A novel 2-thiazoline-2-thiol functionalized gold (Au-TT) nanosensor was developed for the selective and sensitive detection of toxic heavy metal ions, g(II) and Pb(II) by colorimetry, and for studying its competitive surface reactivity using SERS and XPS. The detection mechanism and urface reactivity is based on competitive binding affinity of thiocarbonyl S or hiazoline ring N/S atom with the metal ions and the nanoparticles (NPs). Due to differences in the binding affinities of the metal ions towards the active binding sites of TT, addition of different metal ions resulted in variation of colour and SERS spectral features. Of the various metal ions studied, viz. Cu (II), Cd(II), Co(II), Zn(II), Ni(II), Hg(II), Pb(II), Ca(II), Fe(II), Mn(II) and Sn(II), only Hg(II) and Pb(II) showed distinctive colorimetric and SERS spectral response that was quantified using XPS. This study, thus, realizes a selective and sensitive visual based nanosensor for Hg(II) and Pb(II) with limit of detection (LOD) of ~ 0.1 ppm. SERS revealed the formation of Hg(TT)₂ and Pb(TT)₂ complexes on the Au NPs surface with the former remaining bonded to the NPs, resulting in enhanced Raman intensity for the 1026^-1cm band, while the latter gets desorbed from the surface leading to reduced SERS intensity.