Fluorescent sensors optode

Membrane-covered optochemical sensors (optodes) with O2 sensitive or pH sensitive fluorescence indicators (e.g. pyrene butyric acid or hydroxypyrene trisulfonic acid) have been coupled with different enzyme reactions, such as the conversion of glucose, lactate, ethanol, or xanthine, and with antigen-antibody couples (Opitz and Lubbers, 1987). 

Opitz, N., Lubbers, D. W., Electrochromic Dyes, Enzyme Reactions and Hormone-Protein Interactions in Fluorescence Optic Sensor (Optode) Technology , Thlanta 35 (1988) 123-128. 

Figure 4 Schematic cross-section of an optochemical sensor (optode) for glucose based on indicator fluorescence quenching by Oa-...

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. 

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... 

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]. 

Recently, GOD was immobilized on the tip of a fluorescence pH sensitive optode (Trettnak et al., 1989). This sensor responded to 0.1-2 mmol/1 glucose within 10 min. 

Optodes provided with non-fluorescent esters of fluorophores have been used for the determination of external enzyme activities. The fluorophores are liberated by the enzymes and then seen by the optical Ober [214], As ecamples of p(02)-modulated optical biosensors, a glucose probe [213] and an ethanol probe [216] can be mentioned sensors based on glucose, alcohol, and other oxidases were reviewed by Opitz and Lttbbers [217]. The advantages of these 02-dependent optical biosensors are that, unlike corresponding amperometric sensors, they do not consume O2 and that they are strictly diffusion limited in their response. Fiber-optical devices are also available for the determination of substrates of dehydrogenases the NADH fluorescence produced by the immobilized enzyme is measured as a function of time [218, 219]. 

A potassium opto-sensor was recently described [75] for the continuous determination of electrolytes. Certain fluorescent dyes respond to an electric potential at the interface between the aqueous and lipid phases. This potential is created by the neutral ion carrier. The lipid layer is formed on a glass support by the Langmuir-Blodgett thin-fllm technique. This layer incorporates Rhodamine B as a dye and valinomycin as the carrier. The lipid membrane is made of arachidic acid. The fluorescence intensity decreases when this layer is exposed to potassium ions (linearity between 0.01 and 10 mM). This optode is also sensitive to sodium ions [76]. The selectivity factor of potassium in comparison with sodium ions varies from 10 - to 10 , and in relation to ammonium ions by 10. Interferences can be compensated for by a reference optode. However, better selectivity is obtained with new lipid membrane compositions (octadecan-l-ol-valinomycin) [77]. Tetralayers (Figure 17-9) give a maximum response for K". The K /Na selectivity is about 10 in a wide range (0.01-100 mM). 

Three methods have been described for three halogens, two based on fluorescence and one on absorption. In the first [87], the fluorescence of rubrene in polystyrene is quenched by traces of iodine. This method is nonselective and the optode is also sensitive to oxygen. In another sensor, naphthoflavone in solution in a material of the silicone or PVC type serves as a sensitive layer for free halides [88]. The absorption technique uses a fiber with a liquid CS2 core [89] to detect 10 ng of iodide using a S m long capillary cell with sample circulation. The Hber itself constitutes the active optode (total reflection in the liquid core). A comparison of optodes based on dynamic quenching of absorbed Rhodamine 6G by iodide was reported [90]. Three solid supports for immobilization were used PTFE tape, XAD resin beads and crushed XAD-4 resin. The limits of detection are 0.18-0.30 and 1.1 mM respectively. Some anions (eg. Cl , Br , CN ) interfere at the 1-M level. 

Many of the optodes referred to here employ sensors operating on the basis of the 3R scheme (see Section 13.1), the relay mechanism being photoinduced electron transfer, PET. Due to their applicability in various chemical and biological processes, they have received much attention in recent years [1, 7, 8, 10]. Of note in this context are sensors that become fluorescent upon complexation of an analyte because the binding of the analyte within the sensor prevents the PET that suppresses fluoresence in the absence of the analyte [38]. Anthryl aza-crown-ca-lix[4]arene, a iC-selective sensor (see Chart 13.4), exhibits such behavior. It selectively binds potassium ions, and this triggers a substantial increase in anthryl fluorescence through disruption of the PET quenching process [9, 39]. 

Along with simple optical sensors that measure transmission, so-called optodes [169] play an important role. In these, either the color change of an indicator dye in a membrane is monitored by using diffuse reflection [170], or the effect of an analyte on the fluorescence spectrum is observed. In addition. many other effects are used, such as surface... 

However, colorimetric sensors have at least one disadvantage with respect to other analytical methods (electrochemical, potentiometric, etc.), namely, they are generally characterized by low sensitivity. However, lower limits of detection can be obtained using fluorescent- or luminescent-based approaches. Therefore, in an effort to improve the sensitivity of an optode, fluorophores are often coupled to the substrate binding subunit. Using this strategy, it is often possible to obtain sensors that produce both colorimetric and luminescent responses that allow analyte detection via instrumental analysis, as well as in certain cases via the naked eye. Currently, the number of colorimetric sensors for lanthanide ions is still limited. Reviewed below, is recent progress in this area. 

Optodes for anion determination are less common than those for cations, hi Table 8.3, the active agent of the chloride optode is silver fluoresceine, which itself does not fluoresce. By interaction with dissolved chloride, silver chloride is formed, which enhances fluorescence significantly. The controlling precipitation reaction is slow. Hence, the reversible response of the sensor cannot be expected. A similar mechanism operates also with the sulphate-selective optode Hsted in Table 8.3. 

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