Solvent-free synthesis of photoluminescent carbon nanoparticles from lignin-derived monomers as feedstock

ABSTRACT Photoluminescent carbon nanoparticles (CNPs), such as carbon dots (CDs), have attracted much attention owing to a unique set of properties, like high and tunable fluorescence. In this way, the use of carbon-rich lignin has been demonstrated to be a sustainable approach to producing a broad range of photoluminescent CNPs. However, the valorization of this complex polyphenol is limited when it comes to green and efficient ways of conversion. In addition, the existing solvothermal approaches using lignin often result in CDs with low photoluminescence, while flammable and/or toxic solvents are employed. Here, we depolymerized technical lignins, i.e. kraft and soda, through electroreductive cleavage in two different sustainable media: deep eutectic solvent and levulinic acid. After depolymerization, lignin-derived monomers were generated, with a predominance of aryl ether and phenolic groups, which were further combined with 1,2-Phenylenediamine to produce N-doped CNPs in a solvent-free approach. Photoluminescent CNPs with varied sizes were generated (5–50 nm), which presented a wide photoluminescence emission, from blue to red, depending on solvent polarity. These results demonstrate a feasible and sustainable route for the solvent-free synthesis of photoluminescent CNPs using lignin-derived monomers as carbon source, which may find applications in a wide range of fields. GRAPHICAL ABSTRACT

In recent years, researchers have embraced bottom-up approaches due to the availability of different carbon sources and ease of preparation conditions (17), whilst the photoluminescence properties of the synthesized CDs can be easily tuned (13,17).Nevertheless, a scaleup, which usually requires high pressures in enclosed reactors, is still hindered because most of the solvents used as reaction medium are costly, flammable, and/or toxic (4,(17)(18)(19).Additionally, the CDs formed often require long post-treatments and there is a considerable waste production (17).Additionally, the low quantum yield and product yield of synthesized CDs make the cost of raw materials and the availability of cheap raw materials crucial factors for future scale-up.While biomass and its derivatives offer the advantages of availability, low cost, and renewability, their complex composition often results on CDs with a wide range of structural heterogeneity.Therefore, the consistency of the products may be affected by the diversity of the composition of the same biomass from different sources and/or origins.Although significant advances have been made in the field, very little further attention has been given to solvent-free methods for the generation of CDs.
Recently, biomass as a carbon source has been investigated as an alternative to synthesizing eco-friendly CDs, including palm kernel shell (20), soy meal (21), rose-heart radish (22), orange peel (23), rice bran (24), cellulose (25), and lignin (26).Nevertheless, biomass generates low photoluminescence CDs, as a result of a low heteroatom content in their molecular structure, which limits further applications (27).Therefore, there is an urgent need for new approaches while improving the photoluminescence properties of biomass-derived CDs.
The photoluminescence can be enhanced by altering the surface state of CNPs and changing the energy gap width and electron density.One way to do it is by doping with nitrogen heteroatoms, which improves the photoluminescence quantum yield (28).In recent studies, o-phenylenediamine (oPDA), also a carbon precursor, has been successfully used to enhance the photoluminescence of CDs (17,29,30), in spite of its known toxicity (31)(32)(33).Chao et al. and Ding et al. (17,30) reported on the use of oPDA as a acarbon source for the synthesis of CNPs (17,30).Yang et al. (34) used oPDA as both carbon and nitrogen sources, (34) while Tan, et al. (28) utilized it as a nitrogen source and doping (28).Although oPDA has demonstrated to be a powerful carbon and nitrogen sources for CDs generation and doping, respectively, there is an urgent need for investigations aiming at diminishing the amount of oPDA used.
Finding a sustainable carbon source for the synthesis of CDs in a free-solvent system is a promising way to design an eco-friendly process.Lignin, the second most abundant renewable macromolecular material after cellulose (35)(36)(37)(38)(39)(40), is a valuable option due to its high carbon content, aromatic structures, and distinctive chemical composition (18,39).However, its utilization is hindered due to the recalcitrance of the macromolecule composed of a three-dimensional macromolecular structure, summed up to the organizational variability dependable on the origin and extraction methods (26,41,42).
Studies on lignin valorization derived from the pulping and/or biorefinery industries have a long history, although there is still plenty of room for development toward the sustainability of the processes (43)(44)(45)(46)(47)(48).Industrially, this resource is underutilized, with only 2% of the yearly estimated 100 million tons generated as waste being used to obtain useful bio-chemical compounds, while the remaining material is mostly burned for energy generation (49).For further lignin valorization, depolymerization strategies have been investigated to generate functionalized aromatic compounds at room temperature and with good yields.Among the different depolymerization methods for lignin, electrocatalysis has been highlighted as the most promising one due to greater convergence with the principles of Green Chemistry.
The latest publications present the study of ECH of technical lignins aiming to determine the obtained products and needed improvements when handling the complexity of the macromolecule (77)(78)(79).Our recent publication showed a varied range of monomers and dimers as a result of the ECH of kraft lignin in levulinic acid, a biobased solvent (80).Fang et al. (81) addressed the electroreductive cleavage of four pre-oxidized lignin obtained from four pre-treatment methods on alkali media: kraft, soda, γ-valerolactone/sulfuric acid (GVL) and Cu-catalyzed alkaline hydrogen peroxide process (Cu-AHP).GVL and Cu-AHP provided higher monomer yields compared to kraft and soda.The prior oxidation treatment did not enhance the obtained soluble fraction after the ECH of both kraft and soda when compared to the literature.
Several applications of the products generated from the depolymerization of lignin have been reported, including polyurethane foams (82), phenolic resins (82), phenol-formaldehyde resins (82), productions of jet fuel (83), coating in metallic surfaces (80), and polyurethane feedstock (83).However, these applications are usually limited due to the strain in industrial scaleup, and costly pathways of separation of the obtained products into value-added fuels or chemicals (84,85).
In this scenario, carbon dots synthesis is seen as an application that can make use of the complex mixture produced by the lignin depolymerization without the need of a separation procedure.Herein we investigate the use of technical lignin depolymerization products as a carbon source for the solvent-free generation of photoluminescent CNPs.To explore the particularities of the ECH of technical lignin, we chose kraft and soda lignin as they represent sulfur-containing and sulfurfree extraction processes, respectively.With the aim of developing a greener synthesis method, we made use of sustainable solvents, i.e. levulinic acid and a deep eutectic solvent (DES), while the final monomers were combined to reduced amounts of oPDA to produce a wide range of photoluminescent CNPs.

