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Force Measurement for Interaction between Cucurbit[7]uril and Mica and Self-Assembled Monolayer in the Presence of Zn2+ Studied with Atomic Force Microscopy Young-In Bae, Ilha Hwang, Ikjin Kim, Kimoon Kim, and Joon Won Park Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02168 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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Force Measurement for Interaction between Cucurbit[7]uril and Mica and Self-Assembled Monolayer in the Presence of Zn2+ Studied with Atomic Force Microscopy

Young-In Bae,† Ilha Hwang,‡ Ikjin Kim,§ Kimoon Kim,*,†,‡,§ and Joon Won Park*,† †

Department of Chemistry, ‡Center for Self-Assembly and Complexity (CSC), Institue for Basic

Science (IBS), §Division of Advanced Materials Science, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Korea KEYWORDS Cucurbit[7]uril, atomic force microscopy, self-assembled monolayer, singlemolecule interaction, host–guest interaction, dynamic force microscopy

ABSTRACT Force spectroscopy with atomic force microscopy (AFM) revealed that cucurbit[7]uril (CB[7]) strongly binds to a mica surface in the presence of cations. Indeed, Zn2+ was observed to facilitate the self-assembly of CB[7] on the mica surface, while monocations, such as Na+, were less effective. The progression of the process and cation-mediated selfassembled monolayer were characterized using AFM, and the observed height of the layer agrees


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well with the calculated CB[7] value (9.1 angstroms). We utilized force-based AFM to further study the interaction of CB[7] with guest molecules. To this end, CB[7] was immobilized on a glass substrate and aminomethylferrocene (am-Fc) was conjugated onto an AFM tip. The singlemolecule interaction between CB[7] and am-Fc was monitored by collecting the unbinding force curves. The force histogram showed single ruptures and a unimodal distribution, and the most probable unbinding force value was 101 pN in deionized water and 86 pN in PBS buffer. The results indicate that the unbinding force was larger than that of streptavidin–biotin measured under the same conditions, whereas the dissociation constant was smaller by one order of magnitude (0.012 s-1 vs. 0.13 s-1). Furthermore, a high-resolution adhesion force map showed a part of the CB[7] cavities on the surface.

INTRODUCTION Cucurbit[n]urils (CB[n], n = 5−8, 10, 14) are pumpkin-shaped molecules that consist of a number (n) of glycoluril units linked via methylene bridges.1−8 These macrocycles possess a hydrophobic cavity accessible via two identical carbonyl-fringed portals. Due to their unique properties originating from the cavity and portals, CB[n] can bind a variety of guests, including organic and inorganic molecules and metal cations. In particular, CB[7] has attracted considerable attention in host–guest systems because of its ability to complex with a variety of guest molecules with high binding affinity and selectivity.9,10 The binding affinity of CB[7] for some ferrocene and adamantane derivatives is in the range of 1012−1015 M-1, as high as the wellknown avidin–biotin and streptavidin–biotin complexes.11,12 This property has led to numerous applications ranging from the immobilization of biomolecules on a solid surface to protein purification and supramolecular Velcro for underwater adhesion.13−15 In addition, since CB[7] is stable at elevated temperatures,16 unlike biological systems, CB[7] offers high potentials in a


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wide range of fields. The dipole and electron densities of the carbonyl groups at the portals enable interactions with various materials. For example, the carbonyl-fringed portals not only interact with metal substrates but also capture metal cations.17−20 The former characteristics enable the self-assembly of CB[7] on gold surfaces.21 In particular, a self-assembled monolayer (SAM) of CB[7] on a gold surface has been utilized as a novel platform for immobilizing biomolecules and their applications.22,23 As an extension of such progress, we examined the self-assembly of CB[7] on atomically flat mica. A freshly cleaved mica is a widely adopted surface to image proteins and DNAs due to its flatness. Mica consists of aluminosilicate layers with potassium ions.24 Two aluminosilicate layers are faced with potassium ions which can be dissolved in aqueous solution and turns mica surface to be negative. The cation on the top layer can be exchanged with other cations in solution.25,26 Force-based atomic force microscopy (AFM) is a useful tool for investigating intra- and intermolecular interactions at the single-molecule level.27−42 In addition, adhesion force mappings generated from this approach enables visualizing individual target molecules including DNA, RNA, and proteins and identifying the distribution of specific functional groups on polymer surfaces at the nanoscale.29−34 Moreover, recent successes in overcoming the limits of conventional AFM have greatly widened the scope of its applications.41−45 In this study, we employed force-based AFM to understand the adhesion between CB[7] and mica and the host–guest interaction between CB[7] and ferrocene. Our study showed that CB[7] interacts more strongly with mica in the presence of dications, while monocations are less effective. In fact, CB[7] was observed to form a stable SAM on mica in the presence of various dications. The approach was quite efficient to examine the cases, because the force value can be


