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MoS2 nanosheets-assisted coordination of metal ions with porphyrin for rapid detection and removal of cadmium ions in aqueous media Wenyan Yin, Xinghua Dong, Jie Yu, Jun Pan, Zhiyi Yao, Zhanjun Gu, and Yuliang Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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ACS Applied Materials & Interfaces

MoS2 nanosheets-assisted coordination of metal ions with porphyrin for rapid detection and removal of cadmium ions in aqueous media Wenyan Yin,*,a,# Xinghua Dong,a,c,# Jie Yu,a Jun Pan,a Zhiyi Yao,*,a Zhanjun Gu,*,a and Yuliang Zhao*,a,b a

Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China.

bKey

Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology of China, Beijing, 100190, China

c

College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing, 100049, China.

#These

authors contributed equally.

Corresponding Author: [email protected]

[email protected],

[email protected],

[email protected],

ABSTRACT: Molybdenum disulfide (MoS2) is a two-dimensional (2D) graphene-like material that is gaining great attention due to its potential application in various fields. Here, we reported a self-assembled

nanocomposite

consisted

of

MoS2 nanosheets

and

5,10,15,20-Tetrakis

(1-methyl-4-pyridinio) porphyrintetra(p-toluenesulfonate) (TMPyP), named as MoS2@TMPyP. This nanocomposite can be used as a sensing probe for low cost, rapid, selective detection of cadmium (Cd2+) ions. It is found that a new Soret band at 442 nm in UV-vis absorption spectra represented the coordination of Cd2+ ions into TMPyP of the MoS2@TMPyP. The coordination rates between TMPyP and Cd2+ ions is greatly accelerated from 72 h to 20 min with the assistance of MoS2, which is 200 times faster than in the absence of MoS2. The limit of detection (LOD) of the Cd2+ is as low as 7.2 × 10-8 mol/L. The binding behavior between the cationic TMPyP and MoS2 nanosheets was corroborated by molecular dynamics simulation and various control experiments. The results demonstrated that electrostatic interaction was the main force for driving TMPyP enriching around the MoS2 surface, resulting in an accelerated complexation of Cd2+ and TMPyP. Moreover, MoS2@TMPyP nanocomposite can also be used for removing of Cd2+ in water. The removal efficiency (RF) of the MoS2@TMPyP can reach to 91% for high

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concentrations of Cd2+. This work provides a new insight into detection and removal of Cd2+ ions in water. KEYWORDs: MoS2 nanosheets, optical detection, porphyrin, self-assembly, theoretical calculation

1. INTRODUCTION Two-dimensional (2D) transition metal dichalcogenides (TMDs) (WSe2, MoSe2, WS2, etc.), consisting of hexagonal layers of metal atoms (M) sandwiched between two layers of chalcogen atoms (X) with stoichiometry MX2, have received increasing attentions in recent years.1 As a typical TMD, molybdenum sulfide (MoS2) features excellent mechanical and optoelectronic properties originated from its ultrathin thickness and 2D structure.2-3 In contrast to its analogue graphene, nano-sized MoS2 are chemical versatile and well dispersing in aqueous media without adding any surfactant.4-5 Although great efforts have been made for utilizing MoS2 in electronics, optoelectronics, energy storage, catalysis, and biomedicine, etc., their sensing applications based on the surface property have not got fine exploitation.6 Several strategies based on MoS2 using field-effect transistor (FET), electrochemical, surface plasmon resonance (SPR), chemiluminescence and photoluminescence as signal output, have been developed for detection of DNA, H2O2, glucose, trinitrotoluene (TNT) and metal ions such as Hg2+, Ag+, etc.7-8 Among them, optical sensors are more attractive due to their simple operation, high sensitivity and ease of observation. However, in these sensors, MoS2 are commonly used as fluorescent quenchers, which have not unique advantage to the improvement of sensing performance. Therefore, extending new application of MoS2 as sensors in pure aqueous solutions and further obtaining more insight and comprehensive understanding of 2D nanomaterials are of great significance. In previous reports, MoS2 nanosheets can be synthesized facilely by various methods.9-11 Based on these methods, some groups including ours have investigated systematically covalent or non-covalent functionalization of MoS2 nanosheets with 2 ACS Paragon Plus Environment

