Reversible âOffâOnâ Fluorescence of Zn2+-Passivated Carbon Dots

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Reversible“off-on”fluorescence of Zn2+-passivated carbon dots: mechanism and potential for detection of EDTA and Zn2+ Mingxi Yang, Qiuling Tang, Yang Meng, Junjun Liu, Tanglue Feng, Xiaohuan Zhao, Shoujun Zhu, Weixian Yu, and Bai Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00947 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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Reversible“off-on”fluorescence of Zn2+-passivated carbon dots: mechanism and potential for detection of EDTA and Zn2+

Mingxi Yanga, Qiuling Tangb, Yang Mengb, Junjun Liua, Tanglue Fenga, Xiaohuan Zhaoa, Shoujun Zhuc, Weixian Yub and Bai Yang*a

a

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun 130012, P. R. China b

Department of Periodontology, School and Hospital of Stomatology, Jilin University Changchun

130012, P. R. China c

Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging

and Bioengineering , National Institutes of Health, 35 Convent Dr, Bethesda, MD 20892 USA

1

ACS Paragon Plus Environment

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ABSTRACT Zn2+ passivated carbon dots (named Z-CDs) were synthesized from zinc gluconate for the first time through a one-step pyrolysis treatment. The mechanism of Zn2+ enhanced fluorescence was carefully investigated and a new strategy to passivate the surfaces of CDs by Zn2+ was proposed. Inspired by the complexation reaction between Zn2+ and EDTA, a reversible “off-on” fluorescent nanosensor for detection of EDTA and Zn2+ was constructed based on the de-passivation and re-passivation of Z-CDs, with a limit of detection as low as 3.2 × 10-7 M and 5.1 × 10-7 M, respectively. The proposed Z-CDs based nanosensor had been further utilized for EDTA and Zn2+ monitoring in tap water with excellent recovery. To the best of our knowledge, this was the first report of a fluorescence-based sensor of EDTA and a turn-on sensor of Zn2+ based on CDs with reversible detection capability. Also, benefiting from the low toxicity of zinc, Z-CDs was applied for multicolor bio-imaging and in-vitro detection in cells.

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1. INTRODUCION Carbon dots (CDs), with high anti-bleaching photo-stability1, wide and tunable absorption and emission2, good biocompatibility and low biological toxicity3, have drawn increasing attention in recent years4-5. These unique properties make CDs widely used in many application fields, such as bio-imaging6, drug carrier7, catalysis8, optoelectronic devices9, display10-12 and antibacterial13. As a kind of fluorescent nanomaterials with abundant functional groups, the photoluminescent (PL) emission of CDs could be tuned by surface functionality or passivation, which was the so-called surface/edge state14-15. Many works had shown that some polymers (i.e. branched polyethylenimine (b-PEI), polyethylene glycol (PEG)) and organic molecules (usually N-contained molecules) could provide passivation of surface energy traps on CDs, which could greatly enhance the fluorescence of CDs16-19. However, metal ions passivation was rarely reported for CDs. Although there were some works on enhanced fluorescence of CDs doped with metal ions, their mechanism were unclear, and proof-of-concept applications that utilized the passivated ions on CDs’ surfaces were missing20-21. Ethylenediaminetetraacetic acid and its salts (EDTA), well-known chelating and clarifying agents, are widely used in preservatives, eye-drops, cosmetic creams, and lotions. However, EDTA was reported to have allergic contact dermatitis on human skin22-23, and contamination of EDTA may also cause hypocalcaemia, hypomagnesaemia and hypozincaemia in blood24. Therefore, it is very meaningful to determine the existence of trace amounts of EDTA in solution. Unfortunately, the conventional analytical techniques for detection of EDTA are including titrimetry25, High

Performance Liquid Chromatography (HPLC)26, atomic absorption spectrometry27, ion chromatography28, etc., which are not convenient, due to the unsatisfactory limit of detection (LOD), the complicated sample pretreatment and the requirements of expensive equipment. With the advantages of easy operation, high sensitivity and selectivity, the fluorescent sensor is a good alternative. However, to the best of our knowledge, there are no reports about fluorescence-based probe for EDTA detection, not to mention CDs-based fluorescent sensors. Zinc (Zn), as an essential element in human body, is an indispensable component of many enzymes29. Therefore, sensitive detection of Zn2+ is desired in order to understand the physiological process. So far, several methods had been reported to detect Zn2+, such as 3


