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Article

Molecular-Level Regulation of Nitrogen-Doped Ordered Mesoporous Carbon Materials via Ligand Exchange Strategy

by
Dandan Han
*,
Zhen Quan
,
Congyuan Hu
,
Xiaopeng Wang
,
Lixia Wang
,
Ruige Li
,
Xia Sheng
,
Yanyan Liu
,
Meirong Song
and
Xianfu Zheng
College of Science, Henan Agricultural University, Zhengzhou 450002, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(5), 1558; https://doi.org/10.3390/pr13051558
Submission received: 12 March 2025 / Revised: 8 May 2025 / Accepted: 14 May 2025 / Published: 18 May 2025
(This article belongs to the Special Issue Design and Performance Optimization of Heterogeneous Catalysts)
Browse Figures
Versions Notes

Abstract

:
Ordered mesoporous carbon materials (OMCMs) are widely used as high-performance electrode materials due to their uniform pore structure, excellent electrical conductivity, and good stability. In this paper, three OMCMs with controllable N content were prepared by a nanocasting method using Fe3O4 nanocrystals as the template and organic ligands as the carbon source. By adopting a ligand exchange strategy, oleic acid, oleic amine, and octyl amine were successfully capped onto the Fe3O4 nanocrystals, respectively, which allowed the rational control of the elemental composition of OMCMs at the molecular level. Further characterizations revealed that the nitrogen content of the resulting OMCMs increased as the proportion of nitrogen atoms in the ligand increased, while the order of the porous structure decreased as the hydrocarbon chain length decreased. This study demonstrates that both the N-doping content and the order of the OMCMs are influenced by the N-containing ligand. This finding will provide a fundamental aspect for their further applications as high-performance electrode and catalytic materials in the field of electrochemistry.

1. Introduction

With the depletion of fossil resources and the increasing prominence of environmental concerns, the development of sustainable energy technologies has attracted widespread attention of governments and societies around the world in recent years [1,2,3]. Among the various technologies, electrochemical energy storage technologies, such as lithium-ion batteries and supercapacitors, stand out for their high energy density and good cycling performance [4,5,6,7]. In light of these considerations, electrode materials have emerged as a significant area of interest, given their critical role in the advancement of high-performance energy storage devices.
The use of ordered mesoporous carbon materials (OMCMs) as electrode materials in electrochemical energy storage is widespread due to their favorable metrics in conductivity, stability, and ultra-high specific surface area [8,9,10,11]. Templating is the most common method for the synthesis of OMCMs. For example, using mesoporous silica (e.g., SBA-15) as a sacrificial template, organic carbon sources such as sucrose are filled into the pores of the silica by nanocasting methods, and after high-temperature carbonization and template removal, CMK series OMCMs are obtained [12,13,14].
Generally, there are two main strategies to synthesize N-doped MCMs, one is the post-treatment method [15,16,17,18,19], which involves a thermal treatment of MCMs with nitrogen-containing substance (e.g., ammonia) to allow nitrogen atoms to infiltrate the carbon matrix. Another strategy is the in situ doping method [20,21,22,23,24], where the nitrogen containing substances are mixed with carbon precursors prior to carbonization. Due to the avoidance of additional high-temperature treatment, the in situ doping method has recently received increasing attention.
For example, N-doped OMCMs have been prepared by using phenolic resin as carbon source, cyanamide as nitrogen source, and SBA-15 as sacrificial template [25,26,27]. The obtained N-doped OMCMs exhibited a large pore volume and high specific surface area, enabling a high specific capacitance and cycle stability as electrode material in supercapacitors [26]. Although the in situ preparation of N-doped OMCMs shows good electrochemical properties, there are still some weaknesses regarding their composition and structural properties. First, the carbon wall thickness of OMCMs is less tunable by using mesoporous silica as a critical template, which may lead to unexpected drawbacks in electrochemical performance [28]. Then, it is difficult to precisely control the location of nitrogen elements in the obtained OMCMs. Most of the nitrogen atoms are buried in the carbon matrix, making it difficult to contact with the reaction media, which would possibly impose a reduction in electrochemical performance [29,30]. Therefore, the delicate synthesis of Ni-doped OMCMs with the precise control of composition and structures is still a challenge.
Herein, we report the preparation of OMCMs using organic ligand-capped Fe3O4 nanocrystals (NCs) as building blocks. Evaporation-induced self-assembly of NCs resulted in the formation of a nanocrystal superlattice, which was sequentially carbonized and acid-etched to form OMCMs (Figure 1). By varying the ligands, the N content and carbon wall thickness can be precisely controlled at the molecular level. This study provides a fundamental basis for the rational design and precise control of OMCMs for high-performance energy storage materials.

2. Experimental Materials and Methods

2.1. Materials

Oleic acid (90%), 1-octadecene (90%), and oleyl amine (70%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Anhydrous ethanol, n-hexane, chloroform (CHCl3), N, N-dimethylformamide (DMF), and hydrochloric acid (36.0~38.0%) were provided by Tianjin Fuyu Co., Ltd. (Tianjin, China). Octylamine (99%) and iron oleate (95%) were purchased from Macklin Reagent Company (Shanghai, China). Argon gas (99.99%) was provided by Henan Keyi Co., Ltd. (Zhengzhou, China).

2.2. Sample Preparation

2.2.1. Fe3O4 Nanocrystal Preparation

In a typical run, 9 g of iron oleate, 2.3 g of oleic acid, and 50 g of 1-octadecene were added in a 250 mL three-neck flask. The mixture was heated to 50 °C under vacuum to remove low boiling point organic solvent impurities, and then the temperature was raised to 120 °C and evacuated for 30 min. After that, argon gas was introduced. The evacuation process was repeated for three times. Then, the temperature was raised to 320 °C under an argon atmosphere and maintained for 60 min under vigorous stirring. After the complete pyrolysis of iron oleate, the system was cooled to room temperature, and the particles were washed with ethanol and isopropanol twice by centrifugation. Finally, the obtained Fe3O4 nanocrystals were redispersed in n-hexane at a concentration of approximately 40 mg mL−1.

2.2.2. Ligand Exchange

Next, 10 mL of Fe3O4 nanocrystals dispersed in n-hexane was added to 10 mL of DMF, and the solution was divided into two phases, with DMF at the bottom and n-hexane at the top. Then, 0.1 mL of fluoroboric acid was added dropwise, followed by vigorous shaking, the Fe3O4 nanocrystals transferred from the n-hexane phase to the DMF phase, indicating that BF4− had successfully been modified to the surface of the nanocrystals. The DMF phase was separated and placed in another glass bottle, an appropriate amount of n-hexane was added, and the solution was again divided into two phases with Fe3O4 nanocrystals in the lower phase. Then, 50~100 µL of oleyl amine or octylamine was added dropwise, followed by vigorous shaking. The Fe3O4 nanocrystals were transferred from the lower DMF phase to the upper n-hexane phase, and the BF4− on the surface of the nanocrystals was replaced by nitrogen-containing ligands. The nanocrystals in the n-hexane phase were collected for further use.

2.2.3. Mesoporous Carbon Preparation

Next, 5 mL of Fe3O4 nanocrystals in n-hexane with a concentration of approximately 40 mg/mL was added in a 20 mL porcelain boat and placed in fume hood for evaporation and self-assembly. The self-assembly was collected and calcined at 400 °C for one hour under nitrogen atmosphere with a heating rate of 2 °C per minute. During the calcination process, the surface organic ligands converted into a carbon layer, and nitrogen atoms were incorporated in situ into the carbon layer. The calcined sample was added to a dilute sulfuric acid solution to etch the Fe3O4, and the ordered mesoporous carbon material was obtained by filtration and drying.

2.3. Sample Characterizations

X-ray diffraction (XRD) patterns were conducted on a Bruker D8 Advance X-ray diffractometer (Bruker, Mannheim, Germany) using Cu Ka radiation (λ = 1.5406 Å, 40 kV, 40 mA) in the 2θ range of 20~80°; N2 adsorption–desorption measurement was performed using a Micromeritics ASAP 2460 adsorption analyzer. Before the measurements, all samples were degassed at 150 °C under evacuation for 6 h. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method; Fourier transform infrared spectroscopy (FTIR) was performed on a Thermo Fisher infrared analyzer (model IS10, Thermo Fisher Scientific, Waltham, MA, USA), and samples were tested using potassium bromide tableting method; scanning electron microscope (SEM) images were taken on a Zeiss Ultra-55 instrument (Zeiss, Oberkochen, Germany); a transmission electron microscopy (TEM) analysis was performed on a TecnaiG2 20 TWIN instrument (FEI Company, Hillsboro, OR, USA); X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Thermo SCIENTIFIC Nexsa system using Al Kα monochromatic radiation (Thermo Fisher Scientific, Waltham, MA, USA).

