Recently, Purkayastha and coworkers described a simple, high yield synthesis of unaggregated, CoO NPs using octadecylamine as both a solvent and surfactant coating (Fig. 2).[14] The size distribution for these hydrophobic nanoparticles was reported to be centered around 20 nm, which is ideal for XFM visualization applications. The identity and phase purity of the cobalt oxide-based NPs was verified by PXRD. Both PXRD and TEM data indicate that the NP size distribution is centered around 18–20 nm (electronic material).
The hydrophobic octadecyl amine coating renders these initial nanoparticles completely water insoluble. Two strategies were investigated to make them water soluble: 1) exchanging the amine-anchored coating with a water solubilizing polyethylene (PEG)-based coating having a nitro dopamine (nDOPA) anchoring group, and 2) leaving the amine-anchored coating in place and adding a bipolar copolymer that interdigitates with the octadecyl chains while providing a hydrophilic outer layer to solubilize the particles (Fig. 3).
For the ligand exchange strategy, we chose the nitro dopamine (nDOPA) group to anchor the new PEG-based coating to the cobalt oxide surface due to its reported wide range of pH tolerance and nearly irreversible binding to oxide surfaces.[15, 16] Sonication of the hydrophobic nanoparticles with an excess of nDOPA-PEG1000-OMe (50–90%) and nDOPA-PEG2000-N3 (50 − 10%) followed by extraction into water was successful in creating water soluble nanoparticles but in very low yields (1–5%). The final, water soluble, PEG coated CoO nanoparticles were analyzed by cobalt elemental analysis (ICP-OES/MS), single particle ICPMS (spICPMS), PXRD, and TEM (Fig. 4) (see experimental details in EM to this Letter). Although the synthesis is direct (one step) and leads to very stable (months in aqueous solution) hydrophilic NPs, the very low yields in the ligand exchange step led us to search for higher yielding synthetic strategies.
Schieber and coworkers recently described an interdigitation strategy for producing hydrophilic azide-modified CdSe/ZnS Core–shell quantum dots. [17] This interdigitation strategy involves two steps (Fig. 5). First, the C18NH2-coated NPs are exposed to a commercially available vinyl-maleic anhydride copolymer. The hydrophobic sidechains (phenyl, isobutylene, or octadecyl) of the co-polymer interdigitate or intertwine with the existing octadecyl amine coating developing a double coated nanoparticle. In a second in situ step, the maleic anhydride is ring-opened with an appropriately functionalized short-chain PEG polymer thus creating the desired, outer solubilizing, hydrophilic coating. The final product is extracted into water and purified by filtering through a 100k molecular weight centrifugation filter (Spin-X).
Three commercially available maleic anhydride copolymers were tested as well as several different amine PEG polymers (ethanol amine, H2N-PEG150-OMe, and H2N-PEG550-OMe) to determine the optimum combination that produced the desired water-soluble nanoparticles. No combination of PEG-amine and either the maleic anhydride-co-styrene or -co-isobutylene polymers produced water-soluble NPs. Only the combination of maleic anhydride-co-octadecyl vinyl ether (mal-C18) successfully interdigitated with the octadecylamine NP coating, and ultimately produced water soluble nanoparticles.
After opening of the maleimide ring with ethanol amine and H2N-PEG150-OMe only very low amounts of water-soluble NPs were obtained. However, the longer PEG amine (H2N-PEG550-OMe) succeeded in producing much more concentrated aqueous solutions of nanoparticles. This combination gave a desired water-soluble cobalt oxide nanoparticle product in good yield (50% wt).
TEM images of the water-soluble interdigitated NPs showed that the sizes of the cobalt oxide cores of the NPs had not significantly changed (< d > = 20 nm; Fig. 6). Transmittance IR spectra of aqueous solutions of the NPs with different amounts of the PEG-azide in their coating were obtained. Clear evidence for the presence of the azide group was observed (N3 stretch at ~ 2110–2120 cm− 1) down to 20% loading of the PEG-azide with the bulk of the coating made up of PEG-OMe chains.