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measured in minutes under a certain condition, and this condition can be easily varied while repeatedly using the same tip. In addition, force-based AFM was employed to understand the single-molecule interactions of CB[7] with a guest molecule, aminomethylferrocene ( = FcCH2NHCH2CH2OCH2CH2OCH2COOH, am-Fc), which exhibit high affinity (Ka≈1012) and selectivity.9 Moreover, the tool provided valuable information about the energy landscape and the dissociation constant.35−39,45 Additionally, a high-resolution adhesion force map29−34 was obtained with am-Fc-conjugated AFM tips to visualize the individual cavities of CB[7] on the surface.

EXPERIMENTAL SECTION General. Both triangular shaped AFM tips (MSCT(Si3N4) and MSNL(Si), Bruker Nano, USA) and glass slides (Nexterion Glass B, Schott, Germany) were coated with a cone-shaped dendron of the third generation named 27-acid (custom order, VRND NanobioOrganics, India). All solvents and reagents were purchased from Sigma-Aldrich unless otherwise specified. Cucurbit[7]uril (CB[7]) was purchased from CBTech (Korea). Monoamine-functionalized CB[7] and am-Fc were synthesized based on a method in the literature.14,46 Silanized AFM Tips. The AFM tips were treated with 10% nitric acid at 60 °C for 20 min, followed by thorough rinsing with deionized water (18 MΩ·cm) and then drying in a vacuum chamber (30−40 mmTorr) for 20 min. For silanization, the tips were placed in 20 mL anhydrous toluene mixed with 0.20 mL of TPU (N-(3-(triethoxysilyl)-propyl)-O-poly(ethylene oxide) urethane (Gelest, USA) (1% (v/v)) under a nitrogen atmosphere for 4 h at room temperature. Then, the tips were rinsed with toluene and baked at 110 °C for 30 min. After thorough rinsing with toluene and methanol sequentially, the silanized tips were placed in a vacuum chamber.


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Dendron-coated AFM Tips. First, 94.8 mg of the 27-acid was dissolved in 5.0 mL of dimethylformamide (DMF) and mixed with 15 mL of methylene chloride (MC) to achieve a 1.0 mM solution. Then, 122.4 mg of 1,3-dicyclohexylcabodiimide (DCC) and 2.2 mg of dimethylaminopyridine (DMAP) were dissolved in the 27-acid solution. The tips were immersed in the resulting solution overnight, sequentially washed with MC, methanol, and deionized water, and then dried under vacuum. After deprotecting the anthracene group at the apex of 27-acid with aqueous trifluoroacetic acid (1.0 M) and immersing the tips in N,N-diisopropylethylamine (DIPEA) (20% (v/v) in MC) for 10 min, the tips were rinsed sequentially with MC and methanol and dried in a vacuum chamber. The generated primary amine group was utilized for the conjugation. CB[7]-modified AFM Tips. The above amine-terminated AFM tips were placed in an acetonitrile solution containing N,N’-disuccinimidylcabonate (DSC) (25 mM) and DIPEA (1.0 mM) for 4 h under nitrogen. After the reaction, the tips were gently rinsed with DMF and placed in this solvent with stirring for 30 min to eliminate nonspecifically physisorbed molecules. Subsequently, the tips were gently rinsed with methanol and dried under vacuum for 20 min. The resulting N-hydroxysuccinimide (NHS)-modified tips were immersed in a dimethylsulfoxide (DMSO) solution containing monoamine-functionalized CB[7] (10 µM) and triethylamine (200

