Scintillation proximity assay (SPA) is an assay development and biochemical screening that permits the rapid and sensitive measurement of a broad range of biological processes in a homogeneous system. The type of beads that are involved in the SPA are microscopic in size and within the beads itself, there is a

scintillant A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbe ...

which emits light when it is stimulated. Stimulation occurs when radio-labelled molecules interact and bind to the surface of the bead. This interaction will trigger the bead to emit light, which can be detected using a

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ph ...

Overview

The SPA technique is dependent on the

energy conversion Energy transformation, also known as energy conversion, is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work or moving, (e.g. Lifting an object) or provides heat. ...

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

, which releases light

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

s which can be detected via the use of some devices such as the photomultiplier tubes of scintillation counters or CCD imagers. This is a very popular technique in practices that require detecting and quantifying radioactivity.Homogeneous Proximity

Tyrosine Kinase A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions. Tyrosine kinases belong to a larger cla ...

Assays: Scintillation Proximity Assay versus Homogeneous Time-Resolved Fluorescence. ''Analytical Biochemistry'' Volume 269, Issue 1, 10 April 1999, Pages 94-104. The process of converting radioactivity to light requires a liquid medium of scintillation combination consisting soluble

organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...

scintillators and

organic solvents A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

. During the process of radioactive decay, a

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...

will be released. While this particle travels in the medium, the energy it possesses is dissipated as it collides with the surrounding

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

s in the solvent, exciting them while doing so. The excited molecules will transfer the energy they now possess to the scintillator molecules, where the energy will be emitted as light.

Detail

In more detail, when the radio-labelled molecule is attached or is in proximity to bead, light emission is stimulated. However, if the bead does not become bound to the radio-labelled molecule, the bead will not be stimulated to emit light. This is because the beta particles (high-velocity electrons) released from the unbound molecule are lost to collisions with water molecules if they are too far from the scintillant-containing beads, and so the SPA bead which is not then stimulated to produce a signal. The decay of radioactive atoms releases subatomic particles and/or gamma rays. Tritium releases

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s as one of the

subatomic particle In physical sciences, a subatomic particle is a particle that composes an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a pr ...

s. The energy of these particles influences the distance traveled by the particles itself through a medium such as water, because there is an inverse correlation between particle kinetic energy and strength of interaction with matter. The SPA method depends on the short pathlength of tritium-released beta particles. For instance, the decay of a

Tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...

atom releases a

, which is well-suited to SPA due to a very short (1.5 µm) path length through water. So, when the β-particle is within 1.5 µm of the scintillant bead, there is sufficient energy to stimulate the bead to emit light. If the distance between them is greater than 1.5 µm, then the β-particle has insufficient energy to reach and stimulate the bead. The beads in SPA are formed from the incorporation of scintillant into small beads known as fluomicrospheres. These are specially designed to bind with specific molecules. When the bead is in close proximity to the radioactive molecule, light is stimulated. The photonmultiplier tube (PMT) can be used to detect the emitted photons. This device converts the emitted photon energy into

electrical energy Electrical energy is energy related to forces on electrically charged particles and the movement of electrically charged particles (often electrons in wires, but not always). This energy is supplied by the combination of electric current and electr ...

by a

photocathode A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...

via a series of other

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

s. Another device is known as

CCD Imager A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...

, which is composed of a set of cooled

digital camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile device ...

s with sensitive charge coupled device detectors and with some refined

telecentric lens A telecentric lens is a special optical lens (often an objective lens or a camera lens) that has its entrance or exit pupil, or both, at infinity. Telecentric lenses are often used for precision optical two-dimensional measurements or reproductio ...

es to convert the captured photon energy into high quality images. There is also an assortment of bead coatings available that allows this method to be applied to a broad range of applications, such as

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

assays and radio-immuno

assay An assay is an investigative (analytic) procedure in laboratory medicine, mining, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a ...

Advantages over previous methods

In comparison to the previous over-coated plate-based methods, SPA has a number of advantages that makes it more popular: *Assay flexibility - the concentration of the components in the assay can be adjusted to suit the user due to the higher surface area design of the SPA beads, hence providing the user flexibility in designing assay. * Radioactive label reduction - the SPA beads allows a potential reduction in the quantity of radioactive labeling required due to its higher binding capacity, which gives a higher signal. This method also allows the user to optimize the sensitivity of the assay by altering the quantity of SPA beads. *Convenient - the key component of the SPA assay, the beads, do not depend on a particular plate type or supplier, hence its wide availability. *Bead assortment - there is a diversity of bead types to choose from to suit the need of the user and application. *No separation step - allows binding measurement without separation step associated with earlier methods, which reduces errors and allows higher throughput.

References

{{reflist Biochemistry detection methods