Sensors chemical environment

So far we have discussed the one-sensor/one-analyte approach. However, arrays of independent electrodes can offer much more analytical information and thus hold a great potential for many practical applications. These include the development of intelligent sensing systems capable of responding to changes in the chemical environment of the array. 

The silver clusters can be applied as fluorescent probes to retrieve information about the chemical environment. There are reported three classes of sensors based on silver clusters. First, we discuss silver cluster sensors of which the fluorescence quenches in the presence of the analyte. Second, we discuss a sensor in which fluorescent clusters are formed only in presence of the analyte. Finally, we discuss the shift in the absorption and fluorescence bands of silver clusters while sensing the chemical environment. 

Muscles contract and expand in response to electrical, thermal, and chemical stimuli. Certain polymers, such as synthetic polypeptides, are known to change shape on application of electric current, temperature, and chemical environment. For instance, selected bioelastic smart materials expand in salt solutions and may be used in desalination efforts and as salt concentration sensors. Polypeptides and other polymeric materials are being studied in tissue reconstruction, as adhesive barriers to prevent adhesion growth between surgically operated tissues, and in controlled drug release, where the material is designed to behave in a predetermined matter according to a specific chemical environment. 

As mentioned earlier, chemicals and explosives are usually detected spectroscopically. Fiber-optic-based chemical sensors have application in the real-time tracking of rapidly changing chemical environments. These sensors provide rapid... 

SiC technology provides the opportunity for the exploitation of chemical gas sensors in the near future in environments and applications that are problematic today. The extension of this technology to these areas will enable the reduction of unwanted emissions and will promote reductions in the use of fuel and energy, resulting in positive implications for natural resources and the environment. 

One redox species is chemically insensitive with respect to variation in E1/2, e.g. a ferrocene derivative, and serves as an internal reference in a linear sweep voltammogram. The second species is chemically sensitive, e.g. a pH sensitive quinone or a CO sensitive ferraazetine derivative, which has an E1/2 that varies with the changes in the chemical environment. A linear sweep voltammogram thus shows two waves, one for the reference molecule and one for the indicator molecule. The shift for the indicator wave along the potential or current axis provides a method for analyte detection. Surface derivitization, proof-of-structure, and proof-of-concept sensor functions are demonstrated. 

Ceramic chemical sensors fall into two broad categories, namely those that exploit solid electrolytes and those that exploit electronic conductors. In all cases the sensors respond to changes in the chemical environment. The operational principles and typical applications are described below. 

Despite their advantages of specificity and selectivity, biosensors suffer from a severe disadvantage, the instability of the biological recognition system, which makes storage and operation in harsh chemical environments problematic. This problem has been addressed using a newly developed sensor with injected recognition element biosensors (whose operational principle is based on the one time use... 

Asymmetry in the lipid distribution over the bilayer could also be controlled in a similar way by the lateral packing pressure, which is likely to differ between constituent monolayers, due to the distinct chemical environments inside and outside the membrane. The enzymes involved may also be distributed asymmetrically. A configuration with constant, but nonzero, mean curvature, shown in Fig. 5.7, reflects such a situation. A membrane-spatming protein can then be viewed as a sensor of the lateral packing pressure in both monolayers. This speculation has some experimental justification. In a recent study of chromaffin granules, trans-membrane lipid asymmetry was shown to be induced by an ATP-dependent "flippase" [35]. 

While most measurements of tempemture in industrial processing simply involve the use of thermocouples or platinum resistance thermometers, there are some applications involving microwave fields or chemical environments for which this might not be possible. In that case a fibre-optic sensor is the instrument of choice. These sensors are often also sensitive to stmin, so they may be used to obtain a measure of pressure and local deformation. The operating principle is discussed in the following section. 

The adaptation of tissue blood flow to local metabolic requirements is a dramatic example of the control of smooth muscle contraction by metabolic factors. The exact mechanisms involved are still not known with certainty, and in particular it is not clear how much is due to a direct response of the vascular smooth muscle to its chemical environment and how much is due to "vasodilator metabolites" released from the tissue parenchyma (Sparks, 1980). The possibility that structures other than smooth muscle cells, notably the endothelium, act as oxygen sensors is a further area for investigation. The emphasis here will be on studies relating the metabolic supply of smooth muscle and its contractile activity. 

The success of FET-based CNT gas sensors quickly attracted interest for use in biosensing applications, particularly since biomolecules such as DNA and proteins are heavily charged under normal conditions. SWCNT FETs are expected to be more sensitive than chemisorbed gas molecules to the binding of such charged species. However, the wet chemical environment with the presence of various ions and other biomolecules makes it much more complicated than gas environments. Studies demonstrated that proteins in the solution tend to adsorb irreversibly onto the bare CNT surface and induce significant change in FET characteristics [67,68]. Thus, extensive efforts have been made to passivate CNT with various polymers, such as PEI, poly(ethylene glycol) (PEG), Nafion, or Tween 20 [67,69,70]. It is conunonly accepted that a polymer or surfactant... 

In this chapter, only brief descriptions of traditional probes of the physical and chemical environments will be presented. Instead, our focus will be on the on-line (and quasi on-line) monitoring of bioprocesses. This type of measurement is required for bioprocess development, control, and optimization and places special demands on the types of sensors involved. In addition, special attention will be given to biosensors. These new probes provide on-line measurements of compounds that would otherwise require off-line analysis and have greatly increased the power of automated bioprocess monitoring and control. 

The most frequently used sensors of the chemical environment in biotechnology are poten-tiometric pH and amperometric p02 electrodes (see Figures 22-9 and 22-10). A huge variety of sterilizable electrodes of these types is offered by several companies. Since these sensors are used in situ, problems can arise because recalibration is not possible when no calibration system (see 22.3.1) is used (see also Chapter 7). 

Romain, A.-C., Andre, P., Nicolas, J. Three years experiment with the same tin oxide sensor arrays for the identification of malodorous sources in the environment. Sensors and Actuators B Chemical 84, 271-277 (2002)... 

---