Tracking interfacial single-molecule pH and binding dynamics via vibrational spectroscopy.

Junyang Huang,Marlous Kamp,Jeremy J Baumberg,David-Benjamin Grys,Jack Griffiths,Bart De Nijs,Qianqi Lin

doi:10.1126/sciadv.abg1790

Abstract

Understanding single-molecule chemical dynamics of surface ligands is of critical importance to reveal their individual pathways and, hence, roles in catalysis, which ensemble measurements cannot see. Here, we use a cascaded nano-optics approach that provides sufficient enhancement to enable direct tracking of chemical trajectories of single surface-bound molecules via vibrational spectroscopy. Atomic protrusions are laser-induced within plasmonic nanojunctions to concentrate light to atomic length scales, optically isolating individual molecules. By stabilizing these atomic sites, we unveil single-molecule deprotonation and binding dynamics under ambient conditions. High-speed field-enhanced spectroscopy allows us to monitor chemical switching of a single carboxylic group between three discrete states. Combining this with theoretical calculation identifies reversible proton transfer dynamics (yielding effective single-molecule pH) and switching between molecule-metal coordination states, where the exact chemical pathway depends on the intitial protonation state. These findings open new domains to explore interfacial single-molecule mechanisms and optical manipulation of their reaction pathways.

Highlights

Ligand-nanoparticle surface interactions play key roles in catalysis and nanotechnology, from particle stabilization [1] to functionalization [2] for biosensing [3] and drug delivery applications [4]
Proton activities at the metal interface surface play a central role in ligand tethering [10] as well as catalytic [11,12,13] and electrochemical processes [14], yet the interfacial pH at molecular length scales markedly varies from the bulk environment and is challenging to track [15]
We use long-lived picocavities within a cascaded SERS substrate to enable in situ optical tracking of individual 3–mercaptopropionic acid (MPA) molecules undergoing structural transformations at the surface of a gold nanoparticle (AuNP) under ambient conditions. This reveals distinct three-level chemical oscillations of the carboxylic group. Combining this with theoretical calculations, we identify reversible proton transfer dynamics as well as subsecond switching between single-molecule coordination states, where the chemical pathway observed varies depending on the initial protonation state

Summary

Introduction

Ligand-nanoparticle surface interactions play key roles in catalysis and nanotechnology, from particle stabilization [1] to functionalization [2] for biosensing [3] and drug delivery applications [4]. After AuNP deposition to form either NPoM, NCoM, or SPARK nanocavities, the MPA molecules within the nanogaps can exist in four chemically distinct coordination states: protonated (COOH), deprotonated (COO−), monodentate ( 1-COO−), and bidentate bridge [ 2-( 2-COO−)] coordination to the upper AuNP facet

Results

Conclusion