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Black Phosphorus Quantum Dots as the Ratiometric Fluorescence Probe for Trace Mercury Ion Detection Based on Inner Filter Effect Wei Gu, Xueyu Pei, Yuxiao Cheng, Cuiling Zhang, Jidong Zhang, Yinghan Yan, Caiping Ding, and Yuezhong Xian ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00102 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Black Phosphorus Quantum Dots as the Ratiometric Fluorescence Probe for Trace Mercury Ion Detection Based on Inner Filter Effect †

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Wei Gu, Xueyu Pei, Yuxiao Cheng, Cuiling Zhang,*, Jidong Zhang, Yinghan Yan, Caiping Ding, † Yuezhong Xian*, †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai, 200241, China. ‡ Shanghai Entry-Exit Inspection and Quarantine Bureau, Shanghai 200135, China KEYWORDS: Black phosphorus quantum dots; Fluorescence; Ratiometric; Inner filter effect; Mercury ion

ABSTRACT: In this work, a novel ratiometric fluorescence sensor has been constructed for the selective and sensitive detection of Hg2+, which is based on the inner filter effect (IFE) of tetraphenylporphyrin tetrasulfonic acid (TPPS) towards black phosphorus quantum dots (BP QDs). Highly fluorescent BP QDs were successfully synthesized from bulk BP by sonication-assisted solvothermal method via a top-down route. In the presence of Hg2+, the IFE originating from spectral overlap between the excitation of BP QDs and the absorption of TPPS is inhibited and the fluorescence of BP QDs is restored. At the same time, the red fluorescence of TPPS is quenched due to its coordination with Mn2+. These phenomena result from the rapid coordination between Mn2+ and TPPS in the presence of Hg2+, which leads to the dramatic decrease of the absorption of TPPS. On the basis of these findings, we design a ratiometric fluorescence sensor for the detection of Hg2+. The as-constructed sensor reveals a good linear response to Hg2+ ranging from 1 to 60 nM with a detection limit of 0.39 nM. Furthermore, the sensing assay is applicable to detect Hg2+ in real samples.

Mercury ion (Hg2+) is among the most toxic metal ions and can damage various organs, central nervous system, digest system and so on.1-3 At the same time, Hg2+ is quite recalcitrant to degrade in the environment, which may lead to serious long-term damage because of enrichment through the food chain.4-6 Therefore, selective and sensitive detection of Hg2+ is of great importance for environment protection, food safety and human health. To date, many traditional methods have been constructed to determine Hg2+, including atomic absorption/emission spectroscopy,7-8 inductively coupled plasmamass spectrometry,9 atomic fluorescence spectrometry,10 Xray absorption spectrometry.11 However, these strategies suffer from time-consuming, large sample requirement and expensive costs. Recently, various fluorescent assays have been developed to detect Hg2+ due to unique advantages such as short response time, easy operation and low required sample volume.12-18 Especially, DNA-based fluorescent assays towards Hg2+ detection were widely reported based on the specifically binding of thymine (T) toward Hg2+ and the formation of T-Hg2+-T structure.13-14, 19-23 Sensors based on Hg2+-induced fluorescence quenching were developed for the determination of Hg2+ by employing semiconductor quantum dots (QDs),24 gold nanoclusters,25-26 and carbon nanomaterials.27-29 Compared with T-Hg2+-T model, these assays were much more simple and low-cost. Zang et al. demonstrated to prepare βlactoglobulin-stabilized gold nanoclusters for the measurement of Hg2+ with high sensitivity in beverages, urine, and serum.26 Li et al. reported a label-free sensor for Hg2+ detection in water

and bioimaging in living cells based on the coordination between Hg2+ and N, S-doped carbon dots.28 However, most of these methods adopt the change of single fluorescence peak intensity as the signal output, which is easily affected by instrumental efficiency and environmental conditions.30 Ratiometric fluorescence assay is more effective in the detection of complex samples through the measurement the ratio of two signals, which can minimize the influence of instruments, background and environment signal.31-32 Black phosphorus (BP) QDs have been received significant attention due to their outstanding properties and promising applications in electronic devices,33 vapor sensor34-35 and cancer therapy.36-37 BP QDs were firstly synthesized by Zhang’s group through a top-down route from bulk BP in 2015.33 Recently, Sun et al. reported a liquid exfoliation method to obtain ultrasmall BP QDs and used as NIR photothermal agent.36 BP QDs can also be obtained in ionic liquids through sonication and applied in fluorescent bioimaging38-39. However, the application of BP QDs in biosensors is in the preliminary stage and needs to be further explored. Herein, a label-free ratiometric fluorescence sensor was designed to detect Hg2+ using BP QDs as fluorescent probe (Scheme 1). The BP QDs were synthesized through sonication-assisted solvothermal approach from bulk BP via a top-down route. The as-constructed sensor was based on the inner filter effect (IFE) between tetraphenylporphyrin tetrasulfonic acid (TPPS) and fluorescent BP QDs (Scheme 1A) because of an overlap between the excitation spectrum of BP QDs and the absorption spectrum of TPPS. On the basis of these findings, the fluorescence of BP QDs was

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quenched by TPPS through IFE mechanism. Besides, Hg2+ can accelerate the coordination reaction between Mn2+ and TPPS (Scheme 1B). The formation of Mn2+-TPPS complex results in the decrease of absorption intensity of TPPS, and further induces the recovery of blue-green fluorescence of BP QDs and the reduction of red fluorescence of TPPS. To the best of our knowledge, it is for the first time that BP QDs-based ratiometric fluorescence sensor was reported through the catalytic ability of Hg2+ toward the coordination between Mn2+ and TPPS.

Scheme 1. (A) Schematic illustrating the ratiometric fluorescence detection of Hg2+ based on the IFE between BP QDs and TPPS; (B) the catalytic effect of Hg2+ towards the coordination reaction between Mn2+ and TPPS.

EXPERIMENT SECTION Chemicals. N-methyl-2-pyrrolidinone (NMP, AR), hydrochloric acid (AR) and mercuric chloride (HgCl2, AR) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Manganese chloride tetrahydrate (MnCl2⸱4H2O, 99.99%), Sulforhodamine 101 (98%), boric acid (H3BO3, 99%), sodium tetraborate decahydrate (Na2B4O7⸱10H2O) were purchased from Aladdin Industrial Co. Ltd. (Shanghai, China). TPPS (85%) was purchased from TCI development Co. Ltd. (Shanghai, China). Bulk BP (99.998%) was obtained from XFNANO Materials Tech Co. Ltd (Nanjing, China). Deionized distilled water was used throughout experiments. Characterizations. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on a HT-7700 TEM system (Hitachi, Japan). X-ray photoelectron spectra (XPS) were acquired from an AXIS Ultra DLD X-ray photoelectron spectrometer (Shimadzu, Japan). UV-vis spectra were taken on a UV-2550 spectrophotometer (Shimadzu, Japan). Fluorescence spectra and fluorescence lifetime were recorded on an F-7000 (Hitachi, Japan) and FLS980 (Edinburgh, UK) fluorescence spectrometer, respectively. Synthesis of BP QDs. Fluorescent BP QDs were synthesized through solvothermal method via a top-down route by using bulk BP as the precursor. Firstly, bulk BP was dispersed in NMP solvent and then kept vigorous stirring for 12 h at 140 oC under the protection of N2. During the process, the solution was gradually turned to light yellow. The resultant QDs were obtained by centrifugation and stored at 4 oC. Fluorescence sensor towards Hg2+. As for the sensing of Hg2+, 12 µL TPPS (0.25 mM), 5 µL Mn2+ (0.02 M) and 50 µL Hg2+ with different concentrations were added to 833 µL borate buffer solution (BBS, 10 mM). The solution was incubated at room temperature for 30 min, and then 100 µL BP QDs

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were added. After that, the fluorescence spectra under the excitation wavelength of 412 nm were recorded. In order to investigate the selectivity of the sensor toward Hg2+, various cation and anion ions were examined under the same analytical procedure.

