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.

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Gold nanoparticles in chemical and biological sensing.

Aggregation-induced emission: phenomenon, mechanism and applications.

Charge-Transfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Organic light-emitting diodes (OLEDs) represent a promising technology for large, flexible, lightweight, flat-panel displays. Such devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode into the lowest unoccupied molecular orbital of the adjacent molecules (simultaneously, holes are injected by the anode into the highest occupied molecular orbital). The two types of carriers migrate towards each other and a fraction of them recombine to form excitons, some of which decay radiatively to the ground state by spontaneous emission. Doped pi-conjugated polymer layers improve the injection of holes in OLED devices; this is thought to result from the more favourable work function of these injection layers compared with the more commonly used layer material (indium tin oxide). Here we demonstrate that by increasing the doping level of such polymers, the barrier to hole injection can be continuously reduced. The use of combinatorial devices allows us to quickly screen for the optimum doping level. We apply this concept in OLED devices with hole-limited electroluminescence (such as polyfluorene-based systems), finding that it is possible to significantly reduce the operating voltage while improving the light output and efficiency.

Improving the Performance of Doped pi-Conjugated Polymers for Use in Organic Light-Emitting Diodes

demonstrated an organic EL device based on aligned con-jugated polymers which directly emitted polarized light,and realized that such devices would be particularly usefulas backlights for conventional liquid crystal displays(LCDs). However, typical supertwisted nematic LCDs re-quire a contrast ratio of 40 to 50,

Blue Polarized Electroluminescence from a Liquid Crystalline Polyfluorene

Improving the Performance of Polyfluorene-Based Organic Light-Emitting Diodes via End-Capping

Gold nanoparticle/alkanedithiol films were prepared via layer-by-layer self-assembly. For the assembly process, dodecylamine-stabilized Au nanoparticles with an average size of 4 nm and alkanedithiols with different alkylene chain lengths (C6, C9, C12, C16) were used. The thickness of the films was determined by AFM and ranged between 26 and 34 nm. FE-SEM and TEM images indicate that the particle size within the film materials was similar to that of the dodecylamine-stabilized particles used for film preparation. The composition of the films was analyzed by XPS. The absence of the nitrogen signal indicated that the dodecylamine ligands were quantitatively exchanged by alkanedithiol molecules during film assembly. Two sulfur signals were observed, which could be assigned to sulfur bound to gold (S−Au) and to free thiol groups (S−H). As indicated by the relative signal intensities, about 60% of the alkanedithiol molecules were bound with both ends to the nanoparticles, whereas 40% were bound with only one t...

Self-Assembled Gold Nanoparticle/Alkanedithiol Films: Preparation, Electron Microscopy, XPS-Analysis, Charge Transport and Vapor-Sensing Properties

Circularly polarized electroluminescence (CPEL) has potential application in optical data storage and processing. It was recently discovered that CPEL could be generated from chiral-substituted, conjugated polymers. These authors have synthesized a range of polyfluorenes (e.g., see Figure) that form liquid crystals and emit bluish light with a significant (200 times greater than in the first report) circularly polarized component

Heinz-Georg Nothofer

Papers

Improving the Performance of Doped pi-Conjugated Polymers for Use in Organic Light-Emitting Diodes

Blue Polarized Electroluminescence from a Liquid Crystalline Polyfluorene

Improving the Performance of Polyfluorene-Based Organic Light-Emitting Diodes via End-Capping

Self-Assembled Gold Nanoparticle/Alkanedithiol Films: Preparation, Electron Microscopy, XPS-Analysis, Charge Transport and Vapor-Sensing Properties

Circularly polarized electroluminescence from liquid-crystalline chiral polyfluorenes