Nanoscopic Force Sensitivity of Polydiacetylene 2D Layered Composites with Guest Molecules

2D polydiacetylene (PDA) layered composites with added guest molecules have demonstrated an excellent tunable mechanochromic properties toward the development of mechanochromic force sensors in energy‐related and medical applications. However, its force sensitivity at the nanoscale is left uncharacterized due to a lack of technique to conveniently apply forces to the 2D films in different directions, limiting the quantitative understanding of the sensitivity‐tuning mechanism. In this work, the quantitative friction force microscopy combined with fluorescence microscopy is introduced to measure the force sensitivity of the PDA layered composites with guest organic amines at the nanoscale. Intercalating nonylamine and 1‐aminodecane increase the sensitivity of 10,12‐pentacosadiynoic acid by 127% and 111%, respectively, whereas adding n‐tridecaylamine and stearylamine reduce it by 77% and 86%. This force sensitivity at the nanoscale is inversely correlated to the thermochromic temperature and the Young's moduli of the films, suggesting that the softer film has both higher force‐ and thermo‐sensitivity.

][22][23] Due to the anisotropy that originates from these 2D structures and their flexibility, soft-type layered materials exhibit characteristic dynamic functions. [24]Previously, we have shown that the mechanical sensitivity of PDA can be conveniently tuned by mixing various guest molecules into PCDA, which self-intercalates in between the PCDA lamellar structures. [17]The force in the range of 7.6-25.0N was applied by repetitively moving the bottom of the weight-loaded cage.However, the force response at the nanoscale was left unstudied; therefore, the sensitivity-tuning mechanism remains elusive.
Nanoscopic mechanical properties of PDA have been previously characterized by several groups based on atomic force microscopy (AFM). [25,26]Nevertheless, they were largely qualitative, because PDA reacts against forces lateral to the substrates due to its anisotropic structure, whereas the standard AFM quantifies only vertical forces.Recently, to overcome this issue, we have combined the nano-friction force microscopy (FFM), [27] which is an AFM-based technique that enables the quantification of lateral forces in addition to the vertical ones, and the fluorescence microscopy for reporting the quantitative and anisotropic forcefluorescence correlation at the nanoscale. [28]n this work, we will take an advantage of this unique characterization setup and study the nanoscopic force sensitivity of 2D PDA layered materials with different guest molecules.Four interlayer guests, nonylamine (C9), 1-aminodecane (C10), n-tridecaylamine (C13), and stearylamine (C18) were intercalated in between PCDA lamellar structures and their forcefluorescence correlations will be studied.

Results and Discussions
PDA layered composites were fabricated by the Langmuir-Blodgett method.We used four organic amines, (ii) nonylamine (C9), (iii) 1-aminodecane (C10), (iv) n-tridecaylamine (C13), and (v) stearylamine (C18) as guest molecules (Figure 1d).Each of them was mixed with (i) PCDA monomer in chloroform at 1 : 1 molar ratio, which was spread over the air-water interface in the Langmuir-Blodgett (LB) trough.Their vertical transfer to oxygen-plasma-activated glass coverslips produced micro-sized multilayer crystals in different sizes dispersed over the surface (Figure 1a, zoom-in AFM images in Figure S1, Supporting Information).
X-ray photoelectron spectroscopy (XPS) analysis proves the incorporation of the organic amines.To study whether the organic amines are integrated inside of the PCDA matrix, these samples were characterized by XPS.C1s peaks were found at around 285 eV for all the samples (Figure 1b).N1s peaks from amines were detected around 400 eV for PCDA mixed with C9, C10, C13, and C18 but not for the pure PCDA sample.This suggests that the organic amines are successfully incorporated in the PCDA matrix.The N to C ratio of each sample was estimated from the integration of the peak areas, which was smaller for PCDA-C9, PCDA-C10, and PCDA-C13, whereas slightly larger for PCDA-C18, compared to the theoretical values assuming 1:1 molar ratio.This implies that the guest molecules may not have been integrated exactly at 1:1 molar ratio (Tables S1 and S2, Supporting Information).
