Immunosensing platform based on gallium nanoparticle arrays on silicon substrates

Gallium nanoparticles (GaNPs) of different sizes are deposited on Si(100) substrates by thermal evaporation. Through ellipsometric analysis, it is possible to investigate the plasmonic effects in the GaNPs and exploit them to develop biosensors. The excitation of the resonant modes for certain incidence angles leads to negative values of the imaginary part of the pseudodielectric function (<εi>) obtained in ellipsometry. Furthermore, there is an abrupt sign change when the difference between the phase shifts of p- and s-polarization components reaches 180° at an energy of around 3.15 eV. At that energy, reversal of the polarization handedness (RPH) occurs for an elliptically-polarized input beam. The energy of the RPH condition reduces as the evaporation time increases. The slope of <εi> at the RPH condition is extremely sensitive to changes in the surrounding medium of the NP surface and prompts the use of the GaNP/Si system as sensor platform. Fourier transformed infrared spectroscopy (FTIR) is used before and after functionalization with 3,3'-dithiodipropionic acid di(N-succinimidyl ester) and a glutathione-specific antibody to confirm the chemical modification of the sample surface. The developed immunosensor is exposed to different concentrations of glutathione (GSH) showing a linear relationship between the slope of the pseudodielectric function at the RPH condition and the GSH concentration. The immunosensor shows a limit of detection of 10nM enabling its use for the detection of low GSH levels in different medical conditions.