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Cite This: Environ. Sci. Technol. 2017, 51, 1364-1376
Aggregation Kinetics and Self-Assembly Mechanisms of Graphene Quantum Dots in Aqueous Solutions: Cooperative Effects of pH and Electrolytes Qingqing Li,†,‡,# Baoliang Chen,*,†,‡ and Baoshan Xing# †
Department of Environmental Science, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China # Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States ‡
Environ. Sci. Technol. 2017.51:1364-1376. Downloaded from pubs.acs.org by 5.188.216.223 on 01/28/19. For personal use only.
S Supporting Information *
ABSTRACT: The cooperative effects of pH and electrolytes on the aggregation of GQDs and the aggregate morphologies are characterized. Because GQDs have an average size of 9 nm with abundant O-functionalized edges, their suspension was very stable even in a high electrolyte concentration and low pH solution. Divalent cations (Mg2+ and Ca2+) excelled at aggregating the GQD nanoplates, while monovalent cations (Na+ and K+) did not disturb the stability. For Na+ and K+, positive linear correlations were observed between the critical coagulation concentration (CCC) and pH levels. For Mg2+ and Ca2+, negative, but nonlinear, correlations between CCC and pH values could not be explained and predicted by the traditional DLVO theory. Three-step mechanisms are proposed for the first time to elucidate the complex aggregation of GQDs. The first step is the protonation/deprotonation of GQDs under different pH values and the self-assembly of GQDs into GQD-waterGQD. The second step is the self-assembly of small GQD pieces into large plates (graphene oxide-like) induced by the coexisting Ca2+ and then conversion into 3D structures via π−π stacking. The third step is the aggregation of the 3D-assembled GQDs into precipitates via the suppression of the electric double layer. The self-assembly of GQDs prior to aggregation was supported by SEM and HRTEM imaging. Understanding of the colloidal behavior of ultrasmall nanoparticles like GQDs is significantly important for the precise prediction of their environmental fate and risk.
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INTRODUCTION Graphene quantum dots (GQDs), one novel species of ultrasmall graphene-based nanomaterials that possess exceptional physical and chemical properties, have inspired intensive research efforts since being synthesized very recently.1−4 They are classified as a zero-dimensional graphitic nanomaterial with lateral dimensions of less than 100 nm in a single to a few layers.5,6 GQDs can be viewed from a dimensional perspective as a scaled down version of graphene oxide (GO)3,7 and also as an enlargement of benzene-based molecules, i.e., polycyclic aromatic hydrocarbons (PAHs).8−10 Correspondingly, these types of nanoparticles (NPs) are generally derived through topdown nanocutting methods3,11 and bottom-up organic synthesis routes.12 The surface morphology and edge oxygencontaining groups can be further modified to gain controllable characteristics.13−16 Emerging as superior and environmentally friendly functional materials, GQDs are attracting increasing attention in various research areas of environmental, biological, and energy-related applications.17−24 With such promising potential for many applications, GQDs would probably be released into the environment and transported between environmental media, which may trigger uncertain ecological effects.25−29 For example, when serving as delivery vectors for medicines, GQDs have been shown to be © 2017 American Chemical Society
readily taken up by cells and circulated in the blood system due to their smaller sizes. In the body fluids of higher animals, the exchange of cations such as Na+, K+, Mg2+, and Ca2+ is highly dynamic, and all of these cations are vital to the regulation of stable blood pressure and pH.30,31 Excessive salt intake has been recognized as a major risk for triggering hypertension.32 The stability of delivery vectors, such as GQDs, could be influenced by blood salt concentrations, and their travel path and eventual fates could be unintended. With such concerns, the response of GQDs to solutions with variable levels of electrolytes and pH could be of vital importance, as the GQDs may encounter concentrated cations, leading to variability in their transport and aggregation behavior. The stability of engineered nanoparticles is one of the vital factors that ultimately controls their environmental fate and ecological risks.33−36 Extensive research has been conducted aiming to understand the stability and aggregation kinetics of nanoparticles in an aqueous environment. In numerous publications, the Derjaguin−Landau−Verwey−Overbeek Received: Revised: Accepted: Published: 1364
September 7, 2016 November 17, 2016 January 9, 2017 January 9, 2017 DOI: 10.1021/acs.est.6b04178 Environ. Sci. Technol. 2017, 51, 1364−1376
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Environmental Science & Technology (DLVO) theory has been commonly applied.37 The SchulzeHardy rule was usually utilized to predict the minimum concentration of ions necessary to cause the rapid coagulation of colloids.38 Attachment efficiency (α) was estimated on the basis of the inverse stability ratio (w). The successive suppression of the electric double layer (EDL) leads to the gradual destabilization of nanoparticles, which is reflected by the lifting of attachment efficiency and the intensification of nanoparticle aggregation. The critical coagulation concentration (CCC) value was accordingly determined and utilized as a quantitative indicator for the evaluation of the stability of nanoparticles in the aqueous phase. However, whether the aggregation of ultrasmall nanoparticles, such as GQDs, obeys the traditional colloidal theory, which usually estimates nanoparticles with a spherical shape, remains unclear. The colloidal behavior and aggregation mechanisms of GO have been studied previously. However, limited information concerning GQDs is available. GO dispersion in aqueous environments occurs in the form of single layers interacting via two predominate fundamental patterns: edge-to-edge and faceto-face.34,39 Adding divalent cations could cause aggregations of GO nanosheets in edge-to-edge patterns, ascribed to their strong bridging/cross-linking and intercalating effects.34 Aggregation by lowering of the pH primarily occurs in the face-to-face pattern because of decreased electrostatic repulsion, increased van der Waals interactions, and possible π−π bond interactions. In turn, an increase in pH would stabilize GO dispersion due to the enhanced repulsion between the negatively charged GO nanosheets. The excessive addition of OH− could also cause aggregation, which is referred to as the “salting out” effect.40 By adjusting the solution chemistry, conformational changes to 2D graphene materials could occur,40−45 which would make their colloidal properties more complicated. Similar to their larger relative graphene oxide (GO), the amphiphilicity of GQDs is recognized to be pH dependent due to the protonation/deprotonation of the oxygen-containing groups at the edges.46−51 With large edgeto-area ratios and surface charge densities, GQDs are much more hydrophilic than GO.39,49,52,53 Recently, the selfrecognition of GQDs has sparked attention in the field of smart and fine structure research, and the self-assembly of GQDs is thought to be pH dependent and affected by the coexisting cations and temperature.54 So far, the assembly behaviors of nanoparticles, such as GQDs, from molecule-like materials into large particles before aggregation have been poorly studied. This knowledge gap must be filled to fully recognize the colloidal behavior and nanosize effects of nanoparticles or nanosheets in the environment. The quantum confinement and nanosize effects have vested GQDs with exceptional properties, and varied applications have been developed. However, there are almost no reports on the colloidal behavior and fate of GQDs in the environment. The overall objective of this work is to investigate the stability and aggregation kinetics of GQDs in aqueous systems with a wide range of water chemistries (i.e., cation type/valence, pH, and the combined effects of cations and pH). In this report, the cooperative effects of pH and electrolytes on the self-assembly and aggregation mechanisms of GQDs are unveiled and characterized.
