As used herein, a paramagnetic metal ion is selected from a group consisting of Gd, Mn, Mn, Fe, Cr, Cu, Co, {HEAD ~ Ni}, {TAIL ~ Dy}, Tb, Ho, Er, Eu, and Nd 
As used herein , a paramagnetic metal ion is selected from a group consisting of Gd , Mn , Mn , Fe , Cr , Cu , Co , { HEAD ~ Ni } , { TAIL ~ Dy } , Tb , Ho , Er , Eu , and Nd
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