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= Astatine =
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Introduction
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Astatine is a chemical element; it has symbol At and atomic number 85.
It is the rarest naturally occurring element in the Earth's crust,
occurring only as the decay product of various heavier elements. All
of astatine's isotopes are short-lived; the most stable is
astatine-210, with a half-life of 8.1 hours. Consequently, a solid
sample of the element has never been seen, because any macroscopic
specimen would be immediately vaporized by the heat of its
radioactivity.
The bulk properties of astatine are not known with certainty. Many of
them have been estimated from its position on the periodic table as a
heavier analog of fluorine, chlorine, bromine, and iodine, the four
stable halogens. However, astatine also falls roughly along the
dividing line between metals and nonmetals, and some metallic behavior
has also been observed and predicted for it. Astatine is likely to
have a dark or lustrous appearance and may be a semiconductor or
possibly a metal. Chemically, several anionic species of astatine are
known and most of its compounds resemble those of iodine, but it also
sometimes displays metallic characteristics and shows some
similarities to silver.
The first synthesis of astatine was in 1940 by Dale R. Corson, Kenneth
Ross MacKenzie, and Emilio G. Segrè at the University of California,
Berkeley. They named it from the Ancient Greek () 'unstable'. Four
isotopes of astatine were subsequently found to be naturally
occurring, although much less than one gram is present at any given
time in the Earth's crust. Neither the most stable isotope,
astatine-210, nor the medically useful astatine-211 occur naturally;
they are usually produced by bombarding bismuth-209 with alpha
particles.
Characteristics
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Astatine is an extremely radioactive element; all its isotopes have
half-lives of 8.1 hours or less, decaying into other astatine
isotopes, bismuth, polonium, or radon. Most of its isotopes are very
unstable, with half-lives of seconds or less. Of the first 101
elements in the periodic table, only francium is less stable, and all
the astatine isotopes more stable than the longest-lived francium
isotopes (205-211At) are synthetic and do not occur in nature.
The bulk properties of astatine are not known with any certainty.
Research is limited by its short half-life, which prevents the
creation of weighable quantities. A visible piece of astatine would
immediately vaporize itself because of the heat generated by its
intense radioactivity. It remains to be seen if, with sufficient
cooling, a macroscopic quantity of astatine could be deposited as a
thin film. Astatine is usually classified as either a nonmetal or a
metalloid; metal formation has also been predicted.
Physical
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Most of the physical properties of astatine have been estimated (by
interpolation or extrapolation), using theoretically or empirically
derived methods. For example, halogens get darker with increasing
atomic weight - fluorine is nearly colorless, chlorine is
yellow-green, bromine is red-brown, and iodine is dark gray/violet.
Astatine is sometimes described as probably being a black solid
(assuming it follows this trend), or as having a metallic appearance
(if it is a metalloid or a metal).
Astatine sublimes less readily than iodine, having a lower vapor
pressure. Even so, half of a given quantity of astatine will vaporize
in approximately an hour if put on a clean glass surface at room
temperature. The absorption spectrum of astatine in the middle
ultraviolet region has lines at 224.401 and 216.225 nm, suggestive of
6p to 7s transitions.
The structure of solid astatine is unknown. As an analog of iodine it
may have an orthorhombic crystalline structure composed of diatomic
astatine molecules, and be a semiconductor (with a band gap of 0.7
eV). Alternatively, if condensed astatine forms a metallic phase, as
has been predicted, it may have a monatomic face-centered cubic
structure; in this structure, it may well be a superconductor, like
the similar high-pressure phase of iodine. Metallic astatine is
expected to have a density of 8.91-8.95 g/cm3.
Evidence for (or against) the existence of diatomic astatine (At2) is
sparse and inconclusive. Some sources state that it does not exist, or
at least has never been observed, while other sources assert or imply
its existence. Despite this controversy, many properties of diatomic
astatine have been predicted; for example, its bond length would be ,
dissociation energy <, and heat of vaporization (∆Hvap) 54.39
kJ/mol. Many values have been predicted for the melting and boiling
points of astatine, but only for At2.
