Enhancement of electron scavenging by water-soluble fullerenes

Abstract

Polyhydroxyfullerenes (PHFs) having enhanced electron scavenging capabilities have a ratio of non-hydroxyl functional groups to hydroxyl functional groups that is less than or equal to 0.3. When combined with a semiconductor photocatalyst, such as titanium dioxide nanoparticles, the PHFs provide a photocatalyst for degradation of chemical and biological contaminates with an efficiency of at least twice that of titanium dioxide nanoparticles free of PHFs. The PHFs are included in these catalysts at a weight ratio to titanium dioxide of about 0.001 to about 0.003, whereas significantly lower and higher ratios do not achieve the highly improved photodegradation capability. PHFs outside of the desired structure are shown to be of little value for photodegradation, and can be inhibiting to the photocatalytic activity of TiO.sub.2. The enhanced electron scavenging PHFs can be employed as a component of materials for solar cells, field effect transistors, and radical scavengers.

Claims

1. A polyhydroxyfullerene (PHF) with enhanced electron scavenging properties consisting of a fullerene (C.sub.n) wherein n is 20 to 500 having a multiplicity of hydroxyl (OH) substituents on said fullerene, wherein the PHF contains O, H and monovalent metal atoms with the C.sub.n atoms of the fullerene, wherein a ratio of non-hydroxyl functional groups to hydroxyl functional groups is less than or equal to 0.3 and wherein said PHF has an average of about 27 to about 48 total functional groups per 60 carbon atoms.

2. The polyhydroxyfullerene with enhanced electron scavenging properties of claim 1, wherein the polyhydroxyfullerene is C.sub.n(OH).sub.xO.sub.yM.sub.z; where M is a monovalent cation, n is 60, x+y+z is between 27 to 48, and (y+z)/x0.3.

3. The polyhydroxyfullerene with enhanced electron scavenging properties of claim 1, wherein said fullerene is C.sub.60, C.sub.70 or a mixture thereof.

4. An antioxidant, comprising the polyhydroxyfullerenes (PHFs) according to claim 1, wherein said PHFs have an average of about 27 to about 48 functional groups per 60 carbon atoms.

5. The antioxidant according to claim 4, wherein the PHFs have electron scavenging capabilities and are processing components of a biological system in aqueous solution for electronic devices.

6. The antioxidant according to claim 4, wherein the PHFs have radical scavenging capabilities and are radioprotectants in a biological system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows atmospheric pressure chemical ionization mass spectra (APCI-MS) of PERC PHFs (enhancing) and BuckyUSA PHFs (inhibiting).

(2) FIG. 2 shows Fourier Transform Infrared Spectra of PERC PHFs (enhancing) and BuckyUSA PHFs (inhibiting).

(3) FIG. 3 shows the structure of a C.sub.60(OH).sub.24 molecule calculated by Gaussian simulation.

(4) FIG. 4 shows a vibrational spectrum for C.sub.60(OH).sub.24 calculated by Gaussian simulation.

(5) FIG. 5 shows the C1s signals in XPS spectra of PERC PHFs (enhancing) and BuckyUSA PHFs (inhibiting). Each spectrum is deconvoluted into three signals for non-oxygenated, mono-oxygenated and di-oxygenated carbons.

(6) FIG. 6 shows thermal gravimetric analysis (TGA) spectra of PERC PHFs (enhancing) and BuckyUSA PHFs (inhibiting).

(7) FIG. 7 shows first-order degradation kinetics plots for Procion Red MX-5B under UVA irradiation using TiO.sub.2, TiO.sub.2 with BuckyUSA PHFs (enhancing), and TiO.sub.2 with PERC PHF (inhibiting) as photocatalysts. Error bars represent plus or minus 1 standard deviation.

DETAILED DESCRIPTION OF THE INVENTION

(8) The present invention is directed to PHF compositions that result in an enhanced electron scavenging ability. In embodiments of the invention the enhanced electron scavenging PHFs are combined with other materials or employed in devices that exploit the enhanced electron scavenging PHFs. The functionality of PHF influences its electron scavenging ability. PHFs commonly contain functional groups such as hydroxyl, hemiketal, epoxide and carbonyl groups that modify the electronic properties of the PHFs. It was discovered that PHFs which have a low ratio (<or=0.3) of non-hydroxyl functional groups to hydroxyl functional groups have enhanced electron scavenging ability, whereas PHFs that display a ratio higher than 0.3 have little or no electron scavenging ability. It was also discovered that PHFs where the weight loss to temperatures of about 1,000 C. is less than about 55% exhibit enhanced electron scavenging ability, whereas PHFs which display weight loss of more than 80% have little or no electron scavenging ability and can even promote inhibition of processes requiring an electron scavenger. PHF compositions that achieve higher electron scavenging can enhance processes such as photocatalysis.

