One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition

Abstract

Dynamic nuclear polarization (DNP) ²⁹Si solid-state NMR spectra of a hybrid mesoporous silica material impregnated with aqueous biradical solutions have been acquired with cross-polarization (CP) and cross-polarization Carr–Purcell Meiboom–Gill (CP/CPMG) pulse sequences. The integrated intensities (II) and signal to noise ratios (S/N) of the ²⁹Si solid-state NMR spectra are monitored in order to measure the DNP enhancement factors (ε_Si CP) as well as the overall sensitivity enhancement (Σ_Si CP) available from the combination of DNP and CPMG acquisition. Here, , where θ_Si is a factor which quantifies reduction of the NMR signal by paramagnetic effects (quenching) and κ is the square root of the ratio of nuclear longitudinal relaxation times of the dry material and material impregnated with radical solution. It is found that Σ_Si CP is always substantially lower than the measured value of ε_Si CP due to paramagnetic effects which reduce the II of the ²⁹Si CP solid-state NMR spectra at high biradical concentrations. In this system, it is observed that the sample preparation which provides optimal DNP signal enhancement does not provide optimal overall signal enhancement. Notably, optimal signal enhancements are obtained for CPMG acquisition of the ²⁹Si solid-state NMR spectra when lower radical concentrations are employed due to slower transverse relaxation rates. To the best of our knowledge this is the first study which seeks to quantify the overall sensitivity enhancements available from DNP solid-state NMR experiments.