gold nanoparticle catalyst
Post on 28-Mar-2015
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GOLD NANOPARTICLES FOR CATALYST Few people look beyond gold's glitter and rarity, but chemists have found that its chemical properties are just as interesting, making it a unique catalyst for producing unusual organic molecules. UC Berkeley's Dean Toste, a leader in the area of gold catalysis, attributes these properties to relativistic effects in the gold atom, the same effects that give gold its yellow luster. Essentially, gold catalysts are 'hot' because their electrons are heavy, this chemist proposes. Toste has found a mother lode of new and unique gold-catalyzed reactions by applying Einstein's theory of relativity to the rare and precious metal. Catalysts are metals that speed up chemical reactions, such as when the platinum in a car's catalytic converter instantly converts polluting engine exhaust to oxygen, nitrogen, carbon dioxide and water. Dean Toste, a UC Berkeley associate professor of chemistry, was one of the first chemists to experiment with gold as a catalyst. He opened the door for others interested in gold's versatility and in the potential to generate chemicals of interest for the chemical and pharmaceutical industry more efficiently and using less toxic precursors. "This is a really hot area," Toste said. "If you look at the most-cited articles in the Journal of the American Chemical Society, many are about gold catalysis. "With this class of gold catalysts, you can develop a number of unprecedented reactions that have never been seen before." Toste discusses the new field and proposes a new theory for why gold has such unusual, and practical, catalytic properties. So far, the hypothesis has successfully predicted the behavior of gold catalysts in new chemical reactions. "Our hypothesis really allows us to approach catalysis in a new way, melding the two fields of theoretical chemistry and synthetic chemistry," Toste said. At the heart of his hypothesis is the special theory of relativity, proposed by Albert Einstein 102 years ago and typically thought of as applying only to cosmological questions. But late UC Berkeley chemist Kenneth Pitzer showed some 70 years ago that the theory comes into play in chemistry as well. Other researchers have used so-called relativistic quantum mechanics to explain gold's yellow color and why mercury is a liquid instead of a solid. Toste now takes this explanation a step further, crediting special relativity with making gold and perhaps the related and widely used catalyst platinum - act as both an acceptor and a donor of electrons in a catalytic reaction. Typical metal catalysts do one or the other, but not both. One of the key tenets of relativity is that nothing can travel faster than the speed of light. The reason for this is that objects become heavier, or more massive, the faster they go, with the mass approaching infinity as the object approaches the speed of light. In an atom, where electrons race around the nucleus like buzzing bees, the velocity of an electron doesn't get anywhere near the speed of light until the atomic nucleus fills up with lots of positively charged protons - the negatively charged electrons have to move faster to keep from being pulled into the highly positive nucleus. This occurs in the transition metals of the periodic table of elements, metals ranging from tantalum and tungsten to platinum and gold. In a gold atom, with 79 protons in the nucleus, the 79 electrons whip around the nucleus at about half the speed of light. The net effect is that gold's electrons are much heavier and are pulled in closer to the nucleus, lowering the energy levels and making the atom more compact. According to this hypothesis, gold's s shells, which are its lowest energy spherically symmetric electron shells, contract. This
shields the electrons in outer, asymmetric p and d orbits from the nuclear charge, allowing them to expand slightly. In gold, the contraction of the outermost (6s) shell and the expansion of the next-inner (5p) shell reduces the energy difference between the two to the equivalent of a photon of blue light. This allows gold to absorb blue light and, thus, look yellow. Silver, because it exhibits a much less dramatic relativistic effect, is unable to absorb any visible light and is totally reflective. Toste proposes that this same shielding effect allows the more tightly bound s shell to easily accept electrons from other molecules, while the partly shielded d shell can easily donate electrons to a reaction. Thus, gold is able to participate in reactions both as a donor and as an acceptor of electrons, which makes it particularly useful in catalyzing reactions at carbon-carbon bonds, the backbone of all organic molecules. According to Toste, a gold atom can attach to carbon loosely, with a single bond or a double bond, allowing flexibility in reactions that can lead to novel organic molecules. Using this model, he has accurately predicted the products in various organic reactions. For example, a gold atom attached to the chemical phosphine and dispersed homogeneously in a liquid can efficiently convert alkynes to pyrroles, which are ring structures found in many drugs. Gold-phosphine catalysis also can create an unusual carbon triangle called cyclopropane that is used in industrial organic synthesis. "We can make cyclopropanes without the need for explosive diazo compounds," Toste said. Toste predicts that gold catalysts also will be very useful in producing chemicals with a specific handedness, that is, a left-handed molecule, but not its right-handed or mirror image. Such stereoselective reactions are becoming more important because many drugs come in right- and left-handed forms, but only one form is effective in the body. The most efficient synthesis would produce only the effective form, not its ineffective mirror image. He is tuning the phospine attached to gold to affect this stereoselectivity. "The future of gold catalysis still involves a lot of theoretical work, and we need to understand more about how it works," Toste said. "But already, some of these reactions are being used by medicinal chemists, and it's a really exciting field."
