Cysteine detection using a high-fluorescence sensor based on a nitrogen-doped graphene quantum dot–mercury(II) system

Author’s Accepted Manuscript Cysteine Detection Using a High-Fluorescence Sensor Based on a Nitrogen-doped Graphene Quantum Dot–Mercury(II) System Zhenzhen Liu, Yan Gong, Zhefeng Fan www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(15)30748-1 http://dx.doi.org/10.1016/j.jlumin.2016.01.036 LUMIN13839

To appear in: Journal of Luminescence Received date: 25 November 2015 Revised date: 17 January 2016 Accepted date: 24 January 2016 Cite this article as: Zhenzhen Liu, Yan Gong and Zhefeng Fan, Cysteine Detection Using a High-Fluorescence Sensor Based on a Nitrogen-doped Graphene Quantum Dot–Mercury(II) System, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.01.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Cysteine Detection Using a High-Fluorescence Sensor Based on a Nitrogen-doped Graphene Quantum Dot–Mercury(II) System

Zhenzhen Liu, Yan Gong, Zhefeng Fan* Department of Chemistry, Shanxi Normal University, Linfen 041004, PR China

Abstract A novel and highly sensitive fluorescence sensor, which was based on the recovered fluorescence of a nitrogen-doped graphene quantum dot–Hg(II) system, was developed for cysteine detection. An easy, green, one-pot synthesis of nitrogen-doped graphene quantum dots was established by using citric acid and urea as carbon and nitrogen sources, respectively. The fluorescence of nitrogen-doped graphene quantum dots was significantly quenched by Hg(II) because of the efficient electron transfer between nitrogen-doped graphene quantum dots and Hg(II). Subsequently, fluorescence was recovered gradually upon cysteine addition to form a stable complex with Hg(II). The fluorescence sensor showed a response to cysteine within a wide concentration range of 0.05–30 μmol L−1, with a detection limit of 1.3 nmol L−1. The sensor was successfully applied to detect cysteine in honey and beer samples, with a recovery range of 98%–105%.

Keywords: Nitrogen-doped graphene quantum dots; Hg(II); Fluorescence sensor; * Corresponding author. Fax: (86) 357-2051070. E-mail: [email protected].

Cysteine 1. Introduction The development of graphene quantum dots with distinct properties and wide-ranging

applications

in

electrochemical

luminescence,

bioimaging,

photocatalysis, and ion detection is attracting increased attention [1–5]. From the visible to the near-infrared region, diverse sizes of graphene quantum dots with different photoluminescence colors have been adopted by various synthetic "top-down" and "bottom-up" methods. The graphene quantum dots show satisfactorily low toxicity, good biocompatibility, stable photoluminescence, excellent solubility, and better surface grafting, which make them promising in optoelectronic devices, bioimaging, and sensors [6]. Sensor, which has gained considerable attention, is designed based on the distinctive properties of diverse nanomaterials in linking with natural or artificial molecular recognition units [7]. Doping heteroatoms, such as boron and nitrogen, in graphene and carbon nanotubes have yielded results that are of academic interest and practical value [8–12]. Graphene quantum dot modification has significant application in the field of analysis and biological imaging. Photoluminescent sensors have showed the more important position because of operational simplicity, real-time detection, high-throughput process, and good sensitivity [13]. Cysteine, a water-soluble compound, generally exists in many food and biological fluids; this compound also plays a crucial role in many life processes. Cysteine, a low-molecular weight thiols, is also often applied in reversible redox

