Inorganic Salt Incorporated Solvothermal Synthesis of Multicolor

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Inorganic Salt Incorporated Solvothermal Synthesis of MultiColor Carbon Dots, Emission Mechanism and Anti-bacterial Study Bo Ju, Hui Nie, Xiao-guang Zhang, Qiaonan Chen, Xiaowei Guo, Zhen Xing, Minjie Li, and Sean Xiao-An Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01355 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Applied Nano Materials

Inorganic Salt Incorporated Solvothermal Synthesis of Multi-Color Carbon Dots, Emission Mechanism and Anti-bacterial Study Bo Ju,1 Hui Nie,1 Xiao-guang Zhang,2 Qiaonan Chen,1 Xiaowei Guo,1 Zhen Xing,3 Minjie Li,*1 Sean Xiao-An Zhang1

1

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

University, Changchun 130012, P.R. China. 2

College of Food Science and Engineering, Jilin University, No. 5333 Xi’an Street, Changchun

130062, P.R. China. 3

Key Laboratory for Molecular Enzymology and Engineering of the Ministry of Education,

College of Life Science, Jilin University, Changchun 130012, P. R. China

KEYWORDS inorganic salt incorporation, multi-color carbon dots, emission mechanism, surface emissive domains, antibacterial activity

ACS Paragon Plus Environment

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ABSTRACT

Enriching the synthetic routes of functionalized carbon dots (C-dots) and expanding their application are necessary for promoting the research of carbon-based nanomaterials. In this study, we develop an one-step and efficient method to prepare excitation-dependent multi-color emissive carbon dots (M-C dots) with quaternary ammonium functional groups via solvothermal reaction of chloroform, diethylamine and ammonium carbonate. Through detailed structural characterizations and further reactions of surface functional groups (such as reduction, oxidation and hydrolysis) on M-C dots, the excitation-dependent tunable photoluminescence behaviors are proposed to come from the multiple emission domains on the surface. The prepared M-C dots possess antibacterial activity against Gram-positive bacteria and Gram-negative bacteria. And the antibacterial effects of M-C dots were investigated by destroying the bacterial cell walls/membranes via strongly electrostatic interactions between positively charged M-C dots and negatively charged bacteria. This study provides a novel strategy to prepare functionalized Cdots and reveals the nature of intrinsic multi-color emission of C-dots.

INTRODUCTION

Carbon dots (C-dots) have attracted more and more attention as a category of photoluminescent carbogenic nanomaterial. Due to their fascinating physical and chemical properties, such as abundance of resources, good biocompatibility, low toxicity and controllable surface passivation,1-3 C-dots have been widely used in photocatalysis,4-5 sensors,6-7 optical information storage,8-10 photovoltaic devices,11-12 light emitting diodes,13-15 and medical diagnosis.16-17 What’s more, our group and others has developed a special type of C-dots that can show peculiar


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excitation-dependent multi-color emission on single dots level.18-20 This intrinsic multi-emission properties make C-dots superior in bioimaging,21-25 anti-counterfeiting2 and chemical sensing.26 So far, C dots have been prepared with both bottom-up and top-down methods with a variety of resources.27-31 In these methods, heteroatoms, such as O and N are suggested to play important role in tuning the optical properties.32-34 Compared with organic compounds on preparation of Cdots, low-cost inorganic components have been widely adopted as important dopants that can provide various kinds of heteroatom or metal for synthesizing functionalized C-dots. In most cases, the O element comes from organic reactants. In addition, O2 or water in the air (or in the solution) is also suggested to providing the O resources,26 however, the N elements are inevitably come from the organic reactants, whether inorganic ammonium salts can provide N containing organic functional groups is a question. If inorganic N resources can be applied to synthesize C dots, new functional groups or structure will be introduced because of the different chemistry, which will then lead to new understanding on the synthesis of C-dots and may result in unexplored application potentials.

