Sensor detects incredibly small cancer marker

NYU (US) — Scientists have used a special type of biosensor to detect a single cancer marker protein that is one-sixth the size of the smallest virus.

The achievement shatters the previous record, setting a new benchmark for the most sensitive limit of detection, and may significantly advance early disease diagnostics.

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The team at the Polytechnic Institute of New York University (NYU-Poly) set a record recently using the same technique to detect in solution the smallest known RNA virus, MS2, with a mass of 6 attograms.

The researchers have now detected two proteins: a human cancer marker protein called thyroglobulin, with a mass of just 1 attogram, and the bovine form of a common plasma protein, serum albumin, with a far smaller mass of 0.11 attogram.

“An attogram is a millionth of a millionth of a millionth of a gram,” explains Stephen Arnold, a professor of applied physics, “and we believe that our new limit of detection may be smaller than 0.01 attogram.”

This latest milestone builds on a technique pioneered by Arnold and collaborators from NYU-Poly and Fordham University. In 2012, the researchers set the first sizing record by treating a novel biosensor with plasmonic gold nano-receptors, enhancing the electric field of the sensor and allowing even the smallest shifts in resonant frequency to be detected.

Their plan was to design a medical diagnostic device capable of identifying a single virus particle in a point-of-care setting, without the use of special assay preparations.

At the time, the notion of detecting a single protein—phenomenally smaller than a virus—was set forth as the ultimate goal.

“Proteins run the body,” explains Arnold. “When the immune system encounters virus, it pumps out huge quantities of antibody proteins, and all cancers generate protein markers. A test capable of detecting a single protein would be the most sensitive diagnostic test imaginable.”

To the surprise of the researchers, examination of their nanoreceptor under a transmission electron microscope revealed that its gold shell surface was covered with random bumps roughly the size of a protein.

Computer mapping and simulations created by Stephen Holler, once Arnold’s student and now assistant professor of physics at Fordham University, showed that these irregularities generate their own highly reactive local sensitivity field extending out several nanometers, amplifying the capabilities of the sensor far beyond original predictions.

“A virus is far too large to be aided in detection by this field,” Arnold says. “Proteins are just a few nanometers across—exactly the right size to register in this space.”

The implications of single protein detection are significant and may lay the foundation for improved medical therapeutics.

Among other advances, Arnold and his colleagues posit that the ability to follow a signal in real time—to actually witness the detection of a single disease marker protein and track its movement—may yield new understanding of how proteins attach to antibodies.

Arnold named the novel method of label-free detection “whispering gallery-mode biosensing” because light waves in the system reminded him of the way that voices bounce around the whispering gallery under the dome of St. Paul’s Cathedral in London.

A laser sends light through a glass fiber to a detector. When a microsphere is placed against the fiber, certain wavelengths of light detour into the sphere and bounce around inside, creating a dip in the light that the detector receives.

When a molecule like a cancer marker clings to a gold nanoshell attached to the microsphere, the microsphere’s resonant frequency shifts by a measureable amount.

The National Science Foundation supported the research, which is reported in the journal Nano Letters.

Source: NYU