Color integration in biomass-derived carbon dots to realize one-step white light

ABSTRACT In this work, the extract of mango leaf was chosen as a green raw material, and ethylenediamine (EDA) as a supporting material for a controllable one-step reaction to realizing white light. During the reaction, EDA could balance the precursor consumption, and participated in the formation of the carbon dots (CDs). The final integrated emitting color of the materials could be adjusted to white light with a one-step hydrothermal treatment. The composite solution was directly coated on a commercial 365 nm light-emitting diode (LED) chip without a polymer matrix and showed a stable white light with CIE coordinates of (0.326, 0.328). This work developed an interesting method for tuning the reaction process in a complex system and provided a simple way of realizing an integrated white light with a single wavelength excitation. GRAPHICAL ABSTRACT


Introduction
There have been increasing demands for white lightemitting diodes (WLEDs) as the next-generation lighting source (1).Conventional materials such as rare-earth phosphors (2) and inorganic quantum dots (3,4) usually needed costly raw materials, and their intrinsic toxicity limited their application (5).As a rising fluorescent material, carbon dots (CDs) with superior optical properties have shown great advantages (6) compared with traditional phosphors.Their low cost, high photostability, easy synthesis, and low toxicity revealed their promise as an appropriate replacement for conventional materials in LED applications (7)(8)(9).
So far, tunable photoluminescence (PL) of the CDs has been realized across the full visible region (10,11), and CDs exhibiting blue, green and red light were combined to fabricate WLEDs (12)(13)(14)(15).The integrated color could be adjusted by changing the ratio of the components.Also, yellow-emitting CDs (16,17) or red-emitting CDs (18) were coated on a commercial blue LED chip to achieve the integrated white light.However, a combination of multiple materials increased the complexity and cost of the fabrication and might reduce the stability of the whole device.
It will be useful if a single type of CD can exhibit white light.Unfortunately, although CDs with multi-band emission have been occasionally reported (19)(20)(21), their emission wavelength could not cover the whole spectral region.Meanwhile, due to the complexity of the energy states in the CDs, it was difficult to obtain desired PL bands with a specific synthetic route.An interesting method was to combine blue emission of CDs in their dispersed state, and red emission in their aggregated state (22)(23)(24).However, close aggregation of the CDs easily led to self-quenching of their fluorescence, and the red emission of these CDs always had low fluorescence.It is still in high demand to develop new materials for achieving white light containing sufficient color components and simultaneously with good stability.
In this work, one-step color integration toward white light was designed in another way.Mango leaf was chosen as a raw material, which contained chlorophyll with red fluorescence, and mangiferin with yellow fluorescence.Ethanol was used as the extract solvent to dissolve both components.Ethylenediamine (EDA) was used for tuning the reaction during hydrothermal treatment.With two active amino groups, EDA suppressed the decomposition of chlorophyll and preferentially reacted with mangiferin to generate CDs.The consumption of the precursor and formation of the nanodots could be adjusted, which influenced their integrated emitting color after the reaction.With a proper amount of EDA, one-step heating realized a composite solution exhibiting white light.Moreover, EDA could tie the components in the solution with hydrogen bonds and form a linked structure.The mixed solution could be directly transferred to a solid state without another matrix, and showed a stable white light on a commercial 365 nm LED chip.

Materials and reagents
Fresh mango leaves were picked from the botanical garden in the Institute of Fruit Tree Research, Guangdong Academy of Agricultural Sciences, and washed with deionized water before use.Ethanol and ethylenediamine were purchased from China Chemical Reagents Company without further purification.

Synthesis of the CDs from mango leaf
For CD synthesis, a fresh mango leaf was immersed in 100 mL absolute ethanol and placed at 4°C overnight.The solution turned deep green while the leaf turned yellow, indicating the organic components were excluded from the leaf and dissolved in ethanol.A certain amount of EDA (5-30 μl) was added to the extract solution.Then the mixed solution was transferred to a Teflon-lined autoclave and heated at 200°C for 35 min.After the reaction, the solution was cooled naturally to room temperature and stored at 4°C.

Components' isolation experiments
For rough isolation of the hydrotropic and hydrophobic components in the mango leaves, ethanol was changed with water or acetone as the extracting solvent.The water or acetone extract solution was first dried at 100°C to remove the solvent and re-dissolved in ethanol for further reaction.The hydrothermal treatment of the extract solution was also performed at 200°C for 35 min.

