Far-Field Polarization Optics Control the Nanometer-Scale Pattern of High-Fluorescence Dissymmetry Emission from Achiral Molecules near Plasmonic Nanodimers

Abstract

Chiroptical spectroscopies like circular dichroism report on the handedness of molecules but are often limited to the high-concentration regime because of the mismatch between the wavelength of propagating visible light and the size of molecules. Recent work has demonstrated that the electromagnetic fields generated near chiral plasmonic nanostructures can have high optical chirality that bridges these length scales and can induce high-fluorescence dissymmetry signals from achiral dyes by redirecting their emission. As a step toward single-molecule chiral sources, the induced fluorescence dissymmetry and apparent emission pattern near chiral plasmonic nanoparticles must be quantified. Here, we measure the induced fluorescence dissymmetry from the achiral dye Cy5.5 near a chiral gold nanodimer structure, and we map out the apparent emission pattern of the dyes using super-resolution single-molecule microscopy. Our high-sensitivity approach quantifies the fluorescence dissymmetry from sampling only ∼100 zeptomoles of Cy5.5 and measures from Cy5.5 near gold nanodimers median fluorescence dissymmetry factors that are 2 orders of magnitude greater than those of synthesized chiral fluorescent molecules. We observe a spatial correlation between the simulated optical chirality about a plasmonic gold nanodimer and experimentally obtained super-resolution apparent emission patterns, and we use the incident polarization to control our detection bias for the brightest molecules to sample nanometer-scale regions of high electric flux.

Single-Particle Photoluminescence Measures a Heterogeneous Distribution of Differential Circular Absorbance of Gold Nanoparticle Aggregates near Constricted Thioflavin T Molecules

“…7−9 Fibril chirality can be measured with circular dichroism. 10 This measurement typically requires high concentrations (∼100 μM) and sufficiently large volumes (∼1 mL) for a sufficient enough signal. A recent study has also shown that plasmonic nanoparticles that match the fibril pitch form ordered structures around the fibril, which can be used to detect fibril chirality at low concentrations.…”

“…We designed single-particle fluorescence polarimetry experiments to distinguish between these mechanisms by measuring if there is a significant chiroptical response when a single AuNR is coupled to fibrils and proximal to fibril-bound ThT molecules. 10 The experiments were also designed to investigate the heterogeneity in the circular differential absorption of single particles, a distribution that is hidden in the ensemble measurement (Figure 1a). We find monomeric AuNRs have no detectable induced circular dichroism (CD) signal, and thus, the net CD signal cannot be attributed to the sum of the induced CD from monomeric AuNRs.…”

“…We implemented a polarization-sensitive single-molecule microscope to measure PL-detected CD from single emitters (Figure S2). 10 We compensated for phase changes in the optical path (primarily due to multiple reflections and the dichroic filter) and alternated between left-and right-handed circularly polarized excitations with a variable waveplate (Thorlabs LCC1223T-A). The incident beam was left-handed circularly polarized when the phase between the orthogonal axes was 90°and right-handed circularly polarized when the phase between the orthogonal axes was −90°.…”

“…This signature handedness, or chirality, correlates with their biological function . If fibril formation is disrupted or the fibrils are damaged, the fibril chirality is affected. − Thus, sensitive detection of a fibril’s chirality could improve the early diagnosis of plaque-related diseases. − Fibril chirality can be measured with circular dichroism . This measurement typically requires high concentrations (∼100 μM) and sufficiently large volumes (∼1 mL) for a sufficient enough signal.…”

“…We investigated the molecular-scale mechanism behind the nonresonant changes in the chiroptical signals that are detected on the ensemble level (Figure a and Figure S1) and asked if these chiral signals can be attributed to either (1) plasmon-coupled enhancement of differential absorption by the constricted ThT molecules near the AuNRs or (2) templated aggregation of the AuNRs by the fibrils and ThT. We designed single-particle fluorescence polarimetry experiments to distinguish between these mechanisms by measuring if there is a significant chiroptical response when a single AuNR is coupled to fibrils and proximal to fibril-bound ThT molecules . The experiments were also designed to investigate the heterogeneity in the circular differential absorption of single particles, a distribution that is hidden in the ensemble measurement (Figure a).…”