The influences of surface chemistry and dynamics on elasticity

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Influence of the Surface Chemistry and Dynamics on an ElasticityDependent Macroscopic Supramolecular Assembly Guannan Ju, Mengjiao Cheng,* Qian Zhang, Fengli Guo, Peichen Xie, and Feng Shi* Beijing Laboratory of Biomedical Materials and Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 185.2.32.159 on 10/10/18. For personal use only.

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ABSTRACT: Macroscopic supramolecular assembly (MSA) investigates multivalent supramolecular interactions between large surfaces (exceeding a size of 10 μm) modified with numerous interactive motifs. Because MSA is the first step to initiate noncovalent interactions at the interface, study of MSA mechanism is significant for various interfacial phenomena such as underwater adhesion, cell-material interaction, self-healing etc. Until now, most researches about MSA mainly report some assembly phenomena by applying molecular interactions into macroscopic assembly, insight into the underlying assembly mechanism is still lacking, e. g. fundamental questions such as what kind of building blocks or surface chemistry is requisite to realize MSA remains to be answered. Especially, multiple variables including surface chemistry, substrate effects, interaction dynamics etc. are significant factors to influence MSA behaviors. Previously we have revealed a rule that the MSA probability declines with increasing substrate rigidity (elastic modulus) under similar surface chemistry and a critical modulus of 2.5 MPa is a boundary condition, above which no assembly occurs. To elucidate the versatility of this rule and investigate the influences of surface chemistry (e.g., interaction type or number) or assembly dynamics on MSA, in this work, we have changed the supramolecular interaction type from β-cyclodextrin (CD)/azobenzene (Azo) to CD/adamantane (Ad) and increased the interaction number of both CD/Azo and CD/Ad. The results have demonstrated that regardless of varied surface chemistry or interaction dynamics, the MSA probability still displays a negative correlation with the substrate modulus while the boundary modulus is dependent on the strength of the applied supramolecular interaction. For the weak CD/Azo system, 2.5 MPa is a critical value while for the stronger CD/Ad system this boundary value increases to 3.3 MPa. We envision that these fundamental understandings of the MSA mechanism may favor for establishing a general design principle of MSA systems and interpreting interfacial phenomena. KEYWORDS: macroscopic supramolecular assembly, multivalency, surface chemistry, host/guest interaction, self-assembly

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originated from the perspective of “self-assembly at all scales” by Whitesides and Grzybowski;18 they reported a few assemblies of millimeter-scaled building blocks at oil/water interfaces driven by capillary force and the manufacture of ordered structures based on the principle of minimizing the interfacial free energy.19 However, few noncovalent molecular interactions were involved for these macroscopic assemblies. Only very recently have Harada’s and our groups developed

INTRODUCTION Macroscopic supramolecular assembly (MSA) is a recently developed research topic in supramolecular chemistry1,2 and is defined as a noncovalently multivalent process between numerous interactive moieties anchored on two macroscopic surfaces.3 Because the physical essence of multivalent interaction between large surfaces is similar to that of various interfacial phenomena, MSA has provided an ideal platform to investigate the mechanism underlying these phenomena such as underwater adhesion,4−6 cell−cell or cell−material interactions,7−9 fabrication of 2D/3D-ordered structures10−13 or biomaterials through assembly,14−17 etc. The concept of MSA © XXXX American Chemical Society

Received: July 24, 2018 Accepted: September 12, 2018 Published: September 12, 2018 A

DOI: 10.1021/acsanm.8b01277 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 1. Illustration of the Fabrication and Surface Modification Processes of PHEMA Hydrogels

MSA by applying supramolecular interactions as the driving force to realize the assembly of macroscopic building blocks made of hydrogel1 or glass fiber.2 Harada et al. demonstrated molecular recognition of macroscopic hydrogels decorated with host or guest moieties by shaking them in water.1 Later, other groups used other supramolecular interactions such as DNA hybridization,20 hydrogen bonding,21 and electrostatic interactions16,22 to assemble macroscopic hydrogels. To break the material limit of hydrogel systems, we proposed a concept of “flexible spacing coating” to address the problem of rigid building blocks failing to assemble; this highly flowable coating is modified beneath supramolecular interactive moieties to provide a compliant surface for high molecular motility, which is favorable for the multivalent binding of large building blocks for their macroscopic assembly.23−25 However, at this early stage, most reports focus on the application of various supramolecular interactions to realize macroscopic assembly but lack insight into the underlying assembly mechanism. Fundamental questions such as what kind of building blocks or surface chemistry is requisite for the realization of MSA still remain to be answered. In essence, whether MSA can be realized is largely determined by the binding efficiency of multivalent events between macroscopic surfaces.8,26−29 Quite a few variables could influence the assembly probability, such as surface chemistry, substrate effects, interaction dynamics, etc. On the aspect of substrate effects, recently we have investigated the influence of the substrate elasticity on the assembly probability while keeping other variables constant.4 We found a rule that the assembly probability decreases with increasing elastic moduli of the building blocks; for the applied host/guest system of β-cyclodextrin (CD)/azobenzene (Azo), we found a critical modulus of 2.5 MPa, and building blocks with a

modulus exceeding this value lead to no assembly. However, these conclusions are drawn by limiting variables to the modulus only while keeping other factors constant such as the surface chemistry and interaction dynamics. Therefore, we wonder whether the proposed rule of the assembly probability decreasing with increasing modulus is also applicable to other systems especially when the surface chemistry or assembly dynamics is varied. In this work, by taking poly(hydroxyethyl methacrylate) (PHEMA) hydrogels as macroscopic building blocks, we have changed the supramolecular interaction type from CD/Azo to CD/adamantane (Ad) and improved the interaction number of both CD/Azo and CD/Ad to investigate the assembly probability; meanwhile, we have checked the interaction dynamics of the MSA probability over a long assembly time. The results have drawn several conclusions as follows: (1) regardless of the varied surface chemistry or interaction dynamics, the MSA probability still presents a decreasing trend with the growing moduli of the building blocks; (2) with varying surface chemistry, soft building blocks show a remarkable increase of the assembly probability or interactive forces while rigid building blocks respond very little; (3) for the CD/Azo system, the moduli of the building blocks to realize MSA should be below a critical value of 2.5 MPa; for host/guest systems with stronger binding strength (CD/Ad) or higher surface density of the interactive groups, the boundary value of the building block modulus increases.

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EXPERIMENTAL SECTION

Materials and Instruments. The chemical reagents were used as purchased: 6-amino-β-cyclodextrin (CD-NH2) from Shandong Binzhou Zhiyuan Bio-Technology Co., Ltd. (Binzhou, China); poly(diallyldimethylammonium chloride) (PDDA; aq, 20 wt %, Mw = 400000), poly(acrylic acid) (PAA; aq, 25 wt %, Mw = 240000), and B

DOI: 10.1021/acsanm.8b01277 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 2. Illustration of the Apparatus and Measurement Process of in Situ Interactive Forces

N,N′-methylenebis(acrylamide) (MBA) from Alfa Aesar (Shanghai, China); hydroxyethyl methacrylate (HEMA), ammonium persulfate (APS), tetramethylethylenediamine (TMEDA), a red dye of rhodamine 6G, a blue dye of methylene blue and dialysis bag (MWCO 3500), etc., from Sinopharm Chemical Reagent Beijing (Beijing, China); 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) from J&K (Shanghai, China). The elastic moduli of PHEMA hydrogels were measured with a CMT6203 universal tester from Shenzhen Xinsansi Material Testing Co., Ltd. (Shenzhen, China). The optical photographs of assembled hydrogels were taken with a Nikon camera (D7000, Tokyo, Japan). In situ force measurements of hydrogels were conducted with a dynamic contact-angle measuring device and -tension meter (DCAT21) (Dataphysics, Filderstadt, Germany). Fabrication of PHEMA Hydrogels with Varied Moduli. The building blocks for macroscopic supramolecular assembly are cubic PHEMA hydrogels, which were fabricated with a poly(methyl methacrylate) (PMMA) template with arrays of cubic cavities (3 mm × 3 mm × 3 mm; Scheme 1a). The hydrogel precursor solution was prepared by mixing the following solutions in 5 mL of deionized water: a monomer of HEMA (2.5 mL), a cross-linker of MBA (5 mg), APS (10 mg), which is one component of the redox initiator pair of APS/TMEDA, and 200 μL of a methylene blue solution (aq, 5 mg/ mL) or rhodamine (aq, 1 mg/mL) as blue or red dyes, respectively. After sufficient mixing under ultrasonication, we added TMEDA (10 μL), which is another component of the redox initiator pair of APS/ TMEDA, to the mixture and further poured it into the PMMA template. Then we sandwiched the template between two glass substrates and heated them at 60 °C for 20 min for cross-linking. Separate hydrogel building blocks were harvested after demolding and immersion in copious water for several hours. The hydrogel modulus was varied simply by changing the content of the cross-linker (MBA) from 5 mg to 50, 75, 125, and 150 mg, corresponding to weight ratios of 1.91, 2.86, 3.81, 4.77, and 5.72 wt % in the precursor solutions, respectively. Synthesis of Polyelectrolytes Grafted with Host/Guest Moieties. We used polyelectrolytes grafted with host or guest groups for subsequent surface modification of hydrogels with a layer-by-layer assembled technique,30 which will be described in detail in the next section. To start with, we chose PAA as the polyelectrolyte for further surface modification. PAA grafted with the CD, Azo, and Ad moieties are marked as PAA-CD, PAA-Azo, and PAA-Ad, respectively. PAACD with varied grafting ratios was synthesized following our previously reported EDC/NHS protocol.23 First PAA (218 mg, 0.74 mmol of repeated unit), EDC (38.3 mg, 0.2 mmol), and NHS (23.0 mg, 0.2 mmol) were dissolved in a 10 mL of phosphate-buffered solution (0.2 M, pH = 7.4) and agitated for 30 min. Then CD-NH2 (170.1 mg, 0.15 mmol) dissolved in 10 mL of deionized water was dropped into the mixture containing PAA, followed by 48 h of stirring at room temperature under a nitrogen atmosphere. The reacted mixture was dialyzed in a dialysis bag (MWCO 3500) for 7 days, and the product was freeze-dried. The resulted grafting ratio of PAA-CD was calculated to be 5.6% from its 1H NMR spectrum (Figure S1a,b). Similarly, by changing the CD-NH2 amount to 255.2 and 510.3 mg, we obtained PAA-CD with grafting ratios of 10.4% and 17.3% (Figure

S1c,d). Note that a higher grafting ratio may result in the insolubility of CD or the gelation of PAA-CD solutions because of a weak CD− CD interaction.31 PAA-Azo or PAA-Ad was synthesized through a radical copolymerization of acrylic acid with Azo acrylamide or Ad acrylamide, which were presynthesized following reported methods.1 Acrylic acid (630 μL), Azo acrylamide (225.9 mg, 0.9 mmol), and azobisisobutyronitrile (12.3 mg, 0.075 mmol) were mixed in 15 mL of N,N-dimethylformamide and blown with nitrogen for 30 min. The mixture was heated to 60 °C and kept for 12 h under a nitrogen atmosphere. The reacted solution was dialyzed in a dialysis bag (MWCO 3500) for 7 days, and the yellowish product was freezedried. The grafting ratio of Azo is about 3% (Figure S2a,b). Following similar synthesis methods except changing the amount of Azo acrylamide or Ad acrylamide, we obtained another two PAA-Azo with grafting ratios of 7.5% and 11.4% (Figure S2c,d) and two PAA-Ad with grafting ratios of 3.4% and 9.5% (Figure S3a,b). Note that a higher grafting ratio may result in insolubility of guest molecules. Surface Modification of PHEMA Hydrogels as Host/Guest Building Blocks. Surface modification of the as-prepared PHEMA hydrogels with varied moduli was realized by a layer-by-layer assembled technique,32 which modifies substrates with thin films through alternate deposition of interactive species (e.g., polycations and polyanions). As schematically illustrated in Scheme 1b, we pretreated PHEMA hydrogels in a PDDA solution (aq, 1 mg/mL) for 2 h to reach saturated adsorption of PDDA; subsequently, we carried out alternate immersion of these hydrogels in PDDA and PAA-CD (aq, 1 mg/mL) or PAA-Ad (aq, 1 mg/mL) solutions for cycles to obtain multilayered films: hydrogels dyed blue were alternately immersed in PDDA and PAA-CD for 5 min each, and the resulting multilayer film is marked as (PDDA/PAA-CD)n (n represents the number of alternating cycles); similarly, hydrogels dyed red were alternately immersed in PDDA and PAA-Ad or PAA-Azo for 5 min each, resulting in multilayers of (PDDA/PAA-Azo)n or (PDDA/PAAAd)n. Note that, after each immersion of these solutions, all hydrogels were rinsed with copious water to remove excessive physical adsorption before being transferred to another solution. The hydrogels for MSA experiments were finally modified with (PDDA/ PAA-CD)10 (host hydrogels), (PDDA/PAA-Azo)10 (guest hydrogels), and (PDDA/PAA-Ad)10 (guest hydrogels). To quantify the surface density of modified host or guest groups, we used UV−visible spectra for stepwise characterization of the film formation process of the (PDDA/PAA-Azo)n multilayer because the Azo group has a strong absorption on its UV−visible spectrum. The results summarized in Figure S4 demonstrate that, for one PAA-Azo with a specific grafting ratio (e.g., 5.6%), the increased content of PAA-Azo in each immersion cycle is identical, indicating good control over the content of PAA-Azo through layer-by-layer cycles. Furthermore, we compared the absorbances of (PDDA/PAA-Azo)10 multilayers modified on hydrogels of moduli varied from 0.52 to 2.45 MPa and found that all of these absorbance values remain at a similar level, indicating that the PAA-Azo content is similar regardless of the hydrogel modulus. On the basis of these results, we can tune the surface density simply by changing the grafting ratios of PAA-Azo. Similarly, the group density of PAA-CD, PAA-Azo, and PAA-Ad on C

DOI: 10.1021/acsanm.8b01277 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 3. Illustration of Multivalent Binding Events between Building Blocks To Realize MSA

Figure 1. Assembly results of 100 pairs of PHEMA hydrogels with different elastic moduli (E, MPa; the green cubes show the geometry of the assembled dimers while the blank gray cubes mean no assembly). The hydrogels were modified with 10 bilayers of PDDA/PAA-CD (5.6%; blue, host gels) or PDDA/PAA-Ad (3.4%; red, guest gels). The corresponding original photographs of these assembly results are shown in Figure S5. hydrogel surfaces could be adjusted by layer-by-layer modification of these species of different grafting ratios. MSA of Host/Guest Building Blocks and Dynamics. The MSA behaviors of the as-prepared host and guest hydrogels were investigated by 100 parallel and independent assembly experiments under the same assembly conditions (Figure S5): we placed 100 identical host/guest hydrogel pairs into 100 separate containers, each containing 5 mL of water; these containers were aligned in a 10 × 10 array and then shaken all together for 5 min on a rotating shaker with a set speed of 160 rpm. For a given assembly condition, e.g., PHEMA hydrogels modified with (PDDA/PAA-CD)10 versus those modified with (PDDA/PAA-Azo)10 and grafting ratios of PAA-CD and PAAAzo of 5.6% and 3%, respectively, we calculate the assembly probability under these conditions by counting the number of dimers formed out of 100 original pairs, leading to a statistical result. For better observation of the assembled results in each container, we presented the data after the following treatments: for assembled dimers, we cut the geometry of one assembled dimer from photographs like Figure S5 and placed it into a green cube in a 10 × 10 array, corresponding to the original location of the container; for building blocks that failed to assemble, we left the corresponding cubes blank with a gray background in the 10 × 10 array. Finally, we present an assembly result like Figure 1 corresponding to its original assembly results in Figure S5. In Situ Measurements of the Interactive Force between Building Blocks. We used a DCAT 11 apparatus to measure in situ interactive forces between building blocks following a method

schematically illustrated in Scheme 2: One cubic building block (3 mm × 3 mm × 3 mm) was hung onto the force detector, while the other building block was fixed at the bottom of the container. To ensure a constant contact area between these two building blocks, we changed the bottom hydrogel to a larger sheet (12 mm × 20 mm × 1 mm). Both building blocks were totally immersed in water and kept separate before the test. When the test was started, the container, together with the sheet building block, was moved upward and downward, controlled by a motor, to result in the approaching− contacting−separating processes with the hung cubic building block. During these processes, the in situ force changes on the cubic building block can be recorded. At the separation point when these two interacting hydrogels detach, we could observe a sharp increase of the force values, and the top value indicates the interactive force between these building blocks. After normalization with the contact area of 3 mm × 3 mm, we could obtain a result of interactive forces (N/m2) versus different experimental conditions.

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RESULTS AND DISCUSSION To investigate the influence of the surface chemistry on the MSA probability, we used a model system of PHEMA hydrogels and varied the surface chemistry of either the interaction type or the interaction number with the following controls: (1) the building block materials were limited to one hydrogel system to exclude variables related with the material category; (2) a PDDA/PAA multilayer was used for the surface D

DOI: 10.1021/acsanm.8b01277 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

averaged interactive forces of CD/Ad versus its corresponding modulus, as summarized in Figure 2; for comparison, the

modification of hydrogels, and the only changes were the type and number of groups decorated to PAA (CD, Azo, and Ad); (3) the hydrogel modulus was adjusted by only changing the cross-linker content. Briefly, we synthesized PAA decorated with varied grafting ratios of the host or guest groups, i.e., PAA-Ad (3.4% and 9.5%), PAA-CD (5.6%, 10.4% and 17.3%), and PAA-Azo (3%, 7.5% and 11.4%), and fabricated PHEMA hydrogels with different moduli (0.52 ± 0.05, 0.88 ± 0.06, 1.04 ± 0.06, 1.48 ± 0.10, 1.65 ± 0.04, and 2.45 ± 0.14 MPa). Compared with the previously reported results4 obtained with the combination of PAA-CD (5.6%) and PAA-Azo (3%) modified on PHEMA hydrogels of varied moduli, we increased the binding strength by either using stronger host/guest recognition or increasing the group density. After modifying the hydrogel of varied moduli with different combinations of these PAA derivatives, we checked both the MSA probability and in situ interactive forces correspondingly to investigate the influence of the surface chemistry changes. The general idea between the assembly probability of the building blocks with different rigidities and the surface chemistry is schematically illustrated in Scheme 3: Rigid surfaces modified with minor moieties may lead to failure of MSA, while soft compliant surfaces with dense moieties are more advantageous to realize MSA through efficient multivalency; besides, molecular interaction with a high binding constant should also favor MSA. Influence of the Interaction-Type Change. In our previous work using the combination of PAA-CD (5.6%) and PAA-Azo (3%),4 we obtained a trend of decreasing MSA probability with increasing moduli of the hydrogels; hydrogels with moduli above a critical value of 2.5 MPa lead to no assembly. We wonder whether applying a stronger host/guest interaction, e.g., CD/Ad rather than CD/Azo, will change the above conclusion. Therefore, we modified the hydrogels with a PDDA/PAA-Ad (3.4%) multilayer while keeping the other experimental conditions constant; in this situation, the grafting ratio of PAA-Ad (3.4%) is similar to that of PAA-Azo (3%) and only the guest group is changed from Azo to Ad. We modified (PDDA/PAA-CD)10 or (PDDA/PAA-Ad)10 multilayers onto PHEMA hydrogels with varied moduli from 0.52 to 2.45 MPa. The modulus-dependent MSA experiments of the as-prepared CD and Ad hydrogels were conducted with 100 parallel and independent assembly experiments (Figure S5). The assembly results in Figure 1 show the geometry of assembled dimers in green cubes, while the pairs that fail to form any assemblies are not shown, as indicated by blank gray cubes. Moreover, we can also observe a trend of the assembly probability versus modulus similar to that of the CD/Azo system: under low moduli from 0.52 to 1.65 MPa, MSA was realized after shaking in water for 5 min, but the assembly probability decreases when the modulus increases; at a high modulus of 2.45 MPa, still no assembly was observed even though we prolonged the shaking time. A difference between the current CD/Ad system and the former CD/Azo system4 is that the assembly probability has increased slightly from 99% to 100% at 0.88 MPa, from 85% to 95% at 1.04 MPa, from 52% to 58% at 1.48 MPa, and from 27% to 38% at 1.65 MPa. These increases of the MSA probability may be attributed to the higher intrinsic binding constant of CD/Ad than that of CD/Azo.1,33,34 To quantify the contribution of the CD/Ad interaction in the assembly, we have measured the in situ interactive forces between CD and Ad hydrogels with varied hydrogel moduli. With about 10 similar tests for each modulus, we obtained