(9) PHFs can be of a single size or can be mixtures of different fullerene sizes. The fullerene cage can be C.sub.28, C.sub.32, C.sub.44, C.sub.50, C.sub.58, C.sub.60, C.sub.70, C.sub.84, C.sub.94, C.sub.250, C.sub.540, or any other fullerene. The PHFs have an average of about 1.25 to 3 C atoms per OH group, which is equivalent to about 27 to about 48 OH groups on a C.sub.60 cage. The PHFs are often C.sub.60 molecules due to their commercial availability, but C.sub.70, C.sub.82 or their mixtures or other PHFs can be used in various embodiments of the invention. The PHFs have CC single bonds that can be observed by Fourier transform infrared spectroscopy (FTIR). Other functional groups are primarily carbons of a hemiketal and carbonyl structure. PHFs can also contain epoxy groups and ester groups. As the PHFs of the present invention are water soluble, they can be incorporated into devices that can exploit their enhanced electron scavenging capabilities and permit fabrication or use in an aqueous environment. Hence, the enhanced electron scavenging PHFs can be used in biological systems or permit processing from aqueous solution for electronic devices.

(10) In an embodiment of the invention, a photocatalytic composition of scavenging enhanced PHFs with semiconducting photocatalyst nanoparticles comprises a ratio of PHF/photocatalyst of about 0.001 to about 0.003 in aqueous suspension at about pH 6. In an embodiment employing TiO.sub.2 as the photocatalyst, the photocatalytic activity of the inventive composition is at least two times the photocatalytic activity of TiO.sub.2, absent the PHFs. The TiO.sub.2 concentrations can be from about 10 to about 100 mg/L. The TiO.sub.2 nanoparticles can range from about 2 to about 100 nm in diameter. The PHFs have CC single bonds that are observable by Fourier transform infrared spectroscopy (FTIR). Other functional groups are primarily carbons of a hemiketal and carbonyl structure. PHFs can also contain epoxy groups and ester groups as long as the ratio of non-hydroxyl functional groups to hydroxyl functional groups is 0.3 or less.

(11) Among the semiconducting photocatalysts that can be used for the practice of the invention are particles of titanium oxide, anatase titanium oxide, brookite titanium oxide, strontium titanate, tin oxide, zinc oxide, iron oxide, and mixtures thereof. Particles can range from about 2 to 500 nm maximum cross section or diameter. Particles can range from 2 to 100 nm in average diameter or cross section. The particles can be spherical or any other shape. Another embodiment of the invention including semiconducting photocatalysts with PHFs is a method to decontaminate a surface or a fluid in contact with the surface. The surface is treated with semiconducting photocatalytic nanoparticles with the proper proportions of scavenging enhancing PHFs or other functional fullerenes. The fluid in contact with the photocatalyst can be a liquid, generally an aqueous solution, or a gas. Irradiation of the photocatalyst results in the decomposition of chemical or biological contaminates. The irradiation source can be ultraviolet (UV) or visible and can be from a natural or artificial source. For example, sunlight can be used for the irradiation of the photocatalyst or light from a lamp can be directed to the photocatalyst. The system can be employed on exterior surfaces for passive cleaning in air ventilation systems or in water purification systems where the photocatalyst is restricted to a desired region to function as a decontaminating agent. The fluids can be forced into contact with the photocatalyst and recirculated to promote partial to complete decontamination of the fluid.

(12) In other embodiments of the invention the electron scavenging enhanced PHFs as an electron acceptor are combined with an electron donor to form a heterojunction organic solar cell's active film. The electron donor can be a low band gap conjugated polymer, for example polythiophene, substituted polythiophene, polyvinylenephenylene substituted polyvinylenephenylene, polybenzothiadiazole, substituted polybenzothiadiazole, polypyrroles, substituted polypyrroles and regular or random copolymers thereof. The active film can also have included nanoparticulate semiconducting photocatalyst, such as TiO.sub.2. Conjugated polymers that are water soluble due to substitution can be combined with the water soluble PHFs to fabricate an active film of a heterojunction organic solar cell.

(13) In other embodiments of the invention, the electron scavenging enhanced PHFs are used as the organic semiconductor of n-channel organic thin-film field effect transistors (FETs). In another embodiment of the invention, the electron scavenging enhanced PHFs can be combined with p-channel organic components, such as polythiophenes, to form ambipolar organic field effect transistors.

