Simple Microwave-Assisted Synthesis of Amphiphilic Carbon

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Article

Simple Microwave-Assisted Synthesis of Amphiphilic Carbon Quantum Dots from A/B Polyamidation Monomer Set 3

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Yujin Choi, Seongho Jo, Ari Chae, Young Kwang Kim, Jeong Eun Park, Donggun Lim, Sung Young Park, and insik in ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06066 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Applied Materials & Interfaces

Simple Microwave-Assisted Synthesis of Amphiphilic Carbon Quantum Dots from A3/B2 Polyamidation Monomer Set Yujin Choi1, Seongho Jo1, Ari Chae1, Young Kwang Kim1, Jeong Eun Park4, Donggun Lim*1,4, Sung Young Park,*1,3 and Insik In*1,2 1

Department of IT Convergence (Brain Korea PLUS 21), Korea National University of Transportation, Chungju 27909, Republic of Korea Department of Polymer Science and Engineering, Korea National University of Transportation, Chungju 27909, Republic of Korea 2

Department of Chemical and Biological Engineering, Korea National University of Transportation, Chungju 27909, Republic of Korea. 3

Department of Electronic Engineering, Korea National University of Transportation, Chungju 27909, Republic of Korea. 4

ABSTRACT Highly fluorescent and amphiphilic carbon quantum dots (CQDs) were prepared by microwave-assisted pyrolysis of citric acid and 4,7,10-trioxa-1,13-tridecanediamine (TTDDA), which functioned as an A3 and B2 polyamidation type monomer set. Gram quantities of fluorescent CQDs were easily obtained within 5 min of microwave heating using a household microwave oven. Because of the dual role of TTDDA, both as a constituting monomer and as a surface passivation agent, TTDDA-based CQDs showed a high fluorescence quantum yield of 29% and amphiphilic solubility in various polar and non-polar solvents. These properties enable the wide application of TTDDA-based CQDs as non-toxic bioimaging agents, nanofillers for polymer composites, and as down-converting layers for enhancing the efficiency of Si solar cells. KEYWORDS: Carbon quantum dots; Microwave-assisted pyrolysis; TTDDA; Amphiphilic, Polymer composites, Bioimaging, Solar cells

1. INTRODUCTION 1 ACS Paragon Plus Environment


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In view of their interesting fluorescence performances coupled with their non-toxicity in physiological media, carbon quantum dots (CQDs) have been intensively researched.1-3 For example, the high fluorescence quantum yield (QY) and multi-color emission of CQDs enable them to be exploited as efficient bioimaging agents to visualize cellular media.4 Both topdown and bottom-up methods have been widely studied for the mass production of CQDs for various applications, where they are used as bioimaging agents, nanocatalysts, light-emitting devices, solar harvesting materials, and sensors.5,6 The bottom-up approach is preferred to top-down methods with respect to productivity and material safety, because gram scale synthesis of CQDs can be achieved using low-cost and environment-friendly materials.7-9 An external energy input is indispensable for constructing a carbon framework, which is one of the essential features in fluorescent CQDs. Photochemical,10,11 thermal,12,13 chemical (acid/base),14-16 or hydrothermal treatment17-19 is typically required for the synthesis of fluorescent CQDs from small natural compounds, organic molecules, polymers, carbon-rich sources. Recently, another simple bottom-up method making use of a household microwave oven has been developed for the synthesis of soluble and fluorescent CQDs from small molecules including carbohydrates such as glucose, and polyamidation monomer sets such as citric acid (A3, A: -COOH) and urea (B2, B: -NH2). The time of synthesis is typically within 10 min of microwave operation and produces CQDs with a QY of up to 14%.20-22 Most CQDs reported in previous studies are soluble in polar solvents such as water and alcohol due to the presence of highly oxidized surface functionalities including hydroxyl and carboxylic acid groups. For an example, extraction of CQDs from food caramels has been successfully attempted in aqueous phase.23 In order to impart organic solubility to CQDs, a suitable surface modification is required, mainly through an amidation reaction between the amine-rich surface passivation agent and the highly oxidized surface functionalities of CQDs.24-28 This solubility switching limits the bio-application of organo-soluble CQDs 2 ACS Paragon Plus Environment