Materials and methods
Kraft and soda lignins derived from hardwood black liquor of the supplier pilot pulping plant (WAT Venture sp.z o. o., Poland) were used as samples of sulfur-containing and sulfur-free extraction processes.Levulinic acid, anthracene 99+%, analytical grade ethylene glycol, technical grade ethyl acetate and 4-methyl-2-pentanone (methyl isobutyl ketone, MIBK) were purchased from Sigma-Aldrich Co. Sulfuric acid 95% and ethanol absolute were purchased from VWR Chemicals.Choline chloride (98%), AnalaR NORMAPUR, N, N-Dimethylformamide (DMF) and 1,2-Phenylenediamine (oPDA) were purchased from TCI. Glycerol and acetic acid were purchased from Honeywell and Acros Organics, respectively.The copper foil was purchased from WAT Venture, Poland.

Lignin depolymerization in deep eutectic solvent (DL/DES)
Both technical lignins were depolymerized according to Cruz et al., 2022 (86).In a typical preparation, DES was prepared from the mixture of ethylene glycol and choline chloride (EtGly:ChCl) in a molar ratio of 2:1.The compounds were stirred at 40°C until a transparent solution was achieved.The lignin concentration in DES was 3 g⋅L −1 , which was achieved by mechanical stirring at 800 rpm until the complete dissolution of lignin.Next, 10%vol. of DI water was added to the solution to improve the solution conductivity, while decreasing its viscosity and enhancing diffusion (87).The electrochemical reactions were carried out in a three-electrode undivided cell connected to an Interface 5000E potentiostat (Gamry Instruments).A Cu foil (1 cm 2 ) was used as working electrode, a Pt wire as counter electrode and Ag/AgCl (saturated KCl) as reference electrode.The selected reductive potential was −1.7 V vs. Ag/AgCl (saturated KCl) and the experiments were performed for 20 h at room temperature.

Recovery and liquid-liquid extraction of DL/DES products
1 M H 2 SO 4 was added dropwise to the depolymerized lignin (DL) until pH 2. The mixture was allowed to stand for 1 h until the complete precipitation of the insoluble fraction that was centrifuged at 8000 rpm for 10 min.Subsequently, the liquid-liquid extractions were performed using ethyl acetate as organic extractant and stirring overnight with the remaining solution in an orbital shaker at 250 rpm.The mixture was shaken overnight and then the phases were separated using a separating funnel.The supernatant phase containing the depolymerized products was separated and evaporated under vacuum in a rotary evaporator.

