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Carbon Dots with Red Emission for Sensing of Pt2+, Au3+ and Pd2+ and Their Bio-Applications in Vitro and in Vivo Wenli Gao, Haohan Song, Xiao Wang, Xiaoqiang Liu, Xiaobin Pang, Yanmei Zhou, Bin Gao, and Xiaojun Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16991 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

Carbon Dots with Red Emission for Sensing of Pt2+, Au3+ and Pd2+ and Their Bio-Applications in Vitro and in Vivo Wenli Gaoa, Haohan Songa, Xiao Wanga, Xiaoqiang Liua, Xiaobin Pang c, Yanmei Zhoua, b, *, Bin Gaod, Xiaojun Pengb a

Institute of Environmental and Analytical Sciences, Henan Joint International Research

Laboratory of Environmental Pollution Control Materials, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China b

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024,

China c

Pharmaceutical Institute, Henan University, Kaifeng 475004, China

d

Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL

32611, United States

KEYWORDS: Carbon dots; Red emission; Noble metal ions; Fluorescent probe; Bioimaging; Biosensing.

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ABSTRACT: Red emissive carbon dots (CDs) have drawn more and more attention due to their good organ penetration depth and slight biological tissues photodamage. Herein, the fluorescent CDs with red emission were synthesized by facile one-pot hydrothermal treatment of citric acid and neutral red, and they show red fluorescence both in aqueous solution and solid state. The solution of CDs exhibits the quantum yield (QY) of 12.1%, good stability against photobleaching, and low cytotoxicity. As-prepared CDs can be used as fluorescent probe for peculiar detection of Pt2+, Au3+ and Pd2+. Furthermore, the CDs show excellent biocompatibility, which were successfully used as hopeful bioimaging and biosensing of noble metal ions in PC12 cells and zebrafish (ZF).

1. INTRODUCTION As a new style of zero-dimensional carbon-based nanomaterial, fluorescent CDs have attracted scientific wide attention in the past few years.1-5 Comparing with traditional semiconductor quantum dots and organic fluorescent dyes, CDs hold many unique properties, including superior optical properties, excellent solubility, good biosafety, and multiple functional groups, which indicate that CDs can be applied in various fields such as sensing, bioimaging, drug delivery, printing inks, solar cells, optoelectronics, and catalysis.4, 6-14 However, there still exist some problems for CDs. For example, CDs in most literatures just displayed blue fluorescence, which greatly limit their applications and developments owing to the general blue autofluorescence of biological substrate and photodamage of biotic organization under UV excitation light.15-21 By contrary, red emissive CDs are propitious for their applications, especially in the biomedical fields, which can provide a deeper light seeping into organizations and show low tissues damage in organizations.5 In addition, the fluorescence of CDs is always


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strong quenched in the solid state because of their aggregation. It is rarely reported that pure CDs (no add polymers or use organosilane for functionalization) can present red emission in solid state.5,

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Consequently, it is a great challenge to obtain CDs with red emission in aqueous

solution and solid state. Noble metals play the important roles in various chemical and biological activities. Platinum (Pt), gold (Au) and palladium (Pd) as three kinds of the rarest metal elements, are widely applied in various materials including catalysts, dental crowns, jewelry, medicine, and many others.23-26 Though they are significant in these areas, an important issue is the consequent pollution of the environment that follows with their continual use, because noble-metal Pt, Au, Pd and their compounds are deemed to be potentially dangerous to the people's health.24 Therefore, it is quite meaningful to sense these noble metal ions in biological systems or environments. The fluorescence method has obtained widespread attention because of its high sensitivity, quick response, simplicity, and the application in fluorescence imaging. For example, Sharma et al. reported synthesis of the fluorescent carbon nanoparticles, which was used to response to Pd2+ and Hg2+ sensitively and rapidly, and it could be a bioimaging probe for sensing Pd2+ and Hg2+ in cells.27 Ren et al. synthesized fluorescent nitrogen doped carbon quantum dots as a fluorescent probe for sensitive and selective detection of Hg2+ and Ag+ ions in real environmental water samples.28 However, as far as we know, there has been no previous report about the use of fluorescent CDs for particularly sensing noble metal ions of Pt2+, Au3+ and Pd2+ and visualizing Pt2+ imaging in vitro and in vivo.


