Facile synthesis of a nanocomposite based on graphene and ZnAl layered double hydroxides as a portable shelf of a luminescent sensor for DNA detection

Abstract

Recently, nanocomposites based on graphene and layered double hydroxides (LDH) have been developed and used in many fields. However, to the best of our knowledge, there is no report on the luminescence sensor applications of graphene/LDH composites. Herein, a hybrid graphene–ZnAl-LDH nanocomposite has been developed using a facile one-step process and the presence of LDH in the composite can effectively prevent the restacking of graphene and improve both its luminescence properties and thermal stability. Furthermore, the composite can be used as a portable shelf of the Ru(phen)₃Cl₂ (tris(1,10-phenanthroline)ruthenium(II) dichloride) sensor to selectively discriminate DNA. It was found that the graphene–ZnAl-LDH composite can effectively quench the emission of the Ru(phen)₃Cl₂ sensor. After the addition of a certain amount of DNA into the system, Ru(phen)₃Cl₂ was released from the graphene–ZnAl-LDH composite and it interacted with DNA immediately, leading to the luminescence recovery of the sensor. The results indicate that the RGO–ZnAl-LDH composite displayed an excellent luminescence response and good linear correlation to DNA. Therefore, the composite can be employed as a portable shelf of Ru(phen)₃Cl₂ to discriminate DNA. Moreover, both the shelf and the sensor can be easily collected and made ready for the next sample if there is no DNA in the solution. The proposed method was further applied to detect the immunodeficiency virus gene (HIV), thus providing a new field of application for hybrid graphene/LDH composites.

---

Introduction

Graphene, a two-dimensional monolayer carbon material, has been widely researched because of its large specific surface area, high electrical/thermal conductivity, high flexibility, and unusual mechanical strength.^1–3 However, the restacking of graphene sheets during reduction and drying processes has severely hampered the performance of graphene materials.^4,5 The functionalization of graphene has been widely regarded as an efficient way to improve its dispersibility and stability.⁶ Due to its ease of chemical modification, high surface area, electrical conductivity and high flexibility, graphene could assembly with other functional nanomaterials to form hybrid graphene-based nanocomposites.⁷ Owing to their unique nanostructure, high specific surface area and good biocompatibility, graphene-based nanocomposites have shown potential applications in the field of biological/chemical sensors, cellular imaging, and drug delivery.^8–13 According to research, the presence of other functional nanomaterials in the graphene-based composites can not only effectively prevent the agglomeration and restacking of graphene but also improve the dispersibility, stability and some properties of the materials.^14,15

Layered double hydroxides (LDH) materials can be a good candidate for this type of nanomaterial. To date, some LDH materials (such as MgAl-LDH, ZnAl-LDH) have shown prospective applications in the biological field, due to their high biocompatibility, low cytotoxicity, low cost, opened layer structure, and high anion exchange capacity.^16–22 In recent years, nanocomposites based on graphene and LDH materials have been developed, which were used as supercapacitors, Li-ion batteries, catalysts, photocatalysis, and for heavy metal removal.^23–30 However, to the best of our knowledge, there is no report on the luminescence sensor applications of graphene/LDH composites.

Herein, a hybrid graphene–ZnAl-LDH composite was developed using a simple and green approach. In the synthesis procedure, the exfoliated graphite oxide (GO) is simultaneously reduced to graphene and is accompanied with the homogeneous precipitation of ZnAl-LDH to form a hybrid graphene–ZnAl-LDH composite. The as-prepared graphene nanocomposite was found to effectively quench the emission of Ru(phen)₃Cl₂ (tris(1,10-phenanthroline)ruthenium(II) dichloride), which is a DNA sensor in aqueous solution. After being encountered with CT DNA, Ru(phen)₃Cl₂ was released from the RGO–ZnAl-LDH composite and it interacted with CT DNA immediately, leading to the luminescence recovery of Ru(phen)₃Cl₂. The luminescence increase is linearly proportional to the amount of DNA added, demonstrating that the composite can be employed as a portable shelf of Ru(phen)₃Cl₂ to selective discriminate DNA. If there is no DNA in the solution, both the shelf and the sensor can be easily collected and made ready for the next sample, the proposed method was further applied to detect the immunodeficiency virus gene (HIV), providing a new field of application for the hybrid graphene–ZnAl-LDH composite.

Experimental

Materials

All the chemicals were of analytical grade and were used without further purification. Natural flake graphite (325 mesh) was purchased from Alfa-Aesar Co. Calf thymus DNA (CT DNA), bovine serum albumin (BSA), pepsin, thrombin, casein, RNase myoglobin and concanavalin were purchased from Sigma Chemical Co. Ultrapure Milli-Q water (ρ > 18.0 MΩ cm) was used throughout the fluorescence experiments. The ruthenium(II) complexes (Ru(phen)₃Cl₂) used in the experiments were synthesized according to the previously reported procedure.³¹ The immunodeficiency virus gene (HIV) was synthesized by Beijing AuGCT DNA-SYN Biotechnology Co., Ltd. The sequences of the oligonucleotides are as follows:

Target DNA:

5′-ATCCCATTCTGCAGCTTCCTCATTGATGGTCTC-3′

Probe DNA:

5′-GAGACCATCAATGAGGAAGCTGCAGAATGGGAT-3′

Preparation of the graphene–ZnAl-LDH nanocomposite

Graphite oxide (GO) was synthesized from natural flake graphite by a modified Hummers method.³² The graphene–ZnAl-LDH composite was prepared via a solution approach. In a typical procedure, the as-prepared GO (100 mg) was dispersed in deionized water (500 mL) with the assistance of sonication for 60 min. Subsequently, ZnCl₂, AlCl₃·6H₂O and urea were added into the abovementioned suspension to give the final concentration of 10, 5 and 35 mM, respectively. After vigorous shaking for a few minutes, the suspension was then refluxed at 100 °C for 24 h under continuous magnetic stirring. The resulting black product was filtered, washed with deionized water, and dried in air, which was abbreviated as RGO–ZnAl-LDH. Herein, urea was used as a hydrolysis agent to adjust the pH value and also as a reductant to reduce GO into graphene.

For comparison, pure ZnAl-LDH was also prepared by the same procedure using ZnCl₂, AlCl₃·6H₂O and urea but without GO. To prepare graphene, only the exfoliated GO suspension (0.2 mg mL⁻¹, 500 mL) was refluxed at 100 °C for 24 h in the presence of urea (35 mM) under the same experimental condition. The resulting product was abbreviated as RGO.

Characterization

X-ray diffraction (XRD) analysis was carried out with a D/Max2550VB+/PC X-ray diffractometer with Cu Kα (λ = 0.15406 nm), using an operation voltage and current of 40 kV and 30 mA, respectively. A Quanta 200 environmental scanning electron microscope (SEM) was used to observe the morphologies of the obtained materials. Transmission electron microscopy (TEM) images were collected using a JEM-2100 microscope working at 200 kV. Specimens for observation were prepared by dispersing the samples into alcohol by ultrasonic treatment and they were dropped on carbon–copper grids. The X-ray photoelectron spectroscopy (XPS) measurement was performed with an Axis Ultra, Kratos (UK) spectrometer using Al Kα excitation radiation (1486 eV). Thermogravimetry analysis (TGA) was obtained using a thermal analyzer (Q1000DSC + LNCS + FACS Q600SDT) under air at a heating rate of 10 °C min⁻¹. The absorption and emission spectra were collected using a Shimadzu 1750 UV-visible spectrometer and an RF-5301 fluorescence spectrometer (Japan), respectively.

Luminescence experiments

The stock of CT DNA was prepared by dissolving commercial CT DNA in ultrapure water. The concentration of CT DNA was determined spectrophotometrically. The stock solution of Ru(phen)₃Cl₂ (0.49 μM) was prepared in ultrapure water. The RGO–ZnAl-LDH composite was added in ultrapure water and sonicated for 2 h until the composite was dispersed homogeneously in solution (50 mg mL⁻¹). Then the abovementioned RGO–ZnAl-LDH composite suspension was gradually added into the Ru(phen)₃Cl₂ solution (3 mL) with stirring until the luminescence was almost quenched. Finally, an increasing amount of CT DNA solution was added until the highest luminescence intensity was reached. The sample was stirred for 5 s each time, and the supernatant was determined by fluorescence spectrophotometry. In all the titration experiments, the total volume was maintained, not exceeding 5% of the original volume.

Results and discussion

X-ray diffraction analysis

The X-ray diffraction (XRD) patterns of the prepared GO, pure ZnAl-LDH, RGO and the as-prepared RGO–ZnAl-LDH composite are presented in Fig. 1. The XRD pattern of the GO (Fig. 1a) exhibits a reflection peak at 2θ = 10.7° with an interlayer spacing of 0.82 nm, confirming the formation of graphite oxide.³³ After its reduction with urea, the peak at 10.7° of GO completely disappears (Fig. 1b) and only a abroad diffraction peak at around 24.4° is observed, corresponding to the C (002) reflection of graphene. The result suggests that GO was reduced to graphene after treating with urea. The XRD pattern of pure ZnAl-LDH (Fig. 1c) shows an evident layered structure with a basal spacing of 0.75 nm, which is in good agreement with the characteristic peaks of the standard compound Zn_0.63Al_0.37(OH)₂(CO₃)_0.185·x(H₂O) (JCPDS: 48-1024).³⁴ The XRD pattern of the as-prepared RGO–ZnAl-LDH composite shows that the diffraction peaks are similar to those of the pristine ZnAl-LDH (Fig. 1d). The diffraction peaks of graphene are not distinguishable in the composite, indicating that GO was reduced into graphene in the presence of urea³⁵ and the restacking of the as-reduced graphene sheets is effectively prevented by the introduction of ZnAl-LDH, which results in the shielding of the graphene peaks by those of ZnAl-LDH.^36,37 This result suggests that in company with the homogeneously precipitated ZnAl-LDH platelets on the surface of GO nanosheets, GO nanosheets are simultaneously reduced into graphene to form the hybrid graphene–ZnAl-LDH composite.

Fig. 1 XRD patterns of (a) GO, (b) RGO, (c) pure ZnAl-LDH, and (d) RGO–ZnAl-LDH composite.

X-ray photoelectron spectroscopy analysis

The reduction of GO can be further confirmed by the X-ray photoelectron spectroscopy (XPS) analysis and it is presented in Fig. 2. The C 1s spectrum of GO shows three types of carbon bonds, as shown in Fig. 2a. The peak of the non-oxygenated ring C (C–C) located at 284.4 eV is assigned to the bonds between the sp² hybridized carbon atoms, and the two peaks at 286.5 and 288.0 eV are attributed to the epoxy and alkoxy carbon (C–O) and the carboxylate carbon (O–C=O), indicating a considerable degree of oxidation.^38,39 The C 1s XPS spectrum of RGO (Fig. 2b) and the RGO–ZnAl-LDH composite (Fig. 2c) also exhibits these three types of carbon. However, compared to that of GO, the absorbance band intensities of the epoxy and alkoxy carbon (C–O) and the carboxylate carbon (O–C=O) of RGO and the RGO–ZnAl-LDH composite decrease significantly due to the reduction of GO to graphene. By integrating the area of these peaks, the percentage of oxygen-containing ring C in the GO is calculated to be 64%, whereas that of the RGO–ZnAl-LDH composite is 28%. These results indicate that most of the oxygen-containing functional groups in the RGO–ZnAl-LDH composite are removed, thus confirming the successful reduction of GO to graphene. For the sample of RGO in the control experiment, the percentage of oxygenated ring C is calculated to be 48%, which is higher than that of the RGO–ZnAl-LDH composite. These results indicate that the addition of ZnAl-LDH in the composite can lead to an RGO–ZnAl-LDH nanocomposite with RGO in a higher reductive degree. This result can be explained by the fact that the restacking of the as-reduced graphene sheets is effectively prevented by the introduction of ZnAl-LDH, which is consistent with the XRD analysis. The XPS wide scan spectrum of the RGO–ZnAl-LDH composite (Fig. S1†) not only exhibits the peaks of O 1s and C 1s, but also exhibits the peaks of Zn 2p and Al 2p, suggesting the formation of the RGO–ZnAl-LDH composite.

Fig. 2 C 1s XPS spectra of (a) GO, (b) RGO, and (c) RGO–ZnAl-LDH composite.

Surface morphology and thermogravimetric analysis

SEM and TEM images of pure ZnAl-LDH and the as-prepared RGO–ZnAl-LDH composite are presented in Fig. 3. It can be seen that the pure ZnAl-LDH material consists of thin hexagonal platelets with a mean lateral size of about 5 μm (Fig. 3a and b). In contrast, the SEM and TEM images (Fig. 3c and d) of the RGO–ZnAl-LDH composite exhibit both the relative irregular hexagonal platelets of ZnAl-LDH and the corrugation and scrolling image of graphene nanosheets. The result indicates that ZnAl-LDH platelets grew on the surface of the graphene nanosheets to form the RGO–ZnAl-LDH composite during the urea reflux procedure. From the HRTEM image of the RGO–ZnAl-LDH composite (Fig. 3e and f), three types of contrast fringes can be observed. The lattice fringe with an interplanar distance of 0.34 nm is in accordance with that of the graphite (002) plane,⁴⁰ and the lattice fringes with an interplanar distance of 0.24 and 0.26 nm are ascribed to the (104) and (101) plane of the hexagonal ZnAl-LDH phase.⁴¹ Due to the introduction of ZnAl-LDH nanosheets, the restacking of graphene nanosheets can be effectively prevented, which is consistent with the XRD results.

Fig. 3 (a) SEM and (b) TEM images of ZnAl-LDH. (c) SEM and (d) TEM images of the RGO–ZnAl-LDH composite. (e) and (f) High-resolution TEM images of the RGO–ZnAl-LDH composite.

The thermal behaviors of RGO and the hybrid RGO–ZnAl-LDH composite were investigated by thermogravimetric analysis (TGA) in dry air. As shown in Fig. S2a,† the RGO prepared in the control experiment undergoes a consecutive weight loss of 90%, which corresponds to the release of adsorbed water and the combustion of the carbon skeleton,⁴² whereas the RGO–ZnAl-LDH composite (Fig. S2b†) exhibits a three-step weight loss of only 31%, which is attributed to the release of the interlayer water, the dehydroxylation of the ZnAl-LDH nanolayers in the hybrid RGO–Ni–Fe LDH material, and the combustion of the carbon skeleton, respectively.⁴³ However, the residue percentage of the RGO–ZnAl-LDH composite (69%) is considerably higher than that of RGO (10%), suggesting that the combination of ZnAl-LDH with graphene significantly improves the thermal stability.

Luminescence spectra analysis

The luminescence spectra of Ru(phen)₃Cl₂ (0.49 μM) in aqueous solution upon the addition of different concentrations of the RGO–ZnAl-LDH composite are shown in Fig. 4a. When the RGO–ZnAl-LDH composite was added gradually to Ru(phen)₃Cl₂ in aqueous solution, the luminescence intensity of Ru(phen)₃Cl₂ decreased as the RGO–ZnAl-LDH composite concentration increased, suggesting that strong π–π stacking interactions and electrostatic interactions existed between the RGO–ZnAl-LDH composite and Ru(phen)₃Cl₂. When the concentration of the RGO–ZnAl-LDH composite increases to 0.83 mg mL⁻¹, the luminescence intensity of Ru(phen)₃Cl₂ does not change any further. This means that no free Ru(phen)₃Cl₂ is left in the solution at this point. Then CT DNA was added gradually into the abovementioned solution. Along with the addition of CT DNA, the luminescence intensity increased gradually, as shown in Fig. 4b. This process can be explained as follows: after being encountered with CT DNA, Ru(phen)₃Cl₂ was released from the RGO–ZnAl-LDH composite and it interacted with CT DNA immediately, leading to the luminescence recovery of Ru(phen)₃Cl₂.^44–47 After 23 μg mL⁻¹ DNA was added into the system, more than 17 times luminescence increase was observed, whereas only 2 times luminescence increase was obtained in the control experiment for Ru(phen)₃Cl₂ alone (Fig. S3†).

Fig. 4 Luminescence spectra of Ru(phen)₃Cl₂ in aqueous solution (a) upon the addition of different concentrations of the RGO–ZnAl-LDH composite; (b) upon the addition of different concentrations of CT DNA in the presence of 0.83 mg mL⁻¹ RGO–ZnAl-LDH composite; (c) luminescence signalling change at 592 nm plotted as the function of CT DNA concentration, Ex = 464 nm.

As shown in Fig. 4c, the luminescence intensity was linearly related to the amount of CT DNA added in the range from 0.8 to 18.0 μg mL⁻¹ with a correlation coefficient of 0.9974. The detection limit (3σ/k method)⁴⁸ was 5.9 × 10⁻⁸ g mL⁻¹. It indicates that the hybrid RGO–ZnAl-LDH composite can be employed as a portable shelf of Ru(phen)₃Cl₂ to selectively discriminate DNA.

To assess the selectivity of the detection, different proteins, such as BSA, pepsin, thrombin, casein, myoglobin and concanavalin, were measured. Firstly, the luminescence of Ru(phen)₃Cl₂ (0.49 μM) was completely quenched by the RGO–ZnAl-LDH composite (0.83 mg mL⁻¹), and then, an excess of various proteins were added separately into the cuvettes. The luminescence response of Ru(phen)₃Cl₂ after the addition of an excess of various proteins in the presence of the RGO–ZnAl-LDH composite are shown in Fig. S4.† The highest luminescence enhancement was observed only in the presence of CT DNA, whereas relatively little change can be observed in the presence of the other proteins. It is noted that a small red shift of luminescence was found upon the addition of CT DNA, whereas with addition of various proteins the luminescence peak undergoes a blue shift. The result thus demonstrated that this method had a high selectivity for CT DNA detection.

To investigate the role of the ZnAl layered double hydroxides in the RGO–ZnAl-LDH composite for DNA detection, a comparison between the RGO–ZnAl-LDH composite and RGO as well as ZnAl-LDH alone was made through luminescence experiments, as shown in Fig. S5.† The luminescence spectra of Ru(phen)₃Cl₂ upon the addition of different concentrations of pure ZnAl-LDH and RGO are shown in Fig. S5a and b,† respectively. When pure ZnAl-LDH was added gradually into Ru(phen)₃Cl₂ in aqueous solution, not much luminescence quenching can be observed, suggesting that almost no interaction exists between ZnAl-LDH and Ru(phen)₃Cl₂ (Fig. S5a†). For the sample of RGO in the control experiment, the luminescence intensity of Ru(phen)₃Cl₂ systematically decreases as the RGO concentration increases (Fig. S5b†), suggesting that strong interactions exist between RGO and Ru(phen)₃Cl₂. The luminescence spectrum of Ru(phen)₃Cl₂ upon the addition of different concentrations of DNA in the presence of 5.7 μg mL⁻¹ RGO is shown in Fig. S5c.† As shown in Fig. S5c,† after the addition of a certain amount of DNA, the luminescence intensity of RGO increased by only 5-fold, which is lower than that of the RGO–ZnAl-LDH composite (17 times increased as shown in Fig. 4b). As shown in Fig. S5d,† for the sample of RGO, the luminescence response shows a linear correlation to the DNA added over quite a narrow concentration range from 0.04 to 0.38 μg mL⁻¹. The results indicate that the RGO–ZnAl-LDH composite displayed a more excellent luminescence response, wide concentration range and good linear correlation to DNA than that of RGO and pure ZnAl-LDH. Moreover, the participation of ZnAl-LDH in the graphene composite can be helpful to effectively improve the luminescence properties.

The RGO–ZnAl-LDH composite was also employed as a portable shelf of Ru(phen)₃Cl₂ to discriminate the immunodeficiency virus gene (HIV) as shown in Fig. S6.† When 0.3 mg mL⁻¹ RGO–ZnAl-LDH composite was added to Ru(phen)₃Cl₂ in aqueous solution, the luminescence intensity of Ru(phen)₃Cl₂ was effectively quenched (Fig. S6a†). Fig. S6b† depicts the fluorescence spectra of Ru(phen)₃Cl₂ upon analyzing different concentrations of the HIV target gene in the presence of 0.30 mg mL⁻¹ RGO–ZnAl-LDH composite. As the concentration of the HIV gene increases, the resulting fluorescence is intensified. As shown in Fig. S6c,† the luminescence intensity is linearly related to the HIV target gene added, indicating that the hybrid RGO–ZnAl-LDH composite can be also employed as a portable shelf of Ru(phen)₃Cl₂ to selective discriminate the immunodeficiency virus gene (HIV).

On the basis of the abovementioned results, a schematic representation of the formation process of the RGO–ZnAl-LDH composite and the luminescence detection of DNA using the composite as a portable shelf of Ru(phen)₃Cl₂ is given in Fig. 5. GO has a layered structure with a basal spacing of 0.82 nm and was first synthesized from graphite by a modified Hummers technique. There are different types of oxygen-containing functional groups (–OH, –COOH, C–O, etc.) on the surface of GO. When the exfoliated GO is soaked in a mixed solution of ZnCl₂, AlCl₃·6H₂O and urea, positive Zn²⁺ and Al³⁺ ions can attach to the negatively charged functional groups on GO by electrostatic attraction and serve as nucleation precursors. In the urea refluxing process, a mass of nuclei was formed and the as-prepared ZnAl-LDH crystallites grew in situ and adhered to the surface of GO via an intermolecular hydrogen bond or a covalent coordination area, and moreover, GO was reduced to graphene due to the existence of urea. The as-prepared RGO–ZnAl-LDH composite was found to effectively quench the emission of Ru(phen)₃Cl₂ due to the strong π–π stacking interaction and electrostatic interaction, which exists between Ru(phen)₃Cl₂ and the RGO–ZnAl-LDH composite. After being encountered with CT DNA, Ru(phen)₃Cl₂ was released from RGO–ZnAl-LDH composite and it interacted with CT DNA immediately, leading to the luminescence recovery of Ru(phen)₃Cl₂. It was found that the luminescence increase is linearly proportional to the amount of DNA added. The composite can be employed as a portable shelf of Ru(phen)₃Cl₂ to selective discriminate DNA. If there is no DNA in the solution, both the shelf and the sensor can be easily collected and made ready for the next sample, providing some applications into the hybrid graphene/LDH composite.

Fig. 5 Schematic representation of the formation process of the RGO–ZnAl-LDH composite and the luminescence detection of DNA using the composite.

Conclusions

A hybrid RGO–ZnAl-LDH nanocomposite was developed using a facile and green one-step process and it was used as a portable shelf of Ru(phen)₃Cl₂ sensor to selectively discriminate DNA. In the synthesis procedure, the exfoliated GO is simultaneously reduced to graphene using urea as the reductant accompanied with the homogeneous precipitation of ZnAl-LDH to form a hybrid graphene–ZnAl-LDH composite. The as-prepared RGO–ZnAl-LDH composite was found to effectively quench the emission of Ru(phen)₃Cl₂ in aqueous solution. A luminescence enhancement of approximately more than 17 times can be observed after the addition of a certain amount of DNA into the abovementioned system. Moreover, it was found that the luminescence increase is linearly proportional to the amount of DNA added in the concentration range from 0.8 to 18.0 μg mL⁻¹ with a correlation coefficient of 0.9974. The detection limit is 5.9 × 10⁻⁸ g mL⁻¹. Therefore, the composite can be employed as a portable shelf of Ru(phen)₃Cl₂ sensor to selectively discriminate DNA. Furthermore, the results indicate that the RGO–ZnAl-LDH composite displayed a more excellent luminescence response, wide concentration range and good linear correlation to DNA than that of RGO and pure ZnAl-LDH, and the presence of LDH in the composite can effectively prevent the restacking of graphene and improve both the luminescence properties and thermal stability. The proposed method was further applied to detect the immunodeficiency virus gene (HIV), thus providing a new field of application for the hybrid graphene/LDH composite.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 201205095), the Scientific Research Foundation of Northwest A&F University (Z109021115, Z111021103 and Z111021107), the Fundamental Research Funds for the Central Universities (Z109021204), the Shaanxi Province Science and Technology (no. 2013K12-03-23), and the State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University (no. 2013005).

Notes and references

1. C. N. R. Rao, K. Biswas, K. S. Subrahmanyama and A. Govindaraj, J. Mater. Chem., 2009, 19, 2457.
2. C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj, Angew. Chem., Int. Ed., 2009, 48, 7752.
3. X. Y. Dong, L. Wang, D. Wang, C. Li and J. Jin, Langmuir, 2012, 28, 293.
4. J. H. Lee, N. Park, B. G. Kim, D. S. Jung, K. Im, J. Hur and J. W. Choi, ACS Nano, 2013, 7, 9366.
5. Y. Wang, Y. P. Wu, Y. Huang, F. Zhang, X. Yang, Y. F. Ma and Y. S. Chen, J. Phys. Chem. C, 2011, 115, 23192.
6. L. H. Li, X. L. Zheng, J. J. Wang, Q. Sun and Q. Xu, ACS Sustainable Chem. Eng., 2013, 1, 144.
7. M. Quintana, E. Vazquez and M. Prato, Acc. Chem. Res., 2013, 46, 138.
8. D. Chen, L. H. Tang and J. H. Li, Chem. Soc. Rev., 2010, 39, 3157.
9. Y. X. Liu, X. C. Dong and P. Chen, Chem. Soc. Rev., 2012, 41, 2283.
10. M. Chen, Y. He, X. Chen and J. Wang, Bioconjugate Chem., 2013, 24, 387.
11. Y. X. Ye, P. P. Wang, E. M. Dai, J. Liu, Z. F. Tian, C. H. Liang and G. S. Shao, Phys. Chem. Chem. Phys., 2014, 16, 8801.
12. S. K. Bhunia and N. R. Jana, ACS Appl. Mater. Interfaces, 2011, 3, 3335.
13. W. L. Sun, S. Shi and T. M. Yao, Anal. Methods, 2011, 3, 2472.
14. S. Stankovich, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Carbon, 2006, 44, 3342.
15. Y. X. Xu, H. Bai, G. W. Lu, C. Li and G. Q. Shi, J. Am. Chem. Soc., 2008, 130, 5856.
16. S. He, Z. An, M. Wei, D. G. Evans and X. Duan, Chem. Commun., 2013, 49, 5912.
17. Z. P. Liu, R. Z. Ma, M. Osada, N. Iyi, Y. Ebina, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2006, 128, 4872.
18. M. J. Climent, A. Corma and S. Iborra, Chem. Rev., 2010, 111, 1072.
19. N. Baliarsingh, K. M. Parida and G. C. Pradhan, RSC Adv., 2013, 3, 23865.
20. H. Hu, K. M. Xiu, S. L. Xu, W. T. Yang and F. J. Xu, Bioconjugate Chem., 2013, 24, 968.
21. Z. An, S. Lu, L. W. Zhao and J. He, Langmuir, 2011, 27, 12745.
22. Y. Wang and D. Zhang, J. Mater. Chem. B, 2014, 2, 1024.
23. Z. Gao, J. Wang, Z. S. Li, W. L. Yang, B. Wang, M. J. Hou, H. Yang, Q. Liu, T. Mann, P. P. Yang, M. L. Zhang and L. H. Liu, Chem. Mater., 2011, 23, 3509.
24. L. Wang, D. Wang, X. Y. Dong, Z. J. Zhang, X. F. Pei, X. J. Chen, B. Chen and J. Jin, Chem. Commun., 2011, 47, 3556.
25. L. Yan, R. Y. Li, Z. J. Li, J. K. Liu, Y. J. Fang, G. L. Wang and Z. G. Gu, Electrochim. Acta, 2013, 95, 146.
26. J. Memon, J. H. Sun, D. L. Meng, W. Ouyang, M. A. Memon, Y. Huang, S. K. Yan and J. X. Geng, J. Mater. Chem. A, 2014, 2, 5060.
27. M. Latorre-Sanchez, P. Atienzar, G. Abellάn, M. Puche, V. Fornés, A. Ribera and H. García, Carbon, 2012, 50, 518.
28. J. L. Gunjakar, I. Y. Kim, J. M. Lee, N. S. Lee and S. J. Hwang, Energy Environ. Sci., 2013, 6, 1008.
29. X. Y. Yuan, Y. F. Wang, J. Wang, C. Zhou, Q. Tang and X. B. Rao, Chem. Eng. J., 2013, 221, 204.
30. Z. J. Huang, P. X. Wu, B. N. Gong, Y. P. Fang and N. W. Zhu, J. Mater. Chem. A, 2014, 2, 5534.
31. P. A. Lay, A. M. Sargeson, H. Taube, M. H. Chou and C. Creutz, Inorg. Synth., 1986, 24, 291.
32. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
33. Z.-H. Liu, Z. M. Wang, X. J. Yang and K. Ooi, Langmuir, 2002, 18, 4926.
34. H. M. He, H. L. Kang, S. L. Ma, Y. X. Bai and X. J. Yang, J. Colloid Interface Sci., 2010, 343, 225.
35. Z. B. Lei, L. Lu and X. S. Zhao, Energy Environ. Sci., 2012, 5, 6391.
36. L. J. Zhang, X. J. Zhang, L. F. Shen, B. Gao, L. Hao, X. J. Lu, F. Zhang, B. Ding and C. Z. Yuan, J. Power Sources, 2012, 199, 395.
37. J. Xu, S. L. Gai, F. He, N. Niu, P. Gao, Y. J. Chen and P. P. Yang, J. Mater. Chem. A, 2014, 2, 1022.
38. S. Huang, G. N. Zhu, C. Zhang, W. W. Tjiu, Y. Y. Xia and T. X. Liu, ACS Appl. Mater. Interfaces, 2012, 4, 2242.
39. X. B. Fan, W. C. Peng, Y. Li, X. Y. Li, S. L. Wang, G. L. Zhang and F. B. Zhang, Adv. Mater., 2008, 20, 4490.
40. C. Yu, J. Yang, C. T. Zhao, X. M. Fan, G. Wang and J. S. Qiu, Nanoscale, 2014, 6, 3097.
41. B. Hu, S. F. Chen, S. J. Liu, Q. S. Wu, W. T. Yao and S. H. Yu, Chem.–Eur. J., 2008, 14, 8928.
42. C. Xu, X. Wang and J. W. Zhu, J. Phys. Chem. C, 2008, 112, 19841.
43. F. Kovanda, T. Grygar and V. Dorničcák, Solid State Sci., 2003, 5, 1019.
44. A. B. Tossi and J. M. Kelly, J. Photochem. Photobiol., A, 1989, 49, 545.
45. D. Z. M. Coggan, I. S. Haworth, P. J. Bates, A. Robinson and A. Rodger, Inorg. Chem., 1999, 38, 4486.
46. W. L. Sun, T. M. Yao and S. Shi, Analyst, 2012, 137, 1550.
47. W. L. Sun, J. L. Yao, T. M. Yao and S. Shi, Analyst, 2013, 138, 421.
48. X. B. Zhang, Z. D. Wang, H. Xing, Y. Xiang and Y. Lu, Anal. Chem., 2010, 82, 5005.

---

Footnote

† Electronic supplementary information (ESI) available: The XPS wide scan spectrum and TGA curves of RGO–ZnAl-LDH composite; luminescence spectra of Ru(phen)₃Cl₂ upon addition of different concentration of CT DNA; luminescence response of Ru(phen)₃Cl₂ after addition of various proteins in the presence of RGO–ZnAl-LDH composite; luminescence spectra of Ru(phen)₃Cl₂ upon addition of different concentration of RGO and pure ZnAl-LDH; luminescence response of Ru(phen)₃Cl₂ upon addition of different concentration of DNA in the presence of RGO. See DOI: 10.1039/c4ra15395b

---