Exploration of the adsorption of caffein molecule on the TiO2 nanostructures: A density functional theory study
Amirali Abbasi
1
(
Azarbaijan Shahid Madani University
)
Jaber Jahanbin Sardroodi
2
(
Azarbaijan Shahid Madani University
)
Keywords: Electronic Properties, density functional theory, Tio2 Nanoparticle, Caffein,
Abstract :
First principles calculations were performed to study the adsorption behaviors of caffein molecule on the pristine and N-doped TiO2 anatase nanoparticles. Both oxygen and nitrogen in the caffein molecule can react with the TiO2 nanoparticle strongly. Thus, the binding sites were located on the oxygen or nitrogen atom of the caffein, while on the TiO2 nanoparticle the binding site occurs on the fivefold coordinated titanium atoms. By counting van der Waals (vdW) interactions, it was found that the adsorption on the N-doped TiO2 is more favorable in energy than the adsorption on the undoped one, indicating the high sensitivity of N-doped TiO2 nanoparticles towards caffein molecules. It means a dominant effect of nitrogen doping on the adsorption properties of pristine TiO2. The large overlaps in the PDOS spectra of the oxygen and nitrogen atoms of the caffein and titanium atom of TiO2 represent a forming Ti-O and Ti-N bonds between them. The molecular orbital calculation results indicate that the HOMOs are strongly localized on the caffein. Charge analysis based on Mulliken charges reveals a considerable charge transfer from the caffein to the TiO2 nanoparticle.
Exploration of the adsorption of caffein molecule on the TiO2 nanostructures: A density functional theory study
Abstract
First principles calculations were performed to study the adsorption behaviors of caffein molecule on the pristine and N-doped TiO2 anatase nanoparticles. Both oxygen and nitrogen in the caffein molecule can react with the TiO2 nanoparticle strongly. Thus, the binding sites were located on the oxygen or nitrogen atom of the caffein, while on the TiO2 nanoparticle the binding site occurs on the fivefold coordinated titanium atoms. By counting van der Waals (vdW) interactions, it was found that the adsorption on the N-doped TiO2 is more favorable in energy than the adsorption on the undoped one, indicating the high sensitivity of N-doped TiO2 nanoparticles towards caffein molecules. It means a dominant effect of nitrogen doping on the adsorption properties of pristine TiO2. The large overlaps in the PDOS spectra of the oxygen and nitrogen atoms of the caffein and titanium atom of TiO2 represent a forming Ti-O and Ti-N bonds between them. The molecular orbital calculation results indicate that the HOMOs are strongly localized on the caffein. Charge analysis based on Mulliken charges reveals a considerable charge transfer from the caffein to the TiO2 nanoparticle.
Keywords: Caffein; TiO2 nanoparticle; Electronic properties; Density functional theory.
1. Introduction
Titanium dioxide (TiO2) is one of the most extensively studied transition metal semiconductors, which has outstanding properties such as non-toxicity, chemical stability, abundance and high catalytic efficiency [1-5]. However, due to its wide bandgap (3 and 3.2 eV for rutile and anatase, respectively), pristine TiO2 can be only activated by ultraviolet light, lowering the efficiency of its photocatalytic activity. Doping of TiO2 can introduce energy levels in the bandgap, efficiently modifying its electronic band structure to absorb light in the visible region [6, 7]. Nitrogen doping of TiO2 improves optical sensitivity and makes it possible TiO2 can response to the incoming solar light more appropriately.
TiO2 is widely studied for applications such as photo-catalysis [8], gas sensor devices, heterogeneous catalysis [9] and photovoltaic cells [10]. Over the past few years, a huge surge of interest has arisen in researching for fundamental principles and crucial practical features of TiO2 [11-18]. Several researchers from different fields of science have focused on studying the outstanding properties of TiO2 nanoparticles. For instance, Liu et al. suggested that nitrogen doping of TiO2 strengthens the adsorption of toxic gas phase NO molecules by anatase nanoparticles [14]. Recently, it has been revealed that the N-doped TiO2 anatase nanoparticles react with CO molecules more efficiently, compared to the undoped ones [19]. Furthermore, substituting of nitrogen atom into TiO2 particles enhances its sensing capability in whole range of applications [20-25]. The effects of doping of nitrogen atom on the photo-catalytic activity and energy band gap of TiO2 have been investigated in detail [26-28].