Besides the exploration of broader synthetic resources that can bring in new functional groups, the detailed and concrete study on the emission mechanism is important, especially for the multicolor emission C-dots. Great efforts have been made on the mechanism study, It’s revealed both the carbon core and the surface fragments are suggested to contribute to the emission of C-dots, and the multi-emission of C-dots are commonly attributed to the existence of different emission centers.20,27 However, it is still very hard to relate clearly the chemical structure with the emissive properties in molecular level due to the high complexity of the structure. It needs more clever designs and control experiments, as well as breakthrough in characterization methods. Any new synthetic methods have the chance to provide new insights of the mechanism of emission.


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Herein, inorganic ammonium salt has been utilized as a resource, for the first time, to introduce N elements in preparation of C-dots via a facile solvothermal method with chloroform (CHCl3), diethylamine (DEA) and ammonium carbonate ((NH4)2CO3) as reactants. It results in excitationdependent multi-color emission C-dots (M-C dots) with new functional groups of cyano (C≡N). Through detailed structural characterizations and further reactions of surface functional groups (such as reduction, oxidation and hydrolysis) on M-C dots, the excitation-dependent tunable photoluminescence behaviors are proposed to mainly come from the co-existence of multiple molecular domains on the surface, suggesting engineering only of the surface of the C-dots can also lead to multi-color emission. And the M-C dots have much broader excitation-dependent tunable multi-color photoluminescence behaviors, which is compared with reported C-dots. Due to the one-step inorganic salts (N resource) incorporation, the anti-bacterial quaternary ammonium structure was efficiently introduced into M-C dots. The prepared M-C dots possess antibacterial activity against Gram-positive and negative bacteria. And the antibacterial effects of M-C dots were investigated by destroying the bacterial cell walls/membranes via strongly electrostatic interactions between M-C dots and bacteria.

RESULTS AND DISCUSSION Characterization of M-C dots As shown in Figure 1a, the excitation-dependent M-C dots can be obtained through an inorganic salt incorporated solvothermal method from the reaction among CHCl3, DEA and (NH4)2CO3. The M-C dots have a broad featureless absorption in visible light region (Figure 1 b), which were very common in reported C-dots with different electronic absorption transitions in multiple emission centers.22,35 It’s remarkable that M-C dots have multicolor emission peaks ranging from


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477 nm to 682 nm at different excitations (Figure 1c). Table S1 in Supporting Information shows the detailed fluorescence parameters of M-C dots. The larger the excitation wavelength is, the shorter the △λ (λem-λex, similar to Stokes shift) is. The variation of full width at half maximum of the emission spectra (FWHM) with the excitation wavelengths is irregular and the spectral shapes have long-wavelength small tails with asymmetric characteristic. These results suggest there are multiple emission domains in the C-dots and there are energy transfer among these emission domains.18 The approximate quantum yield of M-C dots calculated at different emission wavelengths was ~ 4% for excitations at 270 nm, 470 nm and 610 nm.

Figure 1. (a) Synthesis route of M-C dots. (b) UV-vis absorption spectra of M-C dots. (c) Excitation-dependent photoluminescence spectra of M-C dots at different excitation wavelengths (0.02mg/mL). Through atomic force microscopy (AFM) and transmission electron microscopy (TEM) characterizations, the morphology and structure of M-C dots were studied. TEM image shows


