The effect of Ho³⁺ doping on ¹³C dynamic nuclear polarization at 5 T

Dissolution dynamic nuclear polarization was introduced in 2003 as a method for producing hyperpolarized ¹³C solutions suitable for metabolic imaging. The signal to noise ratio for the imaging experiment depends on the maximum polarization achieved in the solid state. Hence, optimization of the DNP conditions is essential. To acquire maximum polarization many parameters related to sample preparation can be modulated. Recently, it was demonstrated that Ho³⁺, Dy³⁺, Tb³⁺, and Gd³⁺ complexes enhance the polarization at 1.2 K and 3.35 T when using the trityl radical as the primary paramagnetic center. Here, we have investigated the influence of Ho-DOTA on ¹³C solid state DNP at 1.2 K and 5 T. We have performed ¹³C DNP on [1-¹³C] sodium acetate in 1:1 (v/v) water/glycerol with 15 mM trityl OX063 radicals in the presence of a series of Ho-DOTA concentrations (0, 0.5, 1, 2, 3, 5 mM). We have found that adding a small amount of Ho-DOTA in the sample preparation not only enhances the ¹³C polarization but also decreases the buildup time. The optimum Ho-DOTA concentration was 2 mM. In addition, the microwave sweep spectrum changes character in a manner that suggests both the cross effect and thermal mixing are active mechanisms for trityl radical at 5 T and 1.2 K.