Understanding Pathway Complexity of Organic Micro/Nanofiber Growth in HydrogenBonded Coassembly of Aromatic Amino Acids Pengyao Xing,† Peizhou Li,† Hongzhong Chen,† Aiyou Hao,*,‡ and Yanli Zhao*,†,§ †
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore ‡ Key Laboratory of Colloid and Interface Chemistry of Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China § School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *
ABSTRACT: Rational engineering of one-dimensional (1D) selfassembled aggregates to produce desired materials for versatile functions remains a challenge. In this work, we report the noncovalent modulation of 1D aggregates at the micro/nanoscale using a coassembly protocol. Aromatic amino acids were employed as the model building blocks, and melamine (Mm) behaves as a modulator to form coassembly arrays with aromatic amino acids selectively. The selective self-assembly behavior between aromatic amino acids and Mm allows distinguishing and detecting Mm and aromatic amino acids from their analogues in macroscopic and microscopic scales. Dimensions and sizes of fibrous aggregates prepared from different amino acids show two opposite pathways from pristine assemblies to coassemblies induced by the addition of Mm. This pathway complexity could be controlled by the molecular conformation determined by α-positioned substituents. The developed hypothesis presents an excellent expansibility to other substrates, which may guide us to rationally design and screen 1D materials with different dimensions and sizes including the production of high-quality selfstanding hydrogels. KEYWORDS: aromatic amino acids, coassembly, hydrogen-bonding interaction, organic micro/nanofibers, pathway complexity
N
parameters proposed by Israelachvili et al. decades ago highlights the volume ratio effect of hydrophobic tails and polar heads of simple surfactants (also including block polymer amphiphile assemblies) on topological nanostructures above the critical micelle concentration (CMC), there are barely any protocols developed for a good understanding of the relatively complicated self-assemblies of organic building blocks with diverse functionality.6−8 A second challenge in organic selfassembly is the discovery of programmed stepwise noncovalent synthesis, such as stripping, appending, and merging of subnanostructures (secondary assembly), since this synthetic method would help the fabrication of complex nanostructures for demanding applications.9−15 For example, Aida et al.10 utilized ferrocene−pyridine conjugates to self-assemble into nanotubes that underwent a cut and paste process triggered by
oncovalent modulation of soft matter at the micro/ nanoscale is a promising approach in materials design.1 This method allows for the fabrication of organic nanostructures depending on intermolecular weak forces such as π−π stacking interactions, hydrogen-bonding interactions, hydrophobic forcs, metal−ligand coordination, and electrostatic forces in a rationally designed manner.2 Micro/nanostructures in diverse dimensions and shapes (such as spheres, rods, fibers, tubes, membranes, toroids, and helixes) have been developed with various building blocks, and their dynamic morphological transformation in response to stimuli has been extensively explored.3−5 In spite of this, in contrast to covalent synthesis and inorganic nanocrystal engineering, considerable challenges still remain in the fabrication of organic micro/nanoarchitectures. The most frequently employed protocol in building up organic nanostructures is the bottomup self-assembly process. For most solution-processed selfassembly, however, dimensions and morphologies are easier to define than to predict. Although a theory of molecular packing © 2017 American Chemical Society
Received: February 19, 2017 Accepted: April 3, 2017 Published: April 3, 2017 4206
DOI: 10.1021/acsnano.7b01161 ACS Nano 2017, 11, 4206−4216
Article
www.acsnano.org
Article
ACS Nano
Scheme 1. (a) A concise illustration of two co-assembly pathways including nanofiber thinning and thickening routes; (b) chemical structures of Fmoc group protected amino acids (all amino acids are L-type except D-Ser) and melamine; (c) two plausible coassembled molecular arrays within thinned nanofibers (columnar) and thickened nanofibers (lamellar). Arrows indicate favorable growth dimensions.
π−π stacking would lead to the oriented molecular growth to form 1D stacks. In this situation, molecular packing in assemblies is in accordance with that of the crystal state. In contrast to crystallization-induced self-assembly, some building blocks tend to generate aggregates with less crystallinity.23 Actually, in this case, most organic compounds organize into mesoscale structures, and their molecular level packing modes could refer to a liquid-crystalline phase such as cubic, hexagonal, or lamellar phase.24−26 The production of mesostructures would result in the emergence of a periodic distance sometimes larger than the molecular length. Considering these two self-assembly mechanisms, several protocols have been developed to tailor the size and dimensions of 1D materials for diverse uses. Overall speaking, these protocols can be classified into thermodynamic and kinetic controls in the manipulation of 1D size and dimensions, which highlight the compound design and self-assembly process modulation, respectively.16 Conventionally, thermodynamic factors that have been taken into account are weak interaction sites such as hydrogen-bonding or π−π stacking sites, and kinetic factors are temperature, solvent environment, concentration, etc. In addition, the stimulus-responsive property of the self-assembly enables the structural tuning and transformation by the participation of external stimuli such as invasive chemical triggers or noninvasive stimuli such as sound and light.27,28 Nevertheless, utilizing low molecular weight organic compounds as supramolecular modifiers to manipulate molecular organization behavior via noncovalent interactions is a promising pathway.29−32 The employment of these two methods allows a facile modulation of 1D aggregates to give desired soft materials. For example, physical gel materials including hydrogels, organogels, and ionogels with special functions in a cell culture matrix, crystal growth media, and drug delivery systems are constituted
oxidation to generate nanotoroids with a high fidelity. Complex structures such as heterojunction nanostructures have also been developed relying on secondary self-assembly. By controlling the solvent polarity, we have successfully synthesized helix− toroid-linked superstructures,11 and others have also contributed to the stepwise construction of complex nanostructures.12−14 In these cases, the application of electrostatic repulsion, conformational similarity, and crystal plane matching allows for the nanoscale modulation rather than the modulation at a molecular scale. On the other hand, methods for chemically stimulus-responsive cutting or merging nanostructures are still limited. Toward the applications of self-assembled organic materials, engineering topological morphologies and dimensions is the key to determine the function scopes, known as structure− function relationships.11,15 One-dimensional (1D) micro/ nanostructures with periodic and long-range ordered molecular packing arrays are one of the most important materials candidates, as they show interesting properties in mechanical, optical, electrical, and biomedical aspects.16 Similar to other dimensional matter, 1D objects reflect a remarkable size effect on favorable functions, but it is still quite difficult to precisely control their dimension/size/shape parameters.17 For instance, relatively rigid microrods from crystallization-induced selfassembly are needed for wave-guiding research,18 while flexible nanoscale fibers are a necessity in designing physical gel-based materials. 19−21 Under certain conditions, the size and dimensions of 1D assemblies display a great dependence on molecular organization route, which could be reflected by molecular packing arrangements.15 A common molecular organization pathway for most low molecular weight building blocks is crystallization-induced self-assembly, especially for building units with aromatic moieties including linear fused or planar extended aromatic rings.22 Aromatic interactions such as 4207
DOI: 10.1021/acsnano.7b01161 ACS Nano 2017, 11, 4206−4216
Article
ACS Nano
Figure 1. TEM, SEM, and AFM images of (a) Gly and (a1) Gly−Mm complex nanoribbons, respectively; TEM, AFM, and SEM images of (b) Ser and (b1) Ser−Mm complex nanoribbons/fibers, respectively; TEM, SEM, and AFM images of (c) Ala and (c1) Ala−Mm complex nanoribbons, respectively; TEM and SEM images of (d) Gln, (d1) Gln−Mm complex, (e) Trp, (e1) Trp−Mm complex, (f) Asp, and (f1) Asp− Mm complex nanofibers. Concentrations of all Fmoc-amino acids and Mm are fixed at 5 mM (molar ratio: 1:1). Scale of AFM images (a), (a1), (b), (c), and (c1): 3 μm × 1.5 μm, 10 μm × 4.5 μm, 1 μm × 0.6 μm, 10 μm × 5 μm, and 10 μm × 5 μm, respectively.
by entangled 1D nanofibers.33−35 Formation and properties of physical gels including mechanical strength, thermostability, transparency, and solvent retention ability are largely determined by the size and length (related to capillary force) of self-assembled fibers.36,37 Hence, the prediction of gel formation is still an ongoing challenging task. In this regard, limited studies have been reported so far. Xu and co-workers have proposed that, for aromatic peptides, linker groups between aromatic rings and a linear backbone may influence the gelation behavior.38 Computational methods have been developed by Berry et al. to predict the gelation of peptides.39 Recently, McNeil et al. used the crystal morphology prediction to screen potential gelators for lead detection.40 Herein, we report cooperative self-assembly directed 1D micro/nanofiber engineering. Commercially available 9-fluorenylmethoxycarbonyl (Fmoc)-protected amino acids with different α-substituents were selected as the main building blocks to self-assemble into micro/nano 1D aggregates by a solvent replacement method in aqueous media. Melamine (Mm) with abundant donor−acceptor (D−A−D) hydrogen-
bonding sites that could form 1:1 complementary hydrogenbonded complexes with Fmoc-amino acids was then introduced. In contrast to other analogues, Fmoc-amino acids show a good selectivity to Mm. It was found that different amino acids adopt two distinct coassembly pathways to form cylinder/ columnar and lamellar packing arrays (Scheme 1). These two pathways could greatly influence the tendency of molecular growth: columnar packing enhances 1D growth to generate thinned nanofibers, while lamellar packing increases 3D growth to form thickened fibers. Along with morphological studies, the mechanism regarding the pathway complexity was revealed by structural analysis. We found that two factors are closely related to the coassembly pathways: molecular conformation and aromatic group. Linear backbone type amino acids have more opportunities to be induced into lamellar structures, while nonlinear molecules with small dihedral angles between the backbone and substituents result in columnar packing. In addition, the existence of aromatic groups would favor columnar formation. Coassembly-directed fibrous thinning leads to gelation, giving high-quality hydrogels with high 4208
DOI: 10.1021/acsnano.7b01161 ACS Nano 2017, 11, 4206−4216
Article
ACS Nano
Figure 2. Powder XRD patterns of (a) Ala, Mm, and Ala−Mm complex assemblies, as well as (b) Trp and Trp−Mm complex assemblies (red stars stand for the main peaks found in small-angle regions). (c) Single-crystal structures of Ala and Trp. (d) Crystal graph of Trp, where dotted lines represent the proposed intermolecular interactions with adjacent molecules. Predicted crystal morphologies of (e) Ala and (f) Trp. Predicted molecular packing arrays of (g) Ala assembly and Ala−Mm complex assembly, as well as (h) Trp and Trp−Mm complex assemblies.
applied range of Fmoc-amino acids, only Asp gives gel rapidly under sonication conditions, whereas Trp needs a longer aging period. Supramolecular gel formation provides a promising method for convenient and effective sensing. Some environmentally toxic analytes such as lead40 and important biological agents such as adenosine triphosphate (ATP)41 could be probed by physical gelators, attributed by distinct phase changes. However, natural α-amino acid supramolecular sensors have not been built up yet. Importantly, Mm, a prohibited food additive with considerable toxicity to infants, could be discriminated and detected (Figure S1).42 Other analogues of Mm including cyanuric acid, miazines, urea, thiourea, and cyanamide that can also form complementary hydrogen-bonded pairs with Fmoc-amino acids only afforded precipitates with Trp under the same self-assembly protocol. The critical gel formation concentrations (CGCs) verified by the inverse tube method are ca. 2 mM (1 molar equiv of Mm) and 1 mM (1 molar equiv of Mm) for Trp and Asp, respectively, giving an estimated limit of detection (LOD) of 126 ppm. Therefore, coassembled gel formation allows bidirectional detection for both amino acids and Mm. Then, the micro/nanomorphology of self-assembled amino acids together with their Mm complexes was examined through electron microscopy (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) and atomic force microscopy (AFM) techniques (Figure 1 and Figure S2). It was found that all samples showed 1D morphology with varied
water content (up to 99 vol %). In the meantime, the gelation advances the bidirectional sensing at the macroscopic scale. Thus, we were able to distinguish melamine or amino acids from their analogues. Hopefully, this research with a high universality could facilitate the engineering of 1D selfassembled aggregates including hydrogel materials for demanding applications.
RESULTS AND DISCUSSION The self-assembly of Fmoc-amino acids was triggered by a typical solvent replacement method.15 A small portion (