methods described in an earlier report.11 Briefly, 6 g of commercial pitch-based carbon fibers (CF, micrometer-sized) was added to a mixture of concentrated H2SO4 (1200 mL) and HNO3 (400 mL). After 2 h of sonication, the mixture was put in an 80 °C water bath and stirred for 24 h under reflux condensation. The resulting solution was then cooled, and the pH was adjusted to 5 with Na2CO3. The final product was dialyzed in a dialysis bag (with a retained molecular weight of 2000 Da) to remove the extra acid and salts. The diluted and clear solution was then concentrated by evaporating water in a 75 °C water bath to make the stock GQD solution. Concentrated HNO3 and NaOH were used to acquire GQD solutions at selected pH. GQD Characterization. The synthesized GQDs were observed by high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and field emission scanning electric microscopy (SEM). The surface functional groups were further characterized through Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The hydrodynamic diameter (Dh) and zeta potential (ζ-potential) were measured with a ZetaSizer Nano ZS90 (Malvern Instruments, Worcestershire, U.K.). The GQDs at selected pH values of 2, 4, and 12 were observed using HRTEM. The Ca(NO3)2-induced aggregation of GQDs was observed using SEM to monitor the surface morphology. Detailed procedures are presented in the Supporting Information (SI). GQDs Aggregation Kinetics. The early stage aggregation kinetics of GQDs can be obtained from the initial rate of Dh change with time t. The time-resolved dynamic light scattering (TR-DLS) method was applied. In the early aggregation stage, the initial aggregation rate constant (kα) is proportional to the initial rate of increase in Dh and inversely proportional to the initial primary GQD concentrations in the suspension (N0). The early aggregation stage is defined as the time period from the initiation of the experiment (t0) to the time when the measured Dh value exceeds 1.5−2 Dh,initial, which is also referred to as the reaction-limited regime. After that, the increase of Dh slows down until the aggregation rate constant reaches 0, and this stage is referred to as the diffusion-limited regime.34 kα ∝
1 ⎛ d D h (t ) ⎞ ⎜ ⎟ N0 ⎝ dt ⎠t → 0
(1)
Particle attachment efficiency (α) is used to quantify particle aggregation kinetics, and it is defined as the initial aggregation rate constant (kα) normalized by the aggregation rate constant measured under diffusion-limited (fast) conditions.34 k 1 α= = α = w kα ,fast
1 N0
dD h (t ) dt
N0,fast
dD h (t ) dt
1
( (
) )
t→0
t → 0,fast
(2)
The GQD concentrations across all samples were identical, allowing for a simplification of eq 2 (i.e., N0 drops out). Therefore, α can be determined directly by normalizing the initial slope of the aggregation profile for a specific background solution chemistry by the initial slope under diffusion-limited (fast) conditions. The critical coagulation concentration (CCC) values of the GQDs were determined from the intersection of the extrapolated lines through the diffusionand reaction-limited regimes.
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EXPERIMENTAL SECTION Synthesis of Graphene Quantum Dots. The GQDs were synthesized through oxidation and cutting according to the 1365
DOI: 10.1021/acs.est.6b04178 Environ. Sci. Technol. 2017, 51, 1364−1376
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Figure 1. Characterization of GQDs. (A) AFM image of the GQDs, and the inset shows the height analysis; (B) HRTEM image of the GQD nanoplate, and the inset represents the 2D FFT pattern; (C) SEM image; (D) FTIR spectra; (E) Raman spectra; and high-resolution XPS C 1s spectra of carbon fibers (CF) (F) and GQDs (G).
Table 1. Critical Coagulation Concentrations (CCC) of Several Carbon-Based Nanoparticles in the Literature CCC, mM
a
NPs
particle size
concn, mg/L
pH
NaCl
GO36 GO35 rGO35 GO34 CNT33 C6059 GQDs (current study)
179.2 ± 111.5 nma not mentioned not mentioned 582 ± 111.2 nma 17.6 ± 7.9 nm (diameter); 1.5 ± 1.5 μm (length) 55.7 ± 1.8 nmb average 9.0 nm (3.9−26.8 nm)a
40 10 10 10 2.54 TOC 5.92 TOC 80
5.5 5.2 5.2 5 6.0 7.5−8.5 4
44 200 30−35 188 25 160 2906
KCl
1007
MgCl2
CaCl2
1.3
0.9 0.7 0.7 2.6 2.6 6.1 1.9
3.9 1.5 8.0 70
The average square root of the area. bHydrodynamic radius of the nanoparticles suspended in deionized water.
nm (average square root of the area of GQD nanoplates, Table 1), with an average value of 9.0 nm according to the AFM particle analysis (Figure 1A). The inset diagram in Figure 1A reveals that GQDs were primarily composed of one or two carbon domain layers with a thickness of approximately 1−2 nm. Clear lattice fringes (1120) with a lattice parameter of 0.242 nm were observed through HRTEM imaging (Figure 1B), indicating the high crystallinity of the GQDs. The corresponding fast Fourier transform (FFT) pattern is shown in the inset of Figure 1B. The edge orientation of the GQDs was predominantly consistent with the zigzag orientation, which was confirmed to possess excellent electrical and optical characteristics.11 SEM showed a slightly larger size distribution of GQDs (Figure 1C), which can be ascribed to the platinum coating that improves the surface conductivity. The surface functional groups were further characterized through FTIR spectra, Raman spectra, and XPS. As shown in the FTIR spectra (Figure 1D), the carbonyl, carboxyl, hydroxyl, and epoxy groups were clearly present in the GQDs. Using the oxidation cutting method, abundant oxygen-containing functional groups were introduced to the edges and onto the basal
In the current study, a GQD concentration of 80 mg/L provided strong DLS signals and was therefore used in all of the aggregation studies. Equal volumes (500 μL) of the GQD solutions with the selected pH and electrolyte solution (NaCl, KCl, CaCl2, or MgCl2) were pipetted into a DLS glass cuvette (Malvern Instruments, Worcestershire, U.K.) to achieve a specific electrolyte and GQD concentration at the selected pH (the pH variation of the mixing procedure was less than 0.2%). After being vortexed for 1 s, the cuvette was immediately placed in the DLS instrument. The intensity of scattered light was measured at 173°, and the autocorrelation function was allowed to accumulate for 15 s during the aggregation study. The Dh measurements were conducted over a period of 400 s. This time span was selected to ensure that the early stage aggregation was complete.
