Construction of Porous Organic Nanostructures Using Cooperative

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Construction of Porous Organic Nanostructures using Cooperative Self-assembly for Lipase Catalyzed Inclusion of Gastrodigenin Sagar Biswas, Rohit G. Jadhav, and Apurba K Das ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00085 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Applied Nano Materials

Construction of Porous Organic Nanostructures using Cooperative Self-assembly for Lipase Catalyzed Inclusion of Gastrodigenin Sagar Biswas,† Rohit G. Jadhav† and Apurba K. Das*,† †

Department of Chemistry, Indian Institute of Technology Indore, Khandwa Road, Indore

453552, India E-mail: [email protected] ABSTRACT. In this report, discotic pyridyl amphiphile was found to co-assemble with complementary acids to form emulsion gels with porous architectures. Cooperative selfassembly between amphiphilic pyridyl derivatives and complementary acids was established by several spectroscopic techniques. The alteration of complementary acids helps to tune the mechanical property of the bicomponent emulsion gels and also leads to nanostructural diversity. Further, the formed supramolecular nanostructures of emulsion gels with large pores and surface area exhibit as a template for lipase (from Candida rugosa) catalyzed inclusion of gastrodigenin (p-hydroxybenzyl alcohol) via esterification reaction. The enzymatic reaction leads the emulsion gel to separate into two phases which results in recyclability of the pyridyl amphiphiles.

Keywords: bicomponent system, co-assembly, emulsion gel, supramolecular organic nanostructure, porous, enzymatic reaction 1 ACS Paragon Plus Environment

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INTRODUCTION. Construction of porous materials with various structural topology through self-assembly has gained key interest to the researcher over last few decades.1-3 Recently, porous architectures have become the center of attraction for manifold applications including molecular storage, separation and catalysis.4-9 Recently, scientists are involved in tuning the pore size of organic frameworks through non-covalent interactions to avail supramolecular organic frameworks (SOFs).10-12 Furthermore, functional two-dimensional (2D) SOFs are quite attractive for larger surface area due to the formation of supramolecular organic nanosheets as 2D material, which exhibit different properties from their bulk counterpart.10 SOF materials are the simplest multicomponent system as SOF materials form porous frameworks through non-covalent interactions using complementary functional groups of two or more different components.13,14 The flexibility, soft nature, guest selectivity and the specificity towards selective gas molecules make the SOF materials significantly noticeable.5,15 The loss of porosity of SOFs through structural deformation was perceived on guest removal and the modification was materialized with permanent cavity formation by the complementary building blocks.16 Therefore, molecular self-assembly is the perfect tool to achieve SOFs with controlled pore size. Molecular selfassembly integrates molecules through self-organization which is governed by non-covalent interactions to provide complex architectures.17-19 Different types of amphiphilic molecules impart distinguished supramolecular architectures which result in the formation of vesicles, toroids, tubes and other porous architectures.20,21 To enhance the simultaneous multitasking ability, researchers are inspired to expand multicomponent self-assembled systems to tune the nanostructures.22,23 Bicomponent materials have been explored as the simplest multicomponent system to acquire controlled molecular self-assembly.24-28 By changing the specific functionality in the bicomponent system, the formation of nanostructures can be tuned by the assistance of co-

