Life Cycle Assessment of "Green'' Nanoparticle Synthesis Methods

In recent years, "green'' nanomaterial synthesis methods that rely upon natural alternatives to industrial chemicals have been increasingly studied. Although the feasibility of synthesizing nanoparticles (NPs) using phytochemicals, carbohydrates, and other biomolecules is well established, environmental burdens of these synthesis processes have not been critically evaluated from a life cycle perspective. Environmental impacts of nanotechnologies may potentially be reduced by applying green chemistry principles. However, doing so without evaluating the life cycle impacts of the processes may be misleading; merely replacing a conventional chemical with a natural or renewable alternative may not reduce environmental impacts. To explore this issue, we conducted a comparative, screening-level life cycle assessment (LCA) of gold nanoparticle (AuNP) synthesis using three conventional reducing agents and 13 green reducing agents. We found that a substantial portion of the energy footprint of AuNP synthesis is due to the embodied energy in gold. As a result of this embodied energy, even green AuNP synthesis methods have significant environmental impacts that are highly dependent upon reaction times and yields. Our results showed that LCA can elucidate the different environmental impacts of AuNP synthesis processes, help in choosing processes with reduced life cycle impacts, and directing decisions for future research and data collection efforts. We also discuss some challenges in conducting LCAs for nanotechnologies and highlight some major gaps in the green nano-synthesis literature that limit the comparability of reported green synthesis protocols. This research showed that screening-level LCAs can direct nanotechnology research toward more environmentally sustainable paths.