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Gold nanoparticles for catalytic and potential biological applications.

Zhang, Jianming (2013). Gold nanoparticles for catalytic and potential biological applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 178 p.

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Gold nanoparticles (AuNPs) have attracted much attention in many applications due to their unique physical and chemical properties, which the bulk material does not possess. Generally, two main approaches can be used for synthesis of AuNPs, namely chemical and physical methods. The chemical reduction method is the most widely used synthesis technique. It involves using various chemical precursors, such as Au salt and a reducing agent. Au ions are reduced to Au atoms followed by the growth of NPs. The laser ablation approach is one of the most used physical synthesis methods, such as pulsed laser ablation in liquid (PLAL), where NPs are synthesized via laser ablation of a solid target placed in a liquid medium without addition of chemicals. AuNPs based nanomaterials used as catalyst have shown highly active catalytic properties in many reactions, such as selective hydrogenation of organic molecules, carbon monoxide (CO) oxidation, and the water-gas shift reaction. The catalytic activity of AuNPs is strongly dependent on their surface chemistry. Various Au-based nanostructures have been successfully synthesized by chemical methods, however, the surface features of AuNPs prepared by these chemical approaches are not optimal for catalysis, due to the existence of surface stabilizing molecules or tightly adsorbed reaction residues, which exerts a “barrier” effect to catalysis or poisons the catalyst. Therefore, AuNPs with relatively “bare and clean” surfaces are highly desired for catalysis. The unique relatively “bare and clean” surface of AuNPs prepared by PLAL makes them a good candidate for catalysis. However, this potential of applications of PLAL-AuNPs in the field of catalysis is still not explored. It is therefore of great interest to investigate the catalytic properties of PLAL-AuNPs and the influence of their surface chemistry on catalysis. AuNPs have also been widely used in the field of biological research. In these applications, small and toxic-chemical-free colloidal AuNPs are highly desired. Nonetheless, the colloidal stability and molecule adsorption ability of the AuNPs are two primary properties for their biological application development, which highly depend on the surface chemistry of AuNPs. Unfortunately, the chemical synthesis of very small AuNPs (less than 10 nm) involves the use of toxic chemicals which renders them unsuitable for biological applications. Another laser technique ─ laser irradiation, which combines chemical reduction and laser methods, is suitable for synthesis of “green” Au colloids with very small sizes. Interestingly, laser irradiation introduces novel surface chemistry to chemically synthesized AuNPs, such as surface oxidation. The effect of the surface chemistry variation introduced by laser treatment on NP colloidal stability and amine molecule- AuNP interaction has not been investigated so far. The work performed in this thesis was therefore driven by two main objectives: 1) PLAL and chemical synthesis of Au and PtAu NPs and characterization of their catalytic properties and 2) investigation of the surface modification of chemically synthesized Au NPs by laser irradiation and its influence on Au colloidal stability and amine-AuNP interaction. The results obtained in this work are summarized in two sections as follows. In the first section, the synthesis and catalytic property of AuNPs prepared using the PLAL technique are investigated. To do so, I prepared a novel nanostructured catalyst, composed of small (~5 nm in diameter) and uniform AuNPs and ceria nanotubes (CeO2 NTs). AuNPs with relatively “bare and clean” surfaces fabricated by PLAL on a bulk Au target in pure water are efficiently assembled onto the surfaces of CeO2 NTs without performing any surface functionalization of either component to promote their coupling, thanks to the presence of the –OH groups (that do not block catalysis) on the surfaces of PLAL-AuNPs. The model reaction of reducing 4-nitrophenol to 4- aminophenol catalyzed by our PLAL-AuNP/CeO2-NT catalyst exhibits a remarkably higher reaction rate than the reaction catalyzed by other supported Au catalysts reported recently by other groups. Meanwhile the study of the effect of surface chemistry on catalysis shows that the catalytic activity of the PLAL-AuNP/CeO2-NT catalyst is much higher in comparison to that of similar catalysts composed of chemically prepared AuNPs (Chem-AuNPs) and/or commercially available CeO2 powder as support. Its superior catalytic activity is found to be due to the unique, relatively “bare” surface of the PLAL-AuNPs as well as oxidized Au species induced by the strong interaction between the “barrier-free” surface of PLAL-AuNPs and surface defects (oxygen vacancies) of CeO2 NTs. The important role of the unique surface chemistry of PLAL-AuNPs in catalysis was further demonstrated in the CO oxidation reaction in the gas phase. Our results suggest that the use of PLAL-AuNPs enables easy and efficient attachment of AuNPs onto the surface of the CeO2 NTs and their unique combination leads to the development of highly efficient catalysts. In order to overcome the aggregation issue induced by centrifugation when recycling AuNPs during catalysis, and to expand the application of PLAL-AuNPs for catalysis in liquid phases, I further developed a novel in situ recyclable AuNP catalyst for 4-NP reduction using PLAL-AuNPs and CO2-switchable polymers. The surface of PLAL-AuNPs is relatively “bare and clean” and therefore favours easy surface functionalization. I modified the surfaces of PLAL-AuNPs by coupling a thiol-terminated poly(N,Ndiethylaminoethylmethacrylate) (SH-PDEAEMA) with Au, to form CO2-switchable AuNPs (PDEAEMA-AuNPs). The structure of PDEAEMA polymer was rationally designed to enable its robust attachment to the NP surface, yet limit the number of polymers that can be anchored onto the NP surface. So the possible, negative “blocking” effect on the catalytic active sites of AuNPs is limited. The dispersion status of PDEAEMA-AuNPs in aqueous solution can be easily