Letter Cite This: ACS Macro Lett. 2018, 7, 431−436
pubs.acs.org/macroletters
Facile Fabrication of CO2‑Responsive Nanofibers from Photo-CrossLinked Poly(pentafluorophenyl acrylate) Nanofibers Shaojian Lin,†,§ Jiaojiao Shang,† and Patrick Theato*,†,‡ †
Institute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesser Str. 18, D-76131 Karlsruhe, Germany
‡
S Supporting Information *
ABSTRACT: CO2-responsive nanofibers were facilely prepared from photo-cross-linked poly(pentafluorophenyl acrylate) (PPFPA) nanofibers via “amine-active ester” chemical modification. Photo-cross-linked PPFPA nanofibers were modified with histamine under mild conditions to generate cross-linked poly(histamine acrylamide) (PHAAA) nanofibers featuring a CO2 responsiveness. As expected, the prepared crosslinked PHAAA nanofibers can exhibit a CO2-responsive behavior to induce a reversible transition from hydrophobic to hydrophilic upon alternating addition and removal of CO2 on the surface of nanofibrous membranes. Based on this finding, we could demonstrate that crosslinked PHAAA nanofibers can be employed for reversible absorption and release of protein using bovine serum albumin (BSA) as a model.
P
other monomers to obtain copolymers with higher Tgs. As a typical example of CO2-responsive nanofibers, Yuan’s group successfully fabricated CO2-responsive nanofibrous mats from poly(methyl methacrylate-co-2-(diethylamino)ethyl methacrylate) (PMMA-co-PDEAEMA) for oil/water separation.30 Interestingly, the wettability of PMMA-co-PDEAEMA nanofibrous mats can reversibly switch from hydrophobic to hydrophilic upon alternating addition and removal of CO2. However, in their case, at least 50 wt % of PMMA was required to maintain stable nanofibers before and after CO2 stimulation. Hence, the number of CO2-responsive moieties on the nanofibers was sacrificed via this strategy. In order to avoid this drawback, it is worth to explore CO2-responsive homopolymers with suitable Tgs, because nanofibers from those homopolymers can maintain their morphologies well without CO2 stimulation and make them suitable for much broader applications. Nevertheless, cumbersome synthesis and purification processes are usually required to prepare novel CO2-responsive monomers.26,31 Fortunately, it has been demonstrated that the facile preparation of functional polymers from reactive precursor polymers via postpolymerization modification is an efficient method to avoid cumbersome synthesis processes.17,26,31−34 Moreover, the postpolymerization modification strategy can also endow corresponding polymers with more complicated structures and properties.33,34 Inspired by the advantages of postpolymerization modification, the facile decoration of nanofibers via postelectrospinning
olymeric nanofibers have attracted considerable attention in the past decades due to their unique features, such as high surface area to volume ratio, controlled size and morphology, and high porosity.1−3 It is noteworthy that stimuli-responsive polymeric nanofibers (SRPNs) have undergone a rapid development due to great progress in the field of stimuli-responsive polymers recently.2,4−7 Based on the characteristics of SRPNs, such nanofibers can undergo reversible physical and chemical transitions upon alternating addition and removal of external stimuli, including temperature,4−6 pH,7,8 light,2,9 redox,2 etc. Therefore, they have been extensively investigated for various applications, such as controlled drug release,7−9 “smart” actuators,4,5 and absorbing materials.2,6 Undeniably, CO2-responsive polymers have been considerably investigated in the past five years due to the “green” feature of CO2 stimulation.10−15 In view of this feature, numerous CO2-responsive assemblies,16−19surfaces,20 hybrid nanoparticles,21,22 latexes,23,24 and hydrogels25−27 etc. have been reported. In contrast to this, to date, CO2-responsive nanofibers have been scarcely explored. The main reason for this can be attributed to the fact that the majority of CO2responsive homopolymers have a low glass transition temperature (Tg), such as poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) (Tg = ∼18 °C),28 poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) (Tg = ∼20 °C),29 and poly(Namidino)dodecyl acrylamide (PADDAA) (Tg is below room temperature).16,24 Hence, the morphology of nanofibers from such homopolymers cannot be preserved well at room temperature. To address this issue, it is a common strategy that such CO2-responsive monomers are copolymerized with © XXXX American Chemical Society
Received: February 12, 2018 Accepted: March 15, 2018
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DOI: 10.1021/acsmacrolett.8b00115 ACS Macro Lett. 2018, 7, 431−436
Letter
ACS Macro Letters
Scheme 1. Illustration of the Fabrication of CO2-Responsive Nanofibers Derived from Photo-Cross-Linked PPFPA Nanofibers via Post-Electrospinning Modification and Their Reversible BSA Absorption and Release Behaviors upon CO2 Stimulation
Figure 1. SEM images of P(PFPA-co-ABP) nanofibers before (A) and after (B) UV irradiation; printable P(PFPA-co-ABP) nanofibrous mats via UV irradiation (C).
modification has received considerable attention recently.35−41 For example, Becker’s group prepared a series of “active”and biodegradable nanofibers based on various functional initiators.35−38 These nanofibers can be postdecorated via “click” chemistry. In another example Hoogenboom’s group fabricated hydrophilic poly(2-oxazoline) nanofibers as precursor nanofibers via electrospinning, and then the thiol−ene toolbox was employed to further postfunctionalize the nanofibrous surfaces.41 As demonstrated by Theato’s group, poly(pentafluorophenyl acrylate) (PPFPA) is an excellent candidate as a precursor polymer to prepare various functional polymers via postpolymerization modification.17,32,33,42−45 Particularly, a series of CO2-responisve polymers have been synthesized from PPFPA via this strategy.17,26,45−47 Nevertheless, utilization of PPFPA as a precursor nanofiber to fabricate CO2-responsive nanofibers is still unexplored. In this case, how to find an appropriate amine compound to generate CO2-responsive nanofibers with a suitable T g is the key issue. Taking this factor into consideration, histamine is explored in this work. To fabricate corresponding CO2-responsive nanofibers via postelectrospinning modification (see Scheme 1), PPFPA precursor nanofibers
have been modified with histamine because the resulting poly(histamine acrylamide) (PHAAA) has a suitable Tg (Tg = ∼102 °C; Figure S3) and a CO2 responsiveness (Figure S4) and can therefore be viewed as a good candidate to fabricate CO2-responsive nanofibers. Additionally, in order to maintain a nanofibrous structure before and after CO2 stimulation, 4acryloyloxybenzophenone (ABP) is utilized as a photo-crosslinker to create a stable nanofibrous structure since it can react highly efficiently with −C−H bonds upon UV irradiation leading to a cross-linked structure (Figure S5). In the present work, the precursor random copolymer, poly(pentafluorophenyl acrylate-co-4-acryloyloxy benzophenone) P(PFPA-co-ABP), was synthesized by free radical polymerization. The chemical structure was confirmed by 1H NMR (Figure S1A), 19F NMR (Figure S1B), and FT-IR measurements. As a result, 10 mol % ABP was incorporated in the random copolymer as confirmed by its 1H NMR spectrum after calculating the integral areas of peaks at 3.10 ppm from PPFPA and at 7.76 ppm from PABP. After successfully obtaining the targeted random copolymer, P(PFPA-co-ABP) nanofibers as facile and versatile precursor nanofibers were fabricated under optimal electrospinning conditions. As shown 432
DOI: 10.1021/acsmacrolett.8b00115 ACS Macro Lett. 2018, 7, 431−436
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ACS Macro Letters
Subsequently, the morphologies of histamine-modified nanofibers were observed by SEM. As illustrated in the SEM images (Figure 3), the obvious nanofibrous morphologies could be observed after conducting postelectrospinning modification for 30 s, 2 min, 5 min, 10 min, and 30 min, respectively. However, some nanofibers stuck together after a longer reaction time of 30 min (Figure 3E). Based on these results, it can be concluded that cross-linked PHAAA nanofibers with a good morphology can be obtained after postelectrospinning modification of PPFPA nanofibers for 10 min, followed by a reduction of the water contact angles (WCAs) of the nanofibers from around 135° to around 96° (Figure 3F). Then, the CO2-responsive behavior of cross-linked PHAAA nanofibers was investigated. Upon CO2 stimulation, the WCAs of cross-linked PHAAA nanofibers decreased from around 96° to almost 0° (Figure 4A) because the imidazole groups in the PHAAA nanofibers were protonated upon CO2 stimulation, resulting in nanofibrous surface transition from hydrophobic to hydrophilic. As expected, this CO2-triggered hydrophobic and hydrophilic transition behavior of cross-linked PHAAA nanofibers was reversible and repeatable via alternating addition and removal of CO2 to protonate and deprotonate imidazole groups (Figure 4A). Moreover, the morphology of cross-linked PHAAA nanofibers was well