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operative self-assembly.29-30 Co-operative self-assembly is driven by hydrogen bonding and π-π stacking interactions between two complementary building blocks which can also form gels.24,31 Therefore, bicomponent system act as a remote control for self-assembly and gelation processes.32,33 However, exploration of the SOF materials with a suitable bicomponent system is still required to tune the pore size and stability towards multiple conditions. Controlling the porosity of SOF materials in gel state is also a challenge to the researchers and very few reports have been inscribed in the literature.14,34 To achieve controlled porous architectures in supramolecular gel, multifunctional (bifunctional and trifunctional) amphiphiles are appropriate due to extended network formation through coordination, ionic or non-covalent interactions.35 Therefore, multifunctional amphiphiles are suitable building blocks to design supramolecular porous architectures in gel medium.36 Various functionalized pyridyl amphiphiles have been developed which self-assemble to form distinct nanoarchitectures.37 Therefore, multi-coordinating cationic amphiphile and complementary acids have been chosen for the development of self-supporting gels.38 Bi-functional (BDC) and trifunctional (BTC) acids are well known for their use in the construction of molecular frameworks.39-41 Multifunctional pyridyl amphiphiles (tpeb and bpeb) also coordinate with metal centers and self-assemble to form various architectures.42,43 Therefore, bi-functional and tri-functional pyridyl amphiphiles were chosen to develop metal free organic nanostructures. Herein, the objective was to prepare gel phase supramolecular organic porous nanostructures by co-assembly of bicomponent system and the use of supramolecular organic nanostructure (SONs) as a template for biocatalysis. Lipase is considered to hydrolyze esters in aqueous and organic media44 whereas; self-assembly assists the inclusion of gastrodigenin (p-hydroxybenzyl alcohol) via esterification reaction in the gel phase.45 Gastrodigenin (p-HBA) exhibits 3 ACS Paragon Plus Environment


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scavenging effect towards free radicals and is being considered as a bioactive component of gastrodin.46,47 Here, lipase is used for the incorporation of gastrodigenin in organic-aqueous media based supramolecular organic porous nanostructure and it is an incomparably significant way to achieve the goal.

EXPERIMENTAL SECTION All the chemicals including 1,4-benzenedicarboxylic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC) were purchased from Sigma-Aldrich. The enzyme lipase from Candida rugosa (CRL) was obtained from Sigma Chemical Company, St. Louis, MO. 1,3,5-Tris[2-(4pyridyl)ethenyl]benzene (tpeb) and ditopic pyridyl 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) were synthesized using Heck reaction. Preparation of Emulsions. Bicomponent systems were prepared using different ratio of amphiphiles and acids (Table S1). To prepare gel, 7.74 mg (20 mM) of tpeb and 4.2 mg (20 mM) of trimesic acid (BTC) were placed in a 5 mL borosilicate vial with a cap containing 700 µL of water and then 300 µL of hexane was added (water : hexane at a ratio 7:3). The biphasic solution was then stirred (using a 1 cm magnetic bar) for 10 min on a magnetic stirrer with a speed of 480 rpm to make the mixture homogeneous and kept it for another 10 minutes to attain the rigidity of tpeb-BTC. Then, test tube inversion study was performed to visualize the formation of self-supporting gels, which were further used for rheological experiments. Similarly, tpeb-BDC (BDC = terephthalic acid) emulsion gel and bpeb-BTC, bpeb-BDC emulsions were prepared using all the same conditions. In case of tpeb-BDC, 7.74 mg (20 mM) of tpeb and 9.96 mg (60 mM) of terephththalic acid (BDC) were placed in a vial containing 700 4 ACS Paragon Plus Environment

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µL of water and then 300 µL of hexane was added. 8.52 mg (60 mM) of bpeb and 4.2 mg (20 mM) of trimesic acid (BTC) were placed in a vial containing 700 µL of water and then 300 µL of hexane was added to prepare emulsion bpeb-BTC. Similarly, 8.52 mg (30 mM) of bpeb and 4.98 mg (30 mM) of terephththalic acid (BDC) were placed in a vial containing 700 µL of water and then 300 µL of hexane was added to make emulsion bpeb-BDC. However, tpeb-BCA (BCA = benzene carboxylic acid), tpeb-SA (SA = succinic acid), bpeb-BCA and bpeb-SA didn't form emulsions under similar conditions. Gelation study was performed with the tpeb-BTC and tpebBDC system in only water and hexane individually. However, gelation was not observed (Figure S1). Preparation of Emulsion-gels for Enzymatic Reaction. The biphasic solutions were stirred for 10 min on a magnetic stirrer to make the mixture. After proper mixing of two components in biphasic system, lipase and p-hydroxybenzyl alcohol (p-HBA) were immobilized in the emulsion-gels by simple handshaking for 1 min. The mixture was then incubated at 37 °C and the progress of the reaction was monitored by HPLC. Separation of Product after Enzymatic Esterification: After completion of esterification reaction, emulsion gel (tpeb-BDC) was separated into two layers. The organic layer was then collected using syringe and it was diluted to 5 mL. The diluted hexane layer was then washed with 1M HCl and organic layer was collected to get the product. The aqueous layer containing lipase and tpeb were recycled up to 5 times. After enzymatic reaction, the solution became acidic in nature (pH = 5) and the pH of the aqueous solution was maintained to neutral after each of the reaction with mild NH4OH solution before using it for next reaction. After completion of fifth reaction cycle, pyridyl amphiphile tpeb was extracted from the aqueous part. The aqueous



