An on-demand, light-triggered drug release method, known as vesicular assembly of small plasmonic nanoparticles, or plasmonic vesicle, could be used to treat disease; support the study of the nervous system in real time; provide insight into how the brain works; and provide rapid clearance of small inorganic particles from the body. Wide ranges of optical properties have been reported for plasmonic vesicles, including characteristic absorption peak in the visible or NIR ranges. However, it is unclear how the interaction among a large number of small plasmonic gold nanoparticles contribute to the collective optical property of a plasmonic vesicle. Cross-plane view of near-field electrical enhancement in plasmonic vesicles. Shown are 10-nm gold nanoparticles around 75-nm vesicle cores. Courtesy of Jaona Randrianalisoa, Xiuying Li, Maud Serre and Zhenpeng Qin. To learn more about plasmonic vesicles, including how to best engineer them, researchers performed computational investigations into their collective optical properties. The research team, from the University of Texas at Dallas (UT Dallas) and the University of Reims in France, focused its study on the dynamics of clusters of small gold nanoparticles with liposome cores and the nanoparticles’ potential applications in diagnostic and therapeutic areas. The team used the Stampede and Lonestar supercomputers at the Texas Advanced Computing Center, as well as systems at the ROMEO Computing Center at the University of Reims Champagne-Ardenne and the San Diego Supercomputing Center (through the Extreme Science and Engineering Discovery Environment) to perform large-scale virtual experiments of light-struck vesicles. “A lot of people make nanoparticles and observe them using electron microscopy,” UT Dallas professor Zhenpeng Qin said. “But the computations give us a unique angle to the problem. They provide an improved understanding of the fundamental interactions and insights so we can better design these particles for specific applications.” Using the discrete dipole approximation (DDA) computation method, the team made predictions of the optical absorption features of the gold-coated liposome systems. Using DDA allowed the team to design new complex shapes and structures and to determine quantitatively what their optical absorption features would be. The team simulated different liposome core sizes, different gold nanoparticle coating sizes, a wide range of coating densities, and random vs uniform coating approaches. The coatings included several hundred individual gold particles, behaving collectively. “It is very simple to simulate one particle. You can do it on an ordinary computer, but we’re one of the first to be looking into a complex vesicle,” said professor Jaona Randrianalisoa from the University of Reims. “It is really exciting to observe how aggregates of nanoparticles surrounding the lipid core modify collectively the optical response of the system.” Geometric features of gold-coated liposomes based on random (A-D) and uniform (E-H) arrangements of gold nanoparticles on the core surface. Courtesy of Jaona Randrianalisoa, Xiuying Li, Maud Serre and Zhenpeng Qin. The team identified four characteristic regimes — the isolated nanoparticle regime, Coulomb interaction regime, black gold regime and nanoshell regime. They found that the small plasmonic nanoparticles needed to be very close together, or even overlapping, to give a broadband absorption (i.e., black gold regime) or form a NIR plasmon peak. They further found that smaller gold nanoparticle or larger core size led to higher NIR peak shift and photothermal conversion efficiency. “We’d like to develop particles that interact with light in the near-infrared range — with wavelengths of around 700 to 900 nanometers — so they have a deeper penetration into the tissue,” Qin said. The researchers anticipate that this study will provide design guidelines for nanoengineers and that it could have a significant impact on further development of complex plasmonic nanostructures and vesicles for biomedical applications. The research was published in Advanced Optical Materials (doi: 10.1002/adom.201700403).
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