Cyclic voltammetry

Cyclic voltammetry was performed using a three electrode setup composed of a glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl reference electrode coupled with a Pine Wavenow USB or a BAS 50W potentiostats. Acquisition parameters for the cyclic voltammogram of Eu^II/III-DOTA-4AmC (Figure 3A) were 8 segments, initial potential of −1.4 V (rising), upper potential of 0 V, lower potential of −1.4 V, final potential of −1.4 V, and a sweep rate of 150 mV/s. Acquisition parameters for the cyclic voltammogram of Eu^II/III-DOTA-4AmC (Figure 3B) were 8 segments, initial potential of 0.5 V (falling), lower potential of −1.4 V, upper potential of 0.5 V, final potential of 0.5 V, and a sweep rate of 100 mV/s. Samples were prepared by dissolving Eu^III-DOTA-4AmC and either tetraethylammonium perchlorate or potassium chloride in water. The pH of the resulting solution was 4 and was adjusted to 7 using an aqueous solution of NaOH (1 M) for the potassium chloride solution. The solutions were sparged with Ar while stirring for 5 min prior to measurements, but were not sparged or stirred during the measurements. The standard potential is reported as mean ± standard error of 3 independently prepared samples.