Electrochemical Preparation and EPR Studies of Lithium

J. Phys. Chem. B 2000, 104, 4047-4059

4047

Electrochemical Preparation and EPR Studies of Lithium Phthalocyanine: Evaluation of the Nucleation and Growth Mechanism and Evidence for Potential-Dependent Phase Formation Govindasamy Ilangovan, Jay L. Zweier, and Periannan Kuppusamy* The EPR Center and DiVision of Cardiology, Department of Medicine, Johns Hopkins UniVersity School of Medicine, 5501 Hopkins BayView Circle, Baltimore, Maryland 21224 ReceiVed: October 1, 1999

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.

Introduction Lithium phthalocyanine (LiPc), an intrinsic semiconductor, has been found to be a useful electron paramagnetic resonance (EPR) oximetry probe for in vivo biological applications.1 The EPR oximetry technique has been demonstrated to provide accurate and reproducible measurements of local concentrations of molecular oxygen in a variety of biological tissues including brain,1a tumor,1d and heart.1b,c However, synthesis of LiPc in the pure and desirable oxygen-sensitive form has been a critical problem and challenge in this field.2 This monomolecular organic semiconductor shows polymorphism, and depending upon the preparation condition, it can crystallize in three different structures, namely, the oxygen-sensitive x form, the commonly obtained R form, or the relatively rare β form. Critical changes in their magnetic behavior are observed for these polymorphs due to their very different crystal structures. Detailed EPR studies3 of these three polymorphs have demonstrated very different magnetic properties that strongly correlate with the nature of molecular packing. Apart from its potential use as an oximetry probe, LiPc has aroused fundamental interest because of its interesting electrical4 and magnetic behavior.3 The EPR oximetry technique is based on the principle of Heisenberg spin-spin exchange between the triplet-state molecular oxygen and spin probes. Among the three known polymorphs of LiPc, such a spin-spin exchange is reported only * To whom correspondence should be addressed. E-mail:kuppu@ welch.jhu.edu. Phone: (410) 550-3229. Fax: (410) 550-2448.

in the oxygen-sensitive x form.3f The oxygen-insensitive R and β structures of LiPc show a monoclinic unit cell with parameters a ) 2.57 nm, b ) 0.38 nm, c ) 2.36 nm, and β ) 91.0° and the space group C2/c3a and a ) 1.94 nm, b ) 0.49 nm, c ) 1.40 nm, and β ) 120.36° and the space group P21/c, respectively.3e However, the x form shows a tetragonal unit cell with parameters a ) b ) 1.385 nm and c ) 0.65 nm and the space group P4/mcc.3e Thus, fundamental structural differences make the x polymorph behave differently from the other two. A notable difference among the three forms is seen in the form of large variations in the EPR signal line width. The line width of the x form has been reported to be extremely narrow, about 26 mG in a high vacuum, while the other two forms exhibit line widths of about 200 and 400 mG, respectively.3g Although it is not completely understood yet why the x form gives such a narrow EPR line compared to the other two forms, the oxygenbroadening effect, which is seen only in the x form, has been understood in terms of crystal structure. Figure 1 illustrates unit cells projected on the stacking axis, which is defined as the c axis in the case of the x polymorph and the b axis in the case of the R and β forms. It is clear from Figure 1 that the tetragonal structure of the x form has microchannels of width about 6 Å since the molecular packing is straight.3a,e,f,j But in the cases of R and β structures, the molecular packing is tilted to some angle (91.0° for R and 120.36° for β structures), and hence these channels are narrowed and partially blocked.3f Thus, oxygen molecules (size 2.8 × 3.9 A2) can easily diffuse into the channels of the x form and hence can perturb spin diffusion along the

10.1021/jp9935182 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/05/2000

4048 J. Phys. Chem. B, Vol. 104, No. 17, 2000

Ilangovan et al. is associated with some experimentally uncontrollable parameters. For example, the applied voltage is varied without the experimenter’s control, and any undesired high voltages induced at the electrode due to concentration polarization can result in overoxidation and decomposition, leading to different product compositions at different experimental times. (ii) Extensive characterization of the LiPc obtained in various deposition potentials using X-ray diffraction (XRD) and EPR line-shape analysis so that the individual contributions of the various forms can be evaluated and quantified. This analysis should enable one to choose the right material for a given application. (iii) Evaluation of the LiPc as an EPR oximetry probe for in vivo biological applications. This comprehensive investigation comprising electrochemical synthesis, fundamental characterization, and applications for EPR oximetry should enable solution of some of the fundamental problems in the preparation and use of this material. Experimental Details

Figure 1. Molecular structure and crystallographic packing of LiPc: (top, left) molecular structure, (top, right) crystallographic packing of LiPc, (bottom) schematic illustration of molecular stacking in LiPc along the stacking axis. An octahedral microchannel is formed between the four stacks in the x form since the packing is straight with a channel bore size of 6 Å. This channel is narrowed in the R and β forms due to a tilt in the packing [after ref 3a,e,f,j].

stacking axis via Heisenberg spin exchange. There is no such possibility for oxygen penetration in the other two forms. As a consequence, the small oxygen effect observed in the case of the other polymorphs is only a surface effect, which is virtually negligible.3f These observations reveal that optimization of the synthetic procedure is very critical to selectively obtain the oxygen-sensitive LiPc, which is required for biological oximetry.1 Moreover, the synthetic method should yield bulk material unlike the thin films extensively reported in prior studies.3 Though this one-dimensional semiconducting solid has been synthesized and characterized as thin films mainly by the vapor deposition method3 and a few electrochemical deposition methods,2b,3g,h there has been no systematic investigation on the electrochemical preparation of the oxygen-sensitive x form of LiPc crystalline powder. It is very important to optimize the deposition conditions and investigate the fundamental characteristics of phase formation to obtain and control desired oxygen sensitivity in the LiPc crystals. It is well-known that the deposition potential is the single most critical parameter (assuming that the other factors, namely, migration, convection, or diffusion, that are associated with transport phenomena are avoided by continuous stirring of the solution during electrolysis) in any electrodeposition to get the desired crystal structure, size, spatial density, and surface texture of the resulting particles. For example, deposition of any low-dimensional molecular solids such as tetrathiofulvalene complexes has resulted in simultaneous growth of different phases with different stoichiometries during electrocrystallization at different potentials.5 The present work was initiated with the following goals: (i) Optimization of the electrochemical preparation of the x form of LiPc by the potentiostatic method where the charge-transfer process can be controlled precisely, unlike the galvanostatic method used previously,6 so that the preparation method is more reliable and reproducible. The galvanostatic deposition method

