Provided herein are methods off functionalizing a carbon nanotube, functionalized carbon nanotubes, methods of forming a suspension, and methods of forming a sensor. The methods may include contacting one or more carbon nanotubes with a dienophile in the presence of a supercritical fluid to form one or more functionalized carbon nanotubes. The one or more functionalized carbon nanotubes may have a degree of functionalization of about 1% to about 5%.