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because most physiological media are water-based. Therefore, the preparation of CQDs with amphiphilic solubility in both aqueous and organic media could play an important role in widening the application area of CQDs.28,29 However, fluorescence QY’s of CQDs showing amphiphilic solubility in the previous literature are often equal or less than 20%,30 which might hinder the facile applications of CQDs in practical usages. In this study, the synthesis of amphiphilic CQDs was attempted by adopting the polyamidation monomer set as the starting material. In addition to the A3/B2 type monomer set described above,21,22 the AB2 type monomer (L-lysine) and A2/B3 type monomer set (succinic acid and tris(2-aminoethyl) amine) have been successfully utilized for the synthesis of highly water-dispersible and strongly fluorescent CQDs with a fluoresce QY up to 49.9% through simple microwave heating.31,32 To achieve amphiphilic solubility, it is much more productive to use either the A3/B2 or the A2/B3 type monomer set because, in these cases, moieties that promote amphiphilicity can be easily incorporated into one of the Ax (x2) and By(y2) type monomers. However, the incorporation of amphiphilic moieties into either AB2 or A2B (L-glutamic acid for example) type monomer requires complex synthetic procedures. 4,7,10-trioxa-1,13-tridecanediamine (TTDDA), an oligo-ethylene glycol (EG)-containing diamine, is an ideal B2 type monomer owing to the presence of EG moieties; EG-containing oligomers or polymers present amphiphilic solubility both in aqueous and organic media.34,35 Therefore, it is very important to evaluate the microwave-assisted synthesis of CQDs from TTDDA as a EG-containing B2 type monomer and another A3 type monomer (citric acid (CA) in this study). In addition, the utilization of TTDDA, a well-known surface passivation agent, can lead to the enhancement of CQD fluorescence through surface passivation via reaction with carboxylic acid groups present on the surface of the weakly fluorescent CQDs.14,36 Finally, the covalent incorporation of EG into CQDs can endow additional physiological nontoxicity to CQDs, being similar to the covalent grafting of poly(ethylene glycol) (PEGylation).37,38 Considering all the above aspects, the resulting TTDDA-based CQDs can 3 ACS Paragon Plus Environment


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present both strong down-conversion fluorescence behavior and can be applied as ultraviolet (UV)-to-visible (Vis) light converting top layers in traditional Si solar cells. Due to their amphiphilic solubility, these materials are applicable as fillers in composite films or fibers with commodity polymers, and as non-toxic bioimaging agents to precisely map biological tissues (Scheme 1).

+ TTDDA

CA

Amphiphilic TTDDA-based CQDs

Fluorescent Composite

Bioimaging

Down-converting layer UV Vis

PMMA

c-Si Solar Cell Al Scheme 1. Illustration for the synthesis of TTDDA-based CQDs from CA and TTDDA and their applications.

2. EXPERIMENTAL SECTION 2.1 Materials and characterization

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TTDDA (97%), CA (≥99.0%), and poly(methyl methacrylate) (PMMA, average molecular weight of up to 120,000 Da) were purchased from Sigma-Aldrich Corp. (USA) and used without further purification. All organic solvents including chloroform (99.5%) and N,Ndimethylformamide (DMF, 99.9%) were purchased from Samchun Chemical Corp. (South Korea). Molecular weight cut-off membrane (MWCO, 3,500 Da) was purchased from Spectrum Laboratories, Inc. UV-Vis spectra were obtained with Optizen Alpha UV-Vis Smart Spectrometer of Mecasys (Republic of Korea). Photoluminescence (PL) spectra were obtained using a Fluoro Mate FS2 luminescence spectrometer. Atomic force microscope (AFM) images were obtained using a Multimode-N3-AM Scanning Probe Microscope of Bruker. Field emission scanning electron microscope (FE-SEM) images were obtained from JSM-7610F Ultra high resolution field emission scanning electron microscope of JEOL. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained from a Tecnai F30 ST field emission transmission electron microscope of FEI Corporation. 1H NMR and

13

C NMR

spectra were obtained using a Avance 400FT 400 Hz spectrometer of Bruker with deuterium oxide (D2O) as the solvent. Raman analysis was done with a RFS 100/S Raman scope of Bruker with the excitation lasser wavelength of 1,064 nm to inhibit strong fluorescence signals from CQDs. X-ray photoelectron spectroscopy (XPS) spectra were obtained by using Sigma Probe Multi Purpose XPS (Thermo VG Scientific Co.). The multimode microplate reader (Varioskan flash, Thermo Fisher Scientific) was used for MTT assay and quantitative cellular accumulation. Confocal microscopy image analysis was performed with a LSM510 confocal laser scan microscope (Carl Zeiss, Germany) with the excitation laser wavelength of 405, 488, 543 nm for blue, green and red fluorescence image, respectively. EVOS FL fluorescent microscope (Thermo Fisher Scientific) was observed to analysize the fluorescence images of nanofibers of CQD/PMMA nanocomposites. 5 ACS Paragon Plus Environment


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2.2 Synthesis of TTDDA-based CQDs 3 g (1 eqv.) of CA and 6.88 g (2 eqv.) of TTDDA were solubilized in 10 mL of deionized water by brief ultrasonic treatment using a bath sonicator (300 W). After loosely covering the top of the flask with a polyethylene wrapping film, the 250 mL flask containing the aqueous solution of CA and TTDDA was exposed to microwave heating for 5 min in a 700 W household microwave oven, resulting in dark brown solids (The whole microwave oven needs to be kept inside the fume hood during the synthesis of CQDs so as avoid exposure to organic or gaseous volatiles possibly formed during the microwave treatment!) After cooling to room temperature, an additional 100 mL of deionized water was added into the flask and a solution of TTDDA-based CQDs that was strongly blue-emitting under UV light was extracted from the dark brown residue by ultrasonic treatment in a bath sonicator (300 W) for 60 min (Figure S1). Any insoluble dark brown solid residues present were removed from the aqueous solution of CQDs by ultracentrifugation at 5,000 rpm for 30 min. Next, the optically clear CQD solution was transferred into a MWCO tube and dialyzed for three days. Finally, powders of CQD1 were obtained after freeze-drying the dialyzed solution. TTDDA-based CQDs with CA to TTDDA molar ratios of 1:3 (CQD2) and 1:4 (CQD3) were prepared in a similar fashion. The maximum mass yield of 66.4% was obtained in the synthesis of CQD1 with a CA to TTDDA molar ratio of 1:2; 6.56 g of purified powders of CQDs were obtained from single microwave-treated batch (Table S1). 2.3. Fluorescence lifetime imaging of TTDDA-based CQDs Fluorescence lifetime imaging (FLIM) was carried out using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany, KBSI Daegu Center) with a 60x (water) objective.39 A single-mode pulsed diode laser (379 nm with ~30 ps pulse width and ~10 W laser power) was used as the excitation source. A dichroic mirror (Z375RDC, AHF), a longpass filter (HQ405lp, AHF), a 75 m pinhole, a band-pass filter, and an avalanche photodiode detector (PDM series, MPD) were used to collect the emitted radiation from the 6 ACS Paragon Plus Environment

