Gold nanoparticles for catalytic and potential
\nbiological applications.

The symbols and special characters used in the original abstract could not be transcribed due 
\nto technical problems. Please use the PDF version to read the abstract.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. 
\nAuNPs 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
\ncatalysis is still not explored. It is therefore of great interest to investigate the catalytic properties of
\nPLAL-AuNPs and the influence of their surface chemistry on catalysis.
\nAuNPs have also been widely used in the field of biological research. In these applications,
\nsmall and toxic-chemical-free colloidal AuNPs are highly desired. Nonetheless, the colloidal
\nstability and molecule adsorption ability of the AuNPs are two primary properties for their
\nbiological application development, which highly depend on the surface chemistry of AuNPs.
\nUnfortunately, the chemical synthesis of very small AuNPs (less than 10 nm) involves the use of
\ntoxic chemicals which renders them unsuitable for biological applications. Another laser technique
\n─ laser irradiation, which combines chemical reduction and laser methods, is suitable for synthesis
\nof “green” Au colloids with very small sizes. Interestingly, laser irradiation introduces novel surface
\nchemistry to chemically synthesized AuNPs, such as surface oxidation. The effect of the surface
\nchemistry variation introduced by laser treatment on NP colloidal stability and amine molecule-
\nAuNP interaction has not been investigated so far.
\nThe work performed in this thesis was therefore driven by two main objectives: 1) PLAL and
\nchemical synthesis of Au and PtAu NPs and characterization of their catalytic properties and 2)
\ninvestigation of the surface modification of chemically synthesized Au NPs by laser irradiation and
\nits influence on Au colloidal stability and amine-AuNP interaction. The results obtained in this work
\nare summarized in two sections as follows.
\nIn the first section, the synthesis and catalytic property of AuNPs prepared using the PLAL
\ntechnique are investigated. To do so, I prepared a novel nanostructured catalyst, composed of small
\n(~5 nm in diameter) and uniform AuNPs and ceria nanotubes (CeO2 NTs). AuNPs with relatively
\n“bare and clean” surfaces fabricated by PLAL on a bulk Au target in pure water are efficiently
\nassembled onto the surfaces of CeO2 NTs without performing any surface functionalization of either
\ncomponent to promote their coupling, thanks to the presence of the –OH groups (that do not block
\ncatalysis) on the surfaces of PLAL-AuNPs. The model reaction of reducing 4-nitrophenol to 4-
\naminophenol catalyzed by our PLAL-AuNP/CeO2-NT catalyst exhibits a remarkably higher reaction
\nrate than the reaction catalyzed by other supported Au catalysts reported recently by other groups.
\nMeanwhile the study of the effect of surface chemistry on catalysis shows that the catalytic
\nactivity of the PLAL-AuNP/CeO2-NT catalyst is much higher in comparison to that of similar
\ncatalysts composed of chemically prepared AuNPs (Chem-AuNPs) and/or commercially available
\nCeO2 powder as support. Its superior catalytic activity is found to be due to the unique, relatively
\n“bare” surface of the PLAL-AuNPs as well as oxidized Au species induced by the strong interaction
\nbetween the “barrier-free” surface of PLAL-AuNPs and surface defects (oxygen vacancies) of CeO2
\nNTs. The important role of the unique surface chemistry of PLAL-AuNPs in catalysis was further
\ndemonstrated in the CO oxidation reaction in the gas phase. Our results suggest that the use of
\nPLAL-AuNPs enables easy and efficient attachment of AuNPs onto the surface of the CeO2 NTs
\nand their unique combination leads to the development of highly efficient catalysts.
