Hydrogels of Superlong Helices to Synthesize Hybrid Ag-Helical


Furthermore, the controlled growth of Ag nanoparticles at spatially arranged locations along the nanohelices (hybrid Ag-helical nanomaterial) is readi...
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Hydrogels of Super-Long Helices to Synthesize Hybrid Ag-Helical Nanomaterials Guihua Li, Yitong Wang, Ling Wang, Aixin Song, and Jingcheng Hao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03052 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 5, 2016

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Table of contents only (TOC):

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Hydrogels of Super-Long Helices to Synthesize Hybrid Ag-Helical Nanomaterials

Guihua Li, Yitong Wang, Ling Wang, Aixin Song, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, China



To whom correspondence should be addressed.

E-mail: [email protected]; Tel: +86-531-88366074; Fax: +86-531-88364750 (o)

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ABSTRACT:

The gelation behavior of mixtures of sodium deoxycholate (NaDC) and

glutathione (GSH) in water is investigated. The system exhibits a structural transition of self-assembled hydrogels from nanofibers to nanohelix structures, and then to helical ribbons with increasing GSH concentration. Super-long helical nanofibers with left- and right-handed orientations are produced by tuning the concentration of GSH at a fixed concentration of NaDC. Random coil and β-sheet structures are significant for the formation of the helical structures, and are indicated by circular dichroism (CD) and Fourier transform infrared (FT-IR) spectra. The mechanical strength of the “weak” hydrogels is enhanced by the introduction of appropriate suitable amount of AgNO3. Furthermore, the controlled growth of Ag nanoparticles at spatially arranged locations along the nanohelices (hybrid Ag-helical nanomaterial) is readily achieved by UV reduction of Ag (I) ions on the supramolecular helical templates.

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1. INTRODUCTION Self-assembled fibers, belts, tubes, and especially, nanohelices have gained great attention over the past decade because of their similarity to biological structures and the possible new functions they may provide in advanced materials. 1-5 Among these supramolecular structures, chiral nanoarchitectures such as turns and helices can be used for mimicking the structures of DNA and proteins, leading to a better understanding of self-assembly processes and the physical forces at play at this scale. 6-8 Chiral nanostructures have been fabricated by the non-symmetric spatial arrangement of components via noncovalent interactions.1,2 In such systems the chirality of the resulting structure is strongly related to the chirality of the components and their assembly manner.6,7 Further, molecular self-assembly has provided an attractive bottom-up approach to obtain chiral structures from both chiral and achiral molecules.9,10 Supramolecular gels, which arise from the self-assembly of low molecular weight organics into entangled nanostructures can immobilize solvents.11-13 Since the gelators are usually chiral molecules, their molecular chirality can be transferred to the supramolecular level during the gelation process. 14 Recently, considerable attention has been focused on the design of component systems to acquire hydrogels with specifically desired structures or functions. The gelator building blocks include peptides,15,16 amino acid derivatives,17,18 sugar-lipids,19-23 nucleosides24-27 and bile acids,28-30 among other molecules. Moreover, the microstructures of the gels can be manipulated by tuning environmental conditions, such as pH, temperature, light, solvent polarity, ionic strength (the addition of salts), or the oxidation/reduction state.31-38 Since gels exhibit solid-like properties on the macro level, new functions that single chiral molecules could not perform may be expected. For instance, hydrogels with stimulus-responsive properties have been used in drug-delivery systems.39 Also, hydrogels fabricated from

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biomolecules have been developed as scaffold materials for three-dimensional (3D) cell cultures or in tissue engineering technology owing to their biocompatibility.40,41 The inclusion of chiral components imparts to hydrogel materials particular functional properties, providing asymmetric aspects of catalysis, chiral recognition, chiroptical switching,42,43 to the material. Moreover, supramolecular gels with tunable microstructures can work as templates to prepare inorganic nanoparticles. Recently, significant effort has been directed toward assembling nanoparticles into sophisticated structures for potential application in fuel cells, biomaterials, photonics and sensing.44-51 As being mentioned above, peptides and bile acid salts are important hydrogelators. For instance, the self-assembly of an ionic self-complementary peptide, RADARADARADARADA (RADA16-I), leads to well-defined nanofibers and eventually a scaffold hydrogel consisting of > 99.5% water.52 The bioactive molecule diphenylalanine self-assembles into various functional nanostructures such as nanotubes, spherical vesicles, nanofibrils, nanowires and ordered molecular chains.53 Under certain conditions, these nanostructures are able to further entangle to form 3D networks, leading to the formation of hydrogels. The formation mechanism of peptide A3K nanotubes, A6K nanofibers and A9K nanorods was investigated, and samples of peptides at a higher concentration displayed gel-like features.54 Finally there are numerous reports that bile acid salts can form gels by coordinating with metals, with adjustment of pH, or with increased ionic strength.28-30, 55 Here we report the gelation behavior of an ultrashort peptide, glutathione (GSH), with sodium deoxycholate (NaDC) in water. The significant advantage of our method is the elimination of the need to synthesize the amino acid surfactant. It was demonstrated that short peptides are favored for practical applications because of the relative ease of their large-scale production and

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purification. GSH is a ubiquitous biological tripeptide with multiple functional groups, diverse chemistry, and possible therapeutic uses.56 While GSH cannot self-assemble into aggregates in water, various self-assembled structures, nanofibers, helices and helical ribbons were obtained in combination with NaDC. Random coil structures and β-sheet structures were found coexisting in helices and helical ribbons, which play an important role for the formation of helical structures. The resulting nanohelices were used as templates to synthesize Ag- nanoparticles helical structures under UV irradiation without added reducing agents, indicating the tenability of spatial arrangements of nanoparticle building blocks for controlling specific material properties.

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials.

Sodium deoxycholate (NaDC) and glutathione (GSH) were

purchased from J&K Chemical Company (China, purity > 98%). AgNO3 was purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used without further purification. Ultrapure water with a resistivity of 18.25 MΩcm was obtained using a UPH-IV ultrapure water purifier (China). 2.2 Sample Preparation.

The samples were prepared by mixing appropriate amounts of

NaDC and GSH in ultrapure water to a final volume of 2 mL. The concentration of NaDC was fixed at 100 mM. The solutions were gently stirred at room temperature until all solids dissolved. All samples were equilibrated at 25.0 ± 0.5 °C for at least 4 weeks before the phase behavior was inspected. 2.3 Transmission Electron Microscopy (TEM).

About 3 μL of gel sample was placed on

carbon-coated copper grids (400 mesh) and freeze-dried. The morphologies of samples were studied on a JEOL JEM-1400 TEM (acceleration voltage, 120 kV) with a Gatanmultiscan CCD for collecting images.

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2.4. Atomic Force Microscopy (AFM). A droplet of sample solution was placed on mica plate and the excess solution was evaporated with nitrogen in order to obtain a thin film. Images were recorded using a digital instrument (NanoScope III) operating in tapping mode. 2.5 Field-Emission Scanning Electron Microscopy (FE-SEM).

For SEM observations, ~

4 μL of gel sample was coated on a silica wafer surface, and most of the colloid gel was removed using small forceps leaving a thin film. The wafers were freeze-dried under vacuum. The samples were observed on a JEOL JSM-6700F FE-SEM at 3 kV. 2.6 Rheological Measurements.

Rheology was carried out on a HAAKE RS6000

rheometer with a cone-plate system (C35/1Ti L07116) for samples with high viscosity. In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was carried out prior to the following frequency sweep in order to ensure the selected stress was in a linear viscoelastic region. 2.7 Small-Angle XRD Measurements. XRD patterns were recorded on a DMAX-2500PC diffractometer with Cu K  radiation (  = 0.15418 nm) and a graphite monochromator. The samples were examined at room temperature over 1-10º in the 2θ mode (1º·min -1). 2.8 Optical Spectroscopy.

Circular dichroism (CD) spectra were obtained using a JASCO

J-810 spectropolarimeter flushed with nitrogen during operation. Wavelength scans were recorded at 0.1 nm intervals from 300 to 180 nm. FT-IR spectra were obtained on a VERTEX-70/70v FT-IR spectrometer (Bruker Optics, Germany). 2.9 Helical-Ag Nanoparticles (Helical-Ag-NPs) Preparation.

