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. 2026 Apr 30;2(8):597–605. doi: 10.1021/polymscitech.6c00039

Upcycling of Post-Consumer Polycaprolactone to Polyamides and Poly(ester amide)s via Aminolysis-Dehydrogenative Polymerization

Yiwei Rao †, Shuo Wen †, Fangxin Zhao †, Shiyu Shen †, Chenglong Yu §, Yanqing Peng †, Yichao Lin ∥, Changguang Yao †,‡,*
PMCID: PMC13520749  PMID: 42662388

Abstract

Upcycling waste polyesters into valuable materials to address plastic pollution and reduce the waste of carbon resources is a necessary and highly challenging task. Herein, we present an “aminolysis-dehydrogenative polymerization” strategy that enables one-pot upcycling of polycaprolactone (PCL) into polyamides (PAs) and poly­(ester amide)­s (PEAs) in high yields with controllable ester-to-amide ratios and tunable functional moieties. Detailed mechanism analysis elucidates that the process involves four kinds of reactions: aminolysis, dehydrogenative cross-coupling of amine and alcohol, transesterification, and dehydrogenative self-coupling of alcohol. Significantly, the upcycling of post-consumer PCL into PAs and PEAs is also achieved, demonstrating a strong industrial application potential of this approach.

Keywords: polycaprolactone, upcycling, polyamides, poly(ester amide)s, dehydrogenative polymerization, aminolysis

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1. Introduction

Owing to their excellent properties, plastics have found widespread application in diverse fields such as clothing, transportation, communications, and biomedical sciences. − The global annual output of plastics in 2024 is approximately 500 million tons (Mt) estimated by the United Nations Environment Program (UNEP). It is forecasted that annual plastic waste will reach 121 Mt by 2050 without intervention. However, less than 10% of plastic waste is recycled and reused. The accumulation and persistence of plastics have had a serious impact on the environment, particularly due to the bioaccumulation of micro- and nano-plastics throughout the food web. To address this issue, biodegradable plastics have been seen as a potential alternative because of their natural decomposability. As one of the most representative biodegradable polyesters, polycaprolactone (PCL) has been widely used in biomedical, three-dimensional (3D) printing, and tissue engineering because of its biocompatibility and biodegradability. , Although the use of polycaprolactone (PCL) can alleviate plastic pollution to some extent, its natural decomposition process is very slow and emits CO2, and the carbon resources in the polymer cannot be effectively recycled and reused. Up to now, various methods have been used to recycle PCL into value-added chemicals (6-hydroxyhexanoic acid, 6-hydroxyhexanoic acid methyl ester, 1,6-hexanediol, ε-caprolactam, γ-caprolactone, etc.) and raw polymers, but they cannot meet the requirements of high-performance materials (Figure ). −

1.

1

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(a) Depolymerization of PCL into valuable compounds or monomers; (b) stepwise upcycling of PCL into new material; (c) this work: one-pot upcycling of PCL to poly­(ester amide)­s (PEAs) and polyamides (PAs).

In recent years, a chemical upgrading strategy that directly converts waste polymers into higher-value materials has gained increasing attention. It bypasses the complex separation and purification of recycled monomers and generates promising high-value-added materials. − In this regard, many reports on polyester upgrading have demonstrated its significant reuse value. − However, the research on upgrading PCL into high-performance materials is still in its infancy (Figure ). ,, In addition, the upcycling process is rather complicated and is unable to directly obtain the product in one step. Therefore, there is an urgent need to develop new technologies to achieve green and efficient upcycling of waste polyesters into high-performance materials.

Dehydrogenative polymerization has the advantages of being green and high efficiency, high selectivity, low cost, and easily available monomers with various functional groups, and the only by-product H2 can be used as an energy source. , Using the commercially available pyridine-based pincer-type ruthenium complex (PNN-Ru), , Guan and Milstein respectively achieved the dehydrogenative coupling of diols and diamines to prepare PAs. Robertson et al. demonstrated the dehydrogenative polymerization of aliphatic diols to prepare polyesters. Kumar directly synthesized polyureas through dehydrogenative coupling of diamines and methanol by using a ruthenium pincer complex. In particular, Miyake utilized dehydrogenative polymerization of the hard and soft oligomeric building blocks to prepare chemically recyclable multiblock polyolefin-like materials with diverse mechanical properties. Very recently, we synthesized a series of chemically recyclable aliphatic and alipharomatic polyesters through dehydrogenative (co)­polymerization of diols and bifunctional hydroxy-aldehyde monomers catalyzed by the PNN-Ru complex.

Herein, we further demonstrate a synergistic “aminolysis-dehydrogenative polymerization” strategy for PCL upcycling (Figure ) into PAs or PEAs with controllable ester-to-amide ratios and tunable functional moieties catalyzed by PNN-Ru. PEAs with different properties could be obtained by simply adjusting the dosage of diamines, and various functional groups can be introduced to the resultant polymer chains through altering the type of diamines. This work provides a one-pot method for PCL upcycling into PEAs and PAs under mild conditions, which realizes the transformation of PCL from “waste” to “treasure”, demonstrating a profound impact on the sustainable circular plastics economy.

2. Materials and Methods

2.1. Materials

All operations were carried out in Schlenk-type glassware in a nitrogen atmosphere or in a nitrogen-filled glove box. Polycaprolactone (PCL) was supplied by Guangdong Jiecheng Plasticized Raw Materials Company. All involved diamine compounds were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Carbonylchlorohydridotris­(triphenylphosphine)­ruthenium­(II) was purchased from Jiangsu Sinoco Catalyst Co., Ltd. Potassium tert-butoxide was purchased from Shanghai Haohong Scientific Co., Ltd. The PCL was dried in a 50 °C vacuum drying oven for 24 h before use. The post-consumer PCL (M n = 16.5 kDa, Đ = 2.2) was dissolved in tetrahydrofuran (THF) and then precipitated in ethanol before drying. All involved diamines were dried over calcium hydride (CaH2) overnight and distilled. 1,4-Dioxane was dried by sodium, with benzophenone as an indicator, and then evaporated after turning purple. Ruthenium complex was synthesized according to the reference method. All dried chemicals and ruthenium complex were stored in a glove box for use. Other chemicals were used directly without further purification unless otherwise noted.

2.2. Instrumentation

1H and 13C NMR spectra were recorded on a Bruker AVANCE NEO 400 spectrometer. Differential scanning calorimetry (DSC) analyses were carried out on a Mettler TOPEM TM DSC Instruments under a nitrogen atmosphere. All samples were heated at 5 °C/min to 250/300 °C and kept for 5 min to remove the thermal history. Then they were cooled down to −70 °C in the second cooling scan and later heated to 250/300 °C in the second heating scan. The molecular weight (M n) and molecular weight distribution (Đ) of the polymers were measured by gel permeation chromatography (GPC) on a TOSOH HLC-8420 GPC instrument at 40 °C against polystyrene standards. The temperature at 5% mass loss (T d,5%) and maximum mass loss rate (T d,max) of the polymers were measured by thermal gravimetric analysis (TGA) on an SDT-Q600, TA Instruments. Polymer samples were heated from ambient temperatures to 800 °C at a rate of 20 °C/min. Tensile tests were performed with a SANSIZONGHENG (CMT6104) analyzer. Dumbbell-shaped samples with a length of 30 mm, a width of 5.0 mm, and a thickness of 2 mm were prepared by hot press molding and cold press at room temperature. The drawing rate was 20 mm/min for the tensile test at room temperature. Ultimate tensile strength, elongation at break, Young’s modulus, and impact strength were reported as values from the measured data. The mechanical properties were obtained through tensile testing using a Shimadzu AGS-X-50N electronic universal testing machine. Long strip samples were prepared with a length of 45 mm, a width of 10 mm, and a thickness of 2 mm through hot pressing and cold pressing at room temperature. A tensile test was conducted at room temperature with a tensile rate of 10 mm/min. Ultra-high-performance liquid chromatography-time of flight mass spectrometer (UHPLC-TOF-MS) was tested by X500R QTOF ExionLC AD, TOF start mass was 50 Da, and TOF stop mass was 1000 Da. The mobile phase was MeOH, with a flow rate of 0.3000 mL/min.

