⇦Chinese Physics Letters, 2020, Vol. 37, No. 6, Article code 066803Express Letter Water-Mediated Spontaneously Dynamic Oxygen Migration on Graphene Oxide with Structural Adaptivity for Biomolecule Adsorption * Yusong Tu (涂育松)1,2**, Liang Zhao (赵亮)1, Jiajia Sun (孙佳佳)1, Yuanyan Wu (吴园燕)1, Xiaojie Zhou (周晓洁)3, Liang Chen (陈亮)4, Xiaoling Lei (雷晓玲)5,6, Haiping Fang (方海平)5,6, Guosheng Shi (石国升)7** Affiliations 1College of Physics Science and Technology, Yangzhou University, Jiangsu 225009, China 2Key Laboratory of Polar Materials and Devices (Ministry of Education), Department of Optoelectronics, East China Normal University, Shanghai 200062, China 3National Facility for Protein Science in Shanghai, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China 4Department of Optical Engineering, Zhejiang A&F University, Lin'an 311300, China 5Department of Physics, East China University of Science and Technology, Shanghai 200237, China 6Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 7Shanghai Applied Radiation Institute, Shanghai University, Shanghai 200444, China Received 7 May 2020, online 29 May 2020 *Supported by the National Natural Science Foundation of China (Grant Nos. 11675138, 11705160, 11605151, U1832150, U1932123 and 11974366), the National Science Fund for Outstanding Young Scholars (Grant No. 11722548), the Key Research Program of Chinese Academy of Sciences (Grant No. QYZDJ-SSW-SLH053), the Fundamental Research Funds for the Central Universities, the Special Program for Applied Research on Supercomputation of the NSFC-Guangdong Joint Fund (the second stage), Supercomputer Center of CAS, and the BL01B Beamline of NFPS at SSRF.
**Corresponding author. Email: ystu@yzu.edu.cn; gsshi@shu.edu.cn Citation Text: Tu Y S, Zhao L, Sun J J, Wu Y Y and Zhou X J et al 2020 Chin. Phys. Lett. 37 066803    Abstract We theoretically and experimentally show that, with water being adsorbed, the graphene oxide (GO) is converted to a spontaneously dynamic covalent material under ambient conditions, where the dominated epoxy and hydroxyl groups are mediated by water molecules to spontaneously break/reform their C–O bonds to achieve dynamic oxygen migration. This dynamic material presents structural adaptivity for response to biomolecule adsorption. Both density functional theory calculations and ab initio molecular dynamics simulations demonstrate that this spontaneously dynamic characteristics is attributed to the adsorption of water molecules, which sharply reduces the barriers of these oxygen migration reactions on GO to the level less than or comparable to the hydrogen bonding energy in liquid water. DOI:10.1088/0256-307X/37/6/066803 PACS:68.37.-d, 68.90.+g, 61.48.Gh, 87.90.+y © 2020 Chinese Physics Society Article Text Large-area dynamic covalent new materials that are elaborately adaptive and responsive to their surroundings have shown great potential in new technologies and can be used in a variety of desired applications,[1–4] such as molecular detection and recognition,[5] (bio)sensors,[6] controlled drug delivery and release,[7] and dynamic-responsive systems.[8] In the past decade, there have been great efforts to exploit various reversible covalent reactions to achieve structural adaptivity in dynamic materials,[9–12] including nucleophilic aromatic substitution,[10] imine condensation,[11,13] disulfide exchange,[12] acetals,[14] olefin,[15] and alkyne metathesis.[16] However, these covalent reactions have usually relied on catalysis[4,9,12,14–16] or temperature[10,11] or pH[12,13] conditions to enable the formation of dynamic reversible covalent bonds. So far, it remains a large challenge to achieve a spontaneously dynamic material with reversible covalent reactions under ambient conditions, leading to a continuous change in constitution and offering structural adaptivity in practical applications. Graphene oxide (GO) is an atom-thick sheet of carbon that is covalently functionalized with rich oxygen-containing groups dominated by epoxy and hydroxyl on its basal plane,[17–20] and it has great potential applications in areas such as molecular detection and sensors,[21–23] electronics and photonics,[24] catalysis,[25] biological and medical technologies,[26–29] and water treatment systems.[30,31] The functional oxygen groups are of fundamental importance to the properties and relevant applications of GO, as they allow for extensive interactions of GO with various molecules including biomolecules.