Fluorescent Molybdenum Oxide Quantum Dots and HgII

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Fluorescent Molybdenum Oxide Quantum Dots and HgII Synergistically Accelerate the Cobaltporphyrin Formation: a New Strategy for Trace HgII Analysis Li Zhang, Zhao-Wu Wang, Sai-Jin Xiao, Dong Peng, JiaQing Chen, Ru-Ping Liang, Jun Jiang, and Jian-Ding Qiu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00351 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Applied Nano Materials

Fluorescent Molybdenum Oxide Quantum Dots and HgII Synergistically Accelerate the Cobaltporphyrin Formation: a New Strategy for Trace HgII Analysis Li Zhang,a Zhao-Wu Wang,b Sai-Jin Xiao,c Dong Peng,a Jia-Qing Chen,a Ru-Ping Liang,a Jun Jiangb,* and Jian-Ding Qiua,* a

College of Chemistry, Nanchang University, Nanchang 330031, China. Fax: (+86) 791-

83969518, E-mail: [email protected]. b

Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation

Center of Chemistry for Energy Materials, School of Chemistry and Materials Science, University of Science

and

Technology of

China,

Hefei

230026, China.

E-mail:

[email protected]. c

School of Chemistry, Biology and Material Science, East China University of Technology

(ECUT), Nanchang 330013, China. Keywords: MoO3-x QDs, HgII, metalloporphyrins, synergistic effect, multi-signals

ACS Paragon Plus Environment

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Abstract

Construction of efficient and sensitive platform for HgII analysis is of great importance due to the potential hazards of HgII to human safety and environmental stability. Herein, we propose a new strategy for HgII sensing based on the synergistic effect of fluorescent MoO3-x quantum dots (QDs) and HgII on the cobaltporphyrin formation, in which the accelerated reaction rate far exceeds the simple sum of the contributions from individuals. A first-principle theoretical study reveals the synergistic mechanism of CoII-porphyrin generation with HgII-porphyrin and CoII being docked together on the MoO3-x QDs platform. The synergistic reaction results in distinct and rapid evolution of the optical properties of porphyrins and MoO3−x QDs, facilitating kinetic HgII sensing and intracellular HgII imaging. Compared with the traditional “substitution reaction” method in the presence of individual accelerator, the present strategy can markedly shorten the analysis time, largely improve the sensitivity, and easily achieve multi-signal HgII determination. Moreover, it is the first proof-of-concept demonstration of the acceleration effect of transition metal oxide nanoparticles on metalloporphyrin formation as well as its application in metal ion analysis.


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Introduction Due to the inherent functionality and biological importance, metalloporphyrins have attracted much interest of researchers in a variety of fields. Hereby, special focus lies on the investigation of metalloporphyrin formation kinetics due to its importance in understanding the process of in vivo metal incorporation that leads to chlorophylls and heme.1 The rate of metalloporphyrin formation is several orders of magnitude lower than that of chelation between open-chain ligand complex and metal ions, which has put the acceleration of metalloporphyrin formation rate in the spotlight.2 Among them, the substitution reaction has been extensively explored for the coordination acceleration as well as for the kinetic determination of hazardous large/heavy metal ions such as Hg2+,3 Pb2+,4 and Cd2+.5 It is supposed that large metal ions can’t fit into porphyrin nuclei, instead they prefer to sit on the top of porphyrins to form a ‘sitting-atop’ (SAT) complex, which can deform porphyrin nuclei and facilitate other metal ions attacking from the back.1,4,6-7 Unfortunately, this procedure relies on high metal ion concentrations and suffers from poor selectivity due to other ions with similar radii. Recently, nanomaterials have been investigated as a new type of reagents to accelerate the coordination reaction between metal ions and porphyrins. For example, Xu and co-workers have demonstrated that chemically converted graphene

(CCG)

can

flatten

5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin

tetra(p-

toluenesulfonate) (TMPyP) molecules, accelerating the coordination reaction between TMPyP and CdII to facilitate the selective CdII detection.8 More recently, we investigated the catalytic effect of nitrogen-doped graphene quantum dots (NGQDs) on the metalloporphyrin generation in terms of the coordination reaction between NGQDs and CdII as well as the formation of the TMPyP/NGQDs assembly.9 Although metal substitution reactions and some special nanomaterials have been applied to accelerate the metalloporphyrin formation for kinetic metal


