Time-efficient syntheses of nitrogen and sulfur co-doped graphene quantum dots with tunable luminescence and their sensing applications

Abstract

Heteroatom doped graphene quantum dots (GQDs) are particularly promising in bioimaging and fluorescent sensing because of their better photoluminescence tunability compared to pristine GQDs. Herein, two nitrogen and sulfur co-doped GQDs (N,S-GQDs) with varied fluorescence emission wavelength were synthesized via HNO₃ vapour cutting route, in which a porous polythiophene-derived carbon served as the sulfur source while the HNO₃ vapour was presented as the scissor and the nitrogen source. The as-prepared N,S-GQDs exhibited blue and yellow-green coloured fluorescence, owing to their varied morphologies and surface states resulted from varied reaction temperature. Compared to the typical top-down syntheses via hydrothermal or solvothermal routes, the present HNO₃ vapour cutting method is prominently efficient in time expense and product separation. An application of the obtained greenish-yellow N,S-GQDs for highly selective and sensitive fluorescent detection of Fe³⁺ was demonstrated, with a linear range of 0–130 μM and a detection limit of 0.07 μM. The protocol reported here can also be readily applied for facial synthesis of other heteroatom doped GQDs.

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Introduction

Graphene quantum dots (GQDs), a recent promising category of graphene with lateral dimension less than 100 nm, has widely extended the applications of graphene in the fields of electronics, optoelectronics, catalysis and sensors.^1–3 In the past decade, GQD has been applied frequently in biomedical imaging,^4,5 analyte detection,^6,7 photovoltaics and light-emitting devices^8,9 as an ideal substitute for organic dye and semi-conductive quantum dot because of its lower toxicity, better photostability and biocompatibility.⁷

Similar to some promising heteroatom doped carbon quantum dots (CQDs) like N-CQDs,¹⁰ recent reports show that optical and electrical properties of pristine GQDs can be modulated by introducing heteroatom into carbon skeleton, which would decrease the band gap, enhance the optical absorption and therefore expand the utility of the GQDs.^11–14 For example, a novel kind of luminescent N-doped GQDs was firstly reported in the Qu's group. The reported GQDs exhibited unique optoelectronic features and superior electrocatalytic abilities for oxygen reduction reaction distinctive from those of their N-free counterparts.¹² Li et al. synthesized S-doped GQDs by one-step electrolysis of graphite rod in sodium p-toluenesulfonate electrolyte in 2014.¹³ The as-prepared S-doped GQDs exhibited high selectivity for Fe³⁺ detection as fluorescent probes, with an ultralow detection limit of 4.2 nM and linear regions of 0–0.7 μM.¹³ Dey et al. firstly reported B-doped graphene quantum dots with an average diameter of 5–6 nm by a two-step chemical synthesis route. For comparison, the photoluminescence properties of the B-GQDs, N-GQDs and un-doped GQDs were also studied comparatively.¹⁵ More recently, Ananthanarayanan et al. prepared N,P co-doped GQDs by carbonization and subsequent chemical exfoliation of an adenosine triphosphate precursor.¹⁴ The obtained N,P co-doped GQs showed good biocompatibility, high water solubility and promising potential for real-time molecular tracking in live cells.¹⁴

Usually, the synthesis of heteroatom doped GQDs (H-GQDs) were achieved by top-down method in which bulk carbon materials such as grapheme,^16,17 carbon nanotube,¹¹ carbon nanofiber,¹⁸ graphite rod¹³ and metal–organic framework (MOF) derived carbon¹⁹ are cut into zero dimensional H-GQDs via acidic oxidation,¹⁸ hydrothermal and solvothermal treatment,^16,17 electrochemical treatment¹³ and so on. This strategy possesses excellent merits such as simple operation, abundant precursors and high water solubility of H-GQDs.^3,20 Nevertheless, the disadvantages of low yield, expensive equipment, complicated and tedious separation process caused by top-down cutting are non-negligible.^3,21 So, the development of a rapid, facile and less expensive top-down method is still desirable, especially for the preparation of co-doped GQDs such as N,S-GQDs, N,P-GQDs and so on.

