Surfaces chemical sensing

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

Such approximation is valid when the thickness of the polymeric layer is small compared to die thickness of die crystal, and the measured frequency change is small with respect to the resonant frequency of the unloaded crystal. Mass changes up to 0.05% of die crystal mass commonly meet this approximation. In die absence of molecular specificity, EQCM cannot be used for molecular-level characterization of surfaces. Electrochemical quartz crystal microbalance devices also hold promise for the task of affinity-based chemical sensing, as they allow simultaneous measurements of both tile mass and die current. The principles and capabilities of EQCM have been reviewed (67,68). The combination of EQCM widi scanning electrochemical microscopy has also been reported recently for studying die dissolution and etching of various thin films (69). The recent development of a multichannel quartz crystal microbalance (70), based on arrays of resonators, should further enhance die scope and power of EQCM. 

Vol. 144. Surface-Launched Acoustic Wave Sensors Chemical Sensing and Thin-Film Characterization. By Michael Thompson and David Stone... 

Macrocyclic Compounds in Analytical Chemistry. Edited by Yury A. Zolotov Surface-Launched Acoustic Wave Sensors Chemical Sensing and Thin-Film Characterization. By Michael Thompson and David Stone Modern Isotope Ratio Mass Spectrometry. Edited by T. J. Platzner High Performance Capillary Electrophoresis Theory, Techniques, and Applications. Edited by Morteza G. Khaledi... 

Trouillet, A. Ronot Trioli, C. Veillas, C. Gagnaire, H., Chemical sensing by surface plasmon resonance in a multimode optical fibre, Pure Appl. Opt. 1996, 5, 227 237... 

Polymers are among the most widely used materials for chemical sensing since they are able to collect and concentrate molecules on sensor surfaces by reversible sorption they can be deposited on several substrates as thin adherent films with easy processing techniques they are available in many kinds having different chemical and sorption properties, which can be used to enhance the selectivity of the sensors last but not least, they are of low cost. 

ACN vapor had the most pronounced permutation of the relatively rapid and relatively slow response and recovery kinetics. Such behavior could be due to the combination of physical and chemical adsorption. Physical adsorption effects are typically pronounced with rapid response and recovery kinetics because of the relatively low energies of physical interactions between vapors and the sensing surface. Chemical adsorption effects have much slower recovery kinetics because of the relatively high energies of chemical interactions between vapors and the sensing surface. The recovery from all tested vapors was reversible with the slowest recovery after the exposure to ACN on the order of several hours from the highest tested vapor concentration of 0.1 P/Po-... 

So far, in label-free chemical sensing applications, the interaction of a WGM s evanescent component with an analyte in the ambient or adsorbed on the microresonator s surface has led to the development of two sensing methods. These are monitoring of the WGM resonance frequency shift due to the analyte s change of... 

L. M. Zhang and D. Uttamchandani, "Optical Chemical Sensing Employing Surface Plasmon Resonance," /ectro ici ieffera 23, 1469-1470 (1988). 

Bulk sensors certainly have a role in chemical sensing of explosives, but the subject of this book is the other basic type sensor, one that seeks molecules released from the bulk of the explosive material in an object. We will refer to these as trace chemical sensors. They are sometimes called vapor sensors, but that seems a less accurate description when they are applied to explosive molecules, which may not always be found in a vapor state. As we shall see in Chapter 5, that requires us to understand where and how to look for these molecules. It will become apparent upon a little reflection that the two types of sensors are complementary and are best used in different situations. Furthermore, even when trace sensors are used, in some situations sampling of particles of soil or vegetation or sampling from surfaces may prove to be more productive that vapor sampling. For underwater sources the term vapor sensing is also inappropriate. 

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