Applications of DNA hybrids in biobased medicine and materials
Liu, Qing
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Chapter 5
Highly Stiff and Stretchable DNA Liquid
Crystalline Organogels with Fast
Self-Healing and Magnetic Response Behaviors
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5.1 Introduction
Macromolecular soft materials, such as liquid crystals (LCs),1-3 hydrogels,4-10 and
organogels,11-14 garner significant interest due to their fascinating functional
properties and their various technological applications.15-18 Among them,
DNA-based LCs19-24 and gels25-30 particularly stand out as they combine molecular
recognition capabilities with programmability and can be employed in stimuli-responsive materials31-34 as well as biomedical applications.35-37 While these
attributes hint towards the potential of DNA in soft matter materials, their relatively poor mechanical properties remain a significant challenge38-40 that impedes their
practical application in fields requiring mechanical integrity and tunability. Additionally, most investigations of DNA-based gel systems are currently limited to amorphous materials.25-33, 36-40 This renders it difficult to harness favorable
anisotropic electrical, optical, magnetic, or mechanical properties and dynamic functions due to the absence of cooperative effects of ordered internal structures within the network of gelators.6,12,41 In this context, the realization of mechanically
strong DNA gel materials with LC structures is highly relevant to a variety of scientific and technological pursuits. For instance, a relatively high stiffness, toughness, and stretchability would enable the formation of free-standing DNA structures, thus permitting their use in chemo-mechanical systems and soft DNA actuators. Moreover and from a perspective of tissue engineering, DNA LC gels with dynamic character and adaptive mechanical performance could act as artificial 3D extracellular matrices (ECM) for cell proliferation and differentiation.18 Alongside,
the internal structural ordering of a DNA LC gel matrix may promote cell growth and motility in a predetermined direction thereby opening the pathway towards the formation of hierarchical architectures similar as found in tissues and organs.6,42
Eventually, stimuli-responsive DNA gel materials with highly stretchable, thermoplastic, and self-healing properties show great potential for the future fabrication of artificial skin and muscles,43,44 rendering research in this field highly
attractive.
Here we report mechanically strong DNA-based LC organogels with spatially anisotropic features that are formed by the electrostatic complexation of DNA with cationic surfactants. Inspired by other works relying on supramolecular self-assembly,45,46 we obtained nematic DNA-surfactant organogels in combination with
a series of polar and non-polar solvents. Remarkably, although our DNA organogels are non-covalently crosslinked and contain more than 90 wt% organic solvents, they can be strained more than 3·104%, highlighting their exceptional stretchability.
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MPa and toughness up to 18 MJ/m3 rendering them the current record holder
among DNA gels.25-40 Concomitantly, the nature of the supramolecular bond endows
the DNA-surfactant organogels with thermoplastic properties and recoverable deformability. This also provides complete self-healing capabilities within 5 s. In turn, blending Fe3O4 nanoparticles into the DNA gel matrix allows to slightly alter
the structural ordering of the organogels magnetically expanding the scope of possible applications to a wide range of possible composite materials.
5.2 Result and Discussion
Figure. 5.1. Preparation and characterization of the surfactant LC organogels. (A)
DNA-surfactant organogel materials were formed by electrostatic complexation of DNA and cationic surfactants. Schematic representation of the molecular packing model of the nematic mesophase of the DNA-surfactant organogels (surfactant head groups in red; hydrophobic part of the surfactant in grey and DNA double helix in blue). (B) Photographs of the lyophilized DNA-surfactant complex and the corresponding toluene-swollen organogel captured under UV-light (λex = 365 nm). Note that the sample was stained with SYBR Green I to warrant the homogeneous
distribution of dsDNA within the organogel. (C) POM analysis of the 2000DNA-DDAB organogel in toluene. Scale bar is 100 μm. (D) SAXS profile of the organogel. The formed nematic mesophase shows an average distance of 4.3 nm. The broad diffraction peak at q ≈ 4 nm-1
(labeled with *) is due to the kapton used for sealing the DNA-surfactant complex sample. Inset is the corresponding SAXS 2D pattern.
