A very reliable and reproducible electrochemical preparative procedure to obtain oxygen-sensitive lithium phthalocyanine (LiPc) microcrystalline powder, a critical material for in vivo application of electron paramagnetic resonance (EPR) oximetry to measure the partial pressure of molecular oxygen pO2, is described. Important issues including the effect of preparative conditions on the resulting material and the influence of the deposition mechanism on crystal structure are investigated using cyclic voltammetry, chronoamperometry, X-ray diffraction (XRD), and high- and low-frequency EPR measurements. 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-dimensional growth is controlled by the diffusion of the reactant from the bulk solution. Critical evidence, for deposition potential-dependent electrochemical 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 β structure of LiPc, which is insensitive to molecular oxygen 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-sensitive x form is obtained. A rapidity test showed that while at deposition potentials +0.4 and +0.7 V only the x form is obtained, the +0.4 V sample responds 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 applications, can be prepared.
|Original language||English (US)|
|Number of pages||13|
|Journal||Journal of Physical Chemistry B|
|State||Published - May 4 2000|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry