Letter Cite This: ACS Macro Lett. 2018, 7, 437−441
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Azo Coupling Reaction Induced Macromolecular Self-Assembly in Aqueous Solution Shang Li, Jilei Wang, Jiajia Shen, Bing Wu, and Yaning He* Department of Chemical Engineering, Key Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing, 100084, China S Supporting Information *
ABSTRACT: This communication reported azo coupling reaction induced macromolecular self-assembly in aqueous solution. Diblock copolymer (PEG-b-PSNHBoc) consisting of a hydrophilic PEG block, and a hydrophobic N-Boc protected poly(p-vinylaniline) block was synthesized by RAFT polymerization. Then double hydrophilic diblock copolymer (PEG-bPSN2+) composed of PEG and PS based macromolecular diazonium salts was prepared by the diazotization of PEG-bPSNH2, which was obtained by deprotection of PEG-bPSNHBoc. As N,N-dimethylaniline was gradually added into the freshly prepared PEG-b-PSN2+ aqueous solution, the azo coupling reaction between N,N-dimethylaniline and diazonium salts took place, which would lead to the generation of azobenzene pendants. Due to the poor solubility of azobenzene pendants in water, the formed hydrophobic polymeric chains aggregated to form the self-assembly colloidal particles. By incorporating a fluorescent group into the aniline, the aggregates formed through azo coupling reaction induced macromolecular self-assembly showed enzyme-triggered fluorescent behaviors.
A
was added to the THF solution and reached a critical value, and the hydrophobic azobenzene containing chains started to aggregate. The final self-assembly aggregates suspension was dialyzed against water to remove the organic solvent. It could be seen that the assistant organic solvent was unnecessary, which was removed at the final stage. On the other hand, due to the similar solubility of the azobenzene monomer and its polymer, the polymerization-induced self-assembly of azobenzene containing polymers has not been reported yet. As we know, the water insoluble azobenzene is usually prepared by azo coupling reaction between diazonium salts and derivatives of aniline or phenol in water. This provides the possibility to obtain the self-assembly aggregates induced by azo coupling reaction in aqueous solution. Anyway, to our knowledge, no successful example has been reported in the literature. In this communication, we developed a one-pot strategy to prepare polymeric aggregates via the azo coupling reaction induced macromolecular self-assembly in aqueous solution (Scheme 1). Double hydrophilic diblock copolymer composed of PEG and PS based macromolecular diazonium salts was first prepared. Then N,N-dimethylaniline (DMA) was added into the diazonium salts aqueous solution. Along with the azo coupling reaction, due to the poor solubility of azobenzene group in water, the obtained amphiphilic polymer self-assembled into nano aggregates. This method provides us a convenient way to directly prepare azobenzene polymeric nano
mphiphilic block copolymers constituted of both hydrophilic and hydrophobic chains have been extensively investigated for their easy self-assembly in selected solvents to form polymeric aggregates with spherical, wormlike, or vesicular morphologies.1−7 For this self-assembly in selected solvents method, it is very convenient to prepare functional polymeric aggregates just by incorporating functional units into the block copolymers. For example, stimulus responsive polymeric aggregates have well been studied by incorporating stimulus responsive segments into the polymer chains, which can be potentially used in various applications, such as drug delivery systems, nanoreactors, and many others.8−15 Recently, in order to simplify the polymeric aggregates fabrication process, polymerization-induced self-assembly (PISA) has been developed as a powerful strategy to fabricate well-defined polymeric nano objects.16−20 This efficient strategy combined polymerization and self-assembly in one pot. In the typical PISA process, the reaction solvent should be carefully selected to ensure that monomer was soluble and its polymer was insoluble. Thus, due to the similar solubility of many functional monomers and their polymers, only a few stimulus responsive polymeric nano objects have been prepared by the typical PISA strategy.21,22 In recent years, azobenzene containing polymeric aggregates have attracted much attention for their interesting stimulus responsive properties.23−26 The typical procedure for the fabrication of azobenzene polymeric aggregates was by selfassembly in selected solvents. For example, the amphiphilic azobenzene containing copolymer was first synthesized and dissolved in organic solvent such as THF, which was a good solvent for both hydrophilic and hydrophobic blocks. Water © XXXX American Chemical Society
Received: January 17, 2018 Accepted: March 19, 2018
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DOI: 10.1021/acsmacrolett.8b00049 ACS Macro Lett. 2018, 7, 437−441
Letter
ACS Macro Letters Scheme 1. Fabrication of Polymeric Aggregates via Azo Coupling Reaction Induced Macromolecular Self-Assembly in Aqueous Solution
generate the azobenzene pendants (the experiments details are shown in the Supporting Information). UV−vis absorption spectra were used to confirm the formation of the azobenzene pendants. Figure 1 gives the UV−vis absorption spectra of
