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One-step preparation of single-layered graphene quantum dots for the detection of Fe3+
Journal Pre-proof One-step preparation of single-layered graphene quantum dots for the detection of 3+ Fe Haoqiang Wang, Xingxing Wu, Weilong Dong, Shern-Long Lee, Qunhui Yuan, Wei Gan PII:
S1386-1425(19)31016-9
DOI:
https://doi.org/10.1016/j.saa.2019.117626
Reference:
SAA 117626
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 19 February 2019 Revised Date:
4 September 2019
Accepted Date: 6 October 2019
Please cite this article as: H. Wang, X. Wu, W. Dong, S.-L. Lee, Q. Yuan, W. Gan, One-step preparation 3+ of single-layered graphene quantum dots for the detection of Fe , Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117626. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
One-step preparation of single-layered graphene quantum dots for the detection of Fe3+
Haoqiang Wanga, Xingxing Wua, Weilong Dongb, Shern-Long Leec, Qunhui Yuana,* and Wei Gand,*
a
State Key Laboratory of Advanced Welding and Joining, and School of Materials
Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China b
College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518055,
China c
Institute for Advanced Study, Shenzhen University, Shenzhen 518055, China
d
School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055,
China
* Corresponding author. Tel.:+86 -755-86244014 E-mail address: [email protected] (Q. Yuan) and ganwei@ hit.edu.cn (W. Gan) 1
Abstract Single-layer graphene quantum dots are highly desirable while their facile and controllable preparations remain challenging. Herein, single-layered graphene quantum dots (sl-GQDs) were developed via a facile one-step hydrothermal synthesis, with citric acid and β-cyclodextrin (CD) as starting materials. The sl-GQDs decorated with CD molecules emit green fluorescence with a quantum yield of 5.34%, and exhibit a good response exclusively to ferric ions for their structural oxygenous groups. The linear range of the proposed sensor for ferric ions was found in a wide concentration range of 0-85 µM. The detection limit is about 0.26 µM. The sl-GQDs based sensing platform also demonstrates its feasibility in real water sample analysis with recoveries of 93.8%-101.5%.
Keywords:
Single-layered
graphene
quantum
β-cyclodextrin; Fluorescence sensing; Ferric ion 2
dots;
One-step
preparation;
1. Introduction As an emerging type of fluorescence (FL) materials, graphene
quantum dots
(GQDs) have shown great potentials in bioimaging[1], fluorescent sensing[2], optoelectronics[3], photocatalysis[4] and so on, due to their unique features such as tunable fluorescent emissions, good photo-stabilities, favorable biocompatibilities, easy functionalization and robust chemical inertness[5, 6]. It is noted that the varied lateral dimensions, the discrete layered structures as well as the different functional groups of the GQDs will lead to the varied FL characteristics[7, 8]. To achieve the anticipated performance of GQDs in applications, many efforts aiming on the structural engineering of GQDs including surface capping, post-synthesis modification, ion intercalation-exfoliation, nano-reactor confined synthesis and controlled electrochemical oxidation/reduction have been made[9-13]. However, the syntheses of GQDs with uniform structures are still challenging. In this regard, the single-layered graphene quantum dots (sl-GQDs) with relatively fine structures could be as good model systems for revealing the relationship between structure and property of GQDs. Recently, several studies reported the obtaining and application of sl-GQDs. For example, Dong et al. reported a one-step preparation of single- and multi-layer graphene quantum dots by refluxing CX-72 carbon black in nitric acid for 24 h[14]. In subsequent study, they synthesized another type of sl-GQDs by using single-walled carbon nanotubes (SWCNTs) as precursors, where a multi-step hydrothermal etching approach was applied, including a pre-treatment of SWCNTs, a chemical oxidation of 3
SWCNTs and a followed refluxing in nitric acid[15]. Bai et al. recently synthesized sl-GQDs with the assistance of layered double hydroxides (LDH). Owing to the rigidity of templated LDH layers, the growth of GQDs along z-axis was restricted, thus leads to the formation of sl-GQDs in confined nanospaces[7]. Despite the advantages
mentioned
above,
the
preparations
of
sl-GQDs
are
usually