RESULTS AND DISCUSSION Characterization of BP QDs. Uniform BP QDs were synthesized through solvothermal route from bulk BP, which could be observed from the TEM image (Figure 1A). The average size is about 1.76 ± 0.32 nm (inset in the Figure 1A) and is much smaller than that of previous reports.33-34, 36, 38-39 We further investigated the morphology of the BP QDs by AFM (Figure 1B). The thickness of BP QDs is about 1.2 nm, implying the bilayered structure.40 XPS was performed to confirm the composition of the as-prepared BP QDs. Figure 1C shows two distinct peaks at 129.7 and 130.5 eV, assigning to the characteristic features of bulk BP.41-42 Other peaks at 134.0 and 133.2 eV indicates the partially oxidation of QDs. The lone pair electrons in BP make it sensitive to water and oxygen, thus, the oxidation of BP QDs has been widely observed.33-34, 36, 39 UV-vis absorption spectroscopy was used to investigate the optical property of BP QDs (Figure S1). Figure 1D reveals the excitation-dependent fluorescence of QDs, and the emission peak is red-shifted from 505 to 549 nm with the excitation wavelength varied from 400 to 480 nm. And the maximum emission peak is located at 523 nm under the excitation of 440 nm, which may attribute to the polydispersity of QDs.43-45 In addition, BP QDs display blue-green fluorescence under the excitation of UV light (inset in the Figure 1D). These results reveal the successful preparation of fluorescent BP QDs.

Figure 1. (A) TEM, (B) AFM images, (C) high resolution peakfitting P 2p XPS spectrum and (D) excitation-dependent fluorescence spectra of BP QDs. Inset of A and B show the size distribution of BP QDs and the height profile along the line overlaid on the AFM image, respectively. Inset of D shows the digital photograph of BP QDs colloidal solution triggered by UV light (365 nm).

Quenching BP QDs with TPPS. Porphyrin and their derivatives are widely presented in nature and play important roles in

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physiological and pathological processes. Besides, they can coordinate with metal ions and are applied in the detection of metal ions.46-50 In this work, TPPS with red fluorescence can quench the blue-green fluorescence of BP QDs. As shown in Figure S2, the fluorescence intensity of BP QDs is decreased gradually with the addition of TPPS. Meanwhile, the red fluorescence of TPPS is gradually enhanced. The fluorescent quenching mechanism by TPPS was further studied. It can be seen in Figure 2A that there is a spectrum overlap between the excitation of BP QDs and the absorption of TPPS, suggesting the IFE between BP QDs and TPPS. The IFE was further corrected through Equation S1 based on the cuvette geometry (Figure S3) and the changes in the absorbance, and the results were listed in Table S1. Figure 2B displays the suppressed efficiency of the observed and corrected fluorescence of BP QD, implying the fluorescence quenching is mainly attributed to the IFE of TPPS towards BP QDs. At the same time, some other quenching mechanism might be coexisted in the quenching system. The quenching mechanism was further explored by timeresolved fluorescence spectrometry. Fluorescence lifetime can be used to distinguish the dynamic quenching or static quenching,51 because it changed proportionally with the concentration of quencher for dynamic quenching and kept constant for static quenching.52-54 Figure 2C demonstrates the fluorescence lifetimes of BP QDs are about 2.03 and 1.98 ns without or with TPPS, respectively. The result indicates that the fluorescence quenching might be ascribed to static quenching. What’s more, the Stern-Volmer equation (Equation 1) can be used to describe the fluorescence quenching,  

 1    1  

Equation 1

F and F0 are fluorescence intensities with and without TPPS, respectively. Ksv is quenching constant. cq is the concentration of TPPS. Kq is quenching rate constant and τ0 is the fluorescence lifetime of BP QDs. In our developed sensing system, a good linear relationship is obtained between F0/F and the concentration of TPPS. Ksv is about 5.39×105 M-1 by the slope of the regression line (Figure 2D). And Kq is calculated to be 2.65×1014 M-1 s-1 based on the fluorescence lifetime of BP QDs (2.03 ns) and Ksv, which is much higher than the possible value for dynamic quenching effect (1.0×1010 M-1 s-1).55 Both the fluorescence lifetime data and the high quenching rate constant indicate the existence of static quenching effect.56

corrected (red line) fluorescence quenching efficiency of TPPS towards the fluorescence of BP QDs at 523 nm. E=1-F/F0. F0 and F are the fluorescence intensities of BP QDs in the absence and presence of TPPS, respectively. (C) Fluorescence lifetime of BP QDs in the presence of different concentrations of TPPS (0, 1, 2, 3, 4 µM) by monitoring the emission at 523 nm (λex=412 nm). (D) Stern-Volmer plot for the quenching of TPPS towards the fluorescence of BP QDs at 523 nm

Mechanism for the detection of Hg2+. It is well known that porphyrin derivatives can coordinate with metal ions, but the reaction rate and the stability of metalloporphyrin complexes are quite different from each other. As for Mn-TPPS, the kinetic study revealed that the rate of the incorporation of Mn2+ by TPPS was very slow.57 With the assistance of small amounts of Hg2+ (for example, nM), the reaction could be obviously accelerated. It is because the formation of Hg2+-TPSS complex makes the configuration favorable for the coordination with Mn2+ as well as the releasement of Hg2+ (shown in Scheme 1B). In other words, Hg2+ acts as catalyst in the formation of Mn-TPPS. The phenomenon was verified by the following experiments (shown in Figure 3A). Since the concentration of Hg2+ is two orders of magnitudes lower than that of TPPS in our detecting system, little TPPS can be turned to metalloporphyrin in the presence of Hg2+. With Mn2+ and TPPS in the solution, the reaction is too slow to occur. Thus, few change of the UV-vis absorption of the TPPS can be observed in the presence of Hg2+ or Mn2+, respectively. However, the absorbance at 412 nm for TPPS is dramatically reduced in the condition of the coexistence of Hg2+, Mn2+ and TPPS, which indicates the fast formation of Mn-TPPS complex. In addition, the IFE between TPPS and BP QDs is weakened during the process, which leads to the recovery of fluorescence of BP QDs and the reduction of fluorescence of TPPS (Figure 3B). These results also confirm that Hg2+ can catalyze the coordination of Mn2+ and TPPS. In addition, the feasibility of ratiometric fluorescence detection of Hg2+ is also proved.

Figure 3. (A) UV-Vis spectra of TPPS, TPPS+Mn2+, TPPS+Hg2+, and TPPS+Mn2+ +Hg2+, respectively. (B) Fluorescence spectra of BP QDs mixed with TPPS, TPPS+Mn2+, TPPS+Hg2+ and TPPS+ Mn2++Hg2+. The concentration of BP QDs, TPPS, Mn2+ and Hg2+ are 0.1 µg mL-1, 3 µM, 100 µM and 40 nM, respectively.

Figure 2. (A) UV-vis spectrum of TPPS (black solid line), fluorescence excitation spectrum (red dash line) and emission spectrum (blue dot line) of BP QDs. (B) Observed (black line) and

Ratiometric detection of Hg2+ based on BP QDs. We further evaluated the ratiometric fluorescence assay towards Hg2+ detection. As shown in Figure 4A, the fluorescence spectra of the sensor are recorded after the addition of various concentration of Hg2+ (1-100 nM). With the increase of Hg2+ concentration, the green fluorescence of BP QDs at 523 nm is gradually recovered and the red fluorescence of TPPS at 649 nm is dramatically deceased (Figure 4B). Figure 4C shows the change of the ratio of these two fluorescence intensity (I523/I649) with different concentration of Hg2+, and there is a good linear relationship between intensity ratio and Hg2+ concentration in the range of 1 - 60 nM (R2=0.983) with a detection limit of 0.39 nM (3σ) (inset in the Figure 4C). In addition, the UV-vis ab-

