Chemical species sensors

MAJOR limitation TO research on surface-exchange and flux measurements is the lack of sensitive, reliable, and fast-response chemical species sensors that can be used for eddy correlation flux measurement. Therefore we recommend that continued effort and resources be expended in developing chemical species sensors with the responsiveness and sensitivity required for direct eddy correlation flux measurements. This recommendation (I) was assigned the first priority in the report of the recent Global Tropospheric Chemistry workshop jointly convened by the National Science Foundation, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration. The authors of the report recognized that the limited availability of fast, accurate chemical sensors is a major measurement challenge in the field of atmospheric chemistry. 

The energies in Table 2.1 are listed as enthalpies (AH), but the driving forces in the chemical species/sensor interactions are really the changes of free energy (AG), which include the change of entropy (A5). At constant temperature, the two are related by (2.1). 

Continued advances in analytical instmmentation have resulted in improvements in characterization and quantification of chemical species. Many of these advances have resulted from the incorporation of computet technology to provide increased capabiUties in data manipulation and allow for more sophisticated control of instmmental components and experimentation. The development of rniniaturized electronic components built from nondestmctible materials has also played a role as has the advent of new detection devices such as sensors (qv). Analytical instmmentation capabiUties, especially within complex mixtures, are expected to continue to grow into the twenty-first century. 

In contrast, various sensors are expected to respond in a predictable and controlled manner to such diverse parameters as temperature, pressure, velocity or acceleration of an object, intensity or wavelength of light or sound, rate of flow, density, viscosity, elasticity, and, perhaps most problematic, the concentration of any of millions of different chemical species. Furthermore, a sensor that responds selectively to only a single one of these parameters is often the goal, but the first attempt typically produces a device that responds to several of the other parameters as well. Interferences are the bane of sensors, which are often expected to function under, and be immune to, extremely difficult environmental conditions. 

In the field of chemical sensors, the revolution in software and inexpensive hardware means that not only nonlinear chemical responses can be tolerated, but incomplete selectivity to a variety of chemical species can also be handled. Arrays of imperfectly selective sensors can be used in conjunction with pattern recognition algorithms to sort out classes of chemical compounds and thek concentrations when the latter are mixed together. 

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). 

Narayanaswamy R., Sevilla F., Optical fiber sensors for chemical species, J. Phys. E Sci. Instrum. 1988 21 10. 

Optical sensors (Figure 1) can be defined as devices for optical monitoring of physical parameters (pressure1, temperature2, etc.) or (bio)chemical properties of a medium by means of optical elements (planar optical waveguides or optical fibres). Chemical or biochemical fibre-optic sensors3 are small devices capable of continuously and reversibly recording the concentration of a (bio)chemical species constructed be means of optical fibres. 

Optical sensors rely on optical detection of a chemical species. Two basic operation principles are known for optically sensing chemical species intrinsic optical property of the analyte is utilized for its detection indicator lor label) based sensing is used when the analyte has no intrinsic optical property. For example, pH is measured optically by immobilizing a pH indicator on a solid support and observing changes in the absorption or fluorescence of the indicator as the pH of the sample varies with time1 20. 

Optical sensors for oxygen measurement are attractive since they can be fast, do not consume oxygen and are not easily poisoned. The most common method adopted in construction is based on quenching of fluorescence from appropriate chemical species. The variation in fluorescence signal (I), or fluorescence decay time (x) with oxygen concentration [O2] is described by Stem-Volmer equation91 ... 

Optical biosensors can be designed when a selective and fast bioreaction produces chemical species that can be determined by an optical sensor. Like the electrochemical sensors, enzymatic reactions that produce oxygen, ammonia, hydrogen peroxide, and protons can be utilized to fabricate optical sensors. 

OFDs can be divided into two subclasses (1) optical fiber chemical detectors (OFCD) which detect the presence of chemical species in samples, and (2) optical fiber biomolecular detectors (OFBD) which detect biomolecules in samples. Each subclass can be divided further into probes and sensors, and bioprobes and biosensors, respectively. As a result of the rapid expansion of optical research, these terms have not been clearly defined and to date, the terms probe and sensof have been used synonymously in the literature. As the number of publications increases, the terminology should be clarified. Although both probes and sensors serve to detect chemicals in samples, they are not identical. The same situation exists with bioprobes and biosensors. Simply, probes and bioprobes are irreversible to the analyte s presence, whereas sensors and biosensors monitor compounds reversibly and continuously. 

This sensitivity to slow electron transfer kinetics could, however, prove to be an advantage in sensor applications where a mediator, with fast electron transfer kinetics, is used to shuttle electrons to a redox enzyme [82]. Chemical species that are electroactive in the same potential region as the mediator can act as interferants at such sensors. If such an interfering electroactive species shows slow electron transfer kinetics, it might be possible to eliminate this interference at the NEE. This is because at the NEE, the redox wave for the kinetically slow interferant might be unobservable in the region where the kinetically fast mediator is electroactive. We are currently exploring this possibility. 

Selective. Sensors should have minimum interference from nonanalyte parameters such as temperature, ionic strength, pressure, and other chemical species not being measured. One route to successful sensor design will rely on the identification of appropriate and selective recognition chemistry. 

The potential of an electrode is related directly to the activities, and thus indirectly to the concentrations, of the chemical species involved in the equilibria that establish the potential. The main virtues of potentiometry are simplicity, very low power requirements, and the possibly small size of the sensors. The main drawback is that an unwanted reaction may enter into determination of the potential and sensitivity may be poor. Potentio-metric sensing, as a transducing technique, can be coupled with an infinite... 

Chemical sensing is part of an information-acquisition process in which an insight is obtained about the chemical composition of the system in real-time. In this process, an amplified electrical signal results from the interaction between some chemical species and the sensor. Generally, the interaction consists of two steps recognition and amplification. One common example is the measurement of pH with a glass electrode (Fig. 1.1). 

Generally speaking, we can distinguish two types of interactions between the chemical species and the sensor a surface interaction in which the species of interest is adsorbed at the surface, and a bulk interaction in which the species of interest partitions between the sample and the sensor and is absorbed. The classification of the interaction as either surface or bulk is relative with respect to the size of the species. It is the case of chicken and chicken wire. Obviously, a chicken wire fence is impervious to chickens, but presents no barrier whatsoever to mosquitoes. Similarly, large molecules, such as proteins, may adsorb at the surface of the sensor layer, whereas smaller ions can penetrate and absorb in the bulk. 

Interaction of a chemical species (X) with sensor (S) can be described by the equilibrium ... 

Clearly, a mass-related signal will be obtained only if the species-sensor interaction results in a net change of mass of the chemically selective layer attached to the device. Thus, an equilibrium binding will yield a measurable signal. On the... 

In Fig. 2.10, the boundary between the enzyme-containing layer and the transducer has been considered as having either a zero or a finite flux of chemical species. In this respect, amperometric enzyme sensors, which have a finite flux boundary, stand apart from other types of chemical enzymatic sensors. Although the enzyme kinetics are described by the same Michaelis-Menten scheme and by the same set of partial differential equations, the boundary and the initial conditions are different if one or more of the participating species can cross the enzyme layer/transducer boundary. Otherwise, the general diffusion-reaction equations apply to every species in the same manner as discussed in Section 2.3.1. Many amperometric enzyme sensors in the past have been built by adding an enzyme layer to a macroelectrode. However, the microelectrode geometry is preferable because such biosensors reach steady-state operation. 

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