3. Results and Discussion

3.1. Synthesis of Fe3O4 Nanocrystals and Ligand Exchange

To prepare N-doped OMCMs and tune the N content at the molecular level, monodispersed oleic acid-coated Fe3O4 nanocrystals (OA@Fe3O4) were first synthesized using a high-temperature pyrolysis method. Then, the OA@Fe3O4 nanocrystals were subjected to a two-step ligand-exchange processes to replace the oleic acid with oleyl amine (OAm) and octylamine (Otyl), respectively. The ligand-exchange processes have been reported in our previous publications [22,31,32,33,34] and representative phase transfer photographs during the ligand-exchange process are shown in Figure 2a. The carboxyl group (-COOH) of oleic acid and the amino groups (-NH2) of oleylamine and octylamine can coordinate on the surface of Fe3O4 nanoparticles, while the long hydrocarbon chain can provide steric hindrance and van der Waals forces to stabilize the nanocrystals in non-polar solvents and prevent their aggregation. As expected, Fe3O4 nanocrystals capped with oleic acid are hydrophobic, which can be well-dispersed in non-polar solvent (n-hexane) in the upper phase. However, after the ligand exchange with HBF4, the BF4− ions were grafted onto the Fe3O4 nanocrystals, which show hydrophilic properties and can be dissolved in polar solvent (N, N-dimethylformamide, DMF) in the lower phase. After the second ligand exchange with amine ligand, the Fe3O4 nanocrystals became hydrophobic again and moved up to the n-hexane phase. Therefore, according to the phase transfer phenomenon, the oleic acid on the surface of Fe3O4 nanoparticles was successfully replaced by nitrogen-containing ligands, with BF4− ions serving as a bridge for the exchange.
Figure 2b–e show the TEM images of Fe3O4 nanocrystals with different ligands during and after ligand-exchange process. It can be seen that the obtained monodispersed OA@Fe3O4 nanocrystals have a narrow particle size distribution ranging from 8 to 10 nm (Figure 2b). Figure 2c–e are TEM images of Fe3O4 nanocrystals after the ligand exchange, demonstrating that the size and morphology of the nanoparticles unchanged before and after the ligand exchange.
To demonstrate the successful exchange of ligands, we conducted a Fourier infrared spectroscopy (FTIR) analysis of the Fe3O4 nanocrystals coated with different ligands (Figure S1). After the ligand exchange, the appearance of the C-N bond absorption peak at 1053 to 1070 cm−1 confirmed the successful exchange of oleic acid by nitrogen-containing ligands [35]. Additionally, after the exchange, the absorption peak of C-H (2800 to 3000 cm−1) shifted towards longer wavelengths, indicating that the long-chain oleic acid was replaced by shorter-chain ligand octylamine [36].

3.2. Assembly and Carbonization of Fe3O4 Nanocrystals

Based on the successful ligand exchange of Fe3O4 nanocrystals, the monodispersed Fe3O4 nanocrystals were subjected to evaporation-induced self-assembly process. The resulting self-assemblies were calcined under an N2 atmosphere to carbonize the organic ligands into a carbon framework. The carbonized Fe3O4 assemblies were subjected to XRD and SEM characterization to verify their crystal phase and morphology. Figure S2 shows the XRD patterns of the Fe3O4 assemblies obtained with different ligand-capped nanocrystals. A set of diffraction peak locates at 30°, 35°, 43°, 54°, and 63° correspond to (220), (311), (400), (511), and (440) crystal planes of Fe3O4 [37], respectively, confirming the spinel-derived Fe3O4 phase was obtained. Ligand-exchange processes did not have any effect on the crystal phase of the Fe3O4 assemblies.
The formation of the nanocrystal superlattice was confirmed by scanning electron microscopy (SEM) analysis. As shown in Figure 3a, the initial OA@Fe3O4 assembly evolved into a persistent, well-organized face-centered cubic (FCC) superlattice [38]. After the ligand exchange, the formation of the nanocrystalline superlattice structure was still observed (Figure 3b,c). However, as the length of the ligand reduced, the degree of orderliness slightly decreased. It is well known that the driving force of self-assembly is the van der Waals force [39]. The length of the hydrocarbon chains plays a crucial role in providing van der Waals force to derive the orderly self-assembly process.

