Compared to healthy cells, brain tumors are characterized by a relatively more acidic extracellular fluid compared to healthy cells. Therefore, sensing and monitoring pH can convey critical information about brain tumors. Building on our Miniaturized Neural Drug delivery System (MiNDS), we envisioned a way to incorporate pH sensing ability to the device to detect tumor cells. However, it was not straightforward. Although novel materials and systems for pH sensing have been intensively researched, a proper characterization and assessment protocol is still lacking. This results in an inability to objectively compare various published reports, attributing inconsistent results, and infeasibility of replicating or reproducing the reported results. Hence, we wanted to first experimentally demonstrate the importance on reporting of those vital parameters such as pH sensing repeatability, stability, and inherent properties of the pH system components.

With advancements in microfabrication and miniaturization, researchers have directed great efforts toward exploring new materials and configurations for pH sensing systems and their various applications. However, less attention has been given to the critical parameters and the importance of standard comprehensive assessment of novel materials and systems. Our work aims at providing a standardized protocol for all novel materials, structures, and configurations to enable objective assessment of the investigated new component and still enable us to benchmark the results with previous reports.

We micro-fabricated sensing electrodes using zinc oxide (ZnO) thin film and assessed their performance. We used a configuration with an extended gate field effect transistor (EGFET). Due to the delicacy of our aimed application, we rigorously studied the ZnO films’ reliability. As repeatability, stability, instrumentation input resistance, essential time plots for EGFET configuration, and inherent properties of the pH system components go unreported in many literary works, we experimentally demonstrated the importance of reporting of these parameters, and showed that not assessing them can lead to inaccurate results. Finally, we proposed a standardized protocol to be used as a convention for pH reports, using EGFET setup as an example, to help objectively assess pH materials and systems and benchmark different reports.

We chose ZnO as a sensing material due to its biocompatibility and facile fabrication process, along with a commercial glass reference electrode. We utilized the traditional open circuit potential (OCP) configuration as well as an EGFET configuration, which has the advantage of separating the electronics from the sensing parts of the sensors. We conducted experiments to extract the imperfections of the different setup components, and showed their significant impact on experimental results. Finally, because pH sensors’ full response does not saturate at a certain value (due to drifting), it’s important to agree on a convention for identifying a critical point at which the end of response is reached.

This research is a step towards establishing a standard protocol for reporting on novel pH systems and materials. It will help to increase the impact of pH sensing reports in terms of enabling objective comparisons between various materials and systems reported by different research groups. This, consequently, would make it feasible for the community to identify the best materials and systems for various applications.

In our paper, we report a series of essential characterization tests and parameters to objectively evaluate novel pH systems and demonstrate the pitfalls of using existing non-standardized methods from recent literature. In addition, we use commercial sensing and reference electrodes—i.e., more robust reliable components, and a parameter analyzer with varying shunt resistance to study the effect of instrument’s input resistance on the outputs of the pH system. Based on the experimental results, we provide recommendations for key parameters and methods to accurately report on pH sensing materials and systems.

Gaining insights into all the different factors that need to be reported and identifying the extent to which they can affect the results was challenging. For instance, input resistance of characterization device can significantly affect the outcome for OCP. On the other hand, experiments a few hours long do not exhibit significant shift in pH due to the dissolving carbon dioxide from the environment. Hence, reporting and accounting for the input resistance of the instrument used is critical while accounting for dissolved CO2 from ambient is not (and thus not part of the recommended protocol).

One remaining challenge is for the community to follow the recommended protocol and add other recommendations that would help make new reports more objective, and easier to reproduce and benchmark against previous and future works.

Our collaboration with grad student, Pedro De Souza and Professor Martin Bazant from the Department of Chemical Engineering and the Department of Mathematics at MIT has been very fruitful. We had insightful discussions throughout, integrating chemical, sensing, and microfabrication aspects of this work.

Identifying the proper protocol and following from discussions with Pedro De Souza and Professor Martin Bazant, we have laid the foundation for more objective studies to follow. For instance, by investigating the standard reduction potential value at the sensing electrode, one can hypothesize what reactions are taking place. Extracting the standard reduction potential values from various reported plots helped us understand why some thin films exhibit a more reliable performance than others. To this end, our next step is to use the reported protocol to compare common thin film materials based on the standard reduction-potential values, and to move forward towards biomedical applications once a suitable material is identified.

This work is an essential step towards a standardized pH sensing protocol that would enable accurate assessment and benchmarking of new materials. The results and discussions collectively lead to the recommended pH reporting protocol. In addition, the suggested evaluation of standard reduction potentials at the electrode surface can suggest specific pH electrode materials that would lead to stable and repeatable results. This would enable applications of pH sensors in more interesting and demanding fields, such as biomedical and implantable applications.