2.3. Synthesis of Poly­(ester amide)­s (Taking PCL/1,6-Hexanediamine = 9:1 as an Example) General Synthetic Method

In a glove box filled with nitrogen, PCL (342.3 mg, 3.0 mmol) and 1,6-hexamethylenediamine (38.6 mg, 0.33 mmol, adjust based on a 3 mmol PCL) were weighed into a 25 mL Schlenk flask. PNN-Ru complex (15 mg, 0.03 mmol), potassium tert-butoxide (9.9 mg, 0.09 mmol), and 0.5 mL of 1,4-dioxane were added to a small glass flask, stirred for 5 min for catalyst activation, and transferred to the Schlenk flask with 3× 0.5 mL of 1,4-dioxane. The Schlenk flask was placed in a 120 °C heating jacket and was connected to a balloon filled with nitrogen for protection. After 72 h, the solvent was removed under vacuum, the residue was dissolved in a small amount of tetrahydrofuran (THF) and precipitated in methanol. The resulting solid was washed with cold methanol and dried under vacuum for 2 days, weighing to obtain the yield.

2.4. Synthesis of PEAs under Vacuum (Taking PCL/1,6-Hexanediamine = 9:1 as an Example)

In a glove box filled with nitrogen, PCL (342.3 mg, 3 mmol) and 1,6-hexamethylenediamine (38.6 mg, 0.33 mmol, adjusted based on a 3 mmol PCL) were weighed into a 25 mL Schlenk flask. PNN-Ru complex (15 mg, 0.03 mmol), potassium tert-butoxide (9.9 mg, 0.09 mmol), and 0.5 mL of 1,4-dioxane were added to a small glass flask, stirred for 5 min for catalyst activation, and transferred to the Schlenk flask with 3× 0.5 mL of 1,4-dioxane. The Schlenk flask was placed in a 120 °C heating jacket, and was connected to a balloon filled with nitrogen for protection. After 7 h, the Schlenk flask was cooled to room temperature, the solvent was removed by vacuum, and then the mixture was heated to 120 °C, maintaining the reaction under vacuum. After 24 h, the solvent was removed under vacuum, and the residue was dissolved in a small amount of tetrahydrofuran (THF) and precipitated in methanol. The resulting solid was washed with cold methanol and dried under vacuum for 2 days, weighing to obtain the yield.

3. Results and Discussion

3.1. Preparation and Structure Characterization of PEAs and PAs

The labile ester linkages make polyesters ideal feedstocks for the sustainable production of original monomers and value-added materials. Aminolysis of polyesters provides hydroxy-containing small molecules or oligomers, which are usually used as building blocks for high-performance materials synthesis. Dehydrogenative polymerization of diamine/diol or diol to polyamide and polyester provides a green and efficient method for polymer synthesis. , Therefore, we planned to use the in situ aminolysis products of PCL to afford PEAs and PAs via a dehydrogenative coupling method.

By screening the catalyst, solvent, and reaction temperature (Table S1), we confirmed the optimal conditions for PCL upcycling as follows: PNN-Ru/KO t Bu catalytic system, 1,4-dioxane, 120 °C, 72 h. Under these conditions, a series of PEAs and PAs were synthesized via the reactions of PCL with different diamines, and the relevant data are summarized in Table . The structures of the resulting polymers were analyzed by 1H and 13C NMR spectroscopy. The signals at 4.25 and 2.51 ppm in the 1H NMR spectrum are attributed to –C H 2 OCOR and RC H 2 CO– of the ester bond (Figure ), and the –C H 2 NHCOR and RC H 2 CONH– adjacent to the amide moiety resonate at 3.57 and 2.77 ppm, demonstrating the successful synthesis of both PEAs and PAs through this methodology. The composition of ester and amide in the polymer (calculated by the 1H NMR spectrum) is consistent with the [PCL]/[diamine] feeding ratio. Exceptionally, PA was obtained when the feeding ratio of [PCL]/[diamine] is equal to 1:1. To further explore the structural diversities and properties of PEAs and PAs, the diamine was changed from 1,6-hexanediamine to 1,2-bis­(2-aminoethoxy)­ethane, which could be confirmed by the characteristic resonance at 69.94 ppm (– C H2O C H2–) in the 13C NMR spectrum (Figure S12). The aminolysis reaction exhibits excellent selectivity towards primary amines, which enables the direct utilization of secondary amine-containing diamines without requiring protection–deprotection steps. When diethylenetriamine was used as the reagent, the signal of the resulting polymer at 3.73 ppm in the 1H NMR spectrum (Figure S14) is attributed to the methylene protons of –C H 2 NHC H 2 –. To incorporate aromatic functionality into the polymer, we employed p-xylylenediamine as the aminolysis reagent and monomer, as demonstrated by the characteristic aromatic proton resonance at 7.22 ppm (C6 H 4 ) in the 1H NMR spectrum (Figure S17). The PEA yield decreased under vacuum, possibly resulting from the volatilization of monomer or oligomer. This observation is consistent with the previous report of polyester synthesis, and the difference in M n changes of PEA (Table , Entry 8) and PA (Table , Entry 9) is attributed to the fluidity and crystallinity differences of polymers. Since the obtained PEAs and PAs have poor solubility in THF, even in hot dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), most of M ns of the resultant polymers were calculated by 1H NMR spectra (to verify the accuracy of the calculation method, the soluble PEAs with the amide content of 5 and 10% were synthesized, and the M ns tested by GPC are consistent with the NMR calculation results (Figure A)). For PEA derived from different 1,6-hexanediamine loadings, the M n gradually decreases as the amide content increases, probably arising from the lower solubility of the amide segment. With the expansion of the substrate scope, it was observed that the long-chain 1,10-diaminodecane can enhance the flexibility of the obtained polymer, thereby improving its M n. Notably, the upcycling of PCL with 1,2-bis­(2-aminoethoxy)­ethane exhibited high efficiency. This could be attributed to the enhanced intramolecular hydrogen bonds and the flexibility of the polymer backbone by the ether oxygen atoms, which improve the solubility of the polymer.