Most of the reported GQDs typically emit blue fluorescence.^3,22–25 Considering the practical applications of GQDs in in vivo bioimaging, solution phase biomolecular logic devices and sensing devices, tunable multi-color fluorescence of GQDs is important. Currently, adjustment on the size of GQDs is known as a promising way that can endow GQDs with tailored photoluminescence from blue to longer wavelength.⁵ On the other hand, heteroatom doping may also provide a solution because the GQDs are likely to process tailored or new photoluminescence properties with heterogeneously hybridized carbon network.^26–28 So far, a fast and efficient cutting route for simultaneously achieving the tunable luminescent and co-doping of GQDs is rare and highly desirable.

An important application of the GQDs in sensing is the detection of heavy metal ions. For example, ferric ions play a key biological role in the human body. Its abnormal concentration is closely related to severe diseases.^6,13 Therefore, an efficient determination of Fe³⁺ is of great importance. Recently, fluorescent sensors based on GQDs for the detection of ferric ions have received considerable attentions due to their advantages of rapid responses, high sensitivity and intrinsic operational simplicity of the fluorescent technique²⁹ as well as superiorities of high photostability, low toxicity and good biocompatibility of the GQDs.⁷

In our previous work, GQD with N-doping was prepared by cutting MOF derived carbon for 5 h in the presence of HNO₃ vapor, which avoided complicated and tedious separation process.¹⁹ The obtained N-GQD fluorescent probe was used for the detection of ferric ions, showing good selectivity and a linear range of 1 to 70 μM. In this extended work, a faster (only 1 h) top-down method was demonstrated for an achievement of two kinds of nitrogen and sulfur co-doped GQDs with greenish-yellow and blue luminescence, respectively, by streaming a polythiophene-derived carbon precursor in the presence of very small amount of HNO₃ at different target temperatures. Besides, as a proof of concept, the N,S co-doped GQDs with greenish-yellow fluorescence was further applied as fluorescent sensing probe for determination of Fe³⁺ with excellent sensing performance in a wider linear range (up to 130 μM).

Experimental

Reagents and apparatus

All chemicals were of analytical grade and used without further purification. X-ray powder diffraction (XRD) data were collected on a Siemens D5005 diffractometer (Siemens, Germany). Nitrogen adsorption–desorption experiments were performed on a N₂ adsorption apparatus (Quantachrome Instruments, USA). X-ray photoelectron spectroscopy (XPS) experiments were performed using an Al Kα source, with energy of 1486.6 eV at room temperature. Transmission electron microscopy (TEM) images were recorded on a JEM-3010 (Japan). Atomic force microscopy (AFM) was performed on a Multimode 8 (Bruker, USA). The UV-vis spectra were recorded on a Shimadzu UV-1800 spectrometer (Shimadzu, Japan). The fluorescence intensity of the N-GQDs solutions (pH ≈ 3) were measured on an F-7000 luminescence spectrometer with an excitation slit width of 10 nm.

Acetonitrile, FeCl₃ and 2-thiophenemethanol were purchased from Alfa, Acros, Adamas Reagent Co., Ltd., respectively. Ascorbic acid, cysteine, ethanol and glucose were obtained from Shanghai Chemical Reagents Co. Ltd. Uric acid was purchased from Acros. Stock solutions of cations were prepared from their corresponding salts with ultrapure water (≥18.2 MΩ). All chemicals were of analytical grade and used as received without further purification. Powder X-ray diffraction (XRD) data were collected on a Siemens D5005 diffractometer with Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-3010, Japan. Atomic force microscopic (AFM) characterization was performed on a Multimode 8 (Bruker, USA). The UV-vis spectra were recorded on a Shimadzu UV-1800 spectrometer (Shimadzu, Japan). The fluorescence spectra of the GQDs solutions were recorded on an F-7000 luminescence spectrometer with an excitation slit width of 10 nm. X-ray photoelectron spectroscopic (XPS) measurements were performed using an Al Kα source, with energy of 1486.8 eV at room temperature (Thermo Fisher Scientific, America).

Synthesis of polythiophene-derived carbon

The mesoporous polythiophene-derived carbon (see Fig. S1 ESI†) that exhibited spherical morphology, high surface area (582 m² g⁻¹) and abundant mesopores (3.9 nm) was obtained according to reported procedure,³⁰ except that the carbonization was directly accomplished without the addition of KOH. Briefly, 3 g 2-thiophenemethanol and 20 mL acetonitrile were mixed. After stirring for 30 min, the solution was added to a solution containing 28.9 g FeCl₃ and 100 mL acetonitrile. Afterwards, the mixture was stirred for 15 h at room temperature. The products were washed with water, acetone and ethanol sequentially. Finally, the resulting sample was heated to 800 °C and kept under that condition for 2 h.

