrinsic particle circular dichroism. This emergent mode, which is absent in isolated nanoparticles and orientationally disordered assemblies, is attributed to order-dependent collective near-field plasmonic coupling among aligned chiral building blocks and is structurally tunable: increasing orientational coherence enhances its intensity, decreasing interparticle spacing red-shifts its spectral position, and multilayer stacking further amplifies the optical dissymmetry. As a proof‑of‑concept demonstration, these superlattice films enable reproducible, highly enantioselective surface-enhanced Raman scattering detection of penicillamine enantiomers. This work establishes orientational coherence as a decisive structural parameter for engineering emergent chiroptical activity in chiral plasmonic superlattices, providing a versatile assembly strategy for designing collective optical functionality from chiral building blocks.