time-consuming or less eco-friendly. Besides, it is difficult to achieve successful preparation of sl-GQDs with large scale production. Bayat et al. has recently demonstrated a relative green and facile synthesis of fluorescent sl-GQDs via hydrothermal process by using glucose as precursor[16]. Since the studies on the preparation of sl-GQDs is still in an early stage, more facile and eco-friendly routes for large-scale preparation of sl-GQDs with uniform size distributions are highly desired[7, 14, 15]. Herein, we present a facile preparation of sl-GQDs through a one-step hydrothermal route using to citric acid and β-cyclodextrin (CD) as precursors. Citric acid has been known as a good carbon source for the preparation of GQDs[17, 18] and CD can serve as dispersing agents to weaken the π-π interlayer conjugate interaction of graphitic sheets[19]. Fourier transform infrared spectroscopic (FT-IR) and atomic force microscopic (AFM) characterizations confirmed that the CD molecules were decorated on the surface of the as-prepared sl-GQDs. Because the oligosaccharide skeletons and hydrophilic exteriors of CDs, the irreversible agglomeration between the as-prepared sl-GQDs was effectively inhibited. The sl-GQDs decorated with CD molecules emit green fluorescence, therefore, their usage 4
in the detection of heavy metallic ions in aqueous solution was also studied. In this application, the proposed FL sensor shows exclusive response to Fe3+ in the presence of other interfering metallic ions, benefiting from the specific recognition between hydroxyl, carboxyl and carbonyl groups on the surface of sl-GQDs and decorated CDs. This study allows for a new synthetic route of sl-GQDs in a large scale. 2. Experimental 2.1 Reagents All reagents were of analytical grade and used without further purification. Citric acid, β-cyclodextrin, tetrahydrofuran, (Bu)4NBF4, DMSO-d6, Co(NO3)2, CaCl2, MnSO4, Al2(SO4)3, Cu(NO3)2, FeCl3, KCl, MgSO4, FeSO4, Zn(NO3)2, Ni(NO3)2, Cr(NO3)3, NaCl and HCl were purchased from Aladdin (China). Ultrapure water (≥18.2 mΩ) was used for the preparation of sl-GQDs and testing solutions containing metallic ions. 2.2 Apparatus Morphological features of sl-GQDs were revealed with a TECNAI F30 transmission electron microscopy (TEM) (FEI, USA) and a multimode 8 AFM (Bruker, Germany). The compositional analysis was carried out by using a Nicolet 380 FT-IR spectrometer (Thermo, USA), an inVia Raman spectrometer (Renishaw, England) and a Sigma probe X-ray photoelectron spectrometer (XPS) (Thermo, USA). Optical measurements were performed on an F-7000 FL spectrometer (Hitachi, Japan) and a UV-1800 spectrometer (Shimadzu, Japan). 1H NMR spectra was performed on an AVANCШ400 nuclear magnetic resonance spectrometer (Bruker Biospin, 5
Switzerland). Cyclic voltammetry (CV) was performed on a CHI 660D electrochemical workstation (CH Instruments, China). Thermogravimetric analysis (TGA) of CD was conducted on a STA 2500 Thermal Gravimetric Analyzer (Netzsch, Germany). 2.3 Preparation of sl-GQDs The sl-GQDs were achieved through a one-step hydrothermal process, as illustrated in Scheme 1. Briefly, 1.0 g citric acid and 0.8 g CD were thoroughly dissolved in 20 mL water under stirring. The mixture was then transferred into a Teflon-lined autoclave and heated at 160℃ for 6 h. After reaction, the light brown solution was centrifuged at 11,000 rpm for 10 min to collect the supernatant containing the produced sl-GQDs, which was then stored at 4℃ for subsequent experiments.
Scheme 1 Illustration on the preparation of sl-GQDs. 2.4 Determination of the quantum yield of sl-GQDs FL quantum yield (QY) of the as-prepared sl-GQDs (in water) was determined with quinine sulfate standard (QY = 54% at 360 nm excitation). The calculation was carried out according to the following equation[20]. QYx =QYref (Ix/Iref) (Aref/Ax) (n2x/n2ref) 6
Here I is the integrated emission intensity, A is the absorbance, and n is the refractive index of solvent. The subscripts 'x' and 'ref' refer to sl-GQDs and quinine sulfate, respectively. To minimize the re-absorption effects, the absorbance of the sample and standard were kept under 0.1[20]. 3. Results and discussion 3.1 Structural and compositional characterizations of sl-GQDs TEM image in Fig. 1a reveals that the as-prepared sl-GQDs exhibit uniform size of 3.5-6.5 nm, with an average diameter of 4.8 nm. The high resolution TEM image in Fig. 1b indicates that the sl-GQDs were highly crystalline with lattice spacing of 0.21 nm, which can be ascribed to the (100) facet of graphite[10]. AFM analysis in Fig. 1d demonstrates that typical height of the sl-GQDs range from 0.8 to 1 nm, suggesting that the synthesized GQDs are mostly single-layered since the thicknesses of graphene sheet and CD molecule are 0.34 and 0.5 nm, respectively[21, 22]. To verify the assumption, the quantum dots derived from only citric acid (CA-QDs) were synthesized under the same experimental conditions and analyzed with AFM. As shown in Fig. S1 in the Supporting Information, the typical height of the CA-QDs is ranging from 1.7 to 2.1 nm, suggesting that the absence of CD resulted in different layered structure of obtained quantum dots. Base on this result and the data listed above, the height of sl-GQDs at 0.8-1 nm is in consistence with the total thickness of CD and single-layered graphene. However, it needs to be noted that the existence of GQDs with three layers of graphene cannot be totally excluded.