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luminescent films. Time-correlated single-photon counting (TCSPC) technique was used to count fluorescence photons. FLIM images with 200×200 pixels were recorded using timetagged time-resolved (TTTR) data acquisition. The acquisition time was 1 ms for each pixel. Exponential fitting of fluorescence decay curves, extracted from the FLIM images, was performed using Symphotime software (version 5.3). 2.4 TTDDA-based CQD/PMMA nanocomposite films or fibers CQD/PMMA nanocomposites in the form of thick film or nanofibers were prepared by solution mixing. First, nanocomposite films with thicknesses of ~60 m were prepared by solution casting a mixture of PMMA (5 g) and CQD1 (50 mg) in 15 g of chloroform on petri dishes. After the complete evaporation of solvents, optically clear and bright brown nanocomposite films were obtained in the form of free-standing films. Secondly, nanocomposite fibers with diameters of around 700 nm were prepared by electrospinning (eSrobot, NanoNC Corp., South Korea). 1.667 g of PMMA and 32 mg of CQD1 were dissolved in 5 g of DMF. The resulting CQD/PMMA mixture was loaded in a 5 mL plastic syringe with needle of #24 gauge that was connected to a voltage supply to apply a high voltage of 20 kV. The distance between the needle tip and the aluminum collector was 10 cm. The feed rate of CQD/PMMA solution was 2 mL/hr during the electrospinning process. The morphology of CQD/PMMA nanofibers was examined by fluorescent microscopy and FE-SEM. 2.5 Cytotoxicity and cellular imaging The cell viability of TTDDA-based CQDs was measured by the [3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl-2H-tetrazolium bromide] MTT assay method. 200 L of MDAMB-231 or MDCK cells, with a density of 1105 cells/mL, was placed in individual wells of a 96-well plate. Next, the cells were incubated for 24 h at 37 oC in a humidified 5% CO2 atmosphere. To determine the cellular viability, a stock solution of CQD1 was dissolved in RPMI medium at a concentration of 1 mg/mL and the stock solution was diluted to 0.001 mg/mL The medium 7 ACS Paragon Plus Environment


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was removed and the cells were treated with CQD1 solutions of different concentrations. The cells were then incubated for another 24 hrs as previously described. The media containing TTDDA-based CQDs were then replaced with 180 mL of a fresh medium and a 20 L stock solution containing 15 mg of MTT in 3 mL PBS, and incubated for another 4 hrs. Finally, the medium was removed and 200 L of a solution of MTT solubilizing agent was added to the cells followed by vigorous shaking for 15 min. The absorbance was measured at 570 nm using a microplate reader. The relative cell viability was measured by comparing with a 96well control plate containing only cells. The cellular uptakes of the composite materials were analyzed by confocal imaging. MDAMB-231 or MDCK cells were plated over a cover slide on an eight-well plate at a density of 2105 cells per well and incubated for 24 hrs at 37 ℃ in a humid 5% CO2 containing atmosphere. Afterwards, the cells were treated with 1 mg/mL CQD in a fresh culture medium with pH between 5.0 and 8.0. The pH of the medium was adjusted by adding solutions of 0.1 N HCl or 0.1 N NaOH. After incubation for 1 h at 37 ℃, the cells were washed several times with ice-cold PBS and fixed with fresh 4% (w/v) formaldehyde solution at room temperature. Following cell viability measurements, confocal images were obtained to confirm stained cells (cells imaging) by using confocal laser scan microscope (CLSM) at 10 magnification). 2.6 Down-conversion top coating layer for a p-type Si solar cell Crystalline Si (c-Si) solar cells used in this experiments were prepared as reported in the literature.39 Czochralski p-type (100) Si wafers ( ~200 m thickness and resistivity of ~1.5 •cm) were used in the experiment. The cut wafers were etched with KOH to remove any surface damage caused by the saw wires during cutting. After removing surface damage, various texturing methods including general anisotropic etching were performed, such as etching with a mixture of KOH and isopropyl alcohol, reactive ion etching, and Ag catalytic 8 ACS Paragon Plus Environment

