Bovine-Hemoglobin Detection by Single-Particle Plasmon-Coupled Circular Dichroism

Plasmon-coupled circular dichroism enables chiral molecule detection by inducing circular dichroism at the plasmon resonance through interactions with a plasmonic sensor. Coupled nanoparticles offer potentially higher sensitivities due to stronger plasmonic fields at the junctions. However, ensemble-level sensitivity is limited by signal averaging, and the structural chirality of the sensor itself can obscure the molecular response. To overcome these issues, we combine single-particle dark-field scattering with electron microscopy. Individual gold nanosphere dimers, selected to avoid interference from structural chirality, yield unambiguous plasmon-coupled circular dichroism for hemoglobin as an analyte, while monomers give no detectable signal. We explain these results based on their difference in refractive index sensitivities with respect to hemoglobin's circular birefringence, as supported by electrodynamic simulations. This study sheds new light on the mechanism of plasmon-coupled circular dichroism by isolating the response of individual nanostructures and thereby avoiding ensemble averaging over a heterogeneous mixture of nanostructure geometries.