Detection of Metal Ions Using Ion-Sensitive Field Effect Transistor-Based Sensor

Emmanuel Florido^1*, Judiel Mella¹, Edward Julius Dawal¹, Vince Paul Pantino¹

¹Institute of Mathematical Sciences and Physics, University of the Philippines Los Banos, Los Banos, Laguna, Philippines

* Presenter:Emmanuel Florido, email:eaflorido@up.edu.ph

This study was aimed at developing a low-cost but reliable method of detecting and measuring metal ion concentrations in soil and water soil samples using an ion-sensitive field effect transistor (ISFET) as metal sensor. The ISFET used is an N-enhanced metal oxide field effect transistor (MOSFET) consisting of a p-type silicon base implanted with n-type source and drain contacts. The metal gate is an ion sensitive layer. The gate potential depends on metal-ions in the solution which in turn determines the source-drain current I. The source-drain current can then be calibrated with the ion concentration. The ion sensitive layer can be coated with an ion-selective membrane to make the ISFET selective to a particular ion. This phase of the study is focused only on testing the sensitivity of the ISFET and not on its selectivity.
Metal ions of iron (Fe), copper (Cu), palladium (Pd), and Manganese (Mn) of various concentrations were prepared in the range of 150 – 15,000 ppm using 20-ml solution volume for each concentration. An Ag/AgCl electrode in KCl was used as a reference electrode for the ISFET. A signal processing and conditioning circuit was used supplied with 2 VDC. A multi-meter was used to measure the response of the sensor to ion concentration. A logarithmic trend was obtained from the voltage-concentration response of the sensor. The equations obtained y = -0.018 ln(x) + 0.970 for Fe, y = -0.032ln(x) + 0.843 for Cu, y = -0.016ln(x) + 1.001 for Pd, and y = -0.013ln(x) + 1.114 for Mn. The curves obtained were smooth with coefficient of correlation of 0.99 with no outliers except for Mn with a correlation of 0.72. Results showed a high correlation between voltage response and concentration. At x = 100 ppm, the sensitivity of the sensor is 180 µV/ppm for Fe, 320 µV/ppm for Cu, 160 µV/ppm for Pd, and 130 µV/ppm for Mn. At 1000 ppm the sensitivity is 18 µV/ppm for Fe, 32 µV/ppm for Cu, 16 µV/ppm for Pd, and 13 µV/ppm for Mn. At 10000 ppm the sensitivity is 1.8 µV/ppm for Fe, 3.2 µV/ppm for Cu, 1.6 µV/ppm for Pd, and 1.3 µV/ppm for Mn.. The sensor is more sensitive at lower concentrations for all the ions. The sensor has the highest sensitivity for Cu, followed by Fe, then Pd, and lastly for Mn. Each voltage response corresponds to a gate voltage V which in turn has a corresponding characteristic I vs V curve. The triode and saturation regions were clearly demonstrated in the in the I vs V curves.

Keywords: ISFET, metal ions, gate voltage, sensitivity, sensor