Focused orientation and position imaging (FOPI) of single anisotropic plasmonic nanoparticles by total internal reflection scattering microscopy

Automated Five-Dimensional Single Particle Tracking by Bifocal Parallax Dark-Field Microscopy with Electronic Tunable Lens

We present a novel method for the precise tracking of plasmonic gold nanorods (AuNRs) in live cells, enabling a comprehensive understanding of the nanocargo's cellular dynamics. Traditional single particle tracking (SPT) struggles with accurately determining all five spatial parameters (x, y, z, ϕ, and θ) in live cells due to various challenges. Our innovation combines electronic tunable lens (ETL) technology with bifocal parallax dark-field (DF) microscopy, allowing continuous adjustment of the imaging focal plane for automatic tracking of both translational and rotational movements of AuNRs. This 5D single-particle orientation and rotational tracking (5D SPORT) method achieves remarkable precision, with 3D localization precisions of 9 (x), 10 (y), and 15 nm (z) and angular resolutions below 2°. To showcase its applicability, we investigated intracellular transport of nanocargos using transferrin-modified AuNRs as the imaging probe. Differentiated transport stages, such as active transport and pause period, were clearly unveiled from the observed dynamics in 5D. This advancement in single particle tracking holds promise for a wide range of applications in biomedical research, particularly when combined with other imaging modalities, such as light sheet fluorescence microscopy.

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.

Ultrahigh-Throughput Single-Particle Hyperspectral Imaging of Gold Nanoparticles

Gold nanoparticles (AuNPs) have become increasingly useful in recent years for their roles in nanomedicine, cellular biology, energy storage and conversion, photocatalysis, and more. At the single-particle level, AuNPs have heterogeneous physical and chemical properties which are not resolvable in ensemble measurements. In the present study, we developed an ultrahigh-throughput spectroscopy and microscopy imaging system for characterization of AuNPs at the single-particle level using phasor analysis. The developed method enables quantification of spectra and spatial information on large numbers of AuNPs with a single snapshot of an image (1024 × 1024 pixels) at high temporal resolution (26 fps) and localization precision (sub-5 nm). We characterized the localized surface plasmonic resonance (SPR) scattering spectra of gold nanospheres (AuNSs) of four different sizes (40-100 nm). Comparing to the conventional optical grating method which suffers low efficiency in characterization due to spectral interference caused by nearby nanoparticles, the phasor approach enables high-throughput analysis of single-particle SPR properties in high particle density. Up to 10-fold greater efficiency of single-particle spectro-microscopy analysis using the spectra phasor approach when compared to a conventional optical grating method was demonstrated.

Imaging Dynamic Processes in Multiple Dimensions and Length Scales

Optical microscopy has become an invaluable tool for investigating complex samples. Over the years, many advances to optical microscopes have been made that have allowed us to uncover new insights into the samples studied. Dynamic changes in biological and chemical systems are of utmost importance to study. To probe these samples, multidimensional approaches have been developed to acquire a fuller understanding of the system of interest. These dimensions include the spatial information, such as the three-dimensional coordinates and orientation of the optical probes, and additional chemical and physical properties through combining microscopy with various spectroscopic techniques. In this review, we survey the field of multidimensional microscopy and provide an outlook on the field and challenges that may arise. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Mesoporous silica shell-coated single gold nanorods as multifunctional orientation probes in dynamic biological environments

Mesoporous silica shell-coated gold nanorods (AuNRs@mSiO₂) can be employed as promising multifunctional orientation probes in biological studies owing to their anisotropic optical properties, enhanced stability, excellent biocompatibility, etc. In this study, the optical properties of single AuNRs@mSiO₂ are characterized under dark-field and differential interference contrast (DIC) microscopy. Furthermore, we presented polarization-dependent, periodic DIC images and intensities of single AuNRs@mSiO₂ at their localized surface plasmon resonance wavelength and investigated their use as multifunctional orientation probes in dynamic biological environments. Moreover, the real-time rotational motions of the AuNRs@mSiO₂ on the HeLa cell membranes were tracked with millisecond temporal resolution. Overall, AuNRs@mSiO₂ demonstrated their capacity to act as multifunctional optical probes owing to the combined effect of the Au core, which can serve as an orientation probe and a local heat generator for phototherapy, and the mesoporous silica shell, which can be used as a reservoir of chemotherapeutics owing to its excellent loading capacity.

We presented polarization-dependent, periodic DIC images and intensities of single AuNRs@mSiO₂ at their LSPR wavelength and investigated their use as multifunctional orientation probes in biological environments.

Influence of oxygen plasma treatment on structural and spectral changes in silica-coated gold nanorods studied using total internal reflection microscopy and spectroscopy

This paper shows how oxygen plasma treatment affects the structural, localized surface plasmon resonance (LSPR) spectral, and spatial orientation changes in single gold nanorods coated with a mesoporous silica shell (AuNRs@SiO2) in comparison with bare AuNRs with the same aspect ratio (AR). Single AuNRs@SiO2 subjected to different plasma treatment times were characterized using scanning electron microscopy and total internal reflection scattering (TIRS) microscopy and spectroscopy. The AR of the single AuNRs without a silica shell was decreased by structural deformation, while their LSPR linewidth was increased with increasing plasma treatment time. In contrast, single AuNRs@SiO2 showed much higher structural and spectral stability due to the silica shell under the energetic plasma treatment. Furthermore, there was no noticeable variation in the three-dimensional (3D) orientations of single AuNR cores in the silica shell before and after the plasma treatment. The results support that no significant structural and spectral changes occur in single AuNRs@SiO2 and that the silica coating enhances the stability of AuNR cores against oxygen plasma treatment. Therefore, fundamental information on the relationship among plasma treatment time, structural change, LSPR damping, and defocused orientation patterns is provided at the single-particle level.

Individual Plasmonic Nanoprobes for Biosensing and Bioimaging: Recent Advances and Perspectives

With the advent of nanofabrication techniques, plasmonic nanoparticles (PNPs) have been widely applied in various research fields ranging from photocatalysis to chemical and bio-sensing. PNPs efficiently convert chemical or physical stimuli in their local environment into optical signals. PNPs also have excellent properties, including good biocompatibility, large surfaces for the attachment of biomolecules, tunable optical properties, strong and stable scattering light, and good conductivity. Thus, single optical biosensors with plasmonic properties enable a broad range of uses of optical imaging techniques in biological sensing and imaging with high spatial and temporal resolution. This work provides a comprehensive overview on the optical properties of single PNPs, the description of five types of commonly used optical imaging techniques, including surface plasmon resonance (SPR) microscopy, surface-enhanced Raman scattering (SERS) technique, differential interference contrast (DIC) microscopy, total internal reflection scattering (TIRS) microscopy, and dark-field microscopy (DFM) technique, with an emphasis on their single plasmonic nanoprobes and mechanisms for applications in biological imaging and sensing, as well as the challenges and future trends of these fields.

Single Gold Bipyramid Nanoparticle Orientation Measured by Plasmon-Resonant Scattering Polarimetry

The 3D orientation of a single gold nanoparticle is probed experimentally by light scattering polarimetry. We choose high-quality gold bipyramids (AuBPs) that support around 700 nm a well-defined narrow longitudinal localized surface plasmonic resonance (LSPR) which can be considered as a linear radiating dipole. A specific spectroscopic dark-field technique was used to control the collection angles of the scattered light. The in-plane as well as the out-of-plane angles are determined by analyzing the polarization of the scattered radiation. The data are compared with a previously developed model where the environment and the angular collection both play crucial roles. We show that most of the single AuBPs present an out-of-plane orientation consistent with their geometry. Finally, the fundamental role of the collection angles on the determination of the orientation is investigated for the first time. Several features are then deduced: we validate the choice of the analytical 1D model, an accurate 3D orientation is obtained, and the critical contribution of the evanescent waves is highlighted.

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.

