Gold nanoparticles improve technology to detect hazardous chemicals

The new system can pick out a single target molecule from 10 000 trillion water molecules within milliseconds, by trapping it on a self-assembling single layer of gold nanoparticles.
The team of LCN scientists from the Department of Chemistry at Imperial say this technology opens the way to develop devices that are compact, reusable and easy to assemble, and could have a range of uses including detecting illegal drugs, explosives, pollutants in rivers or nerve gases released into the air. Results of the research are published this week in Nature Materials (“Self-assembled nanoparticle arrays for multiphase trace analyte detection”).

In one potential use, such a device could detect tiny traces of explosives or other illegal substances left behind by criminals on the surfaces they touch. The advances made by this team would help law enforcers to identify and deal with such activities involving illegal substances.
Research co-author, Michael Cecchini, said: “Our system could solve a key problem of reliable and portable chemical testing for use in the outside world. It is very sensitive and could well be used to look for very small amounts of a specific molecule even in busy, public areas.”

The target molecules are identified by an effect called Surface Enhanced Raman Scattering (SERS) of light. This technique, which has been around since the late 1970’s, works because each molecule scatters light in a unique way. Previous research has shown that the signal can be amplified by catching molecules in a particular way on a layer of metal nanoparticles. However, these sheets are complex to manufacture.

The scientists overcame this problem by dealing with interfaces of two liquids that do not mix, such as water and oil, or water and air interface. By manipulating the electrical charge of the gold nanoparticles and the composition of the solution, they were able to create a situation where the particles line themselves up at the interface between the two non-mixable liquids, or between a liquid and the air.

“The trick to achieving this system’s sensitivity to the target molecules was in finding the conditions at which nanoparticles would settle at the interface at close distances to each other without fusing together”, commented another co-author Jack Paget.

If the nanoparticles are disturbed, they spontaneously arrange themselves back in the correct way make the device more robust than those made rigidly arranged particles. Research co-author, Vladimir Turek, said: “The system shows real promise for detectors for use in rough outdoor environmental and defence applications, since the liquids and nanoparticles can be easily replaced to regenerate the device.”

Source: By Joshua B. Edel, London Centre for Nanotechnology

Nanotechnology simplifies hydrogen production for clean energy

In the first-ever experiment of its kind, researchers have demonstrated that clean energy hydrogen can be produced from water splitting by using very small metal particles that are exposed to sunlight. In the article, “Outstanding activity of sub-nm Au clusters for photocatalytic hydrogen production”, published in the journal Applied Catalysis B: Environmental, Alexander Orlov, PhD, an Assistant Professor of Materials Science & Engineering at Stony Brook University, and his colleagues from Stony Brook and Brookhaven National Laboratory, found that the use of gold particles smaller than one nanometer resulted in greater hydrogen production than other co-catalysts tested.
Experimental and theory predicted optical properties of supported sub-nanometer particles.
Experimental and theory predicted optical properties of supported sub-nanometer particles.
“This is the first ever demonstration of the remarkable potential of very small metal nanoparticles [containing fewer than a dozen atoms] for making fuel from water,” said Professor Orlov.
Using nanotechnology, Professor Orlov’s group found that when the size of metal particles are reduced to dimensions below one nanometer, there is a tremendous increase in the ability of these particles to facilitate hydrogen production from water using solar light. They observed a “greater than 35 times increase” in hydrogen evolution as compared to ordinary materials.
Experimental and theory predicted optical properties of supported sub-nanometer particles. In order to explain these fascinating results, Professor Orlov collaborated with Brookhaven National Lab computational scientist Dr. Yan Li, who found some interesting anomalies in electronic properties of these small particles. Professor Orlov noted that there is still a tremendous amount of work that needs be done to understand this phenomenon.
“It is conceivable that we are only at the beginning of an extraordinary journey to utilize such small particles [of less than a dozen atoms in size] for clean energy production,” he said.
“In order to reduce our dependence on fossil fuels it is vital to explore various sustainable energy options,” Professor Orlov said. “One possible strategy is to develop a hydrogen-based energy economy, which can potentially offer numerous environmental and energy efficiency benefits. Hydrogen can conceivably be a promising energy source in the future as it is a very clean fuel, which produces water as a final combustion product. The current challenge is to find new materials, which can help to produce hydrogen from sustainable sources, such as water.”
Source: Stony Brook University

