Gold nanoparticles are conjugated to phosphatidylserine-specific ligands for targeting and binding to surface-exposed phosphatidylserine on tumor cells and tumor vasculature. The ligand may be an annexin (e.g., annexin V). Tumor contrast is significantly increased using the targeted gold nanoparticles. Breast cancer tumors as small as 4 mm, for example, were detectable via computed tomography (CT) within 4 hours after injection of the conjugates, demonstrating usefulness of the conjugates as imaging agents. The targeted gold nanoparticle conjugate may further have a drug conjugated thereto that can be used therapeutically, for example, for cancer treatment. The gold nanoparticle conjugates can also be used for photothermal therapy and can be used in concert with an X-ray radiation treatment for cancer treatment.