Principal Investigator Tomas Palacios
Ion-sensitive field-effect transistors (ISFET) are used for measuring ion concentration in solution. Typical ISFET is silicon-based and suffers stability problems. Graphene is an atomically thin material with excellent electrical, mechanical, optical, and chemical properties. It can be used to replace silicon for biological and chemical sensing with the potential of being light weighted, flexible, and transparent.
This work develops a sensing platformwith 152 individual ISFETs and an automatic data acquisition system. The array is functionalized with an ion-selective membrane and acts as a calcium sensor with excellent selectivity, sensitivity and response time. In particular, only calcium ion can be transported from the solution phase into the membrane via a charge neutral ionophore. At equilibrium, a stable Nernstian interface potential is achieved. With higher calcium concentration, the interface potential increases causing an effective shift in the sensor I-V characteristic. Hence, the sensor can detect and quantify changes in ionized calcium concentration through the shift in sensors I-V characteristic.
The shift in I-V characteristic is quantified by the location of minimum conduction point in graphene’s V-shaped curve, Dirac point. The theoretical rate of change in potential versus calcium concentration at room temperature is approximately 30mV/decade for bivalent ions such as calcium. Our data shows an average slope of 30.1 mV/decade with a standard deviation of 1.9 mV/decade, which agrees very well with thetheory,therefore,indicatesexcellentsensitivity. By matching data from transient response with data from I-V characteristic, we can calculate the concentration of calcium with a single calibration reference. Sensors are capable of quantifying ionized calcium concentrations spanning over five orders of magnitude. This proof-of-concept work represents a milestone in the development of graphene-based sensors for solution-phase chemical detection of analytes such as ionized calcium.