Polymer Self-Assembly to Tailorable and

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From tunable DNA/polymer self-assembly to tailorable and morphologically pure core-shell nanofibers Weichong Wang, Kaka Zhang, and Daoyong Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02992 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 17, 2018

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From

tunable

DNA/polymer

self-assembly

to

tailorable and morphologically pure core-shell nanofibers Weichong Wang, Kaka Zhang* and Daoyong Chen*

The State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan University, 2005 Songhu Road, Shanghai 200433 (P.R. China)

ABSTRACT

In reported experimental studies, DNA/polymer self-assembly is usually kinetically trapped, leading to the encapsulation and irregular collapse of DNA chains within the resultant assemblies. In striking contrast, eukaryotic cells use tetrasome-to-nucleosome pathways to escape possible kinetic trapping for the formation of well-defined 10-nm

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chromatin fibers. Here, we report a novel pathway of DNA and amphiphilic diblock copolymer self-assembly inspired by the tetrasome pathway with highly controllable kinetics. The polymer is an A-b-B diblock copolymer with a hydrophilic and noninteractive block A and a hydrophobic and interactive block B. Below the critical water content for the micellization, B blocks wrap the backbone of a DNA chain by weak electrostatic interactions to form a linear DNA/polymer complex. With a gradual increase in the water content, the diblock copolymer unimers in the bulk solution tend to aggregate on the linear DNA/polymer complex, which induces the originally wrapped DNA chain to change its conformation to wrap around the polymer aggregate, guiding and tailoring the self-assembly. The highly controllable kinetics is achieved via the reduced DNA/polymer electrostatic interactions and the high dynamics of the polymer chains in the system. The DNA/polymer self-assembly leads to tailorable and morphologically pure core-shell nanofibers. Compared to the DNA/micelle self-assembly pathway described in our previous study, the present self-assembly pathway exhibits advantages for the fabrication of flexible nanofibers with lengths in micrometers and


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potential for unique applications in preparing not only 2D networks at extremely low percolation thresholds but also chemiresistors with large on/off current ratios.

Keywords: block copolymers, core-shell nanofibers, DNA, percolation, self-assembly

INTRODUCTION Studies of DNA/polymer self-assembly are significant in many important research fields, including biological technology and science, physical chemistry and nanomaterials. In eukaryotic cells, a genomic DNA chain wraps around histone particles, and the complex folds into the precise hierarchical chromatin structure, which is crucial for life systems.1-5 In biological technology, the manipulation of DNA/polymer self-assembly is an essential process for devising gene and drug delivery systems with enhanced efficiency.6-9 In addition, the interaction between DNA and oppositely charged polyelectrolytes is a fundamental problem of polymer physics and has received tremendous interest from the theoretical aspect.10,11 Therefore, DNA/polymer self-assembly has been widely and intensively studied. However, due to the complicated DNA/polymer interactions, DNA/polymer self-assembly is kinetically trapped in almost all reported systems, mainly


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resulting in the encapsulation and irregular collapse of DNA chains within the resultant assemblies,12 with exceptionally few examples of highly regular assemblies and DNA compaction in systems involving relatively short DNA chains.8,9,13

On the other hand, 1D nanomaterials have attracted much interest during the past decades, and polymeric core-shell nanofibers represent an important class of 1D nanomaterials.14-20 Recent studies have revealed that core-shell nanofibers exhibit intriguing properties when used as drug carriers, polymer additives and bioactive matrices, and their functions are strongly related to their structural parameters.21-24 For example, core-shell nanofibers with lengths on the order of microns exhibit prolonged blood circulation times an order of magnitude longer than those of their spherical counterparts.22 Core-shell nanofibers of peptide amphiphiles with high aspect ratios have been found to promote cell adhesion and survival, while their short counterparts may lead to cell death.24 For further applications of core-shell nanofibers and deep insights into the structure-function relationship, monodisperse core-shell nanofibers, especially those with large aspect ratios and high morphological purity, are required. A


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previous study performed by our group demonstrated the formation of monodisperse nanofibers with lengths of hundreds of nanometers through the self-assembly of DNA and spherical polymeric micelles, in which kilobase pairs (kbp) of DNA were used.13 Because the spherical polymeric micelles have a molecular weight of several millions, when longer DNA chains with tens of kbp were used, the system was kinetically trapped, resulting in irregular DNA/micelle complexes (Figure S1). In striking contrast, biological systems involving much longer genomic DNA chains can achieve highly regular DNA/histone assembly for the formation of chromatin fibers.2,3,25-28 In eukaryotic cells, during the formation of 10-nm chromatin fibers, each DNA segment first binds to a (H3-H4)2 histone tetramer via electrostatic attraction to form a tetrasome; then, two H2A-H2B dimers incorporate into the as-formed tetrasome to form a nucleosome.26-28 The tetrasome-to-nucleosome pathway in biological systems significantly reduces the electrostatic attraction and facilitates the dynamics of self-assembly compared to artificial systems of chromatin reconstitution using DNA and histone octamers as building blocks.


