Two exfoliation approaches for organic layered compounds: hydrophilic and hydrophobic polydiacetylene nanosheets

Two versatile exfoliation approaches generate both hydrophilic and hydrophobic nanosheets from an organic layered compound.

the PCDA monomer unit (RTBAOH/PCDA) was adjusted to RTBAOH/PCDA=4.0. The dispersion liquid was maintained under stirring at room temperature for 3 weeks. The lateral size and the thickness of the resultant PDA nanosheets were measured by atomic force microscopy (AFM, Hitachi, AFM 5000II). The dispersion liquid was centrifuged at 13500 rpm for 10 min to remove the bulk aggregates. However, the precipitate was not collected after the centrifugation.
The aqueous dispersion of the PDA nanosheets was mixed with ethyl acetate (1/1 by volume) for 30 min to remove the remaining PCDA monomers. The separated aqueous phase was used as the dispersion liquid of the hydrophilic PDA nanosheets. The dispersion liquid was dropped on a silicon (Si) substrate for the AFM observation. A Si substrate was cleaned by immersion in the mixture of methanol and hydrochloric acid (1/1 by volume) and then the concentrated sulfuric acid for 30 min. After washing by purified water, the cleaned Si substrate was immersed in 2.0 g dm -3 of polyethyleneimine (Acros Organics, PEI, Mw=6.0×10 4 , 50 wt.-% aqueous solution) aqueous solution for 15 min to obtain the positively charged surface. Then, the excess amount of PEI was removed by washing with purified water. The PEI-coated substrate was immersed in the aqueous dispersion liquid of the PDA nanosheets. The excess PDA precipitates P. S3 were removed by washing with purified water. The photochemical properties of the PDA nanosheets in the dispersion liquid were analyzed by UV-Vis spectrophotometer (Jasco, and spectrofluorophotometer (Jasco, FP-6500). Water in the dispersion liquid was evaporated to measure the X-ray diffraction (XRD, Bruker, D8-Advance) pattern and Fourier-transform infrared (FT-IR, Jasco, FT/IR-4200) spectrum. In addition to the AFM image in Fig. 3b, the hydrophilic PDA nanosheets were obtained throughout the substrate (Fig. S2a,b). The PDA nanosheets were observed after 3 days of the dispersion (Fig. S2c). The PEI-coated silicon substrate was immersed in the aqueous dispersion liquid of the PDA nanosheets. The negatively-charged PDA nanosheets were adsorbed on the surface via electrostatic interaction. The PEI-coated silicon substrate showed no platy objects higher than 1.0 nm in thickness (Fig. S2d). Therefore, the platy objects around 5 nm in thickness are regarded as the resultant PDA nanosheets. Fig. S3. FT-IR spectra of the layered PDA (i), the hydrophilic PDA nanosheets after the exfoliation (ii), the TBAOH aqueous solution after evaporation of water (iii). XRD patterns of the original layered PDA (iv), the collected precipitates after the dispersion in the TBAOH aqueous solution after 10 min (v) and 24 h (vi).

FT-IR and XRD analyses of the hydrophilic PDA nanosheets
The dimerized carboxy groups were changed to carboxylate ones after intercalation of TBA + in the hydrophilic interlayer space consisting of the carboxy groups. The stretching vibration of C=O bond in the intramolecular dimerized carboxy group was only observed on the precursor layered PDA around 1700 cm -1 (the white arrow with the blue background in Fig. S3a). The dispersion liquid of the exfoliated PDA nanosheets was dried with evaporation of water. The PDA nanosheet sample showed the absorption corresponding to the carboxylate group around P. S7 1550 cm -1 (the black arrow with the red background in Fig. S3a), whereas the absorbance around 1700 cm -1 originating from the dimerized carboxy group was decreased. The results indicate that the intercalation of TBA + proceeds in the hydrophilic interlayer space.
When the precursor layered PDA was dispersed in the TBAOH aqueous solution, the peaks corresponding to the layered structure were kept after 10 min (the profiles (iv) and (v) in Fig.   S3b). However, the characteristic peaks disappeared after 24 h (the profile (vi) in Fig. S3b).
These results indicate that the exfoliation with disappearance of the periodic layered structure proceeds after 10 min. Fig. S4. Photographs of the toluene dispersion liquid at 5 days (a), 1 week (b), 2 weeks (c), and 3 weeks (d).

Time-course observation of the exfoliation behavior in nonaqueous media
The dispersion liquid was colored to red with an increase in the immersion time. The bulk precipitate remained on the bottom of the toluene dispersion liquid (Fig. S4), whereas the bulk precipitate was not observed in the aqueous dispersion liquid containing TBAOH (Fig. S1b-e).
The bulk precipitate was removed by centrifugation to obtain the dispersion liquid of the hydrophobic PDA nanosheets (Fig. 3e).

P. S9
AFM images of the hydrophobic PDA nanosheets Fig. S5. Additional AFM images of the hydrophobic PDA nanosheets obtained after dispersion for 3 weeks.
In addition to the AFM image in Fig. 3f, the hydrophobic PDA nanosheets were observed on a cleaned silicon substrate (Fig. S5). The platy objects around 5 nm in thickness were observed throughout the substrate. In addition, the thicker objects than ca. 6 nm were observed on the substrate. Since the hydrophobic PDA nanosheets are prepared in toluene, the dissolution of the remaining monomer and the redeposition respectively cause the broadening of the thickness distribution. The hydrophobic PDA nanosheets exhibiting the long-alkyl chain were dispersed in toluene. Therefore, the disordered arrangement of the alkyl chains causes the broadened distribution of the thickness. P. S10 Fig. S6. FT-IR spectra of the layered PDA (i), the hydrophobic PDA nanosheets after the exfoliation and evaporation of toluene (ii). XRD patterns of the layered PDA (iii), the collected precipitates after the dispersion in toluene after 1 day (iv) and 3 weeks (v).

FT-IR analysis of the hydrophobic PDA nanosheets
While the absorbance corresponding to the dimerized carboxy group around 1700 cm -1 was slightly decreased with an increase in the immersion time (the white arrow with blue background in Fig. S6a), the absorbance of the monomeric carboxy group around 1760 cm -1 appeared (the gray arrow with the purple background in Fig. S6a). Nevertheless, the absorbance originating from the carboxylate group around 1550 cm -1 was not observed after the immersion P. S11 in toluene for 3 weeks (the black arrow with red background in Fig. S6a). The results suggest that toluene molecule was intercalated not in the hydrophilic interlayer space consisting of the dimerized carboxy groups but in the hydrophobic one consisting of the long alkyl chains.
When the precursor layered PDA was dispersed in toluene, the peaks corresponding to the layered structure were drastically weakened after a day (the profiles (iii) and (iv) in Fig.   S6b). The characteristic peaks completely disappeared after 3 weeks (the profile (v) in Fig. S6b).
P. S12 Fig. S7. UV-Vis spectrum of the precursor PDA with the blue color state before application of external stimuli.

UV-Vis spectrum of the precursor PDA
The original layered PDA with blue color showed the broadened absorption peak centered around 590 nm and 650 nm. 26b