**3. Computational Details**

All calculations were carried out using the periodic *ab-initio* code CRYSTAL09 [45]. All the SCF calculations and geometry optimizations were performed using the B3LYP-D\* density functional method, which includes an empirical *a posteriori* correction term proposed by Grimme [46] to account for dispersion forces (missed in the pure B3LYP [47,48] method), but whose initial parameterization (D) was modified for extended systems (D\*) [49], to provide accurate results for the calculations of

cohesive energies of molecular crystals and of adsorption processes within a periodic treatment [49–51]. The adopted Gaussian functions consisted of an all electron triple-ȗ 6-311G\* standard basis set for the B and N atoms of the BNNTs and a TZP basis set from Ahlrichs and coworkers [52] for the atoms of Gly. This basis set combination has been proved to exhibit small basis set superposition error interaction energies [18,50]. The shrinking factor of the reciprocal space net defining the mesh of k points in the irreducible Brillouin zone was set to 5, which requires diagonalizing the Hamiltonian matrix in 3 k points [53]. The accuracy of both Coulomb and exchange series was set to values of overlap integrals of 10<sup>í</sup><sup>7</sup> and 10<sup>í</sup>16, respectively, which ensure a very good numerical accuracy. A pruned (75, 974) grid has been used for the Gauss–Legendre and Lebedev quadrature schemes in the evaluation of functionals. The condition to achieve SCF convergence between two subsequent cycles was set to 10<sup>í</sup><sup>7</sup> Eh. Full relaxations of both lattice parameters and internal atomic coordinates by means of analytical energy gradients [54–56] have been carried out. The geometry optimization was performed by means of a quasi-Newton algorithm in which the quadratic step (BFGS Hessian updating scheme) is combined with a linear one (parabolic fit) [57].

## **4. Conclusions**

Periodic quantum mechanical calculations have been used to simulate the adsorption of glycine (Gly) on different zig-zag (*n*,0) single-walled boron-nitride nanotubes (BNNTs, *n* = 4, 6, 9 and 15) both in the gas-phase and in a microsolvated state (*i.e.*, modeled by the presence of seven explicit water molecules) with the aim of determining the adsorption properties and the effect exerted by water as a function of surface curvature. These calculations are based on the B3LYP-D2\* method, which includes the B3LYP hybrid functional with a revised version of the empirical *a posteriori* correction term (D2\*) to account for dispersion interactions.

Gas-phase results clearly indicate that the most stable interaction between Gly and the (4,0), (6,0) and (9,0) BNNTs takes place through a covalent dative interaction between the NH2 group of Gly and the B atom of the BNNTs, which produce charge transfers from Gly to the BNNTs. In contrast, the interaction between Gly and the (15,0) BNNT is mainly governed by non-covalent dispersive forces based on a S-stacking between the S systems of the Gly COOH group and the B-N hexagon rings of the nanotube. Remarkably, the energy difference between these two adducts decreases when increasing the BNNT radius, in line with the polar/apolar character of the considered nanotubes.

The adsorption of Gly on the BNNTs in the presence of seven water molecules has been studied adopting a progressive microsolvation procedure, in which water solvent molecules are added at the dry Gly/BNNT interfaces, hence progressively removing the direct interaction between Gly and the BNNTs. The obtained results indicate that for the (6,0), (9,0) and (15,0) BNNTs, the most stable microsolvated systems were found to exhibit no direct contact between the two partners; that is, the Gly/BNNT interfaces are fully bridged by the water solvent molecules. In contrast, for the (4,0) one of the most stable systems shows direct contact between Gly and the BNNT through an interaction between the Gly COO<sup>í</sup> group and a nanotube B atom, although entropic effects (not accounted for in this work) might favor water mediated interface. Further energetic results point out that the larger the BNNT radius, the less water affinity. Accordingly, for larger radius BNNTs, the interaction between water and Gly was found to be predominant, in detriment to their interaction with the BNNT. However, it is found that the (4,0) BNNT exhibits a large water affinity, which is reflected by the fact that the replacement of seven adsorbed water molecules by a microsolvated Gly has been found to be an unfavorable process.

The results presented here provide evidence that the adsorption properties of the BNNTs as well as their water affinity can significantly be modulated by controlling the tube diameter, as they are expected to exhibit different physico-chemical features, which may be of interest for the design of bioconjugated systems based on boron-nitride nanostructures and their potential bionanotechnological applications.
