Active optodes

The techniques most frequently employed are absorption, reflectance and fluorescence, with some attempts to exploit the variation in refractive index of the sensitive polymer acting as the cladding for the fiber. The general principle of semi-active and active optodes (Chapter 12) is applied. The optical spectrum of an optode is modified by chemical action between a reagent (indicator), immobilized on the fiber or an inert support at its end, and the activity of the H ions (Oh )=... 

The pH range of the dyes is narrow (3 pH units). The best performance (accuracy <0.01) is obtained in a narrow zone, generally 0.02 units around the pK, . The accuracy decreases sharply with increasing distance from the pK (the relative intensity curves f(pH) are nonlinear). In the outer zones ( 1.5 pK units), the accuracy hardly exceeds 0.1 pH units. The use of several p J, values with the same indicators requires differentiated spectral responses for the two species present [12, 14, 36). New dyes are under development [58, 59] to broaden the pH range with a single indicator for several pIC values and for wider spectra. Also, the pIC is different in solution and for a semi-active optode as presented in Figure 17-6 [35]. A summary of theoretical pK and pK values for some optodes is given in Thble 17-3. 

Dye Theoretical pA", pK measured with passive optode Semi-active optode References... 

With active and semi-active optodes, the chemical species (S) to be measured (analyte) reacts with a reagent phase (R. This reaction forms another species (SR according to the reaction... 

The main research projects for determining cations are on semi-active optodes. The measurement of trace species (Pu , UO, rare earths) in the environment is dealt with in Chapter 18 and optodes for continuous monitoring (Cu"), U and Pu valencies) in Section 17.3. 

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. 

Today, no active optode is capable of application in industrial environments for process control. Indeed, the required properties such as reversibility, durability, and reliability are very difHcult to obtain. Nevertheless, various approaches have been made in the laboratory, eg, pH measurements in acidic or basic media [34], the detection of uranium in phosphate medium [166], and the continuous measurement of the concentration of vapors in polar organic solvents using blue thermal paper placed in a flow-through cell [153]. Moreover, the features of the flow-injection analysis (FIA) technique [69, 70] are being adapted in the laboratory to process control. 

Optical activity, diethyl and dusopropyl tartrate, 395 Optodes... 

Sensing of chlorine is possible with a phthalocyanine-based optode that is elec-trochemically reset [101]. Also a direct electrochemical Clark-type sensor employing carbon electrodes has been investigated [102]. For this type of sensor, the various types of carbon gave different responses and the edge-plane sites of graphitic electrodes were identified as electrochemically active. Both chlorine reduction and chlorine evolution were studied and the effects of the trichloride anion, Ch", were highlighted. 

It has been widely accepted for many years that the LOD of an ISE in an unbuffered solution is at micromolar level. Interestingly, if a com-plexing agent is added into the sample and the concentration of the free primary ions is significantly lowered, the LOD is reduced sometimes to subnanomolar levels [35]. In addition, if halide ions are added to samples in which a silver-selective electrode is immersed, the electrode shows a decrease in potential indicating lowering of the activity of a silver at the sample/membrane phase boundary [36]. Moreover, ionophore-based optodes showed picomolar detection limits [37], even... 

Fig. 2 Schematic representation of optochemical sensors depending on the arrangement of the optically active reagents, a Sin-face optodes reagents are directly immobilized over a solid support by (1) covalent bonding, (2) adsorption or (3) electrostatic interactions or a waveguide or are trapped in a porous matrix (4). b Bulk optodes reagents are dissolved in a plasticizer (5)...

Fig. 3 Basic composition of ion-selective bulk optodes and extraction mechanisms. This type of sensor follows an extraction equilibrium between the aqueous solution and the membrane and the signal is related to the analyte activity in the aqueous solution...

The membrane used to activate this potassium-selective IWAO [134] consists of a potassium bulk optode based on 0.5 wt % chromoionophore ETH 5294, 1.0 wt% ionophore valinomycin, 0.5 wt% ionic additive potassium tetrakis(4-chlorophenyl)borate (KtpClPB), 31.0 wt % polymer PVC, 67.5 wt % organic solvent and plasticizer bis(2-ethylhexyl)sebacate (DOS) [142], This commercially available optode not only acts as an example of the development of an enhanced ion-selective IWAO, but also serves to validate the previously remarked features, because results can be compared with the ones obtained with membranes of the same composition and thickness in a con-... 

Some lanthanide /3-diketonate complexes (11, M = Pr, Eu, Dy, R = n-Pr, R = f-Bu) have selectivity for further coordination with Cl ions, over F , Br , I , C104, SCN , OH, AcO , HCOs, N03 and SOa ", and are the active component of ion-selective electrodes (ISE) for CG. It was proposed to take advantage of the luminescence in the visible range associated with CH complexation, for the development of specific naked eye optodes (M = Eu) for this ion . A sensitive fiuorescence enhancement system for the determination of terbium was developed and studied. This method was applied to... 

It was mentioned before that fiber-optic chemical and biosensors are broadly classified into two categories extrinsic- and intrinsic-type sensors. In the extrinsic-type sensors, the fiber is acting as a link connecting optical signals to (and from) the active material (medium) positioned at the end of the fiber, such as the Optode case. In the intrinsic-type sensors, the fiber is modified in different ways, through construction of the sensing component, which will be explained next. 

Recently, optodes (optical fiber chemical sensors) have been actively studied due to their inherent characteristics such as immunity to electrical noise, ease of miniaturization and the possibility of real time monitoring and remote sensing. For example, a change in absorption peak with humidity is observed using a Nafion membrane ion exchanged with dye, such as crystal violet305 or tri-phenylcarbinol.306 Similarly, a Nafion membrane loaded with the ruthenium complex [Ru(bpy)2(dhphen)]2 (bpy 2,2 -bipyridine, dhphen 4,7-dihydroxy-1,10-phenanthroline) has been used as an optical pH sensor.307... 

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]. 

Ihble 17-3. values for passive (without immobilization of dyes) and semi-active (active-extrinsic) optodes... 

The mediator present at the front end of the optode must be able to form an optically active compound in an amount which definitely depends on sample concentration. Useful optical effects are either spectral light absorption (formation of coloured products) or luminescence. 

In addition to pH-sensitive optodes, fibre sensors for numerous other analytes are available. Among them, sensors for physiologically active cations as well as for dissolved oxygen are worth mentioning in particular. If the principles of ISEs are to be transmitted to optodes, it is necessary to include a chromophoric group in the receptor layer. Commonly, this is an indicator reagent which has been made insoluble (lipophilic) and which responds secondarily to pH changes caused by the complex formation of the primary receptor... 

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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