Abstract
Interfacial interactions between inorganic surfaces and organic additives are vital to develop new complex nanomaterials. Learning from biosilica materials, composite nanostructures have been developed, which exploit the strength and directionality of specific polyamine additive-silica surface interactions. Previous interpretations of these interactions are almost universally based on interfacial charge matching and/or hydrogen bonding. In this study, we analyzed the surface chemistry of bioinspired silica (BIS) materials using solid-state nuclear magnetic resonance (NMR) spectroscopy as a function of the organic additive concentration. We found significant additional association between the additives and fully condensed (Q4) silicon species compared to industrial silica materials, leading to more overall Q4 concentration and higher hydrothermal stability, despite BIS having a shorter synthesis time. We posit that the polyfunctionality and catalytic activity of additives in the BIS synthesis lead to both of these surface phenomena, contrasting previous studies on monofunctional surfactants used in most other artificial templated silica syntheses. From this, we propose that additive polyfunctionality can be used to generate tailored artificial surfaces in situ and provide insights into the process of biosintering in biosilica systems, highlighting the need for more in-depth simulations on interfacial interactions at silica surfaces.
Highlights
Understanding the interactions of small organic molecules with inorganic surfaces is key to controlling the formation and behavior of both biological and artificial nanomaterials
We combined the previously developed acid elution technique with solid-state nuclear magnetic resonance (NMR) experiments probing 1H and 29Si to resolve the local structure and surface chemistry of this bioinspired silica” (BIS) at different levels of activation, assessing how surface bonding is affected by the additive’s initial incorporation and subsequent removal. By comparing this against amine-free industrially precipitated silica (IPS) and IPS, which has been exposed to additive, we show the lasting impact additive has on silica polymerization and how this may relate to other nanocomposite silica materials
These were analyzed by 29Si magic angle spinning (MAS) NMR to study how PEHA removal affected the silica structure itself (Figure 1, below). 29Si MAS NMR enables distinction between unexpected, as polymerization of silica from Q3 to Q4 is generally considered to occur through syneresis[41] the slow condensation of silica to expel water from the framework which should be more prevalent in the industrial silica material because of its significantly longer reaction times (2−3 h vs 5 min)[12] and similar micron-scale aggregate sizes compared to BIS.[35,42]
Summary
Understanding the interactions of small organic molecules with inorganic surfaces is key to controlling the formation and behavior of both biological and artificial nanomaterials. Q4 Si sites in general; Q4 signals were enhanced in both BIS and modified IPS materials This agrees with the results reported by Folliet et al, which proposed amine-Q4 hydrogen bonding as part of quantum chemical simulations.[34] these findings appear to contradict previous simulations and experimental results for artificial templated silica systems, which show electrostatic or ion-pairing interactions only during surface adsorption of PEHA (or other amine molecules), that is, onto deprotonated Q3 Si centers only.[27,35,49,50] Previous studies of dried silica surfaces have shown secondary interactions with surrounding moieties are possible;[26] PEHA-Q4 adducts may be the result of secondary interactions on (partially) dried BIS surfaces. Future research focusing on solidstate NMR experiments directly probing 13C and 15N nuclei on isotopically labeled PEHA and other bioinspired additives, as well as experiments probing 13C−29Si and 15N−29Si internuclear distances (e.g., 13C−29Si and 15N−29Si CP, HETCOR, or REDOR MAS NMR) will be vital to further resolve the surface PEHA−silica interactions and PEHA-mediated silica synthesis
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