Sensing Enhancement of Surface-Based Graphene Nanosensors Using Acoustic Bubbles

There is a high demand for ultrasensitive sensors in many situations, such as biodefense and cancer detection. Current sensors are limited by the slow mass transport process in traditional pressure-driven microfluidic setups. To overcome problems related to diffusion phenomena it is necessary to force movement of target species towards the sensor surface: in a microfluidic environment, this movement can be obtained thanks to the microstreaming flow generated by oscillating microbubbles. The aim of this work is to demonstrate that sensing performance of a nanosensor can be significantly enhanced using the microstreaming flow. The sensor used for the experiments is a graphene sensor, a relatively new device that has a very wide field of applications. Graphene was discovered in 2004 and since then it has received a lot of attention due to its unique chemical, mechanical, electrical and structural properties. In this work the graphene sensor is used to determine the pH of a buffer solution. Values of pH can be determined by analyzing the graphene resistance and thanks to a digital multimeter (DMM), the change in the resistance is taken as a continued measurement. Droplets of fluid under analysis are poured into a particular device placed over the sensor. PMMA plates with micro-drilled cavities are fitted into this device: bubbles are trapped inside these cavities and then activated at their resonance frequency to produce microstreaming flow around the sensing element for sensing performance enhancement. Graphene resistance will reach a saturation value in a certain amount of time, called sensing period: this time interval is expected to be reduced as a result of the additional streaming generated by the oscillating bubbles. The last part of the work consists in the analysis of data, in particular in the evaluation of sensing period reduction. Eventually, experiments are carried out to study the effects of frequency and voltage on bubbles actuation and sensing period reduction.