Enhanced detection of thiophenol adsorbed on gold nanoparticles by SFG and DFG nonlinear optical spectroscopyw Olivier Pluchery,* a Christophe Humbert, b Mehrnoush Valamanesh, a Emmanuelle Lacaze a and Bertrand Busson b Received 2nd February 2009, Accepted 28th May 2009 First published as an Advance Article on the web 30th June 2009 DOI: 10.1039/b902142f Sum frequency generation (SFG) and difference frequency generation (DFG) are applied to study vibrational resonance of the thiophenol molecule adsorbed on two different gold samples. One sample is made of 17 nm gold nanoparticles (AuNPs) fixed on a silicon substrate that has been previously functionalized with a silane monolayer (aminopropyltriethoxysilane, APTES). This sample is fully characterized through visible reflection spectroscopy and AFM. The second sample is a gold monocrystal also covered with thiophenol molecules. From their comparison, an enhancement factor of 21 is deduced for the SFG signal on AuNPs with respect to the Au(111), related to the surface plasmon resonance (SPR). From a combined analysis of the SFG and DFG spectra, we demonstrate that SFG/DFG spectroscopy is able to identify the nature of the substrate where the molecules are adsorbed. This opens new perspectives for this nonlinear spectroscopy by adding to its well-known intrinsic surface specificity, the ability to selectively probe the chemical layer capping the AuNPs. Introduction Gold nanoparticles (AuNPs) are attracting a still-growing interest from the research community because this nano-object combines a series of unique properties with promising applications in various fields: nano-optics, catalysis or biology. AuNPs are characterized by a surface plasmon resonance (SPR) that is being extensively studied and can be used, for example, to track nanoparticles on a surface 1 or use them as biomarkers instead of conventional dyes. 2 Regarding catalysis, AuNPs with a diameter smaller than 5 nm exhibit an unexpected chemical reactivity for the oxidation of carbon monoxide. 3,4 The detailed mechanism of this reaction is not fully understood yet, 5 and a characterization tool able to specifically monitor the chemistry around the gold particles would be most helpful. Regarding biology, successful healing experiments were led in vitro on diseased mouse cells. AuNPs were capped with molecules, and antibodies able to target the diseased cells. By shining a laser tuned at the specific plasmon frequency, nanoparticles were heated up so that the targeted cells were selectively destroyed. 2 This selective thermotherapy offers all the more promising perspectives for the treatment of some types of cancers 6 because AuNPs seem not to present cytotoxicity. 7 In that case as well, the development of biologically-activated AuNPs deeply relies on the ability to monitor the chemistry occurring at the surface of the nanoparticle. To achieve this goal, a characterizing technique with extreme sensitivity and high selectivity is necessary. Sum frequency generation (SFG) can fulfil these requirements. Applying SFG to study nanoparticles has been undertaken by several groups. 8–13 In these works on AuNPs, there is no systematic investigation of the effect of SPR on the SFG signal. The authors were interested in demonstrating other effects. Indeed, Tourillon et al. put in evidence a SFG signal enhancement factor of 100 for thiolated AuNPs in attenuated total reflection with respect to normal reflection. 9 Bordenyuk et al. performed comparison between the SFG signal of commercial pre-capped thiolated AuNPs and AgNPs but had no direct access to intrinsic electronic properties such as interband or intraband contributions due to the use of a femtosecond laser source which produces a beam with photon energy below the interband transistions of gold. 10 Kawai et al. performed comparative SFG measurements of AuNPs films prepared by Langmuir– Blodgett deposition but with respect to the silicon substrate itself, not the Au metal. In our work, we demonstrate how the SFG is enhanced around the AuNPs on the basis of a careful comparison of one given molecule adsorbed either on AuNPs or on bulk gold. An enhancement factor is calculated to be 21 for the thiophenol–AuNP system. Moreover, the combination of sum-frequency generation and difference- frequency generation (DFG) makes possible to selectively obtain the vibrational spectrum of the surroundings of grafted and functionalized AuNPs on silicon. a Institut des NanoSciences de Paris, Universite´ Pierre et Marie Curie, CNRS, 140 rue de Lourmel, 75015 Paris, France. E-mail: olivier.pluchery@insp.jussieu.fr; Fax: +33 1 44 27 46 00; Tel: +33 1 44 27 94 10 b Laboratoire de Chimie Physique, Universite ´ Paris-Sud, CNRS, Ba ˆtiment 201 Porte 2, 91405 Orsay, France w Electronic supplementary information (ESI) available: Nano- particles and sample chemical preparation; other AFM images; details on the fitting model for SFG and DFG accompanied by a short discussion on the degeneracy of the model. See DOI: 10.1039/b902142f This journal is  c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7729–7737 | 7729 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics