It’s the extraction lab this week in the OChem lab I’m TA’ing. It’s a straightforward aqueous base extraction of an acidic unknown from a neutral impurity. Acidify, filter the precipitate, and you’re done. I was trying to come up with a demonstration for the lab. I thought about extracting caffeine from coffee or tea leaves, but that would take a while, and isn’t all that visually appealing. I’ve only got a few minutes in my pre-lab lecture time.
So I looked around for a while, and finally found this paper by James Hutchison from the University of Oregon (doi: 10.1039/b405810k). They suggest a new lab for undergraduates involving the extraction of D-limonene from orange peels using liquid carbon dioxide. That’s right, I said liquid carbon dioxide.
The premise: create a removable filter using copper wire and filter paper to jam into the bottom of a disposable centrifuge tube. Add grated orange peel. Add crushed dry ice. Cap the centrifuge tube tightly (but not TOO tightly! The tube needs to be able to vent so as not to EXPLODE!) and immerse in warm water (T = 40-60 degC). The pressure rises (naturally) and the temperature increases and you jump into the liquid portion of carbon dioxide’s phase diagram (click for larger)
phasediagramco2
The liquid carbon dioxide percolates through the orange peels and extracts the limonene. the oil-in-solvent mixture drains through the filter paper to the bottom of the centrifuge tube. If you leave the tube in the water long enough, eventually the liquid all evaporates and the pressure decreases.
The goal is that the evaporation of the carbon dioxide leaves the pure oil at the bottom of the tube. The authors mention that for approximately 2.5 g of freshly-grated orange peel, 0.1 mL of oil should remain after 3 carbon dioxide extractions. They note this is a yield comparable to typical organic solvent extraction or cold pressing. I did one extraction on day-old chopped orange peel and did not isolate any oil whatsoever. Not a drop. I’m a little disappointed by that, but not really. It’s still an ok teaching point for the students. Not all experiments work all the time. I could examine my starting materials and get better quality reagents and it might work.
Now, inside the tube I don’t think we were past the critical point. I don’t think the temperature inside the centrifuge tube actually makes it up to the temperature of the surrounding water. I say this because after the examining the tube after the experiment, the orange was cold and there were ice crystals in the tube. There are two possible explanations for this. One, the temperature inside didn’t make it past the critical temperature. Two, when I opened the tube after the experiment, some non-trivial amount of pressure was released. PV=nRT tells us that a sudden drop in the pressure simultaneously lowers the temperature, and I could have frozen the water out that way. In fact, the authors note that while exact temperature and pressure readings are impossible with this simple setup, they speculate that the conditions approach the triple point.
In any case, it was a very cool experiment to watch, even if it didn’t do what it was supposed to. Pictures below. These pictures are from Monday night when I was practicing the demonstration. It looked much cooler in person. The first shows the system when first submerged in the water. The second is about 15-30 seconds later. It’s hard to see, but if you look closely, all three phases are apparent in the system. The third is after the dry ice has completely liquified. Click for larger.
before3phasesliquidco2
by azmanam
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Chemistry Lab Demonstrations: LIQUID CO2 Extraction!
Polyethylene bags
Politicians in Switzerland have suggested to ban the polyethylene plastic bags used in supermarkets for environmental reasons. I am no expert, but I guess a thin plastic bag cannot be so bad as long as it’s properly disposed of. What really grabbed my attention was the statement of one politician, who said that the combustion of these bags releases dioxin.
Of course he was talking about polychlorinated dibenzodioxins (PCDDs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which was involved in the Seveso disaster and was a contaminant of Agent Orange that caused many of its severe health effects. Any chemist can see immediately that there is no way polyethylene will release PCDDs because it contains no chlorine. Such a statement immediately indicates how little research the politician has done.
I think this is indicative of a way of thinking predominant among a large part of the population here. Let me put it in a formula: Chemistry = Evil. It really annoys me when I see how little knowledge there is to support this general damnation of all things related to a scientific branch. In addition, because chemistry is bad, some people not only refuse to learn about it, but they are even proud of their lack of knowledge! I’m sure you have heard sentences like “You know, I never understood chemistry at school”, meaning “chemistry is for nerds and you don’t really need it in daily life”. Consider the same statement about art or literature, and you can see how little chemistry is appreciated. After all, modern life as we know it would be unthinkable without the advances of the chemical industry in the last century. But still, because polyethylene is made in a chemical plant, politicians will jump to the conclusion that it must be environmentally disastrous and should be banned.
by Phil
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Objectives and skills checklists
Objectives and skills checklists
The scientific method
* State the central objectives of chemistry (and this course).
* Outline the scientific method.
o Classify statements and explanations as observations, experimental data, laws*, hypotheses*, or theories*. Quiz Quiz
o Understand the importance of making controlled comparisons and obtaining reproducible data.
Measurement
* Use the SI* system.
o Know the SI base units*.
o State rough equivalents for the SI base units in the English system.
o Read and write the symbols for SI units.
o Recognize unit prefixes and their abbreviations.
o Build derived units* from the basic units for mass, length, temperature, and time.
o Convert measurements from SI units to English, and from one prefixed unit to another.
o Use derived units like density* and speed as conversion factors.
o Use percentages, parts per thousand, and parts per million as conversion factors.
* Use and report measurements carefully.
o Consider the reliability of a measurement in decisions based on measurements.
o Clearly distinguish between
+ precision* and accuracy*
+ exact numbers and measurements*
+ systematic error* and random error*
o Count the number of significant figures* in a recorded measurement. Record measurements to the correct number of digits.
o Estimate the number of significant digits in a calculated result.
o Estimate the precision of a measurement by computing a standard deviation*.
Matter
* Classify material properties as extensive properties*, intensive properties*, chemical properties*, and physical properties*. Give examples of each. Quiz Quiz
* Distinguish between gases*, liquids*, and solids*. Explain how these states differ at the molecular level.
* Classify samples of matter as pure substances*, homogeneous mixtures*, heterogeneous mixtures*, compounds*, and elements*. Quiz Quiz
* Use sketches to show how elements, compounds, and mixtures differ at the molecular level.
* Describe six different techniques for separating mixtures.
* Relate the names of elements to their international element symbols*.
* Describe the periodic table*. Name the major groups* and regions on the periodic table, and identify elements belonging to these groups.
* Distinguish between metals, nonmetals, and metalloids using the periodic table.
Atoms and ions
* Describe early milestones in the development of modern atomic theory.
* State and apply the law of conservation of mass* and the law of definite proportions* .
* State the premises of Dalton's atomic theory.
* Describe J. J. Thomson's experimental evidence for the existence of electrons*.
* Describe Rutherford's scattering experiments and show how the results of the experiments imply the existence of atomic nuclei*.
* List the three most important particles that all atoms are composed of, and describe their charges and relative masses.
* Understand the concept of atomic weight*.
* Describe how isotopic masses* and isotopic abundances* are measured experimentally using mass spectrometry*. Use a mass spectrum* to compute an average atomic mass. Given a table of isotopic masses and abundances, sketch a mass spectrum.
