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3D MoS2 Composition Aerogel as Chemosensors and Adsorbents for Colorimetric Detection and High-Capacity Adsorption of Hg2+ Lihua Zhi, Wei Zuo, Fengjuan Chen, and Baodui Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00409 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 13, 2016

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ACS Sustainable Chemistry & Engineering

3D MoS2 Composition Aerogel as Chemosensors and Adsorbents for Colorimetric Detection and High-Capacity Adsorption of Hg2+ Lihua Zhi,† Wei Zuo,† Fengjuan Chen,† and Baodui Wang†,* †

Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province

and State Key Laboratory of Applied Organic Chemistry Lanzhou University Gansu, Lanzhou, 730000, P.R. China. Fax: +86-931-8912582

ABSTRACT：Precise detection, and effectively eliminate the mercury pollution in aqueous solutions remains a onerous task for protecting the public health and environment. In this paper, a porous MoS2 composite aerogel-supported Au nanoparticles with strong mercury affinity has been fabricated to deal with this problem. Such composite aerogels are fabricated using graphene oxide (GO) -doped MoS2 sheets as the feedstock by hydrothermal assembly, and then the Au and Fe3O4 nanoparticles (NPs) embedded between GO-doped MoS2 sheets, respectively, through the coordination. The resultant porous Au/Fe3O4/MoS2CAs aerogel not only can sensitively detect mercury(II) in aqueous solution by a colorimetric method with a low detection limit (3.279 nM), but also exhibits a super mercury adsorption capacity (~1,527 mg g-1) and fast desorption ability. After the magnetic separation, the Hg2+ levels decreased from 10 ppm to the 0.11 ppb within a few minutes, which is far below 2 ppb. In addition, Au/Fe3O4/MoS2CAs could be successively recycled more than 10 times with high removal efficiency (>95%). The excellent performance of the composition aerogel profits from its 3D interconnected macroporous framework as well as

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strong couple between Au nanoparticles and MoS2 nanosheets, rendering it a potential detection and adsorbent materials for mercury(II) from contaminated water for environmental remediation. KEYWORDS: Three-dimensional, MoS2 composition aerogel, Detection, Removal, mercury (II)

INTRODUCTION From the view of human health and ecosystem protection, finding a fast and efficient way to separate, enrich and recognize the contaminants in water is very significant. Among all the toxic metals, Hg2+ is a widespread and one of the most toxic cation due to its strong affinity to the enzymes and proteins which content of sulphydryl groups (−SH).1-4 Hg2+ can accumulate in human and animal livers through bioaccumulation, which causes disorders and damages to kidney, brain, liver, immune and nervous system, the endocrine system and other organs.5-7 Also, recent research showed that genes damage and impair mitosis may be attributed to excessive exposure and accumulation of mercury.8 Consequently, it is vital to enrich, separate, and detect Hg2+ in water simultaneously. Searching for novel Hg2+ recognization and removal approaches that are fast convenient enrichment, separation, low-cost, sensitive, and suitable for water solution has become a critical demand to protect our health and ecosystem. By virtue of its simplicity, high efficiency and low-cost,9 adsorption holds great promise for mercury removal over the years, Most of materials including hydrogels, polymers, activated carbon, zeolites, resins, MOF and porous silica have been fabricated and used for eliminating Hg2+ from waste waters.10-12 However, the physical adsorption process usually suffer from low adsorption capacity, time-consuming, and hard recovery. Furthermore, the vast majority of above materials couldn’t rapidly build up a visual signal indicating the presence of mercury at the same