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sorption spectra were also unitized to assess the sensing assay. Figure S3A displays that the absorption peak of TPPS is decreased with the increase of Hg2+ concentration, and tends to be stable at high Hg2+ concentration (40 nM). The relationship between absorption intensity of TPPS versus Hg2+ concentration is shown in Figure S3B with a linear response ranging from 2 to 25 nM (inset in the Figure S3B). The results demonstrate that the ratiometric fluorescence technique possesses more excellent performance than that of UV-vis absorption. The maximum contaminant level of Hg2+ in drinking water is 10 nM by the United States Environmental Protection Agency58. Thus, the ratiometric fluorescence can be used to detect Hg2+ in water samples. To further investigate the selectivity of the ratiometric fluorescence sensor, various potential interfering ions were investigated in the BP QDs-TPPS-Mn2+ system. As shown in Figure 4D, few ions have influence on the fluorescence responses except for Hg2+. It is worth mentioning that Pb2+ and Cd2+ are universal interfering ions in the detection of Hg2+. However, in this system, Pb2+ and Cd2+ do not show obvious interference, even the concentration of Pb2+ is 10-fold higher than that of Hg2+. It might ascribe to the ionic radius-dependent catalytic effect for the formation of metalloporphyrin. It was reported that Hg2+ with relative larger radius displayed better catalytic ability.57 Based on above-mentioned results, the ratiometric fluorescence sensor possesses excellent selectivity towards Hg2+ detection.

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ery is over 102.3-104.8% with RSD of 1.76-2.14%. And Hg2+ is successfully detected in the river water, which is about 3.03 nM with the RSD of 2.30%. These results indicate the ratiometric fluorescence sensor has great potential for the Hg2+ detection in environmental water samples. Table 1. Recoveries for the detection of Hg2+ in the water samples (n=3). Samples

Hg2+ (nM)

drinking pure water

0

tap water

river water

Addition (nM)

Detected (nM)

recovery (%)

RSD (%)

10

9.81±0.22

98.1

2.28

0

20

19.71±0.35

98.5

1.76

0

30

31.09±1.25

103.6

4.02

0

10

10.48±0.18

104.8

1.76

0

20

20.47±0.37

102.3

1.80

0

30

30.85±0.66

102.8

2.14

3.03 ±0.07

-

-

-

2.30

CONCLUSION In summary, BP QDs with blue-green fluorescence were successfully synthesized and used to construct of a ratiometric sensor for Hg2+ detection. The sensing system was based on the catalytic activity of Hg2+ toward the coordination of Mn2+ and TPPS as well as the IFE between TPPS and BP QDs. The fluorescence of BP QDs can be quenched by TPPS due to the significant overlay of the emission spectrum of BP QDs and the absorption spectrum of TPPS. Mn2+ can coordinate with TPPS rapidly in the presence of Hg2+, resulting in the decrease of the absorption and fluorescence intensity of TPPS as well as the recovery of fluorescence of BP QDs. The ratiometric fluorescence sensor exhibits a good performance towards Hg2+ detection with high selectivity and sensitivity. In addition, this strategy can be applied in the determination of Hg2+ in real water samples with satisfactory results.

ASSOCIATED CONTENT

Figure 4. (A) Fluorescence responses of BP QDs-based assay towards different concentration of Hg2+. (B)The fluorescence intensity changes for BP QDs at 523 nm (black line) and TPPS at 649 nm (red line). (C) The plot of the fluorescence ratio versus the concentration of Hg2+. Inset of C shows the linear plots of fluorescence ratio versus Hg2+ concentration. (D) Fluorescence responses of BP QDs-TPPS-Mn2+ system towards different ions. Hg2+ and Cd2+ concentration are 40 nM, and other ions are 400 nM. Black bars represent the addition of appropriate metal ions. Red bars represent the addition of Hg2+ and appropriate metal ions.

Hg2+ detection in real samples. To investigate the practicality of the ratiometric fluorescence sensor, Hg2+ was detected in real samples, such as drinking pure water, tap water and river water (Table 1). Due to the very low concentration, Hg2+ can not be detected in drinking pure water and tap water. We then employed the standard addition method for Hg2+ determination. As for the drinking pure water samples, the recovery of Hg2+ is over the range of 98.1-103.6% with the relative standard deviation (RSD) of 1.76-4.02%. For tap water samples, the recov-

UV-vis spectrum of BP QDs, fluorescence quench of TPPS towards BP QDs, IFE correction, Absorption response of TPPS towards Hg2+. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported from the National Natural Science Foundation of China (21605050), Shanghai Natural Science Foundation (15ZR1411600), the Postdoctoral Science Foundation of China (2015M570349), National Key Research and Development Program of China (2016YFF0203704), and General Administration of Quality Supervision, Inspection and Quarantine of China (No. 2016IK223).