(14) In another embodiment of the invention, the electron scavenging enhanced PHFs can be employed as a radical scavenger or antioxidant. The electron scavenging enhanced PHFs can be employed as a radioprotectant for an organism that experiences exposure to ionizing radiation, such as X-rays.

Materials and Methods

(15) Two types of polyhydroxyfullerenes, both synthesized by an alkali route, were tested: 1) PHF from BuckyUSA (BuckyUSA PHF) and 2) PHF synthesized in the laboratory (PERC PHF). The laboratory synthesis was carried out in a manner derived from that disclosed in Li et al. J. Chem. Soc.-Chem. Commun., 1993, 1784. A solution of non-derivatized fullerenes was prepared by adding 80 mg of C.sub.60 (95%, BuckyUSA, Houston Tex.) to 60 mL of benzene (HPLC grade, Fisher). A mixture of 2 mL of NaOH solution (1 g/mL) and 0.3 mL of tetra butyl ammonium hydroxide (40% solution) was prepared in a separate Erlenmeyer flask. The fullerene solution was added to the alkali-surfactant solution under vigorous stirring. The stirring was stopped after 30 minutes and the mixture was allowed to phase separate. The top clear phase was decanted and the remaining slurry was stirred with an additional 12 mL of deionized water for 24 hours. The mixture was filtered through Whatman 40 filter paper and the filtrate was concentrated to 5 mL in a vacuum oven at 60 C. The resultant slurry was washed four times with 50 mL of methanol by alternate centrifugation (5000 g, 10 min) followed by resuspension in methanol. After the final wash, PHF were suspended in 20 mL of methanol and dried under vacuum at 60 C. The mass of PHF obtained was 120 mg. The PHF samples were analyzed with atmospheric pressure chemical ionization (APCI) mass spectroscopy (MS) (ThermoFinnigan, San Jose, Calif.).

(16) Both the BuckyUSA and PERC PHFs were characterized by Diffuse Reflectance Infrared Fourier Transform (DRIFT) and x-ray photoelectron spectroscopy (XPS). DRIFT experiments were carried out with Thermo Electron Magna 760 unit with potassium bromide as the background. XPS experiments were performed with Kratos Analytical Surface Analyzer XSAM 800 in survey and multiplex mode. The carbon 1s (C1s) spectrum was subjected to peak fitting analysis using Grams 7.01 software (Thermo Fisher Scientific, Waltham, Mass.) to determine the oxidation states of carbon.

(17) Characterization of PHF Samples

(18) Mass Spectroscopy

(19) Mass spectroscopy was employed to investigate the stability of the fullerene cages of the BuckyUSA and PERC PHF samples. The portions of the MS spectra in the range of 700-850 m/z for the two samples are presented in FIG. 1. In both samples, the base fullerene (C.sub.60) peak at 720.6 m/z is present at a relative intensity of 100, indicating that the fullerene cage is intact.

(20) FTIR Analysis

(21) FTIR spectroscopy has been employed previously to identify functional groups present on a fullerene cage. The FTIR-DRIFT spectra for BuckyUSA and PERC PHF are presented in FIG. 2. The PERC PHF exhibits five peaks at 3300, 1591, 1450, 1062, 881 and 471 wavenumbers along with shoulders at 1661, 1357 and 1165 wavenumbers. The BuckyUSA PHF has peaks at 3300, 1595, 1408 and 1074 wavenumbers with a shoulder at 1680 cm.sup.1. The vibrational modes of PHF were assigned to FTIR peaks of PHF based on the information obtained from a Gaussian simulation and the literature, as shown in Table 1, below. The peak at 1591 was not observed in the simulation and was ascribed to CC vibrations as reported in the literature. The shoulders at 1658 cm.sup.1, 1357 cm.sup.1 and 1165 cm.sup.1 were attributed to hemiketal, epoxides and esters respectively.

(22) TABLE-US-00001 TABLE 1 Assignments of FTIR peaks for PERC and BuckyUSA PHF based on results from Gaussian simulation of C.sub.60(OH).sub.24 and literature. Peak Assignments in cm.sup.1 based on Peak Location Gaussian simulation in cm.sup.1 based on and/or literature Vibration Gaussian BuckyUSA modes Simulation Literature PERC PHF PHF OH stretching 3450 3420, 3430, 3300 3300 3410, 3300 Hemiketal 1658 1661 1680 CC stretching 1595, 1600, 1591 1595 1593, 1585 COH 1450 1392, 1412 1450 1408 bending Epoxides 1376 1357 CO stretching 1070 1084, 1070, 1062 1074 1065 Esters 1197 1165 CC 490 471