µM) overnight. After the reaction, the tips were gently rinsed with DMSO and placed in this solvent with stirring for 30 min. Finally, the tips were washed with methanol and kept under vacuum. Cation-mediated Self-assembled Monolayer of CB[7]. Freshly cleaved mica (Ted Pella, USA) was prepared by the taping method. A piece of mica was placed in a 1.0 mM ZnCl2 (8.0 mL) solution with 4.0 mg of CB[7] for 10 min, 12 h, or 24 h. After the assembly, the mica was


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washed with deionized water (2.0 mL), tilted to allow the remaining water to drip off, and blown by a stream of N2. Am-Fc-modified AFM Tips. Carboxylic acid-terminated am-Fc was prepared to conjugate the am-Fc to the tips.14 First, 2.5 mg of carboxylic acid-terminated am-Fc was dissolved in 0.20 mL of DMF, and 1.5 mg of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 1.0 mg of NHS were added. After allowing the solution to stand for 2 h at room temperature, the DMF solution was mixed with PBS buffer (pH 7.4, 2.0 mL), and the resulting solution was diluted by a factor of 100 with serial dilutions. The amine-terminated tips were placed in the final solution with stirring for 2 h at room temperature and then rinsed sequentially with a copious amount of PBS buffer (pH 7.4) and deionized water. The prepared tips were stored in deionized water before starting force measurements. CB[7]-modified glass slides. 27-acid-coated glass slides (1” × 3”) were purchased from NB POSTECH, Inc. (Korea). These slides were cut to dimensions of 13 mm × 12 mm. The cut slides were placed in an acetonitrile solution of bis(NHS)PEG9 (bis-N-succinimidyl-(nonaethylene glycol) ester) (25 mM) and DIPEA (1.0 mM) for 4 h at room temperature. After the reaction, the slides were gently rinsed with DMF and placed in this solvent with stirring for 30 min to eliminate nonspecifically physisorbed molecules. Subsequently, the slides were washed with methanol, blown with a stream of dry N2 and dried in vacuum for 1 h. The resulting NHSmodified slides were immersed in a DMSO solution containing dissolved monoaminefunctionalized CB[7] (10 µM) and triethylamine (200 µM) overnight. After the reaction, the slides were gently rinsed with DMSO, and placed in this solvent with stirring for 30 min. Finally, the slides were washed with methanol, blown with a stream of dry N2, and kept under vacuum.


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Biotin-modified AFM Tips. 27-acid coated AFM tips were immersed in a pH 8.5 PBS buffer dissolving NHS-PEG12-biotin (Thermo Scientific, USA) (25 mM). After the coupling reaction, the tips were thoroughly rinsed with pH 7.4 phosphate buffered saline with Tween-20 (PBST), pH 7.4 PBS buffer, and deionized water in a sequential manner. Then, the biotin-modified AFM tips were placed in a vacuum chamber for 30 min. For blocking residual NHS group, the tips were immersed and stirred in pH 8.5 PBS buffer dissolving ethanolamine (50 mM) for 30 min. Subsequently, the tips were stringently rinsed with pH 7.4 PBST, pH 7.4 PBS buffer, and deionized water in a sequential manner. Finally, the resulting biotin-modified AFM tips were kept in pH 7.4 PBS buffer until the force measurements. Streptavidin-immobilized Glass Slides. On 27-acid coated glass slides, NHS-PEG4-biotin (Thermo Scientific, USA) (10 mM) in pH 8.5 PBS buffer was placed to make millimeter-sized spots manually. The biotinylated slides were washed 20 times with deionized water. Subsequently, the slides were centrifuged at 1000 g for 100 s. A solution of streptavidin (100 µM) was placed on top of the spots for 2 h to allow the reaction, and the spots were washed thoroughly with pH 7.4 PBST, pH 7.4 PBS, and deionized water in a sequential manner. The resulting slides were stored in pH 7.4 PBS buffer until the force measurements. Topographic Imaging with AFM. Topological AFM images were obtained with a NanoWizard III (JPK Instruments, Germany) under ambient conditions. Silicon probes (PR-T190, BudgetSensors, Bulgaria and SSS-NCLR, Nanosensors, Germany) were used for the air tapping with 512 × 512 pixels at a line rate of 0.60−1.0 Hz. With SSS-NCLR (nominal curvature (r) of 2 nm) topographic images were recorded over areas of 0.20 × 0.20 µm2 and 0.010 × 0.010 µm2. The mica substrates were attached to glass slides using epoxy glue before scanning.