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biocompatibility

polymers

in

the

applications

for

drug

photothermal/photodynamics therapy, and gene therapy of cancers.6,

11-15

delivery, We thus

expected that non-covalent functionalization strategy based on the intrinsic property of the MoS2 nanosheets could also be used for extending their application. Recently, Shi’s group has reported a quite nice work about porphyrin-graphene composites and their sensing application.16 Porphyrin derivatives are well known as a kind of functional dyes.17 The large-conjugated systems of porphyrin and unique cyclic structure endow them with excellent optical properties and coordination ability of metal ions. That is, porphyrins could act as receptor and reporter for metal ions.18 Therefore, it will be very interesting that assembly of porphyrin onto MoS2 nanosheets to form nanocomposite and then investigation of the spectroscopic behaviors of the nanocomposite in aqueous solutions can lead to the development of a new sensing system with special performance. In this work, we firstly report a nanocomposite self-assembled by MoS2 nanosheets and a cationic porphyrin, 5,10,15,20-Tetrakis(1-methyl-4-pyridinio) porphyrintetra(p-toluenesulfonate) (TMPyP), named as MoS2@TMPyP (Scheme 1). This nanocomposite can assist coordination of metal ions with TMPyP and be used for rapid detection of cadmium ions (Cd2+) ions within 20 min along with an unexpected 200 times’ acceleration of chelating Cd2+ ions into porphyrin rings. The interaction between TMPyP and MoS2 was confirmed by molecular dynamics simulation in combination with various experiments, implying that electrostatic interaction was the main force for driving TMPyP enriching around the MoS2 surface. The local high concentration of TMPyP on the MoS2 surface induces a remarkable accelerated Cd2+-TMPyP complexation. Furthermore, the nanocomposite can be effectively used for removing of Cd2+ because of the easy separation of the nanocomposite from water. Consequently, a low cost, rapid and selective sensor for the detection and removal of Cd2+ ions in aqueous media was developed.

2. RESULTS AND DISCUSSION 3 ACS Paragon Plus Environment


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MoS2 nanosheets were synthesized by a modified oleum-assisted exfoliation process with a mild and efficient manner in the absence of surfactant (Figure S1).11 Atomic force microscopy (AFM) measurement reveals that the thicknesses of MoS2 nanosheets are ~0.6-0.8 nm (Figure 1a). Moreover, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) image of the synthesized MoS2 (Figure S2a-b) indicate the sizes of MoS2 nanosheets are in the range of 50-150 nm and the sizes of a small amount of MoS2 nanosheets are less than 25 nm. Raman measurements were also introduced to characterize the MoS2 nanosheets (Figure 1b). Two characteristic Raman bands of A1g and E12g modes, respectively, exhibited in the sample, corresponding to the counterpart of the layered 2H-MoS2.19-20 Especially, the A1g and E12g modes are broadened and shifted compared to the bulk sample, implying that the lateral dimensions of these layers are in the nano-regime after the exfoliation process.21 And then, TMPyP was introduced to functionalize MoS2 nanosheets. SEM image shows that the size of the MoS2@TMPyP nanosheets is 40-110 nm (Figure S3a), which is consistent with the result of TEM measurement (Figure S3b). A slight aggregation was observed for the MoS2@TMPyP, which was mainly due to the strong interaction between the MoS2 and TMPyP. AFM image indicates that the thickness of MoS2@TMPyP nanosheets (Figure S3c) increases to ~1.1 nm compared with the non-functionalized single-layer MoS2 nanosheets (Figure 1a), which could be due to the attachment of a large amount of TMPyP on both planes of the MoS2 nanosheets. Figure 1c shows Fourier transform infrared (FTIR) spectra of MoS2 nanosheets, TMPyP and MoS2@TMPyP nanocomposite. It can be seen that the FTIR spectrum of TMPyP exhibits five peaks at 694 cm-1, 817cm-1, 1030 cm-1, 1126 cm-1, and 1636 cm-1, assigning to porphyrin in-plane vibration deformation, out-of-plane C–H bending, in-plane N–H bending, the stretching of C-N, and C=C bonds, respectively

22-23

whereas no apparent signal was

obtained in pristine MoS2. The characteristic peaks of TMPyP can also be observed in the MoS2@TMPyP and slight red shifts appear as compared to pure TMPyP, implying that TMPyP was adsorbed on the surfaces of MoS2 nanosheets.24 X-ray 4 ACS Paragon Plus Environment

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photoelectron spectroscopy (XPS) was employed to further confirm the formation of MoS2@TMPyP nanocomposite. The XPS measurement in Figure 1d is conducted after centrifugation and washing with water to exclude the interference of free TMPyP. Compared with the XPS data of pure MoS2 nanosheets (Figure S4) and pure TMPyP (Figure S5), survey of the MoS2@TMPyP shows the N1s peaks at 401.2, 399.6, 397.5 eV, S2p peak of SO32- centered at 166.9 eV (Figure S6), and C1s peaks at 284.7 and 286.4 eV (Figure S7) originating from TMPyP, indicating that TMPyP has been non-covalently assembled onto the MoS2 surfaces. UV-vis absorption spectroscopy of MoS2 nanosheets before and after loading with TMPyP was also investigated (Figure 1e). MoS2 nanosheets exhibit strong and broad absorption from 300 to 700 nm. After interacting with TMPyP, an absorption peak at ~422 nm, which originated from Soret band of TMPyP,25 was observed. Notably, compared with pure TMPyP, the relative peak intensity of TMPyP became weaker after adding MoS2, implying that MoS2@TMPyP nanocomposite have been formed. To further confirm this point, we centrifuged the MoS2@TMPyP and collected the supernatant. The absorbance intensity of TMPyP in supernatant was much weaker than that of the original intensity before loading, indicating the successful assembly of TMPyP onto MoS2 nanosheets (Figure 1f). Cd2+ is one of the major toxic and carcinogenic pollutants.26-28 Exposure to Cd2+ with a high level could increase the risks of some diseases such as cancer mortality, renal dysfunction, cardiovascular diseases, and so on.29 Thus, it is of great importance to develop a rapid and simple method for detection and removal of Cd2+ in water. Although several methods including electrochemical assay30-35 and optical probes based on synthetic organic receptors36 have been devoted to detect of Cd2+, some drawbacks associated with these approaches still existed. For example, extensive sample preparation, high cost, tedious synthesis and requirement of expensive equipment. As mentioned above, there are various optical sensors based on water-soluble porphyrins, which could act as both receptor and reporter for metal ions. We thus hypothesized that MoS2@TMPyP nanocomposite could be applied to the 5 ACS Paragon Plus Environment


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rapid detection and removal of Cd2+ ions with improved sensing performance. First, detection of Cd2+ by MoS2@TMPyP was investigated by comparing the absorption and emission spectra of MoS2@TMPyP in the absence and presence of Cd2+. When the MoS2 concentration was kept at 11.5 ppm, the relative intensity of absorption spectra of MoS2@TMPyP decreased gradually after the addition of TMPyP into the MoS2 aqueous solution and a slight red-shift was observed (Figure 2a). The absorption intensity remained unchanged after 2 h, indicating the interaction of MoS2 and TMPyP reached equilibrium. Therefore, we used the MoS2@TMPyP after mixing MoS2 and TMPyP for at least 2 h. Then, upon addition of Cd2+, the intensity of the absorption bands at 422 nm decreased gradually accompanying with the increase of the absorbance at 442 nm as shown in Figure 3a. The equilibrium was reached within a short time of 20 min. The absorption spectrum of TMPyP@Cd is shown in Figure 3b and the chelation of TMPyP with Cd2+ ions in the absence of MoS2 is much slower than that in the presence of MoS2 under the same conditions. The reaction of the free TMPyP and Cd2+ required at least 72 h. These results substantiate that MoS2 can accelerate the coordination of Cd2+ ions into the porphyrin moieties of the MoS2@TMPyP more than 200 times. Similar results were obtained in the photoluminescence (PL) spectra of the MoS2@TMPyP@Cd2+ (Figure 3c-d). It can be seen that Cd2+ induced 21% quenching of the fluorescence of TMPyP within 72 h (Figure 3d), whereas in the presence of MoS2, the quenching efficiency could be reached 60% within 20 min (Figure 3c). Moreover, concentration-dependent absorption spectra of MoS2@TMPyP were recorded. The absorbance at 442 nm of the MoS2@TMPyP dispersion increased upon increasing MoS2 concentrations from 6.5, 9.0, 11.5, to 14.0 ppm, while Cd2+ concentration was kept constant and the test time was kept at the same point (Figure 2b). Figure 4a shows UV-vis absorption changes of MoS2@TMPyP after adding different concentrations of Cd2+ within 10 min. The final concentrations of MoS2 and TMPyP are 11.5 ppm and 1.5 µM, respectively. Upon increasing amounts of Cd2+, the absorption maximum of MoS2@TMPyP was red-shifted from 422 nm to 442 nm with an isosbestic point at 432 nm. Even if a low 6 ACS Paragon Plus Environment

