Platinum-based chemotherapy drugs have had an enormous impact on cancer treatment, but cancers often become resistant to these treatments through mechanisms which are still poorly understood. To clarify these issues, there is a need for sensors that can detect active cisplatin drug exposure. As optical sensors, single-walled carbon nanotubes (SWCNTs) have properties that make them ideally suited for detection of biologically important analytes. Carbon nanotubes exhibit bandgap fluorescence which does not photobleach or blink, and their fluorescent properties can be modulated by changes in their immediate environment. Tissue transparent emission, biocompatibility, and versatile modes of optical response make SWCNTs uniquely valuable optical biosensors. Recent work by our group has yielded insight into the mechanism of optical changes induced by analyte interaction with single-stranded DNA-wrapped SWCNTs. Using these findings, we designed SWCNT-based sensors for controllable red or blue-shifting responses to platinum-based chemotherapy drugs. We employed these sensors for the purpose of quantifying exposure of tissue to active drug in vivo.