Sections (5 μm thick) of nodule or bacteria samples collected onto
coated glass slides were stained with methylene blue solution (1%
methylene blue and 4% sodium metaborate) and imaged with a Zeiss
Axioskop 2 microscope. Thin sections of bacteria or nodule samples
(70 nm thick) were collected onto pre-coated gold grids and counterstained
with 1% aqueous uranyl acetate and lead citrate (Reynolds,
1963). For Electron Energy Loss Spectroscopy (EELS) and elemental
mapping analysis of bacteria exposed to nZnO, thin sections were collected
on un-coated gold gridswithout being stained with either uranyl
acetate or lead citrate. All samples were visualized and imaged on a
Zeiss Libra 120 TEM (Carl Zeiss, Inc., Germany).