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ratiometric fluorescence30, UV-Vis spectroscopy31, capillary electrophoresis32, fluorescent nanosensor based on quantum dots (QDs) and CDs/fluorogenic zinc(II) probe quercetin (QCT-Zn2+)33-35. However, there were no reports about CDs sensor of Zn2+ by turning on their own fluorescence possessing reversible detection capability. Herein, we demonstrated the preparation of a novel Zn2+ passivated CDs (named Z-CDs) through a one-step pyrolysis treatment of zinc gluconate for the first time. By comparing with CDs synthesized from glucose (named G-CDs) where Zn2+ was absent, we proposed a possible mechanism to understand the much stronger photoluminescence (PL) property of Z-CDs. The Zn2+ could prevent the aggregation during the process of carbonization, also itself passivated the surfaces of Z-CDs to further stabilize CDs and improve PL intensity. Inspired by the complexation reaction between Zn2+ and EDTA, a Z-CDs-based turn-off fluorescent sensor for detection of EDTA and a turn-on fluorescent sensor for detection of Zn2+ was constructed, and the limit of detection was as low as 3.2 × 10-7 M and 5.1 × 10-7 M, respectively. The quenching and recovering process could be repeated without obvious changes, indicating its practical application in reversible “off-on” detection of EDTA and Zn2+. Then, the proposed Z-CDs based sensor had been successfully utilized for EDTA and Zn2+ monitoring in tap water with excellent recovery. Moreover, benefitting from the low toxicity of zinc and the bright fluorescence, Z-CDs were then utilized for multicolor bio-imaging and in-vitro detection in cells. To the best of our knowledge, this was the first report of a fluorescence-based sensor of EDTA and a turn-on CDs sensor of Zn2+ with reversible detection capability. The present work might provide a new strategy for both synthesis and applications of metal ions-CDs hybrid nanomaterials.

2. EXPERIMENTAL SECTION 2.1 Reagents and materials D-(+)-Glucose was purchased from Aladdin. Zinc gluconate, sodium gluconate and copper gluconate were purchased from Macklin. Ethylenediaminetetraacetic acid disodium salt (EDTA) was purchased from Beijing Chemical Reagent. Ethylenediamine (EDA), sodium citrate, 2,2-dipyridyl, 8-hydroxyquinoline and 1,10-phenanthroline monohydrate was purchased from Aladdin. NaCl, AlCl3, KCl, CaCl2, CrCl3, FeCl3, CoCl2, NiCl2, CuCl2, ZnCl2, CoCl2, PbCl2, HCl and NaOH were purchased from Beijing Chemical Reagent. All reagents were of analytical grade 4


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and used directly without any further purification. Deionized water was used during the whole experiments.

2.2 Synthesis and purification of G-CDs and Z-CDs A quartz boat filled with glucose or zinc gluconate (1.0 g) was placed in a tube furnace and calcined at 160 °C for 1 h at a heating rate of 5 °C min-1 under a N2 atmosphere. The sample was ground and dissolved in 80 mL water, ultrasonically treated for 30 min at room temperature, and then centrifuged at a high speed of 12000 rpm for 15 min. The upper brown solution was filtered slowly using a 0.22 µm millipore filter to remove the aggregation product or deposited salts. After the filtration process, the solution was dialyzed against deionized water with a 1000 Da dialysis tube for 24 h to remove the small fragments and raw materials. Pure CDs powder can be obtained by lyophilized for one day.

2.3 Instrumentation Fluorescence spectroscopy was performed on Shimadzu RF-5301 PC spectrophotometer. UV-Vis absorption

spectra

were

obtained

using

Shimadzu

3100

UV-Vis

spectrophotometer.

High-resolution transmission electron microscope (HRTEM) was recorded on JEM-2100F. FTIR spectra were measured on Nicolet AVATAR 360 FTIR spectrophotometer. The fluorescent images were taken on Olympus IX81 at different light excitations. X-ray diffraction (XRD) measurements were performed on a PANalytical B.V. - Empyream Diffractometer with Cu Kα radiation. X-ray Photoelectron Spectroscopy (XPS) was investigated using ESCALAB 250 spectrometer with a mono X-Ray source Al Kα excitation (1486.6 eV). Absolute PLQY measure was recorded on calibrated integrating sphere in FLS920 spectrometer. Nanosecond fluorescence lifetime experiments were measured on FLS980 spectrometer.

