Research Article www.acsami.org
Atomic Oxygen Tailored Graphene Oxide Nanosheets Emissions for Multicolor Cellular Imaging Qingsong Mei,*,† Jian Chen,† Jun Zhao,‡ Liang Yang,‡ Bianhua Liu,‡ Renyong Liu,‡ and Zhongping Zhang*,‡,§ †
School of Medical Engineering, Hefei University of Technology, Hefei, Anhui 230009, China Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China § State Key Laboratories of Transducer Technology, Chinese Academy of Sciences, Hefei, Anhui 230031, China ‡
S Supporting Information *
ABSTRACT: Graphene oxide (GO) has been widely used as a fluorescence quencher, but its luminescent properties, especially tailormade controlling emission colors, have been seldom reported due to its heterogeneous structures. Herein, we demonstrated a novel chemical oxidative strategy to tune GO emissions from brown to cyan without changing excitation wavelength. The precise tuning is simply achieved by varying reaction times of GO nanosheets in piranha solution, but there is no need for complex chromatography separation procedures. With increasing reaction times, oxygen content on the lattice of GO nanosheets increased, accompanied by the diminution of their sizes and sp2 conjugation system, resulting in an increase of emissive carbon cluster-like states. Thereby, the luminescent colors of GO were tuned from brown to yellow, green, and cyan, and its fluorescent quantum yields were enhanced. The obtained multicolored fluorescent GO nanosheets would open plenty of novel applications in cellular imaging and multiplex encoding analysis. KEYWORDS: graphene oxide, atomic oxygen, multicolored emission, emissive carbon cluster, cellular imaging
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also have been developed for synthesis of GQDs.27 Another alternative method for modulating band gap to realize PL is achieved through creating discrete energy levels by coupling of edge-functional groups on GO nanosheets.28−30 We previously found that alkylamine-modified GO nanosheets could generate highly efficient blue fluorescence.31 Yang et al. also demonstrated the surface modification could reduce nonradiative recombination and lead to blue emission.32 Unfortunately, the emissions of graphene and its derivatives are always limited to blue fluorescence, the maximum peaks of which are centered at 420−450 nm. Although many reports have presented excitation-dependent multicolored fluorescence of graphene derivatives, their intensities significantly decreased to invisible when excitation shifted to longer wavelength regions. Such an emissions shift cannot be labeled as tunable fluorescence. To date, the means to perfectly tune optical emissions of graphene has been seldom reported, and the preparation methods always suffer from complex procedures such as column chromatography separation.33,34 Herein, we proposed an entirely novel strategy to modulate the fluorescence of GO nanosheets spanning from brown to cyan simply by controlling reaction times. GO nanosheets prepared
INTRODUCTION Graphene and its oxygenated derivatives (GO) have recently attracted increasing attention in the analytical community due to their unique optical properties and excellent capabilities for surface functionalization. Initiated from Yang et al.’s research,1 graphene and GO have been widely explored as photoluminescence (PL) acceptors for constructing energy transfer mechanism based sensing platforms, where a variety of fluorophores such as quantum dots,2 upconversional nanoparticles,3 fluorescent conjugated polymers,4 and organic dyes5,6 acted as energy donors for the detection of nucleic acids, proteins, and so forth.7−17 This universal application should be credited to their rigid π-conjugated planar structures used for easily anchoring various fluorophores or analytes. However, these properties should also endow luminescent properties in addition to PL quenching. Inspired by this speculation, many efforts have been made to exploit their fluorescence emissions. However, graphene is a zero-bandgap semiconductor, which makes it impossible to emit any visual fluorescence.18,19 Many endeavors have been pursued to modulate its band structure for expecting new optical phenomena. One promising method is to convert twodimensional graphene nanosheets into zero-dimensional graphene quantum dots (GQDs), in which top-down strategies are mostly used, such as solvothermal means,20 electrochemical cutting methods,21 and so on.22−26 Some bottom-up strategies © XXXX American Chemical Society
Received: January 20, 2016 Accepted: February 29, 2016
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DOI: 10.1021/acsami.6b00791 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces through modified Hummer’s method were reoxidized in piranha solution (H2SO4/H2O2, v/v = 7/3) for different times, resulting in cutting into small fragments with different emissive colors. The obtained fluorescent GO nanosheets were characterized in detail to unveil their structural evolutions for systematically illustrating the luminescent origin.
