Optode

Later on, such S-layer-based sensing layers were also used in the development of optical biosensors (optodes), where the electrochemical transduction principle was replaced by an optical one [97] (Fig. 10c). In this approach an oxygen-sensitive fluorescent dye (ruthenium(II) complex) was immobilized on the S-layer in close proximity to the glucose oxidase-sensing layer [97]. The fluorescence of the Ru(II) complex is dynamically quenched by molecular oxygen. Thus, a decrease in the local oxygen pressure as a result of... 

Holst G., Glud R. N., Kuehl M., Klimant I., A micro-optode array for fine-scale measurement of oxygen distribution, Sensor Actuat B-Chem 1997 B38 122, and refs, cited. 

Lubbers D.W., Opitz N., The pC02-/p02 optode, a new probe for measurement of partial pressure of carbon dioxide or partial pressure of oxygen in fluids and gases, Z. Naturforsch. 1975 30C 532. 

Morf W.E., Seiler K., Lehmann B., Behringer C., Hartman K., Simon W., Carriers for chemical sensors design features of optical sensors (optodes) based on selective chromoionophores, Pure Appl. Chem. 1989 61 1613. 

Seiler K. and Simon W., Theoretical aspects of bulk optode membranes, Anal. Chim. Acta 1992, 266 73. 

Huber Ch., Werner T., Krause Ch., Wolfbeis O.S., Novel Chloride-Selective Optode Based on Polymer-Stabilized Emulsions Doped with a Lipophilic Fluorescent Polarity-Sensitive Dye. Analyst 1999 124 1617. 

The absorption maximum for the Ca2+-complexed form of KBC-002 is observed at 550 nm (Figure 24a). When Ca2+ measurements were performed at a pH of 9.0 using the optode membrane, a dynamic response range between 10 pM and 10 mM was observed for the sensors as illustrated by the calibration function shown in Figure 24b. 

Figure 24. (a) Absorption spectra of a calcium-selective optode membrane (DOS/PVC) based on KBC-002 after equilibration with pH-buffered (0.1 M boric acid/0.1 M TMAC1 pH 9.0) calcium solutions and (b) calibration curve for the optode at 550 nm for Ca2+ ( ). 

Kosch U., Klimant I., Werner T., Wolfbeis O.S., Strategies To Design pH Optodes With Luminescence Decay Times In The Microsecond Time Regime, Anal. Chem. 1998 70 3892-3897. 

Seiler K., Wang K., Bakker E., Morf W.E., Rusterholz B., Spichiger U.E., Simon W., Characterization of sodium-selective optode membranes based on neutral ionophores and assay of sodium in plasma, Clinical Chemistry 1991 37 1350-1355. 

Oehme I. et al., LED-compatible copper(II)-selective optode membrane based on lipophilized Zincon, Fresenius Journal of Analytical Chemistry 1994 350 563. 

As it is shown in Figure 8, the enzyme can be immobilized in the vicinity (membrane, beads, etc) or on the surface of the fiber for optical fiber sensor development. Alternatively, it can be placed in a reactor and use the optode... 

Hobel W., Papperger A., Polster J., Penicillinase optodes-substrate determinations using batch, continuous-flow and flow injection analysis operation conditions, Biosens. Biolectron. 1992 7 549-557. 

Applications exploiting porous silica to encapsulate sensor molecules, enzymes and many other compounds are developing rapidly. Nowadays, sol-gel technology is being used in various fields of modem technology, as for example the basis for optodes, integrated systems, fiber optics, lasers, and new materials. 

The optode transduces the non-optical signal from the environment to the optical one, readable by the photodetector. Various indirect optical sensors and theirs applications are described in literature35. The optode can work as a chemical sensor that detects certain analytes in aqueous solutions or in air on chemical way. It means that changes in the environment cause the changes in the photosensitive material, which is immobilized in the optode matrix. These chemical changes influence the observed light intensity (for example, due to absorption) or one can analyze the intensity or time decay of luminescence. There are numbers of publications devoted to the family of optical chemical sensors36. 

Construction of the optode for optical biosensor requires immobilization of sensitive compounds in the host matrix. There are several methods enabling molecules entrapment. One can use gels, polymers, saccharose, various meshes and membranes78. In case of fiberoptic indirect sensors optode must be attached to the fiber tip. Nowadays, there are two commonly used optode host materials sol-gel materials and polymers. 

Silica gels seem to be ideal materials for construction of optodes for indirect fiberoptic sensors. Their visible transparency, porosity enabling the... 

In order to act as a transducer optode must be attached to the optical fiber. Bulks (>1 mm) of sol-gel matrix can be easily glued to the fiber tip, especially, if the polymer fiber is used39. The smaller optodes can be attached to fiber end by dip-coating method or simply by direct painting of the fiber-tip with a liquid gel. 

Depending on the optode dimension two constructions are possible micro- or macrooptodes. If the optode diameter is comparable with the diameter of optical fiber or bigger, it is defined as a macrooptode. If it is smaller, then we have a microoptode construction. Typical sizes of macrooptodes vary from 125 pm up to 2 mm and of microoptodes from 50 pm down to 25 pm or even smaller. 

The most common shape of the sensor fiber tip is the flat one. It is easy to prepare, since only a fiber cutter is needed. However, the bounding of optode is not very robust and it can be destroyed mechanically. 

The way of enzyme entrapment has been described79 proposing the application of sol-gel matrices. The optodes of urea sensor were prepared by the sol-gel method and were stored in a refrigerator. As the pH sensitive dye the bromothymol blue was used. Since it is best acting in pH range 6 to 7.6, the pH of sol-gel bulks obtained in the experiment was chosen as pH 6. Before measurements, the optodes were incubated in the temperature 36.6°C. 

Andrzejewski D., Podbielska H., Examination of various shape of sol-gel optodes for indirect fiberoptic sensors, OPTIK 112, 158-162, 2001. 

Koncki R., Mohr G., Wolfbeis O.S., Enzyme sensor for urea based on novel pH bulk optode membrane, Biosens. Bioelectr. 1995 10 653-659. 

Figure 2. Optode with phenol red bound to polyacrylamide microsphere for blood pH monitoring.

All the problems associated to an invasive application are clearly avoided in a system working in an extracorporeal blood circuit, developed by CDI-3M, which has been commercially available since 1984 (now distributed by Terumo)21. A disposable probe which uses the same chemistry as the previously described intravascular optode, is inserted on line in the blood circuit on one side and connected to the fibre bundle on the other (see chapter 2, Figure 5). 

Other optodes have been developed and tested in-vivo, all of them using a fluorophore, the fluorescence of which is quenched by oxygen. In the intravascular sensor developed by CDI, previously described, a specially synthesised fluorophore, a modified decacyclene ( Lexc=385 nm, em=515 nm), is combined with a second reference-fluorophore that is insensitive to oxygen, and is incorporated into a hydrophobic silicon membrane that is permeable to oxygen. 

Ion-selective electrodes are now well understood in terms of the underlying theory, and this has made it possible for new sensing principles to emerge that make use of the thousands of chemical receptors originally developed for ion-selective electrodes. One is the field of optical sensors, which has not been discussed here because it is outside the focus of this chapter. Such so-called bulk optodes do not require electrical connectivity between the sensing and detection unit and are therefore more easily brought into various shapes and sizes, including particle formats, which suit the need of modem chemical analysis. 

E. Bakker, P. Buhlmann, and E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics. Chem. Rev. 97, 3083-3132 (1997). 

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