Preparation and EPR studies of lithium phthalocyanine radical as an oxymetric probe

Abstract

The electron paramagnetic resonance (EPR) spectrum of the paramagnetic center in solid lithium phthalocyanine, LiPc, exhibits a pO₂ (partial 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 tissue pO₂, such probes can be used to obtain physiological information. 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 potential ranging from 0.05 to 0.75 V versus Ag⁺//AgCl(s) was used for obtaining LiPc. The EPR spectral studies were carried out using spectrometers operating at X-band and at radiofrequency (RF) at different pO₂ values to characterize the spectral response of these crystals. The results indicate that, depending on the electrolysis conditions, the products contain mixtures of crystals exhibiting pO₂-sensitive and pO₂-insensitive line widths. Electrolysis conditions are reported whereby the pO₂-sensitive LiPc crystals were the predominant product. The influence of the working surface of the electrode and the electrolysis time on the yield were also evaluated. The crystals of LiPc were also studied using a time-domain RF EPR spectrometer. In time-domain EPR, the signals that survive beyond the spectrometer dead time are mainly the narrow lines corresponding to the pO_2-sensitive crystals, whereas the signals arising from the pO₂-insensitive component of LiPc were found not to survive beyond the spectrometer dead time. This signal survival makes the time-domain EPR method more sensitive for pO₂ measurements using LiPc because the line width becomes very narrow at very low pO₂ and, concomitantly, the relaxation time T₂ longer, with no modulation or power saturation artifacts that are encountered as in the continuous wave (cw) mode. Further, minimal contributions from object motion in the spectral data obtained using time-domain methods make it an advantage for in vivo applications.