Presently, fluorescence microscopy represents the standard technique for living cells analysis. However, it still presents critical disadvantages such as bleaching or the necessity of labels, which limit the amount of observable features. A promising alternative could be Raman spectroscopy, which, differently from fluorescence, has the potential to give a complete picture of the cell chemical environment without using target labels. The present main limitation of Raman spectroscopy is its extremely low sensitivity that leads to unsuitable detection limits in biology. In order to increase Raman sensitivity, we propose to use three-dimensional (3D) plasmonic nanoantennas directly fabricated on cell culture substrates. It has already been proven that 3D plasmonic nanoantennas present significantly high plasmonic field enhancement1 when coated with silver. Nevertheless, silver is a cytotoxic material; therefore, here we evaluates the performance of gold-silver 3D nanoantennas for SERS analysis of living cells.
Arrays of nanoantennas placed with various arrangements were fabricated on silicon nitride membranes. A silver/gold bilayer (30 + 7 nm) was deposited homogenously on the membrane/nanoantennas system. Neuro 2A and fibroblast 3T3 cell lines were cultured on these substrates to study biocompatibility. Results were observed by electron and confocal microscopy. Both cell lines adhered and proliferated on the membranes.
Raman spectroscopy was performed directly on living cells in their cell media. A near-IR source (785 nm) was used for reducing light absorption and cell photo-damage. Raman spectra acquired at low light intensities on nanoantennas showed distinctive peaks of relevant biomolecules, whereas spectra acquired on the gold surface with the same acquisition parameters did not show any significant peak. These results show that 3D nanoantennas are extremely promising tools for the analysis of living cells.
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Acknowledgment
The research has received funding from the European Research Council under the European Union`s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [616213], CoG: Neuro-Plasmonics.
francesco.deangelis@iit.it