Sustainable Carbon Nanodots with Tunable Radical Scavenging

Nov 25, 2015 - The application of polymers as an essential class of material was greatly inhibited due to the aging failure of these versatile materia...
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Research Article pubs.acs.org/journal/ascecg

Sustainable Carbon Nanodots with Tunable Radical Scavenging Activity for Elastomers Siwu Wu, Peijin Weng, Zhenghai Tang, and Baochun Guo* Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, P. R. China S Supporting Information *

ABSTRACT: The application of polymers as an essential class of material was greatly inhibited due to the aging failure of these versatile materials during normal use. Hence, it is generally recognized that stabilization against thermo-oxidative aging is indispensable to extend the service life of polymers for long-term applications. However, toxicity and pollution of the state-of-the-art antiaging technologies have long been puzzles in the polymer industry. Herein, sustainable carbon nanodots (CDs), synthesized by facile and cost-effective microwaveassisted pyrolysis, are used for first time as radical scavengers to resist the thermo-oxidative aging of elastomers. We have demonstrated that incorporation of the resultant CDs could be green and generic radical scavengers toward highly aging-resistant elastomers. Furthermore, by controlling the photoluminescent quantum yield of the CDs with various passivated agents, tunable radical scavenging activity was achieved. We established for the first time that the aging resistance originates from the prominent reactive radical scavenging activity of the CDs, which was rationally controlled by their photoluminescent quantum yield. KEYWORDS: Carbon nanodots, Radical-scavenging, Antioxidants, Photoluminescence, Elastomer

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INTRODUCTION The aging of polymeric materials greatly increases resource consumption and may be disastrous in some cases. As an important class of polymers in modern industries, dienic elastomers such as natural rubber (NR) and styrene butadiene rubber (SBR) are susceptible to thermo-oxidative aging due to the unsaturated chains and allylic hydrogens in their backbone chains.1 To increase the aging resistance and hence the reliability of polymers, a general solution is to include antioxidants to prolong their lifetime. However, most of the antioxidants are congenitally toxic inherited from their raw materials.2−7 Moreover, the industrialized processes for the manufacture of most antioxidants are quite complicated, timeconsuming, and eco-unfriendly. These inevitable puzzles inspire researchers to seek sustainable antioxidants with generic processing methods.8−15 As an alternative solution, several kinds of nanofillers, such as clays, carbon nanotubes, fullerene, and graphene, have been explored as new antioxidants. This approach has offered us an alternative insight into the design of the antiaging polymeric materials.16−19 Unfortunately, the high fabrication costs and sophisticated processing required for these materials (e.g., graphene-based composites) impede their applications in real-world application. Recently, carbon nanodots (CDs), as quasi-spherical carbonbased nanoparticles, have been widely reported because of their fantastic properties, such as strong photoluminescence (PL) emission, low toxicity, excellent biocompatibility, facile synthesis, and wide materials sources.20,21 Some studies have © XXXX American Chemical Society

demonstrated that CDs can not only act as electron donors but also as electron acceptors and accelerate the electron transfer process that originates from the high electron density and mobility of the π−π inner structure and peripheral functionalized moieties.22 Furthermore, CDs exhibited efficient radical scavenging property and antioxidant activity through in vitro assay.23 Herein, inspired by the unique characteristics of CDs, we synthesized several kinds of CDs via facile microwaveassisted pyrolysis and, for the first time, utilized CDs as novel radical scavengers for typical dienic rubber. Furthermore, the mechanism of antioxidation by amine-passivated CDs was analyzed by correlating the aging resistance of the rubbers to the CDs’ PL properties which originated from their inherent structure.

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EXPERIMENTAL SECTION

Materials and Chemicals. Citric acid (CA, 99%), 1,2-ethylenediamine (EDA, 98%), urea (99.3%), 1,4-butanediamine (BDA, 98%), triethylamine (TEA, 99%), L-ascorbic acid (≥99%), quinine sulfate (≥95%), and 2,2-diphenyl-1-picrylhydrazyl (DPPH, 95%) were purchased from Beijing InnoChem Science & Technology Co., Ltd., Beijing, China. All chemicals were used as received without further purification. Styrene−butadiene rubber (Trademark: SBR1502, styrene content 23.5 wt %) was purchased from Jilin Chemical Industry Co., Jilin, China. Nature rubber (Trademark: NR RSS3) was Received: September 13, 2015 Revised: November 8, 2015

