Abstract
This PhD thesis focuses on the development and evaluation of transistor-based biosensors for the detection of ammonium in the sweat, covering every aspect from design and fabrication to electrical characterization, as well as functionalization with specific bio-recognition elements (such as ion-selective membranes) to achieve the desired biosensor sensitivity and selectivity to the analyte of interest. Due to the advantages offered by nanomaterials, especially in terms of increasing overall biosensor sensitivity by amplification of their conductivity and catalytic activity, a significant part of this PhD work focuses on carbon nanotubes (CNTs), in terms of ink preparation and integration into the biosensing platform of choice. The first part of this research work, summarized in Chapter 5 and 6, focuses on the fabrication of the transducing platform. Planar electrolyte-gated field-effect transistors (EG-FETs) were chosen due to their manifold advantages, such as large specific capacitance, low operating voltage, and intrinsic signal amplification. In particular, semiconducting CNTs were employed as electroactive material. The fabrication of the carbon nanotube EG-FET (EG-CNTFET)-based biosensors was carried out by means of microfabrication and printing. In this respect, the first part of the work focuses on the design, development, and optimization of the fabrication protocol, including the photolithographic process for the deposition of the electrodes, the preparation and the spray deposition of the CNT ink. EG-CNTFETs were first fabricated on standard rigid Si/SiO2 substrates and then on flexible polyimide (EG-CNTFET) foils. As proof of concept, the detection of the NH+ 4 ions in water with flexible EG-CNTFETs functionalized with nonactin ion-selective membrane was demonstrated: the calibration curve of the fabricated sensors showed a linear detection range for ammonium from 0.01 mM to 10 mM, covering the entire range of physiological concentrations of interest (0.12 mM to 2.17 mM), with an average sensitivity of 0.346 µA/decade. Moreover, the extensive literature research carried out during this first part of the PhD work led to the publication of an invited and featured review paper in the Applied Physics Reviews journal, which covers the working principle, the fabrication techniques, the bio-functionalization strategies, the present issues, and the challenges faced for the EG-CNTFET-based biosensors. The second part of this PhD work focuses on a thorough step-by-step evaluation of the fabrication protocol of the developed EG-CNTFET-based NH+ 4 sensors, and it is described in 7. Different fabrication workflows were compared, to investigate the impact of the conditioning step of the NH+ 4 -selective membrane, as well as the composition of the membrane cocktail, on the sensing properties of the devices. A facile, and reliable data analysis protocol to obtain a highly stable baseline response (i.e., 60 min), which is required for reliable sensing applications, was established. The fabricated EG-CNTFET-based sensors were successfully employed for the detection of NH+ 4 in complex artificial sweat medium, which was used to better mimic the real working conditions of the proposed sensors. Conditioning the membranes in artificial sweat significantly reduced the variability of the sensors, in accordance with the reported literature for other classes of sensors employing ion-selective membranes. Sensitivity as high as 1.797 µA/decade was achieved, with the linear range of the sensors entirely covering the physiological range of concentrations of NH+ 4 in sweat as reported in the literature. In an attempt to further improve the sensitivity, the concentration of the nonactin in the membrane cocktail was increased (from 0.2 %wt to 1 %wt), leading to an improvement of the sensors’ sensitivity by a factor of 2, which was however not enough to justify the extra cost derived from the higher quantity of material employed. Additionally, a novel design for the flexible and planar EG-CNTFET-based sensors was proposed, with a U-shaped gate electrode instead of the well-established square-shaped gate electrode. The devices with the new U-shaped design showed a more than double sensitivity and a 55 % reduction in the variability. The devices with the U-shaped gate electrode benefited from the higher uniformity of the applied gate-source voltage VGS throughout the active area of the device. These results are summarized in Chapter 8. The final part of this PhD work, described in Chapter 9, is dedicated to the design of a flexible readout circuitry for the characterization of the EG-CNTFET-based sensors. Starting from the electrical specifications of such sensors, the focus was centered on enabling low-power operation with a single coin-cell battery with compact wireless data transmission. To maximize the bendability of the flexible printed circuit board (PCB), particular attention was taken in layout routing as well as in selecting small-sized packages for the commercial integrated circuits and all-in-one systems such as programmable Bluetooth system-on-chip. A high sensitivity of 4.516 µA/decade was recorded for sweat ammonium level analysis between 0.1 mM to 100 mM. The characterization was carried out with the introduced front-end readout, and the results were benchmarked with a "gold standard" instrument, showing good and reliable performance of the developed fully flexible bioelectronic system. This first working prototype proved the feasibility of a holistic approach, in which sensors and circuitry are fabricated using the same flexible substrate.