SERS reveals counterion-controlled Au(I)/Au(III) speciation in thiocyanate-mediated gold nanowire growth

Understanding how coordination chemistry governs nanoparticle growth requires direct identification of the metal-ligand complexes that precede reduction. Here, we show that thiocyanate-mediated gold nanowire formation proceeds through distinct Au-SCN complexes whose oxidation state is determined by the counterion. Using surface-enhanced Raman scattering (SERS) calibrated by density functional theory (B3LYP/def2-TZVPP/D3(BJ)), we resolve the ν(CN) signatures of the active Au-SCN intermediates that persist in the as-synthesized colloids. With the imidazolium thiocyanate ionic liquid (EMISCN), the band at 2121 cm^-1 matches the computed signature of [Au(SCN)₂]^- (Au(I), 2119 cm^-1); under sodium-based conditions (NaSCN), the band at 2128 cm^-1 aligns with [Au(SCN)₄]^- (Au(III), 2131 cm^-1). Despite producing wire-like nanostructures in both cases (mean diameters 40 nm and 14 nm, respectively), these systems follow different oxidation-state pathways. Control experiments show that thiocyanate added after nanoparticle formation gives ν(CN) positions distinct from both synthesis-derived signatures and induces no anisotropic growth, indicating that the structure-directing species forms prior to reduction in solution. These findings establish SERS as a practical probe of reactive speciation and identify the counterion as a controlling parameter for selecting mechanistic pathways in anisotropic nanoparticle synthesis.