Simultaneous and Reversible Triggering of the Phase Transfer and

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22851−22857

Simultaneous and Reversible Triggering of the Phase Transfer and Luminescence Change of Amidine-Modified Carbon Dots by CO2 Rui Xiong, Meiling Chen, Xin Cui, Qi Wang, Xiaowang Liu,* and Baoyou Geng* College of Chemistry and Materials Science, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Centre for Nano Science and Technology, Anhui Normal University, Wuhu 241000, P. R. China

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S Supporting Information *

ABSTRACT: The ability to reversibly manipulate the surface nature of luminescent nanoparticles upon external stimulation enables the development of advanced optical probes for biological sensing and data encoding. Herein, we report the synthesis of a new class of smart carbon dots (CDs) via surface modification of amine-enriched CDs with CO2-responsive groups of amidine. We present that alternative CO2 and N2 bubbling can not only lead to a reversible phase transfer of the CDs between an organic phase and an aqueous phase but also give rise to a corresponding reversible luminescence change between blue and cyan-green. We attribute these observations to changes in both the surface chemistry and the emission states of the CDs triggered by the alternative CO2/N2 introduction. We also find a similar luminescence change of the CDs upon alternative exposure to a humid vapor of CO2 and a mixture of NH3 and N2 at room temperature, allowing them to be used as a new class of optical materials for optical encoding. KEYWORDS: surface modification, carbon dots, reversible phase transfer, switchable luminescence, document encoding

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INTRODUCTION The surface chemistry of nanoparticles is the most fundamental characteristic that determines their applications in bioimaging, drug delivery, and catalysis.1−4 Both hydrophobic and hydrophilic nanoparticles can be readily obtained through the use of appropriate ligands (or surfactants) in the synthetic procedures.5−8 In addition, postsynthetic modification has proven effective as a complementary tool to regulate the surface feature of nanoparticles by modification with desirable ligands.9,10 Among the surface-modification strategies, methods of stimuli-responsive group conjugation have shown the ability to develop smart nanoparticle platforms with a set of stimuli-switchable attributes, such as solubility, color output, and catalytic capacity.11−16 These switchable attributes can largely extend advantages of the nanoparticles in the applications ranging from drug delivery to tumor-specific therapy.17−19 More importantly, there is considerable flexibility in the selection of stimuli when designing smart nanoparticle platforms, including ionic strength, pH, light, temperature, and gases.20−24 These stimuli sometimes can be assembled at the single-particle level to produce multiple-responsive nanoparticle platforms.25−27 When compared with other stimuli, CO2 has been found to be inexpensive, highly biocompatible, and easily removable and has been suggested to be an excellent trigger to switch the surface chemistry of nanoparticles. As a result, a variety of CO2-responsive nanoparticles, including Au, CdSe/ZnS, SiO2, © 2019 American Chemical Society

carbon dots (CDs), and polymer beads, have been synthesized by making use of CO2-sensitive molecules/functional groups in the surface-modification step.28−33 In these cases, reversible phase transfer of the nanoparticles can be readily achieved between an organic and an aqueous phase upon alternative bubbling of the system with CO2 and N2 (or Ar). Despite the notable achievements, achieving reversible CO2triggered phase transfer of luminescent nanoparticles between an organic and an aqueous phase, accompanied by a corresponding luminescence change, is still a daunting challenge.34−36 The main reason is that the luminescent properties of the studied nanoparticles are insensitive to the changes in their environment. As inspired by previous findings that CDs exhibit surface-state-controlled luminescence behaviors, we reason that surface modification of CDs with CO2responsive groups is possible to produce dual-response of CDs via CO2 triggering,37−42 that is, the in situ generated acidity upon CO2 treatment not only enables the transformation of the CDs from hydrophobic to hydrophilic but also allows the surface emission states of the modified CDs to be changed.43,44 Subsequent removal of the dissolved CO2 by passing N2 into the system may render reversible phase transfer and luminescence recovery of the CDs. Such CO2-switchable Received: March 27, 2019 Accepted: June 5, 2019 Published: June 5, 2019 22851

DOI: 10.1021/acsami.9b05421 ACS Appl. Mater. Interfaces 2019, 11, 22851−22857

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic representation of achieving reversible phase transfer of the amidine-modified CDs, accompanied by a corresponding reversible luminescence change between blue and cyan-green, upon alternative stimulation of CO2 and N2. The amidine-modified CDs can only disperse in organic solvents, including toluene, dichloromethane, chloroform, and 1-octanol. Upon bubbling CO2, the modified CDs initially dispersed in an organic phase can be transferred into an aqueous phase because of the formation of ionic moieties at the surface via the reaction of amidine with the in situ formed H+. Meanwhile, the surface emission states of the CDs can also be changed by the in situ formed H+, allowing the emergence of a corresponding luminescence change from blue to cyan-green during the phase transfer. By bubbling the mixture with N2 to completely remove the in situ formed H+, both the surface chemistry and the emission states can be recovered, enabling a reversible phase transfer of the CDs from water to 1-octanol, together with a corresponding luminescence change from cyan-green to blue.

dual-responsive luminescent nanoparticles can find widespread applications in sensing, bioimaging, and optical data encoding.

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in 1-octanol (2 mL) to form a colorless dispersion. Deionized water (2 mL) was then added into the mixture to produce two phases. Next, CO2 was bubbled into the mixture, and the luminescence of the two phases was measured at different intervals. N2-stimulated reversible phase transfer and luminescence change were performed in the same way, except that N2 was used to replace CO2. Use of a Mixture of N2 and NH3 to Accelerate the Phase Transfer of the CDs from Water to 1-Octanol. The protocol is essentially similar to that of the use of N2, except that a small-sized cotton ball was placed into a plastic tip used for N2 bubbling. Before the experiment, a small amount of NH3·H2O (36 wt %) (8 μL) was added into the plastic tip. To exclude the impact of the salt residues on the reversibility of the phase transfer process, the water phase was replaced by fresh deionized water upon the completion of the phase transfer of the CDs from water to 1-octanol in each cycle. Impact of Added Salts on the Reversible Phase Transfer and the Corresponding Luminescence Change. The procedure is similar to that used for studying the CO2/N2-stimulated reversible phase transfer of the CDs, except that (NH4)2CO3 (27 mg) was added into the aqueous phase (2 mL) to form a homogeneous solution. Reversible Luminescence Change of the CDs in the Solid State. A two-dimensional covert pattern was first fabricated on a filter paper by stamping a dispersion of the CDs in 1-octanol (2.5 mg mL−1). To perform the luminescence reversibility study, the plastic tip used to bubble gas was loaded with a small-sized cotton ball. Prior to the experiments, H2O (400 μL) and NH3·H2O (8 μL) were added into the plastic tip to create corresponding stimulating atmospheres. Synthesis of 2,6-Diaminotoluene-Derived CDs. 2,6-Diaminotoluene-derived CDs were also synthesized by a hydrothermal method. In brief, a precursor solution was first prepared by dissolving 2,6-diaminotoluene (0.25 g) in ethanol (25 mL). The resulting solution was then transferred into a Teflon-lined autoclave (45 mL) and heated at 200 °C for 12 h. Thereafter, the product was purified by silica column chromatography using a mixture of methylene chloride and methanol (v/v = 3:1) as an eluent. The purified 2,6diaminotoluene-derived CDs were afforded by removing the solvent under vacuum. Surface Modification of 2,6-Diaminotoluene-Derived CDs with Amidine. The procedure was similar to that used for surface modification of m-PD-derived CDs. Typically, a methanol dispersion of the as-prepared CDs was first prepared with a concentration of 1.4 mg mL−1 (20 mL). Next, DMADMA (1.5 mL) was added into the CD mixture. The resulting mixture was stirred for 10 h at room temperature. Quantitative Determination of the Content of AmidineModified m-PD-Derived CDs in 1-Octanol by Fluorescence

EXPERIMENTAL SECTION

Chemicals. m-Phenylenediamines (m-PD, 99.5%), 2,6-diaminotoluene (98%), and N,N-dimethylacetamide dimethylacetal (DMADMA, ≥ 90.0%) were purchased from Aladdin. Other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All chemicals were used as received, except as noted. Physical Measurements. Transmission electron microscopy (TEM) was carried out with a field emission transmission electron microscope (JEOL-JEM 2010F) operated at an acceleration voltage of 200 kV. The Fourier transform infrared (FT-IR) spectra were recorded on Varian Excalibur 3100. UV−vis absorption spectroscopy was performed with a Shimadzu UV-2401 spectrometer. The luminescence and lifetime measurements were performed on a fluorescence spectrophotometer (Edinburgh FLSP920) equipped with a nanosecond hydrogen flash-lamp. The absolute quantum yield was also measured on the same fluorescence spectrophotometer equipped with an integrating sphere. Synthesis of m-PD-Derived CDs. The amine-enriched CDs were synthesized via a slightly modified method of Lin et al.45 In a typical experiment, a precursor solution was first prepared by dissolving mPD (0.25 g) in ethanol (25 mL) to form a transparent solution. The resulting solution was then transferred into a Teflon-lined autoclave (45 mL) and heated at 180 °C for 12 h. The product was purified by silica column chromatography using a mixture of methylene chloride and methanol (v/v = 5:1) as an eluent. Purified m-PD-derived CDs were obtained by removing the solvent under vacuum. Surface Modification of m-PD-Derived CDs with Amidine. The surface-modification reaction was carried out at room temperature. In brief, a methanol dispersion (15 mL) of the as-prepared CDs was first prepared with a concentration of 1.33 mg mL−1. DMADMA (1.2 mL) was then added into the mixture. The resulting mixture was stirred for 10 h at room temperature. Purification of the As-Prepared Amidine-Modified m-PDDerived CDs. The above-mentioned reaction mixture was dried directly under vacuum before the addition of deionized water (2 mL). The mixture was then ultrasonicated for 5 min to disperse the CDs. Thereafter, methylene chloride (6 mL) was introduced, and the resulting mixture was ultrasonicated for another 15 min. The organic phase was finally collected and dried to produce amidine-modified CDs. CO2/N2-Stimulated Reversible Phase Transfer and Luminescence Change of the As-Prepared Amidine-Modified mPD-Derived CDs. The as-prepared CDs (2.5 mg) were first dissolved 22852


Research Article

ACS Applied Materials & Interfaces Spectroscopy. A standard working profile was first obtained by measuring the dependence of the luminescence intensity of the modified CDs on their concentrations in 1-octanol.32 In the phase transfer study, 1-octanol phase was separated from the mixture at different time intervals. The luminescence intensity in 1-octanol was then measured to evaluate the content of the CDs in the organic phase according to the standard working profile. Quantitative Determination of the Content of the AmidineModified m-PD-Derived CDs in Water by Fluorescence Spectroscopy. Similar to the method used for the quantitative determination of the content of amidine-modified m-PD-derived CDs in 1-octanol, a standard working curve was first established in an aqueous phase. The mass change during the phase transfer was evaluated on the basis of the variations in the luminescence intensity. Quantitative Determination of the Content of the AmidineModified 2,6-Diaminotoluene-Derived CDs in 1-Octanol and in Water Phases by Fluorescence Spectroscopy. The quantitative measurements were carried out by a similar method. In the first step, standard working curves in both phases were established. The concentrations of the modified CDs in the two phases were estimated by a combination of the emission intensity in each phase and the corresponding working curves.

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Figure 2. Characterization of the as-prepared amidine-modified CDs. (a) FT-IR profile comparison of the unmodified and the amidinemodified CDs. (b) Optimal excitation and emission profiles of the amidine-modified CDs in 1-octanol. The inset shows a typical luminescent photograph of the amidine-modified CDs in a mixture of 1-octanol and water upon excitation at 365 nm. (c) Luminescence decay curve at 437 nm of the modified CDs in 1-octanol dispersion.

