Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a fluorescent sensing probe for Cu2+ detection

Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a fluorescent sensing probe for Cu2+ detection

Journal Pre-proof Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a 2+ fluorescent sensing probe for Cu detection Bo ...

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Journal Pre-proof Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a 2+ fluorescent sensing probe for Cu detection Bo Gao, Da Chen, Bingli Gu, Ting Wang, Zihao Wang, Feng xie, Yongsheng Yang, Qinglei Guo, Gang Wang PII:





CAP 5144

To appear in:

Current Applied Physics

Received Date: 24 November 2019 Revised Date:

21 January 2020

Accepted Date: 30 January 2020

Please cite this article as: B. Gao, D. Chen, B. Gu, T. Wang, Z. Wang, F. xie, Y. Yang, Q. Guo, G. Wang, Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a fluorescent 2+ sensing probe for Cu detection, Current Applied Physics (2020), doi: https://doi.org/10.1016/ j.cap.2020.01.018. 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. © 2020 Published by Elsevier B.V. on behalf of Korean Physical Society.

Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a fluorescent sensing probe for Cu2+ detection

Bo Gaoa, Da Chena, b, *, Bingli Gua, Ting Wanga, Zihao Wanga, Feng xiea, Yongsheng Yanga, Qinglei Guoc, *, Gang Wanga, * a

Department of Microelectronic Science and Engineering, School of Physical Science

and Technology, Ningbo University, Ningbo 315211, P. R. China. b

Division of Physics and Applied Physics, School of Physical and Mathematical

Sciences, Nanyang Technological University, Singapore 637371. c

School of Microelectronics, Shandong University, Jinan 250100, P. R. China.

*Corresponding author: E-mail address: [email protected] (Da Chen); [email protected] (Qinglei Guo); [email protected] (Gang Wang);


Abstract: : Nitrogen-doped graphene quantum dots (N-GQDs) with high blue fluorescence efficiency were synthesized by the hydrothermal method from p-Phenylenediamine and p-Coumaric acid. The N-GQDs possess several superiorities, most significantly in excellent solubility and superior photostability. Besides, the as-prepared N-GQDs exhibit a uniform size distribution with a diameter of about 3.8±0.5 nm. After dispersing the N-GQDs in water, the formed aqueous solution still presents a stable and homogeneous phase even after 2 months at room temperature. The N-GQD dispersion was further utilized as sensing probes for the selective detection of copper ions (Cu2+), which is realized by the photoluminescence (PL) quenching of N-GQDs after adding Cu2+. The detection limit for Cu2+ was found to be 57 nM L-1, with superior selectivity in the presence of other commonly interfering metal ions. The presented results in this study provide a facile and high-efficiency method for synthesizing N-GQDs, with ultra-high detectivity and selectivity for Cu2+ detection, offering numerous opportunities for the development of biosensing, bioimaging, environment monitoring, and others.

Keywords: Nitrogen-doped graphene quantum dots, Hydrothermal method, Photoluminescence quenching, Cu2+ detection


