Adsorption of nucleobases on 2D transition-metal dichalcogenides and graphene sheet: a first principles density functional theory study

Abstract

Adsorption characteristics of DNA/RNA nucleobases on monolayers of transition-metal dichalcogenide (TMD) such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) have been studied using first principles density functional theory (DFT) with vdW-DF method. The same calculations have been performed with PBE and DFT-D2 method for comparison. In addition, a comparative study has been done for adsorption with graphene (GRA) also to compare with MoS₂ and WS₂. We have found that all nucleobases are physisorbed on MoS₂ and WS₂ due to van der Waals interaction, which is similar to that of nucleobases on GRA. The order of binding energy of the nucleobases with MoS₂ and WS₂ is G > A > T > C > U using vdW-DF and DFT-D2 method, which is also similar to that of GRA–nucleobases. Without the inclusion of vdW interaction (PBE only), the order of the binding energies is calculated to be G > C > A > T > U for MoS₂ and WS₂–nucleobase complexes and G > C > A=T > U for GRA. We have analyzed changes in the electronic structures due to adsorption and the consequences in the calculated optical absorption spectra. Moreover, we have found that the calculated work functions of MoS₂, WS₂ and GRA decrease after the adsorption of nucleobases. Our results demonstrate that apart from graphene, transition metal dichalcogenides may also be used to detect biomolecules for medical science and biotechnology.

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Introduction

The role of DNA is vital in life science and its detection is very important in the field of genomics, diagnosis of disease and forensic sciences.¹ Using nanomaterials such as carbon nanotubes (CNT's), quantum dots (QD's)^2,3 and graphene oxide (GO),^4–6 DNA molecules have been sensed based on calorimetric^7,8 and fluorescent assays.^4,9 The important advantage of using 2D materials in the sensing of molecules is due to the high surface-to-volume ratio.¹⁰ For example, 2D sheets of graphene (GRA),^11–15 graphene oxide (GO)^16,17 and hexagonal boron nitride (h-BN)^18,19 are used to sense the gas molecule and biomolecules. Transition metal dichalcogenides (TMD) constitute a class of two dimensional (2D) materials with intrinsic band gaps of 1–2 eV used for various applications.²⁰ Molybdenum disulfide (MoS₂) is one of the well known metal dichalcogenides used in nanoelectronics, optoelectronics, energy harvesting and catalysis processes.^21–25 2D MoS₂ is a layered semiconducting material with a hexagonal arrangement of Mo atoms in the middle layer and S atoms on the surface. Mo atoms are located in between the surface S atoms. Mo and S atoms are bonded covalently with a typical distance of about 2.40 Å with S–Mo–S angle of 80.6°.²⁶ 2D MoS₂ and WS₂ are being studied extensively as some of their properties are similar to GRA.^27–31 Pachter and coworkers have studied chemical sensing of MoS₂ with planar and non-planar molecules using DFT.³² Using first principles calculations, Yue et al. have demonstrated that the adsorption of gas molecules such as H₂, O₂, NH₃ and NO on MoS₂ surfaces increases in the presence of external electric field.³³ Experimental studies on chemical vapor sensing of laboratory solvents, such as dichlorobenzene, dichloropentane, nitromethane, nitrotoluene with MoS₂ are available in the literature.³⁴

Tungsten disulfide (WS₂) is used in solid lubricants, catalysis and tips in the scanning microscopy. WS₂ nano sheets have been used as nanoquenchers for DNA detection.,^35–37 e.g., by fluorescence quenching method.³⁷ Electronic and mechanical properties of WS₂ have been studied theoretically by several groups.^38–40 Amin et al. have studied the effect of strain on WS₂, WSe₂ and WTe₂, where it was found that the direct band gaps change to indirect ones due to the application of compressive strain.³⁸ Wei et al. have studied the effect of defects on the optical properties of WS₂ and found that the presence of defect increases the intensity of absorption.⁴⁰

Although the application of MoS₂ and WS₂ is explored in electronics, catalysis and vapor molecule detection, the study of sensing of biomolecules can hardly be found in literature. Even though, graphene and its analogues have been used for this purpose, alternative 2D materials should be sought because of the poor ON/OFF current ratio in graphene.^40–43 It has been demonstrated that a monolayer of MoS₂ exhibits a better ON/OFF current ratio²¹ than graphene and hence, can potentially be used for sensing. Recently, Zhu et al. have studied the homogeneous detection of DNA with MoS₂ based fluorogenic nanoprobes.⁴⁴ Density functional theory and molecular dynamics study by Farimani et al. showed that the DNA sequencing ability of nanopore and nanoribbon–MoS₂ with four different nucleobases is better than that of graphene.⁴⁵

Understanding of organic molecules and biomolecular interaction with nanosurfaces is important for designing of better sensors with the selectivity. Numerous theoretical studies have been carried out to understand the interaction of DNA/RNA nucleobases, e.g., adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U) with 2D surfaces of GRA, GO, h-BN^11–19 for sensing of biomolecules. Also, monolayer GRA and h-BN for Li-ion battery electrode materials have been studied.^46,47

To gain insights on the absorption characteristics of nucleobases on 2D surfaces, the objective of the present investigation is to study the.

1. Geometries and energetics of MoS₂ and WS₂–nucleobase complexes.

2. Modifications of the electronic structures.

3. Signatures of adsorption in the optical adsorption spectra of the complexes.

4. Changes in the work function of pristine MoS₂, WS₂, GRA and its complexes with nucleobases.

5. Comparison of the properties with complexes of nucleobases with 2D graphene (GRA).

Computational methods

DFT calculations were performed using Vienna ab initio simulation package (VASP)^48,49 in a plane wave basis with the electron–ion interactions described by projector augmented wave method. We have used non-local van der Waals density functional (vdW-DF)^50,51 to describe the interactions between nucleobases and 2D substrates. The exchange–correlation functional in vdW-DF has three terms: exchange energy from GGA, local correlated energy from LDA and nonlocal correlation energy, which approximates vdW interaction. Calculations were performed with 6 × 6 lateral supercells of MoS₂, WS₂ (108 atoms) and GRA (72 atoms). To minimize the interaction between the supercell images, a 20 Å spacing was used in the direction perpendicular to the substrate. Brillouin zone integrations were performed with a 5 × 5 × 1 Monkhorst–Pack k-point mesh to calculate total energies and densities of states. The Hellman Feynman forces on each atom were allowed to converge to 10 meV Å⁻¹ with a plane wave cutoff of 400 eV. Binding energies of the nucleobases with substrates (MoS₂ or WS₂ or GRA) were calculated using the equation:

E_BE = E_Sub+Nuc − (E_Sub + E_Nuc),

where, E_Sub+Nuc, E_Sub and E_Nuc are total energies of the complex, substrate and nucleobase respectively. Apart from vdW-DF, binding energies of the complexes have also been calculated using PBE and DFT-D2 methods.

To study the optical properties of pure substrates and substrate–nucleobase complexes, frequency dependent dielectric functions were computed. The imaginary part of the dielectric function was calculated by the following equation:

where α and β are Cartesian components, e_α and e_β are unit vectors, c and ν represent conduction and valence band states respectively and u_c,k is the cell periodic part of the wave function at each k-point. ε′′_αβ is the imaginary part of the dielectric constant, e is the electronic charge, and Ω is the volume of the supercell. ε_ν,k and ε_c,k represent the eigenvalues of valence and conduction band states respectively.

The work functions of the 2D substrates before and after adsorption have been calculated using the following equation:

ϕ = V(ϕ) − E_f

where V(ϕ) and E_f are the electrostatic potential at the vacuum level and Fermi energy respectively.

Results and discussion

We have placed nucleobases on different adsorption sites of MoS₂, WS₂ and GRA as the starting geometries for the calculation. The center of each nucleobase was placed on top of hexagon (A site), M (Mo or W) (B site) and S (C site) of MoS₂ and WS₂. For GRA, the center of the nucleobases was placed on top of hexagon (A site), C (B site) and C–C (C site) bonds. All of these starting configurations were relaxed using vdW-DF method and the minimum energy configuration for each system has been taken for further calculations. The total energies of all configurations are listed in Table S1.† The optimized M (Mo or W)–S bond lengths are obtained as 2.40 Å and 2.43 Å for MoS₂ and WS₂ whereas S–M–S angles are calculated to be 81.9° and 81.7° for MoS₂ and WS₂ respectively. Our calculated geometries for MoS₂ and WS₂ match very well with earlier studies.^52,53 Similarly for GRA, the optimized C–C distance is 1.42 Å, which is in good agreement with previous reports.⁵⁴

Adsorption of nucleobases on MoS₂ and WS₂ surfaces

In this section the structure and energetics of MoS₂–nucleobase and WS₂–nucleobase will be discussed.

Force minimized structures of MoS₂–nucleobase and WS₂–nucleobase complexes are shown in Fig. 1 and 2 respectively. For each system, the minimum energy configuration has been chosen among the various possibilities (Table S1†). The optimized structures for GRA–nucleobases are presented in the ESI (Fig. S1†). The calculated vertical distances between nucleobases and MoS₂, WS₂ and GRA are presented in Table 1.

Fig. 1 Optimized structures of nucleobases adsorbed on monolayer MoS₂ (purple: Mo, yellow: S, red: O, blue: N, gray: C and white: H).

Fig. 2 Optimized structures of nucleobases adsorbed on monolayer WS₂ (purple: Mo, yellow: S, red: O, blue: N, gray: C and white: H).

Table 1 Calculated vertical distances between nucleobases and substrates (MoS₂, WS₂ and GRA)

	Vertical distance (in Å)
	MoS2	WS2	G
G	3.55	3.61	3.56
A	3.53	3.59	3.55
T	3.64	3.59	3.58
C	3.52	3.53	3.55
U	3.60	3.54	3.54

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The calculated vertical separations between MoS₂, WS₂ and GRA and nucleobases are in the ranges 3.52–3.64 Å, 3.53–3.61 Å and 3.54–3.58 Å respectively. Our calculated distances for GRA–nucleobase complexes are in good agreement with a previous report.¹⁴ From Table 1, it is clear that the difference in the vertical distances for these three substrates is very small. Similar results have been obtained for MoS₂–heterocyclic composites with a vertical distance of about 3.50 Å.³⁷ The presence of hetero atoms like N and O tilts the molecules slightly on MoS₂ and WS₂ whereas the molecules have a parallel orientation on GRA. The orientation and position of the nucleobases on the GRA surface agree quite well with an earlier study.¹⁴ The nucleobases are parallel to GRA due to π–π interactions. In TMD, in addition to the hetero atoms, the hydrogen atoms in the tail (–NH₂ and –CH₃ group) part of nucleobase interacts with MoS₂ and WS₂ surfaces. In TMD–A complex, the H atom of CH₃ group in adenine is in close contact with S atom of the surface with a typical distance of about 3.20 Å and for TMD–C, H of methyl group and O of cytosine are relatively closer to substrates with distances of about 2.72 Å and 3.33 Å respectively.

For TMD–G, H of NH₂ and O of guanine interact strongly than the other atoms and the vertical distance for this close contact is about 3.22 Å. In the case of MoS₂/WS₂–T, H atoms of two methyl groups of thiamine have a close contact (3.12 Å) with surface and for MoS₂/WS₂–U, similar to other complexes, H of CH₃ of uracil is in close contact (3.15 Å) than other atoms. It should be noted that the presence of nucleobases does not modify the surface geometrical parameters such as M–S bond distances and S–M–S angles significantly. The unchanged geometrical parameters of TMD–nucleobase complexes clearly show that the nucleobases are physisorbed on monolayer TMDs. This physisorption is mainly due to van der Waals interaction between nucleobases and monolayer TMDs. The calculated vertical distances between MoS₂, WS₂ and GRA surfaces and nucleobases with PBE and DFT-D2 methods are shown in Table S2.† It is clear from Table 1 and S2† that the vertical distances between the substrates and nucleobases are larger with PBE and shorter with DFT-D2 with the values obtained with vdW-DF in between the PBE and DFT-D2 ones. Our obtained trend is similar to the one reported in literature.⁵⁵

The comparison of binding energies of complexes of nucleobases with MoS₂, WS₂, and GRA is shown in Fig. 3. The order of binding affinity of nucleobases with MoS₂ and WS₂ is G > A > T > C > U, which is similar to that of GRA in the present study and also in earlier studies.¹⁴ Among all nucleobases, G has the highest binding affinity, which is similar to the nucleobase adsorption with GRA and h-BN sheets.¹⁹ We have considered another method, viz. DFT-D2 to treat vdW interactions to compare the results with vdW-DF method. The results are presented in Table S3† along with the results of PBE calculations without the inclusion of any vdW interaction. First of all, the binding energies predicted by PBE are very small along with relatively large vertical distances, which indicates the necessity of the treatment of vdW. Secondly, the vdW contributions to total binding energies of G, A, T, C and U adsorbed on MoS₂ are 75.89, 71.36, 67.62, 61.48 and 58.88 kJ mol⁻¹ respectively and for WS₂ the values are 77.59, 71.24, 67.54, 61.01, 58.01 kJ mol⁻¹ respectively. For GRA, the vdW contributions to the binding energies of G, A, T, C and U are 81.19, 70.41, 70.55, 64.40 and 63.52 kJ mol⁻¹ respectively. The vdW contribution to the total binding energy clearly indicates physisorption of base molecules on the substrates. Finally, we want to comment that the binding energy difference between the nucleobases are 8–25 kJ mol⁻¹, which is small. However, G has a higher binding affinity and it may be recognized by substrates similar to silicene.⁵⁶

Fig. 3 Comparison of binding energies for nucleobases adsorbed on MoS₂, WS₂ and GRA.

The calculated work functions for pristine MoS₂, WS₂ and GRA and nucleobase–substrate complexes are shown in Table 2.

Table 2 Calculated work functions for pristine MoS₂, WS₂, GRA and with nucleobases

	Work function (in eV)
	MoS2	WS2	GRA
Pristine	5.74	5.57	4.00
G	5.18	5.06	3.62
A	5.40	5.10	3.80
T	5.50	5.24	3.78
C	5.60	5.42	3.84
U	5.64	5.29	3.81

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The calculated work function of pristine graphene is 4 eV, which agrees well with work function of CVD-grown monolayer graphene (∼4.3 eV).⁶⁰ Similarly, the calculated work function of MoS₂ is closer to the experimental value of 5.25 eV.⁶¹ A previous DFT calculation⁶² reported the work function for WS₂ to be 5.89 eV, which is close to our value of 5.57 eV. Due to adsorption of nucleobases, the work functions decrease by 0.1–0.56 eV, 0.15–0.51 eV and 0.16–0.38 eV for MoS₂, WS₂ and GRA respectively. The changes are not so dramatic as our calculated charge transfer by Bader analysis^57–59 is around 0.01e between the nucleobases and the substrates.

Electronic structure of MoS₂/WS₂–nucleobases

To understand further the interactions of nucleobases with MoS₂, WS₂ and GRA, we have calculated total density of states (TDOS) of pure substrates and their complexes with nucleobases.

