Chemical sensor, definition

Hulanicki A., Glab S., Ingman F., Chemical sensors definitions and classification. Commission on General Aspects of Analytical Chemistry, Pure Appl. Chem. 1991 63 1247. 

Hulanicki, A., Glab, S., and Ingman, FA. (1991) Chemical sensors - Definitions and classification. Pure Appl Chem. (Tech. Rep.), 63, 1247-1250. 

Hulanicki A, Glab S, Ingman F (1991) Chemical sensors definitions and classificatioiL Pure Appl Chem 63(9) 1247-1250... 

Conductometric sensors, measuring the variatimi of the solution conductance due to electrical charge concentrations changes. The method is simple but not selective, since the conductance depends on the ionic concentration of all of the present species. In view of the fact that no electrochemical processes take place, conductometric sensors are not strictly electrochemical ones. They are considered, according to lUPAC chemical sensors definitions and classification [1], as a subclass of electrical devices in which the signal results from the change of electrical properties caused by the interaction of the analyte. Nevertheless, electrical devices are frequently put into one category with the electrochemical devices [1]. 

Definition the electronic tongue is an analytical instrarment including an array of low-selective chemical sensors and appropriate pattern recognition tool, capable to recognize quantitative and qualitative compositions of simple and complex solutions . 

Let us start with a definition. Semiconductor chemical sensor is an electronic device designed to monitor the content of particles of a certain gas in surrounding medium. The operational principle of this device is based on transformation of the value of adsorption directly into electrical signal. This signal corresponds to amount of particles adsorbed from surrounding medium or deposited on the surface of operational element of the sensor due to heterogeneous diemical reaction. 

We apply some of the above definitions to practical examples related to temperature and chemical sensors. 

Janata, J., Principles of Chemical Sensors, Plenum Press, New York, 1989. Some sensor specialists regard this as the definitive work on the subject. While extremely dated, its introductory sections provide a clear, uncluttered introduction to the different modes of sensor operation. 

Bio)chemical sensors can be active or passive according to whether they use a sensing microzone to accommodate a chemical or biochemical reaction and/or a biochemical e.g. immunological) or physico-chemical separation e.g. sorption). It should be noted that passive sensors e.g. a fibre-optic tip immersed in an industrial process stream) do not meet one of the essential requirements included in the definition of sensors as regeirds composition... 

The twofold definition given in Section 1.3 encompasses several properties a (bio)chemical sensor must have in order to fulfil the objectives demanded by the analytical quality level to be achieved. Some such properties are mandatory, whereas others are only desirable. Figure 1.15 shows advantageous and essential sensor properties arranged in fom groups that are discussed below. 

The thin layer technology is also used in the fabrication of chemical sensors. Clechet58 defines a chemical sensor as a physical device, called a transducer which delivers an electric signal, which is controlled by its sensitized part, called the detector. This detector must be selective i.e. must be prepared, or functionalized in order to respond only to the substance or group of substances to be detected. Since sensor technology goes beyond the scope of this work, we will limit ourselves here to this definition. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

Chemical sensors are by definition small, inexpensive and preferably hand-held devices, capable of continuously monitoring chemical constituents in liquids or gases. MIP sensors usually consist of an imprinted sensitive layer and a transducer to convert the chemical information, in real time, into an electrical or optical signal which is further evaluated electronically [12]. Figure 21.1 shows the set-up of chemosensors and two typical mass-sensitive devices. 

Biosensors can be defined as chemical sensor systems in which an analyte is detected based on biochemical processes or biochemical utilization. A biosensor is mostly composed of a biological element responsible for sampling and tracing, and a physical element called a transducer responsible for sample transmission and further processing (see also Part V, Chapters 8 and 9). The term biosensor does not really meet the lUPAC definition, in which sensors are defined to be self-containing, perform continuous monitoring and are reversible. For the purpose of this chapter, the term biosensor will not be so strictly used as in the traditional context. 

In this text, we distinguish between chemical sensors and biosensors according to the nature of their reactive surface. By this definition, chemical sensors utilize specific polymeric membranes, either per se or containing doping agents, or are coated with non-biological (usually low-molecular-weight) materials. These polymeric layers or specific chemicals, attached to the layers or directly to the transducer, interact with and measure the analyte of interest. The nature of the analyte or the reaction which takes place is not limited with such chemical sensors. 

Given these definitions, we can further define the basic components of a chemical sensor or biosensor. These include the active surface, the transducer, and the electronics/software as shown in figure 1.1. 

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