Sensors solution interface

An inner filling solution and internal reference electrode are used in macro ISEs due to a very good stability of the potential at the inner membrane-solution interface in such a setup (see Fig. 4.4). However, the presence of a solution inside a sensor could be a serious limitation for development of microelectrodes and may be undesired for a variety of other reasons, including ionic fluxes in the membrane and limited temperature range of sensor operation. There are several requirements for such an inner contact. First of all, a reversible change of electricity carriers ions-electrons must take place at the membrane-substrate interface. The potential of the electrochemical reaction, ensuring this transfer, has to be constant, stable, and must not depend on the sample composition. At last, the substrate must not influence the membrane analytical performance. 

One other difference lies in the type of detection technique used, which dictates the flow-cell design. Thus, a distinction can be made in this respect between optical (absorptiometric, luminemetric) sensors, which make measurements of the bulk solution where the flow-cell is immersed, and electroanalytical (amperometric, potentiometric) sensors, where measurements are based on phenomena occurring at the electrode-solution interface. 

The essential difference between the biosensors described in Section 3.2.1.1 and those dealt with in this Section is that, while the former must have a minimal flow-cell void volume in order to provide adequate sensitivity because measurements are made on the solution held in the cell, electro-analytical sensors rely on measurements during the process that takes place at the electrode/solution interface — the solution must be in contact with the sensing surface, so the cell volume is not a limiting factor. 

For sensors, we separate the NP roles into two broad categories based upon the location of their electrochemical signal or response exo-active and core based. Exo-active surfaces describe NPs that generate an electrochemical response that occurs at the ligand-solution interface at the periphery of the NP (Fig. 11.1a). Exo-active surfaces are widely used for sensing applications due to the large number of molecular receptors and their accessibility to target molecules. 

The obvious advantage of the symmetrical arrangement is that the processes at all internal interfaces can be well defined and that most nonidealities at the mem-brane/solution interface tend to cancel out. Because the volume of the internal reference compartment is typically a few milliliters, the electrode does not suffer from exposure to electrically neutral compounds that would penetrate the membrane and change the composition of this solution. This type of potentiometric ion sensor has been used in the majority of basic studies of ion-selective electrodes. Most commercial ion-selective electrodes are also of this type. The drawbacks of this arrangement are also related to the presence of the internal solution and to its volume. Mainly for this reason, it is not conveniently possible to miniaturize it and to integrate it into a multisensor package. 

This type of diffusion/reaction mechanism has been treated semi-analyti-cally by Albery et al. [42, 44, 45], under steady-state conditions and its applications to amperometric chemical sensors has been described by Lyons et al. [46]. In both models, only diffusion and reaction within a boundary layer is considered, while the effect of concentration polarisation in the solution is neglected. Thus, to apply the model to an experimental system it is necessary to be able to accurately determine the concentration of substrate at the polymer/solution interface. Assuming that the system is in the steady state, the use of the rotating disc electrode allows simple determination of the substrate concentration at the interface from the bulk concentration and the experimentally determined flux using [47]... 

For ion-selective membrane electrodes, the potential development is directly correlated with the stability of the compound formed at the membrane-solution interface.262 Therefore, the selectivity of the electrodes is correlated with the stability of the compounds formed by the analyte and interference at the interface. Actually, the difference between their stabilities determines the selectivity of the electrodes.263 The piezoelectric sensors are known as the most sensitive class of sensors, but their selectivity is very low. 

Rojas, M.T. Kaifer, A.E. Molecular recognition at the electrode-solution interface. Design, self-assembly, and 127. interfacial binding properties of a molecular sensor. J. 

A good conductivity, high surface active area and high binding capacity are some of the properties that make the EPs attractive for the construction of amperometric sensors. In some cases higher electron transfer rates at the EP/solution interface have been reported although the mechanistic details of such enhancement are not completely understood. 

---