Two-Dimensional Folded Nanosheets Lead to an Unusual Circular

Article pubs.acs.org/Langmuir

Two-Dimensional Folded Nanosheets Lead to an Unusual Circular Dichroism Effect in Aqueous Solution Fei Li, Qiao Song, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: We found that two-dimensional (2D) folded nanosheets with crystalline structure in aqueous solution could induce an unusual time-dependent circular dichroism (CD) effect in the CD measurements after shaking or stirring the solution. In addition, a critical damping curve was observed in the CD spectrum after stop of stirring the solution. The apparent CD effect actually originates from linear dichroism (LD) changes in the oscillations of two-dimensional folded nanosheets and the inherent optical imperfections of the CD instruments. The oscillations of the folded nanosheets should induce this CD effect by the conformational change. Although an unusual apparent CD effect in alignment of one-dimensional (1D) dye-containing supramolecular nanofibers has been documented, this study reports for the first time that 2D folded nanosheets can lead to such a CD effect. It is anticipated that the method may provide a way to spectroscopically visualize the dynamic conformational change of self-assemblies in solutions.

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aligned in the propagation direction of audible sound.14 To date, although 2D assemblies can be torqued with conformational change in the flow which induces the transient emergence of true CD, we wondered if 2D assemblies could lead to an apparent CD.15,16 Instead of that the apparent CD effect induced by alignment of 1D nanofibers between themselves, it is assumed that self-assembly with 2D structure should induce this effect by conformational change of itself.17,18 Therefore, it may be possible to build a new supramolecular model to visualize the conformational change of 2D nanosheets, even those that lack chiral sense. In this respect, the work is very meaningful to the study of biomimetic simulations and on the preparation of new nanostructures and materials from achiral starting materials. Recently, we reported that 2D ordered nanostructure could be prepared by charge-transfer (CT) complexation of an azulene-based oligomer (PAL) and water-insoluble pyrene molecules.19,20 As a straightforward idea, we supposed that

INTRODUCTION Liquid motion and vibrations are important in chemistry because of their influence on the motion of polymers, large supramolecular assemblies, and some particles.1,2 Interestingly, such hydrodynamic effects can offer a symmetry-breaking source on the induction of forming chiral self-assemblies, transferring chirality from stirring vortexes down to the level of electronic transitions.3−9 In recent years, macroscopic chirality, which is induced by chiral alignment of achiral one-dimensional dye-containing supramolecular nanofibers in vortexes, has been spectroscopically visualized by circular dichroism.10 Another independent investigation reported that the partial alignment of those fibers could lead to a monosignate CD effect caused by convective flow, compared with the bisignate CD effect, the sign of which depends on the stirring direction of a vortex.11 These studies have revealed that the CD response is an artifact attributed to the LD in the partially aligned solution together with the inherent optical imperfections of CD instruments.12,13 More importantly, it has provided a way to visualize the changes in alignment of achiral self-assemblies.10 Furthermore, this phenomenon was used to sense the vibrations of solutions containing supramolecular nanofibers that are preferentially © 2014 American Chemical Society

Received: April 7, 2014 Revised: May 10, 2014 Published: May 13, 2014 6064

dx.doi.org/10.1021/la501340e | Langmuir 2014, 30, 6064−6070

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Scheme 1. (a) Amphiphiles Consisting of Azulene-Based Oligomer (PAL) Complexes with 1-Hydroxypyrene (1-PyOH) To Form a 2D Folded-Self-Assembly; (b) Model Showing the Oscillation of Such a 2D Folded Self-Assembly in Solution after Stop of Shaking or Stirring

Figure 1. (a) TEM image of PAL−1-PyOH coassemblies after complexation for 24 h. (b) TEM image of PAL−1-PyOH coassemblies after complexation for one month. (c) AFM image of PAL−1-PyOH coassemblies adsorbed on a carbon-coated grid. (d) Section analysis of (c). The concentration of the solution was 0.2 mM.

relation process in CD measurements after stop of stirring because of the high viscosity of the solution.23 In this report, a critical damping curve was observed in CD spectrum after stop of stirring the solution in very dilute solutions. Considering that folded structure and the preferred orientation of the 2D nanosheets in the solution, we may propose an interesting possibility that this time-dependent apparent CD effect originates from the oscillations of the large 2D folded nanosheets in the solutions. Through the combination of CD and LD experiments, we visualize the conformational change of assemblies lacking any chiral sense.

