Iron Oxide Nanomaterials at Interfaces for Sustainable Environmental Applications

Surface active iron oxide nanoparticles (NPs) belong to a novel class of nanomaterials with an inherent ability to adsorb at interfaces and perform diverse applications. Over the last several years, bulk soluble iron oxide NPs have emerged as one of the most prominent materials for environmental and biological applications. Bulk solubility unintentionally contributes toward the toxicity of nanomaterials with largely unknown consequences. Surface active NPs provide a viable solution and limit the toxicity by restricting their action to the interface. That enhances their applicability in the extraction processes across the immiscible interfaces frequently used in water purification as well as in biological systems. This Account summarizes the characteristic features of these applications elegantly accomplished by the surface active iron oxide NPs without even being incorporated in the aqueous bulk.

Surface activity of iron oxide NPs is achieved through hydrothermal synthesis by carefully selecting ionic Gemini surfactants that meticulously control crystal growth as well as provide colloidal stabilization. Both headgroup polarity and hydrophobicity of Gemini surfactants adsorbed on the surface of magnetic NPs are instrumental in generating precise surface activity, which mainly depends on the appropriate hydrophilic–lipophilic balance (HLB). Such a protocol produces highly surface active small crystalline iron oxide NPs of ∼10 nm functionalized with Gemini surfactants that only adsorb at immiscible interface and do not incorporate in bulk.

Surface active iron oxide NPs efficiently extract Au and Ag NPs as model nanometallic pollutants from aqueous bulk, which are otherwise difficult to extract by conventional filtration techniques. Extraction can be accomplished through specific and host–guest interactions operating between functionalized surface active magnetic NPs and nanometallic pollutants. Gemini surfactant functionalized magnetic NPs act as excellent vehicles for the extraction of protein fractions from aqueous bulk. Amphiphilicity of such NPs very well differentiates between the extraction efficiency of predominantly hydrophobic and hydrophilic protein fractions. Remarkably, magnetic NPs are also fully capable of extracting blood cells without inducing hemolytic anemia when functionalized with cyclodextrins (CD), which encapsulate sugar moieties of membrane proteins or the lipid bilayer of the cell membrane.

Extraction can be quantitatively monitored with simple techniques such as UV–visible and dynamic light scattering in real time. Highly sophisticated imaging and spectroscopic studies elucidate the mechanistic steps traced by the surface functionalities of both magnetic NPs and extracted species. Surface activity of magnetic NPs also makes their separation and quantification much easier under the effect of an external magnetic field for their reusability as sustainable nanomaterials. Separation of nanometallic pollutants and protein fractions from magnetic NPs can be achieved through vortex dispersion or pH variation, while ferromagnetism facilitates the rejuvenation of purified magnetic NPs to achieve sustainability. Thus, a novel class of surface active iron oxide NPs possesses enormous potential to explore their versatile and diverse interfacial chemistry that spans environmental to biological applications.