We have investigated human-stress monitoring by making use of salivary nitrate, which can be a candidate for stress markers, with ion-selective field-effect transistors (ISFETs). ISFETs are suitable for on-site single-drop analysis of salivary nitrate within 10 s. However, when ISFETs are used for salivary nitrate, ISFETs have a problem that is called the initial drift. The initial drift makes accurate nitrate monitoring difficult. Thus, the purpose of this study is to prevent the initial drift and to search for a new, simple polymer to possess a better performance of sensor responses than conventional matrix membranes, such as PVC. In this research, we investigated ISFETs using specific matrix membranes, for example KP-13, Pellethane®­­, and P7281-PU. The initial drift was evaluated from the fluctuations of the response values generated by the ISFETs when immersed in saliva or aqueous solution. As a result, P7281-PU showed a prevention effect on the initial drift, both in the whole saliva and in various solutions. Furthermore, the cause of drift may be H+ diffusion, and the drift prevention effect of P7281-PU may be affected by urethane bond capturing H+ in the ion-selective membrane. This result suggests that a continuous nitrate monitoring is feasible and may be applied to wearable sensors.

We systematically investigated the drift of ion-selective field-effect transistors (ISFETs) using a series of polyurethanes as a polymer matrix. Drift is one of the shortcomings of ISFETs that adopt a polymeric ion-selective membrane. The drift is caused by the pH change in the aqueous layer between the ion-selective membrane and the pH-sensitive gate of the ISFET. We previously reported NO 3 − -ISFETs with a novel polyurethane P7281-PU, which give specific stable values. This polyurethane showed less drift than conventional polyvinyl chloride. We considered that the urethane bonds in the ion-selective membrane might capture the diffused H + or OH − ions that induce the drift. In this study, we investigate a polyurethane composed of an ion-selective membrane and consider how the condition and performance of the ion-selective membrane affect the drift. We selected eight types of polyurethane with almost the same composition as P7281-PU for comparison. We confirmed a narrower variation range of 4.5 mV/h from ISFETs with polyurethane P7293-PU than those of 33.7 mV/h from PVC and 5.0 mV/h from P7281-PU through drift tests using artificial saliva. From the tests, we also confirmed that the drift prevention effect might be caused by the ion-trapping sites generated with the hydrolysis of the urethane bonds.

We studied stable nitrate ion-selective field-effect transistors (ISFETs) to detect human salivary nitrate using direct potentiometry. We investigated several biocompatible polymer based nitrate ISFETs as polymer matrix material. The prepared NO 3 --ISFETs showed almost the theoretical Nernst response over 10 -5.5 M to 10 -0.5 M with a response time of less than a few seconds except for Tecoflex ® . As we applied whole human saliva using direct potentiometry, we obtained good relationship with conventional ion chromatography in case of KP-13 based ISFETs. We will also introduce several acute stress subjects for healthy volunteers using wearable heart rate monitor and salivary nitrate by prototype of nitrate FET checkers.