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Posted: Jun 30, 2006

Electrochemically programmed release of biomolecules and nanoparticles

(Nanowerk Spotlight) The controlled release of biomolecules or nanoparticles is a problem of general interest for a wide range of applications. Researchers at Johns Hopkins University in Baltimore have demonstrated the programmed release, by applying a small voltage pulse, of biomolecules and nanoparticles chemically tethered to patterned electrode arrays.
"These devices are small, low power, low cost, easily loaded,and can be regenerated" says Professor Peter Searson, of the Department of Materials Science and Engineering at Johns Hopkins University. "They have relatively fast response times, have no moving parts, and are biocompatible."
Searson, together with first author Prashant Mali, a graduate student in his group, recently published their findings in the May 18, 2006 online edition of Nano Letters ("Electrochemically Programmed Release of Biomolecules and Nanoparticles").
Current approaches for drug delivery include diffusion-controlled release of a drug from a host (e.g. polymer) matrix and release from a microfabricated solution reservoir or well. In the diffusion-controlled release technique, after implantation, the drug diffuses out of the host matrix at a rate determined by the properties of the drug and the matrix. In the microfabricated solution reservoir, release is achieved by dissolving the lid.
In contrast, the idea of electrochemically programmed release is based on the fact that molecules that form self-assembled monolayers (e.g. alkanethiols on gold) can be electrochemically desorbed from the surface at sufficiently negative potentials.
"This phenomenon offers a powerful tool for the programmed release of immobilized molecules (e.g. drugs, peptides, proteins, DNA, and viruses) and nanoparticles from an electrode surface by the application of an appropriate voltage" says Searson. "Since the molecules or particles are anchored to a surface in a monolayer, patterning techniques can be used to provide both spatial and temporal control over the release of one or more molecules or particles, and of very small quantities. Furthermore, simple coupling chemistry techniques can be used to attached a wide range of biomolecules or nanoparticles to the surface. Importantly, the electrodes can be regenerated and hence these devices can be used for multiple release cycles."
Schematic illustration of surface functionalization, loading of molecules or nanoparticles, electrochemically programmed release, and electrode regeneration. (Reprinted with permission from American Chemical Society)
The above illustration shows how a molecule or nanoparticle of interest is tethered to the gold electrode using a thiol linkage. The researchers explain how in the first step, a self-assembled monolayer (SAM) of a thiol terminated with a coupling group (e.g., amine) is formed on a gold electrode. A suitable receptor with a variable length spacer can then be attached via the coupling group (e.g., using a terminal succinimide group). Finally, the molecule or nanoparticle is attached to the receptor. The thiol bond is strong but is electrochemically reversible, i.e., thiols can be electrochemically desorbed from the gold surface. Applying a potential negative to the desorption potential releases the bound SAM.
"We anticipate potential applications in drug delivery, lab-on-a-chip devices, protein purification systems, micro-reactors and fundamental research requiring spatially controlled release and the ability to achieve ultralow concentrations" Searson says.
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