* Predict the most common ion formed by a main group element by consulting a periodic table.
* Name and write the formulas for common transition metal ions.
Molecules and compounds
* Describe two fundamental types of chemical bonding*.
* Compare properties of ionic compounds* and covalent compounds*.
Quiz Quiz
classifying compounds
* Classify compounds as ionic or covalent.
* Define and distinguish between empirical formulas*, molecular formulas*, and structural formulas* for compounds.
* Define, calculate, and relate formula weights* and molecular weights*.
* Name and write the formulas for
Quiz Quiz
names and formulas of polyatomic ions
o polyatomic ions*
o binary compounds*(covalent or ionic)
o simple ionic compounds*
o inorganic acids
o addition compounds*
* Explain the mole concept, and convert between grams, moles, and atoms and molecules.
* Determine mass percent composition of a sample from experimental data.
* Determine mass percent composition of a compound from its formula.
* Determine empirical formula of a compound from its mass percent composition.
Chemical change
* Write balanced chemical equations from descriptions of chemical changes.
* Classify chemical reactions as synthesis, formation, decomposition, thermolysis, electrolysis, displacement, single displacement, metathesis, precipitation, neutralization, redox, and combustion reactions.
* Write formation and combustion reactions for given compounds.
* Understand the concept of equilibrium solubility* and use it to recognize saturated* and supersaturated* solutions.
* Explain why water can dissolve polar* and ionic substances.
* Visualize the link between a solution's ability to conduct electricity and the degree of ionic dissociation*.
* Classify solutes as strong acids*, weak acids*, strong bases*, and salts*.
* Classify solutes as strong electrolytes*, weak electrolytes*, and nonelectrolytes*.
skills to master for exam II
Molarity
* Distinguish between saturated and supersaturated solutions.
* Predict amounts of reactants or products involved in a reaction involving solutions by using solution molarities as conversion factors.
* Use molarity as a conversion factor in dilution problems.
* Use molarity as a conversion factor in titration problems.
Gases
* Understand the definition of pressure. Use the definition to predict and measure pressures experimentally.
* Describe experiments that show relationships between pressure, temperature, volume, and moles for a gas sample.
* Use empirical gas laws to predict how a change in one of the properties of a gas will affect the remaining properties.
* Use empirical gas laws to estimate gas densities and molecular weights.
* Use volume-to-mole relationships obtained using the empirical gas laws to solve stoichiometry problems involving gases.
* Understand the concept of partial pressure in mixtures of gases.
* Use the ideal kinetic-molecular model to explain the empirical gas laws.
* List deficiencies in the ideal gas model that will cause real gases to deviate from behaviors predicted by the empirical gas laws. Explain how the model can be modified to account for these deficiencies.
Energy and chemical change
* Describe, distinguish, and relate the following properties. Predict whether these properties increase, decrease, or stay the same over the course of a given chemical or physical change.
o temperature
o thermal energy
* Understand heat on both theoretical and experimental levels.
o Relate heat transferred to changes in thermal energy when no work is done.
o Relate heat to an object's mass and initial and final temperatures. Clearly distinguish heat and temperature.
o Explain how heat can be measured experimentally (calorimetry).
o Estimate the final temperature when hot and cold objects are brought into contact.
o Define heat capacity and specific heat. Describe how these quantities can be measured experimentally.
* Define enthalpy. Distinguish enthalpy from thermal energy.
* Describe how changes in enthalpy and thermal energy accompanying a chemical reaction can be measured calorimetrically.
* Define bond energy. Use tables of bond energies to estimate the enthalpy of a reaction.
* Write and manipulate thermochemical equations.
o Combine a set of step thermochemical equations to obtain a net thermochemical equation (Hess's Law)
o Write thermochemical equations for combustion and formation reactions.
The quantum theory
* Relate wavelength*, frequency*, and velocity of waves.
* Explain how electromagnetic radiation* is produced by an oscillating charge.
* Explain how electromagnetic radiation carries energy from a transmitter to a receiver.
* Describe the collapsing atom paradox.
* List wave behaviors, and distinguish them from particle behaviors.
* Cite experimental evidence that implies that electromagnetic radiation can display both wave and particle behaviors.
* Cite experimental evidence that implies that electrons display both wave and particle behaviors.
* Connect particle and wave properties of matter using de Broglie's hypothesis.
* Explain what a standing wave is.
* Compare a wave on a wire, a particle on a wire, and an electron on a wire.
* Show how de Broglie's hypothesis implies the existence of quantized energy states for standing electron waves.
* Show how quantum numbers arise for standing electron waves.
* State Heisenberg's uncertainty principle, and explain why it resolves the collapsing atom paradox.
skills to master for exam III
Electrons in atoms
* Explain the difference between a continuous spectrum and a line spectrum.
* Explain the difference between an emission and an absorption spectrum.
* Use the concept of quantized energy states to explain atomic line spectra.
* Given an energy level diagram, predict wavelengths in the line spectrum, and vice versa.
* Define and distinguish between shells, subshells, and orbitals.
* Explain the relationships between the quantum numbers.
* Use quantum numbers to label electrons in atoms.
* Describe and compare atomic orbitals given the n and ell quantum numbers.
* List a set of subshells in order of increasing energy.
* Write electron configurations* for atoms in either the subshell or orbital box notations.
* Write electron configurations of ions.
* Use electron configurations to predict the magnetic properties of atoms.
The periodic table
* Understand the rationale behind the DEFINE[periodic table]; view the table as an ordered database of element properties.
* Explain how the periodic table reflects the quantum mechanical structure of the atom.
* Explain and use DEFINE[periodic trends] in:
o DEFINE[atomic radius]
o DEFINE[ionic radius]
o DEFINE[ionization energy]
* Explain the connection between ionization energy and metallic character.
Chemical Bonding
Intermolecular Forces
Liquids
Solids
Solutions
* Relate the following solution concentrations.
o molarity
o percentage (w/w, w/v, and v/v)
o molality
o mole fraction
o ppt, ppm, and ppb
o pX
* Explain how a dilute solution with specified volume and concentration can be prepared from a stock solution.
* Define the following colligative properties, and give a molecular explanation of each. Show how the properties can be measured experimentally.
o vapor pressure lowering (Raoult's Law)
o freezing point depression
o boiling point elevation
o osmotic pressure
* Use basic relationships involving colligative properties to estimate the molecular weight of nonelectrolyte solutes.
o Relate the vapor pressure of a solution with concentration and solvent vapor pressure.
o Use the relationship between freezing point depression and solution molality to predict the molecular weight of a solute.
o Use the relationship between osmotic pressure and solution molarity to predict the molecular weight of a solute.
About the practice exams
The tests available online and the tests on reserve at the library are actual tests given in other semesters.
The tests should be viewed as a study aid. They are not a list of questions that might reappear on future tests. Use the tests to diagnose trouble spots and topics that require further study.
The content of our general chemistry is continually being improved and modified, and the textbook is changed from time to time. Some of the tests may contain questions that are inappropriate for your course, and some areas covered in current lectures and labs are not represented in older tests.
To take a practice exam, follow any of the links below. The page will ask you for your local alias and email, but these are only kept to maintain the 'high score' file; your performance on practice exams does not affect your grade in the course at all! When you finish taking the test hit the Submit button on the bottom of the page to see how well you did.
Take Exam IA
Take Exam IIIA
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..What is Chemistry ?
Chemistry (from Egyptian kēme (chem), meaning "earth"[1]) is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions.[2] It is a physical science for studies of various atoms, molecules, crystals and other aggregates of matter whether in isolation or combination, which incorporates the concepts of energy and entropy in relation to the spontaneity of chemical processes. Modern chemistry evolved out of alchemy following the chemical revolution (1773).
Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include inorganic chemistry, the study of inorganic matter; organic chemistry, the study of organic matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the energy related studies of chemical systems at macro, molecular and submolecular scales; analytical chemistry, the analysis of material samples to gain an understanding of their chemical composition and structure. Many more specialized disciplines have emerged in recent years, e.g. neurochemistry the chemical study of the nervous system.Wikipedia..
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Chlorinated solvents industry regrets most widely used solvent in paint stripping will be partly banned
20 Jan 2009 - The European chlorinated solvent industry, represented by ECSA, regrets the EU Parliament backs the ban on Dichloromethane in paint stripping. This ban will not necessarily enhance the safer use of paint strippers because the alternative substances have not yet been submitted to an adequate risk assessment. Here, the legislators missed an opportunity to benefit from the recent REACH legislation on the risks of chemicals.
Tthe European Parliament confirmed an agreement with the Council to ban the substance dichloromethane from use in paint strippers. It foresees a blanket EU-wide ban for consumer use, and also a ban for professional use that allows Member States to derogate under certain conditions. The obligatory protection requirements for industrial use have been strengthened.
The European Chlorinated Solvent Association (ECSA) is disappointed by this decision and warns the ban will not necessarily reduce health risks for users. There is compelling evidence that the most popular alternative chemicals and methods of paint removal present a greater risk than DCM paint strippers.
Unfortunately the actual risks with DCM paint strippers and alternatives have not been quantified as there has not yet been an adequate risk assessment. In fact, here an opportunity was missed to apply the chemical regulation REACH, which would have provided the comparative risk assessments that are needed in order to make well informed decisions to enable risk reduction.
As the alternative chemical paint strippers to DCM are demonstrably less effective we are also concerned that the use of blow torches and heat guns for paint removal will increase dramatically. This would result in a major increase in serious accidents.
Nevertheless, the European Chlorinated Solvent Association takes the decision adopted by the European Parliament into full consideration and will support a correct implementation in the Member States. Selengkapnya...
Borealis awards innovative polymer prizes to Austrian and Italian scholars
21 Jan 2009 - Two students carrying out important research on polyolefins have been recognized by Borealis under its new Student Innovation Award scheme. The students will receive their awards at the annual Innovation Day that Borealis is celebrating in Linz, Austria, on January 21, 2009.
For his doctoral degree on norbornene-based polyolefins by postmetallocene catalysts, Italian student Dr. Andrea Ravasio will receive 5,000 euros, while a masters degree thesis on scavengers and chemical bonding for defined volatile components in polyolefins yields Austrian student Andreas Fuchs 3,000 euros.
"The Borealis Student Innovation Award recognises the two most innovative research papers at masters and doctorate level on polyolefins, olefins or melamine sciences,” says Dr. Christian Paulik, Manager for External Research and Funding at Borealis. "Our aim is to engage young people in our vision of Shaping the Future with Plastics. These two important research projects symbolize exactly the spirit that we are trying to capture.”
Ravasio's doctoral dissertation was carried out at the University of Pavia, under Professor Maurizio Licchelli and Incoronata Tritto.
"What singled out Andrea was his comprehensive knowledge of catalysis for olefin and cyclo-olefin copolymerization,” says Christian Paulik in explaining the decision to give the doctorate award to Dr. Ravasio.
Andreas Fuchs studied for his Masters degree at the Johannes Kepler University, Linz, under the tutelage of Professor Oliver Brüggemann, who describes him as "one of the best co-workers I have worked with in my career”.
Fuchs' thesis shows new ways to reduce emissions of volatile organic compounds (VOCs) from polyolefins. "This work is set in the real world,” says Christian Paulik. "Andreas's findings might well help in the development of future polymers.” Selengkapnya...
The molecular basis for flavor
A similar lock-and-key type of model has been used to explain why different substances have different flavors. The stereochemical theory of odor suggests that a molecule that fits into an olfactory receptor can fire nerve cells, ultimately producing a particular odor perception. Five basic odors were associated with different molecular shapes. Football shaped molecules fit in to a "camphoraceous" receptor, and smell like mothballs. Necklace-shaped molecules have a musky odor because they fit into a "musky" receptor. Wedged-shaped molecules have a pepperminty odor, tadpole-shaped molecules smell like flowers, and long thin ether molecules are, well, ethereal. Putrid and pungent smells were explained on the basis of partial charges on atoms within the molecule, rather than by shape alone. Putrid molecules have a buildup of negative charge somewhere in the molecule that's strongly attracted to a partially positive site on the "putrid" receptor. Pungent molecules (like acetic acid, in vinegar) are just the opposite: they have an electron-deficient region that is strongly attracted to an electron-rich site on the "pungent" receptor. These seven receptors were believed to be the only letters in the olfactory alphabet in Amoore's version of the theory, published in the early 1970's. Molecules that can lock into more than one receptor have more complex odors. For example, Amoore explained the almondy odor of benzaldehyde by showing that it could fit comfortably into the postulated shapes for the camphoraceous, floral, and pepperminty receptors. Amoore's stereochemical theory is now known to be an oversimplification, but it's still useful in relating smells to molecular shapes. There are over a thousand olfactory receptors, not just seven. The molecule's ability to move through tissue containing layer after layer of receptors also determines how its odor is perceived. For example, attaching a hydrocarbon tail to a molecule improves its solubility in fats and alters its behavior at cell membranes. Perfume chemists have long known that adding a hydrocarbon tail to some perfume molecules increases their potency. Let's look at some specific examples. The vanilloids (vanillin, eugenol, zingerone, and capsaicin) are molecules with distinctive flavors but obviously similar molecular structures. All contain a six-sided hexagonal ring of carbons (called a benzene ring). Subtle changes in the sizes or positions of groups of atoms attached to the ring dramatically change the compound's flavor.
