Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Abstract

•Surface-enhanced Raman spectroscopy reveals that aromatic hydrocarbons adsorb spontaneously to Au and Ag colloids•Density functional theory reveals that aromatic hydrocarbons co-adsorb alongside capping ligands•The π-metal interaction is prohibited by surface oxidation of the Ag nanosurfaces•Recognizing π-metal interaction on IB metals paves the way for new nanotechnologies The interactions between aryl molecules and metals are ubiquitous in nanotechnology and have direct consequences for the formation and applications of metal nanomaterials. The current consensus is that π-metal interactions with IB metal nanomaterials are negligible in the presence of solvents and capping ligands. As a result, work based on π-metal interactions with IB nanomaterials under ambient conditions has hardly been pursued. Here, by combining surface-enhanced Raman spectroscopy and density functional theory modeling, we show that significant π-metal interactions between aromatic hydrocarbons and Ag/Au nanosurfaces do in fact exist under these conditions. Rationalizing this effect immediately led to the design of plasmonic sensors with greatly improved sensitivity. This work is expected to have a broader significance in areas across chemistry and nanotechnology because it completely alters the way in which interactions between aryl molecules and IB nanomaterials are understood. The interactions between aromatic molecules and metal nanoparticles are ubiquitous in nanotechnology. Currently, π-metal interactions under ambient conditions are well appreciated for VIIIB but not for IB metals. Here, we demonstrate the direct probing of π-metal interactions under ambient conditions with molecular specificity using surface-enhanced Raman spectroscopy (SERS), which reveals that aromatic hydrocarbons adsorb strongly from solution onto the surfaces of Ag and Au nanoparticles, provided there is no surface oxidation. Theoretical modeling shows that this adsorption is driven by dispersive π-metal interactions and that the aromatic molecules co-adsorb alongside surface ligands that are present initially on the nanosurface. Finding direct evidence for this long-neglected interaction has significant implications for various applications. This is demonstrated with SERS sensing of an aromatic drug molecule whose structure would typically suggest it to be weakly adsorbing, and indeed it is not detected on conventional Ag colloids due to surface oxidation, but it is found to adsorb strongly onto Au particles. The interactions between aromatic molecules and metal nanoparticles are ubiquitous in nanotechnology. Currently, π-metal interactions under ambient conditions are well appreciated for VIIIB but not for IB metals. Here, we demonstrate the direct probing of π-metal interactions under ambient conditions with molecular specificity using surface-enhanced Raman spectroscopy (SERS), which reveals that aromatic hydrocarbons adsorb strongly from solution onto the surfaces of Ag and Au nanoparticles, provided there is no surface oxidation. Theoretical modeling shows that this adsorption is driven by dispersive π-metal interactions and that the aromatic molecules co-adsorb alongside surface ligands that are present initially on the nanosurface. Finding direct evidence for this long-neglected interaction has significant implications for various applications. This is demonstrated with SERS sensing of an aromatic drug molecule whose structure would typically suggest it to be weakly adsorbing, and indeed it is not detected on conventional Ag colloids due to surface oxidation, but it is found to adsorb strongly onto Au particles. IntroductionStrong attractive π-metal interaction on VIIIB metals is well recognized and serves as the foundation for important applications in the industrial production of fuels and chemicals with typical examples including hydrocracking and biomass gasification using Pt nanocatalysts.1Ghadami Yazdi M.G. Moud P.H. Marks K. Piskorz W. Öström H. Hansson T. Kotarba A. Engvall K. Göthelid M. Naphthalene on Ni (111): experimental and theoretical insights into adsorption, dehydrogenation, and carbon passivation.J. Phys. Chem. C. 2017; 121: 22199-22207Crossref Scopus (13) Google Scholar,2Zhang F. Zeng M. Yappert R.D. Sun J. Lee Y.-H. LaPointe A.M. Peters B. Abu-Omar M.M. Scott S.L. 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A potential way to address this challenge is to take advantage of the plasmonic properties of IB metal nanomaterials by using surface-enhanced Raman spectroscopy (SERS), which allows molecule-metal interactions to be probed in-situ with ultra-high sensitivity and specificity.16Dick S. Konrad M.P. Lee W.W.Y. McCabe H. McCracken J.N. Rahman T.M.D. Stewart A. Xu Y. Bell S.E.J. Surface-enhanced Raman spectroscopy as a probe of the surface chemistry of nanostructured materials.Adv. Mater. 