Cancer is one of the leading causes of death worldwide and represents a significant burden on global health systems. Many existing chemotherapy treatments come with severe side effects, ranging from hair loss to cardiotoxicity, and many types of cancer express chemotherapy resistance, such as triple-negative breast cancer. This study presents a novel boron/nitrogen-doped carbon nano-onion (BN-CNO) based nanocarrier system that can deliver doxorubicin (DOX) to cancer cells via a pH-dependent drug release mechanism. The nanocarrier formulation consists of a hyaluronic acid/phospholipid conjugate (HA-DMPE) that is non-covalently bound to the BN-CNOs upon which DOX is loaded via π–π stacking interactions. The HA-DMPE/BN-CNO/DOX system enhances the uptake and anticancer effects of DOX in MDA-MB-468 and MDA-MB-231 TNBC cells whilst reducing the cardiotoxicity of DOX in AC-16 human cardiomyocytes.