Highly sensitive fluorescence sensor for mercury(II) based on boron- and nitrogen-co-doped graphene quantum dots

Journal Pre-proofs Highly sensitive fluorescence sensor for mercury(II) based on boron- and nitrogen-co-doped graphene quantum dots Zhenyu Liu, Zunli Mo, Xiaohui Niu, Xing Yang, Yangyang Jiang, Pan Zhao, Nijuan Liu, Ruibin Guo PII: DOI: Reference:

S0021-9797(20)30106-5 https://doi.org/10.1016/j.jcis.2020.01.092 YJCIS 25965

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

9 November 2019 22 January 2020 24 January 2020

Please cite this article as: Z. Liu, Z. Mo, X. Niu, X. Yang, Y. Jiang, P. Zhao, N. Liu, R. Guo, Highly sensitive fluorescence sensor for mercury(II) based on boron- and nitrogen-co-doped graphene quantum dots, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.092

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Highly sensitive fluorescence sensor for mercury(Ⅱ) based on boronand nitrogen-co-doped graphene quantum dots Zhenyu Liu a, Zunli Mo , *, Xiaohui Niu a, Xing Yang a, Yangyang Jiang a,Pan Zhao a, Nijuan Liu a, Ruibin Guo a (a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Research Center of Gansu Military and Civilian Integration Advanced Structural Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070 China)

Abstract: In this work, we invented a new method to synthesize boron- and nitrogenco-doped graphene quantum dots (B, N-GQDs). This synthesis method was more reliable, simple, and environmentally friendly than the bottom-up methods reported in the current literature that use citric acid or other organic materials as the carbon source. Instead, B, N-GQDs were synthesized from graphene oxide (GO) by a top-down method. The obtained B, N-GQDs showed a bright blue luminescence under a UV lamp at 365 nm, and the absolute photoluminescence quantum yield (PLQY) was 5.13%. Because mercury(Ⅱ) ions (Hg2+) are highly toxic and can seriously affect the ecological environment and human health, the detection of low-concentration Hg2+ and the establishment of rapid, accurate and sensitive methods for detecting Hg2+ are of great significance to the activities of life, including those of humans. Therefore, using the good luminescence properties of B, N-GQDs, we invented a new type of fluorescence sensor that can detect Hg2+. The sensor was very sensitive to Hg2+, had a good linear relationship in the concentration range of 0.2~1 μM, and exhibited a minimum limit of detection of 6.4 nM. Furthermore, this approach was successfully applied to the detection of Hg2+ in real environmental water samples.

Keywords: Boron- and nitrogen-co-doped graphene quantum dots; Graphene oxide; Absolute photoluminescence quantum yield; Fluorescence sensor; Mercury ions

Zunli Mo [email protected] a

Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, and Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China.

1. Introduction Mercury is one of many heavy metals. However, unlike other heavy metals, mercury is the only metal in nature that exists in liquid form at ambient temperature. Mercury has a very low melting point, melting as the temperature increases and evaporating to form mercury vapor. The spread of mercury vapor in water, soil, air and other media can cause serious environmental pollution [1~3] and human health issues [4], and it is difficult to eliminate; therefore, the best way is to control the source of the pollution. In view of this situation, it is urgent to develop an effective method to detect mercury ions efficiently, quickly and sensitively and to understand its toxicity mechanism [5]. There are many familiar techniques for detecting mercury ions, including electrochemistry [6~9], atomic absorption [10~12], ICP-MS [13, 14], and fluorescence sensor methods. In addition, emerging photoelectrochemical (PEC) methods have also attracted great interest from researchers in environmental monitoring. These methods have a low background noise and high sensitivity and provide a new platform for the fast and accurate monitoring of related pollutants. The use of electronic readings render PEC instruments simple, cost-effective and miniaturized [15]. However, any technology has its advantages and disadvantages, for example, although the PEC approach has a good sensitivity and low detection limit, it has difficulty achieving the simultaneous determination of multiple elements, the instrument is expensive, and it necessitates high operator requirements.To summarize, this article used a fluorescence sensor method to monitor mercury ions because this method presented a wide linear response range, high sensitivity and selectivity [16], and the simultaneous detection of multiple heavy metal elements. In addition, the fluorescence spectrophotometer instrument and equipment required by this method were relatively inexpensive and could simultaneously detect solid and liquid samples, with simple sample processing. This technology belongs among the most researched detection methods and is expected to be widely used. In recent years, GQDs have become a newcomer to the carbon material family and is in many fluorescent sensor materials, and GQDs have attracted the attention of many

researchers. Due to the synthetic elements of traditional quantum dots (QDs), this nanomaterial poses a risk to biosafety and the environment [17]. However, GQDs have the advantages of a low biotoxicity, biocompatibility, good water solubility, and the regulation of fluorescence and light stability. In addition, GQDs have a very stable structure, corrosion resistance to light, and do not contain highly toxic metal elements; they are environmentally friendly materials. Therefore, these properties suggest that GQDs can be of benefit in the fields of biological imaging and sensors. There are also certain novel nanomaterials, such as mixtures of GQDs, that can also play an important role in sensors and other applications. Yola et al. developed a voltammetric sensor for the simultaneous determination of ascorbic acid, dopamine, uric acid and tryptophan on a rod gold-platinum nanoparticles (rAu-PtNPs) /GQDs/glassy carbon electrode (GCE) [18]. In addition, Yola et al. developed a new imprinted sensor based on GQDs incorporating two-dimensional hexagonal boron nitride nanosheets and applied it to serotonin detection in urine samples [19]. Akyıldırım et al. developed a new electrochemical

