Sensors conductometric chemical

Kriz D, Kempe M, Mosbach K. Introduction of molecularly imprinted polymers as recognition elements in conductometric chemical sensors. Sens Actual B 1996 33 178-181. 

Solid-state electrochemistry, as a subsection of electrochemistry, emphasizes phenomena in which the properties of sohds play a dominant role. This includes phenomena involving ionically and/or electronically conducting phases (e.g., in potentiometric or conductometric chemical sensors). As far as classical electrochemical cells are concerned, one refers not only to all-solid-state cells with sohd electrolytes (e.g., ceramic fuel cells), but also to cells with hquid electrolytes, such as modern Li-based batteries in which the storage within the sohd electrode is crucial [1-3]. 

E.S. Forzani, X. Li, and N. Tao, Hybrid amperometric and conductometric chemical sensor based on conducting polymer nanojunctions. Anal. Chem., 79, 5217 5224 (2007). 

Kriz, D. Kempe, M. Mosbach, K. Introduction of Molecularly Imprinted Polymers as Recognition Elements in Conductometric Chemical Sensors. Sens. Actuators B 1996, 33, 178-181. 

Metal oxides are among the most used active materials for conductometric chemical sensors. They have a wide variety of electrical properties spanning from insulator to quasi metallic behavior. The discovery of their sensing properties was made more than five decades ago, thereafter the interest of researchers was focused on nanostructured materials. These materials may give a greater modulation of the electrical properties for the interaction with the surrounding atmosphere thanks to the higher surface to volume ratio. 

Oxides are normally stable at the operating temperatures necessary to enhance the interaction between their surface and the gas phase, much more stable compared to organic materials. They are normally operated between 500 and 800 K where the conduction is electronic and oxygen vacancies are doubly ionized. Different oxides have been proposed for conductometric chemical sensors, the most studied is by far tin dioxide that has also been commercialized in form of thick film sensors. Other oxides studied are titanium oxide, tungsten oxide, zinc oxide, indium oxide and iron oxide, first in form of thick and then in form of thin films. Furthermore, the use of mixed oxides, as well as the addition of noble metals, has been studied to improve not only selectivity but also stability. 

For the preparation of conductometric chemical sensors the nanowires electrical properties have to be monitored during the exposure to gases, therefore electrical contacts have to be deposited over the nanowires bundles or the single nanowire. Moreover the nanowires have to be kept at the desired working temperature for the target chemical species. The most used substrates are alumina or silicon. For the latter since the substrate is not insulating an additional layer is needed to measure the electrical properties of the oxide layer. The simplest configuration is achieved with... 

This method is primarily based on measurement of the electrical conductance of a solution from which, by previous calibration, the analyte concentration can be derived. The technique can be used if desired to follow a chemical reaction, e.g., for kinetic analysis or a reaction going to completion (e.g., a titration), as in the latter instance, which is a conductometric titration, the stoichiometry of the reaction forms the basis of the analysis and the conductometry, as a mere sensor, does not need calibration but is only required to be sufficiently selective. 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

Chapter 10 deals with composite films synthesized by the physical vapor deposition method. These films consist of dielectric matrix containing metal or semiconductor (M/SC) nanoparticles. The film structure is considered and discussed in relation to the mechanism of their formation. Some models of nucleation and growth of M/SC nanoparticles in dielectric matrix are presented. The properties of films including dark and photo-induced conductivity, conductometric sensor properties, dielectric characteristics, and catalytic activity as well as their dependence on film structure are discussed. There is special focus on the physical and chemical effects caused by the interaction of M/SC nanoparticles with the environment and charge transfer between nanoparticles in the matrix. 

Physical and chemical properties of PVD-produced composite films with M/SC nanoparticles including dark- and photo-induced conductivity, conductometric sensoring properties, dielectric characteristics, and catalytic activity. 

Experimental data relating to the conductivity of composite films with M/SC nanoparticles are described by the classical percolation model in terms of tunnel processes. Chemisorption of chemical compounds on the surface of M/SC nanoparticles in films and the subsequent reactions with participation of chemisorbed molecules change the concentration of conducting electrons and/or barriers for their tunnel transfer between the nanoparticles with the result of strong influence on the film conductivity. Such films are used as conductometric sensors for detecting various substances in an atmosphere. 

Electrochemical transducers work based on either an amperometric, potentio-metric, or conductometric principle. Further, chemically sensitive semiconductors are under development. Commercially available today are sensors for carbohydrates, such as glucose, sucrose, lactose, maltose, galactose, the artificial sweetener NutraSweet, for urea, creatinine, uric acid, lactate, ascorbate, aspirin, alcohol, amino acids and aspartate. The determinations are mainly based on the detection of simple co-substrates and products such as 02, H202, NH3, or C02 [142]. 

From numerous results achieved using combinatorial and high-throughput methods, the most successful have been in the areas of molecular imprinting, polymeric compositions, catalytic metals for field-effect devices, and metal oxides for conductometric sensors. In those materials, the desired selectivity and sensitivity have been achieved by the exploration of multidimensional chemical composition and process parameters space at a previously unavailable level of detail at a fraction of time required for conventional one-at-a-time experiments. These new tools provided the opportunity for the more challenging, yet more rewarding explorations that previously were too time consuming to pursue. 

Schematic representation of the continuous conductometric sensor. Reprinted with permission from S Bruckenstein J S Symanski. Anal Chem 58(1986), 1766. Copyright 1986 American Chemical Society.

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