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ion determination, the sensitivity and selectivity of these methods are limited and little is known about the synergistic effect of the two processes, much less its application in special metal ion analysis. The developing of nanotechnology has prospered the nanosensors for determination of environmental contaminants such as antibiotics10 and mercury ions.11-15 Among the popular nanomaterials, molybdenum oxide-based nanoparticles, as important transition metal oxides, have attracted great attention due to their unique electronic structures and wide applications in energy storage and photoelectric devices.16,17 Regrettably, thus far, there have been few reports about utilizing molybdenum oxide-based nanomaterials for construction of chemical contaminant sensing platform.18,19 Herein, we find that molybdenum oxide quantum dots (MoO3−x QDs) combined with a trace amount of HgII can co-accelerate the cobaltporphyrin formation. Surprisingly, the reation rate far exceeds the simple sum of the contributions expected from individual accelerators. A first-principle theoretical study reveals the synergistic mechanism of CoII(TMPyP) generation with HgII(TMPyP) and CoII being docked together on the MoO3-x QDs platform. It should be noticed that although synergistic effects of two or more nanoparticles have been investigated previously, most of them focused on the enhancement of electrocatalytic activity for the sensitive detection of analytes20-22 and few were about optical sensors.23

In

our

present

contribution,

the

synergistic

reaction

results

in

rapid

fluorescence/absorption spectral change of porphyrins upon metalloporphyrin formation as well as distinct fluorescence spectral evolutions of MoO3−x QDs due to the inner filter effect (IFE) of porphyrins on MoO3−x QDs, which favors the kinetic, sensitive, selective HgII sensing and intracellular HgII imaging. Experimental Details


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Materials:

5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin

tetra(p-toluenesulfonate)

(TMPyP), MoS2 powder, glutathione (GSH), cysteine (Cys), and homocysteine (Hcy) were commercially

obtained

from

Sigma-Aldrich

(USA).

The

5,10,15,20-tetrakis(4-

sulfophenyl)porphyrin hydrate (TPPS) was commercially obtained from TCI Development Co., Ltd (Shanghai). Other chemicals such as hydrogen peroxide (H2O2), sodium hydroxide (NaOH), glucose (Glu), ascorbic acid (AA), and metal salts were bought from Sinopharm Chemical Reagent Co. Ltd. (China). 18.2 MΩ.cm ultrapure water was used throughout. Instrumentation: A JEOL JEM-2010 transmission electron microscope (TEM) was used to determine the morphology and size of MoO3−x QDs. A Bruker MultiMode 8 atomic force microscope was used to capture atomic force microscopy (AFM) images. X-ray diffraction (XRD) analysis was conducted on a Bruker AXS D8Focus diffractometer with Cu Kα radiation (Germany). X-ray photoelectron spectroscopy (XPS) spectra were measured on a Thermal Electron spectrometer (USA). A Nicolet 5700 FTIR spectrometer was used to collect Fourier transform infrared (FTIR) spectra. Quantaurus-QY and Quantaurus-Tau spectrophotometers were employed to measure fluorescence quantum yield and fluorescence lifetimes, respectively (Hamamatsu Photonics, Japan). A Hitachi F-7000 fluorescence spectrophotometer was used for fluorescence measurements (Tokyo, Japan). A Shimadzu UV-2450 spectrophotometer was employed to record absorption spectra (Tokyo, Japan). A LabRAM HR800 Laser confocal Raman spectrometer was applied for Raman spectra. Zeta potentials were measured on a Malvern Nano ZS90 (England). The fluorescence imaging was performed on a confocal laser microscope (Zeiss 710, Germany). HgII sensing by synergistic effect: For the sensing of HgII, 40 µL of 20 mM pH 7.0 phosphate buffer (PB), 100 µL of 50 µM TMPyP solution, 20 µL of 1 mM CoII solution, 20 µL


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of 1 mg mL−1 MoO3−x QDs solution and different amounts of HgII solution were sequentially added to a 1.5 mL centrifugal tube, and then the mixture was diluted with ultrapure water to a volume of 400 µL. The reaction mixture didn’t require any other treatments except for incubation at room temperature (25 °C) for 30 min, and then UV-vis absorption and fluorescence spectra of the resulting solution were recorded. Cytotoxicity and cellular imaging: The toxicity of MoO3−x QDs to cells was measured by MTT assay. Briefly, A549 cells were seeded at 7 × 103 cells/well in 100 µL DMEM in 96-well plate and were grown for 24 h. A series of concentrations of MoO3−x QDs were added and incubated with cells for 48 h. Controls were cultivated under the same conditions without the addition of MoO3−x QDs. After addition of 20 µL of 5 mg mL-1 MTT and incubation for 2.5 h, the optical density (OD) was measured at 450 nm. The following equation was used to calculate the cell viability: Cell viability (%) = ([OD]treated/[OD]control) × 100% The assay for the cytotoxicity of HgII to cell viability was conducted similarly. For cellular imaging, A549 cells were grown in DMEM supplemented with 10% FBS, streptomycin (100 µg mL-1), and penicillin (100 µg mL-1) at 37 ºC under 5% CO2. When the cells reached confluence, they were harvested with trypsin and plated at a cell concentration of 1 × 106 cells/mL to grow for 48 h. For HgII sensing, the cells were washed with phosphate-buffered saline (PBS) solution for three times and incubated with HgII at a final concentration of 100 nM in DMEM for 30 min. After that, the cells were rinsed three times with PBS, and then incubated with 40 µL of 20 mM pH 7.0 PB, 100 µL of 50 µM TMPyP, 20 µL of 1 mM CoII solution, 20 µL of 1 mg mL−1 MoO3−x


