Continuous Monitoring of pH and Blood Gases Using Ion-Sensitive and Gas-Sensitive Field Effect Transistors Operating in the Amperometric Mode in Presence of Drift.

Shahriar Jamasb

doi:10.3390/bios9010044

Abstract

Accurate and cost-effective integrated sensor systems for continuous monitoring of pH and blood gases continue to be in high demand. The capacity of ion-selective and Gas-sensitive field effect transistors (FETs) to serve as low-power sensors for accurate continuous monitoring of pH and blood gases is evaluated in the amperometric or current mode of operation. A stand-alone current-mode topology is employed in which a constant bias is applied to the gate with the drain current serving as the measuring signal. Compared with voltage-mode operation (e.g., in the feedback mode in ion-selective FETs), current-mode topologies offer the advantages of small size and low power consumption. However, the ion-selective FET (ISFET) and the Gas-sensitive FET (GasFET) exhibit a similar drift behavior, imposing a serious limitation on the accuracy of these sensors for continuous monitoring applications irrespective of the mode of operation. Given the slow temporal variation associated with the drift characteristics in both devices, a common post-processing technique that involves monitoring the variation of the drain current over short intervals of time can potentially allow extraction of the measuring signal in presence of drift in both sensor types. Furthermore, in the amperometric mode the static sensitivity of a FET-based sensor, given by the product of the FET transconductance and the sensitivity of the device threshold voltage to the measurand concentration, can be increased by adjusting the device design parameters. Increasing the sensitivity, while of interest in its own right, also enhances the accuracy of the proposed method. Rigorous analytical validation of the method is presented for GasFET operation in the amperometric mode. Moreover, the correction algorithm is verified experimentally using a Si3N4-gate ISFET operating in the amperometric mode to monitor pH variations ranging from 3.5 to 10.

Highlights

The health care system will encounter a major challenge in the near future due to the ageing of the population
As indicated, excluding sizeable steps in pH corresponding to changes in pH of 3 units or higher as well as the 9.0→10.0 pH transition, the absolute value of the change in pH as determined based on the proposed method was within 0.2 pH units of that indicated by the pH meter with an average relative error of 5.7%
Considering the similarity between the drift behavior of ion-selective field effect transistor (ISFET) and Gas-sensitive FET (GasFET), a method for correction of instability in these field effect transistors (FETs)-based sensors was proposed. This method was analytically developed for GasFETs and verified experimentally using a Si3 N4 -gate pH-sensitive ISFET operating in the amperometric mode

Summary

Introduction

The health care system will encounter a major challenge in the near future due to the ageing of the population. Biomedical sensors will potentially be able to address this challenge by providing the ability to monitor important body functions as part of preventive medical practices or by serving as an enabling technology for telemedicine to reduce healthcare costs. The pH-sensitive ion-selective field effect transistor (ISFET) and the FET-based oxygen-sensitive gas sensor e.g., with biocompatible hydrogel Nafion, represent important FET-based biomedical sensors. Monitoring of plasma carbon dioxide level can be accomplished by utilizing the change in pH coinciding with the change in the liquid-phase CO2 concentration as a result of formation or dissociation of carbonic acid. The changes in pH corresponding to changes in the partial pressure of CO2 can be measured using an ISFET.

Methods

Results

Conclusion