DFT calculations on molecules are based on the Kohn–Sham approach, two robust and efficient Hohenberg and Kohn theorems are provided in order to describe the DFT formalism. The first Hohenberg–Kohn theorem states that all the properties of a molecule in a ground electronic state are calculated using the ground state electron density function, 
, that is, using 
we can calculate any ground state property related to the system under study, e.g. the energy. This can be represented as:
(1)
This relationship means that 
is a functional of
. The first Hohenberg–Kohn theorem, then, says that any ground state property of a molecule is a functional of the ground state electron density function. The second Hohenberg–Kohn theorem says that any trial electron density function will give an energy higher than (or equal to) the true ground state energy. In DFT calculations, the electronic energy from a trial electron density is defined as the energy of the electrons moving under the potential of the atomic nuclei. We can call this nuclear potential as the “external potential” and designate it by v(r). The second Hohenberg–Kohn theorem can therefore be specified by the following equation:
(2)
Caffeine is a drug and can affect people in a different way just like any other substance. Caffein is important that consumers realize how caffeine interacts with their bodies in regards to their personal health histories. It is considered to be the most commonly used psychoactive drug in the world. Coffee, soda, and tea are the most common sources of caffeine in the world. A majority of adults use it in our daily life, and research is being completed on its health benefits and concerns. There are a minority of research studies that address the potentially injurious effects of caffeine. The risks of suffering from any of the harmful effects of caffeine are reduced by doing research about how much is personally being consumed daily. It is also essential to be informed about any pre-existing medical conditions that may contribute to caffeine’s negative effects. In this work, we studied the interaction of caffein drug with undoped and N-doped TiO2 anatase nanoparticles. We provided various adsorption configurations of the caffein towards the nanoparticles. The electronic properties of the adsorption systems were examined in view of the density of states, and molecular orbitals. This work aims at providing a theory basis on how caffein drug interacts with biocompatible TiO2 nanoparticles.
2. Computational details and models
2.1. Computational Methods
Density Functional Theory (DFT) calculations [29, 30] were performed with the Open source Package for Material eXplorer (OPENMX3.8) [31]. The pseudo atomic orbitals were utilized as basis sets in the geometry optimizations. The considered cutoff energy was set to the value of 150 Ry in our calculations. The exchange-correlation energy functional was treated using the generalized gradient approximation (GGA) parameterized by Perdew–Burke–Ernzerhof (PBE) [32]. To fully describe the effects of long range van der Waals (vdW) interactions, we have employed DFT-D2 method, which was developed by Grimme et al. [33]. For self-consistent field iterations, the convergence criterion of 1.0 × 10-6 Hartree was used, while for energy calculation the criterion was set to 1.0 × 10-4 Hartree/bohr. The crystalline and molecular structure visualization program, XCrysDen [34], was employed for displaying molecular orbital isosurfaces. The Gaussian broadening method for evaluating electronic DOS was used. When caffein interacts with TiO2 nanoparticle, the adsorption energy was calculated according to the following equation.
Ead = E (adsorbent + drug) – E adsorbent – E drug (1)
where E(adsorbent + drug), E adsorbent and E drug are the energies of the complex system, the free TiO2 nanoparticle without any adsorbed caffein molecule and the isolated caffein molecule, respectively. The charge transfer between caffein molecule and TiO2 nanoparticle was estimated based on the Mulliken charge analysis.