Structural and optical characterizations
Transmission electron microscopy (TEM) images were recorded by a JEOL JEM 2100 electron microscope (JEOL, Tokyo, Japan) operated at 200 KV.Fourier transform infrared (FTIR) spectra were recorded using a Thermo Nicolet Fourier transform infrared spectrometer (Nexus 670).Xray photoelectron spectroscopy (XPS) analysis was carried out on a VG ESCALAB 220i-XL surface analysis system.UV-vis absorption spectra were recorded with a Shimadzu UV-vis spectrophotometer (Columbia, MD, USA).Photoluminescence spectra were obtained using a Shimadzu RF-6310PC spectrofluorometer (Shimadzu Corporation, Columbia, MD, USA).The PLQY was measured using an FLS980 fluorescence spectrometer (Techcomp, China).

Fabrication of white LED
GaN LED chips were purchased from New Star Photoelectric CO., Ltd.The emission wavelength of the LED chip was centered at 365 nm with a working voltage of 3.0 V.The CD solution was directly dropped onto a glass slide above the LED chip and dried at 80°C for 2 h.A spectra scan PR-650 spectrophotometer was used to collect the PL spectra of LEDs.The color coordinate (CIE) was calculated from the color-calculator software.

Component analysis of the leaf extract
Here, ethanol extract from mango leaves was used as the precursor for CD synthesis.According to the references, phenolic compounds are the major constituents of the mango leaf, including a variety of phenolic acids, phenolic esters, and flavanols (25-28).The representative phenolic compounds in mango leaves are mangiferin, gallic acid, methyl gallate and penta-O-galloylglucoside.Some sample compounds are shown in the supporting information for a rough description of their molecular structure (Figure S1).
The absorption spectrum of the leaf-extract solution was obtained from UV-vis analysis.As shown in Figure 1a, absorption peaks at 433, 470, and 663 nm belonged to chlorophyll (29, 30), while the 260, 315, and 370 nm peaks belonged to the polyphenolic compounds (31,32).The high-energy 260 nm peak originated from ππ* transition of the conjugated C=C bonds in aromatic rings, while the 315 and 370 nm peaks were from n-π* transition of C=O bonds in carbonyl groups.The photoluminescence (PL) spectrum of the extract solution was recorded with 365 nm excitation.As shown in Figure 1b, chlorophyll exhibited a characteristic peak at 670 nm (29).Besides, two broad peaks at 550 and 490 nm appeared.These PL peaks showed an excitation-independent property (Figure S2a), which was usually a feature of a fluorescent dye.In Figure S2b, the 550 and 490 nm peaks showed the same excitation peak at 380 nm, which matched well with the 370 nm absorption peak (Figure 1a).Therefore, the 550 and 490 nm PL should belong to a certain kind of polyphenolic compound in mango leaves.Some other kinds of plant leaves were tested but did not show this characteristic fluorescence (Figure S3).
Considering that mangiferin is a special component in the mango leaf, we investigated the energy states of the mangiferin molecule through density functional theory (DFT) calculations.The ground-state geometries of the molecules were optimized using the TD-APFD at 6-311+G (2d, p) basis set.The predicted absorbance and PL spectra (Figures S4c,d) matched well with the experiment results (Figure 1).Therefore, mangiferin was regarded as the main component that contributed to the 550 nm fluorescence and used as the sample molecule in the following discussion.

Structural reconstruction of the leaf-extract solution during hydrothermal treatment
The hydrothermal heating of the leaf-extract solution caused a great change in its UV and PL spectra (Figure 2a,b).After direct heating at 200°C for 35 min, both the 670 and 550 nm peaks disappeared (Figure 2b, red curve), indicating the structural decomposition of the chlorophyll and mangiferin molecules.The PL of the treated solution was composed of a broad peak at 440 nm, and a small peak at 520 nm. Figure 2c and d showed the morphology characterization of the heated solution.Sphere dots with sizes of 5-10 nm can be found, suggesting the formation of the carbon dots, which should be responsible for the 440 and 520 nm PL.In Figure S5a, the 440 nm peak showed excitationdependent property, which was a common feature of the surface states from the CDs (33).The 520 nm peak showed excitation-independent properties, and it might belong to a single type of chromophore structure formed during the hydrothermal treatment (34).The broad 325 nm peak in the absorption spectrum should belong to the π-π* transition of C = C bonds, which widely existed in the aromatic structures in CDs (35).These results indicated that carbon dots could be generated from the precursor molecules in the leaf-extract solution, which showed a common blue fluorescence (36,37).The heated leaf-extract solution was purified by dialysis with a 0.5 KD membrane in ethanol for 2 days.The CDs inside the dialysis bag were collected for the PLQY measurement.The PLQY of the CDs obtained by heating the leaf-extract solution was 16%.Then, we added 30 μl EDA into the leaf-extract solution and did the same heating at 200°C for 35 min.As seen in Figure 3b, the 670 nm peak of chlorophyll still had high PL intensity after heating, with an additional shoulder at 650 nm.As for the CDs, the emission in the blue region was further broadened and red-shifted.The weak emission at 520 nm changed to a strong and broad peak at 530 nm.Actually, this 530 nm peak was the combination of the 550 and 520 nm peaks (peakdifferentiation shown in Figure S6).Therefore, in the heated solution, there were unreacted chlorophyll and mangiferin molecules and new-generated carbon dots.The TEM images (Figure 3c,d) also indicated the formation of the CDs.The lattice space of the CDs was 0.24 nm, which belonged to the (100) facet of graphitic carbon.As expected, the 460 nm PL was excitationdependent, and the 530 nm PL was excitation-independent (Figure S5b).Similar dialysis of the mixed solution was carried out, and the PLQY of the CDs was 36%, indicating a different type of CDs compared with that synthesized without EDA.
The results showed that by adding EDA in the precursor solution, chlorophyll and mangiferin molecules were preserved to some extent after heating.Meanwhile, carbon dots were formed, providing additional emission peaks in the blue region.Interestingly, the whole solution exhibited an integrated white light when excited at 365 nm.The Commission International d'Eclairage (CIE) coordinates are located at (0.326, 0.328).