7
Fig. 1 (a and b) TEM and HRTEM images of sl-GQDs. Inset in (a) is the corresponding particle size distribution. (c and d) AFM image and height profile of sl-GQDs. 1
H NMR experiment using DMSO-d6 was carried out to characterize the structure
of CD after hydrothermal treatment. For comparison, quantum dots derived from cyclodextrin (CD-QDs) solely was also prepared under the same experimental condition as for sl-GQDs. As shown in Fig. S2, nearly identical characteristic proton peaks were observed in the 1H NMR spectra of CD, CD-QDs and sl-GQDs, respectively[23-25]. This result indicates that the structure of CD was remained after the hydrothermal treatment. To further confirm its structural integrity at high 8
temperature, TGA of CD was carried out. As shown in Fig. S3, from room temperature to 200℃, the mass ratio of CD maintained at 100%, indicating that the CD has good stability under the experiment temperature (160℃). The presence of CD on the surface of the sl-GQDs hinders the agglomeration of quantum dot sheets effectively, due to the abundant hydroxyls groups on both conical edges of CD molecules[19, 22, 26]. The compositional information of the sl-GQDs were studied by FT-IR, Raman and XPS surveys. As shown in Fig. 2a, two sets of featured peaks at similar positions are observed in the FT-IR spectra of CD and sl-GQDs. The broad peak at 3420 cm-1 and 3452 cm-1 can be ascribed to the stretching vibration of –OH whereas the weak peak at 1630 cm-1 can be ascribed to the skeletal vibrations of aromatic C=C[27, 28]. In the FT-IR spectra of CD there can be observed another two featured peaks at 1153 and 1029 cm-1, which can be ascribed to the stretching vibration of C-OH and C-O-C, respectively[29, 30]. This result implies that the skeleton structure of CD molecule was preserved, while a part of functional groups may be regressed during the hydrothermal treatment. The hydroxyl groups of the CD enables its adsorption on the as-prepared sl-GQDs via hydrogen bonding, benefiting from the presence of abundant oxygen containing groups on the sl-GQDs. The red-shifted O-H stretching vibration peak in the FT-IR spectrum of the sl-GQDs also implies the formation of hydrogen bonding between CD and some oxygen-containing groups of the sl-GQDs[19, 22, 26]. Raman spectrum in Fig. 2b exhibits a disordered (D) band at 1360 cm-1 and a crystalline (G) band at 1580 cm-1, demonstrating the presence of sp3 defects and 9
ordered sp2 carbon, respectively[31]. An ID/IG value of 0.76 also discloses the highly crystallized structures of sl-GQDs[31]. In the XPS survey, two prominent peaks at 281.16 eV for C 1s and 528.11 eV for O 1s are observed, with relative proportions of 77.4% for carbon and 22.6% for oxygen. The high resolution C 1s spectrum can be divided into four peaks at 284.8, 286.3, 287.6, 288.7 eV (Fig. 2c), which correspond to sp2 hybridized C, O-C-O, C=O, O=C-O[32, 33]. The O 1s spectra in Fig. 2d has two characteristic peaks at 531.4 eV for C=O and 532.7 eV for C-OH, respectively[34]. These data, together with the results of FT-IR characterization, imply the presence of abundant oxygenous groups, e.g. hydroxyl, carboxyl and carbonyl, on the surface of the as-prepared sl-GQDs.
Fig. 2 (a) FT-IR spectra of sl-GQDs and β-CD. (b) Raman spectrum of sl-GQDs. (c and d) High-resolution XPS C 1s and O 1s spectra of sl-GQDs. 3.2 Optical characterization of sl-GQDs 10
Prior to the optical characterization of sl-GQDs, the original supernatant containing sl-GQDs was diluted to 0.75 mg mL-1 with ultrapure water. In UV-vis spectrum, a broad adsorption peak around 280 nm (Fig. 3a) were observed, due to the π-π* transition of C=O[28, 35]. The solution containing sl-GQDs shows light brown color under daylight and emits green luminescence under UV light (see the inset in Fig. 3a). The change in the FL behavior of sl-GQDs upon the adjustment of excitation wavelengths was also studied. As shown in Fig. 3b, the sl-GQDs shows an excitation-independent emission as the excitation wavelengths increasing from 330 to 380 nm, with its optimum reached under 360 nm excitation. The photo-stability of sl-GQDs was also studied. In Fig. 3c, it was found that the FL intensity of sl-GQDs only slightly decreased (< 5%) under continuous UV irradiation for 2 h, implying the good photo-stability of the obtained sl-GQDs.
Fig. 3 (a) UV-vis absorption spectrum of the sl-GQDs solution. Inset shows the photographs of the solution under 365 nm UV lamp (left) and visible light (right). (b) Normalized FL emission spectra of the sl-GQDs solution at different excitation wavelengths from 330 nm to 380 nm. (c) Time-dependent FL spectrum of the sl-GQDs solution. 3.3 Sensitivity characterization of sl-GQDs for cations To evaluate the specificity of sl-GQDs toward metallic ions, the effects of cations 11
Fe3+, Al3+, Ca2+, Co2+, Cr3+, Cu2+, Fe2+, K+, Mg2+, Mn2+, Na+, Ni2+, Zn2+ on the FL signals of the testing solutions were studied. It was shown in Fig. 4 that the presence of other cations (60 µM) only caused small fluctuations on the FL signals of the sl-GQDs, while the presence of ferric ions could significantly quench the signal. This result suggests that the obtained sl-GQDs have remarkably high affinity toward Fe3+. The FL intensity of sl-GQDs decreased proportionally to the concentration of Fe3+ over
0-85
µM.
The
relationship
between
them
can
be
expressed
by
(I0-I)/I0=-0.0007+0.0076*CFe3+ (R2=0.9973), where I and I0 are the FL intensities in the presence and absence of Fe3+.
Fig. 4 FL response of the sl-GQDs solution in the presence of different metal ions of 60 µM.
Fig. 5 (a) FL emission spectra of the sl-GQDs solution in the presence of different 12
concentrations of Fe3+. (b) Calibration curve for Fe3+ detection. The fluorescence quenching induced by Fe3+ can be ascribed to the coordination or chelation between Fe3+ and the -OH groups on the quantum dots via a non-radiative electron-transfer FL quenching mechanism[36] (Scheme 2).