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the M-C dots have excellent dispersibility (Figure 2a). High-resolution TEM images reveals they have crystalline graphitic core and lattice fringes with d-spacing of 0.25 nm and 0.32 nm which are corresponding to the (020) and (002) crystal plane of graphilic carbon (HRTEM, Figure 2a).13 The size statistic of M-C dots was calculated by counting more than three hundred particles, which indicates their sizes center at 8-9 nm (Figure 2b). Similarly, AFM image (Figure S1) also shows M-C dots are monodispersed and the average height centeres at 2-6 nm. 1H NMR spectrum (Figure S2) was employed to detect sp2-C (signals from 7.0 to 9.0 ppm) and sp3-C (broad peaks from 1.0 to 4.0 ppm).36 Moreover, there are defects and disordered carbon structures in both graphilic cores from the co-existence of D band (sp3-C, 1360cm-1) and G band (sp2-C, 1590cm-1) from Raman analyses (Figure 2c).15 And Electron spin resonance (ESR) spectra analysis of M-C dots shows a strong signal peak centered at g~ 2.0045 (Figure 2d), which is attributed to the defects and amorphous carbons.37 The above results indicate that M-C dots possess graphite-like structure with some carbon defects.


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Figure 2. (a) TEM image of M-C dots. Inset: HRTEM images of M-C dots (Scale bar=5 nm). (b) Size distribution of M-C dots, the results of statistics more than 300 particles. (c) Raman spectra of M-C dots. (d) ESR analysis of M-C dots.

The chemical composition and chemical bonding of M-C dots are fully investigated. The element analysis shows that M-C dots mainly contain C, H, N, O elements (C: 69.89%, H: 5.73%, N: 16.57%, O: 7.81%). In the Fourier transform infrared spectroscopy (FT-IR) spectrum (Figure 3a), the following functional groups are observed in M-C dots. N-H: deformation vibration at 795 cm-1, stretching vibration at 3441 and 3203 cm-1; C-H: stretching and bending vibrations at around 2975, 2933, 2872 cm-1 and 1447, 1362 cm-1; C≡N: the vibration absorption band of group at 2202 cm-1; Aromatic C=C: the characteristic absorption band at 1609 cm-1; C=N and C=O: stretching vibrations at 1668 cm-1 and 1737 cm-1; C-N and C-O: vibration at 1260 cm-1; C-O-C: the symmetric and asymmetric stretching vibrations at 1074 cm-1 and 1116 cm-1;26,38 Then high resolution X-ray photoelectron spectroscopy (XPS) was performed to analyze these surface functional groups on M-C dots. Typical peaks of O1s (532 eV), N1s (399 eV), C1s (284 eV) can be seen in XPS full spectra (Figure S3).39,40 High-resolution C1s spectra (Figure 3b) shows the existence of C=O/C=N (287.4 eV), C-O (286.0 eV), graphite-N/C-N/C≡N (285.7 eV) and C=C (284.7 eV). High-resolution N1s spectra (Figure 3c) displays the presence of graphite-N (400.1 eV), C≡N (398.4 eV), C=N (401.2 eV) and C-N (399.1 eV). High-resolution O1s spectra (Figure 3d) can be separated into double peaks at 532.1 eV and 531.1 eV, which are representative binding energies of C-O and C=O groups. Above XPS studies in M-C dots confirm the FT-IR analysis for certain. Because of these diverse functional groups, the M-C dots can be dispersed in an overwhelming majority of common solvents, such as ethanol, chloroform, DMF, DMSO,


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actonitrile, and et al (Figure S4).41 Owing to the existence of hydrogen-bond interactions and dipole-dipole interactions between solvent environment and M-C dots, the absorption of M-C dots dispersed in different polarity solvents are discriminating, which is common in solventdependent fluorescence and absorption behaviors.

Figure 3. (a) FT-IR spectra of M-C dots. (b-d) High-resolution XPS spectra of the C 1s / N 1s /O 1s

peaks of M-C dots respectively.

