432 Symmetric Chiral Gold Nanoparticles for Enantioselective Sensing

Characterizing molecular chirality and understanding their roles in physiochemical situations has been important in broad research scope such as, biology, chemistry, and pharmaceutics. Despite its importance for biochemical principles and relevant applications, it has been considered impossible to in-situ monitor the chirality change of a few molecules with simple optics because of the scale mismatch with the incident light. Previously we demonstrated strong polarization controllability in 432 symmetric chiral gold nanoparticles (432 helicoids) [1,2], synthesized by exploiting chirality transfer between peptide and high-Miller-index gold surfaces. Based on the 432 helicoids, here we discovered a new mode of circular dichroism (collective CD) that can emerge as a result of collectively resonant oscillations (collective resonance) of hexagonally assembled 432 symmetric chiral gold nanoparticles (2D helicoid crystal) and admit extremely low-density enantioselective detections [3]. The slanted incidence of circularly polarized light (CPL) on the 2D helicoid crystal results in collective oscillation and spinning behavior of optically induced electric dipoles of helicoids, and these spinning dipoles leads to strong and uniform chiral electromagnetic field (optical helicity) across the array. The chiral molecules interact with this field modulate the electromagnetic energy of the 2D helicoid crystal depending on the molecular handedness, which substantiated by the asymmetric shift of transmission spectra. Furthermore, we found the opposite optical responses in 2D helicoid crystal for illumination of oppositely handed CPL which leads drastic contrast in CD spectra. Utilizing the new mode of circular dichroism, we achieved the fluidity-based, in-situ monitoring of molecular chirality at an ultrahigh sensitivity. Then, we further elucidated the importance of collective circular dichroism by translating it into the proof-of-concept devices such as a colorimetric cuvette chiral sensor, an on-chip chiral sensor, the total reflection based SPR system, which enable in-situ, enantioselective, and picomolar monitoring of the DNA/RNA hybridization and the protein folding of the SNARE complex. We believe that our collective circular dichroism-based strategy offers a new paradigm and rational design rules of chiral plasmonic structures for robust enantioselective sensing.