Visualization of Adsorption: Luminescent Mesoporous Silica-Carbon

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Visualization of Adsorption: Luminescent Mesoporous Silica-Carbon Dots Composite for Rapid and Selective Removal of U(VI) and in Situ Monitoring the Adsorption Behavior Zhe Wang, Chao Xu, Yuexiang Lu,* Fengcheng Wu, Gang Ye, Guoyu Wei, Taoxiang Sun, and Jing Chen*

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Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing, People’s Republic of China, 100084 S Supporting Information *

ABSTRACT: The removal and separation of uranium from aqueous solutions are quite important for resource reclamation and environmental protection. Being one of the most effective techniques for metal separation, adsorption of uranium by a variety of adsorbent materials has been a subject of study with high interest in recent years. However, current methods for monitoring the adsorption process require complicated procedures and tedious measurements, which hinders the development of processes for efficient separation of uranium. In this work, we prepared a type of luminescent mesoporous silica-carbon dots composite material that has high efficiency for the adsorption of uranium and allows simultaneous in situ monitoring of the adsorption process. Carbon dots (CDs) were prepared in situ and introduced onto amino-functionalized ordered mesoporous silica (SBA-NH2) by a facile microplasma-assisted method. The prepared CDs/SBA-NH2 nanocomposites preserved the high specific surface area of the mesoporous silica, as well as the fluorescent properties of the CDs. Compared with bare SBA-NH2, the CDs/SBA-NH2 nanocomposites showed much improved adsorption ability and excellent selectivity for uranyl ions. Moreover, the fluorescence intensity of the composites decreased along with the increase of uranium uptake, indicating that the CDs/SBA-NH2 nanocomposites could be used for onsite monitoring of the adsorption behavior. More interestingly, the adsorption selectivity of the composites for metal ions was in good agreement with the selective fluorescence response of the original CDs, which means that the adsorption selectivity of CDsbased composite materials can be predicted by evaluating the fluorescence selectivity of the CDs for metal ions. As the first study of CDs-based nanocomposites for the adsorption of actinide elements, this work opens a new avenue for the in situ monitoring of adsorption behavior of CDs-based nanocomposites while extending their application areas. KEYWORDS: carbon dots, adsorption, uranium, fluorescence, in situ monitor

1. INTRODUCTION With the rapid increase in the number of nuclear power plants worldwide, the consumption of uranium is predicted to continuously increase.1,2 In the meantime, the uranium wastewater produced during the nuclear fuel mining and recycling presents high hazardous risks to human beings and the environment.3−5 The monitoring and removal of uranium is of great significance for both economic benefits and environmental protection.6 Among many separation techniques, adsorption has been recognized as a promising method due to its high efficiency and ease in operation.7 A significant amount of research efforts have been devoted to exploring efficient adsorbent materials.8−10 Recently, carbon materials such as activated carbon,11 carbon nanotubes,12 graphene,13 and mesoporous carbon14 have been used for separation of uranium because of their high specific surface area, structural diversity, and accessibility for surface functionalization. However, to monitor the adsorption process, complicated (pre)treatments are often required, since a series of samples © 2017 American Chemical Society

must be collected continuously during the adsorption process and taken for analysis using sophisticated techniques such as ICP-MS or ICP-AES.14−16 It is highly needed to develop materials and facile strategies that allow simultaneous in situ monitoring of the adsorption process while achieving high efficiency of adsorption. Carbon dots (CDs) have raised substantial research enthusiasm in recent years,17−21 because they have a few advantages including (1) fascinating fluorescence emission properties, (2) chemical/biological stability, (3) the ease of dispersion in common solvents,22 and (4) the ease in the preparation of CDs with various functional groups on the surfaces by changing raw materials or reaction conditions. With the functional groups, CDs could specifically interact with metal ions so that they have been widely applied in the areas Received: October 21, 2016 Accepted: February 6, 2017 Published: February 6, 2017 7392

