ARTICLE pubs.acs.org/est
Physicochemical and Toxicological Properties of Commercial Carbon Blacks Modified by Reaction with Ozone Brian C. Peebles,† Prabir K. Dutta,*,† W. James Waldman,‡ Frederick A. Villamena,§ Kevin Nash,§ Michael Severance,† and Amber Nagy‡ †
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States Department of Pathology, The Ohio State University, 4160 Graves Hall, 333 West 10th Avenue, Columbus, Ohio 43210, United States § Department of Pharmacology, The Ohio State University, 390 Biomedical Research Tower, 460 West 12th Avenue, Columbus, Ohio 43210, United States ‡
bS Supporting Information ABSTRACT: Ozonation of two commercial carbon blacks (CBs), Printex 90 (P90) and Flammruss 101 (F101), was carried out and changes in their morphology, physical properties, and cytotoxicity were examined. The hypothesis examined was that different methods of manufacture of CBs influence their chemical reactivity and toxicological properties. Structural changes were examined by X-ray photoelectron spectroscopy, infrared spectroscopy, Raman spectroscopy, and electron paramagnetic resonance spectroscopy (EPR). Introduction of surface oxygen functionality upon ozonation led to changes in surface charge, aggregation characteristics, and free radical content of the CBs. However, these changes in surface functionality did not alter the cytotoxicity and release of inflammation markers upon exposure of the CBs to murine macrophages. Interaction of macrophages with F101 resulted in higher levels of inflammatory markers than P90, and the only structural correlation was with the higher persistent radical concentration on the F101.
’ INTRODUCTION Carbon black (CB) is the product of incomplete combustion of hydrocarbon feedstock and is an important technological material. It is used as a pigment in paints, inks, and toners, a reinforcing agent in rubber and polymers, a conductivity enhancer in polymers, an adsorbent, and a support for catalysts. The many industrial uses of CB and carbonaceous materials make it necessary to tailor their physical and chemical properties. Surface functionalization of CB is an active area of research; in particular, using ozone to introduce oxygen functionalities.17 Carbon black is also chemically similar to the carbonaceous component of environmentally relevant particles such as soot and coal fly ash.2,8 There is considerable research activity in the use of nanoparticles in biological systems,9 and the physiological effects of CBs, particularly their toxic and inflammatory response, are of interest.10,11 In this study, two commercial CBs, Flammruss 101, a lamp black (F101, prepared by burning liquids with a restricted air supply and quenched by deposition on a cool surface), and Printex 90, a furnace black (P90, made by partial combustion of residual aromatic oils, and quenched by water) were examined. The hypothesis behind this study is that methods of manufacture of CBs determine their chemical reactivity, as evidenced by oxidative functionalization and toxicology properties, as measured by interaction of the CBs with murine macrophages. Because of their technological relevance, there have been numerous studies on toxicity and effects of workplace exposure to CBs. Of particular interest is the report of a lack of r 2011 American Chemical Society
lung tumor formation in mice upon inhalation exposure to P90, as compared to controls.12 For intratracheally administered F101 and P90 in rats, the tumor incidence was higher in rats exposed to P90 at 21% (10 out of 48 rats) compared to 8% (4 out of 48 rats) exposed to F101.13 In this study, we report that the evolution of surface oxygen functionality upon ozonation influences surface charge, morphology, size, and toxicity. The cytotoxicity and inflammatory properties of CBs and correlations with the surface chemistry are also relevant to occupational and environmental exposure.
’ EXPERIMENTAL SECTION Physicochemical Characterization. F101 and P90 CB particles (from Degussa) were characterized as obtained, and after ozonolysis. Ozonolysis of F101 and P90 was performed over a 4-h period using an Enaly EOZ-300Y corona discharge ozone generator (Ozone Solutions, Hull, Iowa) that produces approximately 40 mg of ozone per hour in a stream of 20% oxygen. The generator was supplied with a flow of approximately 100 mL/minute compressed air that had been passed through a hydrocarbon trap and a desiccant. The ozone generator was connected to the tip of a buret, which was loaded with 500 mg of CB (fluid bed geometry). Received: March 17, 2011 Accepted: November 4, 2011 Revised: October 31, 2011 Published: November 04, 2011 10668
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Figure 1. TEM images of (a) untreated P90 and (b) untreated F101.
