Ag, Cu, and Se-doped ultrasmall iron oxide colloidal gels: Revealing potential for photo/electrochemical applications

Synthesis and application of stable nanofluids are of great interest due to their self-assembly ability and collective behaviours that can influence optical, electrical, catalytic, and mechanical responses. We report on the one-step synthesis of the stable colloidal gels designed with a sustainable chemistry approach, using low-toxic metal salts and non-toxic heteropolysaccharide as reactants and water as medium. Monodisperse ultrasmall iron oxide (USIO) (2 nm in size) and dopants (Ag, Cu and Se) constitute the colloids. This study explores the main physical–chemical aspects of these colloids, such as their rheological, optical, magnetic, and general structure, focusing on their potential applications in electrocatalysis and photocatalysis. Rheological measurements indicate non-Newtonian shear-thinning and viscoelastic behaviour, characteristic of weak gel systems. Doping with Ag, Cu, and Se increases the viscosity by an order of magnitude, promoting gelation and structuring processes in the colloids. Moreover, adding the dopants significantly improved plasmon properties and fluorescence intensity. Microstructural investigations revealed a well-ordered microstructure that combines tubular and layered features and reveals multi-level complexity. Among the doped variants, Cu-doped USIO colloid shows the most promising electrocatalytic and photocatalytic performance, indicating that USIO colloidal gels are versatile platforms with potential for multiple areas, including biosensor and solar energy conversion applications.