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concentration of 1.5 μM of Cd2+ were used, the red-shift of the absorption spectrum was so obvious. This distinct shift of 20 nm could be attributed to the conformational change of TMPyP induced by Cd2+.37 There is a linear relationship between the absorbance at 442 nm and the concentration of Cd2+ from 0 to 7.5 µM (R2=0.998) (Figure 4b). All these results indicate that MoS2@TMPyP can be used as a probe for rapid and quantitative detection of Cd2+. The limit of detection (LOD) and the linear range of the Cd2+ based on the MoS2@TMPyP nanocomposite sensor were calculated (Table S1). As shown in Table S1, the LOD of the Cd2+ is as low as 7.2 × 10-8 mol/L (8.14 ppb) and the linear range is from 0 to 11.5 × 10-6 mol/L (Figure S8). The LOD is much lower than that of the assay using TMPyP alone as the probe and some Au nanoparticles based methods although it is not as low as reported typical methods such as voltammetric detection techniques and some fluorescent probes (Table S1). Moreover, compared with voltammetric detection techniques and some fluorescent probes38, the merits of our designed method include: i) it is a quite simple and low cost method to prepare such sensor system; ii) the sensing media is 100% aqueous media which is environment friendly; iii) this method could be used not only for the rapid detection of Cd2+, but also for effective removal of Cd2+. Next, to evaluate the selectivity of the sensing system toward Cd2+, we examined the absorption spectra of MoS2@TMPyP in the presence of various metal ions under identical conditions, including Ca2+, Mg2+, Zn2+, Fe3+, K+, Cu2+, Pb2+, and Mn2+ (Figure 5). It was found that all metal ions had very little effect on absorption spectra of MoS2@TMPyP except Cd2+ which induced another absorption peak at 442 nm. We used the ratio of the absorbance at 442 nm to that at 422 nm (A442/A422) to estimate the selectivity of MoS2@TMPyP toward Cd2+ (Figure 4c). The A442/A422 value corresponding to Cd2+ is much higher than that of other metal ions, implying the specificity of this approach toward Cd2+. To address the mechanism of the accelerated coordination of MoS2@TMPyP with Cd2+, both experimental and theoretical studies were conducted. First, zeta potential measurements were further used to evaluate the surface charge status of 7 ACS Paragon Plus Environment


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MoS2, TMPyP and MoS2@TMPyP. The initial zeta potential of TMPyP and MoS2 were +15 mV and -25 mV, respectively, in aqueous solution. However, the zeta potential of MoS2 changed into -3.64 mV after the addition of TMPyP (Figure 4d). This result confirms that electrostatic interactions play a key role in the formation of MoS2@TMPyP nanocomposite and the TMPyP is easy to self-assemble onto the surface of MoS2. Another proof was provided by the introduction of branched polyethylenimine (PEI), a commonly used cationic surface modification agent,39 which we used to modify and shield the negative charge on the surface of MoS2. Figure 4e-f show the absorption and emission spectral responses of TMPyP to Cd2+ in the presence of PEI modified MoS2 (named as MoS2@PEI). It can be seen that upon addition of Cd2+, the profile of absorption and emission spectra changes slightly, implying that the acceleration effect of MoS2 was almost eliminated by PEI. Next, to better understand the interaction process in detail, we performed a density functional theory (DFT) calculation to obtain the partial charge distributions of the porphyrin molecule (Figure S9) and MoS2 using the Dmol3 package in Material Studio。40 Then, the adsorption of TMPyP on the surface of MoS2 nanosheets together with the changes in the TMPyP conformation were studied by means of molecular dynamics (MD) simulation. Figure 6a shows the process of adsorption of TMPyP on the surface of MoS2 substrate in solution. When TMPyP approaches the MoS2 surface, six kinds of conformational equilibrium between MoS2 surface and the TMPyP molecules were constructed (mode I to VI) (Figure 6b). Mode I, II, and III are when one, two and three N-methyl pyridine groups of the porphyrin are adsorbed on the MoS2 surface, respectively. Because of the bending of the planar porphyrin molecule, Mode III transition state is unstable. Therefore, the probability of the mode III is the lowest in all modes (Table S2). Mode V is the most probable conformation and the first layer of adsorbed water molecules exist in between TMPyP and MoS2. Mode VI is the least probable conformation. The TMPyP molecule is parallel to the MoS2 and the distance between TMPyP and the MoS2 is about 7.5 Å. To further evaluate the 8 ACS Paragon Plus Environment

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effect of electrostatic interaction of TMPyP and MoS2, the absorption spectra of TMPyP in the absence and presence of different concentration of MoS2 nanosheets were recorded. As seen in Figure 7a, the Soret band of the TMPyP in aqueous solution is centered at 422 nm, and 1.5, 2, and 6-nm red-shift relative to that of the increased concentrations of MoS2 from 6.5, 11.5, to 18 ppm are possibly due to the increased electrostatic interaction between TMPyP and MoS2. All these results indicate that cationic TMPyP is easily enriched around/on the surface of excess negative charged MoS2 driving by electrostatic interaction and this process is equivalent to increase the TMPyP concentration. That is, the local concentration of TMPyP on the surface of MoS2 (MoS2@TMPyP) is much higher than the actual used concentration before using the MoS2. After adding low concentration of Cd2+ to the MoS2@TMPyP nanocomposites, this composite structure can obviously increase the contact opportunities of low concentration Cd2+ and low concentration TMPyP, resulting in an accelerated complexation of Cd2+ and TMPyP compared with the system Cd2+-TMPyP only containing an equal dose of Cd2+ and TMPyP. The existence of the MoS2 nanosheets in the system can ensure enough concentrated TMPyP were enriched on the surface of these nanosheets even if the concentration of TMPyP is equal to that of the concentration of pure Cd2+-TMPyP detection system. In contrast, for the Cd2+-TMPyP detection system without MoS2, the contact opportunity of TMPyP with Cd2+ is only equal to the homogeneous solution with a lower concentration. Further study about the accelerated complexation mechanism of Cd2+ and TMPyP would be done in our future work. Prompted by the above results, we further applied this system to remove Cd2+ from aqueous media by the coordination of Cd2+ with TMPyP assisted by MoS2 nanosheets. In this experiment, two drinking water samples were spiked with low (10 ppb) and high concentrations (280 ppb) of Cd2+, respectively. Then, TMPyP and MoS2 were added to capture the Cd2+. After a simple centrifugation, the concentration of residual Cd2+ in the solution media was tested by inductively coupled plasma mass spectrometry (ICP-MS). Figure 7b demonstrates the removal efficiency (RF) of 9 ACS Paragon Plus Environment


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TMPyP and MoS2@TMPyP toward Cd2+. It can be seen that in the absence of MoS2, TMPyP is almost ineffective (RF < 8%), whereas the RF values of MoS2@TMPyP containing 20 ppm MoS2 is up to 87% and it is 91% for high concentrations of Cd2+. These results demonstrate that the MoS2@TMPyP was useful in rapid removal of Cd2+ from drinking water. To the best of our knowledge, this is the first example for the application of MoS2 for sensing and removal of metal ions from water. The MoS2-based nanostructures are now emerging as alternatives to graphene, which are beginning to prove their own competitive worth in sensing fields including various metal ions detections7.

3. CONCLUSION In summary, MoS2@TMPyP nanocomposite has been obtained by self-assembly of cationic TMPyP and negatively charged MoS2 nanosheets in aqueous solution through a simple mixing process. This MoS2@TMPyP nanocomposite can be used for the detection and removal of Cd2+. The introduction of MoS2 can accelerate the coordination reaction between TMPyP and Cd2+, which is 200 times faster than in the absence of MoS2. The zeta potential, various contrast experiments and theoretical simulations calculations indicated that the electrostatic interaction between MoS2 nanosheets and TMPyP increases the local concentration of the porphyrin molecules, consequently accelerating the Cd2+-TMPyP complexation process. The RF toward Cd2+ can reach 91%. The self-assembly of MoS2 and porphyrin extends the sensing applications of TMDs. This work provides new insight into detection and removal of Cd2+ and opens the door for constructing new systems to detect toxic ions in water. 4. EXPERIMENTAL SECTION 4.1 Materials Bulk molybdenum disulfide (MoS2) was purchased from Sigma Aldrich. Ethanol, KNO3, Cu(NO3)2, Cd(NO3)2, Mg(NO3)2, Ca(NO3)2, Mn(NO3)2, Pb(NO3)2, Fe(NO3)3, Zn(NO3)2 were purchased from Alfa Aesar. 5,10,15,20-Tetrakis (1-methyl-4-pyridinio) porphyrin tetra(p-toluenesulfonate) (TMPyP) was purchased from Beijing InnoChem 10 ACS Paragon Plus Environment

