Potentiometric deposition

In electro-gravimetric analysis the element to be determined is deposited electroly tically upon a suitable electrode. Filtration is not required, and provided the experimental conditions are carefully controlled, the co-deposition of two metals can often be avoided. Although this procedure has to a large extent been superseded by potentiometric methods based upon the use of ion-selective electrodes (see Chapter 15), the method, when applicable has many advantages. The theory of the process is briefly discussed below in order to understand how and when it may be applied for a more detailed treatment see Refs 1-9. 

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

A potentiometric technique was used in Ref 50 to measure composition, found to be Cui.gsS. In this deposition, stirring the deposition solution resulted in nonuniform and poor-quality films, while good films were obtained in unstirred films. The bandgap was measured to be direct, with a value of 2.58 eV. This is a particularly high value. Examination of the transmittance spectrum showed a sharp drop in transmission at ca. 1.7 eV, which is more likely to be the true bandgap. 

Solid electrolyte electrochemical cells can be operated in a variety of ways (the three modes of operation are illustrated schematically in Figure 2). Such a cell may be operated potentiometrically in order to investigate the behaviour of a catalyst of interest. This technique has become known as solid electrolyte potentiometry (SEP). The catalyst itself is deposited in the form of an electrode... 

Potentiometric stripping analysis (PSA) is another attractive version of stripping analysis [7]. The preconcentration step in PSA is the same as for ASV that is, the metal is electrolytically deposited (via reduction) onto the mercury electrode (usually a film). The stripping, however, is done by chemical oxidation, for example, with oxygen or mercuric ions present in the solution ... 

Figure 24.4 Potentiometric stripping analysis of a solution containing 1.5 x 10 6 M Zn2+, Cd2+, Pb2+, and Cu2+, using 3-min deposition and mercury as an oxidant.

A nitrate-selective potentiometric MIP chemosensor has been devised [197, 198]. For preparation of this chemosensor, a polypyrrole film was deposited by pyrrole electropolymerization on a glassy carbon electrode (GCE) in aqueous solution of the nitrate template. Potentiostatic conditions of electropolymerization used were optimized for enhanced affinity of the resulting MIP film towards this template. In effect, selectivity of the chemosensor towards nitrate was much higher than that to the interfering perchlorate ( o3 cio4 = 5.7 x 10-2) or iodide ( N03, r = x 10 2) anion. Moreover, with the use of this MIP chemosensor the selectivity of the nitrate detection has been improved, as compared to those of commercial ISEs, by four orders of magnitude at the linear concentration range of 50 pM to 0.5 M and LOD for nitrate of (20 10) pM [197]. 

Y. Mourzina, T. Yoshinobu, J. Schubert, H. Liith, H. Iwasaki and M.J. Schoning, Ion-selective light-addressable potentiometric sensor (LAPS) with chalcogenide thin film prepared by pulsed laser deposition, Sens. Actuators B Chem., 80(2) (2001) 136-140. 

Ag+ Urease Deposition onto electrode area and covering with poly(4-vinylpyridine and Nafion) Potentiometric LOD = 3.5 x 10 8moir1 Irreversible [13]... 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

In order to maintain the advantage of the microfabrication approach which is intended for a reproducible production of multiple devices, parallel development of membrane deposition technology is of importance. Using modified on-wafer membrane deposition techniques and commercially available compounds an improvement of the membrane thickness control as well as the membrane adhesion can be achieved. This has been presented here for three electrochemical sensors - an enzymatic glucose electrode, an amperometric free chlorine sensor and a potentiometric Ca + sensitive device based on a membrane modified ISFET. Unfortunately, the on-wafer membrane deposition technique could not yet be applied in the preparation of the glucose sensors for in vivo applications, since this particular application requires relatively thick enzymatic membranes, whilst the lift-off technique is usable only for the patterning of relatively thin membranes. 

Potentiometric stripping can be regarded as -> chronopotentiometry under open-circuit conditions (-> open-circuit potential) with chemical oxidation of preliminarily deposited metals. 

PEVD was developed initially in the course of fabricating type III potentiometric sensors for gaseous oxide (CO, SO, and NO ) detection. Three kinds of PEVD products (NaNOj, Na2C03, and Na SO ) were deposited as the auxiliary phases at the working electrode of NO, CO, and SO sensors, respectively. Because of the underlying similarities, all discussion here will focus on CO gas sensors. Cases of depositing NaNOj and Na SO auxiliary phases for type III NO and SO potentiometric sensors, respectively, can be treated analogously. 

Polarized Electrochemical Vapor Deposition to Deposit Auxiliary Phases at the Working Electrode of Type III Potentiometric CO Sensors... 

NaNOj and Na SO auxiliary phases can be deposited by a similar PEVD method for type III NO2 and SO2 potentiometric sensors, respectively. 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

Improvement of the geometric structure of the working electrode by a well-controlled PEVD process benefits the performance of a CO sensor in many ways. To optimize kinetic behavior, the response and recovery times of CO potentiometric sensors were studied at various auxiliary phase coverages. This was realized by a unique experimental arrangement to deposit the Na COj auxiliary phase in-situ at the working electrode of type III potentiometric CO sensors by PEVD in a step-wise fashion. Since the current and flux of solid-state transported material in a series of PEVD processes can be easily moiutoredto control the amount of deposit... 

According to previous applications to solid state potentiometric sensors, PEVD seems to be a possible technique to deposit in-situ a layer of an oxygen ion conducting phase on a metallic anode of an SOFC. Thus, a composite anode with both a PEVD product and a metallic anode could be realized to overcome the aforementioned anodic limitations. 

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