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Figure 1. Schematic showing the synthesis of CDs with various passivation agents. Characterization. High-resolution transmission electron microscopy (HRTEM) was performed on a Philips Tecnai F20 TEM microscope (Philips, Netherlands) operated at an accelerating voltage of 200 kV. The samples for HRTEM were prepared by dropping an aqueous solution onto a copper grid coated with carbon film. UV−vis absorption spectra were recorded at room temperature using a Scinco S3100 UV−vis spectrophotometer (Scinco, Korea). Photoluminescence emission spectra were measured on an Edinburgh FL920 fluorescence spectrometer (Edinburgh Instruments, Britain). Isothermal differential scanning calorimetry (DSC) analysis was performed on a Q200 differential scanning calorimeter (TA Instruments, USA) at 180 °C following ISO standard 11357-6:2005. Electron spin resonance (ESR) spectra were obtained using a JES-FA 200 X-band electron spin resonance spectrometer (JEOL, Japan) with a temperature accessory (JEOL, Japan). To collimate the center and strength of magnetic field, manganese (Mn2+) was inserted beside the specimen as external standard. Fourier transform infrared spectra (FTIR) were measured on a Bruker Vertex 70 FTIR spectrometer (Bruker, German). X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 equipped with an Al Kα radiation source (Thermo Fisher Scientific, USA). Tensile tests were conducted at room temperature with a Gotech AI-7000 S servo control system Universal testing machine (Gotech, Taiwan) following ISO standard 37-2005. The cross-link density of the composites was determined by the equilibrium swelling method and was calculated using the Flory− Rehner equation. Dynamic mechanical analysis was performed on a TA Q800 dynamic mechanical analyzer (TA Instruments, USA). The tests were performed in tensile mode at a frequency of 1 Hz and a strain of 0.5%. The scanning temperature ranged from −80 to 100 °C with a heating rate of 3 °C/min under flowing liquid nitrogen. The quantum yield (QY) of the resultant amine-passivated CDs was determined by a comparative method involving quinine sulfate with a known QY (54%) as a standard sample. All samples were dissolved in distilled water at different concentrations, and the corresponding UV− vis absorbance was not allowed to exceed 0.1 in the 10 mm fluorescence cell to minimize reabsorption effects. The fluorescence spectra of solutions were recorded at their maximum excitation wavelength (for EDA-CDs, 360 nm; UREA-CDs, 340 nm; BDA-CDs, 360 nm; TEA-CDs, 380 nm). The integrated fluorescence intensity was calculated from the whole range of the spectra. A graph of the integrated fluorescence intensity vs the corresponding absorbance was plotted, and a trend line was added with an intercept of 0. The absolute values were calculated according to the following equation:

purchased from Guangzhou Rubber Institute, Guangzhou, China. The other rubber additives were industrial grade and were used as received. Synthesis Procedure for the Amine-Passivated CDs. The amine-passivated CDs were synthesized by a facile microwave-assisted pyrolysis method that has been described elsewhere.24,25 Briefly, 1.0 g of CA was dissolved in a 100 mL beaker containing 10 mL phosphate solution, and 0.313 g of EDA was added under stirring. The mixture was heated in a domestic 750 W microwave oven for 3 min, during which the colorless solution changed to a red−brown solid, indicating the formation of amine-passivated CDs. The resulting solid was dissolved in the minimum amount of distilled water. The red−brown aqueous solution was then precipitated and washed with excess anhydrous ethanol several times to remove soluble residue. The purified resultant CDs were dried in vacuum at 50 °C and were stored for further use. Other CDs passivated with urea, BDA, or TEA were synthesized through a similar procedure by microwave-assisted pyrolysis of a mixed solution containing CA and urea, BDA or TEA, respectively; the molar ratio between −COOH and −NH2 was maintained at 3:2. The reaction time of the UREA-CDs, BDA-CDs, and TEA-CDs was 4, 3, and 4 min, respectively. Preparation of Rubber Composites. Various purified aminepassivated CDs were mixed with SBR in an open two-roll mill. The aforementional compounds were then subjected to isothermal DSC analysis to measure the oxidative induction time (OIT). The curing agents were then incorporated with the remaining compounds, followed by hot-pressing at 150 °C for the optimum vulcanizing time, as determined by a U-CAN UR-2030 vulcameter. The samples are referred to as SBR/xy, where x denotes x parts of the aminepassivated CDs (relative to 100 parts of SBR: phr) and y corresponds to the type of amine-passivated CDs. The specific formulations are expressed as parts per hundreds of rubber and are listed as SBR 100, amine-passivated CD variable, zinc oxide 5, stearic acid 2, Ncyclohexyl-2-benzothiazole sulfonamide (CZ) 1.5, and sulfur 1.5. In addition, NR composites with various EDA-CDs were prepared according to the aforementioned process and basic recipe. The wellmixed compounds were hot-pressed at 143 °C for the optimum vulcanizing time determined by a U-CAN UR-2030 vulcameter. For comparison, SBR and NR were mixed with 1.5 phr commercial antioxidant N-isopropyl-N′-phenyl-p-phenylenediamine (4010NA); the resulting samples were SBR/1.5 phr 4010NA and NR/1.5 phr 4010NA, respectively. All the rubber samples suffered thermo-aging in a UA-2071A aging oven tester with hot air at 100 °C for various times. B

