Yellow-Emissive Carbon Dot-Based Optical Sensing Platforms: Cell

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Yellow-Emissive Carbon Dots based Optical Sensing Platform: Cell Imaging and Analytical Applications for Biocatalytic Reactions Hongxia Li, Xu Yan, Shanpeng Qiao, Geyu Lu, and Xingguang Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17619 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Materials & Interfaces

Yellow-Emissive Carbon Dots based Optical Sensing Platform: Cell Imaging and Analytical Applications for Biocatalytic Reactions

Hongxia Lia,†, Xu Yana,†,*, Shanpeng Qiaob, Geyu Lua and Xingguang Suc,* a State Key Laboratory on Integrated Optoelectronics, College of Electron Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, China

b State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P.R. China

c Department of Analytical Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012, China

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ABSTRACT Carbon dots (CDs) attract increasing interest in bioimaging and sensing recently. Herein, we present a simple synthetic strategy to prepare yellow-emission CDs (λem = 535 nm) by one-pot hydrothermal treatment of p-phenylenediamine and aspartic acid. The as-prepared CDs possess outstanding optical features, excellent biocompatibility and low cytotoxicity, especially for fluorescence cellular imaging. Interestingly, by combining the quenching and recognition ability of AgNPs with the optical capacity of CDs, a label-free strategy for specifically monitoring H2O2-generated biocatalytic processes was proposed, such as glucose oxidase-induced conversion of

glucose,

cholesterol

oxidase-catalyzed

oxidization

of

cholesterol,

bi-enzyme

of

acetylcholinesterase and choline oxidase-mediated reaction of acetylcholine. In this process, AgNPs act as a “nanoquencher” to decrease the fluorescence intensity of CDs via surface plasmon enhanced energy transfer mechanism. The enzymatic oxidation product (H2O2) subsequently etches the AgNPs to silver ions, thus restoring fluorescence of CDs, which enabled this proposed nanosensor to sensitively detect H2O2-generated biocatalytic processes. The above results pave the way to implement CDs as fluorescence labels for biosensor and biological imaging.

Keywords: Carbon dots, Silver nanoparticles, Surface plasmon enhanced energy transfer, Hydrogen peroxide, Enzyme

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INTRODUCTION Fluorescent carbon dots (CDs), emerging as a potent alternative to classical fluorescent nanomaterials, have gained ever-increasing attention owing to their outstanding merits, including outstanding chemical stability, excellent biocompatibility, good environmental friendliness, and unique photoluminescence properties.1-3 Compared with dye and fluorescent protein, CDs presents many advantages in terms of their low cost and fabrication simplicity. Different synthetic routes, including calcination,4 ultrasonication,5 electrochemical oxidation,6 microwave-assisted methods,7 and hydrothermal synthesis strategies8, have been efficiently developed for the fabrication of CDs. Meanwhile, numerous carbon precursors, such as polymers,9-11 biomaterials12,13 and natural product,14,15 are successfully utilized to prepare CDs. Various synthesis methods and numerous precursors provide convenient synthesis condition for CDs preparation. Owing to their splendid properties, CDs act as promising fluorescent probes with a broad application for sensing16,17 and bioimaging.18,19 In particular, CDs have been successfully employed for sensing various enzymes including phosphatase,20 tyrosinase,21 cholinesterase,22 galactosidase23 and glucosidase,24,25 et al. Thus, associations of enzyme molecules and CDs have provided great opportunities for advanced development of fluorometric assay. Manipulating ideal optical nanosensor by combination of fluorophore and nanomaterial-based quencher is the most fascinating strategy to optimize their performance and tackle the weaknesses of individual components.26-29 Silver nanoparticles (AgNPs) hold unique optical characteristics, such as high extinction coefficient, size-dependent color change and facile chemical modification, gaining special interest in the development of strategies.30-33 More attractively, the structure of AgNPs can be destroyed by hydrogen peroxide (H2O2), which provided a new sight for the 3



