In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

Electrogenerated chemiluminescence (also called electrochemiluminescence and abbreviated ECL) is the process whereby species generated at electrodes undergo high-energy electron-transfer reactions to form excited states that emit light. The first detailed ECL studies were described by Hercules and Bard et al. in the mid-1960s, although reports concerning light emission during electrolysis date back to the 1920s by Harvey. After about 40 years study, ECL has now become a very powerful analytical technique and been widely used in the areas of, for example, immunoassay, food and water testing, and biowarfare agent detection. ECL has also been successfully exploited as a detector of flow injection analysis (FIA), high-performance liquid chromatography (HPLC), capillary electrophoresis, and micro total analysis (μTAS). Figure 1 illustrates a time line of various events in the development of ECL. A literature survey using SciFinder Scholar reveals that more than 2000 journal articles, book chapters, and patents on various topics of ECL have been published. The overall number of publications, as shown in Figure 2, has increased exponentially over the past 20 years, of which 40–50% were biorelated. Similar amounts of ECL papers could be also found from the Thomson ISI Web of Science as well as † Telephone (601) 266 4716; fax (601) 266 6075; e-mail wujian.miao@ usm.edu. Wujian Miao received his undergraduate diploma in chemistry from Nantong University (Nantong, China) in 1982, his M.Sc. degree in analytical chemistry from Zhongshan University (Guangzhou, China, with Jinyuan Mo) in 1991, and his Ph.D. degree in electrochemistry from Monash University (Melbourne, Australia, with Alan M. Bond) in 2000. He then served as a Research Scientist in CSIRO (Melbourne, Australia), followed by a postdoctoral fellowship at the University of Texas at Austin with Allen J. Bard in 2001. Since 2004 he has served as an assistant professor of chemistry at the University of Southern Mississippi.

Electrogenerated Chemiluminescence and Its Biorelated Applications

Silica aerogel; synthesis, properties and characterization

This review is devoted to the most recent developments (2000–2005) of sol–gel materials at the interface with biology. In the context of bioencapsulation in mineral hosts, novel synthetic approaches have been designed, allowing the immobilization of numerous proteins, enzymes and immune molecules as well as poly-saccharides, phospholipids and nucleic acids. These efforts have led to the development of new biosensors and bioreactors. A similar trend was also observed for whole cell encapsulation, survival periods over several weeks now being achieved. This has opened the possibility of designing hybrid hosts for cell-based biosensing and bioproduction, ultimately allowing the development of artificial organs. Indeed, applications of sol–gel processes are not restricted to bioencapsulation, as demonstrated by recent progress in drug release systems and bioactive materials. Finally, the considerable efforts devoted to the biomimetic elaboration of mineral structures suggest that they might be the key for future development of improved sol–gel materials for bio-applications.

Recent bio-applications of sol–gel materials

Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles, which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by compounding them with different organic polymers or different fiber networks. Recent studies showed that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be readily achieved if proper functional groups are introduced on the surface of silica aerogels and then co-polymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their very open texture, can be an outstanding thermal insulator material for different industrial and aerospace applications.

This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with cost effectiveness of each methodology.

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An overview on silica aerogels synthesis and different mechanical reinforcing strategies

A proof-of-principle study is reported in which bacteria were immobilized within macroporous, supercritically dried silica sol–gel discs and signal induction was demonstrated by aerosolized virus particles Escherischia coli (pET-gfp) bacteria-doped gels were used as an aerosol collector to detect bacteriophage The bacteriophage (105 and 108 plaque forming units/ml) (pfu/ml) were aerosolized through the discs for 10 min, at a flow rate of 175 l/min and aerosol humidity of 70% The discs were then incubated in bacterial growth media for 4 h and green fluorescent protein (GFP) expression monitored The induction of GFP indicated that both bacteriophage and bacteria survived the stressful desiccating conditions of the aerosol challenge Scanning confocal laser microscopic (SCLM) analysis demonstrated that the bacteriophage contacted viable bacteria and induced expression of the GFP in 35–95% of the bacterial cells These findings indicate that virus particles can penetrate the structure of macroporous silica gels and trigger a detectable response in immobilized bacteria The goal is to use microorganisms immobilized within these materials to facilitate the detection of chemicals and organisms within the environment

Aerogels as biosensors: viral particle detection by bacteria immobilized on large pore aerogel

Aerogels have an extraordinarily large internal surface area which is accessible via open pores, making them candidates for filters and gas adsorption media. The idea is frequently included in lists of potential applications, but little research has been conducted on the use of aerogels as filters or adsorption media. Consideration of aerogels for use as aerosol collection media requires knowledge of the behavior of gas flow through them. Nitrogen and helium gases at a range of pressures from 6–200 kPa were flowed through aerogel and the flow was shown to be a superposition of molecular diffusion and laminar viscous flow.

Gas flow through aerogels

A novel, high hydrolysis ratio sol-gel route for the biocompatible production of macroporous silica gels is presented. This route exploits the two step nature of the gelation reaction to remove undesired alcohol by-products from an acidic aqueous sol prior to gelation. These alcohol-free sols will gel when the pH is raised to the physiologic range in a two-step, acid/base catalyzed process. Furthermore, monolithic macroporous samples can be produced in a controlled manner by introducing water-soluble organic polymers into the sol.

/pdf/cells-in-sol-gels-i-a-cytocompatible-route-for-the-554wcqq6ou.pdf

Cells in sol-gels I : A cytocompatible route for the production of macroporous silica gels

The pore size of macroporous silica aerogel can be controlled by varying the concentration of water-soluble organic polymers in the sol. These gels demonstrate a wide array of mechanical and acoustical properties in proportion to the organic polymer concentration in the sol. Presented in this paper are the resulting mechanical dependencies upon the concentration of high molecular weight polyethylene glycol (PEG) in the initial sol. Physical properties studied include density, surface area, pore structure, acoustic velocity, and mechanical strength. Most of these properties exhibited a large change when a small concentration of PEG was added to the initial sol, correlating with a strengthening of the solid matrix. Still higher concentrations of PEG progressively weakened the solid matrix. Through examination of the gels containing PEG, experiments have shown an inverse relation of acoustic velocity to PEG concentration while density remains relatively constant.

Mechanical and acoustical properties as a function of PEG concentration in macroporous silica gels

Atmospheric Samplers are disclosed using aerogel filter material. The samplers include one or more cascaded impact sample stations with discs of aerogel as the targets. Ambient air carrying particulate matter is introduced into the sampler and the particles are captured by the aerogel in its pore structure. In certain embodiments, the aerogel filter is next transported to a position where it is fragmented by a spray of water. The fragmented aerogel and the captured particles are transported in the fragmenting water from the sampler for analysis. In one embodiment, the sampler is segmented with alternate nozzle and filter segments for easy assembly, sampling and disassembly for analysis of the captured particles in aerogel filters positions below the nozzles.

Bouvard Hosticka

Papers

Aerogels as biosensors: viral particle detection by bacteria immobilized on large pore aerogel

Gas flow through aerogels

Cells in sol-gels I : A cytocompatible route for the production of macroporous silica gels

Mechanical and acoustical properties as a function of PEG concentration in macroporous silica gels

Multi-stage sampler concentrator