An optode or optrode is an optical sensor

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-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

 device that optically measures a specific substance usually with the aid of a chemical transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

.

Construction

An optode requires three components to function: a chemical that responds to an analyte

Analyte

An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...

, a polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 to immobilise the chemical transducer and instrumentation (optical fibre

Optical fiber

An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

, light source

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, 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 wave

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 applies...

 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

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

, absorption

Absorption (electromagnetic radiation)

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...

, evanescent wave, luminescence

Luminescence

Luminescence is emission of light by a substance not resulting from heat; it 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 emitted by a...

 (fluorescence

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 and phosphorescence

Phosphorescence

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...

s), chemiluminescence, surface plasmon resonance

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 for nanometer-sized metallic structures....

. By far the most popular methodology is luminescence.

Luminescence in solution obeys the linear Stern–Volmer relationship. Fluorescence of a molecule is quenched

Quenching (fluorescence)

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...

 by specific analytes, e.g., ruthenium

Ruthenium

Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...

 complexes are quenched by oxygen. When a fluorophore

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...

 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

Nonlinearity

In mathematics, a nonlinear system is one that does not satisfy the superposition principle, or one whose output is not directly proportional to its input; a linear system fulfills these conditions. In other words, a nonlinear system is any problem where the variable to be solved for cannot be...

 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

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.It has particular significance in medicine and...

 in water, and the calibration is done the same way as with the Clark type sensor

Clark electrode

The Clark electrode is an electrode that measures oxygen on a catalytic platinum surface using the net reaction:- History :Leland Clark had developed the first bubble oxygenator for use in cardiac surgery...

. 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

Tübingen is a traditional university town in central Baden-Württemberg, Germany. It is situated south of the state capital, Stuttgart, on a ridge between the Neckar and Ammer rivers.-Geography:...

2006, OFS 18, Cancun 2006.