Plant-Mediated Green Synthesis of Cerium Oxide Nanoparticles: A Critical Perspective of Some Unclear Issues

Nanotechnology is undoubtedly our century's leading scientific and technological breakthrough since it encounters applications in several fields. In parallel, modern societies are facing drawbacks related to environmental concerns. For this reason, several areas are attempting to make the production process eco-friendly. In this direction, the first work using plant extracts to obtain cerium oxide nanoparticles (CNPs) was published in 2011. After more than 10 years, many advances were achieved regarding this green synthesis (GS) route, using a variety of plant types. However, some critical issues remain unclear or with less experimental evidence, such as the role of plant extracts during the synthesis (reducing, oxidizing, chelating, stabilizers, or capping agents) and difficulties in obtaining CNPs with shapes different from spherical ones. Considering the broad possible applications of these nanoparticles in many fields and the importance of their properties, this review aims to critically discuss some uncovered/unsolved issues related to the role of phytocompounds during the CNPs green synthesis and the shape of the produced nanoparticles. Environmental and scale-up aspects are also briefly discussed. The main limitations found are the lack of agreement about the mechanisms of formation of CNPs by plant-mediated green synthesis and the difficulty in obtaining morphologies like nanocubes, nanotubes, and nanowires. Despite the challenges, the various suggestions and insights presented may help overcome these limitations and expand the potential of CNPs.