Electrostatic Assembly with Poly(ferrocenylsilanes)
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- Ma, Y., Hempenius, M.A. & Vancso, G.J. J Inorg Organomet Polym (2007) 17: 3. doi:10.1007/s10904-006-9081-4
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New frontiers in polymer science involve the incorporation of functional species into material systems. The electrostatic layer-by-layer (LBL) self-assembly technique constitutes a versatile tool for nano- and microscale fabrication of devices and novel material structures. Although a rapidly growing attention has been paid to this area, most of the studies conducted were based on organic polymeric electrolytes. Due to synthetic developments, new polymeric structures with inorganic elements and transition metals incorporated in the main chain have become accessible. With the development of new synthesis routes, organometallic poly(ferrocenylsilane) polycations and polyanions emerged. Their charged nature and water-solubility made them excellent candidates for the extension of electrostatic multilayer assembly to organometallic polymeric materials. The present review gives a concise summary on the LBL fabrication of organometallic thin films and microcapsules based on water-soluble poly(ferrocenylsilane) polyions. The unique functions of these structures come from the molecular structure of poly(ferrocenylsilanes), in which silicon atoms and redox-active ferrocene units are present. In this context, the diverse application potentials of these organometallic multilayer structures are also discussed.
KeywordsPoly(ferrocenylsilanes) layer-by-layer self-assembly responsive polymers redox multilayer microcapsules water-soluble selective deposition
The development and multidisciplinary broadening of materials science necessitates the design and synthesis of new functional polymeric materials. Macromolecules containing inorganic elements or organometallic units have attracted growing attention since these materials present very interesting properties, as they may combine novel optical, electrical, magnetic and chemical characteristics with the processability of polymers [1, 2].
Poly(ferrocenylsilanes) (PFS), composed of alternating ferrocene and silane units in the polymer main chain, belong to this class of materials. Although oligomers with degrees of polymerization up to 10 were first obtained by Rosenberg in a condensation polymerization , the real breakthrough has been realized by the group of Manners. Fast and impressive progress in the chemistry of these polymers followed after the discovery of a thermal ring-opening polymerization (ROP) route of silicon-bridged ferrocenophanes in the early 1990s . Later on, living anionic and transition metal-catalyzed polymerization methodologies were also established, giving access to well-defined, monodisperse poly(ferrocenylsilane) homo- and block copolymers [5, 6].
The distinctive structural features of poly(ferrocenylsilanes) come from the silicon and iron atoms in the main chain, which make them valuable in the development of surface nano- and microstructuring strategies . PFS were found to be effective resists in reactive ion etching processes due to the formation of an etch resistant iron/silicon oxide layer . This resulted in the surface patterning applications of PFS using soft lithographic techniques as well as block copolymer lithography [9, 10]. Phase separated PFS block copolymer thin films were also reported to be catalytically active for the synthesis of carbon nanotubes . One particularly important property of PFS is their unique redox-activity. Oxidation and reduction of PFS can be performed by chemical reactions, where the extent and reversibility of oxidation were only reported recently . Alternatively, the oxidation and reduction process of PFS is more precisely controlled using electrochemistry . Oxidation of iron centres in PFS changes its electrical, optical and physical properties. Previous studies in our group on self-assembled end-functionalized PFS monolayers on gold revealed electrochemically induced morphology and volume/thickness changes . Atomic force microscopy (AFM) based single molecule force microscopy (SMFS) measurements on poly(ferrocenylsilane) single chains showed significantly increased Kuhn length and segment elasticity after oxidation, demonstrating redox-induced changes of the torsional potential energy landscape .
With their significant industrial and commercial value in bio-related and medical applications, water-soluble polymers are becoming increasingly important in recent years. Although water-soluble organic macromolecules have been widely studied, their inorganic and organometallic counterparts are largely unexplored . The macromolecular properties of poly(ferrocenylsilane) depend, to a large extent, on the substituents on silicon. Along with other characteristics, such as the crystallinity [13a, 16] and glass transition temperature , the solubility of PFS [17, 18, 19, 20, 21, 22] can be tuned by varying the size and type of the substituents. Modification of the polymer side groups has been shown to give access to water-soluble PFS [18, 19, 20, 21, 22]. Homopolymers of water-soluble PFS have potential uses as electrode modifiers and redox-active polymeric electrolytes for which the ionic conductivity might be tuned by oxidation of the iron centres. These materials may also be useful in redox-controlled drug delivery applications since various water-soluble ferrocenium salts have been shown to display anti-cancer activity . Water-soluble block copolymers with a poly(ferrocenylsilane) polyelectrolyte block, on the other hand, can self-assemble in aqueous media and the resulting micellar and vesicular aggregates may demonstrate redox-tunable encapsulation properties and emulsifying abilities for heterogeneous catalysis .