sensor,

palladium-nanoparticle-functionalized

GQDs

with

a

molecularly imprinted polymer and applied it to citrinin detection [20]. Yola et al. developed a molecularly imprinted voltammetry sensor based on glassy carbon electrodes modified by carbon nitride nanotubes decorated with GQDs for the determination of chlorpyrifos [21]. Çolak et al. developed 3D polyoxometalatefunctionalized GQDs with mono-metallic and bi-metallic nanoparticles that could be used directly in methanol fuel cells [22]. It can be seen that the applications of GQDs and their mixtures are very extensive, which further illustrates their importance. In this article, we focused on the optical properties of GQDs to develop a fluorescent sensor to detect mercury ions. Overall, we know that how to synthesize GQD is the most important thing. At present, there are two approaches for synthesizing GQDs, top-down and bottom-up methods. This work used a top-down synthesis method to synthesize B, N-GQDs. From the existing literature reports, we know that Yang et al. synthesized B, N-GQDs using citric acid, ethylenediamine, and phenylboronic acid through a one-step hydrothermal method, and the product was used for the detection of Hg2+ with a minimum detection limit of 0.16 μM [23]. Ye et al. synthesized B, N-CQDs from three

borates, and the minimum detection limit for Hg2+ was 7.3 nM [24]. It can be clearly seen that there are very few synthesis methods for B, N-GQDs or B, N-CQDs, and the resultant minimum detection limits for Hg2+ were higher than that of the B, N-GQDs synthesized in this work. The previous syntheses were all bottom-up methods, and there were no reports on the synthesis of B, N-GQDs using the top-down method. There were also some fluorescent sensors for Hg2+ detection, such as S, N-GQDs [5, 25], N-GQDs [26]. Although their synthesis methods were relatively simple and the products were also used for the detection of Hg2+, there were too many synthesis methods regarding nitrogen doping and nitrogen-sulfur co-doping, which were not as innovative as the B, N-GQDs in this work. Therefore, the method we invented has a very good novelty. This method of synthesis was simple, green, environmentally friendly and low cost. The synthesized B, N-GQDs were very sensitive to Hg2+ with a minimum detection limit of 6.4 nM. Additionally, this minimum detection limit was lower than many other GQDsbased fluorescent sensors. In addition, this method was successfully applied to the detection of Hg2+ in water samples from real environments. The synthesized B, NGQDs exhibited many other excellent properties, so it has very broad application prospects for use in other fields such as biological cells and optoelectronic devices. In this paper, we synthesized B, N-GQDs using a top-down approach. Specifically, the prepared GO, boric acid and ammonia solution were used as raw materials to synthesize B, N-GQDs by a one-pot hydrothermal method (Scheme 1). The synthesized B, N-GQDs had a very good sensitivity and rapid response for Hg2+ detection. From Scheme 2, we can see that B, N-GQDs showed a bright blue luminescence under a 365 nm UV lamp, and the fluorescence was significantly quenched when Hg2+ was added. The quenching mechanism of the B, N-GQDs-Hg2+ system was due to the inner filter effect (IFE) and static quenching between B, N-GQDs and Hg2+. Additionally, the Hg2+ detection had a good linear relationship in the range of 0.2~1 μM. In addition, B, NGQDs were successfully applied to the detection of Hg2+ in real water samples as a fluorescent sensor.

2. Experimental section

2.1. Materials Expandable graphite, H3BO3, an ammonia solution (25~28%), KCl and CaCl2 were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). CuCl2, AlCl3, FeCl2, BaCl2 and C4H6O4Zn were obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). H3PO4 was obtained from HongYan Chemical Reagent Factory (Tianjin, China). H2O2, NaCl and NiCl2·6H2O were obtained from Xi'an Chemical Reagent Factory. CoCl2·6H2O, PbC4H6O4·3H2O, SnCl4·5H2O, and CdCl2·5/2H2O were purchased from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). FeCl3 was obtained from Xilong Chemical co., Ltd. (Shantou, China). Cr(CH3COO3)3 was obtained from Suzhuang Chemical Reagent Factory (Tianjin, China). MgCl2·6H2O was obtained from Shuangshuang Chemical Co., Ltd. (Yantai, China). MnCl2·4H2O was obtained from Tianjin Kaixin Chemical Industry Co., Ltd. All of the materials used above were of analytical grade, and they were not further purified prior to use. 2.2. Characterizations With an acceleration voltage of 200 KV, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were acquired by using field-emission transmission electron microscopy (FEI TECNAI G2 F20, USA). The sample was prepared by dropping a B, N-GQD dispersion in ultrapure water onto an ultra-thin microgrid and letting the water evaporate. The Raman spectrum was measured in the spectral range of 800 to 2500 cm-1 using a HORIBA LabRAM HR Evolution manufactured by Jobin Yvon S.A.S., France. X-ray diffraction (XRD) was measured using a D8 Advance X-ray powder diffractometer (Bruker, Germany), operated at 40 mA and 40 kV. The sample was scanned in the 2θ range of 10~80°, and Cu Kα provided a source of radiation for experimental measurements. The Fourier transform infrared (FT-IR) spectrum was obtained by compressing KBr powder and the sample into a sheet and placing it in an EQUINOX 55 FT-IR spectrometer manufactured by BRUKER, USA. The wavenumber range was from 4000 cm-1 to 400 cm-1. X-ray photoelectron spectroscopy (XPS) spectra were acquired using an Escalab 250 xi photoelectron spectrometer (Thermo, USA). Ultraviolet-visible (UV-vis) absorption spectra were recorded using a UV-2550 spectrophotometer (Shimadzu, Japan). Fluorescence spectra:

excitation and emission photoluminescence spectra and PLQY were measured using a FluoroMax-4 Compact Spectrofluorometer (HORIBA, USA). 2.3. Preparation of GO First, an appropriate amount of expandable graphite was heated in a muffle furnace at 800 °C to obtain expanded graphite. The expanded graphite was poured into a DMF solution, ultrasonically dispersed, washed repeatedly with ethanol and water, and vacuum dried at 60 °C to obtain the liquid phase of ultrasonically exfoliated graphene. Using the improved Hummers’ method [27], the typical steps were as follows. The first step of pre-oxidation: 1.2 g of K2S2O8 and 1.2 g of P2O5 were added to 12 mL of concentrated H2SO4, then 1 g of liquid-phase ultrasonically exfoliated graphene was added, and the mixed solution was heated to 80 °C under magnetic stirring and refluxed for 5 h. After cooling, the mixed solution was diluted with 200 mL of ultrapure water, washed to neutrality, and dried under vacuum at 60 °C. The second step was a further oxidation process: the above dried graphite oxide was weighed to 1 g and dispersed in 120 mL of mixed concentrated H2SO4 and H3PO4 acids (volume ratio of 3:1) under ice bath conditions (0 to 5 °C), following which 9 g of KMnO4 was continuously added with stirring. The temperature was then raised to 50 °C and stirred for 12 h. After the temperature of the system was cooled to room temperature, 200 mL of ultrapure water and 5 mL of 30% H2O2 were added, individually. After stirring, the mixed solution turned bright yellow, and finally, the product was washed to neutrality and dried to obtain GO. 2.4. Synthesis and formation mechanism of B, N-GQDs The synthesis of B, N-GQDs was a top-down method in which GO, boric acid and an ammonia solution underwent a one-pot hydrothermal reaction in a Teflon-lined autoclave. First, 30 mg of GO (1 mg·mL-1) was weighed, dissolved in 30 mL of ultrapure water, and sonicated for 10 min to obtain a uniformly dispersed GO solution. Then, 1 mL of NH3·H2O (25~28%) and 30 mg of H3BO3 were added to the GO solution, and the solution was completely mixed by sonication for 1 min. Next, the above mixed solution was charged into a Teflon-lined autoclave and heated at 180 °C for 20 h. After the reaction was completed, the supernatant, B, N-GQDs, and the sediment, reduced

graphene oxide (RGO), were filtered through a 0.22 μm microfiltration membrane to obtain a clear aqueous solution of B, N-GQDs. The colour of the aqueous solution of these B, N-GQDs was slightly yellowish (Scheme 1). Finally, the obtained aqueous solution of B, N-GQDs was dialyzed for two days in a dialysis bag with a cut-off amount of 3000 Da to remove excess ammonia. The purified B, N-GQD aqueous solution was collected and freeze-dried to obtain a powder of B, N-GQDs. According to this synthetic method, approximately five to six milligrams of B, N-GQDs could be collected each time before dialysis. When dialysis was performed, the quality of the obtained B, N-GQDs depended on the time and water used for dialysis. A poor grasp of the water volume and time inevitably caused unnecessary losses. In this synthesis method, the very important carbon source for the synthesis of B, N-GQDs was GO, which was further oxidized with strong acids and strong oxidants. In this way, rich oxygen-containing functional groups and carboxyl groups were introduced at the edge of GO, and GO could be further cut and peeled, resulting in the smaller size of B, N-GQDs obtained in the subsequent one-pot hydrothermal method. In addition, H3BO3 was used to provide a boron source, and NH3·H2O not only provided a nitrogen source but also created good alkaline conditions that could be used to reduce GO carbon precursors [28, 29]. We clearly knew that GO still maintained the original graphite sheet structure. When the above NH3·H2O and H3BO3 were introduced, after the ultrasonic treatment, for the high-temperature and high-pressure hydrothermal reaction, the large-sized GO carbon precursor could be reduced to small-sized nanoscale B, N-GQDs. In the end, we obtained B, N-GQDs. Through a series of detailed subsequent characterizations, we could further indicate that boron and nitrogen were successfully doped into the framework of GQDs. This was the complete formation process of B, N-GQDs.

Scheme 1. Synthesis steps of B, N-GQDs.

Scheme 2. Detection of Hg2+ as a fluorescent sensor.

2.5. Measurement of absolute PLQY The PLQY of B, N-GQDs was tested with a FluoroMax-4 compact spectrofluorometer, which obtained the absolute PLQY of B, N-GQDs. Compared with the traditional test, this measurement had the advantages of a good stability, excellent repeatability and little error caused by the standard. In addition, the PLQY measured using an integrating sphere. 2.6. Fluorescent detection of ions The Hg2+ test was carried out by fluorescence spectroscopy at room temperature in an aqueous solution of B, N-GQDs with an excitation wavelength of 322 nm. The concentration of the B, N-GQD aqueous solution was 90 μg·mL-1, and it was applied for the selectivity investigation with the interfering ions and for the sensitivity towards Hg2+. The sensitivity study of Hg2+ was typically performed as follows: Different concentrations of Hg2+ (0.1 μM~100 μM) were separately added to an aqueous solution of B, N-GQDs (90.0 μg·mL-1), and the resulting mixed solution was incubated at room

temperature for 2 min. Then, the fluorescence intensity of the B, N-GQD solution in the presence and absence of Hg2+ was recorded. Here, the relative fluorescence ratio (F/F0) was usually used to evaluate the induced quenching degree of B, N-GQDs by Hg2+, where F and F0 represent the fluorescence intensity values of B, N-GQDs in the presence and absence of Hg2+, respectively. The typical operation with interfering ions was as follows: the interfering ions used were Ag+, Al3+, Ba2+, Ca2+, Cd2+, K+, Cr3+, Cu2+, Fe3+, Mg2+, Na+, Ni2+, Sn4+, Zn2+, Hg2+, Fe2+, Co2+, Pb2+ and Mn2+ all of which had the same ion concentration of 20 μM. They were incubated at room temperature for 2 min, and their fluorescence spectrum was recorded separately under the same conditions. 2.7. Detection of Hg2+ in real water samples To prove B, N-GQDs were effective and practical as a fluorescent sensor in the real environment. We chose the Yellow River water and tap water (Lanzhou, China) as real water samples for the experiment. We had pretreated all the real water samples. First, the samples were centrifuged at 10,000 rpm for 10 min and then filtered through a 0.22 μm microporous membrane to remove suspended solid particles and impurities. Finally, the Hg2+ in the real water samples was evaluated by the fluorescence spectrum, indicating the practical significance of the B, N-GQD fluorescent sensor.

3. Results and discussion 3.1. Structural characterization of B, N-GQDs To completely determine the structural features of B, N-GQDs, we characterized them by TEM, Raman spectroscopy, XRD, FT-IR spectroscopy and XPS. The morphology and microstructure of B, N-GQDs were observed by TEM and HRTEM. Fig. 1a shows that the synthesized B, N-GQDs were approximately spherical in shape, uniform in distribution with no clear agglomeration, and had a good dispersibility. The particle size distribution was relatively narrow, ranging from 2.2 to 7.6 nm, with an average diameter of approximately 3.8 nm (inset of Fig. 1a). The lattice fringe distance of B, N-GQDs observed from HRTEM was 0.22 nm (Fig. 1b), corresponding to the (1120) lattice plane of graphene [30]. These results clearly showed that B, N-GQDs had

a high degree of crystallinity and were essentially highly nanocrystalline. Fig. 1c shows the XRD pattern of B, N-GQDs. There was a diffraction peak at 13.2° 2θ due to the GO (002). The diffraction peak appearing at 27.5° indicated that the graphene in-plane characteristics were more apparent when the GQDs scale was smaller. The results showed that B, N-GQDs had the common characteristics of GO and graphene. Fig. 1d shows the Raman spectrum of B, N-GQDs. The characteristic peak at 1578 cm-1 was attributed to the G band, while the characteristic peak at 1389 cm-1 was attributed to the D band, and the intensity ratio ID/IG was 0.95. Theoretically, the D band represents the disordered carbon atoms at the edges, and the G band represents the inplane stretching vibration of the sp2 hybrid carbon atoms [31~33]. Therefore, based on the intensity values, we knew that the B, N-GQDs had more crystalline graphite components, which further confirmed the test results of HRTEM and XRD [34].

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Fig. 1. (a) TEM and (b) HRTEM of the as-synthesized B, N-GQDs. The inset of (a) showed the particle size distribution histogram. (c) XRD of B, N-GQDs. (d) Raman spectrum of B, N-GQDs.

Fig. 2a is the FT-IR spectrum of B, N-GQDs, which can roughly reflect the internal

structure of B, N-GQDs and information about functional groups. There were two distinct broad peaks at 3395 cm-1 and 3326 cm-1, attributed to the stretching vibration of O-H and N-H bonds, respectively. B, N-GQDs had a good hydrophilic property due to the large amount of amino and hydroxyl groups on its surface [35]. The peaks at 1636 cm-1 and 1101 cm-1 were designated as the stretching vibrations of the C=O bond and the C-O bond, respectively, indicating that the synthesized B, N-GQDs surface contained abundant carboxyl groups. In addition, the peak at 1355 cm-1 was attributed to the stretching vibration of the C-N bond. Finally, the peak at 1432 cm-1 corresponded to the B-O bond stretching vibration. All the functional groups mentioned above could be used to verify that the skeleton of GQDs was successfully doped with boron and nitrogen. XPS was used to characterize the chemical composition of the synthesized B, NGQDs. The full scan of the XPS spectrum shown in Fig. 2b showed four different typical peaks at 284.76 eV, 532.63 eV, 401.73 eV and 192.73 eV attributed to carbon (C 1s), oxygen (O 1s), nitrogen (N 1s) and boron ( B 1s), respectively. It was known that the main elements and their corresponding atomic percentages of B, N-GQDs synthesized by this method were C (49.77%), O (34.10%), N (4.56%) and B (11.57%). From the high-resolution spectrum of the C 1s, it was observed that the C 1s can be deconvoluted into five peaks: the peak appearing at 283.9 eV was attributed to the C-B bond, the peak at 284.8 eV was attributed to C-C (sp3)/C=C (sp2), the peak at 286.5 eV belonged to the C-O bond, and the peaks at 288.5 eV and 285.1 eV were attributed to the O=C-O bond and the C-N bond, respectively (Fig. 2c). The two peaks centred at 533.0 eV and 531.7 eV in the O 1s spectrum (Fig. 2d) were attributed to the presence of both C-OH/C-O-C and C=O groups. The N 1s spectrum (Fig. 2e) was deconvoluted into four peaks at 399.8 eV, 398.2 eV, 401.3 eV and 402.9 eV. These peaks corresponded to four nitrogen forms: pyrrole N, pyridine N, graphite N and oxidized N. Finally, three forms of boron were observed in the B 1s spectrum, which were attributed to the B-N, B-O and B-C groups at 191.6 eV, 192.4 eV and 193.3 eV, respectively. The XPS results supported the successful synthesis of B, N-GQDs by the one-pot hydrothermal method using GO as the carbon material. At the same time, these

