One-Pot Green Synthesis of High Quantum Yield Oxygen-Doped

Jun 30, 2014 - This work reports on a facile, economical, and green preparative strategy toward water-soluble, fluorescent oxygen-doped, nitrogen-rich...
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One-Pot Green Synthesis of High Quantum Yield Oxygen-Doped, Nitrogen-Rich, Photoluminescent Polymer Carbon Nanoribbons as an Effective Fluorescent Sensing Platform for Sensitive and Selective Detection of Silver(I) and Mercury(II) Ions Zhong-Xia Wang∥ and Shou-Nian Ding*,∥ School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, China S Supporting Information *

ABSTRACT: This work reports on a facile, economical, and green preparative strategy toward water-soluble, fluorescent oxygen-doped, nitrogen-rich, photoluminescent polymer carbon nanoribbons (ONPCRs) with a quantum yield of approximately 25.61% by the hydrothermal process using uric acid as a carbon−nitrogen source for the first time. The asprepared fluorescent ONPCRs showed paddy leaf-like structure with 80−160 nm length and highly efficient fluorescent quenching ability in the presence of mercury(II) (Hg2+) or silver (Ag+) ions due to the formed nonfluorescent metal complexes via robust Hg2+-O or Ag+-N interaction with the O and N of fluorescent ONPCRs, which allowed the analysis of Hg2+ and Ag+ ions in a very simple method. By employing this sensor, excellent linear relationships existed between the quenching degree of the ONPCRs and the concentrations of Hg2+ and Ag+ ions in the range of 2.0 nM to 60 μM and 5.0 nM to 80 μM, respectively. By using ethylenediaminetetraacetate and ammonia as the masking agent of Hg2+ and Ag+ ions, respectively, Hg2+ or Ag+ ions were exclusively detected in coexistence with Ag+ or Hg2+ ions with high sensitivity, and the detection limits as low as 0.68 and 1.73 nM (3σ) were achieved, respectively, which also provided a reusable detection method for Hg2+ and Ag+ ions. Therefore, the easily synthesized fluorescent ONPCRs may have great potential applications in the detection of Hg2+ and Ag+ ions for biological assay and environmental protection.

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analysis, simplicity, and being nonsample destructing or less cell damaging. Carbon materials are getting more and more attention because of their wide applications in daily life and specialized fields such as bioimaging, catalysis, sensors, and photoelectronics. Recently, carbon materials containing heteroatoms such as nitrogen, sulfur, and boron have been actively pursued and considered as the most promising candidates to complement carbon in materials applications.17 Particularly, the nitrogen (N) atom, having a comparable atomic size and five valence electrons for bonding with carbon atoms, has been widely used for the preparation of N-doped carbon materials (N-CMs). However, until now, considerable research efforts have been focused on the preparation of N-CMs,18−20 as a novel type of fluorescent nanoprobe; research in the detection application of N-CMs is still rare. Especially, fluorescent Ndoped carbon quantum dots (N-Cdots) as new promising NCMs have recently aroused great interest in the detection of heavy metal ions as a possible means to fulfill the abovementioned requirements due to their intrinsic advantages such

ollution by heavy metal ions has become a serious and urgent problem due to their potential to do great damage to the environment and the human body even at low concentrations. Heavy metal ions such as mercury(II) (Hg2+) and silver (Ag+) ions, the most ubiquitous heavy metal ion pollutants, have long-term adverse effects on liver, kidney, and the central nervous systems, and so on.1−3 Therefore, developing an effective analytical method for the sensitive and selective detection of trace amounts of Hg2+ and Ag+ ions is especially important for clinical diagnosis, effective toxicity monitoring, and ultimately successful treatment of human health. To date, a plethora of methods, including atomic absorption/emission spectroscopy, auger-electron spectroscopy, inductively coupled plasma mass spectrometry, and polarography,4−8 have been applied to detect heavy metal ions. However, these methods often require time-consuming analysis, complicated procedures, large sample volumes, sophisticated instrumentation, and/or specialized skills. To overcome these drawbacks, various sensor platform systems for the detection of heavy metal ions have been developed, including electrochemical sensors,9,10 colorimetric detection,11,12 fluorescent sensors,13,14 and plasma mass spectrometry.15,16 Among them, the fluorescent sensors have aroused great attention due to their high-accuracy, sensitivity, fast © XXXX American Chemical Society

Received: March 25, 2014 Accepted: June 30, 2014

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Scheme 1. Schematic Diagram of the Mechanism of the Detection of Hg2+ or Ag+ Ions by the Fluorescent ONPCRs

(NH4OH) which allow the selective detection of Ag+ and Hg2+ ions or repeated detection of Hg2+ and Ag+ ions. The principle of the proposed Hg2+ and Ag+ ions sensing concept is shown in Scheme 1. To the best of our knowledge, this is the first example of the construction of a fluorescence sensing platform for Hg2+ and Ag+ ions based on highly efficient ONPCRs.

as ease in preparation and conjugation, favorable biocompatibility, low toxicity, and unique optical properties.21,22 For example, a simple and sensitive fluorescence assay has been reported for Hg2+ ion detection based on the fluorescence quenching of N-Cdots.23 Recently, Yan et al. designed a novel fluorescent nanoprobe, N-Cdots, to act as a fluorescence sensing platform to detect the spatial and quantitative distribution of Hg2+ ion in an aqueous system.24 Nevertheless, nearly all the reported fluorescent N-Cdots sensors for heavy metal ions detection are of a type with a “one-to-one” readout signal. In view of material utilization and detection cost, undoubtedly, this single detection scheme will greatly limit the further applications of the fluorescent N-CMs sensors in practical detection. Up to now, to the best of our knowledge, only a few approaches have been reported that can be used to detect different metal ions by fluorescent carbon material or other sensing platforms.25,26 Herein, as a new doping carbon material, low-cost oxygendoped, nitrogen-rich, photoluminescent polymer carbon nanoribbon (ONPCR) fluorescent nanoprobes with superior water dispersibility, high-stability, and unique optical properties were successfully synthesized for the first time by hydrothermal treatment of uric acid with quantum yield (QY) of approximately 25.61%. Meanwhile, the hydrothermal method was selected to prepare the ONPCRs in our study because of the uniform heating in solution and environment-benign nature of the process. It need not use special equipment, and it is facile to control the experimental conditions of the ONPCRs just by temperature, amount of the uric acid, and time regulation. This low energy consumption and green heating style are comparable to the classical heating mode such as microwaveassisted methods or sonochemical methods.27 Moreover, on the basis of the strong affinity of Hg2+ and Ag+ ions to the O and N elements on the surface or edge of the fluorescent ONPCRs, respectively, we demonstrate that the easily synthesized fluorescent ONPCRs as a fluorescent nanoprobe can be directly quenched by Hg2+ or Ag+ ions in a highly selective and sensitive method, which can be utilized to directly detect low concentrations of Hg2+ and Ag+ ions in aqueous media, which show excellent and wide linear relationships from 2.0 nM to 60 μM and 5.0 nM to 80 μM with detection limits as low as 0.68 nM and 1.73 nM for Hg2+ and Ag+ ions, respectively. In addition, Hg2+ and Ag+ ions can be successfully masked by the addition of ethylenediaminetetraacetate (EDTA) and ammonia

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EXPERIMENTAL SECTION Reagents and Chemicals. Uric acid (UA) and protamine (PTM) were purchased from Sigma-Aldrich. Bovine serum albumin (BSA) and glucose oxidase (GOD) were purchased from Shanghai Sangon Biological Co. Ltd. (Shanghai, China). Silver nitrate (AgNO3) was purchased from Nanjing Chemical Reagent Co. Ltd. (Jiangsu, China), and mercuric nitrate (Hg(NO3)2) was purchased from Taixing Chemical Reagent Co. Ltd. (Jiangsu, China). Quinine sulfate dihydrate ((C20H24N2O2)2·H2SO4·2H2O) was obtained from Shanghai Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Phosphate buffer solutions (PBS, pH 6.0−8.0, 50 mM) were prepared by varying the ratio of Na2HPO4 to NaH2PO4. All chemicals and solvents were of analytical grade and were used without further purification. Double distilled water was used throughout. Apparatus. UV−vis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan). The fluorescence (FL) measurements were performed on a Fluoromax-4 fluorescence spectrofluorometer (Horiba, USA). Dynamic light scattering (DLS) measurements were carried out at 25 °C on Malven Zetasizer NanoZS instrument (Malvern Instruments Ltd., Worcestershire, UK). Transmission electron microscopy (TEM) measurements were conducted on a JEM2100 transmission electron microscope (JEOL Ltd.). Energydispersive X-ray spectroscopy (EDX) was carried out using a FEI Sirion 200 scanning electron microscope (FEI). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo ESCALAB 250 X-ray photoelectron spectrometer (USA). The Raman spectrum was collected using a T64000three grating Raman spectrometer (Horiba, USA) with the excitation wavelength at 632.8 nm. The FT-IR spectrum was obtained from a Nicolet 5700 (USA) IR spectrometer in the range of 400−4000 cm−1. The FL lifetime measurements were performed on an FL-TCSPC fluorescence spectrophotometer (Horiba Jobin Yvon, USA) with the excitation wavelength at 340 nm. The quantum yield (QY) of ONPCRs was measured B

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Each sample was filtered through a 0.45 μm membrane and centrifuged at 12 000 rpm for 20 min to remove particulate matter, and the supernatants were diluted with 5 mM PBS (pH 7.0) by 10-fold for subsequent analysis.

according to an established procedure.28 The optical densities measured on the UV−vis spectrum were obtained on a Shimadzu UV-2450 spectrophotometer. Quinine sulfate in 0.1 M H2SO4 (literature quantum yield 0.54) was chosen as a standard. Absolute values were calculated using the standard reference sample that has a fixed and known FL QY value. In order to minimize reabsorption effects, absorbencies in the 10 mm FL cuvette were kept under 0.05 at the excitation wavelength (360 nm). Preparation of Fluorescent ONPCRs. ONPCRs were prepared by hydrothermal treatment of UA. In a typical experiment, 1.1 g of UA was dissolved in 25 mL of deionized water and 25 mL of ethanol to form a homogeneous suspension solution under sonication. Then, 25 mL of asprepared solution was transferred into an autoclave and heated at 180 °C for 4.5 h and then cooled to room temperature naturally. The bright yellow solution was extracted with dichloromethane. The water phase solution was placed in a refrigerator for about 5 days to remove all large ONPCRs. At last, the solution was centrifuged at 7000 rpm for 10 min, and a bright yellow ONPCR aqueous solution was obtained. Optimizing Experimental Conditions. In order to obtain a highly sensitive response for the detection of Hg2+ or Ag+ ions, the optimization of the different pH values of PBS solutions was carried out in our experiment. Briefly, 5 μL of ONPCRs (2 mg mL−1) and 25 μL of Hg2+ or Ag+ ions (1 mM) were incubated for 30 min in different pH values of 50 μL PBS (50 mM) solutions; then, the final volume of the mixture was adjusted to 500 μL with double distilled water. The resulting solutions were studied by FL spectroscopy at room temperature with excitation at 355 nm; both the excitation and emission slit widths were 3 nm. Fluorescence Assay of Hg2+ and Ag+ Ions. A typical metal ion detection procedure was conducted as follows. In a typical run, water samples, which were spiked with various concentrations of Hg2+ or Ag+ ions, were mixed with 5 μL of ONPCRs (2 mg mL−1) in a 50 μL PBS solution (pH 7.0, 50 mM) by a vortex mixer for a few seconds. The final volume of the mixtures was adjusted to 500 μL with double distilled water. The mixtures were equilibrated at room temperature for 30 min before the FL spectroscopy measurements were recorded. The resulting solutions were studied by FL spectroscopy at room temperature with excitation at 355 nm; both the excitation and emission slit widths were 3 nm. Sensor Selectivity Investigation. In the selectivity experiment, a series of competitive metal ions and anions, including Ca2+, Cd2+, K+, Fe3+, Mg2+, Sn2+, Zn2+, Au3+, Cu2+, Pb2+, Mn2+, Co2+, Ni2+, Fe2+, Br−, ClO4−, SCN−, Cl−, NO2−, PO43−, SO42−, I−, and H2PO4−, were mixed with 5 μL of ONPCRs (2 mg mL−1) in a 50 μL PBS solution (pH 7.0, 50 mM) by a vortex mixer under the same conditions for a few seconds, respectively. The final volume of the mixture was adjusted to 500 μL with double distilled water. The mixtures were equilibrated at room temperature for 30 min before the FL spectrum measurements were recorded. The concentration of Hg2+/Ag+ ions was 100 μM; the concentrations of other interference ions were 1 mM, but the concentration of Ni2+, Cu2+, or Au3+ ions was 100 μM, respectively. The resulting solutions were studied by FL spectroscopy at room temperature with excitation at 355 nm; both the excitation and emission slit widths were 3 nm. Real Samples. The urine and serum samples were obtained from the Southeast University Affiliated Zhongda Hospital.

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RESULTS AND DISCUSSION Synthesis and Characterization of Fluorescent ONPCRs. The synthesis procedure is illustrated in the Experimental Section. UA and ethanol were used as the carbon−nitrogen source and stabilizer, respectively, without any other toxic reducing agents, which makes the whole experimental process green and environmentally friendly. The FL QY of ONPCRs increased with the content of UA in the precursor solution from 0.2 to 1.1 g and then decreased as the content of UA in the precursor solution went over 1.1 g (Table S1, Supporting Information). Moreover, the maximum emission intensity of ONPCRs (UA, 1.1 g) at 420 nm increased with the reaction time from 1 to 4.5 h and then decreased in the period from 4.5 to 5.5 h (Figure S1, Supporting Information). Thus, from the above discussions, it is reasonable to speculate that the ONPCRs may be formed gradually through multistep cross-linked polymerization. Namely, the FL intensity enhancement might originate from more polyaromatic structures induced by incorporating N/O atoms and protonation of N/O atoms on nanoribbons,25 and when the reaction time continues for 4.5 h, the more polyaromatic structures of the ONPCRs may be of a structurally rigid plane, leading to the QY of the polymer nanoribbons being the highest. Too long or short reaction time would affect/destroy the structurally rigid plane of the ONPCRs, as result of the decreased FL QY of ONPCRs. However, there is still no convincing synthesis mechanism for photoluminescence ONPCRs until now; despite the aboveproposal of several theories, the practical reaction polymerization mechanism of N/O in ONPCRs remains unclear. Morphologies of the as-prepared fluorescent ONPCRs were characterized by TEM. Typical TEM images of the fluorescent ONPCRs supported on polymer synthesized using 1.1 g of UA in the precursor solution are shown in Figure 1. It clearly shows

