A highly controllable method that uses UV light to direct particle motion and assembly within liquids could assist in drug delivery and chemical sensing. Developed by the Sen Lab at Pennsylvania State University, the new technique utilizes freely suspended nanoparticles to pump fluid toward desired point sources. The pumping rates are dependent on particle concentration and light intensity, making the pumping easy to control. “Many applications related to sensors, drug delivery, and nanotechnology require the precise control of the flow of fluids,” said professor Ayusman Sen. “Researchers have developed a number of strategies to do so, including nanomotors and fluid pumps, but prior to this study we did not have an easy way to gather particles at a particular location so that they can perform a useful function and then move them to a new location so they can perform the function again.” A new method uses ultraviolet light and small amounts of gold or titanium dioxide nanoparticles to gather larger particles at the point of light. The method was used to gather polystyrene particles, which form a well-packed structure called a colloid crystal. Courtesy of the Sen Lab, Penn State. To apply the method, the researchers begin by adding a small amount of titanium dioxide (or gold nanoparticles) to a liquid that contains larger particles of interest such as microbeads carrying a payload. When the researchers direct the light at a specific point in the liquid, the metal nanoparticles heat up and the heat is transferred to the fluid. The warm fluid rises at the point of light, just as warm air rises in a chilly room. The cool fluid fills the space that is left when the warm fluid rises. Researcher Benjamin Tansi said that the larger particles collect at the point of UV light, where they form colloidal crystals. “Changing the intensity of the light or the amount of titanium dioxide or gold particles alters how quickly this process occurs,” he said. When the researchers remove the light, the larger particles disperse randomly throughout the liquid. If they move the light to another spot, the larger particles move toward the new point of light, maintaining most of their structure in the move. This dynamic assembly, disassembly, and movement of organized particles could have important implications for sensing and drug delivery, the team believes. Using the new method, the researchers gather the particles of interest into an organized structure at the point of light. When the light is moved to a new location, the particles move toward the new point of light. Courtesy of the Sen Lab, Penn State. The team tested their light-driven pumping method using an inexpensive metal nanoparticle. “This process is most efficient when gold nanoparticles are used, but we wanted to find an alternative that was less expensive and more accessible,” Tansi said. “We were pleased to find that this method also works with titanium dioxide, an inexpensive and harmless nanoparticle used in cosmetics and as a food additive.” The researchers demonstrated the effectiveness of their fluid pump in hexadecane, an organic liquid, in addition to water. “Particles usually don’t assemble very well in salty or nonaqueous environments because everything sticks together, but here we show that particles can assemble using this method in hexadecane, which suggests we may be able to apply this technique in, for example, biological fluids,” Sen said. Researchers at the University of Pittsburgh used mathematical models to describe the dynamics of the system. In addition to describing how particles move in the system, the models confirm that only a minor change in the temperature of the UV light — less than 1 °C — is required to induce the fluid flow. The team is currently testing the limits of its method — for example, whether particles can move uphill toward the light source or the method can be used to sort particles according to size. The method could be used to gather pollutant particles or silica or polymer beads that carry antibodies or drugs at particular locations within a fluid. “Because ultraviolet light and titanium dioxide are so easy to control, we think this method could be harnessed in various technologies in the future,” Tansi said. “For example, a fluid pump that relies on this method could potentially replace the bulky and more expensive traditional pumps that require a power source or that rely on magnetics or mechanical movement to function." The research was published in Angewandte Chemie (https://doi.org/10.1002/anie.201811568). Using the new method, the researchers gather the particles of interest into an organized structure at the point of light (left). When the light is moved to a new location, the particles move toward the new point of light (right). Courtesy of the Sen Lab, Penn State.