1. PCL Upcycling into PEAs and PAs through Aminolysis-Dehydrogenative Polymerization .

3.1.

3.1.

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a

Reaction conditions: 0.03 mmol PNN-Ru and 0.09 mmol KO t Bu, 3.0 mmol PCL (repetitive unit), 120 °C, 2.0 mL of 1,4-dioxane, in a nitrogen atmosphere for 72 h.

b

After 7 h in the nitrogen atmosphere, the reaction was carried out in vacuum.

c

Isolated yield.

d

Calculated by 1H NMR spectrum.

e

Determined by gel permeation chromatography (GPC) using polystyrene standards.

f

The M n of PCL is 2000.

g

The M n of PCL is 14,000.

h

The M n of PCL is 45,000.

i

Post-consumer PCL.

j

Reaction conditions: 0.15 mmol PNN-Ru and 0.45 mmol KO t Bu, 30.0 mmol PCL (repetitive unit) 120 °C, 20.0 mL of 1,4-dioxane, in a nitrogen atmosphere for 72 h.

2.

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1H NMR spectra of polymers with different amide contents.

3.

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(A) GPC traces of a–c (corresponding to Entries 6, 7, and 10 in Table ); (B) DSC curves for a–e (corresponding to Entries 1–5 in Table ); (C) TGA curves for a–e (corresponding to Entries 1–5 in Table ); (D) X-ray diffraction (XRD) patterns for a–e (corresponding to Entries 1–5 in Table ); (E) mechanical properties of polymer (Table , Entry 1); (F) mechanical properties of polymer (Table , Entry 11).

In contrast, the introduction of aromatic rings increases the steric hindrance along the polymer chain, impeding further chain propagation and leading to a decrease in the molecular weight. To investigate whether the M n of PCL influences the upcycling outcome, the polymer with M n of 2000, 14,000, and 45,000 was chosen to react with 1,6-hexanediamine (Table , Entries 18–20), respectively. The resulting PAs exhibited comparable molecular weights, indicating that the initial

PCL molecular weight has a negligible impact. This observation aligns with the proposed mechanism; the reaction proceeds via initial polyester aminolysis to release hydroxyl chain-ends, which subsequently undergo dehydrogenative polymerization with amino or hydroxyl groups. Notably, the PAs still contain a small amount of ester bonds, which gradually decrease as M n of PCL increases, probably attributing to the easier self-coupling reaction of the new formed hydroxyl chain-ends in the low molecular system. ,