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time. Hence, it is extremely significant to explore new approaches for detection, adsorption and decontamination of Hg2+ ion. Nowadays, large number of nanomaterials based on noble metal have been constructed and applied to detection of metal ions like Hg2+ ion13-15 that could stimulate their catalytic activity through forming a solid amalgam-like structure,16-18 which could significantly affect the stability and the surface properties of the colloid. There are many advantages to detect target analyte by using catalysis-based analysis method compared with the traditional approaches, which possess excellent detection stability and anti-interference ability in the complicated systems. However, the noble metal nanomaterials usually suffer from instability in water. During the catalytic process, they are tend to aggregate because of their high surface energy and result in the loss of active sites.19 In order to overcome these shortcomings, nano-catalyst carriers such as graphene, aptamer, and Tween 20 have been utilized to increase the active sites of catalysts and enhance their cycling performance and catalytic activity. 17-19 As a typical example of graphene analogues, molybdenum disulphide (MoS2) nanosheets (NSs) have recently been attracted increasing attention due to their particular structure and special properties. This material possesses a sandwich structure (S-Mo-S) where Mo atoms are stuck in the middle of two layers of S atoms by covalent bonds.23-25 The prominent mechanical and electrical properties make it undoubtedly an radiant star among 2D materials. To date, MoS2 has been extensively used in the domains of lithium ion batteries, energy storage, optoelectronic devices, nanotribology, catalysis and sensors.26-27 In contrast with the one-dimensional and twodimensional nano-materials, 3D nanostructure possess excellent porosity, large specific surface area and other charming characteristics, which provides a new way for improving the catalytic activities.28–30 Hence, it is predicted that catalysts supported on MoS2-based aerogels would


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dramatically enhance their catalytic activities. As far as we know, there have been no reports of the controllable assembly of precious metal nanoparticles supported on three-dimensional MoS2 networks as enzyme-like catalyst for colorimetric detection and capture of Hg2+. Our previous work has shown that introduction of Au Nps and Fe3O4 NPs to graphene oxide can achieve the aim of detecting and removing in aqueous solutions. However, the absorption capacity of this material is not so good.31 To overcome this problem, in this study, for the first time we report the fabrication of graphene-doped MoS2 aerogel (MoS2CAs) and MoS2CAs supported

Au/Fe3O4

nanoparticles

(Au/Fe3O4/MoS2CAs). Particularly,

the intertwined

macropores, which are rooted in MoS2-based aerogels (MoS2CAs), could be fabricated by hydrothermal method, and then the Au and Fe3O4 nanoparticles (NPs) embedded between the sheets of GO-doped MoS2 through coordination to form the Au/Fe3O4/MoS2CAs. The asprepared MoS2CAs and Au/Fe3O4/MoS2CAs exhibit interconnected macroporous networks, and the morphology and macroporous size of MoS2CAs could be regulated by simple change the doping amounts of GO in MoS2 NSs. In addition, integrating Fe3O4 NPs into MoS2CAs renders the aerogels to possess magnetic property, thus conducive to the recovery of the catalyst and multiple repeated uses on account of easy magnetic separation.32-34 Introduction of Au NPs to MoS2CAs make the aerogels possess mercury-stimulated peroxidase mimetic activity and can be used to detect Hg2+ in water. Benefiting from their large surface area, porous structure, superparamagnetic nature, and high dispersity of embedded Au NPs, these hybrids exhibit significantly improved nanozyme activity for sensitively colorimetric detection and efficient capture of Hg(II), with a detection limit of 3.279 nM and adsorption capacity of 1,527 mg g-1, which outperforms bare Au NPs alone and other reported materials. EXPERIMENTAL SECTION


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Materials. α-thioctic acid (LA), dopamine hydrochloride and N-Ethoxycarbonyl-2-ethoxy1,2-dihydroquinoline (EEDQ) were purchased from Sigma-Aldrich. Molybdenum(IV) sulfide (MoS2, 99%)was purchasedfrom Alfa Aesar. 1.6 M n-butyllithium solution, graphite powder,H2SO4, KMnO4 and H2O2 were purchased from J&K Scientific Ltd. (China). Graphene oxide (GO) was prepared using similar approach according to the literature.35,36 MoS2 nanosheet solution was synthesized by chemical exfoliation method based on the published method.37 Iron oxide NPs and Au NPs were prepared according to the literature.38,39 Measurements. 1H NMR spectra were obtained on Varian 400 MHz NMR. Lakeshore 7404 high-sensitivity vibrating sample magnetometer (VSM) was applied to study the magnetic properties of nanomaterials. FT-IR spectra were measured on a Nicolet FT-170SX spectrometer . Field-emission scanning electron microscope (FE-SEM, FEI, Sirion 200) was used to study the morphology of the nanomaterials. TEM images were taken on Tecna i-G2-F30 (FEI) transmission electron microscope and the acceleration voltage was 300 kV. X-ray powder diffraction (XRD) patterns of the nanomaterials were obtained on a Bruker AXS D8-Advanced diffractometer with Cu Kα radiation (λ=1.5418Å). Confocal microprobe Raman system (Renishaw, RM2000) was utilized to record the Raman spectra. N2 adsorption-desorption isotherms were surveyed at 77 K by Micromeritics TriStar II 3020 analyzer. The specific surface areas was determined by Brunauer-Emmett-Teller method. The Barrett-Joyner-Halenda model was applied to calculate the pore volumes and pore size distributions. Au and Fe contents are determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Varian VISTA-MPX). By using AlKα radiation, X-ray photoelectron spectroscopy were carried out on a PHI-5702 multifunctional spectrometer. UV-visible adsorption spectra (UVvis) were carried out


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on a UV 1750 spectrometer. All of the reactions which need to be shaked were performed in a temperature-controlled water bath shaker (SHA-C). Synthesis of N-(3,4-dihydroxyphenethyl)-5-(1,2-dithiolan-3-yl) pentanamide (LA-DPA). Triethylamine (0.1012 g) was added drop-wise to 20 mL ethanol under nitrogen, which containing 0.1896 g dopamine hydrochloride (DPA). The reaction was carried out at 25 0C for 1h. Then, α-thioctic acid 0.2064 g and EEDQ 0.2473 g were added to the mixture. After stirring for 24 h at 25 0C under nitrogen atmosphere, the insoluble product was filtered, and the filtrate was evaporated. The product was purified by column chromatography to give LA-DPA as yellow oily liquid. (0.2425 g, yield, 71%). ESI-MS: m/z = 342.1721 for [M+H]+. 1HNMR (400MHz, CDCl3), δ(ppm): 8.73 (s, 2H), 7.85-7.82 (t, 1H), 6.65-6.63 (d, 1H), 6.59 (s, 1H), 6.466.42 (dd, 1H), 3.62-3.50 (m, 2H), 3.21-3.07 (m, 4H), 2.44-2.36 (m, 1H), 2.07-2.04 (t, 2H), 1.891.81 (m, 1H), 1.67-1.60 (m, 1H), 1.56-1.47 (m, 3H), 1.36-1.28 (m, 2H). IR (KBr plate, cm-1): 3351 (s), 2924 (s), 2860 (m), 2721 (w),1754 (m), 1707 (m), 1636 (s), 1522 (s), 1442 (s), 1367 (s), 1282 (s), 1197 (s), 1143 (m), 1115 (m), 1048 (m), 812 (m). Synthesis of MoS2-DPA. LA-DPA (10 mg) was dissolved in DMF (10 mL) and then mixed with 20 mL MoS2NSs (2 mg/mL) aqueous dispersion. After the mixture sonicated for 1h and stirred overnight, the formed MoS2-DPA was gathered by centrifuging and washed thoroughly with DMF and water. IR (KBr plate, cm-1): 3431 (m), 1754 (w), 1626 (m), 1344 (m), 1142 (s), 1013 (s), 874 (s), 668 (s). Preparation of MoS2CAs. 12 mL GO (5 mg/mL) aqueous dispersion and 15 ml MoS2-DPA (2 mg/mL) nanosheet aqueous dispersion were added into a round flask (50 mL). Then, 3 mL diluted H2O2 aqueous solution (0.3% H2O2) was added. After sonication for 1h, the mixture was added into a 50 mL Teflonlined autoclave. The mixture was heated at 180 0C for 8h and naturally