observations further proved the results of the FT-IR spectrum, which could fully explain the successful doping of boron and nitrogen into the framework of GQDs. a

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Fig. 2. (a) FT-IR spectrum of B, N-GQDs. (b) Full-scan XPS spectrum, (c) High-resolution C 1s, (d) High-resolution O 1s, (e) High-resolution N 1s and (f) High-resolution B 1s spectrum of the B, N-GQDs.

3.2. Optical properties of B, N-GQDs To obtain the photophysical properties of B, N-GQDs, UV-vis absorption and PL spectra were acquired. Fig. 3a and its illustration show the UV-vis spectrum of B, NGQDs and the PL of B, N-GQDs illuminated with a benchtop 365 nm UV lamp. Due to the π→π* transition of sp2 carbon in the benzene ring, a typical absorption peak of

B, N-GQDs was detected at approximately 228 nm [36, 37]. Another broad absorption peak at approximately 301 nm was attributed to the C=O bond n→π* transition [38]. Additionally, from the illustration, it could be observed that B, N-GQDs emitted a bright blue PL. The fluorescence spectrum in Fig. 3b indicated that the maximum excitation and maximum emission wavelengths of B, N-GQDs were 322 nm and 422 nm, respectively. As shown in Fig. 3c, when the excitation wavelength was changed from 292 nm to 352 nm, the PL peaks of B, N-GQDs did not significantly shift with the increase in the excitation wavelength. It was known from the above results that the B, N-GQDs exhibited excitation-independent PL characteristics, indicating that the prepared B, NGQDs were uniformly stable and had strong fluorescence properties. This excitationindependent fluorescence property may have depended on the surface state of the GQDs rather than the morphology [39, 40]. Furthermore, when the excitation wavelength was 322 nm, the fluorescence intensity reached its maximum. Fig. 3d shows the change in fluorescence intensity of B, N-GQDs under different pH conditions at room temperature. It was clearly observed that when the pH was in the range of 2~7, the fluorescence intensity increased with the pH, and when the pH value was neutral, the fluorescence intensity of B, N-GQDs reached its maximum. However, when the pH was alkaline, in the range of 8~12, the fluorescence intensity decreased with the increasing pH, especially under the strong alkali condition. This result showed that a strongly acidic or strongly alkaline environment was very unfavourable for the fluorescence emission of B, N-GQDs. In the actual test, to ensure that the measured B, N-GQDs fluorescence intensity at acidic and alkaline pH had no error and that the data had good reliability, we needed to ensure the following factors. The temperature had significant effects on the fluorescence intensity. In general, as the temperature increased, the fluorescence intensity of GQDs decreased. As the temperature increased, intermolecular collisions increased, and radiation-free transitions increased, thereby reducing the fluorescence efficiency of GQDs. However, as the temperature decreased, the fluorescence intensity of GQDs increased due to the decrease in intermolecular collisions. Therefore, we chose to detect the fluorescence

intensity of B, N-GQDs at room temperature. Additionally, GQDs were also affected by a long-term exposure to UV light and air oxidation, which could reduce the fluorescence intensity of GQDs. Therefore, after adjusting the acidic and alkaline pH of B, N-GQDs, the fluorescence intensity should be tested immediately, and it should be measured one time. As the results obtained will have errors, we should not test with different batches. The fluorescence intensity of B, N-GQDs should be measured in a dilute solution of the same concentration. When the concentration of B, N-GQDs solution was high, the fluorescence of B, N-GQDs was quenched and self-absorption occurred. Finally, in order to keep subsequent test conditions consistent, the solvent used to dissolve B, N-GQDs must be ultrapure water. Because GQDs had a stronger fluorescence intensity when dissolved in organic reagents, such as ethanol, than in aqueous solution, it was necessary to ensure the consistency of the solvent. To determine the stability of PL in B, N-GQDs in a high salt environment, we added a 0~1 M NaCl salt solution to B, N-GQDs and tested the fluorescence intensity. From Fig. 3e, we knew that adding different concentrations of the NaCl solution hardly affected the PL of B, N-GQDs. Therefore, B, N-GQDs had a strong salt tolerance and are of great significance for practical applications. b

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1

Fig. 3. (a) UV-vis absorption spectrum of B, N-GQDs. The inset of (a) showed the PL of B, NGQDs illuminated by the benchtop 365 nm UV lamp. (b) The fluorescence maximum excitation and emission spectrum of B, N-GQDs. (c) Emission spectrum of B, N-GQDs obtained by changing the excitation wavelength from 292 to 352 nm. (d) Effect of pH on PL of B, N-GQDs. (e) Effect of NaCl concentration on PL Intensity of B, N-GQDs.