Fluorescence mechanism of M-C dots

Until now, the photoluminescence behaviour of C-dots has been widely investigated and the emission mechanism are mostly focused on surface states, defect states, molecular states and etc.42,43 To reveal the mechanism of the full color emission, the excitation spectra of M-C dots are recorded at different emission positions first, very surprisingly, the excitation spectra have three distinct bands (I, II, and III) with fixed positions (Figure 4a). These three distinct bands


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indicate the types of emission domains are at least three or more, which contribute to three emission bands in blue-green (I), green-yellow (II) and orange-red(III) regions, respectively. Therefore, multiple emission domains are proposed to dominate these intrinsic multi-color emissions, which is consistent with the research results that multiple emission centres can be integrated into single carbon dots.18 In order to figure out the relationship between these emission domains and multi-color emissions, reduction, oxidation and hydrolysis treatment of the C-dots were carried out. First, different surface states can cover the whole emission region. When these functional groups on surface were reduced with NaBH4 or oxidization and hydrolysis with HNO3, the emission in blue to green region both enhance and the emissions in longer color region delimitate (Figure S5, S6). And change of the surface functional groups are proved with infrared measurement. As shown in Figure 4c, the N-H bending vibration (795 cm-1) and the stretching vibrations of C-O-C/C-O (1116 cm-1), N-H/O-H (3441 cm-1) increase obviously after reduction; simultaneously, the vibrations of C=N/C=O at 1668 cm-1 and 1737 cm1

decrease greatly. The similar variation of FT-IR spectrum is also observed in M-C dots with

oxidation treatment (Figure S7). In accordance, the intensities and shapes of the excitation spectra in I, II, and III changed too after surface modification (Figure 4b, S8). The above results indicate the different surface functional fragments with C-N/C-O/C=N/C=O groups can responsible for the multi-color emission. Whether there are emission in the graphite core is not known so far, if there are, their emission is buried in the surface emission. Then emission lifetime measurements were done to see whether there are emissions from the core. The M-C dots presents a three-exponential decay with the lifetime of 1.418 ns (18.14%), 4.214 ns (65.7%), 11.13 ns (16.16%) and the average lifetime is 6.64 ns. After introducing NaBH4 to reduce most of C=N/C=O groups in M-C dots, the photoluminescence (PL) lifetime of reduced M-C dots are


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converted to a double-exponential decay with the lifetime of 2.106 ns (55.98%), 6.788 ns (44.02%) and the average lifetime are shorten to 5.46 ns (Figure 4d). From the fact that there is not conserved lifetime after reduction, the emission from the core can be excluded. From the above study, we suggest multi-color emission all come form the surface states. Energy transfer also efficiently occurred between these emission domains, which the energy overlap between the donor and acceptor during energy transfer process can be seen by comparing the excitation spectra and the emission spectra of M-C dots (Figure S9). Then, single particle fluorescence images was adopted to further investigate the presence of energy transfer between these emission domains. High multiple emission overlap ratios from the same field of view in single M-C dots can be observed (Figure S10). This indicates that energy transfer from high energy emission domains to low energy emission domains has occurred.

Figure 4. Excitation spectra of M-C dots (a) and reduced M-C dots (b) monitored at various emission wavelengths (0.02mg/mL). (c) Infrared spectra of reduced M-C dots. (d) Photoluminescence decay profiles of M-C dots treated by NaBH4 at 400 nm excitation.


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In previous reports, the molecular states emission mechanism are suggested to be more related to these fascinating optical behaviors of C-dots.44-47 We also tried to find whether there are emissive molecular by-products in solvothermal preparation route. Dialysates have three differentiable absorption peaks at about 273 nm, 317 nm, 406 nm that corresponding to small molecular byproducts (Figure S11a, c). PL spectra show their emissions can also overlap the whole visible spectra, however, most emissions concentrate in the blue to green region (Figure S11b, d). Mass spectra was then used to reveal the types of partial by-products in the dialysate, which manifest that they are oxygen/nitrogen-related unsaturated small molecules (Figure S12). Interestingly, although the purified M-C dots have similar fluorescence spectra with dialysate, the slight shifts and distinction of band width in emission peaks can also be seen (Figure S13). The slight shifts in PL spectra may be caused by the integration of these molecular by-products on graphitic-like skeleton structure, and carbon core as well as energy transfer between these emission domains may contribute to the broader emission bands of M-C dots. On the basis of the above results, the multiple emissions of M-C dots have more relationships with multi-emission domains including different emissive graphitic fragments and fluorescent molecular residues on the surface. Thus the continuously adjustable multi-color emissions of M-C dots are also proposed to originate from both electron transitions of multiple emission domains separately and energy transfer between these emission domains (Figure 5).


