Thomas A. Klar

Properties and Applications of Colloidal Nonspherical Noble Metal Nanoparticles

Nanoparticles of noble metals belong to the most extensively studied colloidal systems in the field of nanoscience and nanotechnology. Due to continuing progress in the synthesis of nanoparticles with controlled morphologies, the exploration of unique morphology-dependent properties has gained momentum. Anisotropic features in nonspherical nanoparticles make them ideal candidates for enhanced chemical, catalytic, and local field related applications. Nonspherical plasmon resonant nanoparticles offer favorable properties for their use as analytical tools, or as diagnostic and therapeutic agents. This Review highlights morphology-dependent properties of nonspherical noble metal nanoparticles with a focus on localized surface plasmon resonance and local field enhancement, as well as their applications in various fields including Raman spectroscopy, fluorescence enhancement, analytics and sensing, photothermal therapy, (bio-)diagnostics, and imaging.

Label-free Biosensing Based on Single Gold Nanostars as Plasmonic Transducers

Gold nanostars provide high sensitivity for single nanoparticle label-free biosensing. The nanostars present multiple plasmon resonances of which the lower energy ones, corresponding to the nanostar tips and core-tip interactions, are the most sensitive to environmental changes. Streptavidin molecules are detected upon binding to individual, biotin-modified gold nanostars by spectral shifts in the plasmon resonances. Concentrations as low as 0.1 nM produce a shift of the tip related plasmon resonances of about 2.3 nm (5.3 meV).

Fluorescence Enhancement in Hot Spots of AFM-Designed Gold Nanoparticle Sandwiches

We observe an enhancement of fluorescence from a single fluorescent sphere, which is sandwiched between two individual gold nanoparticles, forming a hot spot of strong field enhancement. The fluorescence enhancing hot spot is custom-designed by the deliberate assembly of gold nanoparticles with an atomic force microscope cantilever. The fluorescence intensity is monitored while the separation between the two gold nanoparticles is reduced by gradually pushing the gold nanoparticles closer to the fluorescent sphere. The fluorescence enhancement is maximal when the distance between the two gold nanoparticles is smallest, when the excitation polarization is parallel to the axis of the sandwich, and when the fluorescent sphere is positioned exactly on the axis connecting the two gold nanoparticles.

Long-Range Fluorescence Quenching by Gold Nanoparticles in a Sandwich Immunoassay for Cardiac Troponin T

We report the first homogeneous sandwich immunoassay with gold nanoparticles (AuNPs) as fluorescence quenchers. The sandwich assay is designed for the detection of the protein cardiac troponin T (cTnT) by its simultaneous interaction with two different antibodies, one attached to AuNPs and the other labeled with fluorescent dyes. We demonstrate the working principle of the assay and using time-resolved fluorescence spectroscopy, we determine the quenching efficiency of the gold nanoparticles. In spite of the relatively large separation distance between dye molecules and AuNPs, ranging from 3 to 22 nm, the AuNPs quench the fluorescence with efficiencies as high as 95%. A limit of detection of 0.02 nM (0.7 ng/mL) was obtained for cTnT, which is the lowest value reported for a homogeneous sandwich assay for cTnT. These results illustrate the use of metallic nanoparticles as fluorescence quenchers in immunoassays where the large biomolecules involved impose distances for which energy transfer between fluorophores would be inefficient.

Energy transfer with semiconductor nanocrystals

Forster (or fluorescence) resonant energy transfer (FRET) is a powerful spectroscopic technique to study interactions, conformational and distance changes, in hybrid nanosystems. Semiconductor nanocrystals, also known as colloidal quantum dots, are highly efficient fluorophores with a strong band-gap luminescence tuneable by size as a result of the quantum confinement effect. Starting from a short summary on the FRET formalism and on the basic properties of semiconductor nanocrystals, this Feature Article provides an overview of the major classes of hybrid FRET systems with semiconductor nanocrystals as at least one component. Systems under consideration include thin solid films containing differently sized semiconductor nanocrystals, solution-based complexes of differently sized semiconductor nanocrystals, nanocrystal-based bioconjugates, and hybrid structures of semiconductor and gold nanoparticles. We focus in particular on the directional energy transfer in layer-by-layer assembled multilayers of differently sized CdTe semiconductor nanocrystals and on the energy transfer from individual rod-like semiconductor CdSe/CdS nanoantennae to single dye molecules, which can be efficiently controlled by external electric fields leading to the realisation of the FRET optical switch.

Electrically controlled light scattering with single metal nanoparticles

A concept to electrically control the scattering of light is introduced. The idea is to embed noble metal nanoparticles in an electro-optical material such as a liquid crystal in order to induce a spectral shift of the particle plasmon resonance by applying an electric field. Light scattering experiments on single gold nanoparticles show that spherically shaped nanoparticles become optically spheroidal when covered by an anisotropic liquid crystal. The two particle plasmon resonances of the optically spheroidal gold nanoparticles can be spectrally shifted by up to 50 meV when electric fields of more than 10 kV/cm are applied.

Self-Assembled Binary Superlattices of CdSe and Au Nanocrystals and Their Fluorescence Properties

Different types of Binary Nanoparticle Superlattices (BNSLs) have been self-assembled from monodisperse 8.7 nm CdSe and 5.5 nm Au nanocrystals. Fluorescence spectroscopy studies of AlB2-type BNSL of CdSe and Au nanocrystals revealed considerably decreased fluorescence and a shortened fluorescence lifetime of the CdSe NCs in BNSLs compared to the CdSe-only sample.

Negative-Index Metamaterials: Going Optical

The race toward engineering metamaterials comprising of negative refractive indexes in the optical range started with the realization of negative-index materials for gigahertz frequencies six years ago. Sheer miniaturization of the gigahertz resonant structures is one approach. Alternative designs make use of localized plasmon resonant metal nanoparticles or nanoholes in metal films. Following this approach, a negative refractive index has been realized in the optical range very recently. We review these recent results and summarize how to unambiguously retrieve the effective refractive index of thin layers from data accessible to measurements. Numerical simulations show that a composite material comprising of silver strips and a gain-providing material can have a negative refractive index of -1.3 and 100% transmission, simultaneously

Gold nanostoves for microsecond DNA melting analysis.

In traditional DNA melting assays, the temperature of the DNA-containing solution is slowly ramped up. In contrast, we use 300 ns laser pulses to rapidly heat DNA bound gold nanoparticle aggregates. We show that double-stranded DNA melts on a microsecond time scale that leads to a disintegration of the gold nanoparticle aggregates on a millisecond time scale. A perfectly matching and a point-mutated DNA sequence can be clearly distinguished in less than one millisecond even in a 1:1 mixture of both targets.

Negative index metamaterial combining magnetic resonators with metal films

We present simulation results of a design for negative index materials that uses magnetic resonators to provide negative permeability and metal films for negative permittivity. We also discuss the possibility of using semicontinuous metal films to achieve better manufacturability and enhanced impedance matching.