Chemical
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The chemistry of astatine is "clouded by the extremely low
concentrations at which astatine experiments have been conducted, and
the possibility of reactions with impurities, walls and filters, or
radioactivity by-products, and other unwanted nano-scale
interactions". Many of its apparent chemical properties have been
observed using tracer studies on extremely dilute astatine solutions,
typically less than 10−10 mol·L−1. Some properties, such as anion
formation, align with other halogens. Astatine has some metallic
characteristics as well, such as plating onto a cathode, and
coprecipitating with metal sulfides in hydrochloric acid. It forms
complexes with EDTA, a metal chelating agent, and is capable of acting
as a metal in antibody radiolabeling; in some respects, astatine in
the +1 state is akin to silver in the same state. Most of the organic
chemistry of astatine is, however, analogous to that of iodine. It has
been suggested that astatine can form a stable monatomic cation in
aqueous solution.
Astatine has an electronegativity of 2.2 on the revised Pauling scale
- lower than that of iodine (2.66) and the same as hydrogen. In
hydrogen astatide (HAt), the negative charge is predicted to be on the
hydrogen atom, implying that this compound could be referred to as
astatine hydride according to certain nomenclatures. That would be
consistent with the electronegativity of astatine on the Allred-Rochow
scale (1.9) being less than that of hydrogen (2.2). However, official
IUPAC stoichiometric nomenclature is based on an idealized convention
of determining the relative electronegativities of the elements by the
mere virtue of their position within the periodic table. According to
this convention, astatine is handled as though it is more
electronegative than hydrogen, irrespective of its true
electronegativity. The electron affinity of astatine, at 233 kJ mol−1,
is 21% less than that of iodine. In comparison, the value of Cl (349)
is 6.4% higher than F (328); Br (325) is 6.9% less than Cl; and I
(295) is 9.2% less than Br. The marked reduction for At was predicted
as being due to spin-orbit interactions. The first ionization energy
of astatine is about 899 kJ mol−1, which continues the trend of
decreasing first ionization energies down the halogen group (fluorine,
1681; chlorine, 1251; bromine, 1140; iodine, 1008).
Compounds
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Less reactive than iodine, astatine is the least reactive of the
halogens; the chemical properties of tennessine, the next-heavier
group 17 element, have not yet been investigated, however. Astatine
compounds have been synthesized in nano-scale amounts and studied as
intensively as possible before their radioactive disintegration. The
reactions involved have been typically tested with dilute solutions of
astatine mixed with larger amounts of iodine. Acting as a carrier, the
iodine ensures there is sufficient material for laboratory techniques
(such as filtration and precipitation) to work. Like iodine, astatine
has been shown to adopt odd-numbered oxidation states ranging from −1
to +7.
Only a few compounds with metals have been reported, in the form of
astatides of sodium, palladium, silver, thallium, and lead. Some
characteristic properties of silver and sodium astatide, and the other
hypothetical alkali and alkaline earth astatides, have been estimated
by extrapolation from other metal halides.
The formation of an astatine compound with hydrogen - usually referred
to as hydrogen astatide - was noted by the pioneers of astatine
chemistry. As mentioned, there are grounds for instead referring to
this compound as astatine hydride. It is easily oxidized;
acidification by dilute nitric acid gives the At0 or At+ forms, and
the subsequent addition of silver(I) may only partially, at best,
precipitate astatine as silver(I) astatide (AgAt). Iodine, in
contrast, is not oxidized, and precipitates readily as silver(I)
iodide.
Astatine is known to bind to boron, carbon, and nitrogen. Various
boron cage compounds have been prepared with At-B bonds, these being
more stable than At-C bonds. Astatine can replace a hydrogen atom in
benzene to form astatobenzene C6H5At; this may be oxidized to
C6H5AtCl2 by chlorine. By treating this compound with an alkaline
solution of hypochlorite, C6H5AtO2 can be produced. The
dipyridine-astatine(I) cation, [At(C5H5N)2]+, forms ionic compounds
with perchlorate (a non-coordinating anion) and with nitrate,
[At(C5H5N)2]NO3. This cation exists as a coordination complex in which
two dative covalent bonds separately link the astatine(I) centre with
each of the pyridine rings via their nitrogen atoms.
