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.

Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules. For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10 14 to 10 15 , much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.

http://science.sciencemag.org/content/sci/275/5303/1102.full.pdf

Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering

Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from solution can be used only with certain classes of molecules. An alternative approach—fabrication of multilayers by consecutive adsorption of polyanions and polycations—is far more general and has been extended to other materials such as proteins or colloids. Because polymers are typically flexible molecules, the resulting superlattice architectures are somewhat fuzzy structures, but the absence of crystallinity in these films is expected to be beneficial for many potential applications.

https://science.sciencemag.org/content/sci/277/5330/1232.full.pdf

Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

12.2.2. Four-Wave Mixing (FWM) 4849 12.2.3. Dye Aggregation 4850 12.2.4. Optoelectronic Nanodevices 4850 12.3. Sensor 4851 12.3.1. Chemical Sensor 4851 12.3.2. Biological Sensor 4851 12.4. Catalysis 4852 13. Conclusion and Perspectives 4852 14. Abbreviations 4853 15. Acknowledgements 4854 16. References 4854 * Corresponding author E-mail: tpal@chem.iitkgp.ernet.in. † Raidighi College. § Indian Institute of Technology. 4797 Chem. Rev. 2007, 107, 4797−4862

Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications

The self-assembly of monodisperse gold and silver colloid particles into monolayers on polymer-coated substrates yields macroscopic surfaces that are highly active for surface-enhanced Raman scattering (SERS). Particles are bound to the substrate through multiple bonds between the colloidal metal and functional groups on the polymer such as cyanide (CN), amine (NH(2)), and thiol (SH). Surface evolution, which can be followed in real time by ultraviolet-visible spectroscopy and SERS, can be controlled to yield high reproducibility on both the nanometer and the centimeter scales. On conducting substrates, colloid monolayers are electrochemically addressable and behave like a collection of closely spaced microelectrodes. These favorable properties and the ease of monolayer construction suggest a widespread use for metal colloid-based substrates.

https://science.sciencemag.org/content/sci/267/5204/1629.full.pdf

Self-Assembled Metal Colloid Monolayers: An Approach to SERS Substrates.

Covalent attachment of nanometer-scale colloidal Au particles to organosilane-coated substrates is a flexible and general approach to formation of macroscopic Au surfaces that have well-defined nanostructure. Variations in substrate (glass, metal, Al2O3), geometry (planar, cylindrical), functional group (−SH, −P(C6H5)2, −NH2, −CN), and particle diameter (2.5−120 nm) demonstrate that each component of these assemblies can be changed without adverse consequences. Information about particle coverage and interparticle spacing has been obtained using atomic force microscopy, field emission scanning electron microscopy, and quartz crystal microgravimetry. Bulk surface properties have been probed with UV−vis spectroscopy, cyclic voltammetry, and surface enhanced Raman scattering. Successful application of the latter two techniques indicates that these substrates may have value for Raman and electrochemical measurements. The assembly method described herein is compared with previous methods for controlling the na...

Two-dimensional arrays of colloidal gold particles : A flexible approach to macroscopic metal surfaces

The process of Ag colloid self-assembly onto organosilane-functionalized glass, carbon, and Au surfaces has been studied using UV−visible spectrophotometry, quartz crystal microgravimetry, and atomic force microscopy. Ag colloids prepared by citrate reduction and by ethylenediaminetetraacetic acid reduction of Ag+ yield different deposition kinetics, primarily a result of differences in particle concentration and monodispersity. The sticking probabilities and equilibrium constants for each type of colloid were measured. Surface-enhanced Raman scattering (SERS) and electrochemical properties of Ag colloid monolayers were compared to bulk Ag. Deposition of Ag colloid onto carbon electrodes improves the heterogeneous rate constant for electron transfer to [Ru(NH3)6]3+, but the modified electrodes still fall short of values obtained for bulk Ag. Likewise, Ag colloid monolayers are less SERS-active than roughened bulk Ag. The effects of particles polydispersity on these phenomena are discussed.

Preparation and Characterization of Ag Colloid Monolayers

Two approaches to preparation of Ag-clad Au colloid monolayers are described. Each begins with a preformed monolayer of colloidal Au particles on glass, Au-coated quartz, or In-doped SnO2. Chemical reduction of Ag+ from a commercial Ag coating formulation (LI Silver) or electrochemical reduction of Ag+ leads to surfaces for which the amount of Ag deposited can be controlled. On transparent substrates, Ag deposition can be followed in real time by UV−visible spectroscopy. When the substrate is an electrode, electrochemical deposition can be monitored by coulometry; for the chemical process, anodic stripping voltammetry yields accurate values of the amount of Ag present, as confirmed by quartz crystal microgravimetry (QCM). Chemical reduction yields surfaces that are extremely enhancing for surface enhanced Raman scattering (SERS), although the deposition process required must be precisely tuned. In contrast, electrochemical deposition, while affording more accurate control of the reduction rate, produces o...

Chemical and Electrochemical Ag Deposition onto Preformed Au Colloid Monolayers: Approaches to Uniformly-Sized Surface Features with Ag-Like Optical Properties

The effect of colloidal Au particle aggregation on surface-enhanced Raman scattering (SERS) spectra was probed by SERS filtration experiments. In this approach, SERS and optical spectra were recorded for trans-1,2-bis(4-pyridyl)ethylene (BPE)-aggregated solutions of colloidal Au filtered through straight-channel membranes with successively smaller diameters. This allowed the overall SERS intensity to be factored into aggregate size-dependent contributions. Experiments were carried out as a function of adsorbate concentration (0.5–2.5 µM BPE) and initial particle size (12–50 nm diameter colloidal Au). The key findings are as follows: (i) under conditions of minimal aggregation, appreciable SERS intensity derives from aggregates with effective diameters less than 200 nm; (ii) the amount of aggregant clearly controls the average aggregate size; and (iii) similarly aggregated solutions based on different diameter colloidal Au particles give different distributions of aggregates. These studies provide an insight into the dynamics of colloidal Au aggregation, suggest a procedure for signal optimization in colloid SERS experiments, and set the stage for controlled surface confinement of SERS-active particle clusters. Copyright © 1999 John Wiley & Sons, Ltd.

Robin M. Bright

Papers

Self-Assembled Metal Colloid Monolayers: An Approach to SERS Substrates.

Two-dimensional arrays of colloidal gold particles : A flexible approach to macroscopic metal surfaces

Preparation and Characterization of Ag Colloid Monolayers

Chemical and Electrochemical Ag Deposition onto Preformed Au Colloid Monolayers: Approaches to Uniformly-Sized Surface Features with Ag-Like Optical Properties

Size selection of colloidal gold aggregates by filtration: effect on surface‐enhanced Raman scattering intensities