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Science & Technology Co., Ltd. Branched polyethylenimine (PEI, M.W. ≈25 000) was acquired from Sigma Aldrich Co. LLC. The deionized water was purified using Milli-Q System (Millipore, Billerica, MA, USA). All the materials were used as received without any further purification. 4.2 Preparation of single-layer MoS2 nanosheets Monolayer MoS2 nanosheets was prepared by a modified top-down exfoliation method reported by our previous work.11 Briefly, the bulk MoS2 materials (50 mg) were ground for 3 h by grinding machine. Then, the MoS2 were re-ground by mortar for 30 min. After that, the as-obtained powder was treated by 50 mL of oleum for 12 h at 90oC via water bath, followed by centrifugation and washing for three times using deionized water to remove residual oleum. The oleum-treated MoS2 dispersed in water was transferred to a glass vial and sonicated for 1 h under ice-bath ultrasonication at the power of 100W. Finally, the suspension was re-sonicated by probe-ultrasonication under ice-bath for another 2 h at the power of 320 W. The suspension was centrifuged at 8,000 rpm for 20 min. After that, the supernatant was gently transferred from the top dispersion and washed several times with deionized water and ethanol. 4.3 Preparation of MoS2@TMPyP MoS2 nanosheets with various concentrations (6.5 ppm, 9 ppm, 11.5 ppm, 14, and 18 ppm) were incubated with 1.5 μM of TMPyP aqueous solution (4 mL) for 2 h at room temperature in the dark under stirring, and then the MoS2@TMPyP nanocomposite were obtained. 4.4 Preparation of PEI modified MoS2 aqueous dispersion In a typical synthesis, 10 mg of as-obtained MoS2 nanosheets were dispersed into 20 mL of deionized water and 10 mL of PEI waters solution (1 mg/mL) was then added dropwise to the obtained dispersion under bath sonication treatment for 30 min. After stirring at room temperature for 12 h, the mixture was collected by centrifuging and washed thoroughly several times with deionized water to obtained PEI modified MoS2 (MoS2@PEI) nanosheets. 11 ACS Paragon Plus Environment


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4.5 Experimental procedures for detection of Cd2+ ions in aqueous solutions Detection procedure for Cd2+ ions was implemented as follows. Various concentrations of Cd2+ ions solutions were prepared via serial dilution with distilled water. The as-prepared MoS2@TMPyP nanosheets (final MoS2 concentration = 6.5 ppm, 9 ppm, 11.5 ppm, 14, and 18 ppm) were mixed with 30 µL of 1.0 mM Cd2+ ions solutions with different concentrations (final Cd2+ ions concentration = 7.5 µM). The mixtures were then measured by the UV-vis spectrophotometer and fluorometer. All the measurements were performed for three times. Various kinds of metal ions including Cd2+, Ca2+, Mg2+, Zn2+, Fe3+, K+, Cu2+, Pb2+, and Mn2+ (1.0 mM in stock solutions) were diluted to definite concentrations with deionized water. For detection, 4 mL solution with 7.5 μM for each metallic salt was incubated with the as-prepared MoS2@TMPyP nanosheets (final MoS2concentration = 11 ppm). Subsequently, the

UV-vis absorption and fluorescence intensities of the mixed solutions were measured and analyzed. 4.6 Capture, remove and detection of Cd2+ ions in drinking water MoS2 nanosheets with various concentrations of 11.4, 15, and 20 ppm were

respectively mixed with 1.5 μM of TMPyP to obtain MoS2@TMPyP. Then, drinking water containing different concentrations of Cd2+ ions was mixed with the MoS2@TMPyP nanocomposite for 30 min. After that, the supernatants were effectively separated by centrifugal force at 12000 rpm for 30 min. Finally, these water samples containing MoS2@TMPyP@Cd were dissolved in HNO3 (BV-III grade reagent) at 160 °C. The transparent solutions were volatilized to a volume of 0.5 mL and then diluted with 2% HNO3 up to 3 mL for Cd analysis by inductively coupled plasma mass spectrometry (ICP-MS, Thermo X7, Thermo Electron Corp., USA). 4.7 Characterization Topologies of the MoS2 nanosheets were examined by atomic force microscopy (AFM, Agilent 5500, Agilent, USA) in tapping mode in air after dropping the sample onto clean Si. Morphologies of the MoS2 samples were obtained by a field emission scanning electron microscopy (FE-SEM, Hitachi High Technologies, Japan) and 12 ACS Paragon Plus Environment