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RESULTS AND DISCUSSION Characterization of GQDs. The synthesized GQDs were examined by HRTEM, AFM, and SEM (Figure 1A−C). Small polygonal pieces were present under AFM and SEM. The physical dimensions of the GQDs were between 3.9 and 26.8 1366
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Environmental Science & Technology planes of the GQDs in comparison with the CF. The Raman spectra of the CF and GQDs were recorded (Figure 1E). The relative intensity of the “disorder” D band to that of the crystalline G band (ID/IG) for the GQDs was 0.82, which was lower than the reported value of 0.91 for the GQDs synthesized at 120 °C11 and much lower than the reported values for GO and reduced GQ (rGO) synthesized through Hummer’s method.43,55 The relatively small ID/IG ratio reflected the high crystallinity of the GQDs, suggesting that the conjugated-π network of the GQD basal planes was retained to a large extent. XPS measurements were also taken, and the high-resolution spectra of C1s for the CF and GQDs are presented in Figure 1F and G, respectively. The peak at 284.4 eV was attributed to the presence of C−C/CC, which is a dominant graphitic C1s peak. C−O (285.4 eV) and CO (288.1 eV) were also identified at the corresponding peaks. After the O-groups were introduced, O−CO (288.7 eV) appeared in the XPS spectra of the GQDs compared to that of the CF. The atomic ratios of C to O in the CF and GQDs were 11.80 and 0.61, respectively. The very distinct C/O atomic ratios indicate a high degree of oxidation of the GQDs. The relative contents of each bond were calculated using the XPS-peak-differentiation-imitating analysis. In the spectra of CF, there was 78.50% C−C/CC, 15.21% C−O, and 6.31% CO. Conversely for GQDs, there was 66.44% C−C/CC, 18.62% C−O, 10.43% CO, and 4.52% O−CO. The presence of these O-groups makes the GQDs highly stable in water.11 Effects of Cation Types on GQD Aggregation. Attachment efficiencies (α) in the presence of four selected cations (Na+, K+, Mg2+, and Ca2+) at an unadjusted pH (approximate 4) are presented in Figure 2A. There were significant aggregation capability gaps between these cations, as the attachment efficiencies (α) were very distinct from each other without overlapping. The aggregation of the GQDs in the four selected cations could be divided into reaction-limited and diffusion-limited regimes according to the DLVO theory. In the reaction-limited regime, GQDs were gradually destabilized, which could be a consequence of the gradual elevation of the degree of charge screening and the reduction of the repulsive electrostatic interactions. In the diffusion-limited regime, the charge of the GQD nanoplates was completely screened, so aggregates formed. The CCC values of the GQDs were determined to be 2900 mM (Na+), 1000 mM (K+), 70 mM (Mg2+), and 1.9 mM (Ca2+). The divalent cations (Mg2+ and Ca2+) were far more effective at aggregating GQDs. According to the Schulz-Hardy rule, CCC is positively proportional to 1/z6 (z represents the cation valence),38,56 and there have been studies that verified this experiential model.34,36 However, the CCC values determined with the same cation valence in this study differed greatly, especially for the CCC determined with Ca2+, which deviated largely from the predicted value (Figure 2B). This inconsistency suggests that there are other interactions in addition to charge screening. The CCCs of the GQDs in monovalent electrolytes (NaCl and KCl) were much higher than their environmentally relevant concentrations (usually less than 10 mM)57 and even approached the upper limits of their solubility in the case of NaCl.58 For graphene oxide, the reported CCC values were 44−200 mM for Na+ and 1.3−3.9 mM for Mg2+ at an unadjusted pH (approximate pH 5),34−36 which was much lower than those of the GQDs at the unadjusted pH (∼pH 4) measured in the current study. This result indicates that it is much more difficult to aggregate
Figure 2. Attachment efficiencies (α) of the GQDs as a function of electrolyte concentration (NaCl, KCl, MgCl2, or CaCl2) at an unadjusted pH (approximately pH 4) (A); comparison between the determined CCC values in the current study and the CCC values predicted by the Schulze-Hardy rule (B); aggregation profiles of the GQDs with selected electrolytes at the CCC (C).
GQDs, as they are much smaller in size than GO. The CCC of GO was reported to be 0.7−2.6 mM for Ca2+,34−36 similar to that of the GQDs (1.9 mM) in the current study. This result indirectly verified the universal efficacy of Ca2+-aggregating nanoparticles, such as GO and GQDs. Although the sizes of GO (∼1 μm) and GQD (∼9 nm) nanoparticles are quite different, their colloidal behaviors are almost the same in the presence of Ca2+. This interesting observation may be attributed to the self-assembly of GQDs into GO-like nanosheets with the assistance of Ca2+, which will be explained later. The CCC values of several carbon-based nanomaterials are listed in Table 1. Obviously, the CCC of the GQDs is greatest with the selected electrolytes, demonstrating the high stability of this small nanomaterial. It is facile to correlate the aggregation behavior to the charging degree of GQDs and attribute the effective 1367
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Figure 3. Photo illustrating the influence of pH on the GQD solution (A); ζ-potentials of the GQDs as a function of pH (B); morphology of the GQDs at pH 2 (C−E), pH 4 (F−H), and pH 12 (I−K). The scale bar corresponds to 20 nm in parts C, F, and I and to 5 nm in parts D, E, G, H, J, and K.
weights of 23 and 24 g/mol, respectively). However, no differences in the kfast and hydrodynamic radius in the presence of NaCl, MgCl2, and CaCl2 were observed in the aggregation kinetics of CNTs, GO, and C60.33,34,59 The inconsistency of the results in the current study may result from variations in the dimensions of these examined nanoparticles. The small size of GQDs allowed them to be easily disturbed and their movement to be more dynamic and random in an aqueous system. Figure S-1C, F, G, and M represents the aggregation profiles of four selected cations at an unadjusted pH. The differences in aggregation rates and aggregate sizes among the selected cations are reflected in all of these tested samples. During the same period, the heavier cations allowed the GQDs to aggregate faster and form larger aggregates. The aggregation rate and the size of the aggregates in the early aggregation stage might be less dependent on the cation valence but may have a strong correlation with the cation atomic weight. These results may offer insight into the removal of various small nanoparticles in water, as the aggregates with larger sizes are favorable for precipitation. Note that the GQD particle size is 3.9−26.8 nm derived from the AFM image (Figure 1A), and the initial particle size derived from aggregation profiles at the corresponding CCC of each
destabilization of divalent cations to the reduced negativity of the surface of the nanoparticles in the divalent electrolytes compared to the monovalent electrolytes. The degree of difficulty or effectivity in aggregating the GQDs was reflected by the CCC values. However, little attention has been paid to the aggregates forming during the destabilization procedure. For this reason, the aggregation profiles of the GQDs in the presence of the selected cations at their corresponding CCC are presented in Figure 2C. Interestingly, the aggregation profiles were sorted into two groups: aggregation profiles in the presence of KCl and CaCl2 and aggregation profiles in the presence of NaCl and MgCl2. There was a larger increase in Dh in the presence of either KCl or CaCl2 than in the presence of either NaCl or MgCl2. However, there were no significant differences between the results with KCl and CaCl2 during the measured period, as well as between those with NaCl and MgCl2. Correspondingly, the estimated kfast values for the selected cations were 0.92 (Na+), 2.15 (K+), 1.07 (Mg2+), and 2.42 (Ca2+). It appears as though a greater aggregation rate could be achieved during the early aggregation stage in the presence of heavier cations (K+ and Ca2+, with atomic weights of 39 and 40 g/mol, respectively) than in the presence of those with relatively lighter weights (Na+ and Mg2+, with atomic 1368
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Figure 4. Attachment efficiencies (α) of GQDs as a function of electrolyte concentrations at selected pH levels. The inset represents the relationship of the CCC values with the solution’s pH levels for each given electrolyte. (A) NaCl, (B) KCl, (C) MgCl2, and (D) CaCl2.
