Hierarchical in Situ functionalized TiO2 Nanotubular Substrate for Sensing Volatile Organic Compounds Associated with Tuberculosis
Authors: Saffary Yalda , Willis Christina Nicole , Misra Mano , Mohanty Swomitra K. 
Published: February 2020

Mycobacterium tuberculosis (MTB) is a devastating disease that effects over 10 million people worldwide with approximately 1.8 million deaths1. One major issue contributing to this wide spread disease is lack of low cost point-of-care methods for screening/diagnosis of the disease. One method that has gained a significant amount of attention is the use of volatile organic compounds (VOCs) found in the breath of patients as a biomarker for the disease. These VOCs include methyl nicotinate and methyl p-anisate which are unique metabolic byproducts of the microorganism.
To take advantage of the presence of these VOCs, a hierarchical in situ functionalized TiO2 nanotubular substrate has been synthesized to specifically detect methyl nicotinate and methyl p-anisate in a complex sample. 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 a specifically applied voltage. When a VOC of interest approaches the sensing substrate, it binds preferentially with the metal on the surface of the TiO2 nanotubes causing a large change in current (from nanoamp to microamp range). Our previous research has shown that a TiO2 nanotube array substrate functionalized with cobalt using wet incipient methods is capable of specifically detecting the MTB VOCs2-4. However, the sensor performance in this prior work, while promising, can be improved in terms of reliability, response time, and signal to noise ratio. The structure and consistency of the TiO2 sensing substrate is extremely important and methods for its improvement are constantly explored.
In this work, we improve the MTB VOC sensors response time and accuracy by using a two-step method in the anodization process to create a hierarchical in situ functionalized TiO2 nanotubular substrate. In the first step, titanium metal was anodized for 30 minutes at 60V. In the second step the substrate was anodized at 30 V for 60 minutes. This process results in a hierarchical nanostructure composed of nanotubular and honey comb shapes (Figure 1). The smaller nanotubes inside each honeycomb shape structure are the result of anodizing at lower voltage. The two-step method of anodizing makes highly organized interconnected nanotubes and allows for improved sensor reliability when measuring current. In addition, we can improve our sensor respond to methyl nicotinate and methyl p-anisate by functionalizing the sensor while anodizing it (in situ functionalization). In this work cobalt was added to the anodizing solution resulting in the cobalt metal being integrated into the hierarchical nanostructure. The advantage of this approach is the reduction in sensor synthesis steps when compared to our previous work, and improved sensor response. Detailed analysis of nanostructured synthesis and sensor performance will be presented.
1. Tuberculosis (TB) Data and Statistics. https://www.cdc.gov/tb/statistics/ Accessed 12/16/2016, 2016.
2. Bhattacharyya D, Smith YR, Misra M, Mohanty SK. Electrochemical detection of methyl nicotinate biomarker using functionalized anodized titania nanotube arrays. Materials Research Express 2015;2(2):025002.
3. Bhattacharyya D, Smith YR, Mohanty SK, Misra M. Titania Nanotube Array Sensor for Electrochemical Detection of Four Predominate Tuberculosis Volatile Biomarkers. Journal of The Electrochemical Society 2016;163(6):B206-B214.
4. Smith YR, Bhattacharyya D, Mohanty SK, Misra M. Anodic functionalization of Titania nanotube arrays for the electrochemical detection of tuberculosis biomarker vapors. Journal of The Electrochemical Society 2016;163(3):B83-B89.

Figure 1