For the preparation of Ag

nanoparticles, appropriate amounts of AgNO3 were added to nanofiber or helix samples followed by stirring at room temperature until the sample was homogeneous. Photoreduction of AgNO3 was performed using a 50 mV Hg lamp (365 nm) for 15 minutes until the samples turned pink.

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3. RESULTS AND DISCUSSION 3.1 Gelation Behavior.

The aqueous gelation behavior of the short peptide, GSH, with the

facially amphiphilic bile acid, NaDC, was studied in detail. The phase boundaries were mainly delineated by visual observation and the minimum gelation concentration of GSH was confirmed by the inverted test tube method. The phase transition process of this system was examined at a fixed NaDC concentration of 100 mM, and is shown in Figure 1a. With the addition of GSH to the NaDC solution, one can see changes in the phase sequence from the L1 phase (micelles), to a two-phase consisting of the L1 phase and the sol phase (labeled L1/sol), the sol phase, the gel phase, and precipitates.

This phase behavior is to the previous report for the system of NaDC

with organic acids in aqueous solution.57 The pH of typical samples is included in Figure 1a. In the absence of GSH, the pH of NaDC is relatively high due to the sodium carboxylate group.58 When GSH is added, the pH decreases gradually, and the gel is formed within the pH range of 6.0-7.0. As shown in Figure 1b, the hydrogels with varying GSH concentration vary in appearance, reflecting the different microstructures.

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Figure 1. (a) Phase transitions with the addition of GSH to 100 mM NaDC solution at 25.0 ± 0.5°C (aq, aqueous region; P, precipitates region). (b) Photographs of typical samples at cNaDC = 100 mM with changing cGSH (mM): from left to right; 0 (aq), 10 (L1/sol two-phase), 20 (sol), 40 (gel with low viscosity), 50 (opaque blue gel), 60 (turbid gel), 70 (turbid gel) and 100 (P).

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3.2 Microstructures.

TEM, AFM and FE-SEM observations were employed to determine

the microstructures of the resulting NaDC/GSH hydrosols and hydrogels. Figure 2 shows the images of the hydrosols and hydrogels at cNaDC = 100 mM with different amounts of GSH. From the images, the self-assembly and structural transitions of the hydrogels from nanofibers, to nanohelix structures, and finally to helical ribbons is clearly observed with the increase of GSH concentration. One can find flexible nanofibers with lengths extending to several micrometers formed in the 100 mM NaDC/20 mM GSH mixture (Figure 2a). The diameters of the nanofibers are not uniform, ranging from 50 nm to 500 nm, and some nanofibers are found to aggregate into bundles. As for hydrogels formed in 100 mM NaDC/40 mM GSH mixtures, abundant stiff nanofibers with relatively uniform diameters of about 70 nm are observed (Figure 2b). The AFM images of the stiff nanofibers yield relatively wider diameters of about 85 nm (Figure S1a, Supporting Information). TEM analysis (Figure 2c) and AFM (Figure S1b) clearly demonstrate the formation of helixes in 100 mM NaDC/50 mM GSH hydrogels. The nanofiber helices exhibit twisted shapes s with both right- or left-handed bias, and with the helical pitches ranging from 50 to 75 nm and diameters ranging from 20 to 30 nm. In opaque hydrogels of 100 mM NaDC/60 mM GSH, a large amount of flexible helical nanoribbons with widths ranging from 30 to 100 nm are observed both in TEM (Figure 2d) and SEM (Figure S1c). Enlarged images of TEM and SEM results indicate the formation of both left- and right-handed helical nanoribbons.

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Figure 2.Microscopic images of hydrosols and hydrogels. TEM images of samples formed at constant cNaDC (100 mM) with differing cGSH (mM): 20 (a), 40(b), 50(c) and 60(d). 3.3. FT-IR and CD Characterization of β-Sheet and Random Coil Structures.

To

further characterize the features of nanostructures at various GSH concentrations, CD and FT-IR spectra were measured (Figure 3). CD spectroscopy has employed to examine the various types of peptide secondary structure and the optical activity of self-assembled structures in solution.59-61 Typical α-helical structures have a characteristic positive band around 190 nm and two negative lobes at 208 and 222 nm. The β-sheet structures exhibit a positive CD band around 195 nm and a negative feature at 215 nm. In the case of random coil structures, a weak negative band around 195 nm is observed. As shown in Figure 3a, the aggregates in the NaDC/GSH system exhibit different characteristics in CD spectra with the variation of composition. The solution of 100 mM NaDC in water gives no CD signal. In 100 mM GSH solution, a positive Cotton effect at 215 nm is observed, which is attributed to the n-π* transition of the GSH carboxyl group next to the chiral α-carbon,62,63 no β-sheet or random coil structures are observed. In FT-IR spectra (Figure 3b), the sharp peaks at 3278 and 3350 cm-1 (curve 1) indicate the unassociated –NH2 (or NH3+) groups in GSH molecules. When GSH is added to 100 mM NaDC solution, both CD and FT-IR signals change. The N-H stretching vibrations, amide I and II bands within 3463 and 3278 cm-1 (curves 2-5) are broader and weaker than those arising from the –NH2 group, indicating the formation of intra- or intermolecular hydrogen bonding. 64 For nanofibers of 100 mM NaDC/20 mM GSH and 100 mM NaDC/40 mM GSH, a negative band at around 195

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nm is found, which is characteristic of the random coil structures. The FT-IR spectra display a dominant amide I band around 1654 cm-1 (curves 2 and 3), also being characteristic of random coil conformations,65,66 which is in good agreement with CD results. For hydrogels from 100 mM NaDC/50 mM GSH, one finds a positive band around 195 nm and a negative band at 210 nm in CD spectrum. In FT-IR spectrum (curve 4), the amide I band shows a shift to 1651 cm-1, indicating the coexistence of β-sheet and random coil structures in the helixes.67 Compared with the helices, the helical ribbons of 100 mM NaDC/60 mM GSH mixtures exhibit weaker Cotton effects with a hypsochromic shift of the negative band to 185 nm and positive band to 203 nm. On the other hand, the amide I band shift to 1649 cm-1 and a weak peak around 1602 cm-1 in FT-IR spectrum suggest that more of β-sheet structure than random coil exists in the resulting helical ribbons. 67 To the best of our knowledge, the direct observation of helices and helical ribbons by TEM or AFM images have been rarely reported in NaDC systems,58,68 indicating the significance of the random coil and β-sheet structures for the formation of helical structures. cNaDC

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(b) FT-IR spectra of GSH (curve 1) and

samples with constant cNaDC (100 mM) with changing cGSH (mM): 50 (curve 4), and 60 (curve 5).

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3.4. Proposed Solution Processes.

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More information regarding the chiral microstructures was

revealed in small-angle X-ray diffraction (SXRD) patterns. From Bragg’s law, one obtains d values from XRD reflection peaks that can be related to structural lengths found in repeated units induced by rearrangements of the NaDC and GSH components.55,57 Two strong reflection peaks at 2θ = 5.6 and 14.0 (Figure S2) appear in pure NaDC powder, with d values of 1.57 and 0.6 nm, which are comparable with a cholate backbone length (1.5 nm) and width (0.5 nm), respectively.30 Different peaks were found for varying chiral microstructures, as shown in Figure 4. For nanofibers of 100 mM NaDC/20 mM GSH and 100 mM NaDC/40 mM GSH mixtures, two well-resolved reflection peaks are found at 2θ = 2.46 and 4.79, corresponding to d1 and d2 spacing of 3.6 and 1.8 nm, respectively. This d1 spacing distance is comparable with twice the length of deoxycholate backbone (1.5 nm × 2) plus the width of GSH backbone (0.7 nm); the d2 spacing distance is slightly larger than the GSH backbone length (1.5 nm).30 In our previous report on the NaDC/tartaric acid (TA) system, the d1 spacing distance is slightly larger than in this system. We consider that the interaction between NaDC and GSH species is stronger than that between NaDC and TA molecules, which makes the molecules arrange more tightly. As demonstrated by CD and FT-IR spectra, random coil structures exist in the nanofibers. Based on these results, we assumed that in random coil structures, one peptide molecule interacts with five NaDC molecules through hydrogen bonding (see Figure S3a). Regarding the helices in the 100 mM NaDC/50 mM GSH mixtures and helical ribbons of 100 mM NaDC/60 mM GSH mixtures, the XRD results show some difference. Two strong peaks appear at 2θ = 2.2 and 4.1, from which the d1 and d2 spacing distance are calculated to be 4.14 and 2.07 nm, respectively, and are slightly larger than those of the nanofibers described above. The data suggest that cyclic units in helical structures differ from the nanofibers, because random coil structures and β-sheet structures coexist in helices and helical ribbons. For β-sheet structures, we proposed that two