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Figure 2. (a) HRTEM image of EDA-CDs. (b) UV−vis spectrum and the PL emission spectrum of EDA-CDs aqueous solution. (c) UV−vis spectra of DPPH ethanol solution upon the addition of EDA-CDs solution. (d) ESR spectrum of EDA-CDs.

⎛ GradX ⎞⎛ ηX2 ⎞ ΦX = ΦST⎜ ⎟⎜⎜ 2 ⎟⎟ ⎝ GradST ⎠⎝ ηST ⎠

sustainable. More importantly, the EDA-CDs has been demonstrated to have low or even no toxicity toward cells.24 The HRTEM image (Figure 2a) clearly shows that the asprepared EDA-CDs are narrowly distributed and essentially amorphous, with diameters of 2.5−3.5 nm. The UV−vis spectra of EDA-CDs show an absorption at approximately 240 nm, which is attributable to the π−π* transition, and a typical peak at approximately 340−360 nm, which is characteristic of graphite structure.23 Meanwhile, the aqueous solution of EDACDs exhibited bright blue fluorescence under excitation at 360 nm (Figure 2b). Detailed PL study of EDA-CDs with vary excitation wavelengths ranging from 300 to 420 nm were carried out (Figure S1). The excitation-independent-emission phenomenon of EDA-CDs is detected, which also indicates relatively uniform size and structure of EDA-CDs are achieved. Moreover, as confirmed by FTIR and XPS, a large amount of −NH2 groups and amide carbonyl groups are detected on the surface of the resultant EDA-CDs (Figure S2 and S3). These results, which are consistent with those of a previous study,24 indicate that EDA-CDs were successfully prepared. As a wellknown test for the evaluation of the radical scavenging ability of compounds, DPPH radical assay with certain alterations was performed to estimate the radical scavenging ability of the EDA-CDs.17,26 After the addition of EDA-CDs aqueous solution, the purple DPPH solution turned yellow and the absorbance peaks at 517 nm in the UV−vis spectrum apparently decreased in intensity (Figure 2c). The radical inhibition calculated according to the previous studies17 revealed that the corresponding radical scavenging ability of EDA-CDs was distinctly higher than that of L-ascorbic acid (a standard radical scavenger), which indicated the resultant EDACDs were highly efficient radical scavengers (Figure S4). The ESR spectra of EDA-CDs showed a Lorentzian shaped signal at g-value ∼ 2.001 (Figure 2d), which indicated a singly occupied orbital in ground-state carbon nanodots.27,28 This indicated that

where the subscripts ST and X represent the standard sample and test samples, respectively, Φ is the fluorescence quantum yield, Grad is the gradient from the plot of the integrated fluorescence intensity vs UV− vis absorbance, and η is the refractive index of the solvent. The radical inhibition activity of EDA-CDs was determined using DPPH free radical assay with certain alteration involving L-ascorbic acid as a standard sample according to the previous studies.17,23 The DPPH solution in ethanol and EDA-CDs aqueous solution were freshly prepared with concentration of 0.05 mg/mL. The radical scavenging activity was measured by monitoring the decline of absorption peak of DPPH at 517 nm in UV−vis spectra with incremental amounts of EDA-CDs solution. The radical inhibition activity was calculated as follows:

⎡ A − Ai ⎤ inhibition = ⎢1 − m ⎥ A0 ⎦ ⎣ Where Am is the absorbance of the DPPH solution (2 mL) with diluted EDA-CDs solution (1 mL) at various concentrations (ranged from 0 to 0.05 mg/mL, concentration step: 0.005 mg/mL), Ai is the absorbance of corresponding diluted EDA-CDs solution (1 mL) with the addition of 2 mL ethanol, A0 is the absorbance of DPPH solution (2 mL) with the addition of 1 mL distilled water. All the reaction mixtures were left in dark place for 40 min then receiving UV−vis test.