Electrochemical Synthesis and Current-Time Transient Measurements. All the electrochemical measurements were carried out in dry acetonitrile (Aldrich, HPLC grade) with a CHI electrochemical analyzer controlled by a personal computer (PC). The conventional three-electrode setup was used. In the chronoamperometric and cyclic voltammetric (CV) studies, a platinum (Pt) disk with geometric area 0.07 cm2 was used after its surface was cleaned with dilute nitric acid. A Ag/AgCl reversible redox couple was used as the reference electrode. The Pt working electrode (BAS, West Lafayette, IN) was not subjected to any special electrochemical pretreatment except washing it with dilute nitric acid (0.1 M) and deionized water successively, since electrochemical pretreatment complicates the electron-transfer event at the electrode-solution interface.7 Further, the geometric area of the working electrode was used in all the quantitative calculations, and it was assumed to be equal to the electrochemical area since the Pt surface appeared very smooth and correspondingly the roughness factor was expected to be close to unity. A Pt foil was used as the auxiliary electrode, and the ratio of the geometric area between the counter and working electrodes was more than 100, so that activation overpotential is minimized. An excess concentration of tetrabutylammonium perchlorate (TBAP) (ICN Biochemicals, Aurora, OH) was used as the supporting electrolyte. Digitized chronoamperometric (i-t) transients at 50 ms intervals were further analyzed through the commercial Sigma Plot software (SPSS, Chicago, IL) to evaluate the nucleation and growth mechanism. For bulk electrochemical synthesis of LiPc, a bigger platinum mesh electrode with very high surface area (BAS, West Lafayette, IN) was used as the working electrode in a largevolume electrolysis cell. The electrochemical cell setup and electrolysis procedure were similar to those described by Afeworki et al.2b Briefly, to 100 mL of acetonitrile containing 0.1 M TBAP was added 200 mg of Li2Pc with constant stirring so that all Li2Pc added was completely dissolved. The resultant dark blue solution was transferred to one of the two compartments of the cell. A Pt wire with relatively less surface area was used as the counter electrode in the second compartment, which was connected to the main working electrode compartment through a fritted disk. The reference electrode was inserted into the working electrode compartment directly. During electrolysis, no special effort was used to thermostat the cell. Since the cell was closed and airtight, no loss of solvent by evaporation was noticed. Four different electrolysis potentials,

EPR Studies of Lithium Phthalocyanine namely, +0.1, +0.2, +0.4, and +0.7 V, were used to prepare the LiPc. The electrolysis time was fixed as constant (1 h) irrespective of the electrolysis potential since the time of electrolysis may influence the size of the crystal and hence the line width of the EPR spectrum.3g Since the resulting LiPc is insoluble in acetonitrile, the electrolysis product was either precipitated out at the bottom of the cell or sticking to the electrode surface with poor adherence. It was observed that the LiPc samples obtained in the presence of dissolved oxygen gave complex EPR characteristics irrespective of the deposition potential (data not shown). Thus, during electrolysis, the solution was continuously purged with argon to remove dissolved oxygen and constantly stirred using a magnetic stirring bar to facilitate electrolysis and to avoid mass-transfer limitation. The constant stirring also assisted to effectively remove the LiPc crystallites from the electrode surface easily and exposed most of the electrode surface during the entire period of electrolysis. The i-t curves for this long-time bulk electrolysis were followed and ensured that by the end of the electrolysis time the current remains constant at the level of the background current. After the electrolysis, even the small fraction of LiPc sticking to the electrode surface was removed by shaking it in the solution after additional solvent was added, and the solid microcrystalline particles were filtered and dried at 90 °C under atmosphere to obtain a shiny dark-green powder. X-ray Diffraction and EPR Measurements. X-ray diffraction measurements were carried out with a Rgaku diffractometer (model D/Max) for a wide angle typically for 2θ values from 4° to 50° using Cu KR radiation with λ ) 1.542 Å. The other experimental conditions included 0.5° divergence and scatter slits, 0.15 mm receiving slits, step scans with 0.04° steps and 6 s counting time at each step, and intensity measured in counts. The X band, 9.77 GHz EPR measurements were carried out using a Bruker ER 300 spectrometer with a TM110 microwave cavity. Data acquisition and analysis were performed using an IBM-compatible personal computer interfaced to the spectrometer. Instrument control and data acquisition and processing were performed using SPEX, a personal computer software developed in our laboratory. L-band EPR measurements were carried out using a custom-built reentrant loop-gap resonator. For EPR measurements, the sample was prepared as follows. The crystalline powder was encapsulated in a 2 cm long and 0.8 mm diameter gas-permeable Teflon tube (Zeus Industrial Products, Columbia, SC), and the tube was sealed at both ends. The sealed tube was inserted into a 3 mm diameter quartz EPR tube capped at both ends. The EPR tube was carefully positioned in the cavity with the sample at the center of the active volume. The tube was continuously and gently flushed with purified gas/ gas mixtures. The first-derivative EPR spectra were integrated and subjected to area and line-shape analysis. The effect of O2 on the EPR spectra of LiPc was studied by purging with a mixture of purified O2 and N2 gases prepared in the desired ratio using a precalibrated gas flow meter (Cole-Parmer, IL) and subsequently passed through gas-impermeable silicone tubes into the EPR tube containing the gas-permeable Teflon tube embedded LiPc, made as described above. EPR Absorption Line-Shape Analysis. The EPR absorption line shapes were analyzed as pure Lorentzian lines using commercial software PeakFit (SPSS, Inc., Chicago, IL). No dispersion component of the resonance signal was necessary to be included in the analysis, since in normal CW EPR spectroscopy only the absorption component of the resonance signal is observed, with the dispersion component being suppressed by suitable phasing of the microwave bridge. In the fitting of the

J. Phys. Chem. B, Vol. 104, No. 17, 2000 4049

Figure 2. Cyclic voltammograms of 25 mM Li2Pc in dry acetonitrile containing 0.1 M TBAP as supporting electrolyte: (A) Scan rate 0.1 mV s-1. The inset shows a magnified view of the first redox peaks (B) Scan rate 0.01 mV s-1 in the potential range -0.4 V to +0.4 V covering the first redox peaks alone. (C) Variation of peak currents with scan rate (D) Variation of charge with deposition time for the first redox peaks.

data, both the full width at half-maximum (fwhm) and the amplitude were allowed to change for all the individual spectra under the envelope. The best fit was assessed from the minimum chi-square (χ2) and the maximum correlation coefficient (r2). Results and Discussion Electrochemical Oxidation of Li2Pc on the Pt Electrode and Evidence for Nucleation of LiPc Deposition. Figure 2 shows the cyclic voltammogram of 25 mM Li2Pc (dilithium phthalocyanine, Aldrich) at the potential sweep rate (V), 100 mV s-1, in dry acetonitrile. The entire CV pattern encompassing the potential range from -0.5 to +0.9 V shows basically two processes. The first process showing an oxidation peak at +0.06 V was quasi-reversible, as discussed later in detail, since a cathodic peak is observed in equal magnitude as the counterpart at -0.08 V in reverse sweep. The second process is an irreversible oxidation, which occurs beyond 0.5 V (Figure 2A), although there is no clear-cut peak shape. This peak is extended over 0.3 V, and there is no cathodic counterpart. In a separate experiment, the potential sweep in the anodic direction was reverted at +0.4 V (Figure 2B; the sweep rate is 10 mV s-1), encompassing only the first redox couple. It can be seen from Figure 2B that the current steeply rises at the start of the anodic peak, with the full width at half-maximum (obtained as 2(Ep EHM), where Ep is the peak potential and EHM is the potential at half of the peak height) unusually narrow, about 38 mV. This quantity is comparable neither with 90.6 mV reported for the surface wave without any interaction among the adsorbate8 nor with 28 mV reported for the solution-phase redox processes.9 Moreover, during the cathodic sweep, an overlap between the forward and reverse sweeps (as seen in the Figure 2B) leading to a “current loop” is noticed. In addition, a large potential separation of cathodic and anodic peaks is also observed. The overlap of the forward current-potential curve with the reverse curve during potentiodynamic sweep resulting in the current


Ilangovan et al.

