Proteins take the plunge on microscopic cantilever

Scientists at the University of California have reported a technique for detecting proteins by inducing them to stick to and bend a microscopic cantilever.

The technique is said to be sensitive enough to serve as a diagnostic assay for the protein markers characteristic of prostate cancer. These protein markers, called PSA (prostate-specific antigen), are found at elevated levels in the blood of men with prostate cancer.

‘The technique is sensitive enough to detect levels 20 times lower than the clinically relevant threshold,’ said Arun Majumdar, professor of mechanical engineering at the University of California, Berkeley. ‘This is currently as good as and potentially better than the ELISA (enzyme-linked immunosorbent) assay, which is the standard today for detecting protein markers like PSA.’

The microcantilever technique has far broader applications, however. Any disease characterised by protein or DNA markers in blood or urine could conceivably be assayed by arrays of these microcantilevers.

A microcantilever array would be one of the first ‘protein chips,’ analogous to the DNA chip used broadly today in research labs and the biotechnology industry to conduct hundreds of DNA analyses simultaneously.

UC Berkeley graduate student Guanghua Wu fabricated the cantilevers from silicon nitride using techniques identical to those employed by the semiconductor industry to make microprocessors.

He worked closely with his Oak Ridge National Laboratory colleagues, who perfected a way to coat the top surfaces of the cantilevers with antibodies.

When proteins bind to these antibodies, they elbow one another apart and force the lever to bend downward. The cantilevers can also be coated with single-strand DNA for binding to complementary DNA.

The higher the concentration of the protein or DNA being measured, the greater the deflection of the cantilever, so that the chips not only detect the presence of the protein, but also its amount. Majumdar and his colleagues measured the deflection with a laser.

The cantilevers themselves are about 50 microns wide, 200 microns long and half a micron thick. When molecules bind to the surface, the cantilever moves only about 10-20 nanometers – the diameter of 100-200 hydrogen atoms. Lasers can detect a deflection as small as a fraction of a nanometer, however.

‘The primary advantage of the microcantilever method originates from its sensitivity, based on the ability to detect cantilever motion with sub-nanometer precision, as well as the ease with which it may be fabricated into a multi-element sensor array,’ said Wu. ‘No other sensor technology offers such versatility.’

Majumdar and his UC Berkeley colleagues have found a way to put several hundred cantilevers onto a single silicon chip, and have developed a way to measure the deflection of all simultaneously with a single low-power laser or light emitting diode.

‘It’s not trivial to go from one cantilever to hundreds of them on a chip, a millimetre apart, detecting hundreds of different biomolecules,’ Majumdar said. ‘But that’s what we need to do low-cost, high-throughput, label-free assays.’