Self-assembled organic and fullerene monolayers characterisation by two-colour SFG spectroscopy: a pathway to meet doubly resonant SFG process

Two-colour sum-frequency generation (two-colour SFG) spectroscopy was used to probe both vibrational and electronic properties of 1-dodecanethiol/Ag(1 1 1), Au(1 1 1), and Pt(1 1 1), of 5-[p-(6-mercaptohexoxy)-phenyl]-10,15,20-triphenylporphin/Pt(1 1 1), and of C60/Ag(1 1 1). The role of the various physical parameters determining the sum-frequency generation (SFG) intensity equation is highlighted. The enhancement of the non-linear second order susceptibility in the aforementioned interfaces is explained in terms of metal interband transition, molecular electronic transition and of electron–phonon coupling, respectively

Synthesis and characterization of aromatic self-assembled monolayers containing methylene and ethyleneglycol entities by means of sum-frequency generation spectroscopy

We use infrared-visible sum-frequency generation (SFG) spectroscopy in order to investigate the adsorption properties on Pt(111) of molecules having CH3–C6H4–(O–CH2–CH2)n –O–(CH2)m –SH as general chemical formula. We synthesized three molecules defined by the values m=5 n =4, m =11 n =4, m=11 n =8 and characterized them by Nuclear Magnetic Resonance spectroscopy. Thanks to spectroscopic measurements, we show that these molecules build self-assembled monolayers on Pt(111). First, the weak SFG signals arising from the ad-layer indicate low order and surface coverage of the substrate by these molecules. Next, the vibrational fingerprints of the aforementioned molecules are determined between 2825 and 3125 cm 1 and the observed SFG spectral features are ascribed on the basis of the analysis of shorter and simpler molecules (1- dodecanethiol, 4-methylbenzenethiol and CH3–C6H4–O–(CH2)11–SH) also adsorbed on Pt(111). The occurrence of methylene vibration modes indicates a significant amount of chain defects whatever the n and m numbers are. Finally, the identification of a particular vibration mode, characteristic of the aromatic ring, enables us to qualitatively discuss the effect of the number of methylene and ethylene glycol entities on its orientation. More precisely, higher these numbers, more tilted (with respect to the substrate normal) the aromatic ring plane is

The interaction between a protein and a ligand molecule studied by linear and non-linear optical spectroscopies

peer reviewedWe use infrared reflection-adsorption (IRRAS) and sum-frequency generation (SFG) spectroscopies to study the interaction between a protein (avidin) and a ligand molecule (biocytin or its thiol derivative) adsorbed on metals (Au and Ag) and insulators (CaF2 in two separate spectral ranges 2800-3500 cm-1 and 1400-1800 cm-1, respectively. No specific interaction is detected by SFG when the measurements are carried out on metals although IRRAS measurements attest the presence of a protein ad-layer. This is explained by the disordered character of the adsorbed film of avidin which is therefore SFG inactive. When the experiments are performed on an insulating substrate, no change of the biocytin/CaF2 SFG spectrum is detected in the low spectral range (1400-1800 cm-1) after immersion in an avidin solution in contrast with what was clearly demonstrated in the 2800-3500 cm-1 spectral range. Finally, we report, for the first time, the observation by SFG of the CH2 scissor vibration mode at 1465 cm-1

Electronic and Molecular Properties of an Adsorbed Protein Monolayer Probed by Two-Color Sum-Frequency Generation Spectroscopy

Two-color sum-frequency generation spectroscopy (2C-SFG) is used to probe the molecular and electronic properties of an adsorbed layer of the green fluorescent protein mutant 2 (GFPmut2) on a platinum (111) substrate. First, the spectroscopic measurements, performed under different polarization combinations, and atomic force microscopy (AFM) show that the GFPmut2 proteins form a fairly ordered monolayer on the platinum surface. Next, the nonlinear spectroscopic data provide evidence of particular coupling phenomena between the GFPmut2 vibrational and electronic properties. This is revealed by the occurrence of two doubly resonant sum-frequency generation processes for molecules having both their Raman and infrared transition moments in a direction perpendicular to the sample plane. Finally, our 2C-SFG analysis reveals two electronic transitions corresponding to the absorption and fluorescence energy levels which are related to two different GFPmut2 conformations: the B (anionic) and I forms, respectively. Their observation and wavelength positions attest the keeping of the GFPmut2 electronic properties upon adsorption on the metallic surface

Comparative study of decyl thiocyanate and decanethiol self-assembled monolayers on gold substrates

In a recent paper Ciszek et al. [J.W. Ciszek, M.P. Stewart, J.M. Tour, J. Am. Chem. Soc. 126 2004) 13172] showed that organic thiocyanates may be an interesting alternative to the use of thiols for thiolate assemblies. We use scanning tunnelling microscopy (STM), infrared reflection absorption and sum-frequency generation spectroscopies (IRRAS and SFG) in order to study the adsorption properties of decyl thiocyanates (DTCN) and compare them to the decanethiol (DT) ones. Firstly, IRRAS measurements show that DTCN molecules form self-assembled monolayers (SAMs) on gold via a thiolate link with the metallic substrate. Secondly, the DTCN SAM on gold is less ordered than the DT one as highlighted by SFG spectroscopy. Indeed, the intensities of the methyl vibration modes vanish while the methylene ones increase when DTCN molecules are adsorbed on the substrate instead of DT. We explain the differences in SAMs quality on the basis of STM measurements which reveal differences in molecular order and packing

Study by sum-frequency generation spectroscopy of a model biosensor : the biocytin-avidin complex

peer reviewedInfrared-visible sum-frequency generation (SFG) spectroscopy is used to study the recognition of a protein (avidin) by a derivated vitamin (biocytin). No interaction is observed when the measurements are performed on metallic substrates in the normal co-propagative configuration. When the biocytin layer is deposited on a CaF2 substrate and the SFG measurements are performed in the total internal reflection configuration, drastic modifications in the C-H and N-H spectral ranges attest the molecular recognition of avidin by biocytin. The specificity of the recognition process is confirmed by replacing avidin by other proteins