Vanillin Vanillin has a soothing, pleasant aroma. Its molecular weight is relatively low, and it is fairly volatile. Cooking with vanilla vaporizes some of the vanillin molecules and fills the room with its aroma. Molecules containing only carbon and hydrogen are mostly insoluble in water. The oxygen-containing groups attached to the ring in vanillin can form strong hydrogen bonds with water, making it water soluble (about a gram of vanillin can be dissolved in 100 mL of cold water). Vanillin's solubility in water is responsible for the "finish" acquired by wines aged in oak casks. Vanilla present in the wood lignin of the wine barrels slowly leaches into the wine over time. Eugenol Eugenol is found in bay leaves, allspice, and oil of cloves. Eugenol has a short hydrocarbon chain attached to the ring, which makes it much less water-soluble than vanillin. Although it is practically insoluble in water, it freely mixes with fats and oils. Its fat solubility allows it to penetrate tissues and bind more tightly to the vanilloid receptor, which is believed to have a fatty side chain. The tail gives eugenol a stronger odor than vanillin has. One bay leaf is enough to season a pot of soup; more than one or two ground cloves overpower a pumpkin pie. Eugenol has a numbing, analgesic effect. It is used as a dental antiseptic (it's one component of that strange smell some dentist's offices have). Why is the molecule an antiseptic? Apparently the hydrocarbon tail in combination with the polar OH group on the ring make eugenol rather soap-like, and it can disrupt the cell membranes of bacteria the way soap disrupts a spot of grease. Zingerone Zingerone puts the zing in ginger and is also a flavor ingredient in mustard oil. The hydrocarbon tail attached to its vanillin foundation ring doesn't lower the solubility of zingerone much because it contains a carbonyl group (C=O) that can form strong hydrogen bonds with water molecules. Zingerone is sparingly soluble in water, but also freely soluble in fats and oils. The higher molecular weight of zingerone in combination with the polar side-chain carbonyl group makes zingerone molecules attract each other more strongly than eugenol and vanillin molecules do. As a result, zingerone is less volatile than either eugenol or vanillin. The odor of ginger isn't strong, but the hydrocarbon tail gives it a more intense flavor when it does come into contact with its receptor. Ginger root is a popular folk medicine. Some of the beneficial medicinal qualities claimed for ginger may stem from zingerone's effectiveness as an antioxidant. Zingerone reacts with free radicals that can cause tissue damage and inflammation. Studies by researchers at Case Western University show that a topically applied extract containing zingerone may help prevent some skin cancers. Capsaicin Most (though not all) of the hot in hot peppers comes from capsaicin and a closely related compound, dihydrocapsaicin. It occurs in much lower quantities in oregano, cinnamon, and cilantro. The compound's molecular weight is the highest of any of the vanilloids we've looked at so far, and the side chain contains a polar amide (-NHCO-) group. That makes capsaicin's volatility very low, and it is completely odorless. (A very good thing!) Even without a telltale fragrance, capsaicin's presence in foods is hard to miss. A solution that contains only 10 parts per million produces a persistent burning sensation when placed on the tongue. It is tasteable at much lower concentrations. The intense flavor results from the molecule's long hydrocarbon tail. The chain allows it to bind very strongly with its lipoprotein receptor, which has some hydrocarbon side chains of its own (like dissolves like!) The fatty tail also allows the molecule to slip through lipid-rich cell membranes, making the burn more pervasive and persistent. Several capsaicin-like compounds found in chiles have slight structural variations in the hydrocarbon tail, which changes their ability to bind to the receptors and their ability to penetrate layers of receptors on the tongue, mouth, and throat. That may explain why some chiles burn in the mouth, while others burn deep in the throat. The perception that peppers are "hot" is not an accident. The capsaicin key opens a door in the cell membrane that allows calcium ions to flood into the cell. That ultimately triggers a pain signal that is transmitted to the next cell. When the cells are exposed to heat, the same events occur. Chile burns and heat burns are similar at the molecular, cellular, and sensory levels. One expects that the long hydrocarbon tail will make capsaicin less water soluble than vanillin. This is indeed the case. Capsaicin is insoluble in cold water, but freely soluble in alcohol and vegetable oils. This is why drinking water after munching an habanero pepper won't stop the burning. A cold beer is the traditional remedy, but the small percentage of alcohol will not wash away much capsaicin. For relief from a chile burn, drink milk. Milk contains casein, a lipophilic (fat-loving) substance that surrounds and washes away the fatty capsaicin molecules in much the same way that soap washes away grease.
High concentrations are toxic. Exposure is painful and even incapacitating. Capsaicin prevents nerve cells from communicating with each other by blocking the production of certain neurotransmitters; at high concentrations it destroys the cells! Capsaicin's toxicity makes chiles more than just a food- they're also a weapon. The Mayans burned chiles to create a stinging smoke screen, and threw gourds filled with pepper extract in battle. Nowadays, capsaicin is the active ingredient in pepper sprays, used to ward off attacking muggers, dogs, and bears. Paradoxically, capsaicin's ability to cause pain makes it useful in alleviating pain. Exposure to capsaicin lowers sensitivity to pain, and it is applied as a counter irritant in the treatment of arthritis and other chronically painful conditions. People that eat lots of spicy capsaicin-rich foods build up a tolerance to it. The incentive: a small jolt of capsaicin excites the nervous system into producing endorphins, which promote a pleasant sense of well-being. The endorphin lift makes spicy foods mildly addictive (and for some, an obsession). References The Theory of Odor
Vanillin and Eugenol
Zingerone
Capsaicin
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What do chocolate cravings, forgetful mice, and blissful pigs have in common? The answer is anandamide, a recently discovered messenger molecule that plays a role in pain, depression, appetite, memory, and fertility. Its name comes from ananda, the Sanskrit word for "bliss". Anandamide's discovery may lead to the development of an entirely new family of therapeutic drugs. Anandamide chemistry provides a rare glimpse of processes that affect human behavior at the molecular level. Nerve cells communicate by releasing special 'key' molecules that are intercepted by other nerve cells downstream. There are many different types of molecular keys, each with its own distinctive shape. The surfaces of the target nerve cells are studded with receptors, which are like locks to fit the keys. When the key fits into the receptor, the surface of the nerve cell changes in some way. For example, when the key molecule at right locks into the receptor on the surface of a nerve cell, it opens a door in the membrane that allows chloride ions to flood into the cell. This equalizes charges inside and outside the cell and prevents the cell from firing. The keys must be removed again from the lock somehow, or the nerve cell will be permanently prevented from firing. Certain enzymes are produced that remove (by degrading and destroying) the keys after a certain amount of time, so that the nerve cell can go back to work. Drugs that have a powerful effect on the central nervous system often mimic natural molecular keys. For example, morphine is a potent pain killer that was found to lock into an 'opiate receptor' present on nerve cells. Scientists began to look for receptor sites to explain the action of other drugs and toxins in a similar way. In 1988, specific receptors were discovered for THC (tetrahydrocannibol, the active ingredient in marijuana). THC isn't found naturally in the body. The existence of a specific 'bliss receptor' for THC implied that it was actually just a forgery of a hitherto unknown natural molecular key. The key was isolated by Israeli scientist Raphael Mechoulam in 1992: arachidonyl ethanolamide, later called 'anandamide': Anandamide's long hydrocarbon tail makes it fat-soluble and allows it to easily slip across the hydrocarbon-rich curtain that isolates the brain from the bloodstream. Notice that its three-dimensional shape strongly