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etching. It was prepared with an n-type emitter of 60 /square achieved using common POCl3 diffusion. Phosphosilicate glass layers on the surfaces of the wafers were removed by dipping in buffered oxide etching solution for 1 min. Plasma enhanced chemical vapor deposition (PECVD) was used to deposit a 76-nm-thick silicon nitride (SiNx) layer on the front surface at 300 ℃ for passivation, which functioned as an antireflection coating. The refractive index of the SiNx films was maintained at 1.95. Metallization was carried out by screen printing using common Ag paste for the front side and Al paste for the back side. Metal contacts were formed by rapid thermal process (RTP) firing with a peak temperature ~640 ℃. CQD1 was coated by spin casting (3,000 rpm, 60 s) a 0.3 wt.-% methanolic solution of CQD1 directly on the surface of as-formed c-Si solar cells. Both internal quantum efficiency (IQE) and external quantum efficiency (EQE) values of the c-Si solar cells with or without CQD top layer were measured in the wavelength range 300–1100 nm under 1 sun intensity (AM 1.5G) using K3100 Incident photon-to-current efficiency (IPCE) from McScience Inc.

Figure 1. a) UV-Vis spectra of aqueous solutions (0.1 mg/mL) of TTDDA-based CQDs and b) PL spectra of aqueous solution of CQD1 with different excitation wavelength (insets are photo images of CQD1 in the daylight or under dark with 365 nm UV irradiation).

3. Results and Discussion 9 ACS Paragon Plus Environment


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The synthesis of CQDs was performed by the simple microwave-assisted pyrolysis of CA and TTDDA as A3 and B2 type polyamidation monomers in a household microwave oven following a previously published procedure (Figure S1).32,33. UV-Vis spectra of TTDDAbased CQDs showed peaks at 210 nm due to  transition of C=C moieties and at 300 nm due to n transition of C=O moieties (Figure 1a). In the low energy absorption range > 400 nm, the optical absorption of CQDs was negligible. Interestingly, the absorption intensity of the n-* transition of CQDs decreased with increase in TTDDA equivalent (CA to TTDDA molar equivalent ratio of 1:2, 1:3, and 1:4 for CQD1, CQD2, and CQD3, respectively), possibly revealing that lateral dimension of TTDDA-based CQDs is decreased.41 Fluorescence spectra of all TTDDA-based CQDs were very similar in their shape and position. The maximum emission peak was observed at 440 nm (blue) under UV irradiation (360 nm) (Figure 1b). Upon excitation with 440 nm light, the maximum emission peak appeared at 510 nm (green), showing that the fluorescence emission of TTDDA-based CQDs is dependent on excitation wavelength as reported for most CQDs; this has been attributed to surface-defectrelated fluorescence.19 In the case of CQD3, a slight blue-shift (3–13 nm) of the emission peaks was observed depending on the excitation wavelength (Figure S2). Fluoresce QYs of TTDDA-based CQDs (determined by a comparative method using quinine sulfate in 0.1 M H2SO4, which showed a QY of 54%) continuously decreased (29%, 20.6%, and 8.6% for CQD1, CQD2, and CQD3, respectively) with increase in TTDDA equivalent. Increasing or decreasing of CA to TTDDA molar ratio out of the ratio tested in this work dramatically reduced both the mass yield and QY of obtained CQDs.33 Fluorescence QYs of TTDDAbased CQDs are lower than those for other CQDs prepared by hydrothermal treatment of polyamidation monomers,18 but evidently exceed those of typical CQDs prepared by microwave treatment of glucose (Table S1).20 Next, the photo-bleaching stability of the TTDDA-based CQD was examined by FLIM. A highly diluted aqueous solution (1 ng/mL) of 10 ACS Paragon Plus Environment

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CQD1 was spin coated on a cleaned glass slide. FLIM image of TTDDA-based CQDs distributed on the glass slide showed the presence of both networks of CQD aggregates and individual CQDs distributed over the entire glass substrate (Figure 2a). Very interestingly, it was observed that the greater the lateral size of CQD aggregates, the more uniform was the average fluorescence decay time (avg). Networks of CQD aggregates showed uniform avg in the range 4–7 ns and individual CQDs (partially single particles) showed different avg values ranging from 1 to 20 ns (Figure 2b). Recently, it was shown by single-molecular FLIM analysis of CDs that the presence of excitonic H-aggregates functioned as integral quantum emitters.42 The broad absorption/emission spectra and the large Stokes shift of TTDDA-based CQDs in solution state also match with the typical optical properties of H-aggregates. We believe that the integrated emission properties of CQD aggregates may have a similar origin. In addition, the fluorescence intensities of both CQD aggregates and individual CQDs were traced with respect to their time trajectories. CQD aggregates showed fast photo-bleaching within 10 s followed by continuous on-off intermittency (photoblinking) of fluorescence (Figure 2a). In the case of individual CQDs, photoblinking was observed even up to 10 min of laser irradiation (Figure 2b). Therefore, it is estimated that the initial photobleaching in the case of CQD aggregates is related to the initial desaturation of excitonic H-aggregates by laser excitation. Actually, CQD(s) with a lower avg (2.60 ns) showed a nonlinear exponential decay profile. Multi-exponential tailing with three components of 1, 2, and 3 of 0.77, 2.55, and 7.76 ns, respectively, was observed (Figure S3), showing that the fluorescence decay of TTDDA-based CQDs might result from the significant electronic interaction between the emission sites of individual CQDs or between excitonic H-aggregates.43,44



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Figure 2. a) FLIM image of CQD1 deposited on glass slide and b) magnified FLIM image of a) in the background region with the time-dependent fluorescence intensity profile of each FLIM image in the bottom.