X-ray diffraction (XRD) shows the formation of new periodicities in the layered structure.To investigate the structure of these PCDA-organic amine complexes, the samples were characterized by XRD.The pure PCDA crystals had the first diffraction peak at 2 = 1.70°, which corresponds to the lattice spacing of d 0 = 5.19 nm (Figure 1c).For the PCDA-organic amine composites, the peak shifted towards the larger angles at 2 = 1.82°forPCDA-C9, 2 = 1.79°forPCDA-C10, 2 = 1.94°forPCDA-C13, and 2 = 2.00°for PCDA-C18 (Figure 1c).This indicates that the guest molecules were intercalated in between the PCDA lamellar layers, altering their periodicities.The estimated lattice spacings d 0 were 4.85 nm, 4.92 nm, 4.54 nm, and 4.42 nm for PCDA-C9, PCDA-C10, PCDA-C13, and PCDA-C18, respectively.Compared to the pure PCDA, the addition of the guest molecules declined d 0 for all the samples.This has been previously explained by our work as the detected periodicity originates from the lipid tail-to-tail distance as shown in Figure 1d. [17]Previously, samples made through solvent evaporation showed that d 0 increased (2 decreased) with the increased length of the alkyl-chain for C6-C18, whereas it was random for C1-C5. [17]The samples in the current study fabricated by the Langmuir-Blodgett trough yielded a slightly different result, where PCDA, PCDA-C9, PCDA-C10 are in this trend (2 PCDA-C9 > 2 PCDA-C10 > 2 PCDA ) while PCDA-C13 and PCDA-C18 are not.These outliers suggest that for some samples PCDA and the guest molecules are interdigitated with an overlap in their alkyl chains or tilted against the lamellar plain, as it was suggested before. [17]In addition, the XRD peaks became sharper when C13 and C18 were mixed compared to the pure PCDA (see also the half width analysis in Figure S2, Supporting Information), suggesting the improvement of the periodicity by the guest molecules.
Δfluorescence versus lateral force plot at the nanoscale was obtained by the quantitative friction force microscopy combined with fluorescence microscopy.Next, these samples were UV polymerized for forming PDA and their force sensitivities were studied by the quantitative friction force microscopy combined with an inverted fluorescence microscope (Figure 2a).The lateral laser deflection in AFM was calibrated by the wedge calibration method [27,29] to enable the quantitative friction force measurement.For the experiment, we used the hover mode (it scans during the trace and hovers back during the retrace), which enables the unidirectional application of lateral forces for the simplification.Here, we present the results from PCDA-C9, whereas the ones from PCDA, PCDA-C10, PCDA-C13, and PCDA-C18 are shown in Figures S3-S6 (Supporting Information).PCDA-C9 formed micro-sized crystals on glass with the average height of 13.68 ± 2.84 nm, observed in the bright field microscopy image (Figure 2b) and the AFM height (Figure 2c).These crystals were scanned by the friction force microscopy at different set points (vertical forces) between 30 and 100 nN (Figure 2d).During the scanning, the tip applied lateral forces on the sample, which were quantified by the friction force microscopy (Figure 2e).Fluorescence images were captured before and after scanning and their difference (Δfluorescence) was plotted (Figure 2f).Δfluorescence correlates better with the lateral force F // map (Figure 2e) than the vertical force F ⊥ map (Figure 2d).This suggests that PDA reacts primarily to the lateral forces, as we [28] and others [15] have reported before.Δfluorescence and the lateral force map were correlated pixel by pixel by the home-made MATLAB program for obtaining the Δfluorescence versus lateral force plot (Figure 2g).This plot shows how much lateral force triggers which level of the fluorescence intensity, where it's slope coefficient represents the force sensitivity of the PDA.
The force sensitivity is higher in the order of PCDA-C9 > PCDA-C10 > PCDA > PCDA-C13 > PCDA-C18.All the rest of the samples (PCDA, PCDA-C10, PCDA-C13, PCDA-C18) were characterized in the same way as described above.Fluorescence images after scanning showed that some samples presented higher fluorescence intensity than others (Figure 3a), indicating that their force sensitivities are different.Since the sample thickness was different (18.07 ± 0.58 nm, 13.68 ± 2.84 nm, 14.31 ± 1.17 nm, 28.56 ± 6.16 nm, 29.42 ± 3.25 nm for PCDA, PCDA-C9, PCDA-C10, PCDA-C13, and PCDA-C18, respectively), the accurate comparison of their fluorescence intensity required a normalization procedure.In brief, after scanning, the samples were heated at 100 °C for 10 min to induce the complete blue-to-red transition.Fluorescence images were taken from four regions and their average was used to normalize the Δfluorescence.This normalization process converts Δfluorescence from the absolute fluorescence intensity to the percentage of the polymer activated by AFM, which enables the comparison of various samples with different thickness as shown in Figure 3b.The same method has been previously used to compare 5,7-docosadiynoic acid (DCDA), 10,12-tricosadiynoic acid (TRCDA), and PCDA. [28]The slope coefficients of the normalized Δfluorescence versus lateral force plots from PCDA, PCDA-C9, PCDA-C10, PCDA-C13, and PCDA-C18 revealed that the force sensitivity is high in the order of PCDA-C9 > PCDA-C10 > PCDA > PCDA-C13 > PCDA-C18.Pre-viously, the force sensitivity of PCDA with organic amines with different chain lengths has been studied by applying forces in the range of several newton at the macroscopic scale in the context of, e.g., writing or tooth brushing.Those data indicated that the force sensitivity in the macroscopic scale was in the order of PCDA > PCDA-C6 > PCDA-C12. [17]In another macroscopic work, the order was PCDA-C8 > PCDA > PCDA-C16 for a linear force, whereas it was PCDA > PCDA-C8 > PCDA-C16 for a rotational force.These results show a similar trend to the present data as the mixture of the guest with shorter alkyl chains yielded a higher force sensitivity.However, the exact order seems to be slightly different from work to work, which could be attributed to the different methods of the force application.An extra amine in the guest molecule has also been found to reduce the force sensitivity (PCDA-C8 > PCDA-C8-(NH2)2). [30]DA layered composites with higher force sensitivity also have higher temperature sensitivity.Next, we studied the link between the observed mechanochromism and the thermochromism of the PDA layered structures.To estimate the thermochromic temperature of these samples, all the layered composites were heated from 30 °C to 100 °C, where UV-VIS spectra were obtained at each temperature (Figure S7, Supporting Information).From the EC50 of the colorimetric response (C.R.) versus temperature curve (Figure S8, Supporting Information) (see Experimental Section for the details), the thermochromic temperature was determined as 56.9 °C (PCDA), 52.9 °C (PCDA-C9), 54.8 °C (PCDA-C10), 60.5 °C (PCDA-C13) and 63.3 °C (PCDA-C18), which were overall slightly lower than our previous report based on a macroscopic study, [17] in which the samples were fabricated by solvent evaporation.This is probably due to the variation in the used sample fabrication technique, since the thermochromic temperature is known to be influenced by the crystal size, the amorphous to crystal ratio, the thickness etc. [31,32] For example, the film thickness of our samples is 1000 times thinner than our former report. [17]These data show that the mixture of C9 and C10 reduced, whereas the addition of C13 and C18 elevated the thermochromic temperature.A similar tendency has been previously reported, where the use of low-molecular-weight amines, such as alkyl amines and diamines, as guest molecules decreased the thermochromic temperature. [18]In comparison, the ones with longer alkyl chains have increased the thermochromic transition temperature. [17,33]The slope coefficients extracted from the linear fitting of these Δfluorescence versus lateral force plots (Figure S9, Supporting Information) inversely correlated with the thermochromic temperature (Figure 3c).This illustrates that the layered composites that are highly sensitive against forces also have a higher thermosensitivity, implicating a strong link between the thermochromism and the mechanochromism at the nanoscale, as a similar correlation has been reported. [17,28]n the case of AFM, the tip is breaking the hydrogen bonds between the PCDA headgroups, the hydrophobic attractions between the alkyl chains, disturbs the alignment, induces the torsion in the backbone, and triggers structural transition.The breaking of hydrogen bonds requires approximately 6-30 kJ mol −1 [34,35] and the dissociation of hydrophobic attractions between molecules requires approximately 10 kJ/mol/nm 2 . [36]In the case of mechanochromism, these energies are provided by work, W = FΔx.Previously, we have estimated that the force required to break hundreds of hydrogen bonds is in the nanonewton range, which is in agreement with the force range used in the experiment. [28]In the case of thermochromism, these energies are provided by heat.

