The adsorption of azomethine + anticancer Pt(IV) drug complex (AzPt) onto the surfaces of Graphene (GRN) and Boron Nitride (BNN) nanoflakes was considered using the SIESTA suite of program i.e. using periodic Kohn-Sham DFT calculations [14,15]. We used the Perdew–Burke–Ernzerhof (PBE) Generalized Gradient Approximation (GGA) for the exchange correlation density functional as implemented in the SIESTA package to determine the total energies [16,17]. All calculations were performed without spin polarization. We used the self-consistency mixing rate of 0.1, a maximum force tolerance of 0.02eV.Å-1
and a mesh cut off of 100 Ry (the variations of these parameters showed a very low perturbation of the total adsorption energies by less than 0.2%, i.e. at most 9x10-3
eV for cut off of 350 Ry). The self-consistent cycles were stopped when variations of the total energy per unit cell and band structure energy were both less than 10-4
The 2D nanoflakes envisioned and AzPt molecules were studied using the same unit cell (i.e. 60x60x60 Å3) as depicted in figure 1. Note that the cisplatin form was not considered because this form is less stable in vacuum than the transplatin one, due to strong Cl/Cl repulsion. Note also that the great interest to use an amino-derivative chain is to have a spacer permitting to pull the anticancer molecule away from the nanovector and let the system forming chemical bonding facilities. Given the large unit cell, the Brillouin zone was sampled using a single k-point at the center Γ. We studied different nanoflake systems with or without atom defect. The calculated average C–C (B-N) bond lengths for graphene and boron nitride nanoflakes were 1.42 Å and 1.44 Å, respectively. The GRN lateral size was 22.80x25.25 Å2
that corresponded to 180 C atoms when perfect GRN was simulated (or 179 C atoms when one atom defect was considered) plus 38 H atoms to avoid boundary effects at the dangling bonds. For BNN, the lateral size was 25.78x23.34 Å2
and 90 B, 90 N and 38 H atoms (to avoid, as aforementioned, boundary effects) were necessary. To consider defective BNN, two cases were envisaged (the first (second) one was built with one B (N) atom defect).Figure 1:
Geometry of (a) AzPt molecule, (b) GRN with H termination, (c) BNN with H termination (B, N, C, O, H, Pt, and Cl atoms are represented as, green, blue, cyan, red, white, ochre, and large cyan spheres, respectively).
A basis set of localized atomic orbitals (double-ζplus polarization functions), and norm-conserving pseudopotentials were employed.
To understand the nanoflake/drug interactions, the adsorption Energy (Eads) of the adsorbed molecules (AzPt) was defined as: Eads =E(AzPt + nanoflake) - E(nanoflake) - E(AzPt)..
A negative Eads value denotes a more favorable interaction between the drug and the nanoflake surface. In this work, the Van der Waals (VdW) interactions were not taken into account (even if in physisorption case, they could be preponderant) in order to avoid unrealistic parameter calibration in the DFT framework. Moreover, the choice to use periodic type program instead of localized one was motivated by the total atom number in the studied system and the inherent time computations. A detailed comparison of the performance of three different codes, namely i) the Plane Augmented Wave (PAW) approach implemented in VASP, ii) the Full-Potential Linearized Augmented Plane Wave (FP-LAPW) plus local orbital (lo) method implemented in the WIEN2k code and iii) the Gaussian-type orbitals approach, gives similar results for different systems [18-21]. The charge transfers between the nanoflake and the AzPt were analyzed through a partial charge approach (i.e., valence electrons) in the Bader scheme [22-25]. According to this theory, the formation of a bond path is indicated by an accumulation of electron density, ρ(r), between the nuclei of the bonded atoms, which is necessary for bond formation. In this way the path manifests itself in an electron density map through a pair of bonded atoms as a ridge of electron density. Also, the Laplacian of ρ(r)(i.e., ∇2
ρ(r)) marks the boundaries of regions where ρ(r) is locally above or below its average value in the vicinity of r. In the regions where ∇2
ρ(r) is negative, the electronic density is shared by both nuclei (shared interactions). Otherwise, the electrons are concentrated in each of the atomic basins separately and the interaction belongs to the closed-shell type [26,27].
In the simulation, different angular orientations of AzPt were investigated although perpendicular orientation favors the chemical cycloaddition reaction (when possible) as observed on nanotubes [11-12].