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QDs, and 220 µL H2O for 30 min. After washing with PBS for three times, the cells were fastened for microscopy imaging under an excitation wavelength of 405 nm. Results and Discussion Characterization of MoO3-x QDs. MoO3-x QDs were obtained from bulk MoS2 powder using our reported method that employs H2O2 as both the precursor and oxidant (details see the Supporting Information).18 H2O2 as an excess oxygen supplier results in the spontaneous exfoliation and oxidation of MoS2.24,25 During the oxidation process, the van der Waals forces between adjacent layers will be weakened as a result of the formation of abundant oxygen gas, leading to spontaneous exfoliation. Besides, the Mo-S bonds are broken, and S atoms are released from the lattice which can be refilled by excess oxygen atoms owing to weaker bonding affinity of Mo-S than that of Mo-O (Figure 1a).26 TEM and AFM images show that MoO3-x QDs mainly consist of nanosheets with a lateral dimension of ∼2.3 nm and an uniform thickness of ∼1.40 nm (Figure 1b, 1c), indicating double-layered structure of the MoO3-x QDs,27-29 which is consistent with the unit cell thickness of α-MoO3 and is rarely observed in previously prepared molybdenum oxides. The strong absorption band of MoO3-x QDs between 200-400 nm is ascribed to the charge transfer of Mo-O bands in MoO66- octahedrons (Figure 1d).30 Moreover, MoO3-x QDs exhibit an excitation-dependent photoluminescence behavior with a maximum band at 430 nm upon 290 nm excitation (Figure 1d). Under UV light irradiation (365 nm), the brownish aqueous solution of MoO3−x QDs emits intense yellowish fluorescence (Figure 1d inset), and the absolute fluorescence quantum yield is 2.50%. In addition, the Raman spectrum, FTIR spectrum and the powder XRD pattern confirm the main phase of the MoO3-x QDs as orthorhombic MoO3 (Figure S1a-c in the Supporting Information). The XPS spectra clearly demonstrate that the Mo4+ of MoS2 is oxidized to the higher oxidation states Mo5+/Mo6+ (233.1


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and 230.7 eV corresponding to 3d3/2 and 3d5/2 of Mo5+ while 235.3 and 232.1 eV corresponding to 3d3/2 and 3d5/2 of Mo6+, respectively) accompanied by the existence of oxygen vacancies (Figure 1e). The percentage of Mo5+ is 18.6%, therefore, the average oxidation value of Mo is 5.81, and the formula MoO3−x (x ≈ 0.095) is proposed for the obtained QDs (The survey XPS pattern as well as the high-resolution S 2p and O 1s spectra of MoO3-x QDs are shown in Figure S2-S4 in the Supporting Information).