DOI: 10.1021/acsami.6b13427 ACS Appl. Mater. Interfaces 2017, 9, 7392−7398

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustrative Adsorption and Monitoring of Uranium with CDs/SBA-NH2 Nanocomposites

obtained from J&K Scientific Co. Other analytical chemicals were received from Beijing Chemical Works, including sodium hydroxide (NaOH), nitric acid (HNO3), ethanol, hydrochloric acid (HCl), and all the metal salts including KNO3, Mg(NO3)2, Sr(NO3)2, Ba(NO3)2, Cr(NO3)3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2, Cd(NO3)2, Pb(NO3)2, and Ce(NO3)3. All the solutions were prepared with deionized water, and all reagents with the analytical grade were used as received. 2.2. Characterizations. Surface morphology of the aminofunctionalized SBA-15 was characterized by an LEO 1530 scanning electron microscope (SEM). Transmission electron microscope (TEM) images were recorded on an HT-7700 microscope at an accelerating voltage of 120 kV. Elemental analysis of C, H, and N was performed on an Elementar Vario EL III. The composition of the nanocomposite materials was characterized with the 250XI X-ray Photoelectron Spectroscopy (XPS) spectrometer equipped with a mono Al Kα X-ray source (1361 eV). The Rigaku D/max-2400 X-ray powder diffractometer with Cu Kα radiation was applied for the smallangle X-ray diffraction (SAXRD) patterns of the nanocomposite materials. Specific surface areas were evaluated by the Brunauer− Emmett−Teller (BET) method, and the Barrett−Joyner−Halenda (BJH) method was employed for the analysis of the pore size distribution. The FluoroMax-4 spectrophotometer was used for evaluating the fluorescent behaviors of the nanocomposite materials. FT-IR spectra were recorded by a Nicolet Nexus 470 FT-IR spectrometer with KBr pellets method. The concentration of uranium and other metal ions was measured by the arsenazo III method with a 721 type spectrophotometer at 652 nm and inductively coupled plasma-atomic emission spectrometry (ICP-AES, SPECTRO ARCOS SOP), respectively. 2.3. Preparation of NH2-Functionalized SBA-15 (SBA-NH2). The large-pore and short-channel −NH2-functionalized ordered mesoporous silica SBA-15 was prepared through a modified cocondensation according to the previous report.28,29 Typically, 2.96 g (0.5 mmol) of template agent Pluronic P123 and 5.71 g (0.06 mol) of MgCl2 were dissolved in aqueous solution containing 14.8 mL of 37 wt % HCl and 100 mL of deionized water. Then, TMOS (3.88 g, 0.025 mol) was added and the resultant solution was prehydrolyzed at 40 °C for 2 h under vigorous agitation before APTMS (0.80 g, 0.0045 mol) was slowly added in the mixture. After that, another agitation was needed for 24 h at 40 °C, and then the solution was moved to a Teflon-lined stainless steel autoclave where it was aged at 100 °C under static condition for another 24 h. The molar ratio of the mixture was 0.85 TMOS/0.15 APTMS/0.017 P123/2.04 MgCl2/5.9 HCl/ 206.5 H2O. After filtering, the as-prepared product denoted as SBANH2 was washed with DI water and ethanol and Soxhlet extracted with ethanol/HCl (V/V = 49/1) solution to remove the template Pluronic P123. Finally, the SBA-NH2 powder was dried at 80 °C under vacuum for 24 h. 2.4. Preparation of CDs/SBA-NH2 Composite Materials. The CDs modified SBA-NH2 nanocomposite material (CDs/SBA-NH2) was synthesized using the method established in our previous work.27 In that work, a facile and fast method was developed to prepared carbon dots with the assistance of atmospheric-pressure microplasma. Herein, the same method was used to synthesize the CDs/SBA-NH2

such as metal ion detection with high sensitivity and selectivity and effective adsorption of metal ions. To overcome the technical challenge of using water-dispersed CDs in adsorption application,23 recent research efforts have been devoted to developing CD-based composite materials for the decontamination of hazardous metal ions or organic dyes. For example, Zhang et al. prepared the layered double hydroxide-carbon dot composite for the adsorption of anionic organic dye.24 A 2D hexagonal mesoporous silica assembled with carbon dots composite was synthesized by Zhang et al. and applied for the competitive adsorption between metal ions and 2,4dichlorophenol.25 Also, a magnetic ferrite-MoS2-carbon dot nanohybrid adsorbent was used for efficient Pb(II) removal.26 In these previous studies, however, CDs were used only to provide additional sites for improved adsorption performance. The fluorescence property, the most important property of CDs, was not utilized or even characterized. Furthermore, up to now, there have been no reports on utilizing CDs-based materials for uranium adsorption. On the basis of our previous study where CDs prepared with the assistance of microplasma displayed high sensitivity and good selectivity in the detection of uranium,27 we have conducted the present study to prepare a kind of mesoporous silica-carbon dots nanocomposite, CDs/SBA-NH2 (Scheme 1), by in situ synthesis of CDs onto amino-functionalized ordered mesoporous silica (SBA-15 type). The prepared CDs/SBANH2 nanocomposites preserved the high specific surface area of the mesoporous silica as well as the optical properties of the carbon dots. The composites showed good fluorescence response to U(VI) as well as enhanced adsorption performance over the bare SBA-NH2. Interestingly, the fluorescent response exhibited a close connection with the adsorption behavior of the composites toward U(VI), making the adsorption process “visible” so that it is possible to monitor the adsorption process in situ. Moreover, the selectivity of CDs/SBA-NH2 is high, manifested by both the adsorption ability and the fluorescent response in solutions of uranium with competing metal ions. These results suggested that the fluorescence response might be used for screening CDs for the selective adsorption of different metal ions, before the fabrication of CDs-based composites. This is the first study on CDs-based nanocomposites that are used for adsorption of actinides while allowing in situ monitoring of the adsorption behavior with the fluorescent response.

2. EXPERIMENTAL SECTION 2.1. Materials. Citric acid and ethylenediamine were purchased from Alfa Aesar. Surfactant Pluronic P123 (Mav ∼ 5800), tetramethoxysilane (TMOS), magnesium chloride (MgCl2), and [3-(2aminoethyl)aminopropyl]trimethoxysilane (APTMS, 97%) were 7393

DOI: 10.1021/acsami.6b13427 ACS Appl. Mater. Interfaces 2017, 9, 7392−7398

Research Article

ACS Applied Materials & Interfaces nanocomposite. In brief, the pores of SBA-NH2 were first filled with the raw materials of CDs by immersing SBA-NH2 into the solution of citric acid and ethylenediamine. Citric acid (0.96 g), ethylenediamine (500 μL), SBA-NH2 precursor (30 mg), and NaNO3 (50 mg) were dissolved in deionized water (10 mL). The obtained mixture was dispersed by ultrasonication for 10 min. Then, under the treatment of an atmosphere microplasma, CDs were formed in situ in the pores with the polymerization of citric acid and ethylenediamine. An “H” type glass reactor was used for the preparation of the nanocomposite materials, in which microplasma acted as anode and Pt as cathode. The current between two electrodes was fixed to 10 mA, and the reaction time lasted for 40 min. Finally, the products were centrifuged and washed with water and ethanol and dried at 80 °C under vacuum. The obtained powder showed a yellow-brown color instead of the white color of the original SBA-NH2. 2.5. Uranium Adsorption. Solutions of uranium with varied concentrations at different pHs were prepared by adding appropriate volumes of HCl or NaOH solutions, and the pH of solutions was monitored with a PHS-3C model pH meter. Batch experiments were performed at 25 °C in a thermostatic air bath with a 0.5 mg/mL phase ratio (5 mg of adsorbent and 10 mL of U(VI) solution). After 24 h contact time, the solutions were separated and filtered with a 0.45 μm micropore filter membrane for analysis. The batch adsorption experiments were done in triplicates with a relative error of less than 5%. The initial and residual concentrations of U(VI) were determined by the Arsenazo III method30 at a wavelength of 652 nm, and the adsorption capacity (qe) was determined as follows qe =

