Björn M. Reinhard

Deterministic aperiodic arrays of metal nanoparticles for surface-enhanced Raman scattering (SERS)

Deterministic Aperiodic (DA) arrays of gold (Au) nanoparticles are proposed as a novel approach for the engineering of reproducible surface enhanced Raman scattering (SERS) substrates. A set of DA and periodic arrays of cylindrical and triangular Au nanoparticles with diameters ranging between 50-110 nm and inter-particle separations between 25-100 nm were fabricated by e-beam lithography on quartz substrates. Using a molecular monolayer of pMA (p-mercaptoaniline) as a Raman reporter, we show that higher values of SERS enhancement factors can be achieved in DA structures compared to their periodic counterparts, and discuss the specific scaling rules of DA arrays with different morphologies. Electromagnetic field calculations based on the semi-analytical generalized Mie theory (GMT) fully support our findings and demonstrate the importance of morphology-dependent diffractive coupling (long-range interactions) for the engineering of the SERS response of DA arrays. Finally, we discuss optimization strategies based on the control of particles sizes and shapes, and we demonstrate that spatially-averaged SERS enhancement factors of the order of approximately 10(7) can be reproducibly obtained using DA arrays of Au nano-triangles. The ability to rigorously design lithographically fabricated DA arrays of metal nanoparticles enables the optimization and control of highly localized plasmonic fields for a variety of chip-scale devices, such as more reproducible SERS substrates, label-free bio-sensors and non-linear elements for nano-plasmonics.

Sulfidation of Cadmium at the Nanoscale

We investigate the evolution of structures that result when spherical Cd nanoparticles of a few hundred nanometers in diameter react with dissolved molecular sulfur species in solution to form hollow CdS. Over a wide range of temperatures and concentrations, we find that rapid Cd diffusion through the growing CdS shell localizes the reaction front at the outermost CdS/S interface, leading to hollow particles when all the Cd is consumed. When we examine partially reacted particles, we find that this system differs significantly from others in which the nanoscale Kirkendall effect has been used to create hollow particles. In previously reported systems, partial reaction creates a hollow particle with a spherically symmetric metal core connected to the outer shell by filaments. In contrast, here we obtain a lower symmetry structure, in which the unreacted metal core and the coalesced vacancies separate into two distinct spherical caps, minimizing the metal/void interface. This pattern of void coalescence is likely to occur in situations where the metal/vacancy self-diffusivities in the core are greater than the diffusivity of the cations through the shell.

Scavenger Receptor Mediated Endocytosis of Silver Nanoparticles into J774A.1 Macrophages is Heterogeneous

We investigated the scavenger receptor mediated uptake and subsequent intracellular spatial distribution and clustering of 57.7 ± 6.9 nm diameter silver nanoparticles (zeta-potential = -28.4 mV) in the murine macrophage cell line J774A.1 through colorimetric imaging. The NPs exhibited an overall red-shift of the plasmon resonance wavelength in the cell ensemble as function of time and concentration, indicative of intracellular NP agglomeration. A detailed analysis of the NP clustering in individual cells revealed a strong phenotypic variability in the intracellular NP organization on the single cell level. Throughout the observation time of 24h cells containing non- or low-agglomerated NPs with a characteristic blue color coexisted with cells containing NPs with varying degrees of agglomeration, as evinced by distinct spectral shifts of their resonance wavelengths. Pharmacological inhibition studies indicated that the observed differences in intracellular NP organization resulted from coexisting actin- and clathrin-dependent endocytosis mechanisms in the macrophage population. Correlation of intracellular NP clustering with macrophage maturity marker (F4/80, CD14) expression revealed that differentiated J774A.1 cells preferentially contained compact NP agglomerates, whereas monocyte-like macrophages contained non-agglomerated NPs.