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transmission electron microscopy (TEM) at an acceleration voltage of 200 kV (TEM, Tecnai G2 20 S-TWIN). Samples for TEM characterization were prepared by dropping the MoS2 suspensions onto copper grids with ultrathin carbon-coated holey carbon support film. Raman spectra (RenishawinVia Raman spectroscope) were performed under ambient conditions with 514 nm excitation from an argon ion laser. Fourier transform infrared (FT-IR) spectra were recorded on a micro-Fourier transform infrared spectrophotometer (iN10-IZ10, Thermal Fisher). Ultravioletvisible (UV-vis) absorption spectra were recorded at room temperature by UV-vis spectrophotometer (HitachiU-3900 spectrophotometer). Zeta-potential (ζ) analyses were performed using zeta-potential analyzer (Nicomp380 ZLS plus ZETADi, PSS, USA). XPS measurement was carried out on a VG ESCALAB 220I-XL system. The energy calibration was made against the C1s peak at 284.6 eV during analysis. The photoluminescence (PL) properties were studied using a HORIBA JOBNYVON fluorolog3 spectrometer. 4.8 Computational systems and methods The adsorption of TMPyP molecules on the surface of MoS2 sheets was subsequently studied by means of molecular dynamics (MD) simulation. The size of MoS2 substrates are 4.935 × 4.749 × 0.931 nm3, including two layers of MoS2. The MoS2 substrate was solvated in TIP3P water with three TMPyP molecules. And the

sodium chloride was added to neutralize the system. A 202 ns MD simulation was carried out after energy minimization, and the last 200 ns trajectory was used to characterize the adsorption of TMPyP molecules on MoS2surface. The structure of TMPyP molecule is based on previous density functional theory (DFT) calculation. We performed DFT calculation to obtain the partial charge distributions of the porphyrin molecules using the Dmol3 package in Material Studio, version

8.0,

employing

the

exchange-correlation

functional

local

density

approximation (LDA) with the PWC function.40 There are four positive charges in TMPyP molecule and the calculation was carried out in spin-restricted fashion. The electronic wave functions were expanded in a double numerical basis set (DN) 13 ACS Paragon Plus Environment


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truncated at a real space cut-off of 3.0 Å. One k-point samplings in Brillouin zone were used. Fully relaxed structures were obtained by optimizing all atomic positions until energy minimized. The tolerance of energy, gradient, and displacement convergence was less than 1 × 10-4 Ha, 0.02 Ha/ Å, and 0.05 Å, respectively. The other parameters of the TMPyP molecules were adapted from CHARMM22 force filed.41-42 The MoS2 substrate is restrained to its initial position by harmonic potential. The Lennard-Jones parameters of Mo and S atoms are r0, Mo = 1.6454 Å, εMo= 0.0339 kcal/mol, r0,

S

= 1.8911 Å, εS = 0.0606 kcal/mol and the Mo and S atom are

adopted +0.76e and -0.38e respectively.43 The simulation was carried out using NPT ensemble at 300 K temperature and 1 bar pressure. Period boundary conditions were applied in three directions. A spherical cutoff of van der waals interactions was at 1.2 nm. A time step of 2 fs was adopted, and the data were collected every 1 ps. All the MD systems were performed on the NAMD 2.8 package and then analyzed by VMD.44-45 ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications Website. Schematic representation of the synthesis of MoS2 nanosheets; SEM and TEM images of MoS2 nanosheets; SEM, TEM and AFM images of MoS2@TMPyP nanosheets; XPS survey spectrum and high-resolution peak-fitting spectra of N1s, S2p, and C1s of pure TMPyP; XPS high-resolution peak-fitting spectra of Mo3d, Mo3p, S2p, C1s and magnified Mo3P3/2 of the MoS2@TMPyP; Linear fitted approximation of Cd2+ concentration from 0 to 11.5 μM; Partial charge distributions of the TMPyP; Some reported sensors for detection of Cd2+; Probabilities of the six conformations of porphyrin adsorbed on the surface of MoS2 (PDF).

