Claude Nogues

Interaction of Self-Assembled Monolayers of DNA with Electrons: HREELS and XPS Studies

We present results from high-resolution electron energy loss spectroscopy (HREELS) and XPS studies of self-assembled monolayers of DNA. The monolayers are well-organized and display sharp vibrational peaks in the HREEL spectra. The electrons interact mainly with the backbone of the DNA. The XPS results indicate that, in most of the samples studied, the phosphates on the DNA are not charged.

Electrical characterization of self-assembled single- and double-stranded DNA monolayers using conductive AFM

We recently reported electrical transport measurements through double-stranded (ds)DNA molecules that are embedded in a self-assembled monolayer of single-stranded (ss)DNA and attached to a metal substrate and to a gold nanoparticle (GNP) on opposite ends. The measured current flowing through the dsDNA amounts to 220 nA at 2 V. In the present report we compare electrical transport through an ssDNA monolayer and dsDNA monolayers with and without upper thiol end-groups. The measurements are done with a conductive atomic force microscope (AFM) using various techniques. We find that the ssDNA monolayer is unable to transport current. The dsDNA monolayer without thiols in the upper end can transport low current on rare occasions and the dsDNA monolayer with thiols on both ends can transport significant current but with a much lower reliability and reproducibility than the GNP-connected dsDNA. These results reconfirm the ability of dsDNA to transport electrical current under the appropriate conditions, demonstrate the efficiency of an ssDNA monolayer as an insulating layer, and emphasize the crucial role of an efficient charge injection through covalent bonding for electrical transport in single dsDNA molecules.

Electrical properties of short DNA oligomers characterized by conducting atomic force microscopy

Complementary, single-strands of DNA (ssDNA), one bound to a gold electrode and the other to a gold nanoparticle were hybridized on the surface to form a self-assembled, dsDNA bridge between the two gold contacts. The adsorption of a ssDNA monolayer at each gold interface eliminates non-specific interactions of the dsDNA with the surface, allowing bridge formation only upon hybridization. The technique used, in addition to providing a good electrical contact, offers topographical contrast between the gold nanoparticles and the non-hybridized surface and enables accurate location of the bridge for the electrical measurements. Reproducible AFM conductivity measurements have been performed and significant qualitative differences were detected between conductivity in single- and double-strand DNA. The ssDNA was found to be insulating over a 4 eV range between ±2 V under the studied conditions, while the dsDNA, bound to the gold nanoparticle, behaves like a wide band gap semiconductor and passes significant current outside of a 3 eV gap.

Vertically Coupled Polymer Microracetrack Resonators for Label-Free Biochemical Sensors

We report on the efficient design and fabrication of polymeric microracetrack optical resonators for label-free biosensing purposes. Vertically-coupled microresonators immersed in deionised water display high Q-factors (>;35000)and finesses up to 25. A surface sensing experiment performed with these microresonators using 5-TAMRA cadaverine as a test molecule demonstrated both the high sensitivity and low detection limit of our device.

SAMs functionalised by α-quaterthiophene as model for polythiophene grafting

Abstract A model of polythiophene grafting on a SAM functionalised by α-quaterthiophene is presented. The elaboration and characterisation of the dodecanethiol monolayer terminated by α-quaterthiophene are reported. Blocking level of substrates, structure of the alkyl chains and organisation of the quaterthiophene units as well as their chemical behaviour are analysed. HREELS spectra showed that the extreme surface of the monolayers is entirely covered by quaterthiophene units after typical adsorption times of 12 h. PM-IRRAS spectra allow to follow the orientation of molecules within the monolayer versus adsorption time. It is shown that the long axis of quaterthienyl groups stand slightly tilted with respect to the perpendicular. In addition, interfacial reactions of the SAM layer with a model molecule, the end-capped terthiophene, were electrochemically induced via the formation of radical cations. The number of grafted molecules that depends on the 4T film structure is discussed in regard to the 4T moiety reactivity.

Efficient Antifouling Surface for Quantitative Surface Plasmon Resonance Based Biosensor Analysis

Non-specific binding to biosensor surfaces is a major obstacle to quantitative analysis of selective retention of analytes at immobilized target molecules. Although a range of chemical antifouling monolayers has been developed to address this problem, many macromolecular interactions still remain refractive to analysis due to the prevalent high degree of non-specific binding. In this manuscript we explore the dynamic process of the formation of self-assembled monolayers and optimize physical and chemical properties thus reducing considerably non-specific binding while maintaining the integrity of the immobilized biomolecules. As a result, analysis of specific binding of analytes to immobilized target molecules is significantly facilitated.

Two-Dimensional Crystal Structure of a Quaterthiophene−Alkanethiol Self-Assembled Monolayer on Gold

We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by alpha-quaterthiophene on gold (alpha-4TC 12H 24SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The alpha-4T moieties arrange in the upright position on the surface as the adsorption time increases, while the alkyl chain organization remains poor. Here we quantify the orientation of the self-assembled monolayer and, more significantly, reveal through surface X-ray diffraction that after a long incubation period (12 h) the alpha-4T on the gold surface adopts a 2D crystal structure.

Nanoengineering the second order susceptibility in semiconductor quantum dot heterostructures

We study second-harmonic generation from single CdTe/CdS core/shell rod-on-dot nanocrystals with different geometrical parameters, which allow to fine tune the nonlinear properties of the nanostructure. These hybrid semiconductor-semiconductor nanoparticles exhibit extremely strong and stable second-harmonic emission, although the size of CdTe core is still within the strong quantum confinement regime. The orientation sensitive polarization response is analyzed by means of a pointwise additive model of the third-order tensors associated to the nanoparticle components. These findings prove that engineering of semiconducting complex heterostructures at the single nanoparticle scale can lead to extremely bright nanometric nonlinear light sources.

β-functionalized oligothiophenes for molecular self-assembly

Oligothiophenes carrying an alkylthiol group on one of the internal β-positions have been synthesized and adsorbed onto platinum and gold. The adsorbed assemblies are not very well organized, probably due to the geometry of the single molecules. The thiophene-based layers exhibit electrochemical activity in non aqueous electrolytes. The monomer units within the layers can be polymerized by a single anodic sweep. The obtained polymers show a high electrochemical and thermal stability. In some cases the polymer shows an increase of the structural order compared to the monomer layers.

Shape and size separation of gold nanoparticles using glucose gradient density

We synthesized a mixture composed of gold nanoparticles of various shapes using the wet chemistry method. The final solution contained long nanorods, balls, disks and different spherical nanoparticles. To separate particles of individual shapes from the reaction mixture, the solution was centrifuged in a glucose density gradient. A distribution of nanoparticles based on their diameters was observed and each section was collected independently and each type of nanoobjects was characterised separately. Finally, the difference in nanoparticle shapes depending on the presence of Ag+ ions in the growth solution is reported and its influence on the separation is discussed.