“ADDLs are small proteins that we discovered in 1998 in test-tube experiments,” Bill Klein, professor of Alzheimer’s at Northwestern told nanotechweb.org. “Based on their properties, we formulated a new hypothesis for Alzheimer’s disease in which ADDLs were the ‘hidden toxins’ in the disease’s pathology.”

Alzheimer’s disease is the leading cause of dementia in people over the age of 65. Previously, scientists thought that the amyloid-β plaques seen in the brains of people with Alzheimer’s disease were the cause of the symptoms. But it seems that amyloid-β, which is a 42-amino acid peptide, can self-assemble into small soluble oligomers, known as ADDLs, as well as into plaques.

Under experimental conditions ADDLs have caused “neurological dysfunctions relevant to memory”. And researchers have found that ADDL levels were elevated in brain samples from people with Alzheimer’s disease. So a test able to measure ADDL levels in body fluids could become a diagnostic test for the disease.

“Since the early 1900s, Alzheimer's and other pathologies have shown the presence of plaques and tangles, which are huge and the obvious aspects of the pathology,” said Klein. “ADDLs, however, aren’t much bigger than a molecule like insulin. You need the right tools to determine whether they actually exist in brain tissue.”

According to Klein, the scientists have recently partnered with Merck to develop therapeutics that target ADDLs. “Given the interest of Merck (and other big pharmaceutical companies) in ADDLs, it is crucial to develop assays that can measure ADDL levels in patients,” he explained. “The extraordinary sensitivity of the nanotech approach developed by our collaborator Richard Van Duyne and his students is a very important step in this direction. The LSPR approach is at least two orders of magnitude more sensitive than the assays previously developed by our lab, which were at the time the most sensitive anywhere.”

The LSPR nanosensor takes advantage of the unusual optical properties of nanoparticles of noble metals such as silver, gold and copper. The conduction electrons in such nanoparticles are able to oscillate collectively in response to specific wavelengths of light.

The maximum extinction wavelength of the phenomenon depends on the composition, size, shape and interparticle spacing of the nanoparticles, as well as the dielectric properties of their environment - the substrate, solvent and any surface-confined molecules. Researchers can monitor any shift in this wavelength using ultraviolet-visible spectroscopy.

To create the nanosensor, Van Duyne and colleagues used nanosphere lithography to make nanotriangles of silver on a glass substrate, employing a thin (0.4 nm) layer of chromium to promote adhesion. The nanotriangles were about 28 nm tall.

Next, the researchers formed a self-assembled monolayer of 11-mercaptoundecanoic acid/1-octane-thiol on the surface of the nanotriangles. Then they used a coupling agent to covalently link ADDLs to the resulting surface-confined carboxyl groups. Finally, they added anti-ADDL antibodies to the mix.

They measured the maximum extinction wavelength at each stage of the process - the addition of the monolayer, the ADDLs and the anti-ADDL antibodies. Adding the ADDLs and the antibodies caused a red shift in the extinction wavelength, meaning that the device could act as a sensor.

By measuring the wavelength shift after adding different concentrations of anti-ADDL, the researchers also used the technique to make the first measurement of the surface-confined binding constant for the interaction of ADDL and anti-ADDL. They also found that the chromium adhesion layer limited the detection capabilities of the nanosensor. As a result, they are now investigating other adhesion promoters and different substrates.

The researchers reported their work in Nano Letters.