Toxic element-free quantum dots as nanocarriers in drug delivery: the role of functional ligands in hybrid systems development

Abstract

Among numerous types of nanocarriers which can be used as drug delivery systems, such as polymeric, solid lipid and metal nanoparticles, colloidal nanocrystals of inorganic semiconductors (quantum dots, QDs) play an important role. QDs are, in fact, hybrid nanoobjects consisting of an inorganic core capped with organic ligands inducing their colloidal stability. This structure endows QDs exhibiting properties unmatched by other types of nanomaterials. For example, hydrophobic nanocrystals can be rendered hydrophilic by exchanging the primary hydrophobic ligands for hydrophilic ones, yielding colloidally stable aqueous dispersions. The introduced ligands can be further functionalized by covalent grafting of drug molecules or by exploiting non-covalent interactions between the drug and the surfacial ligand. Other bioactive molecules can also be attached to the surface of nanocrystals, so-called “targeting ligands", ensuring efficient transport of drugs to their final destination. Over the last 15 years, a number of synthetic procedures have been developed for the preparation of nanocrystals free of toxic elements that emit not only in the NIR-I biological window (650-950 nm), but also in the NIR-II window (1000-1400 nm). More recently, carbon dots (CDs) have emerged as a class of nanomaterials that, in certain aspects such as size range and size-dependent photoluminescence, exhibit strong similarities to inorganic semiconductor quantum dots (QDs). The aim of this short review is to critically evaluate these two types of nanomaterials, with particular emphasis on their functionalization for application as drug nanocarriers.