High-throughput in-focus differential interference contrast imaging of three-dimensional orientations of single gold nanorods coated with a mesoporous silica shell

Plasmonic gold nanorods (AuNRs) have been widely applied as optical orientation probes in many biophysical studies. However, characterizing the various three-dimensional (3D) orientations of AuNRs in the same focal plane of the objective lens is a challenging task. To overcome this challenge, we fabricated single AuNRs (10 nm × 30 nm) coated with either an elliptical or spherical mesoporous silica shell (AuNRs@mSiO₂). Unlike bare AuNRs and elliptical AuNRs@mSiO₂, spherical AuNRs@mSiO₂ contained randomly oriented AuNR cores in 3D space, which could be observed on the same focal plane within a single frame by differential interference contrast (DIC) microscopy. The spherical AuNRs@mSiO₂ thus achieved high-throughput detection. The proposed approach can overcome the limitations of the current gel-matrix method, which requires vertical scanning of the embedded AuNRs to capture different focal planes.

Spherical AuNRs@mSiO₂ have randomly oriented AuNR cores in 3D space, which could be resolved on the same focal plane by interference-based DIC microscopy.

Tracking the rotation of single CdS nanorods during photocatalysis with surface plasmon resonance microscopy

Rotational dynamics of anisotropic nanomaterials reveals and regulates their behaviors and functions in diverse fields ranging from nanomotors, biomechanics, and enzymatic catalysis to microrheology. An optical imaging technique that is suitable for all kinds of anisotropic nanoobjects, regardless of its inherent optical property, is thus highly desirable and it is yet to be demonstrated. In the present work, by taking a nonfluorescent and nonplasmonic CdS nanorod as an example, we demonstrate the capability of a recently developed surface plasmon resonance microscopy for determining the orientation of single anisotropic nanomaterials with arbitrary chemical composition and morphology.

While rotational dynamics of anisotropic nanoobjects has often been limited in plasmonic and fluorescent nanomaterials, here we demonstrate the capability of a surface plasmon resonance microscopy (SPRM) to determine the orientation of all kinds of anisotropic nanomaterials. By taking CdS nanorods as an example, it was found that two-dimensional Fourier transform of the asymmetrical wave-like SPRM image resulted in a peak in its angular spectrum in k space. Consistency between the peak angle and the geometrical orientation of the nanorod was validated by both in situ scanning electron microscope characterizations and theoretical calculations. Real-time monitoring of the rotational dynamics of single CdS nanorods further revealed the accelerated rotation under appropriate reaction conditions for photocatalyzed hydrogen generation. The driving force was attributed to the asymmetric production of hydrogen molecules as a result of inhomogeneous distribution of reactive sites within the nanorod. The present work not only builds the experimental and theoretical connections between the orientation of anisotropic nanomaterials and its SPRM images; the general suitability of SPRM also sheds light on broad types of nonfluorescent and nonplasmonic anisotropic nanoobjects from semiconductors to bacteria and viruses.

Homogeneous localized surface plasmon resonance inflection points for enhanced sensitivity and tracking plasmon damping in single gold bipyramids

The most polarizable localized surface plasmon resonance (LSPR) longitudinal mode of anisotropic metallic nanoparticles, such as gold bipyramids (AuBPs), is of high prominence. This optical response has tremendous applications from spectroscopy to photonics and energy devices to sensing. In conventional LSPR-based sensing, broadening and asymmetry in peaks due to chemical and instrument noise hinder obtaining a precise insight on shift positions, accordingly limiting the effectiveness and impact of LSPR sensors. Further, when investigating LSPR properties, utilizing more simplistic frequency dependent dielectric-type models can aberrantly impact the reliability of fundamental properties used for designing and fabricating efficient optical devices. For instance, more approximations can effectively limit screening intra-band and inter-band (IB) electronic transition contributions and other related optical properties. With an aim to find alternative methods to further improve their efficiency, as a first report, we devoted a particular focus on LSPR scattering inflection points (IFs) of single AuBPs. The findings reveal that tracking LSPR IFs exhibit high sensitivity over their counterpart LSPR peak shift locations. In addition, we newly detected IB transition contributions near the resonance energy in the range (1.50 eV-2.00 eV) dominated by intra-band transitions. A small increase in the local RI effectively enhances the LSPR quality factor due to IB transitions. Therefore, while neglecting IB transitions in the range below 2.4 eV can work for local air refractive index (RI), in high local RI media it can be aberrantly underestimated. Demonstrated by the use of the dielectric function based on Kramers-Kronig consistent Lorentz oscillators, our findings are in good agreement with the enhancing RI sensitivity effect. The results of this investigation support the idea that tracking curvature changes of an optical signal can be effectively used for LSPR longitudinal peak RI sensing as well as damping in the local RI environment of a single AuBP.

Plasmonic sphere-on-plane systems with semiconducting polymer spacer layers

Scattering color changes are investigated in plasmonic sphere-on-plane samples containing resonant and non-resonant conjugated polymer spacers.

Imaging the chemical activity of single nanoparticles with optical microscopy

Chemical activity of single nanoparticles can be imaged and determined by monitoring the optical signal of each individual during chemical reactions with advanced optical microscopes. It allows for clarifying the functional heterogeneity among individuals, and for uncovering the microscopic reaction mechanisms and kinetics that could otherwise be averaged out in ensemble measurements.

Background-Free Imaging of a Viral Capsid Proteins Coated Anisotropic Nanoparticle on a Living Cell Membrane with Dark-Field Optical Microscopy

Exploring the diffusion dynamics of a viral capsid proteins (VCP)-functionalized nanocarrier on a living cell membrane could provide much kinetic information for the better understanding of their biological functionality. Gold nanoparticles are an excellent core material of nanocarriers because of the good biocompatibility as well as versatile surface chemistry. However, due to the strong scattering background from subcellular organelles, it is a grand challenge to selectively image an individual nanocarrier on a living cell membrane. In this work, we demonstrated a convenient strategy to effectively screen the scattering background from living cells for single-particle imaging with a polarization-resolved dual-channel imaging module. By taking advantage of the polarization of anisotropic gold nanoparticles (gold nanorods, GNRs), the signals from cell components could be counteracted after subtracting the sequential images one by one, while those transiently rotating GNRs on the cell membrane still exist in the processed image. In contrast to the previously reported methods, this method does not require a complicated optical setup alignment and sophisticated digital image analysis process. According to the single-particle imaging results, the majority of VCP-GNRs were anchoring on the cell membrane with confined diffusion. Interestingly, on further inspection of the diffusion trajectories, the particles displayed anomalous confined diffusion with randomly distributed large walking steps during the whole track. Non-Gaussian step distribution was noted, indicating heterogeneous binding and desorption processes on the cell membrane. As a consequence of the robust background screening capability, this approach would find broad applications for single-particle imaging under a noisy environment, e.g., living cells.

Full-colour carbon dots: integration of multiple emission centres into single particles

The single particle emission behaviours of our previously reported excitation-dependent full-colour carbon dots (F-C dots) have been analyzed by a single-particle fluorescence imaging technique. The co-localization of the F-C dots excited with different wavelengths shows that single F-C dots can also be excited with multiple energies. The co-localization of the F-C dots that emit at different colour regions under the same excitation wavelength or different excitation wavelengths shows that single F-C dots have a broad emission band from blue to red, but the emission intensities in different colour regions vary from one particle to another. So this study concretely proves that the full colour emissions are single particle behaviours; they are different from the other type of excitation dependent full-colour emission carbon dots whose full-colour behaviour originates from the large heterogeneity in both particle size and the structures of the ensemble. Then the origination of the full-colour emission at the single dot level was further studied by comparing the emission properties of the F-C dots and the small molecular byproducts, and it is found that the emissions of the dozens of molecular byproducts can also cover the full visible regions. And the emission positions of F-C dots are very similar to those of the byproducts at the same excitations, but they show different lifetimes. So a mechanism for the full colour emissions of F-C dots is proposed to originate from the hybridization of multiple small emissive molecules on the emissive carbon cores. This single particle level understanding of full-colour emission properties will pave the way towards the development of single dot imaging or tracking.

