A method of detecting a condition in a subject comprises the steps of contacting cells of the subject with single-walled carbon nanotubes (SWCNTs), monitoring photoluminescence emitted by SWCNTs internalized into the cells and generating an SWCNT emission profile, comparing the SWCNT emission profile to a control emission profile for the SWCNTs to produce a result, and determining a likelihood of having the condition in the subject based on the result from the comparing step. Also disclosed is a method for screening agents capable of changing endocytic environment using SWCNTs.