Materials Science and Technology of Polymers at the University of Twente

Smart Organometallic polymer platforms for redox sensing and as metal nanoparticle foundry

Xueling Feng thesis cover: Smart Organometallic polymer platforms for redox sensing and as metal nanoparticle foundryThe aim of the work described in this Thesis was to develop novel redox responsive organometallic poly(ferrocenylsilane)s (PFSs) for the decoration of electrode surfaces and the fabrication of redox-active hydrogels. The produced films and gels were explored as redox active platforms for electrochemical sensing and as metal nanoparticle foundry.

PFSs are a fascinating class of processable materials with redox characteristics suitable for the modification of surfaces and electrodes and have significant potential in the electrochemical detection of biological analytes. We fabricated several kinds of PFS grafts on electrodes through different approaches (electrografting, simple grafting to, covalent layer-by-layer assemblies, hydrogel thin films) and investigated the sensing abilities of these redox-active interfaces.

PFSs can be turned into excellent building blocks for forming 3D structures such as hydrogels by introducing crosslinkable moieties to the PFS main chain. The intrinsic redox-active properties of PFSs enable corresponding hydrogels to be used as noble metal nanoparticle foundry for the in-situ fabrication of a variety of metal nanoparticles.

Chapter 2 is giving a broad overview on the role of organometallic polymers as active component in electrochemical sensors.

In Chapter 3, the approaches for fabricating polymer thin films on solid substrates are discussed.

A simple and fast electrografting method for direct immobilization of PFS chains to Au surfaces from ionic liquids is introduced in Chapter 4. Robust, dense, redox active organometallic PFS grafted films were formed within 5 min by direct cathodic reduction of Au substrates. The electrografted polymer films were employed as an electrochemical sensor, exhibiting high sensitivity, stability and reproducibility.

In Chapter 5, a “grafting to” approach was utilized for chemically tethering PFS chains to silicon or gold substrates, employing an amine alkylation reaction. PFS grafts with a thickness of 9 nm were obtained. The electrochemical properties of the grafts were studied thoroughly. Additionally, the adherence between silicon nitride AFM tips and PFS grafts was investigated in situ by pull-off force measurements using electrochemical AFM. The pull-off force was found to depend on the oxidation state of the grafts. Hydrophobic tips showed a stronger adherence with PFS grafts in the unoxidized, hydrophobic state, and displayed lower pull-off forces from oxidized, more hydrophilic PFS graft layers. The PFS grafts served as an electrochemical sensor for ascorbic acid, exhibiting high sensitivity and stability.

Chapter 6, describes the first example of a redox-active multilayer film, fabricated from a main-chain redox-active polymer (PFS) and a redox-inert polymer (PEI) interconnected by covalent bonds. The multilayer films, formed on silicon, ITO, quartz and Au substrates, were fabricated by covalent layer-by-layer assembly showing a well-defined growth process. Owing to the formation of covalent bonds between the layers, these covalently interconnected layers do not disassemble upon oxidation and reduction, in contrast to PFS layers featuring similar backbones held together by electrostatic forces. PFS/PEI multilayers were successfully used in the electrochemical sensing of ascorbic acid and hydrogen peroxide and show improved sensing performance at higher bilayer numbers.

In Chapter 7, a biosensor based on cross-linked PFS and glucose oxidase (GOx) multilayers was demonstrated, in which PFS acted as redox mediator. The synthesis of cationic PFS bearing crosslinkable side groups, layer-by-layer assembly of PFS and GOx, crosslinking of the layer constituents and sensor characteristics are presented. However, reproducibility of the sensor still needs to be improved.

In Chapter 8, the formation of a redox active PFS hydrogel in a single step is introduced. The hydrogel served as a noble metal nanoparticle foundry. Given that the PFS has an electrode redox potential of around +0.4 V, it is possible to reduce certain metal salts into metal nanoparticles through spontaneous reduction without the aid of other reducing agents. Au, Ag, Pt, Pd, Ir, Rh nanoparticles were fabricated by the PFS NP foundry by this clean and facile method.

In Chapter 9, the fabrication of a stimuli-responsive hydrogel thin film based on materials discussed in Chapter 8 was reported. The thin gel film could be detached from the surface and used as a free-standing film. Compared to bulk gels, hydrogel thin films have a fast response to external stimuli and may be more suitable for applications in sensing and catalysis.