cation (Figure 2C) is around 200 nm. This inconsistency suggested that the GQDs maybe already form aggregates before the DLS measurement began because GQDs aggregated in a relative fast mode at CCC of cations. In comparison, the hydrodynamic sizes of GQD aggregates below CCC (i.e., ∼20 nm at pH 7 in 0.50 mM Ca2+, Figure S-1) when the DLS measurements started are comparable to the GQD particle size derived from the AFM image. In addition, before the DLS measurements begin, there was a 15-s autocorrection as mentioned in the Experimental Section. Concerning this period time, the DLS measurement was unable to record the initial part of the entire aggregation process. Therefore, some aggregation kinetic presented in the manuscript is just the partial of the entire aggregation process. Frankly, the very initial aggregation behaviors of nanomaterials still exist as a big puzzle or blind point. In this study, the morphologies of GQD aggregates were monitored to interpret the possible aggregation process, which will be illustrated in Figure 3. Effect of pH on GQD Aggregation. Six vials of stable GQD suspensions at equal concentrations (80 mg/L) with different pH levels were photographed two months after they were first prepared (Figure 3A). No apparent aggregates were observed, but a darkening of the color of the solution in accordance with an increase in pH was clearly demonstrated (light brown → dark brown → brownish dark). Conversely, it has been reported that GO will form aggregates at a pH of 2 due to the deceased negative charge of the GO sheets.34 The corresponding ζ-potentials of these GQD suspensions were measured (Figure 3B). The well-dispersed GQDs were all negatively charged, while both the addition of an acid and the addition of a base unexpectedly increased the negative charge of GQDs. In the acidic solutions, the ζ-potential increased in
accordance with pH elevation, while in the alkaline solutions, the ζ-potential decreased sharply in accordance with pH elevation. However, quite differently, the variation trend of the ζ-potentials for most other carbonaceous nanomaterials roughly declined with an increase in pH. It has been reported that GO at a pH of less than 5 was always destabilized and formed aggregates without the addition of extra salt.46,51 The hydrodynamic size distributions of the GQDs were measured (Figure S-2), and the GQD Dh was relatively unchanged across the entire pH range, which indicates a high dispersibility of GQDs at different pH levels as well. The reason that ζ-potential increases in accordance with pH elevation in acid solution remains unknown; for further investigation, the morphology of the GQDs at selected pH levels was observed using HRTEM. As shown in Figure 3, GQD nanoplates at different pH levels exhibited different surface morphologies. To the best of our knowledge, this study is the first to report the pH-dependent morphologies of carbonbased nanoparticles such as GQDs. At an unadjusted pH (Figure 3F−H) and a high pH (Figure 3I−K), the aggregated GQDs presented parallel lattice fringes; meanwhile, the GQDs in acidic solutions (pH 2) displayed whorl-like surface fringes (Figure 3C−E). The whorl-like surface fringes of the GQDs were similar to those of soot,60 which is derived from particulate emissions resulting from the incomplete combustion of biomass and fossil fuels. Soot is believed to be composed of a crystallized outer shell and an amorphous carbon-stacked inner core, which is considered to be highly chemically reactive.60−63 The microstructural change of GQD nanoplates could possibly enhance the surface chemical reactivity and consequently alter the surface charging of these GQD nanoplates. Hassanzadeh et al. found that GQDs with concentrations above their CAC 1369
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Figure 5. Aggregation profiles of the GQDs at selected pH levels with the various electrolytes: NaCl (A), KCl (B), MgCl2 (C), and CaCl2 (D).
(Dh) of GQD suspensions was relatively unchanged across the entire pH range (Figure S-2). To further understand the pHdependent stability of GQDs, the aggregation kinetics were examined with all of the above-mentioned mono- and divalent electrolytes (NaCl, KCl, MgCl2, and CaCl2). The results are summed up in Figure 4 with the corresponding CCC values at different pH levels, which are presented as insets. Both reaction- and diffusion-limited aggregation regimes were observed in the measurements. The effects of pH on the CCC values of the GQDs were quite different for the monovalent and divalent cations. For monovalent cations (Na+ and K+), positive linear correlations were observed between the CCC and pH values (insets in Figure 4A and B), which indicate that the aggregation behavior of GQDs is dominantly controlled by the protonation and deprotonation of the oxygen-containing groups. This is consistent with the findings pertaining to GO in the literature.34 It should be noted that we were unable to record the aggregation kinetics at higher pH levels (>4) due to an insufficient count rate. The low count rate indicates that the GQDs may act as highly stable regular salts in alkaline solutions. Moreover, no aggregates of GQDs were observed at pH 7, pH 9, and pH 12 with monovalent cations, even when the added electrolytes had reached their maximum concentrations. In addition, the determined CCC values in the presence of NaCl at pH 2 (2729 mM), pH 3 (2785 mM), and pH 4 (2906 mM) were already very close to the solubility of NaCl.58 Similarly, the CCC values in the presence of KCl were determined to be 590 mM (pH 2), 813 mM (pH 3), and 1007 mM (pH 4). There was an overlap of the pH-dependent aggregation kinetics in the presence of NaCl, but, for now, the exact mechanisms are unknown. The characteristics of GQD stability in the presence of divalent electrolytes were very different and completely unexpected. According to the ζ-potential data (Figure 3B),
(critical association concentration) could self-assemble into porous, irregular, semispherical particles through a nucleationelongation process.54 Additionally, this self-assembly can be tuned by adjusting the pH, ionic strength, and concentration of the GQDs.54 The driving force behind this cooperative selfassembly is believed to be the interplay of secondary interactions, such as electrostatic, π−π, hydrogen bonding, and/or hydrophobic interactions. Therefore, both acidic and alkaline media favor the cooperative self-assembly and formation of multisheet particles of GQDs. According to their results, secondary interactions were enhanced with the addition of acid in the GQD suspension, and this could be the reason that ζ-potential decreased with the increase of acidity of the solution. Basically, aggregation induced by either pH or cations is a process of nucleation that involves radial growth of the assembly. In this study, with the exception of the highly isolated scatter of the GQDs at a pH of 12 (Figure 3I−K), GQDs at lower pH levels were all connected to each other to some degree (Figure 3 C−H), but the connection did not precipitate GQDs out of the acid solution, instead it stabilized GQDs as the measured hydrodynamic sizes were similar across the entire pH range and ζ-potentials decreased in accordance with the pH decrease. It should be noted that due to the drying process for TEM observation, the outcome photos showed instantaneous states of GQDs at extremely high or low pH conditions rather than the accurate morphologies in the solution at the selected pH levels. Combined Effects of pH and Cation Type on the Stability of GQDs. Charge variations induced by pH could play an important role in nanomaterial aggregation.34,36,40 However, it appears as though the stability of the GQDs in the current study was not pH dependent as there were no obvious aggregates observed after quite a long time period (Figure 3A), and the resultant charge variations did not have a severe impact on GQD dispersibility (Figure S-2). The hydrodynamic size 1370
DOI: 10.1021/acs.est.6b04178 Environ. Sci. Technol. 2017, 51, 1364−1376
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Figure 6. Morphologies of GQDs determined using SEM imaging without (A−C) and with (D−I) the addition of CaCl2 at different pH values of 2 (A, D, G), 4 (B, E, H), and 12 (C, F, I).