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GSH molecules are combined in an anti-parallel orientation via hydrogen bonding, in which each GSH molecule interacts with two NaDC molecules (see Figure S3b). At larger diffraction angles, two weak peaks corresponding to d values of 1.23 and 0.97 nm, are ascribed to the ordered arrangement of molecules in other directions. CGSH / mM 20 40 50 60

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Small-angle XRD patterns of samples with constant cNaDC (100 mM) and differing

cGSH (mM). T = 25.0 ± 0.1 C. Based on above results, a process for the formation of the multiple nanostructures in these hydrogels is proposed, as shown in Scheme 1. The fundamental unit of the assemblies is a lamellar structure, which is comprised of random coil structures (Scheme 1a) and β-sheets (Scheme 1b). The outside of lamellar structure is hydrophobic, being beneficial for the further self-assembly processes.65,69 When the molar ratio of GSH to NaDC (r = nGSH /nNaDC) is below 0.4, nanofibers are obtained. In this system, the random coil structures gather to form small fragments via hydrophobic interactions between the rigid steroid rings on NaDC molecules. Further, additional NaDC molecules combine with the backbone functional groups of GSH molecules rather than with the C- or N-terminus, for which the hydrophobic interaction is stronger, resulting in rapid growth of the aggregate fragments along the side direction. 65 The further growth of the aggregate fragments induces the formation of long nanofibers. When the molar

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ratio of GSH to NaDC reaches 0.5, β-sheets are formed. It was demonstrated that β-sheet hydrogen bonding in the one-dimensional growth of peptide assembly plays a crucial role and interplays with hydrophobic interactions.16,70,71 Because the equilibrium structure of the β-sheet is naturally twisted,67,69 hydrogen bonding between the headgroups of peptides leads the bilayer structures to roll into helical structures (Scheme 1e). The formation of more β-sheets with the increase of r results in a limit to the stacking. As a result, the preliminary β-sheets must reduce their twisting in response to the packing constraints imposed by their neighbors,72-74 to finally produce the helical ribbons (Scheme 1f). The results above indicate that the formation of nanostructures is mainly driven by the synergistic effect of hydrogen bonding and hydrophobic interactions.

Scheme 1.

Proposed structural changes in NaDC solution from random coils (a) to β-sheet (b)

helices with addition of GSH (top). Schematic representation of the hierarchical self-assembly process in the NaDC/GSH system:

flexible nanofibers (c), stiff nanofibers (d), helix (e) and

helical ribbon (f).

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3.5. Rheological Properties.

Stress sweep and oscillatory frequency sweep were

performed to investigate the mechanical strength of NaDC/GSH hydrogels. Figure 5 shows the influence of GSH concentration on the mechanical strength of the mixture at cNaDC = 100 mM. When the yield stress is above a critical value (τ*), the solid-like network structures of gels will be suddenly broken. The value, τ*, reflects the strength of the network structures.75 Figure 5a illustrates that the τ* values of these samples are about 2 Pa, and are independent of the concentration of GSH. With increasing GSH concentration, the elastic modulus (G') increases to reach a maximum value at cGSH = 50 mM, that can be ascribed to the microstructural transition from nanofibers to the more tightly entangled networks of helices. When cGSH is higher than 50 mM, G' decreases gradually with continuous addition of GSH. The oscillatory results (Figure 5b) show that for hydrogels formed at 50 mM GSH, G′ and the viscous modulus (G") are about 250 Pa and 60 Pa, respectively, being much higher than those at other concentrations of GSH. These results also indicate that the hydrogels may be categorized as a “weak” gel, which is in agreement with the results reported by Jover, et al.76 2

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3.6. Nanofibers and Helices as Templates to Synthesize Ag Nanoparticles. Ag-nanoparticles exhibit strong antibacterial function and good catalytic activity, thus having

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potential for applications in wound healing and catalytic materials. 77,78 In the present system, Ag nanoparticles were synthesized in situ upon the supramolecular templates, using only UV irradiation and no additional chemical reducing agents. Appropriate amount of AgNO3 was added to the hydrosol or hydrogels, and the microstructures of samples remained the same after UV irradiation (Figure 6). As before, flexible nanofibers, stiff nanofibers, nanohelices and helical ribbons were successively obtained with the increasing GSH concentration and followed by the addition of AgNO3 to the mixture. The mechanical properties of the hydrogels of NaDC/GSH/AgNO3 were also investigated by dynamic rheological measurements. As shown in Figure 7a, the hydrogels exhibited typical solid-like rheological behavior, for which G' and G" are nearly independent of the oscillatory frequency, and G' exceeds G" over the investigated frequency range. From this data we note that the mechanical strength was enhanced by adding AgNO3. The effect of AgNO3 concentration on the mechanical strength of hydrogel was also explored (Figure 7b). In this experiment, the increase of AgNO3 concentration led to the increase of both G′ and G", reaching the highest value at cAgNO3 = 15 mM. We consider that an appropriate amount of AgNO3 is introduced to NaDC/GSH system, the coordination between -COOH, -SH groups and Ag+, as well as electrostatic interactions are the main forces that link the silver cation to the aggregates.79,80 The coordination and electrostatic attraction in the system makes the aggregates arrange more tightly. When AgNO3 was in excess (higher than 15 mM), the mechanical strength of hydrogels decreased sharply. The reason for this may be that the coordination between -COOH and excess AgNO3 could destroy the hydrogen bonding in lamellar structures to damage the original nanofibers and helical structures. In Figure S4, one can see that the concentration of AgNO3 also influences the microstructures of hydrogels. When the

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concentration of AgNO3 was less than 15 mM, flexible helices (Figure S4a) were obtained, which is consistent with the sample without AgNO3. Helices coexist with nanofibers when the concentration of AgNO3 reached 20 mM (Figure S4b). With an additional increase in the concentration of AgNO3, helices disappeared and nanofibers with diameters ranging from 50 to 80 nm were the dominant morphology (Figure S4c). The microstructural changes may be attributed to the coordination affinity between GSH and Ag+ being stronger than the non-covalent interactions between GSH and NaDC. Thus, the β-sheet structures, which play a crucial role in helix structure formation, are probably disrupted. When the AgNO3 concentration was higher than 40 mM, the hydrogels were destroyed, forming precipitates of short nonflexible fragments of nanofibers (Figure S4d).

Figure 6. TEM images of hydrosols and hydrogels of NaDC (100 mM)/AgNO3 (5 mM) at differing concentrations of GSH (mM): 20 (a), 40 (b), 50 (c) and 60 (d).

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Figure 7.

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60 G' 60 G" 1

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Stress sweep plots of hydrogels of NaDC (100 mM)/AgNO3 (5 mM) with changing

concentration of GSH (a), and NaDC (100mM)/ GSH (50 mM) hydrogels with varied concentration of AgNO3 (b). Ag nanoparticles can be obtained as a “beads on a string” structure after UV irradiation of Ag+ with the supramolecular templates for 15 min. The sample turns a pink color to the eye (Figure S5), and the microstructure of the hydrogels were not damaged during the reduction process (Figure 8). For Ag nanoparticles prepared in a sol of 100 mM NaDC/40 mM GSH, as shown in Figure 8a, discrete nanoparticles with an average diameter of 20 nm were found along the surface of the nanofibers. When helical structures from 100 mM NaDC/50 mM GSH mixtures were taken as templates, the controlled growth of hybrid Ag-nanoparticle helices, at spatially arranged locations along the helix, were obtained with average diameters of 15 nm (Figure 8b). The UV-vis absorption spectrum (Figure S6) of Ag-helical hydrogel exhibits one peak around 429 nm, attributed to the characteristic surface plasmon resonance (SPR) of spherical silver nanoparticles.81 It is interesting to find that the Ag nanoparticles are mainly observed as dimers on the helical structures. This may be related to the coordination interaction between -COOH, or -SH groups and AgNO3.79,82 Finally, the Ag and/or Ag-helical nanoparticles

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were mainly observed on the surface of the nanofibers or helices rather than in the bulk solution. The reason may be that most of the GSH molecules in bulk solution formed nanostructures, for which the -COOH and -SH groups were the main interaction sites with the Ag nanoparticles, inhibiting the aggregation of Ag nanoparticles in bulk solution.