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Dynamic Force Spectroscopy. Force−distance curves were obtained using a NanoWizard I (JPK Instrument, Germany). All force measurements were carried out using a liquid cell filled with deionized water or pH 7.4 PBS buffer at room temperature. Before the measurement, the cell was allowed to equilibrate for 30 min. The AFM cantilever (MSCT, r = 10 nm) was calibrated using the thermal noise method, resulting in a range between 0.025−0.032 N/m. Force−distance curves were obtained with a z-length of 200 nm, applying a force of 170 pN, a dwell time of zero second, and different retraction speeds ranging from 0.050 µm/s to 5.0 µm/s. The force values were plotted with respect to the logarithm of the loading rate, and the rate was obtained by multiplying the effective spring constant and the retraction speed. For the interaction between am-Fc and CB[7] in water, five different tips were employed, and the force values were collected at different tip speeds. Typically, 20–30 curves were recorded. The spring constants of the selected cantilevers were close each other, the collected force values were averaged, and the mean values were plotted against the logarithm of the averaged loading rates. For the interaction between am-Fc and CB[7] in PBS buffer, a single tip was employed. Typically, 100 curves were collected at each tip speed, and the median values were plotted against the logarithm of the loading rate. In addition, for the interaction between streptavidin and biotin, a single tip was used for the study. Adhesion Force Map of Cation-mediated CB[7] SAM with am-Fc tethered tips. Adhesion force maps of the SAM in deionized water were obtained using a NanoWizard III (JPK Instrument, Germany). All force measurements were performed in deionized water at room temperature. Before the measurements, the cell was allowed to equilibrate for 30 min. The AFM cantilever (MSNL, r = 2 nm) was calibrated using the thermal noise method. Force−distance curves were obtained with a z-length of 200 nm, applying a force of 170 pN, dwell time of zero


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second and retraction speed of 1.0 µm/s. High-resolution adhesion maps were obtained over an area of 0.010 × 0.010 µm2 at an interval of 0.5 nm (20 × 20 pixels, five measurements per pixel). Data Analysis. Specific curves with stretching were selected for data analysis, and the curves with linear profiles were discarded. The most probable unbinding force was determined by fitting with a Gaussian curve. The statistical error of the mean was estimated by 2σ/√n, where σ is the width of the distribution, and the N is the number of selected rupture events. After the interaction between bis(NHS)PEG9-linked CB[7] and am-Fc was measured, the curves of which stretching distance was over 1 nm were collected for further analysis. For obtaining adhesion frequency force maps for the cation-mediated CB[7] SAM on mica, the filtration process was employed. The process count only the curves of which force value is in the range of 2σ of the most probable force value.

RESULTS AND DISCUSSION Measurement of the Interaction Force between CB[7] and Mica. To confirm the interaction between CB[7] and the mica surface, CB[7] was conjugated onto an AFM probe, and the unbinding force between the macrocycle and mica in aqueous solution was measured (Figure 1a). To avoid multiple interactions, the probe was first coated with a cone-shaped dendron,29,30,34,35 and monoamine-functionalized CB[7] was conjugated with the active group at the apex of the dendron. The force−distance curves showed a single rupture, indicating the interaction of a single molecule. Only the curves with a nonlinear profile (63%, Figure S1) were collected to generate the histogram. The force was 69 ± 1 pN at a tip speed of 1.0 µm/s in deionized water, while this value increased in the presence of cations such as Na+, Ca2+, and Zn2+


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(Figure S1). The increase was most notable in the case of Zn2+, and the force value was 201 ± 3 pN at a Zn2+ concentration of 1.0 mM (Figures 1b−c).

Figure 1. Measurement of the unbinding force between CB[7] and mica. (a) A schematic diagram of the measurement, and force histograms of the specific interaction in (b) deionized water (n=663) and (c) in the presence of 1.0 mM ZnCl2 (n=530). The force value represents the most probable rupture force (median ± standard error of the mean from the Gaussian model).