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Scheme 1. Schematic illustration of DNA/polymer self-assembly with highly controllable kinetics. (a) Molecularly solubilized PEG-b-P4VP. (b) Interaction between PEG-b-P4VP unimers and DNA chains below the critical water content (CWC) for the micellization of PEG-b-P4VP to form a linear DNA/polymer complex with a structure consisting of a DNA chain backbone encapsulated by polymer chains. (c) With a gradual increase in the water content, further aggregation of the P4VP blocks on the DNA chain induce a conformational change in the DNA chain, causing it to wrap around the polymer aggregate and leading to the tailorable fabrication of core-shell nanofibers.


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Inspired by the tetrasome pathway, we modified the previously reported DNA/micelle self-assembly procedure13 and achieved a novel DNA and amphiphilic diblock copolymer self-assembly pathway with highly controllable kinetics. This pathway was confirmed suitable for the versatile fabrication of highly regular core-shell nanofibers, which are flexible, morphologically pure and microns in length. The amphiphilic diblock copolymer poly(ethylene glycol)-b-poly(4-vinylpyridine) (PEG-b-P4VP) was used for the self-assembly (Scheme 1a). Below the CWC for the micellization of PEG-b-P4VP, DNA interacted with the molecularly solubilized and positively charged P4VP blocks to form linear DNA/polymer complexes, in which each DNA chain was encapsulated by P4VP block chains (Scheme 1b). With a gradual increase in the water content, hydrophobic aggregation between the P4VP block chains was allowed. Under well-controlled conditions, aggregation tended to occur between the PEG-b-P4VP unimers in the solution and the PEG-b-P4VP polymer chains complexed on the DNA chain. Significantly, this aggregation induced the DNA chain that was originally encapsulated in the DNA/polymer complex to adjust its conformation such that it wrapped around the resultant polymer aggregate, thus guiding and serving as a template for the self-


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assembly and ultimately resulting in the formation of monodisperse, highly regular coreshell nanofibers (Scheme 1c). The highly controllable kinetics is achieved due to the reduced DNA/polymer electrostatic attraction and the enhanced dynamics of the diblock copolymers in the system, similar to that of the tetrasome pathway. The DNA/polymer self-assembly leads to the formation of tailorable and morphologically pure core-shell nanofibers. On the other hand, at the early stage of the self-assembly, the interaction of the polymer unimers with a DNA chain to wrap around its backbone can be thought as the interaction at the DNA surface, because diameter of double-strand DNA is about 2 nm and DNA chain is highly rigid compared with the polymer unimers. At the later stage, core-shell nanofibers were formed, and the DNA chain guided and tailored the nanofibers by wrapping around the core-shell interface. Compared with the mechanisms of other reported studies,6-9,13 the present self-assembly pathway exhibits advantages for the fabrication of flexible nanofibers with lengths in micrometers, which requires the use of long DNA chains of tens of kbp as the precursor. For the first time, among their analogs, such as polymeric core-shell nanofibers, the as-prepared long, flexible nanofibers were applied successfully for the solution-based fabrication of not only a


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continuous conductive 2D network at an extremely low percolation threshold but also chemiresistors with large on/off current ratios.

EXPERIMENTAL SECTION

Materials

Poly(ethylene glycol)-b-poly(4-vinylpyridine) (PEG-b-P4VP) block copolymers were synthesized via atom transfer radical polymerization of 4-vinylpyridine29 using PEG113Cl30 as a macroinitiator according to reported procedures in the literature. The average degree of polymerization and the polydispersity index (Mw/Mn) of the PEG-b-P4VP obtained were determined by proton nuclear magnetic resonance (1H NMR) and DMFphase gel permeation chromatography (GPC), respectively. PEG113-b-P4VP45 (Mw/Mn = 1.25), PEG113-b-P4VP58 (Mw/Mn = 1.20), PEG113-b-P4VP67 (Mw/Mn = 1.13), PEG113-bP4VP76 (Mw/Mn = 1.25), and PEG113-b-P4VP100 (Mw/Mn = 1.23) were prepared and employed in the present study. Lambda DNA (linear, 48.5 kbp, Sangon Biotech) and calf thymus DNA (20 kbp on average, Sigma-Aldrich) were used as received. Circular plasmid DNAs, pcDNA3.1/myc-His(−) A (5522 bp, Invitrogen) and pcDNA3.1/myc-