[22–28,32,33] For examples, for biomedical applications, these covalent groups afford a vital improvement of GO water solubility and biocompatibility and provide intrinsic advantages to GO as smart platforms for biosensing, biomedical imaging, drug delivery and cancer treatment.[26,32] Generally, these functional oxygen groups seldom change or move along the basal plane of GO under ambient conditions,[34–36] if these functional oxygen groups can spontaneously move on GO, this would significantly change the relevant electrical, mechanical, and thermal properties of GO as well as expand GO-based relevant applications. Here, we combine theoretical calculations and experimental observations to show that, with water being adsorbed, the GO is converted to a spontaneously dynamic covalent material under ambient conditions and presents structural adaptivity for response to biomolecule adsorption, where the epoxy and hydroxyl groups are mediated by water molecules to spontaneously break/reform their C–O bonds to achieve dynamic oxygen migration. To the best of our knowledge, this is the first report of a large-area spontaneously dynamic covalent molecular interface under ambient conditions. We have performed density functional theory (DFT) calculations at the B3LYP/6-31G(d) level of theory (see PS1 in the Supplemental Material (SM)). In previous DFT calculations, the C–O bond breaking/reforming reactions in epoxy and hydroxyl groups on GO without water were found to have relatively high barriers.[34,35] To study the oxygen migration on GO, we further analyzed the C–O bond reaction of epoxy and the proton transfer between epoxy and hydroxyl for the exchange (see PS2 in the SM). As shown in Fig. 1(a), the C–O bond reactions have a much lower barrier of 3.9 kcal/mol for epoxy movement assisted by hydroxyl, but there remains a high barrier of 15.9 kcal/mol for proton transfer between the neighboring epoxy and hydroxyl for the exchange. Both barriers along the pathways indicate the local mobility of epoxy around its neighboring hydroxyl but little mobility of hydroxyl, which is consistent with the previous TEM observations.[36] Therein, an intermediate with one hanging oxygen atom single-bonded onto GO is observed, but this was easy to transform back into epoxy with thermal disturbance.
cpl-37-6-066803-fig1.png
Fig. 1. (a) Oxygen migration pathways on GO without adsorbed water. The epoxy assisted with the hydroxyl moves by C–O bond breaking/reforming (path I, black) and the hydroxyl moves by proton transfer to the epoxy and exchange with each other (path II, blue). Notation: relevant reactants (R), intermediate (M), transition states (TS), and products (P). (b) Pathways III and IV, with one water adsorbed. (c) Pathways V and VI, with three waters adsorbed. The reactions passing through the intermediate in (b) and (c) are omitted for simplification.
We analyzed again the above pathways after adding water to GO and found that the water adsorption induces oxygen migration reactions along the pathways. As shown in Fig. 1, with only one water molecule adsorbed, the epoxy and hydroxyl movements along paths III and IV have energy barriers of $-3.5$ kcal/mol and 5.0 kcal/mol, respectively, and with three water molecules adsorbed, all their barriers are far below zero along paths V and VI. Compared to the barrier without water adsorbed (path II), the adsorption of one, two, and three water molecules reduce the barrier for proton transfer by 4.9 kcal/mol (path IV), 7.7 kcal/mol (Fig. S1 in the SM) and 10.1 kcal/mol (path VI), respectively. Notably, the adsorption of three water sharply reduces the barriers of both hydroxyl and epoxy movements down to 5.8 kcal/mol and 2.2 kcal/mol, respectively (see Fig. 1(c) and Fig. S2 in the SM). We note that the hydrogen bond energy is $\sim $5 kcal/mol in liquid water;[37] thus these barriers are less than or comparable to the hydrogen bond energy, indicating that movements can occur spontaneously under ambient conditions. These results indicate that water activation induces the hydroxyl movement via the exchange with epoxy by proton transfer and further expands the original local mobility of epoxy on GO without adsorbed water. In addition, we have also check possible oxygen migration pathways in between GO layers, and found that water adsorption increases interlayer spacings and prevents direct oxygen migration in between GO layers (see PS4 in the SM).