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19. Cui, X.; Zhu, L.; Wu, J.; Hou, Y.; Wang, P.; Wang, Z.; Yang, M., A Fluorescent Biosensor Based on Carbon Dots-Labeled Oligodeoxyribonucleotide and Graphene Oxide for Mercury (II) Detection. Biosens. Bioelectron., 2015, 63, 506-512. 20. Wang, H.; Wang, Y.; Jin, J.; Yang, R., Gold NanoparticleBased Colorimetric and Turn-On Fluorescent Probe for Mercury(II) Ions in Aqueous Solution. Anal. Chem., 2008, 80, 9021-9028. 21. Huang, D.; Niu, C.; Ruan, M.; Wang, X.; Zeng, G.; Deng, C., Highly Sensitive Strategy for Hg2+ Detection in Environmental Water Samples Using Long Lifetime Fluorescence Quantum Dots and Gold Nanoparticles. Environ. Sci. Technol., 2013, 47 (9), 4392-4398. 22. Kong, L.; Wang, J.; Zheng, G.; Liu, J., A highly sensitive protocol (FRET/SIMNSEF) for the determination of mercury ions: a unity of fluorescence quenching of graphene and enhancement of nanogold. Chem. Commun., 2011, 47 (37), 10389-10391. 23. Li, L.; Wen, Y.; Xu, L.; Xu, Q.; Song, S.; Zuo, X.; Yan, J.; Zhang, W.; Liu, G., Development of mercury (II) ion biosensors based on mercury-specific oligonucleotide probes. Biosens. Bioelectron., 2016, 75, 433-445. 24. Ding, X.; Qu, L.; Yang, R.; Zhou, Y.; Li, J., A highly selective and simple fluorescent sensor for mercury (II) ion detection based on cysteamine-capped CdTe quantum dots synthesized by the reflux method. Luminescence, 2015, 30 (4), 465-471. 25. Zhang, Y.; Yan, M.; Jiang, J.; Gao, P.; Zhang, G.; Choi, M. M. F.; Dong, C.; Shuang, S., Highly selective and sensitive nanoprobes for Hg(II) ions based on photoluminescent gold nanoclusters. Sensor. Actuat. B: Chem., 2016, 235, 386-393. 26. Zang, J.; Li, C.; Zhou, K.; Dong, H.; Chen, B.; Wang, F.; Zhao, G., Nanomolar Hg2+ Detection Using β-Lactoglobulin-Stabilized Fluorescent Gold Nanoclusters in Beverage and Biological Media. Anal. Chem., 2016, 88 (20), 10275-10283. 27. Lu, W.; Qin, X.; Liu, S.; Chang, G.; Zhang, Y.; Luo, Y.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X., Economical, Green Synthesis of Fluorescent Carbon Nanoparticles and Their Use as Probes for Sensitive and Selective Detection of Mercury(II) Ions. Anal. Chem., 2012, 84 (12), 5351-5357. 28. Li, L.; Yu, B.; You, T., Nitrogen and Sulfur Co-doped Carbon Dots for Highly Selective and Sensitive Detection of Hg (II) Ions. Biosens. Bioelectron., 2015, 74, 263-269. 29. Wang, Z. X.; Ding, S. N., One-Pot Green Synthesis of High Quantum Yield Oxygen-doped, Nitrogen-rich, Photoluminescent Polymer Carbon Nanoribbons as an Effective Fluorescent Sensing Platform for Sensitive and Selective Detection of Silver (I) and Mercury (II) Ions. Anal. Chem., 2014, 86 (15), 7436-7445. 30. Dai, Q.; Liu, W.; Zhuang, X.; Wu, J.; Zhang, H.; Wang, P., Ratiometric Fluorescence Sensor Based on a Pyrene Derivative and Quantification Detection of Heparin in Aqueous Solution and Serum. Anal. Chem., 2011, 83 (17), 6559-6564. 31. Grynkiewicz, G.; Poenie, M.; Tsien, R. Y., A New Generation of Ca2+ Indicators with Greatly Improved Fluorescence Properties. J. Biol. Chem., 1985, 260 (6), 3440-3450. 32. Wang, K.; Qian, J.; Jiang, D.; Yang, Z.; Du, X.; Wang, K., Onsite Naked Eye Determination of Cysteine and Homocysteine Using Quencher Displacement-Induced Fluorescence Recovery of the Dual-Emission Hybrid Probes with Desired Intensity Ratio. Biosens. Bioelectron., 2015, 65, 83-90. 33. Zhang, X.; Xie, H.; Liu, Z.; Tan, C.; Luo, Z.; Li, H.; Lin, J.; Sun, L.; Chen, W.; Xu, Z.; Xie, L.; Huang, W.; Zhang, H., Black Phosphorus Quantum Dots. Angew. Chem., Int. Ed., 2015, 54 (12), 3653-3657. 34. Sofer, Z.; Bous, D.; Luxa, J.; Mazanek, V.; Pumera, M., FewLayer Black Phosphorus Nanoparticles. Chem. Commun., 2016, 52 (8), 1563-1566. 35. Zhu, C.; Xu, F.; Zhang, L.; Li, M.; Chen, J.; Xu, S.; Huang, G.; Chen, W.; Sun, L., Ultrafast Preparation of Black Phosphorus Quantum Dots for Efficient Humidity Sensing. Chem. Eur. J., 2016, 22(22), 7357-7362. 36. Sun, Z.; Xie, H.; Tang, S.; Yu, X. F.; Guo, Z.; Shao, J.; Zhang, H.; Huang, H.; Wang, H.; Chu, P. K., Ultrasmall Black Phosphorus Quantum Dots: Synthesis and Use as Photothermal Agents. Angew. Chem., Int. Ed., 2015, 54 (39), 11526-11530.