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cooled down to 25 0C. Then, the obtained material was taken out and immersed in pure water to remove any impurities for the following experiments. At last, the hydrogel was dried by freezedrying for 12 h at -50 0C to form MoS2CAs. IR (KBr plate, cm-1): 3436 (m), 2977 (m), 2901 (m), 1727 (w), 1642 (w), 1573(m), 1401 (m), 1170 (s), 1143 (s), 1075 (s), 1051 (s), 895 (m), 779 (m), 669 (m). Preparation of Au/Fe3O4/MoS2CAs. The MoS2CAs were immersed in the CHCl3 solution containing Fe3O4 Nps. The mixture was shaked for 5 min. Then, the formed Fe3O4/MoS2CAs was took out and immersed in ethanol solution to exchange CHCl3. At last, the Fe3O4/MoS2CAs were immersed in the water containing AuNps. After all of the Au Nps were sucked into the Fe3O4/MoS2CAs, the hybrids were subjected to drying by freeze drying at at -50 0C for 12 h. IR (KBr plate, cm-1): 3435 (m), 2973 (w), 2904 (w), 1721 (w),1571(m), 1401 (m), 1163 (s), 1132 (w), 1107 (w), 1078 (s), 1047 (s), 583 (s), 442 (w). Hg(II) sorption kinetics.The as-prepared Au/Fe3O4/MoS2CAs (25.0 mg) were added to a round-bottomed flask which including 50 mL aqueous solution of Hg(NO3)2 (10 ppm). Then the mixture was shaked for 2h. During the shaking period, the supernatant was taken out at regular intervals and filtered through a membrane filter (0.45-µm), and then the Hg2+ content of the filtrates was obtained by atomic fluorescence spectrometer (AFS-9800). The initial concentration of metal ions The distribution coefficient Kd is an important parameter to measure the performance of the adsorbents. The value of Kd can be determined by:40

where Ci indicates the initial concentration of the contaminant, Cf indicates the final equilibrium concentration of the contaminant, m is the mass of the adsorbent (g) and V is the volume of the


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tested liquid (mL). The Hg2+ contents in the Au/Fe3O4/MoS2CAs was determined by:

where C0 and Ct are the concentration of the heavy metal ions (mg L-1) initially and at time t, respectively, V is the volume solution used (L), and m is the mass of the aerogel used (g). Mathematical Models. The adsorption kinetic of Au/Fe3O4/MoS2CAs for Hg2+ were investigated by pseudo-first-order41 and pseudo-second-order kinetic models,42 and the adsorption kinetic equations are given as eqs 3 and 4, respectively.

where qe and qt are the amount of Hg(II) adsorbed at equilibrium and time t (min), and k1 (min−1) and k2(g mg-1 min-1) are the rate constant of the pseudo-first-order adsorption and the pseudosecond-order rate constant, respectively. The values of k1 and qe can be calculated from the intercepts and slope of the linear plot of log(qe-qt) vs t, and k2can be calculated from the slope and intercept of the plot of t/qt vs t. Hg(II) adsorption isotherm. The as-prepared Au/Fe3O4/MoS2CAs (25.0 mg) were added to different round-bottomed flasks, each of which contains 50 mL aqueous of Hg(NO3)2 with several concentrations. All of the flasks were shaked for 2 h, and then the Au/Fe3O4/MoS2CAs were separated by a magnet. Finally, the atomic fluorescence spectrometer (AFS-9800) was employed to confirm the mercury content of the filtrate. To analyze the adsorption behavior of the Au/Fe3O4/MoS2CAs for Hg2+, the classical Langmuir and Freundlich models were applied for describing and analyzing adsorption equilibrium, the equations are given as eqs 5 and 6, respectively.43,44


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where qe and qm represent the equilibrium and the maximum adsorption capacity (mg/g), respectively. Ce is the Hg2+ concentration (mg/L) at equilibrium, and KL (L/mg) is the Langmuir constant. Kf (L/mg) and n is the Freundlich parameters related to adsorption capacity and adsorption intensity, respectively. Desorption and Regeneration.The as-prepared Au/Fe3O4/MoS2CAs (25 mg) were added to a Hg2+ solution (10 ppm, 50 mL) and shaked for 30 min. Then the 3D hybrids were collected by a magnet and treated with citric acid-disodium to reduce Hg2+. Research has shown that the volatility of Hg0 is 0.056 mg h-1cm-2 (20 °C). Moreover, with the temperature rising, the volatility of Hg0 increases dramatically.31 Thus, the resultant 3D hybrids containing Hg0 were heated at 80 degree to recover the Au nanoparticles. After Hg0 was desorpted from the Au/Fe3O4/MoS2CAs, the regenerated sample was reused for another adsorption test. RESULTS AND DISCUSSION (Figure 1) Preparation and Characterization of MoS2CAs and Au/Fe3O4/MoS2CAs. Figure 1A showed the fabrication process for MoS2CAs and Au/Fe3O4/MoS2CAs. The MoS2 NSs were synthesized by chemical exfoliation using n-butyl lithium to insert and exfoliate the bulk MoS2, and then reacted with N-(3,4-dihydroxyphenethyl)-5-(1,2-dithiolan-3-yl) pentanamide (LA-DPA) to obtain MoS2-DPA NSs. The formed MoS2-DPA NSs were mixed with GO and a certain amount of H2O2 aqueous solution (0.3%) by ultrasonication to form homogeneous dispersion solution, in which GO can dope into MoS2 nanosheets. The dispersion was subsequently heated at 180 °C in a Teflon-lined autoclave for 8 h. During this process, because of the π-π interactions