2.2. Modelling of nanoparticles
TiO2 anatase nanoparticles were modeled by setting a 3×2×1 supercell of pristine TiO2 anatase. The considered unit cell of TiO2 was reported by Wyckoff [35] and taken from “American Mineralogists Database” webpage [36]. The size of the simulation box considered in our calculations is 20×15×30 Å3, being much larger than the nanoparticle size. A vacuum space of about 11.5 Å was set between neighbor particles to avoid the additional interactions between repeated slabs. Two oxygen atoms of pristine TiO2 (twofold coordinated and threefold coordinated oxygen atoms) were substituted by nitrogen atoms to prepare N-doped nanoparticles. Twofold coordinated oxygen atom is denoted by 2f-O and threefold by 3f-O (middle oxygen) in Figure 1 with fivefold coordinated and sixfold coordinated titanium atoms sketched by 5f-Ti and 6f-Ti, respectively [37]. The schematic structure of caffein molecule is represented in Figure 2.
3. Results and discussion
3.1. The interaction of caffein with N-doped TiO2 nanoparticles
Different conformations were simulated for the pristine and N-doped TiO2 nanoparticle + caffein, where the caffein molecule is place perpendicular to the TiO2 surface. Six possible adsorption geometries of caffein towards the nanoparticle were considered. It should be noted that both oxygen and nitrogen atoms of caffein molecule strongly interact with the fivefold coordinated titanium atom of TiO2, and the carbon atom does not contribute to the adsorption any longer. This reaction of active sites of caffein molecule with the fivefold coordinated titanium sites gives rise to a strong binding between the nanoparticle and caffein molecule. Adsorption geometries of caffein molecule on the undoped and N-doped TiO2 nanoparticle were displayed in Figures 3 and 4, as labeled by adsorption configurations A-F.
Each configuration in these figures represents that the caffein molecule was approached to the TiO2 nanoparticles at different positions. From all configurations, it can be seen that the caffein molecule was adsorbed either by its nitrogen or oxygen atom to the undercoordinated titanium sites of TiO2. The nitrogen atom was also substituted into the oxygen vacancy of TiO2 according to two doping positions. In one doping configuration, a nitrogen atom substitutes an oxygen atom in the OC site of the particle, while the other doping configuration represents the replacement of OT site by nitrogen atom. Table 1 lists the bond lengths for caffein molecule adsorbed to the TiO2 nanoparticles. For brevity, we have only reported the newly formed bonds between the drug molecule and nanoparticle. The smaller the bond formed between the nitrogen or oxygen atom of caffein molecule and the fivefold coordinated titanium atom of TiO2 nanoparticle (Ti-N, Ti-O), the stronger the interaction of caffein with TiO2 anatase nanoparticle.
Further analysis of adsorption energies reveals that the interaction of caffein molecule with fivefold coordinated titanium site of TiO2 is strongly favored from the energy point of view. The adsorption energies of the most stable configurations were summarized in Table 1.
Based on the results of this table, we found that the adsorption of caffein molecule on the N-doped nanoparticle is more energetically favorable than the adsorption on the pristine one. Thus, the N-doped nanoparticle can strongly interact with caffein molecule and provide more energy favorable adsorption configurations. The negative sign of adsorption energies indicate the process is exothermic and energy favorable. The higher the adsorption energy of caffein on the TiO2, the stronger the interaction of caffein with TiO2 nanoparticle. Therefore, the N-doped nanoparticles have higher adsorption ability than the pristine ones, suggesting that the nitrogen doping strengthens the interaction between caffein and TiO2 nanoparticle.
As can be seen from Table 1, the highest adsorption energy occurs in configuration A, representing that the interaction of nitrogen atom of caffein molecule with titanium atom is stronger than the interaction of oxygen atom. In contrast, the lowest adsorption energy belongs to configuration E, which shows the interaction of nitrogen atom of caffein with pristine TiO2 nanoparticle is less favorable. By considering these results, we concluded that the nitrogen modified TiO2 nanoparticle is an ideal material to be utilized for sensing of caffein molecule.
The adsorption energies are significantly increased when we take the effects of long range vdW interactions into account. This indicates the prominent effect of van der Waals interaction during the adsorption of caffein on the TiO2 nanoparticles.