1. Introduction Graphene quantum dots (GQDs) have been widely explored for the applications in bioimaging, catalysis, drug delivery, and supercapacitors,[1-4] because of their outstanding properties, including high resistance to photobleaching, superior aqueous solubility, extraordinary chemical stability, excellent biocompatibility and strong photoluminescence (PL) fluorescence quantum yields.[5-8] Recently, various methods, which are divided into top-down and bottom-up strategies,[9] are proposed for the synthesis of GQDs. Top-down methods mainly involve strong acid and alkali, with the complex processes, to cleave carbonaceous materials such as fullerenes, carbon nanotubes, and graphene sheets for the production of GQDs.[10-12] As the complement, bottom-up approaches utilize small organic precursor molecules to synthesize GQDs with high product yields, controllable morphologies, and well-distributed sizes.[13-14] Moreover, doped GQDs with controllable photoluminescence properties,[15-16] for example nitrogen-doped GQDs (N-GQDs), exhibit great potentials in the field of bioimaging or biosensing. Sun et al. demonstrated the high potentials of N-doped GQDs in fluorescence bio-imaging.[17] Wang et al. prepared N-doped GQDs via rapid hydrothermal treatment for the detection of pH and Cu2+ ions through the fluorescence quenching method.[18] Copper (Cu2+) is known as a heavy metal, which is also an essential trace element for the human body, with multiple functions in physiological processes, as iron absorption, hemopoiesis, various enzyme activities and in the oxidation-reduction process.[19] However, when accumulated over a certain amount in the organism, extremely negative health effects such as gastrointestinal disturbance and liver or kidney damage can be induced.[20] Therefore, the ability of sensing Cu2+ with high selectivity is vital for the monitor and diagnosis of physical health. Despite the robust performance of electrochemical Cu2+ sensors, optical Cu2+ sensors based on fluorescent materials have advantages such as high sensitivity and selectivity.[21-23] Therefore, many chemosensors based on semiconductor quantum dots,[24-25] organic dyes,[26] nanoparticles,[27-28] and fluorescent proteins[29] have been widely explored. However, semiconductor quantum dots and proteins generally 3

require specialized synthetic conditions and complicated purification procedures. Sensors based on polymers and small molecules, which are normally toxic, usually have inadequate optical or thermal stabilities, thus limiting their prosperous applications. Therefore, the development of stabile and environmentally friendly materials that are suitable for determining Cu2+ values using fluorescence is highly demanded. In this work, a gentle and low-cost bottom-up strategy is developed for the synthesis of N-GQDs by a facile solvothermal treatment of p-Coumaric acid and p-Phenylenediamine. The aqueous solution of N-GQDs emits brown fluorescence with a visible light illumination, while turns to be strong blue illuminated with an ultraviolet light. In addition, the photoluminescent emission band of the as-prepared N-GQDs has no change with various excitation wavelengths with a range from 350 nm to 415 nm, with a stable fluorescence quantum yield of about 34.4%. Most importantly, the synthesized N-GQDs showed a highly selective and sensitive fluorescence quenching response towards Cu2+ in the range of 0-10 µM L-1 with a detection limit of 57 nM L-1, suggesting great opportunities as a new fluorescent probe for reliable, label-free, and selective detection of Cu2+.

2. Materials and methods 2.1. Materials P-Phenylenediamine (98%), p-Coumaric acid (98%), anhydrous ethanol (EtOH), PdCl2, CaCl2, CdCl2, Al(NO3)3, KCl, HgCl2, NaCl, MgCl2, ZnCl2, Ni(NO3)2, BaCl2, CuCl2, NaOH, and other chemicals were purchased from Aladdin (Shanghai, China) and were used as received without further purification. Water used throughout all experiments was purified by the Millipore system. 2.2. Characterization methods Fourier transform infrared (FT-IR) spectrum was recorded by a Bruker Ten-sor27 spectrometer (Germany) with a KBr disc in the wavelength range of 4000-1000 cm−1. Powder X-ray diffraction (XRD) was measured by Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ=0.154 nm). Transmission electron microscopy 4

