Gold Nanosensors to Track Disease

Gold nanoparticles designed to detect proteins within cells, using just laser light, could enable simple and highly sensitive monitoring tools for blood clots and other disorders. Researchers in Scotland have shown that the novel particles can accurately detect thrombin, a biomarker for blood clots, in blood samples. They ultimately envision tests in which the gold nanosensors are injected directly into the patient, enabling measurement of protein concentrations by shining laser light through the skin. In the nearer term, the technology will allow scientists to directly examine how proteins, such as those involved in viral infections, interact within a cell. 

Internal ingots: Gold nanoshells (black spheres) designed to detect specific proteins cluster in the cytoplasm of a mouse fibroblast cell

The sensor consists of a silica core, 120 nanometers in diameter, encapsulated in a thin layer of gold. Mounted on the gold shell are aptamers, short strands of nucleic acids designed to bind to a specific molecular target. When a laser is shone on the aptamer, the molecule absorbs light and reemits it with a characteristic spectrum, called its "Raman signal." When the aptamer binds with a protein, its conformation changes and subsequently changes the emitted spectrum. The sensor's gold surface amplifies the signal by increasing changes in the electric field in response to the laser light. 

"The gold particle works likes a kind of transducer for the laser," says Colin Campbell, a chemist at the University of Edinburgh, in Scotland, who led the research. Scientists are able to determine the levels of the target molecule in solution by measuring the spectral changes. Using this technique, they were able to detect sub-femtomolar (10-12) concentrations of thrombin in human serum. The work was published online March 2 in the journal Chemical Communications. Researchers say the technology could be adapted to detect a number of different proteins, but they are focusing first on thrombin. In the U.K., a country where blood clots cause an estimated 25,000 deaths each year, clinicians assess each patient admitted to the hospital for risk of blood clotting or thrombosis. The current test requires a blood draw and a multistep assay in which a fluorescent antibody binds to the protein. The new technology would provide easy monitoring for high-risk individuals and prevent clot-related deaths, Campbell says. Rather than provide a single measurement of thrombin levels, which can fluctuate, such a test would allow for continuous monitoring and signal when levels of the protein became dangerously high. 

"It is very important to detect proteins without the use of fluorescent labeling," says Luke Lee, director of the Biomolecular Nanotechnology Center at University of California, Berkeley, who was not directly involved in the research. Fluorescent labeling is complex and prone to bleaching or fading, which can throw off the signal.
The research has established the technology's potential to detect low concentrations of proteins in a mere attoliter of blood. While such sensitivity is not critical for identifying a risk of blood clotting, it is important in monitoring for other diseases. Scientists could record measurements in different cellular organelles--separate compartments within the cell--rather than averaged over the whole cell. "It is remarkable accomplishment that they are able to detect the protein at such low concentrations," says Lee.

"You could actually locate in a cell what is happening at a particular time point in a viral infection," says Michael Ochsenk├╝hn a chemist at University of Edinburgh and one of the researchers on the project. Currently the Edinburgh scientists are using the same technology to look at the biomolecular interactions involved in autoimmune disease. They are also investigating host-pathogen interactions for viral research.

Campbell's team had previously shown that the gold nanoshells appear safe when injected into cells--they don't cause cell death or impede new cell growth. As gold is unreactive, the body will not reject the implant, say the researchers. But the technology still has a number of hurdles to overcome before it can be used for medical applications. 

"The limit of such research is, it needs an aptamer that catches a specific protein," says Jaebum Choo an analytical chemist at Hanyang University in Korea, who was not involved in the study. "While the thrombin aptamers are well known, few known aptamers for other proteins are known at this stage. For the development of this technology, biologists and biochemists need to find the various different kinds of aptamer for capturing valuable proteins." 

By Jane Palmer  
From Technology Review


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