Indicators, chemical sensors

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

Even listing all above problems and requirements leading to their solution indicates that development of the method of semiconductor chemical sensors opens a wide research domain. In order to resolve this problems and implement all capabilities of the method of semiconductor sensors there are two ways now the old trial and error approach and approach related to further studies of physical and chemical properties of surface phenomena, reactions and processes underlying this method. It is quite clear that the second approach is more promising in order to obtain semiconductor sensors designed for the use in accurate scientific studies and for practical gas analysis. 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

This section covers early indirect fiber optic chemical sensors (FOCS) for species that cannot be sensed directly but require the use of indicators, probes, labeled biomolecules, or color-forming reactions. 

Seitz W.R., Chemical sensors based on immobilized indicators and fiber optics, CRC Crit. Rev. Anal. Chem. 1988 19 135. 

An integral part of a fibre optic sensor is the light source. Its primary task is to deliver an appropriate light, which possesses such features as an optical power suitable to interact with an analyte or an indicator from the optrode, a wavelength matched to the spectral properties of the sensors in order to obtain the highest sensitivity, and, in dependence on the construction of the sensor, polarisation, short pulse etc. There are many various light sources utilised in the fibre optic chemical sensors. They differ in spectral properties, generated optical power and coherence. 

Fluorescent pH indicators offer much better sensitivity than the classical dyes such as phenolphthalein, thymol blue, etc., based on color change. They are thus widely used in analytical chemistry, bioanalytical chemistry, cellular biology (for measuring intracellular pH), medicine (for monitoring pH and pCC>2 in blood pCC>2 is determined via the bicarbonate couple). Fluorescence microscopy can provide spatial information on pH. Moreover, remote sensing of pH is possible by means of fiber optic chemical sensors. 

D. C. Sundberg, Z. Zhujun, Y. Zhang, M. Wangbai, R. Russell, Z. M. Shakhsher, C. L. Grant, and W. R. Seitz, Poly(vinyl alcohol) as a substrate for indicator immobilization for fiber-optic chemical sensors, Anal. Chem. 61, 202-205 (1989). 

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

The polymer materials not only act as supports for the dye and other necessary additives in the sensing phase, providing protective covering for the transduction element polymers also play various roles in chemical sensors. They provide a compatible environment for the indicator molecules, maintaining or improving the appropriate photophysical features (compared to those observed in homogeneous solution) on which the sensing principle is based. In many cases they collect and concentrate the analyte molecules on sensor surfaces. In addition, the polymer can play an important role in the sensitivity and selectivity of an optical sensor, and its interactions with indicator and analyte molecules influence the analytical performance of the device. 

Furthermore, these detection devices are based on a wet chemical method which possessed no indicator and could not, therefore, be designated as indicator devices. On the other hand, the gas testers mentioned in the telegram had a physico-chemical sensor connected to a dial (see Fig. 35). 

In the last decade, fiber-optic chemical sensors (FOCS), also known as optrodes, have emerged as alternatives to conventional methods of analysis. FOCS development for a particular analyte depends on the availability of reversible indicating schemes to detect the analyte of interest. Typically, the indicating schemes use commercially available colorimetric or fluorometric indicators (e.g. fluorescein to measure pH (1)). However, the utility of these indicators is limited. Furthermore, indicators may not exist for many analytes. Several reviews discuss the scope of this approach (2,3,4). 

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