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part was diluted to 10 mL with 1M Na2CO3 solution and washed with ethyl acetate to obtain 77% of tpeb.

RESULTS AND DISCUSSION In this article, tri-functional amphiphile 1,3,5-tris[2-(4-pyridyl)ethenyl]benzene (tpeb) and difunctional pyridyl 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) have been used to achieve bicomponent SONs through gelation/emulsification. The emulsions are generally dispersion of droplet (composed of tiny fiber, colloidal particle or fibrillar aggregates) of one liquid in another solution.48-50 Further, emulsification can lead to form self-supporting gel which is referred as emulsion-gel.51 This approach suggests the development of distinct supramolecular assemblies through non-covalent interactions of tpeb with complementary 1,3,5-benzene tricarboxylic acid (BTC) and 1,4-benzene dicarboxylic acid (BDC). Among a series of bicomponent systems (Table 1), only tpeb-BTC and tpeb-BDC form emulsion-gels in water:hexane (7:3) and bpebBTC, bpeb-BDC form emulsions. The gelation was confirmed through test tube inversion study (Figure S1) and the rheological experiments depict the mechanical (viscoelastic) properties of the emulsion-gels. A rigid gel is defined in practice as having a storage modulus (G′) that exceeds to the loss modulus (G′′) by an order of magnitude.28 The storage moduli of both tpeb-BTC and tpeb-BDC emulsion-gels are higher than the loss modulii throughout the frequency regime (Figure S2). The value of storage modulus (G′) of tpeb-BDC emulsion-gel exceeds that of loss modulus (G″) by a factor of 10 throughout the oscillating frequency, whereas the factor is only about 5 (Figure S2) for tpeb-BTC emulsion-gel. However, both signified the formation of strong and rigid gels. The tpeb-BDC emulsion-gel is much stronger than tpeb-BTC emulsion-gel as the


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storage modulus (G′ = 8880 Pa) is much higher for tpeb-BDC compare to the storage modulus (G′ = 872 Pa) of tpeb-BTC. Table 1. Composition of the bicomponent systems and physical appearance

Amphiphile

Complementary acids

Physical appearance Emulsion-gel (tpeb-BTC)

BTC

BDC tpeb

Emulsion-gel (tpeb-BDC) None

BCA SA

None Emulsion (bpeb-BTC)

BTC

BDC

Emulsion (bpeb-BDC) None

bpeb BCA SA

None

BTC = benzene-1,3,5-tricarboxylic acid; BDC = terephthalic acid; BCA = benzene carboxylic acid; SA = succinic acid. “None” signifies two phases that are separated without forming emulsion or emulsion-gel. In all the cases, water:hexane (7:3) was used. Though both emulsion-gels are eminently stable for several days (30 days), acidity and basicity affect the emulsion-gels. tpeb-BTC emulsion-gel shows pH ~7 and tpeb-BDC emulsion-gel shows pH ~6.5. Both emulsion-gels were deformed with the increase in pH at ∼8 (Figure S1). Similar observations were also found for lowering the pH to ∼5 for both the emulsion-gels. The 7 ACS Paragon Plus Environment


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deformation of the emulsion-gels was also observed on treatment with 100 µL of 100 mM NaCl solution (Figure S1).

