Circular Dichroism of Chiral Molecules in DNA-Assembled Plasmonic Hotspots

Abstract

The chiral state of a molecule plays a crucial role in molecular recognition and biochemical reactions. Because of this and owing to the fact that most modern drugs are chiral, the sensitive and reliable detection of the chirality of molecules is of great interest to drug development. The majority of naturally occurring biomolecules exhibit circular dichroism (CD) in the UV range. Theoretical studies and several experiments have demonstrated that this UV-CD can be transferred into the plasmonic frequency domain when metal surfaces and chiral biomolecules are in close proximity. Here, we demonstrate that the CD transfer effect can be drastically enhanced by placing chiral molecules, here double-stranded DNA, inside a plasmonic hotspot. By using different particle types (gold, silver, spheres, and rods) and by exploiting the versatility of DNA origami, we were able to systematically study the impact of varying particle distances on the CD transfer efficiency and to demonstrate CD transfer over the whole optical spectrum down to the near-infrared. For this purpose, nanorods were also placed upright on DNA origami sheets, forming strong optical antennas. Theoretical models, demonstrating the intricate relationships between molecular chirality and achiral electric fields, support our experimental findings. From both experimental measurements and theoretical considerations, we conclude that the transferred CD is most intensive for systems with strong plasmonic hotspots, as we find them in relatively small gaps (5-12 nm) between spherical nanoparticles and preferably between the tips of nanorods.

Chirality Transfer from Sub‐Nanometer Biochemical Molecules to Sub‐Micrometer Plasmonic Metastructures: Physiochemical Mechanisms, Biosensing, and Bioimaging Opportunities

“…DNA origamis have been reported to be an alternative class of connecting molecules for construct plasmonic nanogaps with more flexibilities. [106][107][108] Kneer et al systematically studied the impact of gap size, material composition and particle geometry on the chirality transfer effects in DNA-assembled metal nanostructures, [108] revealing that the CD signal transferred to the plasmonic frequency domain is most intensive for systems with strong near-field hotspots. Interestingly, such plasmonic CD response can be reversed back and forth by temperaturedependent assembly and disassembly of DNA-assembled Au NRs: [106] at low temperature (20 °C), the double-stranded (ds) DNA molecules are complementary to each other, leading to the formation of SBS-assembled Au NRs with strong CD signals in the visible region; when the temperature increases to above 60 °C, dsDNA molecules melt in the solution and the Au NRs disassemble, causing the disappearance of plasmonic CD due to the reduced local field enhancement.…”

Plasmonic Circular Dichroism of Gold Nanoparticle Based Nanostructures

“…In general, the obtained CD signal for discrete nanoparticles is weak, g ‐factor of ≈10 −3 , as strong chiroptical activity requires a high degree of chirality from the well‐oriented, self‐assembled monolayer of chiral molecules on the NP surface and considerable overlap between the plasmon band and the absorption band of chiral molecules. Many efforts have been devoted to enhance this plasmonic CD signal, such as to embed metal NPs in different chiral media, or anchor chiral molecules on the metal NP surface . And “hot spots” that generated at the gap of NP aggregates is found to be an effective way.…”

Chiral Plasmonic Nanostructures Enabled by Bottom-Up Approaches

“…cholate molecules placed in the regions between the nanoparticles, where the plasmonic nearfields were localized. Kneer et al (64) studied the role of the interparticle distance on CD transfer efficiency. The authors assembled Au and Ag nanoparticles on DNA origami, which allowed for precise tuning of structural geometries.…”