Abstract
Before embarking upon any scientific studies on toxicity of nanomaterials, there is a need to understand the effect of inoculum size on the microbial growth. Therefore, in this study, effect of inoculum size in culture medium was investigated for understanding the resultant toxic effect of ZnO nanoparticles with different surface charges on the bacterial culture of gram negative bacteria Escherichia coli (E. coli). ZnO-nanoparticles were synthesized by two procedures. One was through a non-aqueous solution method with zinc acetate dihydrate (10 M) and LiOH (5 M) that resulted in negatively charged ZnO (NZNO). The other was through a non-aqueous solution method with Zinc acetate dihydrate (100 mM) and Isopropyl Amine resulting in positively charged ZnO (PZNO). Morphological observations revealed particles of less than 30 nm in size with almost uniform shape and size distribution with slight agglomeration. The FTIR studies, confirmed IPA coating on ZnO surface evidenced by mixed vibrational bands. In this study, it was observed that with increase in inoculum size the effect of ZnO particles became more apparent. Growth kinetics data indicated that growth rates are recovered with inoculum size and nanoparticles which play an important role in managing the growth characteristics by counter balancing the generated ROS. It is found that positively charged ZnO favours the growth and shows lesser toxicity than negatively charged ZnO owing to the capping effect of amines resulting in reduced ROS generation. The morphological study using AFM shows that the cells are ruptured with nanoparticle at lower inoculum size and it reduces when the inoculum size increases.