Sensor layer

Nyberg 1988 metal oxide sensor layer whose reflectivity reversibly changes with p02... 

As stated above, the beginning of optical pH sensor technology remains hidden. What is nowadays refered to as a sensor layer was formerly mostly refered to as a test strip, a dry reagent chemistry, or an immobilized reagent. 

A novel approach for ion sensing is based on the use of potential-sensitive or polarity-sensitive dyes (PSDs) and was presented first106 in 1987. PSDs are charge dyes and typically located at the interface between a lipophilic sensor phase and a hydrophilic sample phase. The transport of an ion into the lipophilic sensor layer causes the PSD to be displaced from the hydrophilic/hydrophobic interface into the interior of the respective phase (or vice versa), thereby undergoing a significant change in its fluorescence properties107 110. 

Figure 15. Absorbance spectra of dissolved oxidized form of MB (1) and immobilized MB in poly-TMOS sensor layer Li (2).

Figure 16. Response of sensor layer LI to dissolved hydrogen peroxide at pH 7 and its reversibility by exposing to Na2S205 measured at 720 nm.

Figure 6. Mechanism of co-extraction of the analyte anion (X ) together with a proton (H+) into the sensor layer.

Table 14. Optical response of a nitrite sensor layer.

The sensor layer consists of a selective ionophore (e.g. valinomycin for potassium), a lipophilic anionic site (borate) and the cationic PSD. Before interaction with potassium, a lipophilic ion pair between the cationic PSD and borate anion is formed in the polymer layer. When valinomycin (also contained in the layer) selectively extracts potassium into the layer, then the positively charged valinomycin-potassium complex forms an ion pair with... 

Figure 9. Response of the ethanol sensor layer based on ETHt 4004 on exposure to aqueous...

Table 18. Sensor layer for ammonia based on a lipophilic pH indicator dye in silicone.

Sensor layers are mostly attached to a solid support since their mechanical stability is generally quite low. In most cases, all components (polymer, plasticiser, additives and indicator dyes) are dissolved in a common solvent and spin-coated, spray-coated, dip-coated or simply pipetted onto the support material (Figure 10). The solid support can be a glass plate which is mounted in a photometer and exposed to the analyte in a... 

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... 

As described for stopped flow experiments above, all commercially available SPR systems work under (pseudo) first-order conditions as well. This is realized either by a large excess of free ligand (in the large volume of the cuvette) compared with a nanoliter volume of the sensor layer [156] or by continuous replacement of free ligand in a flow injection system (e.g.,BIAcore [157]). 

The results summarized above were obtained by using fluorescence based assays employing phospholipid vesicles and fluorescent labeled lipopeptides. Recently, surface plasmon resonance (SPR) was developed as new a technique for the study of membrane association of lipidated peptides. Thus, artificial membranes on the surface of biosensors offered new tools for the study of lipopeptides. In SPR (surface plasmon resonance) systemsI713bl changes of the refractive index (RI) in the proximity of the sensor layer are monitored. In a commercial BIAcore system1341 the resonance signal is proportional to the mass of macromolecules bound to the membrane and allows analysis with a time resolution of seconds. Vesicles of defined size distribution were prepared from mixtures of lipids and biotinylated lipopeptides by extruder technique and fused with a alkane thiol surface of a hydrophobic SPR sensor. 

Plots in Figure 6.3 show leaching and response of pH sensor layers chemically doped starting from 3-(trimethoxysilyl)propylisocyanate (ICPS) and (glycidyloxypropyl)trimethoxysilane (GOPS) derivatized with the pH indicator AF, or physically doped in SiC>2 (from tetra-methylorthosilicate, TMOS) and in 50% phenyl-modified silica. 

Figure 6.3 Leaching of sensor layers M4, M1, M2 and M3 (from top) on exposure to a flow of buffer solution (left) and titration plots of AF in poly-TMOS (Ml), an organically modified silicate (M4), and covalently immobilized on ICPS (M2) and GOPS (M3) (right). (Reproduced from ref. 4, with permission.)...

Fig. 21. A surface acoustic wave dual-delay line oscillator. The sensitise layer is placed in the propagation path of one of the two SAW devices. The differenee in Ireqnency (At) between the two channels provides a dtrecl result of the mass loading and electric field effects associated w ith the sensor layer...

Generally speaking, we can distinguish two types of interactions between the chemical species and the sensor a surface interaction in which the species of interest is adsorbed at the surface, and a bulk interaction in which the species of interest partitions between the sample and the sensor and is absorbed. The classification of the interaction as either surface or bulk is relative with respect to the size of the species. It is the case of chicken and chicken wire. Obviously, a chicken wire fence is impervious to chickens, but presents no barrier whatsoever to mosquitoes. Similarly, large molecules, such as proteins, may adsorb at the surface of the sensor layer, whereas smaller ions can penetrate and absorb in the bulk. 

Quartz micro balance with dendritic sensor layers... 

Ruthenium complexes with mixed bipyridyl ligands, immobilized inside a Nation film, may also be used as pH-sensitive sensor layers [90]. A completely different approach for a ratiometric imaging of pH sensor foils was developed for diagenetic studies of marine sediments, using the dual fluorescence excitation ratio of the pH-sensitive fluorophore 8-hydroxypyrene-l,3,6-trisulfonic acid (HPTS) [91]. Commonly used dual fluorophors with different absorption and emission maxima in the protonated and basic form for ratiometric measurements are the naphthofluorescein and seminaphthofluorescein derivates (SNARF and SNAFL) [92], It should be noted that ammonia or carbon dioxide can also be detected by some of these pH-sensitive materials [55,93]. 

The Interplay of Indicator, Support and Analyte in Optical Sensor Layers... 

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