Crowding controls whether carbon chains or a hydrogen atom will transfer from transition metal molecular complexes to acceptor molecules.To gain this new insight into the factors governing the onset of hydrogen abstraction from metal alkyls, researchers carefully designed experiments involving series of cobalt and chromium alkyls. The results show that when the alkyl chain is only one carbon long, the alkyl group will transfer to a rhodium acceptor molecule. But, if the chain is made up of two or more carbons, crowding makes it hard for the alkyl group to transfer. In these cases the reaction finds another pathway, one in which a single hydrogen atom, rather than the entire alkyl group, is transferred. The transfer of hydrogen and carbon chains from transition metal molecular complexes to an acceptor molecule is involved in many catalytic reactions of industrial and biological significance. These results provide key fundamental information about these processes.
A new series of catalysts is able to selectively make “left-handed” or “right-handed” nitrogen-containing compounds known as amines. Left-handed and right-handed molecules contain the same components, but are mirror images of each other. Researchers were able to take strings of nitrogen-containing molecules and make five-, six- and seven-membered rings with enantiomeric excess of 90%. Most other catalysts produce a mixture of both enantiomers. Researchers studied hafnium, titanium and zirconium (Group 4) containing catalysts and found the zirconium catalysts to be the best at producing one enantiomer in high yield over the other. The new zirconium catalysts do what no other Group 4 catalyst has done before — they can operate at room temperature and down to minus 20 °F. All other zirconium catalysts operate at 300 °F. These new catalysts are also tolerant of various functional groups attached to the amines. Researchers performed detailed studies of the structure, activity and selectivity of this system of catalysts and were able to characterize the reaction pathways. The pursuit of these optically active amines is important for improved syntheses of commodity and specialty chemicals.
For the first time, researchers can keep multiple nanoparticles in focus while tracking their 3D orientations on a surface with unprecedented angular resolution. The new technique can accurately track anisotropic gold particles that are tilted out of the horizontal plane and has the advantage of not relying on particle interactions with the surface to keep track of them. This technique takes advantage of the optical properties of gold; at certain frequencies of light electrons in the gold are stimulated to collectively oscillate, known as surface plasmon resonance. For gold nanorods the frequency of light needed to induce surface plasmon resonance differs for the long axis of the rod compared to the short axis. This technique takes advantage of this difference. The power of the technique was demonstrated by studying modified gold nanoparticles landing on lipid membrane bilayers. While a significant fraction is "frozen" on the surface, many particles are not. The capability to follow the 3D movements of nanoparticles will greatly enhance our understanding of the way nanoparticles interact with surfaces.
A new technique makes it possible to track not only the location of moving particles to within 10 nanometers, but also their rotation and orientation. This is like watching a football game from the ionosphere and knowing where the football is at anytime within 1.5 inches, how the ball is spinning, and what direction it is moving. This is made possible by inserting an additional arm into the optical path of the specialized instrument known as a differential interference contrast microscope used for visualizing motion. This addition makes it possible to operate in two modes simultaneously; one mode focuses on particle location while the other focuses on rotation and orientation. Images can be taken every 75 milliseconds, 4 times faster than a blink of an eye. The new technique will aid researchers looking to understanding particle movement in nanoengineered environments and within cells involved in important processes such as carbon fixation.
Scientists have made air-stable compounds that should not be stable in air. Custom-designed carbon chains bonded to zinc control the rate and selectivity of reactions with oxygen and lead to the formation of novel stable zinc peroxides. New metal peroxides are desirable because they are highly effective oxidants that are useful for a variety of chemical transformations. Most metal peroxides are made using transition metals and lack the stability necessary to directly study their role in important catalytic industrial processes. Zinc is not a transition metal, but some of its properties are similar to those of transition metals, and this was crucial to the discovery. Common transition metal peroxide decomposition pathways are blocked when zinc is used. Zinc—oxygen and oxygen—oxygen bond cleavage, for example, are not fast whereas they are if transition metals are used. The lack of decomposition pathways make these zinc peroxides remarkably robust compounds. And, zinc has the added benefit of being inexpensive. These new air-stable organozinc compounds may be useful for a variety of catalytic conversions and also provide new routes to metal alkylperoxides.
