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A Unique Approach to Develop Carbon Dot-Based Nanohybrid NearInfrared Ratiometric Fluorescent Sensor for the Detection of Mercury Ions Jingjin Zhao, Mengjiao Huang, Liangliang Zhang, Mengbing Zou, Dongxia Chen, Yong Huang, and Shulin Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01443 • Publication Date (Web): 01 Jul 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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Analytical Chemistry

A Unique Approach to Develop Carbon Dot-Based Nanohybrid Near-Infrared Ratiometric Fluorescent Sensor for the Detection of Mercury Ions

Jingjin Zhao,†,‡,§ Mengjiao Huang,†,§ Liangliang Zhang,*,† Mengbing Zou,† Dongxia Chen,† Yong Huang,† and Shulin Zhao*,†

†

State Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, Guangxi Normal University, Guilin 541004, P. R. China.

‡

Key Laboratory of Ecology of Rare and Endangered Species and Environmental Protection of Ministry Education, Guangxi Normal University, Guilin 541004, P. R. China.

Corresponding author: Professor Shulin Zhao and Liangliang Zhang E-mail: [email protected] [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: Ratiometric fluorescence assay which can eliminate the external effects has attached great attentions. In this work, a carbon dot (CD)-based nanohybrid dual-emission system was simply prepared by a unique approach of solvothermal treating corn bract, and used as a ratiometric fluorescent sensor for Hg2+ detection. Under a single excitation, the obtained nanohybrid sensor had two emission bands around 470 nm and 678 nm which may originate from the intrinsic structure of CDs and chlorophyll-derived porphyrins respectively. In the presence of Hg2+, the fluorescence at 678 nm could be remarkably quenched, while the fluorescence intensity at 470 nm was only slightly altered. The fluorescence intensity ratio at 470 nm and 678 nm exhibited a good linear relationship in the Hg2+ concentration range from 0 to 40 µM with a detection limit of about 9.0 nM. It also had a satisfying assay performance in serum and river water samples. The prepared CD-based nanohybrid sensor here may hold the further potential applications in biomedicine study, environmental protection and food safety.

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Ratiometric fluorescence assay is an analytical method that measures the ratio of the fluorescence intensities at two different wavelengths. As the measured fluorescence ratio is not affected by the intensity of the light source, the sensitivity of the instrument, and the matrix background, this method has high sensitivity, selectivity and a wide linear range, and has attracted the interest of researchers in recent years.1-6 A number of ratiometric fluorescence probes using organic small molecules have been reported based on different signal transition strategies such as fluorescence resonance energy transfer (FRET),7 azide-alkyne cycloaddition,8 excited-state intramolecular proton transfer9 and through-bond energy transfer.10 However, organic small molecule-based ratiometric fluorescence probes require complicated design and synthesis, and some of them are prone to photobleaching which affects the stability of fluorescence emission. Fluorescence nanomaterials such as semiconductor quantum dots (QDs), gold/silver nanoclusters, and carbon dots (CDs, including carbon quantum dots and graphene quantum dots) are relatively easy to synthesize, and do not suffer from photobleaching which have stable emission. Ratiometric fluorescence nanoprobes can be constructed by embedding, self-assembling or conjugating other fluorophores with fluorescent nanomaterials. For example, Wang and co-workers reported that CdTe QDs with different emission wavelengths can be linked together to produce ratiometric fluorescence probes for TNT and Cu2+ detection.11,12 Chu et al. developed a

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ratiometric nanoprobe using gold nanocluster to decorate dye-encapsulated silica particles for the detection of highly reactive oxygen species.13 Among the reported fluorescence nanomaterials, fluorescent CDs have attracted much attention recently because of their simple preparation, low toxicity, good light stability and biocompatibility.14-16 CDs have been used as fluorescence probes in the detection of Fe3+, Hg2+, Cu2+, and glucose, etc.17-20 In the last few years, several CD-based ratiometric probes were developed by covalently conjugating the other fluorophores. For examples, naphthalimideazide derivatives were covalently linked to CDs through the reaction between amine groups and carboxyl groups to produce a FRET-based ratiometric nanoprobe for the detection of H2S.21 By conjugating the CdTe QDs and CDs, Yan et al. prepared a ratiometric fluorescence nanoprobe to detect NO. The NO could quench the fluorescence of CdTe QDs, but it had no obvious effect on the fluorescence of CDs.22 Very recently, researchers also constructed some ratiometric fluorescence nanohybrid sensors by simply mixing CDs and fluorophores with well-resolved emission. In such strategy, CDs and another used fluorophore are present in solution independently. One of their fluorescence acts as reference signal, and another fluorescence uses as reporter signal. Lu et al. mixed CDs with rhodamine B to develop a ratiometric nanosensor. In their work, the fluorescence of CDs could be quenched by Hg2+ and then recovered by glutathione, while the fluorescence of rhodamine B was not affected obviously which was used as the reference signal.23 In a recent work, Zhang’s research group used CdTe QDs and CDs aqueous mixture as the dual-emission nanohybrid system for the ratiometric detection of DNA.24 4


