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Yellow-emissive carbon dots for “off-and-on” fluorescent detection of progesterone
Materials Letters 271 (2020) 127760
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Yellow-emissive carbon dots for ‘‘off-and-on” fluorescent detection of progesterone Lei Cao a,b, Ling Yu a, Juan Yue b, Yi Zhang b, Mingfeng Ge b, Li Li b,⇑, Ru Yang a a
The Affiliated Suzhou Hospital of Nanjing Medical University, 26 Daoqian Road, Suzhou 215008, People’s Republic of China CAS Key Laboratory of Biomedical Diagnostics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Science (CAS), 88 Keling Road, Suzhou 215163, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 31 December 2019 Received in revised form 27 March 2020 Accepted 31 March 2020 Available online 1 April 2020 Keywords: Carbon materials Nanoparticles Fluorescent detection Off-and-on Progesterone
a b s t r a c t Progesterone is one of the most essential elements for women, and it is of great significance to develop new methods to detect progesterone levels. In this work, we explored a novel fluorescent probe to detect progesterone by using carbon dots based on the quenching and recovery (‘‘off and on”) mechanism. The carbon dots prepared via a simple one-step hydrothermal method exhibit bright yellow light and are rapidly quenched by hydrogen peroxide, and when the progesterone is continuously added, the fluorescence of the carbon dots recovers. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Progesterone (P4) is a steroid hormone in the body, most are secreted by the corpus luteum which is in the ovary. In clinical therapy, P4 is used for protecting pregnant women from spontaneous preterm birth (PTB) [1]. Moreover, recent researches show that the concentration of progesterone receptor (PR) is associated with breast cancer [2]. P4 can be used as anti-proliferative medicine in some phenotype breast cancer cells [3]. Thus, it’s significant for detecting the concentration of P4. So far, a large number of technologies such as surface plasmon resonance imaging (SPRi) [4], fluorescent sensors [5], and film electrode [6], etc. have been tried for P4 detection. As a new type of fluorescence probe, carbon dots (CDs) have been widely used in the detection of small biomolecules and biomarkers. CDs possess unique superiorities over traditional organic fluorescent probes, such as low toxicity, low-cost synthesis, good biocompatibility and permeability, tunable bandgaps, good resistance to photo-bleaching, easy clearance from the body and immune system evasion [7]. Then, in this paper, a new method based on CDs is explored to detect P4 through an ‘‘Off-and-On” mechanism. CDs synthesized by o-phenylenediamine are easily
⇑ Corresponding author. E-mail addresses: [email protected] (L. Li), [email protected] (R. Yang). https://doi.org/10.1016/j.matlet.2020.127760 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
quenched by H2O2, and when the P4 is continuously added, the fluorescence of the CDs recovers rapidly. 2. Synthesis Using o-phenylenediamine as raw material, CDs were prepared via a one-step hydrothermal method. (Experimental detail can be seen in Supporting information). 3. Results and discussion 3.1. Characterizations of CDs Transmission electron microscopy (TEM) shows that the synthesized CDs are well separated in deionized water with a nearspherical morphology (Fig. S1), and the size of a single nanoparticle is about 10–20 nm. The FTIR spectrum (Fig. S2) demonstrates that the surface of the CDs is rich in amino and carbonyl groups. 3.2. Optical properties of CDs The optical properties of CDs were employed by FL spectra. From Fig. 1, the CDs show the maximum fluorescence emission (kem) at 573 nm while the excitation wavelength (kex) is around 420 nm. Besides, the CDs exhibit excitation-independent emission when the excitation wavelength change ranging from 400 nm to
2
L. Cao et al. / Materials Letters 271 (2020) 127760
ibration plot [8]. The detection limit indicates that the probe is expected to be used in clinical diagnosis. 3.4. Mechanism of detection The speculated detection mechanism is shown in Fig. 3. The amino groups on the surface of the o-phenylenediamine CDs are oxidized to nitro groups by H2O2 [9,10]. And because nitro groups are strong electron-withdrawing groups, causing the electron group on the surface of CDs to be interpolated, which may bring the fluorescence quenching. While P4 is a strong reducing agent due to its unsaturated double bond structure, it is easy to reduce the nitro groups on the surface of the CDs to amino groups, thereby recovering the fluorescence [11]. 3.5. Selective detection of progesterone
