Article pubs.acs.org/JPCC
Photoluminescent Nanostructures from Graphite Oxidation Suzanne Ciftan Hens,†,⊥,* William G. Lawrence,‡,∥ Amar S. Kumbhar,§ and Olga Shenderova† †
International Technology Center, 8100 Brownleigh Drive, Raleigh, North Carolina 27617, United States Radiation Monitoring Devices, Watertown, Massachusetts 02472, United States § Chapel Hill Analytical & Nanofabrication Laboratory, University of North Carolina, Chapel Hill, North Carolina 27599-3216, United States ⊥ Rivis, 8100 Brownleigh Drive, Raleigh, North Carolina 27617, United States ‡
S Supporting Information *
ABSTRACT: The graphite intercalation compound (GIC) of 3:1 sulfuric to nitric acid mixture was used to produce photoluminescent (PL) nanostructures by oxidizing micrographite, nanographite, nanographite platelets, onion-like-carbon, and highly oriented pyrolytic graphite. The GIC used in this work is a Stage I intercalation compound that expands the graphitic planes to a maximum degree; this expansion was visible as a blue color for these graphitic materials in suspension with the GIC solution. The GIC intercalates into the graphitic layers to facilitate oxidation, resulting in graphite oxide, which may be fully exfoliated to graphene oxide. This treatment produced carbon nanostructures that were colloidally stable and photoluminesced across the visible wavelength range. It was possible to control the PL color of the reaction suspension by tuning the reaction temperature or reaction time; higher temperatures or longer reaction times caused a blue shift in the PL wavelength. Various graphitic oxide nanostructures were observed in the reaction suspension with increasing reaction time, including nanoribbons, graphene-like nanoplatelets, and round single-digit nanoparticles. Using separation methods for a PL orange reaction supernatant solution, it was possible to isolate individual PL colors spanning the visible wavelength region. These PL colors exhibit a blue shift in emission wavelength with filters that decreased in molecular weight or as the migration distanced increased in a denaturing polyacrylamide gel. Chemical functionalization of the PL blue carbon product from the oxidation of nanographite platelet allowed for the fluorescent coating of silica beads.
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INTRODUCTION The study of graphite oxidation has a long history that was initially focused on the transformation of the bulk graphite material, as was first evaluated by Brodie in 1859 using fuming nitric acid and perchlorate.1 This oxidation reaction was modified by Staudenmaier to allow for a single pot synthesis method,2 while Hummers later used concentrated sulfuric acid and potassium permanganate.3 However, in recent years interest has shifted to the supernatant solution produced in these types of chemical oxidation reactions, since these graphite oxidation products are photoluminescent (PL). As early as 1930, Theil showed that the oxidation of graphite, using a modification of the Staudenmaier reaction, produced a colorful residue and a PL reaction supernatant that ranged from yellow to red.4 While many recent reports use methods based upon these early reactions (see reviews),5−8 additional methods for graphite oxidation also include sonication and electrochemical methods. For example, Loh et al. used a one pot electrochemical method to oxidize highly oriented pyrolytic graphite in the presence of ionic liquids. The PL emission wavelength was easily tuned, while the products consisted of various types of carbon nanostructures (nanoribbons, single-digit carbon nanoparticles, and graphene).9 Similar to this work, graphite oxidation studies typically focus on a single reactant graphitic © 2012 American Chemical Society
species, and the trends in PL wavelength as related to the reaction mixture have been analyzed either as a function of temperature10 or composition of the reaction mixture.9 In addition to using graphite-based materials, PL carbon nanoparticles (CNP)have been formed by oxidizing the following varied carbon-based substances: carbon nanotubes,11−13 soot from candles14 and other materials,15−18 highly oriented pyrolitic graphite (HOPG),9 carbon fibers,10 carbon salts,19 and carbohydrates (glucose, sucrose, and starch).20−22 Carbonbased nanoparticles produced from these oxidation reactions have been shown to be biocompatible and useful for fluorescent biolabeling applications.23,24 Previously, we found that it was possible to safely produce PL carbon nanostructures using the chemical oxidation reaction of graphite and a mixture of commonly found chemicals, 95% sulfuric and 68% nitric acid in a 3:1 ratio.18,25 This mixture is commonly used to shorten carbon nanotubes12,26 and to purify nanodiamonds from sp2 carbons;27 it also functions as a graphite intercalation compound (GIC) that has traditionally been used for analyzing microscale oxidation product Received: March 30, 2012 Revised: August 13, 2012 Published: August 22, 2012 20015
