Solvation of Pristine Graphene Using Amino Acids: A Molecular Simulation and Experimental Analysis

We explore the ability of amino acid solutions, of L-Trp, L-Tyr, or L-Val, to solvate pristine graphene flakes in an aqueous environment via atomistic molecular dynamics simulations and experimental characterization. In accord with previous theoretical work, simulations of single amino acid adsorption on graphene predict that L-Trp is most strongly bound, followed by L-Tyr and then L-Val. As the number of amino acids is increased in the simulations, steric hindrance at the graphene interface and amino acid clustering (most pronounced for L-Tyr) reduces the efficiency of interaction with graphene. Using atomic force microscopy and UV-vis absorption spectroscopy, we determine that all three amino acid solutions can exfoliate and suspend pristine graphene flakes in water. However, L-Trp and L-Tyr solutions are considerably more effective than L-Val: L-Trp produces the most stable suspensions and thinnest graphene flakes compared to L-Tyr, with a mean thickness of 6.4 nm, and a narrow distribution of diameter with a mean value of 16 nm, commensurate with the width of cell membranes. At high concentrations of L-Trp and L-Tyr, there was severe instability of the suspensions along with agglomeration and precipitation; this reflects clustering of amino acid molecules observed in molecular dynamics simulations. This study indicates the potential of amino acids to exfoliate and suspend pristine graphene as a step toward developing nontoxic graphene-based vehicles for drug delivery and other in vivo applications.