Working to use sunlight to convert biomass to biofuels, researchers have found a pathway toward reducing the energy costs associated with making renewable biofuels. To achieve this, they designed semiconducting nanorods that act as light harvesting antennas, and attached metal nanoparticles that are activated by energy from the sun. This nanostructured photocatalyst converted bioderived alcohols to benzaldehyde, toluene, and the zero-emission biofuel hydrogen. Benzaldehyde is used as an almond-flavoring agent in foods and as a precursor for many pharmaceuticals, and toluene is a common industrial solvent. The metal nanoparticles, made from platinum or palladium, not only provided photocatalytic activity, but also prevented etching and degradation of the nanorods. By tuning the composition of the nanorods and the amount of metal attached (less being better!) the production of hydrogen could be increased relative to that of benzaldehyde and toluene. Further tuning and new designs of these photocatalytic nanocomposites are expected to lead to additional ways to produce lower-cost biofuels from sunlight.
Researchers have developed a new way to track gold nanorods as they move around and re-orient themselves on metal surfaces, with significantly improved spatial resolution and speed compared with existing methods. Fluorescent dyes are commonly attached to molecules to make it possible to study their orientation and rotation. However this approach has drawbacks, because of limited signal stability and long observation times. One solution is to replace the fluorescent molecules with gold nanoparticles, providing better stability but making it harder to get detailed orientation data. Focused orientation and position imaging (FOPI) overcomes a key limitation of older methods — not being able to distinguish the full 360° orientation of the nanorods. This technique is capable of faster, higher throughput detection of the position and the 3D orientation of the gold nanoparticles, a key step towards allowing researchers to follow the motion of gold-tagged molecules as they move, interact and react on metal surfaces. This could impact a number of technologies ranging from catalysis to corrosion protection.
Light, combined with a novel rhodium catalyst, enables greener production of chemical feedstocks from biorenewables. A key challenge in the utilization of biomass for fuels and fine chemical applications is the control of oxygen and nitrogen-containing functional groups.Unfortunately, current routes such as gasification also generate unwanted by-products such as carbon dioxide and carbonaceous material. Other processes require additional, sacrificial chemicals, increasing costs and decreasing sustainability. Researchers developed a new process for the conversion of primary alcohols into hydrocarbons through tandem catalytic reactions. One reaction removes hydrogen from the alcohol producing valuable H2, while the other removes an atom of carbon and oxygen and produces carbon monoxide. Typically carbon monoxide inhibits the removal of hydrogen, but the use of light and a new rhodium catalyst created specially for this process prevents the inhibition. Interestingly, the catalyst was also found to be useful for controlling reactions with primary amines.The tandem reactions run at room temperature are highly selective and high yielding so these findings have great potential for enabling new industrial biorenewable-based processes.
Capitalizing on the concept that everything proceeds faster with a little cooperation, researchers showed how designing cooperation into solid catalysts leads to enormous benefits.Catalysts attached to a porous solid support are preferred industrially because they are easier to separate from liquid products and reuse. But, these bound catalysts typically do not perform as well and probing their interiors to figure out how to improve them has proved difficult until now. Using new solid-state nuclear magnetic resonance (SSNMR) methods (the equivalent of running an MRI on the catalyst) and innovative synthetic strategies, researchers showed how to probe their inner workings and make optimization possible. Scientists demonstrated this approach on a carbon-carbon bond forming reaction routinely used in chemical manufacturing and biofuel production. Two key insights were revealed. First, access into and out of the pores is blocked by a chemical intermediate. Making the pores a mere 0.8 nanometers wider increased the catalytic activity 20-fold! Knowing the structure of the intermediate, researchers were able to modify the catalyst to eliminate the bottleneck without making the pore wider. This heterogeneous catalyst is significantly more active than the homogeneous catalysts, contrary to expectations. Why? SSNMR showed the support brings the reactants and catalytic groups together, resulting in the enhanced, cooperative activity not possible with the untethered catalyst. This work sets the stage for significant innovations for commonly used catalytic processes.