Lignin depolymerization in levulinic acid (DL/LA)
Lignin was depolymerized according to Cruz et al., 2022 (80).The lignin concentration in levulinic acid solution was 3 g⋅L −1 , which was achieved by stirring until the complete dissolution of lignin.Next, 10%vol. of DI water was added to the solution to decrease the viscosity and enhance diffusion.The electrochemical reactions were carried out in a three-electrode undivided cell connected to an Interface 5000E potentiostat (Gamry Instruments).Cu foil (1 cm 2 ) was used as working electrode, Pt wire as counter electrode and Ag/AgCl (saturated KCl) as reference electrode.The selected reductive potential was −1.7 V vs. Ag/AgCl (saturated KCl) and the experiments were performed for 20 h at room temperature.

Recovery and extraction of DL/LA products
The liquid-liquid extractions were performed using MIBK as organic extractant and stirred overnight with the remaining solution on an orbital shaker at 250 rpm.The mixture was shaken at 250 rpm overnight and then the phases were separated using a separating funnel.The supernatant phase containing the depolymerized products was separated and evaporated under vacuum in a rotary evaporator.
Solvent-free synthesis of carbon dots from DL/ DES products (D-CDs) and from DL/LA products (L-CDs) as substrates In a typical preparation, a solution was prepared by adding 0.1 g of oPDA to 0.5 g of DL/DES or DL/LA.Next, the resulting mixture was transferred into a Teflon-lined stainless-steel autoclave, which was placed in a furnace at 200°C for 10 (D-CDs 10 h and L-CDs 10 h) or 24 h (D-CDs 24 h and L-CDs 24 h).To compare the quantum yield (QY), control CDs (without oPDA) were prepared as follows: 0.5 g of DL/DES or DL/LA was added into a Teflon-lined stainless-steel autoclave, which was placed in a furnace at 200°C for 10 or 24 h.All CDs solutions were filtered using a 0.2 µm cellulose acetate membrane filter before testing.

Quantum yield (QY) measurement
The photoluminescence QY of the CDs was calculated using the slope method by comparing the integrated PL intensities and the values of UV-vis absorbance of the reference with the samples (Equation ( 1)).

ST
Eq. 1where ST and X stand for the standard (anthracene) and sample (CDs), respectively.Φ is the photoluminescence QY, Grad is the gradient from the plot of integrated photoluminescence intensity vs absorbance, and η is the refractive index of the solvent.The standard value of anthracene in ethanol was used (Φ ST = 27% at a wavelength of 360-480 nm) (88).
To calculate the photoluminescence QY, anthracene was selected as a fluorophore reference.The absorbance of anthracene, D-CDs 24 h, and L-CDs 10 h solutions were kept between 0.005 and 0.03 and measured in the wavelength scan range from 200 to 600 nm (medium scan speed and scan step of 0.5 nm).While keeping their absorbance intensities under 0.1, PL was measured at an excitation wavelength of 390 nm.

Characterization
Direct injection high-resolution MS samples were dissolved in a chloroform/methanol mixture (3:7, dilution factor of 1:1000).Samples were introduced into a LTQ Orbitrap XL hybrid tandem high-resolution MS from Thermo Fisher Scientific (Bremen, Germany) by direct infusion applying a flow rate of 5 μL min −1 .The instrument was fitted with electrospray ionization (ESI) ion source and operated in positive or negative ion mode.Nitrogen was used as a sheath gas.Helium was used both as a buffer and a collision gas in the linear ion trap section, where lower energy collision-induced dissociation (CID) was performed.For the identification of chemical structures by tandem MS, product ions were generated in the linear ion trap via CID and detected by the high-resolution orbitrap section of the instrument at a resolution of 60,000 (full width at half maximum, FWHM).The mass measurements were acquired with a mass accuracy of 5 ppm or better.Data processing and interpretation were performed using the software tools Xcalibur version 2.0.7 and Mass Frontier version 6.0 from Thermo Fisher Scientific (Bremen, Germany).m/z describes the mass-to-charge ratio of the detected ions.As all negatively charged ions analyzed were single-charged species, m/z also referred to the monoisotopic molecular masses of the detected ions.It is noted that ionization capability of chemical compounds is dependent on the chemical structure.Fourier-transform infrared spectroscopy (FT-IR) analysis was carried out using a Varian 610-IR FT-IR spectrometer.The samples were analyzed at room temperature in ATR mode, in the range of 400-4000 cm −1 , with 16 scans at 4 cm −1 resolution and 1 cm −1 interval.The 1 H NMR spectra of samples were recorded using a Bruker Avance III 600 MHz instrument (ν L, 1H = 600.13MHz) in 5 mm NMR tubes at 300 K.Chemical shifts are given relative to Me 4 Si.TEM micrographs were obtained using a JEOL 2100F microscope.The optical properties were evaluated using a UV-3100PC spectrophotometer (VWR, USA) and a Varian Cary Fluorescence spectrometer (SPECTRFluor).