(TEM) was carried out on Hitachi H-8100 to determine the microstructure and crystalline quality of the synthesized N-GQDs. Atomic force microscope (AFM, Oxford Instruments, Cypher S) was utilized to determine the size and thickness of the N-GQDs. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD) was employed to determine the surface chemical composition and functional groups. Ultraviolet-visible (Uv-vis) absorption spectra of N-GQDs dispersion was collected from the UV-5800 spectrophotometer. PL and PL excitation (PLE) spectra were obtained from PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at room temperature. Raman spectra (SPLCX40M, Raman) were obtained by utilizing an Ar+ laser with an excitation wavelength of 532 nm. 2.3. Synthesis of N-GQDs 0.82 g (5 mmol) p-Coumaric acid was dispersed into 75 ml anhydrous ethanol (EtOH). Then, 0.75 g (6.94 mmol) p-Phenylenediamine was added. With lightly stirring, the mixture was transferred into a 100 ml Teflon-lined stainless autoclave. Before cooled down to the room temperature, the autoclave was heated at 180 oC for 24 h. Then, the reaction solution was dissolved in the deionized water and condensed by the rotary evaporation to remove the alcohol. To realize the separation and purification, the obtained brown solution was filtered using a 0.22 µm paper filter to remove insoluble precipitate and dialyzed (100-500 Da) against deionized water for 3 days to remove unreacted raw materials. After purification, the solid N-GQDs powders were obtained by using vacuum freeze-drying. 2.4. Determination of Fluorescence Quantum Yield Quinine sulfate in 0.05 M H2SO4 (φ=0.54) was chosen as a standard. The quantum yields of N-GQDs were calculated according to the following Equation,[30]

ϕ = ϕR ×

I AR η 2 × × I R A η R2


Where φ is the quantum yield, I is the integrated emission intensity, η is the refractive index of the solvent, A is the optical density, and the subscript R refers to the reference standard with a known quantum yield. 2.5. Detection of Cu2+ Using N-GQDs. 5

N-GQDs solution (0.1 mg·mL-1) was dispersed in the phosphate buffer solution (PBS, pH=7) and then mixed with different amount of Cu2+. After that, the mixture was shaken for 5 min at ambient temperature. Then, PL emission spectra were recorded by the PL spectrometer with an excitation wavelength of 365 nm. The calibration curve of Cu2+ was determined by measuring the change of fluorescence intensity at various Cu2+ concentrations ranging from 0 to 500 µM. For accuracy, all the measurements were carried out for three times. 3. Results and discussion 3.1. Characterization of N-GQDs Figure 1(a) shows the TEM image of as-synthesized N-GQDs, which are mainly spherical with uniform size distribution. Based on the statistical analysis of more than 100 dots, the average size is about 3.8±0.5 nm. The crystalline structure and quality of the N-GQDs are evaluated by high-resolution TEM (HR-TEM), as shown in Figure 1(b), the lattice spacing is obtained as about 0.24 nm, which corresponds to the (1120) lattice fringe of graphene, suggesting that the N-GQDs contain graphite-like structures.[31] After performing the fast Fourier transform (FFT), as presented in Figure 1(c), typical hexagonal lattices can be observed, indicating that the synthesized N-GQDs have a hexagonal crystal structure. X-ray diffraction (XRD) analysis (Figure S1) of N-GQDs exhibits a broad peak centered at 24.7o, which can be attributed to highly disordered structures.[32] This observation is further confirmed by Raman scattering measurement. As shown in Figure S2, two primary peaks centered at about 1388 cm−1 (D band) and 1596 cm−1 (G band) are found. It is well known that the D band is regarded as a defect-related signal, while the G band is attributed to the sp2 carbon of graphitic domains and the in-plane vibration of sp2 carbon bonded atoms.[33] The intensity ratio of D and G bands (ID/IG) is calculated as about 1.92, indicating a large number of defects present in the N-GQDs due to the existence of nitrogen dopants within graphitic lattice planes.[34] The layer number of N-GQDs is revealed by AFM. As shown in Figure 1(d), N-GQDs have an average height of about 1.2±0.3 nm, suggesting 1-3 layered structures.[35] To analyze the chemical structure of p-Coumaric acid, p-Phenylenediamine, and 6

