Oxidation controlled lift-off of 3D chiral plasmonic Au nano-hooks

Colloidal suspensions of plasmonic nanoparticles (NPs) are a well-established tool for biomedical applications and enhanced spectroscopy because of their strong optical response. The specific response is greatly dependent on the NP shape. The strong optical activity of chiral NPs has created special interest but fabrication of chiral NPs in solution remains challenging. Here, we present an approach whereby three-dimensional (3D) chiral Au nano-hooks, fabricated with the parallel hole-mask colloidal lithography (HMCL) method, can be lifted off from a glass substrate in a controllable manner by using a combined treatment with oxygen plasma oxidation and a reduction step in solution. This method has the advantage of being based on established techniques and not requiring strong acids or complex substrates as in etching based approaches. We furthermore demonstrate the integration of the hook NPs into reversibly cross-linked hydrogels inspired by mussel catechol chemistry but containing an oxidation resistant catechol analogue grafted onto poly(allylamine) crosslinked by coordination of Al³⁺ and how this facilitates the remote analysis of hydrogel microenvironment, e.g. the water content. The suspended particles are promising candidates for optically active surface-enhanced Raman spectroscopy (SERS), asymmetric photo catalysis or aggregation sensing. The integration into hydrogels to produce functional hydrogels holds benefits for applications of metamaterials in optics, sensing or activation in environmental remediation or drug delivery.