“We thought that combining nanotechnology, magnetism and electrochemistry could provide a new approach for information storage,” Itamar Willner told nanotechweb.org. “We demonstrated that the electrochemistry of a redox-active surface-confined quinone monolayer could be switched between two types of electrochemical behaviour. The existence of two distinct states in the absence and presence of the hydrophobic magnetic particles implied an immediate possible application in ‘write’, ‘read’ and ‘erase’ information-processing steps.”
The information-storage device consisted of two gold electrodes functionalized with quinone molecules, an aqueous solution of phosphate buffer, and a layer of the organic solvent toluene containing a suspension of 5 nm diameter nanoparticles of magnetite (Fe3O4). The nanoparticles were capped with a hydrophobic layer of undecanoic acid. The toluene solution floated above the aqueous solution, which was next to the electrodes.
The team used a 12 mm diameter magnet to move the magnetite nanoparticles from the toluene solution to the electrode surface. As the particles moved they brought a small amount of toluene with them, resulting in the electrode becoming coated with a thin non-aqueous film. Transporting the nanoparticles away from the electrode and back into the toluene brought the electrode into contact with the aqueous solution once again.
The quinone monolayer showed different electrochemical behaviour depending on whether it was in an aqueous or non-aqueous environment.
The researchers ‘wrote’ information to the system by applying a voltage to each electrode while the electrodes were surrounded by aqueous solution. They applied a voltage of –0.2 V to one electrode. As a result, the attached quinone molecules remained in an oxidized state. The second electrode received –0.6 V, reducing the quinone to its hydroquinone form.
To ‘read’ this data, the team put the electrodes in a non-aqueous environment by attracting the nanoparticles towards them. The electrode with the oxidized quinone surface had a cyclic voltammogram that showed two redox waves corresponding to one electron reduction and oxidation steps. The electrode with the reduced hydroquinone surface, on the other hand, was in an electrochemically non-active state within the applied potential range. That’s because the hydroquinone state cannot be reduced further and its oxidation in a non-aqueous environment would need a large overpotential.
There were two methods for ‘erasing’ the information. The first involved returning the electrode to an aqueous environment. This enabled the reversible 2e–/2H+ electrochemical process and unlocked the hydroquinone from its electrochemically non-active state. Alternatively, applying a voltage of +0.6 V in a non-aqueous environment to the electrode coated with hydroquinone returned the material to its oxidized quinone form.
The system gave stable and reproducible results, with 50 cycles producing less than a 5% decrease in the peaks of the cyclic voltammograms.
“At present, we use a surface-confined electroactive species, but other approaches, such as the single-electron charging of metallic nanoparticles embedded in a low dielectric environment of the hydrophobic magnetic particles, may be envisaged,” said Willner.
The team says evaluating the practical use of the hydrophobic magnetic particles in information storage will require further basic research. “[We] have demonstrated, however, that the attraction of the hydrophobic magnetic particles to surfaces may eliminate digestive decomposition of biomolecules such as DNA, RNA or proteins,” said Willner. “Thus, in the simplest configuration of the hydrophobic magnetic particles we have a way to stabilize biosensing interfaces.”
The researchers reported their work in Chemical Communications.