⇦Chinese Physics Letters, 2022, Vol. 39, No. 10, Article code 108201 New Insight of Fe Valence State Change Using Leaves: A Combined Experimental and Theoretical Study Zejun Zhang (张泽军)1,2, Yizhou Yang (杨一舟)3*, Jie Jiang (江杰)4, Liang Chen (陈亮)4,5, Shanshan Liang (梁珊珊)3*, and Haiping Fang (方海平)3 Affiliations 1Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 2University of Chinese Academy of Sciences, Beijing 100049, China 3School of Physics, East China University of Science and Technology, Shanghai 200237, China 4School of Physical Science and Technology, Ningbo University, Ningbo 315211, China 5Department of Optical Engineering, Zhejiang Provincial Key Laboratory of Chemical Utilization of Forestry Biomass, Zhejiang A&F University, Hangzhou 311300, China Received 3 August 2022; accepted manuscript online 19 September 2022; published online 30 September 2022 *Corresponding authors. Email: yangyizhou@ecust.edu.cn; liangshanshan@ecust.edu.cn Citation Text: Zhang Z J, Yang Y Z, Jiang J et al. 2022 Chin. Phys. Lett. 39 108201    Abstract Fe$^{2+}$ is of considerable importance in plant growth and crop production. However, most Fe elements in nature favor existing in the trivalent state, which often causes the deficiency of Fe$^{2+}$ in plants. Here, we report the Fe valence state change from Fe$^{3+}$ to Fe$^{2+}$ by using leaves. This valence state change was confirmed by x-ray photoelectron spectroscopy in Fe-Cl@leaves. Fourier transform infrared and ultraviolet-visible spectroscopy demonstrated that aromatic ring groups were included in leaves, and cation-$\pi$ interactions between Fe cations and the components containing aromatic rings in leaves were measured. Further, density functional theory calculations revealed that the most stable adsorption site for hydrated Fe$^{3+}$ cation was the region where hydroxyl groups and aromatic rings coexist. Moreover, molecular orbital and charge decomposition analysis revealed that the aromatic rings took the major part (59%) of the whole net charge transfer between leaves and Fe cations. This work provides a high-efficiency and eco-friendly way to transform the Fe valence state from Fe$^{3+}$ to Fe$^{2+}$, and affords a new insight into the valance change between plant organisms with cations.
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 DOI:10.1088/0256-307X/39/10/108201 © 2022 Chinese Physics Society Article Text Fe element, especially Fe$^{2+}$, is one of the essential components responsible for various biochemical processes of living organisms, including photosynthesis, chlorophyll synthesis, and respiration.[1-4] Lack of Fe$^{2+}$ would cause the decline of photosynthetic components, interveinal chlorosis of young leaves, and poor root formation in plants, resulting in reduced crop yields, which attracts high attention when the whole world is facing a food crisis.[5-7] However, Fe in nature mainly exists at trivalence due to the oxygen partial pressure and soil pH, together with the difficulties in migrating.[5,8] This often causes the lack of Fe$^{2+}$ adsorption and utilization by plants. Even though plants own some strategies to transform Fe$^{3+}$ to Fe$^{2+}$, such as reduction by membrane-bound Fe$^{3+}$ chelate reductases secreted from roots,[9] Fe deficiency still occurs in most cases, which severely affects plant growth and crop yield.[5] On the other hand, inorganic iron fertilizers like FeSO$_{4}$[10] are commonly used to improve Fe$^{2+}$ abundance and availability for plants in soil. However, such applications are sometimes found to be lack of efficiency due to the favor of Fe$^{2+}$ becoming Fe$^{3+}$ oxides form.[5,10] Synthetic Fe chelates[11] like Fe-EDTA show better effects than mineral fertilizers, but more expensive. In recent decades, transgenic approaches have gradually become a powerful molecular engineering technology to improve crops' nutrient uptake of Fe from the soil.[12-14] However, these technics may carry hidden costs in environment and attract concerns of the public at large for safety of food.[15] We noted that the valence state change of Fe during biochemical reactions is based on the charge transfer, which is widely present during the chelation between Fe$^{3+}$ and enzymes secreting by plant roots.[3,16] Among the charge transfer mechanisms, there is a direct charge transfer from donors to cations, i.e., cation-$\pi$ interaction.[17,18] Cation-$\pi$ interactions[19,20] are non-covalent interactions between cations and $\pi$ electrons-rich aromatic ring structures, which have shown vital roles in various discipline areas, including physics, chemistry, biology, and materials sciences.