The most interesting aspect of the processabilities of PFS polyelectrolytes is the use of ionic interactions to deposit these polymers onto substrates. The so-called electrostatic layer-by-layer (LBL) technique is based on sequential adsorption of charged polyelectrolyte species to build up multilayered polymeric thin films with controlled thickness and composition . One of the attractive advantages of this methodology is that thin films can be tailor-made to display desired chemical and physical properties by the choice of specific polyelectrolyte material . Aqueous processing enables synthetic as well as natural polyelectrolytes to be assembled to thin films for a variety of applications, ranging from electroluminescent devices to biosensor arrays . However, in its 15 years of development, the vast majority of polyelectrolyte species studied are organic materials. The accessibility of water-soluble PFS polycations and polyanions enabled the fabrication of organic-organometallic as well as all-organometallic multilayers. A major extension of the LBL method onto curved surfaces resulted in multilayer-coated colloids and eventually, polyelectrolyte hollow capsules . These micro- and nanocapsule systems offer plenty of promising applications in different areas such as biotechnology, medicine, catalysis, optics as well as electronics . In this review, the discussion is focused on organometallic multilayers and microcapsules based on electrostatic self-assembly of water-soluble poly(ferrocenylsilane) polyelectrolytes. First the synthesis and properties of PFS polyelectrolytes as well as their defined self-assembly process are addressed; then the various potential application of these novel materials are summarized, with special emphasis on aspects related to the redox-responsive properties of PFS.
Water-soluble poly(ferrocenylsilane) polyelectrolytes
Both cationic (3, 4) and anionic (8) water-soluble PFS polyelectrolytes have been studied by viscometry measurements in water, with minimal salt concentrations. The results exhibit typical behaviour of polyelectrolyte solutions, i.e. a strong increase of reduced viscosity (ηsp/C, ηsp is the specific viscosity) with decreasing polymer concentrations [12, 20]. Cationic species 6 has been shown to form aggregates in water and display lower critical solution temperature (LCST) near 40°C, due to the presence of the oligo(ethylene glycol) side chains.
Ferrocene (Fc) groups are redox-active. Due to the ferrocene containing backbone, PFS belongs to the class of stimuli-responsive polymers . Many attempts to reversibly oxidize and reduce PFS by chemical [17, 33] as well as electrochemical  methods have already been reported. PFS polyelectrolytes have potential applications as electrode materials and redox-active polymeric electrolytes for which the ionic conductivity could be tuned by oxidation and reduction of the ferrocene units . Although effective chemical oxidation and reducing agents for PFS in organic solvents (such as dichloromethane, THF, and toluene) have been relatively well-developed, identification of good water-soluble oxidizing and reducing agents is not so straightforward.
Water-soluble PFS reducing agents were also reported recently, namely ascorbic acid (vitamin C) and dithiothreitol (DTT) . PFS polycation 4 was used as the test polyelectrolyte species. A FeCl3-oxidized polycation was reduced satisfactorily using ascorbic acid, with an immediate solution colour change back to its original yellow-orange colour. In Fig. 2b, the redox-reversibility was shown as the re-appearance of the ferrocene absorbance at λ = 447 nm. By carefully tuning the pH value of the applied aqueous ascorbic acid solutions, the reduced PFS polyelectrolytes are stable under storage in water and ambient conditions for at least 1 month (Y. Ma, unpublished results).
However, organic solvents based oxidants and reducing agents have also been reported to be able to reversibly change the redox states of water-soluble PFS polyelectrolytes. In the work from the group of Manners , hexane solutions of iodine (as oxidant) and THF solutions of decamethylferrocene (as reducing agent) were used to accomplish multiple redox cycles of the ferrocene units in LBL fabricated PFS-defect colloidal photonic crystals (CPCs) .
LBL electrostatic assembly of poly(ferrocenylsilanes)
Poly(ferrocenylsilane) polyelectrolytes have been applied in the electrostatic LBL self-assembly process to fabricate ultrathin films, both on planar substrates and on colloidal particles. Supported multilayer thin films and free-standing microcapsules, composed of full organometallic as well as organometallic-organic hybrid materials were obtained. The fabrication and properties of these self-assembled superlattice structures are discussed below.
