Optode technique

Since microelectrode measurements are limited to a few centimeters of sediment depth, oxygen penetration depths have been difficult to obtain in strongly oligotrophic areas until the late 1990s. The invention of optode techniques, however, allows measurements up to several decimeters into the sediment (Fig. 6.22). The example is from a station located in the oligotrophic western equatorial Atlantic. 

Fig. 6.22 Oxygen concentration profile measured with an in situ optode technique (after Wenzhofer et. al. 2001b).

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 )=... 

This section attempts to present a broad review of this technique in the light of recent research on fiber-optic chemical sensors (FOCS). A discussion on the advantages and performance of pH optodes will be broached by considering the various applications planned and the resulting pH measurements, including titration and ionic strength. Sensors for low molecular weight electrolytes, particularly optodes for anions and cations in solution, are also considered. 

Table 17-1 for absorption and reflectance techniques and Table 17-2 for fluorescence summarize the pH measurements with the main indicators used so far for biological and industrial media. Apart from the experiments of Attridge et al. [S3], all the investigations are concerned with absorption (and/or reflectance) or luminescence. Both methods proved satisfactory for in vivo measurements, and the accuracies obtained are comparable ( 0.01 near the pA]. The lifetime of the indicator and the optode response time vary according to the authors. In absorption, this response time can differ with pH (see Figure 17-4) and decrease with A pH. 

Many problems still remain to be solved for pH FOCS. Areas still not completely under control are the techniques of immobilization and grafting on the support, permanent monitoring of the concentration of the indicators, ionophore lifetime [60], stability of maintenance of the dye on the support over a wide pH range, determination of the dissociation constant of the immobilization dye [61], optode reversibility, reproducibility of their manufacture (disposal optodes), monitoring of the influence of the medium, and the simplification of calibration procedures. Some progress has been achieved in the understanding of the influence of the ionic strength of the medium. 

A potassium-sensitive optode uses a crown ether labeled with an azo dye and immobilized on Amberlite by encapsulating the tip by a porous FIFE membrane. The absorption is modiHed by chelation with potassium ions [74]. All others cation measurements use the fluorescence technique. 

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. 

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. 

Holst, G. Kiihl, M. Klimant, I. (1995) A novel measuring system for oxygen micro-optodes based on a phase modulation technique. Proc. SPIE 2508(45), 387-398. 

Another area in which PIMs show considerable promise is chemical analysis. The use of PIMs in the construction of ISEs and optodes is well established, but their potential use in analytical separation is only starting to be explored. PIMs are particularly useful in solid-phase extraction (SPE) for preconcentration of analytes [17,18]. Also, Eonths et al. have used a PIM containing Aliquat 336 as the carrier for the preconcentration of Cr(VI) prior to its determination in the membrane by energy-dispersive X-ray fluorescence spectrometry [47]. Flat-sheet PIMs can also be conveniently incorporated into separation modules for use in online analytical techniques such as flow injection analysis. For example, a D2EHPA-based PIM has been used in a separation module incorporated into a flow injection analysis system for the determination of Zn(II) [35]. This new approach has interesting implications for use in automated analysis, particularly in held instruments for the continuous monitoring of pollutants. 

There are several techniques nsed to determine the dissolved oxygen content in a fluid. In practice, five general methods exist chemical, volumetric, tubing, optodes, and the electrochemical electrode (Carroll, 1991 van Dam-Mieras et al., 1992). This section will discuss these methods and some of their limitations and uses the emphasis, however, will be on electrochemical electrodes as they are the most common dissolved O2 sensors. 

Immobilization techniques with optodes are less multifaceted than those with electrochemical sensors, since the selection of sensor materials is smaller. The most common immobilization procedures are the following ... 

Nearly all chemical sensors useful for liquid samples can be utiUzed to indicate titrations. Besides the preferred potentiometric, other electrochemical probes are also used, mainly amperometric and conductometric sensors. The so-called biamperometric titration works with simple wire pairs. Photometric and thermometric indication techniques are less common than electrochemical methods. Miniaturization does not play an important role for titration probes. Classical arrangements predominate to this day. Commercial titration instruments are only slowly starting to make use of the achievements of modern sensor technology. As an example, optodes have achieved a certain popularity in recent years for titration applications. 

Micro-optodes are based on the change of optical properties (fluorescence intensity or fluorescence lifetime) of a layer covering an optical microfiber. Microsensors are developed for O2, pH, and temperature. The presence of the substrate induces quenching of the fluorescence intensity or decrease of the fluorescence lifetime. Klimant et al. (1997) gave a description of the theory and practice of this technique. Advantages of optical sensors are their ease of manufacture, insensitivity to noise, stability of calibration, and mechanical strength. Disadvantages include their size (ca. 20 pm), limited types of sensors available, and cost of the opto-electronics. 

---