Fig. 1. Fluorescence spectra of CDs (the black line represents the excitation curve, and the colored lines represent the emission curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
460 nm, indicating that the CDs have stable photoluminescence. Irradiating CDs solution under 365 nm UV light, and it exerts strong yellow photoluminescence (Fig. S3). 3.3. Detection of progesterone To explore the feasibility of utilizing the fluorescence probe to detect the concentration of P4, we designed an off-and-on strategy in which the fluorescence of CDs solution was quenched by H2O2 and recovered upon the addition of P4. Fig. 2A shows the fluorescence quenching curves of CDs (0.02 mg/mL) with the addition of H2O2. After adding 80 uL of H2O2 (30%) to 2 mL of CDs solution, the fluorescence intensity of CDs (at 573 nm) decreased by 88.7%. And Fig. 2B gives the fluorescence recovery curves of CDs. When P4 was gradually added to the solution (from 0 to 200 lM), the fluorescence intensity of CDs increased simultaneously. The recovery fluorescence intensity is closely related to the amount of P4 and it exhibits a good linear relationship (y = 1.101*x + 148.868) with a correlation coefficient (R2) of 0.998. The limit of detection (LOD) was calculated to be 10.25 nM according to the equation 3r/S, where r is the standard deviation of the signals (n = 6) and S is the slope of the linear cal-
The fluorescence response specificity of CDs towards P4 over various ions and molecules which are common in organisms has been investigated. The quenched CDs solution (treated with H2O2) was tested by various ions and molecules including Na+, Ca2+, K+, Mn2+, Fe3+, Mg2+, Zn2+, Cu2+, P4, ascorbic acid (AA), glutathione (GSH), cysteine (Cys), homocysteine (Hcy), and dopamine (DA). Fig. 4 shows the fluorescence intensity recovery ratio of CDs under different ions and molecules. Here, the fluorescence intensity of CDs after H2O2 treatment is used as a blank control. The reagents with unenhanced fluorescence intensity are marked as black columns, and the reagents that cause fluorescence enhancement are marked as colored columns. From Fig. 4, P4 can easily restore the fluorescence intensity to 86%, while other reducing agents such as GSH, Cys, Hcy, and DA can also restore the fluorescence intensity by 30% 40%. As for metal ions, only Zn2+ and Na+ can slightly increase the fluorescence intensity, and other ions such as Fe3+, Cu2+, etc., cannot restore the fluorescence at all. As the common reducing agent, ascorbic acid (Vitamin C), it has almost no effect on fluorescence recovery. The results confirm that CDs can be used as fluorescent probes to detect P4 levels in vitro, but this process is easily disturbed by strong reducing agents, so it is not suitable for intracellular detection. 3.6. Application of CDs for progesterone in vitro Finally, we performed a simulation experiment to investigate the practical potential of CDs for P4 in vitro. The quenched CDs solution which was contained in EP tube (Fig. S4). Then, different
Fig. 2. (A) The fluorescence quenching curves of CDs (0.02 mg/mL) with the addition of H2O2 (from 0 to 80 lL); (B) The fluorescence recovery curves of CDs with the addition of P4 (from 0 to 200 lM).
L. Cao et al. / Materials Letters 271 (2020) 127760
3
Fig. 3. Schematic diagram of the mechanism of detection of P4.
CRediT authorship contribution statement Lei Cao: Conceptualization, Methodology, Writing - original draft. Ling Yu: Methodology, Data curation. Juan Yue: Methodology, Data curation. Yi Zhang: Methodology, Data curation. Mingfeng Ge: Methodology, Data curation. Li Li: Software, Data curation, Writing - review & editing. Ru Yang: Supervision, Software, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Science and Technology Department of Suzhou City (No. SS201539). Fig. 4. The fluorescence intensity recovery ratio of CDs (0.2 mg/mL) under different ions and molecules (5 mM). F0 represents the initial fluorescence intensity of CDs without H2O2, and F represents the recovered fluorescence intensity of CDs (treated with H2O2) after the addition of ions and molecules.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2020.127760.