TTDDA-based CQDs with diameters less than 10 nm were clearly observed by TEM (Figure 3a). The HRTEM image and selected area electron diffraction (SAED) pattern of CQD1 revealed that TTDDA-based CQDs are composed of an amorphous carbon framework with lattice spacing of 0.21 nm, which corresponds to the (100) in-plane lattice of graphene (Figure 3b).45 Also, lateral dimension of TTDDA-based CQD ranging 3~8 nm was clearly observed in HRTEM image. CQD2 and CQD3 also showed similar lattice spacing but had smaller diameters of 3–6 nm and 1–3 nm (Figure S4), respectively, which is in accordance with the UV-Vis analysis described above. AFM images of CQD1 also revealed the dot-like morphology of TTDDA-based CQDs with vertical dimensions in the range of 3.8–11 nm, in accordance with TEM data (Figure 3c). Raman analysis of TTDDA-based CQDs was 12 ACS Paragon Plus Environment

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performed using an excitation laser wavelength of 1064 nm to minimize strong fluorescence signals from CQDs. The Raman spectrum of CQD1 did not show clear D and G bands, which are often observed for other CQDs or in most graphene quantum dots (GQDs), whose lattice structure is graphene-like (Figure S5).46 Instead, Raman peaks due to aliphatic moieties are clearly observed at 265 cm-1 ((C-C) aliphatic chains), and at 1286 cm-1 ((C-C) alicyclic, aliphatic chains), while Raman peaks of aromatic moieties appear at 1481/1592 cm-1 ((C-C) aromatic ring chain vibration) and 2861/2931 cm-1 ((C-H) aliphatic, aromatic),47 showing that TTDDA-based CQDs have optical properties characteristic of both (hetero)aromatic and aliphatic chains (mainly from TTDDA).

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Figure 3. a) TEM image of CQD1 on carbon coated Cu grid, b) HRTEM image of CQD1 and SAED pattern, and c) AFM image of CQD1 and the height profiles of the selected lines in AFM image. 13 ACS Paragon Plus Environment


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X-ray diffraction profiles of TTDDA-based CQDs showed no graphitic scattering peak at 0.34 nm.48 Instead, broad scattering peaks centered at 0.63–0.66 nm were observed (Figure S6). All these structural analysis data demonstrate that TTDDA-based CQDs possess nongraphitic (or amorphous) carbon framework both in the core and on the surface. Actually, the presence of -conjugated domains in the core of CQDs is not a prerequisite for obtaining highly fluorescent CQDs. Surface defect sites in the amorphous regions on the surface of CQDs are considered to be responsible for the strong fluorescence since they serve as capture centers for excitons.49,50 In-situ polymerization and carbonization through microwave treatment of CA and TTDDA used as A3 and B2 type polyamidation monomers (as performed in this study), are considered to be very effective for the synthesis of highly fluorescent CQDs. This is because control of the surface defect sites is achieved by the incorporation of TTDDA, a well-known surface passivation agent,14 into the carbon lattice framework of CQDs. Another important aspect in the combination of A3 and B2 type monomer set is the presence of a branching point in at least one monomer.32 For comparison, it is interesting to note that the microwave treatment of succinic acid (A2 type monomer) instead CA (A3 type monomer) together with TTDDA failed to produce fluorescent CQDs under similar conditions. Therefore, the branching point in the monomer set seems to play a key role in the formation of a carbonized CQD framework by tailoring the spatial arrangement of the reacting moieties during the CQD formation step. To effectively construct a CQD framework with a 3dimensional morphology, branched or hyperbranched polycondensates are much more efficient

than

linear

polycondensates,

because

the

subsequent

dehydration-based

carbonization is more feasible in polycondensates with a branched morphology. Actually, A2/B3 or A3/B2 type monomer sets form insoluble gels or soluble hyperbranched polymers depending on the initial feeding ratio of the monomers and the monomer concentration.51 It can be therefore concluded that the microwave-assisted synthesis of CQDs from polyamidation monomer(s) requires the presence of branching point(s) in the monomer(s). 14 ACS Paragon Plus Environment

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a)

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O1s (23.9 at.-%)

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N1s (5.9 at.-%)

Raw data Fitted line C-C/C=C (284.5 eV) C-N/C-O (286.0 eV) C=O (287.7 eV)

C1s

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d)

94.8%

Raw data Fitted line Amino N (399.3 eV) Pyrrolic N (400.8 eV)

Raw data Fitted line C=O (532.0 eV) C-O (530.6 eV)

O1s

87.5%

N1s

12.5%

5.2%

526 528 530 532 534 536 538 540 Binding Energy (eV)

396

398

400

402

404

Binding Energy (eV)

Figure 4. a) XPS survey scan of CQD3 and XPS high resolution b) C1s, c) O1s, and d) N1s binding peaks of CQD3.