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Addition of 140 μL of APES (aminopropyltriethoxysilane) was added to the flask. The APES-NGP product was added to TEOS beads under stirring at room temperature. The beads were washed three times with ethanol and two times with water. The solution PL was imaged under UV illumination at 360 nm wavelength. Dialysis and Filtration. The reaction supernatant was collected by removing graphitic carbon residue by centrifugation. The supernatant was either neutralized by the addition of sodium hydroxide or the acid was removed at high temperatures. The neutralized solution was dialyzed for two days against water. Pall filters were used for size separation with centrifugation. Polyacrylamide Gel Electrophoresis. The denaturing 40% polyacrylamide gel was prepared with 34 mL polyacrylamide/bisacrylamide, 33 g urea, 7 mL 10X TBE buffer, and 29 mL water. The solution was cross-linked with initiators tetramethylethylenediamine and ammonium persulfate. The gel was powered with 1000 V for 60 min prior to loading the sample. The sample was prepared with a 50% mixture of glycerol with bromophenyl blue dye. The gel ran for 4 h at room temperature, after which time the gel was wrapped in plastic wrap. The gel was extracted by PL color under a UV lamp and placed in a glass vial with Milli-Q water. After several hours of soaking, the PL solution was collected. Imaging. Samples were placed in glass vials and illuminated by a hand-held UV lamp emitting light at 365 nm. PL samples were photographed using a Lumix DMC-ZS1 camera. Scanning electron microscopy imaging was completed using a Zeiss Supra 25 Scanning Electron Micrograph. Aqueous carbon samples were dried on a conductive silicon wafer. High resolution transmission electron microscopy was performed on JEOL 2010F-FasTEM at Chapel Hill Analytical and Nanofabrication Laboratory using 200 kV accelerating voltage. The samples were dried on carbon Formvar coated copper TEM grids. UV−visible Spectra. Samples were measured in a 1 cm path-length quartz cuvette using a Varian Cary 50 instrument. Excitation and Emission Spectra. Emission and excitation spectra were recorded using the LS-50B Luminescence Spectrometer from Perkin-Elmer (Norwalk CT). This spectrometer uses a pulsed xenon arc lamp illumination source and wavelength selection was accomplished using a scanning monochromator. The sample emission was collected through a second monochromator and detected using a photomultiplier tube. All of the spectra were recorded with 2 mm slits on both the emission and excitation monochromators. All of the spectra were recorded with the same source, monochromator resolution, and PMT gain such that the relative emissions are on the same scale. The photostability of the samples was recorded using the time scan mode of the spectrophotometer to record the emission of the sample over 1800 s. Lifetime Measurements. The emission lifetime was measured using time correlated single photon counting techniques. In these measurements a short pulse laser was used to excite the sample and a fast photon counting avalanche photodiode (APD) was used to detect the emission. A time to amplitude converter (TAC) was used to measure the time delay between the excitation pulse and the detected emission. The TAC value is sent to multichannel analyzer that stores the data and builds a histogram. Two thousand to ten thousand pulses were collected to build the histogram. The histogram represents the probability of a photon being emitted as a
residues.28,29 With an interest in CNP, Peng et al. showed that this mixture can oxidize carbon fibers, and that increasing the reaction temperature causes a blue-shift of the PL emission wavelength.10 In addition to carbon fibers, this GIC is capable of oxidizing carbon nanotubes to produce photoluminescence.12 Furthermore, the reaction supernatants yielded from this sulfuric/nitric GIC produce photoluminescent species using a wide variety of graphitic carbons, including micrographite (MG), nanographite (NG), highly oriented pyrolytic graphite (HOPG), carbon fibers (CFs), carbon nanotubes, detonation nanodiamond soot, and onion-like carbon (OLC).18,25 Continuing this work, we describe results using this GIC to treat MG, NG, HOPG, nanographite platelets (NGP), and OLC, whereby the trends in PL wavelength as a function of temperature, time, and the separation of discrete products are elaborated. We also consider the reaction mechanism used to produce these products that are typical of an exfoliation process that includes carbon nanostructures such as nanoribbons, graphene-like platelets, and single-digit nanometer-sized CNPs.
