This article is not exclusively about DNP-NMR spectroscopy, but describes an interesting approach of measuring cryogenic temperatures using a simple fiber optic-based setup. It is particularly useful for low-temperature MAS NMR experiments, including DNP-NMR spectroscopy.
Tycko, R., Remote sensing of sample temperatures in nuclear magnetic resonance using photoluminescence of semiconductor quantum dots. J Magn Reson, 2014. 244C(0): p. 64-67.
Knowledge of sample temperatures during nuclear magnetic resonance (NMR) measurements is important for acquisition of optimal NMR data and proper interpretation of the data. Sample temperatures can be difficult to measure accurately for a variety of reasons, especially because it is generally not possible to make direct contact to the NMR sample during the measurements. Here I show that sample temperatures during magic-angle spinning (MAS) NMR measurements can be determined from temperature-dependent photoluminescence signals of semiconductor quantum dots that are deposited in a thin film on the outer surface of the MAS rotor, using a simple optical fiber-based setup to excite and collect photoluminescence. The accuracy and precision of such temperature measurements can be better than +/-5K over a temperature range that extends from approximately 50K (-223 degrees C) to well above 310K (37 degrees C). Importantly, quantum dot photoluminescence can be monitored continuously while NMR measurements are in progress. While this technique is likely to be particularly valuable in low-temperature MAS NMR experiments, including experiments involving dynamic nuclear polarization, it may also be useful in high-temperature MAS NMR and other forms of magnetic resonance.