ACKNOWLEDGEMENTS


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This work was supported by the National Basic Research Programs of China (2016YFA0201603, 2015CB932104, 2014CB931900), the Beijing Natural Science Foundation (2162046), the National Natural Science Foundation of China (11621505, 31571015, 21320102003), and Innovation Program of the Chinese Academy of Sciences (QYZDJ-SSW-SLH022). We also thank Dr. Peng Xu in National Center for Nanoscience and Technology of China for her assistance during the XPS measurement.

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Figure Captions

Scheme 1. Schematic illustration the strategy based on TMPyP self-assembly with MoS2 nanosheets to form MoS2@TMPyP nanocomposite for detection of Cd2+ ions in

aqueous solution (a-b) compared with the use of pure TMPyP as probe (c).



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Figure 1. (a) AFM image of MoS2 nanosheets. (b) The comparison of Raman spectra of bulk MoS2 and MoS2 nanosheets. (c) FTIR spectra of MoS2 nanosheets, TMPyP and MoS2@TMPyP nanocomposite. (d) XPS survey spectrum of MoS2@TMPyP. (e) Comparison of the UV-vis absorption of TMPyP, MoS2 and MoS2@TMPyP nanocomposite. (f) UV-vis absorptions of TMPyP before and after loading with MoS2 nanosheets. (Concentration of MoS2: 11.5 ppm, TMPyP: 1.5 μM.)


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Figure 2. (a) UV-vis absorptions of MoS2@TMPyP with concentration of MoS2 11.5 ppm (concentration of TMPyP: 1.5 μM.) after stirring different time at room temperature. (b) Dependence of the absorbance at 442 nm as a function of MoS2@TMPyP aqueous dispersion while Cd2+ ions concentration was kept constant (7.5 µM) and the test time was set in the 10 minute.



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Figure 3. Time-dependent UV-vis absorptions of (a) MoS2@TMPyP@Cd2+ and (b) TMPyP with Cd2+. Inset: Plots of the absorbance at 442 nm versus reaction time. Time-dependent PL spectra of (c) MoS2@TMPyP@Cd2+ and (d) TMPyP with Cd2+ (Concentration of Cd2+: 7.5 μM, MoS2: 11.5 ppm, TMPyP: 1.5 μM.)


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Figure 4. (a) UV-vis absorptions of the MoS2@TMPyP nanocomposite after adding different concentration of Cd2+ ions (MoS2: 11.5 ppm, TMPyP: 1.5 µM). (b) Linear fitting curve of the increased Cd2+ ions concentrations toward the UV-vis absorbance at 442 nm. (c) Relative absorbance of MoS2@TMPyP nanocomposite in the presence of Cd2+ and other metal ions in aqueous media. (d) Zeta potential of TMPyP, MoS2, MoS2+Cd, and MoS2@TMPyP, respectively. (MoS2: 11.5 ppm, TMPyP: 1.5 µM, Cd2+:

7.5 µM) (e) UV-vis absorption of MoS2@PEI after loading TMPyP with prolonged time. (f) UV-vis absorption of MoS2@PEI@TMPyP after adding Cd2+ ions (7.5 µM) with prolonged time.



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Figure 5. UV-vis absorption spectra of MoS2@TMPyP aqueous solutions (MoS2:11.5 ppm) in the absence (0 min) and presence of 7.5 μM of Ca2+, Mg2+, Zn2+, Fe3+, K+, Cu2+, Pb2+, and Mn2+ ions at different time.


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Figure 6. (a) The adsorption of TMPyP on the surface of MoS2 nanosheets using MD simulation from 0 ns to 100.5 ns. White color represents the water molecules. (b) Illustration of six conformations of TMPyP adsorbed on the surface of MoS2sheets (Yellow: S atom, Cyan: Mo atom).



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Figure 7. (a) UV-vis absorptions of MoS2@TMPyP with different concentration of MoS2 (Concentration of TMPyP: 1.5 μM.) after stirring for 2 h at room temperature.

(b) Removal efficiency (RF) of TMPyP toward Cd2+ ions in the absence and presence of MoS2 nanosheets with different concentrations in drinking water. ■: [Cd2+] = 10 ppb; ●: [Cd2+] = 280 ppb.


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