Rapid Detection of Mercury Ions Based on Nitrogen-Doped Graphene

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Rapid Detection of Mercury Ions Based on Nitrogen Doped Graphene Quantum Dots Accelerating Formation of Manganese Porphyrin Dong Peng, Li Zhang, Ru-Ping Liang, and Jian-Ding Qiu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00203 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 22, 2018

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Rapid Detection of Mercury Ions Based on Nitrogen Doped Graphene Quantum Dots Accelerating Formation of Manganese Porphyrin Dong Peng, Li Zhang, Ru-Ping Liang, and Jian-Ding Qiu* College of Chemistry, Nanchang University, Nanchang 330031, China

KEYWORDS: Fluorescence, Mercuric ion, Manganous ion, Graphene quantum dots, Porphyrin

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ABSTRACT: Mercuric ion (HgII) is a stable form of mercury pollution with high toxicity and bioaccumulation ability, and the sensitive and visible determination is of great importance. Herein, a simple method to magically improve the complexation reaction rate between porphyrin and manganous ions (MnII) have been proposed by using the synergistic effect of trace nitrogen doped graphene quantum dots (NGQDs) and Hg(II). A mechanism is proposed in accordance with a substitution reaction in which the deformed porphyrin nucleus by relatively larger Hg(II) ions is favourable for attacking of small divalent metal ions carried by NGQDs from the back. Meanwhile, the formation of metalloporphyrin is accompanied with the absorption red-shift and fluorescence quenching of porphyrins, simultaneously, the fluorescence of NGQDs are gradually enhanced because of the inner filter effect between porphyrins and NGQDs. Thus, the ratiometric fluorescence and colorimetric methods for trace HgII sensing have been proposed based on the distinct absorption/fluorescence spectral changes, which have potential application in complex environmental and biological conditions.


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Mercury is a well-known dangerous pollutant which is extensively existed in the ambient atmosphere, soil, water and bioregion.1-4 It is universal, persistent, and highly toxic that can cause numerous serious negative impacts on human health, including brain damage, kidney failure, motion disorder, and various cognitive.5-7 Mercuric ions (HgII), the typically inorganic species, is one of the most stable existence form of mercury pollution. Because of the high toxicity and bioaccumulation ability of HgII, various regulations and efforts have been exercised to control its concentration in the extremely low levels.8,9 Fluorescence spectrometry and spectrophotometry have attracted tremendous attention owning to the features of simple operation,

low

cost,

excellent

selectivity

and

sensitivity.10-12

Various

ratiometric

fluorescence/colorimetric HgII probes have been fabricated by the self-calibration of two different emission or absorption bonds, which can efficiently eliminate the interferences from the environmental conditions.13-15 However, for the traditional ratiometric fluorescent methods based on the intramolecular charge transfer or fluorescence resonance energy transfer, the energy/charge transfer processes are usually strict between the fluorescer and quencher. Inner filter effect (IFE), resulting from the absorption of the excitation and/or emission beam by absorbers in the reaction system,16-18 is much easier to take place which facilitating the further application in complex biological conditions. Porphyrins are a class of naturally occurring macrocyclic compounds with excellent optic properties (including high molar absorbance and great fluorescence), which exhibit great sensing performance.19-21 To the complexation of small divalent metal ions into porphyrin structure, it occurs extremely slowly for the difficulty in deforming the porphyrin ring plane.22 Tremendous efforts have been made to accelerate the formation rate, and the substitution reaction is one of the most explored.22-26 It is suggested that the large metal ions such as CdII, PbII and HgII cannot fit


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into the porphyrin nucleus but rather sit on the top of porphyrin ring, which deforms its structure for favoring the attack by another small metal ions (like MnII) from the back.24,27-29 However, these acceleration effects are greatly limited by the concentrations of the small metal ions and the severe auxiliary condition.24,27,30-32 Recently, nanomaterials have been investigated for accelerating the formation of metaloporphyrins. It has been reported that the complexation reaction between 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) and CdII can be greatly accelerated with the introducing of chemically converted graphene (CCG) sheets.33 In addition, we found that NGQDs could also promote the coordination reaction rate of TMPyP and CdII.34 However, the simultaneous usage of these two strategies for the small divalent metal ion incorporation is still less studied. Herein, we found that the incorporation rate of TMPyP and small MnII were magically promoted with the coexistence of trace NGQDs and nanomolar HgII. A synergistic mechanism is raised that the relatively larger Hg(II) ions effectively deforme the porphyrin nucleus which is favourable for the attacking of smaller divalent metal ions carried by NGQDs from the back (as shown in Scheme 1). The formation of metalloporphyrin was accompanied with the absorption red-shift and fluorescence quenching of TMPyP, and the fluorescence of NGQDs were gradually enhanced because of the IFE of TMPyP on NGQDs. Based on the interesting phenomenon, the sensitive and selective ratiometric fluorescence and colorimetric methods for determining trace HgII have been proposed. Meanwhile, because of the great cellular compatibility and low toxicity of NGQDs and the wide distribution of porphyrin in plants and animals, we also expect the present method could be applied for monitoring HgII in complex environmental and biological conditions.


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Scheme 1. Schematic illustration of the synergistic effect of NGQDs and HgII in accelerating the coordination rate of MnII and TMPyP (the substitute procedures are shown in the middle circle).

EXPERIMENTAL SECTION Synthesis of NGQDs. The NGQDs were synthesized with a two-step hydrothermal method as described in our previous work.34 The appearance of the N 1s peak in the full range XPS spectrum of NGQDs confirms the nitrogen atoms successfully doped into the graphene lattice, and the fluorescence characterization shows that NGQDs owns excellent fluorescence properties (Figure S1 in Supporting information). Detection procedure. A typical HgII detection procedure was conducted as follows. 160 µL of 50 µM TMPyP solution, 80 µL of 400 µg L-1 NGQDs solution, 160 µL of 20.0 mM phosphate buffer (pH 7.0), 64 µL 1 mM MnII solution, and various amounts of HgII solution were added. Different amount of water were subsequently added to ensure a final volume of 1600 µL. After being incubated for 30 min at ambient temperature, the fluorescence and UV−vis absorption spectra were recorded. It is noteworthy that the detection procedures could be simplified by


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preparing a detection buffer containing PBS, NGQDs, TMPyP and MnII in advance, and then mix the detection buffer with the sample for further detection. Cell Imaging studies. Lung cancer cells (A549) were washed three times with PBS buffer and then incubated with 200 mM HgII in DMEM for half an hour. The cells were washed for three times with PBS, and further incubated with TMPyP (100 µL, 50 µM), PBS buffer (pH 7.0, 40 µL, 20 mM), MnII solution (20 µL, 1 mM), NGQDs (20 µL, 1 mg mL−1), and 220 µL ultrapure water for 30 min. After being gently rinsed three times with PBS, cells were fixed and mounted for microscopy observation. The fluorescence images were obtained at an excitation wavelength of 405 nm. More experimental information is presented in the Supporting Information.

RESULTS AND DISCUSSION The Synergistic Effect for Accelerating the Metalloporphyrin Formation. As mentioned above, the rate for incorporation of MnII into TMPyP was extremely slow as indicated by the trivial change in the fluorescence of TMPyP under room temperature (Figure 1a, black curve). If large divalent metal ion such as HgII ions are present in the solution, they can accelerate the formation of MnIII(TMPyP) by deforming the porphyrin rings which favours being attacked by MnII from the back23. As shown in Figure 1b, the incorporation of MnII into TMPyP were accelerated by adding HgII into the mixture, and the reaction rates were proportional to the HgII concentration. However, a significant acceleration rate could only be observed within 60 min when the concentrations of HgII were higher than 1000 nM (Figure 1b), and the rate were extremely slow under a low Hg(II) concentration (e.g., 200 nM, Figure 1a, red curve). Similarly, NGQDs also could accelerate the formation of MnIII(TMPyP) depending on their concentrations