3.2. Thermal and Mechanical Properties of PEAs and PAs

The thermal properties of the resulting polymers are summarized in Table . The T g and T m values were analyzed by differential scanning calorimetry (DSC) (Figures B, S37–S39), within the ranges of −51.8–87.4 and 182.5–237.2 °C, respectively. The T g and T m values of PA66 (Table , Entry 5) are close to those of the commercial nylon-66. Significantly, most of the PEAs derived from the feeding ratio of 9:1 did not show obvious T m according to powder X-ray diffraction characterization (Figures D, S42, S43). The thermal stability of the obtained polymer was analyzed by thermogravimetric analysis (TGA) at the initial decomposition temperature T d,5% (5% weight loss temperature), and the maximum weight loss rate temperature T d,max. As shown in Figures C and S40, S41, the T d of different polymers is also different. For the 1,6-hexanediamine-based polymer, the change of T d,5% is not consistent with the expectation. When the ratio of PCL (repetitive unit) to 1,6-hexanediamine is 8:2 (Table , Entry 2), the T d,5% (371 °C) of the resulting PEA is highest because the molecular weight is also one of the important factors affecting the thermal stability of polymers. PEA derived from the feeding ratio of 8:2 has a moderate amide content and a high molecular weight. The combined effect of the two factors contributes to this phenomenon. PEA (Table , Entry 10) has a good T d,5% due to its high molecular weight. Most T d,5%s of PAs are also above 300 °C except for PA derived from the diethylenetriamine (Table , Entry 15), maybe originating from the presence of secondary amine groups and the residue of oligomers or monomers. The T d,maxs of these polymers are greater than 400 °C, indicating their good thermal stabilities.

Since mechanical properties represent critical industrial parameters, PEA-1 (Table , Entry 1) and PEA-11 (Table , Entry 11) were selected for evaluation. PEA-1 exhibits a tensile strength of 4.6 MPa, a Young’s modulus of 47.5 MPa, and an elongation at break of 53.5% (Figure E). In contrast, PEA-11 displays significantly declined tensile strength (0.17 MPa) and Young’s modulus (0.18 MPa), but a markedly higher elongation at break of 162.1% (Figure F). These differences were caused by the monomer structure: the introduction of diethylenetriamine into PEA-11 not only disrupts the crystallinity but also enhances the chain mobility and deformability.

3.3. Model Reaction and Kinetics Study

To elucidate the mechanism, three small molecular compounds, ε-caprolactone (ε-CL, 1), n-hexamine (2), and phenylpropanol (3), were used to simulate the reaction process. The reaction mixture was tested after 30 min by ultra-high-performance liquid chromatography-time of flight mass spectrometer (UHPLC-TOF-MS) (Figure S44), and the corresponding structures of the products are summarized in Table . Three of the MS signals at 216.1957, 234.1865, and 313.2868 Da indicated that the aminolysis of ε-CL with n-hexamine, the dehydrogenative coupling of n-hexamine and phenylpropanol, or the newly formed amide-functionalized alcohol occur simultaneously. In addition, the signal at 330.2657 Da is attributed to the aminolysis–transesterification product of ε-CL and n-hexamine, and the peak at 348.2562 Da corresponds to the product of transesterification of phenylpropanol and ε-CL followed by dehydrogenative coupling with n-hexamine. It is worth noting that no newly formed ester was detected in the MS spectrum, suggesting that the dehydrogenative cross-coupling of amine and alcohol occurs prior to the self-coupling of alcohol because of the stronger nucleophilicity of amine. Therefore, we can conclude that the dehydrogenative self-coupling of alcohol occurred after the amino group was completely consumed.

2. HPLC-MS Results of the Model Reaction.

3.3.

3.3.

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A study of the relationship between M n changes and time during the reaction process was conducted. Taking the soluble PEA-10 (Table , Entry 10) as an example, the relationship between M n and time during the reaction is illustrated in Figure S45. The M n decreased rapidly and exhibited first-order kinetics in the beginning because of a swift aminolysis reaction, which subsequently shifted to zero-order kinetics as the amine was consumed. This is consistent with the previous report. The reaction of PCL/1,6-hexanediamine = 9:1 also follows this rule (Figure S46). After 8 h, only a small part of the amino group remained, and the dehydrogenative self-coupling of alcohols began to be dominant.