Scheme 2 Sensing mechanism of the sl-GQDs for Fe3+. To prove this mechanism, CV survey was carried out to reveal the redox properties of sl-GQDs using a standard three-electrode system, with a glassy carbon modified with sl-GQDs as the working electrode, a platinum wire as the counter electrode, and a Ag/AgCl as the reference electrode. The electrolyte was 0.1 mol L-1 (Bu)4NBF4 solution in tetrahydrofuran. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of sl-GQDs could be estimated based on the known equation below[16], EHOMO = -e (Eox +4.75) (eV)
(1)
EHOMO = -e (Ered +4.75) (eV)
(2)
Where Eox and Ered are the onset of oxidation and reduction potentials for sl-GQDs, 13
respectively. Ered was determined to be −0.6 V according to the CV plot in Fig. 6a. Therefore, ELUMO of sl-GQDs was calculated to be −4.15 eV. However, the value for HOMO energy level could not be obtained due to the irreversibility of the oxidation peak. In this case, the Tauc plot derived from the UV-vis spectrum (Fig. 6b) was used to estimate the energy gap (Eg). Based on the Tauc plot, the value of Eg was 4.06 eV. EHOMO was calculated to be -8.21 eV according to the equation below[37]: Eg = ELUMO - EHOMO
(3)
Fig. 6 (a) CV of sl-GQDs in 0.1 mol L-1 (Bu)4NBF4 solution at a scan rate of 100 mV/s. (b) Tauc plot of the absorption spectrum of sl-GQDs. It was reported that the the hydroxyl groups tend to form complex with ferric ions [36, 37]. Thus, upon the addition of Fe3+, the coordination between Fe3+ and the hydroxyl groups on the sl-GQDs occurred (The crystal field stabilization energy of the complex was reported to be 2.70 eV[37]), leading to electron transfer from the excited state of sl-GQDs to the d orbital of Fe3+. Thus the radiated FL emission of the sl-GQDs was hindered, resulting in the observed FL quenching[37, 38]. 3.4 Determination of Fe3+ in tap water samples Based on the good selectivity, high sensitivity and excellent anti-interference performance of the obtained sl-GQDs, we evaluated its potential application in 14
determination of Fe3+ in tap water samples. For this purpose, local tap water was used as replacement for the ultrapure water to prepare the diluted solution of sl-GQDs. The standard addition method was employed by using a stock solution of Fe3+ at 5 mM as mother liquor, which was prepared by using local tap water as solvent as well. The result in Table 1 shows that there is no detectable Fe3+ in the tap water samples. After spiked with certain amounts of Fe3+ standards, the quantitative determination of Fe3+ were achieved. The recoveries of 93.8-101.5% and the relative standard derivations (RSDs) of 0.7-3.2% indicate the feasibility of the present sl-GQDs in real water sample analysis. Table 1 Determination of Fe3+ in tap water samples. (Each sample was tested for three times) Initial
Added (µM)
Detected (µM)
Recovery (%)
RSD (%)
Fe3+
0
1
0.95 ± 0.03
95.6
3.2
Fe3+
0
5
4.69 ± 0.09
93.8
1.7
Fe3+
0
10
9.51 ± 0.11
95.1
1.0
Fe3+
0
20
20.35 ± 0.15
101.5
0.7
4. Conclusion In summary, sl-GQDs with stable green fluorescence emission have been fabricated via a simple and eco-friendly one-step hydrothermal treatment. Attributable to the functionalization and dispersion of CD molecules, the single-layered quantum dots avoid stacking from each other effectively. Besides, their abundant structural oxygenous groups ensure the coordination with ferric ions, thus lead to good specificity of sl-GQDs for the ferric ions. The sl-GQDs based sensor exhibits 15
excellent analytical performance in Fe3+ sensing and a great potential for the determination of Fe3+ in real sample.
Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (21473247), the fundamental research fund from Shenzhen (JCYJ20170811153306372, JCYJ20170307150520453) and the ‘Scientific Research Foundation for the New Teacher’ of the Harbin Institute of Technology (Shenzhen). References [1] P. Yan, R. Li, Y. Yang, Z. Li, Z. Gu, G. Wang, J. Liu, Pentaethylenehexamine and D-penicillamine co-functionalized graphene quantum dots for fluorescent detection of mercury(II) and glutathione and bioimaging, Spectrochim. Acta A 203 (2018) 139-146. [2] C. Qu, D. Zhang, R. Yang, J. Hu, L. Qu, Nitrogen and sulfur co-doped graphene quantum dots for the highly sensitive and selective detection of mercury ion in living cells, Spectrochim. Acta A 206 (2019) 588-596. [3] M. Amjadi, R. Shokri, T. Hallaj, A new turn-off fluorescence probe based on graphene quantum dots for detection of Au(III) ion, Spectrochim. Acta A 153 (2016) 619-624. [4] Z. Zeng, S. Chen, T.T.Y. Tan, F.-X. Xiao, Graphene quantum dots (GQDs) and its derivatives for multifarious photocatalysis and photoelectrocatalysis, Catal. Today 315 (2018) 171-183. 16
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21
Highlights
sl-GQDs were fabricated via a simple and eco-friendly hydrothermal route Functionalization and dispersion of CD inhibit π-π stacking of graphene sheets sl-GQDs contain abundant oxygenous groups and show high affinity for Fe3+ sl-GQDs based sensor show feasibility for tap water sample analysis
S1386-1425(19)31016-9
DOI:
https://doi.org/10.1016/j.saa.2019.117626
Reference:
SAA 117626
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 19 February 2019 Revised Date:
4 September 2019
Accepted Date: 6 October 2019
Please cite this article as: H. Wang, X. Wu, W. Dong, S.-L. Lee, Q. Yuan, W. Gan, One-step preparation 3+ of single-layered graphene quantum dots for the detection of Fe , Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117626. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
One-step preparation of single-layered graphene quantum dots for the detection of Fe3+
Haoqiang Wanga, Xingxing Wua, Weilong Dongb, Shern-Long Leec, Qunhui Yuana,* and Wei Gand,*
a
State Key Laboratory of Advanced Welding and Joining, and School of Materials
Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China b
College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518055,
China c
Institute for Advanced Study, Shenzhen University, Shenzhen 518055, China
d
School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055,
China
* Corresponding author. Tel.:+86 -755-86244014 E-mail address: [email protected] (Q. Yuan) and ganwei@ hit.edu.cn (W. Gan) 1
Abstract Single-layer graphene quantum dots are highly desirable while their facile and controllable preparations remain challenging. Herein, single-layered graphene quantum dots (sl-GQDs) were developed via a facile one-step hydrothermal synthesis, with citric acid and β-cyclodextrin (CD) as starting materials. The sl-GQDs decorated with CD molecules emit green fluorescence with a quantum yield of 5.34%, and exhibit a good response exclusively to ferric ions for their structural oxygenous groups. The linear range of the proposed sensor for ferric ions was found in a wide concentration range of 0-85 µM. The detection limit is about 0.26 µM. The sl-GQDs based sensing platform also demonstrates its feasibility in real water sample analysis with recoveries of 93.8%-101.5%.
Keywords:
Single-layered
graphene
quantum
β-cyclodextrin; Fluorescence sensing; Ferric ion 2
dots;
One-step
preparation;
1. Introduction As an emerging type of fluorescence (FL) materials, graphene
quantum dots
(GQDs) have shown great potentials in bioimaging[1], fluorescent sensing[2], optoelectronics[3], photocatalysis[4] and so on, due to their unique features such as tunable fluorescent emissions, good photo-stabilities, favorable biocompatibilities, easy functionalization and robust chemical inertness[5, 6]. It is noted that the varied lateral dimensions, the discrete layered structures as well as the different functional groups of the GQDs will lead to the varied FL characteristics[7, 8]. To achieve the anticipated performance of GQDs in applications, many efforts aiming on the structural engineering of GQDs including surface capping, post-synthesis modification, ion intercalation-exfoliation, nano-reactor confined synthesis and controlled electrochemical oxidation/reduction have been made[9-13]. However, the syntheses of GQDs with uniform structures are still challenging. In this regard, the single-layered graphene quantum dots (sl-GQDs) with relatively fine structures could be as good model systems for revealing the relationship between structure and property of GQDs. Recently, several studies reported the obtaining and application of sl-GQDs. For example, Dong et al. reported a one-step preparation of single- and multi-layer graphene quantum dots by refluxing CX-72 carbon black in nitric acid for 24 h[14]. In subsequent study, they synthesized another type of sl-GQDs by using single-walled carbon nanotubes (SWCNTs) as precursors, where a multi-step hydrothermal etching approach was applied, including a pre-treatment of SWCNTs, a chemical oxidation of 3
SWCNTs and a followed refluxing in nitric acid[15]. Bai et al. recently synthesized sl-GQDs with the assistance of layered double hydroxides (LDH). Owing to the rigidity of templated LDH layers, the growth of GQDs along z-axis was restricted, thus leads to the formation of sl-GQDs in confined nanospaces[7]. Despite the advantages
