Solid contact sensors

M.N. Abbas, H.S. Amer, A novel solid-contact sensor for flow injection determination of verapamil in pharmaceutical formulations and urine (Review), Current Pharmaceutical Analysis, 4 (2), 90-100, 2008. 

X.X. Sun, H.Y. Aboul-Enein, Internal solid contact sensor for the determination of doxycy-cline hydrochloride in pharmaceutical formulation, Talanta, 58 (2), 387-396,2002. 

The first and very simple solid contact polymeric sensors were proposed in the early 1970s by Cattrall and Freiser and comprised of a metal wire coated with an ion-selective polymeric membrane [94], These coated wire electrodes (CWEs) had similar sensitivity and selectivity and even somewhat better DLs than conventional ISEs, but suffered from severe potential drifts, resulting in poor reproducibility. The origin of the CWE potential instabilities is now believed to be the formation of a thin aqueous layer between membrane and metal [95], The dominating redox process in the layer is likely the reduction of dissolved oxygen, and the potential drift is mainly caused by pH and p02 changes in a sample. Additionally, the ionic composition of this layer may vary as a function of the sample composition, leading to additional potential instabilities. 

M. Fibbioli, W.E. Morf, M. Badertscher, N.F. de Rooij, and E. Pretsch, Potential drifts of solid-contacted ion-selective electrodes due to zero-current ion fluxes through die sensor membrane. Electroanalysis 12, 1286-1292 (2000). 

K.Y. Chumbimuni-Torres, N. Rubinova, A. Radu, L.T. Kubota, and E. Bakker, A universal recipe for solid contact potentiometric sensors for trace level measurements. (Submitted 2005)... 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Solid-contact pH sensors can be constructed by using polypyrrole [45,59] or polyaniline [92,96] as ion-to-electron transducer in combination with pH-selective membranes based on plasticized PVC [45,59,92,96]. The dynamic pH range of the sensors depend on the pH ionophore used in the plasticized PVC membranes, as follows tri-n-dodecylamine (pH 2-12) [45], tris(2-phenylethyl)amine (pH 4.5-12.6) [59], tris(3-phenylpropyl)amine (pH 4.6-13.2) [59], tribenzylamine (pH 2.5-11.2) [92,96], dibenzylnaphtalenemethylamine (pH 0.65-10.0) [96], dibenzylpyrenemethyl-amine (pH 0.50-10.2) [96]. Suggested applications include pH measurements in body fluids such as serum [45,96], whole blood [92], and cow milk [59]. 

Conducting polymers have been studied as potentiometric ion sensors for almost two decades and new sensors are continuously developed. The analytical performance of solid-state ion sensors with conducting polymers as ion-to-electron transducer (solid-contact ISEs) has been significantly improved over the last few years. Of particular interest is the large improvement of the detection limit of such solid-contact ISEs down to the nanomolar level. Further optimization of the solid contacts as well as the ion-selective membranes will most certainly extend the range of practical applications. 

This membrane is normally employed in what is called a symmetrical configuration , to form the classical ISE which employs an internal reference solution, as sketched in Figure 30.8. This membrane can also be used directly fixed on a solid contact, per example a metallic wire or a screen-printed substrate, in the coated-wire configuration [77]. The proposed Procedure 45 (see in CD accompanying this book) suggests the use of coated wire sensors, as these are the easiest to construct. 

Fig. 30.11. Design of the flow-through solid-contact potentiometric sensors to be used in FIA or SIA systems.

Magnetic-field sensors are widely used in automotive applications because of their robust design, their non-contacting and thus wear-free operation and their low manufacturing costs, in particular for solid-state sensors, which can be fabricated using batch processes. Their main fields of application in automobiles are speed and position sensing. In total, about one third of today s automotive sensors are based on the magnetic principle [1],... 

Ionophore-based solvent polymeric membranes are widely used as sensing membranes in ion-selective electrodes (ISEs) [24,25]. This type of potentiometric sensor has attracted great interest in the last decade due to the extraordinary improvement in the detection limit down to picomolar (10-12 M) levels [26,27], Furthermore, solid-contact ISEs have been developed by using various conducting polymers, including PEDOT, as the ion-to-electron transducer [28-31],... 

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