Porous film sensor

T. P. Jones and M. D. Porter, Optical pH sensor based on the chemical modification of a porous film, Anal. Chem. 60, 404-406 (1988). 

Compared to other thin-film-forming processes such as sputtering, evaporation, or chemical vapor deposition (CVD), sol-gel film formation has the advantage that both the composition and microstructure can be controlled on the molecular level. Thus, the films can be tailored for specific applications, for example, dense films for protective or optical applications and porous films for sensors or membranes. 

Cobianu C, Savaniu C, Buiu O, Dascalu D, Zaharescu M, Parlog C, van den Berg A, Pecz B (1997) Tin dioxide sol-gel derived than films deposited on porous silicon. Sensor Actuators B Chem 43 114... 

Drag reduction can be achieved by direct injection of microbubbles through slots or porous skin (193-196) or the generation of hydrogen by electrolysis at the wall (197). The primary parameters, independent of gas type and Reynolds number, appear to be the actual gas flow rate referenced to injector conditions of temperature and pressure (198-200) and the location of the bubbles in the turbulent boundary layer (198,199,201-203). Merkle and Deutsch (196) have provided a comprehensive review on skin friction reduction by microbubble injection. Mahadevan and co-workers (204) postulated that microbubbles like polymer solution destroy turbulence production by selectively increasing the viscosity near the buffer region. They increase the local dynamic viscosity. Pal and co-workers (205) demonstrated that microbubble and polymer solution shear stress statistics as measured by flush moimted hot film sensors are similar at equivalent value of drag reduction. 

Surface reactivity (e.g., acid-base properties, ion-exchange behavior, and ability to selectively complex metal cations) may be modified by appropriate choice of precursor metals, by derivitization with organic ligands, or by reactions of the surface with metal halides, metal alkyls, etc. (See Chapter 10.) Combining microstructure control (demonstrated for silicates) with control of surface reactivity should lead to many appiications for porous films in the areas of sensors and catalysis. 

Yoo HY, Bruckenstein S (2013) A novel quaitz crystal microbalance gas sensor based on porous film coatings. A high sensitivity porous poly (methylmethacrylate) water vapor sensor. Anal Chim Acta 785 98-103. doi 10.1016/j.aca.2013.04.052... 

Composition materials ineluding indieator reagent and bearer have been investigated as sensor materials. It has been found out that nature of beai er (sorbent or polymer film) is of the main signifieanee. Siliea gel, aluminium oxide, porous glass, polyurethane, polyvinylehloride ete have been investigated as beai ers. 

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

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