mentioned
above,
the
preparations
of
sl-GQDs
are
usually
time-consuming or less eco-friendly. Besides, it is difficult to achieve successful preparation of sl-GQDs with large scale production. Bayat et al. has recently demonstrated a relative green and facile synthesis of fluorescent sl-GQDs via hydrothermal process by using glucose as precursor[16]. Since the studies on the preparation of sl-GQDs is still in an early stage, more facile and eco-friendly routes for large-scale preparation of sl-GQDs with uniform size distributions are highly desired[7, 14, 15]. Herein, we present a facile preparation of sl-GQDs through a one-step hydrothermal route using to citric acid and β-cyclodextrin (CD) as precursors. Citric acid has been known as a good carbon source for the preparation of GQDs[17, 18] and CD can serve as dispersing agents to weaken the π-π interlayer conjugate interaction of graphitic sheets[19]. Fourier transform infrared spectroscopic (FT-IR) and atomic force microscopic (AFM) characterizations confirmed that the CD molecules were decorated on the surface of the as-prepared sl-GQDs. Because the oligosaccharide skeletons and hydrophilic exteriors of CDs, the irreversible agglomeration between the as-prepared sl-GQDs was effectively inhibited. The sl-GQDs decorated with CD molecules emit green fluorescence, therefore, their usage 4
in the detection of heavy metallic ions in aqueous solution was also studied. In this application, the proposed FL sensor shows exclusive response to Fe3+ in the presence of other interfering metallic ions, benefiting from the specific recognition between hydroxyl, carboxyl and carbonyl groups on the surface of sl-GQDs and decorated CDs. This study allows for a new synthetic route of sl-GQDs in a large scale. 2. Experimental 2.1 Reagents All reagents were of analytical grade and used without further purification. Citric acid, β-cyclodextrin, tetrahydrofuran, (Bu)4NBF4, DMSO-d6, Co(NO3)2, CaCl2, MnSO4, Al2(SO4)3, Cu(NO3)2, FeCl3, KCl, MgSO4, FeSO4, Zn(NO3)2, Ni(NO3)2, Cr(NO3)3, NaCl and HCl were purchased from Aladdin (China). Ultrapure water (≥18.2 mΩ) was used for the preparation of sl-GQDs and testing solutions containing metallic ions. 2.2 Apparatus Morphological features of sl-GQDs were revealed with a TECNAI F30 transmission electron microscopy (TEM) (FEI, USA) and a multimode 8 AFM (Bruker, Germany). The compositional analysis was carried out by using a Nicolet 380 FT-IR spectrometer (Thermo, USA), an inVia Raman spectrometer (Renishaw, England) and a Sigma probe X-ray photoelectron spectrometer (XPS) (Thermo, USA). Optical measurements were performed on an F-7000 FL spectrometer (Hitachi, Japan) and a UV-1800 spectrometer (Shimadzu, Japan). 1H NMR spectra was performed on an AVANCШ400 nuclear magnetic resonance spectrometer (Bruker Biospin, 5
Switzerland). Cyclic voltammetry (CV) was performed on a CHI 660D electrochemical workstation (CH Instruments, China). Thermogravimetric analysis (TGA) of CD was conducted on a STA 2500 Thermal Gravimetric Analyzer (Netzsch, Germany). 2.3 Preparation of sl-GQDs The sl-GQDs were achieved through a one-step hydrothermal process, as illustrated in Scheme 1. Briefly, 1.0 g citric acid and 0.8 g CD were thoroughly dissolved in 20 mL water under stirring. The mixture was then transferred into a Teflon-lined autoclave and heated at 160℃ for 6 h. After reaction, the light brown solution was centrifuged at 11,000 rpm for 10 min to collect the supernatant containing the produced sl-GQDs, which was then stored at 4℃ for subsequent experiments.
Scheme 1 Illustration on the preparation of sl-GQDs. 2.4 Determination of the quantum yield of sl-GQDs FL quantum yield (QY) of the as-prepared sl-GQDs (in water) was determined with quinine sulfate standard (QY = 54% at 360 nm excitation). The calculation was carried out according to the following equation[20]. QYx =QYref (Ix/Iref) (Aref/Ax) (n2x/n2ref) 6
Here I is the integrated emission intensity, A is the absorbance, and n is the refractive index of solvent. The subscripts 'x' and 'ref' refer to sl-GQDs and quinine sulfate, respectively. To minimize the re-absorption effects, the absorbance of the sample and standard were kept under 0.1[20]. 3. Results and discussion 3.1 Structural and compositional characterizations of sl-GQDs TEM image in Fig. 1a reveals that the as-prepared sl-GQDs exhibit uniform size of 3.5-6.5 nm, with an average diameter of 4.8 nm. The high resolution TEM image in Fig. 1b indicates that the sl-GQDs were highly crystalline with lattice spacing of 0.21 nm, which can be ascribed to the (100) facet of graphite[10]. AFM analysis in Fig. 1d demonstrates that typical height of the sl-GQDs range from 0.8 to 1 nm, suggesting that the synthesized GQDs are mostly single-layered since the thicknesses of graphene sheet and CD molecule are 0.34 and 0.5 nm, respectively[21, 22]. To verify the assumption, the quantum dots derived from only citric acid (CA-QDs) were synthesized under the same experimental conditions and analyzed with AFM. As shown in Fig. S1 in the Supporting Information, the typical height of the CA-QDs is ranging from 1.7 to 2.1 nm, suggesting that the absence of CD resulted in different layered structure of obtained quantum dots. Base on this result and the data listed above, the height of sl-GQDs at 0.8-1 nm is in consistence with the total thickness of CD and single-layered graphene. However, it needs to be noted that the existence of GQDs with three layers of graphene cannot be totally excluded.
