Highly Uniform SERS Substrates Based on Ligand Concentration‐Gradient Modulated Gold Nanobipyramid Self‐Assembled Monolayers

ABSTRACT

Controlling the tip arrangement and interparticle gaps in gold nanobipyramids (AuNBPs) self‐assembled monolayers remains a critical challenge for achieving uniform surface‐enhanced Raman scattering (SERS) hotspots. Herein, we demonstrate a facile liquid–gas interface self‐assembly strategy modulated by a free ligand concentration gradient to precisely engineer tip‐overlapping structures. By tuning the concentration of free thiol‐terminated polystyrene (PS‐SH) ligands (0–1.2 mg/mL), we achieve a continuous evolution of AuNBPs tip arrangements—from separated gaps (∼6 nm) to tip alignment, and further to tunable tip‐overlapping configurations (overlap length up to 18 nm). Combined experimental and finite‐difference time‐domain (FDTD) simulations reveal that the tip‐overlapping structure formed at 1.2 mg/mL induces strong electromagnetic coupling, generating dense and uniform hotspots. Consequently, the optimized substrate exhibits exceptional SERS performance for crystal violet detection, with a wide linear range (10 ⁻³ –10 ⁻⁷  M, R ² = 0.968), and outstanding signal uniformity (RSD = 3.78%). Conversely, excess ligands (>1.2 mg/mL) trigger entropy‐driven three‐dimensional disorder, degrading performance. This work provides a practical strategy for designing sensitive and reproducible SERS substrates through nanoscale structural engineering, while also highlighting the importance of balancing hotspot optimization and ligand‐mediated molecular accessibility.