Here we demonstrate, for the first time, the fabrication of Au nanostructures on polyaniline (PANI) membrane surfaces for surface enhanced Raman spectroscopy (SERS) applications, through a direct chemical reduction by PANI. Introduction of acids into the HAuCl4 solution leads to homogeneous Au structures on the PANI surfaces, which however only show sub-ppm detection levels toward the target analyte, 4-mercaptobenzoic acid (4-MBA), due to limited surface area and lack of surface roughness. Thorny Au nanostructures can be obtained through controlled reaction conditions and the addition of a capping agent poly (vinyl pyrrolidone) (PVP) in the HAuCl4 solution and the temperature kept at 80 0C in an oven. Those thorny Au nanostructures, with higher surface areas and unique geometric feature, show a SERS detection sensitivity of 10-9 M (sub-ppb level) toward two different analyte molecules, 4-MBA and Rhodamine B, demonstrating their generality for SERS applications. These highly sensitive SERS-active substrates offer novel robust structures for trace detection of chemical and biological analytes.