Linker-free Functionalization of Phosphorene Nanosheets by Sialic Acid Biomolecules

The importance of sialic acid on cell functions has been recently unveiled, and consequently, great attention has been paid to its interaction with tumor cells. In this line of research, we have realized phosphorene nanosheets functionalized with sialic acid molecules for biological applications with no need for another linker molecule. The formation of phosphorene sheets is feasible by using hydrogen plasma treatment and conversion of amorphous phosphorus on silicon substrates into highly crystalline nanosheets. Through immersion of these freshly prepared nanosheets into an aqueous solution containing sialic acid molecules, the formation of chemical binding between biomolecules and P atoms is initiated to form a carpet-like coverage. We have studied these structures by using Raman spectroscopy, electron microscopy, FTIR-ATR spectroscopy, and X-ray photoelectron spectroscopy. While XPS supports the passivation of sialic-activated phosphorene nanosheets (SAP) against oxidation in air or aqueous solutions, the FTIR analysis corroborates the evolution of P-O-C and P-C bonds between such biomolecules and the sheet surface. Moreover, the high-resolution TEM images demonstrate a considerable reduction in the lattice spacing from 0.32 nm for pristine phosphorene to 0.30 nm. Similarly, Raman spectroscopy depicts a shift in A² in-plane vibrations, owing to the evolution of stress in the passivated sheets. To investigate their biocompatibility, we examined the toxicity of these bioactivated structures and observed no or little sign of toxicity. For the latter evaluation, we exploited MTT, flow cytometry, and animal models for in vivo investigations.