Nitrogen-Doped Graphene Quantum Dots as a New Catalyst

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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 Li Zhang, Dong Peng, Ru-Ping Liang, and Jian-Ding Qiu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b02450 • Publication Date (Web): 02 Oct 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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Analytical Chemistry

Nitrogen-Doped Graphene Quantum Dots as a New Catalyst Accelerating the Coordination Reaction between Cadmium(II) and

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

for

Cadmium(II) Sensing Li Zhang, Dong Peng, Ru-Ping Liang, and Jian-Ding Qiu* College of Chemistry, Nanchang University, Nanchang 330031, China

ABSTRACT Small molecules or metal ions can be employed as catalysts to accelerate metalloporphyrin formation. Herein, we for the first time report the coordination reaction between cadmium(II) and 5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin can be accelerated by nitrogen-doped graphene quantum dots (NGQDs). This catalytic reaction results in change of the absorption of porphyrins and the fluorescence of NGQDs as a result of the inner filter effect (IFE) of the porphyrins on the assembled NGQDs. Both signals can be used for rapid and sensitive determination of metal ions. The present work promises a novel strategy for constructing sensors for metal ions.

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INTRODUCTION As zero-dimensional carbon material, graphene quantum dots (GQDs) are graphene sheets smaller than 100 nm. They still comprise graphene lattices regardless of the nanoscaled sizes. Most reported GQDs are photoluminescent due to the quantum confinement and edge effects.1 Doping GQDs with heteroatoms (such as nitrogen and boron) can effectively tune their intrinsic properties, including optical characteristics, surface and local chemical features. Recently, we reported a facile hydrothermal approach to the cutting of boron-doped graphene (BG) into borondoped graphene quantum dots (BGQDs) which were applied for selective glucose sensing based on the “abnormal” aggregation-induced photoluminescence enhancement.2 Nitrogen atom, having the similarity of atomic size and five valence-electron structure with carbon atom, can be easily doped into graphene structure via C-N bonds, forming nitrogen-doped graphene (NG)3-7 or nitrogen-doped GQDs (NGQDs).8-11 The resulting NGQDs are attracting building blocks in the field of fuel cells and optoelectronics for their interesting electrocatalytic activity and strong electron-withdrawing effect, respectively.12 Up to now, most of the studies were focused on the electrocatalytic activity and the optical tuning by nitrogen doping. There are no attempts to explore the photocatalytic effect of NGQDs. Porphyrins with unique absorption property have been used as highly sensitive reagents for metals sensing.13 It has been reported that the incorporation rate of metal ions into porphyrin structure for forming metalloporphyrin is 108-109 times slower than that of acylic ligands.14 Additional heating of the reaction mixture is essential to accelerate the coordination reaction.15 Alternative strategies to accelerate the formation of metalloporphyrins have been proposed by using large metal ions such as mercury(II) and lead(II) since they can catalyze the incorporation of medium-sized transition metal ions into porphyrins.14 Ligands such as amino acids, pyridine,


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and imidazole have also been reported to enhance the coordination reaction.16,17 Thus far, fullfledged characterization and investigation of the catalytic effect of nanoparticles, such as metalbased and carbon-based nanoparticles, on the metalloporphyrin formation is almost missing.18 Herein, we report a two-step hydrothermal approach to the preparation of NGQDs. For the first time, we find that the resultant NGQDs can significantly promote the coordination reaction of Cd2+

with

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

tetra(p-toluenesulfonate)

(TMPyP). On the basis of this interesting phenomenon, a simple method for the sensitive, selective, and rapid determination of Cd2+ has been proposed. The catalytic effect of NGQDs is discussed in terms of assembly of NGQDs with porphyrins as well as the coordination reaction between pyridinic N/pyrrolic N of NGQDs with Cd2+ (Scheme 1). To the best of our knowledge, this is the first example of coordination reaction between metal ions and porphyrin derivatives accelerated by NGQDs as the auxiliary coordination agents.

Scheme 1. Schematic illustrating the catalytic effect of NGQDs toward the reaction between Cd2+ and TMPyP.


