This innovation offers a high level of medical diagnostic sensing abilities comparable to more costly clinical instrumentation.
Current blood testing procedures are expensive and time consuming, and the equipment required is often bulky and difficult to transport. A new low-cost, portable technique has been developed to quickly and reliably detect specific proteins in a sample of human blood. The technique, described in the Sept. 1, 2011 issue of the Optical Society’s (OSA) open-access journal, Biomedical Optics Express, could help in a wide range of medical sensing applications, including diagnosing diseases like cancer and diabetes long before clinical symptoms arise.
Human blood contains literally thousands of different proteins. Many are essential for the day-to-day mechanics of life. Others are formed only in response to certain diseases. Knowing which protein is the hallmark of an illness and singling it out of a blood sample leads to earlier diagnosis and more effective treatment. An example of this is the prostate-specific antigen (PSA), which is now routinely tested to help detect prostate cancer and other prostate abnormalities in men.
In this new system, the researchers used artificially created molecules called aptamers to latch on to free-floating proteins in the blood. Aptamers are custom-made and commercially available short strands of nucleic acid. In some ways, they mimic the natural behavior of antibodies found in the body because they connect to one type of molecule, and only one type. Specific aptamers can be used to search for target compounds ranging from small molecules — such as drugs and dyes — to complex biological molecules such as enzymes, peptides, and proteins.
Aptamers, however, have advantages over antibodies in clinical testing. They are able to tolerate a wide range of pH (acid and base environments) and salt concentrations. They have high heat stability, are easily synthesized, and cost-efficient. For their demonstration, the re search ers chose thrombin and thrombin-binding aptamers. Thrombin is a naturally occurring protein in humans that plays a role in clotting.
The researchers affixed the aptamers to a sensor surface, in this case a glass slide coated with a nanoscale layer of gold. As the blood sample is applied to the testing surface, the aptamer and their corresponding proteins latched together.
The next step is to actually determine if the couples pairing was successful. To make this detection, the researchers used a real-time optical sensing technique known as Surface Plasmon Resonance (SPR). A surface plasmon is a “virtual particle,” created by the wave-motion of electrons on the surface of the sensor. If the protein is present and has bound to the aptamer, conditions for which resonance will occur at the gold layer will change. This resonance change is detected through a simple reflectance technique that is coupled to a linear detector.
The ability to monitor these conditions allowed the researchers to quantify the amount of the target protein that was present, even at very low concentrations. The technique is very specific and adaptable for any given application, since unique aptamers for almost any given protein can be identified. This approach also requires less-bulky optics, which is the key to the portability aspect of the design.
Aptamer sensors are also capable of being reversibly denatured, meaning they can easily release their target molecules, which makes them suitable receptors for biosensing applications. This surface plasmon sensor could benefit a number of applications, such as monitoring diabetes, drug research, environmental monitoring, and cancer diagnosis.
For commercial use in medical diagnostics, according to Cameron, the technology is three to five years away, pending FDA procedures and filings.
This technology was done by the University of Toledo, Toledo, OH. For more information, visit http://www.utoledo.edu.