Microwires of Au–Ag Nanocages Patterned via Magnetic Nanoadhesives for Investigating Proteins using Surface Enhanced Infrared Absorption Spectroscopy

The controlled assembly of metal nanoparticles into ordered structures interacting with biological molecules is an emerging concept in surface science. Here, bare magnetite nanoparticles (Fe3O4–NPs) were employed as nanoadhesives to capture hollow metallic nanostructures (Au–Ag nanocages) from aqueous suspensions, and these coupled nanostructures were patterned onto various types of substrate via magnetolithography. Microwires of Au–Ag nanocages patterned onto an Au substrate behaved as optical antennas, providing a plasmonic enhancement exploited in surface-enhanced infrared absorption spectroscopy (SEIRAS) to investigate the proteins cytochrome c, bilirubin oxidase, alcohol dehydrogenase, bovine serum albumin, and glucose oxidase. Chemical maps containing more than 4000 spectra, acquired within only 2 min with a focal plane array detector, indicate that proteins were adsorbed along the microwires with their secondary structure preserved according to the spatial distribution of their amide groups. We believe there are significant practical aspects of the methodology proposed here to develop an alternative label-free assay for investigating biological molecules.