An optode or optrode is an optical sensor A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output device that optically measures a specific substance usually with the aid of a chemical transducer A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute to another for various purposes including measurement or information transfer.
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Construction
An optode requires three components to function: a chemical that responds to an analyte An analyte is a substance or chemical constituent that is determined in an analytical procedure, such as a titration. For instance, in an immunoassay, the analyte may be the ligand or the binder, while in blood glucose testing, the analyte is glucose. In medicine, analyte often refers to the type of test being run on a patient, as the test is, a polymer A polymer is a large molecule (macromolecule) composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties to immobilise the chemical transducer and instrumentation (optical fibre An optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data, light source Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye , or perhaps 380–750 nm. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not, detector and other electronics). Optodes usually have the polymer matrix coated onto the tip of an optical fibre, but in the case of evanescent wave "Evanescent" means "tends to vanish". An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation optodes the polymer is coated on a section of fibre that has been unsheathed.
Operation
Optodes can apply various optical measurement schemes such as reflection, absorption In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat. The absorption of light during wave propagation is often called attenuation. Usually, the, evanescent wave, luminescence Luminescence is light that usually occurs at low temperatures, and is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light generated by high temperatures. Historically, radioactivity was thought of (fluorescence Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer wavelength. The energy difference between the absorbed and emitted photons ends up as molecular rotations, vibrations or heat. Sometimes the absorbed photon is in and phosphorescences Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur less), chemiluminescence Chemiluminescence is the emission of light with limited emission of heat (luminescence), as the result of a chemical reaction. Given reactants A and B, with an excited intermediate ◊,, surface plasmon resonance The excitation of surface plasmons by light is denoted as a surface plasmon resonance for planar surfaces or localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures. By far the most popular methodology is luminescence.
Luminescence in solution obeys the linear Stern-Volmer relationship The Stern-Volmer relationship, named after Stern and Max Volmer, as cited in allows us to explore the kinetics of a photophysical intermolecular deactivation process. Fluorescence of a molecule is quenched Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching. As a consequence, quenching is often heavily dependent on pressure and temperature. Molecular oxygen and the by specific analytes, e.g. ruthenium Ruthenium is a chemical element that has the symbol Ru and atomic number 44. A rare transition metal of the platinum group of the periodic table, ruthenium is found associated with platinum ores and used as a catalyst in some platinum alloys complexes are quenched by oxygen. When a fluorophore A fluorophore, in analogy to a chromophore, is a component of a molecule which causes a molecule to be fluorescent. It is a functional group in a molecule which will absorb energy of a specific wavelength and re-emit energy at a different wavelength. The amount and wavelength of the emitted energy depend on both the fluorophore and the chemical is immobilised within a polymer matrix a myriad of micro-environments are created. The micro-environments reflect varying diffusion co-efficients for the analyte. This leads to a non-linear In mathematics, a nonlinear system is a system which is not linear, that is, a system which does not satisfy the superposition principle, or whose output is not proportional to its input. Less technically, a nonlinear system is any problem where the variable to be solved for cannot be written as a linear combination of independent components. A relationship between the fluorescence and the quencher (analyte). This relationship is modelled in various ways, the most popular model is the two site model created by James Demas (University of Virginia).
The signal (fluorescence) to oxygen ratio is not linear, and an optode is most sensitive at low oxygen concentration, i.e. the sensitivity decreases as oxygen concentration increases. The optode sensors can however work in the whole region 0–100% oxygen saturation Oxygen saturation or Dissolved oxygen is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water in water, and the calibration is done the same way as with the Clark type sensor Leland Clark had developed the first bubble oxygenator for use in cardiac surgery. However, when he came to publish his results, his article was refused by the editor since the oxygen tension in the blood coming out from the device could not be measured. Necessity is the mother of invention, and after a couple of trials, Clark presented his Oxygen. No oxygen is consumed and hence the sensor is stirring insensitive, but the signal will stabilize more quickly if the sensor is stirred after being put into the sample.
Popularity
Optical sensors are growing in popularity due to the low-cost, low power requirements and long term stability. They provide viable alternatives to electrode-based sensors or more complicated analytical instrumentation.
Major international conferences are devoted to their development e.g. Europtrode VIII Tübingen Tübingen, a traditional university town in Baden-Württemberg, Germany, is situated 30 km southwest of Stuttgart, on a ridge between the Neckar and Ammer rivers 2006, OFS 18, Cancun 2006.
See also
Categories: Optical devices | Sensors | Spectroscopy Spectroscopy is the study of the interaction between radiation and matter. Spectrometry is the measurement of these interactions and a machine which performs such measurements is a spectrometer or spectrograph. A plot of the interaction is referred to as a spectrogram, or, informally, a spectrum | Fluorescence
