Nanogap based label-free impedimetric biosensors
Okyay, Ali Kemal
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Despite lots of research going on to find a hope, cancer is still a major cause of death in today‘s world. It has been reported that cancer has some biomarkers in human body and detecting these biomarkers timely can pave the way for early detection and successful treatments. Point-of-care biosensors are highly promising for this mission. If these biosensors can achieve sensitivity and reliability with a low-cost and simple platform, they can address a large mass of people who are at the early stages of cancer without any clear symptoms yet. For this purpose, various biosensing mechanisms can be used to convert the signal coming from the recognition elements on the biosensor surface to the digital domain for signal processing. One of these mechanisms, impedimetric (impedance based) sensing is a very appealing electrical biosensing method since this method can offer label-free, low-cost, low-power requirement, miniaturizable, and chip-integrable detection platforms. However, impedimetric sensing in liquid medium is problematic, since during the electrical measurements, ion-based undesired layers (electrical double layers) are formed over the electrodes in the target liquid. Unfortunately, these layers act like a shield against the applied electric field to the liquid and can prevent the detection of the target biomarkers. In this thesis, a nanogap based label-free biosensor structure is designed and using this design impedimetric sensing in liquid medium is demonstrated at low frequencies (1 kHz – 100 kHz). Low frequency platforms are quite amenable to low-cost applications like point-of-care biosensing. The designed structure utilizes nanometer scale electrode separation (nanogap). Theoretical calculations show that nanogap reduces the undesired effect of electrical double layer. Moreover, nanogap also helps in minimizing the volume of the required liquid for the measurement. Design, fabrication, surface functionalization and biotinylation stages of the biosensor are realized in a cleanroom environment and biomimetic materials laboratory. The fabricated biosensor is tested by introducing the target molecules (streptavidin) in a phosphate-buffered saline solution. A parameter analyzer with a capacitance-voltage unit and a probe station are used for the impedance measurements. With these biosensors, label-free detection of streptavidin is observed for 100 µg/mL, 10 µg/mL, 1 µg/mL, 100 ng/mL and 10 ng/mL concentrations. This is, to the best of our knowledge, the first demonstration of streptavidin detection in nanogap based label-free impedimetric biosensors. The above-mentioned concentrations show that these biosensors are promising for commercial applications. Sensitivity to the dielectric constant of the target medium is measured to be 132 pF per unit change in the dielectric constant at 10 kHz measurement frequency. Reliability tests are performed: stable and repeatable operation of the sensors are checked and verified. In conclusion, this proof-of-concept study shows that nanogap based biosensors would be a suitable and appealing choice for sensitive, reliable, simple, low-power and low-cost point-of care biosensing applications. Next step would be utilizing the platform presented in this work in detecting specific cancer biomarkers like PSA or CA125. Thereby, developed further and commercialized, nanogap based label-free impedimetric biosensors can act in the battle of human being against cancer in the future.
Electrical double layer