Highlights
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
