Miniaturization, chemical sensors

The most frequently used calibration procedure is based on temperature dependence of pressure of saturated mercury vapour [19,39-41]. At 25°C this pressure is of 0.0018 mm Hg height it corresponds to the vapour density of 20 pg/1. To get in the measurement cell a mercury concentration of about 10 ng/1, the saturated vapour should be strongly diluted. Instead of dilution, a lower temperature can be used however, the density of saturated vapour of 10 ng/1 corresponds to the temperature of less than —40°C. Both dilution and temperature decrease can be realized easily in laboratory conditions but their incorporation into a miniaturized chemical sensor is rather complicated. An attempt to develop such a device is reported in Ref. [41]. An additional problem in application of these techniques in portable sensing devices with integrated calibration is the necessity to have a reservoir with mercury in the device it complicates recycling of these devices and does not correspond to modern trends in technology. 

The general tendency to miniaturize chemical sensors and to make them more convenient has led to the developement of potentiometric sensors with solid internal contact. These sensors include coated wire electrodes (CWE), hybrid sensors and ion sensitive field-effect transistors (ISFET). The contact can be a conductor, semiconductor or even an insulator. The price which has to be paid for the convenience of these sensors is in the more restrictive design rules which have to be followed in order to obtain a sensor with the performance comparable to the conventional symmetrical ion-selective electrode. 

Forster RJ (1998) Miniaturized chemical sensors. In Diamond D (ed) Principles of chemical and biological sensors. 

Epstein AJ, Mele EJ (eds) (2001) Symposium proceedings on synthetic meteils, 4-5 May 2(XX). Synth Met 125 138 Eorster RJ (1998) Miniaturized chemical sensors. In Diamond D (ed) Prindples of chemical and biological sensors. Wiley, New York, NY, p 243... 

Optical fibres can be used as single fibres or in the form of fibre bundles. Fibre bundles often are used as carriers for chemical receptor layers in optodes. Single fibres become increasingly meaningful for miniature chemical sensors. 

One of the new trends in chemical analysis appeared in the last decade is that the miniaturization. It becomes apparent in the miniaturization of analytical devices, separation procedures, measuring tools, analyzing samples and as a consequent the term micro have appeared. Further development of this trend have led to transfer from the term micro to nano one (nanoparticles, nanofluides, nanoprobes, nanoelectrodes, nanotubes, nanoscale, nanobarcode, nanoelectrospray, nanoreactors, etc). Thereupon a nanoscale films produced by Langmuir-Blodgett (LB) technique are proposed for modifying of chemical sensors. 

Boisde G., Perez J.J., Miniature chemical optical fiber sensors for pH measurements, Proc. SPIE-Int. Soc. Opt. Eng. 1987 798 238. 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

In recent years, rapid advancements in photonic technologies have significantly enhanced the photonic bio/chemical sensor performance, especially in the areas of (1) interaction between the light and analyte, (2) device miniaturization and multiplexing, and (3) fluidic design and integration. This has led to drastic improvements in sensor sensitivity, enhanced detection limit, advanced fluidic handling capability, lower sample consumption, faster detection time, and lower overall detection cost per measurement. 

Harrison, D. J., Glavina, P. G., and Manz, A. (1993). Towards miniaturized electrophoresis and chemical analysis systems on silicon an alternative to chemical sensors. Sens. Actuators B Chem. BIO, 107-116. 

As regards general technical features, (bio)chemical sensors will foreseeably result in increased automation, simplification and miniaturization, three of their inherent attributes (Fig. 1.8). Operationally, those features assuring quality in the results (Fig. 1.16.B), as well as others including durability, expeditiousness, robustness, cost-effectiveness, compatibility with complex samples, and ease of incorporation into portable analytical systems... 

One of the most topical ways of approaching this type of system, where separation and detection take place sequentially in space and time, to current trends in Science and Technology (e.g. automation and miniaturization) involves integrating both steps. Integrated systems of this type meet the requirements of chemical sensors [7,8] and differ clearly from conventional flow systems, where detection and mass transfer take place at different locations in the continuous configuration. In fact, the characteristic mass transfer of separation techniques takes place simultaneously with detection... 

It is safe to say that any conventional spectrophotometric and colorimetric analysis can be performed in an optical sensing format. That makes the optical sensors probably the most universal type of chemical sensors. Miniaturization of optical components and rapid advances in the development of new optical materials and hardware support this fast-growing area of chemical sensing. 

The number and types of sensors that are available for most sensing situations are very large the computational capacity is abundant and cheap. Miniaturization is a strong trend in the chemical sensor field. The stage is then set for extraction of information from data by means of computational multivariate analysis. 

Significant advances have occurred during the past decade to miniaturize the size of the measurement system in order to make online analysis economically feasible and to reduce the time delays that often are present in analyzers. Recently, chemical sensors have been placed on microchips, even those requiring multiple physical, chemical, and biochemical steps (such as electrophoresis) in the analysis. This device has been called lab-on-a-chip. The measurements of chemical composition can be direct or indirect, the latter case referring to applications where some property of the process stream is measured (such as refractive index) and then related to composition of a particular component. 

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