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.

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

Adsorptive separation is very important in industry. Generally, the process uses porous solid materials such as zeolites, activated carbons, or silica gels as adsorbents. With an ever increasing need for a more efficient, energy-saving, and environmentally benign procedure for gas separation, adsorbents with tailored structures and tunable surface properties must be found. Metal–organic frameworks (MOFs), constructed by metal-containing nodes connected by organic bridges, are such a new type of porous materials. They are promising candidates as adsorbents for gas separations due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability. This critical review starts with a brief introduction to gas separation and purification based on selective adsorption, followed by a review of gas selective adsorption in rigid and flexible MOFs. Based on possible mechanisms, selective adsorptions observed in MOFs are classified, and primary relationships between adsorption properties and framework features are analyzed. As a specific example of tailor-made MOFs, mesh-adjustable molecular sieves are emphasized and the underlying working mechanism elucidated. In addition to the experimental aspect, theoretical investigations from adsorption equilibrium to diffusion dynamics via molecular simulations are also briefly reviewed. Furthermore, gas separations in MOFs, including the molecular sieving effect, kinetic separation, the quantum sieving effect for H2/D2 separation, and MOF-based membranes are also summarized (227 references).

Selective gas adsorption and separation in metal–organic frameworks

Metal–Organic Frameworks for Separations

Metal–organic frameworks (MOFs) have received much attention because of their attractive properties. They show great potential applications in many fields. An emerging trend in MOF research is hybridization with flexible materials, which is the subject of this review. Polymers possess a variety of unique attributes, such as softness, thermal and chemical stability, and optoelectrical properties that can be integrated with MOFs to make hybrids with sophisticated architectures. Hybridization of MOFs and polymers is producing new and versatile materials that exhibit peculiar properties hard to realize with the individual components. This review article focuses on the methodology for hybridization of MOFs and polymers, as well as the intriguing functions of hybrid materials.

Hybridization of MOFs and polymers

Recent developments in polymerizations within the nanochannels of porous coordination polymers (PCPs) are covered in this tutorial review. The characteristic features of PCPs are regular structures, controllable channel sizes and shapes, a designable surface functionality, and flexible frameworks, which can be utilized for precision polymer synthesis and specific polymer confinement. This review discusses promising approaches to multiple controls of polymer structures, analysis systems for low-dimensional polymer assemblies and the construction of PCP–polymer nanohybrids, which are strong enticements to researchers in the areas of inorganic, materials and polymer chemistry.

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Polymerization reactions in porous coordination polymers

Prussian blue (PB) nanoparticles protected by poly(vinylpyrrolidone) (PVP) were prepared by mixing aqueous Fe2+, Fe(CN)63-, and PVP solutions together and were characterized by UV−vis, IR, XRPD, and TEM. Averaged dimensions of the nanoparticles were controlled between 12 and 27 nm depending on initial Fe ion concentrations and feed ratios of Fe ion to PVP. Solubility of PB bulk in organic solvents is considerably low; nevertheless, formations of the PB nanoparticles dramatically increase the solubility in a variety of organic solvents. It is noteworthy that the PVP-protected PB nanoparticles stably maintain the cluster formations without further aggregations and dissociation in CHCl3 over 1 month. Measurement of the critical temperature (Tc) where PB nanoparticles exhibit a ferromagnetic property showed a gradual decrease of Tc for the nanoparticles as the particle sizes become smaller. This result could be ascribed to the reduction of the averaged numbers of magnetic interacted neighbors.

Prussian Blue Nanoparticles Protected by Poly(vinylpyrrolidone)

The development of a new methodology for visualizing and detecting gases is imperative for various applications. Here, we report a novel strategy in which gas molecules are detected by signals from a reporter guest that can read out a host structural transformation. A composite between a flexible porous coordination polymer and fluorescent reporter distyrylbenzene (DSB) selectively adsorbed CO₂ over other atmospheric gases. This adsorption induced a host transformation, which was accompanied by conformational variations of the included DSB. This read-out process resulted in a critical change in DSB fluorescence at a specific threshold pressure. The composite shows different fluorescence responses to CO₂ and acetylene, compounds that have similar physicochemical properties. Our system showed, for the first time, that fluorescent molecules can detect gases without any chemical interaction or energy transfer. The host-guest coupled transformations play a pivotal role in converting the gas adsorption events into detectable output signals.

/pdf/gas-detection-by-structural-variations-of-fluorescent-guest-54lgazukit.pdf

Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer

We show that structural changes of a guest molecule can trigger structural transformations of a crystalline host framework. Azobenzene was introduced into a flexible porous coordination polymer (PCP), and cis/trans isomerizations of the guest azobenzene by light or heat successfully induced structural transformations of the host PCP in a reversible fashion. This guest-to-host structural transmission resulted in drastic changes in the gas adsorption property of the host–guest composite, displaying a new strategy for creating stimuli-responsive porous materials.

Takashi Uemura

Papers

Hybridization of MOFs and polymers

Polymerization reactions in porous coordination polymers

Prussian Blue Nanoparticles Protected by Poly(vinylpyrrolidone)

Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer

Guest-to-Host Transmission of Structural Changes for Stimuli-Responsive Adsorption Property