Single gold bipyramids with sharp tips as sensitive single particle orientation sensors in biological studies

Plasmonic gold bipyramids (AuBPs) with sharp tips are promising orientation probes in biological studies because of their anisotropic shape, strong electric field enhancement at the tips, and convenient manipulation into other shapes. Herein, we elucidate the optical properties of single AuBPs at their localized surface plasmon resonance (LSPR) wavelengths using dark-field (DF) microscopy and differential interference contrast (DIC) microscopy and test their use as orientation probes in a dynamic biological environment. Characteristic scattering field distributions together with a simulation study allowed us to achieve the high-throughput determination of the 3D orientation of single AuBPs within a single frame using defocused DF microscopy. We further present the polarization-dependent, periodic DIC images and intensities of single AuBPs at their LSPR wavelengths with high sensitivity. Finally, we successfully tracked the real-time rotational motions of transferrin-modified AuBPs on live cell membranes using DIC microscopy. Therefore, these results support the use of single AuBPs as sensitive orientation probes in dynamic biological studies using DIC microscopy.

Localization accuracy of gold nanoparticles in single particle orientation and rotational tracking

The Single Particle Orientation and Rotational Tracking (SPORT) technique, which utilizes anisotropic plasmonic gold nanorods and differential interference contrast (DIC) microscopy, has shown potential as an effective alternative to fluorescence-based techniques to decipher rotational motions on the cellular and molecular levels. However, localizing gold nanorods from their DIC images with high accuracy and precision is more challenging than the procedures applied in fluorescence or scattering microscopy techniques due to the asymmetric DIC point spread function with bright and dark parts superimposed over a grey background. In this paper, localization accuracy and inherited uncertainties from unique DIC image patterns are elucidated with the assistance of computer simulation. These discussions provide guidance for researchers to properly evaluate their data and avoid making claims beyond the technical limits. The understanding of the intrinsic localization errors and the principle of DIC microscopy leads us to propose a new localization strategy that utilizes the experimentally-measured shear distance of the DIC microscope to improve the localization accuracy.

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.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Tunable double resonance of silver nanodecahedron on the insulator/conductor film

The generation of double resonance in a nanostructure, thus permitting the modulation of optical field at two frequencies simultaneously, offers new application opportunities for surface enhanced Raman scattering (SERS) and surface enhanced fluorescence (SEF). Here, we present a simple composite nanostructure of silver nanodecahedron (Ag ND)/silica spacer/gold film/glass substrate for achieving double resonance under the normal incidence of polarized light. The optical responses of the composite structure have been theoretically studied by varying the thickness of silica spacer layer from 5 nm to 35 nm for mediating the interaction between Ag ND and gold film. Results indicate that the extinction spectrum of the composite system is strongly dependent on the separation between Ag ND and gold film. The electric field and charge distribution during resonance have been investigated in order to obtain a detailed understanding on the coupling between these two objects. More importantly, due to the anisotropic geometry of Ag ND, double resonance with two plasmonic modes (dipole and gap modes) whose responses can be adjusted through varying the size of Ag ND and mediating its coupling with the gold film respectively, has been achieved in the composite structure under the excitation with polarization parallel to the Ag ND edge adjacent to the spacer surface. The knowledge gained through this work will benefit the development of applications based on local field enhancement.

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.

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

We present characteristic doughnut-shaped image patterns of gold nanorods embedded in a thin layer of a gel matrix observed under a dark-field microscope. The characteristic scattering field distributions allow us to estimate the spatial orientation of single gold nanorods. The measured scattering patterns are further verified by a simulation study.

Multishell Au/Ag/SiO2 nanorods with tunable optical properties as single particle orientation and rotational tracking probes

Three-layer core-shell plasmonic nanorods (Au/Ag/SiO2-NRs), consisting of a gold nanorod core, a thin silver shell, and a thin silica layer, were synthesized and used as optical imaging probes under a differential interference contrast microscope for single particle orientation and rotational tracking. The localized surface plasmon resonance modes were enhanced upon the addition of the silver shell, and the anisotropic optical properties of gold nanorods were maintained. The silica coating enables surface functionalization with silane coupling agents and provides enhanced stability and biocompatibility. Taking advantage of the longitudinal LSPR enhancement, the orientation and rotational information of the hybrid nanorods on synthetic lipid bilayers and on live cell membranes were obtained with millisecond temporal resolution using a scientific complementary metal-oxide-semiconductor camera. The results demonstrate that the as-synthesized hybrid nanorods are promising imaging probes with improved sensitivity and good biocompatibility for single plasmonic particle tracking experiments in biological systems.

