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.

In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.

Surface-enhanced spectroscopy

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.

2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Colloidal particles act in many ways like surfactant molecules, particularly if adsorbed to a fluid–fluid interface. Just as the water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile–lipophile balance (HLB) number, so can that of a spherical particle be described in terms of its wettability via contact angle. Important differences exist, however, between the two types of surface-active material, due in part to the fact that particles are strongly held at interfaces. This review attempts to correlate the behaviour observed in systems containing either particles or surfactant molecules in the areas of adsorption to interfaces, partitioning between phases and solid-stabilised emulsions and foams.

Particles as surfactants—similarities and differences

Two-dimensional interfacial electrodeposition of silver is studied under alternating square-wave electric potential conditions, at the water/dichloromethane interface. The overall rate of growth is found to increase with frequency. The deposit's morphology also depends strongly on the frequency:  at ≤10 Hz compact deposits are formed while at 1−10 kHz highly ramified structures develop, the branches in which tend to broaden out at higher frequencies. At ∼100 Hz a highly disordered growth is observed. A novel pulsating behavior of the deposit size is observed, with alternating contractions and growth phases. The contractions are attributed to a mechanical compaction of the newly-formed deposit, driven by the fast changes in surface tension due to the voltage variation. The mechanical oscillations induce hydrodynamic stirring in the solution, affecting the mass-transport of the reacting silver ions to the electrode, and, consequently, the rate of growth and morphology of the deposit.

AC-Driven Interfacial Electrodeposition of Silver

Alizarin yellow 2G, 5-((3-nitrophenyl)azo)-2-hydroxybenzoic acid sodium salt (AY), strongly interacts with silver ions and adsorbs on silver metal surfaces. It competes with other anions (anisate anion) over adsorption sites and alters the energetics of formation of the small silver metal cluster intermediates involved in the process of colloid production. The increase in the colloid nucleation rate, compared to the growth, results in a larger number of slightly smaller silver particles than that obtained in the absence AY. The larger number of colloidal particles, that function as interfacial colloidal stabilizers of a water/oil macroemulsion, brings about a decrease in the size of the emulsion droplets. The adsorption of AY on the silver colloid is studied by surface Raman spectroscopy. It is found to adsorb in a flat conformation, forming a surface molecular species resembling the complex of AY and silver ions.

Silver Colloids and Interfacial ColloidsAdsorption of Alizarin Yellow 2G and Its Effect on Colloidal Nucleation

We studied Langmuir and Langmuir–Blodgett (LB) films of several strong nonlinear optical (NLO) active amphiphilic derivatives of 1,3-indanedione-5,6-dicarboxylic acid. The surface pressure–area (π–A) isotherms at the air–water interface were investigated at different temperatures and pH values. Non-centrosymmetric Z-type LB films were deposited. UV–visible spectra indicated a uniform film transfer. The orientation of the molecular transition moments was calculated from the polarized UV–visible spectra. Packing within the LB films was observed by UV–visible absorption and fluorescence spectroscopy. The morphology of the LB films was examined by AFM and compared to ellipsometric measurements, and the nature of the films was studied by contact angle measurements. Significant second harmonic generation (SHG) by the LB films was observed. Alternate layer Y-type LB films of two different dyes were fabricated to achieve enhanced stability and SHG.

Studies of Langmuir and Langmuir–Blodgett films of NLO-active amphiphilic 1,3-indanedione derivatives

Outer sphere electron transfer reactions are investigated in the framework of a semiclassical theory with a density matrix representation The coupling of the electron exchange system to the polar solvent is represented as a stochastic Gaussian process Expressions for the transition probability in the nonadiabatic case are derived in terms of the correlation function for the fluctuating coupling Some models for the correlation functions are discussed together with explicit results for the transition probabilities

A stochastic solvent model for electron exchange reactions

Silver electrodeposition at the air/water interface is found to yield a hitherto unreported ring morphology, with silvery, quasi-2D rings alternating with black, 3D-like ones. This is attributed to oscillatory accumulation/detachment cycles of hydrogen at the growing edge of the deposit. With this morphology, the measured morphology diagram closely resembles that of bacterial colony growth, indicating a similarity of the underlying growth mechanism.

Shlomo Efrima

Papers

AC-Driven Interfacial Electrodeposition of Silver

Silver Colloids and Interfacial ColloidsAdsorption of Alizarin Yellow 2G and Its Effect on Colloidal Nucleation

Studies of Langmuir and Langmuir–Blodgett films of NLO-active amphiphilic 1,3-indanedione derivatives

A stochastic solvent model for electron exchange reactions

Ring Morphology in Interfacial Electrodeposition