The DNA-surfactant complexes were prepared by electrostatic complexation of double-stranded (ds) DNA (14 bp, 22 bp, 2000 bp, and 2686 bp) and cationic surfactants containing flexible alkyl chains, including didodecyldimethylammonium
bromide (DDAB), didecyldimethylammonium bromide (DEAB), and
cetyltrimethylammonium bromide (CTAB). As universally representative procedure, simple mixing of an aqueous solution of 2000 bp dsDNA with cationic surfactant of DDAB results in precipitation of the 2000DNA-DDAB complex (Figure 5.1A), which is then obtained in pure form after centrifugation, removal of the supernatant and lyophilization. In order to determine the composition of
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surfactant complexes quantitatively, 22mer single stranded (ss) DNA-DDAB complex was characterized by NMR as a representative example (Figure 5.7). This analysis revealed the stoichiometry of the 22mer ssDNA and the DDAB surfactants to be 1:22 (i.e. ca. one DDAB surfactant molecules per phosphate of the oligonucleotide). Subsequently, the resulting DNA-surfactant complex is immersed in toluene, which results in significant swelling (Figure 5.1B). The free-standing organogel displayed a DNA content of less than 2 wt% confirming that the organic solvent is the dominant component of the bulk material. Polarized optical microscopy (POM) revealed obvious birefringence when analyzing the 2000DNA-DDAB organogel (Figure 5.1C) clearly indicating the ordered alignment of the gelators. Further analysis by small angle X-ray scattering (SAXS) revealed profiles with a broad diffraction peak at q = 1.47 nm−1 (Figure 5.1D) suggesting an ordered
nematic LC mesophase of the 2000DNA-DDAB organogel with an internal structural periodicity of 4.3 nm (d = 2π/q). Considering the dimensions of both 2000 bp dsDNA and DDAB, the organogel mesogen cross section is most likely composed of dsDNA units of ≈2.0 nm thickness separated by regions containing disordered DDAB surfactant molecules of ≈2.3 nm thickness, such as artistically depicted in Figure 5.1A. DNA organogels with nematic LC states were fabricated within a wide series of polar (THF, DMSO, alcohols) and non-polar (CHCl3 and toluene) organic solvents
(Figure 5.8-5.11). Importantly, the DNA LC organogels could even be formed in biocompatible solvents including ethylene glycol and glycerol (Figure 5.11), prospectively opening the path towards biomedical applications and their co-processing with living cells. It should be noted that when oligonucleotides (14mer and 22mer dsDNA) were complexed with the above surfactants, LC organogels were only formed in toluene and DMSO.
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Figure. 5.2. Photographs demonstrating stretchability, deformability, and plasticity of
DNA-surfactant LC organogels. (A-D) Stretchable DNA-DNA-surfactant organogels. In D, the 2000DNA-DDAB-DMSO organogel was stretched 330 times of its initial length without breaking. (E-G) Shaped organogels arranged in different patterns. (H, I) Thermoplastic remolding of DNA-surfactant organogel. The 2000DNA-DDAB-DMSO is injectable after heating. Upon cooling to room temperature the shape is fixated. (J-L) DNA-surfactant LC organogel resisting uniaxial compression. The 2000DNA-DDAB organogel in toluene was placed between two glass slides and when a small force (1.7 MPa) was exerted on the top of the glass, the molded gel was compressed in z-direction without noticeable damage. After releasing the force and a short time immersion in toluene the original shape recovered.
Subsequently, we investigated the macroscopic deformation behavior of the organogels revealing their exceptionally high fracture strains. For example, the 2000DNA-DDAB complex swollen in DMSO can be elongated more than 110 times the initial size (Figure 5.12A) and fracture still was not observable but the measuring range of our tensile tester was exceeded. When the 2000DNA-DDAB organogel was stretched manually, the extensibility was beyond 3·104% (Figure
5.2A-D). This recorded elongation is much higher than that of previously known crosslinked DNA gel systems25-40 as well as other reported polymer gels and
elastomers.11,44,47,48 Moreover, it was found that the largely stretched sample can
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The DNA LC organogels are also plastically deformable allowing the stretched samples to be shaped into multiple patterns, such as triangles, squares, and smiley faces (Figure 5.2E-G). Additionally, the thermoreversible sol-gel transition of the 2000DNA-DDAB complex in DMSO at around 90 °C permits recycling of the samples in a simple way highlighting the organogels’ thermoplastic behavior. After heating above 90 °C, the 2000DNA-DDAB-DMSO organogel was completely melted rendering it injectable. Cooling to room temperature again reestablished the gel-like character providing access to a wide range of shapes (Figure 5.2H-I). Besides uniaxial extension, compression of the organogels in toluene is also possible without noticeable permanent deformation and the macroscopic shape of the material can be fully recovered (Figure 5.2J-L). Notably, SAXS analysis of the compressed DNA organogel indicated a preferential alignment of the DNA-surfactant complex after compression (Figure S10) suggesting that reversible supramolecular gelation of the DNA-surfactant complexes plays an important role for their deformation and plasticity characteristics on a longer time scale.
Figure. 5.3. Investigation of the mechanical properties of DNA-surfactant LC organogels. (A, B)
Dynamic mechanical analysis of the DNA-surfactant organogel materials employing a shear rheometer (strain = 10%, T = 25 °C). (A) Storage (G') and loss (G'') moduli as function of shear frequency of the 2000DNA-DDAB-DMSO organogel. (B) Dependence of the storage modulus (G') of the DNA-DDAB-DMSO organogels with respect to the length of DNA. (C) Tensile test of the 2000DNA-DDAB-DMSO organogel. (D) Stress-strain curves of ds2000-DDAB organogels in DMSO (black line), THF (red line), and CHCl3 (blue line). Inset is the magnified part in the blue
dotted area. (E, F) Young’s moduli and toughness of the corresponding DNA-surfactant organogels in DMSO, THF, and CHCl3.
In order to analyze the intriguing mechanical properties of the DNA organogels in more detail, we subjected them to dynamic mechanical analysis (DMA) employing a
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shear rheometer.49 We determined storage moduli (G') representing the elastic
portion and loss moduli (G'') as a measure for the viscous portion at an applied strain of 10% (in the elastic region). Expectedly, all DNA-surfactant organogels associated with different organic solvents bear viscoelastic properties as evidenced by the larger storage moduli (G'') when compared to the loss moduli (G') over the measured frequency range (0.1-20 Hz) (Figure 5.3A and Figure 5.14). Moreover, it becomes obvious that the elasticity of the DNA-surfactant organogels correlates to the lengths of the employed dsDNA strands. Figure 3B unequivocally reveals that the shear moduli increase from the kPa to the MPa range while increasing the strand length from 14 bp over 22 and 2000 bp to 2686 bp. Alongside, the backbone rigidity (transition from ssDNA to helical dsDNA) also considerably enhances the toughness of the LC organogels (Figure 5.3B).
To cover the mechanical properties beyond the yield point in the plastic region, further mechanical analysis was performed by uniaxial tensile testing (Figure 5.3C and 5.3D). For this purpose, the organogels were molded into fiber shape (e.g. d ≈3 mm, l ≈ 2 cm for 2000DNA-DDAB) and extended at a loading rate of 10 mm∙min-1.
The obtained tensile strengths were in the range of 1-3 MPa and from linear regression in the elastic regime the corresponding Young’s moduli were calculated to be above 20 MPa (Figure 5.3E). We also investigated the toughness of the DNA LC gels by integration of the stress-strain curves, in which the values can be up to 18 MJ/m3 (Figure 5.3F). Stiffness, toughness, fracture and yield strengths of our DNA
LC organogels are significantly improved compared to other reported DNA soft material systems.25-40 The stiffness is even higher or at least comparable to that of
relevant physically and chemically crosslinked polymer gels.11,44,47,48 Evenly
important, the toughness of the samples suggest that the LC gels have the same high level of fracture energy as spider silk materials.50,51 Control experiments involving
the same DNA complexed with another surfactant, i.e. dioctyldimethylammonium bromide (DOAB) showed the formation of an amorphous organogel, which is not free-standing (Figure 5.15). This resulted in a gel with mechanical properties too weak to perform a tensile test. This result strongly suggests that in our supramolecular DNA-surfactant LC organogels, the long macromolecular backbone combined with the multiple intermolecular interactions and the internal structural ordering plays a key role for their outstanding mechanical performance.
