Supramolecular Assembly with Ionic, Redox-Responsive Poly (Ferrocenylsilanes): Engineering of Interfaces and Molecular Release Applications
This thesis describes the synthesis and electrostatic assembly of organometallic poly(ferrocenylsilane) (PFS) strong polyelectrolytes. The distinctive structural features of poly(ferrocenylsilanes) come from the silicon and iron atoms in the main chain. The presence of redox-active ferrocene units in the polymer backbone provides unique redox-responsive properties to PFS. The charged nature and redox-responsiveness of these organometallic polyelectrolytes makes them particularly useful in the electrostatic self-assembly process for the fabrication of novel functional supramolecular nanostructures. A major part of the research described in the thesis (Chapter 4-6) is focused on the electrostatic layer-by-layer assembly featuring poly(ferrocenylsilane) polyelectrolytes.
Chapter 1 gives a short introduction on the thesis and in Chapter 2 a literature overview is given on the electrostatic assembly of polyelectrolytes. Following introductions on the general characteristics of polyelectrolytes, two dominant molecular interactions that are acting between oppositely charged polyelectrolytes are discussed. Electrostatic and hydrophobic interactions often lead to the complexation of oppositely charged polyelectrolytes. As a special case of polyelectrolyte complexation, discussions are mainly focused on the “template-assisted” layer-by-layer sequential assembly of polyelectrolytes. The fabrication, characterization, properties and applications of planar substrate-supported and spherical free-standing polyelectrolyte multilayers are described.
As the first part of the thesis, Chapter 3 deals with the synthesis and redox properties of PFS strong polyelectrolytes used throughout the study. Poly(ferrocenylsilane) polyions and fluorescence labelled polycations were obtained from side group modifications on the precursor polymer poly(ferrocenyl(3-chloropropyl)methylsilane) synthesized with controlled molar mass in the presence of Et3SiH. Fully reversible redox chemistry of these polyelectrolytes was demonstrated following the discovery of several water soluble redox agents. UV/Vis spectroscopy confirmed that poly(ferrocenylsilane) polyelectrolytes could be effectively oxidized by ferric chloride (FeCl3) and potassium ferricyanide (K3Fe(CN)6). Subsequent complete reduction of the oxidized water-soluble polymers was achieved using ascorbic acid (vitamin C) or dithiothreitol (DTT).
Chapter 4 describes the fabrication, characterization and properties of fully organometallic poly(ferrocenylsilane) planar multilayers. Water-soluble poly(ferrocenylsilane) strong polyelectrolytes have been employed in the electrostatic layer-by-layer supramolecular assembly process to give full organometallic polyion multilayer structures on various planar supports. The multilayer film growth was found to follow a linear profile, monitored by UV/Vis spectroscopy, spectroscopic ellipsometry and quartz crystal microgravimetry. Additional ellipsometric studies showed a square root dependence of the film thickness on polyelectrolyte solution salt (NaCl) concentration. On studying the redox properties of PFS multilayers, we found that both burst release and controlled release of PFS polyelectrolytes from their multilayer thin film structures are possible by the choice of different types of stimuli. The redox-responsive nature of the ferrocene-containing polymer backbone renders the multilayers removable by chemical oxidation. UV/Vis spectroscopy and ellipsometry measurements showed fast and complete PFS multilayer removal by exposure to FeCl3 solutions. This oxidation induced dissolution was used to create PFS multilayer patterns by a “reactive” soft lithography procedure using hydrophilic PDMS stamps and FeCl3 solutions as ink. The release of PFS polyelectrolytes from PFS multilayers can also be realized electrochemically by prolonged exposure to a small potential (≤ 0.4 V). The quantity and rate of material release can be precisely manipulated by the on-off switching of the electrical potential and the application of different oxidation potentials. Some successful attempts were made to selectively deposit poly(ferrocenylsilane) multilayers on patterned gold and silicon substrates. The successful fabrication procedure for preparing planar poly(ferrocenylsilane) multilayers was extended to the fabrication of free-standing multilayer microcapsules, whose structure and properties are described in Chapter 5. Free-standing microcapsules were fabricated from the electrostatic layer-by-layer self-assembly of poly(ferrocenylsilane) polyelectrolytes onto curved substrates followed by core dissolution. Optimum preparation conditions were established in order to obtain microcapsules with desired properties. Manganese carbonate (MnCO3) microparticles were chosen as the optimum core material. It was shown that the stability and integrity of the capsules obtained depend mainly on the wall thickness, which is tuned by varying the number of deposited polyelectrolyte bilayers. We demonstrated that by using fluorescence-labelled 4.4 x 103 g/mol dextran as the reference probe molecule, stable and impermeable PFS capsules with an average size of around 10 μm were obtained when the bilayer number was higher than four. The stable and impermeable capsules were very well suited for the subsequent responsive permeability studies. The permeability of stable PFS capsules can be tuned via chemical oxidation. The oxidation agents iodine (I2) and iron chloride (FeCl3) are both effective in oxidizing water-soluble PFS based multilayer capsules. FeCl3 was found to be an effective oxidation agent in that a very dilute solution (in the sub-mM range) was sufficient to cause a fast increase in the capsule permeability. Permeability change was believed to result from increased pore size due to the multilayer expansion along the chain direction and in the multilayer growth direction, which was caused by oxidation-introduced additional positive charges along the polymer backbone. In the later stages of chemical oxidation by FeCl3, capsules made solely from PFS showed a continuous expansion towards their final disintegration. The rate of expansion could be easily tuned by varying the number of deposited polyelectrolyte bilayers and the concentration of the oxidants used. The mechanism of redox-induced capsule expansion and disintegration was studied in detail by the comparison of the responsive behaviour of fully PFS capsules and PFS/organic polyelectrolyte hybrid capsules. The cause of capsule disintegration is likely an alteration of the charge densities of the polyanion and polycation species upon oxidation, leading to a loss of the original charge compensated multilayer structure. The deposition of additional redox-insensitive polyelectrolyte species PSS-/PAH+ as capsule capping layers ensured the shape and integrity of PFS capsules after chemical oxidation. By varying the number of PSS-/PAH+ bilayers, the permeability changing rate could be further manipulated. However, collapsed capsules still showed a decreased capsule wall thickness upon chemical oxidation, indicating some material loss. The electrochemical oxidation and reduction of these composite-wall microcapsules featuring PFS polyelectrolytes was realized by immobilizing them onto gold electrodes. Cyclic voltammograms exhibited redox waves typical for PFS, verifying the reversible electrochemical addressability of these redox-responsive microcapsules.
Chapter 6 describes the hybrid electrostatic assembly of poly(ferrocenylsilane) polycations with the natural anionic polyelectrolyte DNA. Multilayer assembly using double-stranded, high molar mass DNA resulted in unique redox-responsive macroporous thin films and aggregated, free-standing microcapsules with excellent molecular permeability. The redox-active PFS components render the thin films susceptible to chemical oxidation, while their unique macroporous architectures are maintained. The formation mechanism of the peculiar macroporous architectures was proposed as originating in the persistence length and chain length mismatch of the two components, as well as in the extraordinary hydrophobicity of the macromolecular backbone of PFS. The LBL assembly of PFS and DNA with varying molecular weights suggested the existence of a critical molar mass mismatch below which the peculiar macroporous structure could no longer be formed.
Going beyond the above fundamental studies, Chapter 7 describes various applications of organometallic poly(ferrocenylsilane) polyelectrolytes. As a first example, cationic poly(ferrocenylsilane) polyelectrolytes were used in plasmid DNA condensation to give stable nano-sized polymer-DNA complexes with high efficiency. Acting as non-toxic gene carriers, these PFS/DNA polyplexes displayed promising gene transfer properties. In a second approach, poly(ferrocenylsilane) polyanions were self-assembled with positively charged cowpea chlorotic mottle virus (CCMV) capsid proteins. Electrostatic interactions between poly(ferrocenylsilane) polyanions and CCMV capsid proteins resulted in monodisperse polymer-virus nanoparticles with sizes around 18 nm. These particles are potentially useful as nanometer-sized compartments in drug targeting and transportation applications. In the final part (Chapter 8), an outlook was given on some of the possible future research directions. The fabrication of various other forms of multilayer structures based on poly(ferrocenylsilane) polyelectrolytes, such as multilayer nanotubes and free-standing planar films are discussed. The necessity of follow-up studies on the functional structural platforms presented throughout the thesis was described. Many new functions are envisaged by a combination of the unique redox-responsive properties of poly(ferrocenylsilane) as well as these new shapes.