A low-cost, portable technique quickly and reliably detects specific proteins in a sample of human blood, which could benefit a range of medical sensing applications, including diagnosis of cancer and diabetes long before the onset of clinical symptoms. University of Toledo scientists attached artificially created molecules called aptamers to free-floating proteins in the blood. Aptamers are commercially available, custom-made short strands of nucleic acid that resemble antibodies found in the body because they connect to one type of molecule only. 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. However, aptamers have advantages over antibodies in clinical testing. They can tolerate a wide range of pH and salt concentrations, they have high heat stability, and they are easily synthesized and cost-efficient. Aptamer sensors also can be reversibly denatured, meaning that they can easily release their target molecules, making them perfect receptors for biosensing applications. To demonstrate the applicability of the technique, the scientists chose thrombin – a naturally occurring protein in humans that plays a role in clotting – and thrombin-binding aptamers, which they attached to a sensor surface, in this case a glass slide coated with a nanoscale layer of gold. As they applied the blood sample to the testing surface, the aptamer and corresponding proteins latched together. They then used surface plasmon resonance to determine whether the pairing had been successful. If the protein was present and had bound to the aptamer, conditions for which resonance would occur at the gold layer would have changed. This change can be detected through a simple reflectance technique that is coupled to a linear detector. Dr. Brent Cameron (right) and doctoral student Rui Zheng (left) evaluate a custom-developed functionalized localized surface plasmon resonance microchip used in the detection and measurement of albumin protein via a fiber optic light delivery/detection spectral probe. (Inset) A scanning electron micrograph of the gold-based functionalized nanoparticle array used in localized surface plasmon resonance measurements. Courtesy of Brent Cameron. “The demonstrated surface plasmon resonance sensing modality is well suited for the commercialization of portable handheld devices, and the aptamer-based functionalization technique provides targeted selectivity for specific protein detection,” said Dr. Brent Cameron of the department of bioengineering at the university. “A current process based off systematic evolution of ligands by exponential amplification (SELEX) is used by our group as a convenient method for identifying and optimizing unique aptamers specific to a given target (i.e., type of protein or modified protein). Therefore, the sensing possibilities are endless.” The new technique, which Cameron said could be commercially available in three to five years, was described in the Aug. 31 issue of the Optical Society of America’s open-access journal, Biomedical Optics Express (doi: 10.1364/ BOE.2.002731).