Figure 1. (a) Fluorescent intensity of CDs/SBA-NH2 synthesized by using microplasma with different treatment times (λex = 360 nm). (b) Fluorescence spectra of CDs/SBA-NH2 at excitation wavelengths from 320 to 440 nm. m/v = 0.5 mg/mL. (c) The photographs of different emission colors under excitation of UV, blue, and green light, respectively.

clearly observed under fluorescence microscopy by the excitation of UV, blue, and green light, respectively (Figure 1c). The loading of CDs onto SBA-NH2 was further confirmed by the morphology characterization and spectrum analysis. The TEM images of CDs/SBA-NH2 verified that small CDs (2.51 ± 0.49 nm, Figures 2a, S1a) were incorporated into the well-

(C0 − Ce)·V m

where C0 and Ce (mg/L) are the initial and equilibrium concentrations of U(VI). V is the volume of the U(VI) solution (L), and m is the weight of the adsorbents (g), respectively. To evaluate the selectivity of CDs/SBA-NH2 adsorbents for various metal ions, a solution containing different metal ions was prepared and the initial concentration of all of these metal ions was fixed to be 100 mg/L. The competitive adsorption experiments lasted for 24 h at 25 °C, and the concentration of the metal ions after adsorption was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). 2.6. In Situ Monitoring of Uranium Adsorption. To verify the fluorescence property of the CDs/SBA-NH2 toward U(VI), a series of experiments were designed. The solutions were prepared in the same way with the uranium adsorption experiments described in section 2.5. After adding the composites to the solutions, the mixtures were treated by ultrasound for about 2 min to make sure the composites were dispersed in the solutions uniformly. Then, the fluorescence of the mixtures was measured on the FluoroMax-4 spectrophotometer immediately. In the selectivity test, pure metal ion solution with an initial concentration of 100 mg/L was prepared. After being mixed with CDs/SBA-NH2 adsorbent, solutions were shaken for 30 min and the fluorescence was measured on the fluorescence spectrometer as well.

Figure 2. TEM images of (a) CDs and (b) CDs/SBA-NH2. (c) FT-IR analysis of SBA-NH2 and CDs/SBA-NH2 nanocomposite. (d) XPS spectra of CDs/SBA-NH 2 nanocomposite, and inset is the deconvolution analysis of C 1s.

3. RESULTS AND DISCUSSION 3.1. Characterization of CDs/SBA-NH2 Composite. The formation of the CDs/SBA-NH2 composite was first confirmed by investigating the fluorescence property, since the bare SBANH2 does not have detectable fluorescence. As shown in Figure 1a, the fluorescence intensity of the obtained composites increased with longer microplasma treatment time, as more CDs were introduced onto SBA-NH2. The photoluminescence spectra of the CDs/SBA-NH2 solution showed a typical excitation-dependent emission (Figure 1b), which is similar to pristine carbon dots (Figure S1b). As the excitation wavelengths were increased from 320 to 440 nm, the emission peaks showed a red shift from 418 to 500 nm. For the bulk powder, strong fluorescence with different colors was also

ordered parallel-aligned pore channels of SBA-NH2, and the loading of CDs did not compromise the structural regularity of the mesoporous silica frameworks (Figure 2b). In the FT-IR spectra, both SBA-NH2 precursor and CDs/SBA-NH2 show the characteristic peaks at 1090 cm−1 for vibration of Si-O-Si, 1627 cm−1 for bending vibration of O-H,24 while the peaks at 1695 cm−1 for CO stretching appeared and 1395 cm−1 for −CH2− stretching enhanced after the loading of CDs. The peak at 1570 cm−1 (stretching of −NH2) reappeared after the incorporation of CDs, and that may be due to the growing amount of −NH2 7394