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His(−)/lacZ (8592 bp, Invitrogen), were extracted from Escherichia coli using the alkaline lysis method according to standard protocols. Linear DNA comprising 5522 and 8592 bps was prepared by the Hind III digestion of pcDNA3.1/myc-His(−) A and pcDNA3.1/myc-His(−)/lacZ, respectively, followed by phenol/chloroform extraction and precipitation. The DNAs were stocked as solutions in deionized water.

Self-assembly of DNA and block copolymers for the fabrication of polymeric core-shell nanofibers

For small-scale self-assembly, CO2-saturated water (3.0 mL) was dripped at 3.0 mL/h into PEG-b-P4VP diblock copolymer solution in methanol (2.0 mL, 3.0 mg/mL) under gentle magnetic stirring, then DNA aqueous solution (1.0 mL, 0.2 mg/mL) was dripped into the mixture at 2.0 mL/h. After incubation for 4 h, CO2-saturated water (14.0 mL) was added into the mixture slowly at 4.0 mL/h. After incubation for 24 h, 1,4-dibromobutane was added at a Br/pyridine molar ratio of 1.5/1 to allow quaternization reaction for 2 days. For large-scale preparation using calf thymus DNA, CO2-saturated water (300 mL, 150 mL/h), DNA aqueous solution (100 mL, 2.0 mg/mL, 100 mL/h) and CO2-saturated


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water (1400 mL, 400 mL/h) were added sequentially into PEG-b-P4VP solution in methanol (200 mL, 20 mg/mL) under gentle magnetic stirring. After incubation for 24 hours, 1,4-dibromobutane was added at a Br/pyridine molar ratio of 1.5/1 to allow quaternization reaction for 2 days.

2D percolation of the long nanofibers for solution-processable chemiresistors

A high-doped n-type silicon wafer coated with a layer of insulating SiO2 300 nm in thickness was used as the substrate. The substrate was cleaned by sequential washing with acetone, methanol, isopropanol and ethanol and dried under nitrogen flow. Long polymeric nanofibers prepared from calf thymus DNA and PEG113-b-P4VP67 were employed for chemiresistor fabrication. Before fabrication, the long polymeric nanofibers were mixed with potassium iodide into a solution that contained 0.01 wt% nanofibers and 3 wt% potassium iodide. For the fabrication of chemiresistors, the solution of polymeric nanofibers/potassium iodide was spin-coated on the substrate at 3000 rpm for 45 s. Subsequently, the substrate with the polymeric nanofibers was placed under ambient conditions for 2 days to allow the oxidation of I− into I3−, i.e., the iodine doping


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of the polymeric nanofibers. Gold electrodes were prepared by depositing a 100-nmthick gold layer through a shadow mask onto the doped polymeric nanofibers with a thermal evaporator at a vacuum of 2 × 10−6 torr.

Measurement and characterization

1H

NMR spectra were recorded on a 400 MHz Bruker instrument, and the acquired

NMR data were analyzed with Bruker Topspin software. GPC measurements were carried out using a Waters system with DMF (0.2% LiBr, w/w) as an eluent. Poly(ethylene oxide) standards with narrow molecular weight distributions were used for calibration. TEM specimens were prepared by depositing a drop of diluted solution onto a carbon-coated copper grid. The excess solution on the copper grid was absorbed by a filter paper immediately after deposition. To prepare a freeze-dried specimen, the copper grid was immediately transferred into a container in a liquid nitrogen bath. Then, the frozen specimen was transferred immediately into a freeze dryer and dried. To prepare a conventional specimen, the copper grid was allowed to dry under ambient conditions. TEM observations were conducted on a Philips CM120 electron microscope