reactions and cellular functions, including metabolism and detoxification [14–17]. Cysteine at an abnormal level is often related to many diseases. Amino acids often coexist in food and biological samples and have very similar properties, thus to establish a rapid and sensitive method for detecting cysteine is necessary. Current detection assays mainly include high-performance liquid chromatography [18] and capillary electrophoresis [19] through the combination of an efficient separation technique. However, some defects may occur to some extent while employing these methods, such as tedious sample preparation and sophisticated instrumentation. To date, the utilization of nitrogen-doped graphene quantum dot–Hg(II) system as a sensor in the detection of cysteine has not been reported. In the present study, an easy one-pot solid-phase synthesis strategy is adopted to obtain nitrogen-doped graphene quantum dots with a high quantum yield using citric acid and urea as carbon and nitrogen sources, respectively; based on this synthesis, we report a turn-on fluorescent sensor for a selective detection of cysteine on recovered Hg(II)-modulated nitrogen-doped graphene quantum dot fluorescence. Compared with previous reports [20–22], this method showed a lower detection limit, but is simpler, faster, inexpensive, and more efficient than the other methods. The high affinity of cysteine to Hg(II) enables the dissociation of the ion from the surface of nitrogen-doped graphene quantum dots, thereby forming a stable complex with cysteine in the solution, and recovering the fluorescence of the nitrogen-doped graphene quantum dots. The developed nitrogen-doped graphene quantum dot-based photoluminescent sensor was successfully applied in the detection of cysteine in real

sample. 2. Experimental 2.1. Materials Citric acid and all natural amino acids were purchased from Guangfu Chemicals Company (Tianjin, China), urea was purchased from Haohua Chemicals Company (Luoyang, China). NaH2PO4, Na2HPO4 and HgCl2 were purchased from Beijing Chemical Factory (Beijing, China). In addition to amino acids as biochemical reagents, other reagents are analytically pure. Ultrapure water with a resistivity of 18.2 MΩ cm-1 was obtained from the United States Milli-Q purification system preparation. 2.2. Instrument and characterization Fluorescence spectroscopy was recorded on a LS-55 luminescence spectrometer (Perkin Elmer, USA). UV-visible absorption spectra were performed on a Cary 300 (Varian, USA) UV-visible spectrophotometer. The transmission electron microscope measurements were carried out by using a Tecnai G2 F20 (FEI, USA) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy measurements were performed on a ESCALAB 250Xi (Thermo Scientic, USA) spectrometer equipped with monochromatized Al Kα excitation. 2.3. Preparation of water-soluble nitrogen-doped graphene quantum dots Water-soluble nitrogen-doped graphene quantum dots were synthesized according to previous work [1]. 0.21 g (1 mmol) citric acid and 0.18 g (3 mmol) urea were dissolved into 5 mL water, and stirred to form a clear solution. Then the solution

was transferred into a 20 mL Teflon-lined stainless autoclave. The sealed autoclave was heated to 160 ℃ in an electric oven and kept for additional 4 h. The final product was collected by adding ethanol into the solution and centrifuged at 5000 rpm for 5 min. The solid can be easily redispersed into water. 2.4. Analytical procedures To evaluate the quenching effect of Hg(II) on the fluorescence intensity of nitrogen-doped graphene quantum dots was performed at room temperature in phosphate buffered saline buffer (pH 5.0). In a typical, 0.5 mL of nitrogen-doped graphene quantum dots (100 μg mL-1) was added into a 10 mL calibrated test tube solution, subsequently, 0.25 mL of 0.2 mol L-1 phosphate buffered saline buffer solution and Hg(II) standard solution were added to the mixture was further diluted to 5 mL with ultrapure water. The solution was mixed thoroughly and equilibrated for 10 min to measure the fluorescence intensity. To study the effect of cysteine on the fluorescence restoration of the Hg(II)-modulated nitrogen-doped graphene quantum dots, 0.5 mL of 100 μg mL-1 nitrogen-doped graphene quantum dots solution, 0.25 mL of 0.2 mol L-1 phosphate buffered saline buffer solution and 10 μmol L -1 of Hg(II) were sequentially added into a 10 mL calibrated test tube, shaken thoroughly and left for 10 min. Then, cysteine standard solution was added to the mixture was further diluted to 5 mL with ultrapure water. The solution was mixed thoroughly and left for another for 10 min to measure the fluorescence intensity. 3. Results and discussions

