Is black iron oxide nanoparticle always a light absorber?

Here we report the presence of an unexpected fluorescence enhancement zone (E-zone) in the mechanically mixed system of superparamagnetic iron oxide nanoparticles (SPIONs) and upconversion fluorescent nanoparticles (UCNPs). This new finding differs from the traditional consideration that black SPIONs are always strong light absorbers. Our study also showed that the quenching behavior of UCNPs in the quenching zone (Q-zone) by SPIONs is surface state-dependent, with a lower quenching effect when UCNPs was shielded with a hydrophilic silica shell.