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37. Sun, C.; Wen, L.; Zeng, J.; Wang, Y.; Sun, Q.; Deng, L.; Zhao, C.; Li, Z., One-Pot Solventless Preparation of PEGylated Black Phosphorus Nanoparticles for Photoacoustic Imaging and Photothermal Therapy of Cancer. Biomaterials 2016, 91, 81-89. 38. Zhao, W.; Xue, Z.; Wang, J.; Jiang, J.; Zhao, X.; Mu, T., Large-Scale, Highly Efficient, and Green Liquid-Exfoliation of Black Phosphorus in Ionic Liquids. ACS Appl. Mater. Interfaces, 2015, 7 (50), 27608-27612. 39. Lee, H. U.; Park, S. Y.; Lee, S. C.; Choi, S.; Seo, S.; Kim, H.; Won, J.; Choi, K.; Kang, K. S.; Park, H. G.; Kim, H. S.; An, H. R.; Jeong, K. H.; Lee, Y. C.; Lee, J., Black Phosphorus (BP) Nanodots for Potential Biomedical Applications. Small 2016, 12 (2), 214-219. 40. Xia, F.; Wang, H.; Jia, Y., Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun., 2014, 5, 289-305. 41. Kang, J.; D.Wood, J.; Wells, S. A.; Lee, J. H.; Liu, X.; Chen, K. S.; Hersam, M. C., Solvent Exfoliation of Electronic-Grade, TwoDimensional Black Phosphorus. ACS Nano 2015, 9 (4), 3596-3604. 42. Favron, A.; Gaufrès, E.; Fossard, F.; Heureux, A.; Tang, N. Y. W.; Lévesque, P. L.; Loiseau, A.; Leonelli, R.; Francoeur, S.; Martel, R., Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nat. Mater., 2015, 14 (8), 826-832. 43. Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.; Deng, L.; Hou, Y.; Qu, L., An Electrochemical Avenue to Green-Luminescent Graphene Quantum Dots as Potential Electron-Acceptors for Photovoltaics. Adv. Mater., 2011, 23 (6), 776-780. 44. Gu, W.; Yan, Y.; Zhang, C.; Ding, C.; Xian, Y., One-Step Synthesis of Water-Soluble MoS2 Quantum Dots via a Hydrothermal Method as a Fluorescent Probe for Hyaluronidase Detection. ACS Appl. Mater. Interfaces, 2016, 8 (18), 11272-11279. 45. Gu, W.; Yan, Y.; Cao, X.; Zhang, C.; Ding, C.; Xian, Y., A Facile and One-Step Ethanol-Thermal Synthesis of MoS2 Quantum Dots for Two-Photon Fluorescence Imaging. J Mater. Chem. B, 2016, 4 (1), 27-31. 46. Yang, Y.; Jiang, J.; Shen, G.; Yu, R., An Optical Sensor for Mercury Ion Based on the Fluorescence Quenching of Tetra(pDimethylaminophenyl)Porphyrin. Anal. Chim. Acta, 2009, 636 (1), 83-88. 47. Shamsipur, M.; Sadeghi, M.; Beyzavi, M. H.; HashemSharghi, Development of a Novel Fluorimetric Bulk Optode Mmbrane Based on Meso-Tetrakis(2-Hydroxynaphthyl)Porphyrin (MTHNP) for