3.3. Preparation and Characterization of OMCMs

Furthermore, hydrochloric acid etching was carried out on the carbonized assemblies, and nitrogen-doped OMCMs were obtained after the removal of Fe3O4. TEM was performed to analyze the resultant OMCMs (Figure 4a–d). All the OMCMs show a highly ordered mesoporous structures with a pore size of 8~10 nm, inheriting from the Fe3O4 nanocrystals. HRTEM image of the OMCMs derived from Otyl@Fe3O4 assembly indicated that the as-prepared OMCMs are composed of ultra-thin interconnected carbon shells with a thickness of approximately 1.6 nm (Figure 4d). The formation of mesoporous framework is attributable to the close-packed arrangement of Fe3O4 nanocrystal superlattices. During the process of thermal treatment for ligand carbonization, the superlattice undergoes contraction due to the reduction in interparticle spacing, resulting in interconnected adjacent carbon shells, which is robustly intact under the removal of Fe3O4 nanocrystals by HCl. Moreover, STEM-EDS mapping images of OMCMs derived from nitrogen-containing OAm@Fe3O4 and Otyl@Fe3O4 nanocrystal superlattices showed homogeneous distribution of N atoms (Figure 4e–l), confirming that the mesoporous carbon framework is derived from organic ligands. No detectable residual Fe was observed by an EDS analysis (Figure S3).
X-ray photoelectron spectroscopy (XPS) analysis was also performed to verify the surface nitrogen content of the OMCMs. As shown in Figure 5a, it can be seen from the XPS survey spectra that the OMCMs derived from different ligands such as oleic acid, oleylamine, and octylamine exhibit three distinct peaks attributed to C (C 1s), O (O 1s), and N (N 1s), respectively, suggesting the successful incorporation of N atoms into the carbon frameworks [40]. In agreement with the EDS analysis, the presence of residual Fe was not observed. As demonstrated in Figure 5b, the N 1s XPS of the OMCMs derived from OAm@Fe3O4 and Otyl@Fe3O4 superlattices exhibited three fitting peaks at ~397.3, ~398.0, and ~399.4 eV, which were attributed to pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen, respectively [32,33]. The higher concentration of graphitic N in OMCM (OAm) than in OMCM (Otyl) suggests that the short hydrocarbon chain of the ligand leads to less crystallization of the carbon frameworks. Moreover, C 1s and O 1s XPS spectra of the OMCM (Otyl) sample are presented in Figure S4. The C 1s peak can be resolved into four peaks located at 284.0, 284.9, 285.4, and 288.1 eV, which are attributed to C-C, C-N, C-O, and O-C=O, respectively. The O 1s spectrum can be fitted into three components, namely quinone, O-C=O, and C-OH, with binding energies of 530.7, 532.1, and 534.3 eV, respectively.
Subsequent XRD and Raman analysis of the as-synthesized OMCMs was conducted in order to verify the crystallinity of the OMCM frameworks. As demonstrated in Figure 5c, all the OMCM samples display a characteristic broad diffraction peak at 2θ degree ranging from 25° to 26°, indicative of the (002) planes of low-density graphitic materials [32]. The Raman spectra of OMCMs further provide structural information about the material (Figure 5d). It has been established that all OMCMs samples exhibit a D band located in the vicinity of 1395 cm−1, and a G band in the proximity of 1586 cm−1. The D band is indicative of structural defects in the carbon frameworks, while the G band is attributed to the in-plane stretching vibration of sp2-hybridized carbon [33]. The ID/IG value of OMCMs derived from OAm@Fe3O4 and the ID/IG value of Otyl@Fe3O4-derived OMCMs are both significantly higher than that of OMCM (OA) (~0.58). This finding indicates that the introduction of N atoms will result in a lower crystallinity of carbon frameworks.
The N contents of the as-synthesized OMCMs are listed in Table 1. Compared to the proportion of N (calculated by the N/(C + N) ratio) in the original ligand molecules, the N content in the obtained MCMs is reduced, which may be since some of the nitrogen atoms are vaporized during carbonization [41]. Notably, as the proportion of N atoms in the ligand molecules increases, the content of nitrogen elements in the derived MCMs also increases, demonstrating that the N content was successfully tuned by the ligand exchange strategy. Moreover, the N contents obtained from XPS and EDS were almost the same, which is abnormal because EDS analyzes the bulkier phase of the material, while XPS only detects the near surface (first and second layer) of the materials. Thus, the similar N content obtained from both the STEM-EDS analysis and XPS suggests that almost all N atoms are exposed in the obtained N-doped MCMs, which would be significantly important for their applications.
Furthermore, a comprehensive review of the literature on nitrogen-doped ordered mesoporous carbon materials has been conducted. As shown in Table S1, we summarized and compared the nitrogen content, specific surface area and pore size of the N-doped ordered mesoporous carbon materials. Our prepared N-doped OMCMs exhibited competitive advantages in terms of a high BET surface area, large pore size, and relatively high N-doping content. The potential limitation of this molecular-based preparation method could be the difficulty in reconciling the high order and crystallinity of the mesoporous carbon frameworks with the high N-doping content.