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Figure 1. (a) Schematic illustrating the preparation of MoO3-x QDs through exfoliation and oxidation of bulk MoS2 by H2O2. (b) TEM image of MoO3-x QDs; inset: diameter distribution. (c) AFM image of MoO3-x QDs; inset: height profile. (d) UV-vis absorption spectrum and fluorescence spectra of MoO3-x QDs (the excitation spectrum was obtained at the emission wavelength of 430 nm, and the emission spectra were obtained under excitation from 260 nm to 360 nm); inset: photographs of MoO3-x QDs under visible light and 365 nm UV light. (e) Highresolution Mo 3d spectrum of MoO3-x QDs. Acceleration Effect of HgII or/and MoO3−x QDs. As mentioned above, the coordination reaction rate of CoII and TMPyP is quite slow under ambient conditions, and negligible spectral change (by recording the fluorescence intensity of TMPyP at 658 nm) is observed even reacted long for 60 min at room temperature (Figure 2a). As expected, the large metal ions-HgII can catalyze the formation of CoII(TMPyP) via forming a ‘SAT’ complex, which deforms the porphyrin nucleus favourable for attack of CoII from the back.1,4,6-7 Further investigation demonstrates that the catalytic effect of HgII on CoII(TMPyP) formation is dependent on the dose of HgII (Figure 2b). On the other hand, similar to the catalytic effect of NGQDs,9 the CoII(TMPyP) formation can be also accelerated by MoO3−x QDs depending on their concentrations (Figure 2c). Notably, although either HgII or MoO3−x QDs can individually act as accelerators to enhance the formation rate of CoII(TMPyP), introduction of a trace amount of single accelerator (100 nM HgII or 50 µg mL-1 MoO3−x) can hardly achieve this goal (Figure 2a). Surprisingly, the coexistence of 100 nM HgII and 50 µg mL-1 MoO3−x QDs remarkably accelerates the coordination reaction between CoII and TMPyP with the reaction rate far exceeding the simple sum of the contributions from individuals, resulting in a distinct absorption and fluorescence spectral change of TMPyP within 30 min at room temperature (Figure 2a). The


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reaction kinetics is then studied by mixing TMPyP, HgII, CoII and various amounts of MoO3−x QDs at room temperature. The fluorescence change of TMPyP at 658 nm (IF) as a function of time (t, Figure 2d) is monitored. The coordination reaction of CoII(TMPyP) follows a first-order kinetics.31 The reaction rate can be represented by eq 1, where kobsd is the conditional rate constant involving concentrations of MoO3−x QDs, HgII and CoII. -d[TMPyP]/dt = kobsd[TMPyP]

(1)

Plots of IF as a function of t give straight lines, and the slope represents the kobsd. The results show that the absolute value of kobsd increases with MoO3−x concentrations (inset of Figure 2d). While the ratio of [MoO3−x]/kobsd remains constant (-108.8) over the concentration range of MoO3−x QDs, indicating the reaction to be the first order in MoO3−x QDs.6

Figure 2. (a) Fluorescence intensity of TMPyP at 658 nm with excitation at 420 nm versus reaction time in different conditions as indicated. (b) Fluorescence intensity of 12.5 µM TMPyP


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at 658 nm with excitation at 420 nm in the presence of 50 µM CoII and different concentrations of HgII. (c) Fluorescence intensity of 12.5 µM TMPyP at 658 nm with excitation at 420 nm in the presence of 50 µM CoII and different concentrations of MoO3-x QDs. (d) Fluorescence intensity of 12.5 µM TMPyP at 658 nm with excitation at 420 nm versus reaction time in the presence of 50 µM CoII, 100 nM HgII, and different concentrations of MoO3−x QDs. Inset: conditional rate constant (kobsd) plotted against the MoO3−x QDs concentration. Acceleration Mechanism of MoO3−x QDs. The acceleration effect of MoO3−x QDs on the cobaltporphyrin formation can be attributed to the activation entropy effect. In details, MoO3−x QDs can coordinate with CoII to form an activated CoII(MoO3−x) complex and thus the anionic form of cobalt rapidly associates with TMTPyP (4+) to form CoII(MoO3−x)•TMPyP (I) through the electrostatic interaction, resulting in incorporation of CoII into TMPyP with a rate a few orders of magnitude faster than CoII alone.32 It is supposed that the formation of CoII(MoO3−x) can labilize the coordinated water molecules on CoII through electron donation from the coordinated MoO3−x QDs,33,34 and meanwhile the electrostatic interaction between CoII(MoO3−x) and TMPyP results in a larger formation constant of complex I and thus accelerates the overall CoII(TMPyP) formation.33 The formation of CoII(MoO3−x) complex and MoO3−x•TMPyP assembly can be confirmed by zeta potential measurements and AFM images. MoO3−x QDs have a zeta potential of -27.4 mV, while that of CoII(MoO3−x) and MoO3−x•TMPyP increase to -14.2 mV and -13.0 mV (Figure 3a), respectively, which implies that CoII and TMPyP carrying positive charges are associated with MoO3−x QDs. Additional AFM images are employed to further confirm the formation of MoO3−x QDs•TMPyP assembly. Compared to the initial MoO3−x QDs with a thickness of about 1.4 nm (Figure 3b), the assembly of TMPyP with MoO3−x QDs causes a significant topographical change along with a height increment of approximately 1.0 nm


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(Figure 3c, 3d), suggesting that porphyrins are assembled with MoO3−x QDs as a dimer form.35 Notably, due to the small size of MoO3−x QDs and the weak interaction between MoO3−x QDs and TMPyP, neither the flattening nor the aggregation of porphyrin molecules can be induced by MoO3−x QDs,9 which is proved by the negligible evolution of TMPyP absorption spectra upon the supplement of various amounts of MoO3−x QDs (Figure S5 in the Supporting Information).