Figure 1. TEM image (a, b) at different magnifications, HRTEM image (c), SAED image (d), EDS (e), and the element concentrations of C, O, and N (f) of the fluorescent ONPCRs.

that ONPCRs are nearly transparent and have a paddy leaf-like shape; all fluorescent ONPCRs are uniform, monodisperse, and less than 160 nm in length (Figure 1a,b), which is further evidenced by the DLS measurements. The DLS plot (Figure 2A) further demonstrates the monodispersity of the ONPCRs, which possess a typical average dynamic size of ca. 250 nm, and the results have some differences with the TEM results. The slight overestimation in the DLS measured sizes is due to the fact that DLS measures the hydrodynamic diameter/radius and the ONPCRs are suspended in water with loose nanostructure; C

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Figure 2. DLS curve (A) and FT-IR spectrum (B) of the obtained fluorescent ONPCRs.

Figure 3. XPS (A) and C1s (B), O1s (C), and N1s (D) spectra of the as-obtained ONPCRs.

cm−1.20,35,36 Moreover, the bands in the range of 1030−1310 cm−1 suggest the presence of a large amount of C−O groups.37 The surface composition and elemental analysis for the overall composition of the resultant nanoribbons were further confirmed by XPS, as shown in Figure 3. The XPS spectrum shows three strong peaks at 532.0, 401.1, and 286.1 eV, which are attributed to O1s, N1s, and C1s, respectively (Figure 3A).29 The XPS results indicate that these nanoribbons are mainly composed of C (41.26%), N (35.25%), and O (23.50%), as well as a limited amount of the O element (the atomic ratio of C/N/O is 2.15:1.35:1), and the amount of N (35.25%) is much higher than the carbon material reported by Huang et al. and can be used for the surface passivation of carbon materials.23 The C1s spectrum (Figure 3B) shows five peaks at 284.6, 286.0, 287.3, 288.6, and 290.4 eV, which are attributed to C−C/C C, C−N/C−O, CN, CO, and O−CO groups, respectively.38,39 The three peaks at 530.7, 531.6, and 533.1 eV in O1s spectrum (Figure 3C) are attributed to CO, C− OH, and C−O−C groups, respectively.34,40 The XPS spectrum of N1s (Figure 3D) core level electrons of ONPCRs exhibits four fitted peaks at 398.1 (a), 399.1 (b), 399.8 (c), and 400.6 eV (d), which are associated with N in a graphite-like structure, pyridinic-like N, pyrrolic-like N, and N−H groups, respectively.41−43 The C1s and N1s spectrum clearly show the formation of pyridinic-like N, pyrrolic-like N, and amino-like N in the ONPCRs and confirm the successful incorporation of

consequently, the range of average size of ONPCRs is large. The high resolution TEM image in Figure 1c clearly reveals that the diffraction contrast of the ONPCRs is very low and without any obvious lattice fringes, which is indicative of their polymer-like amorphous nature. Meanwhile, Figure 1d shows that the selected-area electron diffraction (SAED) pattern of the fluorescent ONPCRs, which reveals the crystal nature of the obtained fluorescent ONPCRs, is not a crystal structure. The formation of the fluorescent ONPCR polymer was further confirmed by EDS as shown in Figure 1e. The EDS spectrum shows the peaks corresponding to C, N, and O elements and mainly contains the C (43.01%) and N (36.81%) element in the FL ONPCR polymer (Figure 1f), which are much higher than those reported previously.29−31 The surface composition and elemental analysis for the resultant nanoribbons was characterized by the FT-IR spectrum technique. As illustrated in Figure 2B, the peaks at about 3400 and 1050 cm−1 can be ascribed to the characteristic absorption bands of the −OH stretching vibration mode.32 The characteristic absorption band of N−H stretching at 3150 cm−1 is also observed, and the peaks at 2850, 820, and 2940 cm−1 can be assigned to the C−H stretching mode and C−H out-of-plane bending mode.33,34 CO stretching vibrations are at 1750 cm−1, aromatic CN heterocycles stretching vibrations are at 1309−1650 cm−1, and C−N stretching vibrations are at 1405 D

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Figure 4. (A) UV−vis absorption (a, λex = 355 nm) and emission (b, λem = 420 nm) of the ONPCRs. Inset (from left to right): photographs of an aqueous solution of the ONPCRs taken under visible light and 365 nm UV light, respectively. (B) FL spectrum of the ONPCRs at different excitation wavelengths from 320 to 400 nm; both the excitation and emission slit widths were 3 nm.