Figure 1. (a) TEM image of tpeb-BTC shows nanofibrillar network structures and (b) HR-TEM image of tpeb-BTC shows the supramolecular arrangement of fibers having diameter of 3 nm. (c) Supramolecular organic nanofibers show parallel alignment in the tpeb-BDC and (d) higher magnification on the side by side aligned molecular fibers of tpeb-BDC shows the average diameter of 6 nm. To investigate nanostructural insight of the emulsion-gels and emulsions, transmission electron microscopy experiments were carried out. tpeb-BTC shows twisted fibrillar bundle with an average diameter of 200 nm (Figure S3a,b). The fibrillar bundle is composed of small width (2540 nm) fibers (Figure 1a, Figure S3a). The small width fibers are also composed of narrow fibers with an average diameter of 3 nm (Figure 1b). tpeb-BDC emulsion-gel forms aligned 8 ACS Paragon Plus Environment

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supramolecular organic nanofibers which was observed from TEM image (Figure 1c). HR-TEM gives a clear account for side by side alignment of the molecular fibers of diameter ~6 nm (Figure 1d).52 Rod structures with an average diameter of 200 nm are responsible for the formation of bpeb-BTC emulsion (Figure S3c). bpeb-BDC forms linear molecular complex that is unable to form any significant nanostructure on aggregation (Figure S3d). The mechanistic aspect of supramolecular interactions involved in the formation of the nanostructures was investigated using FT-IR experiments. Both BTC and BDC exhibited with a broad band at around 2984 cm-1 due to strong intermolecular hydrogen bonded carboxylic acid groups. Strong intense peaks at 1687 cm-1 and 1700 cm-1 are assigned to stretching vibrations of the C=O group of carboxylic acid in more associated state through intermolecular hydrogen bonding in both BDC and BTC respectively (Figure S4).53,54 The peaks at 1597, 1544, and 1480 cm-1 (Figure S4) are attributed to the ring vibration of the pyridine of aromatic pyridyl amphiphiles (bpeb and tpeb). The pyridinium carboxylate formation was established due to the appearance of a peak at 1634 cm-1 for both tpeb-BTC (Figure S4a) and tpeb-BDC (Figure S4b) xerogels.55,56 The protonation of pyridyl N-atom was also supported by the presence of a peak at 3400 cm-1 for N-H of pyridinium carboxylate of the composites of tpeb-BTC and tpeb-BDC xerogels. Both tpeb-BTC and tpeb-BDC xerogels show a C=O stretching vibration at 1710 cm1

, which revealed that the carbonyl group is in a less associated state as compared to the

precursor acids (BTC and BDC). These results suggest that few pyridyl groups are involved in hydrogen bonding interactions with acid groups. Remaining acid groups are involved in intermolecular hydrogen bonding interactions with another BDC (in case of tpeb-BDC). Here, supramolecular complexes are constructed through protonation of pyridine as well as hydrogen bonding interactions. Both bpeb-BTC and bpeb-BDC (Figure S4c,d) emulsions exhibit two 9 ACS Paragon Plus Environment


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peaks at 1710 cm-1 and 1620 cm-1. These two peaks with equal intensity signify that complexes which formed in the emulsions are equilibrated between protonated as well as hydrogen bonded pyridyl group with an equal extent.55-57 UV-Vis spectroscopic analyses have been implemented to study the aggregation pattern. BTC, BDC (Figure S5) exhibit peaks at 280 nm, 240 nm respectively and amphiphiles tpeb and bpeb (Figure S5) show broad peaks at 300 nm and 310 nm respectively. UV-Vis spectrum of tpebBTC (Figure S5a) shows two peaks at 293 nm, 313 nm with a shoulder at 325 nm whereas tpebBDC exhibits a broad peak at 315 nm with a broad shoulder at 359 nm (Figure S5b). A concomitant bathochromic shift is noticed after gelation for both tpeb-BTC and tpeb-BDC. bpeb-BTC (Figure S5c) complex clearly exhibits a peak at 304 nm whereas a peak is observed at 306 nm for bpeb-BDC (Figure S5d). No significant red-shift is observed for bpeb-BTC and bpeb-BDC emulsions. The red-shift in UV-Vis spectra is attributed to the π-π* transition between aromatic groups in the formed complexes through co-assembly and self-assembly of amphiphiles. According to exciton theory, in-line transition dipole eases to a spectral red-shift for transition in the dimer or complexes formed in the system and higher red-shift in wavelength signifies the higher extent of charge transfer in the aggregated state.58 Bathochromic shift in emulsion-gels indicates that the emulsion-gel is formed through J-type of aggregation.26,58 Powder X-ray diffraction (PXRD) study of tpeb-BTC (Figure S6a) xerogel reveals the appearance of peaks at 2θ of 6.61°, 18.41°, 20.31° and 27.88° which are different from the parent tpeb and BTC molecules. These peaks correspond to the d spacing of 1.33 nm, 0.48 nm, 0.43 nm and 0.32 nm. The disappearance of the peaks in dried tpeb-BTC emulsion-gel within the 2θ range of 10° to 15° corresponding to BTC suggests the formation of new material. The presence of a peak at 2θ of 6.61° (d = 1.33 nm) indicates the diameter of columnar aggregates of the 10 ACS Paragon Plus Environment

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formed complexes whereas the d spacing of 0.48 nm and 0.43 nm indicate inter columnar distances. π-π stacking interaction between the complexes was supported by the peak corresponding to the d spacing of 0.32 nm. PXRD of tpeb-BDC (Figure S6b) complex appeared with peaks at 2θ of 2.82°, 5.5°, 6.76°, 18.37°, 19.48° and 26° which corroborate to the d-spacing of 3.13 nm, 1.6 nm, 1.3 nm, 0.48 nm 0.46 nm and 0.34 nm. The disappearance of sharp peak which were present in both tpeb and BDC suggested the formation of new material in tpebBDC. Similar columnar packing is also described for tpeb-BDC. A peak at 2θ of 26° (d = 0.34 nm) indicates the distance between two aromatic groups (π-π stacking interaction).52 The dried emulsion of bpeb-BTC (Figure S6c) shows peaks at 2θ value of 9°, 18.07°, 19.64° and 26.16° which correspond to the distance of 0.98 nm, 0.49 nm, 0.45 nm and 0.34 nm. The π-π stacking interaction is confirmed by the distance of 0.34 nm. However, the dried emulsion of bpeb-BDC (Figure S6d) attributes only three peaks at 2θ value of 8.9° (d = 0.99 nm), 19.57° (d = 0.45 nm) and 26.16° (d = 0.34 nm). The peak at 2θ of 26.16° (d = 0.34 nm) corresponds to the π-π stacking interaction of the linear complexes and the peaks at 2θ = 8.9° (d = 0.99 nm) and 19.57° (d = 0.45 nm) signify the interlayer distances. The presence of the discotic amphiphiles leads to the formation of a columnar stack.60-62 The combination of linear amphiphile and complementary acids does not form columnar packing.



(b)10

(a)10

6

6

tpeb-BTC

tpeb-BDC

5

5

10

Intensity (a.u.)

Intensity (a.u.)

10

4

10

3

10

d = 3.1 nm

2

10

4

10

3

10

d = 5.6 nm

2

10 1

10

1

2

3

4

2θ (degree)

(c)

5

1

2

(d)

0.030

3

4

5

2θ (degree)

0.005 6.2 nm 2.88 nm

tpeb-BTC

0.025

Volume increment

Volume increment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.020 0.015 0.010

tpeb-BDC

0.004 0.003 0.002 0.001 0.000

0.005 0

2

4

6

8

10

12

14

16

0

2

Pore Size (nm)

4

6

8

10

12

14

16

Pore Size (nm)

Figure 2. (a) and (b) are the SAXS spectra of both tpeb-BTC and tpeb-BDC respectively. Both the data shows the size of the nanostructures formed in the system. (c) and (d) show the pore size distribution obtained from N2 adsorption of tpeb-BTC and tpeb-BDC respectively which matches with the calculated data obtained from SAXS. Small angle X-ray scattering (SAXS) experiments of the dried emulsion-gels of tepb-BTC (Figure 2a) and tpeb-BDC (Figure 2b) were performed to find out the inner diameter of the nanoarchitectures. tpeb-BTC only shows a peak with a calculated d value (Table S2) of 3.1 nm whereas tpeb-BDC shows a peaks at d = 5.6 nm associated with the porous SONs.63-68 The thermogravimetric analysis (TGA) was performed to show the thermal stability of tpeb-BTC


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and tpeb-BDC. Both tpeb-BTC and tpeb-BDC were heated at 10 °C min−1 under nitrogen atmosphere. The tpeb-BTC is stable upto 117 °C whereas tpeb-BDC is stable up to the temperature of 135 °C (Figure S7). The BET N2 adsorption gives type IV isotherm plot (Figure S7c,d) and both emulsion gels (tpeb-BTC and tpeb-BDC) dominant with the presence of mesopores. The surface area is calculated as 11 m2/g for tpeb-BTC and 71 m2/g for tpeb-BDC from N2 adsorption isotherm which is a direct consequence of higher N2 adsorption by tpebBDC. The large hysteresis loop formation in tpeb-BDC is a consequence of capillary condensation. The pore size is calculated on the basis of NLDFT (nonlocal density functional theory) from BET N2 adsorption (Figure 2c,d). The pore sizes are calculated as 2.88 nm for tpeb-BTC and 6.2 nm for tpeb-BDC. Mechanism of Nanostructure Formation: UV-Vis analysis suggests aromatic π-π* transition that is responsible for the formation of complexes through the self-assembly and co-assembly of amphiphiles. The extent of hydrogen bonding (1710 cm-1) is characterized by FT-IR spectroscopy, which is responsible for the formation of molecular complexes in tpeb-BTC and tpeb-BDC emulsion-gels. The PXRD patterns of columnar stacks (d spacing of 1.33 nm, 0.48 nm, 0.43 nm for tpeb-BTC and 1.3 nm, 0.48 nm, 0.46 nm for tpeb-BDC) suggest the elongation of molecular complexes (Figure 3a) and the formation of nanofibers through columnar stacking of molecular complexes. The internal distance of nanofibers for both tpeb-BTC (∼3 nm) and tpeb-BDC (∼6 nm) is considered as the distance between two edges of molecular complexes. The distance between two edges is confirmed from SAXS spectra (3.1 nm for tpeb-BTC and 5.6 nm for tpeb-BDC). However, the extent of protonation is more in case of tpeb-BDC emulsiongel which might have higher aggregation rate compared to tpeb-BTC emulsion-gel. tpeb-BTC complex forms long fibers with average width of 25-40 nm which are evidenced by HR-TEM 13 ACS Paragon Plus Environment


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(Figure 1a, Figure S3a). After formation of long fibers with shorter width, they assemble to form twisted fibrillar bundle with an average diameter of 200 nm (Figure S3). tpeb-BDC molecular complexes (∼6 nm) are elongated to form fibers by π-π stacking (Figure 3c) interactions through J-type aggregation which is understood from UV-Vis spectroscopy. Further, the elongated fibers are aligned side by side (Figure 3c) with a diameter of ∼6 nm which is exhibited from HR-TEM (Figure 1).52 FTIR spectra support equal participation (1:1) of hydrogen bonding and protonation for bpeb-BTC and bpeb-BDC complexes. The molecular complex of bpeb-BTC is elongated vertically through π-π stacking interaction (Figure 3d). Further, elongation led to the formation of supramolecular rods (Figure 3d). bpeb-BDC forms linear molecular complex (Figure 3) and no significant nanostructures were observed.


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Figure 3. (a) Cartoon representation of molecular complexes of emulsions-gel tpeb-BTC, tpebBDC and emulsions bpeb-BTC and bpeb-BDC. (b) The molecular complex of tpeb-BTC elongated vertically through columnar π-π stacking which forms fibril. Then the fibers composed to form nano-fibrillar bundle. (c) Molecular complex of tpeb-BDC with large cavity through 15 ACS Paragon Plus Environment


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columnar stacking forms aligned supramolecular organic nanofibrillar structure. (d) The complex of bpeb-BTC elongates through π-π stacking interactions forms supramolecular rods. Enzymatic Reaction: Micro or nanoporous architectures were used as templates for various enzymatic reactions.69,70 Due to the presence of large pores in the SOFs, various reactions could be done into the cavity. Enzymatic esterification reaction was performed in supramolecular hostguest system.71 However, porous SONs were not yet used as the template for enzymatic esterification reactions. Here, the porous architectures were used for the enzymatic esterification reactions where lipase was used as a catalyst. Lipase from Candida rugosa (CRL) and phydroxybenzyl alcohol (p-HBA) were dispersed for dissolution into tpeb-BTC and tpeb-BDC before attending the formation of emulsion-gels. Initially, 4.56 U-45.6 U of lipase was used for esterification reaction (Figure S8a). However, an efficient result was obtained with 22.8 U of lipase. Higher amount of p-HBA influences the emulsion-gel to deform into sol-emulsion as the emulsion-gel is composed by ionic and hydrogen-bonding interactions. O-H groups of p-HBA have a significant impact on hydrogen bonding of the formed complexes, which weaken the network structures. Therefore, an equivalent amount of p-HBA offers better conversion. The emulsion-gel started to degrade upon enzymatic esterification reaction as the acid group was gradually converted to p-HBA esters. tpeb-BDC emulsion-gel was found to be more efficient for the conversion of terephthalate diester from terephthalic acid. The formation of diester leads to the phase separation (Figure 4a). tpeb-BTC and tpeb-BDC complexes were studied by time dependent rheological experiments during the enzymatic reaction.72 tpeb-BTC emulsion-gel lost its storage modulus significantly at a reaction time of 7.5 hours (Figure S8b) and converted to sol-emulsion whereas tpeb-BDC held the gelation property up to 9.75 hours (Figure 4b). Initially, tpeb-BDC emulsion-gel exhibits 10 times higher storage modulus (G') value than loss 16 ACS Paragon Plus Environment

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modulus (G"). As the reaction time increases, the separation between G' and G" decreases and finally the emulsion-gel was converted to a solution at 9.75 hours with a loss factor or damping factor (tan δ) of 0.5. Time sweep rheological experiment for tpeb-BTC emulsion-gel shows the crossing of G' and G" at 7.5 hours (Figure S8b). The reaction was monitored by HPLC analysis (Figure 4c).

Figure 4. (a) Real time images of tpeb-BDC emulsion-gel for lipase catalyzed esterification reaction. After 10 hours of enzymatic reaction, tpeb-BDC emulsion-gel converted to solemulsion. At 16 hours of the reaction, the emulsion-gel converted into two phases. (b) Rheological analysis of emulsion-gel to sol-emulsion transition. (c) HPLC chromatograms at different reaction time and (d) real-time enzymatic conversion of mono-ester, di-ester. The colors



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in the graph signify different physical states (emulsion-gel, emulsion and separation of two phases). tpeb-BTC emulsion-gel network yielded monoester and other two acid groups of BTC remain unreacted which were also monitored by HPLC. tpeb-BTC emulsion was not separated into two phases. Higher lipase catalyzed esterification reaction within the tpeb-BDC complex in comparison to tpeb-BTC complex is concluded as the diameter of the pore is 6.2 nm for tpebBDC complex and the diameter of lipase is 5 nm.73 Also the SONs formed by tpeb-BDC emulsion-gel have larger surface area to interact with enzyme more as compare to fiber of tpebBTC (S.I. calculations and analysis). The pore diameter of tpeb-BTC complex is 2.88 nm which is calculated from BET isotherm. The optical images of tpeb-BDC complex show emulsion-gel to sol-emulsion transition after completion of 10 hours enzymatic reaction (Figure 4a). Further progress of the reaction to 16 hours, tpeb-BDC emulsion-gel was separated into biphasic layers (Figure 4a) and the conversion of diester was 75%. At 24 hours, the conversion of diester was 80% (Figure 4c,d). On esterification reaction, emulsion-gel (tpeb-BDC) was separated into two phases and the phase separation led to the excellent recyclability of pyridyl amphiphiles (Figure 5) and the enzyme lipase. After 5 reaction cycles, 77% of tpeb was recovered. The biocatalyst lipase was also recycled for 5 times. 42% diester of BDC was obtained after fifth cycle (Figure S8c,d). The turnover frequency (TOF) of lipase is calculated as 1.24 × 104 s-1. In contrast, tpebBTC emulsion-gel turned to emulsion within 7.5 hours with the formation of 47% monoester (Figure S8e,f). The mixture (tpeb-BTC/enzyme/p-HBA hybrid emulsion) was kept for 30 days. However, the conversion was only increased to 49% monoester formation. The BDC diester was also separated and characterized by 1H NMR spectroscopy (Figure S9). The corresponding peaks at 5.23 ppm signifies the esterification of BDC. bpeb-BTC and bpeb-BDC emulsions were 18 ACS Paragon Plus Environment

