Fluorographene sensing membrane in a light-addressable potentiometric sensor

A fluorographene sensing membrane on silicon nitride (Si3N4) for light-addressable potentiometric sensors (LAPS) is investigated. Three layers of monolayer graphene grown using low-pressure chemical vapor deposition (LPCVD) were transferred and stacked on a Si3N4 LAPS and then treated with CF4 plasma with a filter shielding. Clear fluorination of the 3-layer graphene is proven by studying the Raman spectrum, X-ray photoelectron spectroscopy (XPS) and contact angle measurement. The pH sensitivity and linearity of this fluorographene sensing membrane are 56.8 mV/pH and 99.4%, respectively. The drift coefficient and hysteresis are 2.6 mV/h and 2.9 mV, respectively. This sensing performance is comparable to a Si3N4 sensing membrane. A significant negative bias shift in the photocurrent versus gate bias characteristics can be observed in the fluorographene sensing membrane, which could be explained by the fact that the work function increases owing to the dipole effect of carbon-fluorine bonds. A two-dimensional (2D) image of photocurrent measured in a pH 7 buffer solution for a sample with fluorographene and a Si3N4 sensing membrane on the same surface is generated by the scanning controlled by an X-Y stage and LAPS measurement system. The high and the low photocurrents of the fluorographene and Si3N4 sensing membrane can be clearly distinguished in the static 2D pH image with an appropriate gate bias voltage. Through these experimental results, fluorographene is proven to be a suitable pHsensing membrane. Further study on surface immobilization is suggested for biomedical targets in the future.