Single-particle Dark-field Spectroscopy Research Articles

Nonlocal Optical Response of Particle Plasmons in Single Gold Nanorods.

The investigation of particle plasmons in metal nanoparticles has predominantly relied on local optical response approximations. However, as the nanoparticle size approaches the average distance of electrons to the metal surface, mesoscopic effects such as size-dependent plasmon line width broadening and resonance energy blue shifts are expected to become observable. In this work, we compared the experimental spectral characteristics with simulated values obtained by using a generalized nonlocal optical response theory-based local analogue model. Our results show that the nonlocal plasmon damping effects in single nanoparticles are less pronounced than those observed in plasmon-coupled systems. Furthermore, our research demonstrates that single-particle dark-field spectroscopy is an effective tool for investigating the nonlocal optical response of particle plasmons in single nanoparticles. These results have important implications for the rational design of novel nanophotonic devices.

Heterogeneity and Hysteresis in the Polymer Collapse of Single Core–Shell Stimuli-Responsive Plasmonic Nanohybrids

Broad application of polymeric stimuli-responsive smart nanohybrids requires understanding the mechanisms governing active control. Ensemble techniques have identified inhomogeneous polymer collapse in microgels that potentially arise from heterogeneous interchain interactions and differences in core size. A single-particle examination would establish the influence of core size and internal polymer network heterogeneity on local interactions that contribute to the observed inhomogeneous polymer collapse dynamics of nanohybrids. Using single-particle dark-field spectroscopy, we investigated the complex polymer collapse profiles of core–shell plasmonic nanohybrids comprising thermoresponsive poly(N-isopropylacrylamide) (pNIPAM)-encapsulated gold nanorods (AuNRs). We report that the polymer collapse behavior was independent of the core size. For thinner polymer shells, we observed hysteresis in the collapse of AuNR@pNIPAMs, likely related to local pNIPAM aggregation due to interchain hydrogen bonding. For thicker polymer shells, we observed a broad polymer collapse distribution that we attributed to a two-step phase transition that arises from a polymer network density gradient. Our single-particle approach relates the internal heterogeneity of the polymer network of nanohybrids to the mechanisms underlying heterogeneous phase transitions that traditional, ensemble-averaged approaches are unable to discern.

Ultrasensitive detection of lead (II) ion by dark-field spectroscopy and glutathione modified gold nanoparticles

Exposure to trace amounts of lead is known to harm human health, especially for children, which results in irreversible detriment to brain and nerve development. However, most of the sensor platforms using the colorimetric method to detect lead (II) ions (Pb2+) are difficult to reach a decent limit of detection (LOD) and a large detection range. This work presents a practical method to detect Pb2+ in aqueous-medium with the help of single-particle dark field spectroscopy, which allows us to obtain good LOD and wide detection range. Our results illustrate that the glutathione functionalized AuNPs are able to capture free AuNPs in the presence of Pb2+ to form multipliers. Such a process results in a significant spectral redshift of scattering spectra due to plasmonic coupling, which allows us to reach a good LOD of 3.5 pM, and a large linear quantifiable detection range from 10 pM to 10 μM. The performance of our sensor platform provides an alternative way to detect traces of Pb2+ in real-life water samples.

Single particle dark-field spectroscopy of spherical dimers with down to sub-10 nm gaps fabricated by the annealing of nano-pillars

Abstract In this paper we present a method for the fabrication of plasmonic spherical dimers and oligomers with narrow gaps and tunable distances. High-aspect-ratio nano-pillars are created by the two-step evaporation of gold on structured substrates. By using electron beam lithography, it is possible to control the close spacing of the pillars. Rapid thermal annealing causes the pillars to adopt a spherical shape. Since by melting the spheres gain in diameter compared to the initial nano-pillars, the distance between two adjacent spheres can be reduced to below 10 nm. Dimers with different distances were fabricated and optically characterized by single particle dark-field spectroscopy. The characteristic red-shift of the longitudinal mode due to stronger coupling for smaller distances could be clearly observed and follows a general scaling behavior.

Mapping the electric field distribution of tightly focused cylindrical vector beams with gold nanorings.

In this paper gold nanorings (NRs) are applied as particularly well-suited sensing elements for mapping the radially symmetric electric fields in the high numerical aperture focus of cylindrical vector beams. The optical properties of gold nanorings are analyzed by a combination of extinction and single particle dark field spectroscopy as well as confocal photoluminescence (PL) imaging. The results are compared to numerical calculations. The in-plane components in the focus of the cylindrical vector beams are estimated through the PL intensity distributions of the NRs. The optimum overlap between the structure and excitation is visualized by a narrow centre spot in the far-field PL scan.

Optical Magnetism and Fundamental Modes of Nanodiamonds

The optical properties of color centers in nanodiamonds are widely used in various branches of photonics and interdisciplinary studies. Here, we report on an experimental study of the fundamental eigenmodes of subwavelength diamond nanoparticles. The eigenmodes reveal themselves as scattering resonances, which were measured by single-particle dark-field spectroscopy and calculated both numerically and analytically. The resonances experience a red-shift with increasing particle size, and in the case of an anisotropic particle, they change depending on the polarization of the input light. As an example of an application, the Purcell enhancement of the dipole emission from such nanodiamonds is numerically demonstrated. This study demonstrates a simple way to improve the efficiency of diamond-based sensors and single-photon sources by choosing nanoparticles of optimal size and shape.

Dispersive Plasmon Damping in Single Gold Nanorods by Platinum Adsorbates.