Interferon-Inducible Mechanism of Dendritic Cell-Mediated HIV-1 Dissemination Is Dependent on Siglec-1/CD169

Human immunodeficiency virus type 1 (HIV-1) interactions with myeloid dendritic cells (DCs) can result in virus dissemination to CD4+ T cells via a trans infection pathway dependent on virion incorporation of the host cell derived glycosphingolipid (GSL), GM3. The mechanism of DC-mediated trans infection is extremely efficacious and can result in infection of multiple CD4+ T cells as these cells make exploratory contacts on the DC surface. While it has long been appreciated that activation of DCs with ligands that induce type I IFN signaling pathway dramatically enhances DC-mediated T cell trans infection, the mechanism by which this occurs has remained unclear until now. Here, we demonstrate that the type I IFN-inducible Siglec-1, CD169, is the DC receptor that captures HIV in a GM3-dependent manner. Selective downregulation of CD169 expression, neutralizing CD169 function, or depletion of GSLs from virions, abrogated DC-mediated HIV-1 capture and trans infection, while exogenous expression of CD169 in receptor-naïve cells rescued GSL-dependent capture and trans infection. HIV-1 particles co-localized with CD169 on DC surface immediately following capture and subsequently within non-lysosomal compartments that redistributed to the DC – T cell infectious synapses upon initiation of T cell contact. Together, these findings describe a novel mechanism of pathogen parasitization of host encoded cellular recognition machinery (GM3 – CD169 interaction) for DC-dependent HIV dissemination.

HIV-1 incorporation of host-cell–derived glycosphingolipid GM3 allows for capture by mature dendritic cells

The interaction between HIV and dendritic cells (DCs) is an important early event in HIV-1 pathogenesis that leads to efficient viral dissemination. Here we demonstrate a HIV gp120-independent DC capture mechanism that uses virion-incorporated host-derived gangliosides with terminal α2–3-linked sialic acid linkages. Using exogenously enriched virus and artificial liposome particles, we demonstrate that both α2–3 gangliosides GM1 and GM3 are capable of mediating this interaction when present in the particle at high levels. In the absence of overexpression, GM3 is the primary ligand responsible for this capture mechanism, because siRNA depletion of GM3 but not GM1 from the producer cell and hence virions, resulted in a dramatic decrease in DC capture. Furthermore, HIV-1 capture by DCs was competitively inhibited by targeting virion-associated GM3, but was unchanged by targeting GM1. Finally, virions were derived from monocytoid THP-1 cells that constitutively display low levels of GM1 and GM3, or from THP-1 cells induced to express high surface levels of GM1 and GM3 upon stimulation with the TLR2/1 ligand Pam3CSK4. Compared with untreated THP-1 cells, virus produced from Pam3CSK4-stimulated THP-1 cells incorporated higher levels of GM3, but not GM1, and showed enhanced DC capture and trans-infection. Our results identify a unique HIV-1 DC attachment mechanism that is dependent on a host-cell–derived ligand, GM3, and is a unique example of pathogen mimicry of host-cell recognition pathways that drive virus capture and dissemination in vivo.

Photonic-plasmonic mode coupling in on-chip integrated optoplasmonic molecules.

We investigate photonic-plasmonic mode coupling in a new class of optoplasmonic materials that comprise dielectric microspheres and noble metal nanostructures in a morphologically well-defined on-chip platform. Discrete networks of optoplasmonic elements, referred to as optoplasmonic molecules, were generated through a combination of top-down fabrication and template-guided self-assembly. This approach facilitated a precise and controllable vertical and horizontal positioning of the plasmonic elements relative to the whispering gallery mode (WGM) microspheres. The plasmonic nanostructures were positioned in or close to the equatorial plane of the dielectric microspheres where the fields associated with the plasmonic modes can synergistically interact with the evanescent fields of the WGMs. We characterized the far-field scattering spectra of discrete optoplasmonic molecules that comprised two coupled 2.048 μm diameter polystyrene microspheres each encircled by four 148 nm diameter Au nanoparticles (NPs), through far-field scattering spectroscopy. We observed a broadening of the TE and TM modes in the scattering spectra of the optoplasmonic dimers indicative of an efficient photonic-plasmonic mode coupling between the coupled photonic modes of the WGM resonators and the localized surface plasmon modes of the NPs. Our experimental findings are supported by generalized multiple particle Mie theory simulations, which provide additional information about the spatial distributions of the near fields associated with the photonic-plasmonic hybrid modes in the investigated optoplasmonic molecules. The simulations reveal partial localization of the spectrally sharp hybrid modes outside of the WGM microspheres on the Au NPs where the local E-field intensity is enhanced by approximately 2 orders of magnitude over that of an individual Au NP.