3.2. Electronic structures
The total density of states (TDOS) of the complex systems containing caffein adsorbed TiO2 nanoparticles was displayed in Figure 5, which shows that the differences between DOS of bare nanoparticle and caffein adsorbed one are slightly increased by the adsorption of caffein molecule. These differences include both changes in the energies of the peaks and creation of some small peaks in the DOS of N-doped TiO2 at lower-lying energies ranging from -13 eV to -7 eV. Consequently, these changes in the DOS states would affect the electronic transport properties of the nanoparticles.
The projected density of states for caffein molecule adsorbed on the TiO2 anatase nanoparticles were displayed in Figure 6. Panels (a-f) show the PDOSs for configurations A-F, respectively. The significant overlaps between the PDOSs of the interacting atoms (nitrogen or oxygen atom of caffein molecule and titanium atom of TiO2) represent the formation of chemical bonds between them. The PDOSs of the nitrogen atom of caffein molecule, titanium atom and their pertaining d orbitals were presented in Figures 7 and 8 for configurations A and C, respectively.
These figures show the highest overlap between the PDOSs of nitrogen atom and d1 orbital of titanium atom, compared with the other d orbitals. Thus, it can be concluded that the d1 orbital of the titanium has a higher contribution for the formation of chemical bond with nitrogen atom. Figures 9 and 10 show the corresponding PDOSs of the oxygen atom of caffein molecule, titanium atom and different d orbitals, representing considerable overlaps between the PDOSs of the oxygen atom of caffein and d2 orbital (configurations B and D).
Figures 11 and 12 display the isosurfaces of HOMOs and LUMOs for caffein molecule adsorbed on the TiO2 anatase nanoparticles. Interestingly, the HOMOs of the adsorption systems are dominant at the whole surface of caffein molecule, whereas the electronic density in the LUMOs seem to be distributed over the TiO2 nanoparticle. The concentration of electronic density on the adsorbed caffein molecule indicates that the electronic density of adsorption configurations was influenced upon adsorption of caffein molecule. This feature of electronic density (especially HOMO) would be useful to help in the design and development of efficient nanosensors for caffein drug. We have also calculated the total electron densities and Kohn-Sham potentials for the studied adsorption complexes. These results are in accordance with the molecular orbital calculations. Figure 13 shows the calculated total electron densities, while Figure 14 displays the Kohn-Sham potentials for caffein adsorbed TiO2 nanoparticles. As can be seen from Figure 13, the electron density were distributed between the newly formed Ti-N and Ti-O bonds, representing the formation of chemical bonds. This can be clearly understood form Figure 14, which indicates the potential distribution of the considered systems. To further analyze the charge exchange between TiO2 nanoparticle and caffein molecule, we have performed charge analysis based on Mulliken charges. The results indicate that caffein adsorption induces a noticeable charge transfer of about -0.709 e from caffein to the TiO2 nanoparticle for configuration A. This implies that the caffein molecule behaves as a charge donor after the adsorption process. As can be seen from Table 1, the highest value of charge transfer was estimated for configuration A, whereas the lowest charge transfer belongs to configuration E, in accordance with the variations of adsorption energies.
4. Conclusions
In this paper, the interaction of caffein drug with pristine and N-doped TiO2 anatase nanoparticles were investigated using density functional theory calculations. Various adsorption models of caffein on the considered nanoparticles were examined in detail. Both oxygen and nitrogen atoms of the caffein molecule can interact with the fivefold coordinated titanium atom. The calculations predict that caffein presents a stronger interaction with TiO2 nanoparticles containing doped nitrogen atom rather than with pristine or undoped nanoparticles. The interaction of caffein molecule with N-doped TiO2 is more energetically favorable than the interaction with undoped ones, representing that the N-doped nanoparticle is strongly favored. By the inclusion of vdW interactions, the adsorption energies for caffein molecule are considerably increased. The projected density of states of the oxygen and nitrogen atoms of caffein molecule and titanium atom of TiO2 represent considerable overlaps between these atoms and consequently formation of chemical Ti-O and Ti-N bonds at the interface region. After the adsorption, the HOMOs of the adsorption systems were mainly distributed on the adsorbed caffein molecule. Thus, nitrogen doping into TiO2 particle, strengthens the interaction between caffein and TiO2 nanoparticle. The resulting systems suggest that TiO2 anatase, in nitrogen modified form, can be used as caffein sensors due to the sensitivity of the electronic properties around the Fermi energy to the presence of caffein drug.
