SERS as a Probe of Surface Chemistry Enabled by Surface-Accessible Plasmonic Nanomaterials

Conspectus When the size of materials is reduced, theirvolume decreases muchfaster than their surface area, which in the most extreme case leadsto 2D nanomaterials which are all surface. Since atomsat the surface have free energies, electronic states, and mobilitywhich are very different from bulk atoms, nanomaterials that havelarge surface-to-volume ratios can display remarkable new propertiescompared to their bulk counterparts. More generally, the surface iswhere nanomaterials interact with their environment, which in turnplaces surface chemistry at the heart of catalysis, nanotechnology,and sensing applications. Understanding and utilizing nanosurfacesare not possible without appropriate spectroscopic and microscopiccharacterization techniques. An emerging technique in this area issurface-enhanced Raman spectroscopy (SERS), which utilizes the interactionbetween plasmonic nanoparticles and light to enhance the Raman signalsof molecules near the nanoparticles' surfaces. SERS has thegreat advantage that it can provide detailed in situ information on surface orientation and binding between moleculesand the nanosurface. A long-standing dilemma that has limited theapplications of SERS in surface chemistry studies is the choice betweensurface-accessibility and plasmonic activity. More specifically, thesynthesis of metal nanomaterials with strong plasmonic and SERS-enhancingproperties typically involves the use of strongly adsorbing modifiermolecules, but these modifiers also passivate the surface of the productmaterial, which prevents the general application of SERS in the analysisof weaker molecule-metal interactions. In this Account,we discuss our efforts in the development of modifier-freesynthetic approaches to synthesize surface-accessible, plasmonic nanomaterialsfor SERS. We start by discussing the definition of modifiersand surface-accessibility, especially in the contextof surface chemistry studies in SERS. As a general rule of thumb,the chemical ligands on surface-accessible nanomaterials should beeasily displaceable by a wide range of target molecules relevant topotential applications. We then introduce modifier-free approachesfor the bottom-up synthesis of colloidal nanoparticles, which arethe basic building blocks for nanotechnology. Following this, we introducemodifier-free interfacial self-assembly approaches developed by ourgroup that allow the creation of multidimensional plasmonic nanoparticlearrays from different types of nanoparticle-building blocks. Thesemultidimensional arrays can be further combined with different typesof functional materials to form surface-accessible multifunctionalhybrid plasmonic materials. Finally, we demonstrate applications forsurface-accessible nanomaterials as plasmonic substrates for SERSstudies of surface chemistry. Importantly, our studies revealed thatthe removal of modifiers led to not only significantly enhanced propertiesbut also the observation of new surface chemistry phenomena that hadbeen previously overlooked or misunderstood in the literature. Realizingthe current limitations of modifier-based approaches provides newperspectives in manipulating molecule-metal interactions innanotechnology and can have significant implications in the designand synthesis of the next generation of nanomaterials.

Automated summary

When the size of materials is reduced, nanomaterials with large surface-to-volume ratios exhibit remarkable new properties compared to their bulk counterparts. Surface chemistry plays a crucial role in catalysis, nanotechnology, and sensing applications. Surface-enhanced Raman spectroscopy (SERS) is an emerging technique that can provide detailed in situ information on surface orientation and molecule-nanosurface binding. However, the choice between surface-accessibility and plasmonic activity has limited the applications of SERS. This article discusses the development of modifier-free synthetic approaches to synthesize surface-accessible, plasmonic nanomaterials for SERS.