The calculated TDOS of MoS₂–nucleobase and WS₂–nucleobase complexes are shown in Fig. 4 and 5. The same for GRA–nucleobases have been depicted in the ESI (Fig. S2†). For MoS₂–G (−0.26 eV) and MoS₂–A (−0.18 eV), a peak of molecular origin arises near the valence band maximum. For WS₂–G complex, a similar type of peak occurs at −0.24 eV near the valance band. For all complexes with GRA, we have found the same conclusion.

Fig. 4 Total densities of states of pristine MoS₂, MoS₂–nucleobase complexes and nucleobases in the gas phase.

Fig. 5 Total densities of states of pristine WS₂, WS₂–nucleobase complexes and S nucleobases in the gas phase.

The calculated charge densities corresponding to the newly appearing peaks are shown in Fig. 6 for MoS₂–G, MoS₂–A and WS₂–G complexes. It is clear that the charge densities are mainly localized on the nucleobases, once again confirming the molecular origin.

Fig. 6 Partial charge density of the TMD–nucleobase complexes.

The calculated band gaps for pure MoS₂, WS₂ and their complexes with nucleobases are shown in Table 3.

Table 3 Calculated band gaps for pure MoS₂, WS₂, GRA and their complexes with nucleobases

	Band gap (in eV)
	MoS2	WS2
Pristine	1.68	1.85
G	1.16	1.52
A	1.35	1.73
T	1.68	1.85
C	1.53	1.84
U	1.68	1.85

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It can be seen from Table 3 and TDOS plots that pristine MoS₂ and WS₂ have band gaps of 1.68 eV and 1.85 eV respectively, which have a good agreement with experiments and previous theories.^14,40 Changes in the band gap occur when nucleobases are adsorbed on MoS₂, WS₂ surfaces. Among all nucleobases, adsorption of G reduces the band gap of MoS₂ and WS₂ by the largest amount, i.e., 0.52 eV and 0.33 eV respectively whereas the amounts of reduction are 0.33 eV and 0.12 eV for the adsorption of A. T and U do not affect the band gaps. Band gaps of MoS₂ and WS₂ reduce to 0.15 and 0.10 eV respectively after the adsorption of C. For GRA complexes, the DOSs are shown in Fig. S2.† One can easily identify the extra features in the DOS of pristine graphene arising from molecular peaks.

Optical properties of TMD–nucleobase complexes

It is well known that 2D nanomaterials can be used as optical sensors for detecting DNA molecules.^37,44,63,64 We have calculated the optical properties of pure systems and their complexes to explore distinct features in the optical spectra due to adsorption. The imaginary part of the dielectric functions of pure MoS₂ and WS₂ substrates and their complexes are shown in Fig. 7. In MoS₂, the main absorption peak appears at 2.62 eV, which is in very good agreement with a previous study.⁵² The main peak for pure WS₂ appears at 2.92 eV, which also matches well with an earlier report.⁴⁰ Fig. 7 clearly demonstrates that the absorption spectra of MoS₂–nucleobase and WS₂–nucleobase complexes are identical to those of pure MoS₂ and WS₂. Therefore, the addition of nucleobases has no significant effect on the optical properties of pure MoS₂ and WS₂. A slightly different situation occurs for GRA. For GRA–nucleobases, the calculated absorption spectra are depicted in the ESI (Fig. S3†). For pure GRA, we have observed a high intensity peak at 4.9 eV and a low intensity peak at 14 eV, which have also been reported in earlier reports.⁶⁵ Interestingly, a new peak is observed after the adsorption of nucleobases on GRA at 12.5 eV. To analyze the origin of this peak, absorption spectra of nucleobases and 2D GRA sheet in the complex have been calculated (Fig. S3†) separately to find that the new feature originates from the GRA part in the complex. It clearly demonstrates that the GRA sheet is modified due to adsorption and this detection is possible by optical means.

Fig. 7 Imaginary part of the dielectric function of pristine TMD and its complexes with nucleobases.