Preparation of the N,S-GQDs

As shown in Scheme 1, 0.1 g of the obtained polythiophene-derived carbon was homogeneously placed on a porous SiO₂ griddle of a glass steamer. The steamer was transferred into an autoclave containing 1.5 mL concentrated HNO₃. The autoclave was then heated for 1 h at different target temperature (150 °C and 180 °C) in an oven for acid vapor cutting. After the reaction, the steamer in the autoclave was transferred to a glass beaker and washed with ultrapure water for in situ filtration. Finally, the filtrate was obtained and labeled as N,S-GQD₁₅₀ and N,S-GQD₁₈₀ solution, respectively.

Scheme 1 Schematic diagram of the preparation of N,S-GQDs with tunable fluorescence.

Quantum yield measurement

The quantum yield of the N,S-GQDs was measured using quinine sulfate as the standard and calculated with the following equation:

QY_x = QY_std(I_x/I_std)(A_std/A_x)(η_x²/η_std²)

where the subscript “x” designates the N,S-GQDs, the subscript “std” designates quinine sulfate, “QY” stands for the quantum yield, “I” stands for the measured integrated fluorescence intensity, “A” stands for the absorbance, and “η” stands for the refractive index of the solvent. Quinine sulfate (quantum yield: 54%) was dissolved in 0.1 M H₂SO₄ (refractive index: 1.33) and the N,S-GQDs were dissolved in water (refractive index: 1.33). In order to minimize re-absorption effect, the absorbance value of each individual solution was kept below 0.10 at the excitation wavelength of 310 nm.

Results and discussion

Characterization of N,S-GQDs

TEM images in Fig. 1A and C show that the obtained N,S-GQD₁₅₀ and N,S-GQD₁₈₀ exhibit excellent dispersity, due to the innate superiority of top-down cutting method. Representative high resolution TEM (HRTEM) images with graphitic lattices (insets in Fig. 1A and C) reveal that both N,S-GQD₁₅₀ and N,S-GQD₁₈₀ possess relatively good crystallinities, with lattice spacings correspond to the hexagonal lattice plane of d₁₁₀₀ and d₁₁₂₀, respectively.^5,6,31 Based on the statistical analysis of over 100 GQDs, the N,S-GQD₁₅₀ exhibits a narrow size distribution ranging from 1.5 to 4.5 nm, with an average diameter of 2.4 nm (Fig. 1B). While the N,S-GQD₁₈₀ displays a 2-fold larger average diameter of 4.7 nm, with a narrow size distribution ranging from 3.9 to 6.3 nm (Fig. 1D).

Fig. 1 (A) and N,S-GQD₁₈₀ (C). Insets in (A) and (C) are the high resolution TEM images with graphitic lattices, correspondingly. Size distributions of N,S-GQD₁₅₀ (B) and N,S-GQD₁₈₀ (D).

The two as-prepared GQDs were deposited onto freshly exfoliated mica and characterized by AFM. Fig. 2B shows that the N,S-GQD₁₅₀ possesses thickness between 0.5 and 1.1 nm, with an average of approximately 0.8 nm based on statistical analysis of 100 nanodots. As to the N,S-GQD₁₈₀ (Fig. 2D), the thickness fluctuates between 0.6 and 1.8 nm, with a larger average value of 1.1 nm. The AFM results indicate that, (a) the N,S-GQD₁₅₀ and N,S-GQD₁₈₀ consist of several layers of graphene were successfully obtained based on their comparable thickness to previously reported GQDs.^32–34 (b) the thickness of the obtained GQDs is positively correlated with their particle size measured by TEM. This finding agrees well with some reported literatures.^35,36 The thickness of GQDs is demonstrated to be synthetic method dependent and size dependent as well, even though the GQDs were obtained via the same route sometime.³

Fig. 2 AFM images of N,S-GQD₁₅₀ (A) and N,S-GQD₁₈₀ (C) as well as their corresponding height profiles of (B) and (D).