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Figure 5. Schematic illustration for the emission mechanism of M-C dots. It’s worth mentioning that M-C dots exhibit low cytotoxicity (Figure S14) and also have high stability, whose optical property hardly change after month’s storage (Figure S15). Meanwhile, photostability tests indicate M-C dots possess good photostability after continuous UV illumination (12W) for seven hours (Figure S16). Taking these advantages of the intrinsic multicolor emissions, low cytotoxicity and high stability, they have great application potential in multicolor anti-counterfeiting inks.48-49 Here, the M-C dots mixed with commercial black inks are printed on a filter paper to show the badge of Jilin University. The printed pattern shows clear multi-color fluorescence under different light excitation in the fluorescence microscopy. Therefore, the excitation-dependent tunable photoluminescence property can make a better promise in high security information encryption purpose (Figure S17). Formation mechanism of M-C dots The formation mechanism of C-dots has been widely studied in previous researches. Qu et al. have reported the carbonization and dehydration process of C-dots with various surface functional groups from citric acid and urea.13 By varying different reaction solvents, the formation process of these C-dots can also be influenced. In our work, the possible formation


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mechanism of M-C dots was further demonstrated.26 CHCl3 is a precursor of dichlorocarbene by eliminating HCl in the presence of alkaline DEA and (NH4)2CO3. Then the generated dichlorocarbene as active intermediate undergoes linear addition, Diels-Alder cycloaddition and elimination reactions between dichlorcarbene itself to form fused aromatic rings. At the same time, as N resources, DEA and ammonia from the pyrolysis of (NH4)2CO3 undergo multiple reactions including addition, elimination, substitution and N-H insertion reactions into the fused rings formation process of dichlorcarbene. In addition, as important O resources, oxygen in atmosphere and water in reactive materials also have impacts on these formation process through substitution and oxidation reaction. With the further carbonization process, the graphitic carbon core of C-dots with oxygen/nitrogen functional groups are formed. And the carbonization process of the generated C-dots are strongly affected by the variation of reaction temperature or time (Figure S18).

Effect of inorganic salt (NH4)2CO3

Many previous researches have demonstrated the chemical, optical, electrical characteristics of C-dots can be improved through surface functionalization (tuning surface functional groups on C-dots) and doping (incorporating heteroatom into carbon core).50-53 In our system, we further studied the effect of inorganic salt incorporation on structure and properties of C-dots. The optical properties of the M-C dots differ a lot with the results that without using the inorganic salt of (NH4)2CO3 (denoted as dark carbon dots, D-C dots). Although the UV-vis spectrum of the D-C dots is similar to M-C dots and they also have multi-color emissions in similar range, their emission intensities are much weaker than the M-C dots (Figure S19). Detailed investigation reveals that the difference in the emission properties are not from the size or the core structure


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but from the difference in the content of heteroatoms of N and O. Both M-C dots and D-C dots have crystalline graphitic core from HRTEM images (Figure 2a, Figure S20a). Raman spectra also display the co-existence of G band (sp2-C) and D band (sp3-C) in both graphite-like structure (Figure S21). The average sizes of M-C dots and D-C dots are 8-9 nm similarly from TEM images (Figure 2b, Figure S20b). Elemental analyses (Table 1) show the carbon content decrease from 73.81% to 69.89% after introducing (NH4)2CO3, whereas, the nitrogen content doubled from 8.28% to 16.57% and the oxygen content decreases from 11.27% to 7.81%.

Table 1. Element analysis of M-C dots and D-C dots.