3.1. Characterization of nitrogen-doped graphene quantum dots The as-prepared nitrogen-doped graphene quantum dots solution exhibited a long-term homogeneous phase without any noticeable precipitation at room temperature. Fig. 1 shows the transmission electron microscopy image of the monodispersed nitrogen-doped graphene quantum dots. Fig. 2 presents the images of the as-prepared nitrogen-doped graphene quantum dot size distributions with diameters of mainly 4.7 ± 0.6 nm. The nitrogen-doped graphene quantum dots were characterized by X-ray photoelectron spectroscopy, which clearly shows the presence of N besides the original carbon and oxygen (Fig. 3a). The X-ray photoelectron spectroscopy spectrum of nitrogen-doped graphene quantum dots shows three peaks around 284.8, 401.4, and 531.5 eV, which are attributed to C 1s, N 1s, and O 1s, respectively. Moreover, on the basis of the peak intensities of carbon and nitrogen, the doping concentration of nitrogen was calculated to be 36.5%. The two peaks at 284.8 and 288.3 eV in the C 1s photoelectron spectrum (Fig. 3b) can be assigned to the binding energy of carbon in C–C, and C=N/C=O, respectively. The high-resolution N 1s spectrum of the nitrogen-doped graphene quantum dots shows the peaks at 401.4 eV (Fig. 3c), which are attributed to the pyrrolic N (C–N–C) and graphitic N or N–H bands, respectively. Thus, the primary amine molecules play dual roles in the hydrothermal process: as the precursor for N-dopant and passivation agent, which both significantly contribute to the fluorescence enhancement of graphene quantum dots. Fig. 4 shows the UV–visible absorption of nitrogen-doped graphene quantum dots. The graphene

quantum dots exhibit a typical absorption at 360 nm. However, the nitrogen-doped graphene quantum dots absorption band is centered at 334 nm, which has a blueshift of 26 nm compared with that of the graphene quantum dots. These results indicate the successful incorporation of nitrogen atoms into the graphene quantum dots by the present synthetic process. 3.2. Hg(II)-modulation of the fluorescence of nitrogen-doped graphene quantum dots The fluorescence reduction of nitrogen-doped graphene quantum dots by Hg(II) was investigated systematically. Fig. 5 shows that the fluorescence intensity of nitrogen-doped graphene quantum dots is obviously reduced upon increasing the Hg(II) concentration from 0.5 μmol L−1 to 25 μmol L−1. When Hg(II) solutions were added, the unchanged fluorescence emission positioned. The fluorescence was almost totally quenched in the presence of 25 μmol L−1 Hg(II). A good linear relationship was observed between the fluorescence intensity ratio (F/F0) and Hg(II) concentration (CHg(II), μmol L−1) within the range of 0.5–10 μmol L−1 of Hg(II). The linear regression equation is as follows: F/F0 = 0.76836 ‒ 0.06784 CHg(II), (R2 = 0.99467)

(1)

The photoluminescence reduction of quantum dots by transition metal ions is a complicated process, and two main mechanisms have been advanced to explain this process: (1) electron transfer from the photo-excited quantum dots to cations bound at its surface; (2) formation of new non-radiative surface channels for electron annihilation,

which

effectively

competes

with

the

radiative

electron–hole

recombination within the quantum dots. The reduction was concluded to be dynamic

in nature. Both the quantum size effect and surface defects were speculated to contribute to the fluorescence of nitrogen-doped graphene quantum dots, and the luminescence emission of the nitrogen-doped graphene quantum dots arises from the radiative recombination of excitons [23]. Hg(II) was adsorbed onto the surface of nitrogen-doped graphene quantum dots as a surface modifier; Hg(II) was also coordinated with the organic functional groups, which changed the surface traps or electron–hole recombination annihilation via electron or energy transfer process [24,25]. Therefore, the fluorescence could be quenched efficiently. Consequently, we used 10 μmol L−1 of Hg(II) for the subsequent studies. 3.3. Effect of pH The pH of the solution affected not only the fluorescent properties of pure nitrogen-doped graphene quantum dots solution [23], but also the subsequent fluorescence recovery by cysteine. Fig. 6 shows the pH-dependent Hg(II)-induced fluorescence-quenched efficiency (Effq) and fluorescence-recovered efficiency by cysteine (Effr) in the phosphate buffered saline buffer (10 mmol L-1 ). The efficiencies were calculated by the following equations, respectively: Effq (%) = (F0/F)/F0

(2)

Effr (%) = (Fr/F)/(F0 ‒ F)

(3)

where F and F0 represent the fluorescence intensities of nitrogen-doped graphene quantum dots at 445 nm in the presence and absence of Hg(II), respectively; Fr is the recovered fluorescence intensity of nitrogen-doped graphene quantum dots at 445 nm in the presence of cysteine at the same pH.

When 10 μmol L−1 of Hg(II) and nitrogen-doped graphene quantum dot solution was added into different phosphate buffered saline buffer solutions (10 mmol L−1), the Effq decreased with the increase of pH from 5.0 to 8.5 and subsequently decreased. Similarly, Effr decreased gradually in the presence of 10 μmol L−1 of cysteine with pH increase. At a low pH (pH 5.0), Hg(II) presented the highest Effq (85.3%), whereas the Effr was 58.3% of 10 μmol L−1 of cysteine. Therefore, the pH set at 5.0 is appropriate for the nitrogen-doped graphene quantum dot–Hg(II) system of cysteine fluorescence restoration. 3.4. Cysteine-induced fluorescence restoration of nitrogen-doped graphene quantum dots-Hg(II) System α-amino acids are strong Hg(II) chelators [20,21,24,26]. In the presence of cysteine, the fluorescence recovery is attributed to the strong binding preference between Hg(II) and thiol groups, namely, cysteine; cysteine could form stable complexes with Hg(II), and thus Hg(II) is dissociated from the surface of nitrogen-doped graphene quantum dots [21,26,27]. The fluorescence intensity of pure nitrogen-doped graphene quantum dots has a negligible change in the presence of cysteine, and the fluorescence recovery only results from forming Hg(II)–(cysteine)n complexes. The results indicate that Hg(II)-induced fluorescence reduction and subsequent cysteine-induced fluorescent recovery provide a concrete basis for the development of a nitrogen-doped graphene quantum dot-based turn-on fluorescent sensor for detecting cysteine. Fig. 7 shows the fluorescence spectra of the nitrogen-doped graphene quantum

dot–Hg(II) system with different cysteine concentrations added. The fluorescence of the system would be recovered with the increase of cysteine concentration. A good linear relationship was observed between the fluorescence intensity ratio (F/F1) and cysteine concentration (Ccysteine, μmol L−1) within the range of 0.05–30 μmol L−1 for cysteine. The linear regression equation is as follows: F/F1

=

0.16341

+

0.02885

Ccysteine

(5) F is the fluorescence intensity of the nitrogen-doped graphene quantum dot–Hg(II) (10 μmol L−1) system in the presence of cysteine, and F1 is the fluorescence intensity of pure nitrogen-doped graphene quantum dots. The corresponding correlation coefficient of calibration curve is 0.9969, and the sensor detection limit is 1.3 nmol L−1. 3.5. Selectivity of the proposed turn-on fluorescence sensor for cysteine To further verify the applicability of the proposed fluorescence sensor for detecting cysteine in practical applications, the effect of a series of amino acid on the fluorescence based on the present sensor was investigated (Fig. 8). Cysteine exhibited a significant enhancing effect on the fluorescence intensity of Hg(II)-modulated nitrogen-doped graphene quantum dot fluorescence sensor. Under 10 μmol L−1 of cysteine, the fluorescence intensity change of nitrogen-doped graphene quantum dots was not affected by a 1000 μmol L−1 of methionine, tryptophan, and lysine; and 2000 μmol L−1 of glutamic acid, tyrosine, phenylalanine, and proline. Other amino acids did not affect the fluorescence restoration of 4000 μmol L−1.