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EXPERIMENTAL SECTION Apparatus. The fluorescence spectra and the light scattering (LS) spectra were recorded with a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan). The UV-vis absorption spectra were measured by a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan). Except for the inner filter effect (IFE) correction experiment, all the absorption spectra were obtained with a shortpathlength micro cell. The transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2010 transmission electron microscope. The atomic force microscopy (AFM) images were recorded in ScanAsyst mode using a Bruker MultiMode-8 atomic force microscope. X-ray photoelectron spectroscopy (XPS) characterizations were conducted by using a VG Multilab 2000X instrument (Thermal Electron, USA). The Raman spectrum of the as-obtained sample was recorded on a LabRAM HR800 Laser confocal Raman spectrometer at ambient temperature. The X-ray diffraction (XRD) pattern was obtained on a D/MAX2200PC Rigaku powder diffractometer (Rigaku, Japan) equipped with CuKα radiation. The FL lifetime measurements

were performed

on

a Horiba Jobin Yvon FL-TCSPC fluorescence

spectrophotometer (France). The hydrodynamic sizes and zeta potentials were measured on a Malvern Nano ZS90 (England). The NGQDs were prepared using a Nanjing University Instrument Plant TOL1200 Tube Furnace (Nanjing, China). Reagents. The graphite flakes (99.8%, 325 mesh) were purchased from Alfa Aesar (USA). The 5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) was purchased from Sigma-Aldrich (USA). Other chemicals such as ammonium hydroxide (NH4OH), sulfuric acid (H2SO4), sodium hydroxide (NaOH), nitric acid (HNO3), ethanol, and metal salts were bought from Sinopharm Chemical Reagent Co. Ltd. (China) and were used as received


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without further purification. All solutions were prepared and diluted using ultrapure water (18.2 MΩ) from a Millipore Milli-Q system. Preparation of NGO, NGQDs, and N-free GQDs. The NGQDs were prepared by a twostep hydrothermal scheme. Firstly, the nitrogen-doped graphene (NG) was prepared through graphene oxide (GO, synthesized by the modified Hummers method19) in the presence of ammonium hydroxide (NH4OH). 50 mg of GO colloid was first diluted with 25 mL of ethanol. This yellow-brown colloid was ultrasonicated for 2 hours to ensure its homogeneity, then 2 mL of NH4OH was added and the mixture was carefully transferred into a poly(tetrafluoroethylene) (Teflon)-lined autoclave. Nitrogen doping was performed via hydrothermal treatment at 150 °C for 3 h. After cooling down to room temperature, the black precipitates of NG were washed by ethanol and water, and dried in a vacuum at 60 °C. The NG product was then oxidized with concentrated H2SO4 and HNO3 (volume ratio 1:3) for 17 h under mild ultrasonication without any pausing. The solution was diluted with 250 mL of ultrapure water and then filtered through a 0.22 µm microporous membrane to remove the acids. The filter cake was collected, dried and then redispersed in 20 mL of ultrapure water to obtain the nitrogen-doped graphene oxide (NGO) colloid solution. Secondly, for the NGQDs preparation, the pH of the NGO suspension was tuned to 8 with NaOH and then put into a poly-(tetrafluoroethylene) (Teflon)-lined autoclave and heated at 200 °C for 12 h. The resulting solution was filtered through a 0.22 µm microporous membrane to remove the large tracts, and the resultant NGQDs showed strong blue fluorescence. For the synthesis of N-free GQDs, the process was the same as that of NGQDs except for the addition of NH4OH. Detection Procedure. For the sensing of Cd2+, to a 1.5 mL centrifugal tube was sequentially added 40 µL of 20 mM phosphate buffer (pH 7.0), 140 µL of 0.5 µg mL-1 NGQDs solution, 100


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µL of 50 µM TMPyP solution, different amount of Cd2+ solution, and then diluted with ultrapure water to a volume of 400 µL. The reaction mixture is then incubated at room temperature (25 °C) for 2 min, and the UV-vis absorption and fluorescence spectra of the resulting solution were recorded. RESULTS AND DISCUSSION Characterization of the NGQDs. The NGQDs were synthesized from nitrogen-doped graphene oxide (NGO) by a hydrothermal approach described in the experimental section. The transmission electron microscopy (TEM) image shows fairly uniform NGQDs with diameters of ca. 3-5 nm (Figure 1a), and the corresponding atomic force microscopy (AFM) image reveals a typical topographic height of about 1.4 nm (Figure 1b), suggesting that most of the NGQDs consist of 3-4 graphene layers.20-22 The NGQDs show three broad UV absorption peaks located at 214, 265, and 300 nm which are related to the electron transitions from π (or n) to π* of C=C, C=N, and C=O, respectively (Figure 1c).23 The photograph shows that NGQDs solution is paleyellow, transparent, and clear under daylight and exhibits blue fluorescence under an UV irradiation (inset of Figure 1c). The fluorescence excitation bands are close to those of the UV absorption, confirming that the electron transitions for double bonds in the NGQDs occur in the UV range. Similar to previously reported GQDs, the NGQDs exhibit an excitation-dependent fluorescence behavior (Figure 1c), which is possibly attributed to the optical selection of different-size nanoparticles (quantum effect) and different emissive energy trap sites on the surface of NGQDs.24 The X-ray photoelectron spectroscopy (XPS) spectra of the NGQDs confirm the successful incorporation of N atoms into the GQDs (Figure 1d, Figure S1a and S1b in the Supporting Information). Three types of N-related bonding can be identified, namely, pyridinic N, pyrrolic N, and graphitic N (Figure 1d inset). Similar to previously reported NGQDs