cpl-37-6-066803-fig2.png
Fig. 2. Representative AIMD trajectory of oxygen migration in epoxy and hydroxyl groups on the basal plane of GO. Three reaction processes are denoted by reactions I, II, and III, and the numbers indicate the time of snapshots. Atom representations are the same as those in Fig. 1.
Unbiased ab initio molecular dynamics (AIMD) simulations further confirmed the spontaneously dynamic oxygen migration on GO, surrounded by liquid water with a density of $\sim $1.0 g/ml (corresponding to ambient conditions) (see the detailed AIMD Methods, PS5 in the SM). Figure 2 shows the representative AIMD trajectory with three reactions, denoted by I, II, and III (see movie S1 in the SM). In the snapshots from 2.9 ps to 4.2 ps (reaction I), we can see the epoxy moving process assisted directly by water molecules and not assisted by the neighboring hydroxyl (see paths I, III, and V in Fig. 1). The snapshots from 11.5 ps to 18.0 ps present the hydroxyl movement via two successive proton-transfer reactions to exchange with epoxy (reactions II and III). We can clearly see the rearrangement of water molecules with dynamic orientations around the reaction intermediates. Further DFT calculations have also verified the pathway of reactions II and III by optimizing the analogous local reaction conformations of GO (see Fig. S5 in the SM). Interestingly, it can be seen that the intermediate remains with the assistance of water molecules and neighboring hydroxyl (see snapshots from 13.6 ps to 18.0 ps) (see Fig. 1(a)). All these reaction dynamics present in the AIMD simulations agree with the pathways based on DFT calculations, indicating a spontaneously dynamic oxygen migration on GO induced by water adsorption. To check the dynamic oxygen migration on GO, in situ synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy experiments have been carried out (see PS8 in the SM). We used GO membranes from GO suspensions with the drop-casting method[38–40] that has been widely used in characteristic studies and applications of GO.[30,41] The GO membranes were dried under an infrared lamp for $\sim $30 min and then sealed hermetically in dry nitrogen environments (fully dry GO membrane) as a control. The dried GO membranes were also kept under ambient conditions (dry GO membrane) and further were treated in situ by being immersed in a deionized water droplet of 20 µL for $\sim $10 min (wet GO membrane) (see PS9 and PS11 in the SM). These GO membranes were analyzed by in situ SR-FTIR experiments to detect the chemical oxygen distributions above them.
cpl-37-6-066803-fig3.png
Fig. 3. In situ SR-FTIR spectra acquired from the fully dry GO membrane (a), and from the dry GO membrane, the wet GO membrane and the wet GO membrane with cytosine adsorbed (b). Two dotted-box regions of interest in the left subfigures are enlarged to show on the right. Characteristic IR peaks occur in the GO spectra at $\sim $1227 cm$^{-1}$, $\sim $1622 cm$^{-1}$, $\sim $3424 cm$^{-1}$, and $\sim $3594 cm$^{-1}$, and peaks occur in the spectra of GO with cytosine adsorbed at $\sim $3220 cm$^{-1}$ and 1500 cm$^{-1}$. (c)–(g) Sequence of in situ images for 2D chemical distributions over the sampling area, on the fully dry GO membrane, on the dry GO membrane, on the wet GO membrane, on the wet GO membrane with cytosine adsorbed, and on the wet GO membrane for two-hour measurements at 1227 cm$^{-1}$, 1622 cm$^{-1}$, and 3594 cm$^{-1}$. The left numbers denote the time of the sequence in units of minute.
