We have investigated the possibility of utilizing various single-walled pristine and doped carbon nanotubes as adsorbents for the 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124) gaseous molecule. Three candidates, including pristine carbon nanotube (CNT), silicon carbide nanotube (SiCNT) and germanium-doped SiCNT (SiCGeNT) are identified and evaluated theoretically. The quantum simulations have been performed at the density functional theory (DFT) level with four different functionals (i.e., M06-2X, xB97XD, CAM-B3LYP and B3LYP-D3) with a split-valence triple-zeta basis set (6-311G(d)). We found that adsorption on the SiCGeNT is most favourable, while that on the pristine CNT yields the lowest adsorption energy. Adsorption on these nanotubes is not accompanied by an active charge-transfer phenomenon; instead, it is driven by weak van der Waals forces. The HOMO–LUMO energy gaps drastically change when the dopant atom is added to the SiCNT, thereby improving their overall adsorption capability. Among all of the adsorbents investigated here, SiCGeNT shows the most favourable for designing effective HCFC-124 nanosensors.

The study of intermolecular interactions is of great importance. This study attempted to quantitatively examine the interactions between cysteine (C$_3$H$_7$NO$_2$S) amino acid molecule with pristine boron nitride, Al-doped boronnitride and carbon boron nitride nanotubes (BNNT, BNAlNT, BC$_2$NNT, respectively) in vacuum. Quantum mechanical studies of such systems are possible in the density functional theory (DFT) framework. For this purpose, various functionals, such as B3LYP-D3, ${\omega}$B97XD and M062X, have been used. One of the most suitable basis functionals for the systems studied in this research is 6-311G(d), which has been used in both optimization calculations and calculations related to wavefunction analyses. The main part of this work is the study of various analyses that reveal the nature of the intermolecular interactions between the two components introduced above. The results of conceptual DFT, natural bond orbital, non-covalent interactions and quantum theory of atoms in molecules were consistent and in favour of physical adsorption in all systems. Al-doped nanotube provides more adsorption energy than others. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy gaps were as follows: BNNT: 6.545, BNAlNT: 8.127 and BC$_2$NNT: 7.027 eV at B3LYP-D3/6-311G(d) model chemistry. The sensitivity of the adsorption increased when an amino acid molecule interacted with doped BNNT, and could be used to design a nanocarrier for cysteine amino acid.

A high concentration of trimethylamine (TMA) in the body can be converted to trimethylamine N-oxide (TMAO), which is a well-known proatherogenic substance capable of accelerating atherosclerosis disease in humans. Hence, there is a pressing need for the development of fast, accurate and reliable nanomaterials for the detection of TMA. In this study, the interaction of TMA with aluminium nitride nanotube (AlNNT) and gallium-doped aluminium nitride nanotube (AlNGaNT) was investigated using density functional theory at the M06-2X, B3LYP, ${\omega}$B97XD at 6-311G(d)/6- 31G(d) levels of theory. The results of the interaction energy between AlNGaNT and TMA for the 6-31G(d) increased remarkable in the order of M06-2X: -1.57 < B3LYP-D3:1.58 < ${\omega}$B97XD: 1.89 kcal mol$^{-1}$, while the 6-311G(d) showed higher interaction energy in the order of B3LYP-D3: -1.96 < M06-2X: -1.99 < ${\omega}$B97XD: -2.03 kcal mol$^{-1}$. Generally, the interaction of TMA with AlNNT and AlNGaNT increases the global reactivity parameter. From topological, a strong interaction was observed from -0.010 to -0.050 a.u. for sign${\lambda}_2$(r)${\rho}$(r) function and 0.000 to 0.00 to 0.400 for aRDG in TMA/AlNGaNT, where at sign${\lambda}_2$(r)${\rho}$(r) more scattered plot was observed between -0.050 to -0.020 a.u. While the small scattered plot was observed for TMA/AlNGaNT at 0.00 to -0.010 a.u. for sign${\lambda}_2$(r)${\rho}$(r) and aRDG of 0.00 to 0.400. Therefore, it was proposed that an electrostatic interaction is the mechanism between TMA and the AlNNC, and the strength of the interaction increase with the addition of Ga-atom as in AlNGaNT.