objects by using water as the synthesis media. Furthermore, it was very easy to introduce more functionality to the selfassembly aggregates just by modifying the aniline. For example, by introducing fluorescent group into the aniline molecule, the aggregates formed through azo coupling reaction induced macromolecular self-assembly showed enzyme triggered fluorescent behaviors. It has been reported that macromolecular diazonium salts have high reactivity.27−29 The prepared diazonium salt ended poly(ethylene glycol) (PEG) monomethyl ether could be efficiently coupled with aniline ended polystyrene (PS) to prepare diblock copolymer in organic solvent.27−29 Here, we first reported the synthesis of double hydrophilic diblock copolymer composed of PEG and PS based macromolecular diazonium salts (Scheme 1). By esterification reaction between RAFT agent with PEG monomethyl ether, the PEG based macro RAFT agent was prepared (PEG-CTA). Then the diblock copolymer (PEG-b-PSNHBoc) consisting of a hydrophilic PEG block, and a hydrophobic N-Boc protected poly(pvinylaniline) block was synthesized by RAFT polymerization. The diblock copolymer PEG-b-PSNH2 was obtained by the deprotection of PEG-b-PSNHBoc. The 1H NMR spectra of the prepared PEG-CTA agent, PEG-b-PSNHBoc and PEG-bPSNH2 are shown in Figure S1. Figure S2 gives the GPC curves of PEG monomethyl ether, PEG-CTA, and PEG-bPSNHBoc. It can be found that the diblock copolymers were prepared with fine narrow PDI. The length of the PS based part can be well controlled by the feed ratio between the N-Boc protected p-vinylaniline and PEG-CTA. Based on the successful preparation of PEG-b-PSNH2, double hydrophilic diblock copolymer composed of PEG and PS based macromolecular diazonium salts (PEG-b-PSN2+) was prepared by adding NaNO2 aqueous solution into the mixture of PEG-b-PSNH2 and HCl in water cooling with ice bath. The reaction between water-soluble diazonium salts and derivatives of aniline in acidulous water gives the insoluble azobenzene compounds. Thus, in this work, based on the successful preparation of double hydrophilic diblock copolymer containing PS based macromolecular diazonium salts, the azo coupling reaction between the macromolecular diazonium salts and N,N-dimethylaniline will give the amphiphilic azobenzene containing polymer. As N,N-dimethylaniline was added to the freshly prepared PEG-b-PSN2+ solution, the azo coupling reaction between aniline and diazonium salts took place to
Figure 1. UV−vis absorption spectra of PEG-b-PSN2+, N,Ndimethylaniline and azo coupling reaction induced macromolecular self-assembly nano objects suspension. Inset: photographs of polymer solution before and after macromolecular azo coupling reaction.
PEG-b-PSN2+, N,N-dimethylaniline and the aqueous suspension after the azo coupling reaction between PEG-b-PSN2+ and N,N-dimethylaniline. It can be seen that the PEG-b-PSN2+ and N,N-dimethylaniline had no absorption in water in the visible region (380−700 nm). After the azo coupling reaction, the resulting aqueous suspension showed obvious absorption in the visible region (λmax = 415 nm), which was the typical absorption behavior of the amino azobenzene type azo chromophores. This could also be observed with the naked eyes as the color of the solution has changed from colorless to yellow after macromolecular azo coupling reaction. The inset of Figure 1 gives the photographs before and after the azo coupling reaction. Due to the poor solubility of azobenzene pendants in water, the solubility of azobenzene containing PS part chains in water gradually declined along with the azo coupling reaction, which caused the aggregation of the molecular chains to form the self-assembly nano objects. The TEM image of the self-assembly nano objects obtained by the azo coupling reaction between PEG43-b-PSN2+10 and N,N-dimethylaniline in aqueous solution was shown in Figure 2. It can be found that uniform colloid particles with a diameter about 180 nm were obtained (estimated statistically from the TEM image). This result was also confirmed by the DLS experiment (Dh is about 196 nm), which was shown in Figure S3. The length of the PS based part of the block polymer has an effect on the colloidal particle sizes. The longer PS based part can lead to forming larger self-assembly aggregates (Figures S4 and S5). In this way, we developed an efficient one-pot strategy to prepare azobenzene containing polymeric aggregates via the azo coupling reaction induced macromolecular self-assembly. By linking fluorescence group with azobenzene chromophore together, due to the fluorescence resonance energy transfer (FRET) process, the fluorescence of the system will be quenched efficiently.30−34 Under reducing environment, the azobenzene chromophore can be reduced efficiently with azoreductase treatment.26,35−37 After the cleavage of the azo bond, the fluorescence of the system can recover again. The reducible azobenzene containing materials have been used as imaging agents or drug delivery polymeric micelles responsive 438
DOI: 10.1021/acsmacrolett.8b00049 ACS Macro Lett. 2018, 7, 437−441
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ACS Macro Letters
linked with azobenzene chromophore, the emission of TPE would be totally quenched due to the FRET process. Treated with azoreductase in the hypoxic condition, the azobenzene chromophore could be cleaved after the reduction reaction. Thus, no FRET process took place anymore, the strong fluorescence of the TPE molecules appeared again. Figure 3
Figure 2. Typical TEM image of the self-assembly nano objects obtained by the azo coupling reaction (between PEG43-b-PSN2+10 and N,N-dimethylaniline) induced macromolecular self-assembly in aqueous solution.