With oxygen, there is evidence of the species AtO− and AtO+ in aqueous
solution, formed by the reaction of astatine with an oxidant such as
elemental bromine or (in the last case) by sodium persulfate in a
solution of perchloric acid. The species previously thought to be has
since been determined to be , a hydrolysis product of AtO+ (another
such hydrolysis product being AtOOH). The well characterized anion
can be obtained by, for example, the oxidation of astatine with
potassium hypochlorite in a solution of potassium hydroxide.
Preparation of lanthanum triastatate La(AtO3)3, following the
oxidation of astatine by a hot Na2S2O8 solution, has been reported.
Further oxidation of , such as by xenon difluoride (in a hot alkaline
solution) or periodate (in a neutral or alkaline solution), yields the
perastatate ion ; this is only stable in neutral or alkaline
solutions. Astatine is also thought to be capable of forming cations
in salts with oxyanions such as iodate or dichromate; this is based on
the observation that, in acidic solutions, monovalent or intermediate
positive states of astatine coprecipitate with the insoluble salts of
metal cations such as silver(I) iodate or thallium(I) dichromate.
Astatine may form bonds to the other chalcogens; these include S7At+
and with sulfur, a coordination selenourea compound with selenium,
and an astatine-tellurium colloid with tellurium.
Astatine is known to react with its lighter homologs iodine, bromine,
and chlorine in the vapor state; these reactions produce diatomic
interhalogen compounds with formulas AtI, AtBr, and AtCl. The first
two compounds may also be produced in water - astatine reacts with
iodine/iodide solution to form AtI, whereas AtBr requires (aside from
astatine) an iodine/iodine monobromide/bromide solution. The excess of
iodides or bromides may lead to and ions, or in a chloride solution,
they may produce species like or via equilibrium reactions with the
chlorides. Oxidation of the element with dichromate (in nitric acid
solution) showed that adding chloride turned the astatine into a
molecule likely to be either AtCl or AtOCl. Similarly, or may be
produced. The polyhalides PdAtI2, CsAtI2, TlAtI2, and PbAtI are known
or presumed to have been precipitated. In a plasma ion source mass
spectrometer, the ions [AtI]+, [AtBr]+, and [AtCl]+ have been formed
by introducing lighter halogen vapors into a helium-filled cell
containing astatine, supporting the existence of stable neutral
molecules in the plasma ion state. No astatine fluorides have been
discovered yet. Their absence has been speculatively attributed to the
extreme reactivity of such compounds, including the reaction of an
initially formed fluoride with the walls of the glass container to
form a non-volatile product. Thus, although the synthesis of an
astatine fluoride is thought to be possible, it may require a liquid
halogen fluoride solvent, as has already been used for the
characterization of radon fluoride.
History
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In 1869, when Dmitri Mendeleev published his periodic table, the space
under iodine was empty; after Niels Bohr established the physical
basis of the classification of chemical elements, it was suggested
that the fifth halogen belonged there. Before its officially
recognized discovery, it was called "eka-iodine" (from Sanskrit
'one') to imply it was one space under iodine (in the same manner as
eka-silicon, eka-boron, and others). Scientists tried to find it in
nature; given its extreme rarity, these attempts resulted in several
false discoveries.
The first claimed discovery of eka-iodine was made by Fred Allison and
his associates at the Alabama Polytechnic Institute (now Auburn
University) in 1931. The discoverers named element 85 "alabamine", and
assigned it the symbol Ab, designations that were used for a few
years. In 1934, H. G. MacPherson of University of California, Berkeley
disproved Allison's method and the validity of his discovery. There
was another claim in 1937, by the chemist Rajendralal De. Working in
Dacca in British India (now Dhaka in Bangladesh), he chose the name
"dakin" for element 85, which he claimed to have isolated as the
thorium series equivalent of radium F (polonium-210) in the radium
series. The properties he reported for dakin do not correspond to
those of astatine, and astatine's radioactivity would have prevented
him from handling it in the quantities he claimed. Moreover, astatine
is not found in the thorium series, and the true identity of dakin is
not known.
