High-resolution particle size and shape analysis of the first Samarium nanoparticles biosynthesized from aqueous solutions via cyanobacteria Anabaena cylindrica

Samarium (Sm) is one of the most sought-after rare earth metals. Price trends and dwindling resources are making recovery increasingly attractive. In this context, the use of cyanobacteria is highly promising. For Sm it was unclear whether Anabaena cylindrica produces particles through metabolically active Sm³⁺ uptake. High-resolution (HR) imaging now clearly demonstrates microbe generated biosynthesis of Sm nano-sized particles (Sm NPs) in vivo. Furthermore, a simple method to determine particle size and shape with high accuracy is presented. Digital image analysis with ImageJ of HR-TEMs is used to characterize Sm NPs revealing a nearly uniform local size distribution. Assuming round particles, the overall average area size is 135.5 nm², resp. 11.9 nm diameter. In HR, where different cell sections of the same cell are averaged, the mean particle is smaller, 76.7 nm² resp. 8.9 nm diameter. The reciprocal aspect ratio is 0.63. The Feret major axis ratio is calculated as shape factor, with 35% of the particles between 1.2 and 1.4. A roundness classification shows that 38% of particles are fairly round and 41% are very round. Consequently, A. cylindrica represents a suitable microorganism for possible Sm recovery and biosynthesis of roundish nano-sized particles.