Overview of nanomaterials for cleaning up the environment

Researchers have presented an extensive analysis of the role of nanomaterialsin environmental remediation and monitoring. Nanomaterials can be used to clean up toxins and bacteria from natural waters, wastewaters and the air (see paper in Energy & Environmental Science: “A review on nanomaterials for environmental remediation”).
Nanomaterials’ unique properties allow them to remove pollutants from the environment. The extremely small size of nanomaterial particles, typically in the range between 1 and 100 nanometres, creates a large surface area in relation to their volume, which makes them highly reactive, compared to non-nano forms of the same materials.
Silver, iron, gold, titanium oxides and iron oxides are some of the commonly used nanoscale metals and metal oxides cited by the researchers that can be used in environmental remediation. Silver nanoparticles, for example, have proved to be effective antimicrobial agents and can treat wastewater containing bacteria, viruses and fungi. Nanoscale titanium dioxide can also kill bacteria and disinfect water when activated by light.
Gold nanoparticles may potentially be another useful material for removing contaminants, such as toxic chlorinated organic compounds, pesticides and inorganic mercury, from water. They can also be used to remediate air. In combination with titanium dioxide, goldnanoparticles have been shown to convert the toxic air pollutant, sulphur dioxide, to sulphur. Titanium dioxide nanomaterials are commonly used in some processes to disinfect water, in addition to breaking down halogenated compounds, and removing dyes and metal toxins from drinking water and wastewater.
The researchers point to studies that show that carbon nanomaterials are particularly suited to removing a broad range of pollutants. Carbon nanotube clusters, for example, are used to purify water by adsorbing bacteria that contaminate the water. Heavy metals, such as cadmium, as well as organic pollutants including benzene and 1,2-dichlorobenzene can also be removed from water by carbon nanotube materials.
The researchers suggest that nanoparticles can be attached to host polymer materials, such as porous resins, cellulose and silica, to reduce potential harm to human health and the environment derived from the release of nanoparticles into the environment. The nanoparticles fixed to the host material are thus bulkier and can be more easily removed and captured from wastewater. Nanoparticles, such as nanoscale zinc oxide, fixed in this way, are used, for example, to break down organochlorine pesticides, halogenated herbicides and azo dyes.
In addition to remediating pollution, nanoparticles can be used as sensors to monitor toxins, heavy metals and organic contaminants in land, air and water environments and have been found to be more sensitive and selective than conventional sensors. Sensor strips composed of nylon 6 nano-fibre nets are one example. These are used to detect formaldehyde, a toxic air pollutant widely used in the manufacture of household materials and building products. The yellow sensor strips turn red upon exposure to formaldehyde.
The researchers acknowledge that ongoing work is needed to further improve the shape, sizes, structures, functionality and manufacture of nanomaterials that show promise in cleaning up contaminants that enter water, land and air environments from industries, transport and other human activities. A better understanding of the behaviour of nanomaterials and their potential harm to the environment is also required.

First all-carbon solar cell

Stanford University scientists have built the first solar cell made entirely of carbon, a promising alternative to the expensive materials used in photovoltaic devices today.
The results are published in the Oct. 31 online edition of the journal ACS Nano (” Evaluation of Solution-Processable Carbon-Based Electrodes for All-Carbon Solar Cells”).
“Carbon has the potential to deliver high performance at a low cost,” said study senior author Zhenan Bao, a professor of chemical engineering at Stanford. “To the best of our knowledge, this is the first demonstration of a working solar cell that has all of the components made of carbon. This study builds on previous work done in our lab.”