TABLE 1: Cyclic Voltammetric Peak Parameters for 25 mM Li2Pc and 0.1 M TBAP in Acetonitrilea anodic peak

cathodic peak

V/mV s-1

Epa/V (Ag/AgCl)

*Efwhm/mV

ipa/µA

Qt/µC

Epc/V (Ag/AgCl)

*Efwhm/ mV

ipc/µA

Qt/µC

10 25 50 100 200 300 400 500

0.030 0.040 0.049 0.057 0.069 0.070 0.074 0.078

38.4 40.0 50.0 65.0 72.5 50.0 50.0 52.5

12.490 18.752 23.251 28.572 46.820 53.621 62.542 68.721

445.0 228.00 114.0 71.2 55.9 37.0 31.9 28.1

-0.065 -0.067 -0.074 -0.082 -0.082 -0.084 -0.085 -0.087

80.0 80.0 90.0 90.0 95.0 97.5 97.5 97.5

-13.632 -19.460 -25.551 -35.412 -39.971 -50.394 -56.900 -60.682

-62.8 -39.7 -32.3 -25.5 -15.8 -15.2 -13.0 -11.4

a Efwhm ) 2(Ep - EHM), where Ep is the peak potential and EHM is the potential at half peak height. ipa and ipc are the anodic and cathodic peak currents, respectively. Epa and Epc refer to the anodic and cathodic peak potentials.

loop has been shown theoretically to correspond to the electrochemical reaction following the nucleation and growth mechanism.10 However, such an observation of a current loop is also possible in situations where the charge-transfer step is followed by a coupled chemical reaction and the product of the reaction has a lower oxidation potential than the starting material.9 This might not be the case in the present study since in such cases the redox peaks cannot be as sharp as observed here and normally are broader, extending over 120 mV.9 Also it is observed that the anodic to cathodic peak ratio (ipa/ipc, Table 1) is close to unity irrespective of the scan rate, suggesting the absence of any coupled chemical reaction. The current loop, sharp peak, and large potential separation observed in the present work are characteristics of electrochemical phase formation through the nucleation-growth mechanism.10,11 With this mechanism the formation of a critical nucleus on the substrate, which requires high energy, is the slowest and rate-determining step, and this is followed by almost explosive “growth” at large overpotentials. The process at this peak is charge transfer

stacking

Pc2- y\z LiPc y\z (LiPc)crystal After charge transfer, since the resulting neutral LiPc is not soluble in acetonitrile, it is deposited on the surface and stacked to result as crystallites. The observation of a current loop is also an indication of the deposition of conducting LiPc molecules. Due to the extended nature of this one-dimensional solid, significant stabilization of the solid is realized through intermolecular interactions. The adherence of this material on the Pt surface was, however, found to be very poor. The peak potentials, for both the anodic peak (Epa) and the cathodic peak (Epc), were found to increase with sweep rate (Table 1), a typical characteristic of the quasi-reversible redox process. The magnitude of the potential shift, however, was different for anodic and cathodic peaks. In the anodic step, the slope of the Epa versus log(V) plot was 30 mV, which corresponds to a calculated transfer coefficient, Rna, of 0.5. On the other hand, the potential shift in the cathodic sweep was much less, typically around 13 mV. This indicates that the charge transfer in the cathodic stripping process is faster than the anodic deposition. The anodic peak current was found to vary linearly with V1/2 typical of diffusion-controlled deposition of LiPc. Table 1 also includes the full width at half-maximum (Efwhm) of both the anodic and cathodic peaks for various sweep rates. The Efwhm of the anodic peak increases with an increase in sweep rate. This can be interpreted in two ways. Since the charge-transfer process is slightly sluggish unlike any normal nucleation process, peak broadening is possible.9 On the other hand, sharper peaks at low sweep rates may also be due to the fact that more time is provided for the established nuclei to grow steadily, and the

periphery of the growing nuclei expands two-dimensionally along the surface, as well as growing into the solution threedimensionally, so that more LiPc is deposited, leading to the steep rise in current. In other words, the diffusion layer thickness is smaller in the low potential sweep rate ranges. The latter explanation seems to be the most likely case for the following reasons. This is consistent with the observation that at low sweep rates the diffusion tail of the anodic peak does not decline as steeply as in higher scan rates. Also the total charge (Qt) of both anodic and cathodic peaks (Table 1) obtained from the integration decreases with an increase in the potential sweep rate, confirming that the amount deposited in the slow sweep rates is quite high. The deposition and stripping phenomenon is also evident from Figure 2D, where the total charge dispensed during both deposition and stripping is proportional to time. Another interesting aspect observed in the redox behavior of LiPc is that the diffusion control of the reverse cathodic sweep, where the peak current shows linear variation with V1/2. The diffusion control of the anodic peak is understandable in that the electroreactant, namely, Pc2-, is transported to the electrode by diffusion through the diffusion layer in the bulk solution. But the stripping process in any solid electrode is a surface process, and hence the peak current is expected to increase linearly with the scan rate.8 The present case, however, has phenomenological similarity to the metal ion stripping from Hg film electrodes.12 On Hg film, as soon as the reduction of metal ions occurs, they are amalgamated and homogenized into the bulk volume of the Hg film. Thus, in the stripping process, the diffusion layer is set in the Hg, since the surface to volume ratio is relatively higher.12 Since there is no such possibility with the solid Pt electrode, it is more likely that the LiPc formed on oxidation is not stuck on the surface unlike any metal ion deposition; instead it either goes into the solution phase or is adsorbed poorly on the surface, and growth occurs away from the electrode surface. This apparently results in the solutionphase redox-like characteristics. This hypothesis is further supported by the observation that Qt of the forward anodic sweep is higher than Qt of the reverse cathodic sweep (Table 1). It ultimately shows that not all the deposition formed on the forward step is reduced back to LiPc-, as it should be in the case of the surface redox process.8 Thus, LiPc deposition appears more like a conducting polymer phase formation, where the monomeric compound is oxidized, and the oxidized species goes back into solution to chemically react with similarly generated species, and the polymer chain grows.13 Once the concentration of the oligomer near the electrode exceeds the solubility, it is then adsorbed onto the surface. Electrocrystallization and Growth Mechanism of LiPc. In any case of preparation of a desired material through electrodeposition, one has to establish the mechanism of growth, since the very nature of growth can affect both the size and properties

EPR Studies of Lithium Phthalocyanine


Figure 3. Chronoamperograms of 25 mM Li2Pc in dry acetonitrile containing 0.1 M TBAP as supporting electrolyte. (A) Set of i-t curves at various stepping potentials as indicated in the figure. The inset shows a magnified view of t0 and region I. (B) Variation of i - i0 with (t - t0)1/2 corresponding to the rising portion (region II) of i-t curves. (C) Nondimensional plots of (i/im)2 versus t/tm. The two extreme cases (lines a and b) of progressive and instantaneous nucleation mechanisms were simulated using the equation (i/im)2 ) [1.2254/(t/tm)]{1 - exp[-2.3367(t/tm)2]}2 and (i/im)2 ) [1.9542(t/tm)]{1 - exp[-1.2564(t/tm)]}, respectively [ref 17b,c]. The points shown in the figure correspond to the experimental values at stepping potential 0.085 V. (D) Cottrell plot obtained at different stepping potentials as indicated in the figure.