(23) Semi-empirical computation (PM3) has been employed in the literature for structure optimization of various hydroxylated fullerenes and possible stable isomers are reported in the literature. However, no reports are present on theoretical prediction of vibrational peaks for hydroxylated fullerenes. Therefore, hybrid quantum chemical computations were carried out to identify the vibration modes responsible for experimentally observed FTIR peaks. For ease of simulation, 24 hydroxyl groups and no carbonyl, hemiketal and epoxide functionalities on the PHF were assumed. The structure optimization and vibration spectrum generation were performed with hybrid quantum-chemical basis set (B3LYP 6-31G*). The optimized structure, as shown in FIG. 3, is similar to the reported C.sub.60(OH).sub.24 structure obtained by a semi-empirical quantum-chemical optimization (PM3), as reported in Slanina et al., Proceedings of the Electrochemical Society, 1996, 96, 987. The hydroxyl groups are present as intramolecularly hydrogen bonded islands on fullerene cage, similar to the results found in the literature. The average CO and OH bond lengths are 1.43 and 0.98 , respectively, and the average COH bond angle is 107. These results are similar to the values obtained with PM3 optimization for C.sub.60(OH).sub.26 by Guirado-Lopez et al., J. Chem. Phys., 2006, 125. Additionally, weak intramolecular hydrogen bonding was observed with an average bond length of 1.78 , which is similar to the hydrogen bond length in water.

(24) The vibration spectrum generated for C.sub.60(OH).sub.24 with the B3LYP 6-31 G* basis set is presented in FIG. 4, with wavenumbers increasing from left to right on x axis. The simulated FTIR peak exhibits four major peaks. The broad peak at 3450 cm.sup.1 originates from OH stretching. The peak at 1450 cm.sup.1, 1070 cm.sup.1 and 370 cm.sup.1 represents COH bending, CO stretching and OH rocking vibrations, respectively.

(25) Both samples of PHF have hemiketal and epoxide groups in addition to hydroxyl groups, as revealed by FTIR and XPS analysis. This is consistent with reports in the literature. Rodriguez-Zavala et al., J. Phys. Chem. A, 2006, 110, 9459, discloses theoretical studies on hydroxylated fullerenes with different numbers of epoxide groups, where the presence of epoxide groups on hydroxylated fullerenes can have significant effects on their structure and electronic and optical properties.

(26) XPS Analysis

(27) PHFs have hemiketal, epoxide and sometimes carbonyl functional groups in addition to hydroxyl groups. Therefore, extensive characterization of PHF as well as determination of its empirical formula is necessary to compare the properties. Unfortunately, there is a lack of consistency in literature on techniques employed for determining the empirical molecular formula of PHF. Three common methodologies employed are elemental analysis, thermo gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). It is known that the presence of residue can influence the empirical formula determined by TGA and elemental analysis. Therefore XPS data was used to determine the empirical formulas for PERC and BuckyUSA PHF samples.

(28) XPS analysis indicated the presence of C, O and Na in both PERC and BuckyUSA PHF samples. The relative atomic concentrations determined for each element are provided in Table 2, below. The C1s region was deconvoluted as three Gaussian curves revealing the presence of three oxidation states of carbon, as shown in FIG. 5, which is in agreement with the literature. Since no carbonyl peak was present in FTIR spectra, the highest oxidation state was assumed to be due to hemiketal structure as revealed from FTIR. The curve fitting analysis of the three Gaussian peaks was performed for the C1s spectrum of PERC and BuckyUSA PHF to calculate the relative concentration of each carbon oxidation state (Table 3). The relative concentration of mono-oxygenated carbon is three times higher in PERC PHF than BuckyUSA PHF.

(29) TABLE-US-00002 TABLE 2 Elemental Composition of PHFs by XPS Relative Non-Hydrogen Atomic Concentration in Percent Element PERC PHF BuckyUSA PHF Carbon 61.23 47.33 Oxygen 28.44 33.82 Sodium 10.33 18.85

(30) TABLE-US-00003 TABLE 3 Peak Position and Elemental Composition of PHFs by XPS Analysis Peak Position Relative Oxidation in eV Concentration in % State of BuckyUSA PERC BuckyUSA Carbon PERC PHF PHF PHF PHF Non-oxygenated 284.8 284.8 40.2 57.1 Mono-oxygenated 286.16 286.14 47.1 16.1 Di-oxygenated 288.55 287.91 12.7 26.8

(31) The relative atomic concentrations of C, O and Na and along with concentrations of mono- and di-oxygenated states of carbon were employed to deduce the composition of PHF using the assignment as per Husebo et al., J. Amer. Chem. Soc., 2004, 126, 12055. The molecular formula for PERC PHF was calculated as C.sub.60O.sub.8(OH).sub.28Na.sub.10 and for BuckyUSA PHF as C.sub.60O.sub.16(OH).sub.10Na.sub.24. The empirical formulas are consistent with the number of functional groups added to fullerene cage as being in the range of 25 to 42; however, the PERC PHF displayed a higher number of functional groups per molecule than BuckyUSA PHF.