2.4 Turn-off detection of EDTA salts in solution Different volumes (0-1000 µL) of EDTA solution (0.001 M) were separately added into 5 mL of Z-CDs solution (2 mg/mL), mixed well and diluted to 10 mL, the final concentration of Z-CDs was 1mg/mL. The PL spectra were recorded under 365 nm excitation. The fluorescence intensity 5


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ratio (F0-F)/F0 at 460 nm was used to evaluate the quenching effect of Z-CDs caused by EDTA, where F0 and F represented the PL intensity of Z-CDs solution in the absence and presence of EDTA. The anti-interference experiments were evaluated by replacing EDTA solution by 500 µL solution of 0.001 M commonly used chelating agents (ethylenediamine, sodium citrate, 2,2-dipyridyl, 8-hydroxyquinoline and 1,10-phenanthroline monohydrate).

2.5 Turn-on detection of Zn2+ in solution 50 µM EDTA was first added into 5 mL of Z-CDs solution (1 mg/mL) to quenched the PL, and then different volumes of 1 mM ZnCl2 solution were separately added into the Z-CDs with EDTA solution to recover the PL, the final concentration of Zn2+ was estimated as 2-30 µM. The PL spectra were recorded under 365 nm excitation. The fluorescence intensity ratio (F-F0)/F0 at 460 nm was used to evaluate the recovering effect of Z-CDs with EDTA solution caused by Zn2+, where F0 and F represented the PL intensity of Z-CDs solution in the presence of EDTA and after adding Zn2+. The anti-interference experiments were evaluated by replacing ZnCl2 solution by 20 µM solution of other metal salts (NaCl, AlCl3, KCl, CaCl2, CrCl3, FeCl3, CoCl2, NiCl2, CuCl2, CoCl2, PbCl2).

2.6 Cellular toxicity test Mouse osteoblastic cell line (MC3T3-E1) (104 cells/150 µL) was cultured in 96-well plates for 24 h in an incubator (37 °C, 5% CO2), then replaced the culture medium with 100 µL of Dulbecco's modified Eagle's medium (DMEM) containing Z-CDs of different concentrations (0, 50, 100, 200, 400 µg/mL) and cultured for another 24 h. Then, 20 µL of MTT solution (5 mg/mL) was added to each well. The cells were further cultured for 4 h, followed by removing the culture medium, and then 150 µL of DMSO was added. The resulting admixture was shaken for 5 min at room temperature. The optical density (OD) was measured at 490 nm. The cell viability was estimated according to the following equation:

Cell Viability (%) =

ODTreated × 100% ODControl

(Where ODControl was obtained in the absence of Z-CDs, and ODTreated obtained in the presence of Z-CDs.) 6


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2.7 Cellular imaging The cell line was cultured in DMEM added with 10% fetal bovine serum and 1% penicillin/streptomycin. Suspensions (0.25 mg/mL) of Z-CDs were prepared with Dulbecco’s phosphate buffer saline (DPBS) from the stock solution. MC3T3-E1 cell line was cultured with the count of 1×105 cells in the confocal culture dish, 5% CO2, 37 °C co-culture 24h, after adding the culture medium containing Z-CDs concentration at 400 µg/mL, suck out the original culture medium and cultured in 1.5% CO2, 37 °C for another 24h, then terminate the culture, wash 3 times with PBS solution, add 1 mL 10% formaldehyde to fix 15 min, then carefully suck out fixation, after washing 3 times with warm DPBS, 1 mL PBS solution was added to the confocal culture dish and the cell imaging results were observed in a laser scanning confocal microscope at different excitation wavelengths on Olympus IX81.