7
Fig. 1 (a and b) TEM and HRTEM images of sl-GQDs. Inset in (a) is the corresponding particle size distribution. (c and d) AFM image and height profile of sl-GQDs. 1
H NMR experiment using DMSO-d6 was carried out to characterize the structure
of CD after hydrothermal treatment. For comparison, quantum dots derived from cyclodextrin (CD-QDs) solely was also prepared under the same experimental condition as for sl-GQDs. As shown in Fig. S2, nearly identical characteristic proton peaks were observed in the 1H NMR spectra of CD, CD-QDs and sl-GQDs, respectively[23-25]. This result indicates that the structure of CD was remained after the hydrothermal treatment. To further confirm its structural integrity at high 8
temperature, TGA of CD was carried out. As shown in Fig. S3, from room temperature to 200℃, the mass ratio of CD maintained at 100%, indicating that the CD has good stability under the experiment temperature (160℃). The presence of CD on the surface of the sl-GQDs hinders the agglomeration of quantum dot sheets effectively, due to the abundant hydroxyls groups on both conical edges of CD molecules[19, 22, 26]. The compositional information of the sl-GQDs were studied by FT-IR, Raman and XPS surveys. As shown in Fig. 2a, two sets of featured peaks at similar positions are observed in the FT-IR spectra of CD and sl-GQDs. The broad peak at 3420 cm-1 and 3452 cm-1 can be ascribed to the stretching vibration of –OH whereas the weak peak at 1630 cm-1 can be ascribed to the skeletal vibrations of aromatic C=C[27, 28]. In the FT-IR spectra of CD there can be observed another two featured peaks at 1153 and 1029 cm-1, which can be ascribed to the stretching vibration of C-OH and C-O-C, respectively[29, 30]. This result implies that the skeleton structure of CD molecule was preserved, while a part of functional groups may be regressed during the hydrothermal treatment. The hydroxyl groups of the CD enables its adsorption on the as-prepared sl-GQDs via hydrogen bonding, benefiting from the presence of abundant oxygen containing groups on the sl-GQDs. The red-shifted O-H stretching vibration peak in the FT-IR spectrum of the sl-GQDs also implies the formation of hydrogen bonding between CD and some oxygen-containing groups of the sl-GQDs[19, 22, 26]. Raman spectrum in Fig. 2b exhibits a disordered (D) band at 1360 cm-1 and a crystalline (G) band at 1580 cm-1, demonstrating the presence of sp3 defects and 9
ordered sp2 carbon, respectively[31]. An ID/IG value of 0.76 also discloses the highly crystallized structures of sl-GQDs[31]. In the XPS survey, two prominent peaks at 281.16 eV for C 1s and 528.11 eV for O 1s are observed, with relative proportions of 77.4% for carbon and 22.6% for oxygen. The high resolution C 1s spectrum can be divided into four peaks at 284.8, 286.3, 287.6, 288.7 eV (Fig. 2c), which correspond to sp2 hybridized C, O-C-O, C=O, O=C-O[32, 33]. The O 1s spectra in Fig. 2d has two characteristic peaks at 531.4 eV for C=O and 532.7 eV for C-OH, respectively[34]. These data, together with the results of FT-IR characterization, imply the presence of abundant oxygenous groups, e.g. hydroxyl, carboxyl and carbonyl, on the surface of the as-prepared sl-GQDs.
Fig. 2 (a) FT-IR spectra of sl-GQDs and β-CD. (b) Raman spectrum of sl-GQDs. (c and d) High-resolution XPS C 1s and O 1s spectra of sl-GQDs. 3.2 Optical characterization of sl-GQDs 10
Prior to the optical characterization of sl-GQDs, the original supernatant containing sl-GQDs was diluted to 0.75 mg mL-1 with ultrapure water. In UV-vis spectrum, a broad adsorption peak around 280 nm (Fig. 3a) were observed, due to the π-π* transition of C=O[28, 35]. The solution containing sl-GQDs shows light brown color under daylight and emits green luminescence under UV light (see the inset in Fig. 3a). The change in the FL behavior of sl-GQDs upon the adjustment of excitation wavelengths was also studied. As shown in Fig. 3b, the sl-GQDs shows an excitation-independent emission as the excitation wavelengths increasing from 330 to 380 nm, with its optimum reached under 360 nm excitation. The photo-stability of sl-GQDs was also studied. In Fig. 3c, it was found that the FL intensity of sl-GQDs only slightly decreased (< 5%) under continuous UV irradiation for 2 h, implying the good photo-stability of the obtained sl-GQDs.
Fig. 3 (a) UV-vis absorption spectrum of the sl-GQDs solution. Inset shows the photographs of the solution under 365 nm UV lamp (left) and visible light (right). (b) Normalized FL emission spectra of the sl-GQDs solution at different excitation wavelengths from 330 nm to 380 nm. (c) Time-dependent FL spectrum of the sl-GQDs solution. 3.3 Sensitivity characterization of sl-GQDs for cations To evaluate the specificity of sl-GQDs toward metallic ions, the effects of cations 11
Fe3+, Al3+, Ca2+, Co2+, Cr3+, Cu2+, Fe2+, K+, Mg2+, Mn2+, Na+, Ni2+, Zn2+ on the FL signals of the testing solutions were studied. It was shown in Fig. 4 that the presence of other cations (60 µM) only caused small fluctuations on the FL signals of the sl-GQDs, while the presence of ferric ions could significantly quench the signal. This result suggests that the obtained sl-GQDs have remarkably high affinity toward Fe3+. The FL intensity of sl-GQDs decreased proportionally to the concentration of Fe3+ over
0-85
µM.
The
relationship
between
them
can
be
expressed
by
(I0-I)/I0=-0.0007+0.0076*CFe3+ (R2=0.9973), where I and I0 are the FL intensities in the presence and absence of Fe3+.
Fig. 4 FL response of the sl-GQDs solution in the presence of different metal ions of 60 µM.
Fig. 5 (a) FL emission spectra of the sl-GQDs solution in the presence of different 12
concentrations of Fe3+. (b) Calibration curve for Fe3+ detection. The fluorescence quenching induced by Fe3+ can be ascribed to the coordination or chelation between Fe3+ and the -OH groups on the quantum dots via a non-radiative electron-transfer FL quenching mechanism[36] (Scheme 2).
