Surface chemistry: a non-negligible parameter in determining optical properties of small colloidal metal nanoparticles

Surface chemistry can become pronounced in determining the optical properties of colloidal metal nanoparticles as the nanoparticles become so small (diameters <20 nm) that the surface atoms, which can undergo chemical interactions with the environment, represent a significant fraction of the total number of atoms although this effect is often ignored. For instance, formation of chemical bonds between surface atoms of small metal nanoparticles and capping molecules that help stabilize the nanoparticles can reduce the density of conduction band electrons in the surface layer of metal atoms. This reduced electron density consequently influences the frequency-dependent dielectric constant of the metal atoms in the surface layer and, for sufficiently high surface to volume ratios, the overall surface plasmon resonance (SPR) absorption spectrum. The important role of surface chemistry is highlighted here by carefully analyzing the classical Mie theory and a multi-layer model is presented to produce more accurate predictions by considering the chemically reduced density of conduction band electrons in the outer shell of metal atoms in nanoparticles. Calculated absorption spectra of small Ag nanoparticles quantitatively agree with the experimental results for our monodispersed Ag nanoparticles synthesized via a well-defined chemical reduction process, revealing an exceptional size-dependence of absorption peak positions: the peaks first blue-shift followed by a turnover and a dramatic red-shift as the particle size decreases. A comprehensive understanding of the relationship between surface chemistry and optical properties is beneficial to exploit new applications of small colloidal metal nanoparticles, such as colorimetric sensing, electrochromic devices, and surface enhanced spectroscopies.     

Self-assembled metal colloid films: two approaches for preparing new SERS active substrates

In this paper, we propose two new approaches for preparing active substrates for surface-enhanced Raman scattering (SERS). In the first approach (method 1), one transfers AgI nanoparticles capped by negatively charged mercaptoacetic acid from a AgI colloid solution onto a quartz slide and then deoxidizes AgI to Ag nanoparticles on the substrate. The second approach (method 2) deoxidizes AgI to Ag nanoparticles in a colloid solution and then transfers the Ag nanoparticles capped by negatively charged mercaptoacetic acid onto a quartz slide. By transfer of the AgI/Ag nanoparticles from the colloid solutions to the solid substrates, the problem of instability of the colloid solutions can largely be overcome. The films thus prepared by both approaches retain the merits of metal colloid solutions while they discharge their shortcomings. Accordingly, the obtained Ag particle films are very suitable as SERS active substrates. SERS active substrates with different coverages can be formed in a layer-by-layer electrostatic assembly by exposing positively charged surfaces to the colloid solutions containing oppositely charged AgI/Ag nanoparticles. The SERS active substrates fabricated by the two novel methods have been characterized by means of atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) spectroscopy. The results of AFM and UV-vis spectroscopy show that the Ag nanoparticles grow with the increase in the number of coverage and that most of them remain isolated even at high coverages. Consequently, the surface optical properties are dominated by the absorption due to the isolated Ag nanoparticles. The relationship between SERS intensity and surface morphology of the new active substrates has been investigated for Rhodamine 6G (R6G) adsorbed on them. It has been found that the SERS enhancement depends on the size and aggregation of the Ag particles on the substrates. Especially, we can obtain a stronger SERS signal from the substrate prepared by method 1, implying that for the metal nanoparticles capped with stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization in the film is of great use in preparing SERS active substrates. Furthermore, we have found that the addition of Cl- into the AgI colloid solution changes the surface morphology of the SERS active substrates and favors stronger SERS enhancement.     

Flexible ureasil hybrids with tailored optical properties through doping with metal nanoparticles

Hybrid organic-inorganic nanocomposites containing uniform distributions of metal nanoparticles have been prepared by mixing a preformed nanoparticle colloid with the precursors of a ureasil, prior to the sol-gel transition. These nanocomposites possess not only high optical quality and optical features dictated by the size and shape of the nanoparticle dopants but also a high degree of flexibility, which can largely enhance the range of applications in practical devices. The deposition of a uniform silica shell on the nanoparticle surface prior to the sol-gel transition was found to be required to maintain the colloidal stability during the process and, thus, to retain the optical properties in the final nanocomposite material. This method can be readily extended to other materials, such as semiconductor and magnetic nanoparticles.     