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EXPERIMENTAL SECTION
General Materials. All reagents are of analytical reagent grade. Graphite flakes (325 mesh) were purchased from Alfa-Aesar. MTT (3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) was obtained from Sigma-Aldrich. Hydrogen peroxide (H2O2), concentrated H2SO4, and KMnO4 were received from Shanghai Chemical Reagent Corporation (Shanghai, China). Synthesis of Multicolor GO Nanosheets. The starting graphene oxide (GO) nanosheets were first prepared from natural graphite flakes by a modified Hummer’s method. In a typical experiment, 500 mg of graphite flakes was stirred in 20 mL of concentrated H2SO4, which was followed by the slow addition of 1 g of KMnO4 in an ice bath. After the solution reacted for 2 h, 100 mL of water was slowly added, and the resultant continued to react at 95 °C for 0.5 h. Then, 3 mL of H2O2 was sequentially added dropwise into the mixture. The GO nanosheets were then acquired through centrifuging and washing with deionized water. The as-generated graphene oxide nanosheets (200 mg) were then reacted in 20 mL of piranha solution (H2SO4/ H2O2, v/v = 7/3) at room temperature for 1, 4, and 8 h to synthesize yellow, green, and cyan-colored emissive GO nanosheets, respectively. After reaction, the fluorescent GO nanosheets were obtained by centrifuging and further dialyzing in a dialysis bag (retained molecular weight, 500 Da) overnight. Cellular Imaging and Cytotoxity Assays. The living HeLa cells were grown in DMEM (Dulbecco’s Modified Eagle’s Medium) supplemented with 10% FBS (fetal bovine serum) and incubated at 37 °C in 5% CO2 atmosphere. The cells were then incubated with various fluorescent GO nanosheets in medium (2 mL) for 12 h at 37 °C, and washing with phosphate buffered saline (PBS) three times to remove the extracellular GO nanosheets. After that, the cell imaging was then carried out on a Zeiss LSM710 laser confocal scanning microscope imaging system. The cytotoxicity of fluorescent GO nanosheets was examined by MTT assay. First, HeLa cells were seeded in 96-well plates. After incubating for 24 h, the medium was then replaced by the culture solution containing y-GO, g-GO, and c-GO nanosheets with various concentrations (0, 0.1, 0.2, 0.4, 0.8 mg/mL), and the cells were continued to incubate for another 24 h. Next, the cells were washed three times with PBS, and then, freshly prepared MTT (0.5 mg/mL) solution was added to each well. Finally, the MTT medium solution was carefully removed after 4 h incubation, and DMSO was then added into each well. The plate was gently shaken for 10 min at room temperature to dissolve all precipitates, and the absorbance of MTT at 570 nm was monitored in a spectrophotometer. Characterization. The morphology of fluorescent graphene oxide nanosheets were characterized by using a transmission electron microscope (TEM, JEOL-2010). UV−vis absorption spectra and Fluorescence spectra were recorded by a Hitachi U-550 spectrometer and a Hitachi F-2700 spectrophotometer, respectively. The XPS measurements were carried out with a Thermo ESCALAB 250 highperformance electron spectrometer. FT-IR spectra were recorded with a Thermo-Fisher Nicolet iS10 FT-IR spectrometer. Photographs were taken with a canon 350D digital camera.