Discussion
Lipids, such as the host or the guest molecules used in this work, have a characteristic phase transition temperature, where their structure transforms from solid to liquid.The solid-to-liquid phase transition temperature of PCDA is 62-65 °C, which suggests that PCDA is in a solid phase under the ambient temperature (25 °C).Upon UV irradiation, these solid PCDA crystals form blue PDAs, which is so-called in a planar configuration.The application of heat induces thermal motions in the alkyl chains and the subsequent torsion in the backbone, which causes the (planar to non-planar) transformation, making the PDA into red and fluorescent.This color transition temperature and the phase transition temperature have been often found to be close, [37,38] indicating that the phase transformation is the cause of the planar to non-planar structural change, thus the color transition.Now, the solid-to-liquid phase transition temperature of the used guest molecules are −1 °C, 12-15 °C, 26 -30 °C, and 53 °C, for C9, C10, C13 and C18, respectively.This means C9 and C10 are in a liquid state, whereas C13 and C18 are in a solid phase at the ambient temperature (25 °C).The presented result in Figure 3b suggests that mixing liquid guests improves the force sensitivity, whereas adding solid guests reduces it.
Why adding liquid makes it more force sensitive?In general, liquid has a lower density than solid with a famous exception of water.Indeed, XRD (Figure 1c) indicated that the interlayer distance for PCDA-C9 (4.85 nm) and PCDA-C10 (4.92 nm) is larger than PCDA-C13 (4.54 nm) and PCDA-C18 (4.4 nm) despite the fact that their alkyl chains are shorter.This implies that PCDA-C9 and -C10 have a lower density than PCDA-C13 and -C18, although other factors such as interdigitation or tilt angles of the alkyl chains also play a role in the distance.The densely packed systems form more rigid layer structures due to the stronger van der Walls interactions between alkyl chains, whereas the matrix with a lower density is softer.These liquid or solid guests are connected to the host PCDA via the electrostatic interaction between COO − and NH 3 + as the formation of the COO − /NH 3 + complex has been detected by FT-IR spectroscopy before. [17]In addition, lipid tail-to-tail van der Waals interactions may have also played a role.This is how the "softness" of the guest layer affects the sensitivity of the host PDA layers.41][42][43][44] To prove our hypothesis that the rigidity or the softness of the layered structure is the origin of the different force sensitivity, we performed the force spectroscopy [45] experiment to extract Young's modulus from each sample (for the technical details, see Supporting Information).Figure 4a shows a representative force spectrum from the PCDA film.Fitting the curve with the simplest Hertz model extracted the Young's modulus of the PCDA to be 2.71 ± 0.20 GPa.Note that the Hertz model assumes isotropic materials.Its application to anisotropic materials such as PDA may yield an intermediate value for the Young's modulus in different directions.The experiment was repeated for other samples, where the Young's modulus was estimated as 2.37 ± 0.59 GPa (PCDA-C9), 2.58 ± 0.33 GPa (PCDA-C10), 3.76 ± 0.37 GPa (PCDA-C13), and 3.9 ± 0.10 GPa (PCDA-C18).This confirms that mixing solid (C13 and C18) makes the PCDA matrix harder, whereas adding liquid (C9 and C10) slightly softens it (Figure 4b).The slope coefficient of Δfluorescence versus lateral force curve inversely correlates with the Young's moduli, supporting that the softer sample has a higher force sensitivity (Figure 4c).