Figure 3. (a) Zeta potential of MoO3-x QDs, MoO3−x QDs/TMPyP, MoO3-x QDs/CoII, and MoO3x

QDs/HgII, respectively. The concentrations of TMPyP, CoII, HgII, and MoO3-x QDs were 12.5

µM, 50 µM, 100 nM, and 50 µg mL-1, respectively. AFM images of MoO3-x QDs (b), TMPyP (c), and MoO3-x QDs/TMPyP (d), respectively. Synergistic Mechanism of HgII and MoO3−x QDs. The synergistic effect of HgII and MoO3−x QDs on cobaltporphyrin formation can be explained as follows. In the presence of the


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two accelerators, HgII cannot fit well into the plane of the four central porphyrin nitrogen atoms, instead it sits on top of the molecule,6 deforming the porphyrin nucleus to “open the door” favoring other metal ions to “enter” (eq 2). Meanwhile, MoO3−x QDs can easily “hold” the CoII through coordination (eq 3), favourable for “pushing” CoII into the porphyrin “door” (eq 4, Scheme 1).9 The preferential interaction of MoO3−x QDs to CoII rather than to HgII can be confirmed by the increased zeta potential of CoII(MoO3−x) compared with that of MoO3−x QDs or the MoO3−x QDs/HgII mixture (Figure 3a). The HgII released in the reaction (4) can be repeatedly involved in the reaction (2), forming an acceleration cycle. In order to confirm this point, the fluorescence evolution of the sensing system versus reaction time under different concentrations of HgII has been investigated. Apparently, with the coexistence of MoO3−x QDs and CoII in the TMPyP solution, the coordination reaction could also be finished and the cobaltporphyrin can be completely formed even in a low HgII concentration by prolonging the reaction time, which can reveal the repeated use of HgII (Figure S6 in the Supporting Information). HgII + TMPyP ⇌ HgII(TMPyP) (2) CoII + MoO3−x ⇌ CoII(MoO3−x) (3) HgII(TMPyP) + CoII(MoO3−x) ⇌ HgII(TMPyP)•CoII(MoO3−x) → CoII(TMPyP) + MoO3−x + HgII (4)


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Scheme 1. (a) The traditional substitution reaction for HgII sensing. (b) The present strategy for HgII sensing based on the synergistic effect of MoO3-x QDs and HgII on the cobaltporphyrin formation. HgII is known to be easily captured in the center of porphyrin,33,34,36 resulting in a stable HgIIporphyrin structure as demonstrated by computations (Figure 4a). We have also calculated the bonding energy for all cations on porphyrin in our experimental work and the results demonstrate that the bonding energy of Co on porphyrin is larger than that of Hg on porphyrin, providing reasonable theoretical explanation for the substitution reaction that CoII can push HgII out of the porphyrin plane (Table S1 in the Supporting Information). Further first-principle simulations demonstrate that the MoO3-x substrate can effectively capture CoII rather than HgII with a


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stronger bonding energy (Eb for CoII and HgII were 4.95 eV and 0.18 eV, respectively. Figure 4b, Figure S7-S8 and Table S2 in the Supporting Information), which in turn provides an ideal docking platform for the HgII-porphyrin (Figure 4c). The stable adsorption structure of HgIIporphyrin on CoII-MoO3-x can eventually create CoII-porphyrin since CoII is always ready to push HgII out from the porphyrin through an exothermic process releasing much energy of 7.20 eV without energy barrier (Figure 4d). On the other hand, MoO3-x can effectively adsorb HgIIporphyrin (Eb=1.80 eV, Figure S9 in the Supporting Information) and consequently provides a stable platform for surrounding CoII to attack HgII-porphyrin. It means that MoO3-x provides a platform to dock both HgII-porphyrin and CoII, which then interact with each other to create CoIIporphyrin toward efficient optical change.

Figure 4. The optimized structure of HgII-porphyrin (a), CoII-MoO3-x (b), HgII-porphyrin on CoII-MoO3-x (c), and substituting reaction from HgII-porphyrin to CoII-porphyrin when CoII push HgII out of the porphyrin plane (d).


