Conformational changes in the protein caused the nanoparticles to aggregate to a greater or lesser extent, resulting in a colour change.

“What we’ve developed is a simple and inexpensive sensor for determining when a protein changes its conformation,” said Richard Zare of Stanford University. “While we’re not ready to mass-produce this technology, we believe it will eventually be useful for testing other, more complicated proteins.”

Zare and colleagues developed their technique for yeast iso-1-cytochrome c (Cyt c). They covalently attached 19 nm-diameter gold nanoparticles to the protein molecules before adding hydrochloric acid to alter their folding.

As the pH of the solution decreased from 10.1 to 4.0, the protein molecules unfolded. The sample changed colour from red to purple at around pH 6. And by the time the solution had a pH of 4, it was light blue in colour. The scientists measured these colour changes by UV-VIS absorption spectroscopy.

The researchers believe that the unfolding of the proteins enabled the coated gold nanoparticles to clump together. This changed the colour of the solution.

The team also tested the effects of pH changes on a layer of Cyt c bound to a gold film. They monitored changes in the refractive index of the protein layer using a surface plasmon resonance sensor. The refractive index decreased as the pH increased, mimicking the colour change of the nanoparticle solutions. What’s more, these refractive index changes were fully reversible, indicating that the proteins were able to unfold and refold.

The Cyt c-coated nanoparticles did not show such extensive reversibility, on the other hand. Following three pH cycles the solution retained a slight blue tinge at high pH, rather than returning to its original red colour. The scientists believe this is because of the formation of a bonded network of unfolded Cyt c-coated gold nanoparticles.

The researchers reckon the technique may ultimately lead to fast, affordable methods for detecting antibodies and other disease-related proteins. They now plan to repeat their experiments using other protein molecules.

The scientists reported their work in Chemistry and Biology.