An electro-switchable biosurface is a

biosensor A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physical chemistry, physicochemical detector. The ''sensitive biological element'', e.g. tissue, microorganisms, or ...

that is based on an electrode (often gold) to which a layer of biomolecules (often DNA molecules) has been tethered. An alternating or fixed electrical potential is applied to the electrode which causes changes in the structure and position (movement) of the charged biomolecules. The biosensor is used in science, e.g. biomedical and biophysical research or

drug discovery In the fields of medicine, biotechnology and pharmacology, drug discovery is the process by which new candidate medications are discovered. Historically, drugs were discovered by identifying the active ingredient from traditional remedies or by ...

, to assess interactions between biomolecules and binding kinetics as well as changes in size or conformation of biomolecules.

Technological background

The general principle of a biosurface is a solid surface with an additional layer of biological macromolecules. Because this molecular layer will reversibly respond to changes in the environment of the surface, it is also called “stimuli-responsive monolayer”. The external stimuli can be for example changes in temperature, changes in magnetic fields, mechanical forces or changes in electric fields. Different strategies can be used to attach a monolayer of biomolecules to a surface, for example

atomic layer deposition Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...

or layer-by-layer deposition. Another option is the fabrication of

self-assembled monolayers Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact ...

(SAM). The surface used most often with this strategy is a gold electrode. SAM form by spontaneous organization of the molecules, for example alkanethiolates, on the substrate. SAM can be used as surface layers for nanoparticles, e.g. in MRI contrast agents, they can protect metal films from corrosion, and have many other applications in electrochemistry and nanoscience. For their application as a biosensor, one of the most often used molecules self-assembling on gold electrodes is DNA. Due to its molecular structure, double stranded DNA molecules are negatively charged and rigid. By applying an alternating potential to the biosurface, the attached DNA strands can be moved systematically because they will switch between an upright position and a flat position. This enables the usage of the biosurface as a biosensor.

Applications

The ability to control the electrode potential for electro-switchable biosurfaces facilitates several different applications.Shoseyov, O. & Levy, Ilan. (2008). NanoBioTechnology: BioInspired devices and materials of the future. 10.1007/978-1-59745-218-2. One example is the field of

molecular electronics Molecular electronics is the study and application of molecular building blocks for the fabrication of electronic components. It is an interdisciplinary area that spans physics, chemistry, and materials science. The unifying feature is use of m ...

, for instance the investigation of DNA-mediated charge transfer. Another application is the analysis of molecular interactions. To that end, the DNA strand is labeled with a

fluorescent dye A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with sev ...

. Excited fluorescent dyes can transfer energy to metal. Consequently, the fluorescence is

quenched In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ...

in proximity to the metal electrode. To measure interactions, a

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

is additionally attached at the head of the DNA molecule and the interacting analyte is flushed across the biosensor. Two different measurement modes can be performed with the biosensor, a static mode and a dynamic mode. In static mode, the potential applied to the electrode is fixed, keeping the DNA molecule in an upright position. Binding of the analyte to the ligand will change the local environment of the fluorescent dye and thereby quench its fluorescence. The static mode can also be used to measure the activity of enzymes like

polymerases A polymerase is an enzyme ( EC 2.7.7.6/7/19/48/49) that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA template strand using base ...

that influence the structure of the DNA molecule. In dynamic mode, the potential applied to the electrode is oscillating, thus the DNA molecule switches between the upright and the horizontal position. Binding of an analyte will change the size of the attached complex. Consequently, the hydrodynamic friction will change and the DNA molecule will move through the buffer with a different speed. This speed change can be used to investigate size changes or conformational changes induced by the binding of the analyte. The application of electro-switchable biosurfaces as a sensor for molecular interactions is also known as switchSENSE technology. It belongs to the category of microfluidic surface-bound methods to measure molecular interactions. Electroswitchable surface applications

A similar application in this category is surface plasmon resonance (SPR), where a thin gold film on top of a glass slide is the sensor surface. In SPR, the gold film can additionally be modified with SAM or other specific layers. One difference to electro-switchable biosurfaces is that no potential is applied to the SPR surface.Drozd M, Karoń S, Malinowska E. Recent Advancements in Receptor Layer Engineering for Applications in SPR-Based Immunodiagnostics. Sensors (Basel). 2021;21(11):3781. Published 2021 May 29. doi:10.3390/s21113781 In contrast to surface-bound methods, there are also in-solution methods to measure molecule interactions, for example

isothermal titration calorimetry Isothermal titration calorimetry (ITC) is a physical technique used to determine the thermodynamic parameters of interactions in solution. It is most often used to study the binding of small molecules (such as medicinal compounds) to larger macro ...

(ITC). The electric potential cannot only be used to control the movement of the DNA strands, but also to control the release of the molecules into solution. This has possible applications in the field of

gene therapy Gene therapy is a medical field which focuses on the genetic modification of cells to produce a therapeutic effect or the treatment of disease by repairing or reconstructing defective genetic material. The first attempt at modifying human D ...

since it might enable the delivery of genetic material to specific locations.

References

{{reflist Molecular biology

Technological background

Applications

See also

References