Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy

Abstract

The localization of guest molecules at the molecular scale in mesoporous host materials is crucial for applications in heterogeneous catalysis, chromatography, drug delivery, and in different biomedical applications. Here, we present for the first time the precise localization of different guest molecules inside the mesoporous organosilica material UKON2a with a pore size of 6 nm. We exploited paramagnetic probe molecules 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and 4-carboxy-TEMPO in combination with a deuteration strategy. Applying a complementary set of different pulsed electron paramagnetic resonance methods, we obtained information about the dimensionality of the spatial distribution and local concentration via double electron–electron resonance experiments, orientation of the guest molecules with respect to the pore walls via electron spin echo envelope modulation spectroscopy, and about the distance between guest molecules and pore walls via electron nuclear double resonance spectroscopy. This allowed localizing the guest molecules and shows that their spatial distribution in nanopores strongly depends on their polarity.

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

“…Experimental methods for measuring surface diffusion have been reviewed in a number of papers [241-244, 252, 439, 462] and books [3,418], among other ones. Pivtsov et al [463] have presented for the first time the precise localization of different guest molecules inside the mesoporous organosilica material UKON2a with a pore size of 6 nm. Applying a complementary set of different pulsed electron paramagnetic resonance (EPR) methods [249,463], the authors obtained information about the dimensionality of the spatial distribution and local concentration via double electron-electron resonance experiments, orientation of the guest molecules and the pore walls via electron nuclear double resonance spectroscopy.…”

“…Pivtsov et al [463] have presented for the first time the precise localization of different guest molecules inside the mesoporous organosilica material UKON2a with a pore size of 6 nm. Applying a complementary set of different pulsed electron paramagnetic resonance (EPR) methods [249,463], the authors obtained information about the dimensionality of the spatial distribution and local concentration via double electron-electron resonance experiments, orientation of the guest molecules and the pore walls via electron nuclear double resonance spectroscopy. This allowed localizing the guest molecules and shows that their spatial distribution in nanopores strongly depends on their polarity.…”

The Cu(ii) – dietary fibre interactions at molecular level unveiledviaEPR spectroscopy

“…Preferential absorption of copper ions into DFs must result in an increase of the local Cu(II) concentration, which is manifested as increasing the eD interaction in eqn (1) and can be probed by pulse dipolar spectroscopy (PDS). 23,24,[45][46][47] The homogeneous Cu(II) distribution corresponds to the mean distances from one Cu(II) ion to its next neighbor of the order of 17 nm for 1 mM concentration. Thus, the expected dipolar evolution signal is a so-called background signalthe slowly decaying curve, with the decay rate depending on the local concentration of the Cu(II) ions.…”

Tuning molecular adsorption in SBA-15-type periodic mesoporous organosilicas by systematic variation of their surface polarity

“…EPR line-shape analysis provides information about the relative mobility of surface-conned spin labels. [46][47][48] It is inversely correlated with the strength of their interaction with the surface. EPR spectra were acquired for TEMPO radicals attached to aminopropylsilane-modied PMOs (Scheme S1 †).…”