For anisotropic shapes such as gold nanorods, the plasmon mode splits into two bands, the longitudinal one tunable by the aspect ratio (nanorod length/width ratio). This provides the opportunity to tune the resonance band to match external experimental conditions. The elongated shape, on the other hand, makes possible both, a spontaneous self-assembly into a close packed order with a side-by-side and end-to-end alignment and a guided assembly mediated by bifunctional ligands and biomolecules. Additional crystallographic facets, which are not present in a spherical nanoparticles and which exhibit different thermodynamic or catalytic properties, originate from a rod-like shape.
Different “synthetic” pathways like E-beam lithography, photo- and electrochemical techniques, and soft-chemical routes are explored. The latter one being at the same time the most flexible, resulting in single-crystalline particles not restricted to any surface, but as well the most unpredictable one concerning influences of additives and reaction conditions.
Open and discussed questions are: the source and reason for anisotropic growth in general, accompanied by the surprisingly high yield of anisotropic particles. Those fundamental questions are important not only for gold nanorod synthesis, but insights into the growth may also help to tune and understand growth of other materials or different shapes. Furthermore, in the light of the above mentioned effects, it is critical to carefully control and tune the gold nanorod growth in order to achieve a uniform nanoparticle distribution and thereby uniform properties.
Growth Kinetic of a Rod-Shaped Metal Nanocrystal
The crystallization dynamics in wet-chemical synthesis is the key parameter controlling nanoparticlemorphology, which determines the frequency of the optical plasmon resonance in metal nanoparticles. Despiteseveral in situ (dark-field microscopy) and ex situ (electron microscopy) studies of metal nanoparticle growth,the anisotropic growth kinetics are not well-understood mainly because the crystallization is a nonequilibriumprocess. Using simultaneous optical spectroscopy and time-resolved small-angle X-ray scattering at asynchrotron X-ray source, we directly monitor the anisotropic growth kinetics of gold and gold-copper nanorodsand extract the growth parameters for both crystal directions (along the rod’s long and short axes) independently.We find a crossover from 1D to 3D growth modes at 8 and 12 min, respectively, where the nanorods attaintheir maximum aspect ratio. The growth model explains and predicts this crossover point without the need ofa switch for the growth mode and allows for the fine-tuning of the particle shapes.
'Growth Kinetic of a Rod-Shaped Metal Nanocrystal'
A. Henkel, O. Schubert, A. Plech, C. Sönnichsen
J. Phys. Chem. C 113, 10390 (2009)
Gold Nanoparticle Growth Monitored in situ
Size- and shape-dependent optical properties of gold nanorods allow monitoring their growth using a novel fast single-particle spectroscopy (fastSPS) method. FastSPS uses a spatially addressable electronic shutter based on a liquid crystal device to investigate particles randomly deposited on a substrate, orders of magnitude faster than other techniques. We use fastSPS to observe nanoparticle growth in situ on asingle-particle level and extract quantitative data on nanoparticle growth.
'Gold Nanoparticle Growth Monitored in situ Using a Novel Fast Optical Single-Particle Spectroscopy Method'
J. Becker, O. Schubert, C. Sönnichsen
Continuous flow Synthesis of gold and silver nanorods
Continuous flow microfluidic reactors offer some advantages over conventional batch based methods: straight forward up-scaling, on-line monitoring of the products for easy process control, improved reaction yield, selectivity, and kinetics by exploring a wider parameter space for external parameters like pressure and temperature, a
nd improved reactant mixing. Miniaturization of the flow channels allows the change of experimental conditions along the reaction path within microseconds and greatly reduces the volume required to test certain experimental parameters (to microliters or even nanoliters). The wet-chemical synthesis of spherical gold nanoparticles has a long history dating back to Michael Faraday and was recently adapted to continuous flow reactions. During the past 5 years, the high yield batch synthesis of elongated, rod-shaped gold particles was achieved in ‘soft templates’ – tubular micelles formed in concentrated solutions of surfactants. Here, spherical seeds are grown into nanorods in a growth solution. We show here the first example of a continuous flow synthesis of nanoparticles with a rod-like shape, specifically gold and silver nanorods.
'Microfluidic continuous flow synthesis of rod-shaped gold and silver nanocrystals'
J. Boleininger, A. Kurz,V. Reuss, C. Sönnichsen
Phys. Chem. Chem. Phys. 8, 3824 (2006)
Tuning Plasmonic Properties by Alloying Copper into Gold Nanorods
We create rod-shaped single crystalline gold-copper (AuxCu(1-x)) nanoparticles and verify the presence ofcopper in the particles with various direct and indirect optical and electron microscopy techniques. The particlesgrow from small, preformed gold seeds in a growth solution containing both copper and gold ions in thepresence of a surfactant and a mild reducing agent. The presence of copper in the growth solution has apronounced effect on the spectral characteristic of the resulting nanocrystals and reduces the total volume ofthe resulting particles. In contrast to spherical copper particles, our rod-shaped nanocrystals show a strongplasmon resonance and the copper content varies the plasmon resonance frequency and the plasmonic linewidth.Optical single particle plasmon-line-width observations suggest reduced plasmon damping at specificcopper contents corresponding to stoichiometric particle compositions.
'Tuning Plasmonic Properties by Alloying Copper into Gold Nanorods'
A. Henkel, A. Jakab, G. Brunklaus, C. Sönnichsen