[21-26] Shi et al. demonstrated that organic mass with aromatic rings adsorbs cations actively,[27] with charge transfer between aromatic rings and cations.[28] There are plenty of organic masses in leaves, including lignin, polyphenols, and vitamins,[29-32] which contain aromatic rings in their molecular structures.[29] Therefore, studying the cation-$\pi$ interaction between these molecules and Fe cations, with the charge transfer during the reactions in between, may give a new sight into the transformation and adsorption mechanism of Fe elements by plants. In this Letter, we report experimental observation of the Fe valence state change from Fe$^{3+}$ to Fe$^{2+}$ using tea tree, wintersweet, and other leaves. This valence charge change is confirmed by x-ray photoelectron spectroscopy (XPS). Fourier transform infrared (FT-IR) spectroscopy and ultraviolet-visible (UV-vis) spectroscopy reveal that there are strong cation-$\pi$ interactions between hydrated Fe cations and substances with aromatic rings in the leaves. DFT calculations show the inner physical mechanism that the most stable adsorption position for the Fe cation should be the region where hydroxyl groups and aromatic rings coexist. Molecular orbital and charge analyses reveal that the charge transfer from the aromatic ring to the cation takes the major part (59%) of the whole net charge transfer. Overall, this work provides a high-efficiency and eco-friendly way to transform the Fe valence state from Fe$^{3+}$ to Fe$^{2+}$, and gives a new insight into the valance change between plant organisms with cations. The Fe-Cl@leaves sediments were prepared by vacuum filtration method (Fig. 1 and details in PS1 of the Supplementary Information). We first ground 2.0 g of leaves to leaf powder and dispersed them in 40 mL of deionized (DI) water. Then, 20 mL of supernatant of leaves dispersion was added into a bottle containing 20 mL of FeCl$_{3}$ solution (120 mM). The bottle was stored still for 3 h under ambient conditions. The mixture in bottle was vacuum filtered onto an ultrafilter membrane. The Fe-Cl@leaves sediments were dried at 50 ℃ for 12 h in a dry chamber. XPS measurements were carried out to analyze the Fe oxidation state in the Fe-Cl@leaves sample. Figure 2(a) shows that with the full-scan XPS spectra, obvious adsorptions of Fe in the sample were observed, which is consistent with the scanning electron microscopy and energy dispersive spectroscopy analysis (see Fig. S1 for more details in the Supplementary Information). We also note that there are C, O, N, Fe, and Cl elements included in the Fe-Cl@leaves sample, but no Fe and Cl elements are detected in the leaves powder, as shown in Fig. 2(a). C $1s$ with a value of 284.8 eV was used to correct the binding energies of the samples. The peaks at 288.0 eV and 286.2 eV are assigned to C=O and C–O bonds, respectively, according to the C $1s$ partial spectrum of the Fe-Cl@leaves sample [Fig. S2(a) in the Supplementary Information]. The XPS peaks of Fe $2p_{1/2}$ and Fe $2p_{3/2}$ for the FeCl$_{3}$ standard crystals, Fe-Cl@leaves sample, and FeCl$_{2}$ standard crystals are shown in Fig. S2(b). The binding energies of Fe $2p_{3/2}$ for FeCl$_{3}$ crystals, Fe-Cl@leaves sample and FeCl$_{2}$ crystals are 711.3 eV, 710.9 eV and 710.8 eV, respectively. Peak fitting of the Fe $2p$ and quantitative analysis of Fe$^{2+}$/Fe$^{3+}$ for Fe-Cl@leaves sample [Fig. 2(b) and Fig. S2(c)] reveal that the Fe$^{2+}$ component is about 59.5% of the total Fe in the sample. Moreover, we observed a similar shift in the binding energy of Fe $2p$ in other Fe-Cl@leaves samples, such as wintersweet leaves and ginkgo biloba (Fig. S3 in the Supplementary Information). These results indicate that the valence state of the Fe element changed from trivalent to divalent after leaves dispersion being added into FeCl$_{3}$ solution.
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Fig. 1. Schematic diagram of the sample preparation.
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Fig. 2. Spectra of Fe-Cl@leaves samples. (a) Full-scan XPS spectra of leaves powder and Fe-Cl@leaves samples. (b) Partial spectra of the Fe $2p$ peak of Fe-Cl@leaves. The Fe $2p_{3/2}$ peak of the Fe$^{2+}$ component is pointed out by a red arrow, indicating the existence of Fe$^{2+}$ in Fe-Cl@leaves. (c) FTIR spectra of leaves powder and Fe-Cl@leaves samples in the mid-infrared. (d) UV-vis spectra of leaves dispersion, Fe-Cl@leaves dilute solution, FeCl$_{3}$ and FeCl$_{2}$ dilute solution, respectively.
