IR Transducers

After the IR passes through the sample, the resulting IR radiation is monitored using a transducer. An IR transducer changes the received IR signals to electrical signals that can be processed. Thermal, pyroelectric, and photoconducting transducers are common. 

Simply put, a pyroelectric transducer is a piece of pyroelectric material placed between two electrodes. Pyroelectric material produces elechical polarization, such as in a crystal, via a change of temperature. The charge distribution on the pyroelectric material changes when it senses temperature variation. Therefore, the received IR at different intensities will produce electric signals that can be processed. 

Photoconducting transducers use a thin film of saniconductor material. The conductivity of the thin film alters when it receives IR radiation. This change of conductivity is then detected and processed. 

IR transducers are usually enclosed in a vacuum chamber to protect them from environmental variations, such as tanperature fluctuations. 

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IR transducers are of three general types (1) pyroelectric transducers. (2) photoconducting transducers, and (3) thermal transducers. The tirst Ls found in photometers, some FTIR spectrometers, and dispersive speclropholometers. Photoconducting transducers arc found in many F TIR inslrumonis. Thermal detectors arc found in older dispersive instruments but arc too slow to be used in FTIR spectrometers. 

Bolometers. A bolometer is a type of resistance thermometer constructed of strips of metals, such as platinum or nickel, or from a semiconductor. This type of semiconductor device is sometimes called a thermistor. Semiconductor materials exhibit a relatively large change in resistance as a function of temperature. The responsive element in a bolometer is kept small and blackened to absorb the radiant heat. Bolometers are not so extensively used as other IR transducers for the mid-IR region. However, a germanium bolometer, operated at 1.5 K, is an excellent transducer for radiation in the range of 5 to 400 cm (2000 to 25 pm). The response time is a few milliseconds. 

Instruments for measuring IR absorption all require a source of continuous IR radiation and an IR transducer. The desirable characteristics of these instrument components were listed in Sections 7B and 7E. In this section we describe sources and transducers that are found in modern IR instruments. 

The absorption of IR can be detected by using one of the IR transducers or photoacoustic methods. Figure 8.5 illustrates how transducers measure the intensity change of IR absorption after it passes through the sample. The absorbance of the sample can be calculated based on IR radiation beam intensity (Equation 8.1). Since absorbance of the IR follows Beer s law and the path length of the ceU is a constant, substance concentration can be calculated using Equation 8.3. 

Basically, the Michelson interferometer, illustrated in Figure 8.7, is composed of a beam splitter, fixed mirror, and moving mirror. Incoming IR radiation is split into two beams that are reflected by the mirrors and then recombined before reaching the IR transducer. Changing the moving mirror position enables the interference of the two beams to form an interferogram that is used to obtain the spectra. 

Recent advances in instrumentation range from novel (laser) sources and highly compact spectrometers over waveguide technology to sensitive detectors and detector arrays. This, in combination with the progress in electronics, computer technology and chemometrics, makes it possible to realise compact, robust vibrational spectroscopic sensor devices that are capable of reliable real-world operation. A point that also has to be taken into account, at least when aiming at commercialisation, is the price. Vibrational spectroscopic systems are usually more expensive than most other transducers. Hence, it depends very much on the application whether it makes sense to implement IR or Raman sensors or if less powerful but cheaper alternatives could be used. 

Applications of infrared sensors have been reported in the fields of biology/biochemistry, medical diagnostics, environmental monitoring and process control. IR sensors can measure analytes in solid, liquid or gaseous form using one of several different transducer layouts. 

For IR sensing, three transducer principles are standard classical transmission for (sufficiently) transparent samples, (diffuse) reflection for opaque samples, in particular solids and strongly turbid liquids and attenuated total reflection (ATR), in particular for strongly absorbing samples and fluids with varying amounts of suspended solids or gas bubbles. 

JAKs and signal transducers and activators of transcription (STATs) are functionally analogous with IRS and PI3K. JAKs are physically associated with a cell surface receptor (e.g. for leptin, erythropoietin (EPO), growth factors or cytokines) STATs are free monomeric proteins within the cytosol but following phosphorylation by a JAK, individual proteins dimerize and then move into the nucleus of the cell where they control gene expression. 

Signal transduction The binding of insulin to the a-subunits of the insulin receptor induces conformational changes that are transduced to the 3-subunits. This promotes a rapid autophosphorylation of a specific tyrosine residue on each 3-subunit (see Figure 23.7). Autophosphorylation initiates a cascade of cellsignaling responses, including phosphorylation of a family of pro teins called insulin receptor substrate (IRS) proteins. At least four... 

Remember that the interfacial potential is a difference between the inner potential of the electrode material and the potential established in the bulk of the solution. An iR drop can cause the latter to change. One way to minimize this problem is to construct the cell with an auxiliary electrode(s) that is directly across from the working electrodes, resulting in a very short current path and, therefore, a very small iR drop. Multiple-electrode transducers constructed in this way minimize cross-talk between different electrodes. 

The transducers most commonly employed in biosensors are (a) Electrochemical amperometric, potentiometric and impedimetric (b) Optical vibrational (IR, Raman), luminescence (fluorescence, chemiluminescence) (c) Integrated optics (surface plasmon resonance (SPR), interferometery) and (d) Mechanical surface acoustic wave (SAW) and quartz crystal microbalance (QCM) [4,12]. 

Transparency to the incident radiation is a trait of a transducer. Yttrium oxide, doped with lanthanum oxide is transparent to IR radiation. Blocking of undesired UV radiation is achieved by cerium doped glasses. Some examples of modulating transducers are given in Table 12.17. 

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