Components' isolation experiments
To get insight into the detailed reaction process, we did a 'components isolation' experiment.First, deionized water was used as the extract solvent instead of ethanol, to keep the chlorophyll out.Then water was evaporated, and the solid was re-dissolved in ethanol.As seen in the PL spectrum (Figure 4b, black curve), this extract solution contained mangiferin, but no chlorophyll.Then it was subjected to hydrothermal heating at 200°C for 35 min.Without EDA (Figure 4b, blue curve), no obvious fluorescence was found.However, with the addition of EDA, two strong fluorescent peaks at 460 and 520 nm were found (Figure 4b, red curve).These results indicated that mangiferin and other polyphenolic compounds could not generate carbon dots by themselves through direct heating, but succeeded with the help of EDA (also confirmed by TEM images in Figure 4d).
It is well known that dehydration between amino and hydroxyl groups plays an important role in the formation of the CDs (35).Here, the amino groups in EDA could react with hydroxyl or carbonyl groups in mangiferin and form a polymeric-like structure.The further reaction led to aromatization or carbonization, and the intrinsic benzene rings in mangiferin promoted the conjugated π structure in the CDs.
Next, we used acetone to extract the mango leaves for dissolving the chlorophyll (also transferred to ethanol phase).As seen in Figure 5b (black curve), this extract solution mainly contained chlorophyll, with very little mangiferin retained.After direct heating, the PL in the blue region was very weak whether with EDA or not (blue and red curves in Figure 5b).Therefore, CDs could not be generated solely by chlorophyll, nor by chlorophyll and EDA.However, when EDA was added, the 670 nm PL of chlorophyll was maintained to some extent, and an additional 650 nm peak appeared (Figure 5b, red curve).A possible reason was that the amino groups in EDA could bond with carbonyl groups in chlorophyll, and the dispersed chlorophyll molecules in solution were linked together by EDA.This aggregated structure protected chlorophyll from decomposition at high temperatures.Meanwhile, this kind of bonding might also be the reason for the PL position shift of chlorophyll (from 670 nm to 650 nm).

Detail discussion of the possible reaction process
Based on the above experimental results, we can draw a brief picture of the reaction process.For direct heating of the extract solution (containing all the components), the decomposition of chlorophyll occurred to form small molecules such as citric acid, succinic acid, and alanine (38).Then, the amino or carboxyl groups in these small molecules reacted with carbonyl groups in mangiferin and formed covalent bonds through dehydration upon heating.Sphere carbon dots were formed through further aromatization and carbonization.Because of the complete decomposition of chlorophyll, a large amount of small molecules were formed, and mangiferin was all consumed and transferred to CDs, showing two PL peaks at 440 and 520 nm, and exhibited blue light (Figure 6).However, the situation can be greatly changed when EDA is added to the solution.EDA is an organic compound with high alkalinity and two reactive amino groups.As shown in Figure 7, before heating, EDA could form hydrogen bonds with mangiferin and chlorophyll at room temperature, forming a linked structure  and providing a kind of protection for the precursor molecules.During the hydrothermal treatment, the decomposition of chlorophyll was suppressed.Meanwhile, the high activity of the two -NH 2 groups of EDA facilitated its interaction with carbonyl or hydroxyl groups in other compounds.This gave EDA a 'priority' in a complex reaction system.As a result, EDA functionalized in forming CDs with mangiferin, until all the EDA molecules were consumed.As evidence of this explanation, the CDs had different PL peak positions in two situations without or with EDA (440 nm in Figure 2b; 440 and 460 nm in Figure 3b).After the reaction, the unreacted mangiferin, undecomposed chlorophyll, and new-generated CDs all contributed to the fluorescence, and the integrated emission showed a white color.