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In this study, we prepared a CD-based ratiometric nanohybrid sensor by a simple and unique solvothermal treatment of biomass corn bract in anhydrous ethanol. Chlorophyll can be extracted from corn bract by ethanol solvent, and may act as one of the main raw chemicals for the preparation. It was reported that, when organic dyes are used as the one of the starting materials to prepare CDs, the CDs display the similar fluorescence emission as that of the used dyes.25,26 The obtained final product solution in this work exhibits two strong fluorescence peaks at 470 nm and 678 nm under a single excitation at 406 nm. The emission around 470 nm may be from the intrinsic structure of CDs, while the emission around 678 nm may originate from the porphyrins derived from chlorophyll. We found that the fluorescence intensity at 678 nm decreased gradually as the Hg2+ concentration increased, and the fluorescence intensity at 470 nm was only slightly altered. Thus, the obtained system can be used as a CD-based nanohybrid ratiometric sensor for the detection of Hg2+. By skillfully using the chlorophyll extracted from starting material corn bract, the nanohybrid ratiometric sensor constructed here can be obtained during the CDs preparation process, and does not require post-modification of, or further mixing, CDs with other fluorophores.

EXPERIMENTAL SECTION Reagents and Materials. Corn bract was bought from local vegetable market (Guilin, China). Anhydrous ethanol, polyoxyethylenebis (amine) (H2N-PEG-NH2), Na2CO3, HgCl2, FeCl3, CuCl2, SnCl2, FeSO4, Ni(NO3)2, MnCl2, AgNO3, NaCl, MgCl2, 5



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AlCl3, CaCl2, Pb(NO3)2, CrCl3, CdCl2, CoCl2, KCl, ZnCl2, BaCl2 were purchased from Aladdin. Other solvents and reagents were of analytical grade without additional purification. The water for the experiments was ultrapure with a resistance of ≧18.2 MΩ.

Apparatus. Fourier transform near-infrared spectrometer (Perkin-Elmer, USA) was used for recording infrared spectra. Cary 60 UV-visible spectrophotometer (Agilent Technologies, USA) was introduced for UV-vis absorption measurements. Fluorescence

spectra

were

measured

using

Cary

Eclipse

fluorescence

spectrophotometer (Agilent Technologies, USA). Tecnai G2 F20 transmission electron microscope (FEI, USA) was used to obtain transmission electron microscopy (TEM) images. Rigaku D/max2500v/pc X-ray diffractometer (RIGAKU) was used to characterize the lattice structure of the CDs. The X-ray photoelectron spectroscopy (XPS) was tested by Thermo ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo, USA).

Preparation of the CD-based Nanohybrid Sensor. High temperature-dried corn bract (1.0 g) and 20 mL of anhydrous ethanol were added to a 50 mL round bottom flask, and mixed thoroughly. The flask was then sealed with the aid of a nitrogen protection device and fixed in a heating sleeve. Under the protection by nitrogen, the mixture was heated to 100 °C under reflux with stirring for 24 h. After the reaction mixture cooled to room temperature, the supernatant was filtered with a 0.22 µm membrane. 1 mL of 0.6 mg/mL H2N-PEG-NH2/anhydrous ethanol solution 6


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was added into 4 mL of the filtrate and mixed thoroughly in a beaker. After standing at room temperature in the dark for 48 h, 4.0 mL of 0.1 M Na2CO3 solution were added and mixed well. This mixture solution was allowed to stand at room temperature in the dark for a further 24 h. The solution was filtered with a 0.22 µm membrane, and the obtained filtrate was then dialyzed against water. The product was rotationally evaporated under vacuum to obtain a solid sample, which was stored in the refrigerator at 4 °C for future use.

Detection of Hg2+. A volume of 10 µL of CD product solution (2.2 mg/mL) was added to 80 µL HEPES buffer (25.0 mM, pH=7.0), and followed by the addition of 10 µL Hg2+ solutions with different concentrations. The solutions were mixed by agitation and allowed to react at room temperature in the dark for 30 min. Fluorescence measurements were then performed immediately at an excitation wavelength of 406 nm. The calibration curve was plotted with F470/F678 value as the ordinate and Hg2+ concentration as abscissa. For investigating the practicality, the detection was carried out in human serum and real river water samples, respectively. For the detection of Hg2+ in serum samples, human blood samples (obtained from Guilin Fifth People's Hospital) were centrifuged at 12000 rpm for 30 min. The supernatant was diluted by 125 times using HEPES buffer and then stored in a refrigerator at −4 °C. 10 µL of diluted human serum samples and different concentrations of Hg2+ were added into the CDs product-contained HEPES solutions (0.22 mg/mL in final volume of 100 µL). After reacting at room temperature in the dark for 30 min, the fluorescence spectra of final 7



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solutions were recorded. The river water sample was obtained from Lijiang River (Guilin, Guangxi). The water was filtered using 0.22 µm membrane firstly, and the filtrate was centrifuged at 12000 rpm for 30 min. The finally obtained water samples were then spiked with Hg2+ at different concentrations. A volume of 10 µL pretreated river water sample with different Hg2+ concentration added into the CDs product-contained HEPES solutions (0.22 mg/mL in final volume of 100 µL) and reacted at room temperature in the dark for 30 min. The fluorescence spectra of final solutions were recorded after reaction immediately.

RESULTS AND DISCUSSION Characterization. CD-based nanohybrid sensor was prepared using corn bract as raw material via a solvothermal method (Figure 1). The product was dialyzed, rotationally evaporated under vacuum in the dark to obtain solid sample. A series of characterizations were performed to study the morphology, structure, elements and surface groups. As can be seen from the TEM image in Figure 2, CDs were obtained by solvothermal treatment of corn bract, which are nearly spherical and well dispersed. The particle size distribution is in 1.8–3.4 nm range, and about 43% of the CDs have an average particle size of approximately 2.6 nm. A lattice spacing of 0.237 nm was observed in the high resolution TEM image, similar to that of graphite (100) facet.27 The XRD spectrum (Figure S1) displays a broad peak at 2θ=23.43°, which corresponds 8


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to the (002) plane of graphite, further indicating the graphitic carbon domain is present in the prepared CDs.28

The FTIR spectrum of the CDs is shown in Figure S2. The broad peak at 3418 cm−1 corresponds to the O-H and N-H stretching vibrations, whereas the peak at 2976 cm−1 is C-H stretching vibration.29 The peak at 1085 cm−1 is from the stretching vibration of C-O and that at 1642 cm−1 is attributed to stretching vibration of C=O.30 The peak around 1384 cm−1 belongs to the stretching vibration of C-N.29 The above FTIR results indicate that there are some functional groups such as -COOH, -OH, and -N2H on the surface of the obtained CDs. To further study the element compositions and surface states of the CDs, XPS was performed. From Figure 3A, it can be seen that the typical peaks of C1s, N1s, and O1s are present around 285 eV, 400 eV, and 532 eV, respectively. The peaks at 284.7 eV, 285.6 eV, 286.3 eV, and 288.0 eV in high resolution C1s XPS spectrum (Figure 3B) correspond to C-C/C=C, C-N, C-O, and C=O, respectively, which is consistent with the FTIR results. In the N1s XPS spectrum, there are two major peaks at 399.9 eV and 401.6 eV (Figure 3C) which originate from pyrrolic-N and graphitic-N/N-H, respectively. The presence of graphitic-N indicates that N was doped in the framework of the prepared CDs. The O1s spectrum (Figure 3D) has two peaks located at 531.1eV and 532.7 eV, which are assigned to C=O and C-O. In addition, the element content was also analyzed by XPS. The results reveal that the relative atomic percentages of C, N, and O in the obtained CDs product were 78.04%, 1.73%, and 20.23%, respectively.