RESULTS AND DISCUSSION

Our design is shown in Figure 1. We at first carried out the synthesis of amine-enriched CDs via a reported method with modifications (Figure S1).45 The as-prepared CDs were then purified by silica gel column chromatography (Figure S2). TEM (Figure S3) suggested an average size of 8 nm of the asprepared CDs. Optical characterization showed an excitationdependent emission feature of the as-prepared CDs (Figure S4a).46 Under optimal excitation at 353 nm, the purified CDs displayed a strong blue emission at 437 nm in 1-octanol (Figure S4b and the inset). The luminescence decay curve (at 437 nm) suggested a single-exponential behavior, and the lifetime was estimated to be 1.6 ns (Figure S4c). These results are in line with the previous reports in the synthesis of blue luminescent CDs.45 Next, we performed surface modification of the as-prepared CDs with CO2-responsive groups of amidine at room temperature (Figure S5).47−49 After the surface modification, a strong absorption from 330 to 410 nm, originating from the n−π* transition of N−H groups, is decreased tremendously in the UV−visible absorption spectrum. This suggests the participation of the surface amine groups in the surfacemodification reaction (Figure S6).50 The FT-IR spectrum verified the success of surface modification with amidine groups, as evidenced by the emergence of a new and prominent CN stretching absorption at 1642 cm−1 of the modified CDs (Figure 2a).51 Notably, strong stretching vibrations at 2920 and 2855 cm−1 of C−H were unchanged, suggesting that there are still abundant nongraphitizing mphenylenediamine moieties on the surface of the modified CDs. The modified CDs showed a slightly changed excitationdependent emission behavior relative to the unmodified ones (Figure S7). However, the surface modification has a limited impact on the surface emission states of the original CDs as both the modified and unmodified CDs have shown almost the same optimal excitation (357 nm) and emission (437 nm) profiles (Figure 2b) and comparable lifetimes of their blue emissions (Figure 2c). In a further set of experiments, we examined the response of the amidine-modified CDs to CO2 bubbling in a mixture of 1octanol and water. The phase transfer kinetics of the modified CDs was recorded by monitoring luminescence changes in

both phases at different time intervals. Upon bubbling with CO2 for 1 min, a new cyan-green luminescence was generated in the aqueous phase, whereas the luminescence in the organic phase remained blue (Figure 3a and the inset). With passing time, the increase in the intensity of the cyan-green luminescence in the aqueous phase is accompanied by a gradual decrease in the blue luminescence in the organic phase

Figure 3. Phase transfer study of the amidine-modified CDs from 1octanol to water upon CO2 bubbling into the mixture. (a) Luminescence changes in the organic phase and in the aqueous phases upon bubbling CO2 for 1 min. The inset shows the corresponding luminescent photograph upon excitation at 365 nm. (b) Luminescence changes in the organic phase (437 nm) and in the aqueous phase (490 nm) phases upon bubbling CO2 at different time intervals. (c) Three-dimensional spectrum of excitation-dependent emission of the resulting water-dispersible amidine-modified CDs. (d) Luminescence decay curve at 490 nm of the modified CDs in an aqueous dispersion. 22853


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To check the reversibility of the phase transfer and the corresponding luminescence change, we then bubbled N2 into the resulting mixture at room temperature. As designed, the CDs can be almost transferred back to the organic phase quickly within 8 min (Figure 4a). The phase transfer efficiency