Figure 2. In situ interactive forces versus elastic modulus. Aqua-blue column: Hydrogels modified with PAA-CD (5.6%) and PAA-Azo (3%). Orange column: Hydrogels modified with PAA-CD (5.6%) and PAA-Ad (3.4%).

results of CD/Azo interaction are also displayed together. When the modulus of the hydrogel is low at 0.52−1.04 MPa, the force values increase obviously and especially remarkably at 0.52 MPa compared to the interactive forces between the systems of CD/Azo and CD/Ad; however, the force increase slows down at high moduli, which matches well with the observed changes of the MSA probability with a slight increase. Taken together, we have confirmed that the rule of the interactive force decreasing with growing moduli of the building blocks is still applicable for the current CD/Ad system; meanwhile, using a stronger host/guest interaction type is effective in increasing the assembly probability of soft hydrogels but not efficient in enhancing the binding forces of rigid hydrogels. Still, no assembly or force increase is observed at 2.45 MPa. Influence of the Interaction Number Change of Increased Host or Guest Moieties. On the aspect of the interaction number, we have investigated the effects of increased density of either host or guest moieties on the assembly probability, respectively. To check the contribution of the interaction number increase of host moieties only, we kept PAA-Azo (3%) constant and increased the grafting ratio of PAA-CD from 5.6% to 10.4% and 17.3% in layer-by-layer surface modification, leading to two combinations of surface density of (1) PDDA/PAA-CD (10.4%) versus PDDA/PAAAzo (3%) and (2) PDDA/PAA-CD (17.3%) versus PDDA/ PAA-Azo (3%). For the first combination, we modified the host hydrogels with 10 bilayers of PDDA/PAA-CD (10.4%) and the guest hydrogels with 10 bilayers of PDDA/PAA-Azo (3%); for the second combination, the host hydrogels were modified with 10 bilayers of PDDA/PAA-CD (17.3%) and the guest hydrogels with 10 bilayers of PDDA/PAA-Azo (3%). Namely, the surface densities of the CD groups on host hydrogels have been increased, while the guest hydrogels remain unchanged. Then we conducted in situ force measurements of these combinations with about 10 similar tests for each modulus and obtained the modulus-dependent force change summarized in Figure 3a; for comparison, the results of the combination of PAA-CD (5.6%)/PAA-Azo (3%) are also displayed together. With low moduli from 0.52 to 1.65 MPa, there is a slight force increase while solely increasing the grafting ratio of PAA-CD. However, the increasing trend levels off when the modulus values are high. Especially, at a high modulus of 2.45 MPa, the force values are almost unchanged regardless of the grafting ratios of PAA-CD. E

DOI: 10.1021/acsanm.8b01277 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 3. In situ interactive forces between hydrogels modified with (a) PDDA/PAA-CD (5.6%, 10.4% and 17.3%, respectively) versus PDDA/ PAA-Azo (3%), (b) PDDA/PAA-CD (5.6%) versus PDDA/PAA-Azo (3%, 7.5% and 11.4%, respectively), and (c) PDDA/PAA-CD (5.6%) versus PDDA/PAA-Azo (3%) and PDDA/PAA-CD (17.3%) versus PDDA/PAA-Azo (11.4%).

Figure 4. Assembly results of 100 pairs of PHEMA hydrogels with different elastic moduli (E, MPa; the green cubes show the geometry of assembled dimers, while the blank gray cubes mean no assembly). The hydrogels were modified with 10 bilayers of PDDA/PAA-CD (17.3%; blue, host gels) or PDDA/PAA-Azo (11.4%; red, guest gels).

Similarly, the effect of only increasing the interaction numbers of guest moieties is also checked. We kept the surface densities of host groups constant by using PAA-CD (5.6%) for all hydrogels and changed the densities of guest groups by varying the grafting ratios of PAA-Azo from 3% to 7.5% and 11.4%. Thus, we obtained two combinations of (1) PDDA/PAA-CD (5.6%) versus PDDA/PAA-Azo (7.5%) and (2) PDDA/PAA-CD (5.6%) versus PDDA/PAA-Azo (11.4%), which were modified as 10 bilayers of multilayers onto PHEMA hydrogels of varied moduli. With about 10 similar tests of the interactive forces between hydrogels of each modulus, we obtained the modulus-dependent interactive forces between host/guest hydrogels shown in Figure 3b; for comparison, the results of PAA-CD (5.6%)/PAA-Azo (3%) are also displayed together. Similar to the case of only increasing the CD number on the surface, the increase of the Azo moieties on the building block surfaces also results in slight force increases for low moduli of 0.52 and 0.88 MPa but almost nonobservable force increases for high moduli from 1.04 to 2.45 MPa. Taken together, increasing the grafting ratios of either CD or Azo solely leads to slight interactive force

increases, suggesting that synchronized increases of both the host and guest group densities are necessary to enhance the interactive forces and assembly probability. Influence of the Simultaneous Increase of the Host/ Guest Number. The above results suggest that simply improving the interaction number from one side of the host or guest groups is not sufficient to enhance the binding force to a large extent. Therefore, we increase the surface densities of both the host and guest moieties simultaneously by using the maximum grafting ratios available, i.e., PAA-CD (17.3%)/ PAA-Azo (11.4%). Following similar surface modification of PHEMA hydrogels with PDDA/PAA-CD (17.3%) or PDDA/ PAA-Azo (11.4%) multilayers with the above combination, we conducted in situ measurements of the interactive forces summarized in Figure 3c. A remarkable force increase from close to 400 N/m2 to almost 600 N/m2 at a modulus of 0.52 MPa can be observed compared with the combination of PAACD (5.6%)/PAA-Azo (3%). Although the increased amount gradually declines with the growing modulus value, the force values are always larger in the case of PDDA/PAA-CD (17.3%) or PDDA/PAA-Azo (11.4%) than those of PDDA/ F

DOI: 10.1021/acsanm.8b01277 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 5. (a) In situ interactive forces when PAA-CD varies (5.6%, 10.4%, and 17.3%, respectively) with constant PAA-Ad (3.4%) and (b) force comparison between the two combinations of (1) PDDA/PAA-CD (5.6%) versus PDDA/PAA-Azo (3%) and (2) PDDA/PAA-CD (17.3%) versus PDDA/PAA-Ad (9.5%).

from 5.6% to 10.4% and 17.3% for higher interaction number. With constant PAA-Ad (3.4%), we compared the in situ interactive forces with the varied grafting ratios of PAA-CD in Figure 5a. Similarly, we can observe a very slight force increase with the growing grafting ratios of PAA-CD when the hydrogel modulus is low. In the high modulus range from 1.48 to 2.45 MPa, the interactive forces are almost not influenced by the grafting ratios of PAA-CD. These results are quite similar to the cases of the CD/Azo system when solely increasing the host or guest moieties, indicating the limited effects of improving the interactive force simply by increasing the interaction number only from one side of the host/guest interaction, even though here both the interaction type and number are enhanced simultaneously. MSA with the Maximum Host/Guest Interaction Available. Although we tried synchronized change of the interaction type and number by using the CD/Ad system, the control experiments of increasing only the interaction number of PAA-CD while keeping that of PAA-Ad constant still did not show remarkable enhancement of the interaction force. Therefore, we consider the maximum interaction combination available by increasing the interaction number on both the host and guest sides in the CD/Ad system. Note that, with the current synthesis method, CD-NH2 reaches its maximum solubility, possibly caused by hydrogen bonding of many the hydroxyl groups on CD. Besides, with increasing grafting ratios of PAA-CD, the water solution of PAA-CD can form a hydrogel.31 Therefore, the maximum grafting ratio of PAA-CD that we obtained now is 17.3%. Similarly, high grafting ratios of PAA-Azo or PAA-Ad may cause the self-assembly of Azo or Ad as micelles or even insolubility in water, and the maximum grafting ratios that we obtained currently are 11.4% for PAAAzo and 9.5% for PAA-Ad. The averaged results of about 10 tests for each modulus are summarized in Figure 5b. The force difference between the two combinations of (1) PDDA/PAACD (17.3%) versus PDDA/PAA-Ad (9.5%) and (2) PDDA/ PAA-CD (5.6%) versus PDDA/PAA-Azo (3%) is remarkable under each modulus. Especially, at a low modulus of 0.52 MPa, the force value of CD/Ad is twice that of CD/Azo. With increasing hydrogel modulus, the force difference gradually declines. However, at 2.45 MPa, the interactive force of CD/ Ad grows to a value comparable to that of CD/Azo at 1.65 MPa, under which the original MSA assembly reaches 27%. Because of this similar level of interactive forces, MSA of hydrogels with a modulus of 2.45 MPa should be possible. Moreover, this combination of PDDA/PAA-CD (17.3%) versus PDDA/PAA-Ad (9.5%) exhibits force values with almost 1 order of magnitude higher than the control groups,

