Characterizing the fluorescence of single chiral molecules in the near field of gold nanoparticles.

There is considerable interest in increasing the sensitivity of chiral sensing because the intrinsic chirality of biomolecules is an indicator of structural and functional integrity. Single-molecule localization microscopy techniques detect the fluorescence of one molecule at a time and can therefore be leveraged to effectively conduct fluorescence-detected circular dichroism (FDCD) at the single-molecule level. The low differential absorption cross sections of individual molecules yield low signal-to-noise readouts, so we need to reshape the near-field electromagnetic field to enhance the difference between the absorbance of right-handed and left-handed light. Plasmonic Au nanoparticles act as antennas to focus incident plane waves and can be used to engineer optimal fields. However, due to the coupling of the molecular transition dipole and the electric dipole of the particle, the emission intensity does not only depend on absorption cross sections. Moreover, the emission pattern is strongly influenced by the engineered electromagnetic near field and the symmetry breaking results in emission mislocalization. Overall, for plasmon-enhanced FDCD to be a single-molecule chiral-sensing technique, the effect of Au nanoparticles on the fluorescence of a nearby chiral molecule must be characterized. In this talk, I will discuss the intensity and localization distributions of single-molecule fluorescence near plasmonic Au nanoparticle substrates. By varying the geometry of the substrate, the polarization of the incident beam, and the inherent chirality of the fluorescent dye while monitoring competing processes such as photo-bleaching and quenching, I will report on the shift between fluorescence intensity and localization of chiral and achiral molecules. The results, complemented with full-field electromagnetic simulations, will inform the feasibility of FDCD of single biomolecules.