X-ray photoelectron spectroscopy was performed using a Kratos Ultra Axis spectrometer with a monochromatic Al Kα source. Spectral analysis was performed using CasaXPS software. Ultraviolet photoelectron spectroscopy was performed using a He (I) ultraviolet light source with a helium pressure of ∼5 108 Torr. The EPR spectrometer (Bruker EMX X-band) was operated at the X-band (9.87 GHz) with 10 mW microwave power, 1 G modulation amplitude, receiver gain of 1.0 105, 21.5 s scan time, and 42 s time constant with a 120 G sweep width. A Perkin-Elmer Spectrum 400 FTIR spectrometer was fitted with a Pike Technologies 100-mm gas cell with calcium fluoride windows. The carbon samples were coated on the IR window and exposed to ozone. Baseline-corrected spectra were translated to zero at 2000 cm1, a point at which all of the spectra had approximately the same slope and no spectral features. Powders were pressed onto the surfaces of Teflon discs and placed on the stage of a Renishaw InVia Raman microprobe equipped with a 633 nm HeNe laser. Particles were imaged by transmission electron microscopy using a Tecnai F20 transmission electron microscope. Dynamic light scattering (DLS) and zeta potential measurements were made using a Malvern ZetaSizer Nano instrument (sample loading 10 mg/L, 30 s sonication prior to use). Macrophage Toxicity Studies. The murine alveolar macrophage cell line (MH-S) was purchased from the American Type Culture Collection (Manassas, VA), and propagated as described previously.14 Cells were plated in 24 well plates at a density of 1 105 cells/well. Carbon blacks were suspended in PBS, sonicated for 30 s and added to confluent murine macrophage monolayers at a concentration ranging from 2.5 to 10 μg/cm2. Cells treated with 1% Triton-100 (Sigma) served as a positive control for lactate dehydrogenase (LDH) release assays, while cells treated with E. coli lipopolysaccharide (LPS, Sigma) served as the positive control for tumor necrosis factor-alpha (TNF-α) ELISAs. Supernatants were collected from each well and clarified by centrifugation at 16 000 RCF for 2 min to pellet uninternalized carbon nanoparticles. Lactate dehydrogenase and TNF-α secretion was measured via ELISA assays as described previously.14 ELISA assays for IL-1β were also carried out according to manufacturer’s instructions (R&D Systems, Minneapolis, MN). There was no interference of CB on the assays, and these results are shown in Supporting Information.
’ RESULTS Characterization of CB and Its Reaction with Ozone. Morphology. Transmission electron microscope images of the two
Figure 2. Changes in particle diameter (dashed) and zeta potential (solid) for (a) P90 and (b) F101 upon exposure to ozone (particles dispersed in water).
carbon samples are shown in Figure 1. Flammruss 101 comprises well-defined primary particles that have an average diameter of ∼100 nm, with little surface “roughness”, whereas P90 has more “roughness” on its surface. These micrographs are consistent with the Heckman and Harling model of CB structure.15 The morphologies of P90 and F101 were not noticeably changed after 4 h of ozone exposure (Figure S1, Supporting Information). The surface areas of P90 and F101 measured by BET were 320 and 33 m2/g, respectively. Upon ozonation for 4 h, the surface areas changed to 316 and 44 m2/g for P90 and F101, respectively. Size and Surface Charge. Figure 2a shows that the average hydrodynamic diameter of untreated Printex 90 was ∼180 ( 0.7 nm in water at pH 5, with an average zeta potential of ∼26.3 ( 1.9 mV. The positive zeta potential reflects a basic carbon black surface.16 This basicity is also reflected as an increase in the pH of ultrapure water from pH 6.7 to 7.1 with suspended P90 (50 mg/mL). For F101 (Figure 2b), the initial 10669
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Figure 3. (a) Raman spectrum of P90 after ozonolysis, showing bands characteristic of commercial carbon blacks. (b) Relative area comparison of Raman bands of ozonated and untreated P90 and F101.