(32) TGA Analysis

(33) Thermo gravimetric analyses (TGA) of PERC and BuckyUSA PHF samples are presented in FIG. 6. The weight loss generally occurs in 4 or 5 stages. The first stage of weight loss is attributed to desorption of physically adsorbed water and accounted for 16% of the weight of both PHF samples. The second stage is attributed to desorption of hydroxyl functional groups and was 20% and 21% of the weight of PERC and BuckyUSA PHF, respectively. The weight loss in stage 3 is attributed to desorption of hemiketal functional groups and accounted for 11% of weight of PERC PHF and 34% of the weight of BuckyUSA PHF. A small weight loss (6%) in stage 4 was observed for PERC PHF and is attributed to desorption of carbonyl or epoxide groups. No weight loss in stage 4 was observed in BuckyUSA PHF, suggesting an absence of carbonyl or epoxide groups. BuckyUSA PHF exhibited thermal degradation of structure resulting in further 19% weight loss, stage 5. No loss in stage 5 was observed for PERC PHF. The total weight loss up to a temperature of 1,000 C. was 54% for enhancing PHF. TGA shows that the ratio of the weights of hemiketal to hydroxyl groups is higher for BuckyUSA PHF. A higher proportion of hemiketal to hydroxyl groups was also observed by XPS.

(34) Where R is defined as the ratio of other functional groups to hydroxyl groups, as determined by XPS analysis, the values of R for PERC and BuckyUSA PHF are 0.27 and 1.66, respectively. The higher R for the BuckyUSA PHF correlates to a lower stability PHF, as observed by thermal degradation at temperatures greater than 800 C., as indicated in FIG. 6.

(35) Enhancement of Electron Scavenging Ability of PHF

(36) Enhancement of the electron scavenging ability of PHFs by controlling the composition of the PHFs was undertaken by comparing dye degradation in the presence of different PHFs with TiO.sub.2.

(37) Photocatalytic Dye Degradation

(38) Dye degradation experiments were conducted with anatase (5 nm, Alfa-Aesar) titanium dioxide as the photocatalyst. A photocatalyst suspension was prepared by sonicating 30 mg/L of anatase for 1 hour. PHF was added to the suspension to give a final concentration of 0.03 mg/L. The dye, Procion Red MX5B, was added to the photocatalyst suspension to give a final concentration of 3 mg/L. The reaction mixture was transferred to a Petri dish with a magnetic stirrer and placed 100 mm below a bank of 16 UVA lamps (Southern New England Ultra Violet Company, Branfield, CT). The mixture was stirred in the dark for 10 minutes and then exposed to UVA at an intensity of 86 W/m.sup.2. Immediately prior to turning on the lights, two 1.5 mL samples were transferred using a pipette into plastic vials. Subsequent samples were collected at 15 minute intervals for one hour. Each collected sample was centrifuged twice at 10,000 g for 15 minutes and the final supernatant was transferred to a plastic (PMMA) cuvette. UV-Vis spectra were obtained, and the absorbencies at 512 nm and 538 nm were used for data analysis. The log of normalized sample absorbance was plotted against irradiation time and the slopes measured to obtain pseudo-first order degradation coefficients.

(39) Photocatalytic degradation studies of Procion Red dye with TiO.sub.2 free of PHFs, TiO.sub.2 combined with PERC PHFs, and TiO.sub.2 combined with BuckyUSA PHFs are presented in FIG. 7, which plots the first order degradation of the dye over the period of an hour. The ratio of PHF to TiO.sub.2 was set to a value of 0.001. The activity of PHF was determined by its ability to enhance the rate of photocatalytic degradation of the dye. The pseudo-first order rate coefficient for TiO.sub.2 with PERC PHF (0.01280.0029 min.sup.1) was 2.6 times higher than the rate coefficient for TiO.sub.2 without PHF (0.00480.0005 min.sup.1). Presence of BuckyUSA PHF gave a somewhat inhibited photocatalytic rate that was lower but not significantly different (=0.05) than TiO.sub.2 free of PHFs.

(40) All patents, patent applications, provisional applications, and publications referred to or cited herein, supra or infra, are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.

(41) It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.