concentrations of P4 solution were added into. Soaking the filter papers and dried in an oven. The recovery of fluorescence of filter papers was observed under blue light (k = 488 nm). As shown in Fig. S5. The first test strips as a control, the second to fifth test strips were sequentially used with a higher concentration of P4 solution. Obviously, as the concentration of P4 increased, the fluorescence intensity of test strips enhanced. In our experiment, quenched CDs could response to only 20 lL of P4, which indicates the fluorescent probe has a good potential application for detection of P4 in vitro. 4. Conclusion In summary, we have described a fluorescent off-and-on method using CDs as materials to sensitively detect P4 in vitro. The CDs were synthesized by o-phenylenediamine through a one-step hydrothermal process. The prepared CDs exhibit bright yellow light and are easily quenched by H2O2. When P4 is continuously added, the fluorescence of the CDs recovers rapidly. The recovery of fluorescence intensity is closely related to the amount of P4 and there is a good linear relationship between the range from 0 lM to 200 lM with a limit of detection of 10.25 nM. Experimental results determined that the detection of progesterone is a process of oxidation and reduction. The test strip experiment indicates that this work has a good potential application in the sensitive detection of progesterone in vitro.
References [1] W. Goodnight, Clinical application of progesterone for the prevention of preterm birth, 2016, Am. J. Perinatol. 33 (3) (2016) 253–257. [2] P.A. Joshi, H.W. Jackson, A.G. Beristain, M.A. Di Grappa, P.A. Mote, C.L. Clarke, J. Stingl, P.D. Waterhouse, R. Khokha, Progesterone induces adult mammary stem cell expansion, Nature 465 (7299) (2010) 803–807. [3] C.-C. Chen, D.B. Hardy, C.R. Mendelson, Progesterone Receptor Inhibits Proliferation of Human Breast Cancer Cells via Induction of MAPK Phosphatase 1 (MKP-1/DUSP1), J. Biol. Chem. 286 (50) (2011) 43091–43102. [4] E. Zeidan, R. Shivaji, V.C. Henrich, M.G. Sandros, Nano-SPRi aptasensor for the detection of progesterone in buffer, Sci. Rep. 6 (1) (2016) 26714. [5] S.C. Hong, D.P. Murale, M. Lee, S.M. Lee, J.S. Park, J.-S. Lee, Bulk aggregation based fluorescence turn-on sensors for selective detection of progesterone in aqueous solution, Angew. Chem. Int. Ed. 56 (46) (2017) 14642–14647. [6] T. Zidaricˇ, V. Jovanovski, S.B. Hocˇevar, Nanostructured bismuth film electrode for detection of progesterone, Sensors 18 (12) (2018) 4233. [7] K.O. Boakye-Yiadom, S. Kesse, Y. Opoku-Damoah, M.S. Filli, M. Aquib, M.M.B. Joelle, M.A. Farooq, R. Mavlyanova, F. Raza, R. Bavi, B. Wang, Carbon dots: Applications in bioimaging and theranostics, Int. J. Pharm. 564 (2019) 308– 317. [8] T. Wang, A. Wang, R. Wang, Z. Liu, Y. Sun, G. Shan, Y. Chen, Y. Liu, Carbon dots with molecular fluorescence and their application as a ‘‘turn-off” fluorescent probe for ferricyanide detection, Sci. Rep. 9 (1) (2019) 10723. [9] X. Li, X. Jiang, Q. Liu, A. Liang, Z. Jiang, Using N-doped carbon dots prepared rapidly by microwave digestion as nanoprobes and nanocatalysts for fluorescence determination of ultratrace isocarbophos with label-free aptamers, Nanomaterials 9 (2) (2019) 223. [10] M. Pan, Z. Xu, Q. Jiang, J. Feng, J. Sun, F. Wang, X. Liu, Interfacial engineering of carbon dots with benzenediboronic acid for fluorescent biosensing, Nanoscale Adv. 1 (2) (2019) 765–771. [11] F. Feng, J. Ye, Z. Cheng, X. Xu, Q. Zhang, L. Ma, C. Lu, X. Li, Cu–Pd/c-Al2O3 catalyzed the coupling of multi-step reactions: direct synthesis of benzimidazole derivatives, RSC Adv. 6 (76) (2016) 72750–72755.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Yellow-emissive carbon dots for ‘‘off-and-on” fluorescent detection of progesterone Lei Cao a,b, Ling Yu a, Juan Yue b, Yi Zhang b, Mingfeng Ge b, Li Li b,⇑, Ru Yang a a