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construction of sensing platform.34 For example, Li et al. demonstrated a perylene turn-off-on strategy for the detection of glucose based on H2O2-triggered the decomposition of AgNPs.35 Chu’s group reported an exceptionally simple method for the preparation of AgNPs/upconversion nanoparticles hybrid nanocomposite in the sensing of glucose.36 However, the above methods require sophisticated preparation of probe or complicated surface linkage between nanomaterials, restricting their practical applications. Thus, a core challenge still remains to achieve convenient synthesis of probe. To address these limitations, a bright fluorescence (FL) CDs was successfully prepared by utilizing facile hydrothermal synthesis of p-phenylenediamine and aspartic acid. By combining the H2O2-triggered AgNPs decomposition and the specificity of enzyme, we herein presented a sensitive CDs/AgNPs platform for monitoring enzyme catalytic reactions that can generate H2O2. As depicted in Scheme 1, due to the intense electrostatic attractive interactions, the positively charged CDs can be closely attached to the negatively charged AgNPs, resulting in significant FL quenching through surface plasmon-enhanced energy transfer (SPEET). H2O2 with oxidation ability etches the AgNPs to silver ions, accompanying the obvious FL recovery of CDs. Accordingly, this proposed sensing platform was facile and cost-effective to monitor the H2O2 product-related enzyme catalytic system, including glucose oxidase (GOx)-catalyzed oxidation of glucose,

cholesterol

oxidase-triggered

oxidation

of

cholesterol,

and

bi-enzyme

of

acetylcholinesterase/choline oxidase-mediated reaction of acetylcholine. The combination of enzyme-catalyzed reaction and AgNPs-assisted specific recognition not only improved the specificity and sensitivity of sensing platform, but also possessed new sight for monitoring enzyme catalytic reactions. 4


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Scheme 1 The schematic illustration of the SPEET-based platform for H2O2 and enzyme catalytic reactions EXPERIMENTAL SECTION Reagents and Instruments. Glucose, glucose oxidase (GOx), cholesterol, cholesterol oxidase (CLO), acetylcholinesterase (AChE), choline oxidase (CHOx), acetylcholine chlorid (ACh), p-phenylenediamine (PPD) and aspartic acid (ASP) of analytical grade were obtained from Sigma-Aldrich Corporation. Silver nitrate (AgNO3) and sodium citrate dihydrate were purchased from Sinopharm Chemical Reagent Co. Ltd. The water with an electrical resistance of 18.2 MΩ cm-1 were used in this study. The FL spectra and UV-vis absorption were recorded with RF-5301 PC spectrofluorophotometer (Shimadzu, Japan) and UV-1700 spectrophotometer (Shimadzu, Japan), separately. All solution pH was tested by a PHS-3C pH meter (Tuopu Co., China). Transmission electron microscopy (TEM) images were collected by a Philips Tecnai F20 electron microscope. Preparation of CDs. Aspartic acid (ASP, 26.6 mg) and p-phenylenediamine (PPD, 21.6 mg) was dissolved in DI water and transferred into polytetrafluoroethylene autoclaves for heating 12 h at 180 °C. The obtained solution was purified with a dialysis bag (molecular weight cut-off ∼ 1 kDa) for 1 day to dialyze the nonreactive molecules. After that, the solution was freeze-dried and then 5



dissolved with deionized water. Finally, the prepared CDs (10 mg L-1) were stored at 4 °C before use. Preparation of AgNPs. AgNPs were synthesized according to previous methods with slight modifications.37 Briefly, 5 mL silver nitrate (10 mmol L-1) and 5 mL sodium citrate solution (10 mmol L-1) were added to a flask containing 89 mL of ultrapure water for mixing 20 min, then followed by the quick addition of 8.8 mg NaBH4. The reaction was taken in room temperature for 2 h to achieve a yellow solution. The obtained solution was purified with a dialysis bag (molecular weight cut-off ∼ 1 kDa) for 1 day to dialyze the nonreactive molecules. The AgNPs solution was stored at 4 ºC for further study. To calculate the mass concentration, the as-prepared AgNPs solution was freeze-dried and then weighed quality. Thus, the concentration of AgNPs was estimated to be 150 µg mL-1. Detection Procedure. 50 µL of cholesterol with different concentrations was incubated with CLO (50 µL, 5 U mL-1) at 37 °C for 60 min to generate H2O2. Then, the mixture of AgNPs (900 µL, 5.0 nmol L-1), CDs (40 µL, 10 mg L-1) and 100 µL of PBS buffer (7.4, 10 mmol L-1) was added to the above reaction solution and the solution was diluted with deionized water to 1.5 mL. After 5 min, the FL spectrum was recorded for cholesterol detection. RESULTS AND DISCUSSION Characterization and Properties of CDs. To construct the sensing platform, there is a prerequisite to prepared CDs possessing long excitation wavelength and high FL quantum yields (QYs). According to previous studies, aniline and its derivative as significant precursors for producing CDs received more attentions.38,39 Inspired by preceding work, the yellow CDs were prepared facilely through hydrothermal mediated heating of a mixture of ASP and PPD for 12 h at 6