resembles that of THC. But unlike THC, anandamide is fragile. It breaks down very quickly in the body. That explains why anandamide doesn't produce a perpetual natural 'high'. Anandamide is synthesized enzymatically in areas of the brain that are important in memory and higher thought processes, and in areas that control movement. That implies that anandamide's function is not just to produce bliss. Connections between nerve cells are associated with learning and memory. Nerve cells can make new connections and break old ones. Repeated use of a connection makes it grow stronger; lack of use can cause the connection to be lost. Some biochemical evidence suggests that anandamide plays a role in the making and breaking of short term neural connections [Derkinderen, 1996]. And animal studies suggest that anandamide induces forgetfulness. Substances that keep anandamide from binding to its receptor might be used to treat memory loss. USDA agricultural researcher Gary Weesner is studying anandamide's effectiveness as an animal sedative [Pork, 1997]. "Pigs [treated with anandamide] walk less and lay down more," Weesner says. "We see reduced body temperature and slower respiration- all indicators consistent with being calm." Three compounds that strongly resemble anandamide were found in dark chocolate by Daniele Piomelli and co-workers at the Neurosciences Institute in San Diego [Piomelli, 1996]. They also found compounds (N-acylethanolamines) that block the breakdown of anandamide. Piomelli speculates that part of the pleasure of chocolate comes from anandamide and the anandamide-preserving N-acylethanolamines. "We are talking about something much, much, much, much milder than a high", he says. The Nature article has been used by some to equate the effects of chocolate and cannabis to bolster arguments about the legalization of marijuana. "It is not that simple," Piomelli says. The response to THC and to the chocolate anandamides are not at all the same, even if the concentrations could be made comparable. Piomelli was bemused by the spin his research was given in the popular press. "...You may be able to improve mood by blocking the breakdown of anandamide. It's not just something cute that we've done so that now we know more about chocolate. The hope is that it may contribute to helping cure mental disease," he said in a recent interview.[Psychiatric News, 1996] Outside the brain, anandamide acts as a chemical messenger between the embryo and uterus during implantation of the embryo in the uterine wall. As such, it's one of the first communications that occurs between mother and child. The highest concentrations of anandamide in the body were not in the brain, but in the uterus just before embryo implantation (at least, in the animal studies done so far) [KUMC, 1996] The concentration of anandamide changes as the uterus becomes more receptive to embryo implantation. The researchers were able to locate a definite target for the uterus' anandamide signal: mouse embryos contain more anandamide receptors than any tissue known, including the brain. If THC can lock into anandamide receptors, there is the disturbing possibility that it may interfere with signaling between the uterus and the embryo. Indeed, 2-cell mouse embryos exposed to THC-like compounds have a significantly lower survival rate and exhibit a number of abnormalities [Yan, 1996]. More work will have to be done to see if these animal experiments mean that THC can interfere interfere with early pregnancy in humans. Anandamide is not the only THC-like molecule used for signalling in the brain. Piomelli's group has recently reported a new molecular key that closely resembles anandamide [Piomelli, 1997]. Naturally produced sn-2 arachidonylglycerol (2-AG) can also lock into the bliss receptor. 2-AG is present at 170 times the concentration of anandamide in some regions of the brain. Piomelli speculates that 2-AG and anandamide perform complementary functions. Understanding how those functions work may allow some of the positive medicinal effects of anandamide and THC to be exploited therapeutically- without most of the negative effects. The bliss receptor
Scientists reasoned that since morphine is not naturally present in the body, there must be a natural key molecule with a very similar shape that activates this receptor. The natural keys turned out to be molecules called enkephalins. Although morphine is just a forgery of enkephalins, it's much more powerful (and more addictive) than the enkephalins because the key-removing enzymes can't pry it from the receptors. In time, less addictive forgeries (codiene and demerol) were introduced.
Forgetful mice and blissful pigs
Chocolate and anandamide
Mom's first call?
A new key to the bliss receptor
Links and references
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How techniques for separating mixtures helped solve a deadly mystery
One morning in the summer of 1961, hundreds of crazed birds attacked the seaside town of Capitola, California. The birds "cried like babies" as they dove into streetlamps, crashed through glass windows, and attacked people on the ground. Most of the birds were sooty shearwaters, a normally nonaggressive species that feeds on small fish and comes ashore only to breed.
The incident fascinated Alfred Hitchcock, who frequently vacationed in nearby Santa Cruz. He included newspaper clippings about the Capitola attack in his studio proposal for The Birds, which appeared in cinemas two years later.
In the winter of 1987, the agent that is now believed to be responsible for the Capitola incident struck on the opposite shore of the continent. This time, it struck higher on the food chain. Over a hundred people became extremely ill within hours after dining on cultured blue mussels in restaurants around Prince Edward Island in Canada. It quickly became apparent that this was no ordinary outbreak of food poisoning. Vomiting, cramps, diarrhea, and incapacitating headaches were followed by confusion, loss of memory, disorientation, and (in severe cases) seizures and coma. A few exhibited emotional volatility, with uncontrolled crying or aggressiveness. Three elderly victims died. [Perl].
A tragic symptom of poisoning was the destruction of short term memory in about one quarter of the survivors. They could remember nothing that happened after the poisoning. Some were unable to recognize their surroundings or relatives. They could learn no new facts or skills. The most severely affected lost memories several years old. For twelve of the victims, the loss of short term memory was permanent.
The mysterious syndrome was called "amnesic shellfish poisoning". This sort of neurological damage due to food poisoning had never been encountered before. To prevent further injury and loss of life it was imperative that the toxic agent be isolated and identified as quickly as possible. A team of marine biologists and chemists was assembled by Canada's Department of Fisheries and Oceans (DFO) to work on the problem.
But quick resolution of the mystery was unlikely. An initial screening of the sample for known bacterial and viral pathogens revealed nothing. Tests for heavy metals, pesticides, and PCBs also were negative. The mussel samples were extremely complex, containing thousands of different chemical compounds. How can one component be isolated from a such a complex mixture, without knowing anything about its physical or chemical properties? 
Suppose a test could be devised for the presence of the needle in a haystack. The haystack could be divided in half, and the half that tested negative for the needle could be discarded. Repeating this divide-and-discard process over and over again should eventually result in a pile with only one thing left: the needle.
That was the strategy the researchers used to isolate the toxin. A reliable but gruesome biological test was developed. Injection of a small amount of the sample into mice produced a very distinctive neurological reaction if the toxin was present: the mice involuntarily scratched their shoulders with their hind legs. [Teitelbaum]
Standard physical methods for separating complex mixtures were applied to the poisoned mussel samples. At the same time, uncontaminated mussels were subjected to the same separations, to allow the analysts to compare fractions. Any differences in spectra or chromatograms between the control and toxic samples might be valuable clues in the search for the toxic agent. Mice were exposed to each fraction of the separation. Fractions found to be toxic were retained for further analysis. The others were discarded. If chromatograms and spectra indicated that the toxic fraction was still a complex mixture, another separation technique was applied (see Figure 1).