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Herein, we synthesized red emissive fluorescent CDs by a facile one-pot hydrothermal approach using citric acid and neutral red as raw materials and ultrapure water as solvent. Asprepared CDs show red fluorescence in aqueous solution and solid state, respectively. In addition, the CDs exhibit good photostability at UV irradiation, low toxicity and excellent biocompatibility. These CDs were applied to the detection of noble mental ions of Pt2+, Au3+ and Pd2+, and could be successfully used in bioimaging and biosensing in vitro/vivo. 2. EXPERIMENTAL SECTION 2.1 Materials Citric acid (CA) was obtained from Alfa Aesar. Neutral red (NR) was purchased from Aladdin. Silver chloride, potassium chloride, sodium chloride, lead chloride, barium nitrate, calcium nitrate tetrahydrate, zinc chloride, anhydrous magnesium sulfate, copper chloride, mercuric chloride, palladium chloride, platinum chloride, ferric chloride, chromic chloride, and gold trichloride were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Ruthenium trichloride, iridium trichloride, rhodium trichloride hydrate and osmium tetrachloride were purchased from Energy Chemical. Methyl thiazolyl tetrazolium (MTT) was gained from SigmaAldrich. Dimethyl sulfoxide (DMSO) was obtained from Guangdong Xilong Reagents Company. Ethyl 3-aminobenzoate methanesulfonate tricaine (MS-222) was acquired by Nanjing Ezerinka Biotechnology Co., Ltd. 2.2 Characterization High resolution transmission electron microscopy (HRTEM) and TEM were recorded on JEM-2100. Atomic force microscope (AFM) image was performed on Multimode 8. Bruker


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VERTEX 70 was used for research the fourier transform infrared (FT-IR) spectroscopy. X-ray photoelectron spectroscopy (XPS) was conducted by Escalab 250Xi electron spectrometer. Renishaw inVia was used to obtained the Raman spectrum. UV-Vis absorption spectrum of CDs was gained using a U-4100 spectrophotometer. The fluorescent lifetime was measured by Edinburgh FLS980. Edinburgh FS5 spectrofluorometer was used to gain fluorescent spectra of CDs. The pH measurements were obtained by Jingke PHS3D digital pHmeter. Cellular fluorescence imaging was carried out by an Olympus Zeiss 710 laser scanning confocal microscopy (LSCM). An Olympus MVX10 fluorescence microscopy was used for ZF imaging. 2.3 Synthesis of CDs Briefly, 1.5 g CA and 2.1 mg NR with the molar ratio of 1000:1 were dissolved in 5 mL of ultrapure water, and stirred for 2 minutes to form uniform mixture solution. Then the solution was transferred into a 25 mL Teflon equipped stainless autoclave heating at 180 °C for 4 h. The solution of obtained purple CDs was centrifuged at 10000 rpm for 10 minutes to remove black solid deposits and then the supernatant was further filtered by a 0.22 µm pore diameter micropore membrane. Finally, the solid-state CDs was gained by freeze-drying. 2.4 Measurement of photoluminescence QY Rhodamine B in ethanol (QY = 0.56) was chosen as a standard. The QY of CDs was calculated:


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Where ɸ and ɸR are QY of the CDs and standard rhodamine B, respectively; the subscript R represents the reference data; n refers to the integrated emission intensity of CDs; A indicates the absorbance; ƞ is refractive index of the solvent. 2.5 Detection of noble metal ions (Pt2+, Pd2+ and Au3+) using CDs Different concentrations of Pt2+ (0-160 µM) were separately added into 2.5 mL of 0.1 mg mL-1 CDs solution. Then the fluorescence spectra were recorded with excitation wavelength at 530 nm. When detecting for Pd2+ and Au3+, the same process with Pt2+ ions was followed. The selectivity was conducted by adding other metal ions (including Ag+, K+, Na+, Pb2+, Ba2+, Zn2+, Mg2+, Ca2+, Cu2+, Hg2+, Fe3+ and Cr3+) instead of Pt2+, Pd2+ and Au3+ in the similar way. The competition experiments were performed by adding Pt2+, Au3+ or Pd2+ to the CDs solutions with other metal ions, respectively. All experiments were carried out at room temperature. 2.6 Cytotoxicity assay MTT assay was applied to assess the cytotoxicity of CDs in PC12 cells. A 96-well plate was used to seed PC12 cells. Different concentrations (0, 1, 1.5, 2, 2.5, 3 mg mL-1) of CDs were injected into each well. The cells were incubated at 37 °C under 5% CO2 atmosphere for 24 h. 10 µL of 5 mg mL-1 MTT medium was added to every well and then cultured at 37 °C in 5% CO2 for 4 h. Afterward, the medium was removed and 100 µL of DMSO was added to every well. The absorbance at 570 nm of every well was conducted by Thermo Multiskan. 2.7 Cells and ZF imaging PC12 cells were cultured in DMEM medium supplemented with 10% fetal bovine serum. The cells were seeded in the glass bottom culture plate and cultured with various concentrations


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of Pt2+ (0, 25, 50, 150, 300 µM). After incubation at 37 °C for 4 h, cells were rinsed three times with 0.1 M phosphate buffer saline (PBS). Subsequently, PC12 cells were incubated with 0.1mg mL-1 CDs for 24 h at 37 °C. The cells were washed with PBS to remove the remaining CDs and fluorescence images and intensity were recorded using the LSCM. In order to assess the actual applicability of CDs as fluorescent probe in a living organism, the ZF imaging experiment was established. First, wild-type ZF of 4 days old was fed with different concentrations of Pt2+ (0, 30, 60, 100, 150 µM) at 28 °C for 30 min. Afterwards, the ZF was washed with PBS for three times and then it was incubated with 1 mg mL-1 of CDs solution for 4 h. MS-222 was added after washing with PBS again. Finally, the ZF imaging was obtained by an Olympus MVX10 fluorescence microscopy. The fluorescence intensity of CDs in zebrafish was measured by software of Image J. 3. RESULTS AND DISCUSSIONS 3.1 Optimization of synthetic conditions The red emissive CDs were obtained by a facile hydrothermal route from CA and NR. The synthetic conditions of CDs were explored in detail, including the heating temperature and heating time. The fluorescence emission spectra of the synthetic products were researched first at different temperatures ranged from 80 °C to 180 °C. As shown in Table S1, the fluorescence emission peaks of CDs gradually blue-shift with the increasing of the reaction temperature. Table S1 also showed that the heating temperature could have a good effect on QY of the CDs, indicating the high temperature resulted in high QY. The heating time is another key factor influencing the fluorescence emission of the CDs. The samples were chosen to heat at 180 °C for different reaction time. The results were revealed in Fig. S1, demonstrating the fluorescence