4. Conclusions

In this work, we have exploited the self-assembly behavior of Fe3O4 nanocrystals with different ligands by ligand exchange to prepare ordered mesoporous carbon materials with tunable nitrogen content. It has been demonstrated that the proportion of nitrogen atoms in the ligand molecule directly affects the nitrogen content of the resulting mesoporous carbon materials. Moreover, ligands with long hydrocarbon chains are favorable for the formation of highly ordered nanocrystal superlattices. This study provides an adaptable methodology for the preparation of highly ordered mesoporous carbon materials with controllable surface-exposed N atoms at the molecular level.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13051558/s1, Figure S1: FTIR analysis of oleic acid, oleylamine and octylamine capped Fe3O4 nanocrystals; Figure S2: XRD patterns of oleic acid, oleylamine and octylamine capped Fe3O4 nanocrystal superlattices; Figure S3: Energy dispersive spectroscopy analysis of OMCM(OAm) and OMCM(Otyl); Figure S4: C 1s and O 1s XPS spectra of OMCM(Otyl); Table S1: Comparison of the as-synthesized N-doped ordered mesoporous carbon materials with the literature [42,43,44,45,46,47,48,49].

Author Contributions

Conceptualization, D.H.; methodology, D.H. and Z.Q.; validation and formal analysis, Z.Q. and C.H.; resources, X.W., L.W., R.L., X.S. and Y.L.; writing—original draft preparation, D.H. and Z.Q.; writing—review and editing, D.H., M.S., and X.Z.; supervision, D.H., M.S. and X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the financial supports from the National Natural Science Foundation of China (No. 22302058) and Young Talent Program of Henan Agricultural University (111/30500909).

