Rapid, point-of-care (POC) disease diagnostics presents several challenges including cost, detection time, equipment portability, and performance. Detection of volatile organic compounds (VOCs) based disease biomarkers from the breath provides a potential solution to the aforementioned problems. One such tuberculosis (TB) biomarker identified from breath is methyl nicotinate. In this regard, development of smart sensor systems can provide an economically feasible solution and pave the way for rapid disease diagnostics.

In this paper, we present the fabrication of a smart sensor developed by synthesis of self-organized TiO₂ nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) magnetron sputtering. Site-specific interdigitated patterned growth of gold electrode was achieved using photolithography techniques. The gold interdigitated electrodes behave as the working electrode while platinum deposited on the back surface of the Si wafer acts as the back contact. Au on TiO₂ presents a Schottky junction due to differences in their work function and can be overcome by light irradiation. On the other hand, an ultra-thin, inherent SiO₂ layer on Si aids in tunneling electrons across the TiO₂ – Si junction. Thus, Au/TiO₂/SiO₂/Si behaves as a Schottky tunneling diode junction that electronically couples a VOC oxidation catalyst (Au) to a photovoltaic absorber (Si).

A sweep of the bias voltage (-5V to 5V) and associated sensor response at room temperature upon exposure to both air and biomarker vapors (~1000 ppm) was carried out. From the current-voltage plot (I-V), the operating voltage was chosen overcoming the negative differential resistance such that the sensor current response to the biomarker vapor would be maximized under forward bias condition. The sensor current when exposed to the biomarker vapor was higher than that in air. All sensor measurements were carried out under illuminated conditions.

The variation in the output current of the sensor biased at the chosen operating voltage (~ 0.7V) at room temperature was observed for the range of 0.01 ppm to 1000 ppm biomarker concentration. The sensor demonstrated a strong response and the results suggest that the sensor is capable of detecting extremely small concentrations of the biomarker in the vapor phase. This smart sensor presents significant enhancement in sensor response over previously reported metal functionalized TiO₂ nanotube array based sensing platform developed in our group for electrochemical detection of similar disease biomarkers.