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180 °C. The morphology of as-synthesized CDs nanoparticles was measured by transmission electron microscopy (TEM). As shown in Figure 1A, the CDs were homogeneous in size with around 3.3 ± 0.7 nm and possess mono-dispersed with uniform spherical structure. Specifically, high-resolution TEM image clearly revealed that lattice fringes of CDs (0.213 nm) were matched well with the (100) facet of sp2 graphitic carbon.40 In addition, the AFM image illustrated that the CDs possess similar particle heights of 3.4 nm (Figure 1B). XPS and FT-IR were further measured to characterize chemical composition and functionalization of the CDs. As depict in Figure S1, the CDs mainly contain C, N, and O elements that can formation chemical bonds including -NH (3325 cm-1, 1559 cm-1), -CN (1400 cm-1) and C-NH (1285 cm-1) (Figure S2). The optical performance of CDs was collected by utilizing UV-vis absorption and fluorescence spectroscopy. For UV-vis performance, the as-prepared CDs exhibit two characteristic absorption bands around 260 nm and 358 nm which can be assigned to π-π* (aromatic ring structure) and n-π* (C=O) transitions, respectively (Figure 1C). The FL spectra of CDs record maximum emission intensity at 535 nm at excitation wavelength of 410 nm (Figure 1C). The exhibited excitation wavelength of CDs was close to that of the absorption band in the lower energy region. As detailed in Figure 1D, the FL emission bands of the as-prepared CDs do not shift when excitation wavelengths were applied in the range from 370 nm to 460 nm, indicating the CDs possess distinctively excitation-independent FL behavior. These results confirm that the CDs have a well-defined composition and structure as well as uniform surface state, leading to a high QYs.41,42 The QYs of as-prepared CDs were determined to be 8.5 % by using Rhodamine B as reference.

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Figure 1 (A) The TEM image of the CDs. (B) AFM images of the CDs. (C) The UV-Vis absorption spectra, FL excitation and FL emission spectra of CDs. (D) FL emission spectra of the CDs recorded at various excitation wavelengths in the range from 370 to 460 nm.

The chemical stability and photostability of nanomaterials are critical for their meaningful applications. To evaluate its chemical stability, the FL emission intensity of CDs was investigated under different concentration of salt or broad pH ranges. As shown in Figure S3A, the FL signal of CDs kept almost invariable in a wide ionic strength range of 0-150 mmol L-1, which showed good resistance to salt solution. Subsequently, the normalized intensity of CDs merely changes when pH value change from 5.5 to 7.4 (Figure S3B). Although the CDs possess good chemical stability, 8


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their photostability behavior was a concern for us. The FL signal of CDs were measured under the continuous excitation at 420 nm for 60 min. It was obviously found from the photobleaching curves in Figure S3C that CDs preserved ~80% of the initial FL intensity through 60 min continuous irradiation process. Besides, the CDs shown better photostability compared to that of CdTe quantum dots (QDs). As depicted in Figure S3D, the FL of QDs was quickly quenched after 10 min under 225 W xenon lamp irradiation and showed nearly no FL intensity after 60 min irradiation (Figure S3D Inset), while the FL of CDs preserved ∼60% of the initial intensity after 60 min of xenon lamp irradiation, clearly implying that the CDs featured the robust photostability. The excellent FL behavior and fascinate stability enabled the CDs as an ideal candidate to be applied in bioimaging and biosensors. Initially, the characteristic cell cytotoxicity of CDs was evaluated by introducing CDs in MIDA-MB-231, MIDA-MB-68 and RPE1 cells as model cell lines using a standard MTS assay (Figure S4). More than 85% cells could survive after incubating with CDs even at concentrations of 40 µg mL-1 for 24 h. The results showed that CDs poses no marked cytotoxicity to the cells in all treated doses after a long period incubation, clearly revealing that CDs showed promising potential for bioimaging applications. Encouraged by the above-mentioned outcome, we assessed the in vitro imaging performance of CDs towards living cells. The CDs (20 µg mL-1) were incubated with MIDA-MB-231, MIDA-MB-68 and RPE1 cells for 4 h at 37 0C, and FL intensity was imaged using confocal laser scanning microscopy (CLSM). As exhibited in Figure 2, bright yellow-emission were observed in cells under 405 nm laser excitation, which demonstrated that those cells can be homogeneously labeled by as-synthesized CDs. The merged CLSM images illustrated that the yellow FL signal was mainly appeared on the cell cytoplasmic area while no photoluminescence was observed at nucleus, indicating that the 9