The Affiliated Suzhou Hospital of Nanjing Medical University, 26 Daoqian Road, Suzhou 215008, People’s Republic of China CAS Key Laboratory of Biomedical Diagnostics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Science (CAS), 88 Keling Road, Suzhou 215163, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 31 December 2019 Received in revised form 27 March 2020 Accepted 31 March 2020 Available online 1 April 2020 Keywords: Carbon materials Nanoparticles Fluorescent detection Off-and-on Progesterone
a b s t r a c t Progesterone is one of the most essential elements for women, and it is of great significance to develop new methods to detect progesterone levels. In this work, we explored a novel fluorescent probe to detect progesterone by using carbon dots based on the quenching and recovery (‘‘off and on”) mechanism. The carbon dots prepared via a simple one-step hydrothermal method exhibit bright yellow light and are rapidly quenched by hydrogen peroxide, and when the progesterone is continuously added, the fluorescence of the carbon dots recovers. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Progesterone (P4) is a steroid hormone in the body, most are secreted by the corpus luteum which is in the ovary. In clinical therapy, P4 is used for protecting pregnant women from spontaneous preterm birth (PTB) [1]. Moreover, recent researches show that the concentration of progesterone receptor (PR) is associated with breast cancer [2]. P4 can be used as anti-proliferative medicine in some phenotype breast cancer cells [3]. Thus, it’s significant for detecting the concentration of P4. So far, a large number of technologies such as surface plasmon resonance imaging (SPRi) [4], fluorescent sensors [5], and film electrode [6], etc. have been tried for P4 detection. As a new type of fluorescence probe, carbon dots (CDs) have been widely used in the detection of small biomolecules and biomarkers. CDs possess unique superiorities over traditional organic fluorescent probes, such as low toxicity, low-cost synthesis, good biocompatibility and permeability, tunable bandgaps, good resistance to photo-bleaching, easy clearance from the body and immune system evasion [7]. Then, in this paper, a new method based on CDs is explored to detect P4 through an ‘‘Off-and-On” mechanism. CDs synthesized by o-phenylenediamine are easily
⇑ Corresponding author. E-mail addresses: [email protected] (L. Li), [email protected] (R. Yang). https://doi.org/10.1016/j.matlet.2020.127760 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
quenched by H2O2, and when the P4 is continuously added, the fluorescence of the CDs recovers rapidly. 2. Synthesis Using o-phenylenediamine as raw material, CDs were prepared via a one-step hydrothermal method. (Experimental detail can be seen in Supporting information). 3. Results and discussion 3.1. Characterizations of CDs Transmission electron microscopy (TEM) shows that the synthesized CDs are well separated in deionized water with a nearspherical morphology (Fig. S1), and the size of a single nanoparticle is about 10–20 nm. The FTIR spectrum (Fig. S2) demonstrates that the surface of the CDs is rich in amino and carbonyl groups. 3.2. Optical properties of CDs The optical properties of CDs were employed by FL spectra. From Fig. 1, the CDs show the maximum fluorescence emission (kem) at 573 nm while the excitation wavelength (kex) is around 420 nm. Besides, the CDs exhibit excitation-independent emission when the excitation wavelength change ranging from 400 nm to
2
L. Cao et al. / Materials Letters 271 (2020) 127760
ibration plot [8]. The detection limit indicates that the probe is expected to be used in clinical diagnosis. 3.4. Mechanism of detection The speculated detection mechanism is shown in Fig. 3. The amino groups on the surface of the o-phenylenediamine CDs are oxidized to nitro groups by H2O2 [9,10]. And because nitro groups are strong electron-withdrawing groups, causing the electron group on the surface of CDs to be interpolated, which may bring the fluorescence quenching. While P4 is a strong reducing agent due to its unsaturated double bond structure, it is easy to reduce the nitro groups on the surface of the CDs to amino groups, thereby recovering the fluorescence [11]. 3.5. Selective detection of progesterone