In 1936, the team of Romanian physicist Horia Hulubei and French
physicist Yvette Cauchois claimed to have discovered element 85 by
observing its X-ray emission lines. In 1939, they published another
paper which supported and extended previous data. In 1944, Hulubei
published a summary of data he had obtained up to that time, claiming
it was supported by the work of other researchers. He chose the name
"dor", presumably from the Romanian for "longing" [for peace], as
World War II had started five years earlier. As Hulubei was writing in
French, a language which does not accommodate the "-ine" suffix, dor
would likely have been rendered in English as "dorine", had it been
adopted. In 1947, Hulubei's claim was effectively rejected by the
Austrian chemist Friedrich Paneth, who would later chair the IUPAC
committee responsible for recognition of new elements. Even though
Hulubei's samples did contain astatine-218, his means to detect it
were too weak, by current standards, to enable correct identification;
moreover, he could not perform chemical tests on the element. He had
also been involved in an earlier false claim as to the discovery of
element 87 (francium) and this is thought to have caused other
researchers to downplay his work.
In 1940, the Swiss chemist Walter Minder announced the discovery of
element 85 as the beta decay product of radium A (polonium-218),
choosing the name "helvetium" (from , the Latin name of Switzerland).
Berta Karlik and Traude Bernert were unsuccessful in reproducing his
experiments, and subsequently attributed Minder's results to
contamination of his radon stream (radon-222 is the parent isotope of
polonium-218). In 1942, Minder, in collaboration with the English
scientist Alice Leigh-Smith, announced the discovery of another
isotope of element 85, presumed to be the product of thorium A
(polonium-216) beta decay. They named this substance
"anglo-helvetium", but Karlik and Bernert were again unable to
reproduce these results.
Later in 1940, Dale R. Corson, Kenneth Ross MacKenzie, and Emilio
Segrè isolated the element at the University of California, Berkeley.
Instead of searching for the element in nature, the scientists created
it by bombarding bismuth-209 with alpha particles in a cyclotron
(particle accelerator) to produce, after emission of two neutrons,
astatine-211. The discoverers, however, did not immediately suggest a
name for the element. The reason for this was that at the time, an
element created synthetically in "invisible quantities" that had not
yet been discovered in nature was not seen as a completely valid one;
in addition, chemists were reluctant to recognize radioactive isotopes
as legitimately as stable ones. In 1943, astatine was found as a
product of two naturally occurring decay chains by Berta Karlik and
Traude Bernert, first in the so-called uranium series, and then in the
actinium series. (Since then, astatine was also found in a third decay
chain, the neptunium series.) Friedrich Paneth in 1946 called to
finally recognize synthetic elements, quoting, among other reasons,
recent confirmation of their natural occurrence, and proposed that the
discoverers of the newly discovered unnamed elements name these
elements. In early 1947, 'Nature' published the discoverers'
suggestions; a letter from Corson, MacKenzie, and Segrè suggested the
name "astatine" coming from the Ancient Greek () meaning , because of
its propensity for radioactive decay, with the ending "-ine", found in
the names of the four previously discovered halogens. The name was
also chosen to continue the tradition of the four stable halogens,
where the name referred to a property of the element.
Corson and his colleagues classified astatine as a metal on the basis
of its analytical chemistry. Subsequent investigators reported
iodine-like, cationic, or amphoteric behavior. In a 2003
retrospective, Corson wrote that "some of the properties [of astatine]
are similar to iodine ... it also exhibits metallic properties, more
like its metallic neighbors Po and Bi."
Isotopes
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Alpha decay characteristics for sample astatine isotopes
Mass number Half-life Probability of alpha decay Alpha- decay
half-life
207 %
208 %
209 %
210 %
211 %
212 ≈100%
213 %
214 %
219 %
220 %
221 experimentally alpha-stable ∞
There are 41 known isotopes of astatine, with mass numbers of 188 and
190-229. Theoretical modeling suggests that about 37 more isotopes
could exist. No stable or long-lived astatine isotope has been
observed, nor is one expected to exist.
Astatine's alpha decay energies follow the same trend as for other
heavy elements. Lighter astatine isotopes have quite high energies of
alpha decay, which become lower as the nuclei become heavier.