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due to an activation entropy effect (Figure 1c).35 With the co-existence of NGQDs and MnII in the TMPyP solution, the anionic form of manganese ions MnII(NGQDs) could associate with TMPyP through the electrostatic interaction, which successfully overcome the electrostatic repulsion between MnII and TMPyP thus accelerating the coordination rate between TMPyP and MnII for several orders of magnitude. However, a large amount of NGQDs were still failed to accelerate the metalloporphyrin formation in a relatively short time (Figure 1a, the blue curve). Although HgII and NGQDs could accelerate the formation of MnIII(TMPyP) in high concentration individually, neither 200 nM HgII nor 20.0 µg L-1 NGQDs alone could accelerate the incorporation between MnII and TMPyP mainly due to their concentration effect (Figure 1a). Magically, the coordination reaction rate was markedly increased when trace HgII and NGQDs were coexisted, and the reaction could be completed within 30 min (Figure 1a, cyan curve). The coordination rate with the coexistence of HgII and NGQDs was much superior to that of the simply sum of their individual contributions. Besides, the coordination reaction rate was greatly improved accompanied with the increasement of the NGQDs concentration (Figure 1d). The fluorescence intensities of TMPyP were recorded at 658 nm as the reaction time (t) (Figure S2a in Supporting Information). The kinetics of MnIII(TMPyP) formation follows the first-order kinetics,36 which could be represented with eq 1: −݀ሾܶ‫ܲݕܲܯ‬ሿ/݀‫݇ = ݐ‬௢௕௦ௗ ሾܶ‫ܲݕܲܯ‬ሿ

(1)

where kobsd is the coordination rate constant relating with concentrations of NGQDs, HgII, and MnII. The plot of FL intensities as the function of t gives straight lines, and the slope represents the kobsd. Figure S2a shows that the value of kobsd were positively associated with NGQDs concentration, and the value of [NGQDs]/kobsd remained constant (-37.40) over the concentrations of NGQDs.


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Figure 1. (a) FL intensities of TMPyP versus reaction time in various conditions. (b) FL intensities of TMPyP with the addition of MnII and different concentrations of HgII versus reaction time. (c) FL intensities of TMPyP with the addition of MnII and various amount of NGQDs versus reaction time. (d) FL intensities of TMPyP with the addition of HgII, MnII and different amount of NGQDs versus reaction time. All the FL intensities were recorded at 658 nm with the excitation at 420 nm. Except for specifically states, the concentrations of TMPyP, NGQDs, MnII and HgII were 5.0 µM, 20.0 µg L-1, 40 µM, and 200 nM, respectively. The Synergistic Acceleration Mechanism. Based on this interesting phenomenon, a synergistic acceleration mechanism of HgII and NGQDs is suggested in terms of the substitution reaction as well as the accelerative effect of NGQDs for metalloporphyrin formation (Scheme 1). On the one hand, the “deformer” HgII coordinates with TMPyP to form a ‘sitting-atop’ complex