CDs were successfully taken up by the cells through endocytosis and diffuse into cytoplasm.43,44 These results indicated that the CDs with low cytotoxicity and excellent biocompatibility can be applied as a potentials FL candidate for bioimaging.

Figure 2 Confocal FL images of MIDA-MB-231, MIDA-MB-68 and RPE1 cells with 20 µg mL-1 CDs for 4 h. Scale bar = 10 µm. Design of CDs-AgNPs Platform. Taking their excellent optical properties and outstanding biocompatibility into account, the CDs were promising as a candidate to conduct nanosensor for the sensitive sensing of H2O2. The designed platform consists of CDs and AgNPs (Optical 10


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spectrum, TEM and DLS of AgNPs were shown in Figure S5), where CDs worked as fluorometric reporter, and AgNPs served a dual-function of nanoquencher and H2O2-recognizer. As displayed in Figure S6A, AgNPs can effectively decrease the FL intensity of CDs (nearly 80 %, Dot line). Importantly, H2O2 that could not influence the FL intensity of CDs (Figure S7), efficiently induce decomposition of AgNPs, thus the quenched intensity can be recovered by introducing H2O2 (Dash line, Figure S6A), providing the basis for H2O2 detection. For explanation of the quenching mechanism between CDs and AgNPs, the role of SPEET45,46 in the entire suppression process was investigated. As shown in Figure 3A, there was remarkable overlap between the excitation spectrum of CDs and the absorption spectrum of AgNPs, implying the possibility of formation of energy donor-acceptor pairs, which was an indispensable condition for SPEET.46,47 In addition, the CDs showed positively charged (ζ = 24.87 mV) while the AgNPs were negatively charged (ζ = -15.66 mV), which demonstrated the electrostatic interaction between CDs and AgNPs (Figure 3B). To further confirm our assumptions, the FL lifetime of CDs probe in the presence and absence of AgNPs were investigated (Figure 3C), and the decay components and their ratios were shown in Table S1. The average FL lifetime of CDs/AgNPs (1.75 ns) was shorter than that of CDs (3.13 ns), suggesting that the energy transfer process was dominant for quenching mechanism. Therefore, via the efficient SPEET process, FL intensity ratio (FQ/FQ0) at 535 nm of CDs in the sensing system were directly proportional to the increasing concentration of AgNPs from 0 to 120 µg mL-1 (Figure 3D). The chemical stability and photostability of CDs/AgNPs probe was also investigated in Figure S8. Different concentration of NaCl (from 0 to 20 mmol L-1) possess nearly no influence on FL intensity of system. For photostability, the FL emission intensity of system was slightly decrease (~20 %) during a 60-min 11



continuous exposure under xenon lamp with excitation at 410 nm (Figure S8B). All these results suggest CDs/AgNPs probe possess good stability of toward ambient environment.

Figure 3. (A) The excitation spectrum of CDs and absorption spectra of AgNPs. (B) The zeta potentials of CDs and AgNPs. (C) The FL lifetime measurement of CDs probe in absence and presence of AgNPs. (D) The FL spectra of CDs system in the presence of different concentration of AgNPs (0, 3, 7.5, 15, 22.5, 30, 45, 60, 75, 90, 105 and 120 µg mL-1). Inset illustrated that the linear plot of FL intensity ratio FQ/FQ0 versus the concentration of AgNPs. FQ and FQ0 were the FL intensity of CDs probe in the presence and absence of AgNPs, respectively.