Surface modifications of plasmonic nanoparticles with metal adsorbates are essential in applications such as plasmonic sensing, plasmon-enhanced photocatalysis, etc., where spectral broadening is usually observed. A single particle study is presented on plasmon damping by adsorption of platinum (Pt) clusters. Single particle dark-field spectroscopy is employed to measure exactly the same gold nanorod before and after the Pt adsorption. The Pt-induced plasmon damping in terms of linewidth increase is found dependent on the resonance wavelength of the measured nanorod, which is dispersive in nature. The measured dispersion generally matches the theoretical prediction, and it basically exhibits a gradual increase with decreasing resonance energy. This increase can be attributed to the fact that the nanorod as a better resonator is more susceptible to the Pt adsorption than the spherical particles. Moreover, simulated results based on discrete dipole approximation method further indicate that the damping is mainly contributed from the adsorbates on the ends of the nanorod and independent on the type of the metal adsorbed. Knowledge and insights gained in this study can be very important for the design and fabrication of plasmonic heterostructures as functional nanomaterials.

Spectral variation of fluorescence lifetime near single metal nanoparticles.

We explore the spectral dependence of fluorescence enhancement and the associated lifetime modification of fluorescent molecules coupled to single metal nanoparticles. Fluorescence lifetime imaging microscopy and single-particle dark-field spectroscopy are combined to correlate the dependence of fluorescence lifetime reduction on the spectral overlap between the fluorescence emission and the localised surface plasmon (LSP) spectra of individual gold nanoparticles. A maximum lifetime reduction is observed when the fluorescence and LSP resonances coincide, with good agreement provided by numerical simulations. The explicit comparison between experiment and simulation, that we obtain, offers an insight into the spectral engineering of LSP mediated fluorescence and may lead to optimized application in sensing and biomedicine.

Surface asymmetry of coated spherical nanoparticles.

We validate the nonspherical grafting arrangement of isotropically coated spherical nanoparticles as very recently proposed. We utilize localized surface plasmon resonance enhanced dynamic polarized and depolarized light scattering from Au nanoparticles, the spherical symmetry of which was revealed by single-particle dark-field spectroscopy. The same Au nanospheres are grafted with ligands of different chemistry and length. The wavelength dependent depolarization ratio and the two transport coefficients of these nanoparticles, obtained from the dynamic light scattering experiment, can only be reconciled with the TEM data, the single UV/vis extinction spectrum, and the dark-field spectroscopy experiments if their coating is described as asymmetric. Spatially anisotropic graft distribution on spherical nanoparticles impacts their assembly and understanding its origin will help control the structure and properties of polymer nanocomposites.

Uniform gold spherical particles for single-particle surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) benefits from the enhanced electromagnetic field of the localized surface plasmon resonance effect of metallic (especially coinage metals) nanoparticles or nanostructures. The detection sensitivity and reproducibility of SERS measurement appear to be the two critical issues in SERS. To solve the problem associated with traditional nanoparticle aggregates and SERS substrates, we propose in this work single particle SERS. We prepared uniform gold microspheres with controllable size and surface roughness using an etching-assisted seed-mediated method. Single particle dark-field spectroscopy and SERS measurements show that particles with a larger roughness give a stronger SERS signal, but still retain a good reproducibility. This study points to the promising future of the practical application of the single particle SERS technique for trace analysis.

Revealing Plasmonic Gap Modes in Particle-on-Film Systems Using Dark-Field Spectroscopy

Polarization-controlled excitation of plasmonic modes in nanometric Au particle-on-film gaps is investigated experimentally using single-particle dark-field spectroscopy. Two distinct geometries are explored: nanospheres on top of and inserted in a thin gold film. Numerical simulations reveal that the three resonances arising in the scattering spectra measured for particles on top of a film originate from highly confined gap modes at the interface. These modes feature different azimuthal characteristics, which are consistent with recent theoretical transformation optics studies. On the other hand, the scattering maxima of embedded particles are linked to dipolar modes having different orientations and damping rates. Finally, the radiation properties of the particle-film gap modes are studied through the mapping of the scattered power within different solid angle ranges.

Plasmonic Monitoring of Catalytic Hydrogen Generation by a Single Nanoparticle Probe

Plasmonic nanostructures such as gold nanoparticles are very useful for monitoring chemical reactions because their optical properties are highly dependent upon the environment surrounding the particle surface. Here, we designed the catalytic structure composed of platinized cadmium sulfide with gold domains as a sensitive probe, and we monitored the photocatalytic decomposition of lactic acid to generate hydrogen gas in situ by single-particle dark-field spectroscopy. The plasmon band shift of the gold probe throughout the reaction exhibits significant particle-to-particle variation, and by simulating the reaction kinetics, the rate constant and structural information (including the diffusion coefficient through the shell and the relative arrangement of the active sites) can be estimated for individual catalyst particles. This approach is versatile for the monitoring of various heterogeneous reactions with distinct components at a single-particle level.

Scattering Spectra of Single Gold Nanoshells

Single particle dark field spectroscopy has been combined with high-resolution scanning electron and atomic force microscopy to study the scattering spectra of single gold/silica nanoshells. The plasmon resonant peak energies match those calculated by Mie theory based on the nanoshell geometry. The resonance line widths fit Mie theory without the inclusion of a size-dependent surface scattering term, which is often included to fit ensemble measurements. These results suggest that plasmon spectral measurements of nanoparticle ensembles are broadened due to particle inhomogeneity.