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EXPERIMENTAL SECTION Materials. HOPG (SPI Supplies, West Chester, PA), < 20 μm synthetic micrographite (Sigma-Aldrich), ∼400 nm natural nanographite (Nanostructured & Amorphous Materials Inc., Houston, TX), nanographene platelets (Angstrom Materials, Dayton, OH), onion-like carbon (ITC, Raleigh, NC), carbon fibers (Applied Sciences, OH), 95% sulfuric acid (Aldrich), 68% Nitric acid (J.T. Baker), Sodium hydroxide (Fisher), HCl (Mallinckrodt), > 95% fuming nitric acid (Sigma-Aldrich), tetraethyl orthosilicate (Aldrich), aminopropylethoxysilane (Gelest), Millipore filters (10K, 3K), Pall filter (1K MWCO), dialysis membranes (Invitrogen), polyacrylamide/bisacrylamide (Sigma), 10X Tris-borate ethylenediaminetetraacetic acid (Sigma), tetramethylethylenediamine (Sigma), ammonium persulfate (Sigma), and urea (Sigma). Oxidation Reaction. To a 200 mL three neck roundbottom flask, an amount of 200 mg of micrographite or nanographite was added along with 50 mL of 3:1 95−98% sulfuric acid to 68% nitric acid. The flask was fitted with a water-cooled reflux condenser, a thermometer, and argon gas line. Extreme caution should be taken. The reaction mixture is highly oxidizing and all reactions should be performed in a solvent hood. The gas evolved should be neutralized by bubbling through a solution of sodium hydroxide. Using a sand bath, the temperature was elevated to the appropriate reaction solution temperature (55 °C, 95 °C, or 128 °C) while the mixture was stirred. After the mixture was heated for 2 h at the appropriate temperature, the flask was allowed to cool to room temperature. For residue studies, this material was collected by centrifugation at a speed of 5000 rpm for 10 min and washed with water until the supernatant reached pH 5. The residue was dried overnight under vacuum at 50 °C. For the HOPG reaction, an amount of 38 mg material was heated to 120 °C for two hours with 10 mL of the 3:1 acid mixture. For micrographite in sulfuric acid, an amount of 0.2 g was treated with 50 mL of 68% sulfuric acid at 110 °C for 2 h. Inspection of the supernatant under a 365 nm UV lamp did not show any detectable photoluminescence. Alkylsilane Reaction. The blue photoluminescent product from the oxidation of nanographite platelets that was filtered with a 1000 MWCO filter was dried in a glass vial. To this vial, 2 mL of ethanol was added and degassed with N2 for 20 min. 20016
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Figure 1. The mechanism of oxidation illustrates the process of intercalation and dilation that leads to exfoliation and other defects of graphitic planes resulting in nanocarbon materials of graphite oxide and graphene oxide (A). Photographs showing that HOPG, micrographite (MG), nanographite (NG), and carbon fiber (CF) change color from gray to blue when a mixture of 3:1 sulfuric acid/nitric acid is added (B).
