Quantifying the role of redox thermodynamics in the shape-controlled synthesis of gold nanobipyramids

The colloidal synthesis of nanoparticles is governed by both thermodynamic and kinetic factors, yet the role of reduction potential, a key thermodynamic parameter, remains underexplored. In this study, we systematically investigate how the reduction potential of phenol derivatives affects the seed-mediated growth of gold nanobipyramids (Au NBPs). Four structurally similar 4-substituted phenols with varying electron-donating or withdrawing groups (-Cl, -H, -CH₃, -OCH₃) were used as reductants. Their reduction potential was modulated by adjusting the pH of the growth solution, enabling a controlled investigation of redox thermodynamics on product morphology and plasmonic properties. For each reductant, there exists a specific pH window where high-quality Au NBPs are formed, with the longitudinal localized surface plasmon resonance (L-LSPR) peak red-shifting as pH increases. While these effective pH ranges differ across reductants, plotting L-LSPR peak positions against the calculated reduction potentials (via the Nernst equation) collapses all data onto a single trend. This demonstrates that the reaction outcome under the investigated conditions and for the tested reductants is governed by reduction potential, not the identity of the reductant. Further validation using hydroquinone, a structurally distinct reductant with a two-electron oxidation, yields consistent results, underscoring the generality of this relationship. Our findings establish reduction potential as a potential dominant descriptor for guiding the shape-controlled synthesis of Au NBPs, offering a pathway to improve reproducibility and rational design in colloidal nanomaterial synthesis.