First, this can be partially interpreted in terms of the simple change in the force propagation.When PCDA is embedded in a hard matrix (e.g., PCDA-C13, PCDA-C18), only a part of the force from the AFM tip is applied to PCDA, whereas the rest is used for pushing the guest molecules.In contrast, when PCDA is mixed with liquid guests (e.g., PCDA-C9, PCDA-C10), the majority of the force is applied to PCDA as liquid is easily deformable with a small resistance.In such a case, even if the PCDA force sensitivity itself is identical, the softer matrix yields a higher force sensitivity, simply because the effective pressure (the force divided by the effective contact area) changes.Nevertheless, this effect may not be sufficient to explain the 15.7-fold force sensitivity difference between PCDA-C9 and PCDA-C18.In addition, it does not explain the elevated thermosensitivity.This implicates that the force sensitivity of PCDA itself was altered by the presence of the guest.It could have been achieved via the COO − /NH 3 + link between the host and the guest.Alternatively, the host and the guest molecules may have been mixed in the same leaflets or intercalated, unlike the complete separation in layers as shown in Figure 1d, which could have altered the PCDA sensitivity.It is also worth to mention that the absorption spectra for PCDA-C18 have lower intensities than the others (Figures S7 and S10, Supporting Information).This suggests the existence of unpolymerized PCDA monomers left in the PCDA-C18 matrix, which may have affected the softness of the matrix.Nevertheless, PCDA-C18 had the largest Young's modulus, which indicates that these leftover monomers have less influence than the guest molecules probably because of their relatively small amount in the matrix and the guest molecules' large melting temperature differences from PCDA.

Conclusion
We reported the force sensitivity of 2D layered composites made of PCDA and guest organic amines (nonylamine (C9), 1-aminodecane (C10), n-tridecaylamine (C13) and stearylamine (C18) at the nanoscale by utilizing the dual friction force microscopy and fluorescence microscopy setup.First, XPS and XRD confirmed the incorporation of the guest molecules in the PCDA matrix.Second, the force sensitivity was higher in the order of PCDA-C9 > PCDA-C10 > PCDA > PCDA-C13 > PCDA-C18.This implied that mixing liquid makes the matrix more forcesensitive, whereas adding solid reduces its since C9 and C10 are liquid and C13 and C18 are solid at room temperature.This force sensitivity at the nanoscale was inversely correlated to the thermochromic temperature and the Young's moduli of the films, suggesting that the soft film has both higher force-and thermo-sensitivity.The link between the PDA softness and its force sensitivity has been previously proposed, yet the current work quantitatively proved it based on the measured Young's moduli for the first time.The result can be attributed to two effects; i) embedding PCDA inside the liquid matrix improved the force transmission as the majority of the force from the AFM tip is applied to PCDA.ii) The sensitivity of the PDA made of PCDA itself was altered due to the mixture or the intercalation of the guest molecules.The presented work shows that the force response at the nanoscale can be conveniently tuned by the types of the guest molecules without any complex molecular redesigning and synthesis of the DA monomers.Quantitative understanding of the nanoscopic force sensitivity of the 2D PDA layered composites helps with developing an alternative approach to visualize the forces and pressures at nanoscale in various sensing and imaging devices.In addition, the dual nano-friction force microscopy/fluorescence microscopy imaging technique can be applied to other force-responsive materials [46] in future.

Figure 3 .
Figure 3.The force sensitivity of PDA layered composites can be tuned by the addition of guest molecules.a) The fluorescent images of PDA layered structures after AFM scratching, and b) normalized Δfluorescence vs lateral force for PCDA, PCDA-C9, PCDA-C10, PCDA-C13, and PCDA-C18.c) The average slope co-efficient for each sample was plotted as a function of its thermochromic transition temperature (n = 4).n represents the number of the experiments to extract the average and the standard deviations (error bars) for the plot.

Figure 4 .
Figure 4.The force sensitivity responsivity of PCDA layered composite structures depends on the softness and rigidness of the layered structure.a) The extended force spectroscopic curve of PCDA, b) Young's modulus of the layered structures increases with increasing carbon number (n = 4), and c) the average slope co-efficient for each sample structure was plotted as a function of Young's modulus.