Sensitisers for various sensor technologies often owe their sensitivity and selectivity to their nanoscopic structure, either on their surface, or in bulk. Examples include zeolites [1,2], metal- organic frameworks [3], ion exchange resins [4], catalysts [5], composite nanoparticles [6], proteins [7], and aptamers [8]. In addition, sensors also rely on transducers to convert analyte/sensitiser association into a read- out. Transducers may be optical, e.g. [9,10], but often detect the change of electronic properties under analyte/sensitiser association. Transducers for sensors in the aqueous medium include AC capacitive [11] and amperometric methods, e.g. cyclic voltammetry (CV) and differential pulse voltammetry (DPV) [6]. However, potentiometric (voltage- sensitive) transducers based on field effect transistors (FETs) offer sensors with the lowest instrumental footprint. These were first introduced by Bergveld as the ‘ion- sensitive field effect transistors’ (ISFETs) [12]. Later, the ‘extended- gate field effect transistor’ (EGFET) [13] and the ‘water- gate thin film transistor’ (WGTFT) [14] designs joined the FET transducer ‘family’. We here review the classification of ISFET/EGFET/WGTFT [15], discuss their relative merits, and present examples from our work. Particular emphasis will be on our ‘agar- bridged’ EGFET concept that we consider the most convenient and flexible electronic transducer for the aqueous medium. Bridged EGFETs offer both, a universal sensitiser immobilisation strategy without covalent coupling, and DC potentiometric read- out. This ‘liberates’ nanosensor researchers from covalent immobilisation and optical or AC electronic read- out equipment, allowing focus on the preparation of highly selective sensitisers.

Dr. Martin Grell graduated with a PhD from the University of Darmstadt in 1994 and subsequently worked as a researcher at Bangor and Sheffield Universities, as well as the Max Planck Institute for Polymer Research. From September 2000 to August 2019, he served as an academic at the University of Sheffield, UK. His research focused on organic and other solution-processed electronic and photonic devices, particularly thin-film transistors and optical sensors. Since September 2019, he has been working as a consultant, currently collaborating with Riyadh, Taif, and Tabuk Universities in Saudi Arabia, and Chuka University, Kenya, where he also holds an external chair. His recent work is primarily focused on sensor devices, mainly in aqueous media