Scheme 2 Sensing mechanism of the sl-GQDs for Fe3+. To prove this mechanism, CV survey was carried out to reveal the redox properties of sl-GQDs using a standard three-electrode system, with a glassy carbon modified with sl-GQDs as the working electrode, a platinum wire as the counter electrode, and a Ag/AgCl as the reference electrode. The electrolyte was 0.1 mol L-1 (Bu)4NBF4 solution in tetrahydrofuran. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of sl-GQDs could be estimated based on the known equation below[16], EHOMO = -e (Eox +4.75) (eV)
(1)
EHOMO = -e (Ered +4.75) (eV)
(2)
Where Eox and Ered are the onset of oxidation and reduction potentials for sl-GQDs, 13
respectively. Ered was determined to be −0.6 V according to the CV plot in Fig. 6a. Therefore, ELUMO of sl-GQDs was calculated to be −4.15 eV. However, the value for HOMO energy level could not be obtained due to the irreversibility of the oxidation peak. In this case, the Tauc plot derived from the UV-vis spectrum (Fig. 6b) was used to estimate the energy gap (Eg). Based on the Tauc plot, the value of Eg was 4.06 eV. EHOMO was calculated to be -8.21 eV according to the equation below[37]: Eg = ELUMO - EHOMO
(3)
Fig. 6 (a) CV of sl-GQDs in 0.1 mol L-1 (Bu)4NBF4 solution at a scan rate of 100 mV/s. (b) Tauc plot of the absorption spectrum of sl-GQDs. It was reported that the the hydroxyl groups tend to form complex with ferric ions [36, 37]. Thus, upon the addition of Fe3+, the coordination between Fe3+ and the hydroxyl groups on the sl-GQDs occurred (The crystal field stabilization energy of the complex was reported to be 2.70 eV[37]), leading to electron transfer from the excited state of sl-GQDs to the d orbital of Fe3+. Thus the radiated FL emission of the sl-GQDs was hindered, resulting in the observed FL quenching[37, 38]. 3.4 Determination of Fe3+ in tap water samples Based on the good selectivity, high sensitivity and excellent anti-interference performance of the obtained sl-GQDs, we evaluated its potential application in 14
determination of Fe3+ in tap water samples. For this purpose, local tap water was used as replacement for the ultrapure water to prepare the diluted solution of sl-GQDs. The standard addition method was employed by using a stock solution of Fe3+ at 5 mM as mother liquor, which was prepared by using local tap water as solvent as well. The result in Table 1 shows that there is no detectable Fe3+ in the tap water samples. After spiked with certain amounts of Fe3+ standards, the quantitative determination of Fe3+ were achieved. The recoveries of 93.8-101.5% and the relative standard derivations (RSDs) of 0.7-3.2% indicate the feasibility of the present sl-GQDs in real water sample analysis. Table 1 Determination of Fe3+ in tap water samples. (Each sample was tested for three times) Initial
Added (µM)
Detected (µM)
Recovery (%)
RSD (%)
Fe3+
0
1
0.95 ± 0.03
95.6
3.2
Fe3+
0
5
4.69 ± 0.09
93.8
1.7
Fe3+
0
10
9.51 ± 0.11
95.1
1.0
Fe3+
0
20
20.35 ± 0.15
101.5
0.7
4. Conclusion In summary, sl-GQDs with stable green fluorescence emission have been fabricated via a simple and eco-friendly one-step hydrothermal treatment. Attributable to the functionalization and dispersion of CD molecules, the single-layered quantum dots avoid stacking from each other effectively. Besides, their abundant structural oxygenous groups ensure the coordination with ferric ions, thus lead to good specificity of sl-GQDs for the ferric ions. The sl-GQDs based sensor exhibits 15
excellent analytical performance in Fe3+ sensing and a great potential for the determination of Fe3+ in real sample.
Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (21473247), the fundamental research fund from Shenzhen (JCYJ20170811153306372, JCYJ20170307150520453) and the ‘Scientific Research Foundation for the New Teacher’ of the Harbin Institute of Technology (Shenzhen). References [1] P. Yan, R. Li, Y. Yang, Z. Li, Z. Gu, G. Wang, J. Liu, Pentaethylenehexamine and D-penicillamine co-functionalized graphene quantum dots for fluorescent detection of mercury(II) and glutathione and bioimaging, Spectrochim. Acta A 203 (2018) 139-146. [2] C. Qu, D. Zhang, R. Yang, J. Hu, L. Qu, Nitrogen and sulfur co-doped graphene quantum dots for the highly sensitive and selective detection of mercury ion in living cells, Spectrochim. Acta A 206 (2019) 588-596. [3] M. Amjadi, R. Shokri, T. Hallaj, A new turn-off fluorescence probe based on graphene quantum dots for detection of Au(III) ion, Spectrochim. Acta A 153 (2016) 619-624. [4] Z. Zeng, S. Chen, T.T.Y. Tan, F.-X. Xiao, Graphene quantum dots (GQDs) and its derivatives for multifarious photocatalysis and photoelectrocatalysis, Catal. Today 315 (2018) 171-183. 16
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21
Highlights
sl-GQDs were fabricated via a simple and eco-friendly hydrothermal route Functionalization and dispersion of CD inhibit π-π stacking of graphene sheets sl-GQDs contain abundant oxygenous groups and show high affinity for Fe3+ sl-GQDs based sensor show feasibility for tap water sample analysis