We performed FT-IR spectroscopy to identify the functional groups in leaf powder that possibly involved in the interaction between Fe cation and leaves in Fe-Cl@leaves sample. As shown in Fig. 2(c), the strong and wide band at 3285 cm$^{-1}$ are the stretching vibration absorption of –OH functional groups in leaves. Stretching vibration absorption of C–H is attributed to the two bands at 2919 and 2851 cm$^{-1}$. The band at 1734 cm$^{-1}$ in the spectrum is assigned to the presence of C=O groups. The bands at 1623, 1520, and 1445 cm$^{-1}$ are the characteristics of skeletal stretching vibration absorption of C=C groups in aromatic rings. The bands at 763, 823, and 895 cm$^{-1}$ show the deformation vibration absorption of C–H in aromatic rings. These results indicate that there are substances with aromatic ring structures existing in leaf powder samples. After mixing FeCl$_{3}$ dilute solution and leaves dispersion, the bands in the infrared spectrum of Fe-Cl@leaves sample were obviously changed compared to that of leaves [the red curve vs the black curve in Fig. 2(c)]. Clearly band shifts, displacement or disappearance, and transmittance decreases can be observed in Fig. 2(c). The band of –OH functional groups stretching vibrations shifted from 3285 cm$^{-1}$ to 3224 cm$^{-1}$. The band of stretching vibrations of C=C functional groups in aromatic rings shifted from 1623 cm$^{-1}$ to 1606 cm$^{-1}$, and the bands at 1520 and 1445 cm$^{-1}$ disappeared. In addition, some of the bands between 1375 and 600 cm$^{-1}$ disappeared or were displaced. The differences observed in spectra indicate that specific functional groups including aromatic rings and hydroxyl are involved in the adsorption of Fe elements, and there could be interactions between $\pi$ electrons and Fe cations during the adsorption process.[33,34] UV-vis spectroscopy was conducted to investigate the interaction between Fe cation and components in Fe-Cl@leaves solution. As shown in Fig. 2(d), one characteristic peak in 250–300 nm with maxima at 266 nm was observed, which is assigned to the aromatic rings in the component of leaves. As for Fe-Cl@leaves solution, the maxima of the peak shifts to 272 nm, indicating that Fe cation interacts with aromatic rings in leaves. We performed thermal reduction to green tea leaves in a heating oven with a temperature of 180 ℃ for 1 h to increase the relative mass of aromatic rings and to reduce phenolic hydroxyl groups. The quantity analysis of C and O, and oxidation state analysis of Fe in Fe-Cl@reduced leaves sample, were carried out using XPS. The ratio of C/O increased from 2.7 to 3.2, compared to that in leaves. Meanwhile, the valence state of Fe cations changed from Fe$^{3+}$ to Fe$^{2+}$ after reduced leaves were dispersed into DI water and mixed with FeCl$_{3}$ dilute solution (Fig. S4 in the Supplementary Information). This indicates that aromatic rings play a key role in the valence state change in the Fe-Cl@leaves sample.
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Fig. 3. Spectra of Fe-Cl@catechin. (a) The chemical structure of catechin. (b) Partial XPS spectrum of Fe $2p$ peak of Fe-Cl@catechin. The Fe $2p_{3/2}$ peak of the Fe$^{2+}$ component is pointed out by a red arrow, indicating the existence of Fe$^{2+}$ in Fe-Cl@catechin. (c) FT-IR spectra of catechin and Fe-Cl@catechin in the mid-infrared. (d) UV-vis spectra of catechin dispersion, Fe-Cl@catechin solution, FeCl$_{3,}$ and FeCl$_{2}$ dilute solution, respectively.