The structure of AgNPs can be destroyed by H2O2, which easily weakened SPEET process, causing the FL recovery of sensing system. The decomposition of AgNPs was first confirmed by 12


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UV-vis absorption spectra. As depict in Figure 4A, the characteristic absorbance of AgNPs around 410 nm obviouly decrease with the increasing of H2O2 (from 0 to 100 µmol L-1). Furthermore, the TEM images provide convincing evidence to support the strategy rationale. After treatment with H2O2, the AgNPs were etched to smaller NPs (Figure 4B and 4C), which was constant with the dynamic light scattering measurement (Figure S9), clearly indicating that the AgNPs were destroyed by H2O2. The sensing platform for H2O2 detection was established under the optimum conditions (Data in Figure S10 and S11). Figure 4D showed the FL spectra of CDs/AgNPs probe in the presence of various concentrations of H2O2. The FL intensity of system at 535 nm was gradually enhanced with increasing H2O2 concentration, demonstrating that the correlation between recovered FL intensity and H2O2 concentration was dose-dependent. Meanwhile, an excellent linear relationship (correlation coefficient R2=0.992) in the range of 1.0 - 100 µmol L-1 was obtained (Figure 4D Inset), indicating that the constructed CDs/AgNPs probe was appropriate for H2O2 detection.

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Figure 4 (A) The UV-vis absorption spectra of CDs/AgNPs probe in the presence of different concentration of H2O2 (from 0 to 100 µmol L-1). TEM images of CDs/AgNPs probe before (B) and (C) after addition of H2O2. (D) The fluorescence spectra of CDs/AgNPs system in the presence of different concentration of H2O2 (0, 1, 5, 10, 25, 50 and 100 µmol L-1). The inset showed the linear plot of (FR/FR0) versus the concentration of H2O2.

Bestowed with good analytical performance, the SPEET-based platform can be applied as an optical probe for the novel application of biocatalytic transformations of different enzyme systems. Three typical and vital H2O2-related enzymatic reactions were chosen as examples: (1) CLO oxidizes cholesterol and oxygen to generate the cholestenone and H2O2, (2) GOx catalyzes 14


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glucose and oxygen to produce the gluconic acid and H2O2, and (3) bi-enzyme of AChE/CHOx-mediated reaction of acetylcholine (ACh) to produce of H2O2. What in common of these enzyme systems is that H2O2 were produced during the oxidation reaction, causing the etching of AgNPs and restore of FL intensity. As the content of H2O2 in the sensing systems was controlled by the substrates, the resulting FL recovery of the CDs offer a quantitative output signal for the respective substrate. As a proof of concept, the CDs/AgNPs platform was used for monitoring cholesterol. Cholesterol as an important compound of nerve and brain cells is closely associated with the occurrence of atherosclerosis and hypocholesterolemia.48,49 CLO catalyzes the oxidation of the cholesterol to generate H2O2, which triggered the FL restore of sensing platform. For cholesterol detection, the influence of cholesterol and CLO on CDs/AgNPs probe were firstly studied. As shown in Figure 5A, the FL intensity of CDs/AgNPs+ cholesterol and CDs/AgNPs+ CLO was identical to that of CDs/AgNPs system, which indicated the interaction between cholesterol, CLO and CDs/AgNPs can be ignored. While the FL intensity can be recovered with the simultaneous addition of cholesterol and CLO into proposed CDs/AgNPs system. Figure 5B displayed the FL spectra corresponding to cholesterol in the presence of CDs/AgNPs probe and CLO. Evidently, the CLO catalyzed oxidation of substrate was controlled by the concentration of cholesterol, and as its concentration increases, the generation of H2O2 was enhanced. As shown in the inset of Figure 5C, on plotting the relative FL intensity (Fc/Fc0) against the cholesterol concentration, a good linear relationship (R2=0.991) for cholesterol quantification was obtained in the range 1-300 µmol L-1. The linear fitting can be expressed as Fc/Fc0=1.216+0.0064 [cholesterol]. The detection limit was determined to be 300 nmol L-1 (signal-to-noise ratio of 3) 50,51, which was lower than the total cholesterol level found in human blood (~5 mmol L-1).52 It also presented a 15