Next, the compositional features of TTDDA-based CQDs were examined in detail by XPS analysis. The XPS survey scan of CQD1 revealed that the carbon content is ~70.1 at.% (Figure 4a).19 The remaining elements are nitrogen (5.9 at.%) and oxygen (23.9 at.%). With increasing TTDDA content, increased nitrogen inclusion up to 7.0 at.% was observed in the case of CQD3 (Figure S7a). Elemental analysis of CQD3 showed the presence of 52.9, 9.0, 8.9, and 29.2 at.% (calculated) of carbon, nitrogen, hydrogen, and oxygen, respectively, which roughly matches the XPS analysis presented above. The high resolution C1s binding peak in CQD1 showed the presence of 61.2% of CC/C=C, 31.0% of CO/CN, and 7.8% of C=O binding (Figure 4b). In the case of CQD3, the peak due to CO/CN binding energy dramatically increased to 52.3% accompanied by a decrease in CC/C=C binding peak to 15 ACS Paragon Plus Environment


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39.5% (Figure S7b). Thus, it is clear that the increase in TTDDA content induces a much higher incorporation of nitrogen into CQDs. High resolution O1s binding peaks of CQD3 showed the presence of 92.7% of CO together with 7.3% of C=O, as compared to 94.8% of CO together with 5.2% of C=O binding in the case of CQD1 (Figure 4c and Figure S7c). The high percentage of CO bonds in CQD3 originates from the oligo-polyethylene glycol (oligo-PEG) chains of TTDDA, which are incorporated into the carbon framework of the CQD (Figure 4c). High resolution N1s binding peaks of CQD1 showed the presence of 87.5% of amino N together with 12.5% of pyrrolic N (Figure 4d). The presence of oligo-PEG moieties in TTDDA-based CQDs was confirmed by both 1H- and

13

C-NMR spectra of

TTDDA-based CQDs (Figure 5). Unfortunately, the formation of amide linkages could not be clearly detected. Only a weak peak due to carboxyl groups of CA could be observed at around 175 ppm in the 13C-NMR spectrum of CQD1.18 The characteristic peaks of oligo-PEG chains from TTDDA were preserved in the NMR spectra of the CQDs. Therefore, it seems that some oligo-PEG chains of TTDDA are intact even after the microwave-assisted pyrolysis process. However, the major functional groups on the surface of TTDDA-based CQDs are considered to be carboxyl or hydroxyl groups instead of amine groups because the determination of zeta potentials of TTDDA-based CQDs revealed values of -20.4 and -16.5 mV for CQD1 and CQD3, respectively (Figure S8). The less negative zeta potential value of CQD3 supports the conclusion that CQD3 has a smaller amount of carboxyl or hydroxyl groups when compared to CQD1 owing to the higher TTDDA feeding ratio, which in turn results in a much greater inclusion of amide functionalities on the surface of CQD3. FT-IR spectra of CQDs clearly showed the formation of amide C=O stretching modes at 1639 cm-1 (Figure S9). Based on the above discussion, the optimum molar ratio of CA and TTDDA to obtain the highest values of both fluorescence QY and mass yield is 1:2 in the case of CQD1. Further decrease in TTDDA amounts resulted in very low yields of CQDs, showing that the facile microwave-assisted 16 ACS Paragon Plus Environment

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synthesis of CQDs requires the utilization of both carboxylic acid and amine functionalities present in the polyamidation monomer set. In contrast, microwave-assisted CQD synthesis from saccharides such as glucose proceeds through dehydration-based carbonization of neighboring

or

vicinal

hydroxyl

groups

of

saccharides

and

the

self-

condensation/carbonization of the Ax type monomer is much more feasible.20

a)

b)

D2O

4

1 3

2

1 4 a

3

CA

CA 2

3, 4, 5 c

a b

3

1

d

1

4

2 2

e

TTDDA

5

TTDDA

d, e a b

CQD1

CQD1

6

4

2

200

0

150

ppm 1

100

50

0

ppm

13

Figure 5. a) H- and b) C-NMR spectra of CA, TTDDA, and CQD1 (40 mg/mL in D2O as NMR solvent). The incorporation of TTDDA, which contains oligo-EG segments, into the carbon frame of the CQDs leads to a dramatic solubility switch, rendering TTDDA-based CQDs amphiphilic.29 TTDDA-based CQDs are highly soluble in polar solvents such as water, methanol, and ethanol and even in less polar solvents such as chloroform and tetrahydrofuran (Figure S10). Interestingly, the fluorescence intensity of TTDDA-based CQDs is enhanced in methanol, ethanol, and chloroform with blue shift of the emission maximum, which might 17 ACS Paragon Plus Environment


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have originated from the different extents to which the surface defects are stabilized in different solvents, or due to the desaturation of excitonic H-aggregates in non-aqueous solvents.31,42 The emission maxima of TTDDA-based CQDs shift to lower wavelength in all the tested solvents. In particular, the PL intensity dramatically increases in methanol, ethanol, and chloroform. The decreased PL intensity in acetone, THF, and toluene could result from the partial solubility of TTDDA-based CQDs in these less polar solvents. While denaturation of excitonic H-aggregates could explain the fluorescence behavior of TTDDA-based CQDs in non-aqueous solvents, a more detailed microscopic analysis of TTDDA-based CQDs in organic media is required. Most CQDs and GQDs are rich in polar functional groups such as hydroxyl and carboxylic acid groups on their surfaces and are easily soluble in aqueous or alcoholic media. Appropriate surface functionalization (typically accompanied by surface passivation) is a prerequisite to achieve solubility in organic solvents. Amines having bulky organic moieties or long alkyl groups have been utilized for this purpose and the resulting passivated CQDs lose water solubility but are soluble in organic solvents.26 The amphiphilic solubility of TTDDA-based CQDs could dramatically expand the application areas of CQDs because of their increased suitability to different processing conditions.