Genetically engineered plasmonic nanoarrays

In the present Letter, we demonstrate how the design of metallic nanoparticle arrays with large electric field enhancement can be performed using the basic paradigm of engineering, namely the optimization of a well-defined objective function. Such optimization is carried out by coupling a genetic algorithm with the analytical multiparticle Mie theory. General design criteria for best enhancement of electric fields are obtained, unveiling the fundamental interplay between the near-field plasmonic and radiative photonic coupling. Our optimization approach is experimentally validated by surface-enhanced Raman scattering measurements, which demonstrate how genetically optimized arrays, fabricated using electron beam lithography, lead to order of ten improvement of Raman enhancement over nanoparticle dimer antennas, and order of one hundred improvement over optimal nanoparticle gratings. A rigorous design of nanoparticle arrays with optimal field enhancement is essential to the engineering of numerous nanoscale optical devices such as plasmon-enhanced biosensors, photodetectors, light sources and more efficient nonlinear optical elements for on chip integration.

Quantification of Differential ErbB1 and ErbB2 Cell Surface Expression and Spatial Nanoclustering through Plasmon Coupling

Cell surface receptors play ubiquitous roles in cell signaling and communication and their expression levels are important biomarkers for many diseases. Expression levels are, however, only one factor that determines the physiological activity of a receptor. For some surface receptors, their distribution on the cell surface, especially their clustering, provides additional mechanisms for regulation. To access this spatial information robust assays are required that provide detailed insight into the organization of cell surface receptors on nanometer length scales. In this manuscript, we demonstrate through combination of scattering spectroscopy, electron microscopy, and generalized multiple particle Mie theory (GMT) simulations that the density- and morphology-dependent spectral response of Au nanoparticle (NP) immunolabels bound to the epidermal growth factor receptors ErbB1 and ErbB2 encodes quantitative information of both the cell surface expression and spatial clustering of the two receptors in different unliganded in vitro cancer cell lines (SKBR3, MCF7, A431). A systematic characterization of the collective spectral responses of NPs targeted at ErbB1 and ErbB2 at various NP concentrations indicates differences in the large-scale organization of ErbB1 and ErbB2 in cell lines that overexpress these receptors. Validation experiments in the scanning electron microscope (SEM) confirm that NPs targeted at ErbB1 on A431 are more strongly clustered than NPs bound to ErbB2 on SKBR3 or MCF7 at overall comparable NP surface densities. This finding is consistent with the existence of larger receptor clusters for ErbB1 than for ErbB2 in the plasma membranes of the respective cells.

Enhanced light focusing in self-assembled optoplasmonic clusters with subwavelength dimensions.

Compact metallo-dielectric hybrid clusters with subwavelength dimensions are fabricated by template guided self-assembly. Elastic and inelastic scattering spectroscopy and electromagnetic simulations reveal that hybrid clusters comprising TiO(2) nanoparticles on top of a cluster of strongly coupled gold nanoparticles harness synergistic electromagnetic interactions between the building blocks. This results in a boost of the peak electric field intensity and a redistribution of the field in the ambient medium. The complex phase landscape in the clusters features optical vortices that enhance the magnetic field.

Probing subdiffraction limit separations with plasmon coupling microscopy: concepts and applications

Due to their advantageous material properties, noble metal nanoparticles are versatile tools in biosensing and imaging. A characteristic feature of gold and silver nanoparticles is their ability to sustain localized surface plasmons that provide both large optical cross-sections and extraordinary photophysical stability. Plasmon coupling microscopy takes advantage of the beneficial optical properties and utilizes electromagnetic near-field coupling between individual noble metal nanoparticle labels to resolve subdiffraction limit separations in an all-optical fashion. This Tutorial provides an introduction into the physical concepts underlying distance dependent plasmon coupling, discusses potential experimental implementation of plasmon coupling microscopy, and reviews applications in the area of biosensing and imaging.