it can be understood that there is yet no conclusive evidence correlating CNP size with PL emission wavelength. Thus the aim of this work is to focus on the reaction conditions (time and temperature) as well as product separation to elucidate patterns of PL emission wavelength based upon the oxidation mechanism. Also, by using a number of different graphitic carbon starting materials, general observational trends can be made for the oxidation products of these materials as related to PL emission color. Oxidation Process. The GIC used in this work, consisting of a 3 to 1 ratio of sulfuric acid to nitric acid, was heated to elevated temperatures along with different graphitic carbon materials (MG, NG, NGP, HOPG, and OLC). In a typical oxidation reaction, 200 mg of graphitic material was heated for 2 h in 50 mL of the 3:1 acid mixture. For reactions using HOPG, the mass to volume ratio was maintained. This reaction produces a yellow to brown colored supernatant that is photoluminescent when illuminated with UV and visible excitation light. The species responsible for the photoluminescence are the graphitic oxidation products, graphite oxide (GO), and graphene oxide that have epoxides on the surface and acidic groups on the edges of the planar structure, see Figure 1A.5 The reaction solution containing these colloidally stable, graphitic oxide particles are formed by the process of intercalation, dilation, oxidation, and exfoliation.28,29 Intercalation with the GIC causes an increase of the distance between graphene sheets, resulting in an expansion or dilation of the graphite material, see Figure 1A. Depending upon the degree of expansion of the graphene stack, the intercalation is characterized as Stage I, Stage II, or Stage III. The greatest expansion possible occurs with a Stage I intercalation compound, whereby every graphene sheet is separated. The stage index of a GIC can be identified by the graphitic material’s color in the presence of the GIC at room temperature. A Stage I GIC changes the material’s color from gray to blue,29 which was observed for MG, NG, HOPG, and carbon fibers in the 3:1 mixture, shown in Figure 1B. The actual dilation of graphitic
function of time and this probability function is the fluorescence lifetime measurement. Only a fraction of the laser pulses should produce a photon detection event in order to avoid distorting the lifetime measurement and in these studies less than 1 pulse in a thousand produced a photon at the detector. To reduce overhead and maximize collection efficiency, the start pulse of the TAC is generated by the photon counting detector and the stop pulse is generated by the laser such that only laser pulses that generate a photon at the detector are recorded. The excitation source in these experiments was a PicoQuant GmbH (Berlin Germany) 405 nm laser that delivers up to 3 mW output with a pulse width less than 300 ps and a repetition rate of 20 MHz. The detector was an actively quenched Geiger mode avalanche photodiode from ST Microelectronics (Geneva Switzerland) that produces a single output pulse for each detected photon. The emission was collected at 520, 560, and 620 nm using optical filters with 10 nm bandpass. The MCA-8000 multichannel analyzer from Amptek (Bedford MA) was used to collect the TAC data and the MCA was interfaced to a personal computer for data transfer and manipulation.
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RESULTS AND DISCUSSION Similar to other chemical approaches, the GIC of 3:1 sulfuric/ nitric acid causes intercalation into graphitic planes, which results in expansion or dilation of the planes, followed by oxidation. Using this GIC and other chemical approaches to oxidize graphitic materials is relatively easy. However, identifying these products is complex since they are heterogeneously sized and shaped, which are produced simultaneously by a random process of oxidation and defect generation.8 Although it is known that the graphite oxide product resulting from these oxidation reactions have negatively charged surface groups, there still exist several models for its chemical structure.5 The challenge in identifying these oxidation products is caused by their complex surface chemistry, single-digit nanometer size, and the difficulty in collecting significant quantities of purified material. Therefore, 20017
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temperature of 128 °C. Figure 2 shows a typical timedependent progression in PL color for all three graphitic
planes is visible for the millimeter-sized graphitic layers of HOPG in the GIC solution. It follows that the oxidation of graphitic carbon is most efficient using this ratio of 3:1 sulfuric/nitric acid, as shown in the following reaction: 24nC + 3H 2SO4 + HNO3 = C+24nHSO4−·2H 2SO4 + H 2O + NO2
