Nanoporous Gold Leaf: “Ancient Technology”/Advanced Material

Porous silica materials have extensively been used for the immobilization of enzymes aiming at their use as biocatalysts or biosensors. This tutorial review will discuss the benefits and challenges of different immobilization techniques and will provide references to recent papers for further reading. Moreover, novel trends and unsolved problems will be introduced.

Immobilization of enzymes on porous silicas – benefits and challenges

‘Hierarchy’ is a property which can be attributed to a manifold of different immaterial systems, such as ideas, items and organisations or material ones like biological systems within living organisms or artificial, man-made constructions. The property ‘hierarchy’ is mainly characterised by a certain ordering of individual elements relative to each other, often in combination with a certain degree of branching. Especially mass-flow related systems in the natural environment feature special hierarchically branched patterns. This review is a survey into the world of hierarchical systems with special focus on hierarchically porous zeolite materials. A classification of hierarchical porosity is proposed based on the flow distribution pattern within the respective pore systems. In addition, this review might serve as a toolbox providing several synthetic and post-synthetic strategies to prepare zeolitic or zeolite containing material with tailored hierarchical porosity. Very often, such strategies with their underlying principles were developed for improving the performance of the final materials in different technical applications like adsorptive or catalytic processes. In the present review, besides on the hierarchically porous all-zeolite material, special focus is laid on the preparation of zeolitic composite materials with hierarchical porosity capable to face the demands of industrial application.

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Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

Enzymes were first immobilized on inorganic supports through silane coupling agents over 25 yr ago. Since that initial report, literally hundreds of laboratories have utilized this methodology for the immobilization of enzymes, antigens, antibodies, receptors, and other high and low mol wt compounds. Today silane coupling is one of the commonly used techniques in the arsenal of the biochemist for the binding of material of all sorts to inorganic surfaces. Inorganic materials come in a variety of shapes, sizes, and characteristics. Today silane coupling is one of the most used coupling methods for the preparation of biosensing devices. Sol-gel entrapped enzymes are also produced by the application of silane technology by the polymerization of the silane to form glass-like materials with entrapped protein. This review will discuss the general preparation and characterization of silane coupled proteins with special emphasis on enzymes and describe in detail the actual methods for the silanization and specific chemical coupling of proteins to the silanized carrier.

Preparation of immobilized proteins covalently coupled through silane coupling agents to inorganic supports.

The field of coordination polymers and metal–organic frameworks has to date focused on the crystalline state. More than 60,000 crystalline metal–organic framework structures, formed from highly ordered arrays of metal nodes connected by organic ligands in at least one dimension, have been identified. However, interest in non-crystalline systems is growing, with amorphous solids, glasses and liquids identified as possessing similar metal–ligand bonding motifs to their crystalline cousins. In this Review, we provide an overview of the structural design, properties and potential applications of non-crystalline coordination polymers and metal–organic frameworks. In particular, we highlight recent reports of glasses that result from the melt quenching of the liquid states of these topical classes of materials. Finally, we provide a perspective on the future of the non-crystalline domain of coordination polymers and metal–organic frameworks. There is increasing interest in the liquid, glass and amorphous solid states of coordination polymers and metal–organic frameworks. In this Review, we discuss the background and terminology of this emerging field, categorize example structures and provide an outlook for the future direction of the field.

Liquid, glass and amorphous solid states of coordination polymers and metal–organic frameworks

Porous glasses in the 21st century––a short review

The NMR pulsed field gradient technique was used to study molecular transport in porous glasses. The adsorbent materials were produced by leaching phase-separated sodium-boron-silica glasses of different composition, then heat-treating. The pore diameters of the glass samples produced were within the interval 0.8 to 50 nm. Water, methanol, dodecane, and do-decene were used as adsorbates. The coefficients of adsorbate self-diffusion were found to decrease with decreasing pore diameter. In comparison with the neat liquids, a reduction in the adsorbate mobility (up to two orders of magnitude) was observed. For the larger pores, that decrease may be attributed to the tortuosity of the adsorbent, whereas the low diffusivities in the porous glasses with small pores are a consequence of the stabilizing effect of the rigid adsorbent framework.

