Electrochemical preparation and EPR studies of lithium phthalocyanine: Evaluation of the nucleation and growth mechanism and evidence for potential-dependent phase formation

A very reliable and reproducible electrochemical preparative procedure to o btain oxygen-sensitive lithium phthalocyanine (LiPc) microcrystalline powde r, a critical material for in vivo application of electron paramagnetic res onance (EPR) oximetry to measure the partial pressure of molecular oxygen p O(2), is described. Important issues including the effect of preparative co nditions on the resulting material and the influence of the deposition mech anism on crystal structure are investigated using cyclic voltammetry, chron oamperometry, X-ray diffraction (XRD), and high- and low-frequency EPR meas urements. The electrochemical measurements reveal that electrodeposition of LiPc follows a nucleation pathway. Detailed electrocrystallization studies show that the nucleation mechanism is instantaneous and the three-dimensio nal growth is controlled by the diffusion of the reactant from the bulk sol ution. Critical evidence, for deposition potential-dependent electrochemica l phase formation, is presented. The XRD studies indicate that, in certain deposition conditions, namely, deposition at potentials +0.1 and +0.2 V (Ag /AgCl), the beta structure of LiPc, which is insensitive to molecular oxyge n in terms of EPR oximetry, is formed in higher fraction. On the other hand , at deposition potentials +0.4 and +0.7 V, exclusively the oxygen-sensitiv e x form is obtained. A rapidity test showed that while at deposition poten tials +0.4 and +0.7 V only the x form is obtained, the +0.4 V sample respon ds more quickly to oxygen than the +0.7 V sample. From the present work, a variety of LiPc microparticles, suitable for in vivo EPR oximetry applicati ons, can be prepared.