where C is the graphitic compound and n is the stage index.28 In addition to the acid mixture, water from the reactants and product also intercalate between the graphitic layers. A concomitant evolution of the orange gas (NO2) is seen under refluxing conditions.30 Stage I intercalation compounds cause the largest amount of dilation, therefore it has the potential to oxidize graphitic carbon layers more readily in the presence of an oxidant, such as nitric acid. Suspensions made with less sulfuric acid have a higher stage index, since nitric acid is a weaker Brönsted acid as compared to sulfuric acid and thus does not intercalate into the graphitic planes as readily. For example, HOPG is not expanded or colored blue in the acid mixture of 1:1 sulfuric acid to nitric acid (Figure 1B). It follows that the 3:1 acid mixture rapidly oxidizes graphite at elevated temperatures to produce brown supernatant solutions that are PL red, while a reaction mixture with a ratio that is lower in sulfuric acid did not produce PL under the same time and temperature conditions. Similarly, concentrated, fuming nitric acid that was refluxed with graphite over a much longer period of time produced a pale yellow colored solution that was only weakly photoluminescent. In fact, it has been previously shown that the 3:1 acid mixture oxidizes graphite fibers more readily than a 1:1 mixture and that higher temperatures increased the extent of the reaction.31 Properties of the Reaction Product. The supernatant products from the oxidation reaction were found to be colloidally stable from pH 1 to 14. The reaction is known to produce NO2 functional groups on the edges of graphitic carbon, which decompose to produce carboxylic acid groups.31 Therefore, these particles have a high surface charge that facilitate their colloidal stability. It was also observed that the product of the reaction remains colloidally stable when transferred to a variety of solvents (isopropanol, acetone, methanol, dimethylsulfoxide, and dimethylformamide). For the supernatants that are neutralized with NaOH, the PL color was pH-dependent. However, when the acid mixture was evaporated, and the reaction product was transferred to DI water, the solution PL color was green at pH 5 and 7, and pink at pH 14 with the addition of saturated NaOH. Spectroscopy was completed for the oxidation products of NG. The absorbance spectrum shows no unique wavelength peaks in the visible spectrum (Figure S1A of the Supporting Information, SI). However, the fluorescence emission spectrum displayed PL emission wavelengths that ranged from red to blue, shown in Figure S1B of the SI. This reaction product did not show any photobleaching over a two hour period, as determined using a fluorimeter. The fluorescence emission lifetime was measured for the PL blue supernatant isolated using a 3.5 K molecular weight dialysis membrane. The data were fitted to a double exponential, which indicates that there are two different species; their lifetimes are 2 and 20 ns. Role of Time for the Reaction. MG, NG, and HOPG were treated with the GIC acid for different times at a constant
Figure 2. Photograph of the supernatants of HOPG for increasing reaction times treated with a mixture of 3:1 sulfuric/nitric acid at 128 °C. The samples are illuminated with a 365 nm UV lamp.
compounds whereby the supernatant of the reaction changed in order of red, orange, and yellow. X-ray photoelectron spectroscopy (XPS) data revealed an oxygen content of ∼40% for the residue, which is indicative of graphite oxide. Thus, with all three starting materials, the reaction solution turned increasingly brown in color over time with a concomitant blue-shift in the PL emission wavelength. The changes in PL color were observed more closely for the second time-dependent experiment. In this experiment, the solid residue was removed from the reaction of 200 mg MG in the 3:1 acid mixture that was treated for 2 h at 128 °C. After removing the residue, the PL orange colored supernatant was diluted to 20 v/v% with the addition of the GIC, this dilution did not change the PL color of the initial reaction supernatant. Next, the solution was treated at 128 °C and aliquots from the reaction mixture were taken every 30 min to monitor the PL color. The supernatant color changed from PL yellow to green and blue over the course of the reaction (240 min), as shown in Figure 3. At the end of this sequence, the evolution of orange
Figure 3. Time-dependence of PL color and intensity of the supernatant is shown for the reaction product of micrographite. A photograph of the yellow starting solution, solution A, along with supernatant aliquots taken at 30 min intervals during the reaction of 3:1 acid mixture at 128 °C, illuminated with a 365 nm UV lamp.
gas ceased. Since the reactivity of the GIC was expended in this mixture, a fresh solution of GIC was added to the blue reaction using the same dilution ratio (20 v/v%). The mixture was heated at 128 °C and orange gas evolution was observed. This second dilution was further treated for a total experimental time of 150 min, while the PL color changed more gradually from blue to violet (images not shown). Further oxidation with a third addition of fresh GIC caused the PL violet color to become colorless. The observed blue-shift in this reaction series was also observed for NG treated under the same conditions. These colors can also be obtained without the removal of the residue; however, this will result in a more heterogeneous 20018
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Figure 4. SEM images of untreated HOPG (A1), nanographite (B1), and micrographite (C1) as compared to residue collected from the reaction media of HOPG (A2), nanographite (B2), and micrographite (C2) after 2 h reaction time in the acid mixture under refluxing conditions. The treated graphite structures show exfoliation at the edge for HOPG (A2), microcracks and distortions in nanographite (B2), as well as dilation in micrographite (C2).