maintained after three cycles of CO2 stimulation (Figure S6). By the above-mentioned results, the successful fabrication of CO2-responsive nanofibers based on cross-linked PPFPA nanofibers was demonstrated. Moreover, it is worth pointing out that the fabrication process of CO2-responsive nanofibers is time-saving (within 10 min) and energy-efficient (without heating). Owing to the reversible CO2-responsive behavior of crosslinked PHAAA nanofibers, they are expected to be applied in several areas, such as CO2 capture, drug release, and biomolecular separation. To demonstrate potential application of cross-linked PHAAA nanofibers, these nanofibers were employed for bioseparation of bovine serum albumin (BSA) as a model. As mentioned before, the surface of cross-linked PHAAA nanofibers can reversibly undergo a transition from hydrophobic to hydrophilic upon CO2 stimulation. Based on this characteristic, cross-linked PHAAA nanofibers exhibiting a hydrophobic property can absorb BSA from aqueous solution in the absence of CO2 (Scheme 1). The absorbed BSA can be detached from the nanofibrous surface upon CO2 stimulation due to the protonation of imidazole groups, which results in its surface transition from hydrophobic to hydrophilic. This BSA adsorption and release behavior of a CO2-responsive nanofibrous mat was monitored by UV−vis spectroscopy. The absorption capacity of BSA on the PHAAA nanofibers (Qm) in the absence of CO2 was calculated by the equation reported earlier, as following Qm= ((C0 − Ct) × V)/Wnf.6 Briefly, Qm could be calculated from this equation according to the concentration of BSA in aqueous solution, with the details of this procedure described in the Supporting Information. As shown in Figure 4B, the BSA absorption capacity of a PHAAA nanofibrous mat (∼15 mg) can reach its maximum amount (Qm = 256 mg/g) after 3 h, an absorption capacity that is close to that in other published papers.6,48 As expected, approximately 95% of the absorbed BSA can be released from PHAAA nanofibrous surfaces upon removal of CO2 at 50 °C. Interestingly, the BSA adsorption and release capacities of CO2-responsive PHAAA nanofibers can undergo at least three cycles without deteriorating their performance (Figure 4B). The adsorptions of BSA (Qms) on PHAAA nanofibrous
in the SEM image in Figure 1A, the fabricated nanofibers showed a uniform diameter (900 ± 200 nm) and a smooth surface without beads, implying the successful preparation of precursor nanofiber pendant active ester groups. Subsequently, the fabricated P(PFPA-co-ABP) nanofibrous mat was photocross-linked upon UV irradiation (= 365 nm) for 2 h. To verify the successful fabrication of photo-cross-linked nanofibers, UVirradiated P(PFPA-co-ABP) nanofibers were immersed in THF (a good solvent to dissolve un-cross-linked P(PFPA-co-ABP)) for 10 min. As expected, the intact nanofibrous morphology was observed in the SEM image (Figure 1B), demonstrating that the P(PFPA-co-ABP) nanofibers have been photo-crosslinked to form a robust nanofibrous structure. Based on this mechanism, the cross-linked PPFPA nanofibrous mat with flower shape can be printed using a specific pattern via UV irradiation (Figure 1C). It is well-known that the pentafluorophenyl (PFP) ester reacts specifically with primary amines. To meet the purpose of this work, the prepared photo-cross-linked PPFPA nanofibers were employed as a facile platform to fabricate CO2-responsive nanofibers. To achieve this goal, the photo-cross-linked PPFPA nanofibrous mats (around 6 mg, 1.0 cm × 0.6 cm) were immersed in 3 mL of histamine solution (5 mg/mL) to prepare CO2-responsive nanofibers via postelectrospinning modification. The reaction rate and conversion were monitored by FTIR spectroscopy. As illustrated in Figure 2, the characteristic
Figure 2. FT-IR spectra of histamine functional cross-linked PPFPA nanofibers via postelectrospinning modification at different times: 0 min (black line), 30 s (pink line), 2 min (blue line), 5 min (green line), 10 min (orange line), and 30 min (red line).