Fig. 1. Fluorescence spectra of CDs (the black line represents the excitation curve, and the colored lines represent the emission curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
460 nm, indicating that the CDs have stable photoluminescence. Irradiating CDs solution under 365 nm UV light, and it exerts strong yellow photoluminescence (Fig. S3). 3.3. Detection of progesterone To explore the feasibility of utilizing the fluorescence probe to detect the concentration of P4, we designed an off-and-on strategy in which the fluorescence of CDs solution was quenched by H2O2 and recovered upon the addition of P4. Fig. 2A shows the fluorescence quenching curves of CDs (0.02 mg/mL) with the addition of H2O2. After adding 80 uL of H2O2 (30%) to 2 mL of CDs solution, the fluorescence intensity of CDs (at 573 nm) decreased by 88.7%. And Fig. 2B gives the fluorescence recovery curves of CDs. When P4 was gradually added to the solution (from 0 to 200 lM), the fluorescence intensity of CDs increased simultaneously. The recovery fluorescence intensity is closely related to the amount of P4 and it exhibits a good linear relationship (y = 1.101*x + 148.868) with a correlation coefficient (R2) of 0.998. The limit of detection (LOD) was calculated to be 10.25 nM according to the equation 3r/S, where r is the standard deviation of the signals (n = 6) and S is the slope of the linear cal-
The fluorescence response specificity of CDs towards P4 over various ions and molecules which are common in organisms has been investigated. The quenched CDs solution (treated with H2O2) was tested by various ions and molecules including Na+, Ca2+, K+, Mn2+, Fe3+, Mg2+, Zn2+, Cu2+, P4, ascorbic acid (AA), glutathione (GSH), cysteine (Cys), homocysteine (Hcy), and dopamine (DA). Fig. 4 shows the fluorescence intensity recovery ratio of CDs under different ions and molecules. Here, the fluorescence intensity of CDs after H2O2 treatment is used as a blank control. The reagents with unenhanced fluorescence intensity are marked as black columns, and the reagents that cause fluorescence enhancement are marked as colored columns. From Fig. 4, P4 can easily restore the fluorescence intensity to 86%, while other reducing agents such as GSH, Cys, Hcy, and DA can also restore the fluorescence intensity by 30% 40%. As for metal ions, only Zn2+ and Na+ can slightly increase the fluorescence intensity, and other ions such as Fe3+, Cu2+, etc., cannot restore the fluorescence at all. As the common reducing agent, ascorbic acid (Vitamin C), it has almost no effect on fluorescence recovery. The results confirm that CDs can be used as fluorescent probes to detect P4 levels in vitro, but this process is easily disturbed by strong reducing agents, so it is not suitable for intracellular detection. 3.6. Application of CDs for progesterone in vitro Finally, we performed a simulation experiment to investigate the practical potential of CDs for P4 in vitro. The quenched CDs solution which was contained in EP tube (Fig. S4). Then, different
Fig. 2. (A) The fluorescence quenching curves of CDs (0.02 mg/mL) with the addition of H2O2 (from 0 to 80 lL); (B) The fluorescence recovery curves of CDs with the addition of P4 (from 0 to 200 lM).
L. Cao et al. / Materials Letters 271 (2020) 127760
3
Fig. 3. Schematic diagram of the mechanism of detection of P4.
CRediT authorship contribution statement Lei Cao: Conceptualization, Methodology, Writing - original draft. Ling Yu: Methodology, Data curation. Juan Yue: Methodology, Data curation. Yi Zhang: Methodology, Data curation. Mingfeng Ge: Methodology, Data curation. Li Li: Software, Data curation, Writing - review & editing. Ru Yang: Supervision, Software, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Science and Technology Department of Suzhou City (No. SS201539). Fig. 4. The fluorescence intensity recovery ratio of CDs (0.2 mg/mL) under different ions and molecules (5 mM). F0 represents the initial fluorescence intensity of CDs without H2O2, and F represents the recovered fluorescence intensity of CDs (treated with H2O2) after the addition of ions and molecules.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2020.127760.
