Beatriz Alonso

Redox-active ferrocenyl dendrimers and polymers in solution and immobilised on electrode surfaces

Abstract This article reviews the electrochemical properties of different families of redox-active silicon- and amine-based ferrocenyl polymers and dendrimers prepared in our laboratory over the last few years. The electrochemical behaviour of the ferrocenyl polymers both in solution and immobilised on the electrode depends on the polymer structure. Solution electrochemical studies of silicon-based ferrocenyl polymers showed that all the redox centres were electrochemically independent and that polymers containing pendant ferrocenyl moieties undergo oxidative precipitation, yielding polymer films on electrode surfaces. The application of electrodes modified with a ferrocenyl polymer containing octakis(dimethylsiloxy)-octasilsesquioxane as framework, as mediator in the electrocatalytic oxidation of ascorbic acid is described. The electrochemical behaviour of silicon- and amine-based dendrimers in which ferrocenyl moieties behave as electronically isolated centres, and also of dendritic macromolecules containing interacting ferrocenyl units has been studied. The feasibility of modifying electrode surfaces with these redox-active metallo-dendrimers containing a controlled number of metal centres has been demonstrated. The application of ferrocenyl dendrimers as electron transfer mediators in amperometric biosensors and in the electrochemical recognition of anions, as well as the use of dendritic terminal ferrocenyl groups for inclusion complexation by cyclodextrins has been outlined.

Organometallic dendrimers with transition metals

Abstract Dendrimers are a novel class of macromolecules whose hyperbranched symmetrical architectures and special properties fascinate scientists from a wide range of disciplines. One of the most active current areas of research in the chemistry of dendrimers is the incorporation of metal centres into such structures, thus resulting in well-defined molecules with new advantageous properties. This review summarises our contributions in the developing field of organo-transition metal dendrimers, together with relevant works of expert groups on this field. Throughout the article, the synthetic aspects are especially emphasised. Likewise, the electrochemical behaviour exhibited by new families of organometallic dendrimers and some of their relevant applications are highlighted here.

ORGANOMETALLIC SILICON DENDRIMERS

The first series of organometallic silicon dendritic macromolecules, containing a controlled number of redox-active centres, are synthesized and characterized.

Ferrocenyl silicon-based dendrimers as mediators in amperometric biosensors

Abstract The application of ferrocene-containing dendrimers as mediators in amperometric biosensors is described. The steady-state amperometric response of carbon paste electrodes containing these dendritic relay systems and glucose oxidase is investigated as a function of the glucose concentration and the applied potential. The results show that these sensors respond rapidly to the addition of glucose. The dependence of the glucose sensor response on the size, the number of organometallic redox centers and the nature of the organosilicon dendritic framework is discussed.

Cyclic siloxanes and silsesquioxanes as cores and frameworks for the construction of ferrocenyl dendrimers and polymers

Starting from 1,3,5,7-tetramethylcyclotetrasiloxane as core, novel ferrocenyl silicon-containing dendrimers have been constructed up to the third generation, which contains 16 peripheral ferrocene moieties. As precursors of these ferrocenyl dendritic molecules, Si— Cl- and Si— allyl-terminated cyclotetrasiloxane-based dendrimers were also built step by step, by the repetition of hydrosilylation and alkenylation reactions as growing steps. The dendritic structures have been confirmed by 1 H, 13 C and 29 Si NMR, infrared spectroscopy, elemental analysis, mass spectrometry and vapor-pressure osmometry. Solution electrochemical studies showed that all the ferrocenyl redox centers attached to the dendritic surface behave as independent, electronically isolated units, so that the dendrimers exchange all the electrons simultaneously at the same potential. We also describe the application of the first redox-active ferrocenyl polymer containing an octakis(dimethylsiloxy)octasilsesquioxane framework, as a mediator in the electrocatalytic oxidation of ascorbic acid. Copyright # 1999 John Wiley & Sons, Ltd.