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Figure 6. a) Photo images of CQD1/PMMA composite film in the daylight or under dark with UV irradiation, b) excitation-dependent fluorescence images of CQD1/PMMA composite film (excitation wavelength: 35720, 47011, 53020 nm, emission cut-off wavelength: 44730, 52521, and 59320 nm, respectively, from left to right), c) FE-SEM images of porous CQD1/PMMA composite films after dipping in boiling water, d) photo images of CQD1/PMMA composite nanofibers in the daylight or under dark with UV irradiation, e) FESEM images of CQD1/PMMA composite nanofibers, and f) merged multi-colored images of bright filed and fluorescence images of CQD1/PMMA composite nanofibers according to the excitation and emission cut-off according to the parameter of b).

The solubility of TTDDA-based CQDs in organic media such as chloroform and DMF prompted us to attempt the construction of fluorescent CQD/polymer nanocomposites (films or nanofibers). Transparent PMMA composite films having 5 wt.% of TTDDA-based CQDs 19 ACS Paragon Plus Environment


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were prepared by simple solution casting of PMMA and CQD in chloroform (Figure 6a). These composite films also showed the characteristic multi-color fluorescence of CQDs. With the modulation of excitation wavelength, blue, green, or red fluorescence was roughly observed by fluorescence microscopy (Figure 6b). When immersed in boiling water, 20% of the included CQDs were released from the composite film into the aqueous phase, leading to a semi-transparent CQD/PMMA composite film showing clear blue fluorescence under UV light (Figure S11). The FE-SEM image of the semitransparent composite film showed the presence of significant amounts of micro-pores (1~6 um) and nanopores (80–150 nm), which originate from the partial extraction of CQDs (Figure 6c). The resulting fluorescent and porous CQD/PMMA composite film having significant amounts of internal pores could be useful as novel polymer-supported fluorescent CQD sensors or membranes. Nanofibers of TTDDA-based CQD/PMMA composite were also prepared by simply electrospinning DMF solution containing both PMMA and CQD1 (1.9 wt.%) (Figure 6d). 51 The resulting cotton of composite nanofibers revealed clear blue fluorescence under UV light. FE-SEM images of the composite nanofiber revealed a uniform diameter of 700 nm and a smooth surface morphology (Figure 6e). Fluorescent microscopy images of the composite nanofibers also clearly showed blue, green, or red emission under different excitation wavelengths (Figure 6e). The combination of aqueous-solubility and strong fluorescence of CQDs mean that TTDDAbased CQDs can be efficient fluorescent bioimaging agents. MTT assay of TTDDA-based CQDs revealed cell viability of more than 94% for both MDCK and MDAMB cells even at the high concentration of 1 mg/mL, which shows the high physiological nontoxicity and safety of TTDDA-based CQDs (Figure 7a). At a lower concentration of 0.1 mg/mL, CQD1 revealed more than 94.2% cell viability for both types of cells. Incubation of MDCK and MDAMB cells with CQD1 (0.5 mg/mL concentration) clearly revealed the effective cell inclusion of fluorescent CQDs. With increasing excitation wavelength, blue, green, or red fluorescent microscopy images of both types of cells were clearly visualized (Figure 7b and 20 ACS Paragon Plus Environment

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Figure S12). Since TTDDA-based CQDs possess negative charges as previously inferred from zeta-potential analysis, TTDDA-based CQDs could be effectively included inside cell lipid membranes (which also have a net negative charge) possibly through endocytosis.

a)

120

Cell viability (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Control 0.1 mg/mL

0.001 mg/mL 0.5 mg/mL

0.01 mg/mL 1 mg/mL

100 80 60 40 20 0

MDCK

Cell

MDAMB

b)

100 um Figure 7. a) Cell viability tests of CQD1 for either MDCK or MDAMB cells at different concentrations and b) fluorescence confocal images of MDCK cells after incubation with CQD1 (0.1 mg/mL) for 24 hrs.

The strong blue fluorescence of TTDDA-based CQDs in methanol under UV excitation renders TTDDA-based CQDs promising for application as a top down-converting layer for crystalline silicon (c-Si) solar cells.52 A methanolic solution of CQD1 with a concentration of 0.7 wt.% was spin-coated on the top of p-type c-Si solar cells and the optoelectrical performance of the CQD-coated c-Si solar cells was compared with that of bare c-Si solar cells. After incorporation of a top CQD coating, the power conversion efficiency (PCE) of c21 ACS Paragon Plus Environment


