First-Principles Calculations on the Adsorption Behavior of Amino Acids on a Titanium Carbide MXene

Due
to their vast range of promising biomedical and electronic
applications, there is a growing interest in bioinorganic lamellar
nanomaterials. MXenes are one such class of materials, which stand
out by virtue of their demonstrated biocompatibility, pharmacological
applicability, energy storage performance, and feasibility as single-molecule
sensors. Here, we report on first-principles predictions, based on
density functional theory, of the binding energies and ground-state
configurations of six selected amino acids (AAs) adsorbed on O-terminated
two-dimensional titanium carbide, Ti2CO2. We
find that most AAs (aspartic acid, cysteine, glycine, and phenylalanine)
prefer to adsorb via their nitrogen atom, which forms a weak bond
with a surface Ti atom, with bond lengths of around 2.35 Å. In
contrast, histidine and serine tend to adsorb parallel to the MXene
surface, with their α carbon about 3 Å away from it. In
both adsorption configurations, the adsorption energies are on the
order of the tenths of an electronvolt. In addition, we find a positive,
nearly linear correlation between the binding energy of each studied
AA and its van der Waals volume, which suggests an adsorption dominated
by van der Waals forces. This relationship allowed us to predict the
adsorption energies for all of the proteinogenic AAs on the same Ti2CO2 MXene. Our analysis additionally shows that
in the parallel adsorption mode there is a negligible transfer of
charge density from the AA to the surface but noticeable in the N-bonded
adsorption mode. In the latter, the isosurfaces of charge density
differences show accumulation of shared electrons in the region between
N and Ti, confirming the predicted N–Ti bond. The moderate
adsorption energy values calculated, as well as the preservation of
the integrity of both the AAs and the surface upon adsorption, reinforce
the capability of Ti2CO2 as a promising reusable
biosensor for amino acids.