Biocompatible Aluminum-Doped Graphene Quantum Dots for Dual-Modal Fluorescence and Ultrasound Imaging Applications

Aluminum salts are crucial ingredients in vaccine formulations. These salts work as adjuvants, enhancing the immune system's response to an antigen. However, aluminum salt dosages have been limited due to their possible side effects associated with neurological disorders such as Alzheimer's, Parkinson's, and multiple sclerosis. Current vaccines also lack imaging modalities with high spatiotemporal resolution needed to visualize the dynamic response of the immune system at the vaccination site and nearby lymph nodes. To address these shortcomings, in this work, we have developed highly biocompatible aluminum-doped graphene quantum dots (Al-GQDs) as potential adjuvant delivery vehicles with dual-mode fluorescence and ultrasound imaging capabilities. Nanometer-sized (<5 nm) Al-GQDs are biocompatible (up to 80% cell viability) in HeK293 cells at a staggeringly high concentration of 11.2 mg/mL, which is uncommon for nanomaterials. Their structural and chemical features assessed via the FTIR and EDS include a graphitic core with functional groups that are amenable to covalent and noncovalent bonding with molecules for therapeutic applications. Moreover, they also exhibit fluorescence in the visible and near-infrared regions with a pH-dependent response suitable for cellular environment pH sensing. The fluorescence and ultrasound tracking capabilities of Al-GQDs are demonstrated in vitro and in tissue phantoms (agarose gel, vascular phantom, and animal tissue). Al-GQDs internalize and accumulate at a maximum of 24 h in cells and enhance the ultrasound signal in biological environments. Given their pH-dependent fluorescence and ultrasound imaging capabilities, biocompatible Al-GQDs developed for the first time in this work present a unique alternative to current Al-based adjuvants, enabling high-depth ultrasound and high-precision fluorescence tracking as well as sensing and delivery capabilities. Such a multifunctional adjuvant-inspired system is intended to further complement and aid rapidly advancing vaccine research and development.