of the resulting crystals.5 Despite the fact that there has been a great interest shown in such an evaluation of the deposition and growth mechanism of metals, few insights have been gained in the case of organics or organometallics. This is because the issue in question, namely, the formation of a “critical nucleus or ad atom” after the charge-transfer step, is different in the case of deposition of organics. In the case of metal ion deposition, the metal atom simply sticks to the surface as an adatom and forms the critical nucleus. But in the case of organics, the deposits are formed on the surface after charge transfer followed by some chemical steps. These coupled chemical steps in many organic depositions make the processes very complicated. The best example is the case of conducting polymers such as polyaniline and polypyrrole, where the chain growth reaction of the polymer is interposed between charge-transfer and deposition steps as discussed above.13 In the case of electrocrystallization of low-dimensional solids such as LiPc, the solubility product of the depositing material dictates the conditions required for critical nucleus formation. Surface deposition occurs when the concentration near the electrode surface exceeds the solubility product.5 The potential step chronoamperometric technique, where the change in the current

is followed with time after stepping the potential to a desired value, has been established as a superior method for studying phase formation kinetics and the growth mechanism. The advantage of this method in studying phase formation and crystal growth is that the electrode can act as an initiator and facilitate deposition of the substrate.13c This approach has been very successfully used in metal deposition for which various theories and models have been extensively worked out.14 A series of potentiostatic i-t transients were recorded in the 0-10 s window for different stepping potentials (Es), as illustrated in Figure 3A. These experiments were carried out by stepping the potential of the working electrode from an initial value, -0.2 V, where there is no faradaic process, to a set final potential, Es. In accordance with the CV results above, chronoamperograms at different potentials showed the characteristics of the phase formation process following the nucleation step.14 The i-t curves in Figure 3A have three different regions, namely, I-III. When the working electrode potential is stepped up, the current increases instantly due to double-layer charging, and with time it monotonically comes down to the background current (region I; see the inset in Figure 3A) in general for solid electrodes, in less than 1 s.9 If the electroactive species is present


Ilangovan et al.

in solution, then the oxidation/reduction of this species overtakes. In such a case, instead of the current coming down to the baseline in milliseconds, a smooth exponential decrease in current is noticed due to the charge-transfer process over time, and the course of the decreasing pattern of the i-t curve depends on the mechanism of charge transfer.15 As a matter of fact in the present case (Figure 3A), an increase in current in region II is observed after double-layer charging. That is, the i-t transients result in well-defined peaks. This rise in current is due to an increase of the electroactive area due to the formation of conductive deposits, and this new heterogeneous phase behaves as an extension of the electrode surface. In the present case, the extended electrode surface grows three-dimensionally into the solution, and further oxidation of Pc2- to LiPc occurs on the crystal face normal to the stacking axis because of the high degree of conductivity anisotropy in one-dimensional conductors (σ|/σ⊥ = 103). This results in the observation of crystalline needlelike structures of LiPc, as later confirmed by micrographs. After the current rises to a maximum, it starts once again to decrease in region III. It can also be seen that the decreasing portion of the i-t curve in region III is merged into a common line at higher Es, typically at +0.1 and +0.125 V in Figure 3A. The principal nature of the i-t curve is the rising portion (region II) corresponding to the growth of the electroactive area, as established nuclei grow and new nuclei are formed. The nature of this rising portion reflects the mechanism of growth. The relation of i with respect to t in this time domain is different for different mechanisms of growth.16 By establishing the powerlaw relationship of the current and time corresponding to the rising portion, the respective mechanism is predicted. Various relationships have been derived between the variants time (t t0, where t0 is the time at which the rising portion starts; see the Figure 3A inset) and current in this region for various deposition mechanisms. The variation of current in this region of the present study is found to be linear with (t - t0)1/2 (Figure 3B), indicative of instantaneous nucleation and mass-transfercontrolled growth of hemispherical nuclei as required by the equation,17

i - i0 ) nFπN(2Dc)3/2M1/2(t - t0)1/2/F1/2

(1)

where nF is the molar charge of the depositing species, D is its diffusion coefficient, c is its concentration (mol‚cm-3), M is molecular weight, and F is the density. Diffusion control is also evidenced from the fact that the i-t curve in region III merges into a common line at higher stepping potentials. The instantaneous nucleation model represents a situation in which all nucleation sites are activated at the beginning, t0. The characteristic parameter N in the above equation defines the number of nuclei on the surface known as the “number density”, and it depends on the stepping potential (Es), but for a given Es it virtually remains constant throughout the time of the experiment until the peripheries of the nuclei overlap, which is highly improbable as described below, according to this mechanism.17 The above equation is applicable to a single growing nucleus without any other influences such as overlap of similar nuclei and thus considered as an ultramicroelectrode. However, in practice on any conventional electrode getting an isolated nucleus is impossible since the area of any conventional electrode is relatively large, a huge number of similar nuclei appear simultaneously, and their direct or indirect influence on each other is inevitable. Once the nuclei are formed on the surface, all of them grow with the same rate throughout the experiment time. As time progresses, the hemispherical diffusion

TABLE 2: Chronoamperometric Analysis of Nucleation and Growth of LiPc on a Pt Electrodea Es/V

to/s

(tm - t0)/s

106 (im - i0)/A cm-2

10-7N/cm-2

0.065 0.075 0.085 0.100 0.125 0.150 0.175 0.200

0.020 0.020 0.020 0.020 0.020 0.015 0.015 0.010

1.560 1.150 0.590 0.275 0.105 0.050 0.029 0.015

6.4957 20.1000 30.8428 59.5714 110.5713 234.8571 306.8453 438.8345

1.3963 1.8941 3.6919 7.9209 20.7457 43.5649 75.1118 145.2162

The number density for higher stepping potentials beyond +0.2 V could not be obtained since the tm was very less, overlapping with t0. a

or depletion zones (area under which a concentration gradient sets up due to electrochemical reaction) about each nuclei develop since the growth is controlled by spherical diffusion (where the flux depends on the radius of the diffusion zone) of Pc2-. Inside each of the diffusion zones about a nucleus, the concentration of Pc2- decreases exponentially with distance toward the nucleus.9 These diffusion zones around growing nuclei advance readily and much more rapidly than the perimeters of the nuclei themselves, and at the focus of the diffusion zone the nuclei act as a point sink.17b Since the diffusion zones of nuclei grow much faster than the nuclei peripheries themselves, they overlap soon with neighboring diffusion zones, leading to the current as observed in Figure 3A. Once the overlap occurs, the diffusion is no longer spherical and transforms to linear diffusion (where the flux is independent of the radius of the diffusion zone). This problem of overlap of diffusion zones has been treated by different groups, and various conflicting formulations have been presented.17 In all these cases, the major difference of opinion is in quantifying the extended electrode surface area in the case of progressive nucleation. However, all these models agree that in the case of instantaneous nucleation the original theory proposed by Scharifker et al.17a,b is appropriate as recently proved.17f Further validation of instantaneous nucleation is also obtained from commonly used “nondimensional” plots suggested by Scharifker, as illustrated in Figure 3C. The experimental data obtained match the instantaneous nucleation model. However, deviations are seen in Figure 3C in region III. This is presumably due to the fact that this model assumes overlap of hemispherical diffusion zones although in the present case they are not perfect hemispherical diffusion zones as described below. The nucleation parameters are estimated as follows. The time to reach the maximum (tm) and the current at the maximum (im) are dependent on the Es (Table 2). When the Es becomes more positive (or the applied overpotential, Es- E0 gets larger), the number density increases, and this shortens the time required for the overlap of the diffusion zones. As a result, the observed im increases while tm decreases, as the stepping potential becomes more positive. N is obtained from tm as follows:

N ) 1.2564/tmπkD

(2)

where the constant k is defined as k ) 4/3(8πcM/F)1/2. Table 2 summarizes the calculated N values for various stepping potentials. The D value required in the estimation of N was obtained from the current transients at long times of Es of +0.15 and +0.175 V (Figure 3D), as the current decay is described by the Cottrell equation

i ) (nFAcD1/2)/(π1/2τ1/2)

(3)



Figure 4. Micrographs of LiPc obtained at various deposition potentials, magnification 400, electrolysis time 1 h.

where A is the area of the electrode. The average D value was 2.40 × 10-8 cm2 s-1. This value of the diffusion coefficient is smaller but not uncommon as many of the macrocyclic redox proteins have a similar range of D values.17h The N values in Table 2 are on the order of 105-107 cm-2 as in the case of many depositions undergoing this mechanism in aqueous and molten salts.18 The above results show an important conclusion that the LiPc deposition mechanism does not change with Es, as reported in some cases that it switches over from progressive to instantaneous nucleation.17c,e XRD and EPR Analysis of Electrodeposited LiPc. After the redox behavior and electrochemical phase formation mechanism were understood, the LiPc was electrocrystallized at four different electrolysis potentials, namely, +0.1, +0.2, +0.4, and +0.7 V, covering a wide potential range. All four deposition potentials resulted in green-black microcrystalline powders. Microscopic examination of the powders obtained at +0.4 and +0.7 V (Figure 4) showed that needle-shaped crystals were obtained, confirming one-dimensional growth5 of LiPc. Since constant stirring was applied during the electrocrystalization, needles of various sizes are observed in Figure 4. The maximum size of crystals obtained from micrographs was about 80 µm. The needles were very fragile. On the other hand, in the lower deposition potentials, namely, +0.1 and +0.2 V, a considerable amount of the amorphous form was obtained.

EPR measurements were performed at the X-band to investigate the paramagnetic properties of these materials. Figure 5 illustrates the X-band EPR spectra obtained under N2 atmosphere for all four samples prepared at four different deposition potentials. The EPR spectra showed two symmetric components, designated here as A and B, with two different line widths for the samples crystallized at +0.1 and +0.2 V. But in cases of samples obtained at 0.4 and 0.7 V, only one prominent peak, A, was observed. In general, the EPR spectra of the powder samples did not show any significant anisotropy with respect to magnetic field orientation, although all three known crystalline forms show such orientation effects in single crystals.3 Since the total spectrum in any of these cases is symmetrical, showing that the species have slightly different g values and are π radicals, the only factor that differs among these lines is the relaxation time (line width), and this allows one to observe both lines separately. Moreover, as seen in Figure 5, the deposition potential has significant influence on the magnitudes of these peaks. The line, with very sharp line width (about 5 mG), is maximum in the samples obtained at electrolysis potentials +0.4 and +0.7 V. The anoxic line width of 5 mG obtained in the present work is smaller than the value 26 mG reported for the x form in the literature.3g The higher value reported previously might be due either to instrumental broadening or the small deformation of the x form. On the other hand, the powders


Ilangovan et al.

Figure 5. X-band EPR spectra of LiPc under nitrogen for samples obtained using various deposition potentials. Spectral acquisition parameters were modulation frequency 12.5 kHz, modulation amplitude 25 mG, microwave power 0.79 mW, time constant 2.5 ms, number of scans 3, and scan time 5 s.

Figure 6. Powder XRD patterns of LiPc obtained at various deposition potentials. Cu KR radiation with λ ) 1.542 Å was used. The other experimental conditions were scan width 0.04° and counting time 6 s. The peaks are indexed on the basis of refs 3f,g and 19c.

obtained at electrolysis potentials +0.1 and +0.2 V have the maximum proportion of the component with large line width. It is not clear whether the two components observed are due to two different crystalline forms or due to two magnetically different species of a single crystalline phase. Thus, the samples were subjected to XRD analysis to evaluate the crystalline nature. The XRD patterns of the samples obtained are shown in Figure 6. The diffraction peaks were analyzed on the basis of the information available in the literature.3f,4a,19 In general, almost all major reflections are well-resolved, indicating that the resultant powder in all four experiments are with a high degree of uniaxial ordering of the microcrystalline domains. However, comparison of the magnitude of peaks shows that the intensity is 3 times higher for the samples deposited at +0.4 and +0.7 V than the samples deposited at +0.1 and +0.2 V. This is consistent with the micrographs presented in Figure 4. In all four samples one can consistently notice the occurrence of one prominent diffraction peak, at a 2θ value of 6.28° corresponding to the (100) plane reflection of the x form.3f Also it is seen that the magnitude of this is comparable for the samples +0.1 and +0.2 V as well as +0.4 and +0.7 V. Although the formation of the R form is common for phthalocyanines3f in

chemical vapor deposition at room temperature, in the case of LiPc it has been found that the preferential formation of the x form is due to the lower oxidation degree of lithium (+1) as compared to the metal phthalocyanines such as CuPc or NiPc. As a consequence the net negative charge on the phthalocyanine macrocyclic ring is reduced so that the magnitude of the interaction among the macrocycles in the stack as well as in the nearby stacks is affected. However, it is important to notice that the powder patterns obtained for samples of different deposition potentials showed significant variations. Apart from the (100) plane reflection (intensity scaled to 100%), other major reflections corresponding to (200), (300), (221), and (002) planes were also observed as described in Figure 6. On the other hand, samples electrodeposited at +0.1 and +0.2 V showed additional diffraction peaks, which cannot be attributed to any diffraction of the x form (Figure 6). This is contrary to the previous report that electrochemical oxidation of Li2Pc exclusively yields the x form of LiPc films in acetonitrile.3g The potentiostatic deposition used in the present case clearly demonstrates that the LiPc phase formation depends on the electrodeposition potential. This observation is very critical since only the x form is active for Heisenberg spin exchange as described in the Introduction, and useful in EPR



Figure 7. EPR line-shape analysis of the LiPc obtained at various deposition potentials. The theoretical line calculated from the optimized parameters, fwhm and intensity, is shown as the solid line, while the points represent the experimental values, acquired at anaerobic conditions. The fitting program used the Levenberg-Marquardt optimization algorithm. The r2 values were better than 0.99 in all the cases.