Conclusions

In this study, adsorption characteristics of DNA/RNA nucleobases on 2D monolayers of MoS₂ and WS₂ have been investigated using first principles DFT and vdW-DF scheme to treat van der Waals interaction. The results have been compared to those obtained for graphene. The salient outcomes of the present study are:

All the nucleobases are physisorbed on MoS₂ and WS₂ with the order of interactions being G > A > T > C > U, which is similar to that of graphene. Adsorption occurs due to van der Waals interactions. The binding energies for MoS₂–nucleobase and WS₂–nucleobase complexes are lower than that of G–nucleobases due to the different surface electronic clouds of MoS₂, WS₂ and GRA. Charge transfer between the nucleobases and the substrates is very small (∼0.01e). The band gaps of MoS₂ and WS₂ are modified due to the adsorption of G, A and C nucleobases. The calculated work functions of the substrates decrease due to adsorption of nucleobases but not significantly. In general, some new features occur in the densities of states of substrates due to adsorption of nucleobases. As these originate from the nucleobases at different energies, a selective detection may be possible in certain cases.

Therefore, one may conclude from the analysis of geometries, energetics, electronic structures and optical properties of the above mentioned systems that similar to GRA, 2D MoS₂ and WS₂ substrates may also act as potential candidates to sense the nucleobases in DNA sequencing and other biomolecules.

Acknowledgements

This work is supported by VR-SIDA (Swedish Research Links), Linnaeus grant (Uppsala RNA Research Center) and Knut and Alice Wallenberg Foundation (RiboCORE). We gratefully acknowledge supercomputing time allocation by Swedish National Infrastructure for Computing (SNIC) for performing the computations. We acknowledge the supercomputing support from PRACE-3IP project (FP7 RI-312763) resource Cy-Tera based in Cyprus at CaSToRC and PRACE-3IP project (FP7 RI-312763) resource Zeus based in Poland at Cyfronet.

