"Exocytosis is the mechanism by which cells release molecules such as neurotransmitters, hormones and various other compounds," Manfred Lindau, an associate professor at Cornell, told nanotechweb.org. "The molecules to be released are produced inside the cell and packed in vesicles surrounded by a membrane. When the cell is stimulated, one or more vesicles fuse with the plasma membrane and release their contents through what we call a fusion pore."
But it is not yet clear exactly what the molecular mechanism behind fusion is. To find out more, cell biologists must detect the changes that occur when a single fusion pore opens. That's where the nanoscale electrochemical detector comes in.
"The opening and expansion of a fusion pore can be detected by measuring the flux of molecules released from the vesicle," added Lindau. "We are using chromaffin cells that release adrenaline or noradrenaline - these hormones can be detected amperometrically. When an electrode is held at a few hundred millivolts, every molecule that hits the electrode surface is oxidized and two electrons are transferred. It is a very sensitive technique."
The team's detector array consisted of four platinum microelectrodes patterned on a glass coverslip. The platinum wires were about 3 µm wide and 150 nm thick. The detector enabled the scientists to resolve the opening of a fusion pore in both space and time.
"When a fusion pore opens, molecules diffuse to the different electrodes, and from the fraction of molecules detected by the individual electrodes we can determine the position of the fusion pore," said Lindau. "The time course of the currents indicates the dynamics of release and thus information about the size of the fusion pore. So we have an electrochemical imaging device."
The detector array provided time resolution of about 1 ms, which Lindau says is much higher than video imaging, while the spatial resolution is "probably at least as good as with fluorescence imaging". As the device is on a microscope coverslip, the scientists can simultaneously observe the cell surface under a fluorescence microscope.
"The main advantage is that we have the electrochemical movie of fusion pore openings and we can use the fluorescence to study other related phenomena," said Lindau. "Such detector arrays will provide a relatively simple to use method to study transmitter release."
Now the scientists are using the arrays in their research on exocytosis and are also developing new array designs that are more suitable for routine use in pharmaceutical companies. "I hope that we can come up with an inexpensive and practical design within a year or two," said Lindau.
The team reported its work in the June issue of Nanotechnology.