Particle trapping - Light as puppeteer

Researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) have demonstrated a more robust method for controlling single, micron-sized particles with light. Passing light along optical microfibers or nanofibers to manipulate particles has gained popularity in the past decade and has an array of promising applications in physics and biology. Most research has focused on using this technique with the basic profile of light, known as the fundamental mode. Researchers in the OIST Light-Matter Interactions Unit successfully demonstrated that changing the profile of the light distribution into “higher order modes” provides a stronger optical force that can be used to trap and propel tiny polystyrene beads along a microfiber much faster than if they use the fundamental mode. Their findings were recently published in ("Higher order microfibre modes for dielectric particle trapping and propulsion"). The shape of light in the fundamental mode The shape of light in the fundamental mode. “While it was theoretically proposed that higher order modes would produce stronger forces, this is the first time, to our knowledge, that three-dimensional particle manipulation has been experimentally demonstrated,” said Dr. Viet Giang Truong, a physicist at OIST and paper co-author. Light can take different shapes. Usually, in the fundamental mode, the energy is most intense at the center and gradually fades towards the edge of the beam. Any other light shape is called a higher order mode. For example, the energy pattern can look like a doughnut, with most of the energy contained in a ring, and none in the hole or middle. Scientists can create higher order modes by shining light through crystals. To control particles with optical microfibers submerged in water, scientists guide a laser beam through the fiber. The fiber itself starts with a well-defined diameter at each end, but narrows dramatically in the middle “waist” region. As the light travels through the fiber, it cannot fit inside the extremely thin waist, so it spreads out creating an evanescent field around the fiber. This light field can trap particles close to the fiber surface, allowing scientists to control their position and movement. The light propels the particle in the direction the light is moving. OIST researchers compared how particles react to light in the fundamental mode versus higher order modes, which create larger evanescent fields. They observed that when higher order modes were used, the particles moved up to eight times faster along the microfiber. An increase in speed was expected. Part of the increase is likely due to microfluidic dynamics, explained Aili Maimaiti, lead paper author, and a PhD student from University College Cork, Ireland who is conducting his doctoral research at OIST under the supervision of Prof. Síle Nic Chormaic. As the particle picks up speed, it lifts slightly away from the fiber, reducing drag and allowing it to move even faster. One of the keys to making the experiment successful was redesigning the shape of the glass microfiber so that light does not leak out of it before reaching the waist region where the experiments are conducted. The microfiber starts out at 80 microns, and then tapers down to 2 microns at the waist. Researchers painstakingly create this tapered section by positioning the fiber over a flame and, as the glass heats up, gradually stretching it out until the central region reaches the desired thinness. Maimaiti controlled the shape of the tapered section to ensure that there was very little loss of light through the full length of the fiber. This optimizes the amount of light energy used to control particles at the waist. While the microfiber is very, very thin – about 50 times thinner than a human hair – it is still surprisingly robust for its size because of the characteristics of glass. “This experiment proves the capability of higher order modes in microfibers to trap and propel particles,” Truong said. “The next step is to control multiple particles in three dimensions around the microfiber. We are also keen to demonstrate similar behavior of atoms around the nanofibers.” Optical trapping and manipulation of particles using optical micro or nanofibers has the potential to help deliver drugs to specific locations, such as inside a target cell, measure the interaction forces between cell components, and aid in quantum physics research using cold atoms. Researchers are also interested in using this tool to study the proteins involved in the DNA and RNA transcription and translation processes. In this work, researchers used the microfiber in conjunction with optical tweezers, a tool widely used in research to trap and move individual particles. The higher order mode microfiber helps improve the tweezers by increasing the manner in which particles can be manipulated. In the future, researchers expect the inclusion of the microfiber will also increase the optical tweezers’ sensitivity by communicating more precise information about the trapped particle. “The beauty of ultrathin optical fibers is that they are a very non-invasive tool allowing us to probe many different physical systems while only affecting specific parameters that we choose,” said Prof. Síle Nic Chormaic, who heads the OIST Light-Matter Interactions Unit. “While this work focusses on trapping micron-sized particles using higher order light modes in optical microfibers, we can use similar techniques at the atomic level for creating some of the building blocks in quantum networks.”
read more "Particle trapping - Light as puppeteer"