Results and discussion
Reductive electrocatalytic fractionation of lignin One of the main drawbacks of many strategies for lignin valorization relies on both the selectivity and separation of products.The structure of this complex plant polyphenol, which also varies depending on the source and the extraction process, makes the development of a standardized approach for its depolymerization difficult (89).Therefore, the direct use of a mixture of the obtained depolymerized products, which would not require complex, costly, and time-consuming separation steps, is interesting to pave the way for up-scaled processes.
We present here the fractionation of soda and kraft lignin in two different media: DES and levulinic acid.The electrochemical activity towards the cleavage of lignin linkages was confirmed previously (76,80).The ECH was performed during 20 h with an applied potential of E = −1.7 V vs Ag/AgCl (saturated KCl) for both media.The open-cell reactor allows the occurrence of oxidation reactions on the Pt counter electrode.These reactions can lead to partial oxidation of the α-OH on the β-O-4 linkages, which enhances the reactivity of the lignin samples (77,81).
The products were extracted via liquid-liquid extractions using ethyl acetate for the reactions in DES reactions and methyl isobutyl ketone (MIBK) for the ones in levulinic acid.Both solvents are classified as safe and recommended by CHEM 21 (90).The characterization of the lignin-derived oligomers is difficult due to the complex range of various products.Despite the wide spectra obtained from the GC-MS, one can observe the similarities among some of the identified monomers (Figure 1).
It is important to notice that the methoxyphenol ends were mostly preserved in the reactions with kraft lignin, while a mixture of phenols and methoxyphenols is observed for soda lignin.The ethylene glycol present in the DES was partially active during the depolymerization.Compounds derived from the reduction of this compound were observed in the GC-MS spectra, according to results from the literature (76).For the levulinic acid reaction, the solvent was also present in the GC-MS spectra, however not many products of levulinic acid were observed, as previously reported (80).
The lignin samples differ mainly on the presence of sulfur in their process.Kraft pulping uses NaOH and Na 2 S during the delignification, leading to more harsh conditions in the process and producing a technical lignin that is highly crosslinked.Conversely, soda pulping is sulfur-free, which affects less the structure of the raw lignin.Considering that the latter is closer to the structure of raw lignin and provided a mixture of phenols and methoxyphenols monomers among the obtained products, soda lignin was chosen to proceed with the solvent-free synthesis of CNPs.