Figures 3(c), 3(d) and 3(e) show the sequent 2D chemical distributions of the epoxy (C–O–C, left) and hydroxyl (O–H, mid) groups, as well as the mixed contents of aromatic rings (C=C) and water (right), over a sampling area of $30\times 30\,µ$m$^{2}$ (see PS10 in the SM also over a large sampling area of $90\times 90\,µ$m$^{2}$) on the fully dry, dry and wet GO membranes, respectively. As shown in Fig. 3(e) for the wet GO membrane, the red region indicating high epoxy content is frequently stretched, and the red regions indicating high hydroxyl content and mixed content are also stretched, even extending to two or three separated regions in comparison with the corresponding regions on the dry membrane over time. This represents the significant changes in the epoxy and hydroxyl content after adding water to the dry membrane. In contrast, Fig. 3(d) shows that these regions on the dry GO membrane remain almost unchanged with time. Particularly as an important control, on the fully dry GO membrane sealed hermetically in dry nitrogen environments, the 2D distributions of hydroxyl and epoxy contents remain almost unchanged with time (see Fig. 3(c) and PS11 in the SM for details). Further, examining the hydroxyl content on the wet membrane as an example, the top-right red region is converted to green at 21 min, reappears at 30 min, but is shrunk at 60 min. This represents a periodically oscillational change in the hydroxyl contents (see the mid-sequence images of Fig. 3(e)). Figure 3(g) further demonstrates that real-time periodical oscillations in the distributions on the wet membrane have been retained for the whole time period of experimental observations (two hours) (also see PS10 in the SM). These real-time changes and oscillations in the distributions of the epoxy and hydroxyl contents demonstrate that the addition of water induces spontaneously dynamic movements of epoxy and hydroxyl on wet GO membranes. To present the influence of water on the epoxy and hydroxyl spectra, we analyze the characteristic peaks of the corresponding groups. Figure 3(a) and 3(b) together with the relevant enlargements show four characteristic IR peaks, at $\sim $1227 cm$^{-1}$ (C–O–C stretching of epoxy), $\sim $1622 cm$^{-1}$ (C=C stretching of aromatic ring and deformation vibration of adsorbed water), $\sim $3424 cm$^{-1}$ (stretching vibration of adsorbed water), and $\sim $3594 cm$^{-1}$ (O–H stretching of hydroxyl).[42,43] Compared with the spectrum on the dry GO membrane, the peak at $\sim $3424 cm$^{-1}$ is enhanced after adding water to the GO membrane but decreases over time and approaches the corresponding intensity of the dry membrane, indicating a loss of the water from GO due to evaporation. The peak at $\sim $1622 cm$^{-1}$ is also enhanced after adding water, but it maintains a relatively higher intensity, indicating a certain residual of water between the layers of the wet GO membranes. The epoxy spectra do not overlap with water spectra, but the intensity at $\sim $1227 cm$^{-1}$ presents significant oscillations around the corresponding intensity on the dry GO membrane, and the hydroxyl intensity at $\sim $3594 cm$^{-1}$ also shows similar oscillational changes while separated from the band at $\sim $3424 cm$^{-1}$. Also, we have experimentally confirmed that there is no degradation of the wet GO membranes (see PS12 in the SM). In sharp contrast to the real-time changes in the spectra on the wet GO membrane, we see that the spectra on the fully dry GO membrane remain almost unchanged and overlap together over time (see Fig. 3(a)). Thus, these real-time changes induced by the addition of water indicate the occurrence of spontaneous changes in epoxy and hydroxyl contents at the sampling area on the wet GO membrane. Since the oxygen movements in between the GO layers have large barriers and water adsorption increases interlayer spacings and prevents direct oxygen migration in between GO layer (see PS4 in the SM), these real-time changes observed experimentally mainly reflect the oxygen migration along the basal plane of GO, consistent with the above theoretical prediction. Notably, the adaptive structural response to the adsorption of biomolecules was further observed. Similarly, the wet GO membranes were in situ treated by a saturated cytosine solution droplet of 20 µL for 10 min (see Fig. S7 in the SM), and was analyzed in situ by SR-FTIR under ambient conditions. As shown in Fig. 3(b), cytosine has characteristic IR peaks at $\sim $3220 cm$^{-1}$ (H–N–H stretching) and $\sim $1500 cm$^{-1}$ (C–N stretching in C–N ring), and these peaks do not overlap with the above characteristic peaks in the GO spectra, despite overlaps at some other frequencies.