Research Sheds Light on Self-Assembly of Nanoparticles for Drug Delivery and Microfluidics

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Self-assembled nanoparticle ribbons bend and twist after prolonged exposure to light. Image: University of Michigan

Although it's well known that matter manipulates and bends light, scientists at the University of Michigan (Ann Arbor) have found that light can actually induce mechanical twisting of nanoparticle ribbons measuring between one and four micrometers long. This discovery could influence development of future microfluidic and drug-delivery technologies.Three-and-a-half years of research culminated in this realization, according to lead investigator Nicholas Kotov, principal investigator and professor in the departments of chemical engineering, bioengineering, and material sciences and engineering. He acknowledges that while it was known that light could impact matter on the molecular scale, the scientific community had not been aware of light's ability to affect these larger, rigid particles.After dispersing cadmium telluride nanoparticles in a water-based solution, the scientists let the mixture stand, exposing it to light for prolonged periods of time. The scientists observed that after 24 hours of light exposure, self-assembly of the nanoparticles into flat ribbons had occurred; the ribbons twisted and bunched after 72 hours of exposure. When light was removed from the experiment, however, long, straight ribbons formed. The researchers deduced that by increasing the repulsion between nanoparticles, light prompts the ribbon to twist and bend."We discovered that if we make flat ribbons in the dark and then illuminate them, we see a gradual twisting--twisting that increases as we shine more light," Kotov summarizes. "This is very unusual in many ways."With this understanding of light's ability to bend nanoparticle ribbons, nanoscale devices that rotate similar to propellors could be possible. In addition to nanoscale devices for use inside the body, this knowledge could enable light-controllable MEMS, and could potentially be employed in lithography processes.