Role of Reaction Temperature. In order to determine the role of temperature of the GIC on the reaction, MG, NG, and HOPG were treated for 2 h at the following three different temperatures: 55 ◦C, 95 ◦C, and 128 ◦C. At low temperature, the PL color was red for MG and NG, see Figure S3 of the SI. At higher temperatures, the PL color blue-shifted in wavelength, giving PL orange and yellow colors for 95 °C and 128 °C, respectively. These solution PL colors were similar to the changes seen for the above time-dependent experiment, whereby the PL color changed from red to yellow for either increasing temperature or reaction time. Interestingly, the dilution of the PL yellow product in water produced a PL green colored supernatant. Also, significant amounts of NO2 were generated from oxidizing graphite materials in the 3:1 oxidizing reaction mixture, which can be seen as an orange gas. The amount of graphite consumed and the concentration of NO2 produced increased concomitantly with temperature. Nitrogen dioxide is a known oxidant and is expected to contribute to the oxidation of sp2 carbons, particularly at high temperatures. However, the PL color is not caused by changes in the GIC reactant solution due to the breakdown of sulfuric or nitric acid, since the PL supernatant color remained unchanged when it was diluted with fresh aliquots of the GIC acid mixture. The residue for the reactions with HOPG, MG, and NG were collected by centrifugation after treatment at temperatures of 55, 95, and 128 °C, which was followed by washing with water until a pH of 5 was reached. All three graphitic residues were imaged by scanning electron microscopy (SEM), see Figure 4, and all three materials exhibited distortions and etching of their graphite plane edges. For HOPG, the oxidation reaction was least efficient in breaking down sp2 carbons, which is expected since it is more stable toward intercalation/ exfoliation due to the large area of its sp2 graphitic stack. For HOPG, exfoliation was predominantly located at the graphite edges, Figure 4A2, and its surface showed microcracks and uniformly sized holes or pitches, as in Figure S3A2 of the SI.
mixture of products, resulting in numerous PL colors evolving simultaneously. By removing the residue before the reaction series, we have shown that it is possible to directly correlate the PL color with the extent of the reaction, since the amount of reaction products in the mixture remains constant over the course of the reaction. Structural differences in the supernatant reaction solution were compared by transmission electron microscopy (TEM) from the above time-dependent reaction. Using time-dependent series from the initial dilution reaction, we compared the MG supernatant PL product solutions that were orange (t = 0 min), green (t = 150 min), and blue (t = 210 min), see Figure S2A-C of the SI. The particles in the PL orange product consisted of flakes of several hundred nanometers in size with a lattice spacing for the interior region of 0.349 nm, indicative of graphite, see Figure S2A of the SI. Within these graphitic flakes, nanoribbons were present, For the PL green product, the graphitic flakes were smaller and thinner, and they contained a greater density of nanoribbons, Figure S2B of the SI. The graphitic ribbons are attributed to oxidative cleavage of the expanded graphite.9 Under white light, the PL orange product was darker in color than the PL green product. Under white light, the PL blue product was colorless and TEM images of this product consisted of stacked, thin carbon sheets (carbon platelets) that have no visible ribbons (Figure S2C of the SI). The edge of this stack (inset of Figure S2C2 of the SI) has graphene-like character, which was determined by the diffraction pattern from the Fourier transform of the image. These results support an increase in the amount of exfoliation by the GIC over time, an increase in the density of nanoribbons followed by platelet formation, coupled with a concomitant blue shift in the PL emission wavelength. The photoluminescence of these solutions likely arise from graphitic oxide of various quantum confined sp2 structures, such as nanoribbons, and carbon platelets. 20019
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For NG, etched channels were seen on its surface, see Figure 4B2. For MG, the graphitic planes appeared expanded, Figure 4C2. These residues were analyzed by Raman spectroscopy. Variations in acid treatment caused an increase in the intensity of the D band and the D′ band, which is consistent with large amounts of defects generated during the acid treatment.25 Lastly, we compared the amount of recovered residue for HOPG, MG, and NG collected after treatment temperatures of 55, 95, and 128 °C. Surprisingly, the amount of recovered residue was greater for MG than for NG. This result is unexpected since the efficiency of oxidation depends upon the lateral dimensions of the starting material. However, SEM imaging showed that naturally occurring NG (Nanoamor) had a more ordered structure as compared to the synthetically derived MG (Aldrich), see Figure