Fiber optodes

Numerous applications have been reported in the literature on the use of cell membrane receptors which require the use of ligand bilayer or phospholipid vesicles to maintain functionality, but other immobilization methods were also successfully used. The biosensing of e.g. acetylcholine and cholinergies has been reported with different transducers (ISFETs, interdigitated electrodes with measurements of capacity changes and optical fiber optode with fluorescence detection). 

This method is based on the use of fiber optics, sometimes referred to as fiber optic dynamic light scattering (FODLS) and it allows one to measure at high particle number concentrations. FODLS employs either one or two optical fibers. Alternatively, fiber bundles may be used. The exit port of the optical fiber (optode) is immersed in the sample and the scattered light in the same fiber is detected at a scattering angle of 180° (i.e. backscattering). 

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... 

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. 

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. 

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. 

Wygladacz K, Radu A, Xu C, Qin Y, Bakker E (2005) Fiber-optic microsensor array based on fluorescent bulk optode microspheres for the trace analysis of silver ions. Anal Chem 77 4706 1712... 

From a general point of view, a chemical sensor is a device capable of continuously monitoring the concentration of an analyte. The two main classes are electrochemical sensors and optical chemical sensors. The latter are based on the measurement of changes in an optical quantity refractive index, light scattering, reflectance, absorbance, fluorescence, chemiluminescence, etc. For remote sensing, an optical fiber is used, and the optical sensor is then called an optode because of... 

In the passive mode, the optical device measures the variation in fluorescence characteristics (intensity, lifetime, polarization) of an intrinsically fluorescent analyte. The optical device can have different optical configurations involving in most cases an optical fiber (passive optode) (Figure 10.44). 

An optode is a chemical sensor based on an optical fiber. To understand how optodes work, we first need to know a little about refraction of light. 

We can create optical sensors for specific analytes by placing a chemically sensitive layer at the end of the fiber. An optical fiber sensor is called an optode (or opt rode), derived from the words "optical and "electrode. Optodes have been designed to respond to diverse analytes such as sulfites in food, nitric oxide in cells, and explosives in groundwater.12... 

S. L. R. Barker, Y. Zhao, M. A. Marietta, and R. Kopelman, Cellular Applications of a Fiber-Optic Biosensor Based on a Dye-Labeled Guanylate Cyclase, And. Chem. 1999, 71, 2071 M. Kuratli and E. Pretsch, S02-Selective Optodes, AnaL Chem. 1994,66, 85. 

Figure 10. Basic elements for flourescent fiber optic chemical sensors a. Optode Structure Design after Lubbers and Opitz (69) b. Optode Structure Design after Hirshfeld, et al. (71).

Luminescence core-based optodes have been reported as waveguide capillary flow cells in liquid-filled optical fibers [ 103] or with polymers attached to the inner surface of a glass capillary [104],... 

Luminescent evanescent wave-based sensors use optical fibers and planar waveguides [105,106] as fight-guiding structures, and they are more complex than the absorbance ones. However, such optodes have been satisfactorily applied to measure fluorescence of indicators or labels for the measurement of gas molecules, proteins or labeled antigen-antibody interactions as well as directly in solution [24,107] when immobilized in matrices [23,109]. 

Core-based optodes, such as hollow fibers, and direct and coating-based evanescent wave spectroscopic optodes are included in the classification of absorption-type sensors. 

The IWAO proves to be a very promising alternative to the conventional optical sensors, as well as those based on optical fibers such as the conventional flow cell configuration. Fully reversible, reproducible, fast and sensitive bulk optodes are obtained. 

Fiber optic pH optode Peterson, Goldstein, Fitzgerald, and Buckhold14... 

Detection with optodes can be also based on luminescence principles, including emitting or quenching fluorescence of phosphorescence, chemiluminescence, or bioluminescence of conjugates of antibodies or antigens with enzymes. In fluorescent optodes with indicator, the excitation light is introduced into the proximal end of the fiber and travels to the end where it excites immobilized fluorescent indicator. Then some of the isotropically-emitted fluorescence is recaptured by the fiber to a detection system. Many biological molecules show an inherent fluorescence, but... 

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