3.3. Selectivity of B, N-GQDs to Hg2+ To evaluate the detection specificity of B, N-GQDs as a fluorescent sensor, we added 19 kinds of metal interference ions to the B, N-GQDs sensing system under the same conditions. F and F0 indicate the fluorescence intensity of B, N-GQDs in the presence and absence of interfering ions. In general, we used the ratio F/F0, the relative fluorescence intensity (F/F0), to assess the extent to which interfering ions affected the sensing system and the degree of fluorescence quenching of certain particular responding ions. Fig. 4a clearly reveals that there was no significant change after adding Ag+, Al3+, Ba2+, Ca2+, Cd2+, K+, Cr3+, Mg2+, Na+, Ni2+, Sn4+, Zn2+, Fe2+, Co2+, Pb2+ and Mn2+ at a concentration of 20 μM. However, there were different degrees of fluorescence quenching in the presence of Hg2+, Fe3+ and Cu2+. According to the value

of F/F0, Hg2+ presented the strongest quenching ability for B, N-GQDs. That is, B, NGQDs as a fluorescent sensor had the strongest response to Hg2+. Therefore, we chose a suitable chelating agent to eliminate the interference from Fe3+ and Cu2+. Sodium pyrophosphate was used as a chelating agent to mask Fe3+. As shown in Fig. 4b, the addition of Sodium pyrophosphate at a concentration of 360 μM had no significant effect on the fluorescence of the B, N-GQDs itself and on the B, N-GQDsHg2+ systems. However, the agent could eliminate the interference of Fe3+ (20 μM) and restored the fluorescence. Therefore, it was strongly confirmed that sodium pyrophosphate as a chelating agent can effectively mask Fe3+. Additionally, as in the above procedure, EDTA was used as a chelating agent to mask Cu2+. As shown in Fig. 4c, the addition of EDTA (900 μM) had no significant effect on the fluorescence of the B, N-GQDs and B, N-GQDs-Hg2+ systems; however, EDTA did mask Cu2+ (20 μM) and restore the fluorescence. Therefore, EDTA was a suitable reagent for masking Cu2+.

a

1.2

b

1.0

1.0 0.8 0.6

0.6

F/F0

F/F0

0.8

0.4

0.4

0.2

0.2 0.0

0.0

+ 3+ 2+ + 4+ 2+ 2+ 2+ B Ag+ Al3+Ba2+Ca2+Cd2+ K Cr Cu2+Fe3+Mg Na Ni2+Sn Zn Hg Fe Co2+Pb2+Mn2+

c

1

2

3

4

5

6

1.0 0.8

F/F0

0.6 0.4 0.2 0.0

1

2

3

4

5

6

Fig. 4. (a) The relative fluorescence intensity (F/F0) at λex=322 nm of B, N-GQDs aqueous solution (90 μg·mL-1) in the presence of 20 μM of various metal ions. (a) Masking Fe3+ : 1) B, N-GQDs; 2) B, N-GQDs+Sodium pyrophosphate (360 μM); 3) B, N-GQDs+Fe3+ (20 μM); 4) B, N-GQDs+Fe3+

(20 μM) +Sodium pyrophosphate (360 μM); 5) B, N-GQDs+Hg2+ (40 μM); 6) B, N-GQDs+Sodium pyrophosphate (360 μM) +Hg2+ (40 μM). (c) Masking Cu2+ : 1) B, N-GQDs; 2) B, N-GQDs+EDTA (900 μM); 3) B, N-GQDs+Cu2+ (20 μM); 4) B, N-GQDs+Cu2+ (20 μM) +EDTA (900 μM); 5) B, N-GQDs+Hg2+ (40 μM); 6) B, N-GQDs+EDTA (900 μM) +Hg2+ (40 μM).

3.4. Establishment of a fluorescent sensor for Hg2+ The incubation time had a significant effect on the fluorescence quenching efficiency (F/F0) of the B, N-GQDs-Hg2+ system. Therefore, here we set six time periods of 0 min, 0.5 min, 1 min, 2 min, 3 min, 4 min and 5 min to evaluate the optimal incubation time. As observed in Fig. 5a, the quenching efficiency increased rapidly at 0.5 min, while the quenching efficiency was the same at 2 min, 3 min and 4 min, and there was no significant change at 5 min. Therefore, in order to save time, 2 min was selected as the optimal incubation time, and the subsequent experiments employed this time. Fig. 5b is a graph showing the relationship between the fluorescence intensity after different concentrations of Hg2+ were added to B, N-GQDs. As the Hg2+ concentration increased from 0 to 100 μM, the fluorescence intensity of B, N-GQDs gradually decreased. Fig. 5c is the titration curve for F/F0 versus Hg2+. We could observe more intuitively that with the increase in the Hg2+ concentration, the quenching efficiency decreased evidently. When the Hg2+ concentration reached 20 μM, a significant inflection point appeared. Furthermore, when the Hg2+ concentration was increased from 20 to 100 μM, although the quenching efficiency was still reduced, there was no longer a sharp decrease trend, and the reduction was gradual. From Fig. 5d, the fluorescence quenching efficiency (F/F0) showed a good linear relationship with the Hg2+ concentration in the range of 0.2~1 μM. The linear regression equation was: F/F0=-0.1609C+0.7834 (R2=0.9923), where F and F0 represent the fluorescence intensity of B, N-GQDs in the presence and absence of Hg2+, respectively, and C represents the concentration of Hg2+. Based on the above results, we calculated the minimum detection limit (LOD) as 6.4 nM when the signal-to-noise ratio was 3 (S/N=3). This value was calculated from LOD=3 σ/k, where σ is the standard deviation