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Optical Properties of the CDs-based Nanohybrid System. UV-vis absorption and fluorescence spectra were recorded to study the optical properties of the obtained final CDs product solution. The solution of the CDs products exhibits a light yellow color (Figure 4A, inset). In the UV-vis spectrum, an absorption peak was observed around 282 nm (Figure 4A), which originates from π–π* transition of nanocarbon.31 Previously reported CDs that derived from hydrophobic cyanine dye using solvothermal method could have a characteristic absorption band of cyanine dye.26 In our work, chlorophyll can be extracted from corn bract by ethanol, and it will act as one of the main starting chemicals for the solvothermal synthesis. Here, the UV-vis spectrum of the obtained product solution (Figure 4A) shows two weak absorption peaks around 408 nm and 670 nm. These two peaks may respectively be Soret band and one of strong Q band of porphyrins which derive from extracted chlorophyll.32,33 Figure 4B shows the fluorescence emission spectrum of the obtained CDs product solution. Two emission peaks around 470 nm and 678 nm can be observed when excited at 406 nm. The emission around 678 nm is similar to the characteristic emission band of porphyrin derivatives.32 The excitation spectrum for the emission at 678 nm (Figure 4B) indicates a maximum excitation wavelength around 406 nm, which is used in the further experiments. It can be seen from Figure S3, the peak wavelength of another emission displays an excitation-dependent manner, which is a common phenomenon in other reported CDs.31 As a result, it can be speculated that the emission band around 470 nm is from the intrinsic structure of CDs and the emission around 678 nm may originate from the chlorophyll-derived porphyrins. The 10


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fluorescence quantum yield of the CDs was estimated to be about 6.9% using quinine sulfate (quantum yield is 54% in 0.05 M H2SO4) as the reference. Finally, it can be observed from Figure S4 that F470/F678 value shows minimal change after irradiation under a 365 nm UV lamp for 40 min. This implies that the obtained nanohybrid system as a ratiometric fluorescent sensor can eliminate the effect of photobleaching. It should be noted that though the CDs product here was dialyzed against water as that was done in previous paper,26 there may still be some free porphyrin derivatives in the final product. However, the CD-based product solution finally obtained here can still be used as a nanohybrid ratiometric sensor for in vitro detection, because the conjugation of CDs and fluorophores (porphyrins here) is not a must for constructing CD-based ratiometric sensor. For examples, very recently, several CD-based ratiometric fluorescence nanosensors were reported by simply mixing CDs and other fluorophores which were present in the detection solution independently.23,24

Fluorescence Response to Hg2+. In this work, we found that after the addition of Hg2+ into the obtained CD-based nanohybrid system, the fluorescence at 678 nm decreased remarkably while the emission at 470 nm is only slightly affected (Figure S5), making the increase of F470/F678 value. It has been reported that Hg2+ can quench the fluorescence of porphyrin compounds through the interaction with electron-rich aromatic ring,34,35 which induces the decrease of fluorescence at 678 nm here. We also investigated the effect of some other metal ions on the F470/F678value. As shown in Figure 5, after addition of other metal ions such as Na+, K+, Mg2+, Ca2+, Ba2+, Al3+, Sn2+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+, the obtained 11



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F470/F678 values are smaller than that of Hg2+ which induces the largest response. These results imply that the prepared CDs-based nanohybrid system may be used as a ratiometirc fluorescence sensor for Hg2+ detection.

Ratiometric Detection of Hg2+. To improve the sensitivity of Hg2+ detection, the solution pH and reaction time were optimized by recording the F470/F678 value after the addition of 4 µM Hg2+. The results are shown in Figure S6. From the optimization data, the solution pH value and reaction time are selected as 7.0 and 30 min, respectively. Under these optimized experimental conditions, Hg2+ with the concentrations from 0 to 90 µM was tested using the prepared CD-based nanohybrid sensor. With an increase in the Hg2+ concentration, the fluorescence intensity at 678 nm decreases gradually with a slight loss of signal at 470 nm (Figure 6A), and F470/F678 increases (Figure 6B). The value of F470/F678 shows a good linear relationship with the Hg2+ concentration in the 0–40

µM

range

(Figure

6B),

with

a

linear

regression

equation

of

F470/F678=0.2517C+1.4207 and a correlation coefficient of 0.9896 (C is the concentration of Hg2+, µM). Using the 3σ rule (σ=S0/S, S0 is the standard deviation in the blank solution after repeated measurements and S is the gradient of the standard curve), the detection limit was calculated to be about 9.0 nM. We compared this detection limit with that of other methods (Table S1). The comparison result indicates that the prepared CD-based nanohybrid system has a high sensitivity. The mixtures of Hg2+ and some other metal ions were also tested (Figure S7), which further indicates a good selectivity of the prepared CD-based nanohybrid sensor.