FT-IR was performed to determine the chemical composition of the obtained N,S-GQDs. Fig. 3A shows that the FT-IR spectra of N,S-GQD₁₅₀ (upper line) and N,S-GQD₁₈₀ (bottom line) possess similar featured adsorption bands with discrepant intensities, implying that the obtained N,S-GQD₁₅₀ and N,S-GQD₁₈₀ are with similar chemical composition. The absorption bands at 3430, 1720 and 1623 cm⁻¹ indicate the presence of the O–H, C=C and C=O groups.³⁷ The peak between 1376–1496 cm⁻¹ can be attributable to C–N, N–H, and COO⁻ groups.³⁸ The peak at 1229 cm⁻¹ corresponds to C–O group.³⁹ The peak at 1063 cm⁻¹ represents the existence of –SO₃⁻ bond.⁴⁰ Furthermore, the surface chemical states of the obtained N,S-GQDs were investigated by XPS measurements. As shown in Fig. 3B, the XPS spectrum of N,S-GQD₁₅₀ (bottom line) and N,S-GQD₁₈₀ (upper line) contain four distinguishable peaks of O 1s, N 1s, C 1s and S 2p, indicating the accomplishment of both nitrogen and sulfur doping. Meanwhile, as the reaction temperature increases, the nitrogen and sulfur contents of N,S-GQDs decrease from 3.7 to 2.2 at% and from 8.6 to 8.0 at%, respectively, while the oxygen contents increases from 40 to 43%. The core-level XPS spectra of N 1s and S 2p were also displayed in Fig. 3C–F. The N 1s spectra of both N,S-GQDs show the presence of nitrogen functional groups such as pyrrolic-N (N1, binding energy (BE) = 399.8 eV), quaternary nitrogen (N2, BE = 401.3 eV) and the N-oxides of pyridinic-N (N3, BE = 402.1 eV) in the range of 396–404.6 eV.⁴¹ The result significantly indicates that the HNO₃ vapor is supplied as both cutting scissor and nitrogen source. As for the S 2p XPS spectra of N,S-GQD₁₅₀ and N,S-GQD₁₈₀, the two peaks can be ascribed to the sulfur forms of thiophene-S and oxidized-S.⁴² This result is consistent with the conclusions deduced from the FT-IR spectra.

Fig. 3 FT-IR and XPS spectra of the GQDs (A and B). High-resolution N 1s XPS spectra of N,S-GQD₁₅₀ (C) and N,S-GQD₁₈₀ (D). High-resolution S 2p XPS spectra of the N,S-GQD₁₅₀ (E) and N,S-GQD₁₈₀ (F).

Optical properties of N,S-GQDs

As proved by the HR-TEM, FT-IR and XPS measurements, the particle sizes and surface states of the as-prepared N,S-GQDs depends largely on the acidic vapor cutting temperatures. These differences may provide different optical properties of the N,S-GQDs. The particle size and surface state were recently found to be important influence factors for the fluorescence, catalytical activity and sensing property of graphene quantum dots.^5,33,34,43 Fig. 4A shows the UV-vis spectra of N,S-GQD₁₅₀ and N,S-GQD₁₈₀. The N,S-GQD₁₅₀ (bottom line) displays a broad absorption below 500 nm with an obvious peak at 260 nm while the N,S-GQD₁₈₀ (upper line) shows a more broad UV-vis absorption from 750 to 200 nm, with a weaker absorption peak at ca. 270 nm. The red-shift of the absorption peak of N,S-GQD₁₈₀ is probably due to the bigger particle size of N,S-GQD₁₈₀, which is consistent with previous report.⁵ The more broad absorption of N,S-GQD₁₈₀ suggests that it may have potential in the application of optoelectronic devices, as suggested by Gupta et al. and Yan et al.^8,44

Fig. 4 UV-vis absorption spectra (A) and fluorescence spectra (B) of N,S-GQD₁₅₀ and N,S-GQD₁₈₀ under excitation of 365 nm. Insets in (A) are the photographs of N,S-GQD₁₅₀ (a) and N,S-GQD₁₈₀ (b) illuminated by ultraviolet lamp (Ex: 365 nm). (C and D) Fluorescence emission spectra of the N,S-GQD₁₅₀ and N,S-GQD₁₈₀ at different excitation wavelengths from 280 to 350 nm.