FT-IR and XPS measurements show that D-C dots are common in types of functional groups (such as C-N, C-O, C=N and C=O) with M-C dots, but there are absent of C≡N (Figure S22, S23). The generation of C≡N is caused by the introduction of (NH4)2CO3, and specifically from the ammonium, because introduction of inorganic base of Na2CO3 do not lead to the formation of C≡N, and introduction of NH3H2O can also result in the formation of C≡N (Figure S24). And the C-dots prepared by incorporating NH3H2O also exhibit the similar excitation-dependent multicolor fluorescence behaviors with (NH4)2CO3 (Figure S25). The formation of C≡N by dichlorocarbene with ammonium have been reported in literature by insertion and elimination reaction.54 Besides the C≡N, other types of N, such as C-N, C=N can also be formed by the introduction of (NH4)2CO3. Because we haven’t found any C≡N specific reaction that change


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C≡N to other functional groups, we still do not know how C≡N affect the emission. However, we speculate the introduction of cyano groups may provide multiple energy levels in electron transitions and result in the excitation-dependent tunable fluorescence behaviors of M-C dots. Although D-C dots have most types of the functional groups with M-C dots, the content of these functional groups are much less as indicated in the much narrower band width of C1s, N1s and O1s binding energy in XPS (Figure S23). Therefore, the diversified surface states and intrinsic multicolor emissions of M-C dots have close relationship with the introduction of inorganic salt. Antibacterial activity of M-C dots

To investigate the antibacterial effect of M-C dots on Gram-positive S. aureus and Gramnegative E. coli, the corresponding morphological evaluation of bacteria with and without the cultivation of M-C dots were obtained by Scanning electron microscopy (SEM). As shown in Figure 6, comparing with the control groups, bacteria cells with the treatment of M-C dots exhibited severely rough and destructive surfaces. Thus, M-C dots displayed antibacterial activity to a certain degree. In addition, the minimum inhibitory concentrations (MIC) of M-Cdots on S. aureus and E. coli are determined for 40 µg/mL by turbidity measurements (Figure S26). Many Studies have illustrated that surface charge on nanomaterials played a key role in inhibiting the growth of bacteria.55 Generally, the teichoic acids with negative charge on peptidoglycan layer of bacterial cell membrane structure are regarded as good active sites, which can promote the electrostatic interaction between the surface of bacterial cell and cationic substrates. In order to reveal the antibacterial mechanism of C-dots, the surface zeta potential of M-C dots and two types bacteria were measured, which are +29.8 mV, -27.1 mV and -28.6 mV respectively (Figure S27). Therefore M-C dots with positively charged could strongly interact


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with negatively charged bacteria via electrostatic interactions and further destroy the cell walls/membranes of bacteria.56 As we know, the quaternary ammonium groups have been widely adopted as an effective antibacterial structure.57 We further consider that the surface positive charge on M-C dots are originate from the quaternary ammonium moiety of M-C dots. The existence of quaternary ammonium groups in M-C dots were confirmed by alkaline treatment with NaOH. After removing the quaternary ammonium with NaOH, the surface zeta potential of M-C dots alter to negative, which is -11 mV (Figure S28). And the disappearance of infrared band at 971 cm-1 of quaternary ammonium in M-C dots can also be observed from FT-IR analysis (Figure S29).58-59 The formation of quaternary ammonium groups on M-C dots are possibly originate from the N-H insertion reactions between dichlorcarbene-related intermediates and DEA or ammonia. Due to the change of different surface states on M-C dots treated by NaOH, the multi-color emission behaviors of M-C dots can also be affected, which is consistent with the above results on reduction and oxidation treatment (Figure S30). Because of the formation of quaternary ammonium groups, the D-C dots also have similar anti-bacterial activity with surface zeta potential +26.3 mV (Figure S31). Then, we investigated the effect of generated reactive oxygen species (ROS) on bacteria growth. By using 9,10-anthracenediylbis(methylene)dimalonic acid (ADMA), a response probe for ROS, the generation of ROS was efficiently monitored. Owing to the photodegradation caused by generated ROS, the absorption of ADMA from bacterial after M-C dots treatment is reduced, which is compared with the bacterial without M-C dots treatment. Therefore, ROS was also considered as a possible inducement for anti-bacterial (Figure S32). The above fractions demonstrate that M-C dots should be a promising candidate for antibacterial application.