comparable or even better performance comparing with other reported probes for cholesterol (Table S2). In previous studies, carbon dots had been served as signal indicator for cholesterol detection.53-56 Although these strategies possess good performance for analyte detection, most of assays required complicated surface modification. Furthermore, their FL emission were located in the range of 400-500 nm, suffering from biorelated autofluorescence with background interference. In comparison with previous assays, CDs/AgNPs probe did not need material decoration, also exhibited bright yellow FL emissions and well sensitivity, suggesting that CDs/AgNPs sensing platform could be utilized for sensitive detection of cholesterol. The successful analysis of cholesterol by SPEET-based probe suggests that the sensing platform can also be implemented to follow other enzyme cascade catalytic processes (GOx/glucose and AChE/CHOx/ACh) that involve H2O2-generating oxidase. By employing the present SPEET-based strategy, the limit of detection for glucose and ACh were estimated to be 1.0 and 1.2 µmol L-1, respectively (Data shown in Supporting Information). The sensing of cholesterol, glucose and ACh suggested that the combination of different enzyme with CDs/AgNPs probe can be used for development of sensitive biosensor. The selectivity of the SPEET-based platform was further investigated with adding various biologically relevant molecules, including ions (Na+, K+, Ca2+), amino acids (lysine, arginine and histidine) and protein (human serum albumin, lactate dehydrogenase and alkaline phosphatase). As revealed in Figure 5D, the SPEET-based platform possessed a remarkable response toward cholesterol. Even interfering substance (2 µg mL-1 for proten and 200 µmol L-1 for others) exists in system, the platform still showed the same response to substrates, demonstrating that the proposed strategy not only shown high selectivity to its corresponding substrate, but also displayed excellent 16


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ability of resist interference. This promising selectivity of CDs/AgNPs biosensor might be due to the H2O2-induced AgNPs etching and the special catalyzation of enzyme cascade. Those results obtained by the proposed method indicated that the SPEET-based platform possess attractive sensitivity and selectivity, and hold potential applicability. Moreover, combination of different enzyme can be used for development of specific sensor, which can be possible in future by following our strategy. To further test the potential applications, the sensing of cholesterol in human serum sample and urine sample was carried out. The samples were diluted 50-fold to avoid background and interference.57 As displayed in Table S3, recoveries of the known spiked amounts of cholesterol in human serum were obtained in the range of 91.3-104.3 % with the relative standard deviations (RSDs) lower than 4.9 %, suggesting the potential applicability of the proposed strategy for cholesterol detection in real samples.

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Figure 5 (A) Feasibility of detection cholesterol. (B) The FL spectra of CDs/AgNPs/CLO system in the presence of different concentration of cholesterol (0, 1.0, 10, 50, 100, 200 and 300 µmol L-1). (C) the linear plot of Fc/ Fc0 toward the concentration of cholesterol. (D) The FL intensity of the CDs/AgNPs/CLO/cholesterol (200 µmol L-1) system probe with the interfering substances (200 µmol L-1). CONCLUSION In summary, we have designed and synthesized yellow-emission CDs through simple hydrothermal mediated heating of a mixture of ASP and PPD. Owing to its distinguish FL properties and outstanding biocompatibility, the as-prepared CDs have great promise in cell imaging. Moreover, by combining the H2O2-triggered AgNPs decomposition and bright FL 18


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emission of CDs, we have developed a novel SPEET-based fluorescent platform for probing of biocatalytic oxidase-stimulated H2O2-generating transformations. The proposed SPEET-based strategy which do not need surface modification and link procedures, allows the design of optical methods in a cost-effective and time-consuming way. Also, the excellent selectivity of sensing platform obviously demonstrates the potential for the practical application. The present concept not only provides a promising design for sensing of H2O2-related enzymatic reactions, but also represents a new example of wide application of CDs. ASSOCIATED CONTENT Supporting Information Detection procedure and result (glucose and ACh); Stability of CDs; MTS assay of CDs; TEM and UV-vis of AgNPs; lifetime of GQDs with AgNPs; optimization conditions; sensitivity and selectivity of CDs/AgNPs/enzyme system; comparison between methods. AUTHOR INFORMATION Corresponding Author Email: [email protected] (X. Yan) [email protected] (X.G. Su) Author Contributions † These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was finally supported by the National Natural Science Foundation of China (No. 19



21075050 and No. 21275063), the Science and Technology Development project of Jilin province China (No. 20150204010GX). X. Yan is thankful for support from the National Postdoctoral Program for Innovative Talents (BX201700096) and China Postdoctoral Science Foundation funded project (No. 2017M621199).

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Yellow-Emissive Carbon Dot-Based Optical Sensing Platforms: Cell

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