Notes and references

1. D. Klein, Trends Mol. Med., 2002, 8(6), 257–260.
2. C. Y. Zhang, H. C. Yeh, M. T. Kuroki and T. H. Wang, Nat. Mater., 2005, 4(11), 826–831.
3. W. Lu, X. Qin, Y. Luo, G. Chang and X. Sun, Microchim. Acta, 2011, 175(3–4), 355–359.
4. C. H. Lu, H. H. Yang, C. L. Zhu, X. Chen and G. N. Chen, Angew. Chem., 2009, 121(26), 4879–4881.
5. S. He, B. Song, D. Li, C. Zhu, W. Qi, Y. Wen, L. Wang, S. Song, H. Fang and C. Fan, Adv. Funct. Mater., 2010, 20(3), 453–459.
6. S. Guo, D. Du, L. Tang, Y. Ning, Q. Yao and G. J. Zhang, Analyst, 2013, 138, 3216–3220.
7. Y. Guo, L. Deng, J. Li, S. Guo, E. Wang and S. Dong, ACS Nano, 2011, 5(2), 1282–1290.
8. W. Xu, X. Xue, T. Li, H. Zeng and X. Liu, Angew. Chem., Int. Ed., 2009, 48(37), 6849–6852.
9. R. Yang, J. Jin, Y. Chen, N. Shao, H. Kang, Z. Xiao, Z. Tang, Y. Wu, Z. Zhu and W. Tan, J. Am. Chem. Soc., 2008, 130(26), 8351–8358.
10. V. Sorkin, H. Pan, H. Shi, S. Y. Quek and Y. W. Zhang, Crit. Rev. Solid State Mater. Sci., 2014, 39(5), 319–367.
11. F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nat. Mater., 2007, 6(9), 652–655.
12. O. Leenaerts, B. Partoens and F. M. Peeters, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 125416.
13. M. Zhou, Y. H. Lu, Y. Q. Cai, C. Zhang and Y. P. Feng, Nanotechnology, 2011, 22(38), 385502.
14. S. Gowtham, R. H. Scheicher, R. Ahuja, R. Pandey and S. P. Karna, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76(3), 033401–033405.
15. G. H. Lu, L. E. Ocola and J. H. Chen, Nanotechnology, 2009, 20(44), 445502.
16. S. Prezioso, F. Perrozzi, L. Giancaterini, C. Cantalini, E. Treossi, V. Palermo, M. Nardone, S. Santucci and L. Ottaviano, J. Phys. Chem. C, 2013, 117(20), 10683–10690.
17. H. Vovusha, S. Sanyal and B. Sanyal, J. Phys. Chem. Lett., 2013, 4(21), 3710–3718.
18. N. Ding, X. Chen, C. M. Lawrence Wu and H. Li, Phys. Chem. Chem. Phys., 2013, 15, 10767–10776.
19. J. H. Lee, Y. K. Choi, H. J. Kim, R. H. Scheicher and J. H. Cho, J. Phys. Chem. C, 2013, 117(26), 13435–13441.
20. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat. Nanotechnol., 2012, 7(11), 699–712.
21. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6(3), 147–150.
22. K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. Rev. Lett., 2010, 105(13), 136805.
23. Z. Y. Yin, H. Li, L. Jiang, Y. M. Shi, Y. H. Sun, G. Lu, Q. Zhang, X. D. Chen and H. Zhang, ACS Nano, 2012, 6(1), 74–80.
24. J. Liu, Z. Zeng, X. Cao, G. Lu, L. Wang, Q. Fan, W. Huang and H. Zhang, Small, 2012, 8(22), 3517–3522.
25. W. Zhou, D. Y. Yin, X. Huang, Z. Zeng, Z. Fan, H. Liu, J. Wang and H. Zhang, Small, 2013, 9(1), 140–147.
26. E. S. Kadantsev and P. Hawrylak, Solid State Commun., 2012, 152(10), 909–913.
27. J. N. Coleman, M. Lotya and A. O'Neill, et al., Science, 2011, 331(6017), 568–571.
28. K. G. Zhou, N. N. Mao, H. X. Wang, Y. Peng and H. L. Zhang, Angew. Chem., Int. Ed., 2011, 50(46), 10839–10842.
29. Z. Y. Zeng, Z. Y. Yin, X. Huang, H. Li, Q. Y. He, G. Lu, F. Boey and H. Zhang, Angew. Chem., Int. Ed., 2011, 50(47), 11093–11097.
30. Y. M. Shi, C. Hamsen, X. T. Jia, K. K. Kim, A. Reina, M. Hofmann, A. L. Hsu, K. Zhang, H. N. Li, Z. Y. Juang, M. S. Dresselhaus, L. J. Li and J. Kong, Nano Lett., 2010, 10(10), 4134–4139.
31. M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. Loh and H. Zhang, Nat. Chem., 2013, 5(4), 263–275.
32. F. Mehmood and R. Pachter, J. Appl. Phys., 2014, 115(16), 164302.
33. Q. Yue, Z. Shao, S. Chang and J. Li, Nanoscale Res. Lett., 2013, 8, 425–432.
34. F. K. Perkins, A. L. Friedman, E. Cobas, P. M. Campbell, G. G. Jernigan and B. T. Jonker, Nano Lett., 2013, 13(2), 668–673.
35. C. Ataca, H. Sahin and S. Ciraci, J. Phys. Chem. C, 2012, 116(6), 8983–8999.
36. A. Klein, S. Tiefenbacher, V. Eyert, C. Pettenkofer and W. Jaegermann, Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 64, 205416.
37. S. Wang, Y. Zhang, Y. Ning and G. J. Zhang, Analyst, 2015, 140, 434–439.
38. B. Amin, T. P. Kaloni and U. Schwingenschlögl, RSC Adv., 2014, 4, 34561.
39. K. Ashok and P. K. Ahluwalia, Phys. B, 2012, 407(24), 4627–4634.
40. J. Wei, Z. Ma, H. Zeng, Z. Wang, Q. Wei and P. Peng, AIP Adv., 2012, 2, 042141.
41. V. Velusamy, K. Arshak, O. Korostynska, K. Oliwa and C. Adley, Biotechnol. Adv., 2010, 28(2), 232–254.
42. B. M. Paddle, Biosens. Bioelectron., 1996, 11, 1079–1113.
43. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Griorieva and A. A. Firsov, Science, 2004, 306(5696), 666–669.
44. C. Zhu, Z. Zeng, H. Li, F. Li, C. Fan and H. Zhang, J. Am. Chem. Soc., 2013, 135(16), 5998–6001.
45. A. B. Farimani, K. Min and N. R. Aluru, ACS Nano, 2014, 8(8), 7914–7922.
46. Y. X. Yu, J. Mater. Chem. A, 2014, 2, 8910–8917.
47. Y. X. Yu, ACS Appl. Mater. Interfaces, 2014, 6, 16267–16275.
48. G. Kresse and J. Furthmuller, Comput. Mater. Sci., 1996, 6(1), 15–50.
49. G. Kresse and J. Furthmuller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54(16), 11169–11186.
50. J. Klimes, D. R. Bowler and A. Michelides, J. Phys.: Condens. Matter, 2010, 22, 022201.
51. J. Klimes, D. R. Bowler and A. Michelides, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 195131.
52. J. Schutte, J. L. de Boer and F. Jellinek, J. Solid State Chem., 1987, 70(2), 207–209.
53. Y. Li, Y. L. Li, C. M. Araujo, W. Luob and R. Ahuja, Catal. Sci. Technol., 2013, 3, 2214.
54. W. A. Harrison, Electronic Structure and the Properties of Solids: the Physics of the Chemical Bond Freeman, San Francisco, 1980, p. 90.
55. D. Le, A. Kara, E. Schröder, P. Hyldgaard and T. S. Rahman, J. Phys.: Condens. Matter, 2012, 24, 424210.
56. R. G. Amorim and R. H. Scheicher, Nanotechnology, 2015, 26, 154002.
57. W. Tang, E. Sanville and G. Henkelman, J. Phys.: Condens. Matter, 2009, 21, 084204.
58. E. Sanville, S. D. Kenny, R. Smith and G. Henkelman, J. Comput. Chem., 2007, 28(5), 899–908.
59. G. Henkelman, A. Arnaldsson and H. Jónsson, Comput. Mater. Sci., 2006, 36, 254–360.
60. S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong and S. Iijima, Nat. Nanotechnol., 2010, 5, 574–578.
61. Y. Li, C.-Y. Xu, B.-Y. Zhang and L. Zhen, Appl. Phys. Lett., 2013, 103, 033122.
62. C. S. Rout, P. D. Joshi, R. V. Kashid, D. S. Joag, M. A. More, A. J. Simbeck, M. Washington, S. K. Nayak and D. J. Latec, Sci. Rep., 2013, 3, 3282.
63. J. Balapanuru, J. Yang, S. Xiao, Q. Bao, M. Jahan, L. Polavarapu, J. Wei, Q. H. Xu and K. P. Loh, Angew. Chem., 2010, 122(37), 6699–6703.
64. X. J. Xing, X. G. Liu, Y. He, Y. Lin, C. L. Zhang, H. W. Tang and D. W. Pang, Biomacromolecules, 2013, 14(1), 117–123.
65. P. Rani, G. S. Dubey and V. K. Jindal, Physica E, 2014, 62, 28–35.

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Footnote

† Electronic supplementary information (ESI) available: Optimized structures of GRA–nucleobase complexes, TDOS of pristine GRA, GRA–nucleobase complexes, absorption spectra of pristine GRA, GRA–nucleobase complexes and free nucleobases. See DOI: 10.1039/c5ra14664j

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