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Sensing Performance by the Synergistic Strategy. The synergistic effect that exceeds the simple sum of the contributions expected from individuals is promising in the kinetic determination of trace amounts of HgII. The influence of solution pH on the acceleration reaction is investigated, and the results demonstrate that the synergistic effect shows enhancement over the pH range of 6.0-7.0, while it is independent of pH at higher values (pH ≥ 7.0), which is consistent with previously reported conclusions (Figure S10a in the Supporting Information).5,37 Taking into account the synergistic effect and the hydrolysis of metal ions, a pH range of 7.0-8.0 is suitable for the determination of HgII. Under the optimized pH range and CoII concentration (50 µM, Figure S10 in the Supporting Information), the fluorescence intensity of TMPyP at 658 nm linearly decreases with varying concentrations of HgII in the range of 5-100 nM with a detection limit of 0.8 nM (3σ), accompanied by increased fluorescence of MoO3-x QDs at 493 nm (Figure 5a and inset). In order to burst the synergistic effect of the two acceleration processes, control experiments for HgII responses using HgII-accelerated CoII(TMPyP) formation in the absence of MoO3−x QDs were carried out. The results in Figure 5a inset indicate that the response sensitivity of the present assay shows about 20-fold improvement compared to that in the absence of MoO3-x QDs. Moreover, the distinct absorption spectral change corresponding to the CoII(TMPyP) formation can also be applied for HgII sensing. The addition of HgII results in decrease of the Soret band at 422 nm and the Q band at 518 nm (λmax of TMPyP), and increase of the Soret band at 436 nm and the Q band at 552 nm (λmax of CoII(TMPyP)) (Figure 5b). It is worth noting that the Soret band of the cobaltporphyrin is slightly red-shifted compared with that in previous reports, which might be ascribed to partial oxidation of CoII(TMPyP) to CoIII(TMPyP).38,39 The absorbance ratio (A436/A422) increases dramatically in the concentration range of HgII from 5 to 87.5 nM with a detection limit of 0.6 nM (3σ), and the sensitivity is 38-


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fold higher than that only using HgII as an accelerator in the absence of MoO3−x QDs (Figure 5b inset). Notably, the fluorescence evolution of MoO3−x QDs also responds to the metalloporphyrin formation. As shown in Figure 5c, in the absence of HgII, the absorption band of TMPyP centered at 422 nm overlaps integrally with the emission spectrum of MoO3−x QDs. Therefore, the absorption of the emission light by TMPyP results in substantial fluorescence quenching of MoO3−x QDs with a valley at about 430 nm. The unchanged lifetimes of MoO3−x QDs in the presence of TMPyP and/or different metal ions as well as the fluorescence corrections confirm that the fluorescence quenching can be attributed to the inner filter effect (IFE) of TMPyP on MoO3−x QDs (Figure S11a, S12 and Table S3, S5 in the Supporting Information).40,41 In the presence of HgII, the decreased fluorescence lifetime compared to free TMPyP indicates the successful formation of CoII(TMPyP) complex (Figure S11b and Table S4 in the Supporting Information), leading to a bathochromic shift of porphyrin Soret band from 422 to 436 nm (Figure 5c). The integral overlap of MoO3−x QDs emission and CoII(TMPyP) absorption spectra results in IFE and fluorescence quenching of MoO3−x QDs with a valley at about 440 nm (Figure 5c). Interestingly, two well-defined isoemission points at 442 nm and 468 nm similar to that of the absorption spectra can be observed (Figure 5d). The emission intensity at 398 nm linearly increases upon the gradual addition of HgII in the range of 12.5-100 nM with a detection limit of 4 nM (3σ) (Figure 5d inset). The performance of our present method is compared with that in other literatures for sensing of intracellular or in vivo HgII, and the results in Table S6 demonstrate that the performance of our method is better than or comparable to that of other methods (Table S6 in the Supporting Information).


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Figure 5. (a) Fluorescence spectra of 12.5 µM TMPyP in the presence of 50 µg mL-1 MoO3−x QDs, 50 µM CoII and different concentrations of HgII (λex = 420 nm, a-j, 0, 5, 12.5, 25, 37.5, 50, 62.5, 75, 87.5, 100 nM); inset: plot of the fluorescence intensity of TMPyP at 658 nm versus HgII concentrations in the presence of MoO3−x QDs (black curve) or in the absence of MoO3−x QDs (red curve). (b) Absorption spectra of 12.5 µM TMPyP in the presence of 50 µg mL-1 MoO3−x QDs, 50 µM CoII and different concentrations of HgII (a-h, 0, 5, 12.5, 25, 37.5, 50, 62.5, 87.5 nM); inset: plot of the absorbance ratio (A436/A422) of TMPyP versus HgII concentrations in the presence of MoO3−x QDs (black curve) or in the absence of MoO3−x QDs (red curve). (c) The UV-vis absorption spectra of MoO3 − x QDs/TMPyP and MoO3 − x QDs/CoII(TMPyP), and the fluorescence spectra of MoO3−x QDs, MoO3−x QDs/TMPyP and MoO3−x QDs/CoII(TMPyP), λex = 290 nm; concentrations of TMPyP, CoII, HgII, and MoO3−x QDs were 12.5 µM, 50 µM, 100 nM, and 50 µg mL-1, respectively. (d) Fluorescence spectra of 50 µg mL − 1 MoO3 − x QDs in the