The inset in Fig. 4A exhibits the optical images of the N,S-GQD₁₅₀ (a) and N,S-GQD₁₈₀ (b) under UV light (365 nm). The solutions of N,S-GQD₁₅₀ and N,S-GQD₁₈₀ show blue and greenish-yellow emissions, respectively, implying that the luminescence of the N,S-GQDs can be well tuned by the present simple and rapid acid vapor cutting strategy. Correspondingly, the photoluminescent properties of these two N,S-GQDs were investigated. As showed in Fig. 4B, under 365 nm excitation, the emission peak of the N,S-GQD₁₈₀ with larger particle size shifts to relatively longer wavelength compared with that of N,S-GQD₁₅₀, indicating a size-dependent photoluminescence.⁴⁵

The fluorescent property of the N,S-GQDs was studied under various excitation wavelengths ranging from 280 to 350 nm. The results were shown in Fig. 4C and D for N,S-GQD₁₅₀ and N,S-GQD₁₈₀, respectively. The position of the emission peak of the N,S-GQDs remains unchanged when the excitation wavelengths increase gradually, suggesting a nearly excitation-independent behaviour of the obtained N,S-GQDs that is similar to some reported GQDs.^2,46 Meanwhile, we noticed that there are some change in the shape of the emission peaks with the increase of the excitation wavelength, which is similar to previous report.⁴⁷ The mechanism of the variation in the peak shape is still not cleared yet. Besides, the fluorescence intensity of these two GQDs changes at varied excitation wavelengths from 280 to 350 nm, with the highest fluorescence intensity achieved at 330 and 310 nm for N,S-GQD₁₅₀ and N,S-GQD₁₈₀, respectively.

Sensing application of N,S-GQD₁₈₀ for Fe³⁺ detection

The as-prepared greenish-yellow N,S-GQD₁₈₀ was applied as fluorescence probe for the detection of Fe³⁺ ions, considering the abundant of surface phenolic hydroxyl groups and their high affinity to Fe³⁺ ions.^45,48 In this experiment, the pH value of N,S-GQD₁₈₀ solution is less than 3 for inhibiting the hydrolysis of Fe³⁺ and the excitation wavelength of 310 nm was selected for the highest fluorescence intensity. Fig. 5A displays fluorescence spectra of the N,S-GQD₁₈₀ with Fe³⁺ concentration ranging from 0 to 140 μM. With the increased amount of Fe³⁺, the fluorescence was linearly quenched in the ranges varying from 0 to 130 μM (Fig. 5B), respectively. The linear regression can be defined as log (I/I₀) = −0.00398 − 0.00888 × C_Fe³⁺ (R² = 0.998, Fig. 5B), where I and I₀ are the PL intensities in the presence and absence of Fe³⁺, respectively. The detection limit is about 0.07 μM (at a signal-to-noise ratio of 3). The results suggested that the as-prepared N,S co-doped GQD with greenish-yellow fluorescence may be promising for the determination of Fe³⁺ in aqueous media.

Fig. 5 PL emission spectra of the N,S-GQDs solution in the presence of different concentrations of Fe³⁺. Calibration curves (B) for Fe³⁺ detection. I₀ and I are the PL intensities of the N,S-GQD₁₈₀ in the absence and presence of Fe³⁺, respectively (λ_ex = 310 nm).

To examine the selectivity of N,S-GQD₁₈₀ for Fe³⁺, potential interfering ions such as Hg²⁺, Cu²⁺, Co²⁺, Fe²⁺, Na⁺, K⁺, Cd²⁺, Pb²⁺, Al³⁺, Ni²⁺, Ag⁺, Zn²⁺, and Mn²⁺ were examined under concentration of 100 μM. Besides, Ca²⁺ and Mg²⁺ are also used as interferences at a concentration of about 130 and 370 μM, respectively, considering their abundance in the real water sample. The corresponding data was collected and showed in Fig. 6A. Compared with the small FL fluctuations caused by other ions, a remarkably large quenching of about 88% was observed in the presence of 100 μM Fe³⁺. Furthermore, some biomolecules such as ascorbic acid (100 μM), uric acid (100 μM), cysteine (100 μM), glucose (4 mM) and ethanol (10 mM) in the blood plasma⁴⁹ were also investigated. As shown in Fig. S2,† the N,S-GQD₁₈₀ has hardly fluorescent response towards these biomolecules at normal physiological concentration. These results indicate that the obtained N,S-GQD₁₈₀ processes good selectivity towards Fe³⁺. The photostability of the N,S-GQD₁₈₀ was also investigated. As shown in Fig. 6B, the as-prepared N,S-GQD₁₈₀ demonstrates a very good photostability, with 97% of its original luminescence remained after a continuous irradiation of 2 h with a Xe lamp (150 W). Furthermore, the quantum yield of the N,S-GQD₁₈₀ and N,S-GQD₁₅₀ were calculated to be 2.35% and 1.28% respectively, with quinine sulfate as reference.