tuned by simply bubbling CO2 or expelling CO2 from solution using N2 bubbling, inducing novel in situ recyclable AuNPs in liquid. The PDEAEMA-AuNPs were used as a catalyst in a model catalytic reaction of 4-nitrophenol reduction and compared with the PLAL- AuNPs without polymer functionalization and the Chem-AuNPs. Results show that the limited amount of PDEAEMA on the AuNP surface doesn’t noticeably reduce the catalytic activity. More interestingly, this novel catalyst can be easily separated and re-dispersed in solution by CO2 gas- switching and exhibits better colloidal and catalytic stability during successive reactions than PLAL-AuNPs and similarly sized AuNPs synthesized by the conventional chemical reduction approach (Chem-AuNPs). In order to further enhance the catalytic activity of PLAL-AuNPs, it is highly interesting to alloy Au with other transition metals, e.g., platinum (Pt) because of the possible synergistic effect of the alloy structure in catalysis. Using a modified PLAL technique, stable PtAu alloy colloids with a wide range of compositions were prepared successfully on novel metal-mixture targets in water, which are made by compression molding a mixture of Pt and Au powders at different ratios. The concentration of Pt in the alloys can be tuned by varying the Pt/Au ratio in the targets, and their composition basically follows that of their corresponding targets. The effect of aqueous solution pH and ablating laser fluence on the formation and structure of alloy NPs was further investigated. It is found that PtAu alloy colloids of identical composition can be achieved over a pH range extending from 4.0 to 11.0 and at fluences varying from 4 to 150 J cm-2 as long as targets of the same composition are used. This finding suggests that alloy formation is essentially insensitive to both factors in certain ranges and the method developed herein for the alloy NP formation is quite robust. Moreover, the surface composition, estimated from electrochemical measurements, is identical to the overall composition of the NPs estimated from Vegard’s law and X-ray diffraction data, which is a strong indication of the uniform composition on the surface and in the interior of these alloy NPs. The as-prepared PtAu alloy NPs were assembled onto CeO2 nanotubes (NTs) to form hybrid nanocatalysts for 4-NP reduction. As demonstrated in the PLAL-AuNP/CeO2 study, the unique surface features of alloy NPs resulting from PLAL are mainly responsible for their robust adsorption onto the NTs, without any additional surface functionalization to either component. The as-prepared PtAu alloy NPs exhibit exceptional catalytic activity for the reduction of 4-nitrophenol. All PtAu alloy NPs, and in particular the Pt50Au50 sample, outperform the activity of monometallic PLAL-Pt, AuNPs and their mixture, and even outperform most AuNPs reported. Remarkably, the alloying of Au with Pt enhances the catalytic activity by means of a synergistic effect. In the second section, chemically pre-synthesized AuNPs were treated using a laser irradiation technique, and then the effect of the surface chemistry variation introduced by laser treatment on the colloidal stability of NPs and amine molecule-AuNP interactions was investigated. First, monodisperse, AuNPs with diameter ~5 nm were prepared at a relatively high concentration by optimized laser irradiation (without the involvement of any hazardous chemicals) on 20-nm AuNPs synthesized chemically via the “green” Frens method. The citate concentration shows an effect on the final particle size and shape during laser treatment. AuNPs after laser irradiation demonstrate a significant improvement in colloidal stability as compared to similarly sized Chem-AuNPs prepared by a purely chemical approach which involves the use of hazardous NaBH4. The stability has been investigated by adjusting the dielectric constant and ionic concentration of the colloidal solutions. The observed enhanced colloidal stability is confirmed by a significant increase in the Zeta potential of the Au colloids and could be explained by the oxidation of gold surface under laser irradiation. The enhanced stability is a key property for the use of AuNPs in many applications and this study reveals the potential of the laser irradiation treatment (free of hazardous chemicals) for the production of highly-stable ultra-small AuNPs. Then, I investigated the surface chemistry and surface functionality variation introduced by laser irradiation. The effect of the surface chemistry variation of AuNPs on the AuNP-amine (-NH2) interaction was investigated via conjugating an amine probe ─ 1-methylaminopyrene (MAP) chromophore ─ with three Au colloidal samples of the same particle size yet different surface chemistries. The surfaces of laser-irradiated and ligand-exchanged-irradiated AuNPs are covered with acetonedicarboxylic ligands (due to laser-introduced citrate oxidization) and citrate ligands, respectively, and both surfaces contain oxidized Au species which are essentially lacking for the citrate-capped AuNPs prepared by the pure chemical approach. Both laser irradiated samples show inferior adsorption capacity of MAP as compared with the purely chemically prepared AuNPs. Detailed investigations indicate that MAP molecules mainly complex directly with Au atoms via Au-NH2R bonds, and the oxidation of the AuNP surface strongly influences the ratio of this direct bonding to the indirect bonding originating from the electrostatic interaction between protonated amine (-NH3 +) and negatively charged surface ligands. The impact of the oxidized AuNP surface associated with the laser treatment is further confirmed by an aging experiment on AuNP-MAP conjugation systems, which straightforwardly verifies that the surface oxidation leads to the decrease in the MAP adsorption on AuNPs.

Type de document: Thèse
Directeur de mémoire/thèse: Ma, Dongling
Co-directeurs de mémoire/thèse: Chaker, Mohamed
Informations complémentaires: Résumé avec symboles
Mots-clés libres: nanoparticules d’or; propriétés catalytiques; chimie; modification de surface; irradiation laser; Pulsed Laser Ablation in Liquid (PLAL)
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 09 juill. 2014 20:51
Dernière modification: 16 mars 2016 14:54
URI: http://espace.inrs.ca/id/eprint/2179

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