The inventors experimentally demonstrated NO2 gas sensing performance of multilayer black phosphorous (BP) field effect transistors. The BP sensors were sensitive to NO2 concentration down to 5 ppb making them comparable in sensitivity to the best 2D material based sensors. Raman spectroscopy comparison revealed no apparent change in the spectra before and after exposure to NO2, which shows that thick BP flakes can maintain their relative stability after sensing. Moreover, the BP device sensing performance fitted well with the Langmuir Isotherm for molecules adsorbed on a surface, which confirms charge transfer as the dominant mechanism for sensing. The systematic increase in conductance with increasing NO2 concentrations suggests NO2 molecules withdraw electrons and dope BP flakes with holes. These results lay the ground work for BP to be applied to various sensing applications including chemical, gas, and bio-sensors.