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at an acceleration voltage of 60 kV and on an FEI Tecnai G2 20 TWIN electron microscope at an accelerating voltage of 200 kV. FESEM specimens were prepared by a spin-coating technique. First, a drop of the solution was deposited onto a silicon substrate and incubated for several minutes; then, excess solution was removed by spinning. FESEM observations were performed on a Hitachi FE-SEM S-4800 system at an acceleration voltage of 1.0 kV and a Zeiss Ultra 55 system at an acceleration voltage of 1.5 kV. FESEM specimens were observed without gold deposition. The sizes of the nanoobjects were analyzed using ImageJ 1.49v software. Zeta potential measurements were performed at 25 °C on a Zetasizer Nano ZS90 system (Malvern Instrument). Steady-state emission spectra were recorded using an FLS-QM40 fluorescence spectrometer (PTI) from 500 to 800 nm with an excitation wavelength of 482 nm. The change in the light scattering intensity of the mixtures with increasing water content was recorded by an ALV-5000 laser light scattering spectrometer. The solvents and solutions used in the light scattering intensity measurements were filtered through 0.45μm Millipore filters (Millex-LCR hydrophilic PTFE) to remove dust. The current-voltage


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measurements were carried out using a Keithley 2400 multisource meter, and the voltage was set from −0.1 V to 0.1 V, with 501 data points.

RESULTS AND DISCUSSION

Tailorable fabrication of long core-shell nanofibers by DNA/block copolymer selfassembly

Typically, for DNA/PEG-b-P4VP self-assembly, 3.0 mL of CO2-saturated water and 1.0 mL of DNA aqueous solution at 0.2 mg/mL were added sequentially into 2.0 mL of the PEG-b-P4VP solution in methanol at 3.0 mg/mL. The resultant mixture had a water volume fraction (WVF) of 67%, just below the CWC (68%) for the polymer. Then, 14.0 mL of CO2-saturated water was slowly added (at 4.0 mL/hour) under gentle magnetic stirring; the final medium had a WVF of 90%. Due to the presence of CO2, the media were weakly acidic, providing a proper interaction strength for the electrostatic attraction between DNA and PEG-b-P4VP. Then, after further incubation of the mixture under gentle stirring at room temperature for 24 hours, 1,4-dibromobutane was added to crosslink and stabilize the aggregates formed in the mixture.


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Figure 1. TEM images (a, b) and FESEM images (c, d) of the long nanofibers formed by the self-assembly of lambda DNA with PEG113-b-P4VP67 after 1,4-dibromobutane crosslinking.

At a WVF of 90%, the final mixture was bluish, indicating the formation of aggregates in the system. When lambda DNA of 48.5 kbp was used, transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) observations exhibited the formation of long, flexible and morphologically pure nanofibers (Figure 1). The uniform and continuous contrast of the long nanofibers shown by TEM indicates the continuous structure of the condensed P4VP core of the long nanofibers (Figures 1a and b), which is essential for the fabrication of electronic microdevices using nanofibers (see below). The widths of nanofibers resulting from


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lambda DNA and PEG113-b-P4VP67 as determined by TEM and FESEM were 22.2 ± 1.9 nm (Figures 1a and b) and 30.1 ± 1.7 nm (Figures 1c and d), respectively. The remarkable difference between the widths indicates the core-shell structure of the nanofibers (Figure S2).31,32 The increased diameter of the long nanofibers after uranyl acetate staining by TEM further confirms that the nanofibers are core-shell structured with PEG as the shell and P4VP as the core (Figure S2). In solution, the long nanofibers were well solubilized, while in both the TEM and FESEM images, they were heavily entangled. Due to the heavy entanglement, few nanofibers were individually dispersed in the FESEM or TEM images, which increases the difficulty of evaluating the length distribution of the nanofibers. FESEM images show the outline and integrity of the nanofibers despite their heavy entanglement (Figures 1c and d). Furthermore, for the large-scale fabrication of long nanofibers at a relatively low price, concentrated solutions of PEG113-b-P4VP67 (20.0 mg/mL) and calf thymus DNA (2 mg/mL) were used; this commercially available DNA, 20 kbp long on average, is polydisperse but much less expensive than monodisperse DNA. Thus, long, morphologically pure, highly soluble and flexible nanofibers with a uniform width were efficiently obtained (Figure 2).


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Figure 2. TEM (a) and FESEM (b) images of polymeric core-shell nanofibers prepared from the self-assembly of PEG113-b-P4VP67 block copolymers and calf thymus DNA after 1,4-dibromobutane crosslinking.