Spatiotemporal mapping of three dimensional rotational dynamics of single ultrasmall gold nanorods

Spatiotemporal mapping of the position and orientation of nano-machinery inside complex and dynamic cellular environments is essential for the detailed understanding of many bio-physical processes. For the genuine observation of such biomolecular dynamics with high signal to noise ratio and reduced disturbance from the labeling probes, reduction in the size of nano-bio labels and simplification of techniques for their observation are important. Here we achieve this using polarized dark field scattering micro-spectroscopy (PDFSMS), in its simplest form so that it is deployable in several experiments. We not only locate tiny gold nanorods (GNRs) of size 30 (length) × 10 nm (diameter) inside HEK293 cells but also demonstrate mapping of their in-situ polarization patterns using a novel method. Real time observations of rotating GNR with DFSMS and PDFSMS are used to resolve in-plane and out-of-plane rotational modes of GNR. We have shown that PDFSMS itself can provide complete information about the state of GNR. A step ahead, we demonstrate the application of PDFSMS to track three dimensional rotational dynamics of transferrin-conjugated GNRs inside live HEK293 cells. These first-time observations of the three dimensional intracellular rotational dynamics of tiny GNRs using PDFSMS present a new landmark in single particle scattering spectroscopy.

Single Particle Orientation and Rotational Tracking (SPORT) in biophysical studies

The single particle orientation and rotational tracking (SPORT) techniques have seen rapid development in the past 5 years. Recent technical advances have greatly expanded the applicability of SPORT in biophysical studies. In this feature article, we survey the current development of SPORT and discuss its potential applications in biophysics, including cellular membrane processes and intracellular transport.

Accuracy of superlocalization imaging using Gaussian and dipole emission point-spread functions for modeling gold nanorod luminescence

We present a study comparing the accuracy of superlocalization imaging of plasmon-mediated emission from gold nanorods (AuNRs) using both Gaussian and dipole emission point-spread function (PSF) models. By fitting the emission PSF of single AuNR luminescence, we have shown that a 3-axis dipole PSF gives improved localization accuracy over the Gaussian PSF, especially for nonplanar AuNRs, while also allowing the AuNR three-dimensional orientation and emission wavelength to be determined. On the other hand, when a single-axis dipole PSF model is applied to the AuNR emission, the fit estimates converge to values that are inconsistent with their experimentally measured values, affecting both the localization accuracy and precision of the fitted centroid position. These results indicate that when applying superlocalization techniques to plasmonic nanostructures, care must be taken to understand the nature of the emission before a correct dipole PSF can be applied.

Three-dimensional high-resolution rotational tracking with superlocalization reveals conformations of surface-bound anisotropic nanoparticles

The ability to directly follow three-dimensional rotational movement of anisotropic nanoparticles will greatly enhance our understanding of the way nanoparticles interact with surfaces. Herein, we demonstrate dual-color total internal reflection scattering microscopy as a tool to probe the interactions of plasmonic gold nanorods with functional surfaces. By taking advantage of both the short and long axis surface plasmon resonance scattering enhancement, we are able to decipher both in-plane and out-of-plane gold nanorod motion relative to the sample surface with equally high resolution. In combination with superlocalization through point spread function fitting, we overcome the four-quadrant angular degeneracy of gold nanorods in the focal plane of the objective and resolve conformations of surface-bound anisotropic nanoparticles in unprecedented detail.

Three-dimensional orientation determination of stationary anisotropic nanoparticles with sub-degree precision under total internal reflection scattering microscopy

Single-particle and single-molecule orientation determination plays a vital role in deciphering nanoscale motion in complex environments. Previous attempts to determine the absolute three-dimensional orientation of anisotropic particles rely on subjective pattern matching and are inherently plagued by high degrees of uncertainty. Herein, we describe a method utilizing total internal reflection scattering microscopy to determine the 3D orientation of gold nanorods with subdegree uncertainty. The method is then applied to the biologically relevant system of microtubule cargo loading. Finally, we demonstrate the method holds potential for identifying single particles versus proximate neighbors within the diffraction limited area.