A novel proof-of-concept nanosystem for the visualization of tumors generates bright tumor signals by delivering near-infrared quantum dots (QDs) to cancer cells without any toxic effects. The QDs are delivered intravenously into extravascular tumor tissue and cells to gain bright tumor signals. Some QDs leave the bloodstream and cross membranes, entering cancer cells. Fluorescent signals emitted from excess QDs that remain in the bloodstream are made invisible by injecting a membrane-impermeable etchant to eliminate background signals.