size in water was ∼724 ( 71 nm and the zeta potential was negative at ∼ 4 ( 1.2 mV. This is indicative of an acidic surface,16 and reflected in the decrease in pH from 6.7 to 4.2 when 50 mg/mL F101 was added to ultrapure water. These samples were highly polydisperse (polydispersity index PDI ranging from 0.2 for P90 to 0.5 for F101). Though TEM indicates primary sizes of both the CB were 25). Vicinal OH groups are also reported to promote intensity of this band.19,22 Bands formed at 1320 and 1288 cm1 were assigned to CO single bonds in strained and unstrained cyclic ethers, respectively.19,20 Figure 4c is a plot of the intensity of the four major bands as a function of ozonation time. The 1630 and 1720 cm1 bands increase with similar slopes, which might be expected since they are related. The strained carbonyl groups at 1775 cm1 appear rapidly but then saturate, indicating that the sites at which initial carbonylation takes place are the most reactive. Surface Spectroscopy. X-ray photoelectron spectra (XPS) in the C 1s region were similar for P90 and F101. The changes upon ozonation for P90 are shown in Figure S3. The O 1s shows a broad peak at 532.5 eV (shown in the left inset in Figure S3). The C 1s region was deconvoluted into five peaks. They represent CC and CH bonded carbon, CO and ether carbon, carbonyl carbon, carboxyl carbon, and a πfπ* shakeup satellite,1,24,25 with binding energies of ∼284.8, 285.4, 287.5, 288.9, and 291.0 eV, respectively (seen clearly in the inset for the last four peaks in Figure S3). The πfπ* shakeup satellite peak arises from the interaction of the excited C1s electron with the π electrons of the aromatic structure. The quantitative distribution of the various functionalities is shown in Figure 5 for P90 and F101, before and after ozonation. Upon reaction with ozone, there was an increase in CdO functionalities at
Figure 6. X-Band spectra of untreated solid (a) F101and (d) P90, ozonated solid (b) F101and (e) P90, and ozonated samples exposed to PBS buffer (c) F101and (f) P90. 10671
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Figure 7. X-band spectra of (a) carbon-centered adduct of DEPMPO generated from ozonated aqueous F101 (DEPMPOR, 95%, aN = 14.4, aH = 21.4, aP = 46.1, g = 2.0056); (b) hydroxyl adduct of DEPMPO generated from ozonated aqueous P90 (DEPMPOOH, aN = 14.0, aH = 13.2, aP = 47.2, g = 2.0056); (c) α-hydroxy ethyl adduct of DEPMPO generated from ozonated aqueous P90 in ethanol (DEPMPOR, 98%, aN = 14.1, aH = 20.9, aP = 46.4, g = 2.0056). (a)0 , (b)0 , and (c)0 are the corresponding simulations from which the hyperfine constants were obtained.
the expense of the πfπ* shakeup peak, indicating that the graphitic surface was getting oxidized. The oxygenation of the surface was also reflected in the 9-fold O:C ratio increase for P90 and 5.7-fold increase for F101. The decrease in graphitic character upon ozonation was also reflected in the ultraviolet photoelectron spectra (UPS). Figure S4 shows the UPS data with a shoulder on the C 2p band at ∼3 eV attributable to pπ character, which disappeared on ozonation. EPR Spectroscopy. The EPR spectra of P90 and F101 asobtained and after treatment with ozone are shown in Figure 6 (raw data are shown in Table S1, Supporting Information). Stable free radicals were detected on F101 with spin content of 2.8 ( 0.8 108 spins/mg (Figure 6a), whereas for P90, the signal was very weak (