3. RESULTS AND DISCUSSION In the present work, Z-CDs were synthesized by a one-step pyrolysis treatment of zinc gluconate, while G-CDs were synthesized for comparison from sole glucose in the same way (Scheme 1), the preparative yield was 64 % and 80 % for Z-CDs and G-CDs, respectively. As shown in Fig. 1a, the PL intensity of Z-CDs was one order of magnitude higher than that of G-CDs. The fluorescence intensity of G-CDs rose as the pyrolysis temperature increased, while for Z-CDs reached the maximum value at 160°C, thus the optimal reaction temperature was set at 160°C. The PL spectra of Z-CDs were illustrated in Fig. 1b, excitation-dependent PL behavior was observed, which might be due to the different surface chemistry states of CDs36. Both G-CDs and Z-CDs showed two distinct absorption bands around 200 nm and 300 nm, which could be attributed to π→π* transition of aromatic sp2 domains and n→π* transition of the conjugated C=O band, respectively37. Z-CDs had optimal excitation and emission wavelengths at 368 nm and 460 nm (Fig. 1c), both 12 nm redshift than that of G-CDs (Fig. 1d), and it was apparent that the PL emission of Z-CDs solution was much brighter in comparison to G-CDs (insets of Fig. 1c and d). The absolute quantum efficiency measurement showed the similar results that the quantum yield (QY) of Z-CDs was 13.89%, which was comparable to N-doped and polymer passivated CDs based on glucose38-39, while for G-CDs was only 0.78%. It was worth mentioning that Z-CDs 7


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contained no nitrogen element at all, and such QY was quite high among the N-free CDs40-41, indicating the essential role of Zn. Moreover, it was noteworthy that the fluorescence width at half maximum (FWHM) of Z-CDs (76 nm) was obviously narrower than that of G-CDs (142 nm), and narrower FWHMs might be due to more uniform size distribution of CDs. The possible mechanism of the enhanced fluorescence of Zn2+ would be further investigated in detail below. The typical TEM image indicated that Z-CDs had uniform size distribution in 2-3 nm with an average particle size of 2.34 nm (Fig. 2a and b). HRTEM image showed the lattice fringes of 0.21 nm, which was similar to the (100) crystal plane of graphite42. In contrast, G-CDs showed a very uneven size distribution. In addition to discrete small dots, aggregated carbon spheres with diameter greater than 20 nm were observed (Fig. 2c and d). We speculated that the messy components of G-CDs led to lower QY, wider PL peak width, and a more pronounced excitation-dependent PL profile than Z-CDs (Fig. S1). In the process of carbonization, glucose might experience uncontrollable aldol condensation reaction, polymerization and aggregation, while zinc gluconate could carbonize mildly due to the charge repulsion of Zn2+. To understand the differences of the two CDs in TEM, the chemical composition of G-CDs and Z-CDs were further investigated. The XRD patterns of G-CDs and Z-CDs both displayed a broad peak at 20° (Fig. 3a), corresponding to the graphite structure43. In addition, no characteristic peaks of zinc oxide were detected, indicating no zinc oxide generated during the calcination of zinc gluconate perhaps because the temperature was not high enough. In Fourier transformed infrared (FTIR) analysis of CDs (Fig. 3b), stretching vibrations of O-H at 3370 cm-1 , C-H at 2933 cm-1 and C-O at 1083 cm-1 and 1049 cm-1 were observed both in G-CDs and Z-CDs. However, there were a plenty of carboxylate groups on Z-CDs’ surface, showing the vibration bands of C=O at 1609 cm-1 and 1399 cm-1. It was obvious that the content ratio of C=O/OH of Z-CDs was higher than G-CDs. What’s more, stretching vibrations of Zn-O were observed at the range of 460-470 cm-1, which indicated zinc ions’ interactions with carboxylate groups on the surface of CDs (Fig. S2).Thus we could confirm that the zinc element in Z-CDs existed in ionic form, retaining as zinc carboxylate groups, and the zinc carboxylate groups could help stabilize Z-CDs with uniform size44. The surface functionalization of CDs was further studied by X-ray photoelectron spectroscopy (XPS) (Fig. 3c), Fig.3d showed the XPS spectrum of Zn 2p peaks at 1045.3 eV and 8