to the reducing microenvironment of hypoxic tumors.30−34,38,39 In this work, by partially alternating DMA with fluorescent group modified aniline, it is very convenient to incorporate environmental stimuli triggered fluorescent behaviors to the aggregates formed through azo coupling reaction induced macromolecular self-assembly, which can be potentially used in imaging of hypoxic tumors. Scheme 2 gives the fabrication of Scheme 2. Fabrication of Enzyme Triggered Fluorescent Polymeric Aggregates via Azo Coupling Reaction Induced Macromolecular Self-Assembly in Aqueous Solution
Figure 3. UV−vis (a) and fluorescent (b) spectra of the aggregates formed through azo coupling reaction induced macromolecular selfassembly before and after reduction reaction in the hypoxic condition (the insets give the photographs of the suspension of the aggregates before and after reduction reaction in the hypoxic condition).
gives the UV−vis and fluorescent spectra of the aggregates formed through azo coupling reaction induced macromolecular self-assembly before and after reduction reaction in the hypoxic condition (the insets give the photographs of the suspension of the aggregates before and after reduction reaction in the hypoxic condition). From the UV−vis spectra, it could be seen that the peak in the visible region (λmax = 415 nm) decreased to nearly zero after the azoreductase treatment, which meant that all the azobenzene chromophores had been cleaved totally. This could also be observed with the naked eyes as the color of the solution had changed from yellow to colorless after the reduction reaction. On the contrary, no fluorescence peak could be found (blue line of Figure 3b) before reduction reaction due to the FRET process. After the reduction reaction by azoreductase treatment in hypoxic condition, the emission peak at 475 nm due to the TPE was obviously found when excited with 365 nm light (red line of Figure 3b). The inset of Figure 3b shows the photographs of the suspension of aggregates before and after reduction reaction taken under UV illumination. Strong fluorescence could be observed by naked eyes when excited with 365 nm UV light. TEM images of the aggregates before and after reduction reaction were shown
azoreductase triggered fluorescent polymeric aggregates via azo coupling reaction induced macromolecular self-assembly in aqueous solution. In this work, a typical aggregation induced emission (AIE) group tetraphenylethene (TPE) was used to act as the fluorescent part, which has been widely used as the bioprobes.40−43 In aqueous solution, TPE molecules usually show strong fluorescence properties. Anyway, when TPE was 439
DOI: 10.1021/acsmacrolett.8b00049 ACS Macro Lett. 2018, 7, 437−441
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ACS Macro Letters
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in Figure S7. The disassembly of the aggregates can be attributed to the reduction of azobenzene. In conclusion, we have demonstrated a strategy to prepare polymeric aggregates via azo coupling reaction induced macromolecular self-assembly, which efficiently combined the chemical reaction and self-assembly in one pot. Due to the poor solubility of azobenzene pendants in water, the solubility of azobenzene containing polymeric chains in water gradually declined along with the azo coupling reaction, which caused the aggregation of the macromolecular chains to form the selfassembly nano objects. This method provides us a convenient way to prepare azobenzene containing polymeric nano objects in water. By incorporating fluorescent group into aniline, the aggregates formed through azo coupling reaction induced macromolecular self-assembly have shown azoreductase triggered fluorescent behaviors.
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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.8b00049. Experimental procedures and characterization data (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yaning He: 0000-0003-0860-0126 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 21674058 and 21474056) and Fund of Key Laboratory of Advanced Materials of Ministry of Education (No. 2017AML03).
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DOI: 10.1021/acsmacrolett.8b00049 ACS Macro Lett. 2018, 7, 437−441
