Gold nanostars on porous silicon for sensing picric acid, malachite green using SERS

In this work, we report on the trace detection of an explosive molecule, picric acid (PA), and a dye molecule, malachite green (MG), using surface enhanced Raman scattering (SERS) technique. We have synthesized porous Silicon (PSi) by a simple electrochemical etching method and anisotropic gold nanostars (AuNSs) using chemical reduction of the gold salt (HAuCl4). Rough PSi acts as a suitable platform for generating SERS hotspots upon the addition of these AuNSs. The average particle size was found to be <50 nm with a strong absorption peak in the near infra-red (NIR) spectral region. PSi substrates along with AuNSs on its surface are used to explore their detection performance for PA and MG at different concentrations. Furthermore, we have compared the Raman signal intensities of Malachite Green (MG) on Si, PSi with and without Au NSs. Without Au NSs, bare PSi was found to exhibit a low Raman signal as compared to bare Si due to its hindering effect of an analyte molecule in the pore structures. However, this signal is enhanced by employing AuNSs onto the roughened porous surface. A portable Raman spectrometer (BWTEK) was used for all the SERS measurements with an excitation wavelength of 785 nm. We have achieved detection of MG at nanomolar (10-9 M) and PA at micromolar (10-6 M) concentrations using these hybrid SERS substrates. The enhancement factor was estimated to be in the range of 104-105. We believe that the optimization of porosity in PSi and sizes of AuNSs will improve the limit of detection further.