[44] Compared with the spectra on the dry and wet GO membranes, we can see significant changes in the intensities at $\sim $1227 cm$^{-1}$, $\sim $1622 cm$^{-1}$, and $\sim $3594 cm$^{-1}$ on the wet GO membrane with cytosine adsorbed, indicating large changes in the epoxy and hydroxyl contents. Figure 3(f) presents the corresponding sequent 2D chemical distributions. We find that these chemical contents have been redistributed completely on the wet GO membrane with cytosine adsorbed. Over time, their distributions become even more stable than the distributions on the dry GO membrane. DFT calculations show that the oxygen functional groups may contribute around $-30.2$ kcal/mol to the binding energy of one cytosine adsorption onto GO. This indicates that adsorbed cytosine strongly interacts with oxygen functional groups to stabilize their distributions on GO (see PS13 in the SM). The conversion from dynamic to stable distributions indicates that the spontaneously dynamic movements of epoxy and hydroxyl on the wet GO membrane possess an adaptive structural response to the cytosine adsorption onto the wet GO membrane; that is, these functional oxygen groups on GO spontaneously break/reform their covalent bonds to adaptively interact with cytosine molecules around their adsorption loci. In summary, we have theoretically and experimentally shown the water-mediated spontaneously dynamic characteristics of GO under ambient conditions and the structural adaptivity for response to biomolecule adsorption. We note that the isolated local oxygen mobility on GO under electron beam irradiation was observed previously with TEM experiments,[36] and that the spatial structural changes of these oxygen groups on GO can be detected with a room-temperature characteristic relaxation time of about one month[35] or driven with a thermal annealing procedure.[34] Here, we attribute the spontaneously dynamic oxygen migration to the fact that water sharply reduces the barriers of the movements of epoxy and hydroxyl groups to the level less than or comparable to the hydrogen bonding energy in water. Generally, GO structures were observed under a dry or even vacuum condition, e.g., in TEM, STM, or NMR experiments, and a variety of theoretical models of GO structures[17–20] were built, mainly based on static GO structures, e.g., the most popular Lerf–Klinowski model[17] and our recent Shi–Tu model.[19] Clearly, our results present the significant new understanding of GO structures that usually occur in inevitable contact with water in vapor or liquid, e.g., bio-surroundings. Most importantly, the spontaneously dynamic oxygen migration under ambient conditions actuates the structural adaptivity of GO for response to biomolecule adsorption and scarcely disturbs the biomolecular structures and properties. We also note that GO can interact with a wide range of biomolecules to provide either electrical or optical readouts as biosensors[22–24] as well as can be sensitive to a range of stimuli (pH, gas or biomolecules, or thermal, light, magnetic, or ultrasonic stimuli) to extend its applications as smart platforms in clinical diagnosis and treatment.[26,32,45] Considering that these applications occur usually in touch with water, the adaptive interactions indicated here provide new and crucial insights into the understanding of relevant molecular mechanisms on these applications and also open a new avenue to design adaptive dynamic-responsive systems for expanding relevant applications. Moreover, GO-based resistive switching memories have attracted much attention.[46,47] Since oxygen migration is crucial for the resistive switching behaviors of GO,[47,48] the water-mediated dynamic oxygen migration on GO observed here offers new insights for future design and fabrication of GO-based memresistor devices. Therefore, our new fundamental understanding of graphene oxide as a novel spontaneously dynamic covalent material is of essential importance for the further use of this material in practical and desirable applications. We thank Professor Rodney S. Ruoff for his inspiring idea and discussion. We thank Garel Ekoya and Xian Wang for preliminary testing calculations. 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