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and hydrophobic effects between the NSs, the flexible 2D NSs randomly stacked to form 3D bulk materials.45 To study the effect of the doping amount of GO to the formed macro and micro structures of MoS2CAs, the different amount of GO mixed with a certain amount of MoS2 and then heated at 180 °C for 8 h. The obtained MoS2CAs mixed with Au Nps and Fe3O4 Nps to form Au/Fe3O4/MoS2CAs, in which both Au and Fe3O4 NPs could embed between the sheets of GO-doped MoS2 through coordination with corresponding S atoms and O atmos on the MoS2. (Figure 2) The morphology and structure of the as-prepared MoS2CAs and Au/Fe3O4/MoS2CAs were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). SEM images reveal that all the formed MoS2CAs (Figure 1B) posses a well-defined and interconnected 3D macroporous network with the pore diameters ranging from submicrometres to several micrometres and the pore walls composing of single- or few-layer GO-doped MoS2 sheets, which are different from GO and MoS2 nanosheets (Figure S1). After forming Au/Fe3O4/MoS2CAs, SEM images (Figure 1C) reveal that morphology and interconnected macroporous frameworks are similar to the original MoS2CAs. When the doping amounts of GO are 65% and 50%, hollow cylinder MoS2CAs and hollow circular truncated cone MoS2CAs can be obtained, respectively, shown in Figure 2A(ab). However, when this doping amount of GO decreased from 40% to 10%, the hollow structure MoS2CAs were disappeared, and the corresponding morphology changed from the solid cylinder shape to solid circular cone shape, shown in Figure 2A(c-f). In terms of microstructure, as the doping amount of GO decreases, the pore sizes of the formed MoS2CAs become larger (Figure 2B-M). TEM characterization further validated that the highly dispersed Au NPs and Fe3O4 NPs are embedded between the GO-doped MoS2 sheets (Figure 3A-B), which could increase their


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interfacial interaction between NPs within GO-doped MoS2 layers and inhibit the aggregation and dissolution of NPs, and therefore enhancing the catalytic performance of the hybrids. The typical HRTEM image (Figure 3C) clearly shows the (311) crystal plane of Fe3O4 and (111) crystal plane of Au on the surface of GO-doped MoS2 sheets. Moreover, the interlayer spacing of 0.635 nm and 0.27 nm that corresponds to the (002) and (100) crystal planes of MoS2 can also be found. From the elemental mapping and energy-dispersive X-ray (EDX) analysis, the uniform distributions of S, Mo, Au, Fe, C, O and N in the Au/Fe3O4/MoS2CAs are distinctly observed in Figure 3E-N and Figure S2. From the XRD (Figure 4A), MoS2CAs only exhibits the diffractions peaks at 2θ of 22.1 for rGO, and at 2θ of 14.1, 29.7, 33.0, 40.3 and 58.6 for MoS2. For the Au/Fe3O4/MoS2CAs sample, apart from the peaks of MoS2 and rGO phase, the XRD pattern also confirmed the assemble of Au (JCPDS, No. 89- 3697)46 and Fe3O4 (JCPDS, No. 19-629)47 in the hybrids. In addition, the loading amounts of Fe and Au in Au/Fe3O4/MoS2CAs accessed by using ICP-AES test are ~5.3 wt% and ~3.3 wt%, respectively. (Figure 3) The forming of MoS2CAs and Au/Fe3O4/MoS2CAs were confirmed by FT-IR spectra (Figure S3). In the FT-IR spectrum of MoS2CAs, the stretching band at 1143 cm-1 can be assigned as the vibration of phenolic ʋ(C–O) group, which is attributed to LA-DPA. However, after Fe3O4 NPs coordinated with DPA to form Au/Fe3O4/MoS2CAs, the phenolic ʋ(C–O) peak located at 1143 cm1