Self-assembled Monolayer of CB[7] on Cleaved Mica in the Presence of ZnCl2. Based on the strong interactions observed between CB[7] and the mica surface, we formed a CB[7] selfassembled monolayer on the mica surface in the presence of Zn2+. Topographic images of the surface under various dipping conditions showed the progression of self-assembly of CB[7] (0.50 mg/mL) in the presence of ZnCl2 (1.0 mM). Before washing, aggregates were observed as tall as 1.8 nm (Figure 2a), but after washing, either a sub-monolayer or a monolayer was observed. When 10 minustes were allowed for the assembly, a sub-monolayer characterized by a typical island structure was evident after washing (Figure 2b). With the increasing dipping time, the vacancies were filled to generate a monolayer of small voids (Figures 2c−d). The same


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phenomenon was observed for Ca2+, but the resulting voids were larger (Figure S3). A dipping time of 24 h provides a monolayer with the highest occupancy, whereas a layer of the same type was observed after a longer dipping time. The typical height of the layer was 0.7–1.0 nm, and this value agrees well with the calculated height of CB[7], 9.1 angstroms. The height of the aggregates indicates that multiple stacking is possible through the CB[7]−Zn2+−CB[7] interaction, while this interaction is weaker than that of the Zn2+-mediated CB[7] binding to mica (Scheme 1). When the layer was placed in deionized water for 30 min, larger voids were visible, and no significant change was observed after longer exposures to water (Figure S4). In contrast, in the presence of Zn2+ (1.0 mM), the morphological change was not significant after 2 h (Figure S5). In this study, force-based atomic force spectroscopy was revealed to be a useful approach to finding the proper conditions for the self-assembly of CB[7] on a mica. Measuring unbinding force toward a surface can be extended to other systems as long as the molecules of interest can be conjugated onto the tip. Moreover, this approach is efficient and saves time because the force value can be measured in minutes at a given condition, and this condition can be easily varied, while the same tip can be used repeatedly.


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Scheme 1. A structural model of a cation-mediated CB[7] SAM on freshly cleaved mica. Side and top views of the SAM are depicted.

Figure 2. Topographic images of CB[7] SAM recorded in ambient conditions. The images were obtained after dipping a piece of mica in a solution of 0.50 mg/mL CB[7] and 1.0 mM ZnCl2 for (a) 10 min and before washing, (b) 10 min and after washing, (c) 12 h and after washing, and (d) 24 h and after washing. Profiles along the bold section of the line were used to calculate the height difference between the top surface and the bottom of a hole. The height difference is 1.8 nm, 1.0 nm, 0.7 nm, and 0.8 nm for (a), (b), (c), and (d), respectively. Unbinding Force between CB[7] and am-Fc. Force spectroscopy was used to measure the interaction force between CB[7] and the aminomethylferrocene derivative, am-Fc. To this end, a third-generation dendron was coated onto the slides and AFM tips, and am-Fc tethering a


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carboxylic acid group was conjugated with the amine group at the apex of the tip. To immobilize CB[7], a reagent, NHS-oligo(ethylene glycol)-NHS, was used to functionalize the surface, and finally, monoamine-functionalized CB[7] was applied (Figure 3a). The dendron controls the spacing between the immobilized molecules, and as a result, the controlled surface enables mostly 1:1 interactions.29,30,34,35 Flexible poly(ethylene glyocol) (PEG) linkers help distinction of the specific binding unambiguously due to their nonlinear elasticity. Because PEG27 can be stretched by 4–9 nm,47,48 it is predicted that PEG9 can be stretched by 1–3 nm. Also, the total stretchable length including the spacer on the AFM tip should be larger than this. Therefore, we included only the curves of which stretching is longer than 1 nm for further analysis. We observed only two curves of which stretching distance is less than 1 nm out of 408 cases. Moreover, the unit increases the hydrodynamic radius, which enhances the recognition probability. To elucidate the interaction, we used five different tips and measured the interaction at five different positions. The recorded unbinding force values and stretching distances were fitted with the Gaussian model, and the resulting most probable force value was 101 ± 2 pN, with a corresponding stretching value of 2.7 ± 0.2 nm. In addition, the probability of achieving the specific unbinding event was 61% (Figure 3c). Such large force values were also observed from the binary complex between CB[7] and adamantane, and from the ternary complex comprising methyl viologen, naphtol, and CB[8].39,40


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Figure 3. Measurement of the unbinding force between CB[7] and am-Fc. In neutral aqueous solution, the amine group of am-Fc is protonated, and hydrophobic ferrocene interacts with the CB[7] cavity and the protonated amine group interacts with CB[7] portal. (a) A schematic diagram of the measurement, (b) a representative force curve, and (c) a force histogram of the specific interaction from five different positions (n=241). For each position a new tip was employed. In the histogram, the force value (101 ± 2 pN) represents the most probable rupture force (median ± standard error of the mean from the Gaussian model). The probability of obtaining the specific curve is 61%. All force measurements were performed in deionized water, and the tip retraction speed was 1.0 µm/s. The Adhesion Force Map. Adhesion force maps were also obtained (Figures 4a,b and S7). Two thousand measurements were carried out over 2.0 × 2.0 µm2 (20 × 20 pixels, 100 nm interval, five measurements per point) in deionized water, and 75% of the pixels showed the specific rupture event, indicating the high affinity and uniformity of the modified surface. Only nonlinear force–distance curves prior to a rupture event were collected to analyze the specific interaction (Figure S8), and the nonlinear profile attributes to the extension of the linkers on an AFM tip and


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a glass substrate. Again, the histogram showed that the most probable force value is 101 ± 2 pN (Figure 4c), and the probability of obtaining the specific curves ( = number of the specific curves/2,000) is 55%. In addition, the map was obtained in the presence of 1.0 mM am-Fc (Figure 4b). The number of pixels showing the specific curves noticeably reduced (Figure 4), and most of the residual peaks should be associated with the nonspecific interaction.

Figure 4. Adhesion frequency map from the adhesion force map. The maps were obtained over 2.0 × 2.0 µm2 at an interval of 100 nm (20 × 20 pixels, five measurements per pixel) (a) in the absence of free am-Fc and (b) in the presence of 1.0 mM am-Fc. (c) Histograms of the unbinding force values. The most probable force value is 101 ± 2 pN for the interaction between CB[7] and am-Fc. The force value represents the most probable rupture force (median ± standard error of the mean from the Gaussian model). The probability changed from 55% to 19% All force measurements were performed in deionized water, and the tip retraction speed was 1.0 µm/s. Dynamics of the Interaction between CB[7] and am-Fc. To determine how fast the supramolecular pair dissociated, dynamic force microscopy (DFS) was performed. DFS enables


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determining the inherent kinetic parameters and information about the energy landscape associated with the dissociation. The correlation between the unbinding force (F) and the loading rate () is expressed by the following equation. =

+

where kB is Boltzmann constant, and T is the absolute temperature.49,50 The loading rate is obtained by multiplying the retraction speed by the apparent spring constant of the cantilever.35,36 A plot of the most probable unbinding force over the logarithmic value of the loading rate and the Bell−Evans model provides the energy barrier distance (xβ) from the bound state to unbound one and the dissociation rate constant (koff) for the potential barrier.51 By changing the retraction speed from 0.050 to 5.0 µm/s, we measured the unbinding force value for CB[7]−am-Fc pair in deionized water and PBS buffer (pH 7.4) (Figure 5a). The force value increased linearly with the logarithm of the loading rate, ranging from 1.2 × 103 to 1.6 × 105 pN/s in deionized water and from 1.2 × 103 to 1.2 × 105 pN/s in PBS buffer. The linearity shows that the dissociation is governed by a single energy barrier within the examined rates. Fitting the data to the Bell−Evans model yielded xβ values of 0.22 nm in deionized water and 0.29 nm in PBS buffer, and the koff values were 0.024 s-1 and 0.012 s-1, respectively. Both kinetic parameters are comparable to the ones that were previously determined between CB[7] and adamane (0.34 nm and 0.03 s-1).38 The experimental uncertainty of the measurement suggests that the koff and xβ values are comparable in both conditions (Figure 5). For a fair comparison, the unbinding force values for the streptavidin–biotin pair were measured under the same conditions (Figure 5b). The reported unbinding force values of the pair are in the range from 40 pN to 500 pN.52–56 Main reason for the wide variation is that the force measurement has been performed at the different conditions such as pH, salt composition or


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concentration, temperature, and speed. Therefore, we compared the force value of the pair with that of CB[7]-am-Fc side-by-side at the identical condition. The most probable unbinding force value for the former was 64 ± 2 pN at a tip retraction speed of 1.0 µm/s, while the value for the latter was 86 ± 1 pN. In addition, the linear dependency of the unbinding force on the logarithm of the loading rate, ranging from 1.2 × 103 to 1.2 × 105 pN/s in PBS buffer, was observed. Fitting the data to the Bell−Evans model yielded an xβ value of 0.21 nm and a koff value of 0.13 s-1. The results clearly show that the CB[7]−am-Fc pair requires a larger force to unbind and is kinetically more inert against dissociation. Although several studies have been reported on the kinetics of host–guest complexation of CB[n],39,57,58 obtaining kinetic constants of am-Fc and CB[7] is difficult due to their extremely high binding affinity. Dynamic force spectroscopy allowed us to measure the dissociation constant of the CB[7] and am-Fc complex and compare it with that of streptavidin and biotin. This study revealed that the dissociation constant of CB[7]−am-Fc is ten times smaller than that of the biological pair; in other words, CB[7]−am-Fc pair is kinetically more stable. Because CB[7] manifests high affinity constants for various guests, the slower kinetics certainly suggests a large number of new applications, particularly when the high stability of the chemical system at pH values above or below 7.4 and the enzyme resistance are considered.


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Figure 5. Comparison of the dissociation kinetics between the CB[7]−am-Fc pair and streptavidin–biotin pair. Plots of the most probable unbinding force versus the logarithm of the loading rate for (a) CB[7]−am-Fc pair and (b) streptavidin–biotin pair. In all cases, force−distance curves for each were recorded at 6 different retraction speeds. The open circles represent the force values measured in deionized water, and the filled circles and squares represent the values in PBS buffer. The total number of curves for CB[7]−am-Fc pair are (a) n=1,080 in deionized water and n=747 in PBS buffer and (b) n=631 for the streptavidin–biotin pair in PBS buffer.


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Morphology and Adhesion Force Maps of the Cation-mediated CB[7] SAM. Sharp silicon tips (typical radius of curvature is 2.0 nm) were used to map areas of 0.20 × 0.20 µm2 and 0.010 × 0.010 µm2 (Figure 6). A map of the former area showed a CB[7] layer exhibiting a height difference of approximately 0.9 nm (Figure 6a). The scan of the smaller area showed that the surface was smooth (average roughness, Ra = 2.7 × 10-2 nm), whereas individual CB[7] molecules were not visualized (Figure 6b). Next, using an adhesion force map, we analyzed the cation-mediated CB[7] SAM on mica. To see CB[7]’s packing structure in detail, a short am-Fc containing two EG moieties was introduced on the dendron (Figure 3a). A high-resolution adhesion force map was obtained in an area of 0.010 × 0.010 µm2 (20 × 20 pixels, 0.5 nm interval, five measurements per point, in deionized water) (Figure 7a). The interval of 0.5 nm must be sufficient to show individual cavities of CB[7], which exhibit an outer diameter of 1.6 nm. A higher lateral resolution will not help because the hydrodynamic radius of the tether is approximately 2 nm. The pristine maps were treated using two-step filtration processes. First, curves whose force values are outside the range of 2σ of the most probable force value (in other words, smaller than 78 pN or larger than 124 pN) were filtered out. Second, pixels showing a high probability (80% or 100%) were highlighted because the probability should be large in pixels close to the cavities. The resulting adhesion force map (Figure 7b) showed isolated pixels and clusters, mostly in middle and low regions. In the case of isolated pixels and small clusters, their centers were considered the cavities. The circles on the map are likely to represent the cavity of CB[7] assembled on the surface. Pixels forming large clusters are present in two regions, and assigning individual cavities to these clusters was impossible. While the expected distance between the circles is 1.6


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nm, there are evidently cases in which this value is shorter. The discrepancy can be explained by the flexibility of the tethered am-Fc, which lowers the lateral resolution (or accuracy) of the map. The high-resolution adhesion force map showed a potential merit over the morphology map. While the former map did not show all cavities of CB[7] or unveil its packing structure on the surface, the map visualized a part of CB[7] cavities on mica. Further efforts are required to fully explore this opportunity. If these efforts become fruitful through optimization, this approach would be useful for locating individual nanoscale molecular cavities on a surface and can be applied to visualize the direct binding of guests under various conditions. This new capability should facilitate understanding host–guest interactions at the single-molecule level.