Unlike rigid silicon solar panels that adorn many rooftops, Stanford’s thin film prototype is made of carbon materials that can be coated from solution. “Perhaps in the future we can look at alternative markets where flexible carbon solar cells are coated on the surface of buildings, on windows or on cars to generate electricity,” Bao said.
The coating technique also has the potential to reduce manufacturing costs, said Stanford graduate student Michael Vosgueritchian, co-lead author of the study with postdoctoral researcher Marc Ramuz.
“Processing silicon-based solar cells requires a lot of steps,” Vosgueritchian explained. “But our entire device can be built using simple coating methods that don’t require expensive tools and machines.”
Carbon nanomaterials
The Bao group’s experimental solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes. In a typical thin film solar cell, the electrodes are made of conductive metals and indium tin oxide (ITO). “Materials like indium are scarce and becoming more expensive as the demand for solar cells, touchscreen panels and other electronic devices grows,” Bao said. “Carbon, on the other hand, is low cost and Earth-abundant.”
For the study, Bao and her colleagues replaced the silver and ITO used in conventional electrodes with graphene – sheets of carbon that are one atom thick –and single-walled carbon nanotubes that are 10,000 times narrower than a human hair. “Carbon nanotubes have extraordinary electrical conductivity and light-absorption properties,” Bao said.
For the active layer, the scientists used material made of carbon nanotubes and “buckyballs” – soccer ball-shaped carbon molecules just one nanometer in diameter. The research team recently filed a patent for the entire device.
“Every component in our solar cell, from top to bottom, is made of carbon materials,” Vosgueritchian said. “Other groups have reported making all-carbon solar cells, but they were referring to just the active layer in the middle, not the electrodes.”
One drawback of the all-carbon prototype is that it primarily absorbs near-infrared wavelengths of light, contributing to a laboratory efficiency of less than 1 percent – much lower than commercially available solar cells. “We clearly have a long way to go on efficiency,” Bao said. “But with better materials and better processing techniques, we expect that the efficiency will go up quite dramatically.”
Improving efficiency
The Stanford team is looking at a variety of ways to improve efficiency. “Roughness can short-circuit the device and make it hard to collect the current,” Bao said. “We have to figure out how to make each layer very smooth by stacking the nanomaterials really well.”
The researchers are also experimenting with carbon nanomaterials that can absorb more light in a broader range of wavelengths, including the visible spectrum.
“Materials made of carbon are very robust,” Bao said. “They remain stable in air temperatures of nearly 1,100 degrees Fahrenheit.”
The ability of carbon solar cells to out-perform conventional devices under extreme conditions could overcome the need for greater efficiency, according to Vosgueritchian. “We believe that all-carbon solar cells could be used in extreme environments, such as at high temperatures or at high physical stress,” he said. “But obviously we want the highest efficiency possible and are working on ways to improve our device.”
“Photovoltaics will definitely be a very important source of power that we will tap into in the future,” Bao said. “We have a lot of available sunlight. We’ve got to figure out some way to use this natural resource that is given to us.”

Source: Nanowerk News

Graphene-based nanocomposite to adsorb water pollutants

(Nanowerk News) Researchers succeeded in the production of particles with smaller size but higher surface area, and consequently more number of active sites, to adsorb pollutants by synthesizing cerium oxide–titanium dioxide nanoparticles and obtaining cerium oxide–titanium dioxide nanocomposite (see paper in Journal of Hazardous Materials: “Assembly of CeO2–TiO2 nanoparticles prepared in room temperature ionic liquid on graphene nanosheets for photocatalytic degradation of pollutants”).
Among semi-conductive photocatalysts, titanium dioxide (TiO2) is an important candidate to be used in many industries due to its high optical stability and non-toxicity. However, it is impossible to use this component in visible light because its energy gap is placed in the range of ultraviolet.
In this research, researchers from Sharif University of Technology in association with researchers from University of Mohaghegh Ardebili and Nanoscience and Nanotechnology Research Center tried to move the energy gap of TiO2 towards longer wavelengths through the synthesis of carbon-based TiO2 / CeO2 nanocomposite. They also aimed to increase the photocatalytic activity of TiO2.
Results of the research showed that the synthesis of TiO2 in an ionic solution and the addition of cerium oxide to the structure of TiO2 decreased the particle size, increased the surface area, and slowed down the phase exchange from anatase to rutile at higher temperatures. As a result, it caused the creation of nanoparticles with higher thermal stability. High activity of the nanocomposite in the degradation of pollutants is explained by the unique structure of graphene, which increases adsorption on the catalyst surface and decreases the re-composition of ion carriers.

Titanate nanotubes and nanofibers for radioactive waste clean-up in water

Radioactive cesium and iodine ions are products of uranium fission and can be easily dissolved in water during an accident at a nuclear reactor like the one in Fukushima earlier this year. The fear is that these fission products could get into the groundwater and could make their way into the food chain.

