Water Permeates and Plasticizes Amorphous Carbon Dots: Unraveling the Inner Accessibility of the Nanoparticles by Glass Transition Studies

Carbon dots (CDs) are highly versatile nanomaterials with promising applications across catalysis, sensing, biotechnology, and optoelectronics. Yet, despite the widespread interest, their internal structure and permeability to solvents remain poorly understood. In this study, novel insights are provided into the plasticizing effect of water on amorphous CDs, addressing a critical gap in the current knowledge. A new class of polyamide-based CDs is synthesized via a microwave-assisted reaction using an amino acid and a small polyamine in water, systematically varying reaction times (3 - 60 min) to tailor the compactness of the carbon core. These findings reveal that water can permeate the internal structure of CDs and act as a plasticizer, significantly lowering their glass transition temperature, as shown by calorimetric analyses. Less compact CDs absorbed more water, while denser ones showed lower permeability. These trends are further corroborated by X-ray scattering data. This report clearly demonstrates that the carbonaceous core of CDs is accessible to water, challenging the assumption that the nanoparticles are structurally impermeable. These results lay the groundwork for new structural models of CDs and open exciting opportunities for diffusion studies, functional design, and targeted applications in nanoscience.