Astatine-211 has a significantly higher energy than the previous
isotope, because it has a nucleus with 126 neutrons, and 126 is a
magic number corresponding to a filled neutron shell. Despite having a
similar half-life to the previous isotope (8.1 hours for astatine-210
and 7.2 hours for astatine-211), the alpha decay probability is much
higher for the latter: 41.81% against only 0.18%. The two following
isotopes release even more energy, with astatine-213 releasing the
most energy. For this reason, it is the shortest-lived astatine
isotope. Even though heavier astatine isotopes release less energy, no
long-lived astatine isotope exists, because of the increasing role of
beta decay (electron emission). This decay mode is especially
important for astatine; as early as 1950 it was postulated that all
isotopes of the element undergo beta decay, though nuclear mass
measurements indicate that 215At is in fact beta-stable, as it has the
lowest mass of all isobars with 'A' = 215. Astatine-210 and most of
the lighter isotopes exhibit beta plus decay (positron emission),
astatine-217 and heavier isotopes except astatine-218 exhibit beta
minus decay, while astatine-211 undergoes electron capture.
The most stable isotope is astatine-210, which has a half-life of 8.1
hours. The primary decay mode is beta plus, to the relatively
long-lived (in comparison to astatine isotopes) alpha emitter
polonium-210. In total, only five isotopes have half-lives exceeding
one hour (astatine-207 to -211). The least stable ground state isotope
is astatine-213, with a half-life of 125 nanoseconds. It undergoes
alpha decay to the extremely long-lived bismuth-209.
Astatine has 24 known nuclear isomers, which are nuclei with one or
more nucleons (protons or neutrons) in an excited state. A nuclear
isomer may also be called a "meta-state", meaning the system has more
internal energy than the "ground state" (the state with the lowest
possible internal energy), making the former likely to decay into the
latter. There may be more than one isomer for each isotope. The most
stable of these nuclear isomers is astatine-202m1, which has a
half-life of about 3 minutes, longer than those of all the ground
states bar those of isotopes 203-211 and 220. The least stable is
astatine-213m1; its half-life of 110 nanoseconds is shorter than 125
nanoseconds for astatine-213, the shortest-lived ground state.
Natural occurrence
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Astatine is the rarest naturally occurring element. The total amount
of astatine in the Earth's crust (quoted mass 2.36 × 1025 grams) is
estimated by some to be less than one gram at any given time. Other
sources estimate the amount of ephemeral astatine, present on earth at
any given moment, to be up to one ounce (about 28 grams).
Any astatine present at the formation of the Earth has long since
disappeared; the four naturally occurring isotopes (astatine-215,
-217, -218 and -219) are instead continuously produced as a result of
the decay of radioactive thorium and uranium ores, and trace
quantities of neptunium-237. The landmass of North and South America
combined, to a depth of 16 kilometers (10 miles), contains only about
one trillion astatine-215 atoms at any given time (around 3.5 × 10−10
grams). Astatine-217 is produced via the radioactive decay of
neptunium-237. Primordial remnants of the latter isotope--due to its
relatively short half-life of 2.14 million years--are no longer
present on Earth. However, trace amounts occur naturally as a product
of transmutation reactions in uranium ores. Astatine-218 was the first
astatine isotope discovered in nature. Astatine-219, with a half-life
of 56 seconds, is the longest lived of the naturally occurring
isotopes.
Isotopes of astatine are sometimes not listed as naturally occurring
because of misconceptions that there are no such isotopes, or
discrepancies in the literature. Astatine-216 has been counted as a
naturally occurring isotope but reports of its observation (which were
described as doubtful) have not been confirmed.
Formation
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Possible reactions after bombarding bismuth-209 with alpha particles
!rowspan=2| Reaction !colspan=2| Energy of alpha particle
Threshold energy Maximum production
+ → + 2 n 20.7 MeV 30-31 MeV
+ → + 3 n 28.4-28.6 MeV 37 MeV
+ → + 4 n 35.9 MeV
Astatine was first produced by bombarding bismuth-209 with energetic
alpha particles, and this is still the major route used to create the
relatively long-lived isotopes astatine-209 through astatine-211.