As mentioned above, it is very difficult to determine whether the nanofibers resulting from lambda DNA (48.5 kbp)/polymer self-assembly are monodisperse in length because they are too long and heavily entangled. This difficulty can be overcome by the use of relatively short monodisperse DNA chains. We further confirmed that when


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monodisperse but relatively short DNA was used, the nanofibers prepared through this new self-assembly pathway were monodisperse in both length and width (Figure 3). As exhibited in Table 1, both the length and width can be tailored by controlling the structural parameters of the DNA chain and the block copolymer. We also found a large difference between the length of the nanofibers and the contour length of the corresponding precursor DNA. For example, the nanofibers prepared from 5522 bp linear DNA and PEG113-b-P4VP45 exhibited a number-average length (Ln) of 576 nm (Lw/Ln = 1.004, Lw represents the weight-average length) (Figure 3a), much shorter than the contour length of the DNA chain (1877 nm). Because DNA chains did not undergo double-strand breakage under the assembly conditions, the large difference between the length of the nanofibers and the contour length of the precursor DNA demonstrates that the DNA chains in the nanofibers are compacted; the compaction ratios are listed in Table 1. Identical to the abovementioned long nanofibers, these relatively short nanofibers have a core-shell structure with PEG as the shell and P4VP as the core, as revealed by the remarkable difference between the widths observed by FESEM and TEM (Figure S3). Based on the monodispersity of the relatively short nanofibers, it is


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reasonable to believe that the long nanofibers (Figure 1) with a uniform width and high morphological purity formed by the lambda DNA/polymer self-assembly under identical conditions are monodisperse in both width and length. It is also worthwhile to mention here that all the nanofibers are the result of DNA/polymer self-assembly because the PEG-b-P4VP diblock copolymer alone (without DNA) only formed spherical micelles under identical conditions (Figures S4-S6) and because the crosslinking had an insignificant effect on the morphology of the DNA/polymer aggregates (Figure S7).


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Figure 3. TEM images of monodisperse core-shell nanofibers prepared from (a) 5522 bp DNA and PEG113-b-P4VP45, (b) 5522 bp DNA and PEG113-b-P4VP76, (c) 5522 bp DNA and PEG113-b-P4VP100, (d) 8592 bp DNA and PEG113-b-P4VP45, (e) 8592 bp DNA and PEG113-b-P4VP76, and (f) 8592 bp DNA and PEG113-b-P4VP100. These nanofibers (a-e) were crosslinked by 1,4-dibromobutane.


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Table 1. Core-shell nanofibers with controlled diameters and lengths.

DNA[a]

Block

Diameter

Ln/nm

Compaction

/bp

copolymer

/nm

(Lw/Ln)

ratio

1

5522

PEG113-b-P4VP45

13.9 ± 1.5

576 (1.004)

3.3

2

5522

PEG113-b-P4VP76

22.5 ± 2.1

385 (1.006)

4.9

3

5522

PEG113-b-P4VP100

28.9 ± 3.3

277 (1.006)

6.8

4

8592

PEG113-b-P4VP45

13.9 ± 1.6

960 (1.005)

3.0

5

8592

PEG113-b-P4VP76

21.7 ± 2.6

853 (1.009)

3.4

6

8592

PEG113-b-P4VP100

29.7 ± 3.0

593 (1.008)

4.9

Entry

[a]

Linear monodisperse DNA was used.

In the literature, core-shell nanofibers are usually prepared by the self-assembly of amorphous block copolymers in selective solvents.18,33-39 Recently, polymerizationinduced self-assembly (PISA) was developed as a highly efficient method for the preparation of core-shell nanofibers.36-39 However, by either the conventional method or PISA, the resultant core-shell nanofibers are polydisperse in length and width and are usually mixed with spherical micelles or vesicles.33-39 Living crystallization-driven selfassembly (CDSA), pioneered by Manners and Winnik, allows the fabrication of core-


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shell nanofibers with precisely controlled dimensions and diverse architectures with precise structures, which has potential for a range of applications.40-44 The CDSA studies have been limited to crystalline polymers largely based on polyferrocenylsilane block copolymers40-42 and were recently extended to several other types of block copolymers containing crystallizable π-conjugated blocks.43,44 Because the core is highly crystallized, the resultant nanofibers are rod-like and rigid. The nanofibers prepared in the present work feature a core formed by functional noncrystallizable components and can thus be loaded with functional species or be further chemically modified conveniently. In particular, they are very long, highly flexible, uniform in both length and width and have a highly pure morphology; such nanofibers cannot be prepared by existing methods, including the method of DNA/micelle self-assembly we previously reported. These features, as detailed below, are crucial in applications such as the solution-based construction of 2D functional networks at an extremely low percolation threshold.