Solvent-free approach for the synthesis of photoluminescent CNPs
The solvent-free synthesis of a wide range of emissive CNPs, as shown in Figure 2, was achieved by first depolymerizing lignin in DES or levulinic acid and then carbonizing the depolymerization products along with reduced amounts of oPDA (5:1 ratio).
The depolymerized products generated using levulinic acid or DES as medium were characterized using direct injection high-resolution mass spectrometry.Figure 1 shows the products formed in levulinic acid (LA) and in DES for both kraft lignin (KL) and soda lignin (SL).It is important to note that levulinic acid was still present in the products even after the extraction of this solvent.Once again, these products are similar to those obtained from the reductive catalytic fractionation (58)(59)(60).
A feedstock material consisting of a mixture of aromatic compounds, in this case lignin-based monomers, is really valuable (91,92).The design of novel systems for the depolymerization of lignin through the use of electrochemistry has received a great deal of attention in the literature (80,93,94).In the same direction, the number of applications that could effectively make use of the as-obtained depolymerized products has significantly increased (95).Although applications have been traditionally motivated by the coating (80), and fuel industries (80), the need for improved performance and safety-of-operation should encourage the use of this valuable (liquid) in the generation of valuable carbon-based materials.
Up to date, there have been controversies on the mechanism of CDs photoluminescence, which varies depending on the synthesis parameters.The mechanism can be influenced by surface effects (28), such as degree of oxidation and functional groups (28), size-dependent effect (28), the inclusion of fluorescent molecules (28), and heteroatom doping at the CDs core or surface (96,97).Yin et al. (98) reported that nitrogen doping using oPDA improved the photoluminescence performance by introducing a fresh surface state, and therefore capturing a large number of electrons and expanding their radiation recombination from the surface nitrogen atoms (98).This confirms that oPDA is an excellent nitrogen dopant for CDs fabrication.Liu et al., 2020 reported a PL redshift using oPDA as nitrogen dopant in lignin CDs fabrication while improving the photoluminescence QY (99).
Optical properties were evaluated to investigate the solvent polarity of our CDs.Polar solvents (such as water and ethanol) and low-polarity solvents (such as DMF) were used.The UV-vis absorption, photoluminescence (PL), and photoluminescence excitation (PLE) were investigated by dissolving the CDs in the following solvents: DMF, ethanol, water, and 6.1 M H 2 SO 4.
Both L-CDs and D-CDs, generated from the ligninbased monomers after depolymerization in levulinic acid and DES, respectively, presented absorption peaks at about 300 nm in all solvents (Figure 3), which correspond to the Π-Π * transitions in the core of the CDs (12,100).However, D-CDs showed a shoulder peak at 380 nm in ethanol and water and about 500 nm in 6.1 M H 2 SO 4 , suggesting that various types of surface state transitions wherein the energy gaps could be influenced by the π-electron system and the graphitic nitrogen content, may exist in these CDs (17,101).The absorption peaks at 380 and 500 nm would correspond to the n-Π* transitions of the surface states containing  C-O, C=O and C=N or other functional groups (102)(103)(104).However, the absorption in the visible region could be a result of the size distribution or surface effects (105).
Figure 4 shows the PL spectra of CDs at excitation wavelengths of 450 and 370 nm in DMF and water, respectively, for L-CDs and D-CDs; 450 and 480 nm in ethanol and 400 and 600 nm in 6.1 M H 2 SO 4 for L-CDs and D-CDs, respectively.PL emission peak positions at ∼550 and 450 nm were observed for all CDs in DMF and water, respectively.However, in ethanol and 6.1 M H 2 SO 4 , a red shift was observed in D-CDs, with peaks at ∼570 and 635 nm with a shoulder peak at ∼680 nm.L-CDs showed peaks at ∼540 and 475 nm in ethanol and 6.1 M H 2 SO 4 , respectively.
L-CDs exhibited green and blue photoluminescence, while D-CDs exhibited green, yellow, and blue under UV light and red photoluminescence under only white light illumination.The red color (Figure 4(d)) can be observed by the naked eye in close illumination using a white light source, which can be associated to the excitation of CDs by the green component of the light.This is confirmed when the sample is illuminated with a monochromatic green light (Figure S6).A red-shift in the emission wavelength of L-CDs 10 h indicates that its photoluminescence is dependent on the excitation wavelength (Figure 5 and Figure S3).
The excitation-dependent PLE behavior can be a result of carbon charge (106), emissive traps (106), aromatic conjugate structure (106), loose twist sites (106), presence of surface states (106), carboxyl functional groups (107), and particle size (107).The excitationdependent behavior of our CDs could be attributed to the carboxyl group on its surface and larger particle size.This can be observed in the NMR, FTIR spectra, and TEM micrographs, which will be further discussed.D-CDs 24 h showed excitation independent wavelength on emission (Figure 6 and Figure S4), which can be attributed to the surface state which is essentially responsible rather than particle sizes (30,108).This PL behavior indicates uniform surface states (109).
Two PL redshift models for CDs have been suggested describing the mechanism behind: (i) one based on the band gap transitions in conjugated π-domains, and (ii) one related to surface defects on the CDs (17,110).
In our research, surface defect states are considered the primary factor responsible for the tunable PL emissions, because both D-CDs and L-CDs were synthesized using similar precursors and no solvent.
Surface defects are mainly formed through surface oxidation, serving as capture centers for excitons, resulting in surface-state-related photoluminescence (110)(111)(112).As oxidation increases on the CDs surface, more surface defects are formed (110).In a report based on density functional theory (DFT) by Chen et al., (79), it was noted that sp 2 carbon hybridizations from carboxyl groups can produce significant local modifications, narrowing the energy bandgap (110,113).Hence, the redshift and higher QY for L-CDs 10 h could be attributed to the surface carboxyl groups as confirmed in the FTIR and NMR analyses.By adjusting the oxygen-containing functional groups to obtain photoluminescence (PL) emission peaks, the bandgap between the CDs highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) can be altered.Considering changing sizes or chemical properties in carbon particles' surface, the PL effect can be tuned, which can be linked to the bandgap (114).Song et al., 2019 reported that increasing the oxygen content on the CDs surface results in a redshift in the emission wavelength (115).
The passivation effect on the surface of carbon materials is influenced by amide and amino groups.It was suggested that amination and amidation alter the entire electronic system and the molecular orbital distribution in carbon materials.Additionally, the HOMO level could be increased and the energy gap lowered by the lone pair electrons of nitrogen atoms, thereby allowing successful radiative recombination of electrons and holes and enhancing the luminous efficiency of carbon materials (116,117).The band gaps of CDs between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) reduce with an increasing degree of surface oxidation.An increase in the oxygen atoms in the structure results in the reduction of the band gap and in a PL redshift (101,110).
We can hypothesize that the photoluminescent centers of our lignin-based CDs surfaces are primarily made up of conjugated carbon atoms, nitrogen atoms, and bonded oxygen atoms.The oxygen atoms on the surface of our CDs would lead to the observed redshift, while the nitrogen atoms on D-CDs 24 h surface would be responsible for the observed red emission due to an enhancement in its photoluminescence efficiency.
The solvent-dependent tunable PL of our CDs confirms that the emission mechanism is related to surface states (118).To determine CDs solvent interactions, we dispersed the carbon dots in solvents with varying polarities.With increasing solvent polarity, there is a redshift, from green to yellow PL emission.The redshift could be a result of the energy required to change the dipole direction of the solvent molecules due to increasing solvent dipole moment, which results in more energy loss in a solvent with higher polarity due to excited electrons compared to those in a less polar solvent (119,120).It has been reported that acidic conditions notably influence CDs toward higher wavelengths (121,122).The carbon electron network can be distorted by the addition of nitrogen atoms into its lattice, which is homogeneously conjugated.The charge distribution and rotation of doped domains regulate the surface properties by adjusting this network (123,124).We can assume that the nitrogen atoms on the surface of D-CDs are responsible for the red emission under white light.
The optimal PL excitation wavelength is reflected in the absorption spectrum of CDs.Surface passivation or heteroatom doping can alter the surface functional groups and intrinsic structures of CDs, and eventually tune the absorption spectrum of CDs (125).This is confirmed in the FTIR and NMR spectra as nitrogen doping altered the functional groups of the CDs, thereby introducing nitrogen atoms on their surface.
The FTIR spectrum of DL/DES (Figure S1a) shows similar functional groups as reported by Cruz et al., (76), related to stretching vibrations of O-H at 3276 cm −1 , C-H at 2925 cm −1 , and C=O at 1718 cm −1 (76).The spectrum for D-CDs (Figure S1a) shows similar peaks related to O-H/N-H stretching vibrations at 3276 cm −1 , and C-H at 2925 cm −1 , (126) while the peak at N-H 1643 cm −1 is related to bending vibrations due to the nitrogen doping (127).
For DL/LA (Figure S1b), peaks related to O-H at 3359 cm −1 , C-H at 2954 cm −1 , and C=O at 1704cm −1 stretching vibrations were observed.L-CDs 10h and 24 h showed similar peaks related to O-H/N-H stretching vibrations at 3359 cm −1 and C-H at 2954 cm −1 , (126) C=O at 1704cm −1 stretching vibrations (128), while the peak at 742 cm −1 is attributed to the wagging vibration from N-H groups due to the nitrogen doping.
The 1 H NMR analysis (Figure S2) confirmed the functional groups observed in the FTIR analysis.The spectra for DL/DES and DL/LA presented peaks at 11 ppm from the phenolic groups and around 7.2 and 6-8 ppm from aromatic protons, respectively.The 1 H NMR spectra for D-CDs and L-CDs showed peaks in the region of 7-9 ppm corresponding to aromatic groups (129, 130), and around 3 ppm corresponding to the amine group (131), from the nitrogen doping.However, L-CDs showed a peak around 12 ppm corresponding to the carbonyl group, which was not observed for D-CDs.(131), which can be attributed to levulinic acid (132).
According to the PL spectra (Figure 4), D-CDs 24 h and L-CDs 10 h presented higher photoluminescence intensity and, therefore, were selected for further characterization.The photoluminescence QY values were calculated (Figure S5) to be 0.8% and 7.4% for D-CDs 24 h and L-CDs 10 h, respectively.This significant difference could be a result of the carboxyl functional group still present in the L-CDs 10 h, which can react with amino group leading to an increase in the QY (133).These QY values were then compared to CDs synthesized  without oPDA, which were found to be lower (0.6% and 2.6% for 24 and 10 h, respectively).
The TEM micrographs (Figure 7) show that both CDs are roughly spherical particles.
In Figure 7(a,b), we observe that L-CDs 10 h are monodispersed and homogeneous, with particle size in the range of 20-50 nm.Liu et al., 2017, reported on the use of triethylenetetramine (TETA) and p-benzoquinone as CDs precursors and observed similar particle sizes (134).In Figure 7(c), D-CDs 24 h are also observed to be monodisperse and homogeneous, with particle size in the range of 5-10 nm.Souza et al., (135), reported on the use of cellulose as CDs precursor and observed similar particle size (135).

