A Nanowire Microscope

USING A POTASSIUM NIOBATE (KNbO₃) nanowire as their key component, researchers have created an optical probe that potentially combines fluorescence microscopy and force microscopy (Nature 2007, 447, 1098). The probe—developed by scientists at the University of California, Berkeley, and Lawrence Berkeley National Laboratory—is attractive as a bioimaging tool and could also be used in advanced information technology, cryptography, and signal-processing circuits.

The probe makes use of infrared optical tweezers that perform two functions at the same time: They physically grab onto and manipulate the nanowire so that it can be gently pressed against an object to be imaged. The tweezers also make the nanowire emit light like a tiny fiber-optic filament so that it can excite fluorophores. The probe allows the researchers to image objects at the subwavelength level; that is, it resolves features that are smaller than the wavelength of visible light.

"Before this probe, you could, for example, mechanically manipulate a cell using atomic force microscopy, and you could image the cell using confocal or fluorescence microscopy," explains Jan T. Liphardt, who spearheaded the research with colleague Peidong Yang. "But it was difficult to do them both at the same time."

The new nanowire probe, Liphardt says, can be used to characterize material by fluorescence microscopy. The system is particularly interesting for biological imaging because it requires no electrodes or electrical wiring and the niobate is compatible with physiological environments.

Many problems still need to be resolved, though. For example, the researchers found that as they gently pressed the nanowire against a living cell, the cell began to engulf the nanowire.

"This is truly beautiful work???a tour de force???that represents a major advance in nanowire nanophotonics," comments Harvard University professor Charles M. Lieber. "The creation of tunable subwavelength sources opens up many opportunities and will substantially impact imaging in chemistry and biology, as well as other fields."