Figure 1. (a) Photographs of the obtained luminescent GO nanosheets taken under 365 nm UV lamp irradiations. The resulted GO nanosheets with cyan, green, yellow and brown-colored emissions were named as c-GO, g-GO, y-GO and GO, respectively. (b) The photoluminescence spectra of the corresponding luminescent GO nanosheets. From left to right, the curves are ascribed to c-GO, g-GO, y-GO, and GO, respectively.
first gradually increased, and the peak positions did not shift when excitation changed from 360 to 480 nm, while emission peaks would shift from 590 to 660 nm with intensities decreasing after continually elongating excitation wavelength. After reacting in piranha solution for 1 h, the nanosheets showed a bright yellow emission centered at 560 nm, also with a broad excitation ranging from 350 to 450 nm (Figure S2). The PL exhibited a similar property with the pristine GO nanosheets, which could shift from 560 to 620 nm when excitation changed from 470 to 570 nm. Along with extending reaction times to 4 and 8 h, the GO nanosheets were further oxidized and the aqueous solution demonstrated green or cyan colored fluorescence with emission peaks at 520 or 490 nm, respectively. Their fluorescence spectra also possessed the same excitation-dependent characteristics (Figures S3 and S4). By using oxidative reaction in piranha solution, GO nanosheets demonstrated multicolor fluorescence ranging from brown to cyan with emission peaks shifted from 590 to 490 nm (Figure S5). The 100 nm emission shift was simply tuned by varying oxidation times in piranha solution. Hereinafter, for easy description, the GO nanosheets with emission colors of yellow, green, and cyan are denoted as y-GO, g-GO, and c-GO, respectively. To unveil the functions of piranha solution, we also reacted GO nanosheets only in concentrated H2SO4 or H2O2 for the same times. The control experiments showed that both of the resulted GO nanosheets cannot emit any visible fluorescence, indicating the optical tunability was stemmed from the reaction with both concentrated H2SO4 and H2O2 in piranha solution. The luminescent quantum yields of pristine GO was found to be about 0.45%, which was improved to 5 ∼ 6% after reaction, calibrating against quinine sulfate as a standard sample. Moreover, the fluorescence stabilities versus times or pH values were also detailedly studied. Take g-GO nanosheets for example; we found that its fluorescence intensity remained stable during a period of 60 min (Figure S6). Under acidic
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RESULTS AND DISCUSSION As Figure 1 shows, GO nanosheets exhibited four typical fluorescence colors after reacting in piranha solution for different times. The pristine GO nanosheets synthesized from modified Hummer’s method showed a reddish brown color under natural light, and gave an emission located at 590 nm when excited at 360 nm and a broad PL excitation spectrum ranging from 350 to 500 nm (Figure S1). Emission intensities B
DOI: 10.1021/acsami.6b00791 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces conditions, its fluorescence intensity decreased a little, and gGO aqueous solution still emitted strong PL. After the pH changed to alkaline conditions, the fluorescence could be dramatically quenched, as shown in Figure S7a. However, PL intensity could be switched on after pH returned to acidic conditions (Figure S7b). This reversible phenomenon is very similar to previous works and should be attributed to their marginal structures.20 Furthermore, to investigate the effect of surface coating on fluorescence intensities, butylamine was modified onto g-GO nanosheets according to our previously reported method.31 As shown in Figure S8, the obtained GO nanosheets also demonstrated a blue emission centered at 450 nm, as we reported before.31 To uncover the essence under phenomenon of tunable fluorescence, we first monitored the progress of GO nanosheets reacted with concentrated H2SO4 and H2O2 in piranha solution by TEM images. At the beginning, the prepared GO nanosheets demonstrated a flat planar structure with a lateral dimension of micrometers as depicted in Figure 2. After reacting in piranha
Figure 3. Normalized C 1s XPS spectra of the four typical fluorescent GO nanosheets. The signals at 284.6 eV, which was ascribed to the CC bond, were normalized to compare signal intensities of sp3 bonded carbon atoms.