PAA-CD (5.6%) versus PDDA/PAA-Azo (3%). Moreover, the interactive force at 2.45 MPa still stays at the same level regardless of both increases of the PAA-CD and PAA-Azo grafting ratios, indicating that the MSA probability may remain 0% at 2.45 MPa for the current CD/Azo system and 2.5 MPa is still the critical modulus value. To verify whether the quantified result predicts MSA experiments well, we checked the parallel and independent assembly events of 100 interactive host/guest hydrogels for each modulus. After each pair was shaken for 5 min in water, we obtained the assembly result of each pair, which are summarized in Figure 4. We can find that the MSA probability with the maximum grafting ratios of PDDA/PAA-CD (17.3%) versus PAA-Azo (11.4%) is 100%, 100%, 98%, 61%, and 41% in the modulus range from 0.52 to 1.65 MPa, which are at a higher level than those with the surface chemistry of PDDA/ PAA-CD (5.6%) versus PDDA/PAA-Azo (3%), i.e., 100%, 99%, 85%, 52%, and 27%, correspondingly. However, for the hydrogels with a high modulus of 2.45 MPa, the assembly chance still remained 0% even though we prolonged the assembly time. This phenomenon matched well with the quantified results of no observable changes of interactive forces for rigid hydrogels even though the interaction numbers of both host and guest groups were increased. These results demonstrate that, for the CD/Azo system, besides the trend of decreasing MSA probability with increasing modulus being proven applicable, the critical modulus value of 2.45 MPa to determine whether MSA can occur is also applicable regardless of the surface chemistry changes of the interaction number. Influence of the Synchronized Change of Both the Interaction Type and Number. In the above experiments, we changed either the interaction type or the number to investigate the corresponding influences on the assembly probability. The results have demonstrated that increasing the strength of the host/guest interaction or the interaction number can increase the interactive force to some degree, especially for soft building blocks. However, for rigid building blocks, e.g., hydrogels with a modulus of 2.45 MPa, we still observed no assembly regardless of the combinations of varied interaction type or number solely. Considering that these results reveal a trend of increased MSA probability with a stronger interaction type of host/guest interaction and higher surface density of host or guest groups, we wonder whether increasing both the interaction type and number simultaneously may enhance the assembly probability and break the limit of critical modulus. Therefore, we used the CD/Ad interaction as a stronger interaction type while increasing the grafting ratios of PAA-CD G

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Figure 6. Assembly results of 100 pairs of PHEMA hydrogels with varied elastic moduli(E, MPa; the green cubes show the geometry of assembled dimers, while the blank gray cubes mean no assembly). The hydrogels were modified identically with 10 bilayers of PDDA/PAA-CD (17.3%; blue, host gels) or PDDA/PAA-Ad (9.5%; red, guest gels).

reaches 100% and the probability of 2.45 MPa remains 0% even though we prolonged the shaking time, we only checked the kinetics of the following three moduli of 1.04, 1.48, and 1.65 MPa. As shown in Figure 7, the assembly probability of hydrogels with a modulus of 1.04 MPa is increased from 85% to 100% after the assembly time is prolonged from 5 to 25 min; the increase is fast during the early 5−10 min and soon levels off. Under this condition, the soft building blocks require time for surface deformation and the accompanied multivalency between the interactive moieties. At 1.48 MPa, the assembly probability is increased from 52% (5 min) to 94% (270 min), during which the probability growth is fast in the early 50 min but becomes slow in the later 150 min and finally levels off. This result suggests that, with increasing modulus of the building blocks, the difficulty of deformation and multivalency has increased and thus requires longer assembly time. At 1.65 MPa, the assembly probability fluctuated in the range of 20% and 30% because the building blocks are quite rigid to realize multivalency and thus the driving force is not strong against an external shaking disturbance. It was common that with this modulus the already assembled dimers disassembled later with longer shaking time, leading to fluctuation of the assembly probability, as recorded in Figure 7i. The above MSA kinetics dependent on the building block modulus has confirmed the dynamic assembly process related with the surface deformability and accompanied multivalency of the surface groups. These results have further supported the proposed rule of elasticity-dependent MSA from the point of interaction dynamics.