3.6 Detection of cysteine in real sample The proposed turn-on fluorescence sensor based on nitrogen-doped graphene quantum dots modulated with Hg(II) was applied for the determination of cysteine in beer and honey in analyzing real samples. Beer and honey were bought from a local supermarket. About 0.5 g of honey was dissolved in 10 mL of the ultrapure water, filtered, and diluted at about 100-fold before analysis. Beer was directly filtered and subsequently diluted at about 100-fold for analysis. Table 1 shows that the recoveries of spiked cysteine in the diluted samples are within the range of 98%–105%. Table 1 Analytical results for cysteine in honey and beer. Type of samples

Honey

Beer

Found without spiking (μM)

Spiked (μM)

Found (μM)

0.45 0.45 0.45 1.29 1.29 1.29

3.0 5.0 7.0 2.5 4.5 6.0

3.52 ± 0.10 5.36 ± 0.16 7.40 ± 0.14 3.91 ± 0.11 5.84 ± 0.25 7.16 ± 0.19

Relative Recovery standard (%) deviation (n=3, %) 102 98 99 105 101 98

2.7 2.9 1.9 2.9 4.2 2.7

4. Conclusion In summary, nitrogen-doped graphene quantum dots were synthesized via one-pot solid-phase synthesis strategy. Subsequently, a turn-on fluorescence sensor for cysteine was developed based on nitrogen-doped graphene quantum dots modulated with Hg(II); the advantages of a well-known Hg(II)–cysteine affinity pair and fluorescence properties of nitrogen-doped graphene quantum dots were considered. The turn-on fluorescence sensor enabled rapid detection of cysteine with high sensitivity and selectivity. The linear range is 0.05–30 μmol L−1, and the

detection limit is 1.3 nmol L−1. The present study provides a simple, low-cost, and highly efficient route for the production of nitrogen-doped graphene quantum dots for sensing; this sensor can be used as a promising candidate for the determination of cysteine in food industry and diagnostic application.

References [1] S.N. Baker, G.A. Baker, Angew. Chem. Int. Ed. 49 ( 2010) 6726. [2] L. Cao, M.J. Meziani, S. Sahu, Y.P. Sun, Acc. Chem. Res. 46 (2012) 171. [3] H. Li, Z. Kang, Y. Liu, S.T. Lee, J. Mater. Chem. 22 (2012) 24230. [4] L. Li, G. Wu, G.Yang, J. Peng, J. Zhao, J.J. Zhu, Nanoscale 5 (2013) 4015. [5] D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie, Z. Sun, Nanoscale 5 ( 2013) 12272. [6] J.J. Liu, Z T. Chen, D.S. Tang, Y.B. Wang, L.T. Kang, J.N. Yao, Sens. Actuators B 212 ( 2015) 214. [7] P. Wu, X.P. Yan, Biosens. Bioelectron. 26 (2010) 485. [8] S. Dey, A. Govindaraj, K. Biswas, C. Rao, Chem. Phys. Lett. 595 ( 2014) 203. [9] K. Gopalakrishnan, A. Govindaraj, C. Rao, J. Mater. Chem. A 1 ( 2013) 7563. [10] L. Panchakarla, A. Govindaraj, C. Rao, Inorg. Chim. Acta 363 (2010) 4163. [11] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, Science 324 ( 2009) 768. [12] Y. Wang, Y. Shao, D.W. Matson, J. Li, Y. Lin, ACS Nano. 4 ( 2010) 1790. [13] J. Hou, F. Zhang, X. Yan, L. Wang, J. Yan, H. Ding, L. Ding, Anal. Chim. Acta.

