Colorful solution to a chemical industry bottleneck – KAUST Researchers Develop an “hourglass shape” Graphene-Oxide Membrane to rapidly separate chemical mixtures – Application Pharmaceuticals (other chemical mixtures)


KAUST 2 Color 809

A graphene-oxide membrane design inspired by nature swiftly separates solvent molecules.

The nanoscale water channels that nature has evolved to rapidly shuttle water molecules into and out of cells could inspire new materials to clean up chemical and pharmaceutical production. KAUST researchers have tailored the structure of graphene-oxide layers to mimic the hourglass shape of these biological channels, creating ultrathin membranes to rapidly separate chemical mixtures.

“In making pharmaceuticals and other chemicals, separating mixtures of organic molecules is an essential and tedious task,” says Shaofei Wang, postdoctoral researcher in Suzana Nuñes lab at KAUST. One option to make these chemical separations faster and more efficient is through selectively permeable membranes, which feature tailored nanoscale channels that separate molecules by size.

But these membranes typically suffer from a compromise known as the permeance-rejection tradeoff. This means narrow channels may effectively separate the different-sized molecules, but they also have an unacceptably low flow of solvent through the membrane, and vice versa—they flow fast enough, but perform poorly at separation.

Nuñes, Wang and the team have taken inspiration from nature to overcome this limitation. Aquaporins have an hourglass-shaped channel: wide at each end and narrow at the hydrophobic middle section. This structure combines high solvent permeance with high selectivity. Improving on nature, the team has created channels that widen and narrow in a synthetic membrane.

The membrane is made from flakes of a two-dimensional carbon nanomaterial called graphene oxide. The flakes are combined into sheets several layers thick with graphene oxide. Organic solvent molecules are small enough to pass through the narrow channels between the flakes to cross the membrane, but organic molecules dissolved in the solvent are too large to take the same path. The molecules can therefore be separated from the solvent.

To boost solvent flow without compromising selectivity, the team introduced spacers between the graphene-oxide layers to widen sections of the channel, mimicking the aquaporin structure. The spacers were formed by adding a silicon-based molecule into the channels that, when treated with sodium hydroxide, reacted in situ to form silicon-dioxide nanoparticles. “The hydrophilic nanoparticles locally widen the interlayer channels to enhance the solvent permeance,” Wang explains.

When the team tested the membrane’s performance with solutions of organic dyes, they found that it rejected at least 90 percent of dye molecules above a threshold size of 1.5 nanometers. Incorporating the nanoparticles enhanced solvent permeance 10-fold, without impairing selectivity. The team also found there was enhanced membrane strength and longevity when chemical cross-links formed between the graphene-oxide sheets and the nanoparticles.

“The next step will be to formulate the nanoparticle graphene-oxide material into hollow-fiber membranes suitable for industrial applications,” Nuñes says.

References

Wang, S., Mahalingam, D., Sutisna, B. & Nunes, S.P. 2D-dual-spacing channel membranes for high performance organic solvent nanofiltration. Journal of Materials Chemistry Aadvance online publication, 10 January 2019.| article

 

A*STAR team uses graphene oxide to create a cathode for improved li-ion batteries


A*STAR researchers have found that incorporating organic materials into lithium ion batteries could lower their cost and make them more environmentally friendly. The team has developed an organic-based battery cathode that has significantly improved electrochemical performance compared to previous organic cathode materials. The new material is also robust, remaining stable over thousands of battery charge/discharge cycles.

An electron-deficient, rigid organic molecule called hexaazatrinaphthalene (HATN) was previously investigated as an organic cathode material for lithium ion batteries. However, its promising initial performance declined rapidly during use, because the molecule began to dissolve into the battery’s liquid electrolyte. A new cathode material, in which HATN was combined with graphene oxide in an attempt to prevent the organic material from dissolving, has now been developed by Yugen Zhang and his colleagues from the A*STAR Institute of Bioengineering and Nanotechnology.

“Graphene oxide has excellent electronic conductivity, and surface oxygen functionality that may form hydrogen-bonding interactions with HATN,” Zhang says. He explains that this made graphene oxide a promising candidate for forming a HATN-graphene oxide nanocomposite.

The nanocomposite’s performance reportedly exceeded expectations. The materials combined to form core-shell nanorods in which the HATN was coated with graphene oxide. “Graphene oxide and HATN formed a very nice composite structure, which solved the dissolution issue of HATN in electrolyte and gave the cathode very good cycling stability,” Zhang says. A lithium ion battery using this material as its cathode retained 80% of its capacity after 2000 charge/discharge cycles.