A nanoparticle consists of a few atoms forming a cluster with size in the nanometer range. A nanometer is a millionth of a millimeter.Considerable attention has been focused duringthe last few decades on developing and optimising methods for the preparation of gold nanoparticles to size and shape e.g. spherical andnon-spherical (triangles and hexagons) particles,as applications of nano structured materials are dependent on these properties. The colour of ultrasmall gold spheres or clusters, also known as nanoparticles, has been known for centuries as the deep red ruby colour of stained glass windows in cathedrals and domestic glassware. The colour results from the plasmon resonances in the metal cluster. Most gold nanoparticles are produced via chemical routes. Biosynthesis, investigated by Project AuTEK, is an alternative route used to create gold nanoparticles with yeast and bacteria resulting in non-spherical particles. These particles have different properties to conventional spherical particles and could further increase the use of gold.
Gold nanoparticles are easily functionalised to further exploit their properties. The term monolayer protected clusters (MPCs) defines surface functionalisation of these nanoparticles by self-assembled monolayers. A cluster of gold atoms or a gold nano-particle can be stabilized by a monolayer of, for example, alkanethiolatedor phosphine ligands. Gold MPCs provide commercial interest in materials science, biological science and various chemical platforms. One example where gold nanopropertiesare second to none is in the area of medical diagnostics and therapies. Since gold nanoparticles have such good light-scattering properties, as well as easy functionalisation and biocompatibility, they are ideal for a wide rangeof biological and pharmaceutical applicationssuch as bio-labeling and other types of diagnostics. Research conducted at the University of Liverpool in collaboration with Project AuTEK aims to improve these MPC gold nanoparticles even further, by designing stable,water soluble, biocompatible and functionalized nanoparticles. Gold is a critical component in certain therapies, more specifically, in the treatment of cancer by hyperthermia and thermoablation. These two therapies use heat to kill cancer cells. In the case of hyperthermia, the cancerous tissue is heated to enhance conventional radiation and chemotherapy treatments, while in thermoablation the tissue is heated so that the cancer tissue is destroyed by the localised heat. There are two methods one can use to provide heating, infra-red absorption and the application of an oscillating magnetic field to magnetic nanoparticles. At the University of Rice, silica nanoparticles are being developed that are coated with gold nanolayers to form a gold shell around a glass core. By changing the thickness of the gold shell, one can control the wavelength of light that is absorbed by the gold/silica nanoparticle. Tuned to capture infrared light and coated with cancer-specific antibodies, the nanoshell becomes a precision-guided cancer treatment and works as follows (see diagram).
Wide application of gold catalyst nanoparticles (1) To diagnose and treat cancer, thousands of gold-coated nanoshells are injected into the patients bloodstream. Each gold-coated nanoshell is about 10,000 times smaller than a white blood cell. Inside the bloodstream, the nanoshells are taken up naturally by the tumourcells via antibodies stuck to their surface. (2) Each tumour is covered by approximately 20 nanoshells and a brief exposure to near-infrared light by a very simple handheld laser, which passes harmlessly through tissue and illuminates the shells. The doctor then delivers a more intense near-infrared dose resulting in heat generated at the tumo