was disappeared, and a new absorption peak at 1107 cm-1corresponding to C-O-Fe vibration

was observed, indicating that the surface of Fe3O4 NPs were wrapped by the phenolic hydroxyl group of MoS2CAs.48 Moreover,the Fe-O absorption peaks are located at 583 and 442 cm-1.49 The Raman spectroscopy of Au/Fe3O4/MoS2CAs displayed the D, G, 2D and G’ vibrational bands of graphene oxide at 1349, 1596, 2698, and 2930 cm-1, respectively (Figure 4B).50


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Moreover, the characteristic peaks of MoS2 can also be observed at 374 and 402 cm-1.51 These studies further proved the formation of MoS2CAs hybrid. The magnetic measurement reveals that the Au/Fe3O4/MoS2CAs possess superparamagnetic property (Figure S4). The macro- and meso-porous feature of MoS2CAs and Au/Fe3O4/MoS2CAs were further determined by nitrogen adsorption-desorption characterization. The curve displayed in the Figure 4C-D shows the distinct typical of type-IV isotherms. Moreover, in the P/P0 range of 0.4-1.0, a hysteresis loop of H2 can be observed obviously. These results suggest that there are lots of macropores and mesopores existed in the frameworks. Furthermore, the pore size distribution further demonstrates the existence of a large number of mesopores calculated by the BJH method (3.7−5.2 nm for MoS2CAs and 3.6-5.2 nm for Au/Fe3O4/MoS2CAs, inset in Figure 4C-D) and macropores in MoS2CAs and Au/Fe3O4/MoS2CAs. The specific surface area and pore volume of Au/Fe3O4/MoS2CAs based on Brunauer-Emmett-Teller (BET) analysis are 164.6 m2 g-1 and 0.5382 cm3 g-1, which are larger than that of MoS2CAs (116.2 m2 g-1 and 0.3832 cm3 g-1). It is indicated that the introduction of Fe3O4 and Au nanoparticles augments the speciﬁc surface area of the hybrid. (Figure 4) Figure

5A-F

reveals

the

chemical

states

and

compositions

of

MoS2CAs

and

Au/Fe3O4/MoS2CAs confirmed by using X-ray photoelectron spectrometer (XPS). The survey spectrum of MoS2CAs reveals only the existence of C, N, O, S and Mo (Figure 5A). However, the XPS survey spectrum (Figure 5A) revealed the existence of S, Mo, Au, Fe, C, O and N in the Au/Fe3O4/MoS2CAs. In Figure S5A, the C1s spectrum of GO clearly reveals four resolved peaks at 284.4 (C-C/C=C), 286.6 (C-O-C/C-OH), 287.5 (C=O), and 288.6 (O=C-OH) eV.52 While the one C1s peak corresponding to O=C-OH disappeared in the forming MoS2CAs, which confirms


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the reduction of GO to RGO in the forming MoS2CAs process (Figure 5B). Figure 5D depicts the Mo 3d3/2 and Mo 3d5/2 characteristic peaks at 228.3 and 231.8 eV, 229.3 and 232.4 eV, 229.6 and 232.8 eV for MoS2-DPA, MoS2CAs, and Au/Fe3O4/MoS2CAs, respectively. These peaks can be assigned to the quadrivalent Mo.53 The Mo binding energy peaks in MoS2CAs and Au/Fe3O4/MoS2CAs hybrids are positively shifted as compared to MoS2-DPA sample, indicating the presence of electron transfer between rGO and MoS2-DPA as well as between MoS2-DPA and Au/Fe3O4 NPs, suggesting that the formation of MoS2CAs and Au/Fe3O4/MoS2CAs hybrids is not a simple physical mixture of corresponding raw materials.54 It is noteworthy that the Mo6+ 3d3/2 peak in the spectra of Mo3d, which can be assigned to the Mo-O bonding, indicating the presence of O species.55 Figure S5B exhibits the 4f5/2 and 4f7/2 characteristic peaks at 87.6 and 83.9 eV, respectively, which can be assigned to the zero valence state of metallic Au. Figure S5C displays the Fe2p characteristic peaks at 710.5 and 724.1 eV which are consistent with the Fe2p3/2 and Fe2p1/2, respectively. The high-resolution spectrum of O1s (Figure S5E) and N1s (Figure S5D) displays the existence of phenolic oxygen (531.7 eV) and amide (395.5 eV) in the Au/Fe3O4/MoS2CAs.

