Flow-through sensors integrated detection-reaction

Flow-through sensors integrating detection and a chemical or biochemical reaction rely on immobilization in the probe proper or the flow-cell (or a special housing included in it) of a species intended to take part in or catalyse the reaction by which the analyte, viz. the catalyst or reagent, is measured, according to which the sensors described in this Chapter are divided into two broad categories. 

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 5.3 — Types of immobilization and species involved in (bio)chemical flow-through sensors integrating reaction, separation and detection.

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. 

Flow-through sensors based on integrated reaction and detection... 

Sensors based on integrated reaction and detection are the most varied and numerous among flow-through sensors and those that will predictably experience the greatest development in the near future. Both enzyme sensors and immunosensors are bound to become virtually irreplaceable tools in some areas of social interest including clinical and environmental analysis. While other sensors inspired by those discussed in Sections 3.4 and 3.5 may... 

Figure 5.1 — Classification of (bio)chemical flow-through sensors based on integrated reaction, separation and detection according to whether the three processes take place sequentially (A,B) or simultaneously (C) at the sensing microzone. S sample R reagent. (Reproduced from [1] with permission of the Royal Society of Chemistry).

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... 

Figure 5.13 — Irreversible-reusable flow-through sensor for the kinetic multidetermination of phosphate and silicate based on integrated sorption of a reaction product, reaction (/ situ reduction) and photometric detection. (A) Microsensor block (1) and components (2). (B) Continuous-flow configuration coupled on-line to the sensor. P peristaltic pumps SV switching valve W waste. For details, see text. (Reproduced from [39] with permission of the American Chemical Society).

The main important details of these procedures for AA determination are contained in Table 18.8. As can be seen in Table 18.8, among these flow analysis techniques, stopped-flow procedures are the most applied to AA determination, being mostly kinetic enzymatic determinations. BI methodologies for AA determination use spectrophotometric detection and a commercial flow cell, which is filled with appropriate solid beads, works as a flow-through chemical sensor integrating online reaction, retention, and detection on the solid-phase disposable beads. 

Flow-through optical sensors bearing one or more immobilized enzymes at their sensing microzone can be classified according to the type of physical relationship between the microzone and the detection system or instrument used into those using fibre optics (photometric and luminemetric) and those integrating a biochemical reaction and detection (usually photometric). 

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