Tuning the deposition parameters for optimizing the faradaic and non-faradaic electrochemical performance of nanowire array-shaped ITO electrodes prepared by electron beam evaporation.

Nanostructured indium tin oxide (ITO) surfaces present an interesting yet unusual combination of properties (high electrical conductivity and optical transparency) at a high surface-to-volume ratio. Thus, previous studies presented nanostructured ITO electrodes as potentially suitable platforms for electrochemical biosensors, but still there is a lack of research on the optimization of preparation methods for such electrodes. We present a systematic study on the properties of nanostructured ITO electrodes prepared by physical deposition, where the substrate temperature was tuned for achieving the best combination of structural properties (namely electrical conductivity and optical transparency) and electrochemical performance. Analysis of faradaic cyclic voltammetry (CV) was performed to determine the electroactive surface area of the samples, and these results were benchmarked against those obtained by non-faradaic CV and Mott-Schottky (MS) analysis. The latter was useful to determine the dependence of some intrinsic features of the semiconductor on the substrate temperature during deposition. The results show that, out of a wide temperature range covering from 200 °C to 500 °C, there is a two-phase temperature-dependent growth, explained by the Stranski-Krastanov and self-catalytic vapor-liquid-solid (VLS) methods, and, on the other hand, that there is an optimal growth temperature at 300 °C that maximizes the electroactive surface area and sensitivity. This means that cost-effective electrodes can be prepared at low temperatures outperforming in terms of electroactive surface area, surface capacitance and sensitivity. As a proof-of-concept, nanostructured ITO electrodes were electrochemically derivatized with aryl diazonium salts (as a first step towards biochemical functionalization), and the performance of the optimized electrodes was tested in a real scenario.

A low-cost and miniaturized potentiostat for sensing of biomolecular species such as TNF-α by electrochemical impedance spectroscopy.

Miniaturizing potentiostats, keeping their cost low and yet preserving full measurement characteristics (e.g. bandwidth, determination of capacitive/inductive contribution to sensor's impedance and parallel screening) is still an unresolved challenge in bioelectronics. In this work, the combination of simple analogue circuitry together with powerful microcontrollers and a digital filter implementation is presented as an alternative to complex and incomplete architectures reported in the literature. A low-cost acquisition electronic system fully integrated with a biosensors platform containing eight gold working microelectrodes and integrated reference and counter electrodes was developed and validated. The manufacturing cost of the prototype was kept below 300 USD. The performance of the proposed device was benchmarked against a commercial impedance analyzer through the electrochemical analysis of a highly sensitive biosensor for the detection of tumor necrosis factor α (TNF-α) within the randomly chosen range of 266pg/mL to 666ng/mL in physiological medium (PBS). A strong correlation between the outputs of both devices was found in a critical range of frequencies (1-10Hz), and several TNF-α cytokine concentrations were properly discriminated. These results are very promising for the development of low-cost, portable and miniaturized electrochemical systems for point-of-care and environmental diagnosis.