A light-harvesting nanomaterial inspired by the hierarchically structured biominerals in nature could be used to build artificial light-harvesting systems for bioimaging, photovoltaics, and other applications. Created by researchers at Pacific Northwest National Laboratory (PNNL) and Washington State University, the programmable nanomaterial is made from organic and inorganic components. The hybrid material combines the structural and functional complexity of biominerals with the programmability of a protein-like synthetic molecule (a peptoid). Biominerals are organic-inorganic hybrid materials with high mechanical strength. The researchers synthesized a series of organic-inorganic hybrid peptoids by using polyhedral oligomeric silsesquioxane (POSS) nanoclusters as side chains at various peptoid backbone locations. The hybrid peptoids were used as sequence-defined building blocks to assemble programmable 2D nanocrystals. This hybrid material represents a new class of sequence-defined 2D nanocrystals, the researchers said. POSS-peptoid molecules self-assemble into rhomboid-shaped nanocrystals. Courtesy of Stephanie King, Pacific Northwest National Laboratory. After creating the POSS-peptoid nanocrystals, the researchers programmed the material to provide artificial light-harvesting capabilities efficiently. “We wanted to see if we could program our hybrid nanocrystals to harvest light energy — much like natural plants and photosynthetic bacteria can — while achieving a high robustness and processibility seen in synthetic systems,” PNNL materials scientist Chun-Long Chen said. The scientists programmed the nanocrystals to capture light energy in a way similar to the way it is captured in plant pigments. They added pairs of special donor molecules and structures that could bind acceptor molecules at precisely controlled locations within the nanocrystals. The donor molecules absorbed light at a specific wavelength and transferred the light energy to the acceptor molecules. The acceptor molecules then emitted the light at a different wavelength. The system demonstrated an energy transfer efficiency of over 96%, making it one of the most efficient aqueous light-harvesting systems of its kind, the team said. To show how the system could be used as a biocompatible probe for live cell imaging, the researchers inserted the nanocrystals into live human cells and shined light on the cells. When acceptor molecules were present, the cells emitted light at a wavelength that was different from the wavelength that was shined on them. When acceptor molecules were absent, the scientists observed no change in the wavelength emitted by the cells. Although the researchers have only demonstrated the use of this system for live cell imaging, they believe that the enhanced mechanical properties, stability, and programmability of this 2D hybrid material could make it suitable for a number of applications. Materials scientist Chun-Long Chen finds inspiration for new materials in natural structures. Courtesy of Andrea Starr, Pacific Northwest National Laboratory. “Though this research is still in its early stages, the unique structural features and high energy transfer of POSS-peptoid 2D nanocrystals have the potential to be applied to many different systems, from photovoltaics to photocatalysis,” Chen said. He and his colleagues will continue to explore different avenues for the application of this new material. The work could additionally provide a foundation to overcome the challenges involved in creating hierarchical functional organic-inorganic hybrid materials. In nature, hierarchically structured hybrid materials such as bones and teeth typically exhibit a precise atomic arrangement that enhances their strength and toughness. “As a materials scientist, nature provides me with a lot of inspiration,” Chen said. “Whenever I want to design a molecule to do something specific, such as act as a drug delivery vehicle, I can almost always find a natural example to model my designs after.” The research was published in Science Advances (www.doi.org/10.1126/sciadv.abg1448).
Photosensitizers