Enantioselective sensing by collective circular dichroism.

Quantitative determination and in situ monitoring of molecular chirality at extremely low concentrations is still challenging with simple optics because of the molecular-scale mismatch with the incident light wavelength. Advances in spectroscopy 1-4 and nanophotonics have successfully lowered the detection limit in enantioselective sensing, as it can bring the microscopic chiral characteristics of molecules into the macroscopic scale 5-7 or squeeze the chiral light into the subwavelength scale 8-17 . Conventional nanophotonic approaches depend mainly on the optical helicity density 8,9 by localized resonances within an individual structure, such as localized surface plasmon resonances (LSPRs) 10-16 or dielectric Mie resonances 17 . These approaches use the local chiral hotspots in the immediate vicinity of the structure, whereas the handedness of these hotspots varies spatially. As such, these localized resonance modes tend to be error-prone to the stochasticity of the target molecular orientations, vibrations and local concentrations 18,19 . Here we identified enantioselective characteristics of collective resonances (CRs) 20 arising from assembled 2D crystals of isotropic, 432-symmetric chiral gold nanoparticles (helicoids) 21,22 . The CRs exhibit a strong and uniform chiral near field over a large volume above the 2D crystal plane, resulting from the collectively spinning, optically induced dipoles at each helicoid. Thus, energy redistribution by molecular back action on the chiral near field shifts the CRs in opposite directions, depending on the handedness of the analyte, maximizing the modulation of the collective circular dichroism (CD).