Probing sol‐gel matrices microenvironments by PGSE HR‐MAS NMR

We applied Pulsed Gradient Spin Echo diffusion with high‐resolution magic angle spinning NMR to study sol‐gel matrices used to encapsulate enzymes for biocatalysis (TMOS/MTMS and TMOS/BTMS) to gain insight into the local chemical microenvironment. Transport properties of solvents with different polarities (1‐pentanol, acetonitrile and n‐ hexane) were studied through their apparent self‐diffusion coefficients. The spin echo attenuation of the solvents shows two distinct diffusion domains, one with fast diffusion (D fast ) associated with interparticle diffusion and another with slow diffusion (D slow ) corresponding to the displacement inside the pores within the sol‐gel particles. The analysis of the root mean square displacements at different diffusion times showed that the D fast domain has a free diffusion regime in both matrices (the root mean square displacement is linearly dependent of the diffusion time), while the D slow domain shows a different regime that depends on the matrix. We investigated the exchange regime between the two diffusion sites. In both matrices, n‐ hexane was in intermediate exchange between diffusion domains, while the polar solvents were in slow exchange in TMOS/BTMS and in intermediate exchange in TMOS/MTMS. Data were fitted for TMOS/BTMS with the Kärger model, and the physical parameters were obtained. The results add to the evidence that the pores are a hydrophobic environment but that the presence of some free hydrophilic groups inside the pore, as observed in the TMOS/BTMS, has a key role in slowing down the exchange of polar solvents and that this is relevant to explain previously reported enzyme activity in these materials. Copyright © 2016 John Wiley & Sons, Ltd.