Novel IR820-polyethylene glycol-diamine nanoplexes (IR820-PDNCs) have potential multifunctional imaging-hyperthermia applications in cancer. Nanoplexes were formulated by ionic interaction and characterized in vitro for their imaging and hyperthermia capabilities. The resulting nanoplexes were approximately 50 nm diameter, with a zeta potential of 2.0 +/- 0.9 mV, and able to generate heat upon exposure to 808 nm laser. Cytotoxicity studies in SKOV-3, MES-SA and Dx5 cancer cell lines demonstrate comparable cytotoxicity of IR820-PDNCs versus free IR820 after 24 hours. The nanoplexes are able to produce hyperthermic cell growth inhibition in all three cancer cell lines after excitation with laser. The level of cell growth inhibition caused by hyperthermia is significantly higher for IR820-PDNCs compared to IR820 in MES-SA and Dx5 cells. Fluorescent microscope images after 2.5-hour exposure to 5 microM IR820-PDNCs or 5 microM free IR820 show increased uptake for IR820-PDNCs compared to free IR820, especially for SKOV-3 and Dx5 cancer cells. This formulation can potentially be used in multifunctional cancer theranostics.