Conformal cobalt nanoshell deposition on nanoporous gold: A candidate for selective phosphate sensing

Conformal deposition of a Co nanoshell of tailorable thickness (1.5–7.1 nm) on the internal surfaces of nanoporous gold (NPG) was achieved by adapting conventional electroless deposition conditions to circumvent sheath-only deposition on its external surface. Confining electroless deposition onto the ligaments of NPG is challenging as it demands mass transport through the porous network with non-homogeneous deposition likely due to concurrent depletion of the species to be deposited. Spatially-controlled Co electroless deposition into NPG's pores required the sequential addition of dimethylamine borane (DMAB) as reducing agent and an ultra-low concentration of Co ions. DMAB is drawn into the hydrophilic serpentine pores of NPG in a pre-immersion step with temperature elevation prior to addition of Co ions to yield Co@NPG composite. Deposition at planar gold under these conditions was not achieved, emphasising NPG's heightened intrinsic catalytic activity. XPS revealed the presence of cobalt oxide in the nanoshell. Preliminary exploration of oxidized Co@NPG nanocomposite as a potentiometric phosphate sensor revealed a linear dynamic range of 0.10–500 ppm. The facile, exclusively liquid-phase non-conventional electroless deposition approach unveils a new design strategy for a diverse range of core-shell nanoporous catalysts as prospective electrodes in fuel cells, energy conversion devices and sensors.