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1022.1 eV present in Z-CDs, indicating there were zinc ions on the surfaces of Z-CDs, and the content of zinc element was calculated as 2.25%. The deconvoluted C1s spectrum revealed three different types of carbon: graphitic or aliphatic carbon (C-C/C=C) at binding energy of 284.6 eV, and C-O at 286.2 eV in both G-CDs and Z-CDs. However, the peak of C=O in G-CDs was centered at 287.7 eV (Fig. S3a), which was 0.7 eV lower than that in Z-CDs (288.4 eV) (Fig. 3e). It was known that the shift of the peak position in XPS to higher binding energy was usually due to electron withdrawing atoms or groups nearby, we believed that the C=O bonds in Z-CDs were mostly being in the forms of zinc carboxylate, but for G-CDs were aldehyde or carbonyl. The O1s spectra showed similar results (Fig. 3f), G-CDs and Z-CDs both consisted of two conditions of oxygen atoms, corresponding to C-O peak centered at 532.8 eV, C=O peak centered at 531.8 eV for Z-CDs and 532.2 eV for G-CDs (Fig. S3b). To further demonstrate the unique role of Zn2+ in Z-CDs, a comparative experiment was carried out by pyrolysis of sodium gluconate. Surprisingly, the product obtained was no fluorescence (Fig. S4a), and also no typical absorption peaks appeared in UV-Vis spectra like G-CDs and Z-CDs (Fig. S4b), the XPS analysis (Fig. S4c and d) revealed that sodium ions existed but they were unable to passivated the surface to arouse the PL like Zn2+ did, probably because of their different electronic structures. In addition, another transition metal gluconate salt, copper gluconate was also utilized at the same reaction conditions, and the obtained solution also showed no fluorescence (Fig. S5a and b). It was convincing that the hydroxyl groups in glucose would strongly reduce Cu2+ to Cu0 and made the obtained sample with copper color have no PL (Fig. S5c), which could be proved by XPS analysis of the upper solution after centrifugation (Fig. S5d), no Cu 2p signals were detected because they were reduced to copper (0) and precipitated. In contrast, Zn2+ which was hardly to be reduced to zero-valent in the process of carbonation could help form stable carbon dots with abundant functional groups on surfaces. What’s more, Zn2+ could act as passivation agents of surface defects on CDs45. The average fluorescent lifetime (τ) of the two CDs was also calculated (Fig. S6) to be 3.23 ns for G-CDs, and 10.76 ns for Z-CDs (Table S1). Mechanistically, longer PL lifetime of carbon-based photoluminescent materials had been attributed to passivated surface defects (by organic or inorganic compounds via covalent linkages or chemical adsorption) on the surface of CDs acting as excitation energy traps45. To prove that Zn2+ had effectively passivated the surface of Z-CDs, 9


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EDTA, a strong chelating agent of metal ions was added into the solution of G-CDs and Z-CDs. As we expected, zinc ions were removed from Z-CDs after adding a certain amount of EDTA, the PL intensity of Z-CDs decreased sharply (Fig. S7a) and the solution fluorescence was almost quenched (inset of Fig. S7a). In contrast, the fluorescence intensity of G-CDs remained nearly unchanged, only a slight increase (Fig. S7b).It meant that Z-CDs followed a totally different PL mechanism with G-CDs, the Zn2+ on the surface of Z-CDs was indispensable for the rise of the bright fluorescence. It should be mentioned that the quenching phenomenon occurred simultaneously when EDTA was added, which proved that Zn2+ was existed on the surface and ease to be removed. That was to say, in the present work, Z-CDs surfaces were passivated in the presence of Zn2+ and therefore exhibited longer PL lifetimes and higher QY than G-CDs. It could be seen that Zn2+ passivation also enhanced the anti-UV bleaching properties of CDs, as shown in Fig. S8, Z-CDs had better PL stability than G-CDs under 360 nm UV light irradiation. According to TEM, XRD, FTIR, XPS analysis and control experiment results, it had been demonstrated that the zinc element in Z-CDs existed as Zn2+ played two roles: 1) due to the electric charge of zinc carboxylate groups, aggregation could be prevented during the forming of carbon dots to obtain a narrow size distribution; 2) the zinc carboxylate functional groups could further stabilize CDs, what’s more, Zn2+ on Z-CDs’ surface acted as passivation agents of surface defects, resulting in the improvement of PL intensity, as illustrated in Scheme 1. Inspired by the complexation reaction of Zn2+ with EDTA, we then construct a CD-based fluorescent sensor for detection of EDTA. Fig. 5a showed the corresponding PL spectra of Z-CDs solution in the presence of different concentration of EDTA, the fluorescence intensity of Z-CDs decreased gradually as the concentration of EDTA increased, demonstrating the applicability of EDTA detection as a “turn-off” fluorescent probe. Fig. 4b indicated the relationship between fluorescence response and EDTA concentration. The resulting calibration curve for the ratios of (F0-F)/F0 and EDTA concentration displayed good linearity over the range of 2.5-25 µM with a correlation coefficient of 0.99925. The limit of detection (LOD) was as low as 3.2 × 10-7 M at a signal-to-noise ratio of 3, which was comparable with expensive and complicated HPLC methods46, while fluorescence-based sensor was more convenient and simple to implement. Moreover, the Z-CDs-based fluorescent probe had high selectivity towards EDTA, as shown in Fig. 4e, the commonly used chelating agents such as ethylenediamine (EDA), sodium citrate 10