Spectroscopic characterizations of molecularly linked gold nanoparticle assemblies upon thermal treatment

The understanding of surface properties of core-shell type nanoparticles is important for exploiting the unique nanostructured catalytic properties. We report herein findings of a spectroscopic investigation of the thermal treatment of such nanoparticle assemblies. We have studied assemblies of gold nanocrystals of approximately 2 nm core sizes that are capped by alkanethiolate shells and are assembled by covalent or hydrogen-bonding linkages on a substrate as a model system. The structural evolution of the nanoparticle assemblies treated at different temperatures was probed by several spectroscopic techniques, including UV-visible, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that the capping/linking shell molecules can be effectively removed to produce controllable surface and optical properties. The data further revealed that the thermally induced evolution of the surface plasmon resonance property of gold nanoparticles is dependent on the chemical nature of the linker molecule. The spectral evolution is discussed in terms of changes in particle size, interparticle distance, and dielectric medium properties, which has important implications for controlled preparation and thermal processing of core-shell nanostructured metal catalysts.     

Nanostructured metal oxides/hydroxides-based electrochemical sensor for monitoring environmental micropollutants

Nanostructured metal oxides/hydroxides (NMOs/HOs) with unique optical, electrical and molecular properties, chemical and photochemical stability, electrochemical activity, large surface area along with desired functionalities have recently become important as materials to construct electrochemical sensor for monitoring environmental micropollutants. In this review, we present and discuss the NMOs/HOs-based electrochemical sensor for detection of micropollutants including toxic organic micropollutants, heavy metal ions (HMIs), and anions in water. The analytical performance of a NMOs/HOs-based electrochemical sensor can be improved by tailoring the properties of the NMOs/HOs through engineering of morphology, particle size, exposed crystal facets, effective surface area, functionality, adsorption capability and electron-transfer properties. These interesting NMOs/HOs are expected to find potential applications in a new generation of miniaturized, smart electrochemical environmental monitoring devices.     

Au nanoparticle monolayers covered with sol-gel oxide thin films: optical and morphological study

In this work, we provide a detailed study of the influence of thermal annealing on submonolayer Au nanoparticle deposited on functionalized surfaces as standalone films and those that are coated with sol-gel NiO and TiO(2) thin films. The systems are characterized through the use of UV-vis absorption, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and spectroscopic ellipsometry. The surface plasmon resonance peak of the Au nanoparticles was found to red-shift and increase in intensity with increasing surface coverage, an observation that is directly correlated to the complex refractive index properties of Au nanoparticle layers. The standalone Au nanoparticles sinter at 200 °C, and a relationship between the optical properties and the annealing temperature is presented. When overcoated with sol-gel metal oxide films (NiO, TiO(2)), the optical properties of the Au nanoparticles are strongly affected by the metal oxide, resulting in an intense red shift and broadening of the plasmon band; moreover, the temperature-driven sintering is strongly limited by the metal oxide layer. Optical sensing tests for ethanol vapor are presented as one possible application, showing reversible sensing dynamics and confirming the effect of Au nanoparticles in increasing the sensitivity and in providing a wavelength dependent response, thus confirming the potential use of such materials as optical probes.     

Synthesis and applications of silver nanoparticles

Over the past few decades, nanoparticles of noble metals such as silver exhibited significantly distinct physical, chemical and biological properties from their bulk counterparts. Nano-size particles of less than 100 nm in diameter are currently attracting increasing attention for the wide range of new applications in various fields of industry. Such powders can exhibit properties that differ substantially from those of bulk materials, as a result of small particle dimension, high surface area, quantum confinement and other effects. Most of the unique properties of nanoparticles require not only the particles to be of nano-sized, but also the particles be dispersed without agglomeration. Discoveries in the past decade have clearly demonstrated that the electromagnetic, optical and catalytic properties of silver nanoparticles are strongly influenced by shape, size and size distribution, which are often varied by varying the synthetic methods, reducing agents and stabilizers. Accordingly, this review presents different methods of preparation silver nanoparticles and application of these nanoparticles in different fields.     

Formation and Properties of Sol-Gel Films and Glasses with Ultrafine Metal and Semiconductor Particles

A novel method of fabrication of silica-based sol-gel films and glasses containing small semiconductor particles was developed. A series of films and glasses with nanoparticles of copper chalcogenides (CuS, Cu₂Se, CuInS₂) and metal particles (Cu) were fabricated through the chemical transformation of precursors incorporated into a sol-gel derived matrix. The properties of the nanoparticles studied by means of XRD, XPS, TEM and optical spectroscopy are provided both by size effects and the chemical nature of surface states and can be controlled at different steps of chemical treatment.     