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rapidly, which facilitates attacking of MnII to the porphyrin nucleus from underneath. On the other hand, NGQDs functioning as MnII carrier associate with MnII to form MnII (NGQDs) complex. The negatively charged MnII(NGQDs) complex is easily attracted by the positively charged HgII(TMPyP) via electrostatic interaction, and the hydrogen-bond interaction between NGQDs and the ring of TMPyP can also efficiently attracte the MnII(NGQDs) complex around the porphyrin ring. Thus, the molecular complex HgII(TMPyP)•MnII(NGQDs) form rapidly via hydrogen-bond interaction and electrostatic interaction. The electrostatic repulsion between TMPyP and MnII are reduced under the assistance of NGQDs, which facilitates the carried MnII to attack the porphyrin from underneath forming a transition state of HgII-TMPyPMnII•NGQDs.22,23 During the transition state, MnII and HgII coordinate with TMPyP in the two sides of the porphyrin ring respectively. Simultaneously, NGQDs is still attracted by MnII via electrostatic interaction. The HgII is released and the MnII is chelated into the porphyrin ring completely to form MnII(TMPyP) and then be rapidly oxidized to its manganese(III) complex by the oxygen in the aqueous solution.22,37,38 The released HgII is repeatedly associating with the residual TMPyP and accelerates the formation of MnIII(TMPyP). With the addition of NGQDs and MnII into the TMPyP solution, the complexation reaction also could be completed even in the low HgII concentrations by prolong the reaction time, revealing the repeated usage of mercury ions (Figure S2b in the Supporting Information). The assembling of NGQDs with MnII or TMPyP was confirmed by the AFM images and zeta potential determination. As shown in Figure 2, compared with the initial TMPyP, the significant topographical variation and the height increment after the addition of NGQDs intuitively revealed the successfully assembly of NGQDs with TMPyP. Owning to the abundant functional groups (including hydroxyl, carboxyl, and epoxy groups), and better surface grafting via the π–π


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conjugated network for their structural graphene layers, NGQDs are able to associate with large biomaterials, small organic molecules, and inorganic ions.39-43 The zeta potential of NGQDs was -41.0 mV, while the zeta potential for MnII (NGQDs) and NGQDs•TMPyP were increased to 20.6 and -16.8, respectively, which revealing that TMPyP and MnII could bond with NGQDs via electrostatic interaction (Figure S3a in the Supporting Information). The preferential interaction of NGQDs with MnII instead of HgII also can be deduced from the increased value of zeta potential for MnII(NGQDs) compared with initial NGQDs or the HgII(NGQDs) mixture. Besides, the coordination reaction rate was also significantly influenced by the charge on porphyrin periphery. With the addition of NGQDs and different amount of HgII into the negatively charged 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin hydrate (TPPS) solution, no obvious catalytic effect was observed for the complexation reaction of MnII with TPPS under the same reaction conditions (Figure S3b in the Supporting Information). The electrostatic repulsion between TPPS and NGQDs inhibited the formation of a molecular complex MnII(NGQDs)•TPPS, resulting in the poor metal ion coordination rate.

Figure 2. AFM image of TMPyP(a), and AFM image of NGQDs•TMPyP (b).


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Sensing performance. Evolution of the optical signals of the present co-acceleration system was further applied for HgII detection. As shown in Figure S4a, the formation rate of MnIII(TMPyP) in the presence of HgII and NGQDs depended on pH. The synergistic effect became significant at pH>5.0, and then leveled off over the range pH 7.0-10.0, it was much consistent with the reported conclusions.31,32 Thus, pH = 7.0 was selected for the determination of HgII in consideration of the catalytic effect and hydrolysis of metal ions. Besides, the MnII concentration was optimized and the concentration of 40 µM was enough for the formation of MnIII(TMPyP) (Figure S4b in the Supporting Information). Under the optimized reaction conditions (40 µM MnII, 20.0 µg L-1 NGQDs, and pH = 7.0), the TMPyP emission (λex=420 nm) at 658 nm gradually decreased with the HgII concentration in a range of 2-200 nM, while the NGQDs emission band centered at 490 nm gradually increases due to the weaker and weaker ‘primary’ inner filter effect (IFE) (Figure 3a).44 The IFE result from the absorption of the incident beam before it reaches the point in the sample where fluorescence is observed (‘primary’ IFE), and the re-absorption of the emitted light before it leaves the cell (‘secondary’ IFE).44 Because the excitation wavelength (420 nm) is greatly overlapped with the maximum absorption wavelength of TMPyP (422 nm), the incident light is mainly absorbed by TMPyP which hinder the NGQDs excitation. It is mainly attributed to the ‘primary’ IFE between NGQDs and TMPyP.44 And the IFE is gradually reduced accompany with the formation of MnIII(TMPyP), which results in the enhancement of the emission of NGQDs. The IFE was further investigated based on the absorption characteristics of TMPyP and NGQDs and the cuvette geometry, and the results show that the fluorescence quenching effect are mainly caused by the IFE between NGQDs and TMPyP (Figure S5 and Table S1 in Supporting Information).16,45 In order to test whether there was energy transfer between NGQDs and


