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‘Template-free’ hierarchical MoS2 foam as a sustainable ‘green’ scavenger of heavy metals and bacteria in point of use water purification

Molybdenum disulfide (MoS2), with its unique optical and electrical properties, has been explored for a variety of applications in the recent past. Still, its capabilities in point-of-use heavy metal ion removal remain to be explored. Herein, for the first time using a facile approach, we fabricated three-dimensional (3D) MoS2 foam from exfoliated single to few-layered MoS2 sheets for the selective exclusion of heavy metals and stringent bactericidal response. This foam was able to exclude 99.9% of Pb(ii) and 98.7% of As(iii) instantaneously and reduced more than 98% of bacteria E. coli. Moreover, the foam exhibits selective toxicity towards bacterial cells while showing no observable toxicity towards mammalian cells. The foam can be recycled and reused for at least five cycles under accelerated conditions and thus can be used for a promising non-cytotoxic, facile, and environmentally benign process for inline water remediation to remove heavy metal ions from the feed and as a potential antibacterial agent.

The supernatant was further centrifuged to remove unbound protein. The final dispersion was lyophilized to obtain greenish-black foam product.

Material Characterization
The fabricated MoS 2 foam was characterized through Fourier transform infrared (FTIR) spectrum on a Perkin Elmer Frontier (wavelength from 4000 cm -1 to 400 cm -1 ), XRD using X-Pert PRO PANalytical equipment using Cu target. Diffraction intensities were recorded as a function of 2-theta ranging from 10° to 60° at a step width of 0.03° and a count time of 2 s.
The morphology of the foam was assessed using the scanning electron microscopy (SEM) (Ultra55 FE-SEM Karl Zeiss) at an accelerating voltage of 5 kV. To quantify the amount of the bound protein, lyophilized exfoliated samples were re-dispersed in the water at the concentration of 1 mg/mL and subjected to colorimetric Pierce BCA protein assay kit (Thermo Scientific) as per manufacturer's protocol.

Inline Adsorption Studies
For the inline adsorption studies, four different concentrations of lead (Pb II) and arsenic (As III) were formulated in double distilled water i.e. 5 ppm, 10 ppm, 20 ppm and 50 ppm. The stock solution of 100 ppm for Pb (II) was made by dissolving exactly 0.1599 g of lead nitrate to 10 mL of 2 % (v/v) HNO 3 and then diluted in 990 mL of double-distilled water. This formulation was serially diluted to make different specific concentrations. For As (III), 1 L double distilled water was bubbled with nitrogen gas for 15 min to remove dissolved oxygen in it. To this, 175.14 mg of sodium meta arsenite was added. This was 100 ppm arsenic (III) solution. This stock was serially diluted to obtain other concentrations.

Experimental design
The inline adsorption based pre-filtration kit was designed as per the schematics shown in scheme-SI1. Each segment is separated by a retainer membrane of 0.1 µm poresize. In all the inline studies, the active exposed layer was the MoS 2 foam. For control experiments, bulk MoS 2 was taken in the place of MoS 2 foam. The influx flow rate was set at 0.05 LPM using a flow meter. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on the feed and permeate and the heavy metal rejection (in %) was determined using equation (1).

Cell culture and cytotoxicity assays
The mouse lymphoid endothelial cell line (SVEC) was cultured in complete DMEM growth medium containing fetal bovine serum (FBS-10%; Invitrogen Gibco) and penicillin/streptomycin (1%, Invitrogen Gibco). Lyophilised BSA exfoliated MoS 2 was sterilized under UV and reconstituted in complete growth media. Different dilution of the reconstituted material was prepared (0, 5, 12.5, 50, 100, 200 µg/mL). The cells were incubated at 37 °C in a humidified atmosphere of 5 % CO 2 . All experiments were carried out with cells between passages 5-6.
Cell viability was conducted using the LIVE/DEAD Assay Kit (Invitrogen, UK) as per manufacturer's protocol. [2][3][4] In brief, 1000 cells/well concentration were seeded in a 96 well plate and incubated to stabilize for 24h. The next day, the exfoliated material was added at the cell proliferation were analyzed for analysis of variance (ANOVA) and the significance was evaluated using Bonferroni and Turkey's post-hoc analysis at 95% confidence (p < 0.05). All the analysis was carried out using Origin statistical software.

Antibacterial Studies
The antibacterial study was performed using E.coli ATCC 25922 (gram-negative bacteria).
Single colony was isolated from the streak plates of the bacteria and grown in the Luria Bertani (LB) broth at 37 °C and harvested at the mid-exponential growth phase. The cells were centrifuged, washed with 1x PBS three times and re-suspended in 1x PBS. The cells were normalized to 1.5 x 10 7 CFU/mL for plate count and ROS studies.

Standard plate count
The  ppm. For all the other feed concentrations up to 50 ppm >98 % for lead and >97 % of arsenic removal was observed in our study.