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(CANa), sodium gluconate (GlcNa), 2,2-dipyridyl (bpy), 8-hydroxyquinoline (8-OQ) and 1,10-phenanthroline monohydrate (o-P) had no significant quenching effect on PL of Z-CDs. Based on the above results, the Z-CDs could be an ideal fluorescent sensor for trace detection of EDTA in solution. In order to evaluate the applicability of the sensing of EDTA in real environment with Z-CDs, the detection of EDTA in tap water was conducted. As shown in Table 1, 2.5, 10, 25, 50 µM EDTA was added to the tap water. The recovery of EDTA was calculated as 100.1, 98.7, 105.4, 86.7%, respectively, and the RSD ranged from 1.76-3.07%, which was satisfactory for quantitative assays performed in real samples. Interestingly, the quenched PL of Z-CDs by EDTA can be recovered by adding Zn2+ to re-passivate Z-CDs. As shown in Fig. 4c, the fluorescence intensity of Z-CDs with EDTA solution increased gradually as the concentration of Zn2+ increased, and the PL could be totally recovered to original when adding 30 µM Zn2+. Fig. 4d indicated the relationship between fluorescence response and Zn2+ concentration. The resulting calibration curve for the ratios of (F-F0)/F0 and Zn2+ concentration displayed good linearity over the range of 2-15 µM with a correlation coefficient of 0.99835 (insets of Fig. 4d). The LOD was 5.1 × 10-7 M at a signal-to-noise ratio of 3. Moreover, the Z-CDs with EDTA had high selectivity towards Zn2+, as shown in Fig. 4f, the other metal ions such as Na+, Al3+, K+, Ca2+, Cr3+, Fe3+, Co2+, Ni2+, Cu2+, Hg2+, Pb2+ had no PL recovering ability. We considered that after EDTA removing the Zn2+ from Z-CDs, the lack of Zn2+ passivation led to the PL quenching of Z-CDs, and such exposed surface defects would leave corresponding vacant positions for Zn2+, which were able to be re-passivated by adding Zn2+, this could also explain the excellent selectivity of Zn2+ among other metal ions. As we mentioned earlier, because the Zn2+ was existed on the surfaces of Z-CDs, which was ease to be removed by EDTA, the PL intensity could be quenched to constant in a very short time (shorter than 5 s) (Fig. S9a). However, the equilibration time of PL recovery was much longer (after 1 min), which might be due to the lower rate of re-passivation by Zn2+ (Fig. S9b). To evaluate the applicability of the sensing for Zn2+ in real environment of Z-CDs with EDTA, the detection in tap water was conducted. As shown in Table 1, 2, 5, 10, 15 µM Zn2+ was added to the tap water. The recovery of Zn2+ was calculated as 96.1, 88.4, 119.7, 98.4%, respectively, and the RSD ranged from 0.77-2.07%, which was satisfactory for quantitative assays performed in real samples. Also, the reversibility of Z-CDs quenched by EDTA and recovered by Zn2+ was investigated (Fig. S10), 11