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ACS Applied Materials & Interfaces on the composite material (Figure 2c). In the XPS analysis of C 1s peaks (Figure 2d), both CDs/SBA-NH2 and SBA-NH2 showed peaks at around 284.6 and 286.1 eV, that were attributed to the C-C and C-N species. However, the CDs/ SBA-NH2 showed an additional CO bonds peak at 288.0 eV (Figure S2, Table S1) compared with the SBA-NH2 precursor. The appearance of the CO group can be attributed to the CDs on the SBA-NH2. Elemental analysis of C, H, and N suggested that the amount of C increased from 6.24% to 7.35% after the incorporation of the CDs (Table S3), which confirmed the formation of CDs/SBA-NH2 nanocomposite material. All of these analysis results indicated that plentiful amounts of function groups (−COOH, −NH2, −OH, etc.) existed on the surface of the CDs/SBA-NH2 composite material, thus making it a suitable adsorbent for metal separation. The ordered pore structure and large surface area of SBANH2 were maintained in the CDs/NH2 composite. SEM images revealed that the SBA-NH2 precursor exhibited a welldeveloped cylindrical rodlike structure with a short channel and the structure was preserved after the CDs were loaded (Figure S3). Besides, the ordered degree of SBA-NH2 and CDs/SBANH2 was characterized by SAXRD. As shown in Figure 3a, the

Figure 4. (a) Influence of pH in aqueous solutions on the adsorption capacity of SBA-NH2 and CDs/SBA-NH2, [U(VI)] = 100 mg/L. (b) The fluorescent response of CDs/SBA-NH2 in different initial concentrations of U(VI). T = 298.15 K, m/v = 0.5 mg/mL.

enhancement in adsorption capacity of CDs/SBA-NH2 over that of the SBA-NH2 precursor were almost the same at different pHs, suggesting that the adsorption of uranium by the CDs was pH-independent. As can be seen in Figure 4a, at a lower pH (pH = 3), SBA-NH2 exhibited no effective adsorption ability, but the adsorption capacity dramatically increased 7 times from 8.20 mg(U)/g to 61.38 mg(U)/g after the loading of CDs. In a control experiment, the adsorption capacity of the SBA-NH2 precursor maintained almost the same after microplasma treatment (Figure S5). These results demonstrated that the improvement of adsorption capacity was associated with the increase of the functional groups (−COOH, −OH, and so on) after the CDs were loaded on SBA-NH2. When the pH was lower than 3, both materials showed no adsorption ability because most adsorptive active sites were occupied by hydrogen ions (H+) instead of U(VI). The CDs/SBA-NH2 composite also showed sensitive fluorescence response to U(VI), similar to the pristine CDs prepared by microplasma.27 As shown in Figure 4b, the fluorescence intensity reduced gradually with the increase in the initial U(VI) concentration from 0 to 40 mg/L. These results verified that, after the assembling of CDs on SBA-NH2, the obtained composite preserved the fluorescent response of CDs to U(VI), and exhibited higher adsorption capacity than the SBA-NH2 precursor. Thus, the CDs/SBANH2 composite is of great potential for effective adsorption and in situ monitoring of uranium. 3.3. Monitoring the Uranium Adsorption Behavior by Fluorescence Response. To investigate the relationship between the fluorescence response and the adsorption behavior of the CDs/SBA-NH2 composite toward uranium, a solution with pH = 3 was selected for the following experiments,

Figure 3. SAXRD patterns (a) and N2 sorption/desorption isotherms (b) of SBA-NH2 precursor and CDs/SBA-NH2 composite materials. The temperature of N2 sorption/desorption isotherm was 373.15 K.

