pH-Responsive Co-assembly of Oligo(ethylene glycol)-Coated Gold

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pH-Responsive Co-assembly of Oligo(ethylene glycol)-Coated Gold Nanoparticles with External Anionic Polymers via Hydrogen Bonding Yu Torii, Naotoshi Sugimura, Hideyuki Mitomo, Kenichi Niikura, and Kuniharu Ijiro Langmuir, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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pH-Responsive Co-assembly of Oligo(ethylene glycol)-Coated Gold Nanoparticles with External Anionic Polymers via Hydrogen Bonding Yu Torii†, Naotoshi Sugimura†, Hideyuki Mitomo‡§, Kenichi Niikura*,‡§&, Kuniharu Ijiro*,‡§ † Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-Ku, Sapporo 060-8628, Japan ‡ Research Institute for Electronic Science (RIES), Hokkaido University, Kita 21, Nishi 10, Kita-Ku, Sapporo 001-0021, Japan, § Global Station for Soft Matter, Global Institution for Collaborative Research and Education, Hokkaido University, Kita 21, Nishi 11, Kita-Ku, Sapporo 001-0021, Japan

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ABSTRACT

Stimuli-responsive assembly of gold nanoparticles (AuNPs) with precise control of the plasmonic properties, assembly size, and stimuli-responsivity has shown potential benefits with regard to bio-sensing devices and drug delivery systems. Here, we present a new pH-responsive co-assembly system of oligo(ethylene glycol) (OEG)-coated AuNPs with anionic polymers as an external mediator via hydrogen bonding in water. Hydrogen bond-driven co-assemblies of OEGAuNPs with poly(acrylic acid) (PAA) were confirmed by the monitoring of plasmonic peaks and hydrodynamic diameters. In this system, the protonation of anionic polymers on change in pH triggered the formation of hydrogen bond between the OEG-AuNPs and polymers, providing sensitive pH-responsivity. The plasmonic properties and assembly size are affected by both the ratio of PAA to AuNPs and the molecular weight of PAAs. In addition, the attachment of hydrophobic groups to the surface ligand or anionic polymer changed the responsive pH range. These results demonstrated that the co-assembly with an external mediator via hydrogen bonding provides a stimuli-responsive assembly system with tunable plasmonic properties, assembly size, and stimuli-responsivity.


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Introduction Collective properties derived from the assembly of plasmonic nanoparticles1–8, such as gold nanoparticles (AuNPs), provide potential benefits in terms of bio-sensing9–12 and bio-medical devices13–15. To expand these applications, it is essential to develop a system which has both “stimuli-responsivity”, which enables on-demand assembly/disassembly of AuNPs, and “controlled assembly (beyond just aggregation)”, which includes the size or number of AuNPs in the assembly, gap distances, or ordered structure in water, as the plasmon coupling phenomena are quite sensitive to the assembled structures. In the conventional approaches to stimuliresponsive assembly/disassembly, AuNPs are directly modified with stimuli-responsive molecules, producing stimuli-responsive AuNPs.16–20 In these systems, nanoparticle assemblies are triggered by direct colloidal surface-surface interactions. Thus, it is difficult to control the assembly behavior, including assembly size and interparticle distance, simply by tuning surfacesurface interactions. New systems have recently been reported, in which external mediators provide stimuliresponsive assembly of AuNPs through their stimuli-responsivity and role as a nanoparticle assembly switch.21–24 Klajn and coworkers reported that the addition of photoswitchable spiropyran derivatives as mediators allowed the light-responsive assembly/disassembly of AuNPs without surface functionalization of AuNPs with light-responsive ligands.23,24 Moreover, changes in the co-assembly conditions of the AuNPs and polymers produced various changes in the plasmonic properties and assembly size. For example, a co-assembly study of AuNPs with external polymers demonstrated that the mixing ratio or molecular weight (Mw) of polymers had a marked effect on the co-assembly size and plasmonic properties.25,26 Further, polymermediated self-assembly provided two- and three-dimensional nanoparticles networks.27 These


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reports indicate that the co-assembly of AuNPs with external polymers is a potent system for ondemand assembly with desirable structures. To date, electrostatic interactions were widely used as a driving force for co-assembly in water solvent. On the other hand, hydrogen bonds are particularly useful for the assembly of higher order structures because of advantages such as directionality, fidelity, responsiveness to external stimuli, and tunability of the strength based on the multivalent bonds.28,29 Although there are a number of particularly interesting reports on DNA-mediated AuNP assemblies, most are based on thermo-responsiveness.30–33 One of the major stimuli in life science is pH. It is known that pH varies from acidic to neutral at the various organs or organelles in our body, for example, the stomach (pH 1-3), duodenum (pH ~5), and jejunum (pH ~6) in the gastrointestinal tract as well as some endosomes (pH 8000 > 1200 Da. From the point of view of hydrogen bonding, this is quite reasonable in comparison to that for DNAs. DNA hybridization demonstrates that changes in length and sequence change their stability (melting temperature), meaning that changes in the number of hydrogen bonds also produces a change in stability. Thus, when the molecular weight of PAA was 1200 Da, there was little PAA adsorption on the EG6-AuNPs so that only a negligible plasmon shift was observed. It was speculated that PAA with a molecular weight of 8000 Da was partially adsorbed on the EG6-AuNPs in the equilibrium state and induced larger co-assemblies and plasmon shifts than did PAA with a molecular weight of 15000 Da, in the similar manner to the results observed for the different mixing ratios of PAA to AuNPs, based on the SPR results. Although the number of ethylene glycol units on the AuNP surface (EG6- and EG3-AuNPs) did not affect co-assembly, PAA molecular weight (15000, 8000, and 1200 Da) had a significant effect.