pyrene derivative, e.g. 1-hydroxypyrene (1-PyOH), could complex with PAL to form a similar supra-amphiphile, but probably with a chiral sense, because asymmetric binding of the two achiral building blocks could break the symmetry of the supra-amphiphile when we could control their interactions.21,22 However, when we used magnetic stirring to induce chiral selfassembly of PAL and 1-PyOH, no CD response was observed. Instead, surprisingly, a time-dependent CD signal was observed in the control experiments using unstirred aged solutions. Large 2D folded nanosheets were found in these CD-positive solutions. Through the combination of CD and LD experiments in stirring solutions, we confirmed that such CD effect was induced by the 2D nanosheet itself. In other reports, some lyotropic liquid crystal solutions using polymers can show a 6065


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Figure 2. Changes in (a) induced circular dichroism (CD) spectra and (b) linear dichroism (LD) spectra of a PAL−1-PyOH aqueous solution (0.2 mM; 0.4 mL) at 20 °C (time interval is 135 s). (c) CD and (d) LD spectra of a PAL−1-PyOH aqueous solution (0.04 mM; 3 mL) upon rotary stirring at 600 rpm in top clockwise (T-CW; black curve) and top counterclockwise (T-CCW; red curve) directions at 20 °C.

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The sample was prepared by dropwise addition of the solution onto a clean glass substrate at room temperature.

EXPERIMENTAL SECTION

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Materials. Unless noted, water was obtained from the purification machine (Millipore A10). 1-Hydroxypyrene (purity: 97%) was bought from the company of J&K. PAL were prepared by the method in our previous work.19 LD and CD Measurements. Unless noted, CD and corresponding LD spectra were recorded on a ChiralscanTM CD spectrometer from Applied Photophysics Ltd. In addition, the CD spectra were also monitored in another CD machine (π-Pistar 180 from the Applied Photophysics Ltd.) at convenience. It should be noted that the CD spectra are measured with opposite signs between the two CD instruments with same samples because the left circular polarized light may mix with horizontal linear polarized light in one machine, comparing with that left circular polarized light mix with vertical linear polarized light in another machine. The sample solutions (0.2 mM) were prepared by standing in the dark at room temperature, and the solutions (0.04 mM) were prepared by dilution from sample solutions (0.2 mM) when they were used in CD and LD measurements. Other Measurements. UV−vis spectra were measured using a Hitachi U-3010 spectrophotometer; fluorescence spectra were measured using a Hitachi F-7000 apparatus. Unless otherwise noted, excitation and emission slits were 5.0 nm, and the scanning rate was 240 nm/min. Transmission electron microscopy (TEM) images were measured using a JEM 2010 electron microscope operating at an acceleration voltage of 120 kV. Samples were prepared by drop-coating aqueous solutions on a carbon-coated copper grid and drying. This rinsing procedure was repeated three times. Selected area electron diffraction (SAED) were obtained on the same instrument. Atomic force microscopy (AFM) measurements were carried out in tapping mode in air on a commercial multimode Nanoscope IVAFM. Samples were prepared in the same manner as for TEM experiments. CryoTEM samples were prepared at room temperature in a controlled environment verification system (CEVS). The samples were stored in liquid nitrogen until they were transferred to a holder (Gatan 626) and examined by a JEM2200FS TEM (200 kV) at −174 °C. Single crystal XRD data were collected on an Oxford Gemini E diffractometer, and XRD data were recorded on a Rigaku D/max 2500 diffractometer. The Bragg peak was extracted from the XRD data, and the layer thickness d was obtained using the Bragg equation d = λ/2 sin θ, λ = 1.154 05 nm.

