# Biocompatible Cobalt Oxide Nanoparticles for X-ray Fluorescence Microscopy

## TL;DR

Researchers developed biocompatible cobalt oxide nanoparticles that can be visualized using X-ray fluorescence microscopy and are suitable for biological applications.

## Contribution

A novel method for synthesizing water-soluble, non-agglomerated cobalt oxide nanoparticles with stealth coatings for biocompatibility.

## Key findings

- Uniform 20 nm cobalt oxide nanoparticles were synthesized with a CoO core suitable for XFM visualization.
- A maleic anhydride-vinyl copolymer strategy yielded water-soluble NPs in good yield.
- Characterization data confirmed the nanoparticles' compatibility with bioorthogonal reactions for targeting.

## Abstract

The synthesis of water-soluble nanoparticles is a well-developed field for ferrite-based nanoparticles with the majority consisting of iron oxide or mixed metal iron oxide nanoparticles. However, the synthesis of non-agglomerated non-ferrite metal/metal oxide NPs is not as well established. The synthesis and characterization of uniform 20 nm, biologically compatible cobalt oxide (CoO) nanoparticles (NPs) is described. These nanoparticles have two principle components: 1) a CoO core of suitable size to contain enough cobalt atoms to be visualized by X-ray fluorescence microscopy (XFM) and 2) a robust coating that inhibits NP aggregation as well as renders them water-soluble and biocompatible (i.e. stealth coatings). Stable cobalt oxide NPs are obtained with octadecyl amine coatings as reported by Bhattacharjee. Two strategies for solubilizing these NPs in water were investigated with varying degrees of success. Exchanging the octadecyl amine coating for a nitrodopamine anchored PEG coating yielded the desired water-soluble NPs but in very low yield. Alternately, leaving the octadecyl amine coating on the NP and interdigitating this with a maleic anhydride-vinyl copolymer with different hydrophobic sidechains followed by opening the maleic anhydride ring with amine substituted PEG polymers (the water solubilizing component), yielded the desired water soluble NPS were obtained in good yield. Characterization data for the nanoparticles and the components of the coatings required for bioorthogonal reactions to ligate them with biotargeting agents are also described.