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RESULTS AND DISCUSSION The overall synthetic process is illustrated in Figure 1. CA, as a typical renewable resource, was used as the carbon source and EDA was used as a surface passivation agent to fabricate CDs with peripheral amine groups. Furthermore, a series of amine molecules, including urea, BDA, and TEA were utilized as passivation agents to achieve the rational design of CDs with a tunable photoluminescent QY and nitrogen content.24,25 Notably, all of the CDs were synthesized via the microwaveassisted method, which is fast, high-yielding, cost-effective, and C

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Figure 3. (a) DSC of all SBR compounds without curatives after the purge gas was switching from nitrogen to oxygen at 180 °C (OIT is measured by tangent method as shown). (b) ESR spectra of SBR/1.5 phr EDA-CDs before and after in situ thermo-aging. (c) Evolution of the FTIR spectrum for SBR/1.5 phr EDA-CDs during aging. (d) Relative intensity of the CO peak vs the aging time. (e) Evolution of the XPS spectrum for SBR/1.5 phr EDA-CDs during aging. (f) The O/C molar ratio vs the aging time.

verified as a broad asymmetric singlet due to the orbital of the unpaired electron being confined among two oxygen atoms.35−37 The g-values of peroxy radicals are always higher than 2.003 because coupling with heavier oxygen atoms increases the spin−orbit coupling of the electron on peroxy radicals.30,38 According to this, we suggest that contributions from peroxy radicals to the singlet in SBR composites should be taken into account too. After in situ thermo-oxidative aging, the intensity of the signal for blank SBR increased acutely which revealed a sharp growth of radical concentration inside blank SBR matrix during in situ aging (Figure S6a). The addition of 4010NA reduced the intensity of the singlet of radicals compared to that for the blank SBR but it was still not satisfied (Figure S6b). For the SBR/1.5 phr EDA-CDs, the intensity and shape of this singlet were almost unaltered after aging, which meant the radical concentration was not substantially altered, indicating the superior inhibition effect on the reactive radicals of EDA-CDs (Figure 3b). Furthermore, the carbonyl index and oxygen/carbon molar ratio (O/C) were measured by FTIR and XPS, respectively. As shown in Figure 3c, the change of the intensity of the peaks associated with the carbonyl groups (CO) and CC double bonds were significantly inhibited, which reveals that the oxidation reaction was effectively delayed by incorporating EDA-CDs. Taking the bending vibration peak of the stable aromatic C−H bond at 760 cm−1 as the internal standard peak, we calculated the relative CO and CC peak intensities for all SBR composites vs the aging time, as shown in Figure 3d and Figure S7, respectively. With increasing aging time, the SBR/ 1.5 phr EDA-CDs exhibited a weakest increase of the relative peak intensity of CO because the degradation rate of CC was remarkably slowed in comparison to those of blank SBR and SBR/1.5 phr 4010NA (Figure S7c). In addition, the intensity of the O 1s peak centered at about 530 eV in the XPS spectrum of SBR/1.5 phr EDA-CDs (Figure 3e) shows the smallest increase after aging (up to 16 days) compared to those of the blank SBR and the SBR/1.5 phr 4010NA (Figure S8).

EDA-CDs could serve as electron acceptors to stabilize and scavenge reactive radicals during reaction. To achieve a homogeneous dispersion of nanocarbons, such as graphene, in elastomers, previous studies generally utilized solution blending method, which requires a sophisticated process or massive volumes of solvents. However, because the aromatic skeleton of CDs is similar to that of aromatic amine antioxidants, CDs are miscible with dienic elastomer. Hence, all of the CDs were mixed into SBR via traditional two-roll milling, a common rubber processing procedure. To evaluate the antioxidative efficiency of EDA-CDs, OIT of the SBR/EDA-CDs was measured by DSC analysis at 180 °C (Figure 3a). As expected, the addition of EDA-CDs considerably prolonged the OIT of the SBR compounds (Figure S5). In comparison with 4010NA, a well-known antioxidant for its balanced performance of antiaging in tire industry, an equivalent amount of EDA-CDs performed unexpected talent in preventing thermo-oxidative aging, prolonging the OIT to 17 min. A key factor in inhibiting the thermo-oxidation is to capture and stabilize the radicals as soon as possible. To identify the capacity of the radical scavenging activity of the antioxidant in the SBR composite, ESR measurement was carried out during in situ thermo-oxidative aging. All SBR composites exhibited a broad singlet with a gvalue ∼ 2.004 which was typical for free radicals (g-value ∼ 2.0) (Figures 3b and S6). In general, it is inclined to generate alkyl and allyl radicals due to the high reactivity of α-hydrogen in the oxidation process of SBR matrix.29 However, the ESR signals of such radicals will show a multiplet with hyperfine splitting so that can be ruled out in this system.30,31 However, it has been reported that the allyl radicals in SBR tended to transform into polyenyl radicals with a g-value ∼ 2.004 under thermal condition.32−34 Hence, it is reasonable to believe that some polyenyl radicals have contributions to the ESR signals in SBR composites. Moreover, the formation of peroxy radicals is another inevitable section during the oxidation process of polymer. The typical ESR signal of the peroxy radical has been D