N-GQDs, Fourier transform infrared (FT-IR) analyses are performed. The FT-IR spectrum of p-Coumaric acid shown in Figure 2(a) (top) exhibits two primary peaks centered at 3383 and 1449 cm−1, which are ascribed to the stretching vibrations of -OH and -COOH groups.[36] Three characteristic peaks centered at 1517, 3200 and 3308 cm−1 are observed for p-Phenylenediamine (middle, Figure 2(b)), resulting from the bending vibrations of N-H from -NH2.[37] For the FT-IR of N-GQDs, these characteristic peaks that can be observed in p-Coumaric acid and p-Phenylenediamine are strongly suppressed (bottom, Figure 2(c)), suggesting that the content of these functional groups are drastically decreased after the formation of N-GQDs. Besides, the peak centered at 1264 cm−1 can be assigned to the stretching of C-N,[38] demonstrating the successful introduction of N atoms into the GQDs. These observations reveal that the as-synthesized N-GQDs have hydroxyl, amine, and carbonyl functional groups in their surfaces, which facilitates the homogenous and stable dispersion of N-GQDs in water. To determine the surface contents and chemical composition of the N-GQDs, X-ray photoelectron spectroscopy (XPS) is performed. The full scan XPS spectrum of N-GQDs, as shown in Figure 3(a), presents three peaks at ca. 533.3, 399.8 and 284.9 eV, which corresponds to O 1s, N 1s, and C 1s, respectively. As a result, the N-GQDs consist of O 6.4%, N 12.7%, and C 80.9%. Moreover, the high resolution of C 1s, as shown in Figure 3(b), could be deconvoluted into three peaks locating at 284.8, 285.6 and 286.7 eV, which represent C-C/C=C, C-N, and C-O/C=O, respectively.[39] Also, three components (centering at 399.0, 399.8 and 400.7 eV) that are assigned as pyridinic N, amino N, and pyrrolic N can also be found in the high-resolution of N 1s spectrum, as depicted in Figure 3(c), which prove the successful dope of nitrogen atoms into the N-GQDs.[40] The O 1s spectrum (Figure S3) has binding energies at 531.5 and 533.1 eV that corresponded to O-C and O=C, respectively.[40] According to the FT-IR and XPS results, N atoms are successfully doped into N-GQDs, and their related functional groups, including hydroxyl, carboxyl, and amino exist on the surface of N-GQDs, as schematically shown in Figure 3(d). 3.2. Optical properties of N-GQDs 7

For the potential applications of N-GQDs, especially in bioimaging and biosensing, it is important to investigate their optical properties. In this regard, Uv-vis absorption, PLE, and PL analyses are performed. As shown in Figure 4(a), two peaks around 248 and 300 nm, which denote the π-π* transition of the aromatic C-C bond and the n-π* transition of the oxygen-containing functional groups (C-O and C=O) for the shoulder, were observed from the Uv-vis absorption spectrum of the N-GQDs.[41] Moreover, the N-GQDs aqueous solution emits brown fluorescence with a visible light illumination, while turns to be blue illuminated with an ultraviolet light (365 nm), as displayed in the insets of Figure 4(a). For the PL spectrum (as shown in Figure 4(b)), with the excitation wavelength scanning from 350 nm to 415 nm, an obvious peak locating at around 463 nm is observed, indicating the independent of the emission wavelength of N-GQDs.[30-31] The optimal emission peak is about 463 nm when excited with a 365 nm light. Figure 4(c) shows the mapping result of the relationship between fluorescence emission wavelength (λem) and excitation (λex), with a strong emission area (around 463 nm) corresponding to the excitation wavelength around 365 nm. By utilizing an integrated sphere under excitation of 360 nm, the absolute PL quantum yield (QY) of the N-GQDs aqueous solution is determined to be 34.4%, as shown in Figure S4. Figure 4(d) shows the fluorescence decay curves of the N-GQDs aqueous solution, which can be well fitted by the biexponential equation, with a short (τ1) and a long (τ2) lifetime components, evaluated as 2.18 and 8.40 ns, respectively. Except for the excitation-dependent PL behavior of N-GQDs, their fluorescent properties under different conditions are also studied. First, the pH-dependent of PL intensities of N-GQDs are investigated, as shown in Figure S5(a). Compared with the PL intensity with a pH value of 7, an 18.2% increase is found when pH=3.0, followed by a slight change with the increase of pH towards 10. For the influence of NaCl concentrations on PL intensity, no significant change is observed, as shown in Figure S5(b), implying excellent stability of N-GQDs for biomedical applications. In addition, the PL intensity of the synthesized N-GQDs aqueous solution exhibits only a 4.8% decrease after storing for 100 min under constant illumination with UV light 8

