Vibrational Sum Frequency Generation Spectroscopy of Dodecanethiol on Metal Nanoparticles

Abstract

We report studies of metal nanoparticle surfaces using vibrational sum frequency generation (SFG) spectroscopy. SFG is demonstrated as a sensitive and information-rich probe of nanostructured surfaces. Detection of SFG spectra from a few percent of a monolayer of nanoparticles on a transparent substrate was achieved, corresponding to as few as 10 5 isolated particles within the laser beam spot. A new effect arises when the nanoparticle size approaches the molecular scale: the dependence of the molecular conformation on the geometry of the substrate. Conformation of the dodecane chain of the ligand shows systematic variation with the particle diameter in the 1.8-25 nm range. More gauche defects are observed on smaller particles, judged from the relative intensities of the CH 2 and CH 3 stretch transitions in SFG spectra. Similar behavior observed for both gold and silver particles suggests a nanoscale geometric packing effect, whence more volume is available to the chain on a curved surface than a flat surface. Drying-mediated aggregation of gold nanoparticles results in an enhancement of the SFG signal, enabling vibrational SFG spectra of single submicron size aggregates to be obtained. These are different from spectra of isolated particles, indicating a vibrational mode-selective enhancement mechanism. † Part of the special issue "Kenneth B. Eisenthal Festschrift".

Vibrational Spectroscopic Studies on the Formation Processes and Characteristics of Octadecanethiol Monolayers on the Surfaces of Gold Nanoparticles

“…These results can be explained as that the decreasing the curvature of the AuNP (i.e., larger AuNPs) resulted in decreasing the conical volume available for the alkyl chain on the surface of the AuNP (Fig. 2(d)inset) as mentioned by other reports 25,26 . Accordingly, decreasing the interspace of ODT molecules promoted the SAM formation on the surface of AuNP.…”

“…Accordingly, the results of FT-IR and Raman spectroscopy show that the ratio of the gauche conformation depends on the size of AuNP in formation process of SAM. Further, the defects decreased with increasing the curvature, that is, increasing the conical volume available for the alkyl chain 25,26 . Thus, the increased space in the SAM is favorable to form gauche conformation.…”

“…Although the adsorbed structures of alkanethiols have been reported to be affected by alkyl chain length and particle diameter [23][24][25][26][27][28] , the formation process of the SAMs on the surfaces of AuNPs have not yet been studied systematically.…”

Sum Frequency Generation and Catalytic Reaction Studies of the Removal of Organic Capping Agents from Pt Nanoparticles by UV−Ozone Treatment

“…Additionally, the significant presence of methylene contributions is typical of organic adsorbates on rough surfaces evidencing the disorder of the alkyl chains at the surface. 30 The vibrational spectrum of PVP coated nanoparticles ( fig. 5b) shows the asymmetric CH 2 stretch from the ring (2960 cm -1 ), the symmetric CH 2 stretch from the ring (2900 cm -1 ), and the C-H stretch (2860 cm -1 ), in agreement with previous Raman studies on PVP capped platinum nanoparticles.…”

Optical spectroscopy of functionalized gold nanoparticles assemblies as a function of the surface coverage

“…Therefore, the main advantage of this technique for nanotechnological devices characterization lies in its interface intrinsic molecular sensitivity at the sub-monolayer level and its potential coupling to the SPR resonance as recently demonstrated by considering samples based on solid substrate as platforms for nanosensors, an amplification factor of the molecular SFG signal was put in evidence for functionalized AuNps films with respect to a flat gold reference surface. AuNps were deposited either on silicon in external reflection configuration [4] or on glass substrates in total internal reflection configuration [5] to compare the sensitivity and molecular ordering of functionalized AuNp films as a function of the surface coverage on glass [6], to unravel the orientation of grafted molecules on AuNps and AgNps [7] and to extract and deduce the nature of the vibration modes thanks to the SPR amplification on functionalized AuNp films in the fingerprint spectral range of aromatic molecules with density functional theory (DFT) calculations [8]. In a general way, SFG has also proven its efficiency in biomolecular recognition on glass substrate [9], in DNAbased biosensors on Pt(111) single crystal [10] and in enlightening DNA hybridization on glass [11] and (100)-facetted gold films [12].…”