Characteristic image patterns of single anisotropic plasmonic nanoparticles embedded in a gel matrix

Direct Observation of In-Focus Plasmonic Cargos via Breaking Angular Degeneracy in Differential Interference Contrast Microscopy

Breaking the angular degeneracy arising from the 2-fold optical symmetry of plasmonic anisotropic nanoprobes is critical in biological studies. In this study, we propose differential interference contrast (DIC) microscopy-based focused orientation and position imaging (dFOPI) to break the angular degeneracy of single gold nanorods (AuNRs). Single in-focus AuNRs (39 nm × 123 nm) within a spherical mesoporous silica shell were characterized with high throughput and produced distinct doughnut-shaped DIC image patterns featuring two lobes in the peripheral region, attributed to the scattering contribution of the AuNRs with large scattering cross sections. Interestingly, rotation of the lobes was observed in the focal plane for a large AuNR (>100 nm) tilted by more than ∼20° from the horizontal plane as the rotational stage was moved by 10° in a rotational study. From the rotation-dependent characteristic patterns, we directly visualized counterclockwise/clockwise rotations without the angular degeneracy at the localized surface plasmon resonance wavelength. Therefore, our dFOPI method can be applied for in vivo studies of important biological systems. To validate this claim, we tracked the three-dimensional rotational behavior of transferrin-modified in-focus AuNRs during clathrin-mediated endocytosis in real time without sacrificing the temporal and spatial resolution. In the invagination and scission stage, one or two directed twist motions of the AuNR cargos detached the AuNR-containing vesicles from the cell membrane. Furthermore, the dFOPI method directly visualized and revealed the right-handed twisting action along the dynamin helix in dynamin-catalyzed fission in live cells.

Single-particle spectroscopy and defocused imaging of anisotropic gold nanorods by total internal reflection scattering microscopy

Total internal reflection scattering (TIRS) microscopy is based on evanescent field illumination at the interface. Compared to conventional dark-field (DF) microscopy, TIRS microscopy has been rarely applied to the spectroscopic studies of plasmonic nanoparticles. Furthermore, there has been no detailed correlation study on the characteristic optical properties of single gold nanorods (AuNRs) obtained by DF and TIRS microscopy. Herein, through a single-particle correlation study, we compare the spectroscopic and defocusing properties of single AuNRs obtained by DF and TIRS microscopy, which have different illumination geometries. Compared to DF microscopy, TIRS microscopy yielded almost identical single-particle scattering spectra and localized surface plasmon resonance (LSPR) linewidth for the same in-focus AuNRs. However, TIRS microscopy, which is based on evanescent field illumination at the interface, provided a higher signal-to-noise ratio in the defocused image of the same AuNRs compared to DF microscopy. Furthermore, the heavily reduced background noise clarified the defocused scattering patterns of TIRS microscopy, which provided more detailed and accurate angular information than that obtained by conventional DF microscopy.

Three-dimensional defocused orientation sensing of single bimetallic core-shell gold nanorods as multifunctional optical probes

Bimetallic core-shell gold nanorods (AuNRs) are promising multifunctional orientation probes that can be employed in biological and physical studies. This paper presents the optical properties of single AuNRs coated with palladium (Pd) and platinum (Pt) under scattering-based dark-field (DF) microscopy. Strong longitudinal plasmon damping was observed for the bimetallic AuNRs due to Pd and Pt metals on the AuNR surface. Despite the strong plasmon damping, the bimetallic AuNRs yielded characteristic doughnut-shaped scattering patterns under defocused DF microscopy. Interestingly, a solid bright spot appeared at the center of the defocused scattering patterns due to strong damping in the longitudinal plasmon and the increased contribution from the transverse dipoles to the image patterns, which was verified further by a simulation study. Furthermore, the defocused scattering field distributions enabled a determination of the three-dimensional (3D) orientations of single bimetallic AuNRs through a pattern-match analysis technique without angular degeneracy. Therefore, deeper insight into the optical properties and defocused scattering patterns of single bimetallic AuNRs is provided, which can be used to develop multifunctional optical probes that are capable of sensing of the 3D orientation of a probe, biomolecules based on LSPR shift, gas and humidity, etc.

Hybrid plasmonic gap modes in metal film-coupled dimers and their physical origins revealed by polarization resolved dark field spectroscopy

Plasmonic gap modes sustained by metal film-coupled nanostructures have recently attracted extensive research attention due to flexible control over their spectral response and significantly enhanced field intensities at the particle-film junction. In this work, by adopting an improved dark field spectroscopy methodology - polarization resolved spectral decomposition and colour decoding - we are able to "visualize" and distinguish unambiguously the spectral and far field radiation properties of the complex plasmonic gap modes in metal film-coupled nanosphere monomers and dimers. Together with full-wave numerical simulation results, it is found that while the monomer-film system supports two hybridized dipole-like plasmon modes having different oscillating orientations and resonance strengths, the scattering spectrum of the dimer-film system features two additional peaks, one strong yet narrow resonant mode corresponding to a bonding dipolar moment and one hybridized higher order resonant mode, both polarized along the dimer axis. In particular, we demonstrate that the polarization dependent scattering radiation of the film-coupled nanosphere dimer can be used to optically distinguish from monomers and concurrently determine the spatial orientation of the dimer with significantly improved accuracy at the single-particle level, illustrating a simple yet highly sensitive plasmon resonance based nanometrology method.