oximetry. A careful examination of the XRD pattern (Figure 6) shows that the patterns are very similar for materials obtained at +0.4 and +0.7 V, which are far more positive than the formal redox potential (E0) +0.05 V. Also the XRD patterns are comparable for the samples obtained at +0.1 and +0.2 V, which are close to the E0. There are two possible ways to interpret the additional XRD peaks obtained for the latter cases. The first explanation is to consider this powder to be a mixture of two phases, among which one is the x form. The second possibility is to consider the deformation of the tetragonal unit cell of the x form as suggested by Brinkman et al.3f But the second possibility is highly improbable since it cannot explain the highintensity peak at a 2θ value of 9.22°. Moreover, the preparations were carried out in the same solvent (acetonitrile), and hence, the deformation of the tetragonal unit cell due to occlusion of solvent molecules cannot be a valid explanation. Thus, the additional peaks must be due to the presence of an additional phase. These additional peaks are analyzed on the basis of a report by Homborg et al.19c Actually these peaks do not correspond to the R form since the 100% peak is not observed at 2θ equal to 6.78°.3f Instead, it is observed at 9.22°, indicating the presence of the β form.19c The other major peaks of lower angle diffraction at 2θ values of 6.950° for (001), 14.07° for (002), and 18.65° for (101) are observed in accordance with the β crystal structure. The obtained intensities differed by less than 3%, especially in the lower diffraction angle, and this might be due to the absorption of X-ray at these angles as reported previously.3f These results conclusively prove that the samples obtained at +0.4 and +0.7 V are primarily the x form, while the other two obtained at +0.1 and +0.2 V are mixtures of β and x forms. The composition of the mixture is almost the same for samples obtained at both +0.1 and +0.2 V. Thus, the two lines observed in the EPR spectra for the +0.1 and +0.2 V samples are due to two different phases. Correspondingly, the only one extremely sharp EPR line observed for +0.4 and +0.7 V samples corresponds to the x form, and the additional broad line observed for +0.1 and +0.2 V may correspond either to

the β form or to the amorphous form. However, a small amount of the amorphous form of LiPc was present even in the samples obtained at +0.4 and +0.7 V, which is not reflected in XRD but seen in Figure 7 (see later). From the results of electrocrystallization studies of the present work, it is clear that the basic deposition mechanism is the same in all four potentials, while only the rate of deposition of LiPc is different at different potentials as reflected in the resulting number density. It seems the slow deposition of LiPc at lower deposition potentials such as +0.1 and +0.2 V, where the nucleation number density is low, leads to a mixture of x and β forms and also largely to the amorphous form. However, at higher potentials where the number density is higher, the x form is preferably formed. Quantitative analysis of these two phases was performed through EPR peak fitting analysis. The first-derivative EPR spectra were integrated to absorption shape, and the absorption peaks were analyzed. We performed the analysis on absorption profiles and obtained quantitative information regarding the composition of different species. Initial fitting analysis showed that the lines were very close to Lorentzian shape and could be fit to the function20

Y ) Ymax[Γ 2/Γ2 + (B - Br)2]

(4)

where B is the magnetic field and Br is the resonance field. Γ is the half-width at half-maximum. Figure 7 illustrates the curve fitting of the experimental EPR data with the two-peak model for different samples. It is seen from Figure 7 that the theoretical line shape calculated from the fit matches very well with experimental data, indicating that the Lorentzian model of shape describes the absorption profile appropriately. The line-shape analyses of the samples prepared at the different deposition potentials showed evidence for two components (A and B) with slightly different g factors. The relative intensities of these two components obtained from the double integration of the spectra depended on the deposition potential. For example, for samples prepared at +0.1 and +0.2 V, the broad line with a peak-to-


Ilangovan et al.

Figure 8. Effect of microwave power on the intensity and width (fwhm) of the two deconvoluted EPR absorption peaks, peak A and peak B, for samples obtained at +0.4 and +0.2 V, respectively.

peak separation of 0.65 G is maximum with more than 90% of the intensity. On the other hand, the samples prepared at deposition potentials +0.4 and +0.7 V showed the higher ratio of the narrow line width component to the broad peak component. This is consistent with the XRD pattern that the samples prepared at +0.7 and +0.4 V had almost only the x form while the other two samples contained a mixture of both x and β forms. Further experiments on saturation properties of the deconvoluted lines were performed. Such an experiment is very useful to define the right microwave power, to employ in oximetry experiments. Figure 8 illustrates the effect of the microwave power on the peak parameters, namely, the peak width and the intensity, for the samples obtained at +0.2 and +0.4 V. Similar trends were obtained for the other three samples. The peak parameters were obtained by EPR line-shape analysis. The full width at half-maximum (fwhm ) 2Γ) shown in the figures is related to the peak to peak width (∆Bpp) in the first-derivative EPR spectrum as follows:20

∆Bpp ) (2/x3)Γ

(5)

The intensity of the peak is obtained from the integrated area under the peak. It can be seen that the line width of the narrow peak corresponding to the x form remains constant in the lower power region and starts increasing beyond 50 µW. Also the intensity of this peak increases linearly with (power)1/2. However, at high power the intensity of the absorption curve no longer maintains linearity. It is clear from Figure 8 that the saturation of the x form occurs above 50 µW and for the β form understandably linearity is maintained even at higher power.

Figure 9. Effect of room air and nitrogen on the EPR spectrum of LiPc obtained using a deposition potential of +0.4 V. Spectral acquisition parameters were modulation frequency 12.5 kHz, modulation amplitude 2 mG, microwave power 2 µW, time constant 2.5 ms, number of scans 1, and scan time 5 s. The spectrum in room air was obtained with modulation amplitude and 200 mG and microwave power 25 µW.

EPR Oximetry Using LiPc. The four samples were tested for oxygen-induced line-broadening to evaluate their usefulness for EPR oximetry applications. The role of molecular oxygen on the EPR line width in thin films of x LiPc is well established.3 As a first step toward evaluating these samples for oximetry applications, the rapidity of oxygen diffusion (response time) for samples prepared at various deposition potentials was evaluated using the change in EPR line width with time in samples exposed to room air. The samples encapsulated in a gas-permeable Teflon tube inside a 3 mm quartz tube and kept under nitrogen at atmospheric pressure showed very sharp EPR lines. When the flow of nitrogen was stopped, allowing the room air to counterflow into the tube, broadening of the EPR signal was observed, due to diffusion of molecular oxygen inside the microchannels, replacing the nitrogen in thermodynamic equilibrium inside. Since the molecular dimensions of both oxygen (O2) and nitrogen (N2) are 2.8 × 3.9 and 3.0 × 4.1 Å2, respectively,21 they easily go through the microchannels of the x form LiPc with diameter inside the quadratic lattice about 6.0 Å as illustrated in Figure 1. A typical X-band EPR spectrum of the x form of LiPc at very low modulation (2 mG) and power (2 µW), under anoxic conditions, showed a peak to peak line width of 5 mG (Figure 9). When the sample was exposed to room air, the peak broadened and the amplitude decreased in a few seconds and a weak broad signal was observed. This indicates that the spin-spin relaxation time, T2, between O2 and the excited state of LiPc is shortened drastically. However, when the sample was subsequently resaturated with pure