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or N-free GQDs, the XRD pattern of the NGQDs displays a broad peak centered at 25° (dspacing ∼ 0.34 nm), which could be attributed to the effective stacking of tiny graphene layers (Figure S1c in the Supporting Information).12 The Raman spectrum shows a high ID/IG ratio (∼0.9) for NGQDs (Figure S1d in the Supporting Information), indicating the reduction in size of the in-plane sp2 domains and increase in structural distortion.

Figure 1. (a) TEM image of NGQDs, inset: diameter distribution. (b) AFM image of NGQDs, inset: height profile. (c) UV-vis absorption spectrum (left axis) and fluorescence spectra of NGQDs (right axis, the excitation spectrum was obtained at the emission wavelength of 430 nm and the emission spectra were obtained under the excitation at different wavelengths), inset: photographs of NGQDs under visible light and 365 nm UV light. (d) XPS spectrum of NGQDs, inset: high-resolution N1s peaks.


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Catalytic effect of NGQDs. Since NGQDs host pyridinic N/pyrrolic N atoms, they are expected to react with metal ions via coordination bonding. Meanwhile, the negatively charged NGQDs retaining the π-conjugated structure similar to GO would facilitate their interaction with some aromatic molecules (such as cationic TMPyP) via π-π stacking and electrostatic interaction.25-28 Thus, NGQDs may serve as a new catalyst to accelerate the coordination reaction between Cd2+ and TMPyP. By employing NGQDs as the catalyst, the reaction equilibrium between Cd2+ and TMPyP can be reached within 2 min, resulting in a distinct decrease in the absorbance at 422 nm (λmax of TMPyP) and an increase in absorbance at 450 nm (λmax of CdП(TMPyP)) (Figure 2a). The reaction kinetics are studied by mixing TMPyP, Cd2+, and various concentrations of NGQDs at room temperature. The change in the absorption ratio (A450/A422) is monitored as a function of time (Figure 2b). The kinetics of CdП(TMPyP) formation follow first-order kinetics. The reaction rate can be represented by equation (1), where k0 is the conditional rate constant involving concentrations of NGQDs and Cd(II). -d[TMPyP]/dt = k0[TMPyP]

(1)

The plots of ln[(Ao - A∞)/(At- A∞)] as function of t (where Ao, At, and A∞ denote the absorption ratio (A450/A422) of the system at times of zero and t, and infinity, respectively) give straight lines, the slope represents the k0. The results show that the logarithmic value of k0 linearly increases with increasing the concentration of NGQDs (Figure 2b inset).


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Figure 2. (a) Absorption spectra of TMPyP and CdΠ(TMPyP)/NGQDs, the concentrations of TMPyP, Cd2+, and NGQDs were 12.5 µM, 10 µM and 0.175 µg mL-1, respectively. (b) Absorption ratio versus reaction time in the presence of different concentrations of NGQDs (a-g, 0.05, 0.075, 0.1, 0.125, 0.15, 0.175, 0.225 µg mL-1), the concentrations of TMPyP and Cd2+ were 12.5 µM and 10 µM, respectively. Inset: logarithmic conditional rate constant (k0) plotted against the NGQDs concentrations. To further confirm the accelerating effect of NGQDs, other graphene nanostructures including graphene oxide (GO), nitrogen-doped graphene oxide (NGO), and N-free GQDs with the same concentration (0.175 µg mL-1) were also employed as the auxiliary coordination agents. The spectral results are shown in Figure 3, and the calculated values of k0 are summarized in Table S1 in the Supporting Information. In the absence of catalysts, the Soret band of TMPyP at 422 nm will gradually red-shift to 450 nm after chelating with Cd2+. It is found that the coordination reaction between Cd2+ and TMPyP solutions under ambient condition needs about 1.4 h to reach the equilibrium (Figure 3a and 3f). In a parallel experiment, a new band at about 450 nm due to the binding of Cd2+ ions into TMPyP in the presence of GO increases with reaction time. This process under the same conditions takes about 25 min to reach its equilibrium (Figure 3b and 3f), being 3 times faster than that using pure TMPyP solution. Similar results