Dispersions, novel nanomaterial sensors and nanoconjugates based on carbon nanotubes

Nanomaterials are structures with dimensions characteristically much below 100 nm. The unique physical properties (e.g., conductivity, reactivity) have placed these nanomaterials in the forefront of emerging technologies. Significant enhancement of optical, mechanical, electrical, structural, and magnetic properties are commonly found through the use of novel nanomaterials. One of the most exciting classes of nanomaterials is represented by the carbon nanotubes. Carbon nanotubes, including single-wall carbon nanotubes, multi-wall carbon nanotubes, and concentric tubes have been shown to possess superior electronic, thermal, and mechanical properties to be attractive for a wide range of potential applications They sometimes bunch to form “ropes” and show great potential for use as highly sensitive electronic (bio)sensors due to the very small diameter, directly comparable to the size of single analyte molecules and that every single carbon atom is in direct contact with the environment, allowing optimal interaction with nearby molecules. Composite materials based on integration of carbon nanotubes and some other materials to possess properties of the individual components with a synergistic effect have gained growing interest. Materials for such purposes include conducting polymers, redox mediators and metal nanoparticles. These tubes provide the necessary building blocks for electronic circuits and afford new opportunities for chip miniaturization, which can dramatically improve the scaling prospects for the semiconductor technologies and the fabrication of devices, including field-effect transistors and sensors. Carbon nanotubes are one of the ideal materials for the preparation of nanoelectronic devices and nanosensors due to the unique electrical properties, outstanding electrocatalytic properties, high chemical stability and larger specific surface area of nanotubes. Carbon nanotubes are attractive material for supercapacitors due to their unique one-dimensional mesoporous structure, high specific surface area, low resistivity and good chemical stability. Nanoscaled composite materials based on carbon nanotubes have been broadly used due to their high chemical inertness, non-swelling effect, high purity and rigidity. The integration of carbon nanotubes with organics, biomaterials and metal nanoparticles has led to the development of new hybrid materials and sensors. Hybrid nanoscale materials are well established in various processes such as organic and inorganic compounds, nucleic acid detachment, protein separation, and immobilization of enzymes. Those nanostructures can be used as the building blocks for electronics and nanodevices because uniform organic and metal coatings with the small and monodisperse domain sizes are crucial to optimize nanoparticle conductivity and to detect changes in conductivity and absorption induced by analyte adsorption on these surfaces. The highly ordered assembly of zero-dimensional and one-dimensional nanoparticles is not only necessary for making functional devices, but also presents an opportunity to develop novel collective properties.     

Colloidal self-assemblies used as templates to control size,shape and self-organization of nanoparticles

In this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders. Self-assembly of silver metal nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced, and, in these aggregates, the particles are highly organized and form pseudo-crystals with a face-centered cubic structure for various particles sizes. The optical properties of the silver nanoparticles isolated in micellar solution or self-assembled in 2D or 3D supperlattices are reported. Syntheses of magnetic fluids differing in their particle size are presented. The magnetic properties differ with the particle size.     

Modeling interband transitions in silver nanoparticle-fluoropolymer composites

The interband transition contributions to the optical properties of silver nanoparticles in fluoropolymer matrices are investigated. For the materials in this study, nanoparticle synthesis within the existing polymer matrix is accomplished using an infusion process that consists of diffusing an organometallic precursor gas into the free volume of the fluoropolymer and decomposing the precursor followed by metal nanoparticle nucleation and growth. The resulting polymer matrix nanocomposite has optical properties that are dominated by the response of the nanoparticles owing to the broadbanded transparency of the fluoropolymer matrix. The optical properties of these composites are compared to Maxwell-Garnett and Mie theory with results indicating that interband transitions excited in the silver nanoparticles affect the optical absorption over a range of frequencies including the surface plasmon resonance. It is shown that calculations of the optical absorption spectrum using published data for the silver dielectric function do not accurately describe the measured material response and that a classical model for bound and free electron behavior can best be used to represent the dielectric function of silver.     