fractions of carbon atoms in the terms of carbonyl and carboxylic groups obviously increased after reacting in piranha solution. Moreover, in the spectrum of GO, the fraction of sp2 bonded carbon atoms was approximately 85%, which decreased to 67% in the spectrum of c-GO. This distinctive difference should be attributed to the oxidative cutting of CC bonds on GO nanosheets. FT-IR spectra in Figure S9 also demonstrated the variations of functional groups on GO nanosheets. It should be noted that the peak at 1730 cm−1, which is ascribed to the stretch vibration of carbonyl groups, is weak in the spectrum of pristine GO, suggesting a small number of carbonyl groups on GO nanosheets. Along with reacting in piranha solution for different times, this peak became more and more apparent from y-GO to c-GO, indicating the increase of carbonyl groups on GO nanosheets. Another marked changes involve in FT-IR spectra is that the peaks at 1256 cm−1 gradually became distinctive along with the fluorescence blue shift, indicating the formation of epoxy groups during reaction in piranha solution. On the other hand, UV−vis absorption spectra also validated the structural evolutions during the oxidative process. As shown in Figure S10, the spectrum of GO has a characteristic absorption peak at 228 nm as literatures reported, which is often attributed to π → π* transition of CC bonds in sp2 hybrid regions. After oxidation, the maximum absorption peak gradually shifted to 218 nm. This 10 nm blue-shift of absorption should be ascribed to the decreasing of sp2 conjugation domains on GO nanosheets induced by the oxidative cutting. The shoulder peak in the region of 300 nm, which was often thought to be from n → π* transition of C O bonds in sp3 hybrid regions, decreased dramatically from GO to c-GO, further verifying the decrease of π conjugation system. On the basis of above discussions, we proposed a schematic illustration for the structural evolutions. As Scheme 1a shown, the starting material is common GO nanosheets synthesized through modified Hummer’s method from graphite flakes. The obtained GO nanosheets were atomically thin π-network plane owning some oxygen containing functional groups that were put into piranha solution for further oxidation with different times. It is well-known that the mixture of concentrated H2SO4 and H2O2 possesses ultrahigh oxidative capability, which would take place following reaction: H2SO4 + H2O2 → H3O+ + HSO4− + [O]. The generated atomic oxygen has capabilities to oxidize many organic or inorganic components. Herein, the oxidative cutting by atomic oxygen was speculated to initiate
Figure 2. TEM images depicted the morphology variations of graphene oxide nanosheets after reacting in piranha solution for different times: (a) GO, (b) y-GO, (c) g-GO, and (d) c-GO. Scale bars in all images are 200 nm.
solution for about 1 h, many pores appeared on the basal plane of nanosheets, and a porous morphology was presented. This should be assigned to the oxidative breaking of carbon bonds on GO nanosheets. After the reaction time was extended to 4 h, the large planar nanosheets were cut into small fragments with diameters of about tens of nanometers. The nanosheets were eventually converted into quantum dot-sized GO fragments after reacting in piranha solution for about 8 h. The morphology evolutions have clearly validated the oxidative cutting of GO nanosheets in piranha solution. We have further performed XPS experiments to reveal the variations of oxygen content on GO nanosheets before and after reactions (Figure 3). The C 1s signals of GO can be deconvoluted into signals for the CC bond in aromatic rings (284.6 eV), CO bond (287.5 eV), and C(O)−OH bond (289.2 eV) according to the previous assignments.20 The C
DOI: 10.1021/acsami.6b00791 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Scheme 1. Schematic Illustrations of (a) Structural Variations of GO Nanosheets after Reacting with Atomic Oxygen, (b) the Reaction Mechanism of GO with Atomic Oxygen, and (c) Band Gap Variations of GO after Reacting with Atomic Oxygen
To investigate the biocompatibility, HeLa cells were incubated in a medium containing three kinds of fluorescent GO nanosheets (y-GO, g-GO, and c-GO), respectively, and then the cellular images were collected by a confocal microscopy system. As Figure 4 shows, the whole cytoplasm