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presence of 12.5 µM TMPyP, 50 µM CoII and different concentrations of HgII (a-g, 0, 12.5, 25, 37.5, 62.5, 87.5, 100 nM); inset: plot of I398 versus HgII concentrations, λex = 290 nm. The specificity of the present sensing platform is then investigated. The results demonstrate that only HgII can induce distinct fluorescence quenching of TMPyP within 30 min, while other metal ions and some organic molecules in cells such as ascorbic acid, glucose and sulfur alcohols can’t accelerate the coordination reaction between TMPyP and CoII with negligible fluorescence change even in the presence of MoO3−x QDs (Figure 6a). The excellent selectivity can be ascribed to the ionic radii distributions of metal ions (HgII > CdII > PbII > ZnII ∼ CuII)3 as well as the weak binding affinity between MoO3−x QDs and HgII compared with other metal ions (zeta potential in Figure 3a as well as the first-principle simulations discussed above). Notably, when CoII is replaced by MnII or NiII under the same experimental conditions, even the coexistence of HgII and MoO3−x QDs cannot accelerate the coordination reaction between TMPyP and MnII (or NiII) to form manganese porphyrins (or nickel porphyrins) (Figure 6b). Moreover, the charge on the porphyrin periphery can also significantly influence the reaction rate of the MoO3-xaccelerated

metal

ion

incorporation.

If

negatively

charged

5,10,15,20-tetrakis(4-

sulfophenyl)porphyrin hydrate (TPPS) is involved instead of TMPyP, the coexistence of MoO3-x QDs and different concentrations of HgII cannot accelerate the coordination reaction of CoII with TPPS (Figure 6c). What’s more, it is found that MoO3−x QDs can inhibit the CoII(TPPS) formation (Figure 6d), presumably because the activated CoII(MoO3−x) complex in these reactions carries negative charges.32 The electrostatic repulsion between CoII(MoO3−x) and TPPS with

negative

peripheral

substituents

on

the

porphyrin

avoids

the

formation

of

CoII(MoO3−x)•TPPS, leading to decreased rate of metal ion coordination.


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Figure 6. (a) Fluorescence quenching efficiency of 12.5 µM TMPyP in the presence of 50 µg mL −1

MoO3−x QDs, 50 µM CoII and 100 nM different metal ions or 1 µM AA, 200 nM Glu, 5 mM

GSH, 100 µM Cys, 12 µM Hcy. (b) Fluorescence intensity of 12.5 µM TMPyP in the presence of 50 µM CoII (or MnII, NiII), 50 µg mL-1 MoO3-x QDs and different concentrations of HgII. (c) Fluorescence intensity of 12.5 µM TMPyP (or TPPS) in the presence of 50 µM CoII, 50 µg mL-1 MoO3-x QDs and different concentrations of HgII. (d) Fluorescence intensity of 12.5 µM TPPS at 645 nm with excitation at 420 nm versus reaction time in the presence of 50 µM CoII and different concentrations of MoO3−x QDs. Finally, the present porphyrin-based platform is further applied for intracellular HgII imaging and real water sample analysis. It is demonstrated that MoO3-x QDs can bear high salt concentration which can be deduced from the little decrease of the fluorescence intensity even the concentration of NaCl reaches 100 mM (Figure S13 in the Supporting Information).


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Furthermore, MoO3−x QDs confer very low toxicity to A549 cells, with relative cell viability higher than 80% at concentrations up to 200 µg mL-1 (Figure S14 in the Supporting Information). On the basis of the high stability and adequately low cytotoxicity of MoO3−x QDs, the established HgII sensor based on the synergistic effect is used to monitor intracellular HgII using confocal fluorescence microscopy. As shown in Figure 7a-c, intense emission is observed within the A549 cells incubated with 12.5 µM TMPyP for 30 min. With the addition of CoII, the fluorescence of TMPyP in cells is retained, indicating that the cobaltporphyrin can’t form in the absence of accelerators (Figure 7d-f). In the presence of a trace amount of HgII, the cobaltporphyrin formation can’t be efficiently accelerated and thus the fluorescence is not affected (Figure 7g-i). Similarly, the low concentration of MoO3-x QDs also demonstrates negligible effects on the metal incorporation process nor the porphyrins’ fluorescence in cells (Figure 7j-l), and these results are consistent with the fluorescence spectral observations (Figure 2). In sharp contrast, the simultaneous introduction of CoII, HgII, and MoO3−x QDs leads to great intracellular fluorescence quenching of the TMPyP-labelled cells (Figure 7m-o), indicating that the synergistic effect of HgII and MoO3−x QDs results in the formation of CoII(TMPyP) as well as the fluorescence quenching. Bright field optical measurements confirm that the cells are viable with intact morphology throughout the imaging studies, indicating the negligible cytotoxicity of MoO3−x QDs and HgII on the cell viability that is confirmed by the MTT assay (Figure S15 in the Supporting Information). The bioimaging results indicate that the MoO3−x QDs-based sensing platform can easily realize the monitoring of HgII in cancer cells, demonstrating its great potential in the fields of metal-organism interaction system, and also in the studies of chronic mercury toxicity.42 Although a plenty of other QDs such as graphene quantum dots (GQDs) and semiconductor quantum dots (SQDs) have been employed as optical probes in bioimaging,43 this


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is the first investigation of MoO3−x QDs as an accelerator for cell imaging application, providing new insights of the usage of these nanostructures. Moreover, the present method can be used to determine HgII in river water samples due to its good selectivity and sensitivity. The real samples were analyzed by the standard addition method, and the results prove that this method possesses excellent applicability for real sample analysis (Table S7 in the Supporting information).

Figure 7. Fluorescence imaging of intracellular HgII on the basis of the synergistic effect of MoO3−x QDs and HgII on metalloporphyrin formation. Fluorescence (first column), differential


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interference contrast (DIC) (second column), and the overlay of DIC and fluorescence mode (third column). (a)-(c) demonstrate the cells decorated with TMPyP; (d)-(f) demonstrate the cells decorated with TMPyP and CoII; (g)-(i) demonstrate the cells decorated with TMPyP, CoII, and HgII; (j)-(l) demonstrate the cells decorated with TMPyP, CoII, and MoO3 − x QDs; (m)-(o) demonstrate the cells decorated with TMPyP, CoII, HgII, and MoO3−x QDs simultaneously. The concentrations of TMPyP, CoII, HgII, and MoO3−x QDs were 12.5 µM, 50 µM, 100 nM, and 50 µg mL-1, respectively. Scale bar: 20 µm. Conclusion In this work, an interesting phenomenon is studied systematically that MoO3−x QDs combined with HgII exhibit synergistic effect for the coordination reaction between TMPyP and CoII. It is supposed that HgII can deform the porphyrin nucleus favorable for CoII carried by MoO3-x QDs to attack from the back, resulting in distinct optical evolution of TMPyP and MoO3−x QDs, which can be further used for sensitive, selective, and multi-signal HgII sensing as well as intracellular HgII imaging. First-principle simulations reveal the synergistic mechanism that MoO3-x QDs provide the platform to dock both HgII-porphyrin and CoII, creating CoIIporphyrin toward efficient fluorescence/absorption change. Thus far, this is the first investigation of MoO3−x QDs as an accelerator rather than a fluorophore or a quencher for HgII determination and imaging, and the results demonstrate that the synergistic effect-based strategy can markedly shorten the analysis time, largely improve the sensitivity and the selectivity compared with the traditional “substitution reaction” methods. Moreover, the novel detection mechanism has been revealed not only by experimental results but also through extensive theoretical computations, and the two sides coincide with each other very well.


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Associated Content Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional experimental results for the preparation and characterizations of MoO3-x QDs and IFE corrections. Figure S1-S15 and Table S1-S7. Author Information Corresponding Author *E-mail: [email protected]; [email protected]. Phone: +86-791-83969518. Notes The authors declare no competing financial interest. Acknowledgement This work is supported by the National Natural Science Foundation of China (21675078, 21105044, 11404095), and the Key Project of Scientific and Technological Innovation Talents in Jiangxi Province (20165BCB18022). References (1) Tanaka, M. Kinetics of Metalloporphyrin Formation with Particular Reference to the Metal Ion Assisted Mechanism. Pure Appl. Chem. 1983, 55, 151-158. (2) Tabata, M.; Tanaka, M. Porphyrins as Reagents for Trace-Metal Analysis. Trac-trend. Anal. Chem. 1991, 10, 128-133.


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Fluorescent Molybdenum Oxide Quantum Dots and HgII

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