Chemical sensing passive/active

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... 

Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path through the bilayer for specific solutes. Proteins that bring about this facilitated diffusion, or passive transport, are not enzymes in the usual sense their substrates are moved from one compartment to another, but are not chemically altered. Membrane proteins that speed the movement of a solute across a membrane by facilitating diffusion are called transporters or permeases. 

Optical remote sensing (ORS) technologies, employing infrared spectral analysis techniques, have been utilized in the development of chemical agent standoff detection technologies. Within the ORS technologies, there are two types of remote sensing systems passive and active (laser). The section below only looks at the passive system, which employs a Fourier Transform Infrared (FTIR) spectrometer. 

Not only the components of the sensing layer are important but also the strategies of integration between them and with the primary transducer are of paramount importance. Passive adsorption is one of the simplest and most frequently used immobilization techniques, based mainly on weak noncovalent bindings. However, random protein orientation can lead to the obstruction of functional binding sites and can result in the loss of affinity or activity. Thus, covalent attachment of a protein layer on a chemically functionalized surface produces a more stable layer with correct orientation. This influences sensitivity and specificity of the immunoassay as density of immobilized protein can be better controlled and nonspecific adsorption may be decreased [155]. On the other hand, covalent procedures are usually longer and more tedious and are less justified when disposable surfaces can be used. 

The multifunctional textiles such as fashion and environmental protection, ballistic and chemical protection, flame protection are all passive systems. The smart textiles are a new generation of fibers, yams, fabrics and garments that are able to sense stimuli and changes in tiieir environments, such as mechanical, thermal, chemical, electrical, magnetic and optical changes, and then respond to these changes in predetermined ways. They are multifunctional textile systems that can be classified into three categories of passive smart textiles, active smart textiles and very smart textiles [33],... 

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