Natural inorganic cation exchange materials, such as clays and zeolites, have been extensively studied and used in the removal of radioactive ions from water via ion exchange and are subsequently disposed of in a safe way. However, synthetic inorganic cation exchange materials – such as synthetic micas, g-zirconium phosphate, niobate molecular sieves, and titanate – have been found to be far superior to natural materials in terms of selectivity for the removal of radioactive cations from water. Radioactive cations are preferentially exchanged with sodium ions or protons in the synthetic material. More importantly, a structural collapse of the exchange materials occurs after the ion exchange proceeds to a certain extent, thereby forming a stable solid with the radioactive cations being permanently trapped inside. Hence, the immobilized radioactive cations can be disposed safely.

“Based on our earlier work, we have now demonstrated a potentially cost-effective method to remediate cesium and iodine ions from contaminated water by using the unique chemistry of titanate nanotubes and nanofibers to chemisorb these ions,” HuaiYong Zhu, a professor of chemistry at the Queensland University of Technology, tells Nanowerk.

The team, which reported their findings in the September 20, 2011 online edition of Angewandte Chemie International Edition (“Capture of Radioactive Cesium and Iodide Ions from Water by Using Titanate Nanofibers and Nanotubes”), also found that the new sorbents can not only take up these ions but efficiently trap them for safe disposal because of their unique structural and chemical features.

“The sorbents take up cesium ions via an exchange with sodium ions in the nanostructures; the rapid uptake of cesium ions eventually triggers a phase transition of the titanate and traps the cesium ions inside permanently for safe disposal,” explains Zhu. “This is because the fibers and tubes consist of negatively charged thin layers – as thin as two oxygen atoms – and phase transition of the layers with low rigidity can be readily induced.”

In order to capture and immobilize iodine ions from water, the researchers anchored silver oxide nanocrystals on the external surfaces of titanate nanotubes and nanofibers by chemical bonds owing to their crystallographic similarity. These composites can efficiently capture iodine ions forming silver iodine precipitate on the titanates.

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Basics of TiO2 sunscreen

Ultraviolet (UV) Radiation

Ultraviolet radiation can be divided into three regions: UVA (320–400 nm), UVB(290–320 nm) and UVC (200–290 nm). Although UVC is the most damaging UV radiation, it is filtered out by the ozone layer in the stratosphere before reaching the earth’s surface. The radiation in the UVB region (partially filtered by ozone) can penetrate both the stratum corneum and the epidermis of human skin. It has
sufficient energy to cause damage, such as sunburn, to human skin. This is particularly true for fair-skinned individuals. The UVA radiation, which is unfiltered by ozone, has deeper penetration of human skin to the dermis; it, thereby, stimulates the formation of melanin and produces a tan, which acts as the first line of defense for the protection from sunburn. UVA radiation, therefore, is also called the “tanning region.” Although having considerably lower energy than UVB, UVA photons can cause delayed damages to the skin.

The fundamental function of a sunscreen is to serve as a filter that can prevent the penetration of ultraviolet radiation. The substances most commonly used in commercial sunscreen preparations include p-aminobenzoic acids (PABA), cinnamates, oxybenzone, salicylates, and metal oxides, such as TiO2 and ZnO. In addition to their ability to scatter sunlight, inorganic particles, such as TiO2, do
absorb strongly in the UVA and UVB regions.

Rutile and Anatase TiO2

In addition to its amorphous state, two of the most common crystalline forms of TiO2 are rutile and anatase. The two crystalline forms share many similarities, such as their physical appearance, refractive index, density, low toxicity, and high stability in the presence of strong acids and bases. As a physical blocker for sunlight, bot
crystalline forms would serve the purpose well. Their photochemical properties, however, are very different. TiO2 is a semiconductor. When a semiconductor particle absorbs light, it promotes an electron from its valence band to its conduction band, leading to a charge separation. In rutile TiO2, the charge separation is quickly diminished through a charge recombination within the particle and the energy is released as heat. This translates to a low photoactivity and an effective conversion of UV light into heat. As a result, any photoinduced reactions that can pose damage to the skin are avoided. The low photoactivity and its desirable UV absorption spectrum, which cover the entire erythemal curve make rutile TiO2 an ideal choice as UV blocker for sunscreen preparations.

For amorphous and anatase TiO2, the lifetime of the charge separation is much longer than that on a rutile particle. The electron and electron hole, therefore, have greater opportunity to undergo redox reactions on the surface; therefore, anatase has been extensively used for applications involving such electron transfer processes as
photocatalysis for environmental waste treatment and photovoltaic design for solar energy storage. Extensive reviews on these interesting topics are available. Due to their photoactivity, amorphous and anatase TiO2 are not suitable for sunscreen applications.

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