(100), (110), (210) peaks confirm a typical hexagonally ordered mesoporous structure of SBA-NH2.31 After the loading of CDs, these peaks were still observed, indicating that the ordered structure of the SBA-NH2 was retained. Nitrogen adsorption−desorption measurements were employed to examine the surface and pore properties of the CDs nanocomposites. The isotherms (Figure 3b) exhibit typical type IV curves with a H1 hysteresis loop, which is the characteristics of an ordered mesoporous structure.32 The BET specific surface areas were calculated to be 499.7 and 559.9 m2/ g with pore volumes 0.784 and 1.183 cm3/g for the SBA-NH2 precursor and CDs/SBA-NH2, respectively (Table S4). All results of characterization and analysis indicated that the carbon dots were successfully assembled on the SBA-NH2 precursor through a simple treatment of microplasma and the obtained CDs/SBA-NH2 nanocomposite preserved the intrinsic fluorescence properties of the CDs and the ordered mesoporous structures of the SBA-NH2 precursors. 3.2. Highly Efficient Adsorption of Uranium and Fluorescent Response. The CDs/SBA-NH2 nanocomposite showed highly efficient adsorption to uranium at pH 3−6 (Figure 4a). A maximum adsorption capacity (173.60 mg(U)/ g) of the CDs/SBA-NH2 nanocomposite was obtained at pH = 5, a significant enhancement over the bare SBA-NH2 (96.02 mg(U)/g). It is noteworthy that the magnitudes of the 7395

DOI: 10.1021/acsami.6b13427 ACS Appl. Mater. Interfaces 2017, 9, 7392−7398

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the fluorescence property of the CDs/SBA-NH2 could be utilized for in situ monitoring of U(VI) adsorption. 3.3.3. Adsorption Selectivity. The selectivity of CDs/SBANH2 for U(VI) was evaluated by a competitive adsorption experiment in a multicomponent solution containing a series of interfering metal ions (K+, Mg2+, Sr2+, Ba2+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, and Ce3+). As shown in Figure 6a, the

because, at this condition, the adsorption capacity of the CDs/ SBA-NH2 composite was mainly attributed to the CDs. 3.3.1. Adsorption Kinetics. The adsorption capacity of U(VI) as well as the fluorescence intensity at different contact times (2 min to 8 h) was recorded. The uptake amount of U(VI) increased sharply, especially in the initial several minutes, suggesting an ultrafast adsorption kinetics (Figure 5a). After using different kinetic equations, such as the pseudo-

Figure 5. (a) Sorption kinetics and fluorescent intensity changing of CDs/SBA-NH2 at pH = 3. (b) Kinetic curves of CDs/SBA-NH2 fitted by pseudo-second-order. (c) Adsorption capacity and fluorescent intensity of CDs/SBA-NH2 varied with the initial concentration of U(VI) from 0 to 100 mg/L at pH = 3. (d) Fitting curve of qe and fluorescent intensity ((I0 − I)/I0). T = 298.15 K, m/v = 0.5 mg/mL, [U(VI)] = 100 mg/L, for (a), (c), and (d) λex = 360 nm, data at 440 nm was selected for analysis.

Figure 6. Selectivity test of (a) the adsorption capacity (qe) of SBANH2 and CDs/SBA-NH2 in mixed metal ion solutions and (b) fluorescent intensity of CDs/SBA-NH2 in different metal ion solutions (λex = 360 nm, data at 440 nm was selected for analysis) at pH = 3, T = 298.15 K, m/v = 0.5 mg/mL, [M] = 100 mg/L.