(Figure 3b). The phase transfer reaches an equilibrium within 6 min. The phase transfer efficiency from 1-octanol to water was estimated to be 92 wt % (Figure S8). At the equilibrium state, the intensity of the cyan-green emission is about 27 times stronger than that of the blue emission in the organic phase (Figure S9 and the inset). The efficient phase transfer upon CO2 triggering allows us to study the optical properties of the surface-modified CDs in the aqueous phase. We discovered that the water-dispersible CDs show an excitation-dependent emission behavior as well (Figure 3c). However, both the excitation and emission profiles red-shifted significantly when comparing both parameters with the same CDs dispersed in 1-octanol. Specifically, the maximum excitation and emission peaks are red-shifted to 370 and 490 nm, respectively (Figure S10). These findings suggest that the phase transfer is accompanied by remarkable changes in the surface states of the CDs. In fact, the change in the emitting states of the CDs was also confirmed by the observation of a longer lifetime (8.4 ns) of the cyan-green luminescence (Figure 3d) and a considerable increase in the absolute quantum yield from 6.1 to 20.1%. Collectively, these results show that CO2 bubbling has shown the ability to regulate both the surface chemistry and the emission states of the CDs in the liquid phase. We then probed the role of CO2 in triggering the phase transfer of the amidine-modified CDs from 1-octanol to water. Notably, direct N2 (or Ar) bubbling showed inability to stimulate a similar phase transfer under the same conditions (Figure S11a,b). We speculate that in situ formed carbonic acid (H2CO3) by the reaction of CO2 with H2O should exhibit a profound impact on the phase transfer. Indeed, the pH of the water phase was found to decrease from 7.2 to 4.3 after realizing the phase transfer upon CO2 bubbling. The released H+ ions from the in situ formed H2CO3 can lead to ionization of the surface amidine groups, which in turn results in 1octanol-to-water phase transfer of the CDs (inset, Figure 1).52 Notably, the in situ generated acidity in the water phase is also likely the driving force for the changes in the emission states of the CDs. Control experiments showed inability of CO2 bubbling to trigger a similar luminescence change of the unmodified and modified CDs in 1-octanol dispersions (Figure S12). This is due to the difficulty in the production of H+ ions during CO2 bubbling without H2O.53 Similarly, addition of deionized water in the mixture also has no impact on luminescence. However, we discovered that the in situ formed H+ can be directly replaced by other acids. For example, addition of a trace amount of HCl (10 μL, 0.06 M) in the 1octanol dispersion can drive an identical luminescence change from blue to cyan-green within 1s (Figure S13 and the insets). Notably, the aggregate-induced energy transfer mechanism is often used to explain a red-shift phenomenon in the emission profiles of CDs upon external treatment.54,55 This mechanism is associated with a decreased lifetime of the new emission band. As our optical measurement suggested a 4-fold longer lifetime of the resultant cyan-green emission, we can conclude that the luminescence change of the CDs during the phase transfer is a result of acid-induced changes in their emission states rather than an outcome of an aggregation-induced effect.39,40 Moreover, the bulky amidine groups, together with their surface charges, give rise to limited rotation and vibration freedom of the surface moieties, thereby suppressing the occurrence of nonradiative relaxation and enabling a higher quantum yield of the cyan-green emission.

Figure 4. (a) Reversible phase transfer of the amidine-modified CDs from water to 1-octanol upon bubbling N2 into the resulting mixture. Note that for the quantitative study of the phase transfer kinetics the emission intensities at 437 nm in the 1-octanol phase and 490 nm in the water phase were recorded. (b) Reversibility examination of phase transfer and the corresponding luminescence change of the CDs upon alternative stimulation of CO2 and N2 for successive six cycles. Notably, squares and circles present the emission intensities at 490 nm in the aqueous phase and at 437 nm in the 1-octanol phase at each equilibrium, respectively.

was found to be about 98 wt % (Figure S14). This is obviously ascribed to the removal of the dissolved CO2 in the mixture by N2 bubbling. Notably, a luminescence recovery at 437 nm, accompanied by an emission decrease at 490 nm, was also observed during the phase transfer. At the equilibrium state, the blue emission in the 1-octanol phase is about 32 times stronger than the cyan-green emission in the aqueous phase (Figure S15), indicative of the high efficiency of the reverse phase transfer as well. High reversibility of the phase transfer with a corresponding luminescence change was further confirmed by successive six cycles of alternating CO2/N2 bubbling into the modified CD dispersion (Figure 4b). Limited variations were observed in the luminescence intensity of both phases at the equilibrium states. Compared with those in the previous work, our modified CDs need mild conditions, including room temperature and less bubbling duration, to achieve a similar phase transfer efficiency. We attribute this not only to high reversibility of ionization and deionization of the surface amidine groups upon the alternative stimulation but also to a high coverage of the amidine groups on the surface of the modified CDs. To provide a better understanding of this point, we used 2,6-diaminotoluenederived CDs to carried out the same experiments (Figure S16). The results showed a similar luminescence change during the 22854


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the pattern from blue to cyan-green was observed within 2 min (Figure 5b). As expected, a reverse luminescence change was attained within 1 min upon subsequent exposure to a humid vapor of NH3 and N2. More importantly, the luminescence change showed high reversibility in the successive six cycles of experiments upon the alternative stimulation of a humid vapor of CO2 and a mixture of N2 and NH3. When compared with the stimulation study in the liquid phase, less salt will be accumulated in the solid state due to the ease of evaporation of NH3.57,58 This feature permits an efficient and reversible luminescence change between blue and cyan-green upon the alternative stimulation. The solid-state luminescence responsive nature suggests a potential application of the modified CDs in the fields of optical anticounterfeiting and data encoding.