A new technique simultaneously illuminates the location, orientation and rotation in 3D of individual gold nanorods. Gold nanorods have been used as orientation probes in optical imaging because of their shape-induced anisotropic optical properties and now we can do this even better. Gold nanorods have the benefits of being biocompatible and having optical properties that depend on their orientation. This new development provides full 360° rotational information about these nanorods without sacrificing spatial and time resolution. Previous techniques for tracking nanoprobes in the focal plane could only distinguish from 0 to 90°, so clockwise and counterclockwise movements looked the same. Researchers combined a technique known as differential interference contrast microscopy with image pattern recognition to achieve this breakthrough. Assessing the baseline patterns for each rotational angle involved using static, titled nanorods and a 360° rotating stage. As a first demonstration of the power of the technique, researchers followed functionalized gold nanorods on live cell membranes. Resolving the location, orientation and rotational movements of nanoparticles is important for gaining fundamental information about chemical interactions with nanostructured materials.
Scientists have helped solve an 80-year-old puzzle about a widely used chemical process. The Fenton reaction involves iron and hydrogen peroxide and is used to treat wastewater worldwide. Does the reaction involve a radical intermediate? Or, is it the non-radical, iron species known as Fe(IV)? The exact nature of the intermediate has been debated for decades with data to support both theories. The problem is both intermediates will react to form the same products in most cases making the reaction intermediate hard to pin down. Researchers have now proved that both intermediates can be involved — it just depends on the pH. They carefully studied a reaction for which the two intermediates would form different products. They showed that in an acidic environment, the intermediate is an hydroxyl radical, whereas at near neutral pH the intermediate is Fe(IV). This discovery explains the differences in products formed under certain reaction conditions and clears up a decades-old mystery.
Tweaking the chemicals used to form nanorods can be used to control their shape.Controlling a nanorod’s shape is a key to controlling its properties. Researchers used a combined experimental and theoretical approach to show that precursor reactivity determines the relative ease of formation of different nanocrystals. Specifically, photocatalysts made from tiny amounts of cadmium, sulfur and selenium will form selectively into shapes that look like either tadpoles or drumsticks depending on the relative reactivity of the selenium and sulfur precursors. The more strongly bound the selenium or sulfur is to phosphorous in the precursor, the lower the reactivity. The lower the reactivity, the longer the nanorod and the more it is shaped like a tadpole. Purposely altering and modulating chemical reactivity of reactants will contribute to the development of more predictable routes to fabricate nanostructures with highly specific properties.
A new theory shows that reactivity at catalytic sites inside narrow pores is controlled by how molecules move at the pore openings. Like cars approaching a single lane tunnel from which other cars are emerging, the movement of molecules depends on their distance into the pore; near the ends of the pores, exchange is rapid compared to further into the pores. Dynamics at the openings of these pores controls the penetration of reactants and thus overall conversion to products. Overall, the behavior of catalytic reactions in narrow pores is controlled by a delicate interplay between fluctuations at pore openings, restricted diffusion, and reaction. Until now it has been impossible to reconcile analytical theories with the findings of detailed step-by-step simulations. The new theory enables calculations of reactant and product distributions in minutes compared to the hours or days it takes to do the detailed simulations and yields comparable results. Thus, this new theory is a powerful tool for analyzing the catalytic behavior in these systems.
The motion of particles can now be tracked with high precision in three dimensions inside living cells. 3D tracking is made possible by producing images of planes only 40 nanometer apart and then using an optical processing advance to locate and track the gold nanoparticles.The technique relies on changes in the optical paths of two beams when light hits tiny gold particles used as probes. The gold particles can be readily followed because they affect one of the light beams more than the other, and it is the difference between them that is observed. These particles can be modified so that cells will allow them to pass through the membrane and interact with various receptors. Because the gold has minimal impact on biological systems this method is ideal for long-term precision tracking in living systems. This is the first demonstration of tracking 3D gold nanoparticles to nanometer precision within biological samples.
