Impact of Ho³⁺-doping on ¹³C dynamic nuclear polarization using trityl OX063 free radical

We have investigated the effects of Ho-DOTA doping on the dynamic nuclear polarization (DNP) of [1-¹³C] sodium acetate using trityl OX063 free radical at 3.35 T and 1.2 K. Our results indicate that addition of 2 mM Ho-DOTA on 3 M [1-¹³C] sodium acetate sample in 1:1 v/v glycerol:water with 15 mM trityl OX063 improves the DNP-enhanced ¹³C solid-state nuclear polarization by a factor of around 2.7-fold. Similar to the Gd³⁺ doping effect on ¹³C DNP, the locations of the positive and negative ¹³C maximum polarization peaks in the ¹³C microwave DNP sweep are shifted towards each other with the addition of Ho-DOTA on the DNP sample. W-band electron spin resonance (ESR) studies have revealed that while the shape and linewidth of the trityl OX063 ESR spectrum was not affected by Ho³⁺-doping, the electron spin-lattice relaxation time T₁ of trityl OX063 was prominently reduced at cryogenic temperatures. The reduction of trityl OX063 electron T₁ by Ho-doping is linked to the ¹³C DNP improvement in light of the thermodynamic picture of DNP. Moreover, the presence of Ho-DOTA in the dissolution liquid at room temperature has negligible reduction effect on liquid-state ¹³C T₁, in contrast to Gd³⁺-doping which drastically reduces the ¹³C T₁. The results here suggest that Ho³⁺-doping is advantageous over Gd³⁺ in terms of preservation of hyperpolarized state - an important aspect to consider for in vitro and in vivo NMR or imaging (MRI) experiments where a considerable preparation time is needed to administer the hyperpolarized ¹³C liquid.