PREPARATION AND EPR STUDIES OF LITHIUM PHTHALOCYANINE RADICAL AS AN OXYMETRIC PROBE

The electron paramagnetic resonance (EPR) spectrum of the paramagnetic center in solid lithium phthalocyanine, LiPc, exhibits a pO(2) (parti al pressure of oxygen)-dependent line width. The compound is insoluble in water and is not easily biodegradable and, therefore, is a useful spin probe for quantitative in vivo oxymetry. Because EPR spectrometry is potentially a useful technique to quantitatively obtain in vivo ti ssue pO(2), such probes can be used to obtain physiological informatio n. In this paper, a simple experimental procedure for the preparation of LiPc using potentiostatic electrochemical methods is described. The setup was relatively inexpensive and easy to implement. A constant po tential ranging from 0.05 to 0.75 V versus Ag+/AgCl(s) was used for ob taining LiPc. The EPR spectral studies were carried out using spectrom eters operating at X-band and at radiofrequency (RF) at different pO(2 ) values to characterize the spectral response of these crystals. The results indicate that, depending on the electrolysis conditions, the p roducts contain mixtures of crystals exhibiting pO(2)-sensitive and pO (2)-insensitive line widths, Electrolysis conditions are reported wher eby the pO(2)-sensitive LiPc crystals were the predominant product. Th e influence of the working surface of the electrode and the electrolys is time on the yield were also evaluated. The crystals of LiPc were al so studied using a time-domain RF EPR spectrometer. In time-domain EPR , the signals that survive beyond the spectrometer dead time are mainl y the narrow lines corresponding to the pO(2)-sensitive crystals, wher eas the signals arising from the pO(2)-insensitive component of LiPc w ere found not to survive beyond the spectrometer dead time. This signa l survival makes the time-domain EPR method more sensitive for pO(2) m easurements using LiPc because the line width becomes very narrow at v ery low pO(2) and, concomitantly, the relaxation time T-2 longer, with no modulation or power saturation artifacts that are encountered as i n the continuous wave (cw) mode. Further, minimal contributions from o bject motion in the spectral data obtained using time-domain methods m ake it an advantage for in vivo applications. Published by Elsevier Sc ience Inc.