Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Nanomaterials, such as metal or semiconductor nanoparticles and nanorods, exhibit similar dimensions to those of biomolecules, such as proteins (enzymes, antigens, antibodies) or DNA. The integration of nanoparticles, which exhibit unique electronic, photonic, and catalytic properties, with biomaterials, which display unique recognition, catalytic, and inhibition properties, yields novel hybrid nanobiomaterials of synergetic properties and functions. This review describes recent advances in the synthesis of biomolecule-nanoparticle/nanorod hybrid systems and the application of such assemblies in the generation of 2D and 3D ordered structures in solutions and on surfaces. Particular emphasis is directed to the use of biomolecule-nanoparticle (metallic or semiconductive) assemblies for bioanalytical applications and for the fabrication of bioelectronic devices.

Integrated Nanoparticle–Biomolecule Hybrid Systems: Synthesis, Properties, and Applications

Molecular rulers based on Foerster Resonance Energy Transfer (FRET) that report conformational changes and intramolecular distances of single biomolecules have helped to understand important biological processes. However, these rulers suffer from low and fluctuating signal intensities from single dyes and limited observation time due to photobleaching. The plasmon resonance in noble metal particles has been suggested as an alternative probe to overcome the limitations of organic fluorophores and the coupling of plasmons in nearby particles has been exploited to detect particle aggregation by a distinct color change in bulk experiments. Here we demonstrate that plasmon coupling can be used to monitor distances between single pairs of gold and silver nanoparticles. We use this effect to follow the directed assembly of gold and silver nanoparticle dimers in real time and to study the time dynamics of single DNA hybridization events. These ''plasmon rulers'' allowed us to continuously monitor separations of up to 70 nm for more than 3000 seconds. Single molecule in vitro studies of biological processes previously inaccessible with fluorescence based molecular rulers are enabled with plasmon rulers with extended time and distance range.

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A molecular ruler based on plasmon coupling of single gold and silver nanoparticles.

Conventional optical tweezers, formed at the diffraction-limited focus of a laser beam, have become a powerful and flexible tool for manipulating micrometre-sized objects. Extending optical trapping down to the nanometre scale would open unprecedented opportunities in many fields of science, where such nano-optical tweezers would allow the ultra-accurate positioning of single nano-objects. Among the possible strategies, the ability of metallic nanostructures to control light at the subwavelength scale can be exploited to engineer such nano-optical traps. This Review summarizes the recent advances in the emerging field of plasmon-based optical trapping and discusses the details of plasmon tweezers along with their potential applications to bioscience and quantum optics.

Plasmon nano-optical tweezers

A Molecular Ruler Based on Plasmon Coupling of Single Gold and Silver Nanoparticles Carsten Sonnichsen 1,x , Bjorn M. Reinhard 2,x , Jan Liphardt 2, ** , A. Paul Alivisatos 1, ** Department of Chemistry, University of California, Berkeley and Materials Science Division, Department of Physics, University of California, Berkeley and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA ** Correspondence should be addressed to J.L. (liphardt@physics.berkeley.edu) or A.P.A. (alivis@berkeley.edu) x These authors contributed equally to this work

A molecular ruler based on plasmon coupling of single gold and silver nanoparticles

Gold particles with diameters between 25 and 45 nm have been introduced in the inner cavity of an icosahedral brome mosaic virus The optical properties of single gold-marked virions have been tested in-vitro with respect to the characteristic plasmon polariton resonance The shift in the plasmon polariton resonance of a single Au particle encapsulated in a virus with respect to a free particle in solution indicates a close interaction between the basic residues on the inner wall of the capsid and the negative surface charge of the particle Incorporation of a pair of Au particles, ∼4 nm diameter, is shown to be a frequent event In this case, the dependence of the two-particle surface plasmon spectrum on the interparticle distance and the strong particle/capsid wall interaction suggest that, in the future, it will be possible to use encapsulated Au particles to track changes in the viral capsid volume in real-time and in a physiological environment

Gold nanoparticles as spectroscopic enhancers for in vitro studies on single viruses.

In recent years, optical micromachines based on forces exerted by strongly focused beams of light have started to provide unprecedented access to nonintrusive measurement and manipulation of matter on submicron length scales. However, the sharpness of the present optical tweezers is restricted by the spatial gradients of light attainable with diffraction-limited optics. Here, we demonstrate a technique for optical trapping of single particles in a fluid, which is based on the intense near-field gradients around small apertures in a metal film. Our scheme should be able to trap smaller particles with a fraction of the laser intensity required by conventional optical tweezers. Detailed simulations of the electromagnetic fields near apertures and the resulting forces they can produce are described. We also present a proof-of-principle experiment in which the trapping of latex beads is demonstrated by following the time evolution of their fluorescence. Our scheme allows containment of particles in free soluti...

Optical Trapping with Integrated Near-Field Apertures

In-vitro self-assembly conditions known to result in generating infectious virions have been used in vitro to reassemble bromovirus capsid proteins around negatively charged gold nanoparticles cores. We discuss here the optical properties (elastic light scattering) and the influence of the core size and of the functional moiety on the resulting virus-like particles. Our results indicate that the formation of a closed shell, as opposed to an amorphous protein coat, does occur and that the shell/core interactions can be tuned using different coatings on the nanoparticle core. Such studies may lead to real-time monitoring of viral traffic on the scale of a single virus, as well as to the possibility of chemical sensing along the intracellular and intercellular viral pathways and contribute to a better understanding of the virus transport and cellular compartmentalization.

Packaging of gold particles in viral capsids.

We have enclosed an optically active nanoparticle inside a virus coat to create a virus-like, intracellular optical probe. The core-shell interactions of the self-assembled, nanoparticle-enclosing capsids are investigated spectroscopically and compared with intact viruses.

Eun-Soo Kwak

Papers

Gold nanoparticles as spectroscopic enhancers for in vitro studies on single viruses.

Optical Trapping with Integrated Near-Field Apertures

Packaging of gold particles in viral capsids.

In-vitro studies of nanoparticle-virus complexes