3.4. Plausible Reaction Mechanism

Based on the experimental results and relevant reports, a plausible pathway for the upcycling of PCL into PEAs and PAs is proposed in Figure . Firstly, the aminolysis of PCL occurs and yields amide-functionalized oligomers containing −OH and/or −NH2 end groups. Secondly, the newly formed −OH reacts with the dearomatized PNN-Ru complex I to generate the aromatized ruthenium alkoxide II, which transforms into trans-dihydride species III along with aldehyde release via β-H elimination. In the meantime, II can also directly attack the ester bond in PCL to afford the transesterification product and alkoxide compound II′. Thirdly, the newly produced aldehyde reacts immediately with amine to afford a hemiaminal intermediate, which reacts with III to form ruthenium hemiaminal oxide IV, which finally generates the amide moiety via β-H elimination again. When the molar amount of diamine is less than that of PCL (repetitive unit), bishydroxyl-terminated PEA oligomers are obtained by repeating these three steps. Finally, the dehydrogenative self-coupling of these oligomeric diols occurs through the hemiacetal oxide intermediate VI and forms new ester linkages, ultimately leading to the high molecular PEA. When the molar amount of diamine is equal to that of PCL (repetitive unit), PA is obtained through repeating the aminolysis of esters and dehydrogenative cross-coupling of −OH/–NH2 end-groups processes, and the ester linkage derived from the transisterification reaction will also be absolutely consumed. Owing to the presence of the OH group in the system, the ruthenium dihydride complex III is supposed to be the real active species.

4.

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Proposed mechanism for upcycling of PCL using the ruthenium complex.

3.5. Upcycling of Post-Consumer PCL

With this new upcycling method in hand, we set out to verify its practical application by using the post-consumer PCL (3D printing pen filament) as a starting material (Figure ). Under the optimal conditions, PEA and PA (Table , Entries 21 and 22) were successfully prepared, suggesting that the one-pot upcycling of PCL waste into high-performance materials has significant industrialization potential.

5.

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Commercial PCL upcycling to PA66 and PEA.

4. Conclusions

We have demonstrated a novel aminolysis-dehydrogenative polymerization strategy for waste PCL upcycling into PAs or PEAs in high yields (>85%) with controllable ester-to-amide ratios, tunable functional moieties, and narrow molecular weight distribution catalyzed by a PNN-Ru compound. The resulting polymers exhibit high thermal stability (T d,max, 388–452 °C) and tunable thermal properties. PA derived from the upcycling of PCL with 1,6-hexanediamine has T g and T m values comparable to those of the commercial nylon-66. With the effects of flexibility of the chain and hydrogen bond, the mechanical properties of polymers undergo huge changes. Mechanism studies indicate that aminolysis, dehydrogenative cross-coupling of amine and alcohol, and transesterification occur at the same time, and dehydrogenative self-coupling of alcohol subsequently takes place after amine is completely consumed. The kinetics studies show that the aminolysis reaction is first-order dependent in the early stage and zero-order dependent in the latter stage. This new strategy has been successfully used to upgrade the post-consumer PCL to PEAs and PAs. Overall, our findings demonstrate the feasibility of the one-pot upcycling strategy for PCL, offering a novel method for plastic waste transfer into high-value materials. Based on the upcycling of PCL, the strategy can be further extended to other common polyester plastics (polybutylene succinate (PBS), polyethylene terephthalate (PET), polybutylene terephthalate (PBT)) to demonstrate the universal upcycling of polyester waste. Meanwhile, by introducing a wider variety of functional monomers, the structural and performance regulation of the products can be further enriched to meet the application requirements of different fields.

Supplementary Material

ps6c00039_si_001.pdf (3.2MB, pdf)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (no. 22261034), the Natural Science Foundation of Jiangxi Province (nos. 20232ACB213008, 20224BAB203023), and the Open Research Fund of State Key Laboratory of Polymer Science and Technology, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (no. PPCL2024-07).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/polymscitech.6c00039.

  • Experimental details; NMR spectra; DSC and TGA curves; XRD for polymers; UHPLC-TOF-MS for model reaction; curves of kinetics study (PDF)

Y.R. and C. Yao conceived the project, designed the synthesis of polymers, and wrote the manuscript. Y.R. performed experiments and characterization. Y.R., S.W., F.Z., S.S., C.Yu., Y.P., Y.L. analyzed the data. C. Yao supervised this research. All authors discussed and commented on the manuscript.

The authors declare no competing financial interest.

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