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TMPyP, the decay curve measurements were carried out in different reaction condition (Figure S5a, and Table S2 in Supporting Information). The unchanged lifetimes of NGQDs in the presence of TMPyP further verified the IFE-based mechanism. As shown in Figure S5b and Table S3, the fluorescence lifetime were significantly decreased compared with that of the free TMPyP after the addition of HgII, indicating the successful formation of MnIII(TMPyP) complex. The ratio of emission intensities at 490 nm to 658 nm (I490/I658) reached a maximum when the concentration of HgII ions was 200 nM, and the detection limit was calculated to be0.18 nM (3σ) (Figure 3b). A control experiment for HgII detection based on HgII-catalyzed MnIII(TMPyP) formation without NGQDs has also been carried out (Figure 3b). It is obvious that the presence of NGQDs not only can accelerate the formation of MnIII(TMPyP) but also greatly improve the sensitivity of the present assay. Under the 365 nm UV lamp, the bright pink emission of TMPyP was largely quenched upon addition of HgII with different concentrations, and the fluorescence of NGQDs was gradually recovered with a bright blue emission appearing. Thus the colorimetric evolution could be visual monitored under the 365 nm UV irradiation (picture inset of Figure 3b). Meanwhile, the original Soret band at 422 nm and the Q band at 518 nm of TMPyP decreased gradually while a new Soret band at 462 nm and a Q band at 562 nm appeared, confirming the formation of MnIII(TMPyP) instead of MnII(TMPyP) (Figure 3c).46,47 Inset of Figure 3c shows that the value of A462/A422 increase linearly with the HgII concentrations in a range of 2−200 nM, the detection limit was calculated to be 0.32 nM (3σ). The limit of detection for the fluorescence/colorimetric assay are much lower than the threshold levels defined by the U.S. Enviromental Protection Agency (EPA, 10 nM) and the World Health Organization (WHO, 30 nM), which enabling the proposed methods great practical significance for drinking water


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detection.8, 9 Moreover, the proposed fluorescent and colorimetric methods are much comparable with some previous reported ones (Table S4 in the Supporting information). As we know, some other “large metal ions” including PbII and CdII have also been used to accelerate the small divalent ions into the porphyrin rings.27,31 200 nM HgII could efficiently accelerated the formation of metalloporphyrin within 30 mins with the co-existence of NGQDs, however, PbII or CdII couldn’t induce much change in the fluorescence even with a high concentration (Figure S7a in the Supporting Information), which suggested that the ionic radius of metal ions (HgII>CdII>PbII) plays an important role for the corporation reaction.30 The association constants between TMPyP and metal ions are concerned with the ionic radius of the metal ions, the larger ionic radius, the smaller association constants.48 HgII shows a weaker binding affinity with the porphyrin ring, which can be replaced by MnII in a relative faster speed and raising the HgII reusing efficiency. Besides, be different from CdII, HgII shows a weak interaction force between HgII and NGQDs which enable HgII owns a much weaker resistance during its repeated using process.34 The incorporation between TMPyP and some other small divalent ions in the presence of HgII and NGQDs was also investigated (Figure S7b in the Supporting Information). The incorporation rates of MgII, NiII, and FeII into the porphyrin ring were much slower compared with that of MnII. The activity of these metals toward matalloporphyrin formation is following the order of MnII>FeII>NiII>MgII, which is consistence with the reported works.35,49,50 Meanwhile, the reaction between TMPyP and MnII caused the largest absorption spectral shift, thus, MnII is the best candidate to be applied for HgII sensing. To study the selectivity of the proposed method, we further investigated the interference of different ions according to their actual concentrations in real water samples. As shown in Figure 3d, the proposed method exhibits superior selectivity toward HgII compared with the other ions,


