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Plate readers, also known as microplate readers or microplate photometers, are instruments which are used to detect
biological Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...
,
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...
or
physical Physical may refer to: * Physical examination, a regular overall check-up with a doctor * ''Physical'' (Olivia Newton-John album), 1981 ** "Physical" (Olivia Newton-John song) * ''Physical'' (Gabe Gurnsey album) * "Physical" (Alcazar song) (2004) * ...
events of samples in microtiter plates. They are widely used in research,
drug discovery In the fields of medicine, biotechnology and pharmacology, drug discovery is the process by which new candidate medications are discovered. Historically, drugs were discovered by identifying the active ingredient from traditional remedies or b ...
, bioassay validation, quality control and manufacturing processes in the pharmaceutical and biotechnological industry and academic organizations. Sample reactions can be assayed in 1-1536 well format microtiter plates. The most common microplate format used in academic research laboratories or clinical diagnostic laboratories is 96-well (8 by 12 matrix) with a typical reaction volume between 100 and 200 µL per well. Higher density microplates (384- or 1536-well microplates) are typically used for screening applications, when throughput (number of samples per day processed) and assay cost per sample become critical parameters, with a typical assay volume between 5 and 50 µL per well. Common detection modes for microplate assays are absorbance,
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
intensity,
luminescence Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a crys ...
, time-resolved fluorescence, and fluorescence polarization.


Methods


Absorbance

Absorbance detection has been available in microplate readers for more than 3 decades and is used for assays such as
ELISA The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay uses a solid-phase type of enzyme immunoassay (EIA) to detect the presen ...
assays, protein and nucleic acid quantification or enzyme activity assays (i.e. in the
MTT assay The MTT assay is a colorimetric assay for assessing cell metabolic activity. NAD(P)H-dependent cellular oxidoreductase enzymes may, under defined conditions, reflect the number of viable cells present. These enzymes are capable of reducing the t ...
for cell viability). A light source illuminates the sample using a specific wavelength (selected by an optical filter, or a monochromator), and a light detector located on the other side of the well measures how much of the initial (100%) light is transmitted through the sample: the amount of transmitted light will typically be related to the concentration of the molecule of interest. Several conventional
colorimetric Colorimetry is "the science and technology used to quantify and describe physically the human color perception". It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color ...
analyses have been miniaturized to function quantitatively in a plate reader, with performance suitable for research purposes. Examples of analyses converted to plate reader methods include several for
ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaterna ...
,
nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...
,
nitrite The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also ...
,
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
, iron(II), and
orthophosphate A phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. ...
. More recent colorimetric chemistries have been developed directly for use in plate readers.


Fluorescence

Fluorescence intensity detection has developed very broadly in the microplate format over the last two decades. The range of applications is much broader than when using absorbance detection, but the instrumentation is usually more expensive. In this type of instrumentation, a first optical system (excitation system) illuminates the sample using a specific wavelength (selected by an optical filter, or a monochromator). As a result of the illumination, the sample emits light (it fluoresces) and a second optical system (emission system) collects the emitted light, separates it from the excitation light (using a filter or monochromator system), and measures the signal using a light detector such as a
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sh ...
tube (PMT). The advantages of fluorescence detection over absorbance detection are sensitivity, as well as application range, given the wide selection of fluorescent labels available today. For example, a technique known as
calcium imaging Calcium imaging is a microscopy technique to optically measure the calcium (Ca2+) status of an isolated cell, tissue or medium. Calcium imaging takes advantage of calcium indicators, fluorescent molecules that respond to the binding of Ca2+ ions b ...
measures the fluorescence intensity of calcium-sensitive dyes to assess intracellular calcium levels.


Luminescence

Luminescence is the result of a chemical or biochemical reaction. Luminescence detection is simpler optically than fluorescence detection because luminescence does not require a light source for excitation or optics for selecting discrete excitation wavelengths. A typical luminescence optical system consists of a light-tight reading chamber and a PMT detector. Some plate readers use an Analog PMT detector while others have a
photon counting Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). A single-photon detector emits a pulse of signal for each detected photon, in contrast to a normal photodetector, which generates an analo ...
PMT detector. Photon Counting is widely accepted as the most sensitive means of detecting luminescence. Some plate readers offer filter wheel or tunable wavelength monochromator optical systems for selecting specific luminescent wavelengths. The ability to select multiple wavelengths, or even wavelength ranges, allows for detection of assays that contain multiple luminescent reporter enzymes, the development of new luminescence assays, as well as a means to optimize the signal to noise ratio. Common applications include
luciferase Luciferase is a generic term for the class of oxidative enzymes that produce bioluminescence, and is usually distinguished from a photoprotein. The name was first used by Raphaël Dubois who invented the words '' luciferin'' and ''luciferase'' ...
-based gene expression assays, as well as cell viability, cytotoxicity, and biorhythm assays based on the luminescent detection of ATP.


