A polymer-based test for proteins has proved sensitive, selective and much simpler than a traditional assay.

Proteins can act as markers for diseases, so it is important to be able to detect them easily and quickly. They are conventionally detected in clinics using immunoassays called enzyme-linked immunosorbent assays, or ELISAs. Now Jing Wang and Bin Liu from the National University of Singapore have developed a test for proteins that is simpler and faster to use.

Wang and Liu's polymer-aptamer mixture fluoresces if lysozyme is present

Whilst ELISAs are sensitive they are time-consuming. The assays use antibodies on a solid substrate to bind the protein of interest so the test requires numerous surface modification and washing steps as well as detection. Wang and Liu's test uses aptamers instead of antibodies and protein detection occurs in solution rather than at a surface, which makes their assay quicker.

Aptamers are typically oligonucleic acids or peptides and bind a specific target molecule, which can be a protein. Wang and Liu used an aptamer specific for lysozyme - a protein that is widely present in cells. To detect the protein they modified the aptamer molecules with a dye and combined them in solution with a conjugated polymer. 

In Wang and Liu's system, both the polymer and aptamer molecules are negatively charged and repel each other in solution. However, when a positively charged protein specific to the aptamer - lysozyme in this case - is present, the aptamer binds the protein and its surface charge switches to be positive too. It is therefore attracted to the conjugated polymer. Shining light on the solution excites the polymer which transfers energy to the dye on the bound aptamer, causing it to fluoresce.

Wang and Liu found the test's sensitivity was comparable to an ELISA. The test could be used to detect protein selectively in urine, saliva and serum, although the sensitivity does decrease as the sample complexity increases.

Shengqi Wang, an expert on clinical diagnostic techniques from the Beijing Institute of Radiation Medicine, China, says the method 'offers a new way of detecting protein.' Although much work is needed before the method could seriously rival the robustness, sensitivity and repeatability of ELISAs, 'the method is easy to manipulate and saves time,' adds Shengqi Wang.

Wang and Liu are now working to extend the method beyond positively charged proteins. They also have some ideas about how the sensitivity can be improved - for example, by designing more efficient light-harvesting polymers and using more suitable acceptor dyes for better energy transfer. 'We are optimistic that the polymer-based detection strategy could soon be used for testing clinical samples,' they say.

Freya Mearns 

Enjoy this story? Spread the word using the 'tools' menu on the left or add a comment to the Chemistry World blog.