\nIn order to overcome the aggregation issue induced by centrifugation when recycling AuNPs
\nduring catalysis, and to expand the application of PLAL-AuNPs for catalysis in liquid phases, I
\nfurther developed a novel in situ recyclable AuNP catalyst for 4-NP reduction using PLAL-AuNPs
\nand CO2-switchable polymers. The surface of PLAL-AuNPs is relatively “bare and clean” and
\ntherefore favours easy surface functionalization. I modified the surfaces of PLAL-AuNPs by
\ncoupling a thiol-terminated poly(N,Ndiethylaminoethylmethacrylate) (SH-PDEAEMA) with Au, to
\nform CO2-switchable AuNPs (PDEAEMA-AuNPs). The structure of PDEAEMA polymer was
\nrationally designed to enable its robust attachment to the NP surface, yet limit the number of
\npolymers that can be anchored onto the NP surface. So the possible, negative “blocking” effect on
\nthe catalytic active sites of AuNPs is limited. The dispersion status of PDEAEMA-AuNPs in
\naqueous solution can be easily tuned by simply bubbling CO2 or expelling CO2 from solution using
\nN2 bubbling, inducing novel in situ recyclable AuNPs in liquid. The PDEAEMA-AuNPs were used
\nas a catalyst in a model catalytic reaction of 4-nitrophenol reduction and compared with the PLAL-
\nAuNPs without polymer functionalization and the Chem-AuNPs. Results show that the limited
\namount of PDEAEMA on the AuNP surface doesn’t noticeably reduce the catalytic activity. More
\ninterestingly, this novel catalyst can be easily separated and re-dispersed in solution by CO2 gas-
\nswitching and exhibits better colloidal and catalytic stability during successive reactions than
\nPLAL-AuNPs and similarly sized AuNPs synthesized by the conventional chemical reduction
\napproach (Chem-AuNPs).
\nIn order to further enhance the catalytic activity of PLAL-AuNPs, it is highly interesting to
\nalloy Au with other transition metals, e.g., platinum (Pt) because of the possible synergistic effect of
\nthe alloy structure in catalysis. Using a modified PLAL technique, stable PtAu alloy colloids with a
\nwide range of compositions were prepared successfully on novel metal-mixture targets in water,
\nwhich are made by compression molding a mixture of Pt and Au powders at different ratios. The
\nconcentration of Pt in the alloys can be tuned by varying the Pt/Au ratio in the targets, and their
\ncomposition basically follows that of their corresponding targets. The effect of aqueous solution pH
\nand ablating laser fluence on the formation and structure of alloy NPs was further investigated. It is
\nfound that PtAu alloy colloids of identical composition can be achieved over a pH range extending
\nfrom 4.0 to 11.0 and at fluences varying from 4 to 150 J cm-2 as long as targets of the same
\ncomposition are used. This finding suggests that alloy formation is essentially insensitive to both
\nfactors in certain ranges and the method developed herein for the alloy NP formation is quite robust.
\nMoreover, the surface composition, estimated from electrochemical measurements, is identical to
\nthe overall composition of the NPs estimated from Vegard’s law and X-ray diffraction data, which
\nis a strong indication of the uniform composition on the surface and in the interior of these alloy
\nNPs.
\nThe as-prepared PtAu alloy NPs were assembled onto CeO2 nanotubes (NTs) to form hybrid
\nnanocatalysts for 4-NP reduction. As demonstrated in the PLAL-AuNP/CeO2 study, the unique
\nsurface features of alloy NPs resulting from PLAL are mainly responsible for their robust adsorption
\nonto the NTs, without any additional surface functionalization to either component. The as-prepared
\nPtAu alloy NPs exhibit exceptional catalytic activity for the reduction of 4-nitrophenol. All PtAu
\nalloy NPs, and in particular the Pt50Au50 sample, outperform the activity of monometallic PLAL-Pt,
\nAuNPs and their mixture, and even outperform most AuNPs reported. Remarkably, the alloying of
\nAu with Pt enhances the catalytic activity by means of a synergistic effect.
\nIn the second section, chemically pre-synthesized AuNPs were treated using a laser irradiation
\ntechnique, and then the effect of the surface chemistry variation introduced by laser treatment on the
\ncolloidal stability of NPs and amine molecule-AuNP interactions was investigat

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