NMR Study of Adsorbate Self‐Diffusion in Porous Glasses

The structural and textural properties of “Two-Phase Porous Silica” consisting of a mesoporous pore system formed by finely dispersed silica-gel inside the original pores of a macroporous glass framework have been carefully investigated. Nitrogen adsorption at 77 K, Mercury Intrusion, Small Angle X-ray Scattering (SAXS), Temperature-Programmed Desorption (TPD) of ammonia and 27Al MAS NMR spectroscopy were used. Different mesoporous glasses investigated show BET surface areas up to 300 m2/g, a total pore volume of about 0.16 cm3/g and pore sizes of <2–10 nm, depending on the acid leaching conditions of the phase-separated sodium borosilicate initial glass. The texture of the macroporous “frame” glass was characterized after removing the mesoporous finely dispersed silica-gel phase with alkaline solution. Leaching of the phase-separated initial glass with Al(NO3)3 solution and followed by thermal treatment lead to the formation of Bronsted acid sites at the surface of the resulting mesoporous glasses by an incorporation of aluminium in tetrahedral coordination in the silica framework.

Two-phase porous silica: Mesopores inside controlled pore glasses

A porous VYCOR-glass of porosity c ≈ 30% was analyzed by use of nitrogen adsorption (NA), mercury intrusion (MI) and small-angle scattering (SAS). The distribution density of the pore diameter was determined from the SAS experiment, based on the stereological information for a fixed order range L = 40 nm. A pore can be described by use of two random variables, which depend on each other: The pore diameter d and the chord length l. In a first step, an assumption free data evaluation method yields the second derivative of the SAS correlation function γ″(r). Then, based on the intimate connection between γ″(r) with random chord lengths, an interpretation of the first two mean γ″ peaks was performed. These peaks reflect the chord length distributions of pore and wall. The problem of the allocation of the peaks has been solved based on the information of the NA and MI experiments. The transformation of the distribution densities of the pore diameters V
M(d) (obtained by MI a experiment) and V
N(d) (obtained by a MI experiment) into chord length distribution densities A
M(l) and A
N(l) have allowed the clear interpretation of γ″(r). It was possible to separate the chord distributions of the pores from those of the walls. The first γ″(r) peak reflects the chord length distribution density Φ(l) of the pores (first moment l¯ = 10.6 nm) and the second one that of the walls f(m) (first moment m¯ = 21 nm). It follows c ≈ 30%. The average mean chord length is d¯
lm ≈ 15 nm. The second moment of Φ(l) is 108 nm2. Finally, from the separated function Φ(l), the diameter distribution density of the pores V
SAS(d) has been obtained. V
SAS(d) was calculated, neither assuming a defined mathematical function type of the distribution nor a certain shape or dimension of the pore. The first and second moments of V
SAS(d) are 7 nm and 74 nm2. From comparing the three distribution densities V
SAS(d), V
M(d) and V
N(d) it can be concluded that the assumption of cylindrical pores is fulfilled. While the chord length distribution density of the walls is a highly symmetrical function, which can be approximated by a Gauss term, the pores have an unsymmetrical chord distribution density with the PVG.

Pore Size Distribution and Chord Length Distribution of Porous VYCOR Glass (PVG)

During silylation of porous glass with 3-aminopropyltriethoxysilane, the properties of the carrier affected the concentration of bound amino groups, the formation of aminopolysiloxane mono- or multilayer on the surface, and the hydrolytic stability of the layer formed. The influence of the carrier depended on the specific surface area and the size of pores. In contrast to silylation performed in organic solvents, in aqueous solutions a monolayer of aminopolysiloxane of high hydrolytic stability was formed on the surface of porous glass. Tetraethoxysilane modification of porous glass prior to silylation with aminosilane yields carriers of increased hydrolytic stability. Glucoamylase immobilised on carriers, that were modified in aqueous solutions, exhibit higher enzymatic activity.

F. Janowski

Papers

Porous glasses in the 21st century––a short review

NMR Study of Adsorbate Self‐Diffusion in Porous Glasses

Two-phase porous silica: Mesopores inside controlled pore glasses

Pore Size Distribution and Chord Length Distribution of Porous VYCOR Glass (PVG)

Aminopropylsilane treatment for the surface of porous glasses suitable for enzyme immobilisation.