Protein-based nanoparticles are promising candidates to be used as nanomedicine due to their high biocompatibility. On the other hand, functional nanoagents are fabricated using complex, nonscalable, and environmentally unfriendly procedures. An urge to meet multiple biomedical and industrial requirements highlights a need to find a universal, eco-friendly, and flexible synthetic strategy of functional nanoagent engineering. Here, we show a green, ultrafacile, and size-controlled synthesis (swift thermal formation; STF) of monodisperse immunoglobulin-based nanoparticles. The synthesis allows the fabrication of hybrid nanoparticles as well. We show that the nanoparticles retain the specificity of immunoglobulins and can target malignant cells. In addition, we demonstrate that the nanoparticles are biocompatible and applicable in vivo. Tunability, ease, and sustainability of the synthesis may turn the targeted nanoagent design into a routine procedure. This ready-to-use solution could assist a broad spectrum of biomedical applications, e.g., imaging and therapy.