Just like watching boats in the night, seeing movement at the nanoscale is easier when the object you are watching has a beacon.Dynamic three-dimensional tracking with high precision is possible with nanoscale light emitting particles known as quantum dots at better resolution than 10 nanometers in the vertical direction. This opens up the possibility for understanding three dimensional movement in nanoscale structures and biological systems. The quantum dots are followed using the technique known as scanning-angle total internal reflection fluorescence microscopy (SA-TIRFM). Quantum dots hold advantages over other fluorescent probes because they can be tuned to emit various colors of light. Many, however, will spontaneously “blink” meaning the emitted light is suddenly turns off (or on) thus interrupting measurements. Researchers have developed "non-blinking" quantum dots that make them useful for high precision tracking in dynamic environments. This methodology was used to show the potential of motor proteins as components in nanomachines to transport cargo.
Researchers can now analyze how reactions proceed inside porous nanoparticles where the molecules are in such narrow channels that they cannot pass each other. Catalysis within these confined conditions is significantly impacted by restricted transport. Typical pore diameters are in the range of 2 - 10 nm, and with catalyst molecules attached inside them, the pore diameter can be reduced below 2 nm. Traditional computational tools do not capture the evolution of concentrations inside pores so narrow that reactants and products cannot pass each other. The new methods precisely describe this kind of constrained chemical diffusion. Narrow pores with catalytic sites varying in number and location were analyzed. Snapshots of the locations of reactants and products as a function of time show the factors that influence the transient and steady-state behaviors.These studies set the stage for understanding more complex systems and designing new, even better, catalysts.
Scientists have advanced methods to make maps of the locations of molecules within plant materials. Resolution of 10 to 50 microns, less than a quarter the size of a human hair, is routinely possible. The trick with plant materials is to extract the molecules delicately from thin slices with a fine laser moving stepwise across the sample.Many molecules are analyzed at once using a very sensitive mass spectrometer in this technique known as matrix-assisted laser deposition/ionization-mass spectrometry imaging (MALDI-MS). Within cottonseed embryos, which are about 3/16th of an inch in diameter, this method showed a surprisingly non-uniform mixture of lipids whose concentration varies with tissue functionality. These lipids are important for seed development and can affect the chemistry of the cottonseed oil extracted for use in various foods.These findings demonstrate the potential of this technique to provide a new level of understanding of biosynthetic pathways.
Locating a catalyst and reactants in confined spaces makes catalytic reactions go faster in the desired direction. Of course, the reaction products have to be removed from the confined spaces and researchers have developed a new approach to expelling aqueous reaction products. This works for confinement in nanometer-sized pores in silica particles. By lining the insides of the pores with both catalysts and a fluorinated chemical, like that found in Teflon®, reactions with water as a byproduct proceed much faster. This works because certain chemicals just don’t like each other. Oil and water tend to separate. Water on a Teflon®-coated frying pan balls up to minimize its contact with the Teflon®. Combining state-of-the-art characterization and theory, a structure was designed to maximize this effect inside the catalytic pores. The performance of this catalyst surpasses the commercially available ones for a reaction known as esterification, that yields water as a byproduct. This is the first demonstration of enhancing chemical transformations by expelling the byproducts from porous catalytic materials in this manner and just the beginning of essentially a new class of catalysts.
Scientists have discovered a method to fine-tune the shapes of nanorod photocatalyst particles. These materials accelerate reactions when they are activated by light and their shape affects their behavior. Researchers showed that the photocatalysts, made from tiny amounts of cadmium, sulfur and selenium, will form selectively into shapes that look like either tadpoles or drumsticks depending on the selenium concentration. Their optical properties depend strongly on the relative amounts of sulfur and selenium; the sulfur to selenium ratio changes along the length of each particle causing each end to interact with light differently. These nanomaterials are being studied for their potential as light harvesting antennas, as novel optically driven biomass conversion catalysts and to form more complex nanostructures and light harvesting devices.