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Detection of Hg2+ in Serum and River Water Samples. To evaluate the practicality of the proposed ratiometric fluorescence sensor for detecting Hg2+, experiments were carried out in diluted human serum and river water samples. Figure S8A shows the fluorescence spectra of the Hg2+ detection in diluted serum samples. With the calibration curve in serum samples (Figure S8B), four diluted serum samples spiked with Hg2+ were tested and the concentrations were estimated. The results are shown in Table S2. The recoveries of Hg2+ in human serum samples using this nanohybrid sensor were 94.5%–102.0%, with a relative standard deviation (RSD) lower than 5.1%. Compared to the detection result obtained by ICP-MS (Table S2), the proposed nanohybrid sensor here has an acceptable performance. In addition, according to the detection curve in river water (Figure S9), recovery test was further carried out by determining three water samples with different concentrations of Hg2+. The results in Table S3 also indicate satisfactory recoveries and RSDs for detection in river water samples. The above results imply that the proposed ratiometric sensor may be further introduced in health and environmental related applications.

CONCLUSIONS In summary, a unique approach was presented to prepare the CD-based nanohybrid near-infrared ratiometric sensor for Hg2+ detection, using corn bract as raw material through solvothermal method. The obtained nanohybrid sensor exhibited dual fluorescence emission at 470 nm and 678 nm which may originate from the intrinsic structure of CDs and chlorophyll-derived porphyrins respectively. The Hg2+ could 13



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quenched the fluorescence at 678 nm remarkably while the fluorescence at 470 nm changed slightly, thus providing a near-infrared ratiometric detection mode. Using natural biomass corn bract as raw material makes the preparation simple, inexpensive, and environmentally friendly. Compared with previous reported CD-based nanohybrid ratiometric sensors, the construction of this ratiometric sensor here can be achieved during CDs preparation process and does not require post-modification of, or further mixing, CDs with other fluorophores. This nanohybrid sensor has a high sensitivity and a satisfying detection performance in serum and river water samples, which holds the potential applications in the fields related to biomedicine study, environmental protection and food safety.

ASSOCIATED CONTENT Supporting Information Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Additional figures and tables, including XRD and FTIR spectra of the CDs products, excitation wavelength-dependent emission spectra, effect of irradiation under a 365 nm UV lamp, Hg2+-induced fluorescence response spectra, optimization of solution pH and reaction time for Hg2+ detection, the response induced by the mixture of Hg2+ and other metal ions, the fluorescence spectra and analysis curve of Hg2+ detection in diluted serum samples, analysis curve of Hg2+ detection in river water samples, comparison of analytical performance with other methods, and 14


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recovery tests of Hg2+ detection in serum and river water samples.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

These two authors contributed equally to this work.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (No. 21327007, 21575031), the Natural Science Foundation of Guangxi Province (No. 2015GXNSFDA139006) and BAGUI Scholar Program.

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FIGURE CAPTIONS Figure 1. Schematic illustration for the preparation of CD-based nanohybrid sensor for the detection of Hg2+. Figure 2. TEM image of the obtained CDs product (insets are size distribution and high resolution TEM). Figure 3. (A) Full XPS spectrum, (B) C1s XPS spectrum, (C) N1s XPS spectrum, and (D) O1s XPS spectrum of the obtained CDs product. Figure 4. (A) UV-vis spectrum of the CDs product solution. Inset is the photograph of the CDs product solution under natural light. (B) Fluorescence emission spectrum of the CD-based nanohybrid system and the excitation spectrum for the emission at 678 nm. Figure 5. The responses of different metal ions on CD-based nanohybrid system. The concentration of each metal ion was 20 µM. Figure 6. (A) The fluorescence spectra after addition of different concentrations of Hg2+. The Hg2+ concentrations from a to s: 0, 0.01, 0.1, 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 50.0, 70.0, and 90.0 µM. (B) The linearity of response curve.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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