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Figure 4. (a) EAB vs aging time. (b) Cross-link density vs aging time. (c) Evolution of temperature-dependent tan δ curves for SBR/1.5 phr EDACDs during aging. (d) Relative tan δ peak values vs aging time.

Figure 5. (a) EAB retention for the SBR/amine-passivated CDs with increasing aging time. (b) PL QY of amine-passivated CDs. (d) Interplay among the PL QY of amine-passivated CDs, increase in the O/C molar ratio and EAB retention of SBR/amine-passivated CDs (after 6 days of aging). (d) Schematic of the thermo-antioxidative mechanism of amine-passivated CDs.

EAB) and an increase in cross-link density (Figure 4a, b), due to the recombination of reactive radicals generated from aging.39 The retention of EAB of the blank SBR drastically decreased to only 20% after aging for 16 days (Figure S9a). Although the incorporation of 1.5 phr 4010NA resulted in significant improvement of the EAB after aging, the final retention of EAB was still only 30%. Surprisingly, in the case of SBR/1.5 phr EDA-CDs, the final retention of EAB was up to 42% and the corresponding EAB was approximately 50% higher than that of SBR/1.5 phr 4010NA, which suggests that EDACDs are a more efficient antioxidant than 4010NA. Moreover, incremental incorporation of EDA-CDs led to consistent improvement in the retention of EAB after aging but did not

The corresponding O/C molar ratio for all of the SBR composites also confirmed that the increase of the O/C ratio of the SBR/1.5 phr EDA-CDs was the lowest (Figure 3f). This remarkable reduction in the O/C molar ratio is attributed to the superior antioxidant activity of the EDA-CDs, which inhibit the oxidation process of the SBR matrix by capturing and scavenging reactive radicals. Because the addition of EDA-CDs prominently inhibits the oxidation of the SBR matrix, which generates a significant difference within the composite, evaluating the effects of EDACDs on the mechanical properties of SBR during thermooxidative aging is still important. In both cases, the aging process led to a reduction in stretchability (elongation at break, E

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to further demonstrate our proposal, the O/C molar ratios and retentions of EAB for the SBR/CDs composites after aging for 6 days are calculated and listed in Table S2. Obviously, the increase of O/C molar ratio for SBR/CDs composites is inversely proportional to the PL QY of CDs while the corresponding retentions of SBR/CDs composites go the opposite way as shown in Figure 5c (The separated figures can be found in Figure S15). This result is reasonable because the PL performance of amine-passivated CDs is also determined by surface passivated groups and intrinsic energy traps,24,41 both of which can capture and stabilize reactive radicals to prevent the oxidation of the SBR matrix. A schematic mechanism of aminepassivated CDs as superior antioxidants is given in Figure 5d. This confirms our above proposal that the PL QY of CDs can act as the sensitive and specific indicator of their antioxidant activity because they have similar influencing factors. In summary, we have utilized a facile, sustainable, costeffective, microwave-assisted pyrolysis method to synthesize amine-passivated CDs and, for the first time, to incorporate the sustainable CDs with dienic elastomers as novel antioxidants via the commonest two-roll milling process. The resultant aminepassivated CDs exhibited superior thermo-antioxidant activity to suppress the oxidation process of dienic elastomers, as evidenced by the higher retention of the static and dynamic mechanical performances of the SBR composites compared to those of the commercial antioxidant 4010NA. By controlling the nitrogen content and PL QY of amine-passivated CDs with various amine passivation agents, we achieved tunable antioxidative capability for the resultant CDs. We attribute the superior antioxidant activity of the amine-passivated CDs to their prominent reactive radical scavenging activity, which could be rationally controlled by the PL QY of the CDs. The present work provides new insight into the green and generic design of highly aging-resistant polymers with CDs and into understanding antiaging performance through the PL properties of CDs.