RESULTS AND DISCUSSION Before the investigation, the complexation between PAL and 1PyOH and the structure of their assemblies were characterized and confirmed. 1-PyOH in THF solution (20 mM) was added to an aqueous solution of PAL (1 mM) with a molar ratio of 1:1 for CT complexation. TEM images of the complexes showed that disklike nanosheets with diameters of a few hundred nanometers formed after 24 h (Figure 1a). A SAED pattern of this sample displayed two Debye−Scherrer patterns around the discrete diffraction spots, confirming that the nanosheets are of crystalline structure (Figure S1). The outer and inner rings corresponded to distances of 0.38 and 0.44 nm, respectively, which could be associated with the π−π stacking distance and the characteristic average spacing of molecules between PAL and 1-PyOH molecules, respectively. To further prove the CT complexation of PAL and 1-PyOH, UV−vis and fluorescent experiments were performed. After complexation, the absorption bands of 1-PyOH and the characteristic CT band were observed in the solution (Figure S2a). In addition, the emission of 1-PyOH between 380 and 500 nm was quenched markedly, indicating the formation of a CT complex (Figure S2b). In order to prepare the chiral supra-amphiphiles, vortexes were used to break the symmetry of the binding between PAL and 1-PyOH in aqueous solution (0.2 mM). We carried out experiments to control the formation of the coassemblies by stirring the solutions at different stirring speeds and stirring directions (clockwise (CW) and counter clockwise (CCW)), as well as unstirred the controlled experiments. None of the stirred samples showed a CD response. Conversely, some of the control solutions left standing for one month exhibited time-dependent CD signals even with simple addition of solutions to the cuvettes (Figure 2a). Among 12 unstirred samples, more than half samples had the same observation 6066


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Figure 3. Temperature-dependent (a) CD and (c) LD spectra of PAL−1-PyOH solution (4 × 10−5 M; 3 mL) containing large folded nanosheets (CCW stirring; 300 rpm; temperature interval is 5 °C; dT/dt = 60 K/h; measurement time for one spectrum is 135 s), and the signal (back to 20; blue line) obtained after cooling the solution from 65 to 20 °C; temperature-dependent (b) CD and (d) LD measurements at λ = 431 nm (black line) and 290 nm (red line) from spectra a and c, respectively.

solutions with CD activity, compared with much smaller nanosheets without CD activity in control experiments (Figure S6). To determine the difference of the apparent CD effect of 2D folded nanosheets with 1D nanofibers, vortexes were used in the CD measurements. 1D nanofibers showed a bisignate CD effect whose sign depends on the rotary direction or vortex in the ordinary CD measurement.10,11 If the CD effect arose from partial alignment of the coassemblies in the ordinary CD measurement, a bisignate CD effect should be obtained. To this end, a PAL-1-PyOH solution (0.04 mM) with CD activity was stirred first CW and then CCW during CD and LD measurements.24 Figure 2c,d shows that both CD and LD signals are time-independent in stirred solutions. Importantly, they showed a monosignate effect in vortex flows, revealing that the sign of the CD signal is independent of rotary direction which is different from that of 1D nanofibers. In addition, the CD and LD spectra showed identical band shapes in stirred solutions, further confirming that the CD contribution originated from the LD of the coassemblies and birefringence in the optical system. These results suggest that a macroscopic chiral shear force of the flow has no effect on the alignment of the 2D nanosheets in solution but only leads to the orientation change which was shown in the LD spectrum. It should be pointed out that, although stirring during incubation prevented the formation of large assemblies because of the selective growth in the vortex, once large assemblies formed in the unstirred solutions, neither stirring nor dilution decomposed them during measurement.25 For instance, the nanosheets dissociated very slowly in solution after 5-fold dilution because the CD spectra changed slowly after standing for 24 h (Figure S7). In addition, we have not observed the stirring rate dependence of the hydrodynamic effect because of a strong CD signal even at the lowest rotation speed (Figure S8).

(Table S1). Subsequent LD experiments revealed identical band shape (both CD and LD intensity at λ = 431 and 290 nm) and the same trend. Thus, the CD response could be interpreted as an apparent effect resulting from the change in LD response of the solution, possibly caused by change of the alignment or conformation of some assemblies. To clarify what kind of self-assemblies induced this apparent CD effect, TEM and AFM experiments were carried out. It should be noted that such CD effect could be induced only at a macroscopic level, which was validated by Meijer and coworkers’ work that the dye-contained nanofibers could hardly lead to the artificial CD response unless they were long enough.11 In this regard, we focused on whether some large assemblies existed. Strikingly, we did observe some large folded nanosheets of several micrometers across existed in the CD active solutions by TEM (Figure 1b and Figure S10a). In contrast, we did not observe the existence of larger folded nanosheets in solutions without CD activity. Therefore, the CD response should originate from the folded nanosheets. AFM revealed more details of the folded nanostructure. Two sections were selected from vertical and horizontal positions on the nanosheets (Figure 1c, lines A and B). Line A passed through two folded parts containing four layers of the nanosheet, and line B crossed three layers of the nanosheet. The total thickness of seven layers was 27.1 nm, so the average thickness of a single layer was 3.9 nm (Figure 1d). In addition, the samples incubated for 1 day and 1 month exhibited almost identical X-ray diffraction (XRD) patterns, indicating that the macrostructure of the assemblies does not change; only their lateral size does. The calculated thickness of a single layer was 4.0 nm, close to that obtained from AFM (Figure S3). To confirm that the nanosheets folded not only in the bulk state but also in solution, cryo-TEM experiments were performed. The cryo-TEM images clearly showed that large nanosheets formed and maintained their folded structure in aqueous 6067