concentrations of P4 solution were added into. Soaking the filter papers and dried in an oven. The recovery of fluorescence of filter papers was observed under blue light (k = 488 nm). As shown in Fig. S5. The first test strips as a control, the second to fifth test strips were sequentially used with a higher concentration of P4 solution. Obviously, as the concentration of P4 increased, the fluorescence intensity of test strips enhanced. In our experiment, quenched CDs could response to only 20 lL of P4, which indicates the fluorescent probe has a good potential application for detection of P4 in vitro. 4. Conclusion In summary, we have described a fluorescent off-and-on method using CDs as materials to sensitively detect P4 in vitro. The CDs were synthesized by o-phenylenediamine through a one-step hydrothermal process. The prepared CDs exhibit bright yellow light and are easily quenched by H2O2. When P4 is continuously added, the fluorescence of the CDs recovers rapidly. The recovery of fluorescence intensity is closely related to the amount of P4 and there is a good linear relationship between the range from 0 lM to 200 lM with a limit of detection of 10.25 nM. Experimental results determined that the detection of progesterone is a process of oxidation and reduction. The test strip experiment indicates that this work has a good potential application in the sensitive detection of progesterone in vitro.
References [1] W. Goodnight, Clinical application of progesterone for the prevention of preterm birth, 2016, Am. J. Perinatol. 33 (3) (2016) 253–257. [2] P.A. Joshi, H.W. Jackson, A.G. Beristain, M.A. Di Grappa, P.A. Mote, C.L. Clarke, J. Stingl, P.D. Waterhouse, R. Khokha, Progesterone induces adult mammary stem cell expansion, Nature 465 (7299) (2010) 803–807. [3] C.-C. Chen, D.B. Hardy, C.R. Mendelson, Progesterone Receptor Inhibits Proliferation of Human Breast Cancer Cells via Induction of MAPK Phosphatase 1 (MKP-1/DUSP1), J. Biol. Chem. 286 (50) (2011) 43091–43102. [4] E. Zeidan, R. Shivaji, V.C. Henrich, M.G. Sandros, Nano-SPRi aptasensor for the detection of progesterone in buffer, Sci. Rep. 6 (1) (2016) 26714. [5] S.C. Hong, D.P. Murale, M. Lee, S.M. Lee, J.S. Park, J.-S. Lee, Bulk aggregation based fluorescence turn-on sensors for selective detection of progesterone in aqueous solution, Angew. Chem. Int. Ed. 56 (46) (2017) 14642–14647. [6] T. Zidaricˇ, V. Jovanovski, S.B. Hocˇevar, Nanostructured bismuth film electrode for detection of progesterone, Sensors 18 (12) (2018) 4233. [7] K.O. Boakye-Yiadom, S. Kesse, Y. Opoku-Damoah, M.S. Filli, M. Aquib, M.M.B. Joelle, M.A. Farooq, R. Mavlyanova, F. Raza, R. Bavi, B. Wang, Carbon dots: Applications in bioimaging and theranostics, Int. J. Pharm. 564 (2019) 308– 317. [8] T. Wang, A. Wang, R. Wang, Z. Liu, Y. Sun, G. Shan, Y. Chen, Y. Liu, Carbon dots with molecular fluorescence and their application as a ‘‘turn-off” fluorescent probe for ferricyanide detection, Sci. Rep. 9 (1) (2019) 10723. [9] X. Li, X. Jiang, Q. Liu, A. Liang, Z. Jiang, Using N-doped carbon dots prepared rapidly by microwave digestion as nanoprobes and nanocatalysts for fluorescence determination of ultratrace isocarbophos with label-free aptamers, Nanomaterials 9 (2) (2019) 223. [10] M. Pan, Z. Xu, Q. Jiang, J. Feng, J. Sun, F. Wang, X. Liu, Interfacial engineering of carbon dots with benzenediboronic acid for fluorescent biosensing, Nanoscale Adv. 1 (2) (2019) 765–771. [11] F. Feng, J. Ye, Z. Cheng, X. Xu, Q. Zhang, L. Ma, C. Lu, X. Li, Cu–Pd/c-Al2O3 catalyzed the coupling of multi-step reactions: direct synthesis of benzimidazole derivatives, RSC Adv. 6 (76) (2016) 72750–72755.