Sensors transient signals

Figure 2.18 shows the most relevant and/or usual types of transient signals provided by the flow-through (bio)chemical sensors used in the continuous configurations depicted in Figs 2.12-2.16 and the regeneration modes illustrated in Fig. 2.17. The two sequential steps (1 and 2) affecting the sensitive microzone of the flow-through sensor are distinguished.

Reversible sensors afford virtually symmetric transient signals (Fig. 2.18.A) on passage of the sample through the detector (e.g. see [15]). Such is the case with sensors involving a permanently immobilized... 

The transient signals provided by flow-through (bio)chemical sensors can be processed in various ways in order to draw information that can be directly related to the analyte concentration in the sample. Figure 2.19 shows the more frequently used approaches in this respect, classified according to whether they rely on direct (A) or kinetic measurements (B). 

The most common procedure (Fig. 2.19.A) involves measuring the transient signal provided by a reversible or irreversible-reusable sensor at the maximum and the plateau obtained by injecting the sample into a non-regenerating carrier. 

Figure 2.19 — Types of measurements available on the transient signals provided by flow-through (bio)chemical sensors. (A) Ordinary measurements. (B) Kinetic measurements. For details, see text.

One temporal concept to be borne in mind in this context is whether the (bio)chemical reactions and mass transfer separations taking place at the active microzone (one or both of which, by definition, take place simultaneously with detection) are simultaneous or sequential relative to each other. Whether such processes take place at the same or a different time has a marked effect on the sensor performance and type of transient signal obtained. 

On the other hand, its should be emphasized that such basic analytical properties as precision, sensitivity and selectivity are influenced by the kinetic connotations of the sensor. Measurement repeatability and reproducibility depend largely on constancy of the hydrodynamic properties of the continuous system used and on whether or not the chemical and separation processes involved reach complete equilibrium (otherwise, measurements made under unstable conditions may result in substantial errors). Reaction rate measurements boost selectivity as they provide differential (incremental) rather than absolute values, so any interferences from the sample matrix are considerably reduced. Because flow-through sensors enable simultaneous concentration and detection, they can be used to develop kinetic methodologies based on the slope of the initial portion of the transient signal, thereby indirectly increasing the sensitivity without the need for the large sample volumes typically used by classical preconcentration methods. 

Kummer AM, Hierlemann A, Baltes H (2004) Tuning sensitivity and selectivity of complementary metal oxide semiconductor based capacitive chemical microsensors. Anal Chem 76 2470-2477 Kiunmer AM, Burg TP, Hierlemann A (2006) Transient signal analysis using complementary metal oxide semiconductor capacitive chemical microsensors. Anal Chem 78 279-290 Kurzawski P, Hagleitner C, Hierlemann A (2006) Detection and discrimination capabilities of a multitransducer single-chip gas sensor system. Anal Chem 78 6910-6920... 

Fig. 13.7 (Top) transient signals of two QCM sensors with a vic-dioxime coating to pulses of ethyl acetate (EtOAc) vapor. (Bottom) superimposed response curves of a QCM sensor with a vic-diojdme coating to pulses of (Jeff) triethylamine (Et3N) and (right) n-propanol (nPAOH) vapors (Reprinted with permission from Harbeck et aL (2011). Copyright 2011 Elsevier)...

Rule 1. The first rule is the requirement of the closed electrical circuit. This means that at least two electrodes must be present in the electrochemical cell. From a purely electrical point of view, it means that we have a sensor electrode (the working electrode) and a signal return electrode (often called the auxiliary electrode). This requirement does not necessarily mean that a DC electrical current will flow in a closed circuit. Obviously, if we consider an ideal capacitor C in series with a resistor R (Appendix C), a DC voltage will appear across the capacitor, but only as a transient DC current will not flow through it. On the other hand, if an AC voltage is applied to the cell, a continuous displacement charging current will flow. 

The effect of this subtle difference in device function can be seen when the measured signal in the presence of biofouling is modeled. As a model patient, we considered the transient response of an individual with basal insulin provided after each of the three daily meals. Blood glucose dynamics predicted by Sorensen was corrected for diffusion to subcutaneous tissue using the mass transport model of Schmidtke et al.24 25 Figure 11.1 shows a model comparison between the sensor response of an electrochemical sensor and an optical sensor with an assumed... 

In summary, the steady state and transient performance of the poly(acrylamide) hydrogel with immobilized glucose oxidase and phenol red dye (pAAm/GO/PR) demonstrates phenomena common to all polymer-based sensors and drag delivery systems. The role of the polymer in these systems is to act as a barrier to control the transport of substrates/products and this in turn controls the ultimate signal and the response time. For systems which rely upon the reaction of a substrate for example via an immobilized enzyme, the polymer controls the relative importance of the rate of substrate/analyte delivery and the rate of the reaction. In membrane systems, the thicker the polymer membrane the longer the response time due to substrate diffusion limitations as demonstrated with our pAAm/GO/PR system. However a membrane must not be so thin as to allow convective removal of the substrates before undergoing reaction, or removal of the products before detection. The steady state as well as the transient response of the pAAm/GO/ PR system was used to demonstrate these considerations with the more complicated case in which two substrates are required for the reaction. 

The bracketed term in equation (7.25) describes the current transient as an exponential function of time. The rate constant, kE, is a characteristic of the sensor—it describes the fastest rate of change that the output signal is capable of achieving. The constant is a function of the sensor geometry and the diffusion coefficients. From (7.A7) ... 

CAPRI and RASAL. These two proteins have been recently identified as sensors of distinct temporal aspects of the Ca signals. Whereas CAPRI detects the intensity of the Ca signal and nndergoes transient association with the plasma membrane, RASAL senses the freqnency by nndergoing synchronous and repetitive oscillatory associations with the plasma membrane (34). 

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