Figure 1. General strategy for isolation of the toxin responsible for amnesic shellfish poisoning. Based on a diagram by M. Quilliam and J. L. C. Wright (Analytical Chemistry, 61, 1054 (1989)).
Separation by solubility and volatility
Most drugs and poisons are either fat soluble or water soluble, so a logical first step in the isolation was solvent extraction
The extract was concentrated by evaporation. The vapor was not toxic, but the residue after evaporation was. The poison apparently was nonvolatile, which could indicate a high molecular weight compound, or a compound that ionized in solution.
A second extraction was performed by shaking the concentrated extract with a mixture of a nonpolar solvent (dichloromethane
The dichloromethane fractions for the toxic mussels contained several colored substances absent in the control mussels. The visible light absorption spectrum
But the pigments themselves were not poisonous. The dichloromethane fraction gave a negative result in the mouse bioassay. The aqueous layer contained the toxin, indicating that it was probably a polar, ionizable substance. This was a lucky break, because the researchers could discard the complex dichloromethane fraction and concentrate on the much simpler aqueous fraction.
Separation by polarity
Column chromatography was used to separate the aqueous layer into simpler components. The sample was passed through a narrow tube packed with beads of a resin called XAD-2, which grabs the nonpolar parts of passing molecules, but lets ions pass freely. XAD-2 chromatography is particulary effective for separating organic acids and bases. Flushing the resin with a strong base ionizes acids in the sample. The ionized acids will pass through the column before other organic compounds because the resin won't retain them in their polar ionized form. Flushing with a strong acid gives organic bases in the sample extra hydrogen ions (and a positive charge); any organic bases adsorbed onto the resin will be washed out of the column.
Of the many fractions that passed out of the XAD-2 column, only one was toxic. For the final stage of the purification, the toxic fraction was separated with high performance liquid chromatography
Separation by charge, size, and molecular shape
The researchers had to ensure that the final HPLC fraction was indeed the isolated toxic component. They separated the aqueous XAD-2 fraction again, using a completely different technique: high voltage paper electrophoresis.Electrophoresis is a technique for separating ions based on their charge-to-mass ratios. Ions placed between a positive and a negative electrode will move towards the electrode with the opposite charge. Generally, the higher the ion's charge-to-mass ratio, the faster it moves towards the electrode. The smallest and most highly charged ions move ahead of larger ions with lower charges. Molecular shape also affects the rate of migration; shapes with more concentrated charges tend to migrate faster, all other things being equal.
The sample is applied to a piece of blotting paper. The ends of the paper were dipped in pH buffer solutions; an electrode was placed in each buffer solution. Ions for separate substances migrate at different rates and were resolved as separate bands across the paper. A developing agent (ninhydrin) was sprayed on the paper to stain the bands to make them easier to see.
A band very close to the band for glutamic acid
Identification of the toxin
Mass spectrometry was used to determine the compound's molecular weight (312 g/mol) and molecular formula (C15H22NO6). Spectroscopic analysis revealed the presence of conjugated double bonds and features characteristic of an amino acid. By matching the spectra with those from STN International's Registry system, the compound was unambiguously identified as domoic acid, an triprotic amino acid:  |  |  | |
| Domoic acid in acidic solution. | Glutamic acid in acidic solution. | ||
| Click the images for 3D Chime structures. | |||
Some scientists insisted that domoic acid could not be responsible for the poisonings, because it had been used as a folk remedy for intestinal worms in Japan for many years. There were no previous reports of toxicity from seaweed or seaweed extracts known to contain domoic acid in the medical literature. However, the seaweed extracts used in the remedies contained a total dose of no more than 20 mg of domoic acid, while some of the victims of amnesic shellfish poisoning consumed some 290 mg [Perl]. Many substances that are harmless or even beneficial at low dosages can have toxic effects at higher levels.
Domoic acid is a molecular Trojan Horse. Nerve cells mistakenly recognize domoic acid as glutamic acid
It is possible to have too much of a good thing, however. Glutamate at high concentrations acts as an excitotoxin
Domoic acid's structure is obviously similar to glutamic acid. But its five-sided ring makes it less flexible than glutamate, which causes it to bind very tightly to glutamate receptors. As a result, the excitatory effect of domoate is 30 to 100 times more powerful than that of glutamate [Perl].
How did the domoic acid get into the shellfish (and the anchovies eaten by the birds at Capitola)? Remember that phytoplankton pigments were found in the aqueous layer after solvent extraction. This wasn't quite a smoking gun, but it was definitely a fingerprint of the killer. An extensive investigation traced the domic acid to an obscure species of needle-like diatom*, called Pseudo-nitzschia pungens (shown in the title banner at the top of this page). Pseudo-nitzschia has been found in oceans around the world, so further outbreaks are possible in many locations. Commercial shellfish and seafood is now monitored regularly for domoic acid, using HPLC to identify the toxin. The screening and testing procedures have so far been successful- not a single instance of domoic acid poisoning in humans has been reported since the 1987 outbreak.
References and Links
Accounts of domoic acid outbreaks
- Hitch's Birds Deranged by Dodgy Anchovies (New Scientist)
- An article describing the 1961, 1987, and 1991 domoic acid poisonings, by Rosie Mestel (p. 6, July 22, 1995)
- Diatoms Nature's Marble: Hazard (Eureka)
- A brief description with electron micrographs of the diatoms that produce domoic acid. The 1991 incident of domoic acid poisoning in sea birds around Monterrey Bay, California is described.
- An outbreak of toxic encephalopathy caused by eating mussels contaminated with domoic acid. (Perl, et. al., Health and Welfare Canada)
- The field epidemiologists who investigated the original 1987 outbreak report their findings. The destruction of short-term memory experienced by some of the poisoning victims is described.
Perl, T.M., L. Bard, T. Kosatsky, J.C. Hockin, E. Todd, and R.S. Remis, New England J. Med. 322: 1775-1780 (1990). - Neurologic sequelae of domoic acid intoxication due to the ingestion of contaminated mussels (Montreal Neurological Institute)
- A clinical description of the 1987 outbreak of amnesic shellfish poisoning on Prince Edward Island.
J. S. Teitelbaum, R. J. Zatorre, S. Carpenter, D. Gendron, A. C. Evans, A. Gjedde, N. R. Cashman, New England J. Med. 322: 1781-1787 (1990).
Domoic acid chemistry
- Identification of domoic acid, a neuroexcitatory amino acid, in toxic mussels from eastern Prince Edward Island. (Atlantic Research Laboratory, Department of Fisheries and Oceans)
- Details of the analytical procedure used to identify the culprit in amnesic shellfish poisoning.
Wright, J.L.C., R.K. Boyd, A.S.W. de Freitas, M. Falk, R.A. Foxall, W.D. Jamieson, M. V. Laycock, A.W. McCulloch, A.G. McInnes, P. Odense, V. Pathak, M.A. Quilliam, M.A. Ragan, P.G. Sim, P. Thibault, J.A. Walter, M. Gilgan, D.J.A. Richard, and D. Dewar, Canadian Journal of Chemistry, 67 481-490 (1989). - The amnesic shellfish poisoning mystery (M. A. Quilliam, J. L. C. Wright)
- A retrospective account of the analytical approach for the first isolation of domoic acid from the toxic mussels.
