Reliability, chemical sensors

Finally, there is an urgent and growing demand for analytical chemists and engineers skilled in producing reliable chemical sensors delivering results that are not only reproducible but also accurate (free of systematic errors attributable to interferences from the various sample matrices). 

A complete system providing both a sensor and an actuator would be ideal in the field of process control, but because of a lack of truly reliable chemical sensors on the market the concept has not been widely implemented. One exception relates to the analytical method of coulometry, a technique that offers great potential for delivering chemical compounds to a controlled reaction. Especially attractive in this context is the method of constant- current coulometry, which can be carried out with an end-point sensor and a coulomet-ric actuator for maintaining a generator current until the end-point has been reached. In this case both of the required devices can be miniaturized and constructed with the same technology. 

Semiconductor chemical sensors are characterized by low cost, small size, extra high sensitivity (often unattainable in other analytical techniques) as well as reliability. Moreover, concentration of particles detected is being transformed directly into electrical signal and electronic design of the device is the simplest one which can be arranged for on the active part of the substrate. 

The results reported show that the joint use of different spectroscopic techniques allows the complete characterization, both structural and elemental, of the materials under study. The measurements performed with these techniques are fast, reliable and virtually non-destructive. The results obtained are, therefore, encouraging for the use of optical chemical sensors in the field of Cultural Heritage. 

In parallel with improvements in chemical sensor performance, analytical science has also seen tremendous advances in the development of compact, portable analytical instruments. For example, lab-on-a-chip (LOAC) devices enable complex bench processes (sampling, reagent addition, temperature control, analysis of reaction products) to be incorporated into a compact, device format that can provide reliable analytical information within a controlled internal environment. LOAC devices typically incorporate pumps, valves, micromachined flow manifolds, reagents, sampling system, electronics and data processing, and communications. Clearly, they are much more complex than the simple chemo-sensor described above. In fact, chemosensors can be incorporated into LOAC devices as a selective sensor, which enables the sensor to be contained within the protective internal environment. Figure 5... 

The possibility of a reliable representation of a chemical sensor array data set in subspaces of smaller dimension lies in the fact that the individual sensors always exhibit a high correlation among themselves. PCA consists of finding an orthogonal basis where the correlation among sensors disappears. 

Finally, for a (bio)chemical sensor to effectively solve real problems it should require no immediate interpretation of its response (e.g. in order to alter some physical or physico-chemical parameter influencing its operation). In practice, this requires that the sensor be reliably used by unskilled personnel, who often work under stressing conditions, in order to avoid the human factor as a source of error in the results produced by (bio)chemical sensors. 

In the past few decades, a precise methodology has been developed for sensory evaluation and has proved to give reliable results [826]. Increasingly, in recent years chemical sensor systems ( electronic noses ) are used for such purposes [826a]. 

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. 

Experiments to date have shown that a portable instrument incorporating a thoughtfully chosen array of sensors can detect, identify, and quantify a wide variety of chemicals in air. Also, pattern recognition techniques are being used to understand the information content of the arrays and to focus future experimental work. Development of smaller, more sensitive, and more reliable electrochemical sensors will expand the applications of the system described here. 

As with the majority of ISEs, all of the aforementioned receptors are immobilised within close proximity to the transducer element. However, conducting polymers (electroactive conjugated polymers) are now emerging rapidly as one of the most promising classes of transducer for use within chemical sensors. Here, the receptor can be doped within the polymer matrix, i.e. within the transducer element itself. This will facilitate the production of reliable, cost-effective, miniaturised anion-selective sensors, as it will be possible to move away from plasticiser-based membranes, but allow for ion recognition sites in conjunction with all-solid-state ion-to-electron transducers. 

Last, but not least, full application of QDs in chemical sensors would require the immobilization of the nanoparticles into appropriate solid supports in order to develop reliable active phases (able to provide, for instance, convenient fiber optic-based sensing applications). Although only a few reports have been published so far regarding the trapping of the QDs in solid matrices, some important steps have already started towards the realization of the potential of these technologies. There is still plenty of room for further development in all those directions. 

The first group, which is developed in this chapter, use ion selective electrodes (ISE). The principle of these chemical sensors is to create an electric cell in which the analyte behaves in such a way that the potential difference obtained relates to its concentration. Measurement of pH, probably the most common and best known electroanalytical method, is part of this group. Most of the measurements concern the determination of ions in aqueous solution, though particular electrodes with selective membranes also allow the determination of molecules. The sensitivity of these methods is very great for certain ions but matrix is sometime responsible for lack of reliability in these measurements. In such cases, complexometric or titrimetric methods must replace direct potentiometry. It remains however for potentiometry multiple applications in which the instruments range from low-cost pH meters to automatic titrators. 

Chemical sensors and biosensors are relatively new measurement devices. Up until approximately 30 years ago, the glass pH electrode could be considered the only portable chemical sensor sufficiently reliable for measuring a chemical parameter. Even this sensor, which has been under continuous development since it invention in 1922 (table 1.2), needs to be recalibrated on a daily basis and is limited to measurements in solutions or on wet surfaces. 

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

Measurements are divided into two types, physical and chemical. Physical measurements such as temperature or viscosity usually do not require an FIA system. Sensors for these types of physical measurement are both simple and reliable. Chemical measurements involve additional sample manipulations before the analyte can be detected. Given that a sensor has been designed to measure a specific analyte, an automated system is used to pretreat and deliver a reproducible sample to the sensor. 

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