Optrode

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Optrode , or optode , (opto- ancient Greek ὤψ (ops ) for eye) is a name for an optical sensor that uses a chemical energy converter to measure the concentration of substances (such as the oxygen content in a water sample). The names were presented in 1975 by Lübbers and Opitz and are intended to make it clear that optical and non-electrical signals are measured. Optrodes are becoming increasingly popular because of their low cost, low energy requirements, and long stability. They are viable alternatives to electrode sensors or more complex analytical instruments, especially in the field of environmental monitoring.

construction

An Optrode is made up of three main components:

Schematic structure of an optrode
  • A chemical energy converter that reacts with the analyte
  • A polymer to immobilize the chemical energy converter
  • Technical components (a light source, a detector, fiber optic cables or fiber optic cables and other electronic components)

The energy converter is specifically selected for the measurement of the analyte to be examined. The polymer matrix of the optrodes is usually attached to the tip of the fiber optic cable. When using evanescence waves, however, it is applied to a section of the guide cable that has not been sheathed.

functionality

General principle

How an optrode works (schematic): The energy converter (E) immobilized in a polymer matrix (P) is initially in the non-excited state (gray), but is activated by light irradiation so that it fluoresces (red). This fluorescence is now (continuously) quenched by reaction with the analyte (A) so that the subsequently measured fluorescence is weakened (orange).

Optrodes can use different optical measurement principles, such as reflection , absorption , evanescence, luminescence ( fluorescence and phosphorescence ), chemiluminescence or surface plasmon resonance . The most common measuring principle here is the use of luminescence, which follows the linear Stern-Volmer equation in solution .

First, the Optrode emits light of a specific wavelength, which stimulates the fluorescence of the specifically selected energy converter. This fluorescence is partially extinguished by the analyte , which is why the analyte can also be referred to as a quencher . Because the fluorescent energy converter is immobilized in a polymer matrix, myriads of microenvironments are created which have diffusion coefficients that vary in relation to the analyte. This leads to a non-linear relationship between the measurable fluorescence and the quencher (analyte). More quencher molecules lead to a lower emission signal, which is a measure of the concentration in the sample.

Oxygen optrode

If oxygen is used as a quencher, e.g. B. in connection with ruthenium complexes as fluorescent energy converter, one speaks of an oxygen optrode. They were developed by Kautsky in 1939. The actual Optrode is most sensitive when the oxygen concentration is low because oxygen reduces the lifetime of the fluorescence, but the Optrode sensors can operate in the entire range of 0–100% oxygen saturation in water. The calibration is carried out in the same way as with the Clark sensor . Since no oxygen is consumed, the sensor is insensitive to stirring, but this will stabilize the signal more quickly. Measurements can thus be carried out in a stationary system. The pressure during a measurement can influence this in two ways: it destabilizes the fluorescent energy converter and also reduces quenching. A big point of criticism, however, is the lower resolution compared to modern cathodic microsensors. Oxygen optrodes are often used in oxygen measurements in bodies of water. The “free” dissolved oxygen is measured here and not the oxygen atoms contained in the water molecules. These measurements have been of immense importance in environmental monitoring since 2006, especially in low-oxygen regions such as u. a. shallow coastal areas with significant algal blooms, fjords or other areas with limited exchange of water, around fish farms or in areas of interest for dumping of rubbish.

Glucose optrode

Glucose optrodes are biochemical sensors for determining the concentration of glucose in a solution using optical measuring principles. Usually, glucose oxidase (GOD) is immobilized in a matrix or membrane, an enzyme that catalyzes the oxidation of glucose. It is also used in non-optical measurement methods, such as the GOD test .

Thus the decrease in oxygen can be correlated with the glucose concentration. As a result, oxygen is not measured directly, rather than glucose, which is why it can be described more precisely as a modified oxygen optrode. Alternative measurement methods use pH sensitive dyes to measure the change in light absorption of the dye during GOD oxidation. However, these are dependent on the buffer capacity of the medium, which is why the use of oxygen optrodes is preferred, despite their limitations in terms of resolution.

Another measuring principle was developed by Schulz et al. described; the use of immunoaffinity-glucose sensor based on the reversible competitive binding of glucose and fluorescein -labeled dextran to the sugar binding site of the protein Concanavalin A . A separate measuring volume is created in a hollow fiber by membranes. This measuring volume contains fluorescein-labeled dextran molecules which are bound to the protein Concanavalin A, which is immobilized on the inner surface of the hollow fiber and which can be excited to fluoresce. Glucose can penetrate through the membrane into the measuring volume and now competes with the dextran for the places on the protein and an equilibrium is established . The Optrode now measures the change in fluorescence and correlates it with the glucose concentration. The principle can thus be used in practice using hollow fiber dialysis to determine the glucose concentration in the blood, since the dextran cannot penetrate the membrane and gets into the bloodstream. However, measurements with this method only have a very slow response time.

Individual evidence

  1. ↑ W. Göpel, TA Jones, M. Kleitz, J. Lundström, T. Seiyama (Eds.): Chemical and Biochemical Sensors: Part I (= W. Göpel, J. Hesse, JN Zemel [Eds.]: Sensors - A Comprehensive Survey. Vol. 2). VCH Verlagsgesellschaft mbh., Weinheim / New York / Basel / Cambridge / Tokyo 1991, ISBN 3-527-26768-9 , pp. 576 .
  2. ↑ a b c Oxygen Optode 4330 / 4330F. (PDF) In: Aanderaa. Accessed April 20, 2018 .
  3. ↑ A. Tengberg et al .: Evaluation of a lifetime-based optode to measure oxygen in aquatic systems . In: Limnology and Oceanography: Methods . tape 4 , 2006, p. 7-17 , doi : 10.4319 / lom.2006.4.7 .
  4. ↑ a b c d e H. C. Bittig et al .: Oxygen Optode Sensors: Principle, Characterization, Calibration, and Application in the Ocean . In: Frontiers in Marine Science . tape 4 , 2018, p. 429 ff ., doi : 10.3389 / fmars.2017.00429 .
  5. ↑ Oxygen optode. In: Ocean Networks Canada. Accessed April 20, 2018 .
  6. ↑ Microoptode. In: Unisense. Accessed April 20, 2018 .
  7. ↑ P. Hartmann et al .: Photobleaching of a ruthenium complex in polymers used for oxygen optodes and its inhibition by singlet oxygen quenchers . In: Sensors and Actuators B: Chemical . tape 51 , 1998, pp. 196-202 , doi : 10.1016 / S0925-4005 (98) 00188-9 .
  8. ↑ NP Revsbech et al .: Construction of STOX Oxygen Sensors and their Application for Determination of O2 Concentrations in Oxygen Minimum Zones . In: Methods Enzymol. tape 486 , 2011, pp. 325-341 , doi : 10.1016 / B978-0-12-381294-0.00014-6 .
  9. ^ G. Harsanyi: Polymer Films in Sensor Applications . Technomic Publishing AG, Lancaster / Basel 1995, ISBN 1-56676-201-4 .
  10. ↑ a b M. C. Moreno-Bondi et al .: Oxygen Optrode for Use in a Fiber-Optic Glucose Biosensor . In: Anal. Chem. Band 62 , 1990, pp. 2377-2380 , doi : 10.1021 / ac00220a021 .
  11. ↑ JS Schultz: Affinity sensor: a new technique for developing implantable sensors for glucose and other metabolites . In: Diabetes Care . tape 5 , 1982, pp. 245-253 , doi : 10.2337 / diacare.5.3.245 .