Scientists, from London Centre for Nanotechnology at Imperial College London and the University of Vigo, have created a test to detect particular molecules that indicate the presence of disease, even when these are in very low concentrations. There are already tests available for some diseases that look for such biomarkers using biological sensors or ’biosensors’. However, existing biosensors become less sensitive and predictable at detecting biomarkers when they are in very low concentrations, as occurs when a disease is in its early stages.
In today’s study, the researchers demonstrated that the new biosensor test can find a biomarker associated with prostate cancer, called Prostate Specific Antigen (PSA). However, the team say that the biosensor can be easily reconfigured to test for other diseases or viruses where the related biomarker is known.
a, The mechanism involves capturing the target antigen (red) by means of an antibody linked to plasmonic gold nanostars and a secondary antibody conjugated to the enzyme glucose oxidase (GOx; green). b, GOx oxidizes glucose in the presence of the co-factor flavin adenin dinucleotide (FAD), and produces H2O2, which in turn acts as a reducing agent for the formation of metallic silver from silver ions (these are added to the nanostar solution in a final step, akin to the old-fashioned photographic development process). If the sample contains a low concentration of antigen–GOx complexes, the rate of H2O2 production will be low. Silver will then mainly form as a thin coating on the surface of the gold nanostars, resulting in a large shift of the plasmon resonance wavelength (top right). Conversely, a high concentration of antigen–GOx leads to the production of H2O2 at a rate that is so rapid that silver clusters are formed in solution, resulting in a much smaller shift of the nanostar plasmon band (bottom right). Panel a courtesy of Roberto de la Rica and Molly Stevens.
Professor Molly Stevens, senior author of the study from the Departments of Materials and Bioengineering at Imperial College London, said:
“It is vital to detect diseases at an early stage if we want people to have the best possible outcomes – diseases are usually easier to treat at this stage, and early diagnosis can give us the chance to halt a disease before symptoms worsen. However, for many diseases, using current technology to look for early signs of disease can be like finding the proverbial needle in a haystack. Our new test can actually find that needle. We only looked at the biomarker for one disease in this study, but we’re confident that the test can be adapted to identify many other diseases at an early stage.”
The team demonstrated the effectiveness of their biosensor by testing PSA biomarker samples in solutions containing a complex mixture of blood derived serum proteins. Monitoring the levels of PSA at ultralow concentrations can be crucial in the early diagnosis of the reocurrance of prostate cancer, but classic detection approaches are not sensitive enough to carry out this analysis with a high degree of accuracy. The new test could enable more reliable diagnosis, but more research will need to be done to further explore its potential.
In their study, the team detected PSA at 0.000000000000000001 grams per millilitre, which is at the limits of current biosensor performance. By comparison, an existing test called an Enzyme-Linked Immunosorbent Assay (ELISA) test can detect PSA at 0.000000001 grams per millilitre, which is nine orders of magnitude more concentrated.
The biosensors used in today’s study consist of nanoscopic-sized gold stars floating in a solution containing other blood derived proteins. Attached to the surface of these gold stars are antibodies, which latch onto PSA when they detect it in a sample. A secondary antibody, which has an enzyme called glucose oxidase attached to it, recognises the PSA and creates a distinctive silver crystal coating on the gold stars, which is more apparent when the PSA biomarkers are in low concentrations. This silver coating acts like a signal that PSA is present, and it can be easily detected by scientists using optical microscopes.
The next stage of the research will see the team carrying out further clinical testing to assess the efficacy of the biosensor in detecting a range of different biomarkers associated with conditions such as HIV and other infections. They will also explore ways of commercialising their product.
ABSTRACT – Lowering the limit of detection is key to the design of sensors needed for food safety regulations environmental policies and the diagnosis of severe diseases. However, because conventional transducers generate a signal that is directly proportional to the concentration of the target molecule, ultralow concentrations of the molecule result in variations in the physical properties of the sensor that are tiny, and therefore difficult to detect with confidence. Here we present a signal-generation mechanism that redefines the limit of detection of nanoparticle sensors by inducing a signal that is larger when the target molecule is less concentrated. The key step to achieve this inverse sensitivity is to use an enzyme that controls the rate of nucleation of silver nanocrystals on plasmonic transducers. We demonstrate the outstanding sensitivity and robustness of this approach by detecting the cancer biomarker prostate-specific antigen down to 10^−18 g ml−1 (4 × 10^−20 M) in whole serum
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