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there were no obvious changes after four times recycle, indicating their reversible “off-on”

detection capability. The sensitivity of detection for EDTA and Zn2+ was also compared with some reported works and presented in Table 2. This work was the first one to construct a fluorescent nanosensor with detection capability for both EDTA and Zn2+ with reversibility, the LOD of EDTA and Zn2+ was comparable with and even lower than the reported complicated methods or materials. In addition, low-toxicity and biocompatibility would be essential requirements if the nanosensor was utilized in practical application. Firstly, the PL stability of Z-CDs at different solution pH and ionic strengths were tested. The shift of PL peak with the solution pH was probably because of the protonation and deprotonation of the carboxylate groups on the surface47 (Fig. S11a). At the pH range of 5-12, the PL intensity of Z-CDs remained almost unchanged (Fig. S11b). Also, the fluorescent changes of Z-CDs in different ionic strengths (KCl) could be ignored, replied its stability in physiological environments (Fig. S11c). We exploited the cytotoxicity of Z-CDs via the MTT assay (Fig. S11d). It could be seen even the concentration of CDs was as high as 400 µg/mL, the mouse osteoblasts MC3T3 cell viability was still around 90% after 24h incubation, which meant the good biocompatible and eco-friendly properties of Z-CDs. Due to the low-toxicity and bright fluorescence, we investigated the cellular imaging capability of Z-CDs. Fig. S12 showed the fluorescence images of MC3T3 cells incubated with Z-CDs, because of the excitation-dependent feature (the emission wavelength could change with the excitation wavelength) (Fig. S13), different color bio-imaging pictures could be obtained by select different light sources. Green and red fluorescence could be clearly seen when the excitation light wavelength was 488 and 543 nm, respectively. It could be seen that the Z-CDs mainly located within the cell plasma, probably entered into cells through endocytosis via the cell membrane6. Thus, the in-vitro detection capability of Z-CDs was then investigated as shown in Fig. 5, it can be seen the cells incubated with Z-CDs showed bright fluorescence (Fig. 5a), after adding EDTA, the PL was almost quenched (Fig. 5b) and recovered by adding ZnCl2 (Fig. 5c). The results indicated Z-CDs had great application potential in fields of multicolor bio-imaging and in-vitro detection.

4. CONCLUSIONS In summary, for the first time, using zinc gluconate as a single precursor, we had synthesized Zn2+ 12


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passivated CDs through a one-step pyrolysis treatment. The fluorescent emission of Z-CDs was much brighter than G-CDs where zinc was absent. We proposed that Zn2+ as an important factor, which not only prevented the uncontrollable condensation and aggregation during the pyrolysis process, but also served as surface passivation agent that would help stabilize the carbon dots and further improved the PL intensity. In contrast, both sodium gluconate and copper gluconate were unable to form stable and fluorescent carbon dots, which meant the unique electronic structure of Zn2+ was critical for the rise of bright fluorescence. Zn2+ could be complexed from the surface of Z-CDs by adding EDTA, leading to the PL quenching phenomenon, thus a CD-based fluorescent sensor for detection of EDTA was first established with a LOD as 3.2 × 10-7 M. Furthermore, the PL could be recovered by re-adding Zn2+, thus Z-CDs with EDTA could serve as a turn-on fluorescent sensor for Zn2+, with a LOD as 5.1 × 10-7 M. The quenching and recovering process can be repeated for four times without obvious changes, indicating their reversible “off-on” detection capability for EDTA and Zn2+, which was first reported. Also, the proposed Z-CDs based sensor had been successfully utilized for EDTA and Zn2+ monitoring in tap water with excellent recovery. Moreover, Z-CDs was low-toxic to cells and utilized for multicolor bio-imaging and in-vitro detection, indicating their potential in serving as practical in-vitro sensors. This work presents a novel strategy for the construction of a type of metal ions passivated carbon dots with enhanced fluorescence and innovative reversible detection applications, which may provide new ideas for both the synthesis, structure and the application of CDs.

ASSOCIATED CONTENT Supporting Information -1

PL spectra of G-CDs; FTIR spectra in the range of 455-475 cm ; XPS C1s and O1s peaks of

G-CDs; PL spectra, XPS spectra of CDs obtained from Sodium Gluconate and Copper Gluconate; Fluorescence decays and PL lifetimes; Quenching effect of G-CDs and Z-CDs by EDTA; PL spectra and PL stability test of Z-CDs and G-CDs; Equilibration time of Z-CDs-based sensor; The reversible detection capability of Z-CDs; Effect of pH and KCl concentration on PL of Z-CDs; Cell viability and cell-imaging of Z-CDs; PL spectra of Z-CDs under 400-600 nm excitation.

