SERS Reveals Counterion-Controlled Au(I)/Au(III) Speciation in Thiocyanate-Mediated Gold Nanowire Growth

Abstract

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.