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Optical sensors   Print  E-mail
Written by Editor  
Tuesday, 23 March 2004

The idea behind the sol-gel optical sensors (optodes) is based on changes of optical parameters of active (sensing) molecules physically entrapped in (or covalently bound to) porous sol-gel thin films. Those changes are induced by changing external physico-chemical parameters such as temperature, hydrostatic pressure or presence of analyte molecules or, for example, bacteria. There are several kinds of optical signals which could be used as analytical response of such optodes, for instance: intensity of light absorbed or emitted by the sensing molecules, time of luminescence decay or changes in vibrational spectra.

Sol-gel matrices posses good optical characteristics (transparency and high refractive indexes), mechanical stability, high surface area and high porosity which enable the entrapped molecules to interact with the surrounding medium. Other characteristics of sol-gel materials are: the simplicity of the process, the facility of entrapment of sensing agent, the chemical inertness, photochemical and thermal stability. These materials are very attractive for immobilization of sensing molecules. Pore size distribution can be controlled by changing parameters of the synthesis process, the hydrophobicity may be adjusted by selection of the alkoxide and the indicators can be covalently bound to the matrix to prevent leaking.

All these advantages cause that many research groups are interested in obtaining sensors by this method. Much attention is devoted to pH, gas and solvent sensors. But there can be also found information about sensors estimating many others organic and inorganic compounds or physical parameters (for example: metal ions, glucose, hydrazine, air moisture or temperature).

We have described optode for ammonia that utilize absorption as the information carrier. It is based on pH depended color changes of rosolic acid (or bromophenol blue). The silica layers, obtained from organosilicates precursors, were coated onto a glass slide and onto an unclad fragment in the central part of silica fiber. The absorption spectrum changes if the ammonia solution is added to the water in which the coated glass is immersed or the optical fiber is placed in gaseous atmosphere of pure nitrogen with ammonia. The signal is stabilized after few minutes. The process is fully reversible, replacing the ammonia solution with clean water results in a return to the original spectrum of the dye. The gaseous optode may be used even for detection of ammonia traces.

Sensors can use the effect of indicator luminescence. Optical temperature measure­ment system is performed by monitoring the temperature-induced changes of the fluorescence intensity of the tris(2,2’­bipyridine)dichloro­ruthenium(II) ([Ru(bpy)3]2+). This dye was chosen because of its good stability, highly emissive metal-to-ligand charge transfer state, long lifetime and strong absorption in the blue-green region. In this case, the optode can have either planar or fiber-optical form. In general, the complex emission becomes more intense with lowering of the tempera­ture. The optical sensor’s response is fairly linear in the range 100 -- 330 K, fast (the response time below 1 min) and fully reversible, therefore the continuous temperature monitoring is possible.

In this optode, the effect of luminescence decay can be also used as the analytical response signal. Comparison of the two general analytical methods (intensity measurements and excited electronic states lifetime measurements) shows that while the first one is simple and less expensive, it offers less stable and reliable results than the second one. When higher precision and accuracy are needed, the technique based on emission decay data is preferable. The advantages of this method are: (a) leaching and decomposition of the indicator does not change the response, (b) it is insensitive to changes in the source and detector efficiencies, (c) excited electronic states lifetimes of a luminescent molecule are often much more sensitive to environmental changes than shifts/intensities of electronic absorption and emission bands.

The luminescence biosensor based on detection of changes of depolarization of luminescence Optodes can be based on detection of changes of polarization properties of the sensing molecules bound to sol-gel matrices. For example, it is possible to covalently bind large, biological molecules labeled with luminophores to the surface of a sol-gel thin film in such a way that the macromolecule retains partial rotational freedom. If some other molecule recognizes and binds to the labeled biomolecule its rotation can be hampered what leads to changes of the luminophore emission polarization anisotropy. Silicate thin films with terminal amino groups have been prepared via the sol-gel technique in order to covalently attach biological molecules labeled with luminophores to their surfaces. The obtained thin films with covalently bound Concanavalin-A (Con-A) labeled with fluorescein (FITC) were immersed in a buffer and polarized emission spectra were measured. In the next step, the samples were incubated with a lipopolysacharide (LPS) solution and the spectra were collected again. The influence of LPS binding to Con-A on the luminescence depolarization has been proved. The system based on this effect can be employed as a luminescent sensor for detection of certain bacteria. One of the advantages of the sol-gel materials is their very good adhesion to glass and other silica substrates. The glass plates and optical fibers can be easily coated by sol-gel layer with immobilized indicators. The application of optical fibers causes bigger usability of the sensors. The light that excite molecules can be easily introduced to the sensing elements and simultaneously the analytical signal can be send to a detector. The size of complete optical system may be diminished and the very small contact tip can be placed in not easily accessible places. Therefore, we work out the system in which the active sol-gel layer is placed on the tip of the fiber.

Sol-Gel Materials and Nanotechnology Center of Excellence
Institute of Materials Science and Applied Mechanics
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