Superhydrophobicity of elastocapillary engineered re-entrant CNT topography for self-enrichment assisted femto-molar molecular sensing

Abstract

Superhydrophobicity is a wetting state in which surface strongly repels droplets by reducing the interfacial area. In particular, Cassie’s state on rough structure provides low adhesion owing to the decreased contact line length, which is advantageous for anti-fouling surfaces and self-enrichment assisted sensing . However, once droplets experience wetting transition, its interfacial area is significantly increased and the contact line is pinned to the surface. To prevent such unintended wetting transition to Wenzel state, optimal geometrical configurations of the surface are necessary. Accordingly, a number of strategies to maintain stable Cassie’s state has been reported. For example, re-entrant topography is intensively investigated to have better hydrophobicity and oleophobicity. Also, deep reactive ion etching was introduced to provide high-aspect ratio .In the present work, we fabricated an array of high-aspect ratio re-entrant microstructure, using repetitive sequences of chemical vapor deposition of vertically aligned carbon nanotubes (VACNTs) and elastocapillary engineering. Since polydispersity of CNT mass density results in vertically inhomogeneous densification, we can realize re-entrant topography without using additional post processing. In addition, we utilized our microstructures for plasmon-enhanced Raman scattering sensor device to break Brownian diffusion limit in solution type plasmon-enhanced sensing and unintended capillarity-induced mass flow during evaporation in pinned analyte droplet. This device enables close to a single molecular regime detection in 10-15 M aqueous Rhodamine 6G solution.