Compared

to

the

MoS2CAs,

the

phenolic

O1s

peaks

in

Au/Fe3O4/MoS2CAs shift from 532.0 to 531.7 eV (Figure 5E). The downshift of the O1s indicate the formation of Fe-O.56 In the S2p spectrum, the binding energies for sulfur shift from 161.9 eV and 163.2 eV in MoS2CAs to 162.5 eV and 163.7 eV in Au/Fe3O4/MoS2CAs, indicating the coordination of sulfur atoms to gold (Figure 5F).57 (Figure 5) Optimization of the Detection Conditions. 3,3’,5,5’-Tetramethylbenzidine (TMB), a common chromogenic substrates which can be oxidized by H2O2 in the presence of peroxidases or catalysts and display obvious blue colour. Recent studies have shown that the citrate-capped


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AuNPs possessed the mercury-stimulated peroxidase mimetic activity.17,31 Based on this principle, the 3D holey Au/Fe3O4/MoS2CAs were employed to detect mercury(II) concentration in aqueous solution by a colorimetric method. Generally, the pH, incubation time, the concentration of Au, H2O2 and TMB in the citric acid-disodium buffer solution (25 mM) played crucial roles for the detection sensitivity. Therefore, the mercury-stimulated peroxidase-like activity of the Au/Fe3O4/MoS2CAs was measured by changing the pH from 3 to 6.5, the incubation time from 1 to 8 min, the Au concentration from 5.0 to 30.0 nM, the H2O2 concentration from 0.2 to 1.4 M, and TMB concentration from 0.05 to 0.3 mM. According to the relative catalytic performance (Figure S6), the corresponding optimal conditions for pH, incubation time, Au concentration, H2O2 concentration, and TMB concentration were 5.4, 5 min, 20.00 nM, 1.00 M and 0.20 mM. Therefore, we performed the subsequent experiments under above conditions. Sensitivity for Hg2+. As shown in Figure S7, there was no obvious reaction between 3,3’,5,5’Tetramethylbenzidine (TMB) and H2O2 in absence or presence of Au/Fe3O4/MoS2CAs. However, this logjam can be broken by Hg2+. Just adding a small amount of Hg2+, the catalytic performance of the hybrids get significantly improved. For comparison purposes, the catalytic performance

of

GO,

MoS2,

Fe3O4NPs,

Au

NPs,

Au/MoS2,

Au/graphene

aerogel,

Au/MoS2/graphene aerogel, Au/Fe3O4/graphene aerogel, and a control without catalyst were also determined. Among those tested materials, Au/Fe3O4/MoS2CAs exhibited the highest catalytic activity as it displays the strongest absorption at 652 nm (Figure 6A). However, those systems without Au NPs did not show any mercury-stimulated peroxidase activity. In addition, we can see that the nanomaterials contained the MoS2 exhibit higher mercury-stimulated peroxidase