Study of the DNA/block copolymer self-assembly mechanism


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Figure 4. Change in the light scattering intensity of the DNA/PEG113-b-P4VP58 mixture with increasing water content. The initial concentration of DNA was 0.05 mg/mL, and the initial concentration of PEG-b-P4VP diblock copolymer was 1 mg/mL in the system at WVF of 50%. And the light scattering intensities at different WVFs were calibrated based on the slit width and the volume change. Kcps = kilo counts per second.

To study the mechanism, light scattering measurements and TEM observations were performed to monitor the change in the DNA/PEG-b-P4VP mixture with increasing water content (Figure 4). The scattering intensity of the DNA/PEG-b-P4VP mixture increased with increasing water content, first gradually and then abruptly at a CWC of 68%. For comparison, the CWC for PEG-b-P4VP alone was 71%. In addition, the scattering


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intensity of the DNA/PEG-b-P4VP mixture was higher than that of PEG-b-P4VP alone. These findings indicate the complexation between DNA and P4VP blocks in the system.

Figure 5. TEM images of nanoaggregates formed in the 5522 bp DNA/PEG113-b-P4VP76 mixture at a water content of 60% (a), 70% (b), 80% (c) and 90% (d). Freeze-dried specimens were used for TEM observation.

Nanoaggregates in the DNA/PEG-b-P4VP mixtures at several representative water contents were further characterized by TEM (Figure 5). At a WVF of 60% (below the


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CWC), linear nanostructures with low contrast and an average diameter of 6.0 nm were formed (Figure 5a). Based on the measurable nanoaggregates that are fully stretched and individually dispersed in the TEM images, the compaction ratio of the DNA chains in the linear nanostructures was 1.43. Because pristine DNA has a diameter of 2 nm and is usually invisible by TEM without staining, the linear nanostructures should be DNA/PEG-b-P4VP complexes. Increasing the WVF to above the CWC resulted in nanofiber formation, and the nanofibers were highly regular in morphology and monodisperse in width and length. At WVFs of 70%, 80%, and 90%, the widths were 12.0, 18.0 and 22.0, respectively (Figures 5b, c and d), and the corresponding compaction ratios were 3.2, 3.9, and 4.9. After the WVF reached 90%, further increases in the WVF did not change the width or the compaction ratio.

The high compaction ratio and the monodisperse length of the nanofibers demonstrate that the DNA must interact with the polymer in a highly regular manner. To reveal the structure, noncrosslinked nanofibers were further characterized by ethidium bromide (EB) insertion experiments and zeta potential analysis. Fluorescence spectra of the


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mixture of EB and the noncrosslinked nanofibers at a WVF of 90% revealed that DNA chains on the nanofibers are fully accessible for EB intercalation as free DNA chains (Figure 6).45 Zeta potential analysis of the noncrosslinked nanofibers at a WVF of 90% and pH of 6.4 yielded a negative value of −10.8 mV, while the zeta potential of the linear DNA/polymer complex formed at the initial stage at a WVF of 60% and the same pH was +17 mV. The results of the zeta potential measurements and the EB insertion experiments indicate that the DNA chain wraps around the cylindrical P4VP core of the nanofiber and is thus exposed to the medium; the PEG shell consists of soluble PEG chains that allow the diffusion of EB small molecules. Based on the above results, together with the behaviors of DNA wrapping around nanoobjects approximately 10 nm in size,2,3,25-28 we believe that the DNA chain adopts a solenoidal conformation to wrap around the surface of the P4VP core of the nanofiber, given that the PEG chains do not interact with DNA chains and are highly soluble in the medium (Scheme 1c).


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Figure 6. Fluorescence spectra for determining DNA accessibility by EB intercalation (λexi = 482 nm). The spectra were collected after adding EB to a mixed solvent (the negative control) (the water/methanol volume ratio is 9/1), the suspension of the PEG113-b-P4VP76 micelles in the same mixed solvent (another negative control), 5522 bp linear DNA (the positive control) in the same mixed solvent, and noncrosslinked nanofibers prepared from 5522 bp linear DNA and PEG113-b-P4VP76 (the sample) dispersed in the same mixed solvent. The number ratio of EB/DNA base pairs was 1/3.6, larger than the ratio required for the saturated binding of DNA by EB. The


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concentration of DNA in the nanofiber dispersion and the free linear DNA solution was 0.01 mg/mL.

The formation mechanism of the core-shell nanofibers in the present work is illustrated in Scheme 1. Below the CWC for their micellization, the amphiphilic diblock copolymers interact with the DNA chain via weak electrostatic attraction to form a linear DNA/polymer complex, in which the backbone of the DNA chain is encapsulated by polymer chains. With a gradual increase in the water content, the DNA/polymer complexes recruit polymer unimers from the bulk solution because the complexed P4VP block chains are stabilized by the interacting DNA and are thus thermodynamically favorable for further P4VP aggregation. With further water addition, the attractive interaction between the P4VP block chains is enhanced. Considering one nanofiber, we believe that the enhanced P4VP/P4VP attractive interaction and weak DNA/P4VP interaction is responsible for the unwrapping of the DNA chain because when wrapped around DNA, the P4VP chains cannot fully aggregate together; conversely, due to the weak DNA/polymer interaction, the unwrapped DNA chain wraps around the


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aggregated P4VP. The DNA wrapping tailors and guides the aggregation of PEG-bP4VP diblock copolymers, leading to highly regular core-shell nanofibers. Additional explanation of the assembly pathway and the mechanism for the DNA conformational change is given in the SI as Text S1.

It should be mentioned here that the previously reported method for the preparation of monodisperse core-shell nanofibers by DNA/micelle self-assembly cannot be used for the fabrication of long nanofibers with lengths on the order of microns.13 As the control, we conducted the self-assembly of lambda DNA with the PEG-b-P4VP micelles and obtained only irregular aggregates (Figure S1). In the case of the lambda DNA/micelle self-assembly, both the DNA and the micelles possess large molecular weights such that kinetic trapping is inevitable. As mentioned above, in biological systems, very long genomic DNA first binds to a (H3-H4)2 histone tetramer and then recruits two H2A-H2B dimers into the as-formed tetrasome to form a nucleosome. Similarly, in the present study, we started complexation by interacting DNA with polymer unimers and then allowing the P4VP unimers to aggregate from the bulk solution onto the complex slowly.


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As a result, self-assembly with highly controllable kinetics was achieved, as revealed by the fact that the originally wrapped DNA chain can change its conformation to wrap around the polymer aggregate very regularly during the self-assembly.46,47 Due to the highly controllable kinetics, long, flexible, uniform and morphologically pure core-shell nanofibers were prepared at a polymer concentration of 20 mg/mL (Figure 2). The detailed explanation on the novelty of the present approach compared with the DNA/micelle method we reported previously is given in the SI as Text S2.

Nanofiber percolation and chemiresistor fabrication

In materials science, percolation describes the formation of networks by conducting components on 2D substrates or in 3D matrices above the percolation threshold. The percolation threshold is a critical concentration of the conductive components for the insulating-conducting transitions in the 2D or 3D space.46-48 The concept of percolation has been widely used for the fabrication of composite materials with antistatic or electromagnetic shielding properties.48-50 The conductive components may be much more expensive than the matrices and may reduce the mechanical properties of the


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composite materials at higher loading concentrations. Therefore, lowering the percolation threshold is of critical importance for the fabrication of low-cost and highperformance composite materials.48-50 Some studies have reported the use of 1D conductive nanomaterials with large aspect ratios to form continuous percolating pathways at much lower percolation thresholds than both their short and spherical counterparts.51 Inspired by this unique property, we envision that further endowing the worm-like polymeric micelles with conductivity may open up new avenues for the solution-processable fabrication of low-cost and high-performance electronic devices. Percolating networks on 2D substrates with low percolation thresholds would be used for devising electrical sensors with high sensitivity and low detection limits by taking advantage of the insulating-conducting transitions with drastic changes in electrical conductivity at the percolation threshold.

The fabrication of electronic microdevices represents an important application of 1D nanomaterials, and most studies in this area use inorganic nanowires, organic nanowires formed by small molecules, or carbon nanotubes. Compared to these 1D


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nanomaterials, core-shell polymeric nanofibers possess excellent solubility and structural flexibility,14-24,34-44 which make them an ideal candidate for the solutionprocessable fabrication of electronic microdevices. Nevertheless, to the best of our knowledge, the fabrication of electronic microdevices based on polymeric core-shell nanofibers has not been reported.21-24,52-55 The fact that the long nanofibers easily become entangled (as shown in Figure 1) motivated us to investigate the 2D percolation of the long nanofibers on a substrate (Figures 7a and S8-S11). Figure 7a shows the percolation network obtained by spin-coating lambda DNA/PEG113-b-P4VP67 nanofibers at a surface coverage of 1%. In striking contrast, it is still difficult for the short nanofibers prepared from 5522 bp DNA and PEG113-b-P4VP76 to form percolation networks at a surface coverage of ~10% (Figures 7b and S12-S15).51,56-57 These observations indicate the unique property of long nanofibers with high aspect ratios compared to their short counterparts.