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To further evaluate the influence of the folded nanosheets on the CD and LD response, temperature-dependent CD and LD spectra were measured in stirring solutions. As we have studied in the system that the coassemblies of such CT complexes gradually disassembled with increasing temperature, the large folded nanosheets should be easily destroyed via the same process. Both the CD and LD response showed same gradual decrease of the intensity when the temperature increased and disappeared at 60 °C, suggesting that the large folded nanosheets disassemble at this temperature (Figure 3). When the resulting solution was naturally cooled to room temperature (20 °C), no CD or LD responses were observed. Considering that all of the self-assemblies could be disassembled in the heating process at this concentration, we investigated this change at a higher concentration (0.2 mM) with the annealing process. After heating of the complexes solution with strong CD response at 60 °C in 10 min, no apparent CD response was observed in the measurement with shaking at this temperature (Figure S9). When the solution was cooled to room temperature, there was still no CD response. In comparison with the morphology change before and after annealing, the amounts of the larger folded nanosheets disappeared and only smaller disklike nanosheets was observed in the process, further confirming that the apparent CD signals are induced by the folded nanosheets (Figure S10). With validation that the CD and LD signals in still or stirred solutions corresponded to orientations of the folded nanosheets with 2D crystalline structure, we wondered how the folded nanosheets could induce time-dependent signals. To further determine this slow process, CD and LD spectra at a wavelength of 290 nm were measured after stopping stirring in a stable environment. We clearly observed a critical damping curve of approximately one-half in the spectra, which always happens in the case of oscillator motion in water with dragging force (Figure 4a).26 Notably, there was a LD response in the balanced state, which confirmed that the nanosheets adopted a preferred orientation in solution (Figure S11). In some reports, lyotropic liquid crystal solutions using polymers can show a slow relation process in CD measurements after stop of stirring because of the high viscosity of the solution.23 As the concentration of this solution is rather low (0.04 mM), the viscosity could not affect this process at longer time. Furthermore, the time of the oscillation process that lasted nearly over 6000 s is much longer than that (1500 s) in the solution with high viscosity, suggesting that the critical damping curve does not originate from the oscillations of the convective flows but the oscillations of folded nanosheets themselves. Based on our previous results, the folded structure is crucial in the oscillation process of the 2D nanosheets. According to the model proposed by Ribό et al., if a 2D folded nanosheet has a M or P folding sense and a preferred orientation, the apparent CD effect can be described as a function of α, which is the angle between the two folded parts (Figure 4a).17 In the folded nanosheets, we clearly observed that the folded structures were not restricted like those in folded helicoidal ribbons. Therefore, the shear force in the vortex or convective flows could change α by stretching or compressing the two folded parts to some extent, which changes the conformation of the folded nanosheets. Through analysis of model A, we found that the CD effect could be induced by the chiral alignment of two single-layered parts of the 2D folded assembly. Moreover, the macroscopic chirality did not arise from the vortexes, but from the formation of folded structures with an M or P folding sense.

Figure 4. (a) Time-dependent CD spectrum measured at λ = 290 nm by stopping stirring of a PAL-1-PyOH solution (4 × 10−5 M; 3 mL) containing large folded nanosheets (CCW; 300 rpm). (b) Model A: 2D nanosheets folded with a chiral sense of M or P could induce a CD effect when they adopted a preferred orientation (vertical). (c) Model B: 2D nanosheets folded without any chiral sense with a preferred orientation (horizontal) could induce a CD effect by conformational change during oscillation.