from the CC in aromatic rings of GO nanosheets, which would be first converted into epoxy groups (Scheme 1b). As has been previously reported, it is a tendency to form a line of epoxy groups on a carbon lattice of GO nanosheets that are energetically preferable to be further oxidized into more stable hydroxyl groups or carbonyl pairs.35 Therefore, with elongating reaction times, the breakage of carbon double bonds resulted in the formation of many pores on GO nanosheets and eventually small fragments. The apparent phenomenon of this structural variation is that the aqueous solution of resulted GO nanosheets would exhibit different fluorescence colors under the same 365 nm UV lamp illumination. The optical characteristics of GO nanosheets are commonly believed to stem from the π and π* states of the sp2 carbon atoms because π bonding is weaker and has lower formation energy. Therefore, the structural disorder defect states in the large GO nanosheets induced an energy gap of the π → π* transition, contributing a low PL quantum yields and broad emission centered at longer wavelengths (590 nm). In this study, after oxidative reaction, the carbon bonds on the lattice of GO nanosheets were broken into carbonyl groups by atomic oxygen, forming small sp2 conjugated fragments or quantum dot-sized nanosheets, which is in accordance with the changes of UV−vis absorption spectra and TEM images. Therefore, the number of small and isolated carbon cluster like sp2 domains increased, while the disorder induced defect states decreased. With elongating oxidative reaction time, the size of sp2 carbon cluster decreased, leading to the extension of π → π* energy gap (Scheme 1c). The electron−hole recombination among these sp2 cluster-like states thus exhibits size-dependent blue shift of photoluminescence under the same energy excitations.
Figure 4. Confocal fluorescent images of (a) y-GO, (b) g-GO, and (c) c-GO in HeLa cells. Scale bars: 10 μm. (d) Cell viability assay of y-GO, g-GO, and c-GO at different concentrations. D
DOI: 10.1021/acsami.6b00791 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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area of HeLa cells exhibited bright fluorescence under a 405 nm laser excitation, while no fluorescence was exhibited in nucleus areas. This observation indicated that the fluorescent GO nanosheets have been internalized by cells but not adsorbed at the cellular surface. The diverse intracellular fluorescence after incubation with GO nanosheets can be easily distinguished by using different fluorescence channels. Moreover, the cellular cytotoxity of GO nanosheets was also investigated by MTT viability test. Figure 4d shows the evolutions of cell viabilities with incubation different amounts of y-GO, g-GO, and c-GO, and the cell viability remained greater than 80% for these fluorescent GO nanosheets when concentrations increased from 0.1 to 0.8 mg/mL. This result showed that the different colored fluorescent GO nanosheets exhibited low cytotoxities at a relatively high concentration, suggesting their excellent biocompatibility and application potentials as a fluorescence probe for cellular imaging and detections.
CONCLUSIONS In summary, we have demonstrated a novel strategy to prepare optical tunable fluorescent GO nanosheets. Atomic oxygen generated in piranha solution attacked sp2 carbon bonds on GO nanosheets to form carbonyl groups, resulting in cutting of nanosheets into porous morphology and eventually small fragments. This oxidative cleavage of GO nanosheets induced the increased number of isolated carbon clusters like sp2 domains with their bandgaps extensions, and thus, the emission peaks of GO nanosheets could shift from 590 to 490 nm. This newly developed oxygenation strategy for tuning fluorescence of GO nanosheets provided a novel pathway to elucidate their fluorescence origin and presented a new protocol for tuning emissions of many other two-dimensional nanomaterials, such as graphitic C3N4 nanosheets. The multicolored fluorescent GO nanosheets opened a new scope of applications in multiplex biological imaging and detections. ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00791. Additional fluorescence features, FT-IR spectra, and UV−vis absorption spectra of the multicolored GO nanosheets. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2015CB932002), the National Natural Science Foundation of China (21305143, 21335006, 21475135, and 21375131), the China−Singapore Joint Project (2015DFG92510), the Natural Science Foundation of Anhui Province (1408085MKL52), and Fundamental Research Funds for the Central Universities (2014HGCH0002). E
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DOI: 10.1021/acsami.6b00791 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