Because the P4VP core is continuous and in a condensed state (as indicated above), the network can be used for the fabrication of electronic microdevices. We confirmed


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that the percolated networks formed by the long nanofibers with two evaporated gold contacts (the parts with a high contrast and a separation of approximately 4 μm in Figure 8a) are chemiresistors after the P4VP cylindrical core was doped with I3−.58 The chemiresistors were found to be conductive, with linear current-voltage curves (Figure 8b). Upon exposure to SO2 gas, the chemiresistors became insulating (Figure 8b), which can be attributed to the reduction in doped I3− within the nanofibers into I− by SO2 gas. The on/off current ratio of the chemiresistor was as large as 5 × 103, and even a daily use multimeter could monitor changes in electrical resistance before and after SO2 exposure (Figure S16). Interestingly, after removing the SO2 atmosphere, followed by exposure to ambient conditions for several days, the chemiresistors in the insulating state become conductive again, which was attributed to the recovered formation of I3− from the oxidation of I− within the nanofibers. The chemiresistors could be used repeatedly many times without a significant decrease in their performance (Figure 8c).


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Figure 7. FESEM images of (a) lambda DNA/PEG113-b-P4VP67 nanofibers spin-coated on a silicon wafer to form a percolation network at a very low surface coverage (1%) and (b) 5522 bp DNA/PEG113-b-P4VP76 nanofibers spin-coated on a silicon wafer, which cannot form a percolation network even when the surface coverage is ~10%.


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Figure 8. (a) FESEM image of the chemiresistors formed by the I3−-doped percolated network of long nanofibers with two evaporated gold contacts. (b) Current-voltage curves of the chemiresistors before and after exposure to 300 ppm SO2. (c) Current changes during the repeated use of chemiresistors. The inset shows the redox reaction cycles within the nanofibers during repeated use.

CONCLUSIONS

In conclusion, by mimicking the tetrasome pathway, we demonstrate a novel pathway of DNA and amphiphilic diblock copolymer self-assembly with highly controllable kinetics


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for the fabrication of highly regular core-shell nanofibers, especially those with very high aspect ratios. Under well-controlled conditions, the self-assembly of DNA with PEG-bP4VP allowed the originally encapsulated DNA chain to adjust its conformation to wrap around the polymer aggregate and thus guide the self-assembly and tailor the resultant complexes. The highly controllable kinetics are achieved due to the reduced DNA/polymer electrostatic attraction and the enhanced dynamics of the diblock copolymers in the system, similar to that of the tetrasome pathway. The present method can be used to fabricate morphologically pure and monodisperse core-shell nanofibers, especially those with very large aspect ratios, with respect to previously reported methods for core-shell nanofiber preparation.18,33-39 In addition, the two-component coassembly strategy in the present work allows flexibility in the choice of building blocks and facile control over the structural parameters of the resultant core-shell nanofibers. Compared to our previously reported DNA/micelle self-assembly method,13 the present method uses slightly modified preparation procedures but drastically enhances the assembly dynamics and thus exhibits advantages for the fabrication of flexible nanofibers with lengths in micrometers that involve the use of long DNA of tens of kbp.


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Furthermore, the morphologically pure and long, flexible nanofibers were found to be very promising for fabricating networks at extremely low percolation thresholds. Due to the continuous and condensed structure of the cylindrical P4VP cores, the percolated network became conductive after the core was doped with I3−, and the conductive network could be used as chemiresistors with large on/off current ratios. The present work provides an efficient method for the tailorable fabrication of morphologically pure and monodisperse core-shell nanofibers with lengths ranging from nanometers to micrometers. Further studies will focus on illustrating the structure-property relationship or morphology-function relationship based on these well-defined nanofibers and their functional derivatives. We believe these studies will pave the way for the fundamental study and practical application of these soft nanomaterials.

Supporting Information

TEM images of lambda DNA/polymeric micelle aggregates; other supporting TEM and FESEM images.

AUTHOR INFORMATION


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Corresponding Author *Kaka Zhang, *Daoyong Chen.

E-mail: [email protected], [email protected]

Funding Sources NSFC (51721002, 21334001 and 21574025), MOST (2016YFA0203302) and STCSM (16JC1400702). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We also thank Dr. Ming He and Prof. Feng Qiu at Fudan University for their support in chemiresistor fabrication.

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Polymer Self-Assembly to Tailorable and

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