Acknowledgement
This work has been supported by Azarbaijan Shahid Madani University.
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Table 1. Bond lengths (in Ǻ), adsorption energies (in eV) and Mulliken charge values for caffein molecule adsorbed on the TiO2 anatase nanoparticles.
Complex | Ti-N | Ti-O | Adsorption energy | Mulliken Charge |
|
|
| PBE DFT-D2 |
|
A | 2.43 | --- | -4.43 -6.20 | -0.709 |
B | --- | 2.31 | -4.31 -6.11 | -0.588 |
C | 2.38 | --- | -3.72 -5.80 | -0.533 |
D | --- | 2.17 | -3.70 -5.81 | -0.376 |
E | 2.36 | --- | -1.18 -2.22 | -0.320 |
F | --- | 2.13 | -1.23 -2.28 | -0.325 |
OC 3f-O |
OT 2f-O |
6f-Ti |
5f-Ti |
b |
a |
Figure 1. Optimized N-doped TiO2 anatase nanoparticles constructed using the 3×2×1 unit cells, colors represent atoms accordingly: Ti in gray, O in red and N in blue.
Figure 2. Representation of the optimized structure of caffein molecule, colors represent atoms accordingly: C in yellow, N in blue, O in red and H in cyan.
B |
A |
D |
C |
Figure 3. Optimized geometry configurations of N-doped TiO2 anatase nanoparticles with adsorbed caffein molecule.
F |
E |
Figure 4. Optimized geometry configurations of undoped TiO2 anatase nanoparticles with adsorbed caffein molecule.
f |
e |
d |
c |
b |
a |
Figure 5. Density of states for caffein molecule adsorbed on the undoped and N-doped TiO2 anatase nanoparticles, a: Complex A; b: Complex B; c: Complex C; d: Complex D; e: Complex E; f: Complex F.
f |
e |
d |
c |
b |
a |
Figure 6. Projected density of states for caffein molecule adsorbed on the TiO2 anatase nanoparticles, a: Complex A; b: Complex B; c: Complex C; d: Complex D; e: Complex E; f: Complex F.
Figure 7. Projected density of states for the nitrogen atom of the caffein, titanium atom and different d orbitals of the titanium (complex A).
Figure 8. Projected density of states for the nitrogen atom of the caffein, titanium atom and different d orbitals of the titanium (complex C).
Figure 9. Projected density of states for the oxygen atom of the caffein, titanium atom and different d orbitals of the titanium (complex B).
Figure 10. Projected density of states for the oxygen atom of the caffein, titanium atom and different d orbitals of the titanium (complex D).
B-side view |
A-side view |
D-side view |
C-side view |
F-side view |
E-side view |
Figure 11. The isosurfaces of HOMO molecular orbitals of caffein molecule adsorbed on the considered TiO2 nanoparticles.
B-side view |
A-side view |
D-side view |
C-side view |
F-side view |
E-side view |
Figure 12. The isosurfaces of LUMO molecular orbitals of caffein molecule adsorbed on the considered TiO2 nanoparticles.
B |
C |
A |
F |
E |
D |
Figure 13. Isosurface plots of the total electron density for caffein molecule adsorbed on the TiO2 anatase nanoparticles.
C |
B |
A |
F |
E |
D |
Figure 14. Isosurface plots of Kohn-Sham potentials excluding the nonlocal potential for up-spin in a Gaussian cube format.