Substrate-Supported Multilayer Thin Films
Multilayer Formation and Characterization
Before the development of fully organometallic multilayers, examples based on organic polymers and organometallic PFS were first shown around the year 2000 [19, 36]. In these examples, cationic PFS polyelectrolytes (3, 5, 6) were used, while for the anionic polyelectrolyte species poly(styrene sulfonate) (PSS) or poly(sodium vinylsulfonate) were chosen. The successful exploration of these hybrid multilayer thin films was soon extended to the fabrication of fully organometallic superlattices solely composed of PFS polyelectrolytes. Multilayers were deposited on (but are not restricted to) quartz slides, silicon wafers, gold substrates, and quartz crystal microbalance substrates [36, 37, 38]. For gold substrates, normally a predeposited thiol monolayer (such as sodium 3-mercapto-1-propanesulfonate, 11-mercaptoundecanoic acid, or 2-aminoethanethiol hydrochloride) was applied to impart negative or positive charges to the surfaces. Polyelectrolytes were subsequently deposited in an alternative fashion onto the charged substrates from aqueous solutions (2–10 mM, based on the molar mass of the repeating unit) with low to medium ionic strength (0–0.5 M NaCl). The procedure can be continued until a desired number of bilayers is achieved, which can range from a few to hundreds. Until now, for all the planar supported organic–organometallic and fully organometallic multilayer systems studied, linear growth profiles were observed. This suggests the formation of well-defined multilayers . Depending on the type of substrates used, different characterization methods were applied to follow the multilayer formation process.
An alternative way to obtain the PFS surface coverage, i.e. the adsorbed amount of material, is by quartz crystal microbalance (QCM) measurements. Multilayers were prepared on special electrodes (often gold or silver) and frequency changes were monitored for each deposited (bi)layer. The frequency decrease upon material absorption has a linear relationship with the adsorbed amount by the Sauerbrey equation\( \Delta F = - 1.832 \times 10^8 M/A \), where A is the surface area of the resonator . In such way, a linear frequency decrease implies a linear growth profile. However, slight non-linearity was often observed in the deposition of early layers of multilayer systems that had overall linear growth behaviour [36, 48].
Thin Film Properties
It has been reported that the surface wettability of sequentially adsorbed organic polyelectrolyte layers is controlled primarily by the outmost surface layer [50, 51]. Thus, it is possible to create surfaces with molecularly tuneable wetting characteristics by simply changing the nature of the outmost adsorbed polyelectrolyte layer. This can be accomplished via the use of surface layers with different chemical structures or by controlling the surface composition of a single bilayer combination . Moreover, contact angle (CA) measurements also serve as an important technique to gain information about the level of interlayer interpenetration present in sequentially adsorbed polyelectrolyte layers .
It is worth mentioning that AFM can also serve as an alternative tool to estimate the thickness of multilayer films. This can be done by making a depression on the film surface using the AFM tip. The depth of the depression after it becomes constant is treated as the film thickness . However, in many cases piled up material was observed along the edges of the depression, making the whole procedure difficult to control.
A further exploration of the electrostatic LBL technique was pioneered by the group of Möhwald in 1998, when fabrication, structure and properties of polyelectrolyte multilayer capsules were first reported . The method involves colloidal-templated consecutive polyelectrolyte adsorption followed by decomposition of the templating core. The most fascinating aspect of these hollow capsule systems lies in the fact that due to the LBL preparation, the as-made stable microcapsules normally display permeability to small molecules but not to macromolecules . The investigation of PFS polyelectrolyte multilayers on planar substrates provides the fundamental information to further fabricate and study microcapsules made of the same material. The responsive nature of PFS opens up new revenues to explore polyelectrolyte microcapsule systems that could specifically respond to redox-stimuli and correspondingly change their permeability to certain molecules.
Application potentials of poly(ferrocenylsilane) multilayer assemblies
Since the LBL electrostatic self-assembly can be applied to construct and design complex polymeric architectures, numerous potential applications can be envisaged. Thin film applications range from coatings, compatibilization and surface protection , enzyme immobilization , biosensors , gas separation membranes , high charge density batteries , modified electrodes , etc. Polyelectrolyte multilayer microcapsules, on the other hand, may be used in areas such as medicine, delivery systems , catalysis, micro-containers and reactors . The unique molecular structures of organometallic PFS polyelectrolytes also provide some special application potentials, which will be discussed in this section.
Patterned multilayers may have potential applications for producing complex optical or electro-optical devices, such as waveguides and display materials [67, 68]. Early attempts for selective multilayer assembly on specific areas involve microcontact printing through the use of self-assembled monolayers (SAMs) as molecular templates [69, 70]. Later, non-lithographical means such as inkjet printing were introduced . The development of new techniques, such as the “lift-off” approach , and the “polymer on polymer” stamping  were followed. Due to the co-existence of Si and Fe atoms in the main chain of poly(ferrocenylsilanes), PFS polymers are redox-active and resistant towards reactive ion etching [8, 74]. Area-selective deposition of PFS polyelectrolytes can serve as an attractive method to obtain two-dimensionally patterned redox-active films, which have potential applications as electrochemically modulated diffraction gratings . Moreover, patterned organometallic thin films may be also of interest as etch barriers in reactive ion etch processes. For this second purpose, it is essential to fabricate fully organometallic multilayers.
Functional Microphotonic Devices
Photonic crystals (PCs) are synthetic materials that have a spatially periodic dielectric constant modulation on the scale of the radiation wavelength . They can suppress, slow or guide the flow of electromagnetic radiation in certain lattice directions so as to have promising application potentials for a variety of optoelectronic devices . Analogous to the doping process of semiconductors, the functionality of PCs can be extended by the introduction of extrinsic defects, which will lead to optical defect states within the photonic band gaps or stop bands . Recently, polyelectrolyte multilayers (PEMs) have been used to build planar defects into PCs, allowing for chemically active defects responsive to solvent vapour pressure, light, and thermal cycling [35, 79]. To broaden potential applications in this direction, redox responsive poly(ferrocenylsilane) polyelectrolyte multilayers were also incorporated as planar defects in self-assembled CPCs . The idea is to achieve precise and reversible tuning of the intergap transmitting state of the CPC, by simple redox cycling.
Redox-Active Smart Delivery Systems
The development of polyelectrolyte nano- and microcapsules envisages increasingly interesting potential applications, in which biomedical delivery systems is one of the aspects that attracts utmost attention. For multilayer capsules, a delivery system involves the encapsulation of materials into the hollow shells. Normally there are two methods to realize encapsulation. One way is by using materials that are aimed to be encapsulated as the templating core material, e.g. directly employing biological materials such as protein aggregates. An alternative approach is to load preformed capsules by switching the permeability through their walls, or polymerization of monomers in the presence of empty shells . Thus, many investigation efforts have been put into the development of microencapsulation technologies with permeation control by changing environmental conditions. A number of responsive PEMs that employ different stimuli have been proposed based on various “smart” polymers during the last few years . The working principle of these systems often relies on external stimuli such as pH , ionic strength [81c, 82], temperature , light , magnetic field  and specific interactions  to regulate intermolecular interactions in the originally charge compensated multilayer structures.
Macroporous materials refer to the structures that have pore sizes greater than 50 nm . High porosity is of great interest since these systems can display many special physical (optical, mechanical) and chemical (reactivity, catalytic activity) properties. Various potential applications could potentially benefit from these structures as well, such as catalysis, separation and scaffolds . However, most of the methods employed until now involve multi-step procedures . Films containing DNA are also of great interest because they could have applications in sensing, diagnostics, electronics, and gene delivery . As an anionic polyelectrolyte, DNA has successfully been incorporated into polyelectrolyte multilayer systems, for the aim of biocompatibility or the use as gene therapy carriers and sensor devices .
The electrostatic LBL self-assembly technique has proven to be an attractive and versatile tool in the fabrication of thin films bearing specific functionalities. Incorporating poly(ferrocenylsilane) polyelectrolytes into multilayers assembled in this fashion, novel organometallic thin films with controlled thickness and molecular architectures were obtained. Applying the same thin film growth principle and colloidal templates, organometallic polyelectrolyte microcapsules featuring controlled molecular permeability were also successfully fabricated.
The unique molecular structures of PFS impart many special properties to these organometallic thin films. The redox-responsive nature of PFS provides plenty of opportunities for exploring application potentials related to the stimuli-responsiveness of PFS thin films and microcapsules. Selective deposition of PFS multilayers onto patterned substrates may be valuable for creating etch barriers in nanofabrication. Future work is expected to further explore these special functionalities and develop new systems that may bear multiple active molecules and can respond to multiple stimuli.
The University of Twente, the MESA+ Institute for Nanotechnology of the University of Twente, the Dutch Science Foundation for Chemical Research NWO-CW and NanoImpuls, a Nanotechnology Program of the Ministry of Economic Affairs of The Netherlands are acknowledged for financial support.