Data Availability Statement

The original contributions presented in this study are included in the article. The data further supporting this study’s findings are available from the first author, Dandan Han, upon reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Synthesis of OMCMs from organic ligand-capped Fe3O4 nanocrystals.
Figure 1. Synthesis of OMCMs from organic ligand-capped Fe3O4 nanocrystals.
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Figure 2. Illustration and TEM analysis of ligand-exchange process. (a) Photographs of phase transfer phenomena during ligand exchange; TEM images of (b) OA@Fe3O4 nanocrystals, (c) BF4−@ Fe3O4 nanocrystals, (d) OAm@Fe3O4 nanocrystals, and (e) Otyl@Fe3O4 nanocrystals.
Figure 2. Illustration and TEM analysis of ligand-exchange process. (a) Photographs of phase transfer phenomena during ligand exchange; TEM images of (b) OA@Fe3O4 nanocrystals, (c) BF4−@ Fe3O4 nanocrystals, (d) OAm@Fe3O4 nanocrystals, and (e) Otyl@Fe3O4 nanocrystals.
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Figure 3. SEM images of Fe3O4 nanocrystal superlattices. (a) OA@Fe3O4 assembly; (b) OAm@Fe3O4 assembly; (c) Otyl@Fe3O4 assembly.
Figure 3. SEM images of Fe3O4 nanocrystal superlattices. (a) OA@Fe3O4 assembly; (b) OAm@Fe3O4 assembly; (c) Otyl@Fe3O4 assembly.
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Figure 4. TEM images and its corresponding STEM-EDS mapping images. TEM of OMCMs derived from (a) OA@Fe3O4 assembly, (b) OAm@Fe3O4 assembly, and (c) Otyl@Fe3O4 assembly; (d) HRTEM image of OMCMs derived from Otyl@Fe3O4 assembly; STEM and corresponding STEM-EDS mapping images of OMCMs derived from (e–h) OAm@Fe3O4 assembly, and (i–l) Otyl@Fe3O4 assembly.
Figure 4. TEM images and its corresponding STEM-EDS mapping images. TEM of OMCMs derived from (a) OA@Fe3O4 assembly, (b) OAm@Fe3O4 assembly, and (c) Otyl@Fe3O4 assembly; (d) HRTEM image of OMCMs derived from Otyl@Fe3O4 assembly; STEM and corresponding STEM-EDS mapping images of OMCMs derived from (e–h) OAm@Fe3O4 assembly, and (i–l) Otyl@Fe3O4 assembly.
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Figure 5. Analysis of ordered mesoporous carbons materials derived from OA@Fe3O4, OAm@Fe3O4, and Otyl@Fe3O4 nanocrystal superlattices. (a) XPS survey spectra; (b) N 1s spectra; (c) XRD patterns; (d) Raman spectra.
Figure 5. Analysis of ordered mesoporous carbons materials derived from OA@Fe3O4, OAm@Fe3O4, and Otyl@Fe3O4 nanocrystal superlattices. (a) XPS survey spectra; (b) N 1s spectra; (c) XRD patterns; (d) Raman spectra.
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Table 1. Summary of characterization results for OMCM.
Table 1. Summary of characterization results for OMCM.
LigandBET Surface Area
(m2/g)		N Content in LIGANDS
(N/C + N)
(%)		N Content Measured by EDS
(%)		N Content Measured by XPS
(%)
Oleic acid969.6000
Oleyl amine683.55.20.60.6
Octylamine602.810.81.61.7
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MDPI and ACS Style

Han, D.; Quan, Z.; Hu, C.; Wang, X.; Wang, L.; Li, R.; Sheng, X.; Liu, Y.; Song, M.; Zheng, X. Molecular-Level Regulation of Nitrogen-Doped Ordered Mesoporous Carbon Materials via Ligand Exchange Strategy. Processes 2025, 13, 1558. https://doi.org/10.3390/pr13051558

AMA Style

Han D, Quan Z, Hu C, Wang X, Wang L, Li R, Sheng X, Liu Y, Song M, Zheng X. Molecular-Level Regulation of Nitrogen-Doped Ordered Mesoporous Carbon Materials via Ligand Exchange Strategy. Processes. 2025; 13(5):1558. https://doi.org/10.3390/pr13051558

Chicago/Turabian Style

Han, Dandan, Zhen Quan, Congyuan Hu, Xiaopeng Wang, Lixia Wang, Ruige Li, Xia Sheng, Yanyan Liu, Meirong Song, and Xianfu Zheng. 2025. "Molecular-Level Regulation of Nitrogen-Doped Ordered Mesoporous Carbon Materials via Ligand Exchange Strategy" Processes 13, no. 5: 1558. https://doi.org/10.3390/pr13051558

APA Style

Han, D., Quan, Z., Hu, C., Wang, X., Wang, L., Li, R., Sheng, X., Liu, Y., Song, M., & Zheng, X. (2025). Molecular-Level Regulation of Nitrogen-Doped Ordered Mesoporous Carbon Materials via Ligand Exchange Strategy. Processes, 13(5), 1558. https://doi.org/10.3390/pr13051558

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