Fig. 6 (A) Effect of different metal ions on the relative fluorescence intensity of the GQDs solution. From left to right: Hg²⁺, Cu²⁺, Co²⁺, Fe²⁺, Na⁺, K⁺, Cd²⁺, Fe³⁺, Pb²⁺, Mg²⁺, Ca²⁺, Al³⁺, Ni²⁺, Ag⁺, Zn²⁺, and Mn²⁺. (B) Time-dependent photoluminescence spectrum of the N,S-GQD₁₈₀ solution under 310 nm excitation at room temperature (Xe lamp, 150 W).

Real samples analysis

A tap water sample collected in our lab was also used for quantitative analysis. As shown in Table S1 (ESI†), a relatively satisfying recovery of 88.3–101.6% and a low RSD of 1.9–3.0% were obtained, indicating the reliability and feasibility of Fe³⁺ determination by the N,S-GQD₁₈₀ fluorescent sensor.

A comparison of the determination of Fe³⁺ with N,S-GQD₁₈₀ and other similar fluorescent nanoprobes^6,13,50–55 was also carried out and the results were listed in Table 1. It can be seen that the proposed sensor exhibits superior characteristics in terms of linear range, detection limit and selectivity. The present N,S co-doped graphene quantum dot was thus promising for the selective determination of Fe³⁺ in aqueous solution.

Table 1 Performance comparison of the as-prepared N,S-GQD₁₈₀ with other fluorescent nano-probes for the determination of Fe³⁺

Fluorescent probe	Detection limit (μM)	Linear range (μM)	Response towards metal ions	Ref.
GQDs	7.22	0–80	Fe^3+	6
S-GQDs	0.0042	0–0.7	Fe^3+	13
N-GQDs	0.09	1–1945	Fe^3+, Hg^2+	50
C nanodots	0.32	0–20	Fe^3+	51
C3N4 nanodots	0.001	—	Fe^3+, Cu^2+	52
Polymer nanodots	—	0.1–10	Fe^3+	53
Gold nanoclusters	3.5	5–1280	Fe^3+	54
Silver nanoclusters	0.12	0.5–20	Fe^3+	55
N,S-GQDs	0.07	0–130	Fe^3+	This work

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Conclusions

In summary, two N,S co-doped graphene quantum dots with different particle sizes, heights and surface states were successfully synthesized via a facile acidic vapor cutting strategy within an hour, demonstrating a more time-efficient top-down method than other reported top-down routes. Meanwhile, diverse optical properties for these two N,S co-doped GQDs were demonstrated, with different fluorescent emissions in blue and greenish-yellow. The N,S-GQD₁₈₀ with greenish-yellow fluorescence exhibited a high selectivity for Fe³⁺, with a low detection limit of 0.07 μM, a wide linear range of 0–130 μM and a satisfying recovery for real water sample analysis. The present rapid achievement of the dual heteroatoms doped GQDs with tunable fluorescence may broaden the application of GQDs materials in biomedical imaging, energy conversion, light-emitting devices and so on.

Acknowledgements

The authors acknowledge financial support from the ‘One Hundred Talents Project Foundation Program’ and the ‘Western Light Program’ of the Chinese Academy of Sciences (XBBS201317), the National Natural Science Foundation of China (21203244, 21473247), the ‘Young Creative Sci-Tech Talents Cultivation Project (2013711012, 2013711016)’ and the ‘International Science and Technology Cooperation Project (20146003)’ of the Xinjiang Uyghur Autonomous Region.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The SEM image, nitrogen adsorption isotherm and the size distribution of the porous polythiophene-derived carbon precursor. Effects of different biomolecules and Fe³⁺ on the relative fluorescence intensity of the GQDs solution. Determination of Fe³⁺ in tap water samples with the N,S-GQD₁₈₀. See DOI: 10.1039/c6ra05175h

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