Transient Au–Cl adlayers modulate the surface chemistry of gold nanoparticles during redox reactions

nature chemistry Article https://doi.org/10.1038/s41557-025-01989-4 Transient Au–Cl adlayers modulate the surface chemistry of gold nanoparticles during redox reactions Received: 16 September 2024 Accepted: 3 October 2025 Published online: xx xx xxxx Check for updates Sarah May Sibug-Torres 1, Marika Niihori1, Elle Wyatt 1, Rakesh Arul 1, Nicolas Spiesshofer 1, Tabitha Jones 1, Duncan Graham 2, Bart de Nijs 1, Oren A. Scherman 3, Reshma R. Rao 4, Mary P. Ryan4, Alexander Squires 5, Christopher N. Savory5, David O. Scanlon 5, Abdalghani Daaoub6, Sara Sangtarash6, Hatef Sadeghi 6 & Jeremy J. Baumberg 1 Controlling surface chemistry at the nanoscale is essential for stabilizing structure and tuning function in plasmonic, catalytic and sensing systems, where even trace ligands or ions can reshape surface charge and reactivity. However, probing such dynamic interfaces under operando conditions remains challenging, limiting efforts to engineer nanomaterials with precision. Here, using in situ surface-enhanced Raman spectroscopy, we identify a transient Au–Cl adlayer that forms during electrochemical cycling at gold interfaces. The adlayer exhibits significant charge transfer between gold and chlorine, generating an outward-facing dipole that polarizes neighbouring atoms and modulates the local potential. This dipole stabilizes nanogap interfaces and directs oriented ligand rebinding, enabling reversible reconstruction of subnanometre architectures. It also alters interfacial charge distributions and mediates electron transfer between gold oxidation states, acting as a redox-active intermediate. These findings show how transient surface species shape nanoscale reactivity and stability, offering strategies for designing catalysts, sensors and nanomaterials. Gold nanoparticles (AuNPs) are essential in modern healthcare diagnostics, serving as substrates in sensing platforms such as lateral flow assays, where their nanoscale interfaces enable selective biomolecular recognition and colorimetric detection1,2. Their functionality in these assays depends on local surface chemistry, which dictates ligand attachment, aggregation and overall performance3. This dependence on surface interactions underscores the broader importance of understanding nanoscale interfaces, which govern critical processes such as electron transfer4, molecular adsorption5 and colloidal stability6. Even subtle changes in the interfacial environment can dramatically reshape morphology7–10, alter molecular binding10,11 and reactivity10,12,13, and ultimately determine material functionality. Thus, understanding how ligands and ions modulate nanoscale interfaces is essential for designing better sensors, catalysts and nanomaterials. However, probing nanoscale interfaces under operando conditions remains challenging. Techniques such as X-ray photoelectron spectroscopy (XPS)14,15 and X-ray absorption spectroscopies16,17 provide valuable insights into chemical states and coordination NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, UK. 2Centre for Nanometrology, Department of Pure and Applied Chemistry, Technology and Innovation Centre, University of Strathclyde, Glasgow, UK. 3Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Cambridge, UK. 4Department of Materials, Imperial College London, London, UK. 5School of Chemistry, University of Birmingham, Birmingham, UK. 6Quantum Device Modelling Group, School of Engineering, University of Warwick, Coventry, UK. e-mail: 1 Nature Chemistry Article environments but often lack the resolution needed to resolve transient intermediates at the nanoscale. NMR18–21 and mass spectrometry22 offer molecular-level insights but primarily detect reaction products or bulk-phase intermediates. Surface-enhanced Raman spectroscopy (SERS), on the other hand, offers high sensitivity and molecular specificity by probing molecules within electromagnetic hotspots at nanostructured metal interfaces23. This surface sensitivity makes it well suited for real-time monitoring of interfacial processes, especially when coupled with electrochemical techniques24,25. However, conventional SERS substrates prepared by electrochemical roughening exhibit ill-defined morphologies that suffer from instability and poor reproducibility, making it challenging to systematically probe dynamic interfacial transformations26,27. To overcome these limitations, we have developed a multilayered AuNP aggregate (MLagg) platform stabilized by cucurbit[n]uril (CB[n], n = 5–8) scaffolds, which define sub-1-nm gaps between gold facets28,29. The CB[n] scaffold not only establishes uniform gap spacing, which is key for achieving reproducible strong SERS enhancements30,31, but also provides the structural stability needed for systematic studies of interfacial dynamics32,33. Building on this platform, we introduced an electrochemical regeneration protocol (EC-ReSERS) that oxidatively removes adsorbates and restores the CB[n]-defined nanogap structure in situ, enhancing reproducibility and extending substrate lifetime32. During EC-ReSERS, the oxidative step also oxidizes the gold surface, disrupting the nanogap architecture, while a subsequent reduction step ‘rescaffolds’ the interface by precisely reconstructing the nanogap through CB[n] rebinding. This reversible transformation between an oxidized and reconstructed nanogap makes EC-ReSERS a compelling model system for probing dynamic nanoscale surface chemistry. In exploring the dynamic transformations underlying EC-ReSERS, we now identify a transient Au–Cl adlayer that plays a crucial role in this regeneration process. While chloride ions are often regarded as background electrolytes, our findings demonstrate that even at submillimolar levels, they play a crucial role in stabilizing nanogaps and modulating the electronic structure of the gold surface. This highlights how seemingly innocuous species can profoundly influence nanoscale interfaces, impacting both surface properties and reactivity. Here, we systematically investigate the formation and role of the Au–Cl adlayer in EC-ReSERS and extend these insights to chemically driven regeneration (Ch-ReSERS)28. Understanding these transient phenomena offers broader insights into nanoscale interface engineering, with implications for plasmonic sensing, electrocatalysis and the controlled design of nanomaterials. Results and discussion Electrochemical rescaffolding (EC-ReSERS) Unlike electrochemically roughened electrodes with an ill-defined nanoscale geometry27, ligand-stabilized nanogaps survive multiple oxidation–reduction cycles (ORCs). Here, 0.9 ± 0.05-nm-wide nanogaps are defined30 by aggregating 80-nm-diameter AuNPs (Supplementary Note 1) using rigid scaffolding molecules of CB[n]29. CB[n] binds AuNPs through its n = 5–8 carbonyl groups (Fig. 1a,b). We use 1–2 monolayers of these aggregated AuNPs (MLagg)28 assembled in a s (...truncated)