measured CCC values in the presence of MgCl2 were determined to be 69 mM (pH 4), 25 mM (pH 7), 6.5 mM (pH 9), and 1.0 mM (pH 12). No GQD aggregates were observed at pH 2 and pH 3 with the addition of MgCl2 at maximum concentrations. The variation trend of pH dependency for the divalent cations was reversed compared to the results for the monovalent cations, and the trend was also contrary to what was predicted based on the ζ-potential results (Figure 3B). These unusual observations could be partially attributed to the limitations of the ζ-potential measurements in describing particle stability, as it only reflects the surface charge density on the ζ-plane. It is possible that additional processes or novel mechanisms could be involved in the aggregation of GQDs. To take a clear look at the effect of pH on the aggregates, the aggregation profiles of the GQDs at different pH values with the selected cations (at the CCC) were selected and are compared in Figure 5. All of the aggregation profiles are also available in Figure S-1 of the Supporting Information. In the presence of monovalent cations (Figure S-1A−F), the sizes of the aggregates were not significantly affected by pH. Only a slight decline in accordance with the pH increase was observed for NaCl (Figure 5A). This decline is clearly shown in Figure 5B for KCl. On the contrary, a large increase in aggregate size with pH elevation was observed in the case of the divalent cations (Figure S-1G−P). As is clearly shown in Figure 5C, the aggregate size at pH 12 was distinctly large. This is because Mg2+ primarily exists as a Mg(OH)2 colloid at pH 12, which enhanced the heteroaggregation of the GQD nanoplates and Mg(OH)2 colloids. Similarly, Chowdhury et al. recently studied the complex roles of divalent cations, pH, and NOM in the aggregation and stability of rGO.35 An increase in the rGO aggregate size with pH elevation was observed in the presence
the GQDs were more stable in the solutions with higher pH levels, which could be attributed to the enhanced hydrophilicity of GQD nanoplates derived from their functional groups with deprotonated edges.64 However, as shown in Figures 4C and 4D, the CCC variation trend as a function of pH was completely reversed for divalent cations. Negative, but nonlinear, correlations were observed between the CCC and pH values, which could not be predicted by the traditional DLVO theory. The measured CCC values in the presence of CaCl2 were determined to be 81 mM (pH 2), 5.5 mM (pH 3), 1.9 mM (pH 4), 1.3 mM (pH 7), 0.92 mM (pH 9), and 0.65 mM (pH 12). A sharp increase in CCC values occurred when the pH decreased from 4 to 2, and this result was consistent with the ζ-potential variation trend of the GQDs (Figure 3B). The decrease in CCC from pH 2 to pH 4 for Ca2+ was 42.8fold, which was much larger than that from pH 4 to pH 12 (2.9fold). The commonly used simulated body fluid (SBF) contains Na+/K+/Mg2+/Ca2+ in concentrations (mM) of 142.0/5.0/1.5/ 2.5, with the pH adjusted to 7.4.65 According to the results of our current study, the CCC of Ca2+ at pH 7 was determined to be 1.3 mM. Therefore, Ca2+ in SBF may be concentrated enough to precipitate the GQDs if used as a drug vector. In this case, GQDs may not be acceptable candidates for the delivery of therapeutics. This finding calls for the evaluation of the colloidal properties and aggregation behaviors of certain GQDs under various pH conditions before they can safely be used as carriers for therapeutics because the pH values of the human body can be variable. It should be noted that when the measurements were taken in the presence of MgCl2 in the acidic solutions, the count rates were so low that the suspensions were not suitable for DLS measurement, which also suggests that GQD nanoplates are well dispersed in acidic solutions in the presence of MgCl2. The 1371
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Figure 7. Attachment efficiencies (α) of GQDs as a function of cation concentration at selected pH levels with the addition of monovalent cations at pH 2 (A), pH 3 (B), and pH 4 (C) and with the addition of divalent cations at pH 4 (D), pH 7 (E), pH 9 (F), and pH 12 (G).
of NaCl with CaCl2/MgCl2. The authors attributed this phenomenon to the stronger binding of Ca2+ with GO functional groups and the increased surface charge of GO. Furthermore, aggregations were observed using SEM with and without the addition of CaCl2 at selected pH levels (Figure 6). Prior to the addition of the divalent cation Ca2+, the GQDs
at lower pH levels were scattered across the sample plates (Figure 6 A and B), while those at pH 12 were clustered due to the “salting out” effect induced by the extremely concentrated alkaline solution (Figure 6C). After the addition of Ca2+, all three of these samples at different pH values formed aggregates with separated metal salts (Figure 6D−F). Interestingly, the 1372
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Figure 8. Proposed three-step mechanisms for the aggregation behavior of GQDs with and without the addition of salt under acidic (protonation) and alkaline (deprotonation) conditions. The red ‘+’ symbol surrounding the particles of A, B, and C indicates a positive charge; the blue ‘−’ symbol surrounding the particles of D, E, F, and G indicates a negative charge; and the yellow point denotes the Ca2+ cation.
aggregation behavior of GQDs in an aqueous environment. To gain insight into the complex impact on GQD stability, a comparative analysis of the aggregation kinetics in the presence of cations with equal valence at different pH levels was carried out (Figure 7) to exclude the effects of ionic strength. Due to the impact of the CCC on amplification and deamplification, the CCC gaps were successively narrowed, while the pH levels were increased in both the monovalent and divalent cation solutions (Figure 7). With increasing pH, the difference in aggregation capacity between the cations with equal valence was reduced, especially for the divalent cations (Mg2+ and Ca2+). This could be attributed to the increased ionic strength derived from the extra salts used to adjust the pH in the alkaline solution. However, the bridging of Ca2+ to the deprotonated carboxyl groups was enhanced at higher pH levels. The simultaneous narrowing of the CCC gaps among the cations with equal valence led to the expansion of the CCC gaps between the mono- and divalent cations in accordance with increases in the pH (Figure S-4). Aggregation Mechanisms of GQDs. In the current study, complicated aggregation of GQDs was observed by adjusting the cation type and pH of the solutions. GQDs have been shown to be self-recognizable at the molecular level and can potentially self-assemble into different 3D structures.54 The secondary interactions controlled by the solution chemistry could interplay and either favor or disfavor the formation of 3D structures. From this point of view, the complex aggregations of GQDs under variable solution chemistries can be divided into three steps, which are proposed in this study. A diagram of the schematic mechanism of the colloidal behavior of GQDs is presented in Figure 8. The first step is the protonation/ deprotonation of GQDs at different pH values, and the selfassembly of GQD ultrasmall nanoparticles is controlled by the primary charge interaction. The second step is the self-assembly of small pieces of GQD into large plates, which is induced by the coexisting Ca2+, and the plates are then converted into 3D structures by secondary interactions, such as π−π stacking. The
