Abstract
Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for obtaining structural information of molecules in solution at low concentrations. While commercial SERS substrates are available, high costs prevent their wide-spread use in the medical field. One solution is to prepare requisite noble metal nanostructures exploiting natural nanostructures. As an example of biomimetic approaches, butterfly wing scales with their intricate nanostructures have been found to exhibit exquisite SERS activity when coated with silver. Selecting appropriate scales from particular butterfly species and depositing silver of certain thicknesses leads to significant SERS activity. For morphological observations we used scanning electron microscopes as well as a helium ion microscope, highly suitable for morphological characterization of poorly conducting samples. In this paper, we describe a protocol for carrying out SERS measurements based on butterfly wing scales and demonstrate its LOD with a common Raman reporter, rhodamine 6 G. We also emphasize what special care is necessary in such measurements. We also try to shed light on what makes scales work as SERS substrates by carefully modifying the original nanostructures. Such a study allows us to either use scales directly as a raw material for SERS substrate or provides an insight as to what nanostructures need to be recreated for synthetic SERS substrates.
Highlights
Surface-enhanced Raman spectroscopy (SERS) has been receiving increasing attention in recent years as techniques for fabricating requisite noble metal nanostructures mature and commercial SERS substrates become more readily available [1,2,3]
We found that with rhodamine 6G (R6G) as a model compound, there was a drastic increase in the signal intensity with the deposition thickness up to 20 nm
Individual scales traceable specimen transferred onto a glass slide and silver deposited through a metal mesh. (c) Individual scales traceable to initial to initial locations within the wing
Summary
Surface-enhanced Raman spectroscopy (SERS) has been receiving increasing attention in recent years as techniques for fabricating requisite noble metal nanostructures mature and commercial SERS substrates become more readily available [1,2,3]. The ridge-cross rib structure of Eupleoa mulciber, the striped blue crow, ( referred to as E. mulciber) is relatively large, on the scale of hundreds of nm rather than tens of nm in dimension, not necessarily optimal for hot spot generation when coated with a metal as opposed to decoration with noble metal nanostructures It is, crucial to find out which part of the three-dimensional hierarchical microstructure makes significant contributions to the overall SERS activity. When the area of the opening surrounded by ridges and cross-ribs, often referred to as window, was large as with chestnut scales from S. charonda, the reduction in the signal intensity was relatively small, but with white scales from P. thoas, the sequence of upper layer removal and metal evaporation made a difference. While this requires more care in handling, the wing of even a single species contains a wealth of valuable information and it should be carefully combed for optimal microstructures rather than subjected to random sampling
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