The interactions between molecules and noble metal nanosurfaces play a central role in many areas of nanotechnology. The surface chemistry of noble metal surfaces under ideal, clean conditions has been extensively studied; however, clean conditions are seldom met in real-world applications. We developed a sensitive and robust characterization technique for probing the surface chemistry of nanomaterials in the complex environments that are directly relevant to their applications. Surface-enhanced Raman spectroscopy (SERS) can be used to probe the interaction of plasmonic nanoparticles with light to enhance the Raman signals of molecules near the surface of nanoparticles. Here, we explain how to couple SERS with surface-accessible plasmonic-enhancing substrates, which are capped with weakly adsorbing capping ligands such as citrate and chloride ions, to allow molecule–metal interactions to be probed in situ and in real time, thus providing information on the surface orientation and the formation and breaking of chemical bonds. The procedure covers the synthesis and characterization of surface-accessible colloids, the preliminary SERS screening with agglomerated colloids, the synthesis and characterization of interfacial nanoparticle assemblies, termed metal liquid-like films, and the in situ biphasic SERS analysis with metal liquid-like films. The applications of the approach are illustrated using two examples: the probing of π–metal interactions and that of target/ligand–particle interactions on hollow bimetallic nanostars. This protocol, from the initial synthesis of the surface-accessible plasmonic nanoparticles to the final in situ biphasic SERS analysis, requires ~14 h and is ideally suited to users with basic knowledge in performing Raman spectroscopy and wet synthesis of metal nanoparticles.
This protocol covers the synthesis of three types of modifier-free and surface-accessible nanoparticle colloids and of interfacial nanoparticle assemblies (termed metal liquid-like films) as well as their characterization using surface-enhanced Raman spectroscopy.
The plasmonic nanomaterials enable the characterization of molecule–metal interactions in complex environments, avoiding the typical passivation of nanomaterials when using surface-adsorbed molecular modifiers.
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C.L., Z.Y. and Y.X. acknowledge the University Special Research Scholarship (Queen’s University Belfast) for support. S.E.J.B. acknowledges the Engineering and Physical Sciences Research Council (EPSRC) (grant EP/P034063/1) for support. Y.X. acknowledges The Leverhulme Trust Early Career Fellowship (grant ECF2020703) and RSC Researcher Mobility Grant (grant no. RM1602-4142) for support. Y.Z. acknowledges the Chinese Scholarship Council (202008370188) for support.
The authors declare no competing interests.
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Key references using this protocol
Xu, Y. et al. Nano Lett. 16, 5255–5260 (2016): https://doi.org/10.1021/acs.nanolett.6b02418
Li, C. et al. Chem 8, 2514–2528 (2022): https://doi.org/10.1016/j.chempr.2022.06.008
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(a) Plot showing the intensity of the Au-Cl vibration band measured over time. (b) SERS spectra of aggregated CRGC obtained 30 seconds, 150 seconds, 300 seconds, 450 seconds, 600 seconds, 750 seconds and 900 seconds after adding in the aggregating agent.
SERS data obtained from five independent samples of aggregated CRSC.
Extended Data Fig. 3 SEM images of nanoparticle-polymer films formed with incorrect particle concentrations.
(a) A typical CRGC nanoparticle-polymer film formed with low concentrations of particles leading to a loosely packed particle layer. (b) A typical CRGC nanoparticle-polymer film formed with high concentrations of particles leading to the formation of wrinkles in the particle layer. Scale bars, 500 nm.
Extended Data Fig. 4 Optical microscopy image showing a MeLLF sample before and after being damaged by laser irradiation.
(a) A CRSC MeLLF observed with a confocal Raman microscope. (b) The same area of the MeLLF after being damaged with laser irradiation. Scale bars, 100 µm.
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Li, C., Zhang, Y., Ye, Z. et al. Combining surface-accessible Ag and Au colloidal nanomaterials with SERS for in situ analysis of molecule–metal interactions in complex solution environments. Nat Protoc 18, 2717–2744 (2023). https://doi.org/10.1038/s41596-023-00851-6