phase transfer from 1-octanol to water (Figure S17) but a decreased efficiency to 67 wt % (Figure S18). The decrease in the phase transfer efficiency is possibly due to a remarkable coverage of hydrophobic methyl groups on the surface of the modified CDs. The kinetics of water-to-organic phase transfer of the modified CDs is essentially determined by removal rate of the H+ ions in the mixture. This point is supported by the fact that the phase transfer process can be accelerated by bubbling a mixed gas of NH3 and N2 into the mixture. Our result showed that 94 wt % transfer efficiency can be achieved with 1 min, and an equilibrium can be reached within 3 min with a transfer efficiency of 97 wt % (Figure S19). Also, the phase transfer is accompanied by a blue luminescence recovery (Figure S20). However, subsequent reversible phase transfer from 1-octanol to water is inefficient with an efficiency as low as 26 wt %. The serious deterioration in the efficiency is likely due to accumulation of salts in the mixture (Figure S21).43 To prove this hypothesis, phase transfer from 1-octanol to water was further studied in the presence of (NH4)2CO3 (0.14 M) in the aqueous phase. As expected, a similar low phase transfer efficiency (∼22 wt %) was observed upon bubbling the mixture with CO2 for 6 min (Figure S22). On a separate note, the phase transfer efficiency can return to as high as 92 wt % when replacing the used aqueous phase with fresh deionized water. Indeed, no obvious degradations in the phase transfer efficiency were observed in the next successive five cycles of experiments by making use of the same method to change the used water phase (Figure 5a).

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CONCLUSIONS In summary, we have demonstrated the synthesis of a new class of smart luminescent materials via surface modification of amine-enriched CDs with CO2-responsive groups. Upon alternative bubbling of CO2 and N2, reversible changes in both the surface chemistry and the emission state of the CDs are generated. These changes permit efficient and reversible phase transfer between an organic phase and an aqueous phase of the CDs, together with a corresponding luminescence change between blue and cyan-green. The reversible luminescence change can be extended to the solid state upon alternative stimulation of a humid vapor of CO2 and a mixture of NH3 and N2. The emergence of such smart nanoparticles can largely improve the performance of conventional luminescent nanoparticles in the fields of sensing, security inking, and cell tracking. Our findings can also provide insight into the development of other smart luminescent materials by surface modification of surface-state-controlled luminescent nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05421. Synthesis of m-PD-derived CDs; fraction of the asprepared CDs used for surface modification; TEM image of the purified m-PD-derived CDs; optical characterization of the as-prepared and purified CDs; surface modification of the as-purified CDs with amidine at room temperature; comparison of UV−visible absorption profiles; excitation-dependent emission matrix; dependence of the emission intensity on concentration; optimal excitation and emission profiles; luminescence responses and changes; comparison of emission intensities; and reversibility examination of phase transfer (PDF)

Figure 5. (a) Reversibility examination of phase transfer of the amidine-modified CDs between 1-octanol and water phases upon alternative bubbling of CO2 and a mixture of N2 and NH3. It is important to note that the stimulation durations for CO2 and a mixture of N2 and NH3 were fixed at 6 and 2 min in each cycle, respectively, and the water phase was replaced by fresh deionized water after the addition of a mixture of N2 and NH3 prior to performing phase transfer study in the next cycle. (b) Reversible luminescence change of the amidine-modified CDs in the solid state upon alternative exposure to humid vapors of CO2 and a mixture of NH3 and N2. The exposure durations were fixed at 2 and 1 min, respectively.

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AUTHOR INFORMATION

Corresponding Authors

The reversible luminescence change upon the alternating stimulation inspires one to use the modified CDs for document encoding applications. As a proof-of-concept demonstration, we made a two-dimensional covert pattern by a stamping method using the modified CDs as security ink.56 Upon exposure to a humid vapor of CO2, a luminescence change of

*E-mail: [email protected] (X.L.). *E-mail: [email protected] (B.G.). ORCID

Xiaowang Liu: 0000-0002-5126-0414 Baoyou Geng: 0000-0002-3657-0510 22855


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Anhui Province for Distinguished Youth (1908085J06) and the National Natural Science Foundation of China (21471007 and 21871005).

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REFERENCES

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