Bead chemical sensors

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. 

An understanding of ligand-substrate interactions is important to the design of polymer -supported reagents for the selective comple-xation of a single species in a multi-component solution. Such polymers may be applied to the environmental separation of ions and molecules as well as in chemical sensors and as novel catalysts. A series of polymers has been synthesized where substrate selectivity arises through the polymer s bifunctionality. The dual inechanimii bifunctional polyn rs are described and applied as both polymer beads and membranes for the selective complexation of metal ions and molecules. 

Tlie basic study of intermolecular interactions is facihtated by one-bead-one-stRicture libraries which can be powerful tools for the discovery of hgands to synthetic receptors and vice versa. Encoded combinatorial libraries have been useful for disclosing ligands for well-designed macrocyclic host molecules and to elucidate their specificities for peptide sequences. These studies led via receptors with more flexibility to simple host molecules without elaborate design that ai e accessible to combinatorial synthesis. One application is the development of chemical sensors for analytes that are otherwise difficult to detect or only non-specificaUy detected. Such hbraries have been used to find new catalysts and enzyme mimics. 

In 1976, Peterson et al. [20] originated the development of the first fiber optic chemical sensor for physiological pH measurement. The basic idea was to contain a reversible color-changing indicator at the end of a pair of optical fibers. The indicator, phenol red, was covalently bound to a hydrophilic polymer in the form of water-permeable microbeads. This technique stabilized the indicator concentration. The indicator beads were contained in a sealed hydrogen-ion-permeable envelope made out of a hollow cellulose tubing. In effect, this formed a miniature spectrophotometric cell at the end of the fibers and represented an early prototype of a fiber optic chemical sensor. 

Inorganic nanoshell-coated organic polystyrene beads with well-defined nanostructures are attractive because of their applications in the fields of SERS, catalysis, biochemistry, and chemical sensors. The core-shell type composite materials are in the frontier of advanced research, in which the shell component determines the surface properties and the core component indirectly induces the other properties of the system. Bimetallic nanosheUs on functionalized polystyrene beads have been fabricated through a layer-by-layer deposition pathway involving the electrostatic interaction of the polystyrene moiety. 

Major Applications pH Sensors, optical chemical sensors, biochemical sensors, biosensors," fluorescent pH detector system, measuring fluorescence lifetime in cells, determining concentration of a laminar sample stream, fluorescent reporter beads for fluid analysis, measuring ch ical analytes, intracellular pH in human sperms, " multidrug resistance," recording intramitochon-drial pH, fluorescent probes ... 

The main important details of these procedures for AA determination are contained in Table 18.8. As can be seen in Table 18.8, among these flow analysis techniques, stopped-flow procedures are the most applied to AA determination, being mostly kinetic enzymatic determinations. BI methodologies for AA determination use spectrophotometric detection and a commercial flow cell, which is filled with appropriate solid beads, works as a flow-through chemical sensor integrating online reaction, retention, and detection on the solid-phase disposable beads. 

Figure 5. Schematic depiction of a self-encoded bead array. A mixture of three sensor types fills the fiber tip wells randomly. The sensors are identified by their characteristic responses to a test vapor pulse. Reprinted with permission from ref. 9b. Copyright 1999 American Chemical Society.

Figure 8. Seventy-six sensor beads (Jupiter C4/Nile Red) monitored to show that the average responses for three consecutive 0.38-s exposures of 50% saturated vapor levels result in reproducible and high-speed response profiles. The sensors are positioned on the distal tip of an optical imaging fiber and relative analyte concentrations are 0.5 and 18700 ppm for 1,3-DNB and toluene, respectively. Reprinted with permission from ref 12a. Copyright 2000 American Chemical Society.

Andersson, H., van der Wijngaart, W., Enoksson, P., Stemme, G., Micromachined flow-through filter-chamber for chemical reactions on beads, Sensors Actuators B 2000, 67, 203-208. 

M. Shikida, M. Koyama, N. Nagao, R. Imai, H. Honda, M. Okoehi, H. Tsnehiya, and K. Sato Agitation of magnetie beads by mnlti-layered flat coils. Sensors and Actnators B-Chemical 137, 774-780 (2009). 

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