Direct synthesis of nanomaterials by chemical vapour deposition (CVD) can produce high-quality samples, but the cost and complexity of the equipment prevent widespread use of the method. A simpler alternative is to suspend MoS2 powder in water and then separate 2D layers from the bulk material using ultrasound. Usually, this liquid-phase exfoliation would take place in the presence of organic solvents or surfactants, but these toxic substances cause problems for applications in biomedicine.

Now, writing in 2D Materials, a research team at ICN2 and BIST in Barcelona led by Arben Merkoçi, with colleagues from the University of Naples, has come up with an alternative. First authors Marialuisa Siepi and Eden Morales-Narváez explain what prompted the research: "The main disadvantage of methods based on organic solvents or surfactants is the low biocompatibility of the exfoliated material, as some of the most effective organic solvents are toxic. This obviously also causes problems in the disposal of the wastes during the production process."

To avoid the presence of any harmful residues that might prevent the use of 2D MoS2 in a biomedical context, the researchers turned instead to the enzyme lysozyme. Not only is lysozyme non-toxic to cells, but the environmental friendliness of the enzyme makes it inherently less expensive than methods involving more hazardous substances. Furthermore, explains Siepi, "hen egg white lysozyme is a very cheap reagent and, even considering the cost of processing it into a more suitable form, it allows an easy scale-up of the procedure."

The processing Siepi mentions involves denaturing the enzyme to produce a modified, soluble form, that the researchers term aminopropyl-lysozyme (AP-LYS). The newly configured enzyme includes a hydrophobic region that binds to the surface of the MoS2, and a hydrophilic region that interacts with the water in which the material is suspended.

Weakly bound layers

In its bulk state, MoS2 consists of molecule-thick layers bound weakly by Van der Waals attraction. Under ultrasonication, strong forces generated in the material by cavitation cause 2D layers to flake away, and the amphiphilic nature of the AP-LYS molecules that attach to both sides of the particles prevents them from re-aggregating. Siepi and colleagues used atomic force microscopy (AFM) to determine that the thickness of the flakes corresponded to that of a monolayer of MoS2 coated on each side with AP-LYS. Most of the flakes were between 250 and 550 nm across, and the researchers were able to sort them by size by centrifuging the suspension at different speeds.

To evaluate the suitability of the technique for producing 2D MoS2 appropriate for biomedical applications, the group tested the material under physiological conditions and found that it remained stable throughout a 48 h incubation. Two model human cell lines were also exposed to a dispersion of MoS2, and the lack of an effect on the morphology or viability of the cells confirmed the material’s biocompatibility.

Starting with a graphite precursor instead of powdered MoS2, the researchers were able to use the same experimental set-up to produce graphene, demonstrating that the technique can be generalized to other 2D materials. For now, however, the team plans to further explore the possibilities of MoS2. "The very next step is the functionalization of the flakes," says Siepi. "Many chemical procedures are available to immobilize other macromolecules such as proteins and enzymes, DNA and polysaccharides on the lysozyme coating. This will allow adding specific functionalities for applications in the biomedical field, including biosensors and biophotonics."

Full details of the research are reported in 2D Materials.