Hot-Volumes as Uniform and Reproducible SERS-Detection Enhancers in Weakly-Coupled Metallic Nanohelices

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.

Flexible and Transparent Substrates Based on Gold Nanoparticles and TiO2 for in Situ Bioanalysis by Surface-Enhanced Raman Spectroscopy

“…For the purpose of defining the SERS substrates uniformity, for analytical and bioanalytical applications over a large area the analysis should be extended to greater portions of the substrate, and the analyzed area should be stated unequivocally. Low RSD values show remarkable homogeneity [37] and very low values have been recently reported in literature [38]. However, a standard protocol to evaluate spatial homogeneity of SERS substrates is still missing.…”

Modification of Surface Bond Au Nanospheres by Chemically and Plasmonically Induced Pd Deposition

“…Minimum changes to the particle’s environment following changes in the LSPR peak position [ 1 , 2 , 3 ] are shown through surface-enhanced Raman spectroscopy (SERS). This is ascribed to the enhancement of the electric field around the particle due to the LSPR effect of the metal nanoparticle [ 4 , 5 , 6 ]. Cancer treatment is another field of application for plasmon-active metal nanoparticles based on their capacity to transform irradiated light to heat, enabling photothermal tumor therapy in the presence of oxygen to form reactive oxygen species (ROS) and destroy tumor cells [ 7 , 8 , 9 ].…”

SERS background imaging – A versatile tool towards more reliable SERS analytics

“…Furthermore, the reliability can be impaired by the photoinduced formation of signal intense amorphous carbon 11 often occurring in hot spots [12][13][14] . A strategy towards more reliable and reproducible SERS analytics is therefore the development of gap-free or "hot surfaces" rather than "hot spot" SERS substrates 5,7,15 .…”

“…Spectral acquisition delivers average spectra of the diffraction-limited areas. The uniformity of the signal enhancement on a SERS substrate is most often checked by pointwise [22][23][24] or linewise 25 Raman mapping or by randomly selecting different measurement spots on the substrate 15,26,27 . However, the long spectral acquisition times hinder the imaging of large areas in high resolution in short time, for example to study the distribution of hot spots.…”