Development of a Dual Solid-State pH-AT Sensor

Here we report on our progress toward development of a solid state, reagentless sensor capable of rapid and simultaneous measurement of pH and Total Alkalinity (A_T) using ion sensitive field effect transistor (ISFET) technology. The goal of this work is to provide a means of continuous, direct measurement of the seawater carbon dioxide system through measurement of two “master variables” (pH and A_T). ISFET-based pH sensors that achieve 0.001 precision are presently in widespread use on autonomous oceanographic platforms. Modifications to an ISFET allow a nL-scale acid-base titration of total alkalinity to be carried out in ~10 s. Titrant, H⁺, is generated through the electrolysis of water on the surface of the chip eliminating the requirement of external reagents. Initial characterization has been performed titrating individual components (i.e. OH^-, HCO₃^-, CO₃^2-, PO₄^3-) of seawater A_T. Based on previous work by others in simple acid-base systems and our preliminary results in seawater we feel that it is within reach to set a benchmark goal of ~10 μmol kg^-1 precision in A_T. The estimated resolution of this dual pH-A_T sensor translates to approximately 0.5 and 0.7% error in Total Dissolved Inorganic Carbon (C_T) and pCO₂ respectively and would have a number of immediate applications for investigating biogeochemical processes where strong gradients exist over short distances and in rapidly changing environments.