Figure 8.

TEM images of Ag nanoparticles prepared on nanofibers formed in 100 mM

NaDC/40 mM GSH (a), and on helices formed in 100 mM NaDC/50 mM GSH (b). It has been reported that the chiral nanoparticles having well-defined helicity and sufficient interparticle interactions have enhanced optical activity, detected by CD spectroscopy.46 Naik and co-workers found that peptides of differing secondary structures could artificially create optically active hybrid peptide-gold nanoparticle structures.83 In the present system, the chiroptical properties of hydrogels formed with 100 mM NaDC/50 mM GSH and AgNO3 at different concentrations were investigated. As shown in Figure 9, before UV irradiation (solid line), a positive band around 195 nm and a negative band at 210 nm can be observed for the NaDC/GSH/AgNO3 system, which is similar to the hydrogel without AgNO3. However, the CD intensities of the hydrogels decreased gradually with the increase of AgNO3 and are lower than those without AgNO3. When the concentration of AgNO3 reached 30 mM, the CD signal

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disappeared. This observation, along with the loss of mechanical strength of hydrogels at higher Ag+ concentration discussed above may suggest that the helix concentration is below the limit of detection in the instrument, or that the excess Ag+ may be combining with GSH preferentially, thereby disrupting well-formed helical structures. Additionally, after UV irradiation, no CD signal is observed in the Ag-nanoparticle helical hydrogels (dashed line). Since the sign and the amplitude of the Cotton effect are correlated with the absolute configuration of an asymmetric (helical) assembly, 84 but both left- and right-handed helical structures coexist in this hydrogel system (Figure 2), we may anticipate that part the loss of the CD signal is due to the presence of both types of helix. 40

cNaDC cGSH cAgNO3 / mM 100 50 10 100 50 20 100 50 30

20

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100 50 10 100 50 20

0

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-40

Figure 9.

200

220

 / nm

240

Circular dichroism spectra before (solid lines) and after (dashed lines) UV exposure

of a 100 mM NaDC/50 mM GSH hydrogel with at different concentrations of AgNO3 (mM): 10, 20 and 30.

4. CONCLUSIONS In conclusion, we report the detailed gelation behavior of NaDC with GSH in aqueous solution. At a fixed concentration of NaDC, the structural transitions of the self-assembled hydrogels proceeded from nanofibers, to nanohelix structures, and finally to twisted ribbons with the

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increase in GSH concentration. Both left- and right-handed helical structures were found to coexist in this system. Spectroscopy indicated that random coil and β-sheet structures were significant in the formation of the observed helical structures. The self-assembly of “weak” hydrogels occurred in this system, were mainly driven by the synergistic effects of hydrophobic interactions and hydrogen bonding. The mechanical strength of the resulting helical structures was enhanced by the introduction of an appropriate amount of AgNO3, and under UV irradiation, the controlled growth of Ag nanoparticles at spatially arranged locations along the nanofibers and helices was successfully achieved. We envisage that our results will provide useful information in understanding the self-assembly process of surfactants with peptides and have application in catalysis and materials science.

ASSOCIATED CONTENT Supporting Information AFM images of stiff nanofibers and helical structures; SEM image of twisted ribbons; TEM images of hydrogels with AgNO3 at different concentrations; small-angle XRD patterns of pure NaDC powders; the intermolecular interactions in random coil structure and β-sheet structures; UV-visible spectra after UV exposure of hydrogels. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel: +86-531-88366074. Fax: +86-531-88364750. E-mail: [email protected] Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS We thank Prof. J. David Van Horn (University of Missouri-Kansas City) for helpful discussion and editing work. This work is financially supported by the NSFC (Grant Nos. 21420102006 & 21273134) and NSF for Distinguished Young Scholars of Shandong Province (JQ201303).

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