aggregates at pH 12 displayed two patterns: large aggregations with metal salts of large sizes (Figure 6F) and small clusters without apparent metal salts separated out (Figure 6I). Performing the aggregation observations using TEM was more difficult due to the weak photopermeability of the dense aggregates (Figure S-3). As the pH increased, the aggregates became densely interconnected. The aggregates at a pH of 2 formed two patterns: aggregates with specifically polygonal structures and aggregates with interconnected structures. Both of these two patterns existed universally. The different surface morphologies were used to reflect the potential aggregation mechanism, which will be described later. In a previous report, the mechanical properties of GO sheets were significantly enhanced by modification with a small amount of Mg2+ and Ca2+, two divalent alkaline earth-metal ions.61 The results were rationalized in terms of the chemical cross-linking between the functional groups of the GO sheets and the divalent metal ions. Additionally, the edge bonding contributed to the mechanical enhancement.66 In another closely related study, Medhekar et al. found that the large-scale properties of GO sheets were controlled by networks of hydrogen bonds between neighboring GO interlayers.67 The extent and collective strength of these interlayers of hydrogen bond networks were controlled by the water content. Thus, the mechanical properties can be intensified by increasing the density of the functional groups, which can lead to an increase in hydrogen bonding.67 Herein, chemical cross-linking and hydrogen bonding could control the edge-to-edge and face-toface interactions of the GO sheets and further control their aggregation and assembly. A GQD is a microversion of GO, and the aggregation behavior and assembly process would be governed by the above-mentioned two forces (chemical crosslinking and hydrogen bonding). The combined effects of pH and divalent cations could be the main factors that lead to the anomalous aggregation behavior of GQDs. Based on the current evidence, it is obvious that both pH and cation type are significantly involved in controlling the 1373
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After the self-recognition involved in the first two steps, the third and last step of aggregation began with the growth of the GQD self-assemblies into irregular sphere-like structures (Figure 8C and G). According to the DLVO theory, the sphere-like GQD composites would eventually aggregate and precipitate out of the solutions, similar to other spherical nanoparticles. The growth of the GQD self-assemblies is presented in Figure S-3. After the addition of CaCl2, polygonal and interconnected assemblies were clearly present at pH 2 (Figure S-3A−C) and pH 4 (Figure S-3D), and densely assembled aggregates were present at pH 12 (Figure S-3E). In contrast, GQDs could have difficulty assembling into supermolecular composites with the addition of monovalent cations due to the lack of the cross-linking/bridging effect. They aggregate in another path after gradual EDL suppression due to increased ion strength. The cooperative effects of pH and electrolytes on GQD aggregation are highly dependent on the cation type. For example, the cooperative effects adhere to the DLVO theory in the presence of monovalent cations, which is reflected by GQD colloids that become more stable in alkaline solutions (Figure 4A and B). In contrast, when coexisting with divalent cations, GQD nanoparticles are heavily aggregated in solutions with a high pH (Figure 4C and D), which reveals a discrepancy with predictions based on the DLVO theory. Based on the proposed three-step aggregation mechanism, this colloidal discrepancy is attributed to the heavy self-assembly of GQDs (second step) induced by divalent cations, especially Ca2+, under alkaline conditions. Furthermore, the aggregation of nanoparticles, especially those with 2D structures, was always accompanied by conformational changes.43,45 During aggregation, the nanoparticles wrinkled, bended, folded, and stacked.40−43,64,68 A GQD is classified as a zero-dimension graphite nanomaterial, but small GQDs can be assembled into lateral GQDcomposites with increased sizes, which could further bend, fold, and stack into large and dense aggregates. If the Ofunctional groups on the opposite sides of the GQD nanosheet were unevenly distributed, this could induce the basal planes to bend away from the negative field in the inner space of the bending sheet.42 The bending would result in GQD composites that are similar to microenvelopes filled with a negative charge, which can ensnare positively charged ions within them. Assisted by the cross-linking/bridging effect of divalent cations, the aggregation of bended GQD nanosheets could be enhanced in alkaline systems. The surface-cation interactions have inspired potential applications for GQDs in drug delivery and waste material capture.40 In conclusion, GQDs are considered as a type of anisotropic nanosize composite, but its aggregation behavior in the aqueous solution is missing. It is a great challenge to elucidate the fundamental mechanisms of GQD aggregation behavior based on the traditional colloidal science. The first time investigation on GQD aggregation behavior in the current study suggested whether the colloidal approach and knowledge can be translated to this unique nanomaterial regime or not is highly dependent on the suspension system. The complex influences of pH and cation valence on GQD aggregation were investigated, and a three-step mechanism of self-assembly that involved aggregation was proposed for the first time. In addition to the primary charge interaction, the interplay of several secondary interactions led to different interaction modes under varied solution chemistries. The adjustment of pH levels
third step is the aggregation of the 3D-assembled GQDs into precipitates, which is controlled by primary and secondary interactions. The adjustment of the pH of the solution first changed the surface charge of the GQDs and made them act differently in acidic and alkaline solutions. With abundant O-functional groups on the edges, the protonation/deprotonation of the GQD nanosheets was evident in successive procedures. In pace with the protonation, the polycyclic aromatic network of GQDs became more hydrophobic, which facilitated the extensive hydrogen bonding induced by the self-recognition of the neighboring GQD nanosheets, and the GQD-water-GQD sandwich-like structure was therefore assembled (Figure 8A).53,54,67 Consistent with the observed interconnection of protonated GQD nanoplates at pH 2 (Figure 3C−E), GQD aggregates with composite structures stably existed in water instead of precipitating in a face-to-face pattern, which is an energetically favorable interaction mode.53 The protonation of the carboxyl groups facilitated GQD-water-GQD self-assembly, which could be a reasonable explanation for the high stability of the GQDs at low pH levels (Figure 3A). The differences between the stabilization in acid/alkaline solutions was due mainly to the differences in the dominant secondary interactions. In acidic solutions, hydrogen bonding created distance between the neighboring GQD nanoplates (Figure 8A), while in alkaline solutions, the connections between the GQDs were isolated by electrostatic repulsion (Figure 8D). These results were verified by observing the isolated GQD nanoplates at a pH of 12 (Figure 3I−K). After the addition of the divalent cations, the second step of complex aggregation proceeded. In the acidic solution, the added cations may intercalate into the basal planes of the GQDs and compress the GQD-water-GQD self-assembly structures into denser ones. These compressed GQD supramolecules could further interact with each other through crosslinking of the GQD edges (Figure 8B). However, the conditions in the alkaline solution were vastly different and can be further divided into two substeps. The formerly isolated GQD nanoplates were interconnected by the cross-linking/ bridging of divalent