Thus, we should obtain one CD signal with negative sign in one sample, and another signal with positive sign from other samples on the same CD instrument, because the folded nanosheets were grown in standing culture without symmetry breaking. However, all of the samples showed a monosignate CD effect on the same CD instrument, indicating that the data does not fully match this model (Table S2). Inspired by the report that a monosignate CD contribution arose from LD of the partial alignment of 1D fibers,11 the CD effect in this system should only depend on the LD of the anisotropic conformation of folded nanosheets. In this regard, change of α would lead to a LD change of the folded nanosheets without involvement of macroscopic chirality, which induced a monosignate CD effect in both shaken and stirred solutions. In addition, the folded nanosheets had to prefer an orientation in solution; otherwise, randomly orientated nanosheets would diminish LD signals. We found a negative sign of the LD signals in stirred solutions, revealing that a vertical orientation dominated. Supposing that the shear force stretches the folded nanosheets to some extent during stirring, we propose model B to explain our results (Figure 4c). In this model, the folded nanosheets adopt a horizontal orientation in a stable environment, and vibrations of the solution by shaking or stirring are transferred to the folded nanosheets, causing them to stretch in the vertical direction. Based on this model, the folded nanosheets could be visualized as an oscillator in water, and there should be an oscillation 6068


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process when the force is removed. It should be noted that the change of CD signals is induced by the average vibrations and movements of the 2D nanosheets. On the basis of model B, we could distinguish the oscillation states at this curve. Water quickly relaxes in this process and becomes still in minutes, so there should be no disturbing vibrations from the solution for the flowing measurement in a stable environment. First, the nanosheets are stretched to some extent by the shear force in the stirring solution with maximum negative CD and LD intensities. Second, the folded nanosheets vibrate back to the stretching state (S1) and gradually oscillate to the balanced state (S2) through the compressed state (S3) with damped motion behavior after one cycle.

CONCLUSIONS In conclusion, we have shown that the time-dependent apparent CD effect in present work originates from the oscillations of large 2D folded nanosheets in the solutions. Compared with those works of taking advantage of the combination of apparent CD and LD effects to determine the alignment of 1D nanofibers in the solutions, our results have supplied an interesting possibility that the conformational change of a supramolecular self-assembly even without any chiral sense could be characterized and visualized via CD and LD measurements. Therefore, besides the stationary state of the self-assemblies, this line of study could expand to the dynamic process in this field. In addition, the investigation on how to control the formation of the nanosheets with uniform folded structure and size is in progress, which is helpful for the further study of the relationships between oscillation frequencies of assemblies and imposed external vibrations. ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S11 and Tables S1, S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.Z.). Author Contributions

F.L. and Q.S. contributed equally to this paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program (2011CB808200), the NSFC (21121004), and an NSFC-DFG joint grant (TRR 61). We thank Ms Ling Hu (Tsinghua University) for TEM and electron diffraction measurements, Dr. Cheng Liu, Pei Guo (Chinese Academy of Sciences) and Mr. Haijie Zou (Tsinghua University) for CD and LD measurement, and Kai Song (Chinese Academy of Sciences) for cryo-TEM measurement. We also appreciate the helpful discussion and suggestions from Dr. Penglei Chen, Prof. Minghua Liu, Prof. Chunhong Yang and Prof. Zhibo Li (Chinese Academy of Sciences), and Prof. Dongsheng Liu, Guanglu Wu (Tsinghua University). 6069


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(20) Zhang, J. W.; Li, F.; Yuan, B.; Song, Q.; Wang, Z. Q.; Zhang, X. Layer-by-layer assembly of azulene-based supra-amphiphiles: reversible encapsulation of organic molecules in water by charge-transfer interaction. Langmuir 2013, 29, 6348−6353. (21) Zhang, X.; Wang, C. Supramolecular amphiphiles. Chem. Soc. Rev. 2011, 40, 94−101. (22) Wang, C.; Wang, Z.; Zhang, X. Amphiphilic building blocks for self-assembly: from amphiphiles to supra-amphiphiles. Acc. Chem. Res. 2012, 45, 608−618. (23) Okano, K.; Machida, K.; Yamashita, T. E A Sulfonated polyaramide: stir-induced chirality in its aqueous solution. Chem. Lett. J. 2012, 41, 750−752. (24) The PAL−1-PyOH solutions (0.04 mM) were chosen for stirring in CD and LD experiments because the HT (high tension) voltage is less than 600 at this concentration for the requirement of these measurements. (25) Sorrenti, A.; El-Hachemi, Z.; Crusats, J.; Ribό, J. M. Effects of flow-selectivity on self-assembly and auto-organization processes: an example. Chem. Commun. 2011, 47, 8551−8553. (26) Marion, J. B.; Thornton, S. T. Classical Dynamics of Particles and Systems, 3rd ed.; Harcourt Brace Jovanovich: San Diego, CA, 1995.

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Two-Dimensional Folded Nanosheets Lead to an Unusual Circular

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