2016; 28: 5705-5711Crossref PubMed Scopus (36) Google Scholar With SERS, we have recently shown several aromatic molecules, which lack conventional metal-binding functional groups, adsorbing unexpectedly to colloidal Au nanoparticles.17Ye Z. Li C. Xu Y. Bell S.E.J. Exploiting the chemical differences between Ag and Au colloids allows dramatically improved SERS detection of “non-adsorbing” molecules.Analyst. 2019; 144: 448-453Crossref PubMed Google Scholar On the basis of that experimental observation, in this work, we conducted SERS measurements on specifically tailored interfacial plasmonic nanoparticle arrays, which allowed us to monitor the interaction of colloidal Ag and Au nanoparticles with a wide range of hydrocarbons in-situ at room temperature, atmospheric pressure, and in the presence of different solvents, adsorbed surface ligands, and free counterions. This has revealed that contrary to the common belief, aromatic hydrocarbons adsorb readily to colloidal Ag and Au nanoparticles in solution. Density functional theory (DFT) modeling and biphasic SERS showed that the π-metal interaction is dispersive in nature, has significant strength for all the IB metals and aromatic molecules investigated, and that the adsorption strength is positively correlated to the number of aromatic rings present in the molecular structure. The nature of these interactions means that although they are certainly non-negligible, they are highly sensitive to the presence of chemical layers which block access to the nanoparticle surface. For example, we showed that the adsorption was inhibited by a small amount of oxidation which occurred in minutes on freshly prepared Ag surfaces or by the presence of covalently bonded capping layers on Au. 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We have previously demonstrated that metal nanoparticles can be promoted to self-assemble into two-dimensional arrays, which resemble metal liquid-like films (MeLLFs), at water-oil interfaces using sub-micromolar concentrations of non-adsorbing organo-electrolytes, as shown in Figures 1A, S1, and S2.48Xu Y. Konrad M.P. Lee W.W.Y. Ye Z. Bell S.E.J. A method for promoting assembly of metallic and nonmetallic nanoparticles into interfacial monolayer films.Nano Lett. 2016; 16: 5255-5260Crossref PubMed Scopus (68) Google Scholar,49Konrad M.P. Doherty A.P. Bell S.E.J. Stable and uniform SERS signals from self-assembled two-dimensional interfacial arrays of optically coupled Ag nanoparticles.Anal. Chem. 2013; 85: 6783-6789Crossref PubMed Scopus (80) Google Scholar Importantly, this method allows the construction of highly stable and plasmonically active nanoparticle assemblies for SERS analysis without perturbating the surface and local chemical environment of the nanoparticles. Moreover, since the MeLLFs are present at the water-oil interface, this lets us use SERS to monitor even subtle changes to the surface chemistry of plasmonic metal nanoparticles in both water and oil environments (see Figure S2 and “experimental procedures” section for details).Figure 1B, spectrum i shows the SERS signals obtained from Au MeLLFs formed from standard citrate-reduced and stabilized colloidal particles, the signals are dominated by bands at 701 and 1,533 cm−1, which are due to the bulk dichloromethane (DCM) and the citrate derivatives present on the surface of the colloids, respectively. As shown in spectra ii-iii, when naphthalene (NA) was introduced into the Au MeLLF, the SERS spectra clearly showed new vibrational bands, most prominently at 1,378 cm−1, which are characteristic of NA.7Du J. Xu J. Sun Z. Jing C. Au nanoparticles grafted on Fe3O4 as effective SERS substrates for label-free detection of the 16 EPA priority polycyclic aromatic hydrocarbons.Anal. Chim. Acta. 2016; 915: 81-89Crossref PubMed Scopus (44) Google Scholar,17Ye Z. Li C. Xu Y. Bell S.E.J. Exploiting the chemical differences between Ag and Au colloids allows dramatically improved SERS detection of “non-adsorbing” molecules.Analyst. 2019; 144: 448-453Crossref PubMed Google Scholar As shown in spectra iii and iv, the signals of NA remained unchanged even when the bulk MeLLF solution was replaced with ultrapure water and DCM, which showed that the Raman signals observed corresponded only to the SERS signals contributed by NA molecules adsorbed on the plasmonic metal surface and was not from the Raman signals of free NA molecules in solution or the SERS signals of unabsorbed NA molecules sitting physically within the plasmonic hot-spots. The adsorption of NA on Au MeLLFs took place spontaneously and rapidly at room temperature, as shown by the observation that the intensity of the SERS signals of NA was close to its final value even in the first SERS measurement (50 s accumulation time, see Figure S3). The adsorbed NA molecules on the Au nanosurfaces were found to be very stable so that the signals of NA remained unchanged overnight (Figure S4).Currently, it is widely accepted in literature that aromatic hydrocarbons, such as NA, do not adsorb spontaneously to colloidal Ag and Au nanoparticles. Consequently, in applications which require surface adsorption of aromatic hydrocarbons, the surface of Ag or Au nanoparticles are typically functionalized with hydrophobic molecular modifiers, which exhibit favorable intermolecular interactions with the similarly hydrophobic aromatic hydrocarbons.21Carron K. Peitersen L. Lewis M. Octadecylthiol-modified surface-enhanced Raman spectroscopy substrates: a new method for the detection of aromatic compounds.Environ. Sci. Technol. 