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Figure 6. (A) Plasmon peak plots of EG6-AuNPs in the presence of PAA with different molecular weights as a function of pH and (B) hydrodynamic diameter of EG6-AuNPs-PAA coassemblies with different molecular weights at pH 1.5 measured by DLS (Mw of PAA=15000 (blue), 8000 (red), and 1200 Da (green)). Plasmon peak wavelengths and hydrodynamic diameters are summarized in Table S9, S10, S11.

Control of responsive pH range of the co-assembly by attachment of hydrophobic groups. Hydrogen bonding is quite environment-sensitive, particularly hydrophobicity. Thus, we investigated the effect of the hydrophobicity of the surface ligand or anionic polymer on coassembly response. To increase the hydrophobicity of the surface ligand, a new surface ligand with a methyl group at the terminal of the EG6 ligand (referred to as C1EG6) was synthesized (Figure 7A, see supporting information). Also, as an anionic polymer, we chose poly(methacrylic


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acid) (PMAA), which has methyl groups in the PAA main chain (Figure 7A). Figure 7B shows the pH-responsive plasmon shifts for the co-assembly of the C1EG6-AuNP with PAA and that of the EG6-AuNP with PMAA. These results showed the co-assemblies of C1EG6-AuNPs with PAA and EG6-AuNPs with PMAA were formed around pH 3.1, which is higher than that of EG6-AuNPs with PAA (pH 2.3). In both cases, the addition of hydrophobic groups increased the assembly pH. We speculated that the slight difference between the co-assembly of C1EG6AuNPs with PAA and that of EG6-AuNPs with PMAA resulted from the difference in the distance of the methyl groups from the functional groups for hydrogen bonding. Further, we speculated that the hydrophobic environment enhanced hydrogen bonding between the ethylene glycol moieties and carboxylic acid. On the other hand, strengthening the hydrophobic interaction could also directly promote co-assembly. Although we could not determine the actual mechanism, in either case, the slight difference in surface ligand or external mediator structure affected the stimuli-responsivity. We demonstrated the pH-responsivity on the co-assembly could be tuned by the hydrophobicity of the surface ligands or anionic polymers.


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Figure 7. (A) The chemical structures of the C1EG6 ligand and PMAA, and (B) plasmon peak plots as a function of pH for EG6-AuNPs with PAA (blue), C1EG6-AuNPs with PAA (red), and EG6-AuNPs with PMAA (green). Plasmon peak wavelengths and hydrodynamic diameters are summarized in Table S12, S13, S14.


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Conclusion Our present study demonstrated that plasmonic properties, assembly size, and pH-responsivity were controllable in the system established for the co-assembly of OEG-AuNPs with anionic polymers. We revealed that OEG-AuNPs behaved like a high molecular weight PEG, and the coassemblies of OEG-AuNPs with anionic polymers were induced by changes in pH in a reversible manner. In this co-assembly system, the mixing ratio and molecular weight of PAA changed the plasmonic properties and assembly size. Moreover, the hydrophobicity of the surface ligand or anionic polymer changed the responsive pH. This pH-responsive co-assembly system shows tremendous potential for further controllability through changes in the surface ligands or anionic polymers, leading to a range of potential future bio-applications with further improvements. We expect that external mediators with unique features such as shape and chirality would affect the properties of the AuNP assemblies.

ASSOCIATED CONTENT Supporting Information. Figure S1–S9, Table S1-S14 and synthetic procedures and characterizations of the thiol ligands (including Scheme S1 and Figure S10) as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] *E-mail: [email protected] Present Addresses & (K. I.) Department of Innovative Systems Engineering, and Graduate School of Environmental Symbiotic System Major, Nippon Institute of Technology, Miyashiro, Saitama 345-8501, Japan. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI number “15K13261”. This work was supported in part by “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). A part of this work was conducted at Hokkaido University, supported by "Nanotechnology Platform" Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We thank the OPEN FACILITY of Hokkaido University Sousei Hall for the use of Delsa Nano HC system.


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pH-Responsive Co-assembly of Oligo(ethylene glycol)-Coated Gold

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