deteriorate the tensile strength (TS) of the composites (Figure S9). Dynamic mechanical analysis was performed to obtain more detailed information about the effects of different antioxidants on the chain relaxation. After thermo-oxidative aging, the peak value of the loss tangent (tan δ) for blank SBR exhibited a substantial decrease (Figure S10a), which contrasts sharply with the slight reduction of that of the SBR/1.5 EDACDs (Figure 4c). The addition EDA-CDs generated minimal variations in the relative peak value of tan δ (Figure 4d) and in the shift of temperature of the tan δ peak (ΔTg) (Figure S10). In general, reactive radicals are considered to dominate the cross-linked reaction in the whole process of the aging of SBR.39 Hence, the remarkable preservation of the static and dynamic mechanical properties of SBR/1.5 phr EDA-CDs demonstrates that the thermo-antioxidant activity of the EDACDs is definitely superior to that of 4010NA because of the outstanding ability of the EDA-CDs to capture and stabilize the radicals. On the basis of the combined aforementioned evidence, the EDA-CDs are reasonably believed to exhibit superior thermoantioxidant activity for dienic elastomers because of their prominent radical scavenging capability. Although the diameters of the EDA-CDs are only 2−3 nm, their particulate nature could prevent them from blooming during long-term storage, which is a challenging issue for many organic antioxidants (Figure S11). In addition, by incorporating EDA-CDs with NR through a similar process, we have demonstrated that EDACDs are a superior antioxidant to NR on the basis of the retention of EAB and TS of the NR/EDA-CDs composites after aging (Figure S12). However, the mechanism of the superior thermo-antioxidant activity of the EDA-CDs was unclear until now. By utilizing amine molecules as passivation agents, the surface of the EDA-CDs contains abundant −NH2 groups, which may serve as proton donors to terminate reactive radicals, similar to the function of p-phenylenediamine.40 Hence, we fabricated four types of amines-passivated CDs by using different amines with various nitrogen contents as surface passivation agents (Figure 1). By incorporating four aminespassivated CDs with SBR, we verified the antioxidant activity of different amine-passivated CDs on the basis of the static mechanical properties of the corresponding composites after aging. As shown in Figure 5a, the retentions of EAB at various aging times (the corresponding EAB are shown in Figure S13) are generally degressive with decreasing N content of the CDs (Table S1). This result confirms the previous speculation that the amount of peripheral amine groups is one of the major factors affecting the antioxidant activity of CDs. Curiously, the N contents of the EDA-CDs and UREA-CDs were approximately the same; however, the retentions of EAB for the corresponding composites after aging apparently differed. Notably, the amines serve as not only passivation agents but also N-doping precursors. Doped N atoms disturb the intrinsic regularity of the CDs, resulting in a highly defected structure with a mass of energy traps in the intrinsic carbon core which can also stabilize the reactive radicals. These traps and the surface passivated −NH2 groups both contribute to the PL property of CDs.24,41 Figure S14 shows the integrated PL intensity of all CDs vs the absorbance, while the calculated PL QYs of all CDs are shown in Figure 5b and the corresponding numerical values are listed in Table S1. By correlating the retention of EAB at various aging times and the PL QY of different CDs, we speculate that the PL QY of CDs can act as the specific indicator of the antioxidant activity of CDs. In order

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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/acssuschemeng.5b01069. Abbreviation list, PL emission spectra, FTIR spectra and high-resolution C 1s and N 1s spectra of EDA-CDs; Comparison of the radical scavenging activity of EDACDs and L-ascorbic acid; DSC analysis of SBR/EDACDs compounds, ESR and DMA spectra of blank SBR and SBR/4010NA composites, FTIR and XPS spectra of all rubber composites, mechanical properties of SBR/ EDA-CDs and NR/EDA-CDs, and PL QY and elemental analysis of CDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 20 87113374. Fax: +86 20 22236688. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2015CB654703), National Natural Science FoundaF

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tion of China (Nos. 51222301, 51473050 and 51333003), Fundamental Research Funds for the Central Universities (Nos. 2015PT003 and 2015ZM011) and Natural Science Foundation of Guangdong Province (2014A030310435 and 2014A030311051).

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DOI: 10.1021/acssuschemeng.5b01069 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX