One-pot synthesis of sulfur-doped graphene quantum dots as a novel fluorescent probe for highly selective and sensitive detection of lead(II)

Abstract

A novel one-pot synthesis of sulfur-doped graphene quantum dots (S-GQDs) was proposed based on water-phase molecular fusion with 1,3,6-trinitropyrene, Na₂S, and NaOH in a hydrothermal process. A 75% yield was obtained and mass production of S-GQDs with high crystallinity was possible. The prepared S-GQDs gave a stable yellow-green emission within a wide pH range of 2.0–11.0 and exhibited excitation-independent photoluminescence behaviors. As investigated by atomic force microscopy (AFM), the synthesized S-GQDs possessed monolayer-graphene thickness. As illustrated by transmission electron microscopy (TEM), the synthesized S-GQDs exhibited high crystallinity and uniform size (∼3 nm). Successful doping of S atoms in graphene quantum dot lattices was proven by X-ray photoelectron spectroscopy (XPS) characterization. Compared with GQDs, the S-GQDs had drastically changed surface chemistry and showed a selective and sensitive response to Pb²⁺. Ions such as Na⁺, K⁺, Cu²⁺, Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺, Ni²⁺, Co²⁺, Cd²⁺ have no effect on the fluorescence of S-GQDs. Based on the fluorescence quenching of S-GQDs by Pb²⁺ in water, a facile and direct fluorescence sensor for Pb²⁺ detection was developed. Under the optimized conditions, the linear response ranged from 0.1 to 140.0 μM with a detection limit of 0.03 μM.

---

1. Introduction

Nowadays, poisonous transition metal ions in water and soils have aroused worldwide attention due to the adverse effects on ecosystems caused by their non-biodegradability and toxicity.^1,2 The lead(II) ion (Pb²⁺) is one of the most poisonous substances to animals and humans, which damages the central nervous system and causes brain and blood disorders. To prevent lead poisoning, developing easy, fast and sensitive methods for Pb²⁺ detection is of great significance. In spite of traditional methods including graphite furnace atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectroscopy (ICP-MS), fluorescent sensors have witnessed immense progress in recent years because they do not need highly expensive and complex instruments, and well-trained personnel.^3,4

Recently, fluorescent carbon nanomaterials including carbon quantum dots (CQDs) and graphene quantum dots (GQDs) have been attractive in bioimaging,⁵ matrix/catalysis for deposition of surface-clean metal nanoparticles,^6,7 and sensing detection,^5,8–10 and etc. Resulting from the pronounced quantum confinement and edge effects, zero-dimensional (0D) graphene quantum dots (GQDs) exhibited extraordinary optical and electrical characteristics. Compared with semiconductor quantum dots (e.g. CdS, CdSe, CdTe), GQDs possess low cost, low toxicity and stable photoluminescence properties. Consequently, fluorescent chemosensors with GQDs as fluorescent probe show promising potential for the detection of metal ions owing to their intrinsic sensitivity, selectivity and capacity for rapid, real-time monitoring.^11–14,16 As recently demonstrated, the invasion of heteroatoms into the perfect hexagonal carbon sheet of GQDs serves as useful tool to bring new optical properties and selectivity. The structural, physicochemical and electronic properties of GQDs, such as charge transport, Fermi level, bandgap, localized electronic state and spin density, can be drastically altered by heteroatom doping.^15–17 Amongst various heteroatoms (oxygen, boron, nitrogen, phosphor, sulfur, etc.), sulfur atom (S) doping is distinctive because the mismatch of the outermost orbitals of S and C induces an on-uniform spin density distribution, which consequently endows S-doped materials with unique properties and potential for many applications. Unlike B, N and P, S atom is larger than C atom and the C–S bond length (1.78 Å) is 25% longer than that of the C–C bond. Also, the difference of electronegativity between S (2.58) and C (2.55) appears to be too small to offer significant polarization (charge transfer) in C–S composites. Consequently, chemical doping of S atoms into the framework of GQDs appears to be very difficult. Until now, only few reports cover GQDs or CQDs with sulfur atoms doping. In term of S-CQDs, Kwon et al. synthesized S-CQDs by pyrolyzing glucose with thioglycolic acid being sulfur source.¹⁸ Xu et al. reported the preparation of S-CQDs with sodium citrate and sodium thiosulfate by a hydrothermal method.¹⁹ For S-doped or co-doped GQDs, Li et al. prepared S-GQDs by electrolysis of graphite in sodium p-toluenesulfonate medium.²⁰ Li et al. prepared S-GQDs by a hydrothermal method using fructose and sulphuric acid as specific source materials.²¹ Qu et al. synthesized S, N co-doped graphene quantum dots (S, N-GQDs) using hydrothermal synthesis route with citric acid as the C source and urea or thiourea as N and S sources.²² Thus, developing a facile, low-cost and high yield method for the preparation of S-GQDs with high crystal quality is highly desired. Moreover, it is also necessary to further explore novel properties and promising applications of S-GQDs.

In this paper, a facile approach was developed to prepare the S-doped GQDs (S-GQDs) by using water-phase molecular fusion with 1,3,6-trinitropyrene, Na₂S, and NaOH in one-step hydrothermal process. The method is simple, effective and mass production of the S-GQDs with high crystallinity is possible. Excitation-independent yellow-green fluorescence of the S-GQDs could be sensitively quenched in the presence of Pb²⁺ ions, whereas it was non-sensitive to other metal ions, such as Na⁺, K⁺, Cu²⁺, Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺, Ni²⁺, Co²⁺, Cd²⁺. Such selectivity was not observed for the control sulfur-free GQDs. Thus, sensitive detection of Pb²⁺ ion by sulphur doped GQDs was realized. The proposed preparation strategy and the main characteristic features were described and discussed in detail.

2. Experimental section

2.1 Materials and reagents

Pyrene, Na₂S, NaOH, Na₂CO₃, NaHCO₃, NaH₂PO₄ and Na₂HPO₄ were obtained from Aladdin Chemistry Co. Ltd. (China). Aqueous solutions of Na⁺, K⁺, Cu²⁺, Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺, Ni²⁺, and Co²⁺ were prepared from their chloride salts. Aqueous solutions of Cd²⁺ and Pb²⁺ were prepared from their nitrate salts. All chemicals used in our work were of analytical grade and used without further treatment. All aqueous solutions were prepared with ultrapure water (18.2 MΩ cm, Milli-Q, Millipore).

2.2 Instrumentation

Transmission electron microscopic (TEM) photograph was taken on a JEM-2100 transmission electron microscope (JEOL Ltd., Japan) at operating voltage of 200 kV. Atomic force microscopic (AFM) images were obtained by Nanascopy IVA system for height characterization (Digital Instruments Inc, USA). Elemental analysis of the S-GQDs was performed by X-ray photoelectron spectroscopy (XPS) with PHI5300 electron spectrometer using 250 W, 14 kV, Mg Kα radiation (PE Ltd., USA). UV-vis absorption and fluorescence spectra were recorded on UV-2450 spectrophotometry (Shimadzu Corporation, Japan) and RF-5301PC spectrofluorometer (Shimadzu Corporation, Japan).

2.3 Preparation of S-GQDs

Using pyrene as the initial substrate, 1,3,6-trinitropyrene was synthesized according to the literature.²³ Then, 1,3,6-trinitropyrene (2.0 mg mL⁻¹) were dispersed in NaOH (0.2 M) solution containing Na₂S (0.1 M) to form trinary mixture (1,3,6-trinitropyrene, NaOH, Na₂S) by ultrasonic treatment for 1 h. The obtained suspension was transferred into a 50 mL Teflon-lined autoclave and heated at 200 °C for 10 h. After being cooled to room temperature, a brown-black solution containing no solid precipitation was obtained and dialysed in a dialysis bag (retained molecular weight 1000 Da) for 2 days to remove ions and small organic molecules (e.g. sodium salt and unfused molecules). After being further dialyzed in a dialysis bag with retained molecular weight of 3500 Da for 24 h to remove larger impurities, the obtained S-GQDs were freeze dried for further investigation. When the trinary source materials for S-GQDs were changed to binary 1,3,6-trinitropyrene + NaOH or 1,3,6-trinitropyrene + Na₂S under otherwise identical conditions, the obtained GQDs was named as S-free or S-control samples for comparison.

2.4 Measurement of quantum yield (QY)

QY was determined by using rhodamine 6G as the standard sample and was calculated according to the following equation:
where Y_μ is the quantum yield of unknown samples, Y_s is the fluorescence quantum yield of standard substance, F_u and F_s respectively stand for the integral fluorescence intensity of the test sample and the standard dilute solution, A_μ and A_s respectively stand for the maximum absorbance value of the test sample and the standard dilute solution.

2.5 Detection of Pb²⁺ using S-GQDs as fluorescence probe

The detection of Pb²⁺ was carried out in phosphate buffer solution (PBS, 3.0 mM, pH 7.0). In a typical process, 0.95 μL of S-GQDs solution (1.0 mg mL⁻¹) were dispersed in PBS (6.0 mL) followed by the addition of different amount of Pb²⁺ and then the solution was diluted to 10 mL with PBS. Final concentration of Pb²⁺ ranged from 0.1 to 220.0 μM. The resulting solution was shaken well and incubated for 5 min at room temperature before the fluorescence emission spectrum (excited at 460 nm) was recorded. The detection measurements were performed in triplicate. The relative fluorescence intensity (F₀ − F)/F₀ versus Pb²⁺ concentration were used for calibration. Here, F₀ and F are the fluorescence intensities of S-GQDs in the absence and presence of Pb²⁺, respectively.

3. Results and discussion

3.1 Excitation-independent fluorescence properties of the S-GQDs

The obtained S-GQDs solution was pale-yellow and transparent, as illustrated in the inset of Fig. 1 (the left photograph). It emitted yellow-green fluorescence upon being irradiated by a 365 nm lamp (the right photograph, inset in Fig. 1). With rhodamine 6G as a reference, and based on equation, the quantum yield of the obtained S-GQDs was calculated to be 11.6% at 460 nm excitation wavelength. To further explore the optical properties of the S-GQDs, fluorescence spectroscopy was investigated. When the excitation wavelength changed from 410 nm to 490 nm, the emission all centred at 535 nm. The maximum excitation and emission wavelength at 490 nm and 535 nm was revealed, respectively. These results confirmed an excitation-independent fluorescence of S-GQDs. The single-emission fluorescence with excitation-independence indicated high uniformity of S-GQDs. For comparison, the S-free sample and the S-control sample were also prepared by replacing the ternary source materials for S-GQDs (1,3,6-trinitropyrene + NaOH + Na₂S) to binary 1,3,6-trinitropyrene + NaOH (S-free) or 1,3,6-trinitropyrene + Na₂S (S-control), respectively. When excited at 365 nm irradiation, almost no fluorescence was observed for S-control sample (Fig. S1 of the ESI†). However, S-free sample emitted yellow-green fluorescence that was ascribed to the formation of OH-GQDs according to the literature (Fig. S1 of the ESI†).²³ These results may explain the addition of NaOH play a significant role in the formation of the S-GQDs.⁹ A 75% yield was calculated using the dry weight of the obtained S-GQDs as compared with the initial 1,3,6-trinitropyrene. In addition, mass production of the S-GQDs is possible by using Teflon-lined autoclave with large volume.

Fig. 1 Fluorescence excitation and emission spectra of the S-GQDs. Fluorescence excitation spectrum was obtained at an emission wavelength of 480 nm. Fluorescence emission spectra were obtained at excitation wavelength ranged from 410 to 490 nm. The wavelength interval between two adjacent spectra was 10 nm. The inset were the photographs of the S-GQD aqueous solution under visible light (left) and UV light of 365 nm (right).

3.2 Structure characterization of the S-GQDs

Fig. 2 showed the TEM image of the prepared S-GQDs. Obviously, the obtained S-GQDs were well-dispersed and relatively uniform in size (about 3 nm). The high resolution TEM (HRTEM) image (inset of Fig. 2A) clearly depicted the good crystallinity of the S-GQDs. The lattice parameter was 0.23 nm, which was similar to (1120) lattice fringes of graphene.²⁰ These phenomena indicated the water-phase molecular fusion with 1,3,6-trinitropyrene, Na₂S, and NaOH in hydrothermal process resulted in high quality S-GQDs with almost perfect single crystals. That might be ascribed to the special structure of the initial substrate. Briefly, 1,3,6-trinitropyrene consists of four peri-fused benzene rings with a unique carbon skeleton similar to the primitive cell of graphene.²³ Due to three positively charged sites of NO₂ groups, enhanced reaction activity of 1,3,6-trinitropyrene has been demonstrated for the preparation of OH-GQDs (1,3,6-trinitropyrene + NaOH as source material) and amine-functionalized GQDs (1,3,6-trinitropyrene + ammonia or 1,3,6-trinitropyrene + hydrazine hydrate as source material) using hydrothermal process.²³ Inspired by its specific structure and reaction activity, 1,3,6-trinitropyrene was also used in our work to form the lattice of graphene quantum dots. Na₂S was used as the S source to achieve S-doping in molecular fusion of 1,3,6-trinitropyrene in alkaline NaOH medium. XPS measurements were applied to investigate the composition of the obtained S-GQDs (Fig. 3). The XPS survey spectrum of the S-GQDs (Fig. 3A) clearly displayed the presence of C, O, and S with atomic percentages of 61.7%, 37.0% and 1.3%, respectively. Those results confirmed that the successful S-doping via the one-step hydrothermal process. The C1s spectrum of S-GQDs (Fig. 3B) could be fitted into three peaks, which were assigned to C–O at 286.8 eV, C=C at 282.6 eV and C–S at 285.4 eV, respectively.²⁰ The O1s spectrum of S-GQDs (Fig. 3C) stated that there was a peak at 531.2 eV, which corresponded to C–OH or C–O–C.²³ Thus, the obtained S-GQDs were rich in oxygen-containing groups on the surfaces, suggesting good water-dispersibility. The S2p spectrum of S-GQDs (Fig. 3D) could be fitted into three peaks, which were assigned to –C–SO_x–C (x = 2, 3, and 4) sulphone bridges at 167.7 eV, 168.5 eV and 169.3 eV, respectively.^24,25 Thus, S atoms were indeed covalently bonded to the framework of GQDs. In brief, 1,3,6-trinitropyrene has a mother nucleus structure similar to that of graphene. The nitro groups have a strong electrophilicity, and the addition reaction can occur with electron-rich groups. Sulfur ions in Na₂S and hydroxide ions in NaOH reacted with nitro sites pyrene ring in the hydrothermal reaction to achieve the synthesis of S-doped GQDs.

Fig. 2 (A) TEM and (B) HRTEM images of the as-prepared S-GQDs. The number indicated in (B) was the lattice parameter of S-GQDs.

Fig. 3 (A) XPS survey spectrum, high-resolution (B) C1s spectrum, (C) O1s spectrum and (D) S2p spectrum of the S-GQDs.

AFM image in Fig. 4 showed the topographic morphology of S-GQDs. As illustrated in the inset, the heights of S-GQDs were similar and between 0.7 and 0.8 nm. The average height was 0.75 nm, indicating that the fabricated S-GQDs consisted of single graphene layer. Compared with the theoretical height of plain graphene, the increase of topographic height might be ascribed to the presence of rich oxygen-containing and sulfur-containing groups.

Fig. 4 AFM images of the S-GQDs. Inset was the height profile along the blue line.

3.3 Fluorescence quenching of S-GQDs by Pb²⁺

To achieve a fluorescence sensor for metal ion, the effect of pH on the fluorescence of S-GQDs was firstly investigated. The pH value ranged from 2.0 to 11.0 (Fig. 5A). Results showed that the fluorescence intensity of S-GQDs increased dramatically with the increase of pH value in acid medium. When pH was above 7.0, the fluorescence intensity became stable (Fig. S2†). The phenomena might attribute to the change of surface charge density resulting from the ionization of oxygen-containing groups.

Fig. 5 (A) The effect of pH value on the fluorescence intensity of the S-GQDs. (B) The change ratio of fluorescence intensity (F/F₀) in the presence of various metal ions (140.0 μM). (C) The effect of pH value on the response of S-GQDs to Pb²⁺ (140.0 μM). (D) Time-dependent fluorescence response of S-GQDs aqueous solutions upon the addition of Pb²⁺ (95.0 μM).

To assess the selectivity for metal ions, the fluorescence of S-GQDs upon mixing with different metal ions were investigated. The chosen ions were mainly related to environment including Na⁺, K⁺, Cu²⁺, Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺, Ni²⁺, Co²⁺, Cd²⁺ and Pb²⁺. The concentration of each metal ion was all controlled at 140.0 μM. When excited under UV light (365 nm), the photographs of S-GQDs aqueous solution mixed with different ions illustrated the selectivity (Fig. S3 of the ESI†). Compared to other metal ions, Pb²⁺ significantly quenched the fluorescence of S-GQDs, indicating efficient selectivity of S-GQDs for Pb²⁺ over other cations. The change of fluorescence intensity was quantitatively investigated. Fig. 5B showed the difference of fluorescence intensity ratio (F/F₀) of the S-GQDs solution in the absence and presence of various metal ions. Notably, only Pb²⁺ ions gave apparent quenching effect on the fluorescence of S-GQDs, and the effect of other tested metal ions can be neglected. The high selectivity of S-GQDs for Pb²⁺ could be due to the fact that Pb²⁺ ion might have a higher binding affinity with S and O functional groups of S-GQDs than other transition-metal ions. For comparison, fluorescence detection of Pb²⁺ using sulfur-free GQDs (prepared without Na₂S) was studied (Fig. S4 of the ESI†). However, the fluorescence intensity was almost un-changed at the presence of 140.0 μM Pb²⁺ at UV irradiation (365 nm). Therefore, the S dopant in the GQDs structure promoted the selectivity of GQDs towards Pb²⁺.

The effects of detection pH and the kinetic behavior on the reaction were further studied to optimize the detection conditions for Pb²⁺. To avoid the hydrolysis and precipitation of Pb²⁺, weakly acidic and neutral solution was investigated. As illustrated in Fig. 5C, the fluorescence quenching was pH-dependent. Obviously, Pb²⁺ showed the best quenching efficiency at pH 7.0. Thus, the neutral PBS (pH 7.0) buffer was chosen for further investigation. The kinetic behavior of reactions between the S-GQDs and Pb²⁺ was explored by plotting the values (F₀ − F)/F₀ as a function of time. It was evident that the quenching equilibrium reached in less than 5 min (Fig. 5D), suggesting the fast interaction between S-GQDs and Pb²⁺. Thus, 5 min was chosen as reaction time in the following experiments.

3.4 Fluorescence detection of Pb²⁺ using S-GQDs

Under the optimized conditions, a quantitative detection analysis of Pb²⁺ was carried out. As presented in Fig. 6, the fluorescence intensity of the S-GQDs decreased dramatically when Pb²⁺ concentration increased from 0.1 to 220.0 μM. There were two good linear correlation existed in the values of (F₀ − F)/F₀ as the Pb²⁺ concentration from 0.1 to 1.0 μM and 1.0 to 140.0 μM (inset of Fig. 6), giving calibration curves with linear regression equations of (F₀ − F)/F₀ = 0.0048C + 0.0538, and (F₀ − F)/F₀ = 0.0613C + 0.0019, respectively. Where F₀ and F correspond to the fluorescence intensity of the S-GQDs in the absence and presence of Pb²⁺ ions, C is the concentration of Pb²⁺ ions. The detection limit was calculated to be 0.03 μM at a signal-to-noise ratio of 3, which was lower than most of the previous reported assays for Pb²⁺ detection.^{1–4,11,26–33} Table 1 listed the comparison of various fluorescent probes for Pb²⁺ detection, the detection limit (DL) obtained with the present method was lower than those achieved by organic fluorescent probes including rhodamine derivative,¹ phenazine derivative,²⁷ naphthalene derivative,⁴ pyridine derivative,¹¹ thrombin aptamer and pyrene probe system,³⁰ CPA-8-HQL functionalized SBA-15 ³³ and thrombin aptamer and pyrene probe, but higher than those attained from organic fluorescent probes of BODIPY,²⁸ DNA based system including G-quadruplex and graphene oxide composite,³ G-rich DNA and perylene derivative,² graphene oxide and gold nanoparticles²⁹ and semiconductor quantum dots-based system including ZnS@SiO₂ QDs,³⁴ core–shell CdSe/CdS QDs,³⁵ 3-mercaptopropionic acid functionalized CdTe QDs,³⁶ CuInS₂ QDs³⁷ and DNA functionalized CdS QDs.³⁸

Fig. 6 (A) Fluorescence spectra of S-GQDs upon addition of different concentration of Pb²⁺ (from top to bottom: 0, 0.1, 0.3, 0.5, 0.8, 1.0, 5.0, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 80.0, 90.0, 100.0, 110.0, 120.0, 130.0, 140.0, 150.0, 160.0, 170.0, 180.0, 190.0, 200.0, 210.0 and 220.0 μM, respectively). (B) The curve of the fluorescence quenching values (F₀ − F)/F₀ versus Pb²⁺ concentration. Inset was the linear calibration plot for Pb²⁺ detection.

Table 1 Comparison of different fluorescent probes for Pb²⁺ detection^a

Fluorescent probe	DL (μM)	Linear range (μM)	Ref.
a CPA-8-HQL, 5-(4-carboxy-phenylazo)-8-hydroxyquinoline; QDs, quantum dots.
Rhodamine derivative prepared by rhodamine and tris(2-aminoethyl)amine	100	—	1
Phenazine derivative prepared by phenazine, o-phenylene diamine and ninhydrin	1.3	1–63.1	27
Naphthalene derivative prepared by 2-aminophenol, 2-[2-(2-formylnaphthoxy)ethoxy]-naphthaldehyde	0.5	—	4
BODIPY prepared by 1,2-diaminobenzene, ethyl bromoacetate and 2,4-dimethylpyrrole	0.0134	0.05–1.25	28
Pyridine derivative prepared by 2,6-diaminopyridine and 2-hydroxy-3-isopropyl-6-methyl-benzaldehyde	0.49	—	11
G-Quadruplex and graphene oxide composite	0.003	0.005–0.3	3
Thrombin aptamer and pyrene probe	0.8	—	30
G-Rich DNA and perylene derivative	0.005	0.01–1	2
Graphene oxide and gold nanoparticles	0.0001	0.002–0.23	29
CPA-8-HQL functionalized SBA-15	0.49	—	33
ZnS@SiO2 QDs prepared by zinc acetate dihydrate and tetraethyl orthosilicate	—	0.001–260	34
Core–shell CdSe/CdS QDs	0.02	0.05–6	35
3-Mercaptopropionic acid functionalized CdTe QDs	0.00093	0.01–1	36
CuInS2 QDs	0.06	0.10–38.00	37
DNA functionalized CdS QDs	0.01	0.02–2	38
S-Doped GQDs	0.03	0.1–140.0	Present work

---

4. Conclusion

The successful preparation of S-doped GQDs demonstrated the efficiency of water-phase molecular fusion with 1,3,6-trinitropyrene, Na₂S, and NaOH in hydrothermal process. The advantages of the proposed method and the prepared S-GQDs lie in four aspects. (1) Successful doping of S atom in GQDs was achieved in an easy and cheap strategy with the possibility of mass production. (2) The obtained S-doped GQDs exhibited stable and excitation-independent photoluminescence behaviors, monolayer-graphene structure and uniform size. (3) The S-GQDs showed a selective and sensitive response to Pb²⁺. (4) Based on the fluorescence quenching of S-GQDs by Pb²⁺, fluorescence sensor for the detection of Pb²⁺ was developed with fast response, wide linear range and low detection limit. The ease of the one-step hydrothermal process and the promising performance of the prepared S-GQDs make this methodological study and application attractive in bioimaging and sensing.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21305127), the Zhejiang Provincial Natural Science Foundation of China (Y15B050022, LY14E030012), the Science Foundation of Zhejiang Sci-Tech University (13062173-Y) and 521 talent project of ZSTU.

References

1. H. Ju, M. H. Lee, J. Kim, J. S. Kim and J. Kim, Talanta, 2011, 83, 1359–1363.
2. X. H. Zhao, L. Gong, Y. Wu, X. B. Zhang and J. Xie, Talanta, 2016, 149, 98–102.
3. Y. Bai, L. Zhao, Z. Chen, H. Wang and F. Feng, Anal. Methods, 2014, 6, 8120–8123.
4. R. Azadbakht, H. Vaisi, H. Mohamadvand and J. Khanabadi, Spectrochim. Acta, Part A, 2015, 145, 575–579.
5. J. Ju, R. Zhang, S. He and W. Chen, RSC Adv., 2014, 4, 52583–52589.
6. J. Ju, R. Zhang and W. Chen, Sens. Actuators, B, 2016, 228, 66–73.
7. J. Ju and W. Chen, Anal. Chem., 2015, 87, 1903–1910.
8. R. Zhang and W. Chen, Biosens. Bioelectron., 2014, 55, 83–90.
9. X. Gao, Y. Lu, R. Zhang, S. He, J. Ju, M. Liu, L. Lia and W. Chen, J. Mater. Chem. C, 2015, 3, 2302–2309.
10. J. Ju and W. Chen, Curr. Org. Chem., 2015, 19, 1150–1162.
11. K. C. Tayade, A. S. Kuwar, U. A. Fegade, H. Sharma, N. Singh, U. D. Patil and S. B. Attarde, J. Fluoresc., 2014, 24, 19–26.
12. M. S. Hosseini and A. Pirouz, Luminescence, 2014, 29, 798–804.
13. J. Deng, Q. Lu, H. Li, Y. Zhang and S. Yao, RSC Adv., 2015, 5, 29704–29707.
14. F. Wang, Z. Gu, W. Lei, W. Wang, X. Xia and Q. Hao, Sens. Actuators, B, 2014, 190, 516–522.
15. T. V. Tam, N. B. Trung, H. R. Kim, J. S. Chung and W. M. Choi, Sens. Actuators, B, 2014, 202, 568–573.
16. F. Cai, X. Liu, S. Liu, H. Liu and Y. Huang, RSC Adv., 2014, 4, 52016–52022.
17. J. Ju and W. Chen, Biosens. Bioelectron., 2014, 58, 219–225.
18. W. Kwon, J. Lim, J. Lee, T. Park and S. W. Rhee, J. Mater. Chem. C, 2013, 1, 2002–2008.
19. Q. Xu, P. Pu, J. Zhao, C. Dong, C. Gao, Y. Chen, J. Chen, Y. Liu and H. Zhou, J. Mater. Chem. A, 2015, 3, 542–546.
20. S. Li, Y. Li, J. Cao, J. Zhu, L. Fan and X. Li, Anal. Chem., 2014, 86, 10201–10207.
21. X. Li, S. P. Lau, L. Tang, R. Ji and P. Yang, Nanoscale, 2014, 6, 5323–5328.
22. D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie and Z. Sun, Nanoscale, 2013, 5, 12272–12277.
23. L. Wang, Y. Wang, T. Xu, D. Pan, L. Sun and M. Wu, Nat. Commun., 2014, 5, 5357.
24. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. M. Zhou, X. A. Chen and S. M. Huang, ACS Nano, 2012, 6, 205–211.
25. D. Sun, R. Ban, P. H. Zhang, G. H. Wu, J. R. Zhang and J. J. Zhu, Carbon, 2013, 64, 424–434.
26. N. Singla, A. Tripathi, M. Rana, S. K. Goswami, A. Pathak and P. Chowdhury, J. Lumin., 2015, 165, 46–55.
27. K. Aggarwal and J. M. Khurana, J. Lumin., 2015, 167, 146–155.
28. J. Liu, K. Wu, S. Li, T. Song, Y. Han and X. Li, Dalton Trans., 2013, 42, 3854–3859.
29. X. Shi, W. Gu, W. Peng, B. Li, N. Chen, K. Zhao and Y. Xian, ACS Appl. Mater. Interfaces, 2014, 6, 2568–2575.
30. Y. Yang, S. Wang, X. Hu and X. Yang, Chin. Sci. Bull., 2014, 59, 502–508.
31. J. Y. Kwon, Y. J. Jang, Y. J. Lee, K. M. Kim, M. S. Seo, W. Nam and J. Yoon, J. Am. Chem. Soc., 2005, 127, 10107–10111.
32. Z. Q. Hu, C. Lin, X. M. Wang, L. Ding, C. L. Cui, S. F. Liu and H. Y. Lu, Chem. Commun., 2010, 46, 3765–3767.
33. L. Zhao, D. Sui and Y. Wang, RSC Adv., 2015, 5, 16611–16617.
34. H. Qu, L. Cao, G. Su, W. Liu, R. Gao, C. Xia and J. Qin, J. Nanopart. Res., 2014, 16, 1–12.
35. Q. Zhao, X. Rong, H. Ma and G. Tao, Talanta, 2013, 114, 110–116.
36. B. Du, C. Liu, Y. Cao, L. Gao and X. Zhao, Spectrosc. Spectral Anal., 2013, 33, 5.
37. X. Yan, H. Li, Y. Yan and X. Su, Food Chem., 2015, 173, 179–184.
38. S. Liu, W. Na, S. Pang and X. Su, Biosens. Bioelectron., 2014, 5, 17–21.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10836a

‡ These authors contributed equally to this work

---