Time-resolved fluorescence (TRF)

Time-resolved fluorescence (TRF) measurement is very similar to fluorescence intensity (FI) measurement. The only difference is the timing of the excitation/measurement process. When measuring FI, the excitation and emission processes are simultaneous: the light emitted by the sample is measured while excitation is taking place. Even though emission systems are very efficient at removing excitation light before it reaches the detector, the amount of excitation light compared to emission light is such that FI measurements always exhibit fairly elevated background signals. TRF offers a solution to this issue. It relies on the use of very specific fluorescent molecules, called
lanthanides The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and ytt ...
, that have the unusual property of emitting over long periods of time (measured in milliseconds) after excitation, when most standard fluorescent dyes (e.g. fluorescein) emit within a few nanoseconds of being excited. As a result, it is possible to excite lanthanides using a pulsed light source (Xenon flash lamp or pulsed laser for example) and measure after the excitation pulse. This results in lower measurement backgrounds than in standard FI assays. The drawbacks are that the instrumentation and reagents are typically more expensive, and that the applications have to be compatible with the use of these very specific lanthanide dyes. The main use of TRF is found in drug screening applications, under a form called TR-FRET (time-resolved fluorescence energy transfer). TR-
FRET A fret is any of the thin strips of material, usually metal wire, inserted laterally at specific positions along the neck or fretboard of a stringed instrument. Frets usually extend across the full width of the neck. On some historical instru ...
assays are very robust (limited sensitivity to several types of assay interference) and are easily miniaturized. Robustness, the ability to automate and miniaturize are features that are highly attractive in a screening laboratory.


Fluorescence polarization

Fluorescence polarization measurement is also very close to FI detection. The difference is that the optical system includes polarizing filters on the light path: the samples in the microplate are excited using polarized light (instead of non-polarized light in FI and TRF modes). Depending on the mobility of the fluorescent molecules found in the wells, the light emitted will either be polarized or not. For example, large molecules (e.g. proteins) in solution, which rotate relatively slowly because of their size, will emit polarized light when excited with polarized light. On the other hand, the fast rotation of smaller molecules will result in a depolarization of the signal. The emission system of the plate reader uses polarizing filters to analyze the polarity of the emitted light. A low level of polarization indicates that small fluorescent molecules move freely in the sample. A high level of polarization indicates that fluorescent is attached to a larger molecular complex. As a result, one of the basic applications of FP detection is molecular binding assays, since they allow to detect if a small fluorescent molecule binds (or not) to a larger, non-fluorescent molecule: binding results in a slower rotation speed of the fluorescent molecule, and in an increase in the polarization of the signal.


Light scattering and nephelometry

Light scattering and nephelometry are methods for the determination of the cloudiness of a solution (i.e.: insoluble particles in a solution). A light beam passes through the sample and the light is scattered by the suspended particles. The measured forward scattered light indicates the amount of the insoluble particles present in solution. Common nephelometry/light scattering applications include automated HTS drug solubility screening, long-term microbial growth kinetics, flocculation, aggregation and the monitoring of polymerization and precipitation, including immunoprecipitation.


Instruments and assays

Many of the detection modes (absorbance, fluorescence intensity, luminescence, time-resolved fluorescence, and fluorescence polarization) are available stand-alone in dedicated plate readers, but are very often found today combined into one instrument (multi-mode plate reader). There are also instruments for measuring the dynamic or static light scattered from samples in a microplate. The range of applications for multi-mode plate readers is extremely large. Some of the most common assays are: *
ELISA The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay uses a solid-phase type of enzyme immunoassay (EIA) to detect the presen ...
s *Protein and
cell growth Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis (production of biomolecules or anabolism) is greater th ...
assays *Protein:protein interactions *Reporter assays *
Nucleic acid quantitation In molecular biology, quantitation of nucleic acids is commonly performed to determine the average concentrations of DNA or RNA present in a mixture, as well as their purity. Reactions that use nucleic acids often require particular amounts an ...
*Molecular interactions *Enzyme activity *Cell toxicity, proliferation, and viability *ATP quantification * Immunoassays * High throughput screening of compounds and targets in drug discovery *Bead-Based Epitope Assay While "plate reader" usually refers to the devices described above, many variations are available. Some examples of other devices working with the microplate format are: * ELISPOT plate readers, used to count the colored spots that are formed in the course of ELISPOT assays. * High throughput imagers that can measure all the wells of a microplate at once *
High-content screening High-content screening (HCS), also known as high-content analysis (HCA) or cellomics, is a method that is used in biological research and drug discovery to identify substances such as small molecules, peptides, or RNAi that alter the phenotype of a ...
(HCS) systems that image each well with high resolution, to look at cell populations * Label-free instruments that use specialized microplates to measure binding events without the use of chemical markers


References

{{DEFAULTSORT:Plate Reader Molecular biology laboratory equipment