of the blank solution and k is the slope of the F/F0 versus Hg2+ concentration fit. The results indicated that the fluorescence quenching method based on B, N-GQDs could be used to determine Hg2+. In addition, we compared the minimum detection limit and linear range of Hg2+ in this work with other methods reported in the previous literature in detail. The results are shown in Table 1. It could be clearly seen from Table 1 that the B, N-GQDs synthesized by the presented method were more novel and innovative as a fluorescent sensor and had a lower detection limit and a wider linear range than did the other fluorescent sensors for detecting Hg2+. Therefore, in summary, the effectiveness and value of B, N-GQDs as a fluorescent sensor were strongly confirmed. 1.0

b

Concentration:100 M

Fluorescence Intensity (a.u.)

a

0.8

F/F0

0.6 0.4 0.2

2000000

0 M

1500000

100 M

1000000

500000

0

0.0

0

1

2

3

4

350

5

400

c

d

1.0

0.75 0.72

0.6

0.69

F/F0

F/F0

0.8

0.4

0.66

0.2

0.63

0.0

0

20

40

60

CHg (μM) 2+

450

500

550

Wavelength (nm)

Time (min)

80

100

0.60

Y = - 0.1609 X + 0.7834 2 R =0.9923

0.2

0.4

0.6

0.8

1.0

CHg (μM) 2+

Fig. 5. (a) The effect of different incubation times on the fluorescence intensity of B, N-GQDs after addition of 100 μM Hg2+ (The incubation time selected here was 0 min, 0.5 min, 1 min, 2 min, 3 min, 4 min and 5 min). (b) Fluorescence spectrum of B, N-GQDs (90 μg·mL-1) after adding different concentrations of Hg2+. (c) Fluorescence response of B, N-GQDs with the Hg2+ concentration increased. The concentrations of Hg2+ were 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 μM. (d) F/F0 titration curve for different concentrations of Hg2+. (d) Linear relationships between F/F0 and Hg2+ in the range of 0.2~1 μM.

Table 1 Comparison of different types of quantum dot fluorescent sensors for Hg2+ detection. Fluorescent probe

LOD

Linear range

Application

Assay time

Ref

S, N-GQDs

9.14 μM

12~125 μM

deionized water

20 min

[41]

Eu QDs

2 μM

5~1000 μM

deionized water

3 min

[42]

N-GQDs

23 nM

0~4.31 μM

deionized water

5 min

[40]

N, S-CDs

0.062 μM

0.1~20 μM

universal buffer

6 min

[43]

GQDs

0.1 μM

0.8~9 μM

Tris–HCl buffer solution

not mentioned

[44]

CDs

2.47 μM

4~18 μM

double distilled water

not mentioned

[45]

Mg-N-CQDs

0.02 μM

0.05~5 μM

ultrapure water

10 min

[46]

B, N-GQDs

6.4 nM

0.2~1 μM

deionized water

2 min

This work

3.5. Possible mechanism of the B, N-GQD fluorescent sensor for Hg2+ detection To investigate the possible mechanism of B, N-GQDs as a fluorescent sensor for Hg2+ detection, we did the following analysis. Fig. 6a shows the UV-vis spectrum of B, N-GQDs with 100 μM Hg2+ added and without Hg2+ added. It was clearly observed that the UV-vis absorption spectrum of B, N-GQDs changed significantly when 100 μM Hg2+ was added. The original typical absorption peak at 228 nm appeared at 201 nm, with an apparent blueshift. However, the original absorption at 301 nm was weak and almost invisible. These results indicated that the presence of Hg2+ affected the surface state of B, N-GQDs [47, 48]. This effect was because of the static annihilation that caused the fluorescence quenching of B, N-GQDs [49]. Fig. 6b shows the UV-vis absorption peak of Hg2+ and the fluorescence spectrum of B, N-GQDs. Hg2+ had three absorption peaks at 212, 230 and 275 nm. The maximum excitation and maximum emission peaks of B, N-GQDs were at 322 nm and 422 nm, respectively. It could be observed that the excitation and emission spectrum of B, NGQDs overlapped perfectly with the absorption peaks in the UV-vis absorption spectrum of Hg2+. Therefore, the fluorescence of B, N-GQDs was shielded by Hg2+, which meant that the fluorescence of B, N-GQDs was quenched by Hg2+. This result was caused by the IFE fluorescence phenomenon. A highly efficient IFE phenomenon

indicated that the absorption band of the absorber had sufficient spectral overlap with the excitation and/or emission bands of the fluorophore, where the absorber referred to Hg2+ and the fluorophore referred to B, N-GQDs [50]. Thus, that we knew how to properly select the right absorber and fluorophore was important for the fluorescence quenching caused by the IFE phenomenon [51]. Abs

0.3

228 0.2 0.1

Ex

Em

Fluorescence Intensity (a.u.)

