Nanowrinkled thin films for nanorod assembly in microfluidics

Abstract

Microfluidics-based nanoparticle sensors provide a promising platform for both the enrichment and detection of rare analytes at the cellular and molecular scale. A precise control of the physical assembly of the nanoparticles in a flow environment is key toward the screening performance, with applications in optical metasurfaces, control of hydrophobicity, and enhanced control over light-matter interactions. In this paper, we describe a methodology for simultaneously guiding assembly of nanorods through the use of tunable nanowrinkled polymer substrates as templates and microfluidics. Microfluidics can offer some benefit to this process through enhanced control over the colloidal particles over the substrate. The tunable nanowrinkled substrates were created using a simple stretch-and-release process and plasma treatment. The formation of the nanowrinkled substrates was analyzed theoretically to reveal the mechanism of using hyperelastic response to generate the surface patterns. The microfabricated templates were analyzed using atomic force microscopy (AFM), showing the tunable alignment wrinkles dimension ranging from 50 to 300 nm in amplitude and 500 nm to 1.5 µm in pitch period. We demonstrate both theoretically and experimentally that microfluidic channels could be used to selectively align particles along their axis and to make spots of nanoparticles in desired geometries within the flow channel. Nanoparticles were assembled onto the polymer substrates using laminar microfluidic flow, enabling near-field interactions of the nanorods with the substrate. This process allows for the simultaneous assembly of the gold nanorod particles into the primary and secondary orders of geometry—aligned gold nanorods and the formation of microscale sensing spots. Alignment results, calculated from scanning electron microscopy (SEM) images, show that 70% of the nanorods are aligned within 1° of rotation from the nanowrinkle axis, with nearly 85% aligned within 5°. The method of surface-wrinkled substrate combined with microfluidic alignment shows great potential for realizing ordered nanosensor arrays with high throughput.