Gold nanoparticles to boost the gas sensing performance of porous sol–gel thin films

In this paper we review our research work of the last few years on the synthesis and the gas sensing properties of nanocomposite thin films of sensitive materials with a large specific surface area, which consist of porous matrices containing functional nanocrystals of metal oxides and gold. The film porosity provides a path for the gas molecules to reach the active reaction sites on the nanoparticles surface undergoing chemical reactions which nature depends on the nature of the active material. The introduction of Au nanoparticles affects the reactions mechanism improving the sensing process, moreover the Au Surface Plasmon Resonance peak can be used for the realization of selective optical gas sensor. Two different synthetic approaches will be described, each of them characterized by a peculiar control of the final materials morphology, structure and micro-structure.     

Assembly of gold nanoparticles mediated by multifunctional fullerenes

The understanding of the interparticle interactions of nanocomposite structures assembled using molecularly capped metal nanoparticles and macromolecular mediators as building blocks is essential for exploring the fine-tunable interparticle spatial and macromolecular properties. This paper reports the results of an investigation of the chemically tunable multifunctional interactions between fullerenes (1-(4-methyl)-piperazinyl fullerene, MPF) and gold nanoparticles. The interparticle spatial properties are defined by the macromolecular and multifunctional electrostatic interactions between the negatively charged nanoparticles and the positively charged fullerenes. In addition to characterization of the morphological properties, the surface plasmon resonance band, dynamic light scattering, and surface-enhanced Raman scattering (SERS) properties of the MPF-mediated assembly and disassembly processes have been determined. The change of the optical properties depends on the pH and electrolyte concentrations. The detection of the Raman-active vibration modes (Ag(2) and Hg(8)) of C60 and the determination of their particle size dependence have demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Au nm assembly is responsible for the SERS effect. These findings provide new insights into the delineation between the interparticle interactions and the nanostructural properties for potential applications of the nanocomposite materials in spectroscopic and optical sensors and in controlled releases.     

In Situ Formation of Gold Nanoparticles within a Polymer Particle and Their Catalytic Activities in Various Chemical Reactions

Composite materials consisting of nanoscale gold particles and protective polymer shells were designed and tested as catalysts in various chemical reactions. Initially, the systematic incorporation of multiple gold nanoparticles into a poly(N-isopropylacrylamide) particle was achieved by an in situ method under light irradiation. The degree of gold nanoparticle loading, along with the structural and morphological properties, was examined as a function of the amount of initial gold ions and reducing agent. As these gold nanoparticles were physically-embedded within the polymer particle in the absence of strong interfacial interactions between the gold nanoparticles and polymer matrix, the readily-accessible surface of the gold nanoparticles with a highly increased stability allowed for their use as recyclable catalysts in oxidation, reduction, and coupling reactions. Overall, the ability to integrate catalytically-active metal nanoparticles within polymer particles in situ allows for designing novel composite materials for multi-purpose catalytic systems.     

Molecular Alignment-Induced Chemically Patchy Uniaxial Nanoparticles and Their Applications in Anti-Counterfeiting and Self-Assembled Superstructures

We report an unusual strategy for synthesizing patchy nanoparticles (NPs) by controlling the orientation of the molecules that form the NPs. This is realized by synchronous polymerization and crystallization of liquid crystal (LC) monomers during scalable precipitation polymerization. The resulting NPs are cylinders with highly uniform shapes and have only a single LC domain. The patchy properties originate from the discrepancy of surface chemical compositions on flat and side surfaces and can be switched on and off by solvent. Extra colloidal blocks can be grown onto the patches, resulting in highly uniform triblock patchy dumbbells, which have integrated optical properties, and as demonstrated, show triple-mode optical authentication in anti-counterfeiting labels or patterns. We also demonstrate that the triblock patchy cylinders are attractive building blocks for long LC rods or porous colloidal materials through polymerization-induced self-assembly.     