AUTHOR INFORMATION 13


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Corresponding Author * E-mail: [email protected]. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was financially supported by the National key research and development program of China (2016YFB0401701), the National Science Foundation of China (NSFC) under Grant Nos. 51433003, 21774041 and JLU Science and Technology Innovative Research Team 2017TD-06.

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Scheme 1 Illustration of the possible formation and sensing mechanism of Z-CDs.

Fig. 1 (a) PL emission spectra of G-CDs and Z-CDs at different reaction temperature (360 nm excitation). (b) PL spectra of Z-CDs with different excitation wavelength. (c) UV/Vis absorption, PL excitation and emission spectra of G-CDs and (d) of Z-CDs. Insets show photographs of CDs in aqueous solutions (1 mg/mL) under daylight (left) and hand-held 365 nm UV lamp (right).

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Fig. 2 (a) TEM images of Z-CDs (scale bar: 20 nm) and inset is HRTEM image of Z-CDs (scale bar: 5 nm). (b) Size distribution histogram of Z-CDs. (c) and (d) TEM images of G-CDs at different magnifications.

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Fig. 3 (a) XRD patterns of G-CDs and GZ-CDs. (b) FTIR spectra of G-CDs and Z-CDs. (c) XPS analysis of G-CDs and Z-CDs in full spectra. (d) High resolution Zn 2p, C1s (e) and O1s (f) peaks of Z-CDs.

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Fig. 4 (a) Corresponding PL emission spectra of Z-CDs under 365 nm excitation with different concentration of EDTA. (b) The relationship between (F0-F)/F0 and the concentrations of EDTA, insets show photographs of Z-CDs solution after adding 0, 25, 50, 100 µM EDTA under 365 nm UV lamp. (c) Corresponding PL emission spectra of Z-CDs with EDTA under 365 nm excitation with different concentration of Zn2+. (original: raw Z-CDs solution; quenched: Z-CDs solution after adding 50µM EDTA) (d) The relationship between (F-F0)/F0 and the concentrations of Zn2+, insets show photographs of Z-CDs with EDTA solution after adding 0, 10, 25 µM ZnCl2 solution under 365 nm UV lamp. (e) Relative PL intensities (F/F0) of Z-CDs solution with 50 µM of EDTA and various commonly used chelating agents. (f) Relative PL intensities (F/F0) of Z-CDs with 23


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EDTA solution with 20 µM of Zn2+ and various commonly metal ions. Table 1 Practical analysis ability of Z-CDs in tap water. Sample No.

1

2

3

4

EDTA added

2.5

10

25

50

2.50±0.29

9.87±0.69

26.34±0.64

43.34±0.51

RSD (%)

1.76

2.72

2.31

3.07

Recovery (%)

100.1±11.7

98.7±6.9

105.4±2.6

86.7±1.0

Zn2+ added

2

5

10

15

1.92±0.07

4.42±0.22

11.97±0.30

14.76±0.11

RSD (%)

1.33

1.49

2.07

0.77

Recovery (%)

96.1±3.3

88.4±4.4

119.7±3.0

98.4±0.7

(µM) EDTA detected (µM)

(µM) Zn2+ detected (µM)

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Table 2 Comparison of sensitivity of detection for EDTA and Zn2+ with some reported works. Methods or Materials

LOD of EDTA

LOD of Zn2+

Ref.

Flame atomic absorption spectroscopy

2 µg/mL

-

24

Spectrophotometric

0.14 µg/mL

-

25

HPLC

0.023 µg/mL

-

43

CDs/QCT-Zn2+

-

2 × 10-6 M

30

Ag2S Quantum Dots

-

7.6 × 10-7 M

31

QD@SiO2/TSPP

-

0.6 × 10-7 M

32

Z-CDs

3.2 × 10-7 M

5.1 × 10-7 M

This work

(~ 0.12 µg/mL)

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Fig. 5 Cellular imaging of MC3T3-E1 cells incubated with Z-CDs (a), followed by adding EDTA (b) and ZnCl2 (c) under 488 nm (1), 543 nm (2) excitations and overlapped images (3).

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