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revealing its potential application in real water assay. To study the possibility of the proposed method for organic mercury detection, the acceleration effects of methyl mercury and ethyl mercury were further studied. As shown in Figure S8, neither methyl mercury nor ethyl mercury could efficiently accelerate the MnIII(TMPyP), it is mainly due to inhibiting effect of the organic function groups of the organic mercury for deforming the porphyrin structure. Real water samples from the Ganjiang river were studied by the standard addition method (Table S5 in the Supporting information), and the results obtained by the proposed method are comparable with that of the ICP-MS method, proving its good applicability for practical application.

Figure 3. (a) Fluorescence spectra of 5.0 µM TMPyP with the addition of 20 µg L-1 NGQDs, 40 µM MnII and different concentrations of HgII (a to o: 0, 2, 5, 10, 15, 20, 25, 30, 50, 75, 100, 125, 150, 175, 200 nM). (b) Plot of the value of (I490/I658) vs HgII concentrations in the presence (the


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concentrations of HgII are 0, 2, 5, 10, 15, 20, 25, 30, 50, 75, 100, 125, 150, 175, 200 nM) or absence (the concentrations of HgII are 0, 100, 200, 400, 600, 800, 1000, 1400 nM) of NGQDs, respectively. Inset: the visual pictures corresponding to the HgII concentrations of 0, 25, 50, 75, 100, 125, 150, 175 and 200 nM; plot of the fluorescent ratio (I490/I658) vs HgII concentration (0100 nM) in the presence of NGQDs (c) Absorption spectra of 5.0 µM TMPyP with the addition of 20 µg L-1 NGQDs, 40 µM MnII and different concentrations of HgII; inset: plot of the value of (A462/A422) vs HgII concentrations with/without NGQDs, respectively. (d) Fluorescence quenching efficiency of 5.0 µM TMPyP with the addition of 20 µg L-1 NGQDs, 40 µM MnII and100 nM HgII or some other cations and anions (BaII, CdII, CoII, CuII, FeII, NiII, PbII, ZnII, HCO3-, Cl-at a concentration of 500 nM, and MgII, SO42-, NO3- at a concentration of 100 µM, and CaII at a concentration of 200 µM); inset: the visual picture shows fluorescence changes under 365 UV light. Intracellular Hg(II) Imaging. In consideration of the great cellular compatibility and low toxicity of NGQDs and the widely distribution of porphyrin in plants and animals,51-56 we supposed that the proposed method can be further applied to monitor intracellular HgII ions. Figure S9-10 show the cytotoxicity test results of TMPyP, NGQDs and MnII ions toward A549 cells. The cell viabilities are more than 90% after incubated with 5.0 µM TMPyP, 40 µM MnII, or 20 µg L-1 NGQDs for 24 h, indicating their excellent biocompatibilities for cell imaging. Cell images were obtained with the excitation wavelength at 405 nm and two band-pass emission filters at 490 ± 15 and 658 ± 15 nm. A549 cells incubated with TMPyP (5.0 µm) or NGQDs (20 µg L-1) show a clear red/green intracellular fluorescence, which suggests that TMPyP and NGQDs are cell permeable (Figure 4a-h). With the co-existence of TMPyP and NGQDs, the red emission of TMPyP does not show much change while the green emission of NGQDs becomes much darker


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(Figure 4i-l), which suggesting that the IFE between NGQDs and TMPyP can also efficiently occur in the biological environment. By contrast, with the simultaneous addition of TMPyP, MnII, HgII, and NGQDs, the red fluorescence of TMPyP was significantly quenched and a remarkable increase in the blue fluorescence of NGQDs was appeared, demonstrating the successfully formation of MnIII(TMPyP) complex under the synergistic effect of HgII and NGQDs. Be consistent with the fluorescence spectral, these experiments indicate that the established HgII sensor can provide ratiometric fluorescence detection for intracellular HgII ions. Hence, we supposed that it can be a useful molecular probe to study the biological processes involving intracellular HgII ions.


