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The nature of the self-assembled core−shell interface in gold@silica nanoparticles synthesized via a 3-aminopropyltrimethoxysilane (APTMS) route is investigated using materials synthesis as a sensitive tool for elucidating interfacial composition and organization. Our approach involves condensation of the gold@silica nanoparticles within a silica framework for synthesis of a composite gold−silica material containing ∼30 wt % gold. This material contains one of the highest gold loadings reported, but maintains gold core isolation as ascertained via a single surface plasmon resonance absorption band frequency corresponding to that of gold nanoparticles in dilute aqueous solution. The immobilized gold cores are subsequently etched using cyanide anion for the synthesis of templated porosity, which corresponds to the space that was occupied by the gold. Characterization of immobilized amines is performed using probe molecule binding experiments, which demonstrate a lack of accessible amines after gold removal. Solid-state 13C CPMAS NMR spectroscopy on these materials demonstrates that the amount of amine immobilization must be less than 10% of the expected yield, assuming that all of the APTMS becomes bound to the gold nanoparticle template. These results require a core−shell interface in the gold@silica nanoparticles that is predominantly occupied by inorganic silicate species, such as Si−O−Si and Si−OH, rather than primary amines. Such a result is likely a consequence of the weak interaction between primary amines and gold in aqueous solution. Our method for investigating the core−shell interface of gold@silica nanoparticles is generalizable for other interfacial structures and enables the synthesis of bulk imprinted silica using colloidal templates.

Sol-gel materials are often reinforced through the addition of particulate fillers to improve their mechanical properties, such as toughness or stiffness. Related studies of filled polymeric materials suggest that the effectiveness of added reinforcements may depend sensitively on the state of particulate aggregation in the composites. In some systems and situations, aggregates are found to enhance properties, while in others they are deleterious. The objective of the present study was to determine the effect of state of aggregation of alumina nanoparticle fillers on the mechanical properties of a GPS sol-gel hybrid composite. Aggregates of different sizes were produced at loadings of 3.6, 7.5, and 16.8 volume percent in the GPS sol-gel monoliths by inducing aggregation through the addition of varying amounts of tetrasodium pyrophospate (TSPP). The small amounts of TSPP required to induce varying states of aggregation did not affect the properties of the gel in the absence of the particles. Unaggregated particles or small aggregates were found to impart significant improvements in the mechanical properties of the gel, but at a particular critical aggregate size and beyond, the cured material cracked and crumbled. The critical size was found to correspond roughly to an effective aggregate volume fraction of 0.64 in the final composite, suggestive of a randomly close-packed aggregate structure.

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