Surface chemistry of quantum-sized metal nanoparticles under light illumination

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous chemistry at the surface of the nanoparticles. The optical properties of metal nanoparticles, such as light absorption, also exhibit a strong dependence on their size. It is expected that there will be strong coupling of light absorption and surface chemistry when the metal nanoparticles are small enough. For instance, metal nanoparticles with sizes in the range of 2–10 nm exhibit both surface plasmon resonances, which can efficiently produce high-energy hot electrons near the surface of the nanoparticles under light illumination, and the Coulomb blockade effect, which favors electron transfer from the metal nanoparticles to the surface adsorbates. The synergy of efficient hot electron generation and electron transfer on the surface of small metal nanoparticles leads to double-faced effects: (i) surface (adsorption) chemistry influences optical absorption in the metal nanoparticles, and (ii) optical absorption in the metal nanoparticles promotes (or inhibits) surface adsorption and heterogeneous chemistry. This review article focuses on the discussion of typical quantum phenomena in metal nanoparticles of 2–10 nm in size, which are referred to as “quantum-sized metal nanoparticles”. Both theoretical and experimental examples and results are summarized to highlight the strong correlations between the optical absorption and surface chemistry for quantum-sized metal nanoparticles of various compositions. A comprehensive understanding of these correlations may shed light on achieving high-efficiency photocatalysis and photonics.

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous photochemistry at the surface of the nanoparticles.     

Metal Nano Networks by Potential-Controlled In Situ Assembling of Gold/Silver Nanoparticles

Non-spherical Au/Ag nanoparticles can be generated by chemical reduction of silver ions in the presence of preformed gold nanoparticles. The process of particle formation can be controlled by concentrations of ligands and reducing agent. The formation of ellipsoidal, nanorod- and peanut-shaped nanoparticles as well as of more complex fractal nanoassemblies can be explained by changes in particle surface state, electrochemical potential formation and particle-internal self-polarization effects. It is possible to create highly fractal nanoassemblies with sizes between the mid-nanometer and the lower micrometer range. The assemblies are marked by high optical absorption and complex nano-networks of very high surface-to-volume ratios and a granular base structure.     

Nanocomposites of polymers and metals or semiconductors: Historical background and optical properties

Upon transmission of visible light through composites comprising of a transparent polymer matrix with embedded particles, the intensity loss by scattering is substantially reduced for particle diameters below 50–100 nm (nanoparticles, nanosized particles). As a consequence, related materials (nanocomposites) have found particular interest in optical studies. The first part of this article deals with a historical survey on nanoparticles and nanocomposites and the importance of small particle sizes on their optical properties. The second part focuses on results from our laboratory concerning nanocomposites with extremely high or low refractive indices and dichroic nanocomposites and their application in bicolored liquid crystal displays (LCD). The inorganic colloids required for these studies (lead sulfide, iron sulfides, gold, and silver) were prepared in situ in presence of a polymer or isolated as redispersable metal colloids modified at the surface with a self‐assembled monolayer (SAM) of an alkanethiol. The nanocomposites themselves were finally obtained by coprecipitation, spin coating, solvent casting or melt extrusion, with poly(ethylene oxide), gelatin, poly(vinyl alcohol) and polyethylene as matrix polymers.     

Carbon Nanotube and Gold‐Based Materials: A Symbiosis

Carbon nanotubes constitute a novel class of nanomaterials with potential applications in many areas. The attachment of metal nanoparticles to carbon nanotubes is new way to obtain novel hybrid materials with interesting properties for various applications such as catalysts and gas sensors as well as electronic and magnetic devices. Their unique properties such as excellent electronic properties, a good chemical stability, and a large surface area make carbon nanotubes very useful as a support for gold nanoparticles in many potential applications, ranging from advanced catalytic systems through very sensitive electrochemical sensors and biosensors to highly efficient fuel cells. Here we give an overview on the recent progress in this area by exploring the various synthesis approaches and types of assemblies, in which nanotubes can be decorated with gold nanoparticles and explore the diverse applications of the resulting composites.     

Synthesis and optical properties of silver nanoparticles and arrays.

This Minireview systematically examines optical properties of silver nanoparticles as a function of size. Extinction, scattering, and absorption cross-sections and distance dependence of the local electromagnetic field, as well as the quadrupolar coupling of 2D assemblies of such particles are experimentally measured for a wide range of particle sizes. Such measurements were possible because of the development of a novel synthetic method for the size-controlled synthesis of chemically clean, highly crystalline silver nanoparticles of narrow size distribution. The method and its unique advantages are compared to other methods for synthesis of metal nanoparticles. Synthesis and properties of nanocomposite materials using these and other nanoparticles are also described. Important highlights in the history of the field of metal nanoparticles as well as an examination of the basic principles of plasmon resonances are included.