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unable to give any kind of lipase catalyzed esterified product. Upon addition of p-HBA to bpebBTC, and bpeb-BDC, the interaction between the acid and amphiphile gets interrupted due to the involvement of hydrogen bonding (as both don’t form strong network like emulsion-gel) interaction using additional hydroxyl group of p-HBA.

Figure 5. Overall enzymatic esterification cycle of BDC in emulsion-gel (tpeb-BDC). (i) Formation of emulsion-gel with lipase and p-HBA, (ii) phase separation after esterification, (iii) product isolation and (iv) reformation of emulsion-gel after stirring in presence of additional BDC. CONCLUSION In conclusion, we have successfully developed different linear and discotic amphiphiles based bicomponent systems with tunable nanostructures and pore sizes. Different architectural building blocks have been formed with significant pore sizes. The pores formed by molecular complexes traps a large amount of immiscible water and organic solvents to form emulsion-gels. Molecular complexes were co-assembled to form emulsion-gels for tpeb-BTC and tpeb-BDC systems through J-type aggregation. bpeb-BTC and bpeb-BDC emulsions are not strong enough to hold 19 ACS Paragon Plus Environment


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the enzyme and gastrodigenin into the formed microscopic architectures. As a result, the esterification reaction was not observed in emulsions. Whereas, the emulsion-gels are able to produce esters and the porous architectures in emulsion-gel act as a template for lipase-catalyzed inclusion of gastrodigenin. Larger pore size (tpeb-BDC) permits enzyme molecules to insert into the cavity whereas small pores resist the large enzyme to come into the smaller cavity (tpebBTC). As a result, the enzyme functions on the surface of the smaller porous nanostructures. The enzyme lipase fits into the larger cavity and larger surface area of porous SONs of tpeb-BDC to yield the terephthalate diester with an excellent conversion. In contrast, tpeb-BTC emulsion-gel forms only monoester during enzymatic esterification as the enzyme come contact to a small area as compare to tpeb-BDC. Enzymatic esterification reaction leads to the breakdown of the emulsion-gel tpeb-BDC into two phases and phase separation permits for the recyclability of the pyridyl amphiphile. These porous nanostructures open a new path for various enzymatic reactions into the soft material at a mild aqueous-organic condition with a faster rate. ASSOCIATED CONTENT Several experimental results, figures, synthesis and characterization of 1,3,5-tris[2-(4pyridyl)ethenyl]benzene and 1,4-bis[2-(4-pyridyl)ethenyl]benzene. “This material is available free of charge via the Internet at http://pubs.acs.org.”. AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions


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A.K.D. designed and supervised the work, discussed the results and wrote the manuscript. S.B. performed the experiments, discussed the results and wrote the manuscript. S.B. and R.G.J. contributed to the scientific discussion and manuscript revisions. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT AKD sincerely acknowledges NanoMission, Department of Science & Technology (Project No. SR/NM/NS-1458/2014), New

Delhi,

India

for

financial

support.

The sophisticated

instrumentation centre (SIC), IIT Indore is acknowledged for providing access to the instrumentation. SB and RGJ are indebted to UGC for their fellowship. SAIF, IIT Bombay is also acknowledged for the assistance of EM facility.

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Construction of Porous Organic Nanostructures Using Cooperative

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