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.

TiO2 photocatalysis and related surface phenomena

Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13

In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28

Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37



Figure 1

Physical properties of AuNPs and schematic illustration of an AuNP-based detection system.



In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.

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Gold nanoparticles in chemical and biological sensing.

5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

A new class of materials for catalysis have been intensively investigated, that is, ‘bimetallic nanoparticles’. Extensive studies of non-supported bimetallic nanoparticle dispersions, stabilized by polymers or ligands, started only about 10 years ago. Many preparative procedures have been proposed, and detailed characterizations have been carried out on bimetallic nanoparticles, thanks to the rapid improvement of analytical technology on surface and nanoscale materials. In this review, we focus on the preparation, characterization and application to catalysis of polymer- or ligand-stabilized bimetallic nanoparticles in dispersion, emphasizing our own work and introducing recent progress of this area.

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Bimetallic nanoparticles—novel materials for chemical and physical applications

Structural analysis of polymer-protected palladium/platinum bimetallic clusters as dispersed catalysts by using extended x-ray absorption fine structure spectroscopy

Refluxing mixed solutions of palladium(II) chloride and hexachloroplatinic(IV) acid in ethanol–water (1 : 1, v/v) in the presence of poly(N-vinyl-2-pyrrolidone) results in well dispersed and stable colloidal dispersions of polymer-protected palladium/platinum bimetallic clusters. The UV–VIS absorption spectra and transmission electron micrographs indicate that these bimetallic cluster particles are homogeneous in size, each particle containing both palladium and platinum atoms. Dispersions of these bimetallic clusters were used as catalysts for the selective hydrogenation of cycloocta-1,3-diene to cyclooctene at 30 °C under hydrogen gas at atmospheric pressure. The catalytic activity for the partial hydrogenation was found to depend on the metal composition of the particles. A bimetallic cluster with a mole ratio of Pd : Pt = 4 : 1 was the most active catalyst, twice as active as a typical colloidal palladium catalyst, for the diene hydrogenation. The dependence of the activity and the selectivity on the surface structure of the cluster particle strongly indicates that the cluster particle with the highest catalytic activity had a ‘Pd-surrounded Pt core’ structure.

Polymer-protected palladium–platinum bimetallic clusters: preparation, catalytic properties and structural considerations

A transparent, high purity, amorphous titanium dioxide thin film composed of densely packed nanometer-sized grains has been successfully deposited on a glass substrate at room temperature from an aqueous peroxotitanate solution using a simple, inexpensive, reproducible, and environmentally friendly method. The as-deposited thin film was 117 nm thick and composed of closely packed particles of 10–15 nm in diameter, but they aggregated into large grains of 50–100 nm in diameter. The aqueous peroxotitanate solution was obtained by dissolving metatitanic acid (H2TiO3) in a mixture of concentrated H2O2 and NH3·H2O. An anatase TiO2 thin film was obtained by heating the as-deposited thin film at 500 °C for 1 h in air. A chemical composition of TiO1.4(O2)0.5(OH)0.2·1.34H2O is proposed for the as-deposited thin film, based on XPS, FT-IR and TG-DTA data. Band gap energies of 3.20 eV for indirect transition and 3.63 eV for direct transition were obtained for the anatase TiO2 film.

http://www.titanshield.com/pdf/E/articles/TiO2-JMC-(2003(13)).pdf

Room temperature deposition of a TiO2 thin film from aqueous peroxotitanate solution

No-template synthesis of surface clean platinum nanomaterials with thin projections (petals) on the Pt nanoparticle surface, termed Pt nanoflowers, has been developed, and Pt nanoflowers have achieved superior performance on surface-assisted laser desorption/ionization mass spectrometry (SALDI−MS) of biomolecules. With this new material, one can obtain high-resolution mass spectra with lower laser energy and smaller sample amounts, and no organic matrix is needed for laser desorption/ionization. The mass spectra also have fewer obstacle peaks at the lower m/z region. Furthermore, this novel substrate is quite stable against aging and showed no sweet-spot problems, that is, equal sensitivity on the whole substrate surface. Various biomolecules, including peptides and phospholipids, were examined, and Pt nanoflowers showed good performance for matrix-free laser desorption/ionization MS of these materials.

Tetsu Yonezawa

Papers

Bimetallic nanoparticles—novel materials for chemical and physical applications

Structural analysis of polymer-protected palladium/platinum bimetallic clusters as dispersed catalysts by using extended x-ray absorption fine structure spectroscopy

Polymer-protected palladium–platinum bimetallic clusters: preparation, catalytic properties and structural considerations

Room temperature deposition of a TiO2 thin film from aqueous peroxotitanate solution

Platinum Nanoflowers for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry of Biomolecules