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Figure 3. Evolution of the absorption spectra with time recorded during the reaction of 12.5 µM TMPyP and 10 µM Cd2+ in the absence of catalysts (a), in the presence of GO (b), NGO (c), Nfree GQDs (d), or NGQDs (e). (f) Absorption ratio versus reaction time in the presence of different catalysts. The concentrations of all the catalysts were 0.175 µg mL-1.

were obtained for NGO and N-free GQDs, where the absorption ratio increases rapidly during the initial time and then levels off with further reaction time. The equilibrium time for the


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coordination reaction is about 8 min (Figure 3c, 3d and 3f), faster than that of GO. Interestingly, the accelerating efficiency of NGQDs is the highest among the four graphene nanostructures, and the absorption ratio of A450/A422 reaches the maximum within 2 min (Figure 3e) with a k0 of 0.0294 (Table S1 in the Supporting Information), being 40 times faster than that using the pure TMPyP solution. It is worth noting that the catalytic effect of NGQDs on the coordination reaction between Cd2+ and TMPyP depends on the reaction temperature, where the equilibrium time for the coordination reaction decreased with increasing temperature (Figure S2 in the Supporting Information). In our present contribution, the investigation of catalytic effect of graphene nanostructures as well as the following sensing experiments were conducted at room temperature (25°C). Catalytic mechanism of NGQDs. The catalytic effect of NGQDs on the rate for metalloporphyrin formation is discussed in terms of the coordination reaction between NGQDs with Cd2+ as well as the formation of TMPyP/NGQDs assembly. The reaction sequence of TMPyP with Cd2+ in the presence of NGQDs may be described as follows: the CdП(NGQDs) complex as a result of the coordination reaction between pyridinic N/pyrrolic N of NGQDs and Cd2+ could rapidly associate with free porphyrin to form a molecular complex CdΠ(NGQDs)•TMPyP (I) in which CdП(NGQDs) is weakly bound to the porphyrin plane. The formation of CdП(NGQDs) complex can be confirmed by the increased zeta potential after the coordination reaction between NGQDs and Cd2+ (Figure S3 in the Supporting Information). The water molecules coordinated to Cd2+ in complex I would dissociate (the structure of aqua complex of Cd2+ is shown in Scheme 1), and Cd2+ is rapidly incorporated into the porphyrin core (Scheme 1 and eq 2). One possible role of the bound NGQDs is to labilize the water molecules coordinated to Cd2+.16,29 Our results have demonstrated that the rate for the reaction of


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CdП(NGQDs) with TMPyP is about 4 times faster than that of the CdП(GQDs) (Figure 3), which can be interpreted in terms of the increased lability of water molecule by electron donation of nitrogen atoms in NGQDs.16,29 Another issue should be noticed is that the steric hindrance effect resulting from the large sheet of GO or NGO would weaken the stability of CdП(GO) or CdП(NGO) complex and thus unfavor the lability of water molecules. In other words, the catalysts with smaller sizes (NGQDs or GQDs) can facilitate the coordination reaction between TMPyP and Cd2+. Another possible role of bound NGQDs is the interaction of NGQDs with porphyrins. It is reasonable to assume that the larger the formation constant of molecular complex I, the higher the overall formation rate of CdП(TMPyP).16 Since the negatively charged NGQDs involve large π-conjugated structure similar to GO, NGQDs can be assembled with cationic TMPyP via π-π stacking and electrostatic interaction,25-28 leading to distinct rate enhancement of CdП(TMPyP). Notably, because of the small size of NGQDs, the assembly of NGQDs with TMPyP can neither flatten the porphyrin molecules nor induce the formation of Jor H-aggregation of porphyrin molecules, which can be proven by the negligible evolution of the TMPyP absorption spectra with addition of different concentrations of NGQDs (Figure S4 in the Supporting Information). The present mechanism is completely different from the flattening mechanism proposed for the interaction between CCG (chemically converted graphene) and TMPyP,18 where red-shift of the porphyrin Soret band dependent of the degree of molecular flattening on CCG can be observed. CdΠ(NGQDs) + TMPyP ⇌ CdΠ(NGQDs)•TMPyP(I) → CdΠ(TMPyP) + NGQDs (2) In order to confirm the formation of TMPyP/NGQDs assembly via the molecular interactions (π-π stacking and electrostatic interaction) even after the coordination reaction between TMPyP and Cd2+, AFM, TEM, XPS, zeta potential, dynamic light scattering (DLS), and light scattering


12

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(LS) spectra were investigated. Compared with the initial NGQDs with a thickness of about 1.4 nm, the assembly of TMPyP or CdП(TMPyP) on NGQDs surfaces causes a height increment of about 0.9 nm (Figure S5 in the Supporting Information), suggesting the attachment of dimer porphyrins on NGQDs.30 The TEM images were consistent with the AFM results (Figure S6 in the Supporting Information), providing further evidence for the formation of the CdП(TMPyP)/NGQDs assembly. The XPS spectrum of CdΠ(TMPyP)/NGQDs shows the presence of Cd 3d with binding energy centered at 405 eV (Figure S7a and S7b in the Supporting Information),31 indicating the successful Cd incorporation into TMPyP to form a metalloporphyrin. Moreover, the N and C contents in CdΠ(TMPyP)/NGQDs are higher than those of NGQDs (Figure S7c in the Supporting Information) since the porphyrins comprise C and N rather than O. The pyridinc N and pyrrolic N for the CdΠ(TMPyP)/NGQDs are higher than those of NGQDs (Figure S7d in the Supporting Information) since the porphyrins consist of pyridinc N and pyrrolic N rather than graphtic N. The evolution of the content and chemical bonding give another evidence for the successful formation of nanoassembly. Furthermore, other proof such as zeta potential, DLS, and LS spectra also confirm the self assembly of NGQDs with TMPyP or CdΠ(TMPyP) in the solution. Compared with NGQDs, the increased value of zeta potentials and hydrodynamic sizes of TMPyP/NGQDs implies that TMPyP with the positive charge has been assembled on the surface of NGQDs and similar results were obtained for the CdΠ(TMPyP)/NGQDs (Figure S3 and Figure S8 in the Supporting Information). Moreover, the enhanced LS signals could be collected as a supplemental strategy to confirm the assembly formation in solution based on the dependence of LS intensity on the particle size of the assembly species.32,33 Compared with NGQDs or TMPyP alone, the LS intensities of the TMPyP/NGQDs complex were enhanced with the increasing of the TMPyP concentrations


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(Figure S9 in the Supporting Information). Although the DLS and the LS data cannot reveal the actual size of the assembly, the results clearly suggest the different size distribution between NGQDs and TMPyP/NGQDs, indicating that the TMPyP/NGQDs assembly assuredly occurs in the solution but not because of solvent drying during AFM or TEM specimen preparation.34,35 Sensing performance by using NGQDs as catalysts. The accelerating effect of NGQDs combined with the distinct spectral change corresponding to the CdП(TMPyP) formation can be applied for rapid and sensitive Cd2+ sensing. Figure 4a displays the spectral changes of the aqueous solution of TMPyP and NGQDs with addition of different concentrations of Cd2+ ions. All the spectra were recorded after 2 min of adding metal ions based on the results derived from Figure 3. When Cd2+ is added to the probe solution, a new absorption peak at 450 nm appears, and the peak at 422 nm decreases. An isosbestic point at 435 nm is observed (Figure 4a). The inset of Figure 4a shows that the absorbance at 450 nm increases linearly with the concentration of Cd2+ in a range of 0.1-10 µM, and the colorimetric evolution can be easily monitored by naked eyes. The detection limit of 90 nM is obtained based on a 3σ/slope, which is much lower than that of the reported methods based on gold or silver nanoparticles.36,37 The selectivity of the TMPyP/NGQDs probe toward Cd2+ ion was also studied. As shown in Figure 4b, the optical response of the probe toward Cd2+ is much higher than those toward other ions, demonstrating the high selectivity of the proposed sensor, which can be comparable with some fluorescent probes38 and better than that using TMPyP or TMPyP/CCG as the probe.18 The present absorption method can be used to determine Cd2+ in lake water samples due to its good selectivity and sensitivity. The concentrations of Cd2+ in water samples determined by the present sensor coincide well with those monitored by the ICP-MS method (Table S2 in the Supporting Information).


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Figure 4. (a) Absorption spectra of TMPyP in the presence of NGQDs and different concentrations of Cd2+ (a-j, 0, 0.1, 0.3, 1, 2, 3, 4, 6, 8, 10 µM), inset: plot of A450 vs Cd2+ concentration, the visual picture corresponding to the Cd2+ concentration of 0, 1, 4, 7, 10 µM (from left to right). (b) Absorption response of TMPyP/NGQDs to 10 µM different metal ions, inset: the visual picture showing colorimetric changes corresponding to the different ∆A450 values (from left to right). The concentrations of TMPyP and NGQDs were 12.5 µM and 0.175 µg mL-1, respectively. Since NGQDs and TMPyP involve fluorescence emission, the fluorescence evolution of the both reagents during the coordination reaction of Cd2+ with TMPyP can be also monitored for Cd2+ sensing. As shown in Figure 5a, in the absence of Cd2+, the addition of TMPyP leads to a substantial fluorescence quenching of NGQDs, which can be mainly ascribed to the inner filter effect (IFE) of porphyrin on the assembled NGQDs. The spectra in Figure 5a noticeably indicate a strong overlap between the emission spectrum of NGQDs and the absorption band of TMPyP in the visible range. Therefore, the absorbance enhancement of TMPyP can be successfully translated into exponential fluorescence quenching of the NGQDs.39-42 Due to the highest extinction coefficient of TMPyP at the wavelength around 422 nm, the PL intensity of NGQDs near this band will be decreased more efficiently, resulting in splitting of the fluorescence


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spectrum with a valley at about 430 nm. The little changed lifetimes of TMPyP/NGQDs (6.0 ns) compared with NGQDs (5.6 ns) verify the IFE-based quenching mechanism (Figure 5b).41,42 To further investigate the IFE on the TMPyP-induced fluorescence quenching, the IFE was corrected on the basis of the cuvette geometry and the absorption characteristics of the aqueous solution of TMPyP and NGQDs (Figure S10 and Table S3 in the Supporting Information).43-45 The results demonstrate that the majority of the quenching effect came from the IFE of TMPyP on NGQDs. After removing the IFE, the small remaining quenching effect may come from the molecular interactions (π-π stacking and electrostatic interaction) between TMPyP and NGQDs.28 In the presence of Cd2+, the formation of CdП(TMPyP) complex leads to a large bathochromic shift of porphyrin Soret band from 422 to 450 nm (Figure 5a). The integral overlap spectrum of NGQDs and CdП(TMPyP) results in efficient IFE and fluorescence quenching of NGQDs with a valley at around 460 nm (Figure 5a). The emission intensity at 460 nm linearly decreases upon the gradual addition of Cd2+ in the range of 0.5-8 µM with a detection limit of 88 nM (3σ) (Figure 5c). Interestingly, a well-defined isoemission point at 440 nm similar to that of the absorption spectra is observed. Figure 5d depicts the emission spectra of TMPyP upon addition of various concentrations of Cd2+ in the presence of NGQDs. Notably, such low concentration of NGQDs would not significantly affect the fluorescence intensity of TMPyP (results were not shown), while in the presence of Cd2+, the fluorescence intensity linearly decreases with the increase of Cd2+ concentration in a range of 5-12 µM with a detection limit of 0.6 µM (3σ). The present TMPyP fluorescence-based assay exhibits much lower sensitivity and narrower range to Cd2+ than above two methods probably due to the less sensitive responsiveness of TMPyP fluorescence to the formation of CdП(TMPyP) complex compared with that of the absorption spectra.


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Figure 5. (a) The UV-vis absorption spectra of TMPyP/NGQDs and CdΠ(TMPyP)/NGQDs (left axis),

and

the

fluorescence

emission

spectra

of

NGQDs,

TMPyP/NGQDs

and

CdΠ(TMPyP)/NGQDs (right axis, λex = 310 nm), the concentrations of TMPyP, Cd2+, and NGQDs were 12.5 µM, 10 µM and 0.175 µg mL-1, respectively. (b) Fluorescence decay of NGQDs, TMPyP/NGQDs, and CdII(TMPyP)/NGQDs at 430 nm, the concentrations of TMPyP, Cd2+, and NGQDs were 12.5 µM, 10 µM and 0.175 µg mL-1, respectively, inset: PL lifetimes for different conditions. (c) Emission spectra of 0.175 µg mL-1 NGQDs in the presence of 12.5 µM TMPyP and different concentrations of Cd2+ (a-j, 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8 µM), inset: plot of I460 vs Cd2+ concentration, λex = 310 nm. (d) Emission spectra of 12.5 µM TMPyP in the presence of 0.175 µg mL-1 NGQDs and different concentrations of Cd2+ (a-i, 0, 5, 6, 7, 8, 9, 10, 11, 12 µM), inset: plot of I658 vs Cd2+ concentration, λex = 420 nm.


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CONCLUSION In summary, we propose a hydrothermal scheme for the preparation of NGQDs, and for the first time report the catalytic effect of NGQDs on the rate of metalloporphyrin formation. The coordination complex of CdП(NGQDs) can rapidly associate with free TMPyP to form a molecular complex of CdΠ(NGQDs)•TMPyP, thus labilizing the coordinated water molecules and accelerating the incorporation of Cd2+ to form CdΠ(TMPyP). Moreover, the assembly of NGQDs and TMPyP via π-π stacking and electrostatic interaction will facilitate the formation of CdΠ(NGQDs)•TMPyP complex and thus favors the incorporation of Cd2+ into TMPyP. The porphyrin combined with NGQDs can be used as a new optical probe for sensing Cd2+ ions. The sensor exhibits rapid, sensitive, and selective responses toward Cd2+. This work provides an example of using graphene nanostructures for tuning the interaction between porphyrins and metal ions, which can be extended to other molecules or nanostructures. ASSOCIATED CONTENT Supporting Information High-resolution C1s and O1s peaks of NGQDs, XRD pattern of NGQDs and Raman spectrum of NGQDs (Figure S1), temperature dependence of the reaction kinetics (Figure S2), zeta potential analysis (Figure S3), absorption spectra of TMPyP versus NGQDs concentrations (Figure S4), AFM and TEM images of the assembly (Figure S5 and Figure S6), XPS spectrum of CdΠ(TMPyP)/NGQDs as well as the content analysis (Figure S7), DLS analysis (Figure S8), and LS spectra (Figure S9). Comparison of the acceleration effects of NGQDs with other graphene nanostructures (Table S1). Results of the water sample analysis (Table S2). Correction of inner filter effect of TMPyP on NGQDs (Table S3 and Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21105044, 21163014 and 21265017), the Program for New Century Excellent Talents in University (NCET11-1002 and NCET-13-0848) REFERENCES (1) Shen, J.; Zhu, Y.; Yang, X.; Li, C. Chem. Commun. 2012, 48, 3686-3699. (2) Zhang, L.; Zhang, Z.-Y.; Liang, R.-P.; Li, Y.-H.; Qiu, J.-D. Anal. Chem. 2014, 86, 44234430. (3) Sheng, Z.-H.; Shao, L.; Chen, J.-J.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. ACS Nano 2011, 5, 4350-4358. (4) Zhang, C.; Fu, L.; Liu, N.; Liu, M.; Wang, Y.; Liu, Z. Adv. Mater. 2011, 23, 1020-1024. (5) Sun, H.; Wang, Y.; Liu, S.; Ge, L.; Wang, L.; Zhu, Z.; Wang, S. Chem. Commun. 2013, 49, 9914-9916. (6) Chang, D. W.; Lee, E. K.; Park, E. Y.; Yu, H.; Choi, H.-J.; Jeon, I.-Y.; Sohn, G.-J.; Shin, D.; Park, N.; Oh, J. H.; Dai, L.; Baek, J.-B. J. Am. Chem. Soc. 2013, 135, 8981-8988.


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TOC Small molecules or metal ions can be employed as catalysts to accelerate metalloporphyrin formation. Herein, we for the first time report the coordination reaction between cadmium(II) and 5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin can be accelerated by nitrogen-doped graphene quantum dots (NGQDs). This catalytic reaction results in change of the absorption of porphyrins and the fluorescence of NGQDs as a result of the inner filter effect (IFE) of the porphyrins on the assembled NGQDs. Both signals can be used for rapid and sensitive determination of metal ions. The present work promises a novel strategy for constructing sensors for metal ions.


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Nitrogen-Doped Graphene Quantum Dots as a New Catalyst

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