Membrane flow-through sensors

Figure 3.38 — Integrated flow-through sensors. (A) With electrochemical generation of the luminescent reagent. The flow stream path follows the line between the analyte inlet and the outlet to waste. (B) With immobilization of a phosphor (length, 3 cm internal diameter, 2 mm) 1 immobilized phosphor 2 CFG 3 quartz wool plug 4 KEL-F caps 5 hand-tightened screw 6 stainless steel capillaries. (C) Sensor based on reflectance measurements. The sensor membrane is fixed on a Plexiglas disc. Reflectance spectra are measured from the rear side. (Reproduced from [267] and [269] with permission of the American Chemical Society and Elsevier Science Publishers, respectively).

Figure 5.3 shows the different possible ways in which the ingredients of the (bio)chemical reaction can take part in the sensing process. For example, the analyte can be retained temporarily and take part in the separation process. The reagent can be present in the solution used to immerse the sensor or immobilized in a permanent fashion on a suitable support. Also, the catalyst can be introduced directly across a membrane or be permanently immobilized. Finally, the reaction product can be the species transferred in the separation process or also be temporarily immobilized. These and other, more specific alternatives that are described below are all possible in (bio)chemical flow-through sensors integrating reaction, separation and detection. 

Figure 5.7 — Wall-jet potentiometric flow-through sensor including an internal nonactin based ISE furnished with an outer gas-permeable membrane. For details, see text. (Reproduced from [11] with permission of VCH publishers).

There are two possible configurations for this type of flow-through sensor integrating gas diffusion, reaction and detection that differ in whether the reagent is dissolved in the acceptor solution or immobilized on a sensing microzone located near the diffusion membrane. The descriptions below are based on such a difference. 

The sensing microzone of the flow-through sensor depicted in Fig. 5.9.B1 integrates gas-diffusion and detection with two analytical reactions [28], viz. (a) the urease-catalysed formation of ammonium ion by hydrolysis of urea (the analyte), which takes places on a hydrophilic enzyme membrane in contact with the sample-donor stream, which contains a gel where the enzyme is covalently bound and (b) an acid-b reaction that takes place at the microzone on the other side of the diffusion membrane and involves Bromothymol Blue as indicator. This is a sandwich-type sensor including a hydrophilic and a hydrophobic membrane across which the sample stream is circulated —whence it is formally similar to some enzyme electrodes. Since the enzymatic conversion of the analyte must be as efficient as possible, deteetion (based on fibre optics) is performed after the donor and acceptor streams have passed through the sensor. Unlike the previous sensor (Fig. 5.9.A), this does not rely on the wall-jet approach in addition, each stream has its own outlet and the system includes two sensing microzones... 

Simon et al. [47] developed a photometric flow-through sensor for the determination of zinc using a transparent PVC membrane accommodating a lipophilized ligand, viz. l-octadecyloxy-4-(2-pyridylazo)resorcinol, a water-insoluble derivative of PAR. The mechanism by which the analyte is retained in the membrane and eluted from it is similar to a liquid-liquid extraction... 

Figure 5.17 — Fluorimetric flow-through sensors for the determination of (A) and (B) based on immobilization of the reagent in a membrane (A) or on an ion-exchange resin (B). S sample EER ion-exchange reactor IV injection valves P peristaltic pump W waste. For details, see text. (Reproduced from [47] and [51] with permission of Elsevier Science Publishers and the Royal Society of Chemistry, respectively).

Wolfbeis et al. [105-107] developed several fluorimetric flow-through sensors based on a sensing microzone consisting of a multilayer lipid membrane formed on a glass support by using the Langmuir-Blodgett (LB)... 

Many types of Clark sensor have been prepared. An example of a small flow-through sensor fabricated from a printed circuit board is shown in Fig. 9. The gelatin layer contains a depolarizer (KCl) for the reference electrode and polystyrene functions as a gas permeating membrane [126]. 

Frenzel, W., J. Schulz-Brussel, and B. Zinvirt. 2004. Characterisation of a gas-diffusion membrane-based optical flow-through sensor exemplified by the determination of nitrite. Talanta 64 278-282. 

Strongly scattering environments are a special case because aU turbid waters are considered optically thick, but not all optically thick waters are turbid. In highly turbid environments (e.g., greater than 100 NTU), a flow-through sensor deployed with a miCToporous membrane particle filter may be the best choice to avoid interferences from particles (Belzile et al., 2006 Downing et al., 2009 Saraceno et al., 2009). Although flat-faced sensors are very attractive as they do not require the added power requirements of a pump and filter assembly, the effects of both quantity and quality of particle interferences on these sensors is not fully understood. 

An automated electronic tongue consisting of an array of potentiometric sensors and an artificial neural network (ANN) was developed to resolve mixtures of anionic surfactants. The sensor array was formed by five different flow-through sensors for anionic surfactants, based on polyvinyl chloride membranes having cross-sensitivity features. 

Deposition of sensor layers is possible on fibre Flow-through cell allowing the optics, planar waveguides, and test strips simultaneous exposure of the membrane to... 

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