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emission peak of CDs gradually red-shift with the decreasing of the reaction time. It may be because different types of carbon dots had been synthesized that resulted in diverse luminescence. 29, 30 It was selected to further characterize CDs with red emission that use CA and NR as raw materials in the temperature of 180 °C heating for 4 h. 3.2 Characterization of CDs The structure, morphology and optical properties of the prepared CDs with the 180 °C heating for 4 h were investigated. The TEM image of CDs was shown in Fig. 1a, illustrating that they were well-dispersed and uniform with an average particle size of 3.4 nm (Fig. S2a). The HRTEM image of CDs (inset in Fig. 1a) presented an obvious lattice structures with crystal spacing of 0.21 nm, corresponding to the (100) plane of graphitic carbon.31-33 The AFM image (Fig. 1b) revealed similar result to the TEM measurement, the heights of CDs were about 3.5 nm, suggesting that as-synthesized CDs were nearly spherical in shape. The XRD pattern of CDs and NR were shown in Fig. S2b, the diffraction peaks of CDs coincided well with the NR. Compared with the NR, the diffraction peaks of CDs were sharper, demonstrating that the prepared CDs were formed by organic dots leading to high crystallinity.34-36 The XRD pattern of CDs appeared a new sharp diffraction peaks at 24.3°, which was consistent with the (002) lattice spacing of carbon based materials.18 The Raman spectrum of CDs showed two obvious peaks in Fig. 2a, a D band at 1340 cm-1 (sp3 hybrid carbon) and a G band at 1596 cm-1 (sp2 hybrid carbon) were observed, which attributed to a hybridized vibrational pattern (A1g) related with graphene edges and the in-plane vibration pattern E2g of the graphite, respectively. The coexistence of the two bands indicated that the CDs possessed sectional disordered graphite-like structure.6, 37 The FTIR was shown in Fig. 2b, the peaks around 3496, 1720 and 1210 cm-1 coincided with the stretching vibrations of O-H, C=O and C-O-C,38 illustrating the existence of rich oxygen-


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contained groups in the composition of the CDs. The peak at 3296 cm-1 represented stretching vibrations of N-H.39 In addition, two peaks at 1551 cm-1 and 1412 cm-1 illustrated the presence of C=C and C-N bonds.16 As compared to the FTIR spectrum of CDs and NR, the new bond of C-N bond and stretching vibrations of N-H appeared in the CDs, illustrating that N had been successfully doped in CDs. The XPS spectrum (Fig. 2c) showed that the CDs was consisted with three elements: 60.72 at. % of carbon, 38.07 at. % of oxygen and 1.21 at. % of nitrogen. The high resolution C 1s spectrum (Fig. 2d) showed five obvious peaks located at 283.9, 284.2, 285.4, 286.4, and 289.1 eV, which correspond to C=C, C-C, C-N, C-O and C=O bonds, respectively.40 The O1s spectrum (Fig. 2e) could be divided into three components at 532.2, 533.0 and 533.7 eV, associated with C-O, C=O and C-OH/C-O-C bands, respectively.40 The N1s spectrum peaks at 399.7, 401.6 eV in Fig. 2f indicated that the N mainly existed in the manner of C-N and N-H bonds.40, 41 3.3 Optical properties of CDs UV/vis absorption, fluorescence excitation and emission spectra were used to explore the optical properties of CDs. The CDs solution revealed an obvious absorption peak at 530 nm in Fig. 3a. The images of daylight and 365 nm UV light irradiation were inserted in Fig. 3a, showing a bright red fluorescence. Solid-state CDs under day light and UV light were shown in Fig. 3b-c, which displayed that solid-state CDs emitted the similar colour fluorescence with CDs solution under UV light of 365 nm, suggesting the prepared CDs have potential applications in optoelectronic devices.22, 42, 43 Fig. 3d revealed the fluorescence emission of the CDs solution under diverse λex. With increasing the λex from 410 to 610 nm, the fluorescence emission peak nearly unchanged (the λem at around 632 nm). The excitation-independent emission property illustrated a nearly uniform CDs surface.44 The fluorescence excitation spectrum (at λem = 632


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nm) of CDs could be primely overlapped with the absorption band (in Fig. 3a), which demonstrated that the compositions and structures could be primarily lead to the red emission.45, 46

Fluorescence lifetime decay of CDs in solid state and aqueous solution were measured (Fig.

