Thickness controlled sol-gel silica films for plasmonic bio-sensing devices

Abstract

Plasmonics has recently received considerable interest due to its potentiality in many fields as well as in nanobio-technology applications. In this regard, various strategies are required for modifying the surfaces of plasmonic nanostructures and to control their optical properties in view of interesting application such as bio-sensing, We report a simple method for depositing silica layers of controlled thickness on planar plasmonic structures. Tetraethoxysilane (TEOS) was used as silica precursor. The control of the silica layer thickness was obtained by optimizing the sol-gel method and dip-coating technique, in particular by properly tuning different parameters such as pH, solvent concentration, and withdrawal speed. The resulting films were characterized via atomic force microscopy (AFM), Fourier-transform (FT) spectroscopy, and spectroscopic ellipsometry (SE). Furthermore, by performing the analysis of surface plasmon resonances before and after the coating of the nanostructures, it was observed that the position of the resonance structures could be properly shifted by finely controlling the silica layer thickness. The effect of silica coating was assessed also in view of sensing applications, due to important advantages, such as surface protection of the plasmonic structure

Synergic combination of the sol–gel method with dip coating for plasmonic devices

“…The effects resulting from silica coating were assessed through the direct measurement of the sensitivity of the plasmonic structures to refractive index changes in aqueous solution. In the reflectance spectra, the plasmonic absorption feature is shifted to longer wavelengths due to the modification of the dielectric function surrounding the gold [ 1 – 4 15 – 17 ]. Fig.…”

“…Moreover, the resulting silica sol–gel thin film behaved as a host matrix in which suitable vis or NIR fluorophore dopants could be dispersed. This is useful to couple the plasmonic and the emission signals, thus preventing undesirable metal-induced radiative emission quenching [ 17 ]. Finally, using organically modified alkoxides, the silica platform can be directly functionalized with selected functional groups required for specific detection schemes, all in a single-step process.…”

“…In this sense, many parameters affect the sol–gel mechanism reaction [ 23 – 26 ] and, therefore, the sol–gel process and the deposition technique should be optimized. To the best of our knowledge, few examples related to the use of a sol–gel approach for coating a planar nanostructure have been proposed in the literature [ 15 – 17 27 ]. Yet none of these works presented a comprehensive investigation of the effect of the key processing parameters.…”