Abstract
Reproducible and enhanced optical detection of molecules in low concentrations demands simultaneously intense and homogeneous electric fields acting as robust signal amplifiers. To generate such sophisticated optical near-fields, different plasmonic nanostructures were investigated in recent years. These, however, exhibit either high enhancement factor (EF) or spatial homogeneity but not both. Small interparticle gaps or sharp nanostructures show enormous EFs but no near-field homogeneity. Meanwhile, approaches using rounded and separated monomers create uniform near-fields with moderate EFs. Here, guided by numerical simulations, we show how arrays of weakly-coupled Ag nanohelices achieve both homogeneous and strong near-field enhancements, reaching even the limit forreproducible detection of individual molecules. The unique near-field distribution of a single nanohelix consists of broad hot-spots, merging with those from neighbouring nanohelices in specific array configurations and generating a wide and uniform detection zone (“hot-volume”). We experimentally assessed these nanostructures via surface-enhanced Raman spectroscopy, obtaining a corresponding EF of ~107 and a relative standard deviation <10%. These values demonstrate arrays of nanohelices as state-of-the-art substrates for reproducible optical detection as well as compelling nanostructures for related fields such as near-field imaging.
Highlights
The fabrication of metallic nanostructured substrates has considerably evolved with time since the discovery of near-field enhancements at rough silver surfaces[7]
We demonstrate how arrays of weakly-coupled Ag nanohelices have a reproducible, uniform and high near-field enhancement, reaching SERS enhancement factor (EF) values which are sufficient for detection of molecules in low concentrations, even down to the single-molecule level for the here tested SERS-active molecule[6,7]
According to far-field studies, the optical response of a nanohelix is governed by localized surface plasmon resonances (LSPR)[13,14,15,16,17,18,19,20]
Summary
The fabrication of metallic nanostructured substrates has considerably evolved with time since the discovery of near-field enhancements at rough silver surfaces[7]. There are two basic shapes studied to design monomer-based substrates: nanospheres[11,12] and –rods[3,5] Both nanostructures have broad hot-spots, their maximum near-field intensity enhancement is moderate even when placed in an array configuration[3,5,11,12]. We demonstrate how arrays of weakly-coupled Ag nanohelices have a reproducible, uniform and high near-field enhancement, reaching SERS EF values which are sufficient for detection of molecules in low concentrations, even down to the single-molecule level for the here tested SERS-active molecule[6,7] With these near-field qualities, these substrates represent the first monomer-based substrate able to reach the limit of single molecule detection. These values validate these substrates as state-of-the-art substrates for highly enhanced and reproducible SERS detection[4,6,7], as well as functional nanostructures for related fields such as near-field imaging[9]
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