Astatine is only produced in minuscule quantities, with modern
techniques allowing production runs of up to 6.6 gigabecquerels (about
86 nanograms or 2.47 atoms). Synthesis of greater quantities of
astatine using this method is constrained by the limited availability
of suitable cyclotrons and the prospect of melting the target. Solvent
radiolysis due to the cumulative effect of astatine decay is a related
problem. With cryogenic technology, microgram quantities of astatine
might be able to be generated via proton irradiation of thorium or
uranium to yield radon-211, in turn decaying to astatine-211.
Contamination with astatine-210 is expected to be a drawback of this
method.
The most important isotope is astatine-211, the only one in commercial
use. To produce the bismuth target, the metal is sputtered onto a
gold, copper, or aluminium surface at 50 to 100 milligrams per square
centimeter. Bismuth oxide can be used instead; this is forcibly fused
with a copper plate. The target is kept under a chemically neutral
nitrogen atmosphere, and is cooled with water to prevent premature
astatine vaporization. In a particle accelerator, such as a cyclotron,
alpha particles are collided with the bismuth. Even though only one
bismuth isotope is used (bismuth-209), the reaction may occur in three
possible ways, producing astatine-209, astatine-210, or astatine-211.
Although higher energies can produce more astatine-211, it will
produce unwanted astatine-210 that decays to toxic polonium-210 as
well. Instead, the maximum energy of the particle accelerator is set
to be below or slightly above the threshold of astatine-210
production, in order to maximize the production of astatine-211 while
keeping the amount of astatine-210 at an acceptable level.
Separation methods
====================
Since astatine is the main product of the synthesis, after its
formation it must only be separated from the target and any
significant contaminants. Several methods are available, "but they
generally follow one of two approaches--dry distillation or [wet] acid
treatment of the target followed by solvent extraction." The methods
summarized below are modern adaptations of older procedures, as
reviewed by Kugler and Keller. Pre-1985 techniques more often
addressed the elimination of co-produced toxic polonium; this
requirement is now mitigated by capping the energy of the cyclotron
irradiation beam.
Dry
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The astatine-containing cyclotron target is heated to a temperature of
around 650 °C. The astatine volatilizes and is condensed in
(typically) a cold trap. Higher temperatures of up to around 850 °C
may increase the yield, at the risk of bismuth contamination from
concurrent volatilization. Redistilling the condensate may be required
to minimize the presence of bismuth (as bismuth can interfere with
astatine labeling reactions). The astatine is recovered from the trap
using one or more low concentration solvents such as sodium hydroxide,
methanol or chloroform. Astatine yields of up to around 80% may be
achieved. Dry separation is the method most commonly used to produce a
chemically useful form of astatine.
Wet
=====
The irradiated bismuth (or sometimes bismuth trioxide) target is first
dissolved in, for example, concentrated nitric or perchloric acid.
Following this first step, the acid can be distilled away to leave
behind a white residue that contains both bismuth and the desired
astatine product. This residue is then dissolved in a concentrated
acid, such as hydrochloric acid. Astatine is extracted from this acid
using an organic solvent such as dibutyl ether, diisopropyl ether
(DIPE), or thiosemicarbazide. Using liquid-liquid extraction, the
astatine product can be repeatedly washed with an acid, such as HCl,
and extracted into the organic solvent layer. A separation yield of
93% using nitric acid has been reported, falling to 72% by the time
purification procedures were completed (distillation of nitric acid,
purging residual nitrogen oxides, and redissolving bismuth nitrate to
enable liquid-liquid extraction). Wet methods involve "multiple
radioactivity handling steps" and have not been considered well suited
for isolating larger quantities of astatine. However, wet extraction
methods are being examined for use in production of larger quantities
of astatine-211, as it is thought that wet extraction methods can
provide more consistency. They can enable the production of astatine
in a specific oxidation state and may have greater applicability in
experimental radiochemistry.