Figure 5. (A) The FL intensity at 420 nm (excitation at 355 nm) of the ONPCRs in the NaCl solution. (B) Dependence of the FL intensity on the pH values from 1 to 13. (C) Stability of the ONPCRs as a function of the storage time. Both the excitation and emission slit widths were 3 nm.

of ONPCRs, which is consistent with the results of XPS and FT-IR. Figure 4A shows that the ONPCRs in aqueous solution have two typical UV−vis absorption peaks at ∼275 and ∼355 nm, respectively. The absorption peak at approximately 275 nm was assigned to the π−π* transition of aromatic sp2 domains and confirmed the existence of aromatic heterocycle, which leads to nearly no observed FL signal,49 while the other transition centered about 355 nm due to the trapping of excited-state energy by the surface states results in strong emission.50−52 Meanwhile, it is seen that the UV−vis displays two other absorption peaks at about 290 and 335 nm (Figure 4A); interestingly, the latter peak looks like two prominent bands with unsymmetrical shape and adjoining shoulders, and these absorption bands may be ascribed to that originating from the N atoms of UA to aromatic polyimides and/or to amide intramolecular charge transfer states.53,54 The bright yellow aqueous ONPCR solution emits very intense bright blue luminescence under UV light (365 nm) even at a very low concentration (20 μg mL−1), which can be clearly seen in the inset of Figure 4A. An excitation-dependent emission was observed with the red-shifted emission peaks from the long excitation (Figure 4B), showing the multicolor properties of the ONPCRs and characteristics of carbon materials.55,56 The emission intensity increased in the excitation range of 320−355 nm and then decreased gradually. The maximum emission peak was observed at 420 nm for an excitation of 355 nm, with a Stokes shift of 65 nm. The calculated QY was 25.61% by using quinine sulfate as reference (Table S1, Supporting Information).28 The optical properties of the ONPCRs can be easily controlled by varying the reaction conditions, such as the reaction time, the amount of ethanol, and the content of UA in

nitrogen into the ONPCRs. Therefore, the presence of the aromatic heterocycles containing CN species and the hydroxyl, carbonyl, and carboxylic moieties on the surface of ONPCRs were confirmed by the XPS and FT-IR. All these observations indicate these nanoribbons are oxygen-doped, nitrogen-rich photoluminescent polymer carbon nanoribbons and are quite different from previously reported carbon materials. Raman spectroscopy is a widely used tool for the characterization of carbon materials, especially considering the fact that conjugated and carbon−carbon double bonds lead to high Raman intensities. Figure S3 (Supporting Information) shows the Raman spectrum of the as-obtained ONPCRs. The spectrum exhibits three main features in the 200−3000 cm−1 region corresponding to in-plane vibrations of the aromatic polymer,44 which indicates that the obtained ONPCRs are structurally different compared with those graphite-like doped materials.45,46 The results could be due to mixed stretching modes and vibrations involving the structural carbon atoms which are mainly organic quinone-like polymer and aromatic polyimides. The present peaks around 700−2200 cm −1 illustrate that the FL ONPCRs are more likely organic macromolecules rather than graphite-like materials (Figure S3, Supporting Information). The strongest organic bands, commonly occurring around 1250−1550 and 1900−2200 cm−1, could correspond to the ν2 and ν1 stretching vibrations of carbon−carbon double bonds (CC) and carbon− nitrogen/oxygen double bonds (CN/CO), respectively. Bigger and wider peaks around 650−950 cm−1 likely correspond to C−N/C−C stretching and C−O bending modes.47,48 These three regions could be associated with the presence of the aromatic heterocycles containing CN species and the amino, carbonyl, and carboxylic moieties on the surface E

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ONPCRs to 50 μM Hg2+ or Ag+ ions at various pH values. Here, fluctuating pH values in the range of 6.0−8.0 (PBS, 5 mM) were investigated. As shown in Figure S4 (Supporting Information), only a slight change in the FL intensity of ONPCRs is obtained in the pH ranging from 6.0 to 8.0. Highly acidic or alkaline surroundings would negligibly affect the protonation−deprotonation of the ONPCRs due to lots of N/ O atoms of the ONPCRs, leading to the strong stabilization of the forming ONPCRs-Hg2+/Ag+ metal complexes in different pH values. Considering the protonation−deprotonation of the ONPCRs and the stable effect of the Hg2+ or Ag+ ions for pH values, pH 7.0 PBS was selected as the optimum solvent for heavy metal ions detection. Mechanism of Hg2+ or Ag+ Ions Detection by the FL ONPCRs. We explored the feasibility of using such ONPCRs for Hg2+ and Ag+ ions detection. It is seen that the ONPCR solution in the absence of Hg2+ and Ag+ ions exhibits a strong FL peak at 420 nm (Figure 6, curve a). In contrast, the FL