Conclusions
Among different catalytic approaches, electrocatalysis has found an important space due to a better merging with the Green Chemistry concepts.In this investigation, the reductive electrochemical depolymerization of technical lignins proved to be an effective strategy to obtain a feedstock comprised of phenolic and methoxyphenolic monomers, among other products.As the separation of these products is rather difficult due to the low selectivity of the process, we successfully demonstrated how as-prepared depolymerized solutions can be used as feedstocks for the synthesis of CNPs, while keeping the process sustainable.Firstly, we depolymerized technical lignins, kraft, and soda, through electroreductive cleavage in two different sustainable media: deep eutectic solvent and levulinic acid.After depolymerization, lignin-derived monomers were generated, with a predominance of aryl ether and phenolic groups.Next, differently from conventional approaches for the synthesis of CDs, we developed a solvent-free method for the fabrication of tunable emissive CDs using the as-prepared ligninbased monomers as starting materials.Within the framework of Green Chemistry, we provided not only a straightforward method to produce lignin-based monomers using electrocatalysis and sustainable solvents, but also presented a strategy to produce photoluminescent CNPs while eliminating flammable and toxic solvents in the process.The fabricated CNPs showed stability in different solvents and good photoluminescence properties ranging from blue to red under UV light and/or white light illumination.These results demonstrate a feasible and new route for the synthesis of high photoluminescence CDs using biomass as a carbon source, which may find applications in a wide range of fields, including biosensing, catalysis, and anti-counterfeiting.

