Single-walled carbon nanotubes exhibit photostable near-infrared photoluminescence with excitation and emission energies that respond to fluctuations in the micro-environment. In the creation of robust nanotube-based optical sensors, it is desirable to identify shifts in excitation and emission of an ensemble nanotube sample (HiPco) in response to analytes. Here, we constructed a novel instrument to rapidly acquire two-dimensional excitation/emission photoluminescence data from nanotubes bound to the surfaces of live mammalian cells. We found that the process of contacting the surface of the cell induced a red-shift in nanotube emission with magnitudes that varied by cell-type. Removal of membrane proteins further red-shifted the nanotube sample to a common value independent of cell-type. The magnitudes of shifts due to the membrane proteins were correlated to the whole cell electrostatic potential and could be recapitulated in ionic solutions. These findings may enable the optical measurement of surface electrostatic potentials for biophysical measurements and biomedical applications.