859 (2015) 72. [14] S. Chen, J. Tian, Y. Jiang, Y. Zhao, J. Zhang, S. Zhao, Anal. Chim. Acta. 787 ( 2013) 181. [15] S.K. Sun, H.F. Wang, X.P. Yan, Chem. Commun. 47 ( 2011) 3817. [16] Z.Yao, H. Bai, C. Li, G. Shi, Chem. Commun. 47 (2011) 7431. [17] M. Zhang, M. Yu, F. Li, M. Zhu, M. Li, Y. Gao, L. Li, Z. Liu, J. Zhang, D. Zhang, J. Am. Chem. Soc. 129 (2007) 10322. [18] W. Sawuła, Z. Banecka-Majkutewicz, L. Kadziński, J. Jakóbkiewicz-Banecka, G. Wegrzyn, W. Nyka, B. Banecki, Acta Biochim. Pol. 55 ( 2008) 119. [19] A. Zinellu, S. Sotgia, A.M. Posadino, V. Pasciu, M.G. Perino, B. Tadolini, L. Deiana, C. Carru, Electrophoresis 26 (2005) 1063. [20] B. Han, J.Yuan, E. Wang, Anal. Chem. 81 (2009) 5569. [21] K.S. Park, M.I. Kim, M.A. Woo, H.G. Park, Biosens. Bioelectron. 45 (2013) 65. [22] Y. Zhang, Y. Li, X.P. Yan, Anal. Chem. 81 (2009) 5001. [23] J. Hou, J. Yan, Q. Zhao, Y. Li, H. Ding, L. Ding, Nanoscale 5 ( 2013) 9558. [24] X. Qin, W. Lu, A.M. Asiri, A.O. Al-Youbi, X. Sun, Sens. Actuators B 184 (2013) 156. [25] S. Zhu, J. Zhang, C. Qiao, S. Tang, Y. Li, W. Yuan, B. Li, L. Tian, F. Liu, R. Hu, Chem. Commun. 47 ( 2011) 6858. [26] X. Jia, J. Li, E. Wang, Chem. Eur. J. 18 ( 2012) 13494. [27] L. Zhou, Y. Lin, Z. Huang, J. Ren, X. Qu, Chem. Commun. 48 (2012) 1147.

TABLE CAPTIONS: Table 1 Analytical results for cysteine in honey and beer.

FIGURE CAPTIONS: Fig. 1. Transmission electron microscopy image of the as-prepared nitrogen-doped graphene quantum dots. Fig. 2. Size distribution of the nitrogen-doped graphene quantum dots. Fig. 3. X-ray photoelectron spectroscopy spectra of C 1s (a), N 1s (b) and O 1s (c) of the nitrogen-doped graphene quantum dots. Fig. 4. UV-visible absorption spectra of (a) the nitrogen-doped graphene quantum dots and (b) the graphene quantum dots . Fig. 5. Fluorescence spectra of 10 μg mL−1 nitrogen-doped graphene quantum dots in the presence of corresponding Hg(II) concentrations as follows: (a) 0, (b) 0.5, (c) 1.0, (d) 2.0, (e) 4.0, (f) 6.0, (g) 8.0, (h) 10.0, (i) 12, (j) 15, (k) 20, and (l) 25 μmol L−1. All solutions were prepared in 10 mmol L−1 of phosphate-buffered saline buffer at pH 5.0. Fig. 6. pH effect on (a) the nitrogen-doped graphene quantum fluorescence reduction efficiency by 10 mmol L−1 Hg(II) and (b) nitrogen-doped graphene quantum dot–Hg(II) system fluorescence recovered efficiency in the presence of 10 mmol L−1 of cysteine; all solutions were prepared in 10 mmol L−1 of phosphate-buffered saline buffer at pH 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, and 8.5. Fig. 7. Fluorescence spectra of 10 μmol L−1 of Hg(II)-modulated nitrogen-doped