the precursor solution. The emission spectrum of the mixed solutions were recorded under different reaction times. As shown in Figure S1 (Supporting Information), the FL intensity increased significantly as the reaction time increased up to 4.5 h. Beyond that, a longer reaction time decreased the FL intensity. Meanwhile, the effect of the amount of ethanol on the preparation of FL ONPCRs in a volume of 5−35 mL in the precursor solution was tested (Figure S2, Supporting Information). Although a better FL intensity of ONPCRs was achieved at the volume range of ethanol of 5 to 15 mL, the status of ONPCRs tended to be sol−gel in this concentration range of ethanol (inset of Figure S2, Supporting Information). Considering the stable monodispersion of the FL ONPCRs and the high QY of FL ONPCRs, 25 mL of ethanol was selected as the optimum amount for prepared ONPCRs. In addition, the effect of content of UA in the precursor solution on the preparation of FL ONPCRs was tested. As depicted in Table S1 (Supporting Information), the maximum QY (25.61%) of ONPCRs was obtained when the content of UA was at 1.1 g, too less or more content caused negative effects on the QY of ONPCRs. Investigation of the Properties of Prepared ONPCRs. To confirm the stability of ONPCRs under high ionic strength environments, their FL intensities were measured in the presence of different concentrations of NaCl (up to 1 M). As shown in Figure 5A, only a slight change in the FL intensity is observed, indicating high stability of the ONPCRs even under a high ionic strength environment. This finding suggests that ONPCRs have great potential for sensing applications under physiological conditions. Meanwhile, the FL of the ONPCRs is strong and stable in a wide range of pH values (pH 4−11; Figure 5B). However, as the pH value is lower than 4, the FL intensities decrease gradually, but unlike the N-CMs reported previously,23 the FL intensities of the ONPCRs increase rapidly as the pH is over 11.0. Here, fluctuating QY values of the ONPCRs in the pHs of 1.0 and 13.0 were also investigated. As shown in Table S1 (Supporting Information), the QY value of the ONPCRs in pH 1.0 is 0.25%, but it is 31.06% in pH 13.0. We surmised that the effect of the pH values can be understood in terms of the change in surface charge owing to protonation− deprotonation, which could strongly increase the structurally rigid plane of the ONPCRs through preventing the formation of hydrogen bonds between intramolecular and molecular interactions and then lead to the QY of the polymer nanoribbons being the highest; however, when the pH value was less than 4, the reaction of the proton units in solution with the N/O groups on the ONPCR surfaces or edges creates structurally flexible ONPCR-proton quenches, strengthening the intramolecular or intermolecular reactions and thus resulting in a great decrease in the FL intensity. Furthermore, only a slight change in the FL intensity is obtained when the sample is stored for 60 days (Figure 5C). These results indicate excellent stability of the ONPCRs, probably due to the electrostatic repulsions between the negatively charged nanoribbon resulting in electrosteric stabilization. All these properties make the ONPCRs particularly valuable for real applications in biolabeling and bioimaging. Optimizing Experimental Conditions. The pH of the reaction solution could greatly affect the interaction between ONPCRs and the heavy metal ions. Therefore, the pH of the reaction solution is an important parameter for the kinetic binding between heavy metal ions and the ONPCRs. Figure S4 (Supporting Information) shows the FL response of the

Figure 6. FL emission spectrum (λex = 355 nm) of free ONPCRs (a) and ONPCRs in the presence of Hg2+ ions (b), Ag+ ions (c), both Hg2+ ions and EDTA (d), or both Ag+ ions and NH4OH (e). Inset: Photographs under UV light (365 nm). The final concentrations of ONPCRs, Hg2+ ions, Ag+ ions, EDTA, and NH4OH are 20 μg mL−1, 100 μM, 100 μM, 100 μM, and 200 μM, respectively.

intensity of ONPCRs obviously decreases in the presence of Hg2+ or Ag+ ions, indicating that Hg2+ and Ag+ ions can effectively quench the FL of ONPCRs (Figure 6, curve b, c). The most probable explanation for the quenching of FL of ONPCRs in the presence of Hg2+ and Ag+ ions, respectively, is based on the electron or energy transfer mechanism.57,58 That is, Hg2+ ion is an acid of borderline hardness (according to the hard and soft acids and bases principle) and has stronger affinity for oxygen and sulfur donor atoms than the nitrogen atom, as result of forming the more stable metal complexes between Hg2+ ion and O atoms in ONPCRs.59,60 However, as the identical nature element (according to the diagonal principle), the underlying nature of FL decreasing by the Ag+ ion is attributed to a strong chemical bonding of Ag+ ions to N atoms in ONPCRs, because Ag+ ion has a strong affinity to N atoms and can form stable complexes with nitrogen-containing groups.25 Thereby, the formation of the stable metal complexes assembly may be responsible for the diminished luminescent characteristics. On the other hand, mercury is known to be a strong Lewis acid and, therefore, has a strong affinity to accept electrons and has an identical nature to that of silver (according to the diagonal principle). Thus, initially, Hg2+ or Ag+ ions are slowly added to the FL solution of ONPCRs and the electron, which was already transferred in the ONPCRs by the Hg2+ or Ag+ ions, is back-donated in the vacant orbital of Hg2+ or Ag+ F