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Figure 4. Confocal microscopic images of Lung cancer cells (A549). First column: Images obtained through the red channel. Second column: Images obtained through the green channel. Third column: Bright field images of A549 cells. Fourth column: Merged images of the two channels and the corresponding bright field image. (a)-(d) Cells incubated with TMPyP; (e)-(h) Cells incubated with NGQDs; (i)-(l) Cell incubated with TMPyP, and NGQDs; (m)-(p) Cells incubated with TMPyP, MnII, HgII and NGQDs simultaneously. Scale bar: 20 µm.

CONCLUSION In summary, we found that the incorporate rate of the complexation reaction between MnII and TMPyP is magically promoted with the coexistence of trace NGQDs and HgII. Thus, a synergistic effect mechanism was raised which was discussed in terms of the substitution reaction that HgII deforms the porphyrin nucleus favourably for the NGQDs carrying the small divalent metal ions attack from the back. Therefore, this work provides a good example of utilizing the synergistic effect of graphene nanostructures and large divalent ions for the formation of metalloporphyrin. The coordination reaction is accompanied by the variation of absorption/fluorescence of TMPyP, meanwhile, and the fluorescence of NGQDs are varied because of the IFE of TMPyP and NGQDs, thus a sensitive, selective and multi-signals method for determining trace HgII is proposed. Meanwhile, the present method could be applied for monitoring HgII in cells. ASSOCIATED CONTENT Supporting Information. Supplementary of the experimental section, survey XPS pattern and fluorescence spectra of NGQDs, the conditional rate constant plotted against the NGQDs concentration, zetal potential


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determinations and the influence of the charge in the porphyrin for the accelerate system, the optimization of the reaction conditions, the study of the IFE of TMPyP on NGQDs, the study of the fluorescence lifetime of TMPyP and NGQDs under different conditions, the comparison of the assay performances of different analytical methods, the influences of different metal ions to the reaction system, and analytical results in river water samples detection, and (Figure S1-7 and Table S1-5) AUTHOR INFORMATION Corresponding Author *(J.-D.Qiu) Tel/Fax: +86-791-83969518. E-mail: [email protected]. ACKNOWLEDGMENT We gratefully acknowledge the supports from the National Natural Science Foundation of China (21675078). REFERENCES 1. Brown, R. J. C.; Goddard, S. L.; Butterfield, D. M.; Brown, A. S.; Robins, C.; Mustoe, C. L.; McGhee, E. A., Ten years of mercury measurement at urban and industrial air quality monitoring stations in the UK. Atmos. Environ. 2015, 109, 1-8. 2. Yin, X.; Balogh, S. J., Mercury Concentrations in Stream and River Water: An Analytical Framework Applied to Minnesota and Wisconsin (USA). Water Air Soil Pollut. 2002, 138, 79100. 3. Niu, Z.; Zhang, X.; Wang, S.; Zeng, M.; Wang, Z.; Zhang, Y.; Ci, Z., Field controlled experiments on the physiological responses of maize (Zea mays L.) leaves to low-level air and soil mercury exposures. Environ. Sci. Pollut. R. 2014, 21, 1541-1547. 4. Ding, Y.; Wang, S.; Li, J.; Chen, L., Nanomaterial-based optical sensors for mercury ions. Trends. Anal. Chem. 2016, 82, 175-190. 5. Gong, Y.; Liu, Y.; Xiong, Z.; Zhao, D., Immobilization of mercury by carboxymethyl cellulose stabilized iron sulfide nanoparticles: reaction mechanisms and effects of stabilizer and water chemistry. Environ. Sci. Technol. 2014, 48, 3986-3994. 6. Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D., Review: Environmental exposure to mercury and its toxicopathologic implications for public health. Environ. Toxicol. 2003, 18, 149-175.


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TOC


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Rapid Detection of Mercury Ions Based on Nitrogen-Doped Graphene

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