2
Magneto-switchable Electrodes and Electrochemical Systems

2.1 Introduction

This chapter overviews various electrochemical systems with switchable features that can be controlled by external magnetic fields applied to magnetic micro/nanospecies. The use of magnetic micro/nanospecies (e.g., nanoparticles, nanowires, nanosheets) [1-6] has been particularly important for the development of novel magneto-switchable electrodes with unique and unusual properties [7-13]. Applying a magnetic field to magnetic species with redox, electrocatalytic, or bioelectrocatalytic properties and moving them around allowed their different arrangements on electrode surfaces, thus changing their electrochemical responses, switching them ON and OFF resulting in many interesting features, Figure 2.1. While the exact description of the exemplified systems can be found in original research papers, this richly illustrated chapter aims at providing conceptual explanations to summarize the up to date developments in this field. The Appendix organized at the end of the chapter addresses those readers specifically interested in synthetic procedures used for preparation and characterization of nanosized magnetic materials.

Figure 2.1 Magneto-controlled switchable bioelectrocatalytic process - a general concept.

2.2 Lateral Translocation of Magnetic Micro/nanospecies on Electrodes and Electrode Arrays

One of the earliest examples of the magneto-switchable bioelectrochemical systems was based on lateral translocation of deoxyribonucleic acid ()-functionalized magnetic microspheres (super-paramagnetic polystyrene beads, ca. 1?µm, with included Fe3O4 nanoparticles) along an electrode array composed of two conducting areas ("left" and "right" electrodes), both with the applied potential of oxidizing DNA molecules [14], Figure 2.2. The magnetic microspheres chemically modified with DNA oligomers (GGC CGA CTC ACT GCG CGT CTT CTG TCC CGC CTT TTT CG (1); note that the used oligomer is rich with guanine bases) demonstrated chronopotentiometric responses corresponding to the irreversible oxidation of guanine nucleobases in the DNA molecules. When the DNA-magnetic species were collected at the "left" electrode by positioning an external magnet below that electrode, they demonstrated the electrochemical response on the "left" electrode leaving the "right" electrode mute and not responsive to the applied oxidative potential. Repositioning the external magnet to the "right" electrode resulted in the switch of the electrochemical response to that electrode due to translocation of the DNA-functionalized magnetic species following the magnet. The process of switching the positioning of the DNA molecules and their electrochemical responses between the "left" and "right" electrodes was repeatedly cycled and reversible, thus demonstrating effective translocation of the DNA-functionalized magnetic species following the changes in the magnet position. The switchable magneto-controlled DNA-based electrochemical system was suggested by the authors [14] as a platform "for stimulating charge transfer through DNA, and for other genoelectronic applications." While the concept illustrated by this system is indeed very interesting, the drawback of the system is the electrochemically irreversible and chemically destructive oxidation of guanine bases in the DNA oligomer.

Figure 2.2 Reversible switching of the DNA oxidation upon magneto-induced lateral translocation of the DNA-functionalized Fe3O4 magnetic particles. Chronopotentiometric responses of the "left" and "right" electrodes are shown in the presence and absence of the particles. Amount of magnetic particles, 100?µg; DNA-oligomer (1) structure is shown (note that the used oligonucleotide is rich with oxidizable guanine bases); pre-treatment potential, +1.7?V for 10?s; stripping current, +5?µA (between 0.6 and 1.2?V vs Ag/AgCl reference electrode).

Source: Wang and Kawde 2002 [14]. A fragment of this figure is adapted with permission from Elsevier.

Another magneto-switchable system based on the lateral translocation of redox-functionalized magnetic species included Fe3O4 microparticles (ca. 1?µm average diameter) modified with quinone molecules [15]. The advantage of this system, particularly when compared with the DNA oxidation described above, is the reversible electrochemical process of the used quinone redox species. Two different quinone molecules were used to study the magneto-switchable properties of the system. In the first example [15], biologically important pyrroloquinoline quinone () covalently bound to the magnetic particles through an amino-silane shell was reversibly translocated between the "left" and "right" electrodes by moving an external magnet similarly to the system described above. The electrochemically reversible cyclic voltammogram responses were obtained at the "left" and "right" electrodes depending on the magnet position. Since the PQQ molecules are well known catalytic species for ß-nicotinamide adenine dinucleotide () oxidation[16], the primary electrochemical response of PQQ was extended to the electrocatalytic NADH oxidation observed on the "left" and "right" electrodes by moving the external magnet to the corresponding positions below these electrodes [15]. The present example is particularly important because it demonstrated the cascading electrocatalytic reaction that was controlled by the positioning of the external magnet. In the second example [15], amino-naphthoquinone (2) was covalently attached to the magnetic microparticles, Figure 2.3A. Electrochemical reduction of the immobilized naphthoquinone in the presence of oxygen resulted in the electrocatalytic O2 reduction and formation of H2O2, which was coupled with the biocatalytic oxidation of 3,3´,5,5´-tetramethylbenzidine (3) to give the insoluble product (4), Figure 2.3B. This process was biocatalyzed by horseradish peroxidase enzyme (, E.C. 1.11.1.7), which resulted in deposition of the insoluble product (4) on the electrode surface. Importantly, the electro-biocatalytically produced precipitate was deposited at the location of the magnetic particles controlled by the magnet positioning, thus resulting in the magneto-controlled patterning of the electrode surface, Figure 2.3C.

Figure 2.3 (A) Modification of magnetic Fe3O4 microparticles (ca. 1?µm diameter) with naphthoquinone (2). (B) Magneto-controlled patterning of a Au electrode surface upon formation of an insoluble product (4) of a biocatalytic reaction triggered by the electrocatalytic formation of H2O2 in the presence of naphthoquinone (2)-functionalized magnetic particles. (C) Pattern produced on the Au electrode by the electrocatalytic process using the naphthoquinone (2)-functionalized magnetic particles, 10?mg, HRP, 1?mg?mL-1, and substrate (3), 3?×?10-4 M. Background electrolyte: 0.1?M tris-buffer, pH?7.5, was saturated with air. The potential, -0.5?V (vs SCE), was applied for 3?min on the electrode to produce the first spot, then the potential was switched off, the magnetic particles were moved by the external magnet, and the potential -0.5?V was reapplied for 3?min to produce the second spot.

Source: Katz and Willner 2002 [15]. The figure is adapted with permission from Elsevier.

A similar process, but using luminol (5) as an oxidizable substrate for HRP, resulted in biocatalytically induced luminescence controlled by the magnet position [17]. In this study, the amino-naphthoquinone (2)-modified magnetic microparticles (ca. 1?µm average diameter) were reversibly moved between conducting and nonconducting domains by moving the external magnet, Figure 2.4A. When the particles were concentrated on the conducting Au electrode, the naphthoquinone molecules were electrochemically reduced, subsequently producing H2O2 in the electrocatalytic oxygen reduction process. The generated H2O2 activated HRP-biocatalyzed oxidation of luminol to the light-emitting excited state of 3-aminophthalate (6), resulting in the luminescence process. When the magnetic particles were relocated to the nonconducting glass domain, the electrocatalytic process was stopped and the luminescence was not observed. The electrocatalytic oxygen reduction in the presence of the quinone-functionalized magnetic particles on the Au electrode surface was observed by cyclic voltammetry, Figure 2.4B, curve d. The non-catalyzed oxygen reduction with a smaller cathodic current was obtained when the magnetic particles were relocated to the nonconducting glass domain, Figure 2.4B, curve c. The light emission resulting from the bioelectrocatalytic process followed the current transients observed by chronoamperometry, Figure...