S4 of the SI, and thus a more ordered structure of NG would be more stable to oxidation. These experiments support the observation that the GIC reaction temperature can control the extent of graphite oxidation. Fractionation of Reaction Supernatant. The supernatant solutions yielded from the oxidation of graphitic materials in the GIC mixture were PL red, orange, or yellow under UV light. However, these solutions actually have PL colors that span the visible wavelength region, which was observed directly by fractionating the reaction supernatant into discrete PL colors. Fractionation was accomplished using molecular weight cut off (MWCO) filters, dialysis membranes, and electrophoresis of denaturing polyacrylamide gels. First, we compared the product species isolated from the standard reaction using NG with the GIC at 128 °C for two hours. The colloidal supernatant solution was isolated from the unreacted product residue and separated into different fractions with either different MWCO filters or different MW dialysis membranes. For the separation with MWCO filters, the NG product supernatant was neutralized with sodium hydroxide and dialyzed extensively against water. The supernatant was then filtered through 10K, 3K, and 1K MWCO filters. The supernatant solutions from this MW separation under UV illumination showed PL colors of yellow, green, and blue using the 10K, 3K, and 1K MWCO filters, respectively, see Figure 5. Under visible light, the yellow supernatant solution was darker than the green solution, while the blue solution was colorless. This experiment suggests that there is an MW-dependence on the PL wavelength color, whereby the lower molecular weight species emit at shorter wavelengths.
The second NG reaction product separation experiment used dialysis membranes (10K and 2K), and the solution that passed through the membrane was collected in order to exclude large graphitic residues. The species that weigh less than 10K and 2K had a PL green and blue colored emission under UV illumination, respectively, see Figure 6A. Next, we used TEM
Figure 6. Oxidation products from NG treated at 128 °C for 2 h illuminated with a UV lamp (A) shows the supernatant filtered through a 10K and 2K dialysis membrane. A TEM image for the 2K product shows spherical particles less than 5 nm in size (B).
imaging of the PL blue solution and found that it consisted of round, single-digit CNPs that are less than 5 nm in size, see Figure 6B. As expected, larger, varied carbon particles were collected for the solution that passed through the 10K MW membrane. Single-digit CNPs were also seen for the MG material treated in the GIC reaction. The MG reaction product, treated for 80 min, was filtered through a 3K MWCO filter to produce a PL blue suspension. TEM imaging showed that this blue solution also had round particles of sizes 3 to 4 nm (see Figure S5A,B of the SI). Not surprisingly, when the MG material was treated in the GIC mixture for a shorter time (20 min) larger particles were observed. These larger particles consisted of carbon nanoribbons. These separation experiments support a sizedependence of photoluminescence of the oxidized graphite species. Similar to Xu et al., we also used denaturing polyacrylamide gels to separate PL colors using the concentrated, neutralized PL reddish supernatant of NGP and OLC.20 Both supernatant products were separated into PL colored species that spanned the visible wavelength region, as based upon the migration distance in the electric field. This migration distance can be dependent upon the CNP size/molecular weight and/or the negative charge density, so it is not possible to conclude from this separation that the blue-shifted species has the smaller molecular weight. However, this method clearly shows the distinct PL colors within the mixture, see Figure 7A. For both the NGP (Figure 7A) and OLC samples (data not shown), the PL violet colored species migrated most rapidly, while the PL orange species migrated most slowly. These differently colored species were excised from the gel, soaked in water to extract the carbon product for TEM analysis. Comparison of the PL orange and green colored products, see Figure 7B, showed that the PL green colored product was smaller in size and was more exfoliated than the PL orange species. For the PL blue colored supernatant, we recovered single-digit CNPs that were on the order of 5 nm in diameter. Fluorescent Labeling. The PL products outlined in this work may be used as fluorophore dyes. It is well-known that
Figure 5. The PL color dependence for the supernatant reaction solution of NG (far left) separated by size from the reaction mixture using the 3:1 ratio of sulfuric acid to nitric acid treated at 128 °C. The mix was separated by centrifugation with molecular weight cutoff filters of 10K, 3K, and 1K (from left to right). The samples are illuminated with a 365 nm UV lamp. 20020
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Figure 7. Photographs of the 20% polyacrylamide denaturing gel and the PL solutions in water extracted from the gel for the oxidation reaction of NGP.
