Katherine L. McGilvray

Photochemical Strategies for the Synthesis of Gold Nanoparticles from Au(III) and Au(I) Using Photoinduced Free Radical Generation

A comprehensive study of the ketone-photoinduced formation of gold nanoparticles (AuNPs) from gold ions in aqueous and micellar solution has been carried out. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver but rather because of the reducing free radicals they can generate; thus, efficient nanoparticle generation requires a careful selection of substrates and experimental conditions to ensure that free-radical generation occurs with high quantum efficiency and that gold ion precursors do not cause UV screening of the organic photosensitizers. A key consideration in achieving AuNP synthesis with short exposure times is minimizing excited-state quenching by gold ions; this can be achieved by temporal or spatial segregation or a combination of the two. Temporal segregation can be accomplished by using unimolecular precursors, such as benzoins, that yield ketyl radicals from triplet precursors with lifetimes of a few nanoseconds. Spatial segregation can be achieved by using self-assembled structures such as micelles. In this case, the process can be assisted by selecting ketones with n,pi* triplet states and by adding good hydrophobic hydrogen donors such as 1,4-cyclohexadiene. Systems involving bimolecular reactions of ketones are catalytic in that the ketone is recovered at the end of the reductive process. Rate constants have been determined for the quenching of excited triplets and for the scavenging of ketyl radicals by Au(I) and Au(III); in general, these values are within an order of magnitude of the rate constant for diffusion control. This article provides a paradigm for the photochemical production of nanoparticles of gold and other metal ions that highlights ten aspects that must be considered in order to design successful photochemical systems for nanoparticle generation.

Photochemical routes to silver and gold nanoparticles

The photochemistry of aromatic ketones through the Norrish type I cleavage of benzoins and via photoreduction generates ketyl radicals that readily reduce many metal ions, including silver and gold. Reduction to Au(0) and Ag(0) leads to the spontaneous formation of nanoparticles (NPs) in aqueous or micellar solutions. Careful consideration of kinetic factors to minimize triplet quenching by metal ions can lead to rapid NP generation. These materials are quite stable and have interesting reactivities due to the essentially unprotected characteristics of the surface.

Surface Plasmons Control the Dynamics of Excited Triplet States in the Presence of Gold Nanoparticles

Aqueous gold nanoparticles (AuNPs) cause a large increase in the yield of methylene blue triplets ((3)MB*) obtained upon 650 nm laser excitation as a result of surface plasmon field interactions that can be described as transmitter-receiver antenna effects. Two distinct (3)MB* populations are observed; a fast decaying one (tau(T) approximately 25 ns) is believed to be due to molecules on the AuNP surface at the time of excitation and is described as static quenching. A longer lived (3)MB* population has lifetimes in the tens of microseconds but is subject to an anomalously high rate constant for a AuNP quenching of 6.4 x 10(13) M(-1) s(-1). This ultrafast quenching is attributed to a nonrandom distribution caused by the AuNP plasmon field that preferentially excites MB molecules located in the proximity of the AuNP where they are subject to antenna type interactions with the nanoparticle and are spatially predisposed for efficient quenching.

Photochemical strategies for the seed-mediated growth of gold and gold-silver nanoparticles.

Gold nanoparticles (AuNP) can be used as seeds for the synthesis of larger AuNP of controllable size with narrow size distribution by photochemical reduction of additional Au(III) using water-soluble benzoins or H(2)O(2) as sources of reducing radicals. Further, beyond simply enlarging the AuNP, it is possible to add a shell of another metal, such as silver, leading to Au/Ag core-shell structures with controllable dimensions for both core and shell. This strategy illustrates the fine spatial and temporal control achievable using clean photochemical techniques without the addition of hard surface ligands often necessary to control the size and structure of gold-silver nanostructures. The mild nature of the surface coverage makes these nanomaterials ideal for further surface modification.

Tuning plasmon transitions and their applications in organic photochemistry

The ketone-photoinduced formation of Au, Ag, and Cu nanoparticles from their corresponding ions in solution has been carried out using benzoin photoinitiators. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver, but rather because of the reducing free radicals they can generate. Efficient photochemical nanoparticle generation thus requires a careful selection of substrates and experimental conditions such that free radical generation occurs with high quantum efficiency, where metal ion precursors do not inhibit radical formation. A key consideration to achieve nanoparticle synthesis with short exposure times is to minimize excited-state quenching by metal ions. Applications of nanostructures in catalysis require control of the nanoparticle characteristics, such as dimension, morphology, and surface properties. Part of this article describes the strategies to modify photochemically prepared particles. Finally, we illustrate some of the nanoparticle applications that interest us, with some emphasis on plasmon-mediated processes.

A simple and smart oxygen sensor based on the intrazeolite reactions of a substituted anthraquinone

Inclusion of 2-(hydroxymethyl)anthraquinone in zeolite NaY leads to a solid, photoactivated, reusable oxygen sensor capable of reporting and memorizing oxygen contamination events by simple visual inspection.