cations at the edges of the GQDs (Figure 8E), and the repulsive forces between the negatively charged GQDs decreased simultaneously. The edge-to-edge bridging could enlarge the size of the GQD composite, like knitting the small pieces of GQDs into large graphene-like nanosheets with cation-made pins (Figure 8F). After forming this cross-linkingenhanced self-assembly, the GQD composites with large lateral sizes would bend, fold, and stack through π−π interactions (Figure 8F), maybe perhaps with more cations intercalating between the basal planes. These two substeps proceeded in totally different patterns. During the first substep, the GQDs interconnected in an edge-to-edge pattern, while during the second substep, the GQD self-assembly stacked in a face-to-face pattern. The ordered surface morphologies of the selfassembled GQDs under alkaline conditions (Figure 3H and 3K) support the graphene-like π−π stacking mechanisms of assembled GQDs, while the disordered morphologies of the assembled GQDs under acidic conditions (Figure 3D) reflect the GQD-water-GQD interactions. The differences between the second step in acidic and alkaline solutions can be attributed to the strength of the cross-linking, which was reflected by the differences in aggregate size at the selected pH levels (Figure 5D). 1374
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(10) Zhou, L.; Geng, J.; Liu, B. Graphene quantum dots from polycyclic aromatic hydrocarbon for bioimaging and sensing of Fe3+ and hydrogen peroxide. Part. Part. Syst. Char. 2013, 30 (12), 1086− 1092. (11) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L. H.; Song, L.; Alemany, L. B.; Zhan, X. B.; Gao, G. H.; Vithayathil, S. A.; Kaipparettu, B. A.; Marti, A. A.; Hayashi, T.; Zhu, J. J.; Ajayan, P. M. Graphene quantum dots derived from carbon fibers. Nano Lett. 2012, 12 (2), 844−849. (12) Liu, R.; Wu, D.; Feng, X.; Mullen, K. Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology. J. Am. Chem. Soc. 2011, 133 (39), 15221−15223. (13) Li, Q.; Zhang, S.; Dai, L.; Li, L. Nitrogen-doped colloidal graphene quantum dots and their size-dependent electrocatalytic activity for the oxygen reduction reaction. J. Am. Chem. Soc. 2012, 134 (46), 18932−18935. (14) Yan, X.; Li, B.; Li, L. Colloidal graphene quantum dots with well-defined structures. Acc. Chem. Res. 2013, 46 (10), 2254−2262. (15) Jiang, F.; Chen, D.; Li, R.; Wang, Y.; Zhang, G.; Li, S.; Zheng, J.; Huang, N.; Gu, Y.; Wang, C.; Shu, C. Eco-friendly synthesis of sizecontrollable amine-functionalized graphene quantum dots with antimycoplasma properties. Nanoscale 2013, 5 (3), 1137−1142. (16) Li, T.; Li, Y.; Xiao, L.; Yu, H.; Fan, L. Electrochemical preparation of color-tunable fluorescent carbon quantum dots. Huaxue Xuebao 2014, 72 (2), 227−232. (17) Sun, H.; Wu, L.; Wei, W.; Qu, X. Recent advances in graphene quantum dots for sensing. Mater. Today 2013, 16 (11), 433−442. (18) Craes, F.; Runte, S.; Klinkhammer, J.; Kralj, M.; Michely, T.; Busse, C. Mapping image potential states on graphene quantum dots. Phys. Rev. Lett. 2013, 111 (5), 56804−56809. (19) Lee, H.; Park, S.; Park, E.; Son, B.; Lee, S.; Lee, J.; Lee, Y.; Kang, K.; Kim, M.; Park, H.; Choi, S.; Huh, Y.; Lee, S.; Lee, K.; Oh, Y.; Lee, J. Photoluminescent carbon nanotags from harmful cyanobacteria for drug delivery and imaging in cancer cells. Sci. Rep. 2014, 4, 4665. (20) Sanchez-Dominguez, C. N.; Gallardo-Blanco, H. L.; RodriguezRodriguez, A. A.; Vela-Gonzalez, A. V.; Sanchez-Dominguez, M. Nanoparticles vs cancer: A multifuncional tool. Curr. Top. Med. Chem. 2014, 14 (5), 664−675. (21) Wang, C.; Wu, C.; Zhou, X.; Han, T.; Xin, X.; Wu, J.; Zhang, J.; Guo, S. Enhancing cell nucleus accumulation and DNA cleavage activity of anti-cancer drug via graphene quantum dots. Sci. Rep. 2013, 3, 2852. (22) Sun, H.; Gao, N.; Dong, K.; Ren, J.; Qu, X. Graphene quantum dots-band-aids used for wound disinfection. ACS Nano 2014, 8 (6), 6202−6210. (23) Hui, L.; Huang, J.; Chen, G.; Zhu, Y.; Yang, L. Antibacterial property of graphene quantum dots (both source material and bacterial shape matter). ACS Appl. Mater. Interfaces 2016, 8, 20−25. (24) Zhang, Z.; Zhang, J.; Chen, N.; Qu, L. Graphene quantum dots: An emerging material for energy-related applications and beyond. Energy Environ. Sci. 2012, 5 (10), 8869−8890. (25) Akhavan, O.; Ghaderi, E.; Akhavan, A. Size-dependent genotoxicity of graphene nanoplatelets in human stem cells. Biomaterials 2012, 33 (32), 8017−8025. (26) Nurunnabi, M.; Khatun, Z.; Huh, K.; Park, S.; Lee, D.; Cho, K.; Lee, Y. In vivo biodistribution and toxicology of carboxylated graphene quantum dots. ACS Nano 2013, 7 (8), 6858−6867. (27) Dong, Y.; Li, G.; Zhou, N.; Wang, R.; Chi, Y.; Chen, G. Graphene quantum dot as a green and facile sensor for free chlorine in drinking water. Anal. Chem. 2012, 84 (19), 8378−8382. (28) Ran, X.; Sun, H.; Pu, F.; Ren, J.; Qu, X. Ag nanoparticledecorated graphene quantum dots for label-free, rapid and sensitive detection of Ag+ and biothiols. Chem. Commun. 2013, 49 (11), 1079− 1081. (29) Lin, L.; Rong, M.; Luo, F.; Chen, D.; Wang, Y.; Chen, X. Luminescent graphene quantum dots as new fluorescent materials for environmental and biological applications. TrAC, Trends Anal. Chem. 2014, 54, 83−102.
and coexisting cations could lead to anomalous aggregation behavior due to the self-recognition triggered and enhanced by the above-mentioned adjustments. Our understanding of the assembly of GQD aggregates should be updated, as there could be aggregates that are stable in solutions, as well as aggregates that would precipitate out of solutions. These are significantly important findings related to the safety of GQDs as delivery vectors for therapeutics and the health risks of GQDs released into the environment.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b04178. Sample preparation for microscopy, aggregation profiles (Figure S-1), hydrodynamic size distribution (Figure S2), morphology (Figure S-3), and attachment efficiencies of GQDs (Figure S-4) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 0086-571-88982587. Fax: 0086-571-88982587. E-mail:
[email protected]. ORCID
Baoliang Chen: 0000-0001-8196-081X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (Grant 21425730, 21537005, and 21621005), the National Basic Research Program of China (Grant 2014CB441106), the Doctoral Fund of Ministry of Education China (Grant J20130039), and USDA-NIFA Hatch program (MAS 00475).
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REFERENCES
(1) Trauzettel, B.; Bulaev, D. V.; Loss, D.; Burkard, G. Spin qubits in graphene quantum dots. Nat. Phys. 2007, 3 (3), 192−196. (2) Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic dirac billiard in graphene quantum dots. Science 2008, 320 (5874), 356−358. (3) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots. Adv. Mater. 2010, 22 (6), 734−738. (4) Yan, X.; Cui, X.; Li, B.; Li, L. Large, solution-processable graphene quantum dots as light absorbers for photovoltaics. Nano Lett. 2010, 10 (5), 1869−1873. (5) Li, L.; Yan, X. Colloidal graphene quantum dots. J. Phys. Chem. Lett. 2010, 1 (17), 2572−2576. (6) Zhu, S.; Zhang, J.; Qiao, C.; Tang, S.; Li, Y.; Yuan, W.; Li, B.; Tian, L.; Liu, F.; Hu, R.; Gao, H.; Wei, H.; Zhang, H.; Sun, H.; Yang, B. Strongly green-photoluminescent graphene quantum dots for bioimaging applications. Chem. Commun. 2011, 47 (24), 6858−6860. (7) Shen, J.; Zhu, Y.; Chen, C.; Yang, X.; Li, C. Facile preparation and upconversion luminescence of graphene quantum dots. Chem. Commun. 2011, 47 (9), 2580−2582. (8) Yan, X.; Cui, X.; Li, L. Synthesis of large, stable colloidal graphene quantum dots with tunable size. J. Am. Chem. Soc. 2010, 132 (17), 5944−5945. (9) Li, L.; Wu, G.; Yang, G.; Peng, J.; Zhao, J.; Zhu, J. Focusing on luminescent graphene quantum dots: Current status and future perspectives. Nanoscale 2013, 5 (10), 4015−4039. 1375