3e), the results indicated that their fluorescence decay curves were optimum fitted to a single exponential function, the fluorescence lifetime were 1.76 ns and 1.42 ns, respectively. The stability of the as-prepared CDs under different conditions was studied. The fluorescence intensity of CDs was smooth under continuous UV irradiation for 120 min (Fig. S3a), which implied that the CDs possess great photostability for biomedical applications. Fig. S3b showed the fluorescence intensity decreased mildly with increasing concentration of NaCl, but the maximum fluorescence emission peak (at 632 nm) remained steady. Furthermore, fluorescence intensities keep stable in a solution of pH range from 2 to 8 and decrease in a strong acidic (pH = 1) or higher pH solution (pH = 9-12) (Fig. S3c). The pH dependent photoluminescence characteristic could be caused by protonation and deprotonation of the functional groups (-COOH/-OH and -NH2) on the surface of CDs.36, 47 3.4 Fluorescence response of CDs towards Pt2+, Au3+ and Pd2+ It is considerably important to sense noble metals due to expensiveness and toxicity relevant with these noble metal ions. As shown in Fig. 4a, the fluorescence response of CDs was investigated in the presence of various metal ions. It clearly exhibited that F/F0 (F0 and F were the fluorescence intensities of the CDs at 632 nm in the absence and appear of metal ions, respectively.) nearly remains constant except for Pt2+, Au3+ or Pd2+. The results of fluorescence of CDs with increasing concentrations of noble mental ions (Pt2+, Au3+ and Pd2+) were shown in Fig. 4 b-d. The fluorescence of CDs can be effectively weakened by noble mental ions. The F/F0


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exhibited good linear response with the concentration of noble mental ions (insert in Fig. 4 b, c and d). The detection limits (LOD) of Pt2+, Au3+ and Pd2+ were estimated to be 0.886 µM, 3.03 µM and 3.29 µM, respectively (LOD = 3Sd/k, Sd is the standard deviation for the blank solution, and k is the slope of the calibration curve). Competition experiments were performed to analyze the ability of CDs for detection of Pt2+, Au3+ or Pd2+ in the present of other metal ions. As shown in Fig. S4, fluorescent intensity response (F/F0) of CDs towards Pt2+, Au3+ and Pd2+ had nearly no influenced by other metal ions. In order to understand the fluorescence quenching process, time correlated single photon counting experiments were investigated. As displayed in Fig. 3e, the average lifetime time of CDs was 1.41 ns, which was reduced to 0.41 ns with the addition of Pt2+. The obvious reduction in lifetime can indicate a fast electron transfer process between CDs and Pt2+ that caused dynamic quenching.9,

28, 48

In addition, it was no significant changed in fluorescent lifetime

before and after adding Au3+ or Pd2+. Almost the same fluorescent lifetimes indicated the strong inner filter effect occurring between CDs and Au3+ or Pd2+.49 3.5 Bio-applications To assess the CDs as fluorescent probe in biomedical applications, their cytotoxicity to PC12 cells was measured using the MTT assay. Cell viability had not markedly reduction with the addition of various concentrations (0-2.5 mg mL-1) of the CDs (Fig. S5). Owing to their outstanding biocompatibility and low cytotoxicity, these fluorescent probes showed wonderful promise for monitoring noble metal ions in vitro and in vivo. Accordingly, living cells and ZF fluorescence imaging experiments were performed to further prove their practicability in biological applications. After incubating the PC12 cells with CDs for 24 h at 37 °C, a significant


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red fluorescence in the cells was observed (Fig. 5a). The fluorescence intensity of the labeled cells shows no obvious change after 30 min of continuous irradiation (in Fig. S6). This indicates again that synthesized CDs possesses good resistance to photobleaching and can be potentially applied for biomedical fields. Fig. 5b-e exhibited that the fluorescence was gradually decreased with increasing concentrations of Pt2+ in PC12 cell. The result of the experiment was also shown in Fig. S7, illustrating that the CDs could be successfully used for monitoring Pt2+ in the cells. Moreover, the fluorescence intensity of the cells was decreased after treatment with Au3+ or Pd2+ in Fig. S8, indicating that the CDs as an intracellular fluorescent probe could also be applied to Au3+ and Pd2+ detection. The ZF imaging was shown in Fig. 6, and the results illustrated that the fluorescence intensity was gradually decreased with the increasing concentrations of Pt2+ in ZF. The fluorescence intensity of ZF at different Pt2+ concentrations was shown in Fig. S9, which demonstrated that the CDs could be used to monitor Pt2+ in organism. 4. CONCLUSIONS In summary, we have reported a simple hydrothermal method to synthesize CDs with red emission from CA and NR. These CDs exhibit good resistance to photobleaching property, excellent biocompatibility, and low cytotoxicity. The CDs were applied to detection of Pt2+, Au3+ and Pd2+, and correspondingly the LOD were 0.886 µM, 3.03 µM and 3.29 µM. Furthermore, the CDs were successfully used in the biomaging and biosensing of noble metal ions in cells and ZF. As-prepared CDs emit bright red fluorescence in the solid state overcoming the self-quenching. The applications of solid state CDs with red fluorescence are also being tracked in our group. ASSOCIATED CONTENT Supporting Information