Effective recognition of H2PO4− by a new series of dendrimers functionalized with ferrocenyl-urea termini

A new series of dendrimers with poly(propylene imine) backbones and 4, 8, 16, or 32 peripheral ferrocenyl-urea groups were prepared and characterized; their voltammetric behavior in DMSO solution was very sensitive to the presence of hydrogenphosphate anions at submillimolar concentration levels.

Silicon-based ferrocenyl dendrimers as anion receptors in solution and immobilized onto electrode surfaces

Abstract Electrochemical investigations showed that a new silicon-based ferrocenyl dendrimer receptor, containing Si–NH groups, recognises and senses anionic guest species via significant cathodic perturbations of the oxidation potential of the ferrocene couple. This dendrimer exhibits HSO 4 − and H 2 PO 4 − selective preferences. The presence of Si–NH groups in the dendrimer plays a significant role in the electrochemical recognition of anions. Electrodes modified with this ferrocenyl dendrimer are also sensitive to the presence and concentration of anions.

Functionalization via hydrosilylation of linear and cyclic siloxanes with appendent first generation dendrons containing electronically communicated ferrocenyl units

Abstract The synthesis, structural characterization and redox-properties of a new family of siloxane-based poly(ferrocenyl) dendronized molecules are described. Hydrosilylation reaction of the vinyl-functionalized silicon-bridged biferrocene, CH 2 CHSiMe[Fe(η 5 -C 5 H 4 )(η 5 -C 5 H 5 )] 2 ( 1 ) with SiH-containing poly(siloxane) backbones afforded the novel copolymer 6 and homopolymer 7 . To understand the properties of these polymers more thoroughly, we also synthesized the well-defined, closely related tetraferrocenyl-functionalized disiloxane and octaferrocenyl-functionalized cyclotetrasiloxane model compounds 4 and 5 . Solution voltammetric studies of the siloxane-based poly(ferrocenyl) derivatives 4 – 7 showed two reversible redox processes with a wave separation (Δ E ) of 190–160 mV, indicating that significant electronic communication exists between the ferrocenyl moieties bridged by a pendant silicon atom. The neutral cyclotetrasiloxane-based octaferrocenyl 5 , as well as polymers 6 and 7 become insoluble upon complete oxidation to the corresponding polycationic species, yielding electroactive films onto platinum and glassy-carbon electrode surfaces. The well-defined and persistent redox waves of electrode coated with these films are characteristic of electrochemically stable, surface-confined reversible redox couples. The microstructure of these multimetallic electroactive films has been analyzed by scanning electron microscopy.

Electrodes modified with a siloxane copolymer containing interacting ferrocenes for determination of hydrogen peroxide and glucose

Abstract The electrochemical characterization of a hydrogen peroxide sensor based on a ferrocene-containing polymer electrochemically deposited onto a platinum electrode is described. The redox polymer consists of a siloxane-based copolymer, with pendant electronically communicated ferrocenyl moieties. Amperometric biosensors for glucose were prepared by immobilization of glucose oxidase (Gox) onto these modified electrodes. The effects of substrate concentration, the thickness of the polymer layer, interferences and storage on the response of the sensors were investigated.

Silicon-based organometallic dendritic macromolecules containing {η6-(organosily) arene} chromium tricarbonyl moieties

Abstract Novel organosilicon dendrimers peripherally functionalized with aromatic rings, Si[(CH 2 ) 3 Si(CH 3 ) 2 C 6 H 5 ] 4 ( G1Ph 1 ) and Si[(CH 2 ) 3 Si(CH 2 ) 3 · (CH 2 ) 3 Si(CH 3 ) 2 C 6 5 · 2 ] 4 ( G2Ph 1 ), have been constructed employing a divergent synthetic approach. The π-coordinating ability of these surface located arene rings towards transition metals has allowed the synthesis of a new family of organometallic dendritic macromolecules 1–3 , containing η 6 -coordinated Cr(CO) 3 moieties at the periphery of organosilicon dendritic cores.