Figure 1 .
Figure 1.The main monomers of soda (SL) and kraft (KL) lignin depolymerization in levulinic acid (LA) and deep eutectic solvent (DES) identified using direct injection high-resolution mass spectrometry.

Figure 2 .
Figure 2. Scheme showing the synthesis of photoluminescent CNPs using lignin depolymerized products and oPDA as starting materials and their photoluminescence in different solvents: water, DMF, ethanol and H 2 SO 4 solution.(A) water, (B) DMF, (C) ethanol and (D) H 2 SO 4 solution.

Figure 4 .
Figure 4. PL emission spectra of CDs in the following solvents (a) DMF at Ex. 450 nm; (b) Ethanol at Ex. 450 and 480 nm for L-CDs and D-CDs, respectively; (c) Water at Ex. 370 nm; (d) 6.1 M H 2 SO 4 at Ex. 400 and 600 nm for L-CDs and D-CDs, respectively.The luminescence photos of D-CDs 24 h and D-CDs 10 h (from left to right) in a-c were taken under UV light while d was taken under an intense white light emitter.

Figure 5 .
Figure 5. Density map showing the excitation versus emission dependence of L-CDs 10 h in the following solvents (a) DMF (b) Ethanol (c) Water (d) 6.1 M H 2 SO 4 .

Figure 6 .
Figure 6.Density map showing the excitation versus emission dependence of D-CDs 24 h in the following solvents (a) DMF (b) Ethanol (c) Water (d) 6.1 M H 2 SO 4 .

Figure 7 .
Figure 7. TEM micrographs of the two selected samples after dialysis.(a,b) L-CDs 10 h and (c) D-CDs 24 h.