In some embodiments, the present invention pertains to carbon nanotube fibers that include one or more fiber threads. In some embodiments, the fiber threads include doped multi-walled carbon nanotubes, such as doped double-walled carbon nanotubes. In some embodiments, the carbon nanotubes are functionalized with one or more functional groups. In some embodiments, the carbon nanotube fibers are doped with various dopants, such as iodine and antimony pentafluoride. In various embodiments, the carbon nanotube fibers of the present invention can include a plurality of intertwined fiber threads that are twisted in a parallel configuration with one another. In some embodiments, the carbon nanotube fibers include a plurality of fiber threads that are tied to one another in a serial configuration. In some embodiments, the carbon nanotube fibers of the present invention are also coated with one or more polymers. Additional embodiments of the present invention pertain to methods of making the aforementioned carbon nanotube fibers.