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Figure 6. SEM images of E. coli (a, b) and S. aureus cells (c, d) without (left) and with (right) the cultivation of M-C dots, scale bar= 500 nm.

CONCLUSIONS In summary, an inorganic salt of (NH4)2CO3 was applied as a new resource to synthesize excitation-dependent M-C dots.with solvothermal method. Introduction of (NH4)2CO3 increases the N contents and enhance the quantum yields of emission. It also brings in new surface functional group of C≡N, which will provide new reaction pathways of surface modification. Detailed mechanistic study reveals multiple surface emissive domains were definitely responsible for the intrinsic multi-color emission of M-C dots. Quaternary ammonium groupscontaining M-C dots exhibit antibacterial activity towards Gram-positive and negative bacteria via strongly electrostatic interactions between M-C dots and bacteria. We believe this work will inspire more work on introducing inorganic species in C-dots and bring in new structures, properties and understanding on the mechanism of emission.


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METHODS AND EXPERIMENTAL SECTION Preparation of M-C dots.

Typically, M-C dots were prepared as follows: 10 mL CHCl3, 10 mL DEA and 0.5 g (NH4)2CO3 were added into an autoclave and heated at 160 oC for 12 h. After reaction, the products were cooled naturally. A transparent brown-black solution with a large quantity of salts precipitated at the bottom was obtained. The up layer solution was dried under vacuum to remove unreacted CHCl3 and DEA. The resulting products were purified through dialysis in water first for 12 hours and then in ethanol for 6 hours to remove small molecules or salts (weight cutoff of 3500 Da). At last, the products in the dialysis tube were dried under vacuum to get M-C dots powder. The yield of M-C dots was about 11.63 mg/mL CHCl3.

Cytotoxicity test.

The biocytotoxicity of M-C dots is exhibited through the standard MTT method. Human breast cancer cells MCF7 were seeded at a density of 10000 cells/well in 96-well U-bottom plates, and were preincubated for 24 h in a DMEM medium containing 10 % FBS (37 oC, 5 % CO2). Then for another 24 h by dissolving M-C dots in DMEM medium and adjusting to the required concentration (0、 2、 5、 10、 20、 50、 100 µg/ mL). And 0.5 mg/mL MTT solution was added to each cell well for further incubating 4 h, followed by discarding the culture medium with MTT. Then 150 uL DMSO was transferred to every cell well and shaken the plates for 10 min. The optical density (A) of the solution was performed at 492 nm. The cell viability was evaluated as the following equation: cell viability = A/A0 ×100 %, where A is the absorbance of the experimental group (A was obtained in the presence of M-C dots) and A0 is the absorbance of the control group (A0 was obtained in the absence of M-C dots).


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ASSOCIATED CONTENT Supporting Information AFM image, 1H NMR spectra, cytotoxicity test of M-C dots, Fluorescence emission and excitation spectra, FT-IR spectrum of M-C dots treated with reduction, oxidation and hydrolysis, Mass spectra of dialysate, FT-IR spectrum, emission and absorption spectrum, TEM images, Raman spectra, XPS data of D-C dots, Zeta potential of M-C dots, E. coli and S. aureus cells

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID

Minjie Li: 0000-0002-1458-5690 Sean Xiao-An Zhang: 0000-0002-8412-3774 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the financial support of NSFC Grant No. 21574058 for Dr. Li.


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Inorganic Salt Incorporated Solvothermal Synthesis of Multicolor

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