J. Phys. Chem. B, Vol. 104, No. 17, 2000 4057 TABLE 3: Reduced Diffusion Coefficient, Dred, Obtained from the Diffusion Kineticsa Ed/V (Ag/AgCl)

Dred/s-1

Ed/V (Ag/AgCl)

Dred/s-1

+0.1 +0.2

6.73 × 10-4 7.95 × 10-4

+0.4 +0.7

9.57 × 10-4 4.75 × 10-4

2 aD red ) Dl , where D is the absolute diffusion coefficient and l is the length of the diffusion channel.

molecules. When exposed to O2, a fraction (c) of mobile spins are converted as “fixed” spin states by coupling with O2.3k Thus, the observed total Γ is represented by

ΓT ) (1 - c)Γ0 + cΓ1

(7)

where Γ0 and Γ1 are the widths corresponding to c ) 0 and c ) 1. Since Γ0 , Γ1 Figure 10. Diffusion kinetics of molecular oxygen into the microchannels of the x polymorph of LiPc, followed by means of the change in line width as a function of time for various samples.

nitrogen, the signal sharpened again and the amplitude fully returned (Figure 9). This was reproducible for more than 10 times of nitrogen-air cycling, suggesting that the process is reversible and there is no permanent change to the LiPc crystals due to oxygen. Figure 10 illustrates the oxygen responsivity of the samples measured as the change of line width with time. In all cases the EPR line corresponding to the x form was followed, although the fractions were smaller in the cases of the samples obtained at +0.1 and 0.2 V. The EPR line-broadening in the solid state occurs due to the diffusion of O2 into the microchannels and subsequent spinspin interaction through the Heisenberg exchange mechanism.22 Thus, the diffusion characteristics of these materials will be of fundamental interest in evaluating their efficiency for oximetry applications. All four samples were evaluated for oxygen responsivity. In all the cases, the sample was first flushed with N2 and the flow was stopped, allowing the room air (approximately 150 mmHg of pO2) to counterflow into the EPR tube, and the change in line width was followed with time (Figure 10). First, it is assumed that the particle sizes are random and are in the same order since the electrocrystallization was fixed to constant time in all four cases, as it is known that the size critically depends on the time of electrolysis. Second, the diffusion is assumed to be concentration-independent Fickiantype diffusion23 so that the diffusion coefficient D of O2 is expected to be constant at any given concentration. Third, it is also assumed that the microcrystals do not swell upon sorption of O2; i.e., at any time during the sorption the thermodynamic volume is maintained constant. On the basis of these assumptions, an appropriate solution of the diffusion equation may be written as23

Mt/M∝ )

∑[1/(2m + 1)2] exp{-D(2m + 1)2π2t/l2}

1 - (8/π2)

(6)

Here Mt is the total amount of O2 adsorbed by the crystals at time t and M∝ is the equilibrium sorption attained after infinite time. l is the length to which diffusion occurs. It should be remembered that the amount of O2 sorbed in EPR oximetry is obtained indirectly from the broadening of the line width. From the studies of Bensebaa and Andre3k the Mt can be correlated to Γt. In the absence of O2 the spin states exist in the “mobile” state since there is strong exchange between the stacked

ct ) (ΓTt - Γ0)/Γ1

(8)

Also assuming that this is true at any p(O2)

Mt/M∞ ) ct/c∞ ) (ΓTt - Γ0)/(ΓT∞ - Γ0)

(9)

Then eq 6 can be written as3k

(ΓTt - Γ0)/(ΓT∞ - Γ0) )

∑[1/(2m + 1)2] exp{-D(2m + 1)2π2t/l2}

1 - (8/π2)

(10)

The experimental data shown in Figure 10 showed significant deviations when we tried fit them to the above equation, especially in the lower time domains. This is understandable, since the samples have been encapsulated into gas-permeable Teflon tubes so that the diffusion characteristics of the Teflon tube may influence the curve. However, for the purpose of comparative evaluations, the above equation was used to obtain the reduced diffusion coefficient, Dred ) D/l2, to get an idea of which of the four samples is the most suitable for oximetry applications. Dred is estimated analytically as follows. The value of t/l2 for which Mt/M∝ ) 0.5 is defined as follows, with the approximate error being 0.001%:

D ) 0.049/(t/l2)0.5

(11)

The computed Dred values have been summarized in Table 3. It should be noted that the Dred values are not the absolute diffusion coefficients of the O2 into the crystals; it is also influenced by the Teflon surrounded by them. However, the influence is common in all the cases. It can be seen from these values that the samples obtained at deposition potential +0.4 V show a maximum value, indicating that the sample obtained at 0.4 V is the optimum for oximetry application. It is important to note here that while the other characterizations do not show any discernible difference between +0.4 and +0.7 V samples, the diffusion coefficient evaluation proves that the former is superior. As the next step for quantitative use of LiPc in oximetry measurements, a calibration curve was constructed using the change in the line width with partial pressure of oxygen pO2. Since the real application will be in tissues with a large aqueous environment, a calibration curve was obtained in aqueous solution using L-band EPR instrumentation. The LiPc was taken in a gas-permeable tube containing saline solution. The sample was first purged with nitrogen gas, and then the desired pO2 gas mixture was allowed to saturate. All the measurements were made after a constant peak amplitude was reached, typically


Ilangovan et al. at +0.4 V is very sensitive to molecular oxygen and hence is ideal for EPR oximetry. Oximetry experiments reveal that the x form of LiPc obtained by the present method is highly suitable for oxygen measurements in aqueous media. Further studies of the stability of LiPc in biological tissues with development of appropriate pretreatments to maximize biocompatibility will be needed to realize the full potential of this material as a biological oximetry probe. Acknowledgment. We thank Dr. A. Manivannan, Department of Physics, University of West Virginia, for his help in acquiring the XRD data and valuable discussion. This work was supported by National Cancer Institute Grant CA-78886 and NIH Grants GM-58582 and RR-12190. P.K. was also supported by an Established Investigator Award from the American Heart Association during the tenure of this study. References and Notes

Figure 11. Effect of the partial pressure of oxygen pO2 on the EPR spectrum of LiPc electrodeposited at +0.4 V: (a) 0 mmHg, (b) 20 mmHg, (c) 120 mmHg. The inset is a calibration graph in the form of line width versus pO2.

after about 10 min. A linear calibration curve, with the correlation coefficient 0.98 in the 95% confidence level, was obtained (Figure 11) for up to 100 mmHg pO2. The sensitivity estimated from the slope (9.56 mmHg/pO2) is approximately 1.5 times higher than that reported for the LiPc single-crystal oximetry experiments.1b This may be due to the fact that in the microcrystalline powder the surface area is very high and the molecular oxygen has to penetrate only on the micrometer scale to effectively exchange the spin state. These results indicate that this method using the material obtained in the present work can be applied to evaluate pO2 in any in vivo application in the partial pressure up to 100 mmHg, the case in many of the phathophysiological conditions. Further work on the measurements of in vivo oximetry in biological systems is in progress. Summary and Conclusions The present work indicates that the deposition potential plays an important role in the preparation of desired forms of LiPc crystals. Electrodeposition is found to follow the electrochemical phase formation route as in the case of any metal deposition. The detailed insights into the deposition mechanism indicate that the nucleation pathway is instantaneous and the threedimensional growth is controlled by the diffusion of the reactant through the bulk solution. While lower deposition potentials, which are very close to the E0 of the redox process, yield a mixture of x and β phases, the deposition potentials very far from the E0 yield almost exclusively the oxygen-sensitive x form. Consistently, the EPR studies show a single component with a sharp line for the x form. Two lines, a broad line with a line width of about 650 mG and another very sharp line with a line width of about 5 mG, corresponding to the x form and the β or amorphous form are observed for the former case. Quantitative EPR analysis also confirms that the ratio depends on the deposition potential. Needle-shaped microcrystals are formed at deposition potentials +0.4 and +0.7 V, and mostly amorphous powder was obtained at lower deposition potentials, +0.1 and +0.2 V. The EPR line shape of the crystals obtained