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activity than that of nanomaterials without MoS2, which is due to the rich sulfur atom on the MoS2 that can combine and stabilize more Au NPs. The UV-vis absorption of Au/Fe3O4/MoS2CAs (Au, 20 nM) with different concentrations of Hg2+ (0 to 20 µM) is discribed in Figure 6B. With the increasing concentrations of Hg2+, the absorbance peak at 652 nm is dramatically increased, accompanied by the solution color changes from colorless to bright blue. Furthermore, there was a linear relationship between the absorbance intensity and the concentration of Hg2+ (1~14 µM), shown in Figure 6C. Moreover, the detection limit of the Au/Fe3O4/MoS2CAs for Hg2+ is 3.279 nM based on the reported computing method (Figure S8B).38 (Figure 6) Selectivity for Hg (II). The selectivity of the Au/Fe3O4/MoS2CAs for Hg2+ over other anions was investigated by introduction of Na+, K+, Ag+, Ca2+, Ba2+, Mg2+, Fe2+, Cu2+, Zn2+, Ni2+,Cd2+, Al3+, Pb2+, Cr3+ and Hg2+ to the test system. As displayed in Figure 6D, introduction of 10 equiv. of other metal ions hardly had noticeable effect on the absorbance of oxTMB at 652 nm. Furthermore, potential interference from other cations was also researched. As shown in Figure 6F (red bar), 1 equivalents of Hg2+ can give rise to dramatic absorbance response in the absence of 10 equiv. of various interference species. All in all, these results indicate that Au/Fe3O4/MoS2CAs reveals dramatically high selectivity for Hg2+ and can specific recognize Hg2+ over other cations. Investigation of Detection Mechanism to Hg (II). The current reported studies indicated that the surface properties of the Au NPs can be changed by depositing of Hg0 on their surface, thus improving their peroxidase-like activity.47 In order to prove whether the Hg2+ was deposited on the surface of the Au NPs, X-ray photo-electron spectroscopy was used. As shown in Figure S9,


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the spectrum displays the Hg4f characteristic peaks at 100.7 and 104.7 eV (Figure S9B) which are consistent with the Hg 4f7/2 and 4f5/2, respectively, which indicate the existence of Hg2+ and Hg0 on the surface of Au.58-60 In addition, we demonstrated that alone Hg2+ in a citric aciddisodium buffer solution could not catalyze the oxidation of TMB with H2O2 (Figure S10). In view of the above mentioned facts, we speculate that Hg2+ is firstly reduced by citrate sodium to form Hg0, and then Au NPs was wrapped by Hg0 , shown in Scheme 1. In addition, compared with Au NPs and Au/MoS2, the best mercury-stimulated peroxidase activity of 3D Au/Fe3O4/MoS2CAs is due to their rich porous and strong couple between Au NPs and MoS2CAs, in which the TMB and H2O2 concentrated in porous and then thoroughly contacted with the surface of Au NPs embedded in porous (Scheme 1). (Scheme 1) Hg (II) Removal Studies. As shown in Figure 7A, Hg (II) ions can be rapidly captured by the as-prepared 3D hybrids, and adsorption equilibrium is reached within 30 min. In addition, under an applied magnetic field, the obtained Hg2+/Au/Fe3O4/MoS2CAs could be easily removed from the water (Figure 1A and Figure S4) due to superparamagnetism of Au/Fe3O4/MoS2CAs. By this time, the concentration of Hg2+ in the residual solution was 0.11 ppb, which is far lower than that in the water handled with other reported materials under similar condition, such as FMMS (0.8 ppb), Chalcogel-1 (0.04 ppm), PAF-1-SH (0.4 ppm), and the MOF of Zr-DMBD (0.01 ppm).10,6162

That is to say, almost 99.9989% of the Hg2+ ions were removed by Au/Fe3O4/MoS2CAs under

such condition. Also, we found that the removal efficiency of Au/Fe3O4/MoS2CAs to Hg2+ ions is the highest in among other materials such as GO, MoS2, Fe3O4NPs, Au NPs, Au/MoS2, Au/graphene aerogel, Au/MoS2/graphene aerogel, and Au/Fe3O4/graphene aerogel (Table S1), which reveals that the 3D porous assembly structure gives a major push to the improvement of


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the removal efficiency to Hg2+ ions. In addition, the nanomaterials contained both Au NPs and Fe3O4 NPs exhibit higher removal efficiency than that of nanomaterials only contained Au NPs, which is due to their superior magnetic separation capability. Above experiment results indicated that the mercury concentration of highly contaminated water, which were treated with the asprepared hybrids, could be effectively reduced to a low level which is below the limits specified by U.S. Environmental Protection Agency (EPA) for hazardous wastes and drinking water standards (95%).


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3D MoS2 Composition Aerogels as ... - ACS Publications

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