Cell-like biosensor opens ionic floodgates.

Cell-like biosensor opens ionic floodgates

Tiny things become quite apparent when their presence triggers huge effects. Take he pufferfish poison tetrodotoxin. A relatively few molecules deactivate neurons by plugging membrane pores, or channels which control the cross-membrane travel of many thousands of sodium ions during a neural impulse. In most animals, vanishingly small amounts of tetrodotoxin have the extremely obvious effect of death.

Scientists are learning how to build ion channels into silicon-based biosensors that they hope will announce the presence of minuscule amounts of neuro-transmitters, drugs and workplace or battlefield poisons. "We are trying to build a generic sensor for a range of compounds that have specific physiological effects," says biochemist Frances S. Ligler of the Naval Research Laboratory in Washington, D.C. For instance, the poisons tetrodotoxin, saxitoxin and [mu]-conotoxin differ chemically, but they all block sodium ion channels by binding to the proteins that make up the channels. She estimates that several more years of development lie ahead for a reliable biosensor of this type.

Sodium, calcium and other ion channels pepper cell membranes, which are made of two fragile layers of long lipid molecules that mix well with water on one end but are oily on the other. The oily, or hydrophobic, ends formt the interior of the lipid bilayer by bunching together to avoid contact with the watery environments inside and outside the cells. The hydrophilic heads of the lipids make up the membrane's interior and exterior surfaces. Spanning the two-molecule-thick bilayer are the ion channels, whose openings are regulated by a variety of biologically important molecules. With the new type of biosensor, Ligler and her colleague Thomas L. Fare hope to use the bilayer-bound channels as tiny detectors of channel-binding compounds. Such binding causes large changes in ionic current -- monitored by an underlying silicon electrode--and should give away the presence of the binding molecules.

To make the biosensors, the researchers use a strong acid to etch tiny pores into the surface of a silicon electrode on which they assemble an "asymmetric bilayer" made of a standard, cell-like layer of individual lipid molecules and a layer of polymerized lipids. "This makes the bilayer tougher" than regular cell membrane, Ligler says. During the assembly, the researchers also include any of a variety of ion channels--calcium channels from bovine brain tissue, for instance. The porous surface of the electrode provides spaces below the membrane into, and from which, ions can flow through channels.

Using silicon as the current-detecting electrode, the researchers see no obstacles to building their biosensors with complex electronic circuitry that will amplify tiny signals from changing ionic currents, subtract background noise or even recognize exactly which of many possible molecules is binding to the channel proteins at any one time to cause changes in ionic flow.

"We are building a biosensor that will detect a broad class of compounds as opposed to one that is highly specific," Ligler says. Many biosensor developers attach anitbodies or enzymes -- which bind or recognize only one or a few compounds--to electrodes to make biosensors that exclusively detect, say, glucose or dopamine. "Ion-channel proteins bind a variety of compounds," Ligler stresses. And because biosensors made with the channels do not work by binding only specific chemical structures, they should detect even unknown compounds that may have the same physiological consequences. Ligler is describing the new electrode in Hawaii this week at the Molecular electronics -- Science and Technology Conference.