Size-dependent penetration of carbon dots inside the ferritin nanocages: evidence for the quantum confinement effect in carbon dots

The origin of the excitation wavelength (λex)-dependent photoluminescence (PL) of carbon dots (CDs) is poorly understood and still remains obscured. This phenomenon is often explained on the basis of surface trap/defect states, while the effect of quantum confinement is highly neglected in the literature. Here, we have shown that the λex-dependent PL of CDs is mainly due to the inhomogeneous size distribution. We have demonstrated the λex-dependent PL quenching of CDs inside the ferritin nanocages through selective optical excitation of differently sized CDs. It has been observed that Fe3+ ions of ferritin effectively quench the PL of CDs due to static electron transfer, which is driven by favorable electrostatic interactions. However, control experiment with aqueous Fe3+ ions in bulk medium revealed λex-independent PL quenching of CDs. The λex-dependent PL quenching of CDs by Fe3+ ions of ferritin has been rationalized on the basis of a different extent of accessibility of Fe3+ ions by differently sized CDs through the funnel-shaped ferritin channels. PL microscopy of individual CDs has been performed to get further information about their inherent PL properties at single dot resolution. Our results have shown that these hydrophilic CDs can be used as potential iron sensors in biological macromolecules.