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Figure 7. (A) FL emission spectrum of ONPCRs (20 μg mL−1) in the presence of different concentrations of Hg2+ ions (0, 0.002, 0.02, 0.5, 1.0, 5.0, 10.0, 15.0, 20.0, 35.0, 50.0, 60.0, 100.0, 150.0, and 200 μM, top to bottom, excitation at 355 nm). (B) Plot of the enhanced FL signals [(FL0 − FL)/ FL0] versus Hg2+ ions concentration. (C) FL emission spectrum of ONPCRs (20 μg mL−1) in the presence of different concentrations of Ag+ ions (0, 0.005, 0.02, 0.1, 0.5, 1.0, 10.0, 20.0, 40.0, 50.0, 60.0, 80.0, 100.0, 150.0, and 200 μM, top to bottom, excitation at 355 nm). (D) Plot of the enhanced FL signals [(FL0 − FL)/FL0] versus Ag+ ions concentration.

energy donors or/and acceptors to affect/destroy the structurally rigid plane of the ONPCRs, which subsequently leads to the formation of ONPCRs-Ag+ metal complexes that do not fluoresce. Therefore, all the above discussions indicate that the strong dependence of the luminescence quenching by Ag+ ion is good proof for an electron or energy transfer static quenching mechanism. Hg2+/Ag+ Detection Using the ONPCRs as Fluorescent Probe. For a sensitivity study, FL changes were monitored after the addition of Hg2+ or Ag+ ions for a fixed time of 30 min. Figure 7A shows the variance of FL intensity with the concentration of Hg2+ in the range of 0−200 μM. The FL intensity of the ONPCRs at 420 nm was proportionately decreased upon increasing Hg2+ ions concentration. Upon adding Hg2+ (100 μM) to the ONPCRs solution, the blue FL of the ONPCRs was nearly complete quenched, revealing that the sensing system is sensitive to Hg2+ ions concentration. A linear region was obtained in the plots of the value of [(FL − FL)/FL0] for the ONPCRs versus the concentrations of Hg2+ ions, 2 nM−60 μM (Figure 7B). The limit of detection (LOD) for Hg2+ ions, at a signal-to-noise ratio of 3, was estimated to be 0.68 nM (∼0.15 ppb), which is much lower than the maximum contamination level of Hg2+ ions in drinking water permitted by the World Health Organization (WHO, 6 ppb) and the limit of the 2 ppb set by the U.S. Environmental Protection Agency (EPA).64,65 The FL quenching of ONPCRs in the presence of Ag+ is presented in Figure 7C,D, which exhibits a good linear relationship in the range of 5 nM to 80 μM. Under the current experimental conditions, the LOD for Ag+ ions was estimated to be 1.73 nM (3σ, ∼0.2 ppb), which is considerably lower than the maximum allowable level of Ag+ ions (50 ppb) in drinking water as regulated by the WHO66 and the limit of the 0.46 μM set by the U.S. EPA.67 The resulting sensor provided a sensitivity and low LOD that were much better than those previous reported. The methods of detection of Hg2+ or Ag+ by other fluorescent probes are shown in Table S3 (Supporting Information). The above results suggest that the as-prepared

ions, leading to a strong electronic interaction and formed nonfluorescent metal complexes, thereby decreasing its luminescence intensity. Therefore, lots of O and N atoms in ONPCRs are strongly attracted to bind to the surface or edge of the Hg2+−O and Ag+−N in the FL ONPCRs assembly, respectively, and there is a highly efficient FL quenching ability of the fluorescent ONPCRs. Moreover, when adding a strong Hg2+ ion chelator, EDTA, or a Ag+ ion chelator, NH4OH,61 Hg2+ and Ag+ ions are removed from the surface of ONPCRs by forming metal chelates with EDTA and NH4OH, which induces the FL recovery of ONPCRs (Figure 6, curves d,e). The FL intensity of the above solution was decreased again when Hg2+ or Ag+ ions were added, which also provided a reusable detection method for Hg2+ or Ag+ ions (Figure S5, Supporting Information). Meanwhile, the turn-on effect of EDTA and NH4OH on FL of ONPCRs can be easily observed visually (inset of Figure 6). Thus, these observations demonstrate that ONPCRs together with heavy metal ions can be utilized as a novel and simple probe for the analysis of Hg2+ and Ag+ ions. In short, it is worth noting that the content of the N and O atom as active sites play an important role in the detection of the metal ions. To further reveal the mechanism of FL quenching, the timeresolved FL of the ONPCRs as well as that of the ONPCRsAg+ metal complexes was measured. Due to differences in the distribution of complex luminescent pathways resulting from multiple ONPCRs species and/or sites,62 the FL intensity of ONPCRs follows dual-exponential decay kinetics and two lifetimes were acquired (Figure S6 and Table S2 in the Supporting Information). It is found that the FL lifetime of the ONPCRs does not change in any obvious manner with the addition of Ag+ ions. The unchanged FL lifetime and the linear Stern−Volmer plot (inset of Figure 7D) imply that the quenching of ONPCRs by Ag+ ions obeys a simple static quenching mechanism;63 that is, the Ag+ ions coordinate to the nitrogen-donor atoms of the amino/heterocyclic nitrogen groups on the ONPCRs surfaces, acting as an electron and G

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Figure 8. Selectivity of the ONPCRs-based detection system. The concentrations of Hg2+, Ag+, Ni2+, Cu2+, and Au3+ ions and EDTA are 100 μM, but the concentration of NH4OH is 200 μM, while those of the other ions are 1 mM. The final concentration of the ONPCRs is 20 μg mL−1.

