pH Sensors, Biosensors and Systems

The study of biological systems for the protection of the environment, food, and health is among the most important research fields in the scientific literature. Many biological systems are sensitive even to small changes in chemo-physical parameters such as pH, temperature and humidity [1,2].

The concept of using the pH value to express the activity of hydrogen ions on a logarithmic scale dates back to 1909, and since then, many measurement methods have been developed [3]. Paper test strips and glass electrodes are the most used types of pH sensors, but the demand for less fragile, highly accurate, miniaturized devices that can be flexible and wearable has prompted research on alternatives [4]. Most of these pH sensors can be grouped into one of three macro areas, i.e., optical, electrochemical and field effect sensors, depending on the measurement method. Vivaldi et al. reviewed these areas in detail [5]. Optical pH sensors have good photo- and chemostability and are often used for non-invasive measurements, especially for biological tissues or cell cultures, e.g., for detecting tumor regions using pH-triggered contrast agents or the metabolic profiles of cancer lines [6,7]. Optical pH sensors are also popular for colorimetric or photometric analyses for food safety and control, e.g., using chromophores, which are suitable for continuous monitoring [8].

Electrochemical pH sensors measure the electrical potential in two- or three-electrode cells. These pH sensors can be micrometric, flexible, biocompatible, or even knitted into garments [9,10,11]. Many materials have been exploited to fabricate electrochemical pH sensors with different properties. Reduced graphene oxide has been investigated because it contains carboxylic and hydroxyl groups that can react with hydrogen ions [12,13,14]. Metal oxides have high sensitivity and fast responses but can also exhibit considerable drift and hysteresis [15,16]. Among metal oxides, iridium oxide-based microwires stand out, since they are fabricated with a near-Nernstian response and low drift. These microwires can be knitted, integrated into bandages or used for cell analysis [17,18]. Some polymers have ionizable acidic or basic groups and have thus been used as pH sensors [19,20]. Examples of acidic pH-responsive acidic polymers are polycarboxylic, polysulphonic and polyphosphoric acids, whereas for basic pH vinyl, (meth)acrylamide and (meth)acrylate polymers can be used. Polymers can suffer from reduced stability and degradation over time; however, the most recent formulations might have overcome these limitations. For example, Korostynska et al. reported a polypyrrole-based pH sensor with sensitivity of about 60 mV/pH and a 0.25 mV drift per day [21].

Although they belong to the category of electrochemical pH sensors, field effect transistors (FETs) have been so widely investigated that they can form another group. FETs can be mass-produced, have fast responses and can be miniaturized by means of micro-photolithography. The concentration of hydrogen ions is transduced during a change in the electrical current and depends on their interaction with the transistor gate [22,23]. However, this direct interaction can degrade the gate, since in the liquid solution, there are usually other species, such as ions or biological molecules. To overcome this drawback, new technical approaches have been employed. In particular, the dual gate approach consists of fabricating an additional gate that is the only surface exposed to the solution. Cho et al. and Hyun et al. proposed a coplanar-gate AlGaN/GaN FET and a dual-gate a-IGZO FET with an SnO₂ sensing membrane, respectively [24,25]. Both of these FETs achieved remarkable superNernstian sensitivities. Further improvements were achieved by Kim et al., who adopted a strategy for integrating p- and n-type FETs [26]. The p-type FET has lower noise than the n-type, whereas the n-type FET has higher mobility and lower drift than the p-type. An ambipolar FET showed a superNernstian sensitivity of about 170 mV/pH and a drift of 14 mV/h in the n region.

Although the glass electrode is the gold standard for pH measurement, the devices described in this Collection and in the scientific literature show that there is a constant interest in developing new approaches. This interest depends on the ubiquitous presence of pH as one of the main parameters that can control chemophysical reactions for the sensing and fabrication of sensitive surfaces [27,28,29,30]. Therefore, we would like to thank all of the authors who contributed to this Collection for submitting papers demonstrating the broad spectrum of pH sensors and providing new approaches that will pave the way towards further advances in this field.