Nano packages for anti-cancer drug delivery

Cancer stem cells are resistant to chemotherapy and consequently tend to remain in the body even after a course of treatment has finished, where they can often trigger cancer recurrence or metastasis. A new study by researchers from the A*STAR Institute of Bioengineering and Nanotechnology has found that using nanoparticles to deliver an anti-cancer drug that simultaneously kills cancer cells and cancer stem cells significantly reduces the recurrence and metastasis of lung cancer. The drug phenformin is very effective against cancer stem cells. It is related to the popular anti-diabetic drug metformin but is 50 times more potent against cancer cell lines. However, phenformin is too toxic in its free form to be administered to patients at the doses required to kill both normal cancer cells and cancer stem cells. Now, Yi Yan Yang and her colleagues at the Institute of Bioengineering and Nanotechnology in Singapore have found a way to overcome this problem — using self-assembling polymer nanoparticles to deliver the drug ("Phenformin-loaded polymeric micelles for targeting both cancer cells and cancer stem cells in vitro and in vivo"). Phenformin-loaded nanoparticles kill both cancer cells and cancer stem cells Phenformin-loaded nanoparticles kill both cancer cells and cancer stem cells, leading to tumor regression. (Image: A*STAR Institute of Bioengineering and Nanotechnology) In the first study to use polymer nanoparticles to deliver phenformin to target both cancer cells and cancer stem cells, Yang and co-workers found that phenformin-loaded nanoparticles targeted both kinds of cancer cells in a mouse model of human lung cancer. The nanoparticles released the drug in a sustained manner due to their hydrophilic shells, which “prevent enzymatic degradation of the cargo and protein adsorption onto the nanoparticles,” explains Yang. “This also prolongs blood circulation so that the cargo-loaded nanoparticles have enough time to accumulate in tumor tissues.” This delivery method enabled Yang and her team to arrest the growth of cancer and cancer stem cells when the nanoparticles were delivered to implanted human lung cancer in mice. “The results showed that the phenformin-loaded nanoparticles were more effective than free phenformin in inhibiting the growth of both cancer stem cells and normal cancer cells,” Yang says. Moreover, the nanoparticles did not induce the liver toxicity observed in systemically administered phenformin. The method can also be extended to other drugs. The team has used the nanoparticle-based delivery system in a mouse model of human breast cancer to deliver the anti-cancer drug, doxorubicin — another drug that is toxic at certain doses but is capable of killing cancer stem cells. “The combination shrank tumors by more than 40 per cent and was more effective than treatment with either drug alone,” says Yang. The team is now seeking to collaborate with pharmaceutical companies to bring this technology to human clinical trials.
read more "Nano packages for anti-cancer drug delivery"

Batteries made to last

Electrochemical devices are crucial to a green energy revolution in which clean alternatives replace carbon-based fuels. This revolution requires conversion systems that produce hydrogen from water or rechargeable batteries that can store clean energy in cars. Now, Singapore-based researchers have developed improved catalysts as electrodes for efficient and more durable green energy devices ("Dual-Phase Spinel MnCo2O4 and Spinel MnCo2O4/Nanocarbon Hybrids for Electrocatalytic Oxygen Reduction and Evolution"). Electrochemical devices such as batteries use chemical reactions to create and store energy. One of the cleanest reactions is the conversion from water into oxygen and hydrogen. Using energy from the sun, water can be converted into those two elements, which then store this solar energy in gaseous form. Burning hydrogen leads to a chemical explosion that produces water. For technical applications, the conversion from hydrogen and oxygen into water is done in fuel cells, while some rechargeable batteries use chemical reactions based on oxygen to store and release energy. A crucial element for both types of devices is the cathode, which is the electrical contact where these reactions take place. For a well-functioning cathode, the electronic energy levels of the cathode material need to be well matched to the energies required for the oxygen reactions. An ideal material for such reactions is MnCo2O4, a spinel oxide, which has the advantage that its energy states can be fine tuned by adjusting its composition. The research team, which included Zhaolin Liu and colleagues from the A*STAR Institute of Materials Research and Engineering with colleagues from Nanyang Technological University and the National University of Singapore, combined nanometer-sized crystals of this material with sheets of carbon or carbon nanotubes. These composites offer several benefits including low cost and high efficiency. “The cost is estimated to be tens of times cheaper than the platinum/carbon composites used at present,” says Liu. Because platinum is expensive, intensive efforts are being made to find alternative materials for batteries. The researchers fabricated these composites using a scalable chemical synthesis method and studied their performance in oxygen reactions. In these tests, the composites clearly outperformed the platinum-based alternatives. They were more efficient than the platinum-based solutions, with comparable devices in the lab lasting about five times longer, for more than 64 charge-discharge cycles. While these are still research laboratory results, the first results for full battery prototypes are encouraging, comments Liu. “We envisage a 100-watt rechargeable battery stack in one to two years and a 500-watt one in one to three years.”
read more "Batteries made to last"