Graphene Based Ion Sensitive Field Effect Transistor for K+ Ion Sensing

Graphene, discovered in 2004, has drawn great interest in the wide range of applications due to its distinctive 2-dimensional material property. Sensing is the one application has been developed extensively owing to graphene’s ultra-high mobility, low electric noise, very high surface-to-volume ratio, and easy modulation of electrical characteristics. With these excellent properties, the present research explored graphene’s ion sensing potential with the goal of developing graphene-based K+ ion detector which can be applicable in implantable bio-sensors. As the major intracellular cation, potassium has very critical functions in various biological processes. Additionally, recent research found out that elevations in biological levels of K+ ions precede the onset of sudden cardiac death, epileptic seizures, and other clinical problems. Therefore, the development of implantable K+-sensitive sensor devices could be of great use in predicting the onset of those time-sensitive condition breakouts in medical applications. To synthesize high quality graphene to investigate graphene’s property and explore its ion sensing capability, a CVD system was built in the lab along with an automatic control program that can both control and monitor the synthesis process. Characterization was carried out with various tools (AFM, Raman spectroscopy, Hall measurement and etc.) and the results confirmed that the obtained graphene is high quality mono-layer graphene. In order to have a deeper insight into graphene’s sensing mechanism, graphene’s interactions with typical donor/ acceptor gas molecules and strong donor molecules were investigated by observing its three important electrical properties – carrier mobility, carrier density, and sheet conductivity change upon molecular adsorption. From the investigation, an empirical model was proposed to explain an interesting trend of graphene’s transport property changes during its molecular interactions. On the other hand, an advanced surface functionalization method was developed to greatly enhance graphene’s molecular sensing performance. A light oxygen plasma treatment can boost graphene’s electrical response to NH3 molecules both in response magnitude and response time. A systematic analysis was carried out and found an optimum power of oxygen plasma can induce a surface functionalization caused by graphene crystal grain size nano crystallization and oxygen species p-doping effect. Graphene-based ion sensitive field effect transistors (GISFETs) with high sensitivity and selectivity for K+ ion detection was demonstrated utilizing valinomycin based ion selective membrane. The performance of the GISFETs was studied in various media over a concentration range of 1 M – 2 mM. The sensitivity of the sensor was found to be \u3e60 mV/decade, which is comparable to the best Si-based commercial ISFETs, with negligible interference found from Na+ and Ca2+ ions in high concentration. The performance of the sensor also remained unchanged when fabricated on a flexible and biocompatible PET substrate. The sensor performance did not change significantly in Tris–HCl solution or with repeated testing over a period of two months highlighting its reliability and effectiveness for physiological monitoring. From Real-cell based extracellular K+ ion assay, the GISFET sensor demonstrated good detecting performance confirmed the possibility of using in implantable bio sensing application. Additionally, GISFET sensor arrays fabricated using CMOS compatible technique also showed the same sensing performance which proves mass production capability in the future. At the same time, graphene based ion sensitive diode (G-ISDiode) was also developed using graphene/silicon Schottky junction. Because of the exponential relation between diode current and graphene’s Fermi level, the G-ISDiode sensor demonstrated an exponential sensing performance in ionic detection. This is different from G-ISFET’s linear response to ion concentration, which can be a big advantage in certain applications where demand ultra-high sensitivity