Super-resolution optical imaging of Rhodamine 6G surface-enhanced Raman scattering (SERS) and silver luminescence from colloidal silver aggregates are measured with sub-5 nm resolution and found to originate from distinct spatial locations on the nanoparticle surface. Using correlated scanning electron microscopy, the spatial origins of the two signals are mapped onto the nanoparticle structure, revealing that, while both types of emission are plasmon-mediated, SERS is a highly local effect, probing only a single junction in a nanoparticle aggregate, whereas luminescence probes all collective plasmon modes within the nanostructure. Calculations using the discrete-dipole approximation to calculate the weighted centroid position of both the |E|(2)/|E(inc)|(2) and |E|(4)/|E(inc)|(4) electromagnetic fields were compared to the super-resolution centroid positions of the SERS and luminescence data and found to agree with the proposed plasmon dependence of the two emission signals. These results are significant to the field of SERS because they allow us to assign the exact nanoparticle junction responsible for single-molecule SERS emission in higher order aggregates and also provide insight into how SERS is coupled into the plasmon modes of the underlying nanostructure, which is important for developing new theoretical models to describe SERS emission.
Lipid composition dictates membrane thickness, which in turn can influence membrane protein activity. Lipid composition also determines whether a membrane demixes into coexisting liquid-crystalline phases. Previous direct measurements of demixed lipid membranes have always found a liquid-ordered phase that is thicker than the liquid-disordered phase. Here we investigated non-canonical ternary lipid mixtures designed to produce bilayers with thicker disordered phases than ordered phases. The membranes were comprised of short, saturated (ordered) lipids; long, unsaturated (disordered) lipids; and cholesterol. We found that few of these systems yield coexisting liquid phases above 10 °C. For membranes that do demix into two liquid phases, we measured the thickness mismatch between the phases by atomic force microscopy and found that not one of the systems yields thicker disordered than ordered phases under standard experimental conditions. We found no monotonic relationship between demixing temperatures of these ternary systems and either estimated thickness mismatches between the liquid phases or the physical parameters of single-component membranes comprised of the individual lipids. These results highlight the robustness of a membrane’s liquid-ordered phase to be thicker than the liquid-disordered phase, regardless of the membrane’s lipid composition.
The electromagnetic scattering properties of Ag nanoparticle aggregates known to be antennas for single-molecule surface-enhanced Raman scattering are investigated from a continuum electrodynamics perspective. High-resolution mappings of the spatial, spectral, and polarization dependence of the volumes of the aggregate’s electromagnetic hot spots reveal multiple active regions for enhanced Raman scattering activity by molecular chromophores. Further analysis of these regions using maps of polarization surface-charge density shows that some hot spots are due to the collective and phase-coherent excitation of localized surface-plasmon resonances, whereas others derive from interfering plasmonic excitations resulting from scattering from gaps and surfaces. The latter are still capable of generating intense local fields at certain excitation energies, whereas the former tend to provide the most spatially delocalized regions of high electromagnetic-field strength.
Recent results provide evidence that cholesterol is highly accessible for removal from both cell and model membranes above a threshold concentration that varies with membrane composition. Here we measured the rate at which methyl-β-cyclodextrin depletes cholesterol from a supported lipid bilayer as a function of cholesterol mole fraction. We formed supported bilayers from two-component mixtures of cholesterol and a PC (phosphatidylcholine) lipid, and we directly visualized the rate of decrease in area of the bilayers with fluorescence microscopy. Our technique yields the accessibility of cholesterol over a wide range of concentrations (30-66 mol %) for many individual bilayers, enabling fast acquisition of replicate data. We found that the bilayers contain two populations of cholesterol, one with low surface accessibility and the other with high accessibility. A larger fraction of the total membrane cholesterol appears in the more accessible population when the acyl chains of the PC-lipid tails are more unsaturated. Our findings are most consistent with the predictions of the condensed-complex and cholesterol bilayer domain models of cholesterol-phospholipid interactions in lipid membranes.
We extend our previous quantum many-body Green's function formalism to characterize the deformations induced in the electronic structure of a quantum emitter when it strongly couples with a plasmon-supporting environment at finite frequency. Through infinite-order perturbation theory, we predict the emergence of subtle yet observable changes in the plasmon-dressed molecule's frontier orbitals, orbital energies, and low-lying electronic excitations when the molecular and plasmonic systems are resonantly coexcited. These distortions, which predominately arise from the finitefrequency image interaction, point to new chemical and optical properties beyond those of the vacuum molecule and bear impact upon resonant plasmon-enhanced molecular spectroscopies and hot-electron-driven chemical catalysis. We propose an experiment capable of testing our predictions.
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