(1) (a) Liu, K. J.; Bacic, G.; Hoopes, P. J.; Jiang, J.; Du, H.; Ou, L. C.; Dunn, J. F.; Swartz, H. M. Brain Res. 1995, 685, 91. (b)Liu, K. J.; Gast, P.; Moussavi, M.; Norby, S. W.; Vahidi, N.; Walczak, T.; Wu, M.; Swartz, H. M. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 5438. (c) Smirnov, A. I.; Norby, S. W.; Walczak, T.; Liu, K. J.; Swartz, H. M. J. Magn. Reson. B 1994, 103, 95. (d) Dunn, J. F.; Ding, S.; O’Hara, J. A.; Liu, K. J., Rhodes, E.; Weaver, J. B.; Swartz, H. M. Magn. Reson. Med. 1995, 34, 515. (2) (a) Moussavi, M. U.S. Patent No. 5,112,597 May 12, 1992. (b) Afeworki, M.; Miller, N. R.; Devasahayam, N.; Cook, J.; Mitchell, J. B.; Subramanian, S.; Krishna, M. C. Free Rad. Biol. Med. 1998, 25, 72. (3) (a) Wachtel, H.; Wittmann, J. C.; Lotz, B.; Petit, M. A.; and Andre, J. J. Thin Solid Films 1994, 250, 219. (b) Brinkmann, M.; Wittmann, J. C.; Chaumont, C.; Andre, J. J. Thin Solid films 1997, 292, 192. (c) Brinkmann, M.; Turek, P.; Andre, J. J. Thin Solid Films 1997, 303, 107. (d) Wachtel, H.; Andre, J. J.; Bietsch, W.; von Shulz, J. U. J. Chem. Phys. 1995, 102, 5088. (e) Brinkmann, M.; Turek, P.; Andre, J. J. J. Mater. Chem. 1998, 8, 675. (f) Brinkmann, M.; Chaumont, C.; Wachtel H.; Andre, J. J. Thin Solid Films 1996, 283, 97. (g) Brinkmann, M.; Andre, J. J. J. Mater. Chem. 1999, 9, 1511. (h) Brinkmann, M.; Graff, S.; Chaumont, C.; Andre, J. J. J. Mater. Res. 1999, 14, 2162. (i) Andre, J. J.; Brinkmann, M. Synth. Met. 1997, 90, 211. (j) Wachtel, H.; Wittmann, J. C.; Lotz, B.; Andre, J.-J. Synth. Met. 1993, 61, 139. (k) Bensebaa, F.; Andre, J.-J. J. Phys. Chem. 1992, 96, 5739. (4) (a) Brinkmann, M.; Chaumont, C.; Andre, J. J. Thin Solid Films 1998, 324, 68. (b) Dumm, M.; Lunkenheimer, P.; Loidl A.; Assmann, B.; Homborg, H.; Fulde, P. J. Chem. Phys. 1996, 13, 5048. (5) Ward, M. D. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989; Vol. 16, p 181. (6) Petit, A.; Thami, T.; Even, R. Chem. Commun. 1989, 1059. (7) Pillai, K. C., Thangamuthu, R.; Ilangovan, G. Electroanalysis 1995, 7, 1182. (8) Murray, R. W., Ed. Molecular Designing of Electrode Surfaces; John Wiley and Sons: New York, 1992. (9) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications; Wiley: New York, 1980. (10) Fletcher, S.; Halliday, C. S.; Gates, D.; Westeoldt, W.; Lwin, T.; Nelson, G. J. Electroanal. Chem. 1983, 157, 267. (11) Suarez, M. F.; Marken, F.; Compton, R. G.; Bond, A. M., Miao, W.; Raston, C. L. J. Phys. Chem. B 1999, 103, 5637. (12) Wang, J. Stripping Analysis. Principles, Instrumentation and Applications; VCH: Deerfield Beach, 1985; p 24 (13) Bade, K.; Tsakova, V.; Schultza, J. W. Electrochim. Acta 1992, 37, 2255. (b) Li, F.; Albery, W. J. Electrochim. Acta 1992, 37, 393. (c) Li, F.; Albery, W. J. Langmuir 1992, 8, 393. (d) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 229. (14) Harrison, J. A.; Thirsk, H. R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1971; Vol. 5, p 67. (15) Bard, A. J.; Santhanam, K. S. V. In Electroanalytical Chemistry Bard, A. J., Ed.; Dekker: New York, 1970; Vol. 4, p 215. (16) Fleishmann, M.; Thirsk, H. R. In AdVances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Wiley-Inter Science: New York, 1963; Vol. 3, p 123. (17) (a) Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B. R. J. Electroanal. Chem. 1982, 138, 225. (b) Scharifker, B. R.; Hills, G. Electrochim. Acta 1983, 28, 879. (c) Scharifker, B. R.; Mostany, J. J. Electroanal. Chem. 1984, 177, 13. (d) Scharifker, B. R. J. Electroanal. Chem. 1998, 458, 253. (e) Sluyters-Rehbach, M.; Wijenberg, J. H. O. J.; Bosco, E.; Sluters, J. H. J. Electroanal. Chem. 1987, 236, 1. (f) Heerman,

EPR Studies of Lithium Phthalocyanine L.; Tarallo, A. J. Electroanal. Chem. 1999, 470, 70. (g) Scharifker, B. R.; Mostany, J.; Palomar-Pardave, M.; Gonzalez, I. J. Electrochem. Soc. 1999, 146, 1005. (h) Armstrong, F. A.; Cox, P. A.; Hill, A. L.; Lowe, V. J.; Oliver, B. N. J. Electroanal. Chem. 1987, 217, 331. (18) (a) Jacob, S. R.; Compton, R. G. J. Electrochem. Soc. 1999, 146, 2598. (b) Salinas, D. R.; Cobo, E. O.; Garcia, S. G.; Bessone, J. B. J. Electroanal. Chem. 1999, 470, 120. (c) Legrand, L.; Tranchant, A.; Messina, R. J. Electrochem. Soc. 1994, 141, 378. (d) Carlin, R. T.; Crawford, W.; Berch, M. J. Electrochem. Soc. 1992, 139, 2720. (e) Brouwner, D. H.; Baker, M. D. J. Phys. Chem. B 1997, 101, 10390. (f) Gunawardena, G.; Hills, G.; Montenegro. J. Electroanal. Chem. 1982, 138, 241. (g) Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B. J. Electroanal. Chem. 1982, 138, 255.

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Electrochemical Preparation and EPR Studies of Lithium

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