Hydrogen bond sensing ability of CdSe/ZnS colloidal quantum dots in ionic medium

The potential of water-soluble CdSe/ZnS core-shell colloidal quantum dots (QDs) as sensors of hydrogen bond molecules for their electrolytes environment was explored by measuring the influence of the surrounding media refractive index on their absorption cross section. The hydrodynamic size, surface charge (zeta potential) and absorption cross section of three different sized QDs in three non-adsorbent media, namely NaCl, NaOH and phosphate buffered saline (PBS) are expressed in terms of concentration (polarity) and the optical properties of the surrounding media (refractive index). Three different models for the absorption cross-section of the non-adsorbent medium as a function of refractive index, namely, the ellipsoidal empty cavity (EEC), virtual cavity (VC) and empty cavity (EC) models, were compared using experimentally measured data. It is found that our experimental data do not fit well to the VC and EC models because (i) they do not include the depolarization factor of the nanoparticle, and (ii) these models assume a fixed geometrical constraint on the (spherical) shape of the cavity. On the other hand, the experimental data are well fitted using the EEC model which includes additional degree of freedom in the form of the depolarization factor L-mu. The type and polarity of the surrounding media has a strong influence on geometrical shape of the cavity models due to the presence or absence of hydrogen bonds in these media.