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Figure. 5.4. Fast self-healing behavior of the DNA-surfactant LC organogels. (A) The
2000dsDNA-DDAB-DMSO organogel was cut into two parts. (B) The surfaces of the two parts were held together without any additional energy input. (C) The healed organogel was obtained after 5 seconds. (D) The healed sample can be stretched without any fracture. The two parts of DNA organogel were stained with ethidium bromide (EtBr, purple red) and SYBR Green I (orange) respectively, for visualization purpose. (E) Stress-strain curve of the healed ds2000-DDAB organogel in DMSO. (F) Stiffness and toughness of the healed sample.
Subsequently, the self-healing behavior of the DNA-surfactant LC organogels was investigated. For this purpose, the 2000dsDNA-DDAB-DMSO organogel was cut into two parts. As soon as the surfaces of the two parts were joined for 5 seconds without any external energy input, a robust and healed organogel was obtained, which can be stretched extensively without any apparent fracture (Figure 5.4A-D and Figure 5.16). Moreover, the stiffness and toughness of the healed samples were comparable to that of the original DNA organogels (Figure 5.4E and 5.4F). Since a control experiment employing non-complementary DNA-DDAB organogels also showed such self-healing (Figure 5.17), non-specific supramolecular driving forces, such as H-bond formation, electrostatic, hydrophobic, and van der Waals interactions, are most likely the cause of this phenomenon as opposed to Watson-Crick base pairing. In comparison, other reported polymer gel systems only show
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healing behaviors at least after 30 s or a few hours.44,47,48 Therefore, to the best of
our knowledge, the present DNA-surfactant organogels can be regarded as one of the fastest self-healing gel system reported so far.
Figure. 5.5. Investigation of magnetic field-responsive behavior of the DNA-surfactant LC
organogels after introducing Fe3O4 nanoparticles. (A) POM analysis of the
2000DNA-DDAB-Fe3O4 organogel in DMSO (ca. 8.5 wt% Fe3O4). Scale bar is 100 μm. (B) The corresponding SAXS
profile of the organogel. The formed nematic mesophase shows an average distance of 3.3 nm. The broad diffraction ring at q ≈ 4 nm-1 (labeled with *) is due to the kapton which was used for
sealing the DNA-surfactant complex sample. (C) Artistic representation of the molecular packing model of the nematic mesophase of the DNA-surfactant-Fe3O4 organogels (Fe3O4
nanoparticles in black, surfactant head groups in red, the hydrophobic part of the surfactant in gray and double-stranded DNA in blue). (D-F) Photographs showing the magnetic response of the 2000DNA-DDAB- Fe3O4-DMSO organogel. The downward-bent material fiber was fixated
between two glass sticks. After application of a magnetic field, the organogel fiber jumped upwards.
To exemplarily show that our DNA organogels are suitable for the preparation of a wide range of functional composite materials, we endowed one of our gels with magneto-responsive properties for the purpose of external field induced actuation. Therefore, we synthesized oleic acid stabilized Fe3O4 nanoparticles (NPs) with a
diameter of ca. 10 nm (Figure 5.18A, B) and then introduced these into the DNA-surfactant organogels. POM analysis of the organogels formed with DMSO, THF, and CHCl3 indicate that the birefringence and thus the LC properties are preserved
(Figure 5.5A and Figure 5.18C, D). Alongside, the corresponding SAXS profile of 2000DNA-DDAB-Fe3O4-DMSO showed one broad diffraction peak at q = 1.9 nm-1
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(Figure 5.5B). In combination these results suggest that the NPs are well dispersed within the DNA-surfactant complexes, no obvious aggregation occurs, and the nematic phase of the parent organogel is preserved (Figure 5.5C). The alkyl surfactant shell of the NPs allows additional van der Waals interactions between the NPs and the DNA-surfactant complex, which might be responsible for the formation of well-defined DNA-surfactant-Fe3O4 LC organogels. The corresponding average
distance of 3.3 nm-1 of the mesophase was found to be almost the same as the
pristine DNA-surfactant organogel (Figure 5.8A). Subsequently, the NP-containing organogel was molded into a fiber to investigate its response to magnetic fields. The downward-bent fiber was fixated between two glass sticks (Figure 5.5D) and after application of a magnetic field (N50, NdFeB), the organogel fiber jumped upwards instantaneously (Figure 5E-F). Other modes of movement can also be realized non-invasively (Figure 5.18E, F) rendering the robust DNA organogels in combination with their response to magnetism interesting for the development of DNA-based soft actuators.
5.3 Conclusion
In summary, we have successfully developed a new class of DNA LC organogels with nematic ordering based on the electrostatic complexation of DNA with surfactants containing flexible alkyl chains. The obtained materials form supramolecular LC organogels when swollen in organic solvents and exhibit remarkable and for this class of DNA materials previously unknown extensibility, deformability, stiffness, toughness, and plasticity. The DNA-surfactant soft material can be stretched to more than 300 times its original length without breaking. Moreover, the supramolecular nature of the network allows rapid self-healing within 5 seconds recovering its mechanical properties. To the best of our knowledge, this is the first example of gels that can be healed in such a short timeframe. Concomitantly, due to the formation of LC structure, the DNA gels exhibit exceptional mechanical properties with ultimate tensile strengths in the MPa range, elastic moduli of more than 20 MPa, and toughness of up to 18 MJ/m3 – values comparable with the magnitude of covalently
crosslinked polymer gels. Additionally, magnetic NPs can be blended into these tough DNA-LC organogels without compromising the formation of the nematic mesophase. This endows the organogels with the ability to shape-respond to external magnetic fields and exemplarily demonstrates the facile preparation of DNA organogel composites. In this work, it was discovered that the excellent mechanical properties originate from the liquid crystalline ordering of the DNA surfactant complexes within the organic solvent, however, further studies are needed to understand how the molecular interactions determine the bulk features
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of the DNA organogel systems. Undoubtedly, their internal structural ordering, phase behavior, self-healing and stimuli-responsiveness will be the starting point for the construction of novel and functional DNA networks. They will depart from pristine DNA scaffolds in water broadening the scope of DNA nanostructures to achieve operational molecular materials even in organic environments.
5.4 Experimental Section
5.4.1 Materials
The surfactants used for the DNA complex formation, including didodecyldimethylammonium bromide (DDAB) and didecyldimethylammonium bromide (DEAB) were purchased from ABCR (Germany). Cetyltrimethylammonium bromide (CTAB) was purchased from Pro Analysi (Bergen, Norway). UltraPure™ Salmon Sperm DNA (2000bp) and SYBR Green I (N', N'-dimethyl-N-[4-[(E)-(3- methyl-1,3-benzothiazol-2-ylidene)methyl]-1-phenylquinolin-1-ium-2-yl]-N-propylpropane-1,3-diamine) were purchased from Thermo Fisher Scientific (Waltham, United States). Ethidium bromide was purchased from Bio-Red (California, United States). Anhydrous CHCl3 and DMSO were purchased from Acros
Organics (Geel, Belgium) and stored over molecular sieves. Toluene, THF, ethanol, glycerol, and glycol were purchased from Sigma-Aldrich (St. Louis, United States) and used without further purification. Ultrapure water with a resistivity of ca. 18.2 MΩ·cm was used for all experiments.
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5.4.2 DNA synthesis
Figure. 5.6. Characterization of the synthesized DNA by matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) of (A) 14mer, (B) c14mer, (C) 22mer and (D) c22mer and 1% TAE agarose gel of 1µg pUC19 (2686 bp circular dsDNA) digested with NdeI endonuclease enzyme (E).
The oligonucleotides including 14mer (5’-CCT CGC TCT GCT AA-3’, Mw=4175g/mol), 22mer (5’-CCT CGC TCT GCT AAT CCT GTT A-3’, Mw=6612g/mol) and the complementary sequences (c14mer, 5’-TTA GCA GAG CGA GG-3’ Mw=4352g/mol and c22mer, 5’-TAA CAG GAT TAG CAG AGC GAG G-3’ Mw=6857g/mol) were synthesized on a DNA synthesizer using standard β-cyanoethylphosphoramidite coupling chemistry.52 Deprotection and cleavage from the polystyrene support were
carried out by incubation in concentrated aqueous ammonium hydroxide solution overnight at 60 °C. After deprotection, the oligonucleotides were purified by anion exchange chromatography, using a Hitrap Q HP 5 mL column (GE Healthcare) through custom gradient elution. Fractions were desalted using centrifugal dialysis membranes (MWCO 3000, Sartorius Stedim). Oligonucleotide concentrations were determined by UV absorbance using extinction coefficients. Finally, the identity and purity of the oligonucleotides were confirmed by MALDI-TOF mass spectrometry. Regarding 2686bp DNA synthesis, Escherichia coli strain DH5α (Thermo Fisher Scientific) was transformed with the circular vector pUC19 being comprised of 2686 bp (New England Biolabs) as described by Sambrook et al.53 The vector was isolated
from a 2 L bacterial culture in Lennox Broth medium (Sigma-Aldrich) using the GenElute HP Plasmid DNA Maxiprep Kit (Sigma-Aldrich). The identity and purity of pUC19 was confirmed by 1% TAE agarose gel by using 1µg pUC19 digested with
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NdeI endonuclease enzyme (Thermo Fisher Scientific) according to manufacturer’s protocol. The sample was run at 120 V for 40 min and subsequent staining of the bands was performed using EtBr for 15 min. After characterization, the circular DNA was used for organogel fabrication.
5.4.3 Preparation of DNA-surfactant organogels
An aqueous solution of DNA with a concentration of ~10 mg/mL (14 bp, 22 bp, 2000 bp, and 2686 bp) was obtained by dissolving DNA in MQ water. In a second solution of ultrapure water, the concentration of cationic surfactant (DDAB, DEAB, and CTAB) was adjusted to ~50 mM at room temperature. Both solutions were combined in a ratio so that ~5 mol of surfactant equal 1 mol of phosphate residues within the DNA. As a result of mixing, the insoluble DNA-surfactant complexes precipitated from the aqueous phase. After centrifugation, the water and unreacted surfactants were removed, and then the complexes were lyophilized overnight. Finally, the water-free DNA-surfactant complexes were immersed in 100 μL organic solvents for 0.5 h, leading to the formation of DNA-surfactant organogels.
5.4.4 Synthesis of Fe
3O
4nanoparticles
The iron oxide nanoparticles were synthesized according to a reported work.54 2.7g
of FeCl3·6H2O and 9.1g of sodium oleate were dissolved in a solvent mixture which
was composed of 20 mL ethanol, 15 mL distilled water, and 35 mL hexane. Then the solution was heated to 70 °C and kept at this temperature for four hours. After this step, the organic layer containing the iron–oleate complex was washed with distilled water and dried. This procedure resulted in the formation of iron–oleate complex in a waxy solid form. Then 9 g of the obtained iron-oleate complex and 1.4g of oleic acid were dissolved in 50 g of 1-octadecene at room temperature. This solution was heated to 320°C with a heating rate of 3.3°C min–1 and kept at this temperature for
30 min. After that, the reaction solution containing the nanoparticles was cooled to room temperature and ethanol was used to precipitate the inorganic nanoobjects. After centrifugation, the nanoparticles were collected and dissolved in CHCl3 (10
mg/mL) for further use.
5.4.5 Preparation of DNA-surfactant-Fe
3O
4organogels
The as-prepared CHCl3 solution of Fe3O4 nanoparticles (250 μL, 10 mg/mL) was
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to disperse the Fe3O4 nanoparticles. In a next step, 1.75 mL DDAB aqueous solution
(50 mM) was added to the above DNA-Fe3O4 mixture resulting in black precipitate.
After centrifugation, the supernatant was removed and the precipitate was washed 3 times with ultrapure water, and then the DNA-surfactant-Fe3O4 complex was
lyophilized overnight. Finally, the DNA-surfactant-Fe3O4 organogel was obtained
after incubating the complex (15 mg) in 100 μL organic solvent for 0.5 h.
5.4.6 Characterization
Figure. 5.7. 1H-NMR Analysis of the stoichiometry of the 22mer ssDNA-DDAB complex. The stoichiometry of the 22mer ssDNA-DDAB complex was analyzied by 1H-NMR
(400 MHz) in CDCl3. The signals of terminal methyl (marked by m) and aliphatic
groups (marked by c~l) in DDAB and methyl group of thymine in DNA (marked by n) were utilized to estimate the molecular ratio of 22mer ssDNA and DDAB surfactant. The terminal methyl groups in DDAB surfactant were used as an internal standard. The binding stoichiometry can be roughly calculated as the integration of protons difference (at chemical shift between 1.2-2.0) between the DDAB and ssDNA-DDAB complex. Assuming that one DNA molecule could combine with n DDAB molecules (DNA : nDDAB), then after complexation, the total number of
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protons at chemical shift between 1.2-2.0 can be expressed as: (DNA(T11)) 3 + (DDAB(-CH2-)20) × n. According to the integration of the protons of DDAB surfactant
and ssDNA-DDAB in their 1H-NMR as shown above, we have:
11 × 3 + (3.92+36) × n = (3.97+37.46) × n n=21.9
As a result, the stoichiometric ratio of DDAB and 22mer ssDNA is roughly 22:1.
Figure. 5.8. Characterization of the prepared 2000DNA-DDAB organogels associated with
different solvents. (A) SAXS profile of the organogel in DMSO. The SAXS results presented a broad diffraction peak at q = 1.78 nm−1, indicating an ordered nematic LC mesophase of the
2000DNA-DDAB organogel with an internal structural periodicity of 3.53 nm (d = 2π/q). Considering the dimensions of the ds2000 and the DDAB, the organogel mesogen is composed of dsDNA units of ~2.0 nm thickness separated by regions containing DDAB surfactant molecules of ~1.53 nm thickness. (B) The corresponding POM analysis of 2000DNA-DDAB-DMSO organogel. The present birefringence indicates the ordered alignment of the gelators. (C) SAXS profile of the organogel in chloroform. The formed nematic mesophase showed an average distance of 3.63 nm. (D) The corresponding POM analysis of 2000DNA-DDAB in chloroform. Insets in the SAXS profiles are the corresponding SAXS 2D patterns. Scale bar in POM images are 100 μm.
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Figure. 5.9. Characterization of surfactant organogels. (A) SAXS profile of
2000DNA-DEAB organogel associated with toluene. The formed nematic mesophase showed an average distance of 3.36 nm. Considering the dimensions of the diameter of ds2000 and on the ones of the DEAB, the organogel mesogen is composed of dsDNA units of ~2.0 nm thickness separated by regions containing DEAB surfactant molecules of ~1.36 nm thickness. The broad diffraction peak at q≈4 nm-1 (labeled with *) is due to the kapton, which was used for sealing the
DNA-surfactant organogel. Inset is the corresponding SAXS 2D pattern. (B) The corresponding POM analysis of 2000DNA-DEAB in toluene. (C) POM analysis of the prepared 2000DNA-DEAB organogel associated with ethanol. (D) POM analysis of the prepared 2000DNA-CTAB organogel in ethanol. Scale bars in POM images are 100 μm.