graphene quantum dot system in the presence of corresponding cysteine concentration as follows: (a) 0, (b) 0.05, (c) 1.0, (d) 2.0, (e) 4.0, (f) 8.0, (g) 12.0, (h) 16.0, (i) 22.0, (j) 26.0, and (k) 30.0 μmol L−1. All solutions were prepared in 10 mmol L−1 of phosphate-buffered saline buffer at pH 5.0. Fig. 8. Response of 10 μmol L−1 of Hg(II)-modulated nitrogen-doped graphene quantum dot system in phosphate-buffered saline buffer (10 mmol L−1, pH 5.0) to 20 natural amino acids (10 μmol L−1).

Table 1 Comparison of linear range and detect limit for cysteine using different fluorescent probes.

Table 2 Analytical results for cysteine in honey and beer.

Samples

Found without spiking (µmol L−1)

Spiked (µmol L−1)

Found (µmol L−1)

Recovery (%)

RSD (n=3, %)

0.45 0.45 0.45 1.29

3.0 5.0 7.0 2.5

3.52 ± 0.10 5.36 ± 0.16 7.40 ± 0.14 3.91 ± 0.11

102 98 99 105

2.7 2.9 1.9 2.9

1.29

4.5

5.84 ± 0.25

101

4.2

1.29

6.0

7.16 ± 0.19

98

2.7

Honey

Beer

Scheme 1. Schematic illustration of the developed turn-on fluorescence sensor based on nitrogen-doped graphene quantum dots modulated with Hg(II) for detection cysteine.

graphical abstract

Fig. 1. Transmission electron microscopy image and size distribution of the as-prepared nitrogen-doped graphene quantum dots.

Fig. 2. X-ray photoelectron spectroscopy spectra of C 1s (a), N 1s (b) and O 1s (c) of the nitrogen-doped graphene quantum dots.

Fig. 3. UV-visible absorption and emission spectra of the nitrogen-doped graphene quantum dots (a) and the graphene quantum dots (b).

Fig. 4. Fluorescence spectra of 10 μg mL−1 nitrogen-doped graphene quantum dots in the presence of corresponding Hg(II) concentrations as follows: (a) 0, (b) 0.5, (c) 1.0, (d) 2.0, (e) 4.0, (f) 6.0, (g) 8.0, (h) 10.0, (i) 12, (j) 15, (k) 20, and (l) 25 μmol L−1. All solutions were prepared in 10 mmol L−1 of phosphate-buffered saline buffer at pH 5.0.

Fig. 5. pH effect on (a) the nitrogen-doped graphene quantum fluorescence reduction efficiency by 10 µmol L−1 Hg(II) and (b) nitrogen-doped graphene quantum dot–Hg(II) system fluorescence recovered efficiency in the presence of 10 µmol L−1 of cysteine; all solutions were prepared in 10 mmol L−1 of phosphate-buffered saline buffer at pH 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, and 8.5.

Fig. 6. Fluorescence spectra of 10 μmol L−1 of Hg(II)-modulated nitrogen-doped graphene quantum dot system in the presence of corresponding cysteine concentration as follows: (a) 0, (b) 0.05, (c) 1.0, (d) 2.0, (e) 4.0, (f) 8.0, (g) 12.0, (h) 16.0, (i) 22.0, (j) 26.0, and (k) 30.0 μmol L−1. All solutions were prepared in 10 mmol L−1 of phosphate-buffered saline buffer at pH 5.0.

Fig. 7. Response of 10 μmol L−1 of Hg(II)-modulated nitrogen-doped graphene quantum dot system in phosphate-buffered saline buffer (10 mmol L−1, pH 5.0) to 20 natural amino acids (10 μmol L−1).