1992; 26: 1950-1954Crossref Scopus (86) Google Scholar,24Vasilyuk G. Maskevich S. Sveklo I. Zanevsky G. Gachko G. Strekal N. Modified silver films as a new SERS-active substrata.J. Mol. Struct. 1997; 410–411: 223-227Crossref Scopus (11) Google Scholar,28Olson L.G. Uibel R.H. Harris J.M. C18-modified metal-colloid substrates for surface-enhanced Raman detection of trace-level polycyclic aromatic hydrocarbons in aqueous solution.Appl. Spectrosc. 2004; 58: 1394-1400Crossref PubMed Scopus (50) Google Scholar,30Mosier-Boss P.A. Lieberman S.H. Surface-enhanced Raman spectroscopy substrate composed of chemically modified gold colloid particles immobilized on magnetic microparticles.Anal. Chem. 2005; 77: 1031-1037Crossref PubMed Scopus (53) Google Scholar,32Costa J.C.S. Sant’ana A.C. Corio P. Temperini M.L. Chemical analysis of polycyclic aromatic hydrocarbons by surface-enhanced Raman spectroscopy.Talanta. 2006; 70: 1011-1016Crossref PubMed Scopus (73) Google Scholar,50Gu H.-X. Hu K. Li D.-W. Long Y.-T. SERS detection of polycyclic aromatic hydrocarbons using a bare gold nanoparticles coupled film system.Analyst. 2016; 141: 4359-4365Crossref PubMed Google Scholar Therefore, to study whether NA was driven onto the surface of Au MeLLFs by hydrophobic forces, contrast SERS experiments where NA was allowed to adsorb onto the Au nanoparticles from either the aqueous or organic phase of the MeLLF were performed. As shown in Figure 1B spectra iii and v, the SERS intensity of NA was comparable, regardless of the adsorption taking place in water or DCM. This showed that the π-metal interaction was not driven by the environment but was characteristic of the molecule and the metal. This is reinforced by the fact that the NA adsorption was not affected when organic solvents that have very different polarities from DCM, such as hexane (vi) and chloroform (vii), were used as the oil phase.NA is not a special case, for example, benzene and phenanthrene also generated strong SERS signals on Au MeLLFs, as shown in Figures 2A and 2B . The band positions in the SERS signals of both aromatic hydrocarbon molecules showed notable shifts compared with their Raman signals acquired under the same experimental conditions (see “experimental procedures” section for details), which is a direct evidence of the PAHs interacting with the Au nanosurface.51Liu G.-K. Ren B. Wu D.-Y. Duan S. Li J.-F. Yao J.-L. Gu R.-A. Tian Z.-Q. Effect of intrinsic properties of metals on the adsorption behavior of molecules: benzene adsorption on Pt group metals.J. Phys. Chem. B. 2006; 110: 17498-17506Crossref PubMed Scopus (28) Google Scholar In addition to the frequency shifts, clear changes in the relative intensities of the vibrational bands in the SERS and Raman spectra were also observed, particularly for phenanthrene. For example, the out-of-plane vibrations at 422SERS (not observed in Raman), 492SERS (497Raman), 587SERS (not observed in Raman), and 898SERS (896Raman) cm−1 are much more enhanced than the in-plane vibrational bands, for example, those at 823SERS (831Raman) and 1,031SERS (1,039Raman) cm−1.52Cyvin S.J. Neerland G. Brunvoll J. Cyvin B.N. Condensed aromatics. XII. Phenanthrene.Spectrosc. Lett. 1981; 14: 37-45Crossref Scopus (17) Google Scholar The surface-selection rules in SERS dictate that vibrational modes with a large Raman tensor component normal to the enhancing surface, and therefore parallel to the local field polarization, are more enhanced than those with smaller tensor components.53Le Ru E.C. Meyer S.A. Artur C. Etchegoin P.G. Grand J. Lang P. Maurel F. Experimental demonstration of surface selection rules for SERS on flat metallic surfaces.Chem. Commun. 2011; 47: 3903-3905Crossref PubMed Scopus (87) Google Scholar Therefore, the fact that the out-of-plane vibrations of phenanthrene are much more enhanced compared with in-plane vibrations suggests that the PAHs are adsorbed flatly on the Au nanosurface.Figure 2An atomistic picture of π-metal interactions on ligand-protected Au surfacesShow full caption(A and B) Raman (i) and SERS (ii) spectra of benzene and phenanthrene, respectively.(C) Schematic illustration of the adsorption model of naphthalene on a Cl− capped Au (111) surface in an aqueous environment predicted by DFT. Free chemical species including Cl−, Na+, and unadsorbed naphthalene, which are present in the experimental system but not in the calculations, w