first-order model and pseudo-second-order model, to examine the adsorption process for CDs/SBA-NH233 (Table S5, Supporting Information), the latter equation was found to show a better prediction of the kinetic process (Figure 5b). The fitting results implied that, in the adsorption process of CDs/ SBA-NH2, the rate of adsorption might be controlled by the number of active surface sites. More interestingly, the fluorescence intensity of CDs/SBA-NH2 decreased rapidly when contacting with U(VI), which was coincident with the adsorption process. This fantastic phenomenon suggested that the change in fluorescent intensity of CDs/SBA-NH2 could act as an indicator for monitoring the U(VI) adsorption process. 3.3.2. Influence of Different Initial Concentrations of U(VI). To further verify that the fluorescent property of CDs/SBANH2 could be used to monitor the adsorption process of uranium, the influence of different initial concentrations of U(VI) were investigated with U(VI) solutions at concentrations varying from 0 to 100 mg/L. Figure 5c shows the U(VI) adsorption capacity as a function of U(VI) concentration. With the increase of U(VI) concentration, the adsorption capacity of CDs/SBA-NH2 increased, along with a decrease of the fluorescence intensity, and both of them reached to near saturation at the concentration of 40 mg/L. The relationship between the adsorption capacity (qe) and the fluorescence intensity (F) could be well fitted with a polynomial: F = −1.41 × 10−4q2e + 0.018qe + 0.001, with R2 = 0.998 (Figure 5d). These results further demonstrated that

SBA-NH2 precursor showed nonspecific adsorption of U(VI), as there was no obvious difference in the adsorption capacity of U(VI) and for other metal ions. In contrast, the adsorption capacity (qe) of the CDs/SBA-NH2 composite for U(VI) was much higher than those for other metal ions. In brief, the results clearly show that the selectivity for U(VI) over the competing metal ions was significantly enhanced after the loading of CDs onto SBA-NH2. Results in Figure 6b also show that the fluorescence sensing property of CDs/SBA-NH2 is different toward different metal ions, with U(VI) possessing a much higher quenching efficiency than other competing metal ions. This suggests that the fluorescence property of CDs/SBA-NH2 can also be utilized to selectively differentiate U(VI) from other metal ions. This fluorescence-based selectivity of CDs/SBA-NH2 for U(VI) was similar to that of the microplasma-prepared CDs alone in our previous study.27 More interestingly, the pattern of fluorescence response in Figure 6b parallels that of the adsorption capacity of the composite in Figure 6a. This result demonstrated that it should be possible to predict the adsorption selectivity of CDsbased composite materials by evaluating the fluorescence selectivity of CDs for metal ions.

4. CONCLUSIONS In conclusion, we have prepared luminescent CDs/SBA-NH2 composite materials with the assistance of microplasma. Characterizations with multiple spectroscopic techniques such 7396

DOI: 10.1021/acsami.6b13427 ACS Appl. Mater. Interfaces 2017, 9, 7392−7398

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as TEM, FT-IR, and XPS analysis verified that the carbon dots were successfully assembled on the SBA-NH2 precursor and the obtained CDs/SBA-NH2 nanomaterial maintained the original fluorescence properties of CDs and the ordered mesoporous structures of SBA-NH2 precursors. Uranium adsorption experiments with the composite material showed that the loading of CDs remarkably enhanced the adsorption capacity in solutions with pH from 3 to 6. The CDs/SBA-NH2 also possessed a fascinating selectivity against a series of competing metal ions. Besides, data from this work showed, for the first time, that the fluorescence response of the CDs/SBA-NH2 to uranium can be used to monitor the adsorption behavior of uranium. This work has provided not only an excellent CDsbased adsorbent material for U(VI) but also a new strategy for the in situ monitoring of the adsorption process for uranium as well as other metal ions.

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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/acsami.6b13427. Figures and tables showing the characterizations of carbon dots, SBA-NH2 and CDs/SBA-NH2 as well as the adsorption behavior studies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (J.C.). ORCID

Chao Xu: 0000-0001-5539-4754 Yuexiang Lu: 0000-0003-2755-7733 Gang Ye: 0000-0002-7066-940X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Grant Nos. 51425403, 21390413, 21405090, and 91426302), the Program for Changjiang Scholars and Innovative Research Team in University (IRT13026), and Tsinghua University Initiative Scientific Research Program (2014z22063).

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REFERENCES

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DOI: 10.1021/acsami.6b13427 ACS Appl. Mater. Interfaces 2017, 9, 7392−7398