Detection of Tuberculosis Volatile Organic Biomarkers Methyl Nicotinate and Methyl Phenyl Anisate Using Two-Step Anodized Titanium Dioxide Nanotube Arrays

Rapid screening of tuberculosis (TB) by evaluation of associated volatile organic compounds (VOCs) in breath has been shown to be a promising technology that is significantly faster and more convenient than traditional sputum culture tests. Methyl nicotinate and methyl p-anisate have been isolated as specific biomarkers for the mycobacterium tuberculosis, as they are not found in high concentrations in ambient air or the breath of healthy patients, but should be readily present in all patients with active TB. These VOCs can be detected at very low levels using a solid-state sensor based on metal-functionalized 3D titanium dioxide (TiO₂) nanotube arrays that bind the biomarkers at specifically applied voltages. The change in current when the sensor is exposed to the target VOC is caused by binding to the TiO₂ nanotubes. The characteristic large resistance of the nanotube arrays on the titanium surface is extremely important in measuring these small (on the order of µA) current changes. Therefore, the structure and consistency of the TiO₂ is extremely important and methods for its improvement are constantly explored. The method described here is a two-step anodization, an improvement on the currently used one-step method. Previously, the nanotube layer has been grown in a single step on a polished titanium surface using a single voltage over a prescribed amount of time. It was found that anodizing at a higher voltage grew longer, more brittle tubes of larger diameter, the bulk of which can be easily removed using a simple adhesive, leaving only the strong base layer behind. This provides a consistent template on which the second layer of nanotubes can be deposited. The resulting arrays appear more highly organized when examined under SEM than their one-step method counterparts, displaying a honeycomb effect with fused sidewalls. Mimic testing in the lab can be used to test a sensors response to a given biomarker at known concentration, in this case sensors functionalized with cobalt chloride allow binding of methyl nicotinate at a specific voltage. After testing, it was found that sensors anodized using the two-step method not only provided more consistent current readings over the entire face of the sensor, but also showed faster response to the biomarker. Consistency over the surface is especially important in point of care TB sensors. In addition, the stronger more organized arrays allow for a reduction in noise when measuring current.