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Si solar cells increased from 12.2% to 14.2%. I-V characteristics of the CQD-coated solar cell showed an increased fill factor of 69.0 as compared to 58.1 for bare solar cells (Figure 8a and 8b). Interestingly, the short-circuit current (ISC) and the open-circuit voltage (VOC) did not change significantly. The CQD-coated c-Si solar cell revealed a slightly lower short-circuit current density (JSC) of 33.9 mA/cm2 compared with the corresponding JSC value of 34.3 mA/cm2 for the bare cell. Both cells showed the same VOC of 0.61 V. Therefore, it is believed that the top layer of TTDDA-based CQDs do not lead to poor electrical conductivity and a corresponding decrease in solar cell efficiency, which is often observed in the case of solar cells where a thick GQD top layer is incorporated. It is inferred therefore, that the 2% PCE enhancement in the CQD-coated c-Si solar cell results from the down converting performance of TTDDA-based CQDs. It has been reported that c-Si solar cells show poor light harvesting performance under UV irradiation due to the photogeneration of electron-hole pairs near the surface and the subsequent recombination of these carriers with defect sites in the depletion region.53 The TTDDA-based CQDs top layer can down-convert UV irradiation to visible light through the fluorescence QYs of TTDDA-based CQDs. External quantum efficiency (EQE) measurement revealed that the CQD-coated c-Si solar cell shows enhanced EQE performance at wavelengths from 300 to 500 nm (Figure 8c). The highest EQE enhancement was achieved at the wavelength maximum of 400 nm, where TTDDA-based CQDs have weak optical absorption and show a strong fluorescence. All these results prove that TTDDA-based CQDs can enable c-Si solar cells to absorb more photons and lead to a dramatic enhancement of the solar cell efficiency.


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b)

100 80

Power (mW)

Current (mA)

a)

60 40

Bare CQD coated

20 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

40 30 20

0 0.0

0.7

Bare CQD coated

10

0.1

Voltage (V)

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (V)

d)

100 80

5 0

EQE (%)

c) EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40

Bare CQD coated

20 400

600

800

-5 -10 -15 -20

1000

400

600

800

1000

Wavelength (nm)

Wavelength (nm)

Figure 8. a) I-V and b) P-V characteristics of p-Si solar cell with or in the presence of CQD top layer (insets are photoimages of solar cells (left: bare, right:CQD-coated) in the daylight or under dark with UV irradiation, c) EQE spectra of bare and CQD-coated Si solar cells, and d) plot of EQE enhancement spectrum of CQD-coated Si solar cell (inset is schematic illustration explaining the increase of cell efficiency with the introduction of CQD layer).

4. CONCLUSIONS In conclusion, gram quantities of CQDs were synthesized in a short time (within 5 min) by the microwave-assisted pyrolysis of CA (A3 type) and TTDDA (B2 type), a polyamidation monomer set, using a household microwave oven. TTDDA-based CQDs showed a high mass yield up to 66.4%, high fluorescence QY values up to 29%, and amphiphilic solubility in various solvents through the dual role of TTDDA, which functions both as a constituting monomer and as a surface passivation agent. TTDDA-based CQDs are entirely non-toxic to cellular media up to a concentration of 1 mg/mL and multi-color B/G/R fluorescence imaging of cells was feasible. The unique organo-solubility of TTDDA-based CQDs enabled the 23 ACS Paragon Plus Environment


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construction of multi-color fluorescent films or nanofibers of CQD/PMMA nanocomposites through simple wet processes. Due to the high down-converting performance of TTDDAbased CQDs, a 2% enhancement of c-Si solar cell efficiency can be achieved simply by incorporating a CQD top layer. Using this simple microwave-assisted synthesis of CQDs from A3 and B2 type polyamidation set, CQDs with different compositions, performances, and target applications can be effectively realized.

ASSOCIATED CONTENT Supporting Information. Summary table of the mass synthesis of TTDDA-based CQDs, photo images of crude TTDDA-CQDs showing intense blue fluorescence upon UV irradiation, UV-Vis spectra of CQD2 and CQD3, PL decay profile of CQD1 in single molecular level, TEM, HRTEM, and SAED analysis of CQD2 and CQD3, Raman spectrum of CQD1 with 1,064 nm excitation laser wavelength, XRD spectra of TTDDA-CQDs, XPS analysis of CQD3, Zeta-potential analysis of CQD1 and CQD3, FT-IR spectra of CA, TTDDA, CQD1, and CQD3, UV-Vis spectra and fluorescence images of CQD1 in various organic solvents, Photo images, fluorescence images, and PL spectra of CQD1/PMMA composite film, fluorescence cellular images of MDAMB cells incubated with CQD1. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (I.I.). * E-mail: [email protected] (S.Y.P.). * E-mail: [email protected] (D.L.). Notes 24 ACS Paragon Plus Environment

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The authors declare no competing financial interests. ACKNOWLEDGEMENTS This research was supported by Basic Science Research Program through the National Research

Foundation

of

Korea(NRF)

funded

by

the

Ministry

of

Education(2015R1D1A3A01020192) and by Radiation Technology R&D program through the

NRF

funded

by

the

Ministry

of

Science,

ICT

&

Future

Planning(2017M2A2A6A01019289). And this work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and NRF(NRF-2015H1C1A1035903).

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Table of Contents (TOC)

+ TTDDA

CA

Amphiphilic TTDDA-based CQDs

Fluorescent Composite

Bioimaging

Down-converting layer UV Vis

PMMA

c-Si Solar Cell Al


Simple Microwave-Assisted Synthesis of Amphiphilic Carbon

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