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A proper metalization of bio-derived dielectric nanostructures allows obtaining very complex three-dimensional platforms for plasmonic applications such as Surface Enhanced Raman Spectroscopy and production of efficient nanoantennas.

In particular we exploited our knowledge of the photonic properties of frustules (silica, micro- and nano-patterned shells of diatom microalgae) in order to obtain efficient SERS substrates whose complexity is hardly reproducible even by the most recent lithographic techniques and whose production on large scale is made possible by the replication process of the algae itself.

Our attention focused on Pseudonitzchia multistriata diatom frustules, characterized by an elongated, bilateral simmetry and whose system of ridges, costae, and pores constitutes what we can call a nanometric supergrating, i.e. the superposition of several periodic nanostructures whose mutual interaction allows a very efficient coupling with external optical radiation.


Morphological characterization of a single valve of Pseduonitzchia multistriata diatom by means of Transmission Electron Microscopy (a) and Atomic Force Microscopy (b).
 

 

Numerical simulations performed on CAD replicas of metallic, three-dimensional nanostructures having the same geometry of P. multistriata frustules allowed to estimate plasmonic resonances in the near IR associated to enhancement factors of the order of 106. Furthermore, frustules have been tested, after metalization by thermal evaporation of gold, as SERS substrates on byphenyl-4-thiole (BPT) monolayers. This allowed the optimization of the gold film thickness, in order to obtain the best value of the enhancement factor (in our case EF= 4.6±0.9 *

106, in good agreement with numerical simulations).

 

The presence, in the frustules, of a lateral, extruded edge whose thickness reaches about 10 nm, allows the analysis of the chemical composition of cellular membranes avoiding steric hindrance. In particular, our bio-derived SERS substrates have been succesfully tested on red-blood and B-leukemia REH cells, for which the retrieval of membrane biochemical composition with no interference from the bulk of the cell is of fundamental importance in the assesment of several pathologies.

 


Optical image and relative SERS intensity map of a BPT monolayer on a metalized diatom valve (left). Raman spectra of BPT onto (red) and outside (black) the metalized valve.

These activities are in collaboration with the Institute of Protein Biochemistry and the Institute of Biosciences and Bioresources, National Research Council.

Contact: Edoardo De Tommasi

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