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Highly Sensitive and Selective Monitoring of Trace Amounts of Hg2+ Ions. Mater. Sci. Eng. C-Mater., 2015, 48, 424-433. 48. Ding, Y.; Zhu, W. H.; Xie, Y., Development of Ion Chemosensors Based on Porphyrin Analogues. Chem. Rev., 2017, 117(4), 2203-2256. 49. Zhang, L.; Peng, D.; Liang, R.-P.; Qiu, J.-D., Nitrogen-Doped Graphene Quantum Dots as a New Catalyst Accelerating the Coordination Reaction between Cadmium(II) and 5,10,15,20Tetrakis(1-methyl-4-pyridinio)porphyrin for Cadmium(II) Sensing. Anal. Chem., 2015, 87 (21), 10894-10901. 50. Zhao, L.; Li, M.; Liu, M.; Zhang, Y.; Wu, C.; Zhang, Y., Porphyrin-functionalized porous polysulfone membrane towards an optical sensor membrane for sorption and detection of cadmium(II). J. Hazard. Mater., 2016, 301, 233-241. 51. Yuan, Y.; Li, R.; Liu, Z., Establishing Water-Soluble Layered WS2 Nanosheet as a Platform for Biosensing. Anal. Chem., 2014, 86 (7), 3610-3615. 52. Chen, Y.; Rosenzweig, Z., Luminescent CdS Quantum Dots as Selective Ion Probes. Anal. Chem., 2002, 74 (19), 5132-5138. 53. Xiao, Q.; Huang, S.; Qi, Z. D.; Zhou, B.; He, Z. K.; Liu, Y., Conformation, Thermodynamics and Stoichiometry of HSA Adsorbed to Colloidal CdSe/ZnS Quantum Dots. BBA-Proteins Proteom., 2008, 1784 (7-8), 1020-1027. 54. Gao, Z.; Wang, L.; Su, R.; Huang, R.; Qi, W.; He, Z., A Carbon Dot-Based “Off–On” Fluorescent Probe for Highly Selective and Sensitive Detection of Phytic Acid. Biosens. Bioelectron., 2015, 70, 232-238. 55. Lakowicz, J. R., Principles of Fluorescence Spectroscopy, 3nd ed. Springer: 1999. 56. Zhai, W.; Wang, C.; Yu, P.; Wang, Y.; Mao, L., Single-Layer MnO2 Nanosheets Suppressed Fluorescence of 7-Hydroxycoumarin: Mechanistic Study and Application for Sensitive Sensing of Ascorbic Acid in Vivo. Anal. Chem., 2014, 86 (24), 12206-12213. 57. Tanaka, M., Kinetics of Metalloporphyrin Formation with Particular Reference to the Metal Ion Assisted Mechanism. Pure Appl. Chem., 1983, 55 (1), 151-158. 58. Agency, U. S. E. P., National Primary Drinking Water Regulations, EPA 816-F-09-004. Washington, D.C, 2009.

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A ratiometric fluorescence sensor has been constructed for the selective and sensitive detection of Hg2+ based on IFE of tetraphenylporphyrin tetrasulfonic acid (TPPS) towards fluorescence black phosphorus quantum dots (BP QDs) and the rapid coordination between Mn2+ and TPPS in the existence of Hg2+. The as-constructed sensor reveals a good linear response to Hg2+ ranging from 1 to 60 nM with a detection limit of 0.39 nM and can be applied to detect Hg2+ in real samples.

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