Improving nuclear magnetic resonance and electron spin resonance thermometry with size reduction of superparamagnetic iron oxide nanoparticles

Thermometry based on magnetic resonance has been extensively studied in the context of biomedical imaging. In our previous work, we showed that superparamagnetic iron oxide nanoparticles induce a strong temperature dependence to the spin-spin relaxation time (T₂) of nuclear magnetic resonance (NMR) in water because T₂ scales with the highly temperature-sensitive self-diffusion constant of water. In this work, in addition to the self-diffusion constant of a fluid, we utilize the temperature-dependent magnetization of 4-nm diameter superparamagnetic iron oxide nanoparticles (SPIONs) to enhance T2 sensitivity (sensitivity (ζ_T2T= =5.0) by 1.4 times over self-diffusion (ζDT=3.5) alone in hexane between 248 and 333 K. We extend the application of this NMR thermometry approach to engineering systems by investigating the temperature dependence of T₂ in mineral oil, which exhibits a remarkably high sensitivity (sensitivity (ζ_T2T=12) ) between 273 and 353 K. NMR thermometry, however, is not generally applicable to solids. Therefore, we also evaluate the potential of electron spin resonance (ESR) thermometry with SPIONs in the temperature range of 100 to 290 K. The temperature-dependent linewidth follows a T^-2 law for 4-nm SPIONs. The linewidth at 290 K at 9.4 GHz is 11 mT. For both NMR and ESR thermometry, SPIONs with a small magnetic moment, i.e., a small volume and reduced magnetization, enhance the temperature sensitivity of magnetic resonance thermometry.