Continuous-wave (CW) ESR spectra were recorded at X-band (9.4 GHz) using Bruker EMXnano Benchtop Spectrometer. For MTSL, spectra were acquired with 100 kHz modulation frequency, 0.15 mT modulation amplitude, 2 mW microwave power, 5.12 ms time constant, 22.5 ms conversion time, and 18 mT sweep width. Quantification was performed using the reference-free concentration determination as implemented in the Bruker Xenon software.