Christophe Methivier

Optimized immobilization of gold nanoparticles on planar surfaces through alkyldithiols and their use to build 3D biosensors.

This paper describes a controlled way to immobilize gold nanoparticles on planar gold surfaces and the use of the resulting 3D platform to build up a 3D biosensor. The surface was first functionalized by grafting hexanedithiol, this molecule has 2 thiol end groups, which enables its chemical grafting to planar gold while retaining a free thiol group to attach nanoparticles. This step was optimized by varying experimental parameters such as solvent, temperature and immersion time. The grafting was monitored by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and X-ray photoelectron spectroscopy (XPS). The high resolution XPS sulfur peak made clear the existence of two contributions, S bound to gold and free S, thus led us to determine the optimal conditions to graft hexanedithiol in an extended conformation. 15 nm spherical gold nanoparticles were then immobilized on the resulting surface and their presence was evidenced by surface enhanced Raman spectroscopy (SERS) and atomic force microscopy (AFM). The resulting gold layer was used to build up a 3D biosensor by grafting protein A (PrA), rabbit immunoglobulin (rIgG), and bovine serum albumin (BSA), respectively. Each step was characterized by PM-IRRAS then compared to the results on planar gold surface. Despite the small size of particles and their rather low density on the planar surface, the amount of immobilized proteins, starting from PrA, was almost doubled. The amount of rIgG fixed on the 3D layer was also significantly increased ( approximately 4 times higher than on planar surfaces), however accompanied by a slight decrease of their accessibility, checked by assaying the recognition of a secondary IgG. This work demonstrates the feasibility and interest of building arrays of nanoparticles to immobilize molecular receptors; it also shows that controlling the conditions of elaboration of the biosensor at each step is determining for optimizing the number of molecular receptors.

Surface Segregation of Pd from TiO2‐Supported AuPd Nanoalloys under CO Oxidation Conditions Observed In situ by ETEM and DRIFTS

A TiO2‐supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared by deposition‐precipitation with urea, and its activity in CO oxidation at room temperature was compared to the one of a monometallic Au/TiO2 catalyst. X‐ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggest that Au‐Pd/TiO2 contains bimetallic nanoparticles after reduction under H2 at 500 °C although the presence of monometallic Au particles cannot be totally excluded. The evolution of the AuPd nanoparticles surface composition during exposure to O2 and CO/O2 was studied in situ by environmental high resolution electron microscopy (ETEM) and DRIFTS. Pd segregation at the surface of the bimetallic nanoparticles was evidenced by DRIFTS and directly observed by ETEM under O2 and CO/O2 with the formation of Aucore‐Pdshell structure. The changes in the surface composition of the Au‐Pd nanoparticles under CO/O2 was paralleled with the higher rate of deactivation in the first reaction stages observed for Au‐Pd/TiO2 compared to Au/TiO2, which could be related to the possible replacement of Au in low coordination sites, at the origin of the high activity in CO oxidation, by Pd atoms. These results noticeably underline the modifications induced by the reactant that can undergo a bimetallic AuPd catalyst.