Uses and precautions
======================================================================
Several 211At-containing molecules and their experimental uses
Agent Applications
[211At]astatine-tellurium colloids Compartmental tumors
6-[211At]astato-2-methyl-1,4-naphtaquinol diphosphate Adenocarcinomas
211At-labeled methylene blue Melanomas
Meta-[211At]astatobenzyl guanidine Neuroendocrine tumors
5-[211At]astato-2'-deoxyuridine Various
211At-labeled biotin conjugates Various pretargeting
211At-labeled octreotide Somatostatin receptor
211At-labeled monoclonal antibodies and fragments Various
211At-labeled bisphosphonates Bone metastases
Newly formed astatine-211 is the subject of ongoing research in
nuclear medicine. It must be used quickly as it decays with a
half-life of 7.2 hours; this is long enough to permit multistep
labeling strategies. Astatine-211 has potential for targeted
alpha-particle therapy, since it decays either via emission of an
alpha particle (to bismuth-207), or via electron capture (to an
extremely short-lived nuclide, polonium-211, which undergoes further
alpha decay), very quickly reaching its stable granddaughter lead-207.
Polonium X-rays emitted as a result of the electron capture branch, in
the range of 77-92 keV, enable the tracking of astatine in animals and
patients. Although astatine-210 has a slightly longer half-life, it is
wholly unsuitable because it usually undergoes beta plus decay to the
extremely toxic polonium-210.
The principal medicinal difference between astatine-211 and iodine-131
(a radioactive iodine isotope also used in medicine) is that
iodine-131 emits high-energy beta particles, and astatine does not.
Beta particles have much greater penetrating power through tissues
than do the much heavier alpha particles. An average alpha particle
released by astatine-211 can travel up to 70 μm through surrounding
tissues; an average-energy beta particle emitted by iodine-131 can
travel nearly 30 times as far, to about 2 mm. The short half-life and
limited penetrating power of alpha radiation through tissues offers
advantages in situations where the "tumor burden is low and/or
malignant cell populations are located in close proximity to essential
normal tissues." Significant morbidity in cell culture models of human
cancers has been achieved with from one to ten astatine-211 atoms
bound per cell.
Several obstacles have been encountered in the development of
astatine-based radiopharmaceuticals for cancer treatment. World War II
delayed research for close to a decade. Results of early experiments
indicated that a cancer-selective carrier would need to be developed
and it was not until the 1970s that monoclonal antibodies became
available for this purpose. Unlike iodine, astatine shows a tendency
to dehalogenate from molecular carriers such as these, particularly at
sp3 carbon sites (less so from sp2 sites). Given the toxicity of
astatine accumulated and retained in the body, this emphasized the
need to ensure it remained attached to its host molecule. While
astatine carriers that are slowly metabolized can be assessed for
their efficacy, more rapidly metabolized carriers remain a significant
obstacle to the evaluation of astatine in nuclear medicine. Mitigating
the effects of astatine-induced radiolysis of labeling chemistry and
carrier molecules is another area requiring further development. A
practical application for astatine as a cancer treatment would
potentially be suitable for a "staggering" number of patients;
production of astatine in the quantities that would be required
remains an issue.
Animal studies show that astatine, similarly to iodine--although to a
lesser extent, perhaps because of its slightly more metallic
nature--is preferentially (and dangerously) concentrated in the
thyroid gland. Unlike iodine, astatine also shows a tendency to be
taken up by the lungs and spleen, possibly because of in-body
oxidation of At- to At+. If administered in the form of a radiocolloid
it tends to concentrate in the liver. Experiments in rats and monkeys
suggest that astatine-211 causes much greater damage to the thyroid
gland than does iodine-131, with repetitive injection of the nuclide
resulting in necrosis and cell dysplasia within the gland. Early
research suggested that injection of astatine into female rodents
caused morphological changes in breast tissue; this conclusion
remained controversial for many years. General agreement was later
reached that this was likely caused by the effect of breast tissue
irradiation combined with hormonal changes due to irradiation of the
ovaries. Trace amounts of astatine can be handled safely in fume hoods
if they are well-aerated; biological uptake of the element must be
avoided.
See also
======================================================================
* Radiation protection
External links
======================================================================
* [
http://www.periodicvideos.com/videos/085.htm Astatine] at 'The
Periodic Table of Videos' (University of Nottingham)
*
[
http://quantumchymist.blogspot.com.au/2014/02/astatine-halogen-or-metal-part-1.html
Astatine: Halogen or Metal?]
License
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Original Article:
http://en.wikipedia.org/wiki/Astatine