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graphite oxidation reactions produce oxidized groups on the surface of graphitic oxide particles, such as carboxylic groups. These groups on GO and graphene oxide may be linked to alkylsilane APES (aminopropyltrimethoxysilane), for example. For the GIC reaction of NGP, the PL blue product was isolated by filtration and dried under nitrogen. Addition of dry ethanol and APES to the vial produced a PL orange solution of oxidized NGP-APES that was then linked to TEOS beads, see Figure S6 of the SI. These beads were PL pink under UV light after extensive washing. Thus, graphite oxide particles can be conjugated using typical dye-based reactions. Also, these dyes can be used to make photoluminescent polymer films. For example, PL orange product added to polyacrylic resin retained its emission color.
ASSOCIATED CONTENT
* Supporting Information S
Spectra from the blue photoluminescent product (Figure S1); TEM images of the supernatants yellow, green, and blue (Figure S2); photograph of the reaction supernatant solutions showing the temperature dependence of the PL emission wavelength (Figure S3); SEM images taken at low magnification for untreated HOPG, nanographite, and micrographite compared to their residue (Figure S4); TEM images of MG treated for 80 min in the GIC and filtered using a 3K MWCO filter (Figure S5); and photographs for the labeling of silica beads with PL blue product (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.
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CONCLUSIONS Graphitic carbons can be oxidized safely, simply, and inexpensively from commercially available starting materials to produce PL nanostructures using the GIC acid mixture of 3:1 sulfuric to nitric acid. We found that the PL reaction supernatants are colloidally stable and consist of various species that have different emission wavelengths. These species can be separated using molecular weight filters, dialysis membranes, or gel electrophoresis. A blue shift in the PL wavelength is observed for the reaction supernatant either by increasing the reaction time or increasing the reaction temperature. In addition, this blue-shift in PL color is observed for products with different structural morphologies that depend upon the extent of oxidation. The trend in the blue-shift of PL emission wavelength is independent of the type of graphitic material used. This PL emission is typically size-dependent, which is related to either a quantum confinement effect or to the particle’s electronic structure, for the graphene conjugated system.32 The nanostructures responsible for photoluminescence may consist of graphite oxide and/or graphene oxide in the form of particles, platelets, and nanoribbons.7,9,33 These graphitic oxide nanoparticles are promising PL labels for bioapplications due to their expected nontoxic carbon-based structure and their range of fluorescent colors that span the visible wavelength spectrum.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ∥
Dr. Lawrence is currently with MIT Lincoln Laboratory, 224 Wood St, Lexington MA. Dr. Lawrence’s contribution to the publication was performed at Radiation Monitoring Devices. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.H and O.S. are thankful for the support by the Space and Naval Warfare Systems Centers under Grant N66001-04-18933. S.H. is thankful for support by the United States Air Force under Contract No. FA9550-11-C-0017. Gary McGuire is acknowledged for helpful discussions. Disclaimer: Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Space and Naval Warfare Systems Center (SSC).
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