DOI: 10.1021/acs.est.6b04178 Environ. Sci. Technol. 2017, 51, 1364−1376
Article
Environmental Science & Technology (30) Ames, A. I.; Tsukada, Y.; Nesbett, F. B. Intracellular Cl−, Na+, K+, Ca2+, Mg2+, and P in nervous tissue; response to glutamate and to changes in extracellular calcium. J. Neurochem. 1967, 14 (2), 145−159. (31) Altura, B. M.; Altura, B. T. Interactions of Mg and K on blood vessels–aspects in view of hypertension. Review of present status and new findings. Magnesium 1983, 3 (4−6), 175−194. (32) Iwamoto, T.; Kita, S.; Zhang, J.; Blaustein, M. P.; Arai, Y.; Yoshida, S.; Wakimoto, K.; Komuro, I.; Katsuragi, T. Salt-sensitive hypertension is triggered by Ca2+ entry via Na+/Ca2+ exchanger type-1 in vascular smooth muscle. Nat. Med. 2004, 10 (11), 1193−1199. (33) Saleh, N. B.; Pfefferle, L. D.; Elimelech, M. Aggregation kinetics of multiwalled carbon nanotubes in aquatic systems: Measurements and environmental implications. Environ. Sci. Technol. 2008, 42 (21), 7963−7969. (34) Wu, L.; Liu, L.; Gao, B.; Muñoz-Carpena, R.; Zhang, M.; Chen, H.; Zhou, Z.; Wang, H. Aggregation kinetics of graphene oxides in aqueous solutions: Experiments, mechanisms, and modeling. Langmuir 2013, 29 (49), 15174−15181. (35) Chowdhury, I.; Mansukhani, N. D.; Guiney, L. M.; Hersam, M. C.; Bouchard, D. Aggregation and stability of reduced graphene oxide: Complex roles of divalent cations, pH, and natural organic matter. Environ. Sci. Technol. 2015, 49 (18), 10886−10893. (36) Chowdhury, I.; Duch, M. C.; Mansukhani, N. D.; Hersam, M. C.; Bouchard, D. Colloidal properties and stability of graphene oxide nanomaterials in the aquatic environment. Environ. Sci. Technol. 2013, 47 (12), 6288−6296. (37) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Mutual coagulation of colloidal dispersions. Trans. Faraday Soc. 1966, 62, 1638−1651. (38) Overbeek, J. T. G. The rule of Schulze and Hardy. Pure Appl. Chem. 1980, 52 (5), 1151−1161. (39) Cote, L. J.; Kim, F.; Huang, J. Langmuir− Blodgett assembly of graphite oxide single layers. J. Am. Chem. Soc. 2009, 131 (3), 1043− 1049. (40) Whitby, R. L.; Korobeinyk, A.; Gun’ko, V. M.; Busquets, R.; Cundy, A. B.; Laszlo, K.; Skubiszewska-Zięba, J.; Leboda, R.; Tombacz, E.; Toth, I. Y. pH-driven physicochemical conformational changes of single-layer graphene oxide. Chem. Commun. 2011, 47 (34), 9645− 9647. (41) Zhou, J.; Song, H.; Zhang, X.; Chen, X. Conformational changes of graphene nanosheets induced by metal: Melting metal can spin a graphene cocoon to encapsulate itself. Chem. Commun. 2014, 50 (15), 1886−1889. (42) Whitby, R. L.; Gun’ko, V. M.; Korobeinyk, A.; Busquets, R.; Cundy, A. B.; László, K.; Skubiszewska-Zięba, J.; Leboda, R.; Tombácz, E.; Toth, I. Y. Driving forces of conformational changes in single-layer graphene oxide. ACS Nano 2012, 6 (5), 3967−3973. (43) Wang, J.; Chen, Z.; Chen, B. Adsorption of polycyclic aromatic hydrocarbons by graphene and graphene oxide nanosheets. Environ. Sci. Technol. 2014, 48 (9), 4817−4825. (44) Wang, J.; Chen, B.; Xing, B. Wrinkles and folds of activated graphene nanosheets as fast and efficient adsorptive sites for hydrophobic organic contaminants. Environ. Sci. Technol. 2016, 50 (7), 3798−3808. (45) Yang, K.; Chen, B.; Zhu, X.; Xing, B. Aggregation, adsorption and morphological transformation of graphene oxide in aqueous solutions containing different metal cations. Environ. Sci. Technol. 2016, 50, 11066−11075. (46) Kim, F.; Cote, L. J.; Huang, J. Graphene oxide: Surface activity and two-dimensional assembly. Adv. Mater. 2010, 22 (17), 1954− 1958. (47) Compton, O. C.; Nguyen, S. T. Graphene oxide, highly reduced graphene oxide, and graphene: Versatile building blocks for carbonbased materials. Small 2010, 6 (6), 711−723. (48) Paredes, J.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. Graphene oxide dispersions in organic solvents. Langmuir 2008, 24 (19), 10560−10564. (49) Kim, J.; Cote, L. J.; Kim, F.; Yuan, W.; Shull, K. R.; Huang, J. Graphene oxide sheets at interfaces. J. Am. Chem. Soc. 2010, 132 (23), 8180−8186.
(50) Kim, J.; Cote, L. J.; Huang, J. Two dimensional soft material: New faces of graphene oxide. Acc. Chem. Res. 2012, 45 (8), 1356− 1364. (51) Luo, J.; Cote, L. J.; Tung, V. C.; Tan, A. T.; Goins, P. E.; Wu, J.; Huang, J. Graphene oxide nanocolloids. J. Am. Chem. Soc. 2010, 132 (50), 17667−17669. (52) Wang, X.; Bai, H.; Shi, G. Size fractionation of graphene oxide sheets by pH-assisted selective sedimentation. J. Am. Chem. Soc. 2011, 133 (16), 6338−6342. (53) Shih, C. J.; Lin, S.; Sharma, R.; Strano, M. S.; Blankschtein, D. Understanding the pH-dependent behavior of graphene oxide aqueous solutions: A comparative experimental and molecular dynamics simulation study. Langmuir 2012, 28 (1), 235−241. (54) Hassanzadeh, S.; Adolfsson, K. H.; Hakkarainen, M. Controlling the cooperative self-assembly of graphene oxide quantum dots in aqueous solutions. RSC Adv. 2015, 5 (71), 57425−57432. (55) Eigler, S.; Dotzer, C.; Hirsch, A. Visualization of defect densities in reduced graphene oxide. Carbon 2012, 50 (10), 3666−3673. (56) Sano, M.; Okamura, J.; Shinkai, S. Colloidal nature of singlewalled carbon nanotubes in electrolyte solution: The Schulze-Hardy rule. Langmuir 2001, 17 (22), 7172−7173. (57) Crittenden, J. C.; Trussell, R. R.; Hand, D. W.; Howe, K. J.; Tchobanoglous, G. MWH’s water treatment: principles and design; John Wiley & Sons: 2012. (58) Bharmoria, P.; Gupta, H.; Mohandas, V.; Ghosh, P. K.; Kumar, A. Temperature invariance of NaCl solubility in water: Inferences from salt-water cluster behavior of NaCl, KCl, and NH4Cl. J. Phys. Chem. B 2012, 116 (38), 11712−11719. (59) Chen, K. L.; Elimelech, M. Influence of humic acid on the aggregation kinetics of fullerene (C60) nanoparticles in monovalent and divalent electrolyte solutions. J. Colloid Interface Sci. 2007, 309 (1), 126−134. (60) Chen, B.; Huang, W. Effects of compositional heterogeneity and nanoporosity of raw and treated biomass-generated soot on adsorption and absorption of organic contaminants. Environ. Pollut. 2011, 159 (2), 550−556. (61) Su, Z.; Zhou, W.; Zhang, Y. New insight into the soot nanoparticles in a candle flame. Chem. Commun. 2011, 47 (16), 4700− 4702. (62) Liati, A.; Brem, B. T.; Durdina, L.; Vögtli, M.; Arroyo Rojas Dasilva, Y.; Dimopoulos Eggenschwiler, P.; Wang, J. Electron microscopic study of soot particulate matter emissions from aircraft turbine engines. Environ. Sci. Technol. 2014, 48 (18), 10975−10983. (63) Parent, P.; Laffon, C.; Marhaba, I.; Ferry, D.; Regier, T.; Ortega, I.; Chazallon, B.; Carpentier, Y.; Focsa, C. Nanoscale characterization of aircraft soot: A high-resolution transmission electron microscopy, Raman spectroscopy, X-ray photoelectron and near-edge X-ray absorption spectroscopy study. Carbon 2016, 101, 86−100. (64) Chen, X.; Chen, B. Direct observation, molecular structure, and location of oxidation debris on graphene oxide nanosheets. Environ. Sci. Technol. 2016, 50 (16), 8568−8577. (65) Lu, X.; Leng, Y. Theoretical analysis of calcium phosphate precipitation in simulated body fluid. Biomaterials 2005, 26 (10), 1097−1108. (66) Park, S.; Lee, K.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. Graphene oxide papers modified by divalent ions-Enhancing mechanical properties via chemical cross-linking. ACS Nano 2008, 2 (3), 572−578. (67) Medhekar, N. V.; Ramasubramaniam, A.; Ruoff, R. S.; Shenoy, V. B. Hydrogen bond networks in graphene oxide composite paper: structure and mechanical properties. ACS Nano 2010, 4 (4), 2300− 2306. (68) Whitby, R. L. Chemical control of graphene architecture: Tailoring shape and properties. ACS Nano 2014, 8 (10), 9733−9754.
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DOI: 10.1021/acs.est.6b04178 Environ. Sci. Technol. 2017, 51, 1364−1376
