Functionalized nanotube arrays, sensors, and related methods of detecting target compounds are presented. A functionalized nanotube array can include a plurality of metal oxide nanotubes. The metal oxide nanotubes can be formed of a metal oxide and can have an interior or exterior surface that is optionally functionalized with at least one metal ion. These metal nanotubes can be used in a sensor for detecting target compounds such as volatile organic compounds, and biomarkers in a fluid environment. The sensor can further include a power source configured to apply a voltage to the nanotube array and a current sensor configured to monitor and detect changes in a response current which varies upon binding with the target compounds.