(365 nm), as presented in Figure S5(c). Long-term stable (for two months) PL properties of the synthesized GQDs are also demonstrated, as shown in Figure S5(d). All the presented results imply that the synthesized N-GQDs possess outstanding stability of optical properties, which are extremely applicable for biomedical applications. 3.3. Fluorescence Sensing of Cu2+ by Utilizing the N-GQDs In order to demonstrate the capability of N-GQDs that effectively detect Cu2+ with strong selectivity, screening experiments that involve 12 metal ions including Pd2+, Ca2+, Cd2+, Al3+, K+, Hg2+, Na+, Mg2+, Zn2+, Ni2+, Ba2+ and Cu2+ are carried out. The concentration of each metal ion is 100 µM L-1. Figure 5(a) shows the PL intensity ratio (F/F0) of the N-GQDs with the addition of various metal ions (black bars). Competitive experiments are performed by adding 100 µΜ·L-1 Cu2+ to each solution (red bars). The obtained results indicate that the added ions had no obvious influence on the PL intensity of the N-GQDs except for Cu2+, which can quench the fluorescence significantly, demonstrating outstanding selectivity to Cu2+ of the as-prepared N-GQDs. Previous studies demonstrated that the PL quenching mechanism was associated with the strong binding affinity and fast chelating kinetics of Cu2+ ions with N functional groups of N-GQDs.[42-43] Through the chelating interaction, the irradiation recombination of photoexcited electron-hole pairs in the N-GQDs is strongly suppressed through a fast electron transfer process from N-GQDs to Cu2+ ions,[18, 44] where N-GQDs serve as the electron donors that combine with Cu2+ to generate nonfluorescent complexes. To further assess the sensitivity of the N-GQDs in terms of detecting Cu2+ in water, the PL properties of various N-GQDs solutions with different concentrations of Cu2+ are measured. The excitation wavelength is set as 365 nm. As shown in Figure 5(b), as the concentration of Cu2+ increases, a remarkable decrease in PL intensity is observed. The calibration plot of fluorescence intensity change (F0-F)/F0 varying with the Cu2+ concentration is shown in Figure 5(c). Among the low Cu2+ concentration range, a rapid increase of the fluorescence intensity change is observed, followed by a gradual saturation as the Cu2+ concentration approaching 500 µM. The inset of Figure 9

5(c) shows a good linear relationship between (F0-F)/F0 and Cu2+ concentration in the range of 0-10 µΜ·L-1. According to the equation, 3δ/S, where δ is the standard deviation of the blank signal and S is the slope of the linear calibration plot, a detection limit of 57 nM L-1 was obtained from our prepared N-GQDs. Figure 5(d) displays the digital images of N-GQDs solutions, irradiated by visible light (up) and 365 nm UV lamp (down), after adding Cu2+ with various concentrations. With an increased Cu2+ concentration, the color of the original N-GQDs aqueous solution becomes much darker because of the enhanced PL quenching effect. Comparing the performance of the prepared N-GQDs with the previously reported studies in sensing Cu2+ (Table 1), the sensitivity and selectivity of this sensor were comparable in both fluorescent and colorimetric manners. These presented results indicate that our prepared N-CQDs with superior PL properties have promising application in highly-sensitive and highly-selective detection of Cu2+. 4. Conclusions In summary, N-GQDs in forms of lattice doping were synthesized in a simple and convenient approach via p-Coumaric acid and p-Phenylenediamine. Due to the quantum confinement effect and the edge effect of surface states, the as-obtained brown solution emits blue under 365 nm UV lamp irradiation with several superior physicochemical properties, including good solubility, long-term photostability, and high resistance to photobleaching. Systemic studies demonstrate that our prepared N-GQDs can be utilized a new fluorescent probe for reliable, label-free, and selective detection of Cu2+, with a linear range of 0-10 µM L-1 and a low detection limit of 57 nM L-1, which offers numerous opportunities for biosensing, bioimaging, environment monitoring, and many others.

