Voltammetric transducers

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

Figure 3.3 Sonoelectrochemical cell used for electrosynthesis and voltammetric studies. 1, Sonic horn 2, transducer 3, to control unit of sonic horn 4, graphite counter electrode 5, argon inlet for degassing 6, Pyrex reservoir 7, platinum-disk macro- or microelectrode 8, copper cooling coil connected to the thermostatted water bath 9, titanium tip 10, platinum resistance thermocouple 11, SCE reference. (From Ref. 557, reproduced with permission.)...

Lucarelli et al. describe a disposable indicator-free screen-printed genosensor applied to the detection of apoE sequences in PCR samples [26]. The biosensor format involved the immobilization of an inosine-modified (guanine-free) probe onto a SPE transducer and the detection of the duplex formation in connection with the square-wave voltammetric measurement of the guanine oxidation peak of the target sequence. 

In principle, electrochemical transducers can be used to detect the formation of a surface-bound affinity complex when the affinity-binding reaction is associated with a change in electrical properties (e.g., ion permeability or capacitance) of the layer immobilized onto the electrode surface. For example, the so-called ion-chemnel sensors detect permeabilily changes of a film immobilized on an electrode surface to an electroactive molecule, which is used as a redox marker. The formation of a surface-bound affinity complex results in a permeability change, which can be monitored by the change of cyclic voltammetric response of the redox marker. 

The probe in SECM produces a signal that must be transduced and amplified prior to recording. At a voltammetric tip, electrolysis of either a mediator or a substrate-produced substance produces a faradaic current signal. At a potentiometric tip, the activity of a solution phase species generates a voltage signal. 

Limitations for amperometric and voltammetric transducer include potential interferences to the response if several electroactive compounds can generate false current values. These effects have been eliminated through the use of selective membranes, which carefully control the molecular weight or the charge of compounds that have access to the electrode. 

Mousavi et al. have used PEDOT-CNT composite as ion-to-electron transducer in the fabrication of potassium ISEs [19]. In this work, PEDOT was electrochemically S5mthesized using negatively charged multi-walled CNTs (MWCNTs) as counterions. Results from cyclic voltammetric (CV) and electrochemical impedance spectroscopic (EIS) measurements shown in Fig. 11.1, reveal that the PEDOT-MWCNT film exhibits higher redox capacitance than a film based on PEDOT doped with chloride [Cr) ions, i.e., PEDOT-Cl. This sufficiently high redox capacitance is one of the conditions necessary for stable potential in all-solid-state ISEs having an ECP as the solid contact [23]. 

The measurements of electrochemical impedance, voltammetric (po-larographic) analysis, and spectroelectrochemistry represent a basis for analysis of molecules of biological significance in bulk of solution and at interfaces. These principles are reviewed in the first four chapters. The next three chapters demonstrate how these principles are utilized in voltammetric and interfacial analysis of biomacromolecules such as nucleic acids, proteins, polysaccharides, and viruses in vitro, in the development of biosensors with electrochemical transducers and in in vivo voltammetry. The last two chapters of this volume are devoted to the principles of electrophoresis used for separation analysis of biomolecules and to the theoretical principles and practical description of the patch-clamp technique to an extent suitable for those wishing to initiate research in electrophysiology. 

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