There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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.

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.

/pdf/gold-nanoparticles-in-chemical-and-biological-sensing-5e2qojkdzn.pdf

Gold nanoparticles in chemical and biological sensing.

http://users.encs.concordia.ca/~mojtaba/elec6271/silver%20nanoparticles.pdf

Silver nanoparticles: green synthesis and their antimicrobial activities.

Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Directed Self-Assembly of Nanoparticles

We demonstrate a simple templateless and surfactant-free wet-chemical method of preparing silver (Ag) nanostructures with different dendritic morphologies at room temperature. This has been accomplished by aging the aqueous mixture of AgNO3 and citrate salts, carrying different cations for different time periods. Transmission electron microscopic (TEM) and X-ray diffraction (XRD) studies confirmed the formation of single-crystalline dendritic Ag nanostructures. It has been found that the nature and the concentration of citrate salt have a significant effect on the morphology of the formed Ag nanostructures. A possible formation mechanism has also been discussed on the basis of monitoring the surface plasmon resonance properties and TEM images with time during the course of formation of silver dendrites. The formed dendritic silver nanostructures showed excellent catalytic activity in the borohydride reduction of p-nitrophenol to p-aminophenol compared to the spherical silver nanoparticles.

Synthesis and Catalytic Application of Nanostructured Silver Dendrites

Magnetite nanoparticles of nearly uniform size have been prepared by precipitating ferrous ions in the presence of two different polyelectrolytes, viz., poly(acrylic acid) and the sodium salt of carboxymethyl cellulose at high pH (∼13). The size of the magnetite nanoparticles can be controlled easily by varying the concentration of the polyelectrolyte in the medium. Transmission electron microscopy study indicates that the average particle size varies from 5 to 15 nm, depending on the concentration and the nature of the polyelectrolyte. X-ray diffraction study shows the presence of only magnetite phase. FTIR spectroscopy and thermogravimetric analysis confirmed the presence of polyelectrolyte on the magnetite surface. The magnetization and Mossbauer studies were performed on two samples with mean diameters 7.0 and 14.7 nm. Magnetization measurements suggest that both of these particles are of single magnetic domain. The measurements also estimate the superparamagnetic blocking temperature, TB = 145 K for ...

Size-Controlled Synthesis of Magnetite Nanoparticles in the Presence of Polyelectrolytes

Nanometer sized core−shell particles containing a gold core and poly(methyl methacrylate) (PMMA) shells were prepared by surface-confined living radical polymerization on gold nanoparticles. Gold nanoparticles were prepared in the presence of 11-mercaptoundecanol (MUD) and subsequently esterified with 2-bromoisobutyryl bromide (BIB). Atom transfer living radical polymerization was conducted on the Br-terminated gold nanoparticles using copper(I) bromide/1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (Me4Cyclam) as a room-temperature catalyst. FTIR spectroscopy confirmed the presence of polymer on the nanoparticle surfaces. The resulting gold/polymer nanocomposite particles are stable in suspension. Transmission electron microscopy and UV−vis spectroscopy were performed to characterize the gold nanoparticles (GNPs).

Preparation of Polymer Coated Gold Nanoparticles by Surface-Confined Living Radical Polymerization at Ambient Temperature

Synthetic oligopeptides with a tryptophan residue at the C-terminus have been used for the synthesis of gold and silver nanoparticles at pH 11. The tryptophan residue in the peptides is responsible for the reduction of metal ions to the respective metals, possibly through electron transfer. A mechanistic pathway has been proposed to explain the reductive properties of the tryptophan moiety of the peptide based on some spectroscopic techniques, such as UV-visible and fluorescence spectroscopy. This study reveals that some of the peptide molecules are converted to its corresponding ditryptophan, kynurenine form and some cross-linked products, all of which are highly fluorescent species. The resultant peptide-functionalized metal nanoparticles have also been characterized by UV-visible spectroscopy, transmission electron microscopy, and Fourier transform IR spectroscopy and thermogravimatric analysis.

Tryptophan-based peptides to synthesize gold and silver nanoparticles: a mechanistic and kinetic study.

A simple and easier chemical method for preparing spongy gold nanocrystals has been developed on the basis of a modified-citrate reduction technique of the corresponding gold salt at 25 °C in the absence of template. These nanocrystals possessed autocatalytic behavior and exhibited pronounced catalytic activity in the borohydride reduction of 4-nitrophenol due to their unique spongy morphology.

Tarun K. Mandal

Papers

Synthesis and Catalytic Application of Nanostructured Silver Dendrites

Size-Controlled Synthesis of Magnetite Nanoparticles in the Presence of Polyelectrolytes

Preparation of Polymer Coated Gold Nanoparticles by Surface-Confined Living Radical Polymerization at Ambient Temperature

Tryptophan-based peptides to synthesize gold and silver nanoparticles: a mechanistic and kinetic study.

Synthesis of Spongy Gold Nanocrystals with Pronounced Catalytic Activities