The team saw even better performance when they combined graphene oxide with a HATN derivate called hexaazatrinaphthalene tricarboxylic acid (HATNTA). A battery made from this material retained 86% of its capacity after 2,000 charge/discharge cycles. The improved performance is probably due to the polar carboxylic acid groups on the HATNTA molecule, which attached the molecule even more strongly to the graphene oxide.

The team is continuing to develop new materials to improve the performance of organic cathodes, Zhang says. Aside from investigating alternatives to graphene oxide, the team also is working on HATN-based porous polymers for use as organic cathode materials, which should enhance the flow of ions during battery charge and discharge.

Nanotechnology Sensor for 1 – Step Detection: Combining Biology & Nanoscale Technology into Sensors


BPA Nano Sensors 041315 id39729Detection of very small amounts of a chemical contaminant, virus or bacteria in food systems is an important potential application of nanotechnology. The exciting possibility of combining biology and nanoscale technology into sensors holds the potential of increased sensitivity and therefore a significantly reduced response-time to sense potential problems.

“Graphene oxide has potential applications in a variety of biological fields because of its unique characteristics, In addition, due to large absorption cross-sections and the non-radioactive electronic excitation energy transfer from a fluorophore to GO, GO has been employed to construct fluorescence resonance energy transfer (FRET) biosensors,” Professor Chuanlai Xu from the State Key Lab of Food Science & Technology, and Director, Joint Lab of Biointerface and Biodetection, JiangNan University, tells Nanowerk. Xu and his collaborators have developed a novel, rapid, and sensitive fluorescence sensor to detect BPA. The team reported their findings in ACS Applied Materials & Interfaces (“Building An Aptamer/Graphene Oxide FRET Biosensor for One-Step Detection of Bisphenol A”).

Schematic illustration of the biosensor for BPA based on the target-induced conformational change of the anti-BPA aptamer and the interactions between the FAM-ssDNA probe and graphene oxideSchematic illustration of the biosensor for BPA based on the target-induced conformational change of the anti-BPA aptamer and the interactions between the FAM-ssDNA probe and GO. (Reprinted with permission by American Chemical Society) (click on image to enlarge)

Bisphenol A (BPA) is a chemical produced in large quantities for use primarily in the production of polycarbonate plastics and epoxy resins. Polycarbonate plastics have many applications including use in some food and drink packaging, e.g., water bottles, food packaging materials, impact-resistant safety equipment, and medical devices. Epoxy resins are used as lacquers to coat metal products such as food cans, bottle tops, and water supply pipes. Some research has shown that BPA can seep into food or beverages from containers that are made with BPA. Exposure to BPA is a concern because of possible health effects of BPA on the brain, behavior and prostate gland of fetuses, infants and children. While the actual toxicity of BPA is still debated, the direct measurement of BPA is difficult because of the weak response given by conventional electrochemical sensors, and current optical analysis methods are susceptible to the influence of interfering substances. The novel BPA biosensor developed by the Chinese team now provides a method for the rapid detection and risk assessment of BPA with high sensitivity and selectivity. Aptamers – single-stranded oligonucleotides that can be generated for a target molecule with high affinity – are highly suitable receptors for the selective and high-proficiency detection of a wide range of molecular targets. For instance, researchers have previously shown that aptamer-functionalized graphene can detect mercury in mussels. “Our sensor is based on water-soluble and well-dispersed graphene oxide, which was used as the fluorescence quenching agent, and a specific anti-BPA aptamer labeled by FAM (FAM-ssDNA),” Xu explains. “In the absence of BPA, FAM-ssDNA can be adsorbed onto the GO surface, leading to FRET between GO and FAM-ssDNA. Subsequently, the fluorescence can be quenched quickly. Conversely, BPA can interact with FAM-ssDNA and switch its conformation to prevent the adsorption of GO, resulting in fluorescence recovery in the sensing system.” Under different concentrations of BPA, based on the target-induced conformational change of anti-BPA aptamer and the interactions between the fluorescently modified anti-BPA aptamer (FAM-ssDNA) and GO, the team’s experimental results show that the intensity of the fluorescence signal was changed. They say that these results are comparable to traditional ELISA as well as other instrument-based methods, suggesting that this novel sensor might find applications in food safety testing and the monitoring of industrial production processes. The researchers suggest that their GO-based assay offers several advantages:

  • Fluorescent sensors tend to have higher sensitivity compared to most of the colorimetric sensors;
  • The relationship between GO and FAM-ssDNA provide theoretical support for the experiment;
  • GO can be easily chemically synthesized with large quantities. Besides, the method avoids the dual label of ssDNA with fluorophore and quencher units, which significantly lowers the detection cost.

By Michael Berger