201 Absorbance (a.u.)

b

B, N-GQDs 2+ B, N-GQDs + 100 M Hg

0.4

Absorbance (a.u.)

a

301

0.0 200

250

300

350

400

450

500

200

250

350

300

Wavelength (nm)

400

450

500

550

Wavelength (nm)

Fig. 6. (a) UV-vis absorption spectrum of aqueous B, N-GQDs solution and the B, N-GQDs solution containing 100 μM Hg2+ . (b) The overlap between UV-Vis absorption spectrum of Hg2+ and excitation and emission spectrum of B, N-GQDs.

3.6. Real water sample detection To further confirm the feasibility of using synthetic B, N-GQDs as a fluorescent sensor to detect Hg2+, we tested two water samples from real environments, tap water and the Yellow River water. The recoveries were carried out by using the standard addition method, and the results are listed in Table 2. As seen from Table 2, the recoveries of Hg2+ in tap water and the Yellow River water ranged from 98% to 104%, and the standard deviations (RSDs) were in the range of 1.9% to 4.3%. The results showed that fluorescent sensor based on B, N-GQDs has potential applicability for detecting Hg2+ in real environments. Therefore, this kind of fluorescent sensor can be promoted for the detection of Hg2+ in real water samples. Table 2 Recovery of Hg2+ in real water samples (n = 3) Samples Tap water

Original samples Added (μM) Total found (μM) Not found

0.1

0.102±0.002

Recovery (%)

RSD (%)

102±2

1.9

Not found

1

0.98±0.011

98±0.011

2.8

Not found

10

10.1±0.064

101±0.064

2.3

Not found

100

102.2±0.038

102.2±0.038

3.6

Not found

0.1

0.103±0.003

103±3

2.4

Not found

1

1.04±0.085

104±0.085

3.3

Not found

10

10.26±0.12

102.6±0.12

3.7

Not found

100

103.8±0.087

103.8±0.087

4.3

The Yellow River water

4. Conclusion In this work, novel B, N-GQDs were synthesized via a top-down one-pot hydrothermal method using GO as the carbon precursor material and boric acid and an ammonia solution as doping molecules. The invention has good novelty and creativity, and it is of great practical significance to and inspiration for other researchers to explore the synthesis method of B, N-GQDs. The B, N-GQDs obtained by this method showed a bright blue light luminescence and an average size of 3.8 nm. Most importantly, the B, N-GQDs exhibited a high degree of crystallinity, which was properly confirmed from HRTEM. Moreover, based on B, N-GQDs, we applied their fluorescence properties as a fluorescent sensor for detecting Hg2+, with a minimum detection limit of 6.4 nM. Furthermore, due to their high sensitivity and fast response, the sensor was also successfully used for the determination of Hg2+ in real water samples. Therefore, B, N-GQDs present a good practical significance and application prospects as a fluorescent sensor.

Acknowledgements This work was funded by the National Natural Science Foundation of China (51262027); the State Key Laboratory of Solidification Processing in NWPU (SKLSP201754); the Science and Technology Project Gansu Province (17YF1GA017); the Research Project of Higher Education in Gansu Province (2017A-002); and the Science and Technology Project Gansu Province (17JR5RA082). Compliance with ethical standards

The author(s) declare that they have no competing interests.

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Highly sensitive fluorescence sensor for mercury(Ⅱ) based on boron- and nitrogen-co-doped graphene quantum dots Zhenyu Liu a, Zunli Mo , *, Xiaohui Niu a, Xing Yang a, Yangyang Jiang a,Pan Zhao a, Nijuan Liu a, Ruibin Guo a (a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Research Center of Gansu Military and Civilian Integration Advanced Structural Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070 China) Graphical Abstract We propose a simple green one-pot hydrothermal method to synthesize boron and nitrogen co-doped graphene quantum dots (B, N-GQDs). The B, N-GQDs prepared by this method can detect Hg2+ very quickly and sensitively.

Scheme 1. Synthesis steps of B, N-GQDs.

Scheme 2. Detection of Hg2+ as a fluorescence sensor.

Zunli Mo [email protected] a

Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, and Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China.

Highlights ·B, N-GQDs was synthesized by one-pot hydrothermal method using GO as a carbon material. ·This invention had a good novelty, which will be a good inspiration for the future exploration of synthesizing B, N-GQDs by top-down method. ·It had fast response and high sensitivity for the detection of Hg2+. ·The minimum detection limit for Hg2+ was 6.4 nM. · B, N-GQDs as a fluorescence sensor can successfully detect Hg2+ in real water samples.

Credit Author Statement Zunli Mo, Zhenyu Liu: Conceptualization. Zhenyu Liu: Methodology, Formal analysis, Visualization, Investigation. Zhenyu Liu: Data Curation, Writing-Original draft, Writing-Review & Editing. Zunli Mo, Ruibin Guo, Nijuan Liu: Supervision. Xiaohui Niu, Xing Yang, Yangyang Jiang, Pan Zhao: Project administration.