Plasmon-induced CD response of oligonucleotide-conjugated metal nanoparticles

Abstract

Non-chiral metal nanoparticles conjugated with chiral oligonucleotide molecules demonstrated a circular dichroism (CD) at the plasmonic wavelengths due to aggregation effects.Metal nanoparticles (NPs) have raised much interest in recent years due to their unique optical and electronic properties. Functionalising their surface with biomolecules has led the way to applications in fields as various as biosensing, 1 drug delivery 2 and Surface Enhanced Raman Spectroscopy (SERS). 3The induction or enhancement of a Circular Dichroism (CD) signal using metal nanoparticles (NPs) could be a new tool for highly responsive sensing. While biomolecules created by nature have a very high degree of homogeneity in a macroscopic ensemble and therefore produce amazingly strong and consistent CD signals (mostly in the UV range), it is still very challenging to create chiral nanocrystals because it seems impossible to control all positions of atoms in a nanocrystal. In many experiments, new CD signals in the visible range in metal nanostructures were found for metal nano-clusters with a relatively small number of atoms and with the help of chiral molecules. [4][5][6][7][8] An example is Ag clusters (Bfew hundreds of atoms) synthesized on DNA templates and giving new plasmonic CD lines. 5 In few other experiments, plasmonic CD lines were found for conjugated NPs of larger sizes, having well-defined plasmonic lines. 9-13Suggested mechanisms of new CD bands in conjugated plasmonic nanocrystals include the formation of chiral surface states of the metal component, orbital interactions between the chiral adsorbate and the metal, and dipolar Coulomb interactions between chiral optical dipoles of conjugated molecules and plasmons.14 The latter mechanism was recently proposed theoretically 14 and involves Coulomb interactions between nanoscale objects (chiral molecules and non-chiral nanocrystals), enabling plasmonic CD lines in the visible range. The main motivation for our research here was to create an artificial chirality coming from an interaction between well-defined nanoscale building blocks. In our work, we used assemblies of interacting chiral and non-chiral nanoscale elements, namely chiral oligonucleotide molecules and metal nanocrystals. Using initially non-chiral metal nanocrystals of various shapes, we first conjugated them with oligonucleotide molecules and then created aggregates of the new species. In a set of aggregated samples, we observed strong CD signals at the plasmonic wavelengths. It should be noted that aggregates of nanocrystals conjugated with non-chiral molecules did not show the optical chirality (CD signals). The plasmonic CD signals in aggregates composed of oligonucleotide-nanocrystal conjugates may come from the creation of hot plasmonic spots in aggregates and from the Coulomb interactions between chiral molecules and plasmons. Importantly, in contrast to many previous studies, the plasmonic CD mechanism found in this communication for chiral conjugated nanocrystals appeared only after aggregation. Additi...

Chiral Electromagnetic Fields Generated by Arrays of Nanoslits

“…First, adsorbed molecules may influence the electronic states of the metallic nanostructures. 20 Furthermore, molecular alignment at the substrate interface may play an important role, giving rise to aligned dipole–dipole interactions 21 or contributions to chiroptical phenomena through electric dipole–electric quadrupole coupling. 6 Finally, for large biomolecules which are of the same order as the spatial decay length of the inhomogeneous local electromagnetic fields, nonlocal effects may be important.…”

Giant circular dichroism of a molecule in a region of strong plasmon resonances between two neighboring gold nanocrystals

“…For example, it was a powerful method to differentiate a secondary structure of proteins which can appear in the forms of α-helix, random coil, or β-sheet. Recently the concepts of chirality and CD have been brought to the field of plasmonics [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] . One of the motivations for this development is plasmon-enhanced sensing of chiral molecules.…”

Synthesis and Plasmonic Chiroptical Studies of Sodium Deoxycholate Modified Silver Nanoparticles

“…This is confirmed by the TEM images of NaDC modified Ag NPs containing dimers and linear chain assemblies (Figure 2c). It has been reported that the plasmonic CD signals can be enhanced in aggregates composed of chiral molecule-metal nanoparticle conjugates, which creates hot spots between Ag NPs and remarkably amplifies the electromagnetic fields [24]. The mechanism of plasmonic CD signals is based on the Coulomb interaction between chiral molecules and plasmonic modes in the hot spot.…”