bands from the PFP ester at 1780 cm−1 and PFP moiety at 1510 cm−1 gradually disappeared, respectively, while the new bands at 1650 cm−1 assigned to an amide bond appeared with prolonged reaction time. It is noteworthy that the majority of active ester groups from PPFPA nanofibers can be substituted by histamine within 5 min. This rapid “active ester-amine” chemistry reaction can be attributed to the high reactive character of PFP esters and to the high surface area to volume ratio of the nanofibers. Moreover, active ester groups can be completely consumed by excess histamine within 10 min (Figure 2). 433
DOI: 10.1021/acsmacrolett.8b00115 ACS Macro Lett. 2018, 7, 431−436
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ACS Macro Letters
Figure 3. SEM images of histamine functional cross-linked PPFPA nanofibers via postelectrospinning modification at different times: 30 s (A), 2 min (B), 5 min (C), 10 min (D), and 30 min (E). The optical images of nanofibrous mats and their corresponding WCAs: cross-linked PPFPA nanofibers (F1, left) and cross-linked PHAAA nanofibers (F2, right) derived from postelectrospinning modification of cross-linked PPFPA nanofibers by applying histamine for 10 min.
surfaces were 235 mg/g in the second run and 243 mg/g in the third run. Meanwhile, around 91% of the absorbed BSA was released from PHAAA nanofibrous surfaces in the second and third run upon removal of CO2. In summary, CO2-responsive poly(histamine acrylamide) (PHAAA) nanofibers were facilely prepared via “active esteramine” chemistry based on photo-cross-linked PPFPA nanofibers. The wettability of cross-linked PHAAA nanofibers can reversibly switch from hydrophobic to hydrophilic upon CO2 stimulation and can, thus, reversibly realize BSA absorption and release. Now, it can be concluded from all the aforementioned results that photo-cross-linked PPFPA nanofibers can be employed as a facile and versatile platform to potentially fabricate various functional nanofibers via postelectrospinning modification due to pendant highly active PFP ester moieties.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00115. Materials, characterizaion, synthesis of poly(pentafluorophenyl acrylate-co-4-acryloyloxybenzophenone) P(PFPA-co-ABP), synthesis of poly(histamine acrylamide) (PHAAA), preparation of precursor P(PFPA-co-ABP) nanofibers, post-electrospinning modification of P(PFPA-co-ABP) nanofibers, and BSA adsorption and release performance evaluation (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shaojian Lin: 0000-0002-9179-9190 Patrick Theato: 0000-0002-4562-9254
Figure 4. Reversible water contact angle (WCA) transition (A) and corresponding BSA absorption and release behavior (B) of the crosslinked PHAAA nanofibrous mat upon alternating addition and removal of CO2.
Present Address §
College of Light Industry, Textile and Food Engineering, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu, Chengdu 610065, China. 434
DOI: 10.1021/acsmacrolett.8b00115 ACS Macro Lett. 2018, 7, 431−436
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ACS Macro Letters Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S. Lin and J. Shang are grateful to the China Scholarship Council (CSC) for the financial support of this work (CSC grants: 201306240132 and 201406240131).
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