Supporting Information Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. Figures (Figures S1-S5). See DOI:

Acknowledgment 10

This work was supported by projects from National Natural Science Foundation of China under Grant (Nos. 11704204, 61604084, and 51802337). K. C. Wong Magna Fund in Ningbo University and the Natural Science Foundation of Ningbo under Grant (No. 2017A610104). The authors declare no competing financial interest.


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Figure Captions: Scheme 1. Schematic diagram of synthesizing N-GQDs and their fluorescence quenching response to the presence of Cu2+ ions. Figure 1. (a) TEM image of the N-GQDs, the inset shows particle size distribution. (b) The high-resolution TEM image of N-GQDs. (c) FFT pattern of N-GQDs. (d) AFM image of N-GQDs (the inset shows the height profiles generated from a line-cutting by the white dotted line). Figure 2. FT-IR spectra of (a) p-Coumaric acid, (b) p-Phenylenediamine, and (c) N-GQDs. Figure 3. (a) XPS spectra of N-GQDs and the high-resolution peaks of C 1s (b), N 1s (c). (d) The schematic diagram of the structure of N-GQDs. Figure 4. (a) Uv-vis absorption spectrum (pink line), PL excitation (PLE) spectrum (red line), and photoluminescence (PL) spectrum (blue line) of the synthesized N-GQDs. The insets show optical images of N-GQDs aqueous solution under the irradiation of visible light (left) and 365 nm UV light (right). (b) PL emission spectra with different excitation wavelengths from 350 to 415 nm. (c) The mapping result of the relationship between fluorescence emission wavelength (λem) and excitation (λex). (d) PL decay time of N-GQDs. Figure 5. (a) Competitive effects of various metal ions (black bars) on the detection of Cu2+ ions (red bars). F and F0 are PL intensities of the N-GQDs with and without Cu2+. (b) Fluorescence spectra of N-GQDs with the presence of Cu2+ with various concentrations from 0 to 500 µΜ·L-1. (c) The relationship between fluorescence quenching values (F0-F)/F0 and Cu2+ concentrations. The inset shows the magnified plot at low concentrations of Cu2+ (0 to 10 µΜ·L-1). (d) Images of N-GQDs solution with several concentrations of Cu2+ (from left to right: 0, 1, 5, 10, 50, 100 and 500 µM·L-1) under visible light (up) and under 365 nm UV lamp irradiations (down).


Scheme 1


Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Table 1. Comparison of Different Fluorescent Probes for Sensing Cu2+ Methods FL FL Electrochemistry Electrochemistry FL Colorimetry Atomic Absorption Spectrometry Raman Spectrometry FL

Detection range (µM) 0-8 0.25-10 0.3-1.4 0.5-50 10-150 0.25-14

Detection limit (µM) 0.29 0.076 0.3 0.35 3.5 0.25

[45] [46] [47] [48] [49] [50]




0-50 0-10

0.5 0.057

[52] This work



Highlights 1. We reported a rapid and sensitive fluorescence nanomaterials sensor with satisfactory selectivity. 2. The as-prepared N-GQDs exhibited excitation-independent behavior and high optical stability. 3. The fluorescence intensity of the N-GQDs could be greatly quenched by adding a small amount of Cu2+ ions. 4. The linearity range is 0-10 µM with a detection limit of 57 nM.

The authors declare no competing financial interest.