Figure 6. High-resolution topographic images of a self-assembled monolayer of CB[7] recorded in ambient conditions ((a) 0.20 × 0.20 µm2 and (b) 0.010 × 0.010 µm2). The height difference of 0.9 nm between the void and the region occupied by CB[7] SAM in the image of (a) was observed, and only a wavelike artifact was observed in the zoomed-in image (b), which is an enlargement of the black square in (a). Sharp silicon tips (typical radius of curvature is 2.0 nm) were used, and Ra (average roughness) for the image of (b) is 2.7 × 10-2 nm.


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Figure 7. Adhesion frequency maps on a self-assembled monolayer of CB[7]. The maps were obtained on 0.010 × 0.010 µm2 at an interval of 0.5 nm (20 × 20 pixels, five measurements per pixel) (a) before and (b) after the data treatment. The probability of obtaining specific curves in each pixel is indicated by different colors. In (b), only curves with force values within the range of 78 to 124 pN were counted. In addition, pixels with a high probability (80% or 100%) were colored. Red circles were placed at the center of isolated pixels or clusters. The adhesion force curves were recorded in deionized water, and the tip retraction speed was 1.0 µm/s.

CONCLUSIONS As evident from the force-based AFM study, CB[7] binds to mica surface more strongly in the presence of cations, and forms a monolayer through self-assembly process. It was observed that thus-formed SAM is stable in water in the presence of Zn2+. Further study with dynamic force spectroscopy revealed that the CB[7]–am-Fc pair is kinetically more stable by one order of magnitude compared with streptavidin–biotin pair, suggesting various applications of the former system. High-resolution adhesion force map for the monolayer revealed a part of the CB[7] cavities. It is believed that such analytical approach can be applied for other systems to find right conditions for the self-assembly on surface, characterize structural and functional features of the


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molecular assembly, and reveal binding phenomena of host-guest pairs at the single-molecule level. ASSOCIATED CONTENT Supporting Information. The supporting information contains AFM images, histograms of unbinding forces, and adhesion force maps, which accompanies this paper at pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions K.K. and J.W.P. conceived and supervised the project. Y.-I.B. prepared the CB[7] SAM and performed AFM experiments. I.H. and I.K. synthesized monoamine-functionalized CB[7] and am-Fc. Y.-I.B., K.K. and J.W.P. wrote the paper. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2017008478) and the Brain Research Program through the National Research Foundation of


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Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016903232) (J.W.P.). In addition, this work was also supported by Institute for Basic Science (IBS) [IBS-R007-D1] (K. K.). We thank Erik Hamming (University of Twente) for providing the MATLAB program to analyze the adhesion force map data. REFERENCES (1) Mock, W. L. Cucurbituril. Top. Curr. Chem. 1995, 175, 1-24. (2) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H.-J.; Kim, K. Cucurbituril Homologues and Derivatives: New Opportunities in Supramolecular Chemistry. Acc. Chem. Res. 2003, 36, 621−630. (3) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. The Cucurbit[n]uril Family. Angew. Chem. Int. Ed. 2005, 44, 4844−4870. (4) Masson, E.; Ling, X.; Joseph, R.; Kyeremeh-Mensah, L.; Lu, X. Cucurbituril Chemistry: a Tale of Supramolecular Success. RSC Adv. 2012, 2, 1213-1247. (5) Liu, Y.; Yang, H.; Wang, Z.; Zhang, X. Cucurbit[8]uril-Based Supramolecular Polymers. Chem. Asian J. 2013, 8, 1626–1632. (6) Isaacs, L. Stimuli Responsive Systems Constructed Using Cucurbit[n]uril-Type Molecular Containers. Acc. Chem. Res. 2014, 47, 2052−2062. (7) Assaf, K. I.; Nau, W. M. Cucurbiturils: from Synthesis to High-Affinity Binding and Catalysis. Chem. Soc. Rev. 2015, 44, 394−418. (8) Barrow, S. J.; Kasera, S.; Rowland, M. J.; del Barrio, J.; Scherman, O. A. CucurbiturilBased Molecular Recognition. Chem. Rev. 2016, 116, 12320−12406.


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