Interfacial Charge Sensors

These impedimetric biosensors suffer from the virtually irreversible binding discussed in Chapter 2. This is the main reason why they do not qualify as direct biosensors. They have been used in conjunction with the ferro/ferricyanide redox couple as the indicator of the blocking of the surface (Radi et al., 2005). Unfortunately, the binding event is so strong that the analysis has to be run in an assay format. The result of such a procedure is shown in Fig. 8.2. 

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Theoretical insight into the interfacial charge transfer at ITIES and detection mechanism of this type of sensor were considered [61-63], In case of ionophore assisted transport for a cation I the formation of ion-ionophore complexes in the organic (membrane) phase is expected, which can be described with the appropriate complex formation constant, /3ILnI. 

A notable difference between these two relationships is that the Gibbs-Lippmann equation contains one more independent variable parameter, the interfacial charge. It cannot be determined directly. Several unsuccessful attempts to design chemical sensors (e.g., the immunosensor) based on the measurement of adsorbed surface charge have been made. There are no ideally polarized interfaces that are sufficiently ideal to allow such direct measurement of interfacial charge. 

When a periodically changing excitation signal is chosen for the operation of chemiresistors, they can be used to detect changes of capacitance (Fig. 8.1b). Therefore, the dielectrometric sensors rely on the chemical modulation of one or more equivalent circuit capacitors, either through the change of the dielectric constant of the chemically sensitive layer or through the chemical modulation of the interfacial charge. 

In label-free or direct immunoassays, antibodies are immobilized on the sensor surface plate and subjected to the binding interaction with the antigen of interest. Upon specific molecular recognition of the ant n by the immobilized capture antibody, there will be changes in the interfacial charge, current, capacitance, impedance, mass, and thickness at the immunosensor surface, which in turn has a direct effect on the electron transfer... 

It was demonstrated that reproducible gas-sensitive silicon Schottky sensors could be produced after terminating the silicon surface with an oxide layer [71, 72]. This interfacial oxide layer permits the device to function as a sensor, but also as a diode, as the charge carriers can tunnel through the insulating layer. The layer made the Schottky diode behave like a tunneling diode, and the ideality factor could be voltage-dependent [73]. 

Rule 3 This is not so much a rule as it is an important general point regarding the nature of the interfacial reaction in all electrochemical sensors. Charge transport within the transducer part of the sensor, and/or inside the supporting instrumentation, is electronic. On the other hand, the charge transport in the sample can be electronic, ionic, or mixed (electronic/ionic). In the latter two cases, an electron... 

In other words, at the nonpolarized interface, the interfacial potential Eeq is uniquely tied by the Nernst equation (5.8) to the activity ai of the charged species crossing the interface. This is the key relationship in potentiometric sensors (Chapter 6). 

The modification of electrode surfaces with electroactive polymer films provides a means to control interfacial characteristics. With such a capability, one can envisage numerous possible applications, in areas as diverse as electronic devices, sensors, electrocatalysis, energy conversion and storage, electronic displays, and reference electrode systems [1, 2]. With these applications in view, a wide variety of electroactive polymeric materials have been investigated. These include both redox polymers (by which we imply polymers with discrete redox entities distributed along the polymer spine) and conducting polymers (by which we imply polymers with delocalised charge centres on the polymer spine). 

The resistance of potentiometric sensors does not seem to be a cracial point a priori. Membranes with a very low conductivity (lower than 10S cm- ) are sometimes used in ISE devices. Nevertheless, a too-high resistance leads to a capacitive behavior of the sensor. The parasitic charges which appear in the circuit or electrical noise (intrinsic or extrinsic) may lead to erroneous results. To avoid this disadvantage, thin sensitive membranes may be used (pH or pNa glass membranes), but they are very fragile. Generally, a conductivity of about 10- to 10- S cm- is required. Miniaturization of devices allows a lower resistance of the SIC to be obtained, but the interface impedance is not fundamentally modified because it depends on the electrochemical kinetics, which is a function of the interfacial area. 

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