M. A. Quilliam, J. L. C. Wright, Analytical Chemistry, 61 (18) 1053A-1060A, 1989. - Domoic Acid and Pseudo-Nitzschia References (Stephen Bates, Dept. of Fisheries and Oceans, Gulf Fisheries Centre)
- An extensive bibliography about domoic acid and the organisms that produce it.
- Marine Biotoxins Chemical Structures (National Oceanic and Atmospheric Administration)
- Part of NOAA's Harmful Algae Bloom site, this page discusses the structure and biological activity of domoic acid and the saxitoxins, which cause paralytic shellfish poisoning.
- Receptor Binding Assays and Marine Biotoxins (National Oceanic and Atmospheric Administration)
- Domoic acid's toxicity stems from its ability to bind tightly to glutamate receptors. Scientists at the Northwest Fisheries Science Center at NOAA have used domoic acid's ability to displace kainic acid from the receptor to developed a very sensitive technique for detecting domoic acid in water samples.
Glutamate and domoic acid pharmacology
- Domoic Acid Toxicity in Cerebellar Granule Neurons (Journal of Neurochemistry)
- Domoic acid kills nerve cells by causing them to fire until they die. The toxin works by causing the affected cells to uncontrollably release excitatory amino acids that influence nearby cells. The details may be found in this paper by two researchers at the College of Pharmacy and Toxicology Program at Oregon State University.
- F. W. Berman, T. F. Murray , J. Neurochem. 69, 693-703 (1997).
Metallosupramolecular assemblies: Unravelling the truth
Usually when trying to examine intramolecular rearrangements within assemblies the reactivity is always superimposed by the intermolecular exchange processes. Schalley explains ‘no clear distinction can be made between them in solution. However, in the gas phase, no reactions can occur because isolated ions are investigated. Consequently, a mass spectrometer is the perfect tool to study such processes, in the absence of any subunit exchanges.’
This is the first time that tandem MS (ESI-FTICR) has been applied to 3D assemblies. Schalley says ‘our mechanistic study is the first one to show that a two-fold contraction from a larger metallo-supramolecular bowl to a smaller cage can occur.’
It seems that there is huge potential for MS to be used by supramolecular chemists.
‘MS adds a completely new view on the reactivity when compared to the results obtained from other methods’ explains Schalley. This is certainly an exciting development, which should have a big impact on the future investigations of intramolecular reactivity in other supramolecular systems.
Original publication: Boris Brusilowskij, et. al., Chem. Commun., 2009. Selengkapnya...
Creating chiral carbons
Although there are many ways to make chiral carbon centres by alkylating carbonyl compounds, until now scientists have been unable to asymmetrically mono-alkylate at the 2-position of 1,3-dicarbonyls because the compounds racemise easily under basic or acidic conditions.
Hyeung-geun Park, at Seoul National University, and colleagues converted one of the two ester groups on malonyl esters to an amide to reduce the acidity of the hydrogens at the 2-position. He showed that the resulting malonamic esters could be mono-alkylated under basic conditions at the 2-position with high enantioselectivity, indicating that the malonamic esters are racemisation-resistant.
'To the best of our knowledge, this is the first report to accomplish direct mono-alkylation of the 1,3-dicarbonyl system,' says Park. As well as reducing the acidity of the hydrogens, Park says he may have inhibited enolisation of the products by introducing strain between the N-substituent on the amide group and the new 2-substituent.
Chiral mono-alkylated malonyl derivatives are useful synthetic intermediates, explains Park, because they can be converted into diverse chiral building blocks. He has already selectively reduced the malonamic esters to give a range of products and plans to investigate more chemoselective transformations in the future.
Original publications: Mi-hyun Kim et al, Chem. Commun., 2009. Selengkapnya...
Self-assembled spherical nanostructures
Nature creates spectacular nanoarchitectures through specific supramolecular assemblies of various components. Complementary hydrogen bonds are often utilised, which leads to the optimization of solvophilic interactions.
The use of hydrogen bonding also enables the tuning of the size and shape of the nanoparticles. The complementary hydrogen bonds promote self-organisation of the nanoparticles into uniform aggregates, and also enable a morphological change to occur from nanoparticle to vesicles.
Armaroli has also shown that the nanoaggregation can be reversed with temperature, which suggests possible applications of these vesicles in molecular delivery.
‘The ultimate aim of this work is to create a library of nanoarchitectures, which may exhibit potential applications as drug carriers, in biological imaging and in optoelectronic devices,’ says Armaroli.
However, Armaroli acknowledges that one of the main challenges to overcome is to design nanostructures with the desired molecular functionality, without compromising key features such as chemical stability and photoluminescence.
Original publication: Nicola Armaroli et al., Chem. Commun. 2009. Selengkapnya...
Plasmonic whispering gallery microcavity paves the way to future nanolasers
Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the California Institute of Technology have developed a "whispering gallery microcavity" based on plasmons - electromagnetic waves that race across the surfaces of metals. Such a plasmon wave has very small wavelength compared with the light, enabling the scaling down optical devices beyond diffraction limit of the light. Cavities are the confined spaces in lasers where light amplification takes place and this new micro-sized metallic cavity for plasmons improves on the quality of current plasmonic cavities by better than an order of magnitude.
"We have shown for the first time that metallic microcavities based on surface plasmons can have a large quality factor and can thereby enable ultra-small device fabrication and strong enhancement of the light," said Xiang Zhang, a mechanical engineer who holds a joint appointment with Berkeley Lab's Materials Sciences Division and the University of California (UC) Berkeley where he directs the NSF Nano-scale Science and Engineering Center.
"Plasmonic microcavities have uniquely different physical properties when compared to dielectric cavities and can extend microcavity research in entirely new ways, particularly at nanoscale dimensions," said Kerry Vahala, a physics professor at Cal Tech and authority on photonic devices. "Our work shows that the full potential of this new class of device can be realized with careful design and material control."
Zhang and Vahala led this collaborative research which is reported in Nature . The paper is entitled: "High-Q surface-plasmon-polariton whispering-gallery microcavity." In addition to Zhang and Vahala, other authors of the paper were Bumki Min, Eric Ostby, Volker Sorger, Erick Ulin-Avila and Lan Yang.
Just as the energy in waves of light is carried through space in discrete or quantized particle-like units called photons, so, too, is the energy in waves of charged gas (plasma) carried in quantized particle-like packets called plasmons, as they travel along metallic surfaces. When photons excite the collective electron oscillations at the interfaces between metal and dielectric (insulator) materials, they can form yet another quasi-particle called a surface plasmon polariton(SPP). Such polaritons play an important role in the optical properties of metals and can be used to manipulate light on a nanoscale.
"Metal-dielectric materials, also known as plasmonics, can be used to confine an optical field to a very small scale, much smaller than conventional insulators," said Min, lead author on the Nature paper and former postdoctoral researcher in Zhang's Lab, now an assistant professor at the Korea Advanced Institute of Science and Technology (KAIST). "This capability, often termed as breaking the light diffraction, is unobtainable with dielectric materials alone."
The main obstacle to working with plasmonic materials for creating nanoscale lasers has been a low quality or "Q" factor, which is a measure of power loss in the lasing cavity - a laser cavity with a high-Q factor has a low power loss. Enter the whispering gallery phenomenon, which Cal Tech's Vahala has used to boost the Q factor of dielectric microcavities. Whispering galleries are found in circular or elliptically shaped buildings, such as St. Paul's Cathedral in London, where the phenomenon was first made famous, or Statuary Hall in the U.S. Capitol building.
The prevailing theory behind why whispering galleries work (first proposed in 1871 by British astronomer George Airy to explain St. Paul's cathedral) is that sound originating at one point along the circumference of an enclosed sphere is reflected to another point along the circumference opposite the source. Vahala and his group applied this idea to dielectric microcavities, and Zhang and Min along with Ostby, Sorger and Ulin-Avila applied the idea to plasmonic microcavities.
"In these sphere-shaped microcavities, optical waves propagate in a similar way that sound waves propagate in a whispering gallery," said Zhang. "They continue to circle around the edge of the cavity sphere and the smoothness of the edge enhances or boosts the cavity's Q factor."
In this study, Zhang and his collaborators created a high-Q SPP whispering gallery microcavity by coating the surface of a high-Q silica microcavity with a thin layer of silver.
Explained Zhang, "Whenever light propagates in a metal it experiences some loss of power and this obviously reduces the performance of a device. Silver is the metal with the lowest loss, that is available."
Whereas previous plasmonic microcavities achieved a best Q factor below 100, the whispering gallery plasmonic microcavity allows Q factors of 1,376 in the near infrared for SPP modes at room temperature.
"This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method," said Min, who believes that there is still room for plasmonic Q-factor improvement by geometrical and material optimizations.
Min said one of the first applications of the whispering gallery plasmonic microcavity is likely to be the development of a plasmonic nanolaser.
"To build a working laser, it is essential to have both the laser cavity (or resonator) and the gain media," Min said. "Therefore, we need a good, high-Q plasmonic microcavity to make a plasmonic nanolaser. Our work paves the way to accomplish the demonstration of a real plasmonic nanolaser. In addition, fundamental research can also be pursued with this plasmonic cavity, such as the interaction of a single light emitter with plasmons." Selengkapnya...
A crystal clear view of chalk formation
Calcium carbonate is ubiquitous: everyone has probably held a stick of blackboard chalk in the hand at one time or another, or railed against the deposits it forms in washing machines. It is the main constituent of marble, dolomite and many types of sediment, and it is also found in the shells of crabs, mussels, snails, sea urchins and in single-celled organisms. These biomaterials have properties that make them interesting for applications in medicine and building materials technology. The ingenious structure of their crystals at nanoscopic level makes them particularly robust. Materials scientists would like to know how organisms produce these structures, so that they can copy them.
Researchers at the Max Planck Institute in Golm near Potsdam have now made a contribution to achieving this aim by demonstrating that calcium carbonate crystals are created differently from the way they were previously thought to form. When calcium and carbonate ions come together in a solution, they form stable nanoclusters consisting of around 70 calcium and carbonate ions - and they do that even in very soft water, a dilute solution from which chalk does not normally precipitate. If the concentration of dissolved calcium carbonate is increased, the clusters clump together and the mineral crystallizes. "It seems that it is already decided when the clusters form, which of the three anhydrous crystal structures calcium carbonate will assume," says Helmut Cölfen who headed the study. "We also observed that the crystal structure depends on the pH level." Under low alkaline conditions, calcium carbonate forms calcite, its most stable crystalline structure. In a more alkaline environment, it creates vaterite, a non-stable crystalline structure.
"Our results suggest that the pH level influences the way the ions group together in these clusters, which are just two nanometers in size," explains Denis Gebauer, who played a crucial part in the study. At this stage, they do not form a regular crystal structure, but it is highly likely that the rudimentary arrangement of the crystal is already recognizable. If the clusters then group together into increasingly larger aggregates, this arrangement can remain in place. A transient amorphous form, that is, a non-crystalline solid, is initially created, which then changes into a crystal.
If crystallization really did take this route, it would be easier to understand how mussels, for example, construct their shells or a sea urchin forms its spines. As the tiny clusters with which crystallization starts are stable, organisms would have to intervene only at this early stage to influence the structure. They might use the pH level or biomolecules to do this.
The theory of crystallization that has prevailed so far leaves little room for influencing the arrangement of the ions in the regular crystal lattice early on. It assumes that the ions do not group together until a certain concentration has been exceeded. If these clusters do not reach a minimum size, they break apart. It is only when they can get beyond the size of the "critical crystal nucleus" that it becomes possible for the nucleus to grow into a crystal. The earliest point at which the crystal structure could be influenced would therefore be the critical nucleus.
The researchers in Golm used calcium phosphate and calcium oxalate to test whether other minerals also follow this crystallization pathway. Calcium phosphate is the main constituent of bones and teeth; kidney stones are predominately made up of calcium oxalate. The scientists subjected these materials to the same test as calcium carbonate. Drop by drop they added a solution containing calcium ions to a solution with the other component - that is, carbonate, phosphate or oxalate ions. With a special electrode they measured how many of the added calcium ions were present in the solution. It turned out that also in the experiments with calcium phosphate and calcium oxalate, there were fewer ions available than the researchers had added - they must therefore have been fixed into clusters, as was the case with the calcium carbonate.
The newly proposed mechanism of crystallization also has consequences for technology. "The stable clusters offer a new point at which to tackle lime scale deposits - not only in washing machines and dish washers, but also in industry," says Helmut Cölfen. This problem causes around 50 billion dollars worth of damage each year in industrial nations. Initially, traditional scale inhibitors fish out the calcium ions from the water, and secondly they bind the tiny precipitated crystals and stop them growing. "Now it will be possible to develop new types of scale inhibitors that prevent the nanoclusters from joining up to form larger structures," says Cölfen. "This is more effective than the traditional approaches."
These discoveries about crystallization also have consequences for climate change. The clusters bind carbon dioxide as carbonate. Up to now it has only been known that calcium carbonate mineral stores this greenhouse gas. As the nanoscopic clusters also form in the oceans, they prevent more carbon dioxide from entering the atmosphere than previously assumed for calcium carbonate minerals. However, there is a problem: the oceans are acidifying because a considerable proportion of the carbon dioxide from the atmosphere is dissolving into them as carbonic acid. "When the pH level falls, less carbonate can be bound in the clusters," says Helmut Cölfen. This allows more carbon dioxide to be released into the atmosphere, where it turns the global heating system up another notch.
Original work: Denis Gebauer, Antje Völkel, Helmut Cölfen; "Stable Prenucleation Calcium Carbonate Clusters Science"; Science 2008 .
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