ISFET pH Sensitivity: Counter-Ions Play a Key Role

Abstract

The Field Effect sensors are broadly used for detecting various target analytes in chemical and biological solutions. We report the conditions under which the pH sensitivity of an Ion Sensitive Field Effect transistor (ISFET) sensor can be significantly enhanced. Our theory and simulations show that by using pH buffer solutions containing counter-ions that are beyond a specific size, the sensor shows significantly higher sensitivity which can exceed the Nernst limit. We validate the theory by measuring the pH response of an extended gate ISFET pH sensor. The consistency and reproducibility of the measurement results have been recorded in hysteresis free and stable operations. Different conditions have been tested to confirm the accuracy and validity of our experiment results such as using different solutions, various oxide dielectrics as the sensing layer and off-the-shelf versus IC fabricated transistors as the basis of the ISFET sensor.

Flexible potentiometric pH sensors for wearable systems

“…For these reasons the development of solid-state pH sensors on various exible substrates using sensitive materials ranging from metal oxides (MO x ) and semiconductors to polymers and carbonbased materials is a growing area of research in electrochemical sensing technologies. 8 Electrochemical pH sensors of this kind can generally be categorized as potentiometric, 36,37 conductimetric/chemi-resistors [38][39][40][41] or ion sensitive eld effect transistors (ISFET) [42][43][44] etc. The potentiometric pH sensors are discussed here in detail (Section 3 given design) as they offer stable performance, high sensitivity, less interference, easiness in wireless system, low power, long lifetime and fast response as compared to other type of pH sensors.…”

Ultrafast Large-Scale Chemical Sensing With CMOS ISFETs: A Level-Crossing Time-Domain Approach

“…where α is the coefficient for site binding model and β is the reduction coefficient for counter-ion effect [13], [15]. Compared to non-CMOS based ISFETs or modified CMOS ISFETs, ISFETs in unmodified CMOS boast a lower cost, simplified integration [13], and the ability to implement additional floating gate nodes -this enables the sensor to be decoupled from the electronic biasing [8], and provides extra control path [11] for further compensation.…”

“…It is arguable that noise and linearity performance of readout circuits will be dominant in the sensing system, when chemical noise and drift are much higher than electronic noise [8]. Moreover, chemical systems already introduce large non-linearity due to double layer, site-bonding models, and crowd ion effect [15]. However, for large-scale systems with high sampling rate, the power consumption of the total sensing arrays can easily exceed mW [17], where a significant resource is required for real time non-linearity correction even when the pH change is small.…”

Lysine-based amino-functionalized lipids for gene transfection: the protonation state in monolayers at the air–liquid interface

“…Further equations, taking the influence of the counter ion and the surface potential into account, can be found elsewhere. 44,45 The pK a depends strongly on the properties of the chemical structure such as charge, the choice of atoms (their electronegativity values), their ability to be stabilized through resonance, inductive effects to stabilize conjugated bases and their orbital hybridization (increasing acidity with increasing s character sp 3 , sp 2 and sp for hydrocarbons). For the transfection lipids, the pK a increases slightly with increasing chain fluidity (decreasing packing density).…”