i.e., hydrogels modified with PDDA/PAA multilayers without host or guest groups or totally blank hydrogels (Figure S6). To check the MSA behavior with this maximum interaction combination, we have investigated the MSA probability under varied moduli, as summarized in Figure 6. From 0.52 to 1.65 MPa, the MSA probability has increased to 100%, 100%, 100%, 87%, and 48%, all of which are much higher than the values of 100%, 99%, 85%, 52%, and 27%, respectively, in the combination of PDDA/PAA-CD (5.6%) versus PDDA/PAAAzo (3%). Especially, MSA of rigid hydrogels with a modulus of 2.45 MPa is surprisingly realized, and the assembly probability reaches 24%. In all of the other systems with lower interaction strength or group density, no assembly was observed at 2.45 MPa. This result demonstrates that synchronized enhancement of both the interaction type and number favors for the assembly chance and breaks the limit of the critical boundary value of 2.45 MPa. Therefore, exact values of the critical modulus should have applicable range depending on the host/guest system regarding the interaction strength (interaction type and number). For this combination of PDDA/PAA-CD (17.3%) versus PDDA/PAA-Ad (9.5%), the critical modulus value has increased to 3.3 MPa (Figure S7). However, the trend of the MSA probability decreasing with increasing modulus still fits both the CD/Azo and CD/Ad systems well. Influence of the Interaction Dynamics on MSA. Besides the influence of the above surface chemistry change on the MSA probability, the assembly dynamics in MSA also differs from that at the molecular level or nanoscale because of dramatically increased kinetic paths along with increased contact area and interactive moieties.35 To interpret the dynamics of the MSA process, we recorded the MSA kinetics by counting the assembled dimers after certain time intervals until 100% assembly or leveling off of the assembly probability. Because the assembly probability of 0.52 and 0.88 MPa already

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CONCLUSIONS In summary, we investigated the influence of the surface chemistry and dynamics on elasticity-dependent MSA by varying key parameters of the interaction type, interaction number, and assembly time. The results demonstrated that the H

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Figure 7. Assembly probability of PHEMA hydrogels with different moduli of (a−c) 1.04 MPa, (d−f) 1.48 MPa, and (g−i) 1.65 MPa. The shaking times are (a, d, and g) 5 min, (b) 25 min, and (e and h) 300 min. The assembled number versus shaking time for each modulus is summarized in parts c, f, and I correspondingly.

the underlying mechanism favor the development of efficient adhesion or self-healing methods and interpretation of the corresponding phenomena. With the conclusions in this work, we hope to provide more understanding of the assembly mechanism, which should contribute to the design of systems applicable for macroscopic supramolecular assembly and insight into related interfacial phenomena.

rule of assembly probability decreasing with increasing modulus is still applicable when the surface chemistry is changed from CD/Azo to CD/Ad or the interaction number is increased. For soft building blocks with low moduli, the changes of the MSA probability and interactive forces respond to the surface chemistry change to some degree; for rigid building blocks, the influence of the surface chemistry change is not remarkable. The critical modulus value of 2.5 MPa applies well to the CD/Azo system but increases to 3.3 MPa for the stronger CD/Ad system with high interaction number. On the basis of these results, we can further predict that, with weaker host/guest systems, the critical modulus value will be lower. Although the current work can hardly cover all of the assembly mechanism of MSA, we try to move forward by providing a platform to study the molecular assembly behavior between large surfaces through visible macroscopic assembly in a statistical summary manner. Supramolecular interactions between large surfaces (surface size exceeding micrometers) are widely involved in interfacial phenomena such as underwater adhesion,5 cell−material interaction, and bottomup construction of 3D scaffolds.15 MSA provides the first step to draw surfaces into the molecular interactive distance. Therefore, developing strategies to realize MSA and clarifying

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01277. 1 H NMR spectra of PAA-CD, PAA-Azo, and PAA-Ad with varied grafting ratios, UV−visible spectra of PDDA/PAA-Azo multilayers, photographs of the MSA experiments of hydrogels of different moduli, interactive forces of control groups without host or guest groups, and critical modulus of the CD (17.3%)/Ad (9.5%) system (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.C.). *E-mail: [email protected] (F.S.). I

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Guannan Ju: 0000-0002-2993-7986 Mengjiao Cheng: 0000-0002-1137-3545 Qian Zhang: 0000-0002-2928-9093 Fengli Guo: 0000-0002-5597-836X Peichen Xie: 0000-0002-2604-500X Feng Shi: 0000-0001-5897-5116 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21674009 and 21604002), Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201816), and Postdoctoral Innovation Talent Support Program under Grant BX20180030.

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