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.

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Photoinduced reactivity of titanium dioxide

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.

The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads to anion desorption. Underpotential deposition (upd) of electropositive metal layers results in dramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in t...

Surface Plasmon Spectroscopy of Nanosized Metal Particles

Dendrimer-metal (silver, platinum, and palladium) nanocomposites are prepared in aqueous solutions containing poly(amidoamine) (PAMAM) dendrimers with surface amino groups (generations 3, 4, and 5) or poly(propyleneimine) (PPI) dendrimers with surface amino groups (generations 2, 3, and 4) The particle sizes of the metal nanoparticles obtained are almost independent of the generation as well as the concentration of the dendrimer for both the PAMAM and the PPI dendrimers; the average sizes of silver, platinum, and palladium nanoparticles are 56-75, 12-16, and 16-20 nm, respectively It is suggested that the dendrimer-metal nanocomposites are formed by adsorbing the dendrimers on the metal nanoparticles Studies of the reduction reaction of 4-nitrophenol by these nanocomposites show that the rate constants are very similar between PAMAM and PPI dendrimer-silver nanocomposites, whereas the rate constants for the PPI dendrimer-platinum and -palladium nanocomposites are greater than those for the corresponding PAMAM dendrimer nanocomposites In addition, it is found that the rate constants for the reduction of 4-nitrophenol involving all the dendrimer-metal nanocomposites decrease with an increase in the dendrimer concentrations, and the catalytic activity of dendrimer-palladium nanocomposites is highest

Preparation of PAMAM− and PPI−Metal (Silver, Platinum, and Palladium) Nanocomposites and Their Catalytic Activities for Reduction of 4-Nitrophenol

Gold nanoparticles were prepared in the presence of poly(amidoamine) (PAMAM) dendrimers (generations 3.0 and 5.0) or poly(propyleneimine) (PPI) dendrimers (generations 3.0 and 4.0) with surface amino groups by laser irradiation. UV−visible absorption, dynamic light scattering, and transmission electron microscopy have been used to study the formation and structure of the nanocomposites. The reduction of Au3+ ions proceeded with an increase of the irradiation time, and the average diameters of the gold nanoparticles obtained tended to decrease with an increase of the dendrimer concentration. It is suggested that the dendrimers are adsorbed on the nanoparticles as a monolayer. Studies of the reduction reaction of 4-nitrophenol by the gold nanoparticles with NaBH4 reveal that the rate constants for PPI dendrimers are higher than those for PAMAM dendrimers, whereas the size of the PPI dendrimer of the same generation is considerably smaller than that of the PAMAM dendrimer.

Preparation of Gold−Dendrimer Nanocomposites by Laser Irradiation and Their Catalytic Reduction of 4-Nitrophenol

Au colloids have been prepared by reduction of metal salt with UV irradiation in the presence of dendrimers. Poly(amidoamine) dendrimers with surface amino groups for generations 0, 1, 2, 3, 4, and 5 (G0−5) have been used. The average particle sizes decrease with an increase of concentration of surface amino group of the dendrimers. In particular, Au colloids with a diameter less than 1 nm are obtained in the presence of the later generation dendrimers(G3−5) at above the molar ratio of surface amino group of dendrimer/HAuCl4 (4:1). These concentrations are extremely small compared with that of other polymers, indicating that the dendrimers have an effective protective action for the formation of Au colloids.

Preparation of Gold Colloids with UV Irradiation Using Dendrimers as Stabilizer

Colloidal gold was prepared by irradiation of HAuCl 4 solutions with 253.7 nm light in the presence of rodlike micelles of hexadecyltrimethylammonium chloride. Rodlike colloidal gold was obtained at above some concentrations of HAuCl 4 with increasing irradiation time. This result suggests that the rodlike micelle operates as a template for the preparation of anisotropic gold particles.

Preparation of rodlike gold particles by UV irradiation using cationic micelles as a template

Nanoparticles of gold, platinum, and silver were prepared by reduction of their metal salts with NaBH4 in the presence of poly(amidoamine) dendrimers. The dendrimers used were generation 3, 4, and 5 having surface amino group (G3, G4, and G5) as well as generation 3.5, 4.5, and 5.5 having surface carboxyl group (G3.5, G4.5, and G5.5). The metal nanoparticles obtained were characterized by UV−vis spectroscopy and transmission electron microscopy. By using G3−G5, gold nanoparticles in the 1.5−4.0 nm size regime were obtained where their size decreased with increasing concentration of the dendrimers as well as the generation of the dendrimers. Similarly, platinum nanoparticles with a diameter of 2.4−3.0 nm were obtained, and their size was insensitive against the concentration as well as the generation of G3−G5. In the case of silver nanoparticles, very small silver particles using G3.5−G5.5 were obtained. In addition, the dendrimer concentration required for obtaining stable nanoparticles was found to be de...

Kunio Esumi

Papers

Preparation of PAMAM− and PPI−Metal (Silver, Platinum, and Palladium) Nanocomposites and Their Catalytic Activities for Reduction of 4-Nitrophenol

Preparation of Gold−Dendrimer Nanocomposites by Laser Irradiation and Their Catalytic Reduction of 4-Nitrophenol

Preparation of Gold Colloids with UV Irradiation Using Dendrimers as Stabilizer

Preparation of rodlike gold particles by UV irradiation using cationic micelles as a template

Role of Poly(amidoamine) Dendrimers for Preparing Nanoparticles of Gold, Platinum, and Silver