Control of the EDA amount for tuning the integrated emitting color
The strong interaction of EDA with the precursor molecules in the solution made it possible for us to tune the balance of the reaction system, as well as the final integrated emitting color.As shown in Figure 8a, increasing EDA suppressed the decomposition of chlorophyll, leading to a gradually enhanced 670 nm peak.The reaction of EDA with mangiferin gradually shifted the blue PL of CDs to 460 nm.Meanwhile, unreacted mangiferin molecules were preserved in the solution, causing the integrated 530 nm PL.Briefly, increasing the amount of EDA caused a red-shift of the emitting color of the heated solution and showed white light with 30 μl EDA.
Finally, solid-state LED was fabricated by directly dipping the mixed solution onto a commercial 365 nm LED chip and dried at 80°C.Usually, the solid-state fluorescence of the CDs suffered from aggregation-caused quenching, and solid matrices such as starch (39), silica xerogel (40), and polymers (41,42) were needed for dispersion of the CNDs.Interestingly, the solid fluorescence of our mixed solution was identical to that in the solution phase and did not need a polymer matrix (Figure 9).The CIE coordinates of the LED was (0.326, 0.328).The color   To find a possible reason for this phenomenon, we did a dialysis experiment with the mixed solution.The dialysis was carried out by a 0.5 KD membrane in ethanol for 2 days.As seen in Figure S7, the spectra of the solution only changed a little after dialysis.It seemed most of the mangiferin and chlorophyll molecules could not be filtered through dialysis.It is deduced that EDA could serve as a 'linking bridge' through the amino groups at two ends of its carbon chain.The CDs, unreacted mangiferin, and undecomposed chlorophyll could be tied closely together by EDA through covalent or hydrogen bonds.Therefore, the elimination of small molecules became difficult, as seen in the dialysis experiment.In fact, it has been reported that hydrogen bonding can keep solid-state CDs in a dispersed state and inhibit the fluorescent quenching (43).Here, the linked structure in the mixture might be the reason for its good fluorescent stability, which made the device fabrication simple and easy.

Conclusions
In this work, a balanced state in a multi-component reaction was achieved to realize the desired integrated emitting color in a mixed solution.EDA served as a regulator in the reaction, which influenced the consumption of the precursors, and also participated in the formation of the CDs.The CDs with blue fluorescence combined the intrinsic red and yellow fluorescence of the natural components in the mango leaves, and the integrated emitting color could be adjusted to white by a proper amount of EDA.This work showed a simple and convenient way of achieving white light by one-step synthesis and might expand the application of CDs in the WLEDs by a new way of thinking.

Figure 1 .
Figure 1.(a) UV-vis and (b) PL spectrum of the mango leaf extract solution.

Figure 2 .
Figure 2. (a) UV-vis and (b) PL spectra of the original leaf extract solution (black curves), and the heated solution without EDA (red curves).(c) TEM and (d) HRTEM of the heated solution.The heating temperature was 200°C, and the heating time was 35 min.

Figure 3 .
Figure 3. (a) UV-vis and (b) PL spectra of the original leaf extract solution (black curves), and the heated solution with 30 μl EDA (red curves).(c) TEM and (d) HRTEM of the heated solution.The heating temperature was 200°C, and the heating time was 35 min.

Figure 4 .
Figure 4. Heating of the water-extract solution (transferred to ethanol phase) of the mango leaves at 200°C for 35 min.(a) Absorption and (b) PL spectra (excited at 365 nm) of the original and heated solution (with or without EDA).(c) Different wavelength excited PL spectra and (d) TEM and HRTEM images of the heated solution with EDA.

Figure 5 .
Figure 5. Heating of the acetone-extract solution (transferred to ethanol phase) of the mango leaves at 200°C for 35 min.(a) Absorption and (b) PL spectra (excited at 365 nm) of the original or heated solution (with or without EDA).

Figure 6 .
Figure 6.Direct heating of the mango leaf-ethanol extract solution without EDA.

Figure 7 .
Figure 7. Heating of the mango leaf-ethanol extract solution with 30 μl EDA.