Published online 23 March 2010 | Nature | doi:10.1038/news.2010.143

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Nanoparticle kit could diagnose disease early

Colour change shows the presence of minuscule amounts of key enzymes.

Nanoparticle clusters for detecting diseaseEnzymes snip apart the links between nanoparticles, prompting a colour change.Laromaine, A. et al.

A detection kit that uses nanoparticles to seek out tiny amounts of disease-related enzymes could offer sensitive and fast diagnoses of cancer, HIV and other diseases.

The diagnostic test has been developed and refined by Molly Stevens, a biomedical materials scientist at Imperial College London, and her colleagues. Stevens presented recent work on the test at the American Chemical Society meeting in San Francisco, California, on 21 March.

In earlier experiments, Stevens made a test for enzymes that would warn of the recurrence of cancer in men that have previously had a diseased prostate removed. To do this, she and her team took gold particles, 10 nanometres wide, and stuck short peptide chains to their surface. These peptides help link the gold nanoparticles together because they are attached at the other end to aromatic chemical groups called Fmoc that stack on top of one another and stick together.

When the nanoparticles are linked together in this way, they form a blue solution. But when the solution is exposed to nACT-PSA, an enzyme related to prostate cancer, it turns red1.

This colour change happens because the nanoparticles disperse after the enzyme — a protease — munches its way through the peptides in the linking groups. This frees the nanoparticles from the peptide links and means they speedily scatter. "Once you cleave the peptide, at the end of the peptide you'll be left with a positive charge," says Stevens, and these positive charges make the particles repel each other.

Detecting recurrence

The colour changes within minutes, and can happen with just zeptograms (10-21 grams) of enzyme per millilitre — although these concentrations take longer to bring about the colour change. The test is more sensitive than anything previously reported, says Stevens, and can almost detect a single enzyme molecule.

“We're actually using the protein itself as an inherent amplification step.”


"Normally when you detect a biomarker related to a disease, you're detecting the presence of that protein," says Stevens. "But we're actually using the protein itself as an inherent amplification step, because each enzyme will cut through many molecules time and time again so you're getting an amplification of the signal." At the moment the tests are run on purified proteins, but in future a small drop of blood would be enough.

The sensitivity of the test will be especially useful in detecting the recurrence of prostate cancer, Stevens says, because recurring cancer initially only produces tiny amounts of nACT-PSA, often evading current detection methods. "We can go down to zeptograms per millilitre. That should detect it really early," she says.

The marker that Stevens is looking for with her test — PSA or prostate specific antigen — has come under fire for being an unreliable indicator for the presence of prostate cancer (see 'Markers of dispute').

But Stevens says that as her test focuses patients that have had their prostates removed altogether, any hint of PSA is bad news as it signals their cancer may be recurring.

Speed test

The test is a proof of principle for this method of disease diagnosis. The peptide linking groups can be changed so that they include amino acids that other enzymes can sever. So far the team has made peptides specific for HIV protease and hopes to use the kit to detect HIV infection early.

She is now working with clinicians to investigate detecting markers for oral cancer in saliva, and she also wants to try and use the test to screen drug candidates that work as inhibitors of protease enzymes, or for testing for enzymes in river water that are indicative of certain bacteria.

The sensitivity of the system is remarkable says Vincent Rotello, a chemist from the University of Massachusetts at Amhurst. "If I were looking at it I wouldn't have expected it to work as well as it does," says Rotello. "I would have expected it to be much slower."

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Stevens has also exploited a technique called surface-enhanced Raman scattering (SERS) by using slightly larger nanoparticles for the tests. SERS is an amplification of a spectroscopy signal that comes about when metal surfaces are close to each other. In this case, the area where the 40-nanometre gold particles touch produces a large signal, but once the enzyme snips them apart. the signal's intensity is reduced2. That can be detected by SERS, allowing quantitative information about the disease enzymes to be gleaned, says Rotello.

Stevens admits that there are plenty of other ways to detect proteins, including the enzyme-linked immunosorbent assay, or ELISA, technique, and a 'bio-barcode' being developed by chemist Chad Mirkin at Northwestern University in Evanston, Illinois. But these systems are much more complicated and less portable than her test, she says. "The difference with our technology is the simplicity and also the fact that we are measuring enzyme activity as opposed to just enzyme presence," she says. 

  • References

    1. Laromaine, A., Koh, L., Murugesan, M., Ulijn, R. V. & Stevens, M. M. J. Am. Chem. Soc. 129, 4156-4157 (2007). | Article | ChemPort |
    2. Maher, R. C. et al. J. Phys. Chem. C advance online publication doi:10.1021/jp905493u (2010).

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  • #61497

    I really like the way this article was written. (Seemingly) very accurate and to the point.

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