Direct observation of muonium reacting with uncapped gold nanoparticles in porous silica and nature of the final state.

We report the reaction of muonium (Mu = [µ+e-]), a light isotopic analog of hydrogen, with uncapped gold nanoparticles embedded in mesoporous silica. Using the radio-frequency muon spin rotation (RF-µSR) technique, we directly observe and characterize the resulting final state on the nanoparticle surface, showing conclusively its diamagnetic nature. The magnetic environment experienced by the reacted muons is only weakly perturbed compared to that of muons in a silica reference, consistent with the surface of the gold nanoparticles being metallic and non-magnetic. We demonstrate the potential of RF-µSR for the investigation of the surface properties of nanoparticles and show the feasibility of Knight shift measurements of muons on metal surfaces.

Communication: Chemisorption of muonium on gold nanoparticles: A sensitive new probe of surface magnetism and reactivity.

Chemisorption of muonium onto the surface of gold nanoparticles has been observed. Muonium (µ+e-), a light hydrogen-like atom, reacts chemically with uncapped 7 nm gold nanoparticles embedded in mesoporous silica (SBA-15) with a strong temperature-dependent rate. The addition rate is fast enough to allow coherent spin transfer into a diamagnetic muon state on the nanoparticle surface. The muon is well established as a sensitive probe of static or slowly fluctuating magnetic fields in bulk matter. These results represent the first muon spin rotation signal on a nanoparticle surface or any metallic surface. Only weak magnetic effects are seen on the surface of these Au nanoparticles consistent with Pauli paramagnetism.

Novel bifunctional periodic mesoporous organosilicas, BPMOs: synthesis, characterization, properties and in-situ selective hydroboration-alcoholysis reactions of functional groups.

A new class of bifunctional periodic mesoporous organosilicas (BPMOs) containing two differently bonded organic moieties in a mesoporous host has been synthesized and characterized. By incorporating bridge-bonded ethylene groups into the walls and terminally bonded vinyl groups protruding into the channel space, both the chemistry and physical properties of the resulting BPMO could be modified. The materials have periodic mesoporous structures in which the bridging ethylene plays a structural and mechanical role and the vinyl groups are readily accessible for chemical transformations. The vinyl groups in the material underwent hydroboration with BH(3).THF and the resulting organoborane in the BPMO was quantitatively transformed into an alcohol using either H(2)O(2)/NaOH or NaBO(3).4H(2)O. The materials retained ordered structures after subsequent in situ reactions with largely unchanged pore volumes, specific surface areas and pore size distributions. Other organic functionalized BPMO materials may be synthesized in a similar manner or by further functionalizing the resulting borylated or alcohol functionalized BPMO materials. The thermal properties of the BPMO materials have also been investigated and are compared to those of the periodic mesoporous organosilica (PMO) materials. Noteworthy thermal events concern intrachannel reactions between residual silanols or atmospheric oxygen and organics in BPMOs. They begin around 300 degrees C and smoothly interconvert bridging ethylene to terminal vinyl groups and terminal vinyl to gaseous ethene and ethane, ultimately producing periodic mesoporous silica at 900 degrees C that exhibits good structural order and a unit-cell size decreased relative to that of the parent BPMO.