Engineered Vertically-Aligned CNT for Plasmon-Enhanced Optical Sensing with Programmable Molecular Delivery

Advanced synthesis and post processing techniques of vertically aligned carbon nanotubes (VACNT) provide effective routes for manufacturing of multi-functional surfaces. Here, we introduce engineered VACNT based composite surface structures which enables programmable and spontaneous delivery of molecules for plasmon-enhanced optical sensing. Plasmon-enhanced optical sensors comprising surface-enhanced infrared absorption spectroscopy (SEIRA), surface plasmon resonance (SPR), and surface-enhanced Raman spectroscopy (SERS) have been attained great success in applications requiring high sensitivity. The sensibility of such sensors strongly depends on the morphology and distribution of metallic nanostructures, which enhances the Raman scattering. In addition, during deposition of a low concentration analyte in realistic fluid, effective delivery of the target molecules into the nanogap is also essential. Accordingly, we realize vertically-aligned CNT (VACNT) based nanocomposite surface structures with three-dimensional SERS-activity for the enhanced Raman detection and self-analyte concentration characteristics yet capable to program its analyte-enrich dynamics for effective molecular delivery. We first fabricate arrays of robust three-dimensional microstructures using VACNT synthesis via thermal chemical vapor deposition (CVD) followed by surface etching oxygen plasma and atomic layer deposition of ZnO. Then, Ag and fluorinated polymer deposition via spin-coating enables the SERS activities and hydrophobicity of microstructure, respectively. In virtue of our strategical deposition enabled by the unique structural characteristics, our surface self-derives the enrichment of the target analyte to be deposited on a small spot area yet in sufficiently thin thickness. As a result, we enable 1nM molecular detection even in a practical fluid which cannot be realized by the pre-reported Cassie deposition technique. In addition, we develop a theoretical model and experimental results to confirm the deposition programmability of the microstructure. Finally, we discuss the effect of surface three-dimensional nanostructures in both Cassie and Wenzel deposition. We observe Wenzel deposition of analyte allows the infiltration into the submicron gap with width of 170 nm yet Cassie deposition experiences contact failure.