Technology : The sensitive sensor pipes up

时间:2019-02-27 10:02:04166网络整理admin

By Bob Johnstone Victoria HEART attacks and indigestion share so many symptoms that doctors often have trouble telling them apart. But this and many other diagnostic dilemmas could be resolved in minutes with a drop of blood and an exquisitely sensitive testing device devised by Australian scientists. The sensor, developed by Bruce Cornell and colleagues at the Cooperative Research Centre for Molecular Engineering and Technology at the University of Sydney, achieves its sensitivity by using an artificial cell membrane that mimics the body’s natural recognition mechanisms. The scientists created their membrane by chemically tethering a thin layer of organic molecules called lipids to a plastic sheet. Spread out within the lipid layer are thousands of tiny pipes 4 nanometres long and 1 nanometre wide. Brownian movement, the result of bombardment by surrounding molecules, jostles the pipes around. Because of the jostling, some pipes in the lower half of the lipid layer connect with those in the top half, forming channels through the membrane. An applied AC voltage causes a current of ions to flow through the channels formed by the pipes, and this current is picked up by a gold electrode fixed to the sensor’s plastic base. The sensor plugs into a companion device the size of a pocket calculator, which is used to measure any changes in the current. To make the membrane act as a detector, specific receptors such as antibodies or nucleotides are built into the pipes in the upper half of the lipid layer. To make a detector for the symptoms of heart attack, for example, the receptors could be chosen to respond to minuscule fragments of cardiac muscle. When the receptors locate their target, they bind to it, anchoring their host pipe in place so it can no longer form half an ion channel. With a channel switched off, the current flowing across the membrane falls. The drop in current tells the sensor it has found what it is looking for and in what concentration. Cornell says the device will be far more sensitive than existing biosensors that use enzymes to induce a measurable change in pH. Enzyme sensors suffer from a limited range and sensitivity. They can only deal with small molecules and cannot bind very tightly to them. Enzyme-based biosensors cannot detect molecules at lower than micromolar concentrations. In contrast, antibodies can pick up nano- and even picomolar concentrations. “It’s the difference between detecting a sugar cube in a cup of tea, versus a sugar cube in Sydney Harbour,” says Cornell. He aims to have the first commercial versions of the sensor ready by the end of the decade. These will have between a million and a hundred million ion channels. The centre’s long-term plans are for smaller, multielectrode arrays in which each element would be about 10 micrometres across. The sensors would be able to perform a battery of tests simultaneously. They will be made at Australia’s national nano-fabrication facility, which is due to open in Sydney later this year. Cornell says the first application for the biosensor is likely to be to test for growth hormone deficiencies and various forms of cancer. The biosensor is described in this week’s Nature (vol 387,