Thanks to the innovation of “single particle orientation and rotation tracking” (SPORT), we now can watch the distinctive movements of drug delivering nanoparticles in real time. Nanoparticles have the potential to revolutionize drug delivery. When these particles interact with cell membranes they move in all sorts of ways. They spin, they tumble, they move along and through the membrane. At least that’s what we think. But what’s really going on? Until now, we could track how these nanoparticles rotated only by taking a series of still photographs making studies of fast rotations beyond our reach. Nano-sized rods made from gold were modified with drug delivery agents, like transferrin, and watched via SPORT. For the first time, the distinctive rotational behaviors of these modified nanorods were attributed to specific binding sites on the cell membrane. This new technique will lead to a better understanding of nanoparticle-based drug delivery mechanisms and provide guidance on how to improve existing nanoparticle drug delivery technology.
Chemists have synthesized a highly selective and highly efficient zirconium catalyst that makes new carbon-nitrogen bonds by adding a nitrogen-hydrogen bond to a carbon-carbon double bond. Nitrogen-containing chemicals are important as agrichemicals, pharmaceuticals, and specialty chemicals. These zirconium catalysts are expected to show greater tolerance to other functionality than the well-known and highly sensitive rare earth catalysts. The new catalysts are more efficient than previously reported zirconium catalysts, promoting the reaction at room temperature. This high activity may be related to its ability to access a new mechanistic pathway that was proposed based on unique kinetic and selectivity observations. In this mechanism, carbon-nitrogen and carbon-hydrogen bond formation occurs in a concerted fashion.
Researchers systematically blocked key chemical reaction pathways to get unambiguous information about how carbon-nitrogen bonds are formed in a catalytic reaction known as hydroamination. Understanding a multi-step catalytic mechanism is like a solving a puzzle where you can’t see the pieces. However, you can add your own “pieces” with known shapes to figure out what other pieces of the puzzle then will (or will not) fit. Hydroamination reactions are catalyzed by several different metal catalysts. The researchers studied magnesium-based hydroamination catalysts because they have stable, potential intermediates in the catalytic process that could be synthesized separately, can be used to understand the catalytic mechanism, and provide alternatives to traditional rare earth catalysts. Blocking the common insertion mechanism showed that a second route for hydroamination is possible, indicating that the catalyst can work in at least two distinct ways. This information is key to understanding this class of catalyst, which is used for carbon-nitrogen bond reforming reactions, and to guiding general strategies for replacing rare earths in catalysts.
Researchers have discovered how the geometry of gold nanoparticles affects their images. Gold nanoparticles can be imaged optically and their movements can be seen using a technique known as differential interference contrast (DIC) microscopy. How gold nanoparticles appear in these images depends upon their environment. This can be used to learn about time-dependent nanoscale processes. However, an outstanding question has been whether or not the geometry of the gold particles affects how they are imaged. Researchers looked at three common nanoparticle geometries: a single rod, two rods stuck together and two rods separated but close to each other, so-called proximate rods. Trapping differently positioned nanoparticles and characterizing them with an electron microscope enabled comparison with the DIC images to see how each geometry is imaged in the optical system. Unlike other techniques, DIC produces images that uniquely distinguish these different geometries to even as they move around. Even at the nanoscale, geometry matters.
Until now, watching the detailed spinning motion of nano-objects within living cells has been impossible. Combining an existing technique, known as Differential Interference Contrast (DIC) Microscopy, with nanotechnology, researchers can now see how nanoparticles spin when they move across the interiors of living cells. Nano-sized rods made of gold are non-toxic to living cells and they scatter light differently depending on their orientation. DIC microscopy captures the orientation of gold nanorods in addition to the optical image of the cell. Gold nanorods (25 x 75 nanometers) were used to show particle movement within living cells. Researchers were even able to demonstrate the rotational motions of the host structure using gold nanorod probes. This new technique opens up doors to understanding living nanomachines by revealing their complex internal motions