We choose catechin as a model to verify the valence change phenomena and discuss the underlying physical mechanism as catechin is one of the important constituents in tea leaves, presenting at 8 to 15% of dry leaf weight.[31,35] Moreover, catechins are structurally simple but similar to most of the reducing substances in leaves, with aromatic rings and several hydroxyl groups in their molecular structures shown in Fig. 3(a). Surface analysis of the Fe oxidation state of Fe-Cl@catechin was carried out using XPS [Fig. 3(b)]. There is a minor peak of Fe $2p_{3/2}$ at 709.9–710.30 eV, demonstrating the existence of Fe$^{2+}$. Peak fitting and quantitative analysis show that the Fe$^{2+}$ component occupies $\sim $57.9% of the total Fe element [details are shown in Fig. S2(d) of the Supplementary Information]. This indicates that the valence state of the Fe element is shifted to a divalent state. FT-IR spectroscopy was performed to verify the vibration band changes of characteristic functional groups in catechin and Fe-Cl@catechin. As shown in Fig. 3(c), peaks at 1605, 1514 and 1438 cm$^{-1}$ are the typical skeletal stretching vibrational bands of C=C functional groups in aromatic rings of Fe-Cl@catechin. Compared with typical C=C bands of catechin (1613, 1514, and 1445 cm$^{-1}$), all bands except 1514 cm$^{-1}$ in Fe-Cl@catechin were red-shifted. The strong absorption band in the range of 650–1275 cm$^{-1}$ shows the deformation and bending vibration absorption of C–H and C–O in the aromatic rings. These results strongly suggest that aromatic rings in catechin were involved in the adsorption of Fe cations and there are interactions between aromatic ring structures and Fe cation in Fe-Cl@catechin. UV-vis spectra show that the characteristic absorption peak of aromatic rings at 277 nm was red-shifted to 293 nm after adding catechin into the FeCl$_{3}$ solution [Fig. 3(d)], indicating that there is an interaction between Fe element and aromatic rings in catechin.
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Fig. 4. (a) An adsorption configuration with the strongest adsorption energy of a Fe(H$_{2}$O)$_{6}^{3+}$@catechin cluster. (b) Adsorption energies for Fe(H$_{2}$O)$_{6}^{3+}$ adsorbed: at the aromatic ring and two –OH groups of the catechin; at the aromatic ring and one –OH group of the catechin; at two –OH groups of the catechin; at one –OH group of the catechin. (c) Molecular orbital surface of orbital 86, orbital 111 (the highest occupied molecular orbital-7, HOMO-7), and orbital 113 (HOMO-5). (d) Contribution of orbitals for the net charge transfer from the catechin to the Fe(H$_{2}$O)$_{6}^{3+}$.
We further performed density functional theory (DFT) calculations to illustrate the underlying physical mechanism of the charge transfer in Fe cations on leaves, with a module of a Fe(H$_{2}$O)$_{6}^{3+}$@catechin cluster. As shown in Fig. 4(a), the most stable adsorption configuration for the Fe$^{3+}$ cation was in the region where hydroxyl groups and aromatic rings coexist, which has the strongest adsorption energy among the adsorption configurations [Fig. 4(b), more details about the adsorption configurations can be seen in Fig. S5 of the Supplementary Information]. This result shows that the hydrated cation-$\pi$ interaction has great effect on adsorption of the hydrated Fe$^{3+}$ cation on the catechin molecule. Furthermore, through Multiwfn package,[36] charge decomposition analysis reveals that, in the Fe(H$_{2}$O)$_{6}^{3+}$@catechin cluster, orbital 86, orbital 111 (the highest occupied molecular orbital-7, HOMO-7), and orbital 113 (HOMO-5) contribute the major part ($\sim 59$%, 0.28 e) of the net charge transfer from catechin to Fe(H$_{2}$O)$_{6}^{3+}$. Considering that all these orbitals have orbital composition at the aromatic ring of catechin, we suggest that the aromatic ring should be the main net charge donor. Thus, the cation-$\pi$ interaction between the hydrated cations and aromatic rings takes an essential role in the valence state change of Fe$^{3+}$ at a catechin molecule, and on leaves. In summary, we have experimentally observed a reliable state change from Fe$^{3+}$ to Fe$^{2+}$ directly using leaves, by XPS measurements. The binding energies of the Fe $2p_{3/2}$ peak are 710.9 eV for Fe-Cl@leaves sample and 710.10 eV for Fe-Cl@catechin. FT-IR and UV-vis spectra reveal the interactions between Fe cations and aromatic rings in Fe-Cl@leaves samples and Fe-Cl@catechins. DFT calculation shows that cation-$\pi$ interactions, between the Fe$^{3+}$ and aromatic rings in leaves, play an essential role in the adsorption of Fe cations, and more importantly, in the charge transfer from aromatic rings to Fe cations. Our study provides a step towards the understanding of the relation between the leaves and soil mineral cations and may give us new insight into the field of biological organic matter utilization on trace elements balancing, soil quality improvement, crop yields, and other agricultural applications. Acknowledgments. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11974366, 12004110, and 12147169), and the Fundamental Research Funds for the Center Universities. 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