Low-noise Instrumentation for the Measurement of Piezoresistive AFM Cantilever Deflection in Robotic Nanobiocharacterization Applications

Characterization of biological samples with nano-resolution is an extremely growing research area due to the evolution of the technology for developing nanosensors for biological measurements. One of the most important nanobiosensors is the AFM tip used in scanning probe microscopes. The AFM tip interacts with the biological sample and gets topographic, mechanical, electrical and chemical information with extremely high resolution. Advances in piezoelectric actuators allow robots to position with nm resolution. If they are equipped with AFM tips, these nanorobots overcome some limitations of the classical AFM microscopes: nanorobots can cooperate between them and with microscopes to perform more complex nanobiocharacterization experiments. The main technological limitation of AFM-tip equipped robots is the need of use an alternative to the classical laser-photodiode cantilever deflection detection system. Piezoresistive cantilevers seem to be the adequate solution but they are noisier than laser-photodiode and then the robots' force sensitivity is limited. In this work we present the design of low-noise instrumentation for accurate sensing of piezoresistive cantilevers and the test-bench to characterize and compare the resolution in force of the robot equipped with a commercial AFM tip as nanosensor. Results show a noise of 5 nN in the measured force and a resolution of 6 nN in the force applied by the robot, which is good enough for cell studies but it's still high for interacting with biomolecules.