Biosensors that are based on Atomic force microscope (AFM) cantilevers have recently been attracting interest and attention because of its portability and high sensitivity, the gold coated cantilever surface enable researchers to functionalize the cantilever by immobilizing biomolecules like enzyme, DNA, RNA. Reactions between the immobilized molecules and its target molecules on the cantilever can lead to change in measurable signal (e.g. deflection, resonant frequency). By measuring the deflections or resonant frequency change, researchers are able to detect the presence and quantity of target molecules. This property of AFM cantilever allows itself to have potential application in DNA/Protein detection.

We designed an optical system to monitor the deflection of cantilever subjected to electrochemical stimuli in solution. The cantilever was used as the working electrode of a three-electrode cell in order to add an electrical potential onto the functionalized cantilevers for an external stimuli. In my three experiments, Cantilevers with different functionalized surfaces were tested under positive and negative electrical potentials. Several conclusions were obtained based on the measured cantilever deflection. First, immobilized thrombin aptamer can result in a compressive surface stress (compare to gold cantilever coated with alkanethiol) on gold coated cantilever when negative voltage was applied, and a tensile surface stress when positive voltage was applied. Secondly, different molecules immobilized on gold coated cantilever can lead to different deflections under square wave of -100mV and +100mV voltage. The conformational change of thrombin aptamer caused extra compressive surface stress under -100mV and extra tensile surface stress under +100mV. Also the thrombin molecules that bind with aptamers may inhibit the conformational change of aptamer under electrical potential. Because the adenine base pair on poly A does not form a structure like the G-quadruplex form of thrombin aptamer molecules, the poly A immobilized cantilever does not have deflection change as large as thrombin aptamer cantilever under electrical potential. Thirdly, at concentrations lower than 0.1nM, the deflection of aptamer functionalized cantilever under electrical field decays rapidly as the concentration increases. This phenomenon may have potential application in detecting extremely low thrombin concentration.