Biofunctional few-layer metal dichalcogenides and related heterostructures produced by direct aqueous exfoliation using phospholipids

We report a novel, inexpensive and green method for preparing aqueous dispersions of various biofunctional transition-metal dichalcogenides (MoS2, WS2, TiS2 and MoSe2) and their related heterostructures directly via ultrasonic exfoliation mediated by the presence of phospholipids. The dispersions predominantly consist of few-layer flakes coated with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), as confirmed by Raman, photoluminescence and X-ray photoelectron spectroscopies. The phospholipid coating renders the flakes biofunctional, which coupled with the unique properties of transition-metal dichalcogenides and their heterostructures, suggests this method will have great potential in biological applications.

after which the resulting dispersion was left standing for 24 hours to allow for the sedimentation of large aggregates. The supernatant was subsequently removed and centrifuged at 6000 rpm for 30 min, with the process being repeated once more to produce a dispersion. The amount of dispersed material was calculated by carefully separating the supernatants from the sediment after centrifugation. The volume of dispersion was measured and the undispersed mass was dried in a vacuum oven (Gallenkamp, UK) at 120 °C. For the preparation of DOPC/MX 2 :MX 2 heterostructure dispersions, and volume ratios of the desired parent DOPC/MX 2 dispersions were mixed and subsequently sonicated for 10 minutes.
Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2019 Thermogravimetric analysis (TGA) 15 mL aliquots of supernatants were freeze dried in a 50 mL Falcon tube (Fisherbrand, polypropylene) using a Genevac evaporator (EZ-2 series); the resulting solids were transferred to a cellulose filter, repeatedly washed with ultrapure water and allowed to dry in a vacuum oven at 120 °C for 24 hrs. TGA measurements were performed on a Q500 instrument (TA Instruments, USA) under a nitrogen atmosphere and at 10 °C min −1 using a temperature range of 20-1000 °C.

UV-Vis spectroscopy
UV-visible absorption spectra were recorded on a USB2000+UV-VIS fibre-optic spectrometer using a DH-2000-BAL deuterium-halogen light source (Ocean Optics).

Dynamic light scattering (DLS) and Zeta-potential measurements
A Malvern Zetasizer Nano ZS was operated in backscatter mode (173°), using a 633 nm HeNe laser. The 2D-material/DOPC dispersions were diluted in ultra-pure water (1:10 dilution) and equilibrated by the instrument to 25 °C for each measurement. The zeta potential was calculated by Malvern software using the Smoluchowski model.

X-ray photoelectron spectroscopy (XPS)
Samples were prepared by drop casting 100 µL of fresh dispersions onto 1 cm 2 Si/SiO 2 substrates (pre-cleaned by sonication in ethanol and acetone for 10 min), which were allowed to dry overnight. Spectra were measured using an AXIS Nova (Kratos Analytical, UK) with a monochromatic AlKα source operated at 225 W (15 kV and 15 mA). Analysis was performed using CasaXPS software, utilizing Shirley background subtraction and the appropriate relative sensitivity factors.

Fig. S2: (A)
Mo3d XPS spectrum of a DOPC/MoSe 2 dispersion drop cast onto Si/SiO 2 substrate. In the fitting, the purple line corresponds to the Mo 4+ 3d 5/2 peak, the blue line to the Mo 4+ 3d 3/2 peak, the red line to the Mo 6+ 3d 5/2 peak and the green line to the Mo 6+ 3d 3/2 peak. (B) Se3d XPS spectrum of a DOPC/MoSe 2 dispersion drop cast onto Si/SiO 2 substrate. The fitting is for the Se3d doublet, with the red line depicting the Se3d 5/2 peak and the green line the Se3d 3/2 peak. (C) C1s XPS spectrum of a DOPC/MoSe 2 dispersion drop cast onto Si/SiO 2 substrate. In the fitting, the green line corresponds to the C-C sp 2 peak, the red line to the C-C sp 3 peak, the blue line to the C-O/C-N peak and the purple line to the O-C=O peak.

Fig. S3: (A)
W4f XPS spectrum of a DOPC/WS 2 dispersion drop cast onto Si/SiO 2 substrate. In the fitting, the purple line corresponds to the W 4+ 4f 7/2 peak, the blue line to the W 4+ 4f 5/2 peak, the red line to the W 6+ 4f 7/2 peak and the green line to the W 6+ 4f 5/2 peak. (B) S2p XPS spectrum of a DOPC/WS 2 dispersion drop cast onto Si/SiO 2 substrate. The fitting is for the S2p doublet, with the red line depicting the S2p 3/2 peak and the green line the S2p 1/2 peak. (C) C1s XPS spectrum of a DOPC/WS 2 dispersion drop cast onto Si/SiO 2 substrate. In the fitting, the green line corresponds to the C-C sp 2 peak, the red line to the C-C sp 3 peak, the blue line to the C-O/C-N peak and the purple line to the O-C=O peak.

Fig. S4: (A)
Ti2p XPS spectrum of a DOPC/TiS 2 dispersion drop cast onto Si/SiO 2 substrate. In the fitting, the red line corresponds to the Ti2p 3/2 peak and the green line to the Ti2p 1/2 peak. (B) S2p XPS spectrum of a DOPC/TiS 2 dispersion drop cast onto Si/SiO 2 substrate. The fitting is for the S2p doublet, with the red line depicting the S2p 3/2 peak and the green line the S2p 1/2 peak. (C) C1s XPS spectrum of a DOPC/TiS 2 dispersion drop cast onto Si/SiO 2 substrate. In the fitting, the green line corresponds to the C-C sp 2 peak, the red line to the C-C sp 3 peak, the blue line to the C-O/C-N peak and the purple line to the O-C=O peak.

Raman spectroscopy
Samples were prepared by drop casting 100 µL of fresh dispersions onto 1 cm 2 Si/SiO 2 substrates (pre-cleaned by sonication in ethanol and acetone for 10 min), followed by drying at 70 °C for approximately 10 min. Powder samples were used to obtain spectra for the bulk starting-materials. Raman spectra and photoluminescence (PL) measurements were taken on a Renishaw inVia system using 532 nm (2.33 eV) excitation energy (laser power of 1.8 mW) and a 100× objective (numerical aperture of 0.9), giving a spot size of approximately of 1 µm. This system uses a high-resolution grating of 1800 g mm −1 , giving a spectral resolution ≤1 cm −1 . Spectra represent three accumulations at 5 s of exposure time.
Raman and PL Spectra: MoS 2

Atomic force microscopy (AFM)
Samples were prepared by drop casting 100 µL of fresh dispersions onto 1 cm 2 clean mica substrates, which were allowed to dry overnight. AFM measurements were performed in peak-force tapping mode using silicon nitride SNL-10 cantilevers with a Multimode 8 AFM (Bruker, UK). showed "steps" of 5 nm in height (or multiples of 5 nm) (E) which we ascribe to a DOPC bilayer between MoS 2 sheets, as proposed in the schematic presented in (E).