Amperometric sensing

Amperometric sensing of gases is based on solid ion-conducting materials, as described for potentiometric gas sensors. Solid-state amperometric gas sensors measure the limiting current (ij) flowing across the electrochemical cell upon application of a fixed voltage so that the rate of electrode reaction is controlled by the gas transport across the cell. The diffusion barrier consists of small-hole porous ceramics. The limiting current satisfies the relationship  

FIGURE 9.4 Cross section of the laminated-type NOx sensor using a YSZ-based oxidation-catalyst electrode. (From Ono et al., 2004. Solid State Ionics. 175, 503-506, with permission.) 

A represents the area of holes, L their length, and Dqx the coefficient of diffusion of O2 (Tsipis and Kharton, 2008). 

Amperometric sensing of selected species in liquids is widely extended with an enormous variety of materials and applications, which include electrochemical detection coupled with high-performance liquid chromatography and flux injection analysis. In these cases, high sensitivity and high reproducibility are required. These properties are conditioned by electrode fouling associated to formation of solid deposits and adsorbates on the electrode surface. As a result, the electrode experiences memory effects with concomitant loss of analytical performance. 

Correction of such memory effects can often be made by applying electrochemical pre- and posttreatments. These treatments, consisting of the application of successive potential steps, are able to regenerate the electrode surface before/after measuring. 

Effect of Pt black loading on the response current for a P(Py) based glucose sensor. Pt black loading pgicio , (A) 120, ( ) 240, (o) 300, ( ) 600, ( ) 1050. Batch measurement, applied potential 0.4V. no cover membrane. After Reference [816], reproduced with permission. 

This problem was circumvented by the use of poly(o-phenylene diamine) (P(o-PD)), which has a large, electro-inactive potential window (see Sec. 17.2 above) coinciding with the detection potentials used [817]. This yielded a sensor with high sensitivity, a 10 day shelf life, a 20 h active-use lifetime, and insensitivity to interference from ascorbic acid. 

Amperometric P(Py) glucose sensor Response as function of potential step from open circuit potential (225 mV, vs Ag/AgCl) to 350 mV as ftmction of glucose concentration. After Reference [819], reproduced with permission. 

P(Py) based amperometric galactose sensor data at 0.5 V, in 0.1 M phosphate buffer (pH 6.1) at 25 C. After Reference [821], reproduced with permission. 

An amperometric sensor for amino acids based on flow injection analysis (FIA) and using microelectrodes (10 pm diameter) primarily of P(Py) doped with sulfonate dopants such as tosylate and 3-sulfobenzoate was demonstrated by Akhtar et al. [823, 824]. Linear response was demonstrated for analytes such as aspartic acid and glutamic acid over the concentration range 7.5 X 10 to lO with sensitivities in the region of 1.5 nC-M and detection limits of ca. 10 M. These authors also showed the use of a pattern recognition technique using the responses of six detector electrodes. Fig. 17-11 shows typical response of one of their sensors. 

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Because resistance is an intensive property (i.e., size-dependent), it is possible to localize the source of the signal at one electrode by making its contact area with the selective layer smaller than that of the other electrode. This is exactly the same argument as we have used in the discussion of the amperometric sensing circuit (Chapter 7). 

Figure 1. A permselective coating for amperometric sensing (A, analyte P, product Int., interferent).

The array approach has also been developed for amperometric sensing when used in solution. This has been used by us recently to discriminate between simple ions [52] and even proteins [53]. The approach used is similar to the... 

Figure 18. Schematic illustration of gold nanoparticles embedded in MTMOS silicate sol-gel matrix (MTMOS(SG)-Aumno) sensor for amperometric sensing of H202.

Amperometric sensing is based on the record of the current response of an electrode in contact with the system to be analyzed under the application of a given potential input. Amperometric sensors operate under conditions where mass transport is limiting. 

The various modes in which amperometry can be employed have resulted in its extensive utilization in analytical chemistry - in amperometric titration, amperometric sensing, and amperometric detection in flowing systems. The various applications of amperometry are discussed below. 

To best effectiveness, purposefully designed and synthesized materials could emerge from synergic work between electroanalysts/electrochemists and chemists who are experts specifically in synthesis. The importance of gathering suitable expertise from many different scientific and technological fields should be emphasized. Competences as to the matrices on which their work should also contribute, in order to address the properties of the device developed to the goal pursued amperometric sensing is definitely a deeply interdisciplinary field. 

R. Seeber et al., Functional Materials in Amperometric Sensing, Monographs in Electrochemistry, DOI 10.1007/978-3-662-45103-8 l... 

It is worth recalling that amperometric measurements were first employed to follow redox titrations [1] in which (1) either the reactant or the relevant reaction product or (2) either the titrant or the relevant reaction product are electroactive. Although the titrant volume in amperometric titration measurements is an example of independent variable, it is obvious that time or potential of the working electrode constitute the most useful and frequently encountered variables in amperometric sensing. The i vs. t or i vs. E plots, where i is the current flowing in the working plus auxiliary electrode circuit, constitute the signal to analyse in chronoamperometric and voltammetric measurements, respectively. 

Modification of the electrode started with academic studies on physical and chemical adsorption, i.e., with the appearance of fundamental researches on adsorption of different species on electrode surfaces, both under polarization and at open circuit potential [3]. The properties of similar chemically modified electrodes , in which the modifier consists of a monolayer of a variety of chemical species with different characteristics, possessing (or not) particular properties, were initially studied in a purely electrochemical context, aimed at the collection of fundamental physico-chemical data. A small group of electrochemists were among those involved in these basic studies, envisioning the perspectives opened by the novel systems. In the first, really fascinating, work with similar monomolecular layers, cobalt porphyrin and phthalocyanine, as well as deliberately synthesized dicobalt face-to-face porphyrins were adsorbed on Pt or C surfaces to catalyze molecular oxygen reduction [4]. However, similar systems were not always used or adequately tested in proper amperometric sensing by researchers more interested in electroanalysis dicobalt face-to-face porphirins still constitute a rare example of tailored materials for selective amperometric detection. 

It seems correct to affirm that true applications of novel materials to amperometric sensing date to the 1990s, and, further, to conclude that progress has been and still is rather slow. Huge steps forward have been made by material chemistry in the last decades, in the whole field of new conducting or semiconducting, both... 

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