An Introduction to the Ambri ICS™ Biosensor

In general terms, the biosensor converts a biological binding event into a digital electrical signal. This enables the biosensor to employ computer technology to analyse and define this biological event.

By eliminating the visual reading or human interpretation of the result of a biochemical binding event, the biosensor is capable of producing a response which is objective, more accurate and reproducible than alternative technologies.

HOW AMBRI EXTENDED THE 'BIOSENSOR' CONCEPT

Ambri has built a biological switch: a membrane which can detect the presence of specific molecules and signal their presence by triggering an electrical current. This device - the Ambri Ion Channel Switch(ICS™) Biosensor - is a two molecular layer self assembled membrane based on the ion channel gramicidin.

diagram of the AMBRI Biosensor

The diagram above shows the various components of the AMBRI® Biosensor - the molecule to be detected, the antibody fragments, the linker protein steptavidin and biotin linkers, the membrane layers with included gramicidin molecules.

The AMBRI® biosensor operates as a synthetic mimic of a nerve cell membrane.

The key elements in this artificial membrane are:

  • membrane forming molecules chemically tethered to a surface
  • simple ion channels within the membrane which facilitate the transport of ions like sodium
  • an ionically conducting  reservoir space between the electronically conductive gold surface and the membrane to store ions when they have crossed the membrane.
  • receptors such as antibodies attached to the membrane to recognise target molecules.

scematic illustrating the components in the text

The detection mechanism operates by the binding of the target molecule to the antibody fragment,  altering the population of conduction ion channels pairs within the tethered membrane. This results in a change in the membrane conduction. The following cartoon is useful in explaining the mechanism. It has been drawn to scale using single crystal structures of the component proteins.

ferritin crystal structure

IgG crystal structure

Streptavidin crystal structure

TSH crystal structure

Gramacidin crystal structure

Ferritin

450 kD

IgG

150 kD

Streptavidin

60 kD

TSH

23 kD

Gramacidin

2 kD

  

The binding of the analyte (green molecule) to the antibody fragments (red molecules), causes them to cross-link the mobile gramicidin A ion channels to tethered sites on the membrane, preventing the formation of conducting dimers.

 

animation of the effect as described in text

This causes a loss of conduction of ions across the membrane between the outer solution and the inner reservoir space. This is measured as a decrease in current. This is shown on the following three graphs compared to the controls in blue.

scematic shows what drawn object represents ferritin

scematic shows what drawn object represents IgG

scematic shows what drawn object represents Streptavidin

 

 

A competitive assay can  also be fabricated in which the target analyte causes the population of channel dimers to increase.

non conducting monomers

conducting pair of channels

 

The addition of the biotinylated Fab fragment causes the channels to cross-link and the conduction to reduce. The addition of the analyte (such as digoxin) causes the conduction to increase.

Example Responses

graph of conduction over time

The formation of the membrane is in two stages.

The first stage deposits the tethered components of the ICS™ from alcohol solution.   Sulphur containing compounds onto a gold coated surface. This produces the inner leaflet and part of the outer leaflet of the lipid bilayer membrane, all of which is tethered to the gold surface.

Following an alcohol rinse the second stage introduces the non tethered mobile components of the membrane.

illustration of the formation

 

illustration of the effect of water rinsing

Rinsing with water spontaneously forms a molecular bilayer in which the inner lipid leaflet is tethered to the gold and some of which spans the membrane and comprises part of the outer lipid leafet. The remainder of the outer layer leaflet is mobile within the two dimensional plane of the membrane. Streptavidin and antibody fragments are then added in the aqueous solution.