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Fluorescence spectra of the as-synthesized CDs at 180 °C heating various times; size distribution of the CDs and XRD pattern of CDs and NR; stability research of the CDs in the 365 nm UV irradiation time, ionic strength and pH value; fluorescent intensity response (F/F0) of CDs towards Pt2+, Au3+ and Pd2+ in the presence of various metal ions; cell viability of PC12 cells after cultivation with CDs for 24 h; confocal fluorescence images of PC12 cells with CDs under different laser irradiation time (0, 15, 30 min); fluorescence intensities of CDs with different concentrations of Pt2+ in PC12 cells and in zebrafish; confocal imaging of Au3+ and Pd2+ in PC12 cells; the strongest fluorescence λem and QY of CDs of different reaction temperature. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Y. M. Zhou); Tel: +86-371-22868833-3422; Fax: +86-371-23881589 ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (21576071, 21776061, U1504215, 81541072), the State Key Laboratory of Fine Chemicals (KF1514). REFERENCES 1.

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Fig. 1 (a) TEM and HRTEM (inset) images of the CD. (b) AFM image of the CDs (inset: heightprofile along the corresponding line in (b)).


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Fig. 2 (a) Raman spectrum of the CDs. (b) FT-IR of CDs and NR. (c) Survey of XPS spectra of CDs. (d-f) The high-resolution XPS spectra of C1s, O1s and N1s, respectively.

Fig. 3 (a) UV-vis absorption, fluorescence excitation and emission spectra (λex = 530 of the CDs in aqueous solution; Inset: photographs of fluorescence CDs solution under daylight and 365 nm UV light. (b-c) Photograph of solid-state CDs under daylight (top) and 365 nm UV light (bottom). (d) The fluorescence emission spectra of CDs aqueous solution under the different excitation wavelengths ranging from 410 to 610 nm. (e) Fluorescence decay curves of CDs in solid state and aqueous solution, and fluorescence decay curves of Pt2+, Au3+ and Pd2+ in the CDs aqueous solution.


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Fig. 4 (a) Relative fluorescence intensity of 0.1 mg mL-1 CDs solution with excitation wavelength at 530 nm and emission wavelength at 632 nm in the presence of 160 µM of different metal ions. (b-d) Fluorescence spectra of CDs solution on addition of Pt2+, Au3+ and Pd2+ (0-160 µM), respectively. Insert: the linear relationship between F/F0 and the concentrations of (b) Pt2+, (c) Au3+and (d) Pd2+.


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Fig. 5 Confocal imaging of Pt2+ in PC12 cells. (a1-e1) Bright field images. (a2-e2) Black field images of the CDs in PC12 cells with the different concentrations of Pt2+ (0, 25, 50, 150, 300 µM). (a3-e3) Overlay images.


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Fig. 6 Fluorescence imaging of Pt2+ in ZF. (a1-e1) Bright field images. (a2-e2) Fluorescence images of the CDs in ZF with the various concentrations of Pt2+ (0, 30, 60, 100, 150 µM).


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TOC Figure:


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Carbon Dots with Red Emission for Sensing of ... - ACS Publications

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