Epoxide Opening versus Silica Condensation during Sol–Gel Hybrid Biomaterial Synthesis

Abstract

Hybrid organic-inorganic solids represent an important class of engineering materials, usually prepared by sol-gel processes by cross-reaction between organic and inorganic precursors. The choice of the two components and control of the reaction conditions (especially pH value) allow the synthesis of hybrid materials with novel properties and functionalities. 3-Glycidoxypropyltrimethoxysilane (GPTMS) is one of the most commonly used organic silanes for hybrid-material fabrication. Herein, the reactivity of GPTMS in water at different pH values (pH 2-11) was deeply investigated for the first time by solution-state multinuclear NMR spectroscopic and mass spectrometric analysis. The extent of the different and competing reactions that take place as a function of the pH value was elucidated. The NMR spectroscopic and mass spectrometric data clearly indicate that the pH value determines the kinetics of epoxide hydrolysis versus silicon condensation. Under slighly acidic conditions, the epoxy-ring hydrolysis is kinetically more favourable than the formation of the silica network. In contrast, under basic conditions, silicon condensation is the main reaction that takes place. Full characterisation of the formed intermediates was carried out by using NMR spectroscopic and mass spectrometric analysis. These results indicate that strict control of the pH values allows tuning of the reactivity of the organic and inorganic moities, thus laying the foundations for the design and synthesis of sol-gel hybrid biomaterials with tuneable properties.

Functionalizing natural polymers with alkoxysilane coupling agents: reacting 3-glycidoxypropyl trimethoxysilane with poly(γ-glutamic acid) and gelatin

“…After reaction of Na-γ-PGA with GPTMS for 48 h at pH 6 and 40 °C, 1-D 1 H NMR ( Figure 2, and Figures S10-S12 in Supporting Information) showed a reduction in the relative area of peaks f 1 and f 2 at 2.7 ppm and 2.8 ppm in GPTMS/D 2 O and Na-γ-PGA/GPTMS, which means that the epoxy ring opened under the mildly acidic conditions, in line with previous observations 32,51 . The disappearance of the sharp peak at 3.17…”

“…show that when GPTMS was reacted with only water at pH 2 for 24 h, new signals appeared at 62, 69 and 71 ppm as a new diol species forms, in agreement with Gabrielli et al 32,51 . The chemical shift of e' and f' in the new species increased due to the epoxide ring opening and addition of a second electronegative atom.…”

“…In addition, a number of new peaks were observed in the GPTMS/D 2 O and Na-γ-PGA/GPTMS samples between 3.25-4.00 ppm but considerable overlap prevented identification and quantification of the different species. From previous studies where GPTMS is reacted with model small molecules 32 and in water at different pH values 51 where there is no overlap in the NMR signals, these new peaks are due to the formation of diol species as a consequence of the epoxy ring reaction with water. In the Na-γ-PGA/GPTMS solution, further new peaks were observed between 4.00 ppm and 4.33 ppm.…”

“…A pH of 5 was used for gelatin because using pH 6 caused gelation of the gelatin. At pH 5 the ring opening of the epoxide was previously found to be very slow (just 13% of the epoxide ring results hydrolysed after three days) 48 , so pH 5 is unlikely to hamper gelatin reaction with GPTMS compared to pH 6. In literature, there are numerous reports of assigned NMR spectra of different types of polypeptide and these have been used to assign the NMR spectra in Figure 6 52-54 .…”

“…The carbonyl region of PGA (and also gelatin) is particularly crowded due to the presence of the two bands from amide carbonyls, plus the band from the free (unbound) COOH; it is thus difficult to find the ester band with diagnostic significance in such comeplex systems. In order to get a better understanding of these systems, the authors previously investigated by NMR the reactions of GPTMS in water alone under different pH conditions 48 and with various small molecule nucleophiles, such as propanoic acid, propanol, propanthiol and propylamine 32 as a model for larger systems. The epoxide ring opening of GPTMS was found to be catalysed by Broensted acids with an increased rate of ring opening at reduced pH.…”