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Figure. 5.10. Preparation and POM characterization of the 2000DNA-DDAB LC organogels. (A,
B) The stretchability of the 2000DNA-DDAB organogel associated with ethanol. (C) POM analysis of the 2000DNA-DDAB organogel in ethanol. (D) POM analysis of the 2000DNA-DDAB organogel associated with THF.
Figure. 5.11. Preparation and POM characterization of the 2000DNA-DDAB LC organogels
associated with biocompatible solvents. (A) Photograph of the prepared 2000DNA-DDAB organogel in glycol. (B) The corresponding POM analysis. (C) Photograph of the prepared 2000DNA-DDAB organogel in glycerol. (D) The corresponding POM analysis. Scale bars in POM images are 100 μm.
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Figure. 5.12. (A) Stretching test of the 2000DNA-DDAB-DMSO organogel in tensile experiment. The molded 1D shape of the organogel was stretched from its original size to 18 times, 30 times, 48 times, 72 times, and 110 times. The stretching test was interrupted due to the limitation of the tensile tester. (B) Self-recovery test of the 2000DNA-DDAB-DMSO organogel. The original organogel (~4 cm, top) stretched to 14 cm (middle) can be recovered to ~5.3 cm (bottom) in 5 seconds.
Figure. 5.13. Compression experiment of the 2000DNA-DDAB organogel in toluene.
Photographs of the molded 2000DNA-DDAB organogel (A) before and (B) after compression. (C) SAXS analysis of the compressed organogel. The formed nematic mesophase showed an average distance of 4.1 nm, which is very similar to the unpressed sample (Figure 1D). After compression, the 2D SAXS pattern indicates a preferential alignment of the DNA-surfactant complex.
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Figure. 5.14. Rheological analysis of the 2000DNA-DDAB organogels associated with different
solvents. (A-C) Storage (G') and loss (G'') moduli as function of shear frequency of the 2000DNA-DDAB organogels in (A) THF, (B) CHCl3, and (C) toluene, respectively (strain = 0.1, T = 25 °C).
The larger storage moduli (G'') compared to the loss moduli (G') over the measured frequency range (0.1-20 Hz) confirmed their viscoelastic behaviors. (D) Comparison of storage moduli (G’) of the organogels of 2000DNA-DDAB and 2000DNA-CTAB in DMSO.
Figure. 5.15. A DMSO-swollen organogel. The material is a complex of 2000bp dsDNA and
dioctyldimethylammonium bromide (DOAB). (A) Photograph of the liquid-like organogel. (B) POM analysis of the organogel. Scale bar is 100 μm. The formed organogel is amorphous and is not free-standing, which led to the gel’s mechanics too weak to be determined in a tensile test.
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Figure. 5.16. Self-healing behavior of the DNA-DDAB-DMSO organogel. (A) The organogel was
cut into two parts. (B) Once the surfaces of the two halves were brought into contact and held together within 5 seconds without any external energy input, a robust and healed organogel was obtained (C). (D-F) The healed sample can be stretched extensively (~800%) without any fracture.
Figure. 5.17. Self-healing behavior of non-complementary single-stranded 22mer DNA-DDAB
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Figure. 5.18. (A, B) TEM images of the synthesized Fe3O4 nanoparticles with an average
diameter of ~10 nm. POM analysis of the 2000DNA-DDAB-Fe3O4 organogels (Fe3O4 ~8.5wt%)
associated with (C) THF and (D) CHCl3. Scale bar is 100 μm. (E, F, G) Magnetic response of the
2000DNA-DDAB-Fe3O4-DMSO organogel in a magnetic field. The molded organogel fiber was
fixed on a glass stick. After application of a magnetic field, the organogel fiber waved from bottom to top.
Author Contribution
In this chapter, Zhuojun Meng carried out the preparation of DNA organogels and the mechanical characterization of the DNA organogels. Qing Liu synthesized and characterized DNA oligonucleotides and magnetic nanoparticles. He also performed the chemical as well as physical characterization of these materials. Jing Sun performed the Stretching test.
104
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