ONPCRs can be indeed used as robust fluorescent probes for separately sensing Hg2+ and Ag+ ions with a high degree of accuracy and simplicity. Selectivity of the ONPCRs Probe. Except sensitivity, selectivity is another important parameter to evaluate the performance of the sensing system. Under the optimal conditions, we tested the FL intensity changes of the ONPCRs in the presence of competitive metal ions and anions under the same conditions, respectively, including Ca2+, Cd2+, K+, Fe3+, Mg2+, Sn2+, Zn2+, Au3+, Cu2+, Pb2+, Mn2+, Co2+, Ni2+, Fe2+, Br−, ClO4−, SCN−, Cl−, NO2−, PO43−, SO42−, I−, and H2PO4−, as shown in Figure 8. The FL intensities of the ONPCRs in the absence and presence of other ions are denoted by FL0 and FL, respectively. It is seen that a much higher relative FL intensity [(FL0 − FL)/FL0] was observed for ONPCRs upon addition of Hg2+ and Ag+ ions. In contrast, no tremendous increase was observed by adding other ions into the ONPCRs dispersion. These observations suggest that the proposed method is capable of discriminating between Hg2+/Ag+ ions and the interference ions. It is worth noting that the Ag+ ion shows a large interference on the sensor for Hg2+ ion detection due to the strong reaction between Ag+ ions and ONPCRs. In addition, when Ag+ ions are detected using the ONPCRs, the significant interference from Hg2+ ions could be attributed to the formation of nonfluorescent metal complexes (Hg2+ONPCRs). Further investigations demonstrate that EDTA and NH4OH as Hg2+ and Ag+ ions chelators were able to capture Hg2+ and Ag+ ions to form the metal chelates, respectively. As expected, the interference from Ag+ or Hg2+ ions for the ONPCRs probe toward Hg2+ or Ag+ ions is negligible in the presence of NH4OH or EDTA (Figure 8). The excellent selectivity and specificity can probably be attributed to Hg2+ and Ag+ ions having stronger affinity toward the O and N atom groups on the ONPCR surface than other metal ions, respectively.68 Furthermore, the selectivity of this nanoprobe in the presence of other possible interference factors were evaluated considering the sophisticated reaction system, including salts (e.g., NaCl and NaNO3) and proteins (e.g., BSA, PTM, and GOD), as demonstrated in Figure S7 (Supporting Information); only slight changes in the FL intensity of ONPCRs were obtained in the presence of different concentrations of salts (NaCl, ≤20 mM, NaNO3, up to 0.2 M) and different proteins, indicating high selectivity and stability of the ONPCRs for

detection of Hg2+ and Ag+ ions even under the sophisticated biological environment. All these results validate that the present fluorescent sensing platform exhibits the high selectivity requirements for the Hg2+ and Ag+ ions assay in biological and water environmental fields. Real Samples Testing. The developed ONPCRs sensor was applied to the determination of Hg2+ and Ag+ ions in urine and serum samples to realize its applicability for real sample analysis. The real samples were analyzed by the standard addition method. The results are listed in Table S4 (Supporting Information); the recovery of the added known amount of Hg2+ and Ag+ ions to the three different solutions for each sample was in the range of 95.84−106.58%, which indicates the developed ONPCRs sensor possessed excellent applicability for real sample analysis.

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CONCLUSION In summary, for the first time, green and water-soluble fluorescent ONPCRs were synthesized by a facile and onestep hydrothermal method using UA as a carbon−nitrogen source. Furthermore, no further chemical modification of ONPCRs is required, which offers the advantages of simplicity and cost efficiency. Such ONPCRs have been further used as fluorescent probes for the label-free detection of Hg2+ and Ag+ ions. Taking advantage of the high QY of ONPCRs as fluorescent probes, the proposed method can sensitively measure Hg2+ and Ag+ ions with detection limits of 0.68 nM for Hg2+ ions and 1.73 nM for Ag+ ions, which are superior to most current approaches for metal ion analysis (Table S3, Supporting Information). Furthermore, the addition of EDTA or NH4OH to the detection system can successfully mask Hg2+ or Ag+ ions, which allow the recovery selective detection of Hg2+ or Ag+ ions. Therefore, this work provides a good example of a simple and cost-effective system of sensing Hg2+ and Ag+ ions with broad detection ranges and low detection limits by using the fluorescent ONPCRs. For the easy production of stable and strong FL of the ONPCRs, we believe that the ONPCRs would have many potential applications in chemistry and biology.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S7 and